forensic stable isotope biogeochemistry · stable isotopes, forensic, isoscape, geochemistry,...

EA44CH08-Cerling ARI 17 May 2016 15:41

Forensic Stable IsotopeBiogeochemistryThure E. Cerling,1,2,3 Janet E. Barnette,3

Gabriel J. Bowen,1 Lesley A. Chesson,3

James R. Ehleringer,2,3 Christopher H. Remien,4

Patrick Shea,2 Brett J. Tipple,2,3 and Jason B. West5

1Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112;email: [email protected] of Biology, University of Utah, Salt Lake City, Utah 841123IsoForensics, Salt Lake City, Utah 841084Department of Mathematics, University of Idaho, Moscow, Idaho 838445Department of Ecosystem Science and Management, Texas A&M University,College Station, Texas 77843

Annu. Rev. Earth Planet. Sci. 2016. 44:175–206

First published online as a Review in Advance onApril 27, 2016

The Annual Review of Earth and Planetary Sciences isonline at earth.annualreviews.org

This article’s doi:10.1146/annurev-earth-060115-012303

Copyright © 2016 by Annual Reviews.All rights reserved

Keywords

stable isotopes, forensic, isoscape, geochemistry, ecology

Abstract

Stable isotopes are being used for forensic science studies, with applicationsto both natural and manufactured products. In this review we discuss howscientific evidence can be used in the legal context and where the scientificprogress of hypothesis revisions can be in tension with the legal expectationsof widely used methods for measurements. Although this review is written inthe context of US law, many of the considerations of scientific reproducibil-ity and acceptance of relevant scientific data span other legal systems thatmight apply different legal principles and therefore reach different conclu-sions. Stable isotopes are used in legal situations for comparing samples forauthenticity or evidentiary considerations, in understanding trade patternsof illegal materials, and in understanding the origins of unknown decedents.Isotope evidence is particularly useful when considered in the broad frame-work of physiochemical processes and in recognizing regional to global pat-terns found in many materials, including foods and food products, drugs,and humans. Stable isotopes considered in the larger spatial context add animportant dimension to forensic science.

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ANNUAL REVIEWS Further

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http://www.annualreviews.org/doi/full/10.1146/annurev-earth-060115-012303


1. INTRODUCTION

Forensic science uses scientific knowledge to analyze physical evidence. Stable isotopes are wellsuited to this application for several reasons: First, there are large ranges in isotope ratios of bothnatural and manufactured materials; second, stable isotopes provide a quantitative and repeat-able measurement when analyzed independently in different laboratories; and third, many well-understood natural processes cause isotopes to be separated between two different substances.Therefore, stable isotopes can provide additional evidence as to the origin of material of forensicinterest and thus may be an important part of a forensic investigation.

There is a natural tension between how science and law are conducted. In the sciences, everyconclusion is subject to future review and evidence is constantly being reevaluated. In contrast, thelegal method or process is a one-time matter. That is, a judge or jury decides guilt or innocence—one side wins, and one side loses. Although there is a right of appeal in many legal systems, thatappeal is limited to the record established at trial within the US legal system. The tension betweenthe scientific method and the legal method/process thus reflects the need for both judges andlawyers to understand the utility of using stable isotopes for analyzing physical evidence, andthe need for scientists to understand how evidence is introduced and used in the judicial system(Ehleringer & Matheson 2010).

We note that the following text focuses primarily on the use of stable isotope biogeochemistryin US law. Many of the analytical techniques discussed are independent of location and may beapplicable to forensic science investigations in other countries and regions of the world, but thelegal acceptance and consequences of those techniques can vary. For some types of investigations,stable isotope biogeochemistry has been more widely utilized outside the United States than withinthe US legal system. For example, suspected violations of food and beverage regulations in theEuropean Union—related to labeling and source—have been investigated using stable isotopesfor many years (Rossmann et al. 1996, Gonzalvez et al. 2009). This is not the case in the UnitedStates, where food regulations are generally much less strict than those in the European Union.

A review of the state of forensic science in the United States, conducted by the NationalResearch Council (NRC) of the US National Academy of Sciences (NRC 2009), suggested that theforensic field is in need of improvement: The lack of standardization in training and methodologiesworks against the goals of a unified scientific profession. The goals of scientists to constantly changeand improve analytical methods contrast with the needs of the judicial system for widely acceptedanalytical methods adopted by all laboratories. Several recommendations of the NRC study areparticularly relevant for isotope biogeochemistry because of the long traditions in cross-laboratoryvalidation, in understanding the chemical and physical basis for isotope ratio separation, and inthe technical development of methodologies that are published in peer-reviewed journals.

The basic question is this: What is the most reliable (i.e., predictable) means of ascertainingthe truth regarding conflicting testimony as to facts in dispute between paid, dueling scientists?Under the Daubert standard (see the sidebar The Frye and Daubert Standards for Admittance ofEvidence in the US Court System), the US Supreme Court implicitly endorsed the six-centurycommon law tradition of the judge, jury, or both being the ones to definitively weigh the evidenceand make their one-time decision. Scientists will always have a chance to revise their hypothesiswith later experiments and measurements to be reviewed by their peers, but the trier of fact in thelegal system has only one, time-specific chance for a decision.

In recent years there has been a profusion of stable isotope publications that are relevant toforensic science. Table 1 shows some of the studies that have been used to characterize a variety offoods, drugs, synthetic materials, art and antiquities, and other materials of forensic interest. We

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THE FRYE AND DAUBERT STANDARDS FOR ADMITTANCE OF EVIDENCE IN THEUS COURT SYSTEM

For nearly 100 years the US legal system has used a legal precedent in the case of Frye v. United States (1923) torecognize scientific evidence. Frye was a criminal defendant convicted of murder. The trial court had refused toallow the defendant’s counsel to use a scientific expert to testify about the results of a so-called deception test madeupon the defendant; the test purported to show an increase in systolic blood pressure if the person being tested hadlied (this was an early stage of the later polygraph test). The US Supreme Court’s decision in Frye provided thatexpert witness opinion was admissible when the method had “general acceptance within the scientific community.”

Through the twentieth century the application of the Frye standard became increasingly difficult (i.e., whatconstitutes general acceptance?) as more methods were developed; in particular, the question of how to distinguishbetween “science” and “junk science” in the confines of the courtroom increasingly became a problem. Currently, inthe United States, the US Supreme Court decision in the case of Daubert v. Merrell Dow Pharmaceuticals (1993) placesjudges in the role of gatekeepers for admission of scientific evidence: They should consider the acceptance of thescientific approach, including peer review, and whether standards exist for data evaluation (Bell 2009). Ehleringer& Matheson (2010) reviewed the relevant criteria for using stable isotopes in the legal framework for considerationin US courts. The unspoken but operational premise of Daubert is the trial judge will be able to ascertain theprobative value of the expert’s testimony and will then allow the trier of fact, be it the judge or the jury, to weightthe competing (some say “dueling”) experts. The difference between the Frye standard and the Daubert standard isunder the Frye standard the judge determines whether the science is generally accepted, whereas under the currentprecedent the gatekeeper (the judge) decides whether there is probative value to the expert scientific testimony,either for the judge or the jury as the trier of facts.

A recent case in the state of California—City of Pomona v. SQM North America Corp. (2014)—highlights difficultieswith the Daubert standard. In this case stable isotopes were a key part of the argument in establishing the source ofperchlorate contamination in groundwater (Sturchio et al. 2014, Bennett 2015, Sturchio 2015). The lower districtcourt excluded expert testimony indicating in a Daubert hearing (a separate hearing from the trial to determine thequalifications of the expert and whether said expert testimony would have probative value on facts in dispute at trial)that the evidence the expert for Pomona would offer was not generally accepted and would not have probative value.Pomona appealed the trial court’s ruling to the Ninth Circuit Court of Appeals. The appeals court reversed the trialcourt’s decision, determining that it should have been up to the jury to decide the strength of the arguments andthe credibility of the expert testimony. SQM North America Corp. appealed the Ninth Circuit Court of Appealsdecision to the Supreme Court, which declined to consider the case in 2014. Many legal and scientific observerscommented such a hearing could have provided clear direction in the application of the Daubert standard in futurecases involving scientific expert testimony in the United States. The scientific evidence in Pomona v. SQM NorthAmerica Corp. is discussed in Section 4.1.

do not summarize in detail all forensic applications of stable isotopes, as several comprehensivereviews have been published on the topic.

One common theme in the food studies in Table 1 is to show how “authentics” can be distin-guished from “adulterated” products; these methods use simple mixing models between presumedend-members, well-established relationships between sources and products, and well-known rela-tionships associated with the global meteoric water line (GML). A larger biological theme focuseson predictions of isotope distributions across the planet, giving a spatial dimension to isotope foren-sic studies. The use of stable isotopes in forensic studies has evolved from the simple comparison oftwo samples to a robust framework whereby predictive power can be given to results by obtaining

www.annualreviews.org • Forensic Stable Isotope Biogeochemistry 177

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Table 1 Some representative applications of isotopes to forensic studies

Application Reference Principal finding(s)

Art and antiquities

Alabaster works Kloppmann et al. 2014 Tracing the origin of Medieval and Renaissance alabasters

Archaeological applications Degryse & Braekmans 2014 Ceramics and glass

Greek and Roman marbles Herz & Garrison 2014 δ13C and δ18O of marbles from Mediterranean regions

Religious antiquities Ayalon et al. 2014 St. James ossuary and other religious artifactsFood and drink

Beer Brooks et al. 2002 Detection of C4 carbon used in beer production

Carter et al. 2015a Authenticity of bottled and canned beers available in Australia

Beer and soft drinks Chesson et al. 2010c Spatial “footprint” of bottled water, soda, and beer in thecontinental United States

Beverages Calderone & Guillou 2008 Distinguishing the illegal watering of beverages

Bottled waters Bowen et al. 2005b Global survey of bottled waters (δD and δ18O)

Montgomery et al. 2006 Survey of British bottled waters (87Sr/86Sr)

Redondo & Yelamos 2005 Distinguishing natural versus industrial sources of CO2 insparkling waters

Cheese Camin et al. 2004 Characterization of European cheeses using multi-isotopetechniques

Stevenson et al. 2015 Provenancing cheese using stable and radiogenic isotopes

Ciders Carter et al. 2015b Authenticity of European and Australasian ciders

Fast foods Chesson et al. 2008 “Local” versus “national” foods in fast food restaurants

Honey: beeswax Chesson et al. 2010a Geographic variation in beeswax δD

Honey: bulk Doner & White 1977 13C/12C to detect addition of high-fructose corn syrup tohoney

White & Doner 1978 Detection of corn-derived syrups in honey

Honey: compound specific Tipple et al. 2013 Geographic variation in beeswax n-alkane δD values

Juices Dunbar & Wilson 1983 Juices compared with local meteoric water

Milk Chesson et al. 2010b δD and δ18O in milk in the continental United States

Crittenden et al. 2007 Geographic origin assignment of Australasian milk usingmultiple isotopes

Olive oil Camin et al. 2009 Origin of European olive oils

Angerosa et al. 1999 Geographic origin of olive oils

Orange juice Bricout 1973 Authenticity of fruit juices

Bricout & Mouzae 1971 Distinguishing natural juice from diluted concentrate

Bricout et al. 1972 δD and δ18O in orange juices

Doner & Bills 1981 Baseline to establish addition of corn or cane sugar to orangejuice

Rummel et al. 2010 Geographic origin assignment using multi-isotope techniques

Protected origin of foods Gonzalvez et al. 2009 Discrimination of foods with protected designation of origin

Vanilla Bricout et al. 1974 Distinguishing natural from synthetic vanilla

Hansen et al. 2014 δD and δ13C in vanilla for authentication

Whiskey: Scotch Meier-Augenstein et al. 2012 Detection of counterfeit Scotch whisky

(Continued )

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Table 1 (Continued )


Wine Marchionni et al. 2013 Sr isotopes for Cesanese wine area

Martinelli et al. 2003 Detection of C4 carbon used in sparkling wine production

Rossmann et al. 1996 δ13C in ethanol of European Community data bank wines

Rossmann et al. 1999 δ18O in EU data bank wines

West et al. 2007 Isoscape of wines from the western United States

Breas et al. 1994 Wines from different European countriesIllegal drugs/poisons

Anthrax Kreuzer-Martin et al. 2003 δD and δ18O in Bacillus spores for geolocation

Cocaine Ehleringer et al. 2012 Time between coca plant harvest and cocaine seizure using14C

Ehleringer et al. 2000 Geographic origin of cocaine using stable isotopes

Cocaine and heroin Ehleringer et al. 1999 Geolocation of cocaine and heroin using stable isotopes

Cyanide Kreuzer et al. 2012 Characterization of sodium and potassium cyanide (δ13C andδ15N)

Tea et al. 2012 Geographic origin of cyanide using stable isotopes

Heroin Idoine et al. 2005 Isotopic characterization of heroin and cling film wrappings

Marijuana Booth et al. 2010 Geographic patterns of isotopes in marijuana grown in Alaska

Hurley et al. 2010a Geographic origin and cultivation conditions of marijuanaseized in the United States

Hurley et al. 2010b Tracing retail cannabis in the United States

Liu et al. 1979 Growth conditions of marijuana

Muccio et al. 2011 Compound-specific analysis used on cannabis

Shibuya et al. 2006 Sources of marijuana seized in Brazil

West et al. 2009 Growth conditions of marijuana

Ricin Kreuzer et al. 2013 Stable isotopes in ricin preparationsMiscellaneous

Inks Chesson et al. 2015 Inks in questioned documents

Paints Farmer et al. 2009 Isotope survey of white paints

Reidy et al. 2005 δ13C of white architectural paints

Paper Jones et al. 2013 Isotope survey of office paper

Plastic bags Taylor et al. 2008 Use of stable isotopes for examination of grip-seal plastic bags

Tape Carter et al. 2004 Isotope survey of packaging tapes

Dietz et al. 2012 Isotope survey of polyvinyl chloride (PVC) tape backings

Horacek et al. 2008 Isotope survey of adhesive tapesOrganisms

Cotton West et al. 2010 Isoscape of cotton for forensic studies

Nic Daeid et al. 2011 Provenancing undyed cotton using multi-isotope techniques

Meier-Augenstein et al. 2014 Geographic origin of cotton using multi-element isotopesignatures

Human hair Ehleringer et al. 2008 δD and δ18O in human hair

Sharp 2007 δD in human hair

Thompson et al. 2014 Review of stable isotopes in hair

Human physiology Podlesak et al. 2012 Model for δD and δ18O in human body water

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Human remains Bartelink et al. 2015 Region of origin of human remains from past wars

Font et al. 2015 Origin of unidentified World War II casualties

Juarez 2008 87Sr/86Sr to help identify undocumented border crossers

Katzenberg & Krouse 1989 Isotopes applied to human tissue identification

Meier-Augenstein & Fraser2008

Case study using δD and δ18O on a murder victim

Remien et al. 2014 Case study of inverse modeling applied to isotope forensicsand human hair

Wildlife Bowen et al. 2005a Global patterns of δD and δ18O in wildlife

Hobson & Wassenaar 2008 Reconstructing animal migration using stable isotopes

Hobson 1999 Reconstructing animal migration using stable isotopes

Marra et al. 2009 Case study of migratory geese as the cause of an airplane crash

Uno et al. 2013 14C to determine year of death for seized ivoryPollutants

Perchlorates (ClO4−) Bohlke et al. 2005 δ18O, δ17O, and δ35Cl in natural and synthetic perchlorates

Jackson et al. 2010 Natural perchlorates in the western United States

Sturchio et al. 2006 δ18O and δ35Cl in natural and synthetic perchlorates

Sturchio et al. 2003 Chlorine isotope fractionation during perchlorate reduction

Sturchio et al. 2007 Isotope fractionation during perchlorate degradation

Sturchio et al. 2009 36Cl as a tracer of perchlorate origin

Sturchio et al. 2012 Isotope mapping of perchlorates in groundwater

Sturchio et al. 2014 Perchlorates in groundwater near Pomona, CaliforniaReviews

Legal considerations Ehleringer & Matheson 2010 Legal considerations for isotopes used in courts

Holobinko 2012 Legal considerations related to forensic human identification

Targeted reviews Aggarwal et al. 2008 Applications of heavy stable isotopes

Bell 2009 For chemists

Benson et al. 2006 For forensic scientists

Bartelink et al. 2014 For forensic practitioners interested in human provenancing

Chesson et al. 2014 For archaeologists

Gentile et al. 2011 For forensic scientists

Gentile et al. 2015 For forensic scientists specifically interested in sourceinference

Hoffmann & Jackson 2015 For chemists

Meier-Augenstein 2010 Book on isotopes in forensic studies

Philp 2007 For chemists

Spatial relationships Ehleringer et al. 2010 Spatial relationships of isotopes in forensic studiesSynthetic materials: explosives and their components

Ammonium nitrate Benson et al. 2009 Discrimination of ammonium nitrate sources using isotopes

Howa et al. 2014b Methods for separation and analysis of ammonium ions fromammonium nitrate prills

Brust et al. 2015 Isotopic and elemental profiling of ammonium nitrate

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Explosives, general Widory et al. 2009 Multi-isotope approach to distinguishing explosives

Hexamethylene triperoxide diamine(HMTD)

Lock et al. 2012 Linking precursor to HMTD explosive using isotopes

Hydrogen peroxide (H2O2) Barnette et al. 2011 Isotopes in commercially available H2O2

Pentaerythritol tetranitrate(PETN)

Howa et al. 2014c δ13C and δ15N in PETN

RDX Lock & Meier-Augenstein2008

Linking hexamine precursor to RDX product using isotopes

RDX and HMX Howa et al. 2014a δ13C and δ15N in factory-produced RDX and HMX

Semtex Pierrini et al. 2007 δ13C and δ15N to assess links between samples usinglikelihood ratios

Trinitrotoluene (TNT) Nissenbaum 1975 Isotope characterization of TNT for sample differentiation

Urea nitrate Aranda et al. 2011 Isotopic characterization of urea nitrate and precursors

large comparative data sets and by establishing predictive models based on plant and animal physi-ology and the principles of isotope fractionation (Bowen et al. 2005a; Ehleringer et al. 2008, 2010).

In this review we discuss the early groundwork for using stable isotopes in forensic studies,and then show how a framework for stable isotope biogeochemistry can be used to establish arobust means for using stable isotopes in forensic science. We review how stable isotope ratios candiffer in natural systems and how the principles of variation in nature can be used to understandvariations in human-modified systems. We also provide examples of how isotopes can be used inforensic casework.

2. ISOTOPE RATIOS IN NATURAL SYSTEMS

Almost all elements have more than a single naturally occurring isotope; however, many of theseare short-lived isotopes produced either by cosmic rays or in a radioactive decay chain. Althoughradioactive isotopes can be useful for forensic studies, we do not include them in this review exceptfor long-lived radioisotopes that can be used as tracers—in essence, as stable isotopes.

In this review we mainly discuss the light stable isotopes that have their own terminology,including isotopes of the elements H, B, C, N, O, S, and Cl. Other isotopes are commonlyreported as their measured absolute ratios—these include Sr, Hg, and Pb—and have an importantrole in forensic science but are not discussed in this review. The delta (δ) notation is used todescribe small differences in the isotope ratio from a standard for the light stable isotopes in partsper thousand (or per mil, �); we use oxygen isotopes as an example (Sharp 2007):

δ18O = (Rsample/Rstandard − 1) ,

also written as

δ18O (‰) = (Rsample/Rstandard − 1)× 1,000 ,

where Rsample and Rstandard are the 18O/16O ratios in a sample and a standard, respectively. Thestandards used are international reference materials so that all individual laboratories reportmeasurement results on the same scale. Analogous equations are used for other light stableisotopes.


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Two radioisotopes are discussed in this article in the context of their being tracers because oftheir relatively long half-lives. Both 14C and 36Cl have been used in forensic studies; for example,14C is employed in the context of bomb 14C, in which 14C is used as a tracer of the atmospheric14C concentration over the past 50 or so years. 36Cl is produced in the stratosphere by cosmicrays, but also has a significant bomb component due to nuclear weapons testing, principally inthe mid-1950s.

Isotopologues represent substitutions of rare isotopes in molecules for the more common iso-tope. Hydrogen has two isotopes: 1H (hydrogen) and 2H (deuterium, also abbreviated as D), withnatural abundances of 99.9844% and 0.0156%, respectively. Oxygen has three stable isotopes:16O, 17O, and 18O, with natural abundances of 99.7630%, 0.0375%, and 0.1995%, respectively(Hoefs 2013). In water, the two hydrogen atoms are symmetrical with respect to oxygen, and thesubstitution of D for H cannot be distinguished between the two positions. Likewise, carbon,nitrogen, and sulfur have multiple naturally occurring isotopes, and thus all materials (minerals,liquids, manufactured products, organic compounds) containing these elements have isotopo-logues that can be used in geological, ecological, or forensic studies. In this article, we consideronly single substitutions (e.g., 18O for 16O and D for H in the water molecule); multiple substitu-tions of the rare isotope lead to another branch of isotope geochemistry—clumped isotopes (Eiler2013)—which will likely have forensic applications in the near future.

In certain cases, triple isotopes of a single element can be used to distinguish different origins.For example, oxygen has the stable isotopes 16O, 17O, and 18O; most terrestrial processes fractionate18O twice as much as 17O because the mass difference is “twice” as large for 18O compared to 17O;this is known as mass-dependent fractionation (MDF). However, certain processes exhibit mass-independent fractionation (MIF); the MIF 17O anomaly (�17O, pronounced as “Cap-17O”) isexpressed as (Thiemens 2006)

�17O = δ17O− 0.52δ18O .

Stratospheric reactions involving ozone result in very large δ17O anomalies and provide a markerfor such an origin.

Stable isotopes in forensic studies are typically used as tracers of processes and indicators oflocation or origin. The power of stable isotopes in this context lies in the differences in their naturalabundance ratios and the separation of isotopes through chemical and physical mechanisms suchas equilibrium chemical reactions, phase changes (i.e., evaporation), and kinetic processes. Muchof the background for forensic applications of stable isotopes has been developed through decadesof work in physical chemistry, geochemistry, and ecology to understand the relationships of stableisotopes in natural and manufacturing systems. This means much of the background informationneeded for forensic applications of isotope analyses is well understood, but interpretation of stableisotopes in forensic casework requires data from the problem at hand, as well as from the largerworld from which samples could come.

Early applications of stable isotopes to forensic studies are rooted in observations of meteoricwater relationships and in carbon isotope ratios that are vastly different in plants using differentphotosynthetic pathways. We first discuss meteoric water relationships. Meteoric waters fall on awell-defined relationship between δD and δ18O: The GML has the relationship (Craig 1961)

δD = 8δ18O+ 10 .

Figure 1 shows the δD and δ18O values of bottled waters from around the world, which have aδD/δ18O slope and intercept very similar to those of the GML; thus, bottled waters can readily betested to determine whether more than a single source is being sold under a single label (Bowen

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–150

–100

–50

0

50

50–5–10–15–20

δD (‰

)

δD (‰

)

δ18O (‰)

δ18O (‰)

Global meteoric water line:δD = 8 δ18O + 10

100%addedwater 50%

0%

Global bottled water line:δD = 7.8 δ18O + 7 (n=279)

50

0

–10

10

20

Orange juice

Figure 1δD and δ18O of bottled water samples collected globally. The solid line is the global meteoric water line (Craig 1961), and the dashedline is the global bottled water line. Data are from an IsoForensics, Inc., database (n = 279 samples) and include data from Bowen et al.(2005b). The detail shows four samples of orange juice containing 0% to 100% added water; samples fall on a mixing line between alocal water and the pure concentrate (samples analyzed by IsoForensics, Inc.).

et al. 2005b). The relationship between δD and δ18O, on a global scale, has important implicationsfor forensic applications of stable isotopes, as discussed below. Besides meteoric water, many otherliquids have a relationship between δD and δ18O that is closely related to that of the GML, althoughoften with significant offset. Craig (1961) noted that evaporated waters, such as those found inlakes, are enriched in 18O relative to D and deviate from the GML with a slope that varies betweenabout 3 and 6. This relationship provided a solution to the problem of determining the origins offruit juices: Freshly squeezed orange juice or other fruit juices could readily be distinguished fromjuice reconstituted with meteoric water (Bricout & Mouzae 1971, Bricout et al. 1972, Bricout 1973)by measuring its δD and δ18O. Samples well off the GML were likely to be authentic, and thoseon or near the GML were likely to have been reconstituted (Dunbar & Wilson 1983); Figure 1shows an example of reconstituted orange juice as compared with 100% “fresh-squeezed” orangejuice from the same source. Further, the global relationships of the isotope ratios of meteoricwaters are well known and are related to factors such as temperature, distance from the ocean, andaltitude. The spatial pattern defined by global meteoric waters (e.g., Bowen & Revenaugh 2003)has led to the notion of isoscapes (Bowen 2010, West et al. 2010a), whereby local to regional toglobal patterns of isotope ratios can be mapped in a spatial context.

A second fundamental observation relevant to the early use of isotopes in forensic settings hadto do with the recognition of a second major photosynthetic pathway that was distinguishableusing stable 13C/12C ratios. Wickman (1952) and Craig (1953) noted that most plants had δ13Cvalues of about−26�, but certain other plants had δ13C values of about−11� (Figure 2). Withtime it became clear that the 13C-depleted plants used the C3 photosynthetic pathway, whereas the13C-enriched plants used the C4 photosynthetic pathway (Smith & Epstein 1971, O’Leary 1988);


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–10–15–20–25–30–35

C3 plants C4 plants

Naturalhoney

δ13C plants (‰)

Figure 2Histogram of δ13C values for C3 and C4 plants based on >1,000 isotope analyses. The yellow overlay shows the δ13C distribution ofnatural honey samples from the United States (Doner & White 1977). Figure adapted with permission from O’Leary (1988); copyrightOxford University Press.

this distinction has given rise to thousands of applications in ecology and paleoecology. However,Smith & Epstein (1971, p. 384) made a prescient statement that has great implications for forensicapplications: “For instance, one can easily determine if a particular brand of sucrose was obtainedfrom sugarcane (high 13C) or from sugar beet (low 13C)—a distinction difficult, if not impossible,to make using classical chemical techniques.” Thus, sugars derived from different photosyntheticpathways could be distinguished. Further, Bricout et al. (1974) showed that synthetic vanillincould be distinguished from natural vanilla using carbon isotopes, and Doner & White (1977)showed that most honeys have a narrow range of δ13C values, suggesting that C3 plants are thepredominant source of honey in the United States (Figure 2). Doner & White (1977), White &Doner (1978), and Doner & Bills (1981) further discussed using isotopes to detect the additionof C4 sugars (sugarcane or high-fructose corn syrup) to honey or orange juice. By 1980 it wasclear that stable isotopes could be used to detect some food adulteration, especially the additionof water and the admixture of sugars that use a different photosynthetic pathway than do those inthe product being marketed (e.g., honey).

In the early 1980s, technological advances were made that led to the current generation of on-line extraction and separation systems that increased the speed of analytical throughput, therebyallowing large databases to be developed for specific applications of stable isotopes. Early isotoperatio mass spectrometers were dual collectors, and provision needed to be made for isobars of in-terfering isotopes (e.g., 13C16O16O and 12C17O16O are isobaric at mass 45); Craig (1957) discussedthe correction using the MDF assumption for 17O versus 18O. Development of three-, four-, and

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five-collector instruments meant that throughput could be higher. Equally significant, methodswere developed in the 1980s and 1990s for the separation of gases based on gas chromatographyunder positive pressure (Matthews & Hayes 1978; Hayes et al. 1990; Brand et al. 1994; Ricci et al.1994; Merritt et al. 1994, 1995). This replaced methods where extraction and gas cleanup weredone under high-vacuum conditions. These innovations required faster computation and morepowerful pumping systems than were previously available. The switch to online extraction systemsled to the point where the modern usage of stable isotopes in forensic science truly begins: largedata sets intersecting with geochemical models and animal or plant physiological models.

3. FRAMEWORK FOR STABLE ISOTOPES IN FORENSIC STUDIES

There are several important ways in which stable isotopes can be applied in forensic science.The stable isotope ratios incorporated through natural or manufacturing processes often have arange of different values for a specific item or product, and the stable isotope ratio can thereforebe used to determine whether a product is likely to have the same origin as another sample.This leads to the first-order question: Can a sample of interest be distinguished from anothersample based on comparison of isotope ratios? Answering this question is important in variety ofcontexts: explosives (Widory et al. 2009; Aranda et al. 2011; Howa et al. 2014a–c) or componentsof explosives (Barnette et al. 2011), packaging tapes (Carter et al. 2004, Horacek et al. 2008, Dietzet al. 2012), ink (Chesson et al. 2015), olive oil (Angerosa et al. 1999, Camin et al. 2009), paint(Farmer et al. 2009), and many other products. A principal application would be to show whetherevidence at a crime scene could be associated with the same product in a suspect’s possession; thisprovides comparable evidence that the material could have the same origin but is also useful inproviding falsification of a hypothesis whereby the material could be shown to be different or tohave a different origin.

Ehleringer et al. (2010) discussed how to expand the use of stable isotopes in the broader con-text of forensic applications to consider higher-order questions (Figure 3): (a) Is the specimenconsistent with a set of known observations? (b) What are the predictions for specimens seized inthat location? (c) What are the predicted regions from which the specimen could have originated?The first question requires a comprehensive database for comparison. The second and third ques-tions require models to describe the physiochemical processes that lead to isotope ratio differencesin spatial contexts, including an understanding of the natural variations in the starting materialsthat make up the final product. The latter two questions also require Geographic InformationSystems (GIS) modeling to describe spatial variations across a broad range of regions, includingregions for which data do not exist or are rare.

In all cases, the use of stable isotopes to answer these forensically relevant questions requiresa database of authentic observations for comparison. Here we assume that the bulk chemicalcomposition of the material is sufficiently similar that all the samples can be considered to be thesame material. This assumption is critical, and if it is not met, then the comparison test is notappropriate (Meier-Augenstein 2010, Gentile et al. 2011).

Consider, for example, the plastic explosive Semtex, which can contain the explosive compoundsRDX and HMX—in varying amounts—as well as other additives (e.g., binders, oils, dyes). Thesedifferent components are often produced using vastly different processes that can cause differencesin stable isotope ratios. Analysis of all components contained within the explosive together as asingle, bulk measurement is not ideal. Instead, it is preferable to separate the explosive into itsprimary components for isotope analysis. This compound-specific analytical approach reducesheterogeneity and generates multivariate data sets for comparing chemically similar materials(Howa et al. 2014a–c).


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INPUT QUESTIONS

Process-based model

GISmodel

Predictedspatial maps

Inversespatial maps

Specimen:biological, soil,

water, other

Is the specimenconsistent witha set of knownobservations?

What arepredictions for

specimens seizedin that location?

What are thepredicted regions

from which thespecimen couldhave originated?

Environment atlocation wherespecimen was

collected

Soils andelements

spatial layer

Database ofisotope measurements

and observationsrelevant to specimen

Climate andgeographyspatial layer

Waterspatial layer

Figure 3Framework for isotope evidence used for forensic applications. The left side shows the inputs of data, including data needed to developmodels based on physiology or processes. The right side shows the major classes of questions to be addressed in many forensic studies.Abbreviation: GIS, Geographic Information Systems. Figure adapted with permission from Ehleringer et al. (2010); copyright Springer.

Useful comparisons of samples, or their components, to a reference database depend largelyon how completely understood the isotopic variation is within samples (i.e., sample heterogeneitymust be characterized). It is also important to understand whether enough samples contribute tothe database that the likely total variation for the system is understood (i.e., population variationmust be investigated). However, it is not necessary to know the isotope ratio of every exampleof a particular substance on Earth for conclusions drawn from a database to be acceptable, if therules underlying isotope ratio variation in the natural or manufactured materials of interest arewell understood and modeled.

This leads to the second requirement on applications of stable isotopes in answering forensicallyrelevant questions, which includes isotope models that describe expected isotope ratios given astarting substrate. Isotope mixing models fall into this category and have been used extensively inthe geology literature, in hydrology and other types of studies. A general physiological model forbiological systems was presented by Ehleringer et al. (2010); this model has inputs and outputsthat affect the isotope ratio of a biological material or tissue of interest. This physiological modelhas been used in plants, microbes, and animals (Kreuzer-Martin et al. 2003; Ehleringer et al. 2008;West et al. 2008, 2010b; Podlesak et al. 2012). Another example of a model used in stable isotopesystems describes the isotope trends in residual pools of material that are undergoing mass loss dueto degradation or evaporation. The Rayleigh fractionation process (Sharp 2007) is a well-knownmodel for trace element or trace isotope enrichment (or depletion) where the product has a ratio

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that differs from that of the reactant, such as in equilibrium fractionation. Thus, the Rayleighprocess results in evolving isotope ratios for both the reactant and the product as the substrateloses material over time. Evaporation of water from lakes or from leaves is a classic example of thisprocess. The Rayleigh process could be very important in certain manufacturing methods wherethe substrate is not completely consumed.

Spatial information is the final requirement needed to answer higher-order forensic questionsusing stables isotopes. Perhaps most critically, spatial modeling has the potential to extend predic-tions to geographic regions where isotope measurements have not been made. Expected isotoperatio variation in the material of interest can be estimated in this way using a GIS approach withinputs that could include stable isotopes of meteoric water, local climate and topography, andgeological or soil information. We next discuss several examples of large-scale spatial variations,on continental and global scales, and how they are used in forensic studies.

4. CASE STUDIES OF STABLE ISOTOPE FORENSIC SCIENCE

In the sections below, we discuss some of the different ways in which stable isotopes have been usedin the forensic context. These examples do not provide a comprehensive survey of the field butrather demonstrate how databases have been used, how appropriate models can enhance under-standing of the underlying processes of isotope separation and the resulting isotope relationships,and how a GIS approach can extend the range of existing databases to regions where no isotopemeasurements have been made.

4.1. Comparison of Samples to a Database: Groundwater Contaminationin Pomona, California

The City of Pomona v. SQM North America Corp. (2014) case illustrates some of the differencesbetween the scientific perception of adequate and reliable data and the legal interpretation of dataquality and admission of scientific data in a case. This case also illustrates the complexities of theDaubert standard as it occurs in legal practice, which is a useful exercise for scientists involved insupplying data to the court.

At issue is the responsibility for cost of cleanup of groundwater in the vicinity of Pomona,California. That groundwater has high concentrations of perchlorate (ClO4

−), many times higherthan the background levels found in semiarid regions (Sturchio et al. 2014). High concentrationsof ClO4

− are considered hazardous because it can inhibit iodine uptake for proper thyroid func-tion (NRC 2005). The concentration of ClO4

− in the Pomona groundwater samples often exceeds10 μg/L, whereas the concentration in uncontaminated groundwater usually is less than 1 μg/L(Fram & Belitz 2011). California enacted regulations in 2007 that required drinking water sup-pliers to remediate groundwater with ClO4

− concentrations greater than 6 μg/L. The City ofPomona took steps for compliance with the California regulations, which involved constructionof treatment facilities at considerable cost.

Perchlorate is used for the manufacture of explosives, including flares and fireworks, and isa major component of rocket propellant. It also is found in high concentrations in some fertil-izers, such as those extracted from Chile that were used extensively in citrus agriculture in thePomona region before World War II. Other regional sources in the Pomona case could includethe Colorado River, which is imported for agriculture in the Inland Empire, a region of Californianear Pomona. Thus, there is the potential for multiple sources of perchlorate in the regionalgroundwater (Sturchio et al. 2014).

In a series of papers on the isotope chemistry of ClO4−, the isotope differences between syn-

thetic and natural sources were characterized, the isotope characteristics of degradation were


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Atacama

10

–10

–20

0

0

–20 20 3010–10–30 0

Pomona

Synthetic

Synthetic

Synthetic

IndigenoussouthwesternUnited States



AtacamaDeath Valley

Death Valley

Biodegradation trendBiodegradation trend

10

15

5

20

0

–5

Pomona

Mass-dependent fractionationMass-dependent fractionation

a

b

10 –16 10 –15 10 –14 10 –13 10 –12 10 –11 10 –10

Atacama

Pomona

c

δ18O

36Cl/Cl

δ37Cl (‰)

10

–10

–20

δ37Cl (‰)

Δ17O (‰)

Biodegradation trend

Biodegradation trend

Southernhigh plains

Southernhigh plains

Southernhigh plains

Figure 4Isotope characteristicsof perchlorate in thePomona, California,study. (a) δ17O andδ18O relationships forperchlorate. Mass-dependentfractionation results invalues of δ17O � 0;biodegradation resultsin no change in δ17Obut an enrichment in18O. (b) δ37Cl andδ18O values ofperchlorate fromdifferent sources; mostsynthetic perchloratehas values near 0.(c) δ37Cl and 36Cl/Clratios. Note that notall samples in panel care presented in panelsa and b. Figure adaptedwith permission fromSturchio et al. (2014)and Sturchio (2015);copyright Elsevier.

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determined, and these findings were applied to mapping groundwater plumes (Sturchio et al.2003, 2006, 2007, 2009, 2012; Bohlke et al. 2005, 2009; Jackson et al. 2010). Sturchio et al. (2014)then characterized the isotope systematics of the Pomona groundwater (Figure 4) to determinewhether the source of ClO4

− could be identified.Sturchio et al. (2014) and Sturchio (2015) showed that the possible sources of ClO4

− within thePomona groundwater are isotopically distinct. Using different isotope systems (δ18O, δ17O, δ37Cl,and 36Cl/Cl), the authors found the ClO4

− in groundwater from Pomona to be very similar to thatfrom the Atacama Desert (Figure 4). They concluded that the ClO4

− in Pomona groundwater wasconsistent with isotopic values expected from nitrate fertilizers imported to the Pomona regionfor the citrus industry. This conclusion was also supported by known import of Chilean fertilizersto the region before World War II.

This led to a lawsuit against SQM North America Corp. (a fertilizer supplier) by the Cityof Pomona, which hoped to recover the costs of groundwater treatment. The City of Pomonaalleged that the import and sale of Chilean nitrate fertilizer by SQM North America Corp. wasthe source of ClO4

− contamination; the city’s evidence was based on the isotope fingerprintingarguments outlined above, and its principal witness was Dr. Neil Sturchio.

This case illustrates the need for scientists to be more cognizant of when data are allowed to beintroduced as evidence and who qualifies as an expert. In a pretrial Daubert hearing, the districtcourt initially excluded the opinions of Dr. Sturchio on three grounds:

� Dr. Sturchio’s opinions were subject to future methodological revisions and were not yetcertified.

� The procedures Dr. Sturchio used had not yet been tested and were not subject to retesting.� The reference database used by Dr. Sturchio was too small.

Some of these arguments are discussed in detail by Bennett (2015). The district court decisionwas reversed by the Ninth Circuit Court of Appeals, which ruled the following:

Expert testimony may be excluded by a trial court under Rule 702 of the Federal Rules of Evidenceonly when it is either irrelevant or unreliable [emphasis added]. Facts casting doubt on the credibilityof an expert witness and contested facts regarding the strength of a particular scientific method arequestions reserved for the fact finder. (City of Pomona v. SQM North America Corp. 2014).

These court cases led SQM North America Corp. to submit a petition for consideration ofthis case by the US Supreme Court in 2014. This petition was viewed by many legal scholars andscientists as a chance to clear up some of the confusion related to the Daubert standard: What are“accepted scientific methods” (i.e., are only Environmental Protection Agency–approved methodsallowable)? Are the methods used reproducible, especially when the methods are highly specialized,as in stable isotope analysis? When is a reference database adequate? The Supreme Court’s refusalto hear the appeal (City of Pomona v. SQM North America Corp. 2014) was seen by many as a missedopportunity to establish a more definitive standard of when a scientific expert would be allowedto testify.

Pomona v. SQM North America Corp. was first brought to court in 2010. In June 2015, after thereversal by the Ninth Circuit Court of Appeals, the jury as trier of fact rejected the claim by Pomonathat SQM North America Corp. was financially liable for the contamination of groundwater inthe region. The case provides fascinating insight into how the outcome of dueling scientificevidence involving the use of stable isotope analysis, and the resulting conclusion or verdict,remains unpredictable in a trial. For instance, did the jury understand the scientific conclusionas to the traceability of the perchlorate to the Atacama plains in Chile and its subsequent use


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as fertilizer in the orchards around Pomona? Or was the jury unwilling to impose a significantmonetary penalty on SQM North America Corp. because of the passage of time or the otherpossible sources of perchlorate, such as Colorado River water? For six centuries of common law,the US legal system has deferred to the trier of fact to make final, one-time decisions in the contextof a trial between adversaries. It is incumbent on the scientific community to follow scientific rigorin ascertaining the “facts” they present to the one-time event known as a “legal trial.”

4.2. Patterns of Trade: Marijuana in the Continental United States

Many illegal materials are bought and sold across vast trading networks with widely dispersedsources. This includes illegal wildlife (e.g., birds, elephant ivory, rhino horn), drugs (e.g., co-caine, heroin, marijuana), and counterfeits or mislabeled products (e.g., money as paper notes,wine, honey). Because of the geographic variability due to the meteoric water cycle (i.e., H or Oisoscapes), and variations in soil chemistry due to geological differences (e.g., 87Sr/86Sr variability),it is sometimes possible to assign probable regions of geographic origin using large data sets ofmaterials.

Marijuana (Cannabis spp.) provides an excellent example of the recognition of trade patternsover a large geographic area. Liu et al. (1979) recognized that growth conditions of marijuana couldbe distinguished by their δ13C ratios. Marijuana grown indoors (under higher-CO2 conditions,with more negative δ13C values than in the free atmosphere) was significantly more depleted in 13Cthan that grown under more natural conditions (i.e., outdoors); this pattern was again observed inlater studies. More recent work emphasized additional carbon isotope studies but also added δ15Nas a gross indicator of either region of origin or of the use of varying fertilizer methods (Shibuyaet al. 2006, West et al. 2007, Booth et al. 2010). The combined use of δ13C and δ15N provides acoarse indicator of marijuana origin, but additional resolving power lies in use of the large isotopegradients in δD and δ18O. West et al. (2008, 2010b) developed a leaf-water model that could becoupled to GIS-based mapping at the global scale; combining this with the δD and δ18O mapsof global meteoric waters (Bowen & Revenaugh 2003) made it possible to predict δD and δ18Ovalues of different plants based on plant physiology and local meteorological conditions. For theexample of marijuana, the combined isotope suite of δ13C, δD, and δ18O could be used to provideinformation on growth conditions and probable origin within broadly defined regions. With alarge data set, it could then be possible to understand large-scale trade patterns of marijuana—inthis case in the continental United States. Hurley and coworkers (Hurley et al. 2010a,b; Westet al. 2010b) studied >600 marijuana samples of unknown origin seized by US law enforcementofficials in the period from 2005 to 2007; these came from 52 different municipalities locatedin the United States and were aggregated into 22 different regions. Each sample was assigneda probable origin from one of four broad geographic categories: eastern United States, westernUnited States, north of the northern US border, and south of the southern US border. Figure 5shows that certain regions in the United States have a predominantly local origin (e.g., the Seattleto Oregon corridor, with sources in the western United States and western Canada), that certainother metropolitan areas are predominantly sourced south of the US border (e.g., Arizona andNew Mexico, with sources from Mexico and Central America), and that the eastern United Stateshas very little marijuana that is sourced from south of the US border. Using >500 seized samplesfrom known locations, West et al. (2007) were able to construct a deterministic model to assignmarijuana to growth conditions based on δ13C and δ15N values. The results of West et al. (2007)allowed Hurley et al. (2010b) to assign the growth conditions to each of the >600 samples ofunknown origin seized by US law enforcement officials. The assignations were on the isotopecontinuum and recognized: indoor, shade/indoor, and outdoor growth conditions.

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Oregon SeattleKansas City/

Topeka ChicagoDetroit

Buffalo/Rochester

New York City

Philadelphia

Washington, DC

Richmond/Roanoke

NorthCarolina

Orlando/Tampa

Miami

Atlanta

Cincinnati/Dayton/

Columbus

Memphis

St. Louis

AlbuquerquePhoenix/

Tucson

San Diego

Los Angeles

San Francisco

EasternUnited States

WesternUnited States

South of US border

North ofUS border

Figure 5Probable origin for >500 seized marijuana samples in 22 regions of the United States for the years 2005–2007. Origins were assignedbased on leaf-water models for marijuana (from Hurley et al. 2010b). Figure adapted with permission from Hurley et al. (2010b);copyright Elsevier.

Although marijuana production, processing, and sales remain illegal under the federalControlled Substances Act, several states (Colorado, Washington, Oregon, and Alaska) andWashington, DC, have passed ballot initiatives in recent years making it legal within those admin-istrative divisions for individuals to possess and, in some cases, produce small amounts of marijuanafor nonmedical and recreational uses. Although marijuana remains a Schedule I drug accordingto the Controlled Substances Act, these states and Washington, DC, have sanctioned for-profitfirms to supply marijuana products in their emerging markets.

In response to the swell of state-level initiatives for legalization of the possession and culti-vation of marijuana, the US Department of Justice Office of the Deputy Attorney General hasreleased a series of memorandums for federal prosecutors outlining the department’s enforcementpriorities in states that have legalized marijuana possession (Cole 2013, 2014; Wilkinson 2014).Broadly, these priorities seek to avert the public health consequences of marijuana usage; to curb


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criminal enterprise’s trafficking and revenue generation and the violence associated with illegalmarijuana distribution and sales; and to limit transport of marijuana between states with differ-ing marijuana laws. Hurley et al. (2010b) found that Mexican- and Canadian-grown marijuanaand marijuana grown outside the seizure region were commonly observed and comprised a sig-nificant fraction of materials seized in the United States (Figure 5). Although these samples wereseized prior to any state-level initiatives to legalize marijuana usage, it is of interest to note thatthese foreign and long-traveled products were readily available in states where marijuana is nowlegal. These data suggest that a large fraction of marijuana available on the streets for purchasecould have originated outside the state of collection. Given high tariffs on marijuana productsin states where marijuana is legal and the well-established underground economy for marijuanasales and distribution, it is likely that black-market marijuana will remain a major federal andlocal criminal justice issue regardless of its state-level legality; a comparative study done ten yearsafter this initial study would be very informative about changing trade routes that could be due tochanges in the legal status of marijuana in these states.

In this context it may be worth noting the potential similarities between emerging marijuanamarkets and the ongoing evolution of tobacco markets. Anticipating potential new legal markets,suppliers may seek to establish enterprises in illicit markets, as appears to have occurred withtobacco (Collin et al. 2004, Lee & Collin 2006, Joossens & Raw 2008, LeGresley et al. 2008). Itwould be of interest to know how current suppliers may be interacting with both legal and illegalmarkets at the state level or on a larger scale. In addition, we note that an unexpected revelation ofmarijuana legalization was the increase in consumption and sales of Cannabis-infused edibles. Herecompound-specific isotope analysis may be useful to understand the source of marijuana used inedibles (see the sidebar Compound-Specific Stable Isotope Analysis as a Tool to Understand theSource of Marijuana).

4.3. Patterns of Trade: Authentic Versus Counterfeit Currency

Counterfeit materials, especially monies, have plagued societies for thousands of years. Papermoney has cotton as a major constituent; because principal elements of cotton are C, H, andO, the material is well suited for stable isotope studies. One approach is to obtain bona fidematerials globally, but realistically, this is not always possible. West et al. (2010b) used the plantphysiological model described in Section 4.2 to predict the δD and δ18O values of cotton. Thiswas coupled with a GIS model that included climate parameters (e.g., temperature, precipitationamounts, humidity) and the δD and δ18O values of local meteoric water (Bowen & Revenaugh2003). Validation of the model was obtained for selected regions where bona fide samples couldbe obtained.

Figure 6a shows a global cotton isoscape based on plant physiological models that characterizethe cumulative isotope discrimination in cotton. In this case, only regions where cotton is a knownagricultural product are shown; non-cotton-growing regions are masked. Figure 6b shows the δDand δ18O values of genuine US bank notes, along with high-quality counterfeit notes that wereseized by law enforcement agencies over an extended period of time. Series 1 is from a regionwhere the δD and δ18O values are lower than those of the genuine bank notes, suggesting a regionwith a colder climate. Series 2–5 are from regions with higher δD and δ18O values, suggestingregions that are hotter and/or drier than the region in the United States where cotton is producedfor the US government. The cotton isoscape in Figure 6a can be used to suggest possible regionsfor further investigation as to the origin of these high-quality counterfeit notes.

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COMPOUND-SPECIFIC STABLE ISOTOPE ANALYSIS AS A TOOLTO UNDERSTAND THE SOURCE OF MARIJUANA

Between January and December 2014, Cannabis-infused retail/recreational products increased from 69,000 unitsto over 358,000 units sold in Colorado (Brohl et al. 2015). This 5-fold increase in the sales volume of edibles waswell documented in the media, with several cases of overconsumption of Cannabis-infused edibles being implicatedin accidental deaths. Furthermore, emergency rooms have reported dramatic increases in admitting children withreactions to accidentally consuming Cannabis-infused candies (Steffen 2014). These issues sparked concern regardingthe safety and standardization of edibles (Steffen 2014) and prompted the state of Colorado to convene a workinggroup to provide the legislature with guidance toward controlling edibles in both packaging and potency (Brohlet al. 2015). Although regulations in Colorado are being developed for Cannabis-infused edibles, no analytical testis available to regulators to confirm that the infusion actually derives from a plant grown in the state. Currently,the state of Colorado depends on inventory tracking software to monitor a marijuana plant from seed to sale, withno standardized tracking mechanism to trace the origin of derivatives or extracts from marijuana plants that areinfused into edibles (Brohl et al. 2015).

There continues to be a growing need for novel analytical tests to track the origin of marijuana and Cannabis-derived compounds. In this area, the analysis of nonrefractory compounds, particularly the cannabinoids and otherterpenoids, has been an area of significant scientific research (ElSohly et al. 2000, Chandra et al. 2010, Tipparat et al.2014). The distribution of cannabinoids has been used to discriminate between Cannabis strains and to identify theirgeographic origin (Ilias et al. 2005, Fischedick et al. 2010). Recently, compound-specific isotope analysis (CSIA)of the carbon isotope values of cannabinol, cannabidiol, and δ9-tetrahydrocannabinol has been demonstrated asfeasible (Muccio et al. 2011). CSIA of the hydrogen isotope values of Cannabis compounds has yet to be undertaken,but it could supply a possible analytical and regulatory tool to monitor and source plant derivatives in marijuanaplants and infused edibles. CSIA of hydrogen in plant compounds is a well-established method commonly used todetermine plant origin in ecological, forensic, and geological studies (Eglinton & Eglinton 2008, Tipple et al. 2013).However, the cannabinoids and other common secondary metabolites (i.e., terpenoids) observed in Cannabis spp.have known degradation pathways and numerous sites for hydrogen isotopic exchange. Thus, additional Cannabiscompounds must be considered in the development of a CSIA method using hydrogen isotope values to monitorand source Cannabis materials.

4.4. Patterns of Trade: 14C as a “Speedometer” of Trade in Cocaine and Ivory

Carbon is incorporated into plant tissues during photosynthesis, and this is the basis for the applica-tion of 14C dating of plants for archaeological and geological studies (Arnold & Libby 1951). How-ever, due to aboveground weapons testing in the 1950s and early 1960s, the concentration of 14C inthe atmosphere nearly doubled, so that a robust dating method using the “bomb” curve determinesthe age to within approximately 3 to 6 months for organic materials (Uno et al. 2013). A study ofthe 14C abundance of cocaine seized by law enforcement officials (Ehleringer et al. 2012), basedon >500 14C analyses of cocaine obtained from the US Drug Enforcement Administration, showsseized cocaine to have an average age of∼18 months for large (>150 kg) seizures, where the age ofa seized sample is the time between coca leaf harvest and its seizure by law enforcement officials.

This application of 14C could readily be applied to the ivory trade to learn how illegal ivoryis being taken, stored, and transported; the United Nations Office on Drugs and Crime hasrecommended that all large (>500 kg) seizures of ivory be tested for DNA and stable isotopesto determine provenance, and for 14C to determine the age of death of the elephants poached tomake up the shipment (UNODC 2014). Likewise, it could be used to determine whether ivory


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held by traders is, in fact, legal; a Canadian dealer pleaded guilty in 2015 to possessing illegal ivorybased on 14C measurements, which showed the tusks to be younger than claimed (Environ. Can.2015). The age of legal ivory varies from country to country, but elephants were placed on theCITES Appendix I list of species in 1989, effective January 18, 1990; the CITES classification is,in effect, a global ban on the commercial trade of ivory.

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4.5. Isoscapes as a Means of Determining Origin: Steady-State Comparisons

Among the most vexing cases for local police departments are unidentified decedents, especiallyif there are no missing persons reports that can conceivably be related to the case. Often thesecases can languish and eventually become cold cases, sometimes referred to as investigations ofJohn Does or Jane Does. The US National Institute of Justice defines a cold case as “any casewhose probative investigative leads have been exhausted” (Heuritz 2008, p. 21). Practically, thismeans any case that is not the subject of active investigation can be considered a cold case; in manyinstances, the relatively new methods of DNA profiling or stable isotope analysis could providenew information that could be used to reanalyze these cases. In this and the following section, wediscuss the resolution of two cold cases where stable isotopes played a key role in the investigation.In these examples, key questions included the following: Is this a local person? If not local, howlong had the individual been in the region prior to death? If not local, from what regions couldhe/she have come?

Humans (and other animals) often have continually growing tissues, such as hair or fingernails,which could serve as record of the history of movement because of the large differences amonglocal meteoric water sources. A series of studies on the stable isotopes of hair (Ehleringer et al.2008; Bowen et al. 2009; Thompson et al. 2010, 2014) showed that the δD and δ18O in human hairderived from three principal sources: local drinking water; “food water,” which is evaporated fromlocal meteoric water; and metabolic water, represented by the reaction CH2O + O2 = H2O +CO2. Water in this body pool (Podlesak et al. 2012) is determined by the individual sources ofthese waters and is distributed as biological carbonate or as proteins in hair and collagen. Thus,food that is imported from great distances could affect the final isotopic value of the body waterpool, and this impact was seen in the different populations referenced above (Ehleringer et al. 2008;Bowen et al. 2009; Thompson et al. 2010, 2014), which included Americans consuming the mostglobalized diets; individuals along a transect including India and China, who have intermediateglobalization of diet; and indigenous peoples whose hair was obtained from museum collectionsmade in the 1930s. Given the large range of δD and δ18O values in meteoric waters (see Figure 1),a large range in δD and δ18O values is expected in animal tissues. Hobson and colleagues (Hobson1999, Bowen et al. 2005a, Hobson & Wassenaar 2008) have used these wide isotope variationsextensively for wildlife migration studies.

Another important signal in human tissues is found in δ13C, δ15N, and δ34S. Studies at thecontinental scale show significant differences within and between continents in the isotope valuesin hair (Katzenberg & Krouse 1989; Valenzuela et al. 2011, 2012). In these dimensions, theimportant parameters include diet (C3- versus C4-based foods), trophic position (15N isotopesbecome enriched at higher trophic levels, and diets with a high seafood component are veryenriched in 15N), and other local conditions (e.g., 34S is enriched in the tissues of many island andcoastal populations due to the influence of marine δ34S).

←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−Figure 6Isoscape map of cotton and stable isotope values of genuine and counterfeit bank notes. (a) Global map of predicted δ18O values ofcotton based on leaf-water modeling coupled with local climate and isotope values for local meteoric waters in a GeographicInformation Systems framework. Only regions where cotton is produced are shown in the cotton isoscape. Panel adapted withpermission from West et al. (2010b); copyright Springer. (b) δD and δ18O values of cotton from genuine bank notes and high-qualitycounterfeit bank notes confiscated by law enforcement agencies (samples provided by the US Government and analytical supportprovided by IsoForensics, Inc.). Genuine bank notes have a very small distribution in δD and δ18O values, whereas series 1 through 5show large variations as well as a change in cotton source location.


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Figure 7 shows the spatial δ18O relationship for tap water and hair in the continental UnitedStates. The isoscape of human hair (Figure 7b) is based on the δ18O of local drinking waterand of diet; diet-derived amino acids partially exchange oxygen atoms during protein synthesis(Ehleringer et al. 2008). The modeled δ18O of hair is corroborated by field measurements acrossa broad range of δ18O values in the United States. These samples are considered to be at steadystate (i.e., equilibrium) because of Ehleringer et al.’s (2008) sampling strategy; that is,

∂(δ18Ohair)/∂t = 0.

Thus, it is possible to apply the isoscape concept to human provenancing for the steady-state casewhere, because of long residence time in a region, an individual can be compared to the steady-state isoscape maps. Chesson et al. (2014) presented the case of a woman found 1971 in StanislausCounty, California; the case was reopened in 2008, and stable isotopes indicated little history oftravel over the ∼9 months recorded in her hair based on very nearly constant δ18O ratios. Such aprofile is an ideal case to use a simple inversion of the hair isoscape, because the individual wouldlikely have been close to isotope equilibrium with the environment. Stable isotopes suggest thatshe came from a region where the δ18O of tap water was approximately −12�; this is compatiblewith the Stanislaus County region of California but also with the San Francisco Bay Area, becauseboth regions use water from the Hetch Hetchy reservoir in the Sierra Nevada. Subsequent workidentified this person as Mary Alice Wiley, a disaffected young woman who became involved inthe Black Power movement in the San Francisco Bay region and was in the San Francisco BayArea for two years before her death.

4.6. Isoscapes as a Means of Determining Origin: Migration Reconstructions

With movement across the landscape into regions of differing δD, δ18O, δ13C, δ15N, or δ34S,the isotope values are attenuated by the turnover of animal tissue over time (Hobson & Clark1992, Cerling et al. 2007, Podlesak et al. 2008). This complicates the use of isoscape comparisonsbecause the isoscape maps are based on individuals with long-term residence in the region andthus represent a steady-state case. Two complications arise: First, the turnover in certain tissuesmay take weeks to months to come to steady state, and thus the samples are in the transient state[i.e., ∂(δ18Ohair)/∂t �= 0]. Second, mixing of the time dimension in hair occurs when hair samplesare bundled to increase the sample size for analysis. Both of these issues cause attenuation of theδ18O signal and must be incorporated into the problem.

Figure 8 presents this problem: The remains of a young female were found by duck hunters atSaltair, in Utah, in October 2000, near the Great Salt Lake. Isotope analyses of her hair showedthat when she died, the δ18O value of her hair was similar to that of local (i.e., Salt Lake City)residents’ hair, but that about 5 months prior to death she had traveled to a region with lower δ18Ovalues. However, in order to increase temporal resolution, hair samples were bundled so that thesample could be analyzed (typically 10–12 strands for H and O isotope analysis and 25–30 strandsfor C and N isotope analysis). This presented an attenuation problem because, although each hairgrows at a fairly consistent rate of approximately 1 cm/month, individual hairs have a distributionof growth rates (approximately± 10%). In addition, hairs can be in one of several growth stages.Approximately 90% of human head hairs are in the anagen stage, lasting from 48 to 72 months,during which continuous growth occurs; this is followed by a brief catagen stage during whichthe hair ceases to grow; the telogen stage, comprising about 10% of the hairs, is a resting phaseduring which there is no growth but the hair remains in place for 2 to 6 months before being shed.Remien et al. (2014) approached this problem by assigning an average growth rate to a hair bundle,

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a

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Figure 7δ18O isoscapes for (a) tap water and (b) human hair in the continental United States. Panel a adapted with permission from Bowen et al.(2007); copyright American Geophysical Union. Panel b adapted with permission from Ehleringer et al. (2008); copyright Proceedings ofthe National Academy of Sciences.

about which there was a distribution of ages and in which 10% of the hairs could be in the telogenstage. Using forward modeling, the authors were able to show complete attenuation of a highlyvariable stable isotope signal in about 30 months. They treated the system as an inverse problem,similar to the moving camera problem of Menke (2012), and calculated the most probable inputfunction of body water that could produce the observed data. Results still could not be directlycompared to the steady-state isoscape map for δ18O but rather had to be further corrected for the


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Region

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Figure 8(a) δ18O values of measured hair, derived estimated body water signal, and derived steady-state (equilibrium) signal for human hairsamples made by bundling hair from an unidentified decedent cold case. (b) Tap water maps of geographic regions predicted from theestimated equilibrium isotope values of hair. Figure adapted with permission from Remien et al. (2014); copyright Elsevier.

turnover time of δ18O as recorded in hair. The net result of the inversion modeling (Figure 8a)was that the different areas that the decedent visited were separated by much shorter transitiontimes, but most significantly, a new area where she had traveled was identified (compare Remienet al. 2014 with Ehleringer et al. 2010). The newly identified region is shown as the area in redin Figure 8 and includes areas with higher δ18O values than occur in Salt Lake City, such as theSeattle-Portland corridor on the western coast of the United States. Results from the Saltair caseprovided new search areas for the Unified Police Department of Greater Salt Lake. The decedenthas now been identified as Nikole Bakoles, who had visited her family in Washington State withina year before she disappeared; a positive DNA match confirmed the identification.

5. THE FUTURE OF STABLE ISOTOPE FORENSIC STUDIES

A geologist or biogeochemist will likely understand the usefulness of applying stable isotopes toforensic investigations. Indeed, having these scientists involved in cases would help to addresssome of the concerns raised by the NRC in its 2009 review of the state of forensic sciencesin the United States (NRC 2009). A major issue discussed in the report concerned the lack of

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standardization in training and methodologies used within various forensic science disciplines.However, biogeochemistry has a long tradition in conducting cross-laboratory validation studiesand in developing and publishing methodologies. This means that the biogeochemist could playa significant role as an important provider of fact and expert testimony in forensic cases.

Although the presentation of stable isotope data in the US legal system to date is quite rare, thismay be changing. Consider, for instance, the importance of stable isotope data in the 2015 Pomonav. SQM North America Corp. case and the increased use of stable isotope analyses in investigationsof unidentified decedents both within the United States and abroad, including those killed in war(Meier-Augenstein & Fraser 2008, Ehleringer et al. 2010, Chesson et al. 2014, Remien et al. 2014,Bartelink et al. 2015, Font et al. 2015). In many of these cases, analyses and data interpretation wentbeyond straightforward bulk measurement and sample-to-sample comparison to focus instead oncompound-specific measurements and higher-order questions related to the potential source ororigin of a material, as outlined in this review as a framework for applying isotope evidence(Figure 3). Within this context, is the field of biogeochemistry prepared for a migration of tech-niques and models from the classroom to the courtroom?

There are several key concerns the isotope geologist and biogeochemist should consider withinthe judicial context, as outlined by Ehleringer & Matheson (2010). For example, how “generallyaccepted” are different analytical approaches? What are the means by which methods can betested and reviewed? Here, peer-reviewed publication is crucial in establishing the validity ofmethods and should remain a priority for the biogeochemist. Closely related are monitoring andproviding information on known error rates of measurements; these are necessary for calculatingthe probability (i.e., likelihood) of different interpretations—for example, what is the likelihoodthe decedent was local to the area of discovery? The use of likelihood ratios in forensic science hasincreased within the past two decades, and several recent publications have described the methodas applied to isotope forensics (e.g., Wunder & Norris 2008, Farmer et al. 2009, Carter et al. 2014).

Finally, standards are essential for data evaluation, quality assurance, and quality control. Orga-nizations such as the International Atomic Energy Agency, the National Institute of Standards andTechnology, and the US Geological Survey have produced a variety of isotope standards useful forbiogeochemistry—water, oils, and carbonates, to name a few. Standards should be incorporatedinto a laboratory’s quality management plan and used regularly during isotope analyses. In addi-tion, special-interest groups such as the American Academy of Forensic Scientists and the ForensicIsotope Ratio Mass Spectrometry Network are helpful in providing practitioner approval and or-ganizing interlaboratory comparisons or round-robin tests to evaluate the reliability of analyticalapproaches and test results.

DISCLOSURE STATEMENT

The authors are not aware of any affiliations, memberships, funding, or financial holdings thatmight be perceived as affecting the objectivity of this review.

ACKNOWLEDGMENTS

We thank Neil Sturchio for discussions concerning the Pomona v. SQM North America Corp. case.

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EA44-FrontMatter ARI 4 June 2016 13:46

Annual Reviewof Earth andPlanetary Sciences

Volume 44, 2016Contents

Tektites, Apollo, the Crust, and Planets: A Life with Trace ElementsStuart Ross Taylor � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 1

Environmental Detection of Clandestine Nuclear Weapon ProgramsR. Scott Kemp � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �17

From Tunguska to Chelyabinsk via JupiterNatalia A. Artemieva and Valery V. Shuvalov � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �37

The Lakes and Seas of TitanAlexander G. Hayes � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �57

Inference of Climate Sensitivity from Analysis of Earth’sEnergy Budget

Piers M. Forster � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �85

Ocean Basin Evolution and Global-Scale Plate Reorganization EventsSince Pangea BreakupR. Dietmar Muller, Maria Seton, Sabin Zahirovic, Simon E. Williams,

Kara J. Matthews, Nicky M. Wright, Grace E. Shephard, Kayla T. Maloney,Nicholas Barnett-Moore, Maral Hosseinpour, Dan J. Bower, and John Cannon � � � � 107

Lithification Mechanisms for Planetary Regoliths: The Glue that BindsJohn G. Spray � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 139

Forensic Stable Isotope BiogeochemistryThure E. Cerling, Janet E. Barnette, Gabriel J. Bowen, Lesley A. Chesson,

James R. Ehleringer, Christopher H. Remien, Patrick Shea, Brett J. Tipple,and Jason B. West � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 175

Reconstructing Ocean pH with Boron Isotopes in ForaminiferaGavin L. Foster and James W.B. Rae � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 207

Sun, Ocean, Nuclear Bombs, and Fossil Fuels: Radiocarbon Variationsand Implications for High-Resolution DatingKoushik Dutta � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 239

Climate Sensitivity in the Geologic PastDana L. Royer � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 277

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Redox Effects on Organic Matter Storage in Coastal Sediments Duringthe Holocene: A Biomarker/Proxy PerspectiveThomas S. Bianchi, Kathryn M. Schreiner, Richard W. Smith, David J. Burdige,

Stella Woodard, and Daniel J. Conley � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 295

Fracking in Tight Shales: What Is It, What Does It Accomplish, andWhat Are Its Consequences?J. Quinn Norris, Donald L. Turcotte, Eldridge M. Moores, Emily E. Brodsky,

and John B. Rundle � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 321

The Climate of TitanJonathan L. Mitchell and Juan M. Lora � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 353

The Climate of Early MarsRobin D. Wordsworth � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 381

The Evolution of BrachiopodaSandra J. Carlson � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 409

Permafrost Meta-Omics and Climate ChangeRachel Mackelprang, Scott R. Saleska, Carsten Suhr Jacobsen, Janet K. Jansson,

and Neslihan Tas � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 439

Triple Oxygen Isotopes: Fundamental Relationships and ApplicationsHuiming Bao, Xiaobin Cao, and Justin A. Hayles � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 463

Cellular and Molecular Biological Approaches to Interpreting AncientBiomarkersDianne K. Newman, Cajetan Neubauer, Jessica N. Ricci, Chia-Hung Wu,

and Ann Pearson � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 493

Body Size Evolution Across the GeozoicFelisa A. Smith, Jonathan L. Payne, Noel A. Heim, Meghan A. Balk,

Seth Finnegan, Michał Kowalewski, S. Kathleen Lyons, Craig R. McClain,Daniel W. McShea, Philip M. Novack-Gottshall, Paula Spaeth Anich,and Steve C. Wang � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 523

Nuclear Forensic Science: Analysis of Nuclear Material Out ofRegulatory ControlMichael J. Kristo, Amy M. Gaffney, Naomi Marks, Kim Knight, William S. Cassata,

and Ian D. Hutcheon � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 555

Biomarker Records Associated with Mass Extinction EventsJessica H. Whiteside and Kliti Grice � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 581

Impacts of Climate Change on the Collapse of Lowland MayaCivilizationPeter M.J. Douglas, Arthur A. Demarest, Mark Brenner, and Marcello A. Canuto � � � 613

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Evolution of Oxygenic PhotosynthesisWoodward W. Fischer, James Hemp, and Jena E. Johnson � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 647

Crustal Decoupling in Collisional Orogenesis: Examples from the EastGreenland Caledonides and HimalayaK.V. Hodges � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 685

Mass Fractionation Laws, Mass-Independent Effects, and IsotopicAnomaliesNicolas Dauphas and Edwin A. Schauble � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 709

Indexes

Cumulative Index of Contributing Authors, Volumes 35–44 � � � � � � � � � � � � � � � � � � � � � � � � � � � 785

Cumulative Index of Article Titles, Volumes 35–44 � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 790

Errata

An online log of corrections to Annual Review of Earth and Planetary Sciences articlesmay be found at http://www.annualreviews.org/errata/earth

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forensic stable isotope biogeochemistry · stable isotopes, forensic, isoscape, geochemistry,...

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