PalaeoclimatologyJudith Totman Parrish, University of Arizona, Tucson, Arizona, USA

Palaeoclimatology is the study of climates in the Earth’s past.

Introduction

The methods used to study younger (Quaternary) andolder (pre-Quaternary) climates are similar, but climatescan usually be studied in the Quaternary with much higherresolution. Pre-Quaternary palaeoclimatology is more of achallenge, not only because the rocks have been exposed toerosion and tectonics longer but also because thecontinents have moved.The same computer models that have been used to

predict weather and climate changes in the future havebeen adapted to the study of palaeoclimates. These havebecome important tools in palaeoclimatology because theyprovide hypotheses about climate that can be tested withdata collected from the geological record.

Climate Models

Numerical climate models have become increasinglysophisticated in the last 20 years, as has their applicationto the study of palaeoclimates. Themodels are adaptationsof those written originally for weather prediction. Thefoundations for such models are the equations forconservation of momentum, mass and energy, the hydro-static equation, and the atmospheric equation of state (gaslaw). These are calculated from specified starting condi-tions for cells and their neighbours in a three-dimensionalgrid, and the more complex the grid, the larger the numberof calculations. A typical grid is 48 latitude by 78 longitudeand 12 layers deep, resulting in a large enough number ofcalculations that, only with the advent of supercomputers,was the use of such models practicable. Palaeoclimatemodelling studies are, in some ways, more difficult thanmodelling studies of the modern atmosphere because thestarting conditions are not always known with greataccuracy and assumptions that might be made for themodern atmosphere cannot necessarily be made for theancient one.The grid that can be handled even with modern

supercomputers is much coarser than many atmosphericprocesses. For example, thunderstorms, which are im-portant for convection (that is, vertical heat transfer),operate on horizontal scales of a few tens of kilometres,much less than 48 latitude or even the 18 latitude of higher-resolution models. Many other processes also operate onsubgrid scales. Thus, climate modellers must parameterize

these processes, which means that the effects of theprocesses must be assumed mathematically before themodels are run. Differences in the results among differentclimate models are largely the result of different assump-tions, and much of the most active research in climatologyis focused on improving our understanding of subgrid-scale processes in order to improve parameterization.The climate models are modified and tested based on

evidence of past climates. This evidence comes in a varietyof forms, some quantitative, most qualitative. Even thequalitative evidence is useful, however, because it can stillprovide information about trends and amplitudes ofclimate change.

Kinds of Evidence of Palaeoclimate

Information about palaeoclimate comes from the record ofrocks, fossils and, for the Holocene, ice caps. The types ofrocks and fossils that provide the most information onpalaeoclimate are listed in Table 1. For both, simply wherethey are located can tell a lot about what the climate waslike. For example, salt deposits – called evaporites bygeologists – form by evaporation of sea or lake water andare indicative of warm, dry climates or hot climates thathave only a short rainy season. Deposits formed by iceaction occur only where the climate was cold. Although icecan form on mountains, the chances that such glacialdeposits will be preserved are small because mountains areerosional environments. Therefore, glacial deposits tend tooccur in higher latitudes, and how far they extend towardthe equator can be an indication of how large the ice capswere and, by extension, how cold global climate was.Although a few individual types of plants and animals

are indicative of specific palaeoclimates, more often theinformation about climate comes from the organisms’biogeography, that is, their geographic distribution. Forexample, some marine diatom species live only in coldwater whereas others live only in warm water, while stillothers prefer waters of intermediate temperature. Bymapping out the distributions of different groups ofdiatoms through geological time, geologists can get a senseof how the distribution of temperature changed in theoceans through time. Even if the species are extinct, their

Article Contents

Introductory article

. Introduction

. Climate Models

. Kinds of Evidence of Palaeoclimate

. Case Studies

. Major Changes in Climate in Earth History

1ENCYCLOPEDIA OF LIFE SCIENCES © 2001, John Wiley & Sons, Ltd. www.els.net

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mperature tolerances can be inferred from the patterns, sothat at least relative changes in temperature can be studied.Many groups of organisms exhibit latitudinal diversitygradients, inwhich the abundance of species decreaseswithincreasing latitude. The steepness of the diversity gradientcan be an indication of the steepness of the latitudinaltemperature gradient.Some organisms also have morphologies that are

adaptations for particular climates. Perhaps the mostuseful of these is the different forms of the leaves offlowering plants, which varywith temperature and rainfall.The proportion of species with entire (smooth) leafmargins to those with non-entire (serrated, toothed, orlobed) margins is closely related to the mean annualtemperature where the plants were growing. Size is alsorelated to climate. The leaves of plants in tropical

rainforests tend to be large. At the other extreme, leavesof plants in deserts tend to be very small (or absent). Manyother morphological adaptations of leaves are also useful.The major margin types of leaves of flowering plants hadevolved by the Late Cretaceous, so this method is usefulfrom that time to the present.Palaeoclimatologists who study the Quaternary record

have an advantage using fossils in that many species foundin the Quaternary record are still living today. This meansthat palaeoclimatologists can look at the climates in whichthe modern species are living and directly infer the sameclimate where those species occurred in the past. Incontrast, most species of organisms in the older recordare extinct. Palaeoclimatologists sometimes use what iscalled the nearest-living-relative method, in which theclimatic tolerances of the closest relative are taken as theclimatic tolerances of the extinct species. This works wellfor a few organisms, but is not reliable in most cases.An important tool in palaeoclimatology is the use of

stable isotopes, particularly of oxygen and carbon. Shellsof aquatic animals record the isotopic composition of thewaters in which the animals live; in addition, marinelimestones record the isotopic composition of sea water.The shells and limestones are composed of calcite(CaCO3), and the carbon in the calcite is mostly

12C, themost common carbon isotope, with some 13C. The oxygenin the calcite is mostly 16O, the most common oxygenisotope, with some 18O.The ratio of 18O to 16O in sea water is directly related to

the volume of the ice caps. The water in the ice caps comesfrom the oceans, and thewatermolecules in the ice caps arerich in 16O. This means that the greater the ice-cap volume,the less 16O is left in the rest of the ocean. In addition, theratio of 18O to 16O in limestone and shells is also related tothe temperature of the sea water; if the sea water is warm,the ratio will be lower.Foraminifera (Table 1) are particularly good recorders of

the isotopic composition of sea water, and thus manypalaeoclimatologic studies use the stable isotopic composi-tion of foraminiferan shells to understand the fluctuationsin the ice caps and changes in ocean temperatures. The useof oxygen isotopes in terrestrial animals is more compli-cated because evaporation, rainfall, and a number of otherprocesses can affect the isotopic composition of terrestrialwaters.In the oceans, the stable isotopes of carbon can provide

information about the productivity of the overlying watersand large-scale changes in the carbon cycle; on land,carbon isotopes have been useful for understandingfluctuations in the atmospheric concentration of thegreenhouse gas carbon dioxide and the ecology of theplants that lived on the land surface. Organic matter ispreferentially enriched in 12C compared with the totalglobal carbon reservoir. In the oceans, high productivity inthe surface waters will result in the transport of greateramounts of organic matter to the ocean bottom. This

Table 1 Types of fossils and rocks used to interpretpalaeoclimate (the specific source of information for eachis shown in parentheses)

FossilsMarine:Foraminifera (morphology, biogeography, stable iso-topes)Calcareous nannofossils (biogeography)Marine ostracods (biogeography)Marine diatoms (biogeography, stable isotopes)Dinoflagellates (biogeography)Brachiopods (biogeography, stable isotopes)Molluscs (morphology, biogeography, stable isotopes)Corals (biogeography, stable isotopes)

Terrestrial:Pollen (biogeography)Nonmarine ostracods (morphology, stable isotopes)Nonmarine diatoms (biogeography)Plants (biogeography, morphology)Freshwater and terrestrial molluscs (biogeography,stable isotopes)Vertebrates (biogeography, morphology)

RocksMarine:Chalk (distribution, chemistry)Biogenic siliceous rock (distribution)Phosphorites (distribution)Organic matter and organic-rich rocks (distribution,chemistry)Reefs (distribution)Clay minerals (distribution, type)

Terrestrial:Wind-blown deposits (distribution)Salt deposits (distribution, type)Ancient soils (distribution, type)Lacustrine deposits (lake level, type)Glacial deposits (distribution) and ice (chemistry)

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means that the remaining carbon in the surface watersbecomes more and more enriched in 13C and the shells oforganisms living there, such as foraminifera, will also bemore enriched in 13C. By examining the change in thecarbon isotopic composition of these shells through time,changes in productivity can be determined.On land, carbon isotopes are studied from limestone

nodules that form in some soils. These nodules are formedpartially from carbon dioxide respired by plant roots. Thetypes of plants, which can be related to climate, cansometimes be determined from the carbon isotopiccomposition of the nodules. Where the type of plant isconstant, the carbon isotopic composition of the soilnodules can provide information about the atmosphericcomponent of carbon dioxide in the soil, which is related tothe atmospheric concentration of the gas.

Case Studies

Pollen studies are very important for understandingcontinental climate in the Quaternary era. Pollen arecommonly taken from cores in lake sediments. In theyounger part of the era, it is even possible to study climatechange that is expressed along altitudinal gradientsbecause mountain building is rarely significant over suchshort time scales. Thus pollen from lakes at differentaltitudes can provide a third dimension – elevation – to thegeographic picture of climate change. Plant macrofossils,that is, the remains of fragments or organs such as leaves,are not used as commonly in Quaternary studies as in theolder geological record because pollen are more common,easier to sample, and easily related to living taxa. Anexception to this rule is the plant macrofossils found inpackratmiddens. Packratmiddens have proved tobe a richsource of palaeoclimatic information for the youngerrecord. The importance of packratmiddens is that they aremore densely distributed than lakes in dry areas such as theGreat Basin of the western United States and they occureven at high elevations.Packratmiddens are accumulations of crystallizedurine,

faecal pellets and plant parts brought in by the packrats.They are commonly stratified, such that the oldestmaterialis on the bottom and the youngest on the top, but thepatterns within each midden can be determined by radio-carbon dating. Although packrats are North American,small mammals on other continents, such as hyrax in theMiddle East and stick-nest rats in Australia, build similarstructures.Packrat middens are common in the Mojave Desert, an

extremely arid region of California. Through detailedradiocarbon dating and study of the plant fragments inmiddens from the area, workers such as G. R. Spauldinghave found that juniper woodlands, which today arerestricted to high mountain tops, occurred at much lower

elevations before about 8000 years ago. For example, insouth-central Nevada, Juniperus osteosperma is uncom-mon below 1700m, but occurred at elevations as low as800m until 13 000 years ago. Similarly, limber pine (Pinusflexibilis) occurs above 2600m today, but as low as 1600muntil 13 000 years ago. The lowland vegetation calleddesertscrub, which today occurs to elevations as high as1600m, was restricted to elevations below 1000m untilabout 13 000 years ago.A palaeoclimatologic study from themuch older record,

that of the Triassic period, shows how using a variety ofdata can result in a quite clear picture of climate even in thedistant past. Mutti and Weissert (1995), studying rocks inthe Italian Alps, used oxygen isotope data from carbonateplatforms – large deposits of shallow-water marine lime-stone – alongwith other information. The deposit containsa series of high-relief surfaces, called karst, that indicatethat the limestone was partially dissolved at times beforedeposition resumed. The sequence is also interrupted byriver and salt deposits.The oxygen isotopic values of the limestone were lower

than those for normal seawater for the time, andMutti andWeissert determined that thiswas the result of the influenceof rainfall. However, the isotopic values were low even foraverage rainfall but consistent with the rainfall regimefound in monsoonal climates. Palaeoclimate models byother workers had already shown that a strongmonsoonalclimate was likely to have been in effect there at the time.The existence of karst was further evidence of high

rainfall because so much limestone had been dissolved; thekarst surface shows that the relief on the dissolved surfacewas as much as 120m. In addition, some of the limestoneshowed features that indicate that the carbonate platformdried out. These features alternated with karst, and Muttiand Weissert interpreted this to mean that climate hadalternated between wet and dry. The river deposits were ofa type that occur in regions with high but seasonal rainfall,and they contain material that was probably formed insoils under alternating wet and dry climates. Thus all theevidence compiled by Mutti and Weissert led to the sameconclusion: a warm climate that was highly seasonal withrespect to rainfall.

Major Changes in Climate in EarthHistory

Caution must be used in making broad statements aboutglobal climate in that changes observed in one part of theworld may or may not indicate global changes. The majorchanges that affect global climate arewaxingandwaningofthe ice caps. Major glacial episodes were in the LateProterozoic, the end of the Ordovician period, the laterCarboniferous period to the beginning of the Permian

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period, and the Pleistocene. Not surprisingly, these werethe coolest intervals in Earth history.Two times of strong monsoonal climates have also

characterized palaeoclimate since the end of the Proter-ozoic. Monsoonal climate is characterized by reversal ofcirculation between winter and summer and by concentra-tion of precipitation in the summer months. During theLate Carboniferous to the Early Jurassic, when much ofthe continental area was aggregated into a single, largecontinent called Pangea, much of the continent wasaffected by a large-scale monsoonal circulation. About 8million years ago, the strong Asian monsoon began; thiscirculation affects climate in southernAsia,most ofAfrica,and the Indian Ocean.

Further Reading

Bradley RS (1999) Palaeoclimatology: Reconstructing Climates of the

Quaternary. London: Academic Press.

Mutti M and Weissert H (1995) Triassic monsoonal climate and its

signature in Ladinian-Carnian carbonate platforms (southern Alps,

Italy). Journal of Sedimentary Research B65: 357–367.

Parrish JT (1998) Interpreting Pre-Quaternary Palaeoclimate from the

Geologic Record. New York: Columbia University Press.

Spaulding WG (1990) Vegetational and climatic development of the

Mojave Desert: The Last Glacial Maximum to the present. In:

Betancourt JL, Van Devender TR and Martin PS (eds) Packrat

Middens, pp. 166–199. Tucson: University of Arizona Press.

Wolfe JA (1995) Palaeoclimatic estimates from Tertiary leaf assem-

blages. Annual Review of Earth and Planetary Sciences 23: 119–142.

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