Earth and Planetary Science Letters 368 (2013) 69–77

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Earth and Planetary Science Letters

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jstoner@

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Source as a controlling factor on the quality and interpretation of sedimentmagnetic records from the northern North Atlantic

Robert G. Hatfield a,n, Joseph S. Stoner a, Anders E. Carlson a,b, Alberto V. Reyes b,c, Bernard A. Housen d

a College of Earth, Ocean, and Atmospheric Sciences, 104 CEOAS Admin Building, Oregon State University, Corvallis, OR 97331, USAb Department of Geoscience, University of Wisconsin, Weeks Hall, 1215 W Dayton St, Madison, WI 53706, USAc School of Geography, Archeology, and Palaeoecology, Queen’s University, Elmwood Avenue, Belfast, BT7 1NN, UKd Geology Department, Western Washington University, Bellingham, WA 98225, USA

a r t i c l e i n f o

Article history:

Received 10 November 2012

Received in revised form

27 February 2013

Accepted 1 March 2013

Editor: J. Lynch-Stieglitzgreater concentrations of ferrimagnetic minerals than either clay or sand. Fine pseudo-single domain

Keywords:

magnetic susceptibility

paleomagnetism

magnetic grain size

ocean circulation

Iceland

Greenland

1X/$ - see front matter & 2013 Elsevier B.V.

x.doi.org/10.1016/j.epsl.2013.03.001

esponding author. Tel.: þ1 541 737 3037; fax

ail addresses: rhatfiel@coas.oregonstate.edu (

coas.oregonstate.edu (J.S. Stoner),

@coas.oregonstate.edu (A.E. Carlson), a.reyes

@wwu.edu (B.A. Housen).

a b s t r a c t

Magnetic properties of eight particle size ranges from nine locations in Iceland and 26 locations in

southern Greenland reveal the importance of source variation for our understanding of paleomagnetic

and environmental magnetic records in the marine environment. These terrestrial samples show

varying degrees of particle size dependence with all samples showing that the silt fraction possesses

(PSD) size magnetic grains dominate the magnetic assemblage of all Icelandic fractions. In contrast,

Greenlandic samples possess greater variation in magnetic grain size; only fine silt and clay are as

magnetically fine as the Icelandic PSD grains, while Greenlandic silts and sands are dominated by

coarser PSD and multi-domain grains. These observations from potential marine sediment sources

suggest that the silt size fraction is a likely driver for much of the concentration-dependent parameters

derived from bulk magnetic records and that the magnetic grain size of the silt fraction can be used to

discriminate between Icelandic and Greenlandic sources. Using these results to examine magnetic grain

size records from marine sediment cores collected across the northern North Atlantic suggests that

source, not just transport-controlled physical grain-size, has a significant impact on determining the

magnetic grain size at a particular location. Homogeneity of magnetic grain size in Icelandic sediments

at least partially explains the consistent quality of paleomagnetic records derived from cores

surrounding Iceland and their ability to buffer large environmental changes.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Magnetic studies of northern North Atlantic (NNA) deep seasediments have provided invaluable paleo-geomagnetic informa-tion (Stoner et al., 1995a, 2007; Channell and Lehman, 1997;Channell et al., 2002; Mazaud and Channell, 1999; Mazaud et al.,2009), and have contributed to our understanding of past changesin ocean circulation (Rasmussen et al., 1996, 1997; Kissel et al.,1997, 1999, 2009; Snowball and Moros, 2003; Kanamatsu et al.,2009) and the dynamics of surrounding continental ice sheets(Robinson, 1986; Stoner et al., 1995b; Stoner and Andrews, 1999;Evans et al., 2007). The quality of these NNA magnetic records isfacilitated by high rates of sediment transfer from the continents tothe ocean through glacial erosion of the surrounding ferrimagne-tically rich igneous and metamorphosed bedrock. Regional sour-cing, (re)distribution, sorting, and focusing of sediment by ocean

All rights reserved.

: þ1 541 737 2064.

R.G. Hatfield),

@qub.ac.uk (A.V. Reyes),

currents, coupled with a general lack of diagenetic transformations(Stoner and Andrews, 1999; Robinson et al., 2000; Watkins andMaher, 2003), act to define the sediment magnetic characteristicsat any particular location, providing both stratigraphic and envir-onmental information. Magnetic homogeneity is a preferredrequirement for paleomagnetic studies, while variation in mag-netic properties can indicate environmental change. Despite thesedifferent magnetic requirements, the NNA has consistently deliv-ered high quality paleomagnetic and environmental magneticrecords, often from the same core (e.g., Stoner et al., 1995a,1995b, 2000; Kissel et al., 1997, 1999; Evans et al., 2007).

Variation in both sediment source and the extent of currentrelated sorting is reflected in the physical properties of sedi-ment deposited across the NNA (e.g., Robinson, 1986; Groussetet al., 1993; McCave et al., 1995; Revel et al., 1996; Bianchi andMcCave, 1999; Hemming, 2004). While magnetic properties ofsediments commonly possess strong particle size dependence(Oldfield et al., 1985; Oldfield and Yu, 1994; Rosenbaum andReynolds, 2004; Hatfield and Maher, 2008, 2009; Rosenbaumet al., 2012), magnetic records of deep-sea sediments are usuallyacquired through measurement of bulk (whole sample) sedi-ments, either via whole core, u-channel or discrete samples. Bulk

R.G. Hatfield et al. / Earth and Planetary Science Letters 368 (2013) 69–7770

measurement aggregates the contributions from all the differentclastic size fractions and source lithologies into a single measure-ment that can either smooth or amplify source and/or sortinginformation in a way that can make interpretation of the bulkmagnetic record potentially problematic. Similarly, bulk measure-ments make it difficult to ascertain why some paleomagneticrecords work better than others. We know that the best paleo-magnetic records are derived from sediments possessing arestricted magnetic grain-size range (e.g., King et al., 1983;Tauxe, 1993); however, we rarely understand where or how thesegrains are sourced, making it difficult to predict where highquality records are likely to be obtained.

How magnetic properties from different source regions varywith particle-size, how particle-size fractions vary through sedi-mentary processes (weathering, erosion, transportation, anddeposition), and how this influences the sediment magneticrecord at different locations across the NNA has rarely beenconsidered when interpreting bulk environmental and paleomag-netic records. An evaluation of these influences and an under-standing of their potential effects on the bulk sediment record ofthe NNA is therefore critical for any comprehensive evaluation.Here we present grain-size specific magnetic data from terrestrialglaciated terranes of Iceland and southern Greenland (Fig. 1). Wethen use these data to differentiate between these source regions,and begin to discuss the implications that both source andparticle size variation may have for the interpretation magneticrecords from different locations around the NNA.

2. Materials and methods

A large proportion of the lithogenic sediment in the NNAresults from glacial erosion of Iceland and Greenland (Ruddiman,1977; Laine, 1980; Larsen et al., 1994; Prins et al., 2002). Thegeology of Iceland (and east-central Greenland) is dominated byNeogene and Paleogene flood basalts (Jakobsson, 1972; Pedersen

Fig. 1. Location of terrestrial stream sediment samples collected from the Paleogene Vo

squares), the Archean Block (AB; red squares) and the Nagssugtoqidian Mobile Belt (NM

Ice cores sites (GRIP, GISP2, and NGRIP; yellow squares) are also shown for reference.

path of major bottom water currents as components of North Atlantic Deep Water (NAD

the reader is referred to the web version of this article.)

et al., 1997). The Precambrian rocks in southern Greenland can befurther divided into three geological terranes (Fig. 1); the Ketili-dian Mobile Belt (KMB) is primarily juvenile Proterozoic crustconsisting of voluminous granitic intrusions, while the ArcheanBlock (AB) and the Nagssugtoqidian Mobile Belt (NMB) arecrystalline Archean basement rocks that remained undeformedor were metamorphosed during the Proterozoic, respectively(Fig. 1; Kalsbeek and Taylor, 1985; Korstgard et al., 1987; Escherand Pulvertaft, 1995). Throughout the text we use the term‘‘Greenlandic’’ in reference to sediment sourced only from thesethree southern Greenlandic terranes. To characterize these NNAterrestrial sediment sources we collected sediment from streamsdraining actively glaciated watersheds of Iceland and the threesouthern Greenlandic terranes (Fig. 1). Bulk magnetic propertieswere first measured prior to grain-size separation. Sand wasisolated through sieving at 63 mm. Half of the o63 mm fractionwas settled according to Stoke’s Law to attain bulk silt (3–63 mm)and clay (o3 mm) fractions. The remainder of the o63 mmfraction was sieved and settled to create four silt fractions: 45–63 mm, 32–45 mm, 20–32 mm, 10–20 mm and a fine silt/clayfraction o10 mm which is at the boundary defining cohesive/non-cohesive sediment transport (e.g., McCave et al., 1995;McCave and Hall, 2006). All bulk and fractionated samples wereimmobilized in plastic 8 cc discrete sample boxes prior to mea-surement of magnetic susceptibility and magnetic remanence;200 mg gelatin caps were used for hysteresis measurements.Mass normalized magnetic susceptibility (MS) was measured at0.47 kHz on a Bartington MS2B. Mass-normalized anhystereticremanent magnetization (ARM) was imparted at 100 mT in a0.05 mT d.c. biasing field and mass-normalized isothermal rema-nent magnetization (IRM) was acquired at 100, 300, and a1000 mT saturation IRM field (SIRM). ARM and IRM were mea-sured on a 2G Enterprises cryogenic magnetometer along with MSin the Paleo-and-Environmental Magnetism Laboratory at OregonState University. Hysteresis parameters (Mrs, Ms, Hc, and Hcr) wereacquired in a saturating field of 1000 mT on a Princeton MicroMag

lcanics (PV) of Iceland (black squares), and the Ketilidian Mobile Belt (KMB; orange

B; purple squares) terranes of southern Greenland. The location of the Greenland

Core locations (circles) are color coded as per their spatial groupings in Fig. 5. The

W) is also shown. (For interpretation of the references to color in this figure legend,

R.G. Hatfield et al. / Earth and Planetary Science Letters 368 (2013) 69–77 71

VSM at the Pacific Northwest Paleomagnetic Laboratory at Wes-tern Washington University.

MS, ARM and IRM are primarily related to the concentration offerrimagnetic grains. Ratios of these parameters can provideinformation on magnetic mineralogy and magnetic grain size.Magnetic grain-size is related to the mean magnetic domain statemeasured through the magnetic response to applied fields (e.g.,Dunlop, 2002a, 2002b), whereas clastic grain-size is based on thephysical particle size. Hysteresis parameters were corrected forparamagnetic contributions before construction of Day plots (Dayet al., 1977) to accurately distinguish between single domain (SD),pseudo-single domain (PSD) and multi domain (MD) ferrimag-netic grain sizes (see Stoner et al. (1996), Stoner and St-Onge(2007) or Table 1 in the online supplementary material for furtherexplanation of these terms).

Fig. 2. (a) Histograms of bulk wARM/w of terrestrial sediment samples from Iceland and

one standard deviation about the mean for each of the six particle size fractions from

similarity in the four silt fractions (10–20 mm, 20–32 mm, 32–45 mm, and 45–63 mm)

terranes and the sand fraction (463 mm) in the Greenlandic terranes.

3. Results

Magnetites and/or titanomagnetites have been shown todominate the magnetic fraction of sediments in the NNA (e.g.,Stoner et al., 1995a, 1995b; Lehman et al., 1996; Channell andLehman, 1997; Kissel et al., 1997; Kissel, 2005). Our terrestrialsediments are consistent with that interpretation. Bulk MS is40.2�10�6 m3 kg�1, 490% of SIRM is acquired in a field of300 mT, and all hysteresis loops display saturation in fieldso200 mT, indicating that ferrimagnetic phases govern the bulkmagnetic mineral assemblage (Thompson, 1986; Channell et al.,1997; Dearing, 1999). These samples all fall on magnetite mixinglines (Dunlop, 2002a, 2002b) within a Day plot (Day et al., 1977)plot consistent with (titano)magnetite as the major remanencecarrier. In general, bulk Greenlandic samples have slightly lower

the three terranes of southern Greenland. (b) Mean magnetic susceptibility with

Iceland and the three terranes of southern Greenland. Note for each terrane the

and their higher values compared to the fine silt/clay fraction (o10 mm) for all

R.G. Hatfield et al. / Earth and Planetary Science Letters 368 (2013) 69–7772

coercivity, acquiring a greater portion of their SIRM by 100 mTthan bulk Icelandic samples. Icelandic samples have higherwARM/w ratios than the Greenlandic samples (Fig. 2a), suggestingthat higher Icelandic bulk coercivity may be related to finermagnetic grain sizes. All Greenlandic and Icelandic samples showstrong particle size dependence of magnetic susceptibility(Fig. 2b). Greenlandic silts (10–63 mm) possess two to three timesthe MS of the 463 mm fraction and greater than five times themean MS of the o10 mm fraction. The particle size dependence ofIcelandic samples is lower yet distinct, with the MS of theo10 mm fraction two to three times weaker than the 410 mm

Fig. 3. Representative Hysteresis loops of clay (o3 mm), silt (3–63 mm) and sand (463

by paramagnetic contributions above 500 mT. Note the common magnetization values

to MS. Hysteresis loops display ferrimagnetic behavior with wider loops characteristic

Greenlandic samples.

fraction. For each of the four terranes, the four silt fractionspossess similar MS, suggesting lower inter-fraction variationwithin the silt fraction; much of the variance in MS is betweensand, silt, and fine silt/clay. Hysteresis loops of representativesand, silt, and clay fractions for the four terranes are shown inFig. 3. Like MS, Ms is higher in the silt fraction, indicating a greaterconcentration of ferrimagnetic minerals. All Icelandic fractionshave similar shaped, relatively wide hysteresis loops indicatingcontributions of relatively fine magnetic grains. In contrast inGreenland, only clays display this behavior; silts and sands fromthese terranes have much narrower loops, consistent with coarser

mm) fractions from the four different terranes. Magnetic acquisition is dominated

within terranes but different values between terranes with variations in Ms similar

of fine ferrimagnetic grain sizes in all Icelandic samples and the clay fractions of

R.G. Hatfield et al. / Earth and Planetary Science Letters 368 (2013) 69–77 73

magnetic grains. Day plots (Day et al., 1977) of the six particle-size fractions from representative samples are shown in Fig. 4a,and a range of clay (o3 mm), silt (3–63 mm) and sand (463 mm)fractions from different Iceland and Greenland samples are shownin Fig. 4b. As suggested from Fig. 3, Icelandic samples show littlegrain-size dependence between size fractions; clays, fine silts/clays, silts, and sands all cluster within the fine PSD range (Fig. 4aand b), possessing a higher proportion of SD grains (Dunlop andCarter-Stiglitz, 2006). In contrast, Greenlandic silts and sandspossess a higher proportion of MD size grains and plot within thecoarser PSD/MD and MD grain size range; only Greenlandic clayspossess fine Icelandic-sized PSD magnetites. Icelandic PSD grainsalso have slightly higher ratios of Hcr/Hc than Greenlandic PSDsamples, possibly indicating either greater concentrations ofoxidized magnetite and processes of maghematization (e.g.,

Fig. 4. Magnetic hysteresis parameters Mrs/Ms and Hcr/Hc with magnetic grain size SD

after Day et al. (1977) for (a) representative fine silt/clay (o10 mm), four silt (10–20

terrestrial Icelandic and Greenlandic terranes; (b) clay (o3 mm), silt (3–63 mm) and sa

Iceland (black crosses) and the three terranes of southern Greenland (orange triangles,

coarse MD magnetite is consistent with magnetic grain-size variations and with the e

Stiglitz (2006)). Note that only the clay and fine silt/clay sized particles from Greenland

magnetic grain size of silt and sand size fractions from Greenland and Iceland can be u

color in this figure legend, the reader is referred to the web version of this article.)

Ozdemir et al., 1993; Stoner et al., 1995a; Smirnov and Tarduno,2000) or the presence of small concentrations of super-paramagnetic (SP) grains (e.g., Dunlop, 2002b). Subtle, yet distinctvariation exists between Greenlandic silt fractions. MS is highestin NMB sediments followed by KMB with AB samples the weakestmagnetically (Fig. 2). The high-MS NMB silt samples are magne-tically a little coarser than the KMB or AB samples, with the NMBsilts plotting to the bottom right of the Day plot (Fig. 4c).

In summary, differences exist in magnetic concentration andmagnetic grain size both between different terranes, and betweendifferent size fractions within a given terrane. When massnormalized, the silt fraction carries a large proportion of themagnetic signal from all terranes (Fig. 2b) and can be expected todominate the properties of bulk marine records derived fromthem. In Greenlandic sediments this fraction has a coarser

(single domain), PSD (pseudo-single domain) and MD (multi-domain) zonations

mm, 20–32 mm, 32–45 mm, and 45–63 mm) and sand fractions (463 mm) from

nd (463 mm) fractions from Iceland and Greenland; and (c) silts (3–63 mm) from

KMB; red diamonds, AB; purple circles, NMB). The observed trend from fine SD to

mpirical and theoretical defined magnetite mixing line (after Dunlop and Carter-

are as magnetically fine as Icelandic material of any size. Differences between the

sed to distinguish between these sources. (For interpretation of the references to

R.G. Hatfield et al. / Earth and Planetary Science Letters 368 (2013) 69–7774

magnetic grain size than Icelandic sediments (Fig. 4a and b), withthe latter being restricted to relatively homogeneous fine PSDmagnetic grain sizes across all particle-size fractions.

Fig. 5. Day plot with SD (single domain), PSD (pseudo-single domain) and MD

(multi-domain) zonations after Day et al. (1977). Icelandic silt fractions (black

crosses) and the silt Mrs/Ms ranges from both Iceland (black shaded) and

Greenland (green shaded) are shown for comparison against published data from

Iceland proximal cores SU90-33 and PS2644-5 (Kissel et al., 1999), SO80-05, SO82-

07 and LO09-18 (Snowball and Moros, 2003), MD99-2247 (Ballini et al., 2006), and

MD03-2674, MD03-2678, MD03-2684, MD03-2680, and MD03-2676 (Kissel et al.,

2009), Norwegian Sea cores MD95-2009 and MD95-2010 (Kissel et al., 1999),

Greenland proximal cores SU90-24 and SU90-16 (Kissel et al., 1999), MD99-2244

(Ballini et al., 2006) and MD99-2227 (Evans et al., 2007), and a core from the

Bermuda rise MD95-2034 (Kissel et al., 1999). Note that Iceland proximal cores

have characteristically finer magnetic grain-sizes compared to other cores from

the NNA. (For interpretation of the references to color in this figure legend, the

reader is referred to the web version of this article.)

4. Discussion

Our stream sediment data (Figs. 2–4) suggest that variations insediment contributions from different terranes, and the particlesizes eroded from those terranes, have the potential to substan-tially influence the bulk magnetic properties of marine sedimen-tary sequences in the NNA. It is critical to understand why andhow these variations potentially affect the bulk magnetic recordbecause paleomagnetic records demand, and environmental mag-netic records have almost exclusively been derived from, mea-surement of undisturbed bulk sediments.

Because ferrimagnetic minerals dominate all fractions, MS isprimarily sensitive to the concentration of ferrimagnetic grains. Inall samples, silts have higher MS than sand or clay. Lowerconcentrations of ferrimagnetic minerals in the finest clasticfraction may result from a relative resistance to sub-micronweathering and/or chemical weathering, resulting in the genera-tion of paramagnetic clay minerals (e.g., Nesbitt and Wilson,1992; Stefansson and Gıslason, 2001). Relatively few discreteferrimagnetic grains grow to sand size or larger. Thus theferrimagnetic component in the sand fraction may exist asdiscrete ferrimagnetic inclusions or interwoven fabrics withinnon-magnetic host grains (e.g., Hounslow and Morton, 2004;Feinberg et al., 2005), which dilute and lower the overall MS. Siltfractions have the highest magnetic concentration and thereforeappear to be the optimal physical size for discrete, unalteredferrimagnetic particles and/or poly-crystalline clasts dominantlycomposed of ferrimagnetic grains. The similarity between Icelan-dic silts and their clay and sand fractions probably results from amore homogeneous basaltic parent geology compared to themore complex mineralogy of Greenland’s Precambrian terranes,where geochemical processes and cooling rates can be importantfactors in determining mineralogy, physical crystal-size andmagnetic domain state (Thompson and Oldfield, 1986;Radhakrishnamurty, 1990; Zhou et al., 1997; Butler, 1992). Inrapidly cooled basalts, growth of MD grains may be restricted(Radhakrishnamurty et al., 1977, 1978; Radhakrishnamurty,1990), and they commonly contain a greater proportion of fine-grained magnetites (in the SP/SD/PSD grain size range) thanslowly cooled intrusive rocks (Pick and Tauxe, 1993, 1994; Kentand Gee, 1994; Gee and Kent, 1999; Zhou et al., 2000). In contrast,igneous intrusions and metamorphosed gneisses in southernGreenland cooled over a longer time period and at much slowerrates (e.g., 1–7 1C Ma�1 for the NMB; Willigers et al., 2001, 2002;Sidgren et al., 2006), potentially permitting growth of differentsize magnetic domains up to and including larger MD sizecrystals. Subsequent weathering and sorting of this more hetero-geneous mineralogy can cause partitioning of magnetic mineralsinto different clastic size fractions. The dominantly basaltic andsimpler composition of Icelandic samples results in greaterhomogeneity in magnetic concentration and grain size across allclastic fractions (Figs. 2–4). The more complex Greenland geologyand associated variety in magnetic grain size appears to track theclastic distribution, with finer PSD grains and/or a higher propor-tion of SD grains residing in the finest fraction and coarser PSD–MD grains dominant in silts and sands (Fig. 4a and b).

4.1. Magnetic minerals in the northern North Atlantic

Deep sea sediments can have a long, complex and variedhistory. Ice rafting can provide sand-sized grains to these

environments during glacial periods but otherwise many deep searecords, and those outside of the ice-rafted debris belt (e.g.,Ruddiman, 1977) are dominated by relatively fine silts and clays(e.g., Stow and Holbrook, 1984; Revel et al., 1996; Kissel et al., 1999,2009; Snowball and Moros, 2003; Channell and Raymo, 2003;Praetorius et al., 2008). As most magnetic records are acquiredthrough measurement of bulk sediments, they are an aggregatedand averaged assessment of all the magnetic particles in all clasticfractions, reflecting contributions from different sources and/orchanges in sediment transport or deposition. Our data highlightthe importance of understanding subtle variations in source andclastic size composition when interpreting bulk magnetic records.

For example, a compilation of magnetic grain size data from 18bulk NNA records spanning from the Norwegian Sea to theBermuda Rise (Fig. 1) are spatially grouped and compared toGreenlandic and Icelandic silt fractions in Fig. 5. These records areconstructed of mixtures of different source end-members thatvary spatially across the NNA. While PSD grains dominate theaverage magnetic grain size of these records, distinct spatialgroupings are apparent. Iceland proximal cores possess the finestbulk magnetic records. More distal records, which may beincreasingly influenced by other terrestrial sources like Green-land, Norway and North America, fall further along the PSD–MDmixing line, indicating greater concentrations of coarser magneticgrains (e.g., Dunlop, 2002a; Dunlop and Carter-Stiglitz, 2006). Thismay suggest that like Greenland, sediments from Norway and

R.G. Hatfield et al. / Earth and Planetary Science Letters 368 (2013) 69–77 75

North America possess significant grain-size dependence in theirmagnetic properties and that fine magnetic homogeneity acrossall sized fractions is somewhat unique to Iceland within the NNA.Fine magnetic grain sizes from bulk records around Iceland havebeen previously inferred (e.g., Watkins and Maher, 2003; Kisselet al., 2009); however with our data, we demonstrate that thisreflects an intrinsic property of the sediment source regardless ofsediment grain-size.

4.2. Implications for paleomagnetic and environmental magnetic

data

Many high quality paleosecular variation (PSV) and relativepaleointensity (RPI) records have been obtained from the NNA,and in particular from sediment drifts in the vicinity of Iceland(e.g., Channell and Lehman, 1997; Channell et al., 1997, 2002,2004; Laj et al., 2002). These sediments are generally character-ized by consistently well-resolved magnetic vectors (MaximumAngular Deviation (MAD) values generally o21) and homoge-nous, relatively fine magnetic grain sizes. Coupled with relativelyhigh sedimentation rates, these NNA records have driven much ofour understanding of RPI and PSV, and have become importantcomponents in stacked paleomagnetic records (e.g., Laj et al.,2000; Valet et al., 2005; Channell et al., 2009; Ziegler et al., 2011).Interactions with bottom water currents that source and sortbasaltic fragments are generally thought to be responsible for thehomogenous PSD character of these drift records (e.g., Kissel,2005; Kissel et al., 2009). However, homogenously fine magneticgrains in Iceland-proximal records are often maintained throughperiods when bottom current strength is highly variable (e.g.,Channell et al., 1997; Praetorius et al., 2008) and over glacial–interglacial cycles when the magnetic grain size of other driftscoarsens (e.g., Channell and Raymo, 2003; Evans et al., 2007).Seemingly unique in the NNA, the magnetic grain size of Icelandicsediments is independent of physical grain-size; records derivedfrom these sources receive PSD grains despite the clastic fractionsupplied, deposited, or sorted. This buffering of the magnetic grainsize record against environmental change places greater importanceon the source of the material rather than subsequent current relatedsourcing in the generation of these high quality records. Theconsistent quality of Iceland-proximal records had not been speci-fically examined, hindering our ability to predict locations that arelikely to provide continuous high-quality paleomagnetic records.With our new data it seems that the fine PSD character of Iceland-derived sediments likely minimizes the influence of varying sedi-mentary processes, which in other regions could result in thesourcing and deposition of coarser, sub-optimal magnetic grainsizes for developing quality paleomagnetic records.

Excluding diagenetic and authigenic effects, variations inmagnetic parameters in the NNA are often interpreted as varia-tion in the sourcing of material (e.g., Robinson, 1986; Stoner et al.,1995a; Grousset et al., 1993; Watkins and Maher, 2003). Our datashow that differences in bulk records can also potentially bedriven by variation in clastic grain size from the same source,particularly when those sources possess strong particle-sizedependence. Thus for interpretation, it is important to recognizeand evaluate how these different contributions vary and canaffect a record. Figs. 2–4 suggest that the silt size fraction is alikely driver for much of the magnetic parameters derived frombulk magnetic records, with the important implication that themagnetic grain size of the silt fraction can be used to discriminatebetween Icelandic and Greenlandic sources.

These inferences may be able to help explain some complexmagnetic records from the NNA like, for example, the stronglinkage between MS and the Greenland Ice core (e.g., GISP2, GRIP,NGRIP) oxygen isotope record (e.g., Rasmussen et al., 1996, 1997;

Kissel et al., 1999). As greater current speed is often associatedwith a coarser fraction of sortable silt (e.g., McCave et al., 1995;Bianchi and McCave, 1999; Praetorius et al., 2008) and as the siltfraction possesses 2–5� the MS of the clay fraction (Fig. 2),changes in the physical grain size of oceanic records may havesignificant effects on bulk magnetic properties. While magneticgrain size remains relatively constant in Icelandic proximalrecords (Ballini et al., 2006; Kissel et al., 2009) changes in clasticgrain size and their attendant effect on MS may result in thecoupling and strong sensitivity of marine MS records with currentspeed and Greenland ice-core records. Where magnetic grain sizechanges significantly, our data suggest that this may reflect achange in the sediment source. Increased distance from Icelandicsources results in a coarser average magnetic grain size (Fig. 5),suggesting that local sourcing is a basin wide process affecting themagnetic composition of all cores. In areas influenced by manysources, like the Snorri and Eirik drifts (e.g., Innocent et al., 1997;Fagel et al., 2004) and the Reykjanes Ridge (e.g., Prins et al., 2002),the interpretation of magnetic grain-size and magnetic concentra-tion as reflecting circulation may be more complicated than can beassessed with bulk magnetic measurements alone. This is particu-larly true over glacial to interglacial timescales where large changesin sediment source and flux are linked to the waxing and waning ofcontinental ice sheets (e.g., Stoner et al., 1995b; Carlson et al., 2008;Colville et al., 2011). Because both source and/or particle sizevariation can drive the bulk magnetic properties of ocean sediments,it can be difficult to document the degree to which the bulkmagnetic signal is affected by either process. However, we haveshown potential for isolation and unraveling of these competingsignals through grain-size-specific magnetic measurements.

5. Conclusions

Magnetic properties of potential NNA sediment sources fromIceland and Greenland are strongly dependant on sediment particlesize. Icelandic sediments, whether they are sand, silt or clay, showlittle variation in magnetic grain-size, plotting in a restricted regionon Day plots. In contrast, magnetic grain-sizes of Greenlandicsediments vary with physical grain-size; only Greenlandic clayhas Icelandic-like fine magnetic grains, while silts and sandspossess coarser PSD and MD grain sizes. Higher MS of the siltfraction makes it potentially an important driver of bulk concen-tration dependent parameters in sediments derived from Green-land and Iceland, with magnetic grain size variations of the silt-sizecomponent capable of discriminating between these two sources.These remanence and hysteresis measurements provide newinsights for interpreting paleomagnetic records, specifically relatedto the quality of paleo-geomagnetic records obtained from specificlocations and the consistent quality of records through changingsedimentary and environmental regimes. For environmental mag-netic records, these results highlight the importance of particle-size dependence, and how the knowledge of this dependenceimproves the discrimination afforded by magnetic measurements.

Supplementary data

All data are available from the World Data Center for Paleo-climatology (http://www.ncdc.noaa.gov/paleo/paleo.html).

Acknowledgments

This research was supported by National Science FoundationPaleo Perspectives on Climate Change Grants ARC-0902751 and

R.G. Hatfield et al. / Earth and Planetary Science Letters 368 (2013) 69–7776

ARC-0902571 to JSS and AEC, respectively, and Instruments andFacilities Grant EAR 0651585 to JSS. RGH would like to thankJason Dorfman and Chuang Xuan at OSU and Russ Burmester atWWU for assistance with hysteresis measurements, and LisaColville, Kelsey Winsor, and Sarah Strano for assistance withstream sediment sampling on Greenland and Iceland.

Appendix A. Supporting information

Supplementary data associated with this article can be foundin the online version at http://dx.doi.org/10.1016/j.epsl.2013.03.001.

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