Dating ice shelf edge marine sediments: A new approach using single grain quartz luminescence

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1 Click Here for Full Article JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115,, doi: /2009jf001415, 2010 Dating ice shelf edge marine sediments: A new approach using single grain quartz luminescence G. W. Berger, 1 A. S. Murray, 2,3 K. J. Thomsen, 4 and E. W. Domack 5 Received 11 June 2009; revised 20 January 2010; accepted 15 February 2010; published 3 July [1] To develop an alternative dating tool for the Antarctic Peninsula (where the 14 C method requires large, spatially variable reservoir corrections), we tested the clock zeroing assumption of photon stimulated luminescence (PSL) dating using a variety of PSL procedures. At ice shelf edges around the Antarctic Peninsula, sediment water interface ( zero age analogs), silt rich short cores were collected in , originally only for fine silt dating tests. Later access to suitable instrumentation also permitted testing the potential of single grain quartz (SGQ) dating of sand grains from these cores. For the fine silt grains we employed multiple aliquot and single aliquot methods to obtain last daylight exposure age estimates from near core top material. With the sand fraction we employed automated SGQ PSL methods to isolate the youngest grains. Five of six fine silt samples gave unreasonably large age estimates (>20 ka), with the sixth sample yielding a multiple aliquot short bleach age estimate of 1.1 ± 0.6 ka. In contrast, five of seven sand samples yielded geologically reasonable last daylight exposure ages of ka. These SGQ results are also remarkable when compared to published 14 C ages of 1 ka to 9.7 ka from core top living calcite and acid insoluble organic matter. These SGQ results establish the likely utility of this single grain dating approach in such settings to provide chronologies for calving line histories of ice shelves. To take advantage of this utility, core collection should employ large diameter coring devices (e.g., Kasten and multicorers). A caveat is that large numbers (e.g., 10,000) of quartz grains may need analysis to provide acceptable statistics for useful age calculations. Citation: Berger, G. W., A. S. Murray, K. J. Thomsen, and E. W. Domack (2010), Dating ice shelf edge marine sediments: A new approach using single grain quartz luminescence, J. Geophys. Res., 115,, doi: /2009jf Introduction 1.1. Ice Shelves [2] Ice shelves form significant features around Antarctica and also occur in the Arctic [e.g., Darby and Zimmerman, 2008, and references therein], and were more extensive in both regions during past glacial intervals. Ice shelves wax and wane due to several causes, most prominently climatic changes. For example, the sudden collapse of the Larsen B Ice Shelf (herein LIS B) (eastern Antarctic Peninsula, Figure 1) in 2002 [e.g., MacAyeal et al., 2003; Scambos et al., 2003; Glasser and Scambos, 2008] followed decades of gradual warming in the region [e.g., Scambos et al., 2000; 1 Desert Research Institute, Reno, Nevada, USA. 2 Nordic Laboratory for Luminescence Dating, Department of Earth Science, University of Aarhus, Aarhus C, Danmark. 3 Risø National Laboratory for Sustainable Energy, Technical University of Denmark, Roskilde, Denmark. 4 Radiation Research Department, Risø National Laboratory for Sustainable Energy, Technical University of Denmark, Roskilde, Denmark. 5 Department of Geosciences, Hamilton College, Clinton, New York, USA. Copyright 2010 by the American Geophysical Union /10/2009JF Vaughan and Doake, 1996]. This LIS B collapse has stimulated efforts to understand the stability histories of ice shelves, as well as the genesis of the North Atlantic Heinrich events [e.g., Hulbe et al., 2004]. The future behavior of ice shelves in Antarctica has global implications [e.g., Vaughan, 2006; Pritchard and Vaughan, 2007], but the timing and frequency of past advances and retreats for each ice shelf must be known to allow us to fully assess any modeling of future scenarios. Establishing such chronologies requires that (1) we recognize in marine cores the sedimentary signature of each past ice shelf retreat and advance and (2) we date these sediments Sedimentary Signatures [3] Criteria for recognition of a clear sedimentary signature have been slow to develop, and divergent views persist [e.g., Domack and Harris, 1998; Domack et al., 1999a, 2003; Evans and Pudsey, 2002; Ó Cofaigh and Dowdeswell, 2001], likely because of the spatial and temporal complexity of ice shelf retreat and advance. While there may be little disagreement about recognition of grounding line deposits left by shelf advances, retreats often are more difficult to recognize in the sedimentary record. However, during retreat shelves have collapsing edges that can leave recognizable deposits 1of22

2 from sub ice shelf sediments exposed by the recent rapid retreat of the Larsen A and Prince Gustav ice shelves, northeastern Antarctic Peninsula. They described textural traits, applied 210 Pb and 14 C analyses, and inferred a sedimentary signature of the breakup of the ice shelves [Gilbert and Domack, 2003, p. 2]. The key inference that departs from previous similar studies [see also Barrett et al., 2005] is the role of eolian sand and silt in the sedimentary processes. They argue from evidence that eolian sand and silt deposits can accumulate on stable and advancing ice shelves, concentrate in melt ponds and crevasses during regional warming, then be released rapidly during collapse of the shelf (Figure 2). A caveat is that the bottom distribution of this eolian sand would be spatially heterogeneous. One can envisage also the rapid deposition of eolian sand (if the local geology yields eolian material) at the calving lines of small tidewater glaciers and small ice shelves Dating Difficulties [5] Paralleling the complexity of documenting and classifying these regional variations in characteristics of expected shelf edge deposits, the dating of related sediments also is challenging in the Antarctic region. Numerical (inaccurately absolute ) dating of such signature successions around Antarctica depends mainly on the 14 C method, which presents further difficulties. Principal among these is the spatially Figure 1. Satellite image map of the locations of the three study areas and the core sites: Müller Ice Shelf in Lallemand Fjord (black box and bottom left insets); Larsen B Ice Shelf and Larsen A Ice Shelf (top and right inset). Also shown in the Larsen A and Larsen B areas are the recent extents of the ice shelves. This satellite image mosaic is constructed from component images available from the Landsat Image Mosaic of Antarctica project (LIMA, in some areas. Domack and Harris [1998] proposed a distinct facies succession as diagnostic in sediment cores of retreat of the calving line of the Ross Ice Shelf (Antarctica). In particular, at the calving line a concentration of coarse, iceberg rafted debris is deposited [Domack and Harris, 1998, p.284] with different, characteristic facies being preserved beneath and above this in cores. In investigating recently exposed sub ice shelf sediment facies in the Larsen A Ice Shelf (herein LIS A) area (Figure 1) Evans and Pudsey [2002] described a more heterogeneous succession facies. In the same region Evans and Ó Cofaigh [2003] described very coarse grained (up to cobble size) supraglacial deposits that would end up on the seafloor at a calving line. In the North Atlantic, it is now proposed [Hulbe et al., 2004] that Heinrich layers in ocean cores can be diagnostic of ice shelf collapse events, but with facies differences depending upon distance from putative paleoshelves [e.g., Rashid et al., 2003]. [4] Responding to the need to obtain additional diagnostic sedimentary criteria of waxing and waning ice shelves, Gilbert and Domack [2003] studied grab samples and cores Figure 2. Cartoon showing the accumulation of eolian sediment on the surface of an ice shelf, and the existence of ponds and crevasses before final disintegration of the ice shelf (modified from Gilbert and Domack [2003]). Draining of the ponds and crevasses releases eolian sand in a spatially heterogeneous pattern beneath the ice shelf. Also shown is sediment within the base of the ice shelf that would be released before (by underside melting) and during disintegration and mixed with the eolian sand in bottom sediments. As well, during the existence of the ice shelf, bottom currents could carry silt and sand from the grounding line and upslope areas seaward, to be deposited in the same zones that later receive eolian sand. 2of22

3 variable and large 14 C reservoir effect around Antarctica [e.g., Berkman and Forman, 1996; Domack et al., 1999a, 1999b, 1999c; Gordon and Harkness, 1992], as well as the reworking of old organic carbon. These dating difficulties require local assumptions and solutions. Some of the continuing limitations of 14 C dating applied to glacimarine sediments in this region are well summarized by Brachfeld et al. [2003], who report sediment water interface 14 C ages ranging from 1 ka (living bryozoan calcite) to 9.7 ka (acid insoluble organic matter). They also outline down core complexities in application of 14 C dating. Further complexities in the interpretation of 14 C dating results in ice shelf settings are outlined by Licht and Andrews [2002]. Thus, an additional geochronometer is needed for unraveling the retreat/advance histories of ice shelves and tidewater glaciers. [6] Because of the possibility of daylight exposure (during austral summer) of suspended silt grains, we tested multigrain polymineral fine silt photon stimulated luminescence (PSL, also termed OSL ) dating methods [e.g., Aitken, 1998] around the Antarctic Peninsula. Because our cores were originally collected only for fine silt PSL dating, the cores and subsamples were not selected with a view to utilizing sand. However, the report of Gilbert and Domack [2003] and subsequent access to instrumentation for singlegrain quartz (SGQ) PSL dating [e.g., Olley et al., 1999; Bøtter Jensen et al., 2003; Duller, 2004, 2008; Lian and Roberts, 2006] encouraged the testing of this approach on these samples. Because the PSL signal in eolian quartz can be zeroed in a few seconds of full sunlight [e.g., Godfrey Smith et al., 1988], and because quartz PSL is stable over the periods of interest here [e.g., Aitken, 1998], PSL dating of quartz sand has become an established geochronometer for nonpolar sediments [e.g., Murray and Olley, 2002; Bøtter Jensen et al. 2003; Duller, 2004; Fuchs and Owen, 2008; Jacobs, 2008; Jacobs and Roberts, 2007;Preusser et al., 2008; Rittenour, 2008]. [7] Olley et al. [2004a] first reported use of SGQ dating of marine cores. Unlike here, they chose core samples for which almost all the quartz sand was known in advance to be eolian, making PSL measurement and data interpretation easier than in our study. In our study we infer the presence of eolian quartz only indirectly, as explained below. In their study, true single grain measurements were conducted (grain size fraction mm in grain holes 100 mm deep by 100 mm diameter). We were constrained to use grain holes 300 mm deep by either 300 mm or 200 mm diameter, so that for most of our single grain analyses 2 15 grains (depending on the grain size range employed) resided in each hole. However, we determined from multigrain and single grain PSL tests that with our samples <1/5 to <1/100 of the grains emit measurable signals. Thus, our single grain analyses are (excepting perhaps some experiments with mm grains) effectively true single grain dating experiments. [8] In our study region, silt and sand sized quartz and feldspar grains could be exposed to sufficient daylight if deposited during austral summer on later collapsed glacier/ shelf surfaces, or if silt sized grains were transported suitable distances in near surface suspensions before settling to the bottom. [9] Our main questions are as follows: (1) Are there sufficient numbers of daylight exposed silt grains in near ice shelf sediment cores to record near zero PSL ages for nearcore top samples? (2) Are there sufficient numbers of eolian quartz sand grains in calving line deposits and do they have suitable PSL signals to permit their isolation and dating by SGQ procedures? If either or both of these questions has a positive answer, then we would have a new tool for dating calving line events in calendar years ranging from the last few decades [e.g., Ballarini et al., 2007a] to 150 ka [e.g., Murray and Olley, 2002; Rittenour, 2008], thus both circumventing all the problems associated with 14 C dating and reaching well beyond its range (typically ka). Furthermore, PSL dating can circumvent the problems associated with 14 C dating over the last few centuries [e.g., Berger et al., 2009a; Pietsch, 2009]. 2. Depositional Settings [10] For this study, silty and muddy short cores (10 15 cm long) were collected from three areas around the Antarctic Peninsula that recently were at a calving line or under a nowcollapsed ice shelf. The three areas (Figure 1) are (1) the Greenpeace Trough in the area of the former LIS A (covered by the LIS A before 1993), (2) the area of the former LIS B, before its dramatic collapse in February March 2002, and (3) the Müller Ice Shelf in Lallemand Fjord Larsen A Ice Shelf [11] The ice shelves of the northern Larsen region generally have been retreating since historical observations began in 1843 [e.g., Vaughan and Doake, 1996]. The LIS A underwent a major collapse in 1995, retreating from the continental shelf to the coastline [Rott et al., 1996; Doake et al., 1998]. Domack et al. [2001] and Brachfeld et al. [2003] studied cores from the Greenpeace Trough, in areas covered by the LIS A until 1992, to infer the Holocene history of the LIS A. Using a combination of 14 C dating (which proved problematic) and down core profiling of geomagnetic paleointensity variations, Brachfeld et al. [2003] concluded that LIS A had been a floating shelf since 10 ka, and was stable throughout the early Holocene. [12] As mentioned, Gilbert and Domack [2003] documented the existence of eolian sand in several cores from the area formerly under the LIS A, with up to 30% (by volume) concentration in the top 2 cm at their site 5 (SM5 in Figure 1), 15 km from the edge of the year 2000 LIS A. Their 210 Pb dating analyses indicate that the upper 5 cm of most of their core horizons was deposited within the last years. The sand was thus deposited while the site was under the ice shelf. This implies that sand deposition occurred through developing crevasses and cracks [Gilbert and Domack, 2003; MacAyeal et al., 2003]. For reasons outlined below, our subsamples come from core horizons below the core top, and so would contain somewhat older sand grains Larsen B Ice Shelf [13] A large portion (3370 km 2 ) of the LIS B experienced a spectacular collapse in February March 2002 [e.g., MacAyeal et al., 2003; Scambos et al., 2003], about two months after Berger and Domack had collected various short cores from near the front of the extant shelf (U.S. research cruise NBP01 07). Most of these cores are from areas that were under the ice shelf until after As stated above, these 3of22

4 Figure 3. Map of inner Lallemand Fjord showing the location of our core site LLF03 SM1A (Figure 1, bottom left), the location of nearby core KC72 [Domack et al., 1995], the location of the sediment traps of Gilbert et al. [2003], the bathymetry from Domack et al. [1995], and the past recent positions of the front of the Müller Ice Shelf. The location coordinates of core KC72 from Domack et al. [1995] are of pre GPS era and not too accurate. We locate KC72 in one of the more likely positions, given that its depth is known accurately (655 m) and that it was situated between the 1993 and 1974 ice fronts. We also show an estimate of the 2003 ice front based on Berger s photos taken during the March 2003 expedition (figure modified from Domack et al. [1995]). short cores were collected for fine silt dating, not with the goal of maximizing sand collection Müller Ice Shelf [14] The northernmost ice shelf on the western side of the Antarctic Peninsula is the Müller Ice Shelf, within the embayment of Lallemand Fjord [Domack et al., 1995] (Figure 1). The ice shelf, first observed in 1947, experienced a rapid advance of 40% between 1947 and 1956 [Ward, 1995], retreated until 1974, advanced somewhat until 1986, and continued to retreat until 1998 [Ward, 1995; Gilbert et al., 2003], and even to 2003 (G. W. Berger, unpublished observations, research cruise LMG03 03). Consequently, the box cores collected during the 2003 cruise contain sediments deposited under an ice shelf. Water depths in front of the present ice shelf exceed 600 m [e.g., Domack et al., 1995; Gilbert et al., 2003] (Figure 3). Domack et al. [1995] describe core sediment near the ice front (cores KC72, G61 and TC61) as consisting of sandy mud (their ice shelf facies ) in the upper 60 cm, with sand concentration rising from 4% to 8% below 20 cm to well sorted sand of 20% concentration in the upper 5 cm. This well sorted, finegrained sand is interpreted to be partly eolian [Frederick, 1991; Stein, 1992], and is rich in heavy minerals. This agrees with a visual observation of Berger (during cruise LMG03 03) of dark sediment deposited on the surface of ice landward of the ice shelf edge, and close to mafic composition basement rock rising at the western edge of the western arm of the ice shelf. [15] Domack et al. [1995] inferred that their descriptive sedimentology reflects a relatively recent advance of the Müller Ice Shelf resulting in deposition of a well sorted sand rich facies in the vicinity of the modern calving line, consequent upon the transport of aeolian material across the surface of the ice shelf to the calving line and its deposition beneath the proximal fjord waters [Domack et al. 1995, p.167]. This interpretation is buttressed by several traits of the deposits and settings that they observed. They obtained two 14 C ages from acid insoluble organic matter at the sediment water interface in cores KC72 (Figure 3) and KC75 (not shown, 10 km seaward from the Humphrey Ice Rise) of 2.2 ka and 2.6 ka ( 14 C years BP). Taylor et al. [2001] obtained a 14 C age of 1.5 ka cal. BP for a living scaphopod at the sediment water interface at the head of the Müller Ice Shelf. Domack et al. [1995] deduced a depositional rate of 12 mm/a from two at depth 14 C ages on foraminiferal calcite within core KC72. They also inferred that the ice shelf advanced 400 years ago, beginning the deposition of the ice shelf facies presently preserved in the upper 60 cm of ice proximal cores. Subsequently, Gilbert et al. [2003] concluded from an ice shelf proximal sediment trap study that the mean sediment flux was 1.4 to 2.9 mm/a in Of potential significance for our luminescence study, they observed a secondary peak in sand concentration (always <2 5% in the traps) in the austral winter horizons of the sediment traps, and associated this with a putative eolian input during winter storms. [16] Clearly, if a significant fraction of eolian sand is deposited in water during the seasonal darkness, then the quartz PSL signal would not be zeroed during that process. However, sand lying on the surface of a glacier or ice shelf would have ample seasonal opportunity for clock zeroing prior to such winter storms. 3. Experimental Procedures 3.1. Collection of Cores and Preparation of Samples [17] Smith McIntyre box cores were recovered to preserve the sediment water interface. Box cores were recovered in the Greenpeace Trough in May 2000 [Domack et al., 2001] with the U.S. RV Nathaniel B. Palmer during cruise NBP Box cores from in front of the then LIS B were recovered by Berger and Domack in December 2001 from the U.S. RV Nathaniel B. Palmer during cruise NBP Box cores were recovered by Berger at the front of the Müller Ice Shelf in March 2003 with the U.S. RV Lawrence M. Gould during cruise LMG On deck, 10 cm diameter, opaque tubes were pushed into the relatively soft box core sediment and the tube ends were sealed. The tubes were transported upright to Reno, where separate depth zones were extruded under darkroom lighting conditions [Berger and Kratt, 2008] for PSL dating. The samples we used are listed in Table 1. The sample from the subpolar Admiralty Bay at King George Island (Table 1, 130 km north of the tip of the Antarctic Peninsula) (core ADM01 SM2) is included for a comparison experiment (below). 4of22

5 Table 1. Short Cores From Smith McIntyre (SM) Box Cores and Locations Sample a Horizon b (cm) Water Depth (m) Latitude Longitude Area ADM01 SM S W Admiralty Bay LA00 SM S W Larsen A Ice Shelf LA00 SM S W Larsen A Ice Shelf LA00 SM S W Larsen A Ice Shelf LA00 SM S W Larsen A Ice Shelf LA00 SM S W Larsen A Ice Shelf LB01 SM1B S W Larsen B Ice Shelf LB01 SM1B S W Larsen B Ice Shelf LB01 SM3B S W Larsen B Ice Shelf LB01 SM3B S W Larsen B Ice Shelf LB01 SM4A S W Larsen B Ice Shelf LB01 SM4A S W Larsen B Ice Shelf LLF03 SM1A S W Müller Ice Shelf LLF03 SM1A S W Müller Ice Shelf LLF03 SM1A S W Müller Ice Shelf a Sample name YY SMn m indicates year of collection (YY), sequential SM short core number (n) (or for LA short cores, n is the site number), and m is the Reno subsample number from each short core. Sample LB01 SM4A 345 consists of the combined sand fractions of subsamples , because of low concentration of sand. b The depth interval within a short core. Because the sediment water interface was exposed to light during collection, the topmost 4 10 mm was used for dosimetry, with some exceptions. For push core LLF03 SM1A the bottom 5 mm was also employed for dosimetry, and the average values from the top and bottom 5 mm from this core were used for subsample 7 dosimetry. For push cores LA00 SM5, SM7, and SM23 small edge pieces of intermediate depth core slices were added to the dosimetry samples, to generate a core average dosimetry. [18] Silt for multigrain polymineral dating was extracted from all short cores, and later attempts were made to extract sufficient sand for SGQ dating from all but samples ADM01 SM2 6, LB01 SM1B 5, LB01 SM3B 5, LA00 SM7 4, and LA00 SM23 4. Of these attempts, samples LB01 SM1B 2 and LB01 SM3B 2 yielded too little material of >60 mm diameter to proceed to SGQ dating. A critical variable for SGQ dating is the (unknown a priori) quartz concentration. For example, the 2 cm thick sample LB01 SM1A 3+4 (not listed in Table 1) contained 550 mg of nonmagnetic sand grains (>60 mm) but only 36 mg of quartz (the remainder after HF acid treatment), or 7% of nonmagnetic siliciclastic grains. Since our sand samples were typically 30 60% nonmagnetic, the quartz concentration here would be <4% of the sand. Most of our nonmagnetic sand fractions contained 10% quartz. We report results mainly for one sample (near core top) from each core. However, from one core (LLF03 SM1A in Figures 3 and 4), silt and some sand were extracted from 3 separate horizons. Data from Domack et al. [1995, Figure 3] indicate that nearby Kasten core KC72 (Figure 3) has 10% (by volume) sand within the upper 20 cm (upper 10 cm shown in Figure 4), which proved to be adequate for SGQ PSL testing. [19] With core LA00 SM5 (LIS A area, Figure 1) we encountered a problem during subsample extraction in the laboratory. The upper 2 cm of this core began to fragment during laboratory extraction, so to preclude inadvertent mixing of core wall and interior material within the targeted cm interval, this zone was not used for PSL tests, but instead was combined with the topmost 5 mm for use in dosimetry. Consequently the topmost interval from this core that was suitable for PSL dating was the cm zone, which unfortunately does not include the expected maximum concentration in sand (Figure 4, right). The 210 Pb data of Gilbert and Domack [2003] for a companion push core from their Smith McIntyre box core 5 (also named SM5 ) suggests an average deposition age of 75 a for the LA00 SM5 2 horizon shown in Figure 4. Similarly the 210 Pb data suggest an average deposition age of 55 a for the LA00 SM7 2 horizon (Table 1). No 210 Pb data are available for the other LIS A short cores we studied. [20] For luminescence measurements, we prepared both polymineral fine silt (4 11 mm diameter) fractions and fine sand ( mm) fractions where sufficient sand was available. Fine silt fractions were prepared in the usual way [Berger and Doran, 2001], after sequentially removing carbonates (with 1N HCl acid) and organic matter (with 30% H 2 O 2 ). The fine sand fractions from which organic matter and carbonates were removed were magnetically separated Figure 4. Sketch of the core horizons represented by some of the samples in Table 1 for which we have independent evidence of sand concentrations. Percent sand data for core KC72 are from Domack et al. [1995] and for core GD SM5 from Gilbert and Domack [2003] (hence GD ). Domack et al. [2001] provide indirect percent sand data (see our text) for a core related to our LA00 SM23 short core. 5of22

6 (using Frantz isodynamic magnetic separation). As is well known [e.g., Porat, 2006], magnetic separation is effective in removing almost all of the ferromagnesian minerals, which comprise the bulk of the heavy mineral fraction in most geological samples. We did not attempt to remove heavy minerals from fine silt fractions because not only is this an onerous task, requiring difficult heavy liquid separation procedures [e.g., Berger, 1984] or difficult fine silt magnetic separations [e.g., Mulhern et al., 1981], but also almost all nonfeldspar quartz minerals in typical sediments emit insignificant luminescence [e.g., Aitken, 1985, 1998]. [21] To destroy feldspars, the nonmagnetic fractions of sand were subjected to the usual [e.g., Aitken, 1998] HF acid treatment (50 min. at 48% concentration), followed by 20% HCl acid to dissolve any precipitated fluorides. For the mm fraction, an additional treatment with multiday H 2 SiF 6 acid [Berger et al., 1980] was employed to reduce further any potential feldspar contamination. In retrospect, we found that for sand grains the use of H 2 SiF 6 acid may have reduced the concentration of post HF feldspars further, but did not eliminate them. Because the HF acid treatment generally removed such large proportions of material from these samples during the first HF treatment, we did not employ a second HF treatment for <100 mm fractions. For >100 mm fractions, HF acid treatment was repeated until infrared (IR) tests revealed little or no IR signal, or until the fraction of material remaining was too small (e.g., mg) to continue HF treatment. After the HF acid treatment, we tested for the presence of residual feldspar grains by recording (at 80 C) the IR stimulated luminescence (IR PSL) from aliquots of artificially irradiated fractions ( 10 Gy), followed by a 2 day delay to permit decay of any possible interaliquot differences in short term IR PSL instability [e.g., Aitken, 1998]. Typically, only feldspars respond to IR stimulation at temperatures below 100 C, but at higher temperatures there can be a measureable sample dependent IR PSL signal from quartz [e.g., Bøtter Jensen et al., 2003; Inoue et al., 2005; Jain et al., 2005; Polymeris et al., 2008; Fan et al., 2009]. [22] Separate, untreated portions of the sediment (usually the topmost 5 mm of core material) were analyzed for: saturation water concentration; U, Th (using thick source alpha particle counting, Huntley and Wintle [1981]); and K (commercial atomic absorption spectrophotometry). These data are needed to calculate the ionizing radiation dose rate to the PSL sample [e.g., Aitken, 1998] Multigrain Polymineral Luminescence Dating [23] The clock zeroing assumption of the luminescence dating of unheated sediments is that just before deposition the PSL absorbed energy in discrete grains is rapidly zeroed by daylight, the grains are buried (and shielded from further daylight), and they then absorb energy at a constant rate (over the Quaternary) from ambient ionizing radiations from radio elements (K and Rb, U and Th and their decay products) and cosmic rays. This absorption causes charges (e.g., electrons) to be accumulated in crystal lattice defects (traps). In the laboratory, photonic stimulation releases trapped electrons, a portion of which combine with opposite lattice charges to release photons at wavelengths shorter than the stimulation wavelengths (anti Stokes emission). The intensity of postdepositional PSL is thus related to the duration of time since the last daylight exposure of the grains. Coupled with dose rate analyses [e.g., Aitken, 1985, 1998], PSL measurements permit age calculation; a luminescence age estimation is derived by dividing equivalent dose (D E ) by dose rate (D R ). We determined D E values both from fine silt feldspar quartz (other remaining minerals emitting insignificant PSL) [Aitken, 1998] mixtures and from sand sized quartz grains. Any grains not exposed to daylight before final burial will carry some relict light sensitive absorbed energy and yield an age overestimate. [24] We applied the multigrain, multiple aliquot, thermal transfer correction [Ollerhead et al., 1994; Berger and Doran, 2001] (herein, MATT) IR PSL procedure to the polymineral fine silt fractions. Conventionally, this procedure subtracts from each PSL signal in the laboratory dose response curve a PSL signal from unirradiated aliquots (each grain covered disc) that have been given an extended (e.g., hours) optical bleaching. Berger [2006] introduced a short interval opticalbleaching variant of this MATT procedure that we have adopted for most of our fine silt MATT experiments. The MATT procedure assumes that all grains in each aliquot have been well exposed to daylight. This method yields only one D E value per experiment, with tens of aliquots needed per experiment. In the MATT procedure, the feldspardominant PSL is detected at blue wavelengths (centered at 420 nm), an emission from feldspars believed to be more stable than emissions in the ultraviolet (UV) [Berger, 1995; Krbetschek et al., 1997]. [25] We also applied the multigrain, Single Aliquot Regenerative dose (SAR) [e.g., Murray and Wintle, 2000, 2003] PSL methods to the polymineral fine silt fraction of one sample (LLF03 SM1A 2) in the double SAR variant [Banerjee et al., 2001; Wang et al., 2006]. In this approach, infrared diode stimulation (IR PSL, to release light sensitive luminescence from feldspars) is followed by blue diode stimulation (B PSL, to release light sensitive luminescence from quartz) and the post IR emission is detected in the UV, the region of the most common quartz emissions. The modifications of Wang and others (using extended [e.g., 500 s] IR stimulation at 125 C) can minimize the B PSL from feldspar grains within a fine silt feldspar quartz mixture to negligible levels. This is desirable because feldspar B PSL detected in the UV band pass is unstable. Thus, a post IR B PSL signal is more likely to be dominated by quartz PSL. [26] Roberts [2007] found that for some middle USA loess samples, the double SAR procedure she used (with IR for 100 s at 125 C) did not remove all the feldspar signal from the post IR blue diode stimulation of polymineral 4 11 mm grains. A 7 day H 2 SiF 6 acid treatment did remove the feldspar signal from the mm fraction of the loess samples. We did not apply an H 2 SiF 6 acid treatment to our 4 11 mm grains. Use of this acid would provide no benefit for our samples since the post IR B PSL ages for sample LLF03 SM1A 2 (Table 4) are already many times the expected depositional age, and feldspar signals in the UV typically underestimate the sample age. [27] In data processing we applied a procedure outlined by Ballarini et al. [2007b], which is termed here the early light subtraction (ELS) procedure. With multigrain double SAR, we found that the ELS procedure minimizes or eliminates recuperation effects. The ELS procedure can be considered analogous in purpose to the short bleach MATT procedure. 6of22

7 With fine silt samples it can be effective in parsing out signals representing the most recent daylight exposure from relict age signals, though it appears to be less effective in doing this than single grain dating (this study). Of course, the depositional transport paths of fine silt and sand grains in glacimarine settings (and in other deposits resulting from waterborne processes) are likely to be quite different from each other, leading to significant differences in the respective daylight exposure histories of silt and sand grains Optical Bleaching Tests [28] It is well established that the efficiency of optical bleaching of luminescence in quartz and feldspar is wavelength dependent (e.g., summary Figure 7 by Berger [1988]; summary Figure 4 by Wallinga [2002] and references therein; Bøtter Jensen et al. [2003]). That is, the extent of reduction of thermally stimulated luminescence (TSL or commonly TL) and PSL is greater at shorter wavelengths than at longer wavelengths. In turbid waters the intensity of ambient light differs (short and long wavelengths are attenuated preferentially) from that at the water surface [Berger, 1990]. Therefore, the degree of reduction in TL within a given mineral grain will differ between the group of grains exposed to daylight near or at a water surface and the group of grains within or beneath turbid suspensions. Moreover, this difference will be wavelength dependent. This effect and its implications for interpretation of dual band pass laboratory bleaching have been discussed elsewhere [Berger, 1990, 2009; Berger and Doran, 2001; Berger et al., 2010]. [29] More specifically, below a particular depth in a turbid water column, the ambient light generally consists of wavelengths longer than 500 nm [e.g., Berger, 1990]. Fine silt grains transported at depth in suspension, or reworked by bottom currents, would not likely be exposed to light with wavelengths below 500 nm, if exposed to any light. On the other hand, grains within sediment plumes reaching the surface would be exposed to a relatively larger part of the visible daylight spectrum. Therefore, TL optical bleaching time tests using blue red light (herein, nm wavelengths) and orange red light ( nm) should result in some laboratory distinctions between fractions of grains that reached the surface or near surface and those that remained at depth. [30] Berger [1990] showed that for one eolian silt sample (all grains exposed to a full spectrum of daylight), one surface suspension and one lake bottom surface suspension rainout sample (thin clay lamina from a rythmite) the two band pass bleaching response TL curves did not diverge at extended bleaching times. On the other hand, silt from a siltrich part of rythmites (originating in a turbidity flow or deep suspension transport process) exhibited significant divergence of the bleaching response TL curves, indicating that not all grains in this sample were exposed to a full daylight spectrum during transport before collection. [31] We conducted one dual band pass TL optical bleaching experiment on the fine silt fraction from near core top LIS A sample LA00 SM7 2. In this procedure we employed both blue red and orange red filtered laboratory lamps for bleaching of the natural (unirradiated aliquot) TL signals to partly simulate the natural zeroing process and thus to help infer the extent and manner of luminescence zeroing in these silt grains that may have occurred under natural conditions. [32] For comparison, we also carried out a dual band pass TL bleaching experiment on a short core sample (ADM01 SM2 6, Table 1) from a more temperate glacimarine setting: Admiralty Bay, 130 km northward of the tip of the Antarctic Peninsula. In this lower latitude setting, austral summer meltwater surface suspension plumes are common [e.g., Domack and Ishman, 1993]. Thus, significant fractions of the silt grains in bottom mud are expected to have been exposed to daylight during transport. This would lead to relatively flat dual band pass TL bleaching curves. On the other hand, the depositional origin of silt grains in our LIS A and LIS B samples is comparatively unknown, and we may expect less (or no) opportunity for daylight exposure before deposition. [33] We conducted also IR PSL optical bleaching tests on these two samples (LA SM7 2, ADM01 SM2 6) to compare their relative responses, after the manner of Berger et al. [2009b]. Typically [e.g., Berger et al., 2009b], when plotted on a logarithmic time axis, reduction of the first 1 10 s of the IR PSL signal (as for TL) shows an initial slow decline (nearly flat on a log axis), then a decline to zero at extended bleaching times. The implications for dating of such IR PSL bleaching responses are discussed elsewhere [Berger et al., 2009b] Single Grain Luminescence Dating [34] The combination of the glacimarine sedimentation processes illustrated in Figure 2 necessitates the use of singlegrain luminescence dating methods to isolate any eolian subpopulation from other sand grains in any ice edge core. Therefore, we applied single grain modified SAR PSL procedures (Table 2) to the purified quartz fine sand ( mm) fractions. The concepts and advantages of single grain dating are expounded clearly by Olley et al. [1999], Bøtter Jensen et al. [2003], Duller [2004, 2008], and Lian and Roberts [2006]. In the limit of one grain per aliquot, the SAR procedure permits isolating those grains last exposed to daylight from grains not exposed to daylight at deposition (which thus carry a large relict apparent age). Therefore, true deposition ages can in principle be obtained for samples such as glacimarine deposits if any of the grains were exposed to sunlight for more than a few tens of seconds just before burial, and if those grains can be identified during equivalent dose measurement. Any number of grains can be analyzed for a given sample, limited only by automated instrument time and sample availability. A caveat is that as little as 1 5% of the prepared quartz grains may emit PSL in the UV detection window [e.g., Feathers, 2003; Duller, 2006]. Single grain dating is practicable routinely because of use of microfocused lasers and geometric separation of grains in a grid of holes [e.g., Bøtter Jensen et al., 2003; Duller, 2004]. This enables measurement of thousands of grains per sample in comparable time (a few days) to the SAR measurement of 50 multigrain aliquots using lightemitting diodes (LEDs) for stimulation. [35] In our application of SGQ PSL dating to study sites from which quartz grains had not previously been analyzed for PSL, we were confronted by three empirical challenges. Would there be enough quartz within the sand fraction 7of22

8 Table 2. Modified SAR Procedure for Single Grain Signal Readout a Step Comment 1 Regenerative dose ( 90 Sr/ 90 Y b source, skip in cycle i = 0) 2 Preheat (at 200 C for 10 s at 125 C) 3 IR wash with diodes (875 nm, 90% power, 150 s at 125 C) 4 Measure L i using green laser (532 nm) stimulation (90% power for 1 s at 125 C) 5 Test dose (b source, e.g. 40 Gy at room temperature) 6 Cut heat (at 180 C, recording TL) 7 IR wash with diodes (90% power for 150 s at 125 C) 8 Measure T i using green laser stimulation (90% power for 1s at 125 C) 9 Blue wash with diodes (470 nm, 90% power for 40 s at Preheat + 20C ) 10 Repeat from step 1. In last cycle [2nd recycle], skip step 3, stop after step 8. a Cycles i = 0 to n. (>100 mg is desirable)? We mentioned above that some sample sizes were too small to yield useful amounts of sand sized quartz. Would there be a suitably large fraction of these quartz grains emitting PSL in the UV detection window? Finally, would there be sufficient eolian quartz grains within suitable size fractions to provide a statistically useful numerical age? In our experiments in Denmark, we were limited by available instrument time, and by the amount of quartz that could be recovered from targeted core horizons. Consequently, these SGQ experiments typically used about 2400 grain holes. However, similar instrumentation was installed later (early 2008) at the Desert Research Institute (DRI) in Reno so that some additional, longer experiments could be conducted. In both situations there were 2 15 grains per 300 mm diameter hole, depending upon the grain size range employed. However, because only <1/100 to <1/5 of the quartz grains in this project emit significant PSL in the UV detection window, then our SGQ experiments are effectively true single grain analyses. [36] SGQ stimulation employed a green (532 nm) laser, and the main quartz PSL emission peak was detected by an Electron Tubes 9235QA photomultiplier tube fitted with a 7.5 mm (in one Denmark reader, a 5 mm) Hoya U 340 filter. The SGQ PSL signals were measured for 1 s at 125 C. During SGQ runs, irradiations within each reader were conducted using an externally mounted, calibrated 90 Sr/ 90 Y beta source. For all of the samples SGQ D E values were determined using modified SAR procedures (Table 2). [37] Based on preliminary tests, regenerative dose values were chosen to emphasize dose responses from grains with smallest doses. That is, the maximum regenerative dose was less than the possible D E values from the grains with the largest doses. This strategy is similar to that employed by Roberts et al. [1999] in their study of mixed age sand grains in an archeological context. Moreover, we employed relatively low preheating (200 C) and cut heating (180 C) temperatures, as appropriate for younger grains [e.g., Ballarini et al., 2007a; Berger et al., 2009a; Pietsch, 2009]. Finally, depending upon which PSL reader was available in Denmark, we employed either an IR laser [e.g., Bøtter Jensen et al., 2003] or IR diodes, to IR wash each grain hole (if using a laser) or disc (if using diodes) before green laser stimulation to release the quartz signal (G PSL). [38] The practice in the Danish laboratory (employed in this project at DRI for all but one sample) is to use IR wash stimulation at 125 C during SAR dating experiments [e.g., Olley et al., 2004b; Wintle and Murray, 2006], although at DRI the pre SAR feldspar contamination tests use IR PSL at 80 C. For the last SGQ experiment at DRI in this project (sample LB01 SM4A 345), we employed IR wash at 100 C during the SAR run, for reasons stated in section 3.1. In addition to this within SAR IR wash (Table 2), we also employed an end of run IR check in our SGQ experiments. Rather than the comparison of PSL intensities after and before IR wash as introduced by Duller [2003] (his IR depletion ratio test), we compared L/T (test dose normalized luminescence signals) ratios before and after an IR wash step. Comparison of these L/T ratios accounts for sensitivity changes between SAR steps, unlike comparison of only L intensities. With the exception of sample LA00 SM7 2, we rejected D E values having such before/after L/T ratios >1.0 by more than 1s (thus evidencing significant feldspar signals). As discussed in subsection 5.4.1, we compared the effects of the use of the 1s and 2s L/T ratio IR criteria on sample LA00 SM7 2. [39] Because the fraction of grains emitting acceptable signals was low compared to most of the lower latitude sample studies [e.g., Feathers, 2003; Jacobs et al., 2003; Berger et al., 2009a], we relaxed some of the conventional SAR data acceptance criteria [e.g., Feathers, 2003; Wintle and Murray, 2006] from 1s to 2s levels (except for the intra SAR feldspar check using IR). We accepted data having recycle ratios within 2s of unity (equivalent to the criterion of Ballarini et al. [2007a]), recuperation ratios within 2s of zero, and D E values having relative standard errors as high as 50 60% when D E values exceeded 0.3 Gy. For D E values below 0.3 Gy, D E values with errors up to 100% were accepted. Feathers et al. [2006] accepted D E values having errors as high as 100% for D E values larger than 0.4 Gy. We rejected data having test signal errors higher than 30% and signals less than 3s above background. We forced dose response curves through the origin. Depending upon the dose response curve, linear, quadratic, saturating exponential or saturating exponential plus linear regressions (within Analyst software) were used. Poisson statistics were assumed (within Analyst software) when calculating the uncertainty in individual D E values (e.g., see noisy signals in Figures S1 S4 in Text S1). 1 [40] To calculate L/T ratios, typically we employed an ELS procedure for most samples, but used the conventional [Wintle and Murray, 2006] LLS (late light subtraction) in 3 SGQ experiments (identified in section 4). In the singlegrain ELS procedure we subtracted the summed signal of channels (0.1 s total interval duration) from the sum of the first three signal channels (channels 6 8, the first 0.06 s of signal). Examples of these intervals are shown in Figures S1 S4 in Text S1. In one experiment (the second on sample LB01 SM4A 345) we expanded the early light subtracted signal from channels to so that the 1 Auxiliary materials are available in the HTML. doi: / 2009JF of22

9 error in the background could be reduced [e.g., Galbraith, 2002]. By using ELS we attempted to maximize the contribution from the preferred fast component of quartz [Murray and Wintle, 2000, 2003; Wintle and Murray, 2006] and reduce that from any of the less light sensitive medium and slow components that might be present. Although all quartz PSL components may be zeroed during eolian transport, only the fast and medium components appear to be zeroed during fluvial transport [Singarayer et al., 2005]. Indeed, in most of our samples some of the grain hole signals had relatively large apparently medium components (though we conducted no signal analysis because of low intensities). Examples of SGQ shine curves illustrating apparently dominant fast components and apparent mixtures of fast, medium and (perhaps) slow components are shown in Figures S1 S4 in Text S1. Examples of corresponding dose response curves are given in Figures S5 S8 in Text S1. Fortunately, data (not shown) from most of those grain holes having relatively large apparently nonfast components failed one or more of the data rejection criteria (e.g., the recuperation criterion). [41] A 12% additional uncertainty was included during D E calculation within the Analyst software. The use of this internal error 12% is to account for the variability in single grain dose recovery experiments observed by others and in our own dose recovery tests on various samples, including one sample in this project (LB01 SM4A 345). This 12% is conservative compared to the 4 9% maximum variability (mean 2 4%) observed with similar single grain readers by Thomsen et al. [2005, 2007] and several others [e.g., Jacobs et al., 2006; Pietsch, 2009]. Using 500 grain holes, the mm quartz fraction of sample LB01 SM4A 345, an applied dose of 8.2 Gy (following blue LED bleaching for 100 s at 125 C), a preheat and cut heat of 200 C and 180 C respectively (Table 2), and an internal 12% error in Analyst, we obtained 27 acceptable D E values normally distributed with a weighted mean dose recovery ratio of ± (standard error of the mean). This confirms the validity of our use of this preheat/cut heat combination in our single grain experiments. Indeed, for very young sediments, almost all of the single grain PSL literature demonstrates that preheats of C and cut heats of C are valid. Furthermore, because 26 of 27 of the D E values in this dose recovery experiment fall within the ±2s limit of the mean, this indicates that our internal 12% error assignment is too conservative (we would have expected only 18 of the 27 values to fall within 2s of the mean). [42] The aforementioned user selectable (within Analyst software) internal error of 12% functions by increasing the error assigned to each L/T ratio, thus indirectly increasing the D E error estimate. Arnold and Roberts [2009] looked at a variety of reports of multigrain and single grain D E distributions for putatively eolian and demonstrably eolian sand. They listed overdispersion (scatter beyond that expected from assigned errors in D E values) values from 0% to >40%, depending upon the sample, and inferred a mean overdispersion of 20% for eolian sand. Notwithstanding that several of their so called uniformly bleached eolian samples may in fact contain some poorly reset grains (not uncommon in eolian sands, depending on transport distance and whether it is day or night), they recommended adding an 20% uncertainty in quadrature to D E errors, to account for such natural overdispersion when dating eolian sediments by SGQ. [43] Disregarding for the moment that our samples are most probably dominated by noneolian grains (e.g., Figure 2), let us consider one worked example to see what effect such a recommendation might have on our singlegrain D E errors. Looking at the seventh lowest D E value in Figure 10 below, for example, we may ask: what would be the effect on the shown error (derived via our selected internal 12% factor) if we chose a 5% or 9% internal variability? Figure 10 shows that the ±1s error bar for this seventh datum overlaps with the error bar for the next highest D E value (the eighth datum). This might suggest that our identification (resolution) of the preferred youngest age D E values in Table 4 for all our samples is highly sensitive to our choice of the internal 12% factor. The 5% and 9% values are at least double the measured single grain mean reproducibility values reported by the above several authors, so this worked example is conservative. Also, let us choose a conservative overdispersion value of 25% to be added to each D E value (in quadrature), rather than the 20% recommended by Arnold and Roberts [2009]. If we use a mean 5% single grain reproducibility, then the 25% effect would increase the percent error in this seventh datum (by addition in quadrature) from 31% to 40%. The error in the seventh datum in Figure 10 is already 42%, so there is no significant effect. If we choose a 9% internal single grain reproducibility error ( 3 times the measured mean for this single grain instrumentation reported by several authors), then the 25% effect increases the D E error from 37% to 45%. Compared to the 42% we calculate using our preferred internal 12% factor, this change is also insignificant. In fact, our calculated individual D E errors are generally 30 60% because of various factors (including low signals, see Figures S1 S8 in Text S1), so adding 20 30% in quadrature typically changes our D E errors by <10%. Given that this 25% overdispersion is larger than the aforementioned 20% and that our samples are emphatically noneolian, we are satisfied that the recommendation of Arnold and Roberts [2009] is inapplicable to our present data, and that our error calculations for all of our single grain D E values are realistic and conservative. [44] We also have applied the Internal External Uncertainty (IEU) statistical approach [Thomsen et al., 2007] to one of our SGQ data sets (sample LA00 SM5 2, 141/4500) for which we recognize in radial and Transformed Probability Density (TPD) plots (discussed below) 3 distinct lowest D E values. This model requires two or more target data points (in a distribution) to function, so that we could not apply it to all of our SGQ data sets. For this sample, the IEU model identifies the same three D E values as the TPD and radial plots, independently yielding a youngest age D E value of 0.54 ± 0.17 Gy. This is indistinguishable from the weighted mean D E value of 0.55 ± 0.27 Gy derived from our TPD plot selection, reported in section 4. We are therefore further satisfied that our D E error calculations are conservative and appropriate for all of our samples. [45] We note that in the discussions of Arnold and Roberts [2009] and those of others [e.g., Jacobs et al., 2008], a possible source of additional scatter in D E distributions for terrestrial samples has been ignored. Almost all workers extract samples from eolian and other sandy/pebbly terrestrial deposits using metal or PVC plastic tubes with relatively thick walls. Such a sampling method is prone to along tube mixing of sediment surface (daylight exposed) grains with 9of22