Thiede, J., Myhre, A.M., Firth, J.V., Johnson, G.L., and Ruddiman, W.F. (Eds.), 1996Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 151

26. LATE QUATERNARY STABLE ISOTOPIC STRATIGRAPHY OF HOLE 910A, YERMAK PLATEAU,ARCTIC OCEAN: RELATIONS WITH SVALBARD/BARENTS SEA ICE SHEET HISTORY1

Benjamin P. Flower2

ABSTRACT

Ocean Drilling Program Hole 910A on the Yermak Plateau (80°N, 5°E) at 552 m water depth provides one of the firstopportunities for high-resolution studies of Quaternary Arctic Ocean climates. Oxygen and carbon isotopic data based on Neo-globoquadrina pachyderma (sinistrally coiling) from the top 24.5 m at 10 cm sampling resolution provide both stratigraphicage control and a history of middle through late Quaternary surface water environments. The 518O data reveal oxygen isotopeStages 1 through 16 (and possibly 17), with an amplitude of ~l%o. Carbon isotopic values in the section from -19.5 to 24.5 mbelow seafloor (mbsf) exhibit a series of ~l%c fluctuations, followed by a permanent \%o increase at -19.5 mbsf. Above -19.5mbsf, δ1 3C values are relatively stable, except for large spikes of ~0.5‰, mainly during inferred interglacials. Significantly, thesection from -19.5 to 24.5 mbsf is a remarkable "overconsolidated section," marked by high values for bulk density and sedi-ment strength. The large variations in δ1 3C below -19.5 mbsf, and the overconsolidated nature of the section, suggest a funda-mentally different sedimentary environment. Possible explanations include (1) episodic grounding of a marine-based ice sheet,perhaps derived from the Barents Sea ice sheet and buttressed by Svalbard, which overcompacted the sediment and stronglyinfluenced surface water chemistry on the Yermak Plateau, and/or (2) coarser grained glacial sedimentation that allowedenhanced compaction during this interval. Oxygen isotopic stratigraphy suggests that overconsolidation ended near the Stage17/16 boundary (-670 ka), reflecting a major change in state of the Svalbard/Barents Sea ice sheet.

INTRODUCTION

The Yermak Plateau is an aseismic topographic marginal highabout 100 km north of Svalbard. The plateau is an Eocene (?) volca-nic edifice, although basement has not been drilled and, consequent-ly, the age is not well known. The Yermak Plateau lies just north ofthe Arctic Gateway at the Fram Strait, which connects the ArcticOcean and the Norwegian-Greenland Sea. The Yermak Plateau isseasonally ice free in the present day, as a result of warm waters fromthe West Spitsbergen Current (WSC). This current is an extension ofthe warm Norwegian Current that splits south of Svalbard into awestern branch (the WSC, flowing through the Fram Strait) and aneastern branch (flowing through the Barents Sea) into the ArcticOcean.

Low sea-ice conditions in the summer of 1993 allowed theJOIDES Resolution to drill at three sites in a depth transect on theYermak Plateau. Sites 910,911, and 912 were drilled at 552,902, and1037 m water depth (mwd), respectively, during Ocean Drilling Pro-gram (ODP) Leg 151 (Fig. 1). Sediments recovered at Site 910(80°16'N, 6°35'E) range in age from late Pliocene (in Hole 910Conly) to Holocene; the total depth drilled was 506 m below seafloor(mbsf). The sediments are nearly homogeneous, dominantly fine-grained silty clays, with a large ice-rafted debris (IRD) fraction. Car-bonate material was sparse, partly because of dilution by terrestriallyderived material. Dropstones were found throughout the sequence,attesting to a high flux of debris-laden icebergs (Myhre, Thiede,Firth, et al., 1995). Recovery in the Quaternary at Hole 910A wasnearly 100% from 0 to 24.5 mbsf, with good magnetostratigraphicand biostratigraphic age control (Shipboard Scientific Party, 1995;Sato and Kameo, this volume).

'Thiede, J., Myhre, A.M., Firth, J.V., Johnson, G.L., and Ruddiman, W.F. (Eds.),1996. Proc. ODP, Sci. Results, 151: College Station, TX (Ocean Drilling Program).

institute of Marine Sciences and Earth Sciences Board, University of California,Santa Cruz, Santa Cruz, CA 95064, U.S.A. flower@earthsci.ucsc.edu

Drilling in Hole 910A was inhibited by an "overconsolidated sec-tion" below -19.5 mbsf, marked by large increases downcore in wet-bulk density, from -1.3 to 1.7 g/cm3, and in sediment strength, from-150 to 300 mPa (Site 910; Shipboard Scientific Party, 1995). Sedi-ment strength increased gradually from -16 to 19 mbsf and rapidly

100° W 100° E

40°

20° W 20° E

Figure 1. Location map of the Arctic Gateway region showing Leg 151 drillsites and important basins and ridges in the area. Sites 910, 911, and 912 areon the Yermak Plateau at ~80°N. Bathymetric contour interval is 1000 m.

445

B.P. FLOWER

centered at 19.5 mbsf. On board ship, this section was classified assilty clays to clayey silts. The thickness of this section is not wellknown because of core recovery problems below -35 mbsf; the baseis known to be between -70 and 95 mbsf (Site 910; Shipboard Scien-tific Party, 1995). Only limited increases in consolidation were ob-served in the upper parts of Site 911 (902 mwd) and Site 912 (1037mwd) on the Yermak Plateau. Also, Site 910 seismic stratigraphy re-vealed an acoustically chaotic upper section, whereas seismic sec-tions at Sites 911 and 912 are well stratified (Myhre, Thiede, Firth, etal, 1995).

Interpretations by the Leg 151 Shipboard Scientific Party for thecause of the remarkable overconsolidated section at Site 910 include(1) grounding of a marine-based ice sheet, perhaps derived from theBarents Sea ice sheet and buttressed by Svalbard, and/or (2) coarser-grained glacial sedimentation that enhanced compaction (Leg 151Shipboard Scientific Party, 1994; Myhre, Thiede, Firth, et al., 1995).The timing and cause of the overconsolidated section has importantimplications for Svalbard/Barents Sea ice sheet history. In this con-tribution, oxygen isotopic stratigraphy in Hole 910A is used to datethe termination of overconsolidation near the stage 17/16 boundary at-670 ka, suggesting a major change in Svalbard/Barents Sea ice sheethistory at this time.

METHODS

Samples were dried and weighed, with a small subsample ar-chived for additional work. Samples were then washed over a 63-µmsieve with deionized water only, then dried and weighed again. The>250-µm fraction was also weighed. Individual planktonic foramin-ifers were handpicked from the >150-µm to <250-µm size fractionfor isotopic analysis. The species used throughout the sequence wasN. pachyderma (sinistrally coiling).

Approximately 20 specimens for isotopic analysis were sonicatedin methanol to remove adhering particles, roasted under vacuum at375°C to oxidize organic contaminants, and reacted in orthophospho-ric acid at 90°C. The evolved CO2 gas was then analyzed on-line us-ing an isotope ratio mass spectrometer. Analyses were made either atthe University of California, Santa Barbara, in the laboratory ofJames Kennett or in the laboratory of Christina Ravelo and James Za-chos at the University of California, Santa Cruz. Measurements at UCSanta Barbara were made using a Finnigan/MAT 251 isotope ratiomass spectrometer equipped with an Autoprep Systems carbonatepreparation device; those at UC Santa Cruz were made using a FisonsPrism isotope ratio mass spectrometer equipped with an automatedpreparation device.

Laboratory precision, which was run daily and was based onNBS-20 standards (-4.14%0 for δ 1 8 θ, -1.06%0 for δ13C) at UC SantaBarbara and on NBS-19 standards (-1.85%O for δ 1 8 θ, 2.03‰ forδ13C) at UC Santa Cruz, is 0.1%c or better. No significant interlabo-ratory offsets were found, based on analyses of the NBS-19 and -20standards. Replicate analyses on approximately 20% of the samplesdemonstrated a reproducibility of ±0.14%o and ±0.12‰ (mean stan-dard deviation) for δ 1 8 θ and δ13C, respectively. No corrections wereapplied for offsets from isotopic equilibrium in N. pachyderma (s) foreither δ 1 8 θ or δ13C. All isotopic data are expressed using standard δnotation in per mil relative to the Peedee belemnite (PDB) carbonatestandard.

RESULTS

Oxygen isotopic data based on N. pachyderma (s) (Table 1) from0 to 15 mbsf exhibit a series of ~\%o cycles, with values generallyranging from ~3.2%e to 4.2%o (Fig. 2). The mean δ 1 8 θ values from 15to 24.5 mbsf are ~0.3%o lower, and generally range from ~2.9%o to3.8%e. Numerous low-δ18θ events occur from 19.8 to 23.7 mbsf.

The carbon isotopic data from 0 to 19.5 mbsf generally range from-0.2‰ to -0.5%o, except for several spikes of ~0.5%c at 0.06,1.1,4.2,7.9, 9.6, 16.33 and 16.9 mbsf (Fig. 2). Also, the δ13C values for thetop 50 cm are from -0.1 to +0.4%o, significantly higher than the entirelower section. The δ13C values from 19.5 to 24.5 mbsf exhibit largevariations of over \%c (ranging from about -0.5%o to -1.8%c) fol-lowed by a large permanent increase of ~1.3%e centered at -19.5mbsf. The δ13C increase at -19.5 mbsf coincides with the terminationof the overconsolidated section.

The percentage sand fraction (wt% >63 µm) and percentage verycoarse sand (wt% >250 µm) remain relatively constant throughoutthe sequence (-20% and -3%, respectively; Table 2). Notable excep-tions are in the top -50 cm, at 22.0 and 23.8 mbsf.

DISCUSSION

Stable isotopic data based on N. pachyderma (s) provide an isoto-pic stratigraphy for Hole 910A. Almost 100% core recovery allowsgeneration of nearly continuous isotopic records. The oxygen isoto-pic data largely reflect the well-known variations in global ice vol-ume during late Quaternary glacial-interglacial cycles (e.g., Shackle-ton and Opdyke, 1973; Imbrie et al., 1984). Thus, oxygen isotopicstratigraphy provides age control for the sequence, including the topof the overconsolidated section.

Age Control

The oxygen isotopic data clearly show isotope Stages 1 through16 (and possibly 17), including numerous substages (Fig. 2). For ex-ample, three δ 1 8 θ minima are seen during Stage 5, which are inferredto correspond to isotope Stages 5a, -c, and -e. Many of the isotopestages are readily assigned (e.g., Stages 1-5, 11, and 15). Also, thestage 12/11 transition stands out as a large, rapid decrease in δ 1 8 θ,which distinguishes it among late Quaternary deglaciations (Shack-leton and Opdyke, 1973; Imbrie et al., 1984). Interpretation of theisotope stratigraphy is aided by the last occurrence (LO) of Pseudo-emiliania lacunosa at -9.7 ± 0.75 mbsf (Sato and Kameo, this vol-ume), which is independently dated at 408 ka in Stage 12 (Thiersteinet al., 1977; Niitsuma et al., 1991).

An age vs. depth plot showing selected oxygen isotope stageboundaries (ages from Imbrie et al., 1984; Shackleton et al., 1990)and the P. lacunosa LO datum is shown in Figure 3. These strati-graphic datums allow calculation of ages and sedimentation rates forthe section. Sedimentation rates average -1.9 cm/k.y. for the 0-245ka interval and -3.3 cm/k.y. for the 245-620 ka interval.

The age of the section below -19 mbsf is more problematic, as theoxygen isotope stratigraphy is more ambiguous. Normal magneticpolarity for the entire sequence suggests an age within the BrunhesEpoch. The problem is that extrapolating the sedimentation rate forthe 245-620 ka interval would place the Brunhes/Matuyama bound-ary at -22.3 mbsf, but no reversed polarity sediments were recovered.The most likely explanation is that sedimentation rates were higherbelow -19 mbsf; rates of at least 4.5 cm/k.y would be required toplace the Brunhes/Matuyama boundary at or below 24.2 mbsf.

An alternative interpretation calls for at least one hiatus between19 and 24.5 mbsf, such that the normal polarity interval from -19.5to 24.5 mbsf would represent the lower Brunhes and the JaramilloChrons, with much of the upper Matuyama missing (-780-990 ka).Normal polarity sediments in this interval are not assignable to theOlduvai Chron (1.77-1.95 Ma), based on calcareous nannofossil bio-stratigraphy (Sato and Kameo, this volume). In this view, the ~0.3%olower δ 1 8 θ values from -19 to 24.5 mbsf might reflect the well-known lower mean δ 1 8 θ values of the middle Quaternary prior toStage 22 (-900 ka).

However, this alternative requires unreasonable changes in sedi-mentation rates, particularly during the short Jaramillo Chron (0.99-

446

LATE QUATERNARY STABLE ISOTOPIC STRATIGRAPHY

Table 1. Stable isotopic data from Hole 910A.

Core, section,interval (cm)

151-910A-1H-1H-1H-1H-1H-1H-1H-1H-1H-

,6-8 a

, 20-22a

,20-22,30-32,50-52, 80-82a

, 90-92, 100-102",110-112

1H-1, 120-122"1H-1, 130-1321H-1, 140-142a

1H-2, 10-121H-2, 20-22a

1H-2, 30-321H-2, 40-42a

1H-2, 40-421H-2, 50-521H-2, 60-62a

1H-2, 70-721H-2, 80-82a

1H-2, 80-821H-2, 90-921H-2, 100-102a

1H-2, 110-1121H-2, 120-122"1H-2, 120-1221H-2, 130-1321H-2, 140-142"1H-3, 10-121H-3, 20-22a

1H-3, 20-221H-3, 30-321H-3, 40-42a

1H-3, 40-42a

1H-3, 50-521H-3, 60-62a

1H-3, 70-721H-3, 80-82a

1H-3, 90-921H-3, 100-102a

1H-3, 110-1121H-3, 120-122a

1H-3, 120-1221H-3, 130-1321H-3, 140-1423

1H-4, 10-121H-4, 20-22a

1H-4, 30-321H-4, 40-42a

1H-4, 50-521H-4, 60-62a

1H-4, 70-721H-CC, 10-121H-CC, 18-202H-2H-2H-2H-2H-2H-2H-2H-2H-

I, 10-12[, 23-25a

1, 30-321,40-421,50-52[,60-62a

1, 60-621,70-72[,80-82a

2H-1,90-922H-1, 100-102"2H-1,100-102a

2H-1, 110-1122H-1, 120-1222H-1, 120-1222H-1, 140-142a

2H-2, 10-122H-2, 23-35a

2H-2, 30-322H-2, 40-42a

2H-2, 50-522H-2, 60-62a

2H-2, 70-722H-2, 80-822H-2, 90-922H-2, 90-922H-2, 100-102a

2H-2, 110-1122H-2, 120-1222H-2, 130-1322H-2, 140-1423

2H-3, 10-12

Depth(mbsf)

0.060.200.200.300.500.800.901.001.101.201.301.401.601.701.801.901.902.002.102.202.302.302.402.502.602.702.702.802.903.103.203.203.303.403.403.503.603.703.803.904.004.104.204.204.304.404.604.704.804.905.005.105.305.365.445.605.735.805.906.006.106.106.206.306.406.506.506.606.706.706.907.107.237.307.407.507.607.707.807.907.908.008.108.208.308.408.60

Age(ka)

5.5414.7014.7018.5526.2537.8141.6645.5249.3753.2257.0760.8268.0871.7175.3478.9778.9782.6186.2489.8793.5093.5097.13

100.76104.39108.03108.03111.66115.29122.55126.18126.18132.33141.00141.00149.67158.33167.00175.67184.33193.00201.67210.33210.33219.00227.67245.00248.42251.84255.25258.67262.09268.93270.98273.71279.18283.63286.02289.44292.85296.27296.27299.69303.11306.53309.95309.95313.36316.78316.78323.62330.45334.90337.29340.65343.94347.24350.53353.82357.12357.12360.41363.71367.00370.29373.59380.18

δ 1 8 o(%c PDB)

3.184.253.954.224.183.904.174.033.463.643.563.934.013.754.003.583.884.043.963.863.233.714.003.643.933.593.813.713.283.953.372.974.063.713.753.783.773.883.633.803.813.462.963.393.953.783.884.124.033.753.833.833.953.973.743.903.763.984.044.113.243.473.923.883.653.063.663.494.043.883.423.683.823.504.093.983.633.543.733.554.003.663.573.943.883.953.64

δ 1 3 c(‰ PDB)

0.41-0.01-0.13-0.07-0.04-0.47-0.21-0.12-0.67-0.42-0.33-0.26-0.14-0.26-0.15-0.28-0.36-0.23-0.31-0.18-0.46-0.51-0.32-0.05-0.16-0.33-0.39-0.14-0.55-0.36-049-0.61-0.27-0.41-0.23-0.25-0.29-0.39-0.30-0.33-0.33-0.60-1.31-0.43-0.29-0.33-0.38-0.25-0.23-0.36-0.32-0.34-0.22-0.47-0.39-0.37-0.32-0.37-0.22-0.29-0.49-0.25-0.32-0.31-0.23-0.40-0.41-0.51-0.35-0.46-0.30-0.35-0.28-0.39-0.27-0.36-0.35-0.20-0.36-1.11-3.12-0.30-0.41-0.32-0.28-0.37-0.19

Core, section,interval (cm)

2H-3, 23-25a

2H-3, 30-322H-3, 40-^2"2H-3, 40-42a

2H-3, 50-522H-3, 60-62a

2H-3, 70-722H-3, 83-852H-3, 90-922H-3, 100-102a

2H-3, 100-1022H-3, 110-1122H-3, 120-1222H-3, 130-1322H-3, 140-142"2H-4, 10-122H-4, 23-25a

2H-4, 30-322H-4, 40-42"2H-4, 50-522H-4, 50-522H-4, 60-62"2H-4, 70-722H-4, 82-842H-4, 90-922H-4, 100-1023

2H-4,110-1122H-4, 120-1222H-4, 130-1322H-5, 10-122H-5, 23-25a

2H-5, 30-322H-5, 40^122H-5, 50-522H-5, 60-62a

2H-5, 60-622H-5, 70-722H-5, 80-822H-5, 80-822H-5, 90-922H-5, 100-102a

2H-5, 110-1122H-5, 120-1222H-5, 130-1322H-5, 140-142"2H-6, 10-122H-6, 23-252H-6, 23-252H-6, 30-322H-6, 40-12"2H-6, 50-522H-6, 60-62a

2H-6, 70-722H-6, 80-822H-6, 100-102a

2H-CC, 10-122H-CC, 23-25"2H-CC, 30-323H-1, 10-123H-1,20-22"3H-1,20-22"3H-1, 30-323H-1. 40-423H-3H-3H-3H-3H-3H-3H-3H-3H-3H-

,50-52, 60-62a

,70-72, 81-83,90-92,100-102",110-112, 120-122, 133-135, 133-135

3H-1, 140-142"3H-2, 30-323H-2, 30-323H-2, 40-42"3H-2, 40-423H-2, 40-423H-2, 50-523H-2, 60-62"3H-2, 60-623H-2, 70-723H-2, 81-833H-2, 90-923H-2,100-102"3H-2, 100-102"3H-:>, 110-112

Depth(mbsf)

8.738.808.908.909.009.109.209.339.409.509.509.609.709.809.90

10.1010.2310.3010.4010.5010.5010.6010.7010.8210.9011.0011.1011.2011.3011.6011.7311.8011.9012.0012.1012.1012.2012.3012.3012.4012.5012.6012.7012.8012.9013.1013.2313.2313.3013.4013.5013.6013.7013.8014.0014.2014.3214.4015.1015.2015.2015.3015.4015.5015.6015.7015.8115.9016.0016.1016.2016.3316.3316.4016.8016.8016.9016.9016.9017.0017.1017.1017.2017.3117.4017.5017.5017.60

Age(ka)

384.46386.76390.06390.06393.35396.65399.94404.22406.53409.82409.82413.12416.41419.71423.00427.81430.94432.62435.02437.43437.43439.83442.24445.12447.05449.45451.86454.26456.67463.88467.01468.69471.10473.50475.90475.90478.31480.71480.71483.12485.52487.93490.33492.74495.14499.95503.08503.08504.76507.17509.57511.98514.38516.79521.60527.15530.92533.44555.48558.62558.62561.77564.92568.07571.21574.36577.82580.66583.80586.95590.10594.19594.19596.39608.98608.98612.13612.13612.13615.28618.43618.43621.14623.66625.72672.8628.01630.30

δ 1 8 o<‰ PDB)

3.853.593.673.813.803.473.553.633.413.103.423.033.713.733.833.823.923.964.153.413.423.763.804.013.833.643.753.873.553.493.843.704.063.713.313.853.153.943.763.563.763.583.623.523.933.584.023.943.533.383.753.713.253.563.453.863.713.913.993.793.183.633.773.093.263.303.543.673.603.743.713.303.163.133.883.032.733.143.502.993.263.593.413.563.733.513.243.50

δ 1 3 c‰ PDB)

-0.52-0.56-0.46-0.36-0.46-0.52-0.34-0.50-0.79-1.02-1.54-1.36-0.55-0.41-0.47-0.47-0.44-0.43-0.35-0.46-0.92-0.59-0.55-0.41-0.47-0.43-0.39-0.51-0.39-0.41-0.33-0.50-0.38-0.46-0.23-0.47-0.77-0.38-0.48-0.56-0.55-0.40-0.36-0.51-0.36-0.55-0.36-0.46-0.44-0.48-0.31-0.20-0.33-0.19-0.42-0.33-0.43-0.47-0.38-0.44-0.48-0.45-0.36-0.46-0.51-0.56-0.46-0.59-0.45-0.30-0.30-1.21-0.80-0.66-0.36-1.96-1.21-0.72-1.31-0.60-0.54-0.48-0.29-0.37-0.51-0.48-0.53-0.46

B.P. FLOWER

Core, section,interval (cm)

3H-2, 120-1223H-2, 130-1323H-2, 140-1423H-2, 140-1423H-3, 10-123H-3, 20-223H-3, 30-323H-3, 40-42a

3H-3, 48-583H-3, 60-623H-3, 70-723H-3, 80-823H-3, 90-924H-1, 10-124H-l,20-22a

4H-1,30-324H-1, 40-424H-1, 50-524H-l,60-62a

4H-1,70-724H-1, 80-824H-1,90-924H-1, 100-1024H-1, 110-1124H-1, 110-1124H-1, 120-1224H-1, 130-1324H-1, 140-1424H-1, 140-1424H-2, 10-124H-2, 18-20a

4H-2, 30-324H-2, 40-^2a

4H-2, 50-524H-2, 60-62a

4H-2, 70-724H-2, 80-82a

4H-2, 90-924H-2, 110-1124H-2, 110-1124H-2, 120-1224H-2, 130-1324H-2, 138-1404H-3, 10-124H-3, 20-22a

4H-3, 20-224H-3, 30-324H-3, 40-424H-3, 50-524H-3, 60-62a

4H-3, 70-724H-3, 80-824H-3, 90-924H-3, 100-1024H-3, 110-1124H-3, 120-1224H-3, 120-1224H-4, 10-124H-4, 20-22a

4H-4, 20-22

Table 1

Depth(mbsf)

17.7017.80

a 17.9017.9018.1018.2018.3018.4018.4818.6018.7018.8018.9019.6019.7019.8019.9020.0020.1020.2020.3020.40

a 20.5020.6020.6020.7020.80

a 20.9020.9021.1021.1821.3021.4021.5021.6021.7021.8021.9022.1022.1022.2022.3022.3822.6022.7022.7022.8022.9023.0023.1023.2023.3023.40

a 23.5023.6023.7023.7024.1024.2024.20

(continued).

Age(ka)

632.58634.87637.16637.16641.74644.02646.31648.60650.43653.18655.46657.75660.04676.06678.34680.63682.92685.21687.50689.78692.07694.36696.65698.94698.94701.22703.51705.80705.80710.38712.21714.95717.24719.53721.82724.10726.39728.68733.26733.26735.54737.83739.66744.70746.98746.98749.27751.56753.85756.14758.42760.71763.00765.29767.58769.86769.86779.02781.78781.78

δ 1 8 θ(‰ PDB)

3.493.403.243.623.393.593.783.563.773.363.583.683.713.103.052.903.383.643.042.942.983.443.223.733.753.273.402.974.043.303.003.623.423.573.373.713.763.044.524.113.303.163.243.652.793.223.163.543.513.083.383.193.403.743.172.643.243.103.683.90

δ 1 3 c(‰ PDB)

-0.45-0.53-0.53-0.50-0.38-0.73-0.42-0.42-0.58-0.70-0.55-0.65-0.35-1.14-0.91-1.09-1.03-0.86-1.26-1.69-1.15-1.27-1.07-0.93-0.46-1.41-1.22-1.84-0.17-0.96-0.90-0.68-0.88-0.91-0.63-0.66-0.65-0.53-0.40-1.55-1.20-1.77-1.43-1.32-1.31-0.99-1.32-1.40-1.31-1.13-1.21-1.45-1.16-0.40-1.32-1.08-1.38-1.03-0.71-0.41

Note: aSample analyzed at the University of California, Santa Barbara. All other samplesanalyzed at the University of California, Santa Cruz.

1.07 Ma). Further, interpolation between the 15/16 stage boundaryand the first occurrence (FO) of Gephyrocapsa oceanica (1.70 Ma;Raffi et al., 1993) between Sections 151-910C-7R-CC and 8R-CC(Sato and Kameo, this volume) yields a minimum estimate of -4.0cm/k.y. for the 17-60 mbsf interval. Based on this evidence for high-er average sedimentation rates in this interval, I concur with the as-signment of Hole 910A to the Brunhes Chron (Shipboard ScientificParty, 1995) and conclude that no major hiatus exists in the upper24.5 m in Hole 910A and assign ages for the 19-24.5 mbsf sectionaccordingly. As an aside, a hiatus that removed much of the lowerQuaternary to upper Pliocene is found at the base of the overconsol-idated section between -70 and 95 mbsf, based on biostratigraphy(Site 910; Myhre, Thiede, Firth, et al., 1995). These interpretationsawait independent assessment of ages within the overconsolidatedsection, such as strontium isotopic stratigraphy.

Thus, oxygen isotopic stratigraphy in the overlying sequencedates the top of the overconsolidated section near the Stage 17/16boundary (-670 ka).

Carbon Isotopic Record

The carbon isotopic record is remarkably stable during the inter-val from -620 ka to the present, with the exception of large spikes of~0.5%o mainly during inferred interglacials (Fig. 4). Also, the top 20cm features the highest δ13C values observed in the core (-0.1%c to0.4%o), suggesting a fundamentally different hydrography in the Ho-locene. The major features of the δ13C record, however, are two large-amplitude cycles of ~\%o within the overconsolidated section. A13%c increase coincides with the top of this section. Thus, the over-consolidated section is marked by a fundamentally different δ13C sig-nal than the upper Quaternary section. Such large δ13C fluctuationsare unknown in middle Quaternary global δ13C records (e.g., Shack-leton and Hall, 1989; Berger et al., 1993). However, lower magnitudefluctuations are present in middle to late Quaternary benthic δ13Crecords from the North Atlantic (e.g., Ruddiman et al., 1989). Themagnitude of Hole 910A δ13C cycles suggests highly variable condi-tions specific to the Yermak Plateau region prior to -620 ka. Theseδ13C fluctuations do not seem to correlate with any measured param-eter of the Hole 910A sequence, including sedimentological parame-ters such as grain size, density, or porosity. Nevertheless, the ~\%oδ13C fluctuations prior to -620 ka may well be genetically linked tothe same processes that resulted in an overconsolidated section below-19 mbsf.

Assuming that N. pachyderma (s) reflects surface water δ13C val-ues, the ~\%c variations prior to -620 ka may reflect large changes inthe δ13C of Σ C O 2 over the Yermak Plateau. This assumption is basedon recent findings that N. pachyderma (s) accurately records mixedlayer δ13C in the Greenland Sea (Kohfeld et al., 1994; Johannessen etal., in press). Such changes might be produced either by variations inorganic matter concentrations or by changes in "thermodynamic tag-ging" of surface water Σ C O 2 by air-sea exchange (Charles and Fair-banks, 1990). Thermodynamic tagging should increase surface waterδ13C (Charles and Fairbanks, 1990). However, modern surface watersat 80°N show significant thermodynamic depletion (Charles et al.,1993), yet Holocene δ13C values are the highest of the late Quaternaryin Hole 910A. As the δ13C values within the overconsolidated sectionare anomalously low relative to modern values, the potential controlby thermodynamic gas exchange seems unlikely.

Alternatively, large-scale changes in upwelling may explain thelarge \%c δ13C variations prior to -620 ka, as also observed in thecentral Arctic during the last deglaciation (Stein et al., 1994). Or, in-put of terrestrially derived organic carbon (with lower δ13C values)may have been greater, due to more dynamic glacial processes. Inshort, these large δ13C variations still require explanation, and thequestion is open as to how they relate to the processes of overconsol-idation.

Interpretations of the Overconsolidated Section

Interpretations of the overconsolidated section include (1) episod-ic grounding of a marine-based ice sheet, perhaps derived from theBarents Sea ice sheet and buttressed by Svalbard, which overcom-pacted the sediments on the Yermak Plateau, and/or (2) coarser-grained glacial sedimentation that allowed enhanced compaction dur-ing this interval (Myhre, Thiede, Firth, et al., 1995; Rack et al., Chap-ter 21, this volume). Overcompaction may have also resulted fromlarge icebergs grounding at an inferred water depth of at least 400 m.Grounding of recent icebergs on the Yermak Plateau at water depthsof -450 to 850 m has been demonstrated based on sidescan sonar and3.5-kHz profiling, although the age is not well known (Vogt et al.,1994). Also, the Yermak Plateau exhibits a beveled surface, whichmay indicate the presence of grounded ice during the late Quaternary(Vogt et al., 1994, 1995). In areas of ice sheet or iceberg grounding,large-scale erosion under the ice and redeposition are common (e.g.,Vorren et al., 1988). A major hiatus indeed exists at the base of the

448

LATE QUATERNARY STABLE ISOTOPIC STRATIGRAPHY

Figure 2. Hole 910A δ 1 8 θ and δ1 3C data vs. depth,based on N. pachyderma (sinistrally coiling). Oxy-gen isotope Stages 1 through 16 are present in theupper 19 mbsf. The LO of P. lacunosa is at 9.7 ±0.75 mbsf. The overconsolidated section below-19.5 mbsf is marked by large (>\%c) shifts in δ1 3C.Core recovery and normal magnetic polarity areshown in black.

overconsolidated section between -70 and 95 mbsf, but there is noevidence for a hiatus near the top at -19.5 mbsf.

Alternatively, overcompaction may have resulted from coarsergrained sedimentation, because sandy clays are more easily compact-ed than silty clays (Hamilton, 1976). There is some evidence for morenumerous particles >2 mm in diameter from -16 to 25 mbsf, basedon X-ray imaging of Hole 910D cores (Rack et al., Chapter 21, thisvolume). However, coarse grain size data show no mean differencesin grain size (>63- and >250-µm fractions) in the overconsolidatedsection (Fig. 5), arguing against this hypothesis. Geotechnical andgeomagnetic investigations are being conducted to further differenti-ate possible causes of overcompaction (Rack et al., Chapter 21, thisvolume; Solheim et al., unpubl. data; Williamson et al., unpubl. data).

Quaternary Arctic Gateway Paleoceanography

In addition to age control, stable isotopic data from Hole 910Aprovide records of late Quaternary surface water environments in theYermak Plateau region of the Arctic Ocean. The series of ~\%o δ 1 8 θcycles from 0 to 670 ka are inferred to largely reflect the well-knownvariations in Quaternary ice volumes, although the record is compli-cated by minor salinity variations (Fig. 6). Salinity variations are sig-nificantly less than in the Arctic Ocean (e.g., Morris, 1988). Interest-ingly, the Hole 910A δ 1 8 θ amplitude is ~0.2%c to 0.4%o lower thanthe generally accepted \.2%o-\A%o attributed to Quaternary ice-growth and decay (e.g., Shackleton and Opdyke, 1973; Fairbanks andMatthews, 1978; Mix and Ruddiman, 1984; Fairbanks, 1989), al-though this may reflect minor undersampling of glacial/interglacialvalues. With this caveat, salinity effects appear to have been relative-ly minor, as the δ 1 8 θ amplitude is not increased beyond the ice vol-ume effect of -12%o to 1 A%0.

Possible explanations for dampened glacial-interglacial δ 1 8 θ am-plitudes on the Yermak Plateau relate to the regional oceanography.Surface water δ 1 8 θ in this region is very sensitive to salinity differ-ences (e.g., Fairbanks et al., 1992). The warm, high salinity WSC hasa distinctly higher δ 1 8 θ signature than the low salinity East Green-land Current (EGC) (by up to 1.5%e; Johannessen et al., in press). Ahigher influx of WSC waters during interglacials and a higher out-flow of EGC waters during glacials would significantly dampen theglacial-interglacial δ 1 8 θ amplitude. Alternatively, changes in the sea-

sonal and/or depth preferences of N. pachyderma (s) may have result-ed in dampened δ 1 8 θ amplitudes.

In contrast to the last 670 k.y., δ 1 8 θ values prior to -670 ka are~0.3%o lower, whereas maintaining a similar range of ~l%o. This~0.3%o difference is probably not attributable to lower global ice vol-umes of the middle Quaternary, based on the age control discussedabove. Instead, the ~0.3%o difference may reflect a different temper-ature and/or salinity regime in the early Brunhes Chron. However,there is little evidence for warmer surface waters in the NorwegianSea, which might have been advected into the Arctic Ocean via theWest Spitsbergen Current and the Barents Sea branch. Indeed, thewarmest temperatures in the Norwegian Sea are indicated for the en-tirety of the last 1 m.y., based on planktonic foraminifer flux recordsand percentage carbonate records inferred to reflect temperature-de-pendent carbonate production (Jansen et al., 1988; Spiegler and Jan-sen, 1989; Henrich, 1989). Thus the ~0.3‰ difference observed priorto 670 ka may represent slightly lower mean salinities for YermakPlateau surface waters (by ~0.3‰, based on the calibration of Fair-banks et al., 1992).

Lower mean salinities might be due to (1) higher meltwater inputand/or (2) a different glacial meltwater source marked by lower δ 1 8 θ.If the meltwater input were higher, the interval prior to -670 ka mightreflect greater instability in the circum-Arctic ice sheets. One wouldthink, however, that the amplitude of the δ 1 8 θ variations would belarger if such a mechanism were operating, but it was ~l%o both be-fore and after 670 ka. Stated another way, both the glacial maximaand the interglacial minima are offset by ~0.3‰, instead of preferen-tial decreases during interglacials that might suggest stronger melt-water effects. Alternatively, surface water δ 1 8 θ may have been influ-enced by a lower-δ18θ glacial meltwater source. The δ 1 8 θ of modernglacial ice masses in the Northern Hemisphere ranges from —10%oto -30%o, dependent upon the moisture source and the altitude of dep-osition (e.g., Dansgaard et al., 1973). Thus, the ~0.5%o lower δ 1 8 θsurface water values might reflect a different source, such as the larg-er Greenland ice sheet instead of the smaller Svalbard dome.

Relation to the Middle Quaternary Transition

The cause of the middle Quaternary climatic transition (from cli-mates dominated by -41 k.y. periodicity to -100 k.y. periodicity,

449

B.P. FLOWER

Table 2. Size fraction data from Hole 910A.

Core, section,interval (cm)

151-910A-1H-1, 10-121H-1, 20-221H-1, 30-321H-1, 40-421H-1, 50-521H-1, 60-621H-1, 70-721H-1, 80-821H-1, 90-921H-1, 100-1021H-1, 110-1121H-1, 120-1221H-1, 130-1321H-1, 140-1421H-2, 10-121H-2, 20-221H-2, 30-321H-2, 40-421H-2, 50-521H-2, 60-621H-2, 70-721H-2, 80-821H-2, 90-921H-2, 100-1021H-2, 110-1121H-2, 120-1221H-2, 130-1321H-2, 140-1421H-3, 10-121H-3, 20-221H-3, 30-321H-3, 4 0 ^ 21H-3, 50-521H-3, 60-621H-3, 70-721H-3, 80-821H-3, 90-921H-3, 100-1021H-3, 110-1121H-3, 120-1221H-3, 130-1321H-3, 140-1421H-4, 10-121H-4, 20-221H-4, 30-321H-4, 4 0 ^ 21H-4, 50-521H-4, 60-621H-4, 70-721H-CC, 10-121H-CC, 18-202H-1, 10-122H-1,23-252H-1,30-322H-1,40^122H-1,50-522H-1,60-622H-1, 70-722H-1, 80-822H-1, 90-922H-1, 100-1022H-1, 110-1122H-1, 120-1222H-1,140-1422H-2, 10-122H-2, 23-252H-2, 30-322H-2, 40-422H-2, 50-522H-2, 60-622H-2, 70-722H-2, 80-822H-2, 90-922H-2, 100-1022H-2, 110-1122H-2, 120-1222H-2, 130-1322H-2, 140-1422H-3, 10-122H-3, 23-252H-3, 30-322H-3, 40-422H-3, 50-522H-3, 60-622H-3, 70-722H-3, 83-852H-3, 90-922H-3, 100-102

Depth(mbsf)

0.100.200.300.400.500.600.700.800.901.001.101.201.301.401.601.701.801.902.002.102.202.302.402.502.602.702.802.903.103.203.303.403.503.603.703.803.904.004.104.204.304404.604.704.804.905.005.105.205.365.445.605.735.805.906.006.106.206.306.406.506.606.706.907.107.237.307.407.507.607.707.807.908.008.108.208.308.408.608.738.808.909.009.109.209.339.409.50

Age(ka)

9.2314.7018.5522.4026.25930.1133.9637.8141.6645.5249.3753.2257.0760.8268.0871.7175.3478.9782.6186.2489.8793.5097.13

100.76104.39108.03111.66115.29122.55126.18132.33141.00149.67158.33167.00175.67184.33193.00201.67210.33219.00227.67245.00248.42251.84255.25258.67262.09265.51270.98273.71279.18283.63286.02289.44292.85296.27299.69303.11306.53309.95313.36316.78323.62330.45334.90337.29340.65343.94347.24350.53353.82357.12360.41363.71367.00370.29373.59380.18384.46386.76390.06393.35396.65399.94404.22406.53409.82

Dryweight (g)

12.790013.52148.8190

18.21469.0720

17.679910.305013.577012.037017.105811.256013.845113.428014.240910.636013.972315.238014.183212.976015.917015.445014.415412.969017.594711.716014.411510.694015.659717.381016.962813.736016.923213.495016.005916.976019.420914.051018.570015.551017.645515.195016.831615.832018.076012.776015.352016.894020.990118.897017.624018.831813.351112.038513.111713.154510.955613.554414.245216.732610.479011.03279.4925

12.924810.869810.763020.199211.778111.512014.650515.892519.019117.34159.0922

12.486211.819411.364910.509510.462718.600618.148520.317917.521613.957712.247216.072720.618412.388511.4235

>63-µmweight (g)

2.69336.53082.90004.60144.659011.41182.30312.70252.20433.61061.91272.62222.24682.91411.67202.62302.91952.67791.99123.05242.65662.59322.24773.40311.93002.83322.00752.80863.39043.52162.44873.29382.23383.18652.79324.27772.36652.28431.91623.43322.70823.31982.74493.52782.35303.00332.59233.67913.00693.02813.64842.85322.34242.11982.54912.61722.06562.44503.26071.97392.09222.03732.58002.83742.00373.65092.24842.11172.89132.88983.67263.32731.78662.35852.55552.08202.08382.07644.55373.32683.88103.18122.40162.60743.04814.02121.98802.0021

>250-µmweight (g)

0.86243.67930.86230.52291.38754.46160.73820.33590.34470.44800.23420.37990.33180.44970.21780.34100.72580.37350.32900.49750.39730.31780.38000.45550.21900.42990.44680.37460.63480.62730.44940.39890.29080.47300.46251.09400.33190.29810.27620.42610.56810.44290.44960.44030.44180.52850.35100.47330.40080.52280.54150.36880.72130.24390.31090.38290.27810.32750.37160.27750.26240.43060.32650.25950.25640.46280.32700.29470.45680.50970.56570.59240.29370.31680.54690.25890.29320.31540.58360.51260.60560.40680.34760.40490.44610.67920.27990.3891

Core, section,interval (cm)

2H-3, 110-1122H-3, 120-1222H-3, 130-1322H-3, 140-1422H-4, 10-122H-4, 23-252H-4, 30-322H-4, 40-422H-4, 50-522H-4, 60-622H-4, 70-722H-4, 82-842H-4, 90-922H-4, 100-1022H-4, 110-1122H-4, 120-1222H-4, 130-1322H-5, 10-122H-5, 23-252H-5, 30-322H-5, 4 0 ^ 22H-5, 50-522H-5, 60-622H-5, 70-722H-5, 80-822H-5, 90-922H-5, 100-1022H-5, 110-1122H-5, 120-1222H-5, 130-1322H-5, 140-1422H-6, 10-122H-6, 23-252H-6, 30-322H-6, 40-422H-6, 50-522H-6, 60-622H-6, 70-722H-6, 80-822H-6, 90-922H-6, 100-1022H-CC, 10-122H-CC, 23-252H-CC, 30-323H-1, 10-123H-1, 20-223H-1,30-313H-1, 40-423H-1,50-523H-1, 60-623H-1,70-723H-1,81-833H-1,90-923H-1, 110-1123H-1, 120-1223H-1, 133-1353H-1, 140-1423H-2, 10-123H-2, 20-223H-2, 30-323H-2, 40-423H-2, 50-523H-2, 60-623H-2, 70-723H-2, 81-833H-2, 90-923H-2, 100-1023H-2, 110-1123H-2, 120-1223H-2, 132-1343H-2, 140-1423H-3, 10-123H-3, 20-223H-3, 30-323H-3, 40-423H-3, 48-503H-3, 60-623H-3, 70-723H-3, 80-823H-3, 90-924H-1, 10-124H-1, 20-224H-1,30-324H-1,40-424H-1,50-524H-1,60-624H-1,70-724H-1,80-824H-1,90-93

Depth(mbsf)

9.609.709.809.90

10.1010.2310.3010.4010.5010.6010.7010.8210.9011.0011.1011.2011.3011.6011.7311.8011.9012.0012.1012.2012.3012.4012.5012.6012.7012.8012.9013.1013.2313.3013.4013.5013.6013.7013.8013.9014.0014.1914.3214.3915.2015.3015.4015.5015.6015.7015.8015.9116.0016.1016.2016.3316.4016.6016.7016.8016.9017.0017.1017.2017.3117.4017.5017.6017.7017.8217.9018.1018.2018.3018.4018.4818.6018.7018.8018.9019.6019.7019.8019.9020.0020.1020.2020.3020.40

Age(ka)

413.12416.41419.71423.00427.81430.94432.62435.02437.43439.83442.24445.12447.05449.45451.86454.26456.67463.88467.01468.69471.10473.50475.90478.31480.71483.12485.52487.93490.33492.74495.14499.95503.08504.76507.17509.57511.98514.38516.79519.19521.60526.83530.92533.13558.62561.77564.92568.07571.21574.36577.51580.97583.80586.95590.10594.19596.39602.69605.84608.98612.13615.28618.43621.14623.66625.72628.01630.30632.58635.33637.16641.74644.02646.31648.60650.43653.18655.46657.75660.04676.06678.34680.63682.92685.21687.50689.78692.07694.36

Dryweight (g)

11.487811.350210.302510.722614.779819.117518.879618.442014.128716.154817.568318.501414.487615.001313.470312.062111.854114.703419.009112.337917.216314.586918.281416.450018.133316.864414.091411.944312.221711.984315.309917.058919.356817.556812.459613.768615.930116.681318.116518.741114.565216.038518.789517.767416.096919.103818.004820.152416.838216.738816.096215.982516.537515.598016.615216.288216.365014.750315.011418.578414.517415.887016.240415.627817.214316.614315.887115.127618.183416.250916.903217.978518.834017.196916.505615.748818.201315.992716.942822.033617.023717.689221.843920.498714.511117.624018.271617.485619.0938

>63-µmweight (g)

1.89711.96672.46192.21133.09293.71353.65123.47791.02373.01823.67493.20724.36032.38642.79522.24902.10132.69902.69252.26653.20053.05413.45973.04784.48743.96402.55202.32182.12252.31242.54443.23323.62013.32232.28612.36452.71542.96133.19573.59962.65042.99333.64563.64643.07183.57883.51363.56133.60132.68823.11011.93963.20242.48912.23732.68582.32352.82732.69983.80443.43403.28982.87933.13233.26843.25622.40382.38163.04122.93762.64223.29323.53803.45313.21363.26473.74432.76733.39864.10042.90003.00475.78043.48012.79153.12093.68133.60054.0759

>250-µmweight (g)

0.26010.22400.46040.36700.49210.43140.45760.53790.27420.48230.76300.54370.36950.30360.40950.32660.23650.32680.38770.36310.42660.36580.71760.60380.43490.35010.34050.32700.25990.38970.32060.44550.47140.38180.47170.36780.35780.35920.38970.60060.41460.58500.72130.48340.33810.46280.44800.52720.84630.33290.49430.27250.43670.34610.30050.44210.22900.40190.24540.66780.67110.42850.52830.48770.52180.76440.38180.30660.41580.35060.35990.36210.36490.45390.46120.45800.48020.41150.44250.51180.45870.50271.16640.56200.68690.47390.45750.46210.5792

450

LATE QUATERNARY STABLE ISOTOPIC STRATIGRAPHY

Table 2 (continued).

Core, section,interval (cm)

4H-1, 100-1024H-1, 110-1124H-1, 120-1224H-1, 130-1324H-1, 140-1424H-2, 10-124H-2, 18-204H-2, 30-324H-2, 4 0 ^ 24H-2, 50-524H-2, 60-624H-2, 70-724H-2, 80-824H-2, 90-924H-2, 100-1024H-2, 110-1124H-2, 120-1224H-2, 130-1324H-2, 138-1404H-3, 10-124H-3, 20-224H-3, 30-324H-3, 40-424H-3, 50-524H-3, 60-624H-3, 70-724H-3, 80-824H-3, 90-924H-3, 100-1024H-3, 110-1124H-3, 120-1224H-3, 130-1324H-4, 10-124H-4, 20-22

Depth(mbsf)

20.5020.6020.7020.9021.0021.1021.1821.3021.4021.5021.6021.7021.8021.9022.0022.1022.2022.3022.3822.6022.7022.8022.9023.0023.1023.2023.3023.4023.5023.6023.7023.8024.1024.20

Age(ka)

696.65698.94701.22705.80708.09710.38712.21714.95717.24719.53721.82724.10726.39728.68730.97733.26735.54737.83739.66744.70746.98749.27751.56753.85756.14758.42760.71763.00765.29767.58769.86772.15779.02781.78

Dryweight (g)

16.792617.414314.251214.951317.666217.519419.794119.465719.976219.061921.065221.374721.511618.448619.675120.406720.966716.517222.087319.114323.202521.870021.434918.445019.541419.272021.206817.430120.190621.246024.364922.409319.662218.6604

>63-µmweight (g)

3.33443.59072.42534.31403.16714.24953.79413.80953.99123.80783.70793.87052.95033.675114.15633.94153.79922.88193.96263.62474.60214.80894.50443.01853.76823.89233.99633.57973.51694.05844.944011.15943.70043.1935

>250-µmweight (g)

0.49330.50540.36540.48610.48611.17510.50230.56280.42330.53190.69810.51910.61181.15863.79860.61130.56250.37970.59130.51400.60940.67990.75860.79030.51500.56200.79200.88380.49240.66930.78991.30960.52080.5446

from -0.9 to 0.6 Ma) is a central question in Quaternary paleoclima-tology (e.g., Prell, 1982; Shackleton and Hall, 1989; Imbrie et al.,1993; Berger et al., 1993; Berger and Jansen, 1994). Marine shelf icesheets (such as the Barents Sea ice sheet) are thought to play a majorrole in large-amplitude glacial-interglacial cycles of the late Quater-nary (marked by -100 k.y. periodicity), by allowing major ice build-up on marine shelves and rapid deglaciation (e.g., Broecker and vanDonk, 1970; Shackleton and Opdyke, 1973; Vorren et al., 1988; Im-brie et al., 1993). Examination of circum-Arctic ice sheet history canhelp evaluate the possibility that larger, grounded ice sheets en-hanced global δ 1 8 θ amplitudes following the middle Quaternary cli-matic transition. The termination of the overconsolidated section near

0 -

5 -

1 0 -

15 -

20 -

25 -

'"'•-..5/6

1.9 cm/k.y. • 7 / 8

i i i—r

• f c 9 / 1 0

3.3 cm/k

' ' ' ' i i

, . i ,

1 1 / 1 2

•y \

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13/14

1 1 I I I

-

-

_

15/16 ;

i—i-i—|—i—i—i—r- -

100 200 300 400 500

Age (ka)

600 700 800

Figure 3. Age vs. depth plot of selected oxygen isotope stage boundaries usedin an age model for Hole 910A. Sedimentation rates average -1.9 cm/k.y. forthe 0-245 ka interval and -3.3 cm/k.y. for the 245-620 ka interval.

670 ka coincides with the well-known increase in the climatic re-sponse to the -100 k.y. period Milankovitch forcing (e.g., Berger etal., 1993). Conventional interpretation of the increased amplitude atthe 100 k.y. period includes the supposition that grounded ice sheetsbecame more common in the circum-Arctic during the late Quaterna-ry (Hughes et al., 1977; Prell, 1982; Pollard, 1983; Vorren et al.,1988; Imbrie et al., 1993). Extensive ice buildup on the continentalshelves of the Arctic Ocean during glacial intervals may have ampli-fied the 100 k.y. climatic response and enhanced glacial-interglacialδ 1 8 θ amplitudes. In this scenario, we might expect to find evidencefor grounded marine-based ice sheets such as the Barents Sea icesheet during glacial intervals after -800 ka (starting during Stage 22,the first large late Quaternary glacial) and continuing during subse-quent glacial intervals to oxygen isotope Stage 2. Evidence exists forseveral groundings of the Barents Sea ice sheet, probably within thelast 800 ka (Vorren et al., 1988), but the age is not well known.

Significantly, if the overconsolidated section represents ground-ing of a Svalbard/Barents Sea ice sheet, then this grounding wasclearly most pervasive prior to -670 ka, and much less so toward thepresent. Thus, an age of -670 ka for the top of this section may indi-

o.o-

100 200 300 400 500 600

Age (ka)

700 800

Figure 4. Carbon isotopic records for Hole 910A(based on N. pachyderma [sinistrally coiling]) andfor Site 677 (based mainly on Uvigerina; Shackle-ton and Hall, 1989), plotted vs. age. Also shown areoxygen isotope stages. Note the general stability ofHole 910A δ1 3C, except for large variations of ~\%cprior to -670 ka.

451

B.P. FLOWER

Figure 5. Hole 910A coarse fraction (wt%) vs. age. Nosignificant mean changes in percent >63-µm or percent>250-µm fractions are associated with the overconsoli-dated section. Also shown are oxygen isotope stages.

100 200 300 400Age (ka)

500 600 700 800

Figure 6. Oxygen isotopic records for Hole 910A(based on N. pachyderma [sinistrally coiling]) andfor Site 677 (based mainly on Uvigerina; Shackletonand Hall, 1989), plotted vs. age. The top of the over-consolidated section at -19.5 mbsf occurs near theStage 17/16 boundary (670 ka). Also shown are oxy-gen isotope stages.

100 200 300 400 500Age (ka)

600 700 800

cate that grounded Svalbard/Barents Sea ice sheets did not becomemore common during the late Quaternary. One possible resolution isthat the Barents Sea ice sheet may have undergone a transition froma grounded to a floating, marine-based ice sheet (e.g., Hughes et al.,1977) at this time. Such an ice sheet might still have produced thelarge glacial-interglacial variations in δ 1 8 θ without overcompactingsediment on the Yermak Plateau.

Alternatively, the overconsolidated section may represent in-creased intensity of ice-rafting prior to -670 ka. Coarser grained gla-cial sedimentation might indicate greater ice-rafting through theFram Strait, or possibly greater erosional capacity of the circum-Arc-tic ice sheets. The transition near -670 ka may represent a changefrom very dynamic ice sheets (at least in the vicinity of the YermakPlateau) to increased stability of the Arctic cryosphere, including,perhaps, the circum-Arctic ice sheets and the Arctic sea ice. Althoughthe picture is not complete, the available evidence points to largerand/or more dynamic circum-Arctic ice sheets prior to -670 ka com-pared to after -670 ka. Whatever the cause of the overcompaction,the evidence, therefore, seems inconsistent with the idea that largerice sheets became grounded more commonly in the late Quaternary.

Lastly, I speculate that the termination of ice-sheet grounding onthe Yermak Plateau may correspond to a marked change in the char-acter of Svalbard/Barents Sea glaciations during the middle Quater-nary, and perhaps to the middle Quaternary climatic transition. Sol-heim et al. (in press) note a change from net erosion to net depositionon the western Svalbard Margin, which can be traced regionally asthe Upper Regional Uncomformity (URU). This transition is attribut-ed to "a shift from thick, eroding glaciers with steep ice profiles, tolow profile fast flowing ice streams maintained by an increasedamount of interglacial and interstadial sediments" (Solheim et al., inpress). Detailed seismic profiling from the western Svalbard Marginto the Yermak Plateau (Solheim et al., unpubl. data) will test the po-tential link between the URU and the top of the overconsolidated sec-tion on the Yermak Plateau.

SUMMARY

Stable isotopic records from ODP Leg 151 Hole 910A based onN. pachyderma (s) represent some of the first high-resolution, long-

LATE QUATERNARY STABLE ISOTOPIC STRATIGRAPHY

term (middle Quaternary to present) records produced from the Arc-tic Ocean. The δ l 8 0 data reveal oxygen isotope Stages 1 through 16(and possibly 17), although the record is complicated by salinity vari-ations. Amplitudes of only ~l‰ suggest salinity changes dampenedthe glacial-interglacial δ 1 8 θ signal throughout the middle to late Qua-ternary. Termination of a remarkable overconsolidated section below-19.5 mbsf is dated near the Stage 17/16 boundary (= 670 ka). Largeδ13C fluctuations of >l%0 suggest significant changes in the 613C ofΣ C O 2 prior to -670 ka. The large-amplitude variations in δ'3C below-19.5 mbsf, and the overconsolidated nature of the section, suggest afundamentally different sedimentary environment. Possible explana-tions include (1) episodic grounding of a marine-based ice sheet, per-haps derived from the Barents Sea ice sheet and buttressed by Sval-bard, that overcompacted the sediment and strongly influenced sur-face water environments on the Yermak Plateau, and/or (2) coarsergrained glacial sedimentation that allowed enhanced compaction dur-ing this interval. Whatever the cause of the overcompaction, its ter-mination indicates a major change in Yermak Plateau sedimentationhistory near the Stage 17/16 boundary (670 ka). This termination mayrelate to a fundamental change in state of the Svalbard/Barents Seaice sheets, and to the middle Quaternary climate transition. Furtherevaluation of late Quaternary circum-Arctic climate history is neededto better understand the role of the Arctic cryosphere in Quaternaryclimate change.

ACKNOWLEDGMENTS

This work was supported by the JOI/USSSP (151-20794b), alongwith partial support from NSF (OPP-9423485). I thank the OceanDrilling Program for inviting my participation on Leg 151; NaoAhagon for providing samples; D.W. Oppo, W.F. Ruddiman and ananonymous reviewer for helpful reviews; and Melany Manaig, Bhav-na Sahi, and Diana Taylor for laboratory assistance.

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