Coarse-grained terrigenous sediment deposition on continental rise drifts: A record of Plio-Pleistocene glaciation on the Antarctic Peninsula

Palaeogeography, Palaeoclimatology, Palaeoecology 265 (2008) 275–291

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Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / p a l a e o

Coarse-grained terrigenous sediment deposition on continental rise drifts: A record of Plio-Pleistocene glaciation on the Antarctic Peninsula Ellen A. Cowan a,⁎, Claus-Dieter Hillenbrand b, Lauren E. Hassler a, Matthew T. Ake a a b

Department of Geology, ASU Box 32067, Appalachian State University, Boone, NC 28608, United States British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 OET, United Kingdom

A R T I C L E

I N F O

Article history: Received 7 August 2007 Received in revised form 16 March 2008 Accepted 18 March 2008 Keywords: Antarctic Peninsula Sediment drifts Iceberg-rafted debris Plio-Pleistocene glaciation

A B S T R A C T Sediment drifts on the continental rise are located proximal to the western side of the Antarctic Peninsula and recorded changes in glacial volume and thermal regime over the last ca. 15 m.y. At Ocean Drilling Program (ODP) Site 1101 (Leg 178), which recovered sediments back to 3.1 Ma, glacial–interglacial cyclicity was identified based on the biogenic component and sedimentary structures observed in X-radiographs, magnetic susceptibility and lithofacies descriptions. Glacial intervals are dominated by fine-grained laminated mud and interglacial units consist of bioturbated muds enriched in biogenic components. From 2.2 to 0.76 Ma, planktonic foraminifera and calcareous nannofossils dominate in the interglacials suggesting a shift of the Antarctic Polar Front (APF) to the south near the drifts. Prior to 2.2 Ma, cyclicity cannot be identified and diatoms dominate the biogenic component and high percent opal suggests warmer conditions south of the APF and reduced sea ice over the drifts. Analyses of the coarse-grained terrigenous fraction (pebbles and coarse sand) from Sites 1096 and 1101 record glaciers at sea-level releasing iceberg-rafted debris (IRD) throughout the last 3.1 m.y. Analyses of quartz sand grains in IRD with the scanning electron microscope (SEM) show an abrupt change in the frequency of occurrence of microtextures at ~ 1.35 Ma. During the Late Pliocene to Early Pleistocene, the population of quartz grains included completely weathered grains and a low frequency of crushing and abrasion, suggesting that glaciers were small and did not inundate the topography. Debris shed from mountain peaks was transported supraglacially or englacially allowing weathered grains to pass through the glacier unmodified. During glacial periods from 1.35–0.76 Ma, glaciers expanded in size. The IRD flux was very high and dropstones have diverse lithologies. Conditions resembling those at the Last Glacial Maximum (LGM) have been episodically present on the Antarctic Peninsula since ~ 0.76 Ma. Quartz sand grains show high relief, fracture and abrasion common under thick ice and the IRD flux is low with a more restricted range of dropstone lithologies. © 2008 Elsevier B.V. All rights reserved.

1. Introduction There is growing agreement that the Early Pliocene climate history of Antarctica is marked by interglacial periods warmer than today followed by subsequent cooling to develop large polar ice sheets which expanded during Late Pleistocene glacial periods (e.g. Dingle and Lavelle, 1998; Whitehead and Bohaty, 2003; Hillenbrand and Ehrmann, 2005; Whitehead et al., 2005; Hepp et al., 2006; Raymo et al., 2006; Smellie et al., 2006). The details of this scenario are still unknown and questions remaining to be addressed include the nature and timing of this transition. When did the current ice regime on the Antarctic Peninsula become established and was the change abrupt or gradual? What were the conditions during the climate reorganization from 1.0–0.4 Ma (mid-Pleistocene climate transition)? The Antarctic Peninsula is one of the more climatically sensitive areas of Antarctica ⁎ Corresponding author. Tel.: +1 828 262 2260; fax: +1 828 262 6503. E-mail address: [email protected] (E.A. Cowan). 0031-0182/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2008.03.010

because of its high-throughput glacial regime due to high snowfall and small ice catchments (Barker et al., 1999). In the northern part of the Peninsula, meltwater sedimentation occurs, which is unique for the Antarctic continent (e.g. Anderson, 1999). However, a continuous glacial history of the region is impossible to recover from continental locations on the west side of the Antarctic Peninsula. Onshore, for example, steep-sided fjords are eroded into a long narrow plateau covered by an ice sheet with patchy exposures of striated bedrock and erratic boulders that were deposited during the Late Quaternary, particularly during the Last Glacial Maximum (LGM; Bentley et al., 2006). The continental shelf also preserves the record of the LGM, which has been described using subbottom profiling, swath bathymetry and piston cores (e.g. Heroy and Anderson, 2005; Ó Cofaigh et al., 2005). Glacial advances prior to the LGM are recorded as unconformities in seismic reflection profiles because the ice sheet planed off previously accumulated sediment as it advanced across the shelf (Bart and Anderson, 1995; Larter et al., 1997; Bart et al., 2005). Attempts to recover older sediment from the continental shelf by drilling

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have been plagued by very low core recovery (e.g. ODP Leg 178; Barker and Camerlenghi, 2002). Therefore, the most continuous proximal records of glaciation on the Antarctic Peninsula are obtained by seismic profiling and coring sediment drifts from the continental rise (Rebesco et al., 1996, 1997; Pudsey and Camerlenghi, 1998; Pudsey, 2000; Rebesco et al., 2002; Lucchi et al., 2002; Bart et al., 2007; Lucchi and Rebesco, 2007). These large depositional mounds are located close to the edge of the continental shelf (in the case of Site 1101 ca. 94 km) and parallel the Antarctic Peninsula for over 4° of latitude. Previous work on the drift sedimentary record includes description of sedimentary processes that constructed the drifts and paleoenvironmental reconstructions (Pudsey and Camerlenghi, 1998; Pudsey, 2000; Ó Cofaigh et al., 2001; Barker and Camerlenghi, 2002; Lucchi et al., 2002; Hillenbrand and Ehrmann, 2005; Hepp et al., 2006; Lucchi and Rebesco, 2007). The goal of this paper is to use the geological record from the sediment drifts to assess glacial conditions on the Antarctic Peninsula during the last 3.1 m.y. To this end, we present analyses of the coarsegrained iceberg-rafted debris (IRD; i.e. pebbles and coarse sand) recovered from two proximal drift sites drilled during ODP Leg 178. IRD deposition is controlled by many factors, including oceanic conditions such as surface water currents and temperature, ice thermal regime, and bedrock erodibility, as well as the scale of glaciation with alpine glaciers and continental scale ice as end members (Denton et al., 1991). Our study focuses on Sites 1096 and 1101, drilled where sedimentation rates were high with an identifiable glacial–interglacial cyclicity throughout the Quaternary. This allows comparison of the flux and composition of IRD from Antarctic Peninsula glaciers with observed glacial–interglacial cyclicity. In addition, the lithology of pebbles from Sites 1096 and 1101 were used to determine the source areas of IRD and their temporal variability between glacial and interglacial intervals. We have also imaged the surfaces of quartz sand grains with the scanning electron microscope (SEM) from 3.1–0 Ma at Site 1101. Microtextures have been used in Antarctica to identify sedimentary environments and glacial regimes (Mahaney et al., 1996; Strand et al., 2003; Ó Cofaigh et al., 2005). Specifically our goal is to identify the ice volume, basal glaciological conditions, and the residence time of glaciers at the shelf break on the Antarctic Peninsula since 3.1 Ma, encompassing the time between the Early Pliocene described by Hepp et al. (2006) at Site 1095 and the Late Quaternary described in detail by Pudsey (2000) and Lucchi et al. (2002) from piston and gravity cores collected across the drifts. 2. Setting and general depositional processes The continental rise west of the Antarctic Peninsula has 12 finegrained sediment drifts deposited elongate to the continental shelf edge (Fig. 1; Rebesco et al., 2002). Site 1101 is located on the north western flank of Drift 4, ~ 94 km from the shelf edge in 3280 m water depth and Site 1096 is located in Drift 7 in 3152 m water depth, ~ 125 km from the shelf edge (Fig. 1; Barker and Camerlenghi, 2002). Both sites are located close to the drift crests. The drifts have an asymmetrical shape, gentle to the NE and steep to the SW. They are elevated up to 1000 m above the surrounding sea floor and flanked by deep-sea turbidite channels extending from the base of the continental slope. On the continental shelf, troughs were carved by thick, 100-km-wide fast-flowing ice streams that were grounded at the shelf edge during each glacial maxima (Pope and Anderson, 1992; Pudsey et al., 1994; Bart and Anderson, 1995; Ó Cofaigh et al., 2005). Significant volumes of coarse-grained, unsorted sediment were moved across the shelf, in particular through the troughs, and were deposited on the continental slope during glacial periods when ice streams advanced to near the shelf edge. Gravitational transport processes such as slumps, debris flows, and turbidity currents were triggered by failure of this unstable sediment. In conjunction with sediment focusing by a SW-

flowing bottom current and meltwater plumes discharging from beneath the ice streams the downslope transport resulted in high accumulation rates on the drifts (e.g. Pudsey and Camerlenghi, 1998; Pudsey, 2000, 2002a,b; Lucchi et al., 2002; Hillenbrand and Ehrmann, 2005). Once the ice sheet became unstable because of rising sea level and a negative glacial mass balance, rapid disintegration by iceberg calving is predicted because of the few pinning points on the continental shelf beyond the northwest coastline of the large islands (Larter and Cunningham, 1993). During interglacials, the terrigenous sediment supply to the continental rise is drastically reduced because the shelf is too deep to be reworked by wind-generated currents or storms and because of the long distance across the shelf to the continental rise. Therefore, only a few icebergs supply IRD to the rise at present. 2.1. Antarctic Peninsula geology The Antarctic Peninsula predominantly consists of calc-alkaline plutonic and volcanic rocks of a deeply eroded magmatic arc belonging to the Andean Orogen. Magmatic activity associated with the Andean Orogeny started during the Middle Triassic with vast intrusions of felsic to mafic plutonic rocks (Antarctic Peninsula Batholith) and continued into the Neogene. Calc-alkaline volcanic rocks (Antarctic Peninsula Volcanic Group) were extruded from the Middle Jurassic to the Pliocene (Thomson and Pankhurst, 1983; Smellie, 1990). Middle Jurassic to Eocene backarc basins formed in eastern Palmer Land and on islands east of Graham Land (Macdonald and Butterworth, 1990; Pirrie, 1991; Elliot, 1997). The forearc and backarc basins were mainly filled with thick clastic and volcanogenic marine to terrestrial sequences, which predominantly contain debris supplied from the magmatic arc rocks. The pre-Jurassic basement of the magmatic arc includes deformed metasedimentary and crystalline rocks of an older Paleozoic to Triassic consuming plate margin (Thomson and Pankhurst, 1983; Elliot, 1997; Loske et al., 1998). The basement comprises upper Paleozoic to lower Mesozoic very low grade to greenschist facies sandstones and shales that are mainly exposed in northern Graham Land (Trinity Peninsula Group). Lower Paleozoic granitoids and gneisses, which were affected by Late Paleozoic/Early Mesozoic amphibolite facies metamorphic events, occur in scattered outcrops in southern Graham Land and Palmer Land (Elliot, 1997). 2.2. Oceanography Today, Sites 1096 and 1101 are located within the seasonally seaice covered Antarctic Zone of the Southern Ocean. The Antarctic Zone is bounded by the Antarctic Polar Front (APF) in the north and by the southern boundary of the Antarctic Circumpolar Current (ACC) in the south (Orsi et al., 1995). Warm Circumpolar Deep Water (CDW) upwells in the Bellingshausen Sea near the continental slope of the Antarctic Peninsula and intrudes onto the shelf underneath Antarctic Surface Water (AASW) (e.g., Hofmann et al., 1996). The circulation pattern on the shelf west of the Antarctic Peninsula is quite complex, with surface currents mainly toward the southwest. Hofmann et al. (1996) and Smith et al. (1999) assume the existence of weak cyclonic gyres located in the Bransfield Strait, offshore from Anvers Island, and north of Marguerite Bay. A slow southwest current adjacent to the coast of the Antarctic Peninsula and a northeast countercurrent farther offshore are associated with these gyres. Current meters moored on Drift 7 detected a weak bottom-water flow over the continental rise with the bottom-current flow following the bathymetric contours in a generally southwestward direction (Camerlenghi et al., 1997; Giorgetti et al., 2003). The potential temperature of the bottom water is comparable to deep-water temperatures measured at the South Shetland margin. The bottom water in this area is derived from Weddell Sea Deep Water (WSDW),

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Fig. 1. Bathymetric map of the continental margin west of the Antarctic Peninsula in the Bellingshausen Sea (Rebesco et al., 1998) with locations of ODP Sites 1095, 1096, and 1101 (solid dots). Generalized geology on the Antarctic Peninsula is modified from Moyes et al. (1994) and British Antarctic Survey (BAS) geological maps (1979, 1981a,b, 1982). Locations of the present-day Polar Front and recent sea-ice limits in the Bellingshausen Sea are also shown (modified from Pudsey and Camerlenghi, 1998).

which leaves the Weddell Sea through topographic gaps in the Scotia Ridge (Nowlin and Zenk, 1988). 3. Methods and materials Our study presents results of detailed sedimentological analyses at ODP Sites 1096 and 1101 with focus on the IRD record at Site 1101. Previously we have reported on the mass accumulation rate of IRD (IRD MAR) at Site 1101 (Cowan, 2002) and the roundness and shape of dropstones collected from all drift sites (Hassler and Cowan, 2002). This study extends the previous work by additional analyses of the glacial–interglacial stratigraphy observed from 125–0 m below the

seafloor (mbsf) in visual core descriptions and X-radiographs of uchannels from Site 1101. To establish our stratigraphy we also used magnetic susceptibility (MS) data, which were measured at 2-cm intervals during ODP Leg 178 using the shipboard whole-core multisensor track logger (Shipboard Scientific Party, 1999a,b). The data were downloaded from the Janus Web Database http://wwwodp.tamu.edu/database/. The most recent composite depth scales and age models of Barker (2002) were used to develop a chronology. The age of each sample collected from Sites 1096 and 1101 was computed by linear interpolation between magnetostratigraphic datums from Acton et al. (2002). Following the method of Krisseck (1995), the IRD abundance was determined at Site 1101 from the 250-μm to 2-mm

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sections were made for identification of fine-grained rock types. At the British Antarctic Survey (BAS) in Cambridge we compared the lithologies of the dropstones with suites of rocks from the Antarctic Peninsula. Twenty-one samples of sand grains were imaged on a Quanta FEI 200 Scanning Electron Microscope (SEM) in the high vacuum mode at 20 kV. Grains were mounted on aluminum stubs and coated with gold. Each grain was identified as quartz using energy dispersive X-ray (EDX) before a photomicrograph was collected. We attempted to examine at least 25 grains (cf. Mahaney, 1995) from the 250-μm to 2mm sand fraction from each sample. However the number of grains in some samples were less than 25 and all of those quartz grains were imaged (Table 1). The photomicrograph of each grain was evaluated for the presence or absence of 21 microtextures that commonly occur on glacially transported quartz grains (cf. Mahaney, 2002; Van Hoesen and Orndorff, 2004). Wadell Roundness was estimated from a comparison chart in Gale and Hoare (1991, p. 120). The results were calculated as percentage frequency for each sample (Table 1).

sand fraction, which was separated by wet sieving after air drying, rinsing with distilled water to remove salts, and ultrasonic vibration. The 250-μm to 2-mm fraction was dried, weighed, and the abundance of the coarse-sand fraction (in weight percent) was calculated. The sand was then examined with a binocular microscope to estimate the volume of terrigenous ice-rafted sediment (in volume percent) in order to exclude biogenic components, volcanic ash, and manganese micronodules, which usually do not have an ice-rafted origin. In addition, clasts N2 mm were counted at 1-cm spacing on Xradiographs following the method described by Grobe (1987). The mass accumulation rate of IRD (in grams per square centimeter per thousand years) was calculated as IRD MAR = CS% × IRD% × DBD × LSR where CS% = the coarse-sand abundance (multiplied as a decimal), IRD % = the IRD abundance in the coarse-sand fraction (as a volume ratio), DBD = the dry bulk density of the whole sediment sample (in grams per cubic centimeter) determined from the measurement of dry bulk density on the discrete samples taken closest to the depth for IRD (Shipborad Scientific Party, 1999a,b), and LSR = the interval average linear sedimentation rate (in centimeters per thousand years). At Site 1096, IRD MAR was calculated using the sand (N63 μm) data set from Wolf-Welling et al. (2002) as a proxy for iceberg-rafted debris. Even though not every sample was examined to ensure that only IRD was present, we eliminated samples from depth intervals that bear sand beds (turbidites) or abundant foraminiferal sands according to the core descriptions (Shipborad Scientific Party, 1999a,b). A total of 224 dropstones were obtained from Bremen ODP Core Depository from Sites 1096 and 1101. Pebbles N10 mm diameter were sampled from depths where they were observed on the surface of the split core or from within the core during u-channel sampling. The lithology of each pebble was identified in hand specimen. Thin

4. Results 4.1. IRD Mass Accumulation Rates (IRD MAR) The IRD MAR curves from both sites are comparable with numerous peaks and troughs throughout the past 3.1 m.y. (Fig. 2A and B). IRD MAR (g/cm2/kyr) is approximately an order of magnitude higher at Site 1096 than 1101, which we attribute to the additional sand included within the 63–250-μm size fraction at Site 1096. At Site 1101 three large distinct maxima occur at ~ 2.8–2.6, 1.9 and 0.88 Ma (Fig. 2B), which can also be identified to some degree at Site 1096

Table 1 Frequency of occurrence of microtextures recorded from quartz sand grains from 21 samples from Site 1101. Number of grains examined in each sample and the occurrence within a glacial (G) or interglacial (I) interval are also reported Sample number

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

Age (Ma) Wadell Roundness High relief Medium relief Low relief Fracture faces Breakage blocks Subparallel linear fracture Conchoidal fractures Arcuate steps Straight steps Chattermark trails Crescentic gouges Curved grooves Straight grooves V-shaped percussion Edge abrasion Sharp angular features Edge rounding Preweathered surfaces Completely weathered Adhering particles New precipitation Number of grains Glacial/interglacial

0.028 0.400 31 54 15 0 8 23

0.103 0.359 35 53 12 18 12 6

0.142 0.418 41 47 12 18 18 29

0.180 0.378 30 59 11 0 11 0

0.309 0.388 65 35 0 0 4 50

0.428 0.428 36 56 8 52 24 24

0.508 0.404 30 48 22 15 0 11

0.583 0.405 44 48 8 64 21 13

0.717 0.375 33 58 8 0 8 42

0.852 0.430 55 45 0 45 30 20

0.872 0.357 62 31 8 23 15 35

1.209 0.388 36 52 12 20 12 16

1.329 0.377 23 46 31 23 23 0

1.451 0.362 38 43 19 19 8 24

1.729 0.400 50 45 5 55 15 50

1.988 0.422 19 55 26 52 3 29

2.368 0.381 63 25 13 44 0 6

2.575 0.380 33 44 22 14 3 6

2.894 0.403 28 38 34 25 3 0

3.034 0.373 33 47 20 40 0 27

3.081 0.359 26 56 19 0 11 15

54

35

76

41

85

20

48

18

67

15

42

36

15

35

25

23

38

28

19

13

22

23 46 8 0 8 38 8

29 41 12 12 0 12 6

12 6 12 0 29 12 24

22 19 11 0 7 22 4

35 23 0 8 15 19 15

20 4 4 4 12 0 8

7 22 7 4 26 30 15

23 8 8 0 18 15 5

25 0 17 0 8 17 8

25 15 5 5 15 0 5

15 42 0 4 27 19 4

24 28 16 0 16 16 0

31 23 0 0 15 15 0

19 24 5 3 8 22 5

15 0 5 0 0 25 0

23 0 16 0 19 13 6

25 31 13 13 13 19 13

19 25 6 3 11 17 8

16 3 3 0 16 13 13

27 27 0 13 0 13 20

37 19 7 7 0 22 11

8 15

12 18

12 18

22 22

23 46

40 40

30 37

36 33

0 58

15 30

35 50

24 28

23 15

19 32

30 15

39 16

25 31

11 14

38 9

27 27

30 15

23 23

6 53

35 53

26 41

27 62

68 44

41 26

38 51

17 33

45 65

8 38

20 20

15 46

11 41

25 75

35 61

6 56

17 28

31 44

33 47

26 30

0

0

0

0

0

8

0

3

0

10

0

0

0

8

10

19

13

19

22

20

22

46 0 13 G

12 18 17 I

24 6 17 I

11 11 27 G

12 19 26 I

40 8 25 G

30 11 27 I

59 8 39 G

42 8 12 I

0 0 20 G

8 0 26 G

8 16 25 G

8 23 13 I

57 19 37 G

25 0 20 G

19 3 31 G

19 19 16 NI

33 3 36 NI

44 9 32 NI

80 20 15 NI

33 15 27 NI

Sample numbers and depths (in meters below sea floor) from ODP Leg 178, Hole 1101A; 1 = 1H-2,50–52 cm (2.00 mbsf); 2 = 1H-5,125–127 cm (7.25 mbsf); 3 = 2H-1,125–127 cm (9.95 mbsf); 4 = 2H-3,125–127 (12.62 mbsf); 5 = 3H-3,50–52 cm (21.70 mbsf); 6 = 4H-2,85–87 cm (30.12 mbsf); 7 = 4H-6,50–52 cm (35.70 mbsf); 8 = 6H-2,94–96 cm (40.95 mbsf); 9 = 7H-2,124–126 cm (50.44 mbsf); 10 = 8H-3,49–51 cm (60.69 mbsf); 11 = 8H-4,51–53 cm (62.21 mbsf); 12 = 10H-4,125–127 cm (81.95 mbsf); 13 = 11H-4,48–50 cm (90.68 mbsf); 14 = 12H-3,80–82 cm (99.07 mbsf); 15 = 14H-3,94–96 cm (118.14 mbsf); 16 = 15H-5,50–52 cm (128.83 mbsf); 17 = 17X-6,125–127 cm (151.45 mbsf); 18 = 19X-4,84–86 cm (165.47 mbsf); 19 = 22X-5,85–87 cm (195.73 mbsf); 20 = 24X-1,79–81 cm (208.96 mbsf); 21 = 24X-4,78–80 cm (213.38 mbsf). G = glacial, I = interglacial, NI = not identified.

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Fig. 2. (A) Mass accumulation rate of sand N 63 μm as function of age (Ma) at Site 1096. Intervals with turbidites and foraminiferal sands have been removed so this record can be considered a proxy for IRD. (B) Mass accumulation rate of coarse-sand-sized (250 μm–2 mm) IRD as a function of age (Ma) at Site 1101.

(Fig. 2A). Between these prominent maxima, there are many highfrequency cycles produced by small maxima in IRD MAR that alternate with low or zero IRD MAR. One prolonged period of low IRD MAR occurs at both sites in the Late Pliocene from 2.12 to 1.98 Ma (Fig. 2A and B). Many cycles have a sawtooth pattern with the IRD MAR increasing gradually upsection to a relative maximum and then abruptly decreasing to zero. An example of one prominent cycle occurs from 0.65 to 0.55 Ma. 4.2. Sedimentation rates and carbonate content The sedimentation rates at Sites 1096 and 1101 determined from magnetostratigraphic and biostratigraphic age tie-points are fairly

comparable over the last 3.0 m.y. (Acton et al., 2002). At Site 1096 the mean sedimentation rate is 18.1 cm/kyr prior to 2.581 Ma and averages 8.55 cm/kyr afterwards (Fig. 3A). At Site 1101 the average sedimentation rate is fairly constant at 8.89 cm/kyr, although there are two anomalous sedimentation rates; a maximum between 3.0 and 2.6 Ma and a minimum between 1.95 and 1.78 Ma (Fig. 4A). A smaller maximum also occurs between 1.0 and 0.75 Ma (Fig. 4A). However, with the low temporal resolution of these calculated sedimentation rates we are unable to characterize the variability in sedimentation rates between glacial and interglacial intervals. Previous work on gravity and piston cores from the drifts by Lucchi et al. (2002) and Pudsey (2000) showed that maximum sedimentation rates in glacial intervals could be more than three times higher than during interglacials.

Fig. 3. (A) Sedimentation rate (cm/ka) at Site 1096 from Acton et al. (2002) and (B) wt.% carbonate at Site 1096 (Shipboard Scientific Party, 1999a).

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Fig. 4. (A) Sedimentation rate (cm/ka) at Site 1101 from Acton et al. (2002) and (B) wt.% carbonate at Site 1101 (Shipboard Scientific Party, 1999b).

Carbonate was measured shipboard in discrete samples from both sites (Shipborad Scientific Party, 1999a,b; Figs. 3B and 4B). Peaks, with the exception of carbonate-cemented beds coincide with the presence of foraminifers within interglacial intervals. 4.3. Identification of glacial and interglacial sediments At Site 1101 sediments younger than 2.25 Ma are composed of alternating biogenic-bearing bioturbated to massive clayey silts and thinly to thickly laminated muds, which are interpreted to represent interglacial and glacial sedimentary units, respectively (Shipborad Scientific Party, 1999b). We identified these intervals using X-radiographs or from photographs of the split core surface (Fig. 5). The presence or absence of laminated sedimentary structures and an estimate of the biogenic component were most diagnostic. For example, foraminifer-bearing layers with carbonate concentrations up to 30 wt.% alternate cyclically with barren laminated or massive intervals from ca. 2.25 to 0.75 Ma (Fig. 4B). Within the sedimentary sequence younger than 0.75 Ma at Site 1101, interglacial stages are mainly identified by the presence of diatom-bearing layers (Shipborad Scientific Party, 1999b). The biogenic-bearing bioturbated to massive mud appears as a low-density massive unit on X-radiographs with dispersed coarse-sand grains and granules interpreted as IRD. The degree of bioturbation is often high, resulting in complete burrowing of the sediment and loss of primary structures. Biogenic-bearing muds contain 10–90% biogenic components in smear slides and correspond with low MS values (Fig. 5). Transitions between glacial/interglacial sediments were both sharp and gradational with transitions prior to 1.5 Ma mostly sharp (Fig. 5). Glacial intervals are composed of laminated mud with a low biogenic component. Thickly laminated mud younger than ca. 2.25 Ma is composed of parallel silt laminae N2 mm thick that have sharp lower contacts and grade upward into mud. These silt–mud couplets appear as medium-density diffuse laminae interbedded with lowdensity interlaminae on X-radiographs and correspond to high-frequency variations in MS with thicker graded silt laminae corresponding to MS maxima (Fig. 5). Bioturbation within this lithofacies is usually sparse, with burrows parallel to laminae or absent. IRD is present either as isolated granules and pebbles or aligned in layers between sets of laminae. This lithofacies appears similar in u-channel X-radiographs to the X-radiographs in Lucchi and Rebesco (2007) that

are interpreted as turbidites. Thinly laminated mud in the sequence younger than 2.25 Ma appears color banded or variegated on the split core surface and comprises 1- to 2-mm thick discrete, occasionally wavy thin silt and mud laminae with sharp contacts. IRD content is low and variable within this lithofacies. Diamicton occurs in 20- to 60-cm-thick beds that are massive or weakly stratified with clasts concentrated in bands and with silt laminae. This lithofacies corresponds to maxima in IRD MAR and pebble concentration at 23–24 mbsf, 59.8 mbsf and 61 mbsf (Fig. 5). One graded 3-cm thick tephra layer occurs at 9.8 mbsf (Fig. 5). It occurs at a similar depth as tephra layers described from short cores from the drifts described by Pudsey and Camerlenghi (1998) and Pudsey (2000). Sediments older than 2.25 Ma lack the regular alternation in biogenic content. A diatom-bearing massive and laminated facies was deposited at Site 1101 between 3.0 and 2.9 Ma (Shipborad Scientific Party, 1999b). Although X-radiographs were not available for Site 1096, lithofacies descriptions, biogenic content, and MS were used to designate the glacial–interglacial intervals (Shipboard Scientific Party, 1999a). 4.4. Lithology of dropstones All dropstones at Sites 1096 and 1101 can be matched to outcrops that have been described from BAS reports, published BAS maps (1:500,000 and Moyes et al., 1994), and samples in the BAS rock repository collection (Fig. 1). The most abundant rock groups at both sites are sedimentary, dominated by lithic arenite and wacke, finegrained volcanics such as andesite, basalt and rhyolite, and plutonic rocks dominated by granite and diorite (Fig. 6). Metamorphic rock types such as gneiss, phyllite, and schist are less common as are metavolcanics and volcaniclastic rocks. The quartzite category includes both metamorphic and igneous quartz, which were impossible to distinguish in small samples. Most rock types are present through time with the exception of basalt and metamorphic rocks, which do not occur from 1.0–0 Ma at Site 1096 and 0.6–0 Ma at Site 1101. Pebble lithology was compared by deposition either within a glacial or interglacial interval at the two sites (Table 2). Because very few pebbles occurred above 20 mbsf at both sites and because detailed analysis of IRD composition and concentration in up to 12 m long piston cores has already been presented in Pudsey (2000) and Ó Cofaigh et al.

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(2001) we decided to compare rock types in two time slices of 2.2– 0.76 Ma and 0.76–0.29 Ma. On both drifts, twice as many pebbles per million years were recovered from the Late Pliocene–Middle Pleistocene time period than in the Late Pleistocene (Table 2). At Site 1096, sedimentary rocks such as lithic arenite are dominant throughout and at Site 1101, both volcanic and sedimentary rocks prevail. From 2.2– 0.76 Ma the abundance of pebbles within glacial and interglacial sediments, respectively differs between the two sites: at Site 1096 twice as many pebbles accumulated in the interglacials than in the glacials whereas at Site 1101 twice as many pebbles accumulated in the glacials than in the interglacials (Table 2). In the time slice from 0.76–0.29 Ma

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the number of available pebbles is small, but apparently more pebbles accumulated in the glacials than interglacials at Site 1096, whereas the abundance of dropstones seems to be similar in glacial and interglacial sediments at Site 1101. 4.5. SEM analysis The average Wadell visual roundness of sand grains from Site 1101 varies mainly from 0.36 to 0.43, i.e. from subangular to subrounded (Table 1). None of the imaged grains had a roundness value greater than 0.6. Most samples have medium to high relief (Fig. 7) with

Fig. 5. Visual description, extent of bioturbation, clasts N2 mm, magnetic susceptibility, glacial and interglacial cycles from 0–2.2 Ma (0–136 mbsf) at Site 1101.

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Fig. 5 (continued).

only four samples having more than 25% grains with low relief (Table 1). Fracture faces are smooth with cleanly broken surfaces covering at least 25% of the grain surface (Mahaney, 2002). Samples with N40% of the grains having fracture faces were recovered from glacial intervals prior to 0.43 Ma (Table 1). Conchoidal fractures, arcuate and straight steps (Fig. 7) tend to be inversely correlated with the presence of fracture faces. Grains with chattermark trails and crecentric gouges are relatively rare (0–17% in all of the samples: Fig. 7F). Grains with completely weathered surfaces make up 8–23% of

the samples older than 1.45 Ma (Table 1, Fig. 8A–D). Most samples younger than 1.45 Ma contain no grains of this type. All samples contain 20–75% of grains with preweathered surfaces. This microtexture is identified as a weathered surface truncated by fresh fractures or abrasion (Fig. 8E) and suggests a 2-stage history for the grain with weathering followed by glacial transport (Mahaney, 2002). The abundance of grains with V-shaped percussion cracks varies (0–24%) and apparently does not appear to correlate with edge rounding or edge abrasion (Fig. 8F). Adhering particles are present in most samples.

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Fig. 5 (continued).

New precipitation is rarely observed but varies and shows no consistent pattern with age. 5. Discussion 5.1. Lithofacies and glacial–interglacial cyclicity at Site 1101 Laminated mud with high MS alternating with massive or bioturbated mud and low MS is common in Plio-Pleistocene drift sediments west of the Antarctic Peninsula (Shipboard Scientific Party, 1999b; Pudsey, 2000; Lucchi et al., 2002; Hepp et al., 2006). Thickly laminated mud is interpreted to have been deposited during glacial periods by distal low-density turbidity currents, generated by slope failure along the continental slope (Pudsey and Camerlenghi, 1998; Lucchi et al., 2002) or by rapid hemipelagic sedimentation from meltwater plumes originating at glacier margins near the shelf edge (Pudsey, 2000; Lucchi and Rebesco, 2007). This lithofacies is punctuated by intervals of intense iceberg rafting (Fig. 5). The thinly laminated lithofacies is found in glacial intervals, but less commonly (Fig. 5). The sharp contacts and wavy laminae suggest basal erosion by turbidity currents or by contour-current winnowing. They appear to

have characteristics in common with distal turbidites and glacigenic contourites (Stoker et al., 1998; Lucchi and Rebesco, 2007). The two laminated lithofacies suggest that several processes may have interacted on the Antarctic Peninsula continental rise during glacial periods. At present, contour currents are steady but have too low a velocity to erode drift sediment (Camerlenghi et al., 1997). However, contour-current speed on the Antarctic Peninsula continental rise may have varied during the Late Quaternary (Pudsey, 2000; Ó Cofaigh et al., 2001). The thickness of the glacial and interglacial intervals varies throughout the cyclic record at Site 1101 (Fig. 5). Sedimentary units deposited during glacial periods are relatively thin from 1.68–1.35 Ma and relatively thick from 0.97–0 Ma. Because the average sedimentation rate was fairly constant over the period, the thin glacial units may represent a period of time when glaciers were at the shelf edge for a shorter duration. The near absence of thinly laminated mud between 1.68 and 1.35 Ma may signal a general decrease in along-slope current speed and lower frequency of turbidity currents. The number of prominent graded silt beds is more abundant in the thicker glacial units younger than 0.45 Ma (Fig. 5). These beds likely originate from thick turbidity currents that traveled down the channels between drifts and overspilled onto the drifts. These turbidity currents may

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Fig. 6. Dropstone lithology vs. age at Sites 1096 and 1101.

have been triggered by large failures of sediment on the slope caused by rapid supply of subglacial debris to the shelf break. Homogeneous mud with high biogenic content is assumed to correspond to interglacial periods. Foraminifers are abundant in the interglacial units from 2.2 to 0.76 Ma, and they also occur in thin foraminifer-bearing mud beds in the interglacials around 0.28 and 0.10 Ma (Figs. 3B, 4B and 5). Between 2.2 and 0.76 Ma at Site 1101, carbonate contents increased from b1 wt.% before 2.2 Ma to peak maxima between 20 and 30 wt.% within interglacial mud units (Shipboard Scientific Party, 1999b; Fig. 4B). The glacial-age sediments in this period usually have a few percent carbonate. The planktonic foraminiferal assemblage at Site 1101 consists of over 90% Neogloboquadrina pachyderma (s) with rare occurrences of N. pachyderma (d) and Globigerina bulloides and no reworked planktonic foraminifera species (Shipboard Scientific Party,1999b). Calcareous nannofossils are common to abundant within the foraminifer-bearing mud between 1.9 and 0.76 Ma (Winter and Wise, 2002). At Site 1096, carbonate contents in sediments younger than 2 Ma are generally enhanced, but fewer inter-

glacial intervals than at Site 1101 bear calcareous microfossils (WolfWelling et al., 2002; Fig. 3B). 5.1.1. Warm surface water from 2.2–0.76 Ma and widespread distribution of IRD Biogenic production in surface waters west of the Antarctic Peninsula is mainly controlled by water temperature, sea-ice cover and nutrient availability (Hillenbrand and Fütterer, 2002; Villa et al., 2003). For example, the foraminifera-bearing and nannofossil-bearing muds that occur around 0.28 Ma and 0.10 Ma at Site 1101 were deposited during the moderate glacial Marine Isotope Stage (MIS) 8.5 and the later part of interglacial MIS 5, respectively (cf. Pudsey and Camerlenghi, 1998; Pudsey, 2000; Lucchi et al., 2002; Villa et al., 2003). There are several mechanisms that could cause the appearance of calcareous microfossils in drift sediments at Site 1101 around 2 Ma. First, the Calcite Compensation Depth (CCD), which is apparently shallower at Sites 1096 and 1101 at present (Hillenbrand et al., 2003), could

E.A. Cowan et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 265 (2008) 275–291 Table 2 Glacial–interglacial distribution of pebbles at two drift sites Site 1096

Site 1101

0.76–0.29 Ma

Interglacial

Glacial

Interglacial

Glacial

Rock group Volcanic Sedimentary Plutonic Metamorphic Total Total accum. rate

# Pebbles 3 1 0 0 4 34 pebbles/m.y.

# Pebbles Rock group 3 Volcanic 7 Sedimentary 1 Plutonic 1 Metamorphic 12 Total

0.76–0.29 Ma

# Pebbles 2 1 2 0 5 23 pebbles/m.y.

# Pebbles 2 1 2 1 6

2.2–0.76 Ma

Interglacial

Glacial

Interglacial

Glacial

Rock group Volcanic Sedimentary Plutonic Metamorphic Total Total accum. rate

# Pebbles 16 36 6 7 65 69 pebbles/m.y.

# Pebbles Rock group 11 Volcanic 17 Sedimentary 3 Plutonic 3 Metamorphic 34 Total

# Pebbles 6 6 4 2 18 43 pebbles/m.y.

# Pebbles 13 13 11 7 44

2.2–0.76 Ma

have dropped because of a reduction in the flow of modified Weddell Sea Deep Water (WSDW) to the Antarctic Peninsula rise. Modified WSDW is assumed to establish the bottom-current regime around the drifts today (Camerlenghi et al., 1997) and is considered to be corrosive to carbonate (Pudsey, 2000). A reduced advection of modified WSDW to Site 1101, however, should have enhanced the carbonate preservation at the slightly shallower Site 1096 as well. Because fewer interglacial intervals bearing calcareous microfossils are found at Site 1096, we exclude this possibility. Alternatively, the CCD could have dropped in response to a decrease of primary production, triggered by an expansion of sea-ice coverage (in comparison to interglacial periods

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before 2 Ma). A decrease in biological productivity would have resulted in a reduced rain of organic particles to the seafloor, less degradation of organic matter, and thus a decrease of dissolved CO2 in the bottom water. Grobe and Mackensen (1992) and Hillenbrand et al. (2003) suggested such a mechanism to explain temporal and geographical shifts from biosiliceous to foraminifera-bearing muds in continental margin sediments of the Weddell, Bellingshausen and Amundsen seas. Indeed, a decrease in deposition of biosiliceous microfossils during both glacial and interglacial periods between ca 3.1 and 1.8 Ma was reported for all drift sites on the Antarctic Peninsula rise (Hillenbrand and Fütterer, 2002; Hillenbrand and Ehrmann, 2005). However, considering the more southern location of Site 1096, sea-ice expansion should have reduced interglacial productivity more drastically at this site than at Site 1101, which should have resulted in a deeper CCD. Thus, more interglacial intervals at Site 1096 than at Site 1101 should bear calcareous microfossils, which is in obvious contrast with the observations (Figs. 3 and 4). Another mechanism, which may explain the increase of calcareous microfossils in sediments younger than ca. 2 Ma at both Drifts 4 and 7, is a rise in sea surface temperature (SST). Although, high concentrations of the planktonic foraminifera species Neogloboquadrina pachyderma (s) can occur in Southern Ocean sediments deposited as far south as 72°S (e.g., Hillenbrand et al., 2003), calcareous nannofossils are viewed as indicators of warmer SST (e.g., Honjo, 1990). Their occurrence at Sites 1096 and 1101 suggest a maximum SST above ca. 2–3 °C, which is at least ca. 0.5–2 °C higher than today's summer SST in this area, as well as low nutrient levels in the surface waters (Winter and Wise, 2002; Villa et al., 2003). Villa et al. (2003) state that calcareous nannofossils can only thrive when an annual sea-ice season is less than nine months. This requirement, however, is fulfilled for the study area even today. Therefore, we suggest that during interglacial periods after 2 Ma southward shifts of the APF advected warmer,

Fig. 7. Quartz grains with textures showing glacial transport. (A) Arcuate steps (1) and conchoidal fracture (2) indicate crushing. (B) Edge abrasion produces slight edge rounding (1) on this grain with high relief. (C) Straight steps (1) and arcuate steps (2) as well as edge abrasion (3) indicate crushing and abrasion. (D) High relief angular grain with conchoidal fracture (1). (E) Preweathered surface (1) cut by surfaces with arcuate steps (2) and straight steps (3) showing a multiple history for this grain. (F) Edge abrasion (1) and chattermark trails (2) indicate abrasion.

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Fig. 8. Example of grains in the iceberg-rafted population lacking evidence of glacial transport. Completely weathered quartz grains with low relief occur in samples older than 1.45 Ma (A, 1.45 Ma; B, 1.73 Ma; C, 2.58 Ma; D, 2.89 Ma). (E) A grain deposited at 1.99 Ma shows evidence of preweathering and subsequent glacial modification. Fresh surfaces are shown at (1). A few grains have V-shaped percussion cracks which is evidence for meltwater transport (F, 0.43 Ma).

nutrient-poor surface waters to the Antarctic Peninsula rise. Satellite observations have shown that the APF can meander across large latitudinal ranges over deep-ocean basins around Antarctica (Moore et al., 1999). The present-day APF passes just to the north of Drift 4, and during past interglacials the APF may have moved to a position in close proximity to Site 1101, which could have resulted in relatively higher rate of carbonate deposition at Site 1101, compared with Site 1096 (Fig. 1). Iwai et al. (2002) reported that at Site 1095, the interglacial intervals bearing calcareous nannofossils are lacking subantarctic–subtropical diatom species, which should have been deposited, if the site was located north of the APF. As no significant differences in diatom assemblage are reported from Sites 1095, 1096, and 1101 (Iwai and Winter, 2002), we conclude that the APF was near enough to raise SST but did not pass Drifts 4 or 7. At Sites 1096 and 1101, the occurrence of calcareous microfossils roughly coincides with the greatest compositional diversity of icerafted pebbles between 2.2 Ma and 0.76 Ma. Apparently, icebergs from a wide range of West Antarctic source areas were able to reach Drifts 4 and 7, suggesting that iceberg drift was not blocked by sea ice (Table 3). Similarly, a positive correlation between the abundance of calcareous microfossils and the clay mineral kaolinite, which may have been rafted by icebergs drifting within the ACC from as far as the Amundsen Sea, was reported for Late Quaternary sediments from Drifts 4 and 7 during interglacial periods (Lucchi et al., 2002). Also the timing of iceberg rafting within a glacial–interglacial cycle may have been controlled by variability in SST and sea-ice extent. Between 2.2 and 0.76 Ma more iceberg-rafted pebbles were deposited during glacial intervals at Site 1101 and interglacial intervals at Site 1096. If the APF frequently shifted close to Site 1101 during interglacial intervals, the resulting high SSTs may have caused iceberg melting in close proximity to the Antarctic Peninsula coast, so that most IRD had rained out by the time the icebergs reached Site 1101. Because Site 1096 was more distant from the APF, the SSTs were lower and enabled more icebergs to

drift across this site without melting and losing debris. During glacial periods, the decrease of the SST allowed a greater number of icebergs (and thus IRD) to reach Site 1101 compared to interglacial periods, while perennial pack ice between Site 1096 and the shelf may have prevented most icebergs from drifting across Drift 7. 5.2. Quartz sand microtextures All 21 samples imaged from Site 1101 include grains with microtextures that show evidence of glacial transport. Microtextures attributed to glacial crushing and abrasion such as conchoidal fracture, arcuate and straight steps, sharp angular features and high relief (Mahaney, 1995, 2002) are present in all samples (Table 1, Fig. 7). Grains that appear completely weathered and do not show any evidence of either crushing or abrasion decline gradually from 3.0 to 1.45 Ma except for 3 samples collected in glacial intervals at 0.85, 0.58, and 0.43 Ma which include a few completely weathered grains (Table 1, Fig. 8). When transported by the glacier, the completely weathered grains must have been protected from grain to grain interactions, perhaps by supraglacial transport. The source of the weathered grains may be surficial deposits and bedrock on the Antarctic Peninsula subjected to more intense weathering during relatively warm periods before and/or during the Late Pliocene. These low relief quartz grains have clay coatings and show evidence of dissolution that might have originated in a soil profile (cf. Mahaney, 1992). However, evidence for more intense chemical weathering on the Antarctic Peninsula is not shown in the clay mineral assemblage of drift sediments over the past 9 Ma (Hillenbrand and Ehrmann, 2005). The Upper Pliocene sediments at Site 1101 also contain fewer grains with conchoidal fractures, subparallel linear fracture and breakage blocks than the Upper Pleistocene deposits (Table 1). To illustrate this point all the classified microtextures of two representative samples from the Upper Pliocene (2.89 Ma) and from the Upper Pleistocene (0.18 Ma)

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Table 3 Environmental conditions on the west side of the Antarctic Peninsula from 3.1–0 Ma inferred from observations at Site 1101 Age

Observations at Site 1101

3.1–2.2 Ma Late Pliocene

Small polythermal glaciers are present at sea-level calving debris-rich Stratigraphy — no regular glacial/interglacial cycles identified, stratified icebergs with abundant supraglacial debris, meltwater plumes are present, diamicton deposited in beds as thick as 60 cm with massive, barren mud Polar Front is to the north and sea ice gradually builds up. above, highest sedimentation rates Biogenic — decline from highest rates of opal accumulation as sea ice expands, no foraminifers present IRD — high flux with many large peaks, dropstones are volcanic and sedimentary Sand grain microtextures — low relief, V-shaped percussion cracks, weathered grains

Environmental conditions on the western side of the Antarctic Peninsula

2.2–1.35 Ma Late Pliocene– Early Pleistocene

Stratigraphy — thin glacial/interglacial intervals with some sharp boundaries, lowest sedimentation rates Biogenic — foraminifers present in interglacials, calcareous nannofossils abundant in interglacials beginning at 1.9 Ma, low fluctuating opal accum., high bioturbation in both glacial and interglacial intervals IRD — flux is moderately high, debris first occurs in glacials and continues into interglacials, wide range of dropstone lithologies Sand grain microtextures — low relief, weathered grains

Small glaciers advance to the shelf edge and remain for short periods during glacials, the sedimentation rate is low so bioturbation can continue through glacial intervals, Polar Front is to the south resulting in reduced sea ice that does not restrict IRD transport.

1.35–0.76 Ma Middle– Early Pleistocene

Stratigraphy — glacials are thicker than interglacials, average sedimentation rates, thinly laminated mud is common Biogenic — foraminifers and calcareous nannofossils present in interglacials, bioturbation only in interglacial intervals IRD — flux is high, peaks occur in glacials and thin diamicton layers are formed, all dropstone rock groups are present in high abundance Sand grain microtextures — fracture and abrasion are common, loss of weathered grains

Thicker glaciers occupy the shelf edge for a longer period during glacials and calve debris-rich icebergs, only basal debris is present in IRD, stronger contour currents produce thinly laminated mud, Polar Front is to the south and there is minimal sea ice and IRD transport is not restricted.

0.76–0.41 Ma Stratigraphy — thick glacial/interglacial intervals with gradual boundaries, Middle Pleistocene thickly laminated mud occurs in glacials, average sedimentation rates Biogenic — foraminifers and calcareous nannofossils are not present in interglacials, opal accumulation is low IRD — low flux, peaks span glacial into interglacial intervals, smaller range of rock groups present (i.e. no basalt or metamorphic dropstones at Sites 1096 and 1101) Sand grain microtextures — fracture and abrasion are common, V-shaped percussion cracks occur on grains in interglacials 0.41–0 Ma Late Pleistocene– Holocene

Stratigraphy — thick glacial/interglacial intervals with sharp boundaries, thinly laminated mud and many thin graded sand and silt beds occur in glacials, average sedimentation rates, ash bed at 10 mbsf Biogenic — thin foraminifer-bearing beds are present in two interglacials, low opal accumulation IRD — low flux, peaks occur in interglacial intervals, smaller range of rock groups present (i.e. no basalt or metamorphic dropstones at Sites 1096 and 1101) Sand grain microtextures — high relief, fracture and abrasion are common

are compared in a histogram (Fig. 9). Both samples have medium to high relief grains, however, more grains from the Upper Pliocene sample than from the Upper Pleistocene sample have low relief, edge rounding, edge abrasion and V-shaped percussion cracks. V-shaped percussion cracks are often associated with transport of grains by turbulent meltwater (Van Hoesen and Orndorff, 2004). In addition, high relief, grain sharpness and angularity, presence of conchoidal fracture, arcuate and straight steps have been shown to be indicators of subglacial transport under large continental ice sheets (N1000 m thickness) rather than under smaller valley glaciers (Mahaney et al., 1988; Mahaney, 1995). At Site 1101, these microtextures produced by fracture and abrasion were more frequent in the Late Pleistocene (Fig. 9, Table 1) than before 1.45 Ma. Analyses of iceberg-rafted quartz grains in Upper Pliocene and Lower Pleistocene sediments at Site 1101 indicate erosion and transport by smaller glaciers with debris sources from exposed weathered bedrock valleys or nunataks on the Antarctica Peninsula. Meltwater was more common throughout the Late Pliocene than in the Pleistocene as indicated by the higher number of V-shaped percussion marks on these grains. The near absence of completely weathered grains corresponding with the increased frequency of crushing and abrasion after 1.45 Ma suggests that

Growth of thick polar ice streams transporting basal debris. Polar front is to the north and sea ice increases to a maximum. Dropstone lithology is restricted because iceberg drift is impeded by sea ice. Meltwater is present in interglacials only.

Thick polar ice streams advance to the shelf edge. Slope failures from the shelf generate large turbidites that produce graded beds. Bottom currents produce thinly laminated mud. Polar front is to the north except during two interglacials with foraminifers at 0.12 and 0.28 Ma. Sea surface temperatures are cooler and iceberg drift is limited by sea ice especially during glacials.

smaller ice caps and valley glaciers coalesced into a large-scale continental ice sheet on the Antarctic Peninsula but not until the Early Pleistocene. We agree with evidence from seismic profiles (Larter et al., 1997; Bart et al., 2005) and sedimentological data from the ODP Leg 178 Sites 1095 and 1097 (Eyles et al., 2001; Hillenbrand and Ehrmann, 2005) suggesting that grounded ice extended to the shelf edge during glacials since at least the Late Miocene. Therefore, we suggest that our observed changes in quartz surface texture relate to the duration of these intervals and to the extent of ice on the continent within the glacial cycles. Geomorphological evidence suggests that at the LGM, the Antarctic Peninsula ice cap was up to 500 m thicker than today (Bentley et al., 2006). Increased ice thickness must have reduced the amount of supraglacial debris that could be added to the glaciers as the topography became inundated with ice. 5.3. Antarctic Peninsula glaciation and iceberg-rafting patterns Temporal patterns of iceberg rafting can be assessed at Site 1101 from the IRD MAR curve and from the clast counts from the Xradiographs (Figs. 2 and 5). Spatial distributions can be compared across the drifts using the pebble lithology data from Sites 1096 and 1101 (Fig. 6, Table 2). Overall there is a Late Pliocene to Late Pleistocene

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Fig. 9. Histogram comparing frequency of occurrence (percent) of microtextures from a sample from the Late Pleistocene (0.18 Ma) and the Late Pliocene (2.89 Ma) at Site 1101. The Upper Pleistocene sample contains more grains with evidence of glacial abrasion and crushing (medium–high relief, conchoidal fracture, steps and chattermark trails) than the Upper Pliocene sample, which has more grains with low relief, abraided grains and 22% of grains that are completely weathered with no evidence of glacial transport.

decrease in ice-rafted pebble content on both drifts, which is accompanied by a general decrease of IRD MAR at Site 1101 (Fig. 2). Sand grains at Site 1096 are predominantly terrigenous, and their concentration and accumulation rate also decrease from the Late Pliocene to the Late Pleistocene (Wolf-Welling et al., 2002; Fig. 2). The early part of the records at Sites 1096 and 1101 (3.2–2.25 Ma), prior to the development of well defined glacial–interglacial cyclicity, has the highest IRD flux (Fig. 2). Iceberg rafting at ~2.8 Ma was intense enough to deposit clast-poor diamictons on Drift 4. Also on Drift 7 the overall terrigenous sediment flux was higher at this time than afterwards (Hillenbrand and Ehrmann, 2005), probably due to a very high supply rate of IRD. Analyses of quartz sand microtextures and the roundness, shape and surface textures of dropstones from Drifts 4 and 7 indicate that Antarctic Peninsula glaciers transported supraglacial and englacial debris along with subglacial detritus during this period (Hassler and Cowan, 2002). Quartz microtextures document the presence of meltwater plumite in this interval. The opal record indicates that following a minimum in the Early Pliocene the annual sea-ice coverage increased during the Late Pliocene, but still was less than during the Pleistocene (Hillenbrand and Ehrmann, 2005) and thus allowed icebergs to track freely over the drifts (Table 3). Small polythermal glaciers calving debris-rich icebergs effectively transported large sediment loads offshore. Pebbles are dominantly lithic arenites and lithic wackes at Sites 1096 and 1101 that could mainly have originated from Alexander Island (Fig. 1). Volcanic rock fragments are evenly distributed at both sites. Plutonic rocks are more common within this interval at Site 1096, which may reflect the proximity of Drift 7 to Marguerite Bay, which is surrounded by plutonic rocks (Figs. 1 and 6). Seismic reflection profiles crossing the continental slope and the drifts were interpreted to indicate a major regional boundary at 2.9 ± 0.2 Ma at Site 1101 (Rebesco et al., 2002, 2006). Even though this interpretation was recently challenged (Larter, 2007; Rebesco et al., 2007). We note that in the sediment record presented here, an abrupt transition in sedimentation rates and IRD flux occurs at about 2.6 Ma (Fig. 2). However, the quartz sand microtextures and clast characteristics suggest a significant change in ice volume or thermal regime at ~1.35 Ma.

From 2.2–0.95 Ma the overall IRD flux is relatively low (Fig. 2). Clasts (N2 mm) were deposited during the glacial intervals and the early parts of interglacials when their concentration reaches maxima (Fig. 5). Glaciers rather than a thick ice sheet advanced to the shelf edge during glacial periods generating turbidites that are recorded in the drifts as thickly laminated mud (Fig. 5). IRD contents are enhanced at the beginning and the end of glacial intervals, which may record initial advance of grounded ice to the shelf break and glacial instability during its subsequent retreat. Debris continues to be rafted in interglacials when sedimentation rates are lower and biogenic sediment is an important component. During this period, interglacials contain planktonic foraminifers (Table 3). Oxygen isotopic ratios were measured on 53 samples of Neogloboquadrina pachyderma (s) from Site 1096 and on 38 samples from Site 1101 by Barker et al. (2002). The oxygen isotopic ratios of the planktonic foraminifera show lower isotopic ratios before ca. 1.1 Ma at Site 1096 and before ca. 0.9 Ma at Site 1101. These data may be interpreted to indicate smaller ice volumes on the Antarctic Peninsula. However, oxygen isotopic composition of planktonic foraminifers are not only influenced by global ice volume, but also by local surface water salinity and temperature. We point out, however, that oxygen isotopic compositions of benthic foraminifers from elsewhere suggest that around 0.80–1.0 Ma global ice volume increased (e.g. Raymo et al., 2006). From 2.2–1.35 Ma, dropstones deposited at Site 1101 comprise all types of Antarctic Peninsula lithologies suggesting that before the transition to larger ice volumes, rock sources were widely available, and iceberg drift was not hampered by sea ice. During the Early to Middle Pleistocene (1.35–0.76 Ma), the loss of completely weathered quartz grains indicates that glaciers on the Antarctic Peninsula have grown to inundate the landscape and cut off the supply of supraglacial and englacial quartz grains. This time period corresponds to a doubling of the thickness of the glacial sedimentary intervals at Site 1101 while the thickness of the interglacial units remained unchanged (Fig. 5). Ice streams may have advanced to the shelf edge for longer periods (or more often during a single glacial period) than previously. Thinly laminated mud intervals deposited during the glacials may indicate higher frequency of turbidites or/and

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significant contourite deposition (Fig. 5). All lithologies of dropstones are represented in high abundance. Clasts are most abundant within glacials and are also present in the adjacent interglacial intervals. IRD MAR at Site 1101 during the Quaternary was highest at ~ 0.88 Ma (Fig. 2). The IRD maximum occurs during a particularly thick glacial interval with thin diamicton beds (Fig. 5). The amount of coarsegrained debris and the increase in the overall sedimentation rate (Fig. 4B) suggest a significant ice sheet collapse. Terrigenous sand maxima are also recorded at Site 1096 during this time (Wolf-Welling et al., 2002; Fig. 2). The timing of this peak may be related to the midPleistocene Climate Transition (MPCT) dated from 0.92 to 0.90 Ma in the Northern Hemisphere (Berger and Jansen, 1994). In the Southern Ocean and the Argentine Basin, IRD maxima during this time were reported by Ledbetter and Watkins (1978), Bornhold (1983), Ciesielski et al. (1982), Allen and Warnke (1991) and Becquey and Gersonde (2002). Bornhold (1983) attributed high IRD MAR from 0.90 to 0.65 Ma to changes in oceanic circulation and enhanced iceberg melting rather than increased Antarctic glaciation. Ice rafting from the Scandinavian ice sheet was also high in the Norwegian Sea during the MPCT (Berger and Jansen, 1994). Becquey and Gersonde (2002), who observed enhanced IRD deposition in the Sub-Antarctic Atlantic during glacial periods around the MPCT, concluded that the waning and waxing of the Northern Hemisphere ice sheets during that period led to higher amplitudes of global sea-level fluctuations, affecting the position of the grounding lines of the Antarctic ice sheets. Alternatively, modifications of deep-ocean circulation may be responsible for major iceberg-rafting events in both hemispheres. The North Atlantic Deep Water (NADW) feeds the ACC, thus linking the North Atlantic with the Antarctic continent. More recently, Raymo et al. (2006) proposed that at the MPCT marine-based ice sheet margins replaced terrestrial ice margins around the perimeter of East Antarctica as the Antarctic ice volume increased. We suggest that the widespread ice-rafting event at ~ 0.90 Ma may record changes in Antarctic ice sheet dynamics and in ocean circulation so that icebergs could deposit debris at both proximal and distal Southern Ocean sites. Interglacial sediments at Site 1101 during the MPCT are foraminifera-bearing suggesting warm surface water temperatures resulting in minimal sea-ice cover that would favor drift and melting of icebergs. Similar conditions have been shown to increase iceberg transport from Greenland (Reeh et al., 1999). The extent to which the APF swung south to encompass Sites 1101 and 1096 during the period from 2.2–0.76 Ma is unknown. Several factors suggest that Site 1096 was not covered in warm surface waters as commonly or for as long as at Site 1101. At Site 1096 the amount of carbonate is lower than at Site 1101 (Fig. 3; Wolf-Welling et al., 2002). Iceberg-rafted pebbles are at their maximum during glacials at Site 1101 and during interglacials at Site 1096 (Table 2). Open and warm water conditions at Site 1101 allowed icebergs to drift unhampered and to release their debris soon after they were calved from glaciers, in particular during interglacial periods. In contrast, low SSTs and more extensive sea-ice coverage at Site 1096 impeded drift and melting of icebergs, in particular during glacial periods. Consequently, most IRD at Site 1096 was deposited when more moderate sea-ice conditions occurred, i.e. during the interglacials. From the Middle Pleistocene to the Holocene (0.76–0 Ma), thick, grounded ice streams transporting only subglacial debris advanced to the shelf edge during glacials. IRD flux as represented by both coarse sand and pebbles is low (Fig. 2 and Table 2) and basalt and metamorphic dropstones are absent from the drifts. The presence of a larger ice cap on the Antarctic Peninsula during the LGM is supported by cosmogenic exposure dates from the continent (Bentley et al., 2006) and the presence of grounded ice streams on the shelf is confirmed by continental shelf geomorphology (Pudsey et al., 1994; Ó Cofaigh et al., 2005; Heroy and Anderson, 2005). Microtextures on iceberg-rafted quartz sand grains show a high frequency of fracture and abrasion consistent with transport through polar glaciers N1000 m thick ( cf. Mahaney et al., 1988; Mahaney, 1995).

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6. Conclusions Conclusions that can be drawn about glacial conditions on the western side of the Antarctic Peninsula over the last 3.1 m.y. from the record of glacial–interglacial cyclicity and coarse-gained terrigenous sediment deposition on the drifts include: 1. Glaciers on the Antarctic Peninsula advanced to the continental shelf edge periodically during the last 3.1 m.y. and sediment drifts on the continental rise are proximal enough to be faithful recorders of major changes in the glaciology of the hinterland. 2. Characteristics of glaciers in the Early Pliocene persist until ~1.35 Ma. These glaciers were relatively thin and may have had a shorter residence time at the continental shelf edge during glacial periods. Quartz sand grain microtextures include completely weathered grains, and low abundance of crushing and abrasion suggests thin ice that did not inundate the topography. 3. Glaciers expanded in size during glacial periods from 1.35–0.76 Ma. The thickness of the sediments deposited during a particular glacial interval increased on Drift 4 and deposition by turbidity currents and/or contour currents became more pronounced. The IRD flux was relatively high and all groups of lithologies are present in dropstones. 4. The conditions that spawned the LGM ice sheet have existed on the Antarctic Peninsula since ~0.76 Ma. At Site 1101 the flux of IRD during that period of time decreased and occurred mainly during interglacial periods when sedimentation rates were low. The range of dropstone lithologies is smaller compared with previous periods. 5. From 3.1–2.2 Ma deposition at Site 1101 was characterized by increased diatoms/opal because water temperatures were higher and sea ice was at a minimum. From 2.2–0.76 Ma, foraminifera were abundant at Site 1101 during interglacials when the APF shifted south near Drift 4 and warm surface waters favored productivity of planktonic foraminifera and calcareous nannofossils. From 0.76–0 Ma, fewer diatoms suggest extensive sea-ice cover and cold conditions around the Antarctic Peninsula. 6. Glacial conditions on the Antarctic Peninsula were decoupled from oceanographic conditions in the Bellingshausen Sea from 1.35– 0.76 Ma because the glaciers grew to inundate the topography on the Antarctic Peninsula but the SSTs remained relatively warm, at least above Drift 4 due to the proximity of the APF. Sea ice was at a minimum and iceberg rafting was at a maximum around ~ 0.88 Ma. Since this time, iceberg rafting was low during the first part of the glacial interval, increased in the later half and remained high in the following interglacial. This is consistent with rapid disintegration of ice streams at glacial terminations. Sedimentation rates were probably lower during the interglacials resulting in a higher concentration of IRD in the interglacial sediments relative to the glacial intervals. Acknowledgements Samples were provided by the Ocean Drilling Program. We thank the Bremen Core Repository team for sampling dropstones from all of the drift cores for us. Gary Acton and Yohan Guyodo u-channeled the cores from Site 1101 that we X-rayed. We appreciate the invitation and logistical support extended by Carol Pudsey for EAC and LEH to visit the rock repository at BAS. An ESEM in the Microscopy Center at Appalachian State University was funded by the National Science Foundation, EAR-0111471, which supported the quartz microtexture part of this study. Funding was also provided by a JOI-USSP postcruise research grant and a Travel Grant from the Board of Trustees of Appalachian State University to EAC. This work was supported by the British Antarctic Survey (BAS) GRADES-QWAD project. This manuscript benefited from the helpful comments of Rob Larter, Carol Pudsey, and two anonymous reviewers.

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