The Antarctic Peninsula region of the Southern Ocean currently is the most rapidly warming region of Antarctica, with 2.5°C of warming documented for the last 50 yr (Jones et al., 1993; Leventer et al., 1996, and references therein). This climatic warming has been demonstrated in a dramatic way by the collapse of the Larsen A Ice Shelf (eastern Antarctic Peninsula) in January 1995 (Doake et al., 1998). Investigations of the west Antarctic fjord systems have demonstrated that understanding how sedimentary systems have responded to events of rapid climate change in the past is crucial to attempts to predict how this sensitive region will respond to current and future climate change (Domack et al., 1993; Domack and McClennen, 1996).
Palmer Deep is a series of three fault-bounded basins (Rebesco et al., 1998) situated on the inner continental shelf south of Anvers Island (Fig. F1) that act as a natural sediment trap in one of the most sensitive regions of the west Antarctic Peninsula in terms of oceanic conditions, sea ice dynamics, and, hence, biological productivity and climate (Domack et al., 1993; Domack and McClennen, 1996; Hofmann et al., 1996; Leventer, 1991; Leventer et al., 1996; Morris et al., 1998; Parkinson, 1998; Smith et al., 1999). During Ocean Drilling Program (ODP) Leg 178, a ~50-m-long core was recovered from 1012 m water depth in Palmer Deep Basin I (Site 1098) (Barker, Camerlenghi, Acton, et al., 1999). Near the base (43.34-46.34 meters composite depth) of the recovered sedimentary sequence is a ~3-m-thick interval of spectacularly laminated to thinly bedded orange-brown diatom ooze and blue-gray diatom-bearing terrigenous sediments (hereafter referred to as the "laminated unit") overlying a diamict containing prominent dropstones (Fig. F2). The laminated unit has been dated, using accelerator mass spectrometry (AMS) radiocarbon dating of acid-insoluble organic matter, to a calibrated age of 11.8-13.15 ka (Domack et al., 2001). The laminated unit comprises the first deposition following the retreat of the grounded ice sheet from Basin I (Rebesco et al., 1998) during the last deglaciation and, as such, sedimentation during a period of rapid and substantive climate change. The debate over whether this interval correlates with the time of the Antarctic Cold Reversal (Blunier et al., 1998) or is an expression of Northern Hemisphere warming during the Bølling/Allerød Interstadial that lags the Northern Hemisphere by 1500 yr will not be addressed here. However, it is interesting to note that in the more recent past, when most of the rest of west Antarctica was colder and windier during the Little Ice Age, the Antarctic Peninsula climate warmed (Leventer et al., 1993). The age of the diamict below the laminated unit is >13.15 ka (Domack et al., 2001), which is roughly in agreement with the uncalibrated radiocarbon age of ~11 ka given by Pudsey et al. (1994) for the retreat of the ice sheet across the western Antarctic Peninsula shelf.
The deglacial laminated unit has been ascribed an annual mechanism of formation (Leventer et al., 1998) consistent with present-day seasonality in the region. Sea ice coverage during the austral winter is associated with an impoverished flux of sedimentary particles (A. Leventer, pers. comm., 2001; unpublished sediment trap data). Spring heralds the onset of sea ice melting and water column stratification, which is associated with high biological primary productivity (e.g., Hofmann et al., 1996; Leventer, 1991) and flux to the seafloor. This is followed by the summer season of reduced primary productivity and higher input of terrigenous sediments as ice-rafted debris (A. Leventer, pers. comm. 2001, unpublished sediment trap data). However, decadal- to centennial-scale climatic cycles have been recorded using biological proxies from the Antarctic Peninsula (Leventer et al., 1996) and other regions of the Southern Ocean (e.g., Leventer et al., 1993), and the radiocarbon age model for this laminated unit gives an annual sedimentation rate of 0.3 cm/yr (Domack et al., 2001). A significant amount of caution, therefore, should be applied when interpreting this postglacial laminated unit as being composed of annual sedimentary couplets, or varves.
The purpose of this data report is to present the preliminary results of a scanning electron microscope (SEM)-scale sediment fabric analysis of the postglacial laminated unit. An additional goal is to demonstrate the implications of lamina-scale sampling and the need for precise, high-resolution analysis of the phytoplankton assemblages preserved within the sediment layers before a firm annual mode of deposition can be ascribed.