to 34.0) of AASW are driven by seasonal changes associated with the melting and freezing of sea ice (Hofmann and Klink, 1998). Present observations do not record the formation of any dense saline waters that penetrate through the pycnocline.
This paper reports on our investigation of Ocean Drilling Program (ODP) Site 1098 (64°51.7´S, 64°12.4´W), collected in 1011 meters water depth (mwd) in the Palmer Deep (Barker, Camerlenghi, Acton, et al., 1999). The Palmer Deep (Fig. F1) is a small basin located 30 km offshore from the U.S. Palmer Station on the Antarctic Peninsula (AP). The outer edge of the AP continental shelf lies at ~500 mwd, but the shelf has considerable relief because of numerous basins, trenches, and plateaus. One basin, the Palmer Deep, includes subbasins in excess of 1000 mwd that were formed by a combination of glacial deepening and tectonic subsidence (Rebecso et al., 1998). Palmer Deep Subbasin I contains >40 m of sediment deposited since the last glacial maximum (Fig. F2) (Shipboard Scientific Party, 1999). The Holocene sediments are an alternating sequence of laminated and bioturbated diatom-rich sediments. In general, the pervasive laminated interval (24-9 meters below seafloor [mbsf]) corresponds to low magnetic susceptibility (MS) values and the alternating bioturbated and laminated interval (9-1 mbsf) corresponds to relatively high MS values (Shipboard Scientific Party, 1999). Rhythmic high-frequency cycles in MS are superimposed on the entire record (Fig F2). Previous studies of piston cores from the upper part of the sedimentary record of the Palmer Deep (Leventer et al., 1996) suggested the large- and small-scale variation in laminations and MS were related to changes in productivity that were driven by climate variability. In this study, our objective is to better understand the Holocene climate record from the Palmer Deep sequence by conducting multiproxy studies of representative parts of the sequence. Toward that end, we selected four intervals of the Holocene for detailed comparison of MS, foraminifer assemblages, diatom assemblages, and stable isotope values of benthic foraminifers. Intervals A, B, C, and D (Fig. F2) were selected to sample several high-frequency cycles of both the relatively high- and relatively low-susceptibility intervals.
Several water masses occupy this region of the AP including Antarctic Surface Water (AASW) and various forms of Circumpolar Deep Water (CDW). The most variable water mass, AASW, is found above a permanent pycnocline at 150 mwd on the continental shelf. The temperature (0° to -1.8°C) and salinity (33.9 to 34.0) of AASW are driven by seasonal changes associated with the melting and freezing of sea ice (Hofmann and Klink, 1998). Present observations do not record the formation of any dense saline waters that penetrate through the pycnocline.
Below 150 mwd, the continental shelf is composed of various modified forms of CDW formed as CDW mixes with local shelf and surface waters. Oceanic CDW, with temperatures >2°C, travels clockwise around Antarctica within the Antarctic Circumpolar Current (ACC). Meandering of the ACC is believed to cause episodic intrusion of oceanic CDW onto the continental shelf. As the CDW moves onto the shelf of the AP, it cools and freshens along its upper boundary as it mixes with the colder and less saline AASW (Hofmann and Klink, 1998). This modified Upper Circumpolar Deep Water (UCDW) comprises most of the shelf water below 150 mwd (Hofmann and Klink, 1998).
Several studies have reported on the sedimentology of various regions of the AP in relation to biological, glaciological, and oceanographic settings. Sediments along the AP can vary from diatomaceous muds to gravels depending upon the amount of primary productivity, ocean currents, and terrestrial sediment input from sea-ice melting and glacial ice rafting (Domack and Ishman, 1993).
Ishman and Domack (1994) reported on the benthic foraminiferal distribution in modern sediments of the AP. Samples collected in Marguerite Bay, under the influence of CDW, are represented by the calcareous species Bulimina aculeata cluster, which comprises 0%-3% of the dominantly agglutinated assemblage (average = 77%). Other benthic foraminifer species within the B. aculeata cluster include Bolivinella pseudopunctata, Textularia wiesneri, Milliamina spp., and Portatrochammina eltaninae.
Samples elsewhere along the AP continental shelf in areas under the influence of Weddell Sea Water (WSW) are characterized by a lower diversity calcareous assemblage (74%) dominated by Fursenkoina spp., along with Trochammina intermedia (= Deuterammina glabra in this study).
Leventer et al. (1996) summarize diatom, sedimentologic, MS, and foraminiferal evidence from a 9-m-long piston core collected from Palmer Deep (core PD92-30). Radiocarbon dates indicate that the sedimentation rate in the piston core is 260 cm/k.y. The 9-m-long record of MS in core PD92-30 (Leventer et al., 1996) is almost identical to the upper 10 m of Site 1098, seen in Figure F2 (Shipboard Scientific Party, 1999). Both records contain an upper zone of alternating high and low susceptibility values and a lower zone of reduced susceptibility.
Explanations for the susceptibility fluctuations in core PD92-30 include biogenic, geochemical, and microbiological causes. However, Leventer et al. (1996) report that changes in productivity and the influx of biogenic material, which dilute the magnetite concentration of the sediments, exert the strongest control on the variations in MS (Brachfeld, 1999). Leventer et al. (1996) found that low MS values occur in the strongly laminated sediments, and high MS values occur in the more bioturbated massive intervals.
Leventer et al. (1996) interpreted the laminated sediments with relatively low MS values to indicate stratified ocean conditions. Stable stratified water allows the diatoms to utilize nutrients, resulting in diatom blooms. The diatom blooms form rapidly settling mats of well-preserved diatoms that dilute the MS signal of the laminated sediments. Therefore, the laminated intervals are believed to indicate sea-ice melting, stratified stable water conditions, and resulting high primary productivity. The occurrence of the benthic foraminifer B. aculeata in the laminated intervals was believed to indicate that CDW was periodically injected onto the shelf during the times of high diatom productivity (Ishman and Domack, 1994).
Leventer et al. (1996) interpreted the bioturbated massive sediments with relatively high MS values to indicate conditions with a well-mixed ocean and stronger winds. Reduced water-column stratification would result in lower primary productivity and fewer diatom blooms. Slower settling of diatoms results in poor diatom preservation, but a greater accumulation of magnetic minerals results in higher MS values. The presence of a benthic foraminifer assemblage characterized by B. pseudopunctata in the high-MS intervals was interpreted to indicate an absence of CDW in the well-mixed ocean. However, this conclusion is contrary to Ishman and Domack (1994), who recognize B. pseudopunctata along with B. aculeata as an indicator of CDW.
Spectral analysis of the susceptibility record of core PD92-30 indicates a periodicity of 230 yr during the late Holocene (Leventer et al., 1996). A similar 300-yr cyclicity was also recognized in the amount of preserved organic carbon and biogenic silica in Andvord Bay, close to the Palmer Deep on the Antarctic Peninsula (Domack et al., 1993). Leventer et al. (1996) suggest that solar variability is the cause of the observed productivity cycles in the Palmer Deep.
