RESULTS AND INTERPRETATION

Results are listed in Tables T1 and T2 and are shown in Figures F1, F2, and F3.

The elemental abundance and stable isotopic composition of SOM reflects the bulk signal derived from the mixture of all sources of OM that accumulated on the Prydz Bay shelf. In the antarctic marine environment this may include open-water marine phytoplankton, sea-ice algae, and terrestrial sources of OM with a wide range of ¹³C_org and ¹⁵N values (Rau et al., 1991; Villinski et al., 2000). TOC ranges from 0.17 to 6.7 wt% and varies according to the lithostratigraphic unit sampled (Fig. F1). SOM from lithostratigraphic Unit I and the majority of Unit III are characterized by very low TOC (<0.5 wt%) values. Biogenic clays in Unit II have slightly higher values (0.7 to 0.9 wt%). Values in Subunit ID reach 1.5 wt% TOC, and the preglacial carbonaceous clay sediments of Unit IV vary between 3 and 6.7 wt% TOC. TN values range from 0.01 to 0.18 wt% and follow the trends reported for weight percent TOC (Fig. F1).

SOM ¹³C_org values range from -22.7 to -25.8 and vary by up to 2 over short intervals (Fig. F1). All of the ¹³C_org values measured in this study fall within the known ranges for modern particulate organic matter (POM) measured in Prydz Bay (Gibson et al., 1999) and the Weddell (Rau et al., 1991) and Ross (Villinski et al., 2000) Seas of Antarctica. These values also fall on the isotopically heavier end of the known range for C₃ terrestrial plants, which average approximately -27 (Meyers, 1994). Values are most variable in lithostratigraphic Subunit IC and at the contact between Subunit ID and Unit II where glaciomarine sediments (-22.7 to -24.6) alternate with biogenic clays and clayey silts with slightly lower ¹³C_org values (-25 to -25.8) and the lowest carbon/nitrogen (C/N) values (8 to 8.5) (Fig. F1). The TOC and ¹³C_org values measured in sediments of lithostratigraphic Unit I at Site 1166 (with the exception of Subunit IC) are similar to those in the glacial sediments that dominate the stratigraphy (lithostratigraphic Unit II) at Site 1167 drilled on the upper slope (Theissen et al., in press). Additionally, shipboard Rock-Eval analysis of these two units indicates that they are both composed of highly mature recycled OM (Shipboard Scientific Party, 2001). The similarity between the two units suggests that they may have the same source(s) and establishes a potential link between shelf and slope sedimentation during the late Pliocene-Pleistocene.

Variability in ¹³C values may reflect changes in the proportions of different types and amounts of marine algae in Prydz Bay. Gibson et al. (1999) showed that Prydz Bay sea-ice algae are highly enriched in ¹³C (-8.2 to -18), whereas POM from the underlying water column has values ranging from -21 to -26. The variability in ¹³C_org might also indicate changes in the proportions of marine algal and terrestrial sources of OM. Several lines of evidence point to the input of terrestrial organic material at Site 1166:

1. Results of the shipboard sedimentary investigation indicate glacial, proglacial, fluvial/deltaic, and lagoonal depositional settings that have continental provenance (Shipboard Scientific Party, 2001).
2. C/N values measured in this study are mainly high (majority of section >15 and preglacial >30), characteristic of terrestrial OM (Fig. F1) (e.g., Meyers, 1994).
3. Analyses of kerogen, extractable OM, and biomarkers from glacial and preglacial sediments recovered from three Leg 119 shelf sites drilled in Prydz Bay (Sites 739, 741, and 742) indicate a strong terrestrial signal (McDonald et al., 1991; Kvenvolden et al., 1991).

Terrestrial OM, however, is more durable than marine sources of OM and may be overrepresented in the sedimentary record (de Lange et al., 1994). A crossplot of ¹³C_org and C/N illustrates three groupings of Site 1166 SOM that reflect changes in the depositional environment (Fig. F2).

SOM ¹⁵N values range from 0.18 to 6.7 (Fig. F1). To our knowledge, these are the first reported sedimentary ¹⁵N values from Prydz Bay. On average, the glacial sediments in Unit I have slightly higher values than the preglacial samples, although the highest values occur in the organic-rich sediments at the base of both Units III and IV (Fig. F1). A shift to lower values occurs at the boundary between lithostratigraphic Units I and II, and the lowest values are recorded in Unit III. The range of Site 1166 ¹⁵N values is characteristic of both antarctic terrestrial and marine OM. POM samples from the Weddell Sea have a wide range of ¹⁵N values (-5.4 to +41.3), with the highest values occurring in sea-ice algae (Rau et al., 1991). Antarctic terrestrial values (measured in soils and lake sediments) show an even wider range (-49 to +31) (Wada et al., 1981).

¹⁸O (3.4 to 4.5) and ¹³C values (-0.7 to -2.6) of N. pachyderma (s.) foraminifers from lithostratigraphic Unit I (65.5 to 106.3 meters below seafloor [mbsf]) are similar to late Pliocene-Pleistocene values for N. pachyderma (s.) from the slope (Site 1167) (Theissen et al., in press) and rise (Site 1165) (Warnke et al., this volume) in the Prydz Bay region. The low resolution of the Site 1166 N. pachyderma (s.) ¹⁸O and ¹³C records precludes any interpretation of particular paleoenvironmental conditions, but the temporal range in the results (¹⁸O = 1.1 and ¹³C = 1.9) suggests that glacial-interglacial changes in ice volume/temperature and surface water productivity are potentially preserved and warrant a more detailed investigation.