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doi:10.2204/iodp.proc.318.201.2014

Results

The TOC content measured from the Rock-Eval pyrolysis ranges from 1.20 to 2.16 wt% with an average of 1.67 wt%, whereas the TOC content from the IRMS ranges from 1.06 to 1.86 wt% with an average of 1.44 wt%. The vertical discrepancy in the TOC contents between these two methods is on average 0.22 wt%, yet both TOC values are higher than those reported from other regions in the Antarctic Peninsula (Khim et al., 2002). TOC contents usually depend on sediment grain size in marine environments, with a negative correlation between TOC and mean grain size. The TN content varies between 0.16 and 0.35 wt%, with the lowest values observed at a depth of 20.94 meters below seafloor (mbsf). TN contents vertically covary with TOC contents, except at 17–21 and 45–50 mbsf. C/N ratios range from 4.70 to 9.36, with an average value of 6.62 (Table T1; Fig. F1). C/N ratios <10 generally represent the marine environment, whereas values >15 indicate a freshwater environment (Stein et al., 1994). Crosta and Shemesh (2002) report that the C/N ratio of diatom-bound organic matter varies between 3 and 11 in the Southern Ocean.

The δ¹³C_org ratio varies from –28.33‰ to –21.81‰, with an average of –26.23‰. Values display an overall constant vertical trend, except for a distinct excursion in the uppermost part of the core. The vertical variation in the δ¹³C_org ratio is similar to that seen in Core JPC17B from the Adélie Drift (average = 26.6‰; Costa et al., 2007). The δ¹³C_org value is also used as a common proxy for the origin of organic matter. If measured δ¹³C_org values lie in the range –24‰ to –27‰, they indicate a dominance of terrigenous organic matter in the sediment. In contrast, if the values are less than –24‰, the organic matter is thought to be derived from the marine environment (Ruttenberg and Goñi, 1997). The δ¹⁵N_org ratio ranges from 2.44‰ to 12.37‰, exhibiting a gradual upcore decrease in value with the lowest value occurring at the core top (Table T1; Fig. F1). In general, the δ¹⁵N_org ratio of terrigenous organic matter ranges widely from –5‰ to 18‰, with an average of 3‰, and the C/N ratio of marine organic matter varies from 7 to 10 (Peters et al., 1978; Schoeninger and DeNiro, 1984). In addition, a vertical change in δ¹⁵N_org is related to consumption of nutrient nitrate (Jacot Des Combes et al., 2008).

Of the pyrolysis parameters measured during this analysis, S₂ values varies between 2.75 and 6.98 mg hydrocarbon (HC)/g rock, with an average of 4.53 mg HC/g rock, whereas S₃ values range from 1.48 to 4.99 mg CO₂/g rock, with an average of 2.43 mg CO₂/g rock. S₂ values increase slightly upcore. HI and OI values are 232–440 mg HC/g TOC and 100–299 mg CO₂/TOC, respectively. The vertical variation of HI is negatively correlated with the OI along the entire sample depth. High HI values are dominant shallower than 12.34 mbsf after an abrupt decrease in values between 16.37 and 12.34 mbsf (Table T1; Fig. F1). A HI value of <100 mg HC/g TOC in marine sediment implies that the TOC is mainly made up of terrigenous organic matter, whereas a HI value between 200 and 400 mg HC/g TOC indicates marine organic matter (Tissot and Welte, 1984). T_max values were observed to be between 384° and 429°C along the entire sampled depth. T_max provides an estimate of the organic matter thermal maturity, with values <435°C indicative of immaturity, relative to that of the petroleum generation process (Espitalié et al., 1977; Peters, 1986). Thus, the measured values indicate that most of the organic matter is thermally immature (Table T1).

Correlations of the pyrolysis parameters (HI, OI, S₂, and TOC) measured from the Rock-Eval pyrolysis give useful information that can help to elucidate the origin of the organic matter (Type I, Type II, or Type III) through a modified van Krevelen diagram (HI vs. OI) and cross-plot (S₂ vs. TOC). Here, Type I is indicative of organic matter with a lacustrine origin, Type II represents the dominance of marine organic matter, and Type III indicates organic matter with a terrigenous origin (Espitalié et al., 1985). As shown in the modified van Krevelen diagram (Fig. F2A), most of the 276 sample points plot in the Type II area, except for some excursions to Type I, implying that the organic matter primarily originated from a marine source, likely attributed to the surface primary productivity.

Cross-plots of S₂ vs. TOC show that S₂ values are strongly correlated with TOC values (r² > 0.98), having an inclination of 3.13 (Fig. F2B). This means that the organic matter contains >30% hydrocarbon that can undergo pyrolysis. It also shows that the high organic carbon content, ranging from 1 to 2 wt%, belongs to Type II, as shown in the van Krevelen diagram.

In the coastal environment where organic matter sources are mixed, typical ranges of δ¹³C_org values and the C/N ratios can be used to classify the sources of organic matter. Terrestrial plants (C3) using the Calvin photosynthetic pathway have δ¹³C_org values between –32‰ and –21‰, whereas C4 plants utilizing the Hatch-Slack photosynthetic pathway have δ¹³C_org values ranging from –17‰ to –9‰ (Deines, 1980). Terrestrial vegetation normally has C/N ratios >12 (Prahl et al., 1980). The δ¹³C_org and C/N ratios for marine particulate organic carbon (POC) are limited to –24‰ to –18‰ and 5 to 10, respectively, whereas the δ¹³C_org and C/N ratios for freshwater POC vary from –33‰ to –25‰ and 4 to 10, respectively (Lamb et al., 2006).

As shown in Figure F3, most of the data points measured from Hole U1357A were classified as freshwater algae. The results seem to indicate that the organic matter is strongly affected by freshwater algae rather than marine algae. However, the extremely low δ¹³C_org values observed here are related to a general ¹³C depletion in phytoplankton in Southern Ocean surface waters. It is also known that δ¹³C_org values across the Antarctic and Southern Ocean can range from –25‰ to 30‰ (Gibson et al., 1999).

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