CARBON ISOTOPE FLUCTUATION OF N-ALKANES

The carbon isotope values of the odd carbon–numbered C₂₉, C₃₁, and C₃₃ (Table T1) are plotted vs. meters composite depth (mcd) in Figure F2. There is a constant offset of 0.3–0.9 between the compounds. Between 378.21 and 335.89 mcd in the upper Paleocene (see Shipboard Scientific Party, 2004, for age-depth model), n-C₂₉ and n-C₃₁ gradually decline from –26.6 to –28.1 and –27.2 to –28.5, respectively (Fig. F2), with no change in the offsets. Across the P/E boundary, all n-alkanes exhibit abrupt 4–5 negative shifts. The evaluated n-alkanes show nearly identical values in the minimum of the ¹³C excursion at 335.54 mcd. Above this at 335.39 mcd, the values start to recover. A clear inflection is present in the profiles for n-C₃₁ and n-C₃₃ between 334.75 and 334.60 mcd. At this level the offset with n-C₂₉ begins to reappear. This overall pattern is similar to that of the bulk carbonate curve for Holes 1263C and 1263D (Zachos et al., 2005). This suggests that the carbon isotopic composition of these organic compounds directly reflects on changes in atmospheric CO₂ across the P/E boundary and that the actual excursion may have been larger than recorded by marine carbonates.