DISCUSSION

The P/E boundary interval exhibits drastic changes in CaCO3 content, dropping from 80 wt% down to 10 wt% and rising back up again in a 26-cm interval. At the same time, there is no change in biogenic silica or C-org contents. Although there is the possibility of volcanic glass present in the P/E boundary interval, the largest estimates are only ~5%. Even when corrected for 5% glass, the data indicate that there was no change in biogenic silica production across the P/E boundary. Several C-org peaks are present in Section 199-1221C-11X-3. Whereas two of these may be a product of incomplete dissolution of CaCO3, the rest seem to indicate increased C-org preservation, production, or other source. Evidence supporting contamination is the organic geochemical analysis result of Sample 199-1221C-11X-2, 10–12 cm (Lyle et al., this volume), which had 0.66 wt% C-org but only contaminant biomarkers. The issue of high organic carbon spikes is also discussed in Olivarez Lyle and Lyle (this volume), although their elevated C-org levels are not within the P/E boundary interval.

Calcium carbonate and biogenic silica content in marine sediment is controlled by variable rates of production and dissolution; however, if CaCO3 production decreased by 70% it would not be unreasonable to expect a decrease in biogenic silica values as well. This is not the case; the raw data show an increase in biogenic silica. When considering the dilution effect of CaCO3, biogenic silica experiences no change before, during, or after the P/E low carbonate event. Dissolution of CaCO3 up to 30 cm is an expected result of methane hydrate dissociation (Dickens et al., 1995), suggesting that a change in preservation of CaCO3 occurred, rather than a change in CaCO3 production.

There is very little evidence for anoxia in the P/E boundary interval. The C-org content remains constant before, during, and after the P/E boundary interval. High levels of manganese oxides found in Hole 1221C sediments from the P/E boundary interval (Shipboard Scientific Party, 2002) are inconsistent with an anoxic environment because this would favor the dissolution of manganese (Stumm and Morgan, 1981). Typically, high manganese in sediments marks the base of a mild redox interval, a result of moving through the sediments beneath oxygenated water and referred to as suboxic diagenesis (Froelich et al., 1979). Changes in redox gradients can leave relict manganese oxide peaks in the sediments (Finney et al., 1988).

An increase in barium (Ba2+; from smear slides) at the P/E boundary interval supports the idea that this was a time of increased productivity as suggested by Bains et al. (2000). However, large amounts of Ba2+ may not indicate increased production, per se, because alternate sources are possible. For example, dissolved Ba2+ is present in areas with methane hydrates, and methane hydrate dissociation would result in Ba2+ fronts within the sediment column (Dickens, 2001).

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