We developed the concept of Oxygen Exposure Time (OET)(Keil et al 1994 Nature 369, p639-642) after noticing a correlation between organic carbon to mineral surface area ratios (OC:SA; see our OC:SA page) and both bottom water oxygen concentrations and distance from shore. The logical first-order explanation for these obervations was that more organic carbon is buried at lower oxygen concentrations, but no mechanism was apparent from this relationship (and the idea is controversial).
OET theory was developed to provide a large-scale mechanism for our observations. OET is quantified as the length of time organic matter is exposed to oxygen prior to permanent burial, and can be calculated as a function of sedimentation rate and oxygen penetration depth into the sediment.
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Figure at left from Keil et al, 1994: Organic carbon and pollen grain concetrations along the Washington margin. A) Carbon content predicted from the relationship wioth surface area observed on the shelf (open circles) and measured carbon content (closed circles) disagree only far from shore. B) Carbon contents and pollen grain abundances are concordent. Since pollen grains are known to be degraded only in the presence of oxygen, and since their dispersal is decoupled from that of silts offshore (Panel C), we infer that the degradation of carbon further offshore is due to interactions with oxygen. C) Oxygen exposure time estimates for the Washington margin. |
This unpublished image at right shows the relationship between OC:SA and bottom water oxygen content at a variety of locations world-wide. The general trend is that OC:SA ratios are negatively correlated with oxygen content. Note that this relationship is not universal – see Mayer et al., 2002 for a great evaluation of the eastern seaboard, where this relationship does not hold.
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In order to further evaluate the OET theory, we quantified OET values for a series of loactions along the Washington margin and the Eastern Tropical North Pacific near Mazatlan Mexico (Hartnett et al. 1998). We showed that the burial efficiency for organic matter (what proportion of the input gets buried) is strongly dependent on OET.
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Current work on OET theory involves evaluation of changes in specific organic components as a function of OET. We need to determine whether OET action in selective in its destruction of organic matter, and we’d like to develop a tracer to determine the OET of ancient sediments. This work is strongly linked with our current aggregate work.