What is the oxygen exposure time of laterally transported organic matter along the Washington margin?

Abstract

Organic matter preservation along the Washington margin is correlated with its oxygen exposure time (OET). OET is calculated in situ ( isOET) using oxygen penetration depth and sediment accumulation rate estimates. Values for isOET along the Washington margin range from 35–750 years. isOET estimates do not take into account any lateral input of organic matter that had been previously aged elsewhere, and it is unknown whether the transport-related OET ( t OET) is a significant component of the overall OET. Based on the available literature on sediment transport and pollutant dispersion, a best-guess is that t OET≤ isOET. Conversely, t OET≥ isOET when we use Δ 14C estimates for organic matter present just below the sediment–water interface as a proxy to define the potential range of transport OET values across the margin. Using this approach, t OET values are as great as 8000 years depending on the assumptions made about how much material is laterally advected. Assuming ‘most reasonable’ criteria, t OET values are roughly equal to isOET, and range from 0 to 1800 years. t OET may, in fact, effectively double the overall OET ( ΣOET) for the organic matter in the process of being transported and deposited. Offshore decreases in OC/SA correlate more linearly with tOET than they do with isOET or ΣOET. This is consistent with the lateral transport of oxygen sensitive organic material with a slow degradation rate constant, and suggests that the OET hypothesis can be extended to include laterally transported organic matter.