Chlorin distribution and degradation in sediments within and below the Arabian Sea oxygen minimum zone

The concentration of chlorophyll a degradation products, i.e. chlorins, preserved in deep-sea sediments is a function of the amount of primary production input and the rate at which it is subsequently degraded. Sedimentary chlorins can be used as a proxy for paleoproductivity; however, our understanding of the factors controlling their preservation is limited. To study the effects of changes in export of primary production from the euphotic zone and of differences in depositional conditions on chlorin concentration in marine sediments, chlorins were analyzed by high pressure liquid chromatography from sediments taken within and below the oxygen minimum zone on the Oman margin in the Arabian Sea. Among five stations at water depths between 400 and 1250 m, variation in chlorin concentration in surface sediments (0–0.5 cm) was significantly related to water depth (used here as a proxy for chlorin fluxes to the sediments) and bottom-water oxygen concentration; the more important control on chlorin concentration of surficial sediments measured in this study is the amount of chlorins reaching the sediment. Chlorins decayed exponentially downcore (0–20 cm). The degradation of sedimentary chlorins was better described by a model in which chlorins decayed at different rates within and below the sediment mixed layer. The degradation rates within the mixed layer were 0.0280±0.0385 yr−1 (t1/2=73 yr). Below the mixed layer, degradation rates were one to two orders of magnitude less, ranging from 0.0022±0.0025 yr−1 (t1/2=680 yr). Many stations had subsurface chlorin concentration peaks between 6 and 10 cm depth. The most likely explanation for these peaks is a period of increased deposition of chlorins in the past. This could result from changes in local depositional environment or a more general increase in surface production resulting in an increased sedimentation of chlorins to the sediments 500–1000 years ago. Chlorins are a useful indicator of the magnitude of chlorin deposition; however their usage as indicators of paleoproductivity is more complicated.