The effect of oxic degradation on organic temperature proxies: A case study from the coastal shelf off Honshu Island, Japan
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Organic temperature proxies provide powerful tools to reconstruct past sea surface temperatures. An important requirement for those proxies is their stability towards diageneis. The most prominent diagenenic process is oxic degradation of organic matter, which is a selective process and degrades compounds at different rates. As proxies mostly comprise ratios of different compounds, different degradation rates of the individual compounds involved will lead to a bias of proxy based sea surface temperatures. Thus it is important to know how the composition of biomarkers varies in terms of oxic degradation and the possible impacts on organic temperature proxies, i.e. UK'37, LDI and TEX86. To draw constraints on the impact of oxic degradation on those organic proxies, surface sediments and four cores underneath the oxygen minimum zone off Honshu Island (Japan) were analyzed. Regarding the effect of oxic degradation on the organic carbon and biomarker concentrations in the sediment it was observed, that the concentrations increased where the oxygen concentrations of the overlying waters were low and the following order of resistance towards oxic degradation was established: TOC > alkenones > GDGTs. Sea surface temperature reconstructions with LDI could not be established, because diols were detected in only low amounts. An increase in UK'37 and therefore in temperature is found for the surface sediments with high oxygen concentrations in the overlying water column, resulting in an overestimation of sea surface temperatures of 5.5°C. The likely cause for this increase is the preferential degradation of the C37:3 alkenone. BIT values are slightly higher in the oxic environment than in the suboxic environment, suggesting preferential preservation of the continental derived GDGTs under oxic conditions. While TEX86H seemingly is not affected by oxic degradation, the application of TEX86L lead to an underestimation of sea surface temperatures up to 6°C, when the sediment samples are located in the range of 0-1000m water depth. This is probably due to differential abundances of GDGT-3 above and below 1000m water depth. The study demonstrates, that individual degradation rates within a compound class can occur and thus bias the sea surface temperature towards higher temperatures. It is therefore essential to know the oxygenation history of the study area when those proxies are applied, especially reconstructions based on . TEX86L needs to be appplied with caution, when the water depth of the sediment is above 1000m.