The impact of oxic degradation on long chain alkyl diol distributions in Arabian Sea surface sediments

Abstract

Oxygen exposure has a large impact on lipid biomarker preservation in surface sediments and may affect the application of organic proxies used for reconstructing past environmental conditions. To determine its effect on long chain alkyl diol and keto-ol based proxies, the distributions of these lipids was studied in nine surface sediments from the Murray Ridge in the Arabian Sea obtained from varying water depths (900–3000 m) but in close lateral proximity and, therefore, likely receiving a similar particle flux. Due to substantial differences in bottom water oxygen concentration (<3–77 μmol/L) and sedimentation rate, substantial differences exist in the time the biomarker lipids are exposed to oxygen in the sediment. Long chain alkyl diol and keto-ol concentrations in the surface sediments (0–0.5 cm) decreased progressively with increasing oxygen exposure time, suggesting increased oxic degradation. The 1,15-keto-ol/diol ratio (DOXI) increased slightly with oxygen exposure time as diols had apparently slightly higher degradation rates than keto-ols. The ratio of 1,14- vs. 1,13- or 1,15-diols, used as upwelling proxies, did not show substantial changes. However, the C30 1,15‐diol exhibited a slightly higher degradation rate than C28 and C30 1,13‐diols, and thus the Long chain Diol Index (LDI), used as sea surface temperature proxy, showed a negative correlation with the maximum residence time in the oxic zone of the sediment, resulting in ca. 2–3.5 °C change, when translated to temperature. The UK′37 index did not show significant changes with increasing oxygen exposure. This suggests that oxic degradation may affect temperature reconstructions using the LDI in oxic settings and where oxygen concentrations have varied substantially over time.