Formation of mid-chain alkane keto-ols by post-depositional oxidation of mid-chain diols in Mediterranean sapropels

A high resolution (1–3 cm sediment intervals) organic geochemical study was carried out on the upper 50–60 cm of three box-cores (including the S1 sapropel) from eastern Mediterranean Sea sediments. The Corg, Ba/Al and Mn/Al depth profiles were used to define the oxidised and unoxidised part of the S1 sapropel. Post-depositional oxidation affected ca. 50% of the original sapropel in all three cores, resulting in a significant drop in Corg. C30 long-chain 1,15-diol and keto-ols in the extracts of these sediments were quantified by GC/MS. The vertical variation of these biomarkers in the three cores showed a common depth profile. The C30 long-chain 1,15-diol occurs almost exclusively in the unoxidised sapropel. The corresponding C30 long-chain keto-ol follows this depth profile, although the keto-ol concentrations in the unoxidised sapropel are much lower than the diol concentrations. However, the [keto-ol]/{[keto-ol]+[diol]} ratio shows a significant increase at the base of the oxidised sapropel. This suggests that the long-chain diol was transformed upon post-depositional oxidation into the chemically related long-chain keto-ol, and that this keto-ol was subsequently further oxidised. These findings indicate that long-chain keto-ols, which have no reported biological source but occur ubiquitously in marine sediments, may be formed by oxidation of long-chain diols, both in the water column and in the sediment.