Fatty acid biogeochemistry of sediments from the Chilean coastal upwelling region: Sources and diagenetic changes

Sediments from two different depositional regions off northern (23°S) and off central Chile (36°S) covering water depths from 126 to 1350 m were analysed for their fatty acid (FA) composition. Highest total FA concentrations were found at the shallowest sites from the oxygen minimum zone and total organic carbon (TOC) normalized concentrations were generally higher at 36°S (1.8–4.9 mg g TOC−1) than at 23°S (0.8–1.6 mg g−1 TOC). Reaction rate constants k, calculated from the down-core decrease for total and individual FAs, were 2–4 orders of magnitude higher in the bioturbated sediments off central Chile than for the non-bioturbated ones off northern Chile. Based on the estimated k values, polyunsaturated FAs were the most reactive compounds and long chain saturated n-FAs (LC-FAs) the least reactive. Carbon isotopic compositions of individual LC-FAs (−27.1 ± 1.9‰ vs. VPDB) were similar to those of mid-chain saturated n-FAs (−25.1 ± 1.8‰) and bacterial FAs (−26.1 ± 3.0‰), indicating a non-terrestrial source for most LC-FAs. Principal component analysis (PCA) was applied to reveal information on the main factors that control the FA composition. The first component, explaining 48% of the total variance, was assigned to compositional changes during early diagenesis. Calculated site scores were defined as the FA-Index and showed a good correspondence with other FA based quality indicators and C/N ratios of bulk organic matter. The FA-Index revealed diagenetic alteration that was only partly reflected in the pigment based Chlorin Index and not visible in the amino acid based degradation index.