The contribution of prokaryotes and terrestrial plants to Maldives inter-atoll sapropels: Evidence from organic biomarkers

Abstract

The composition of organic matter in early Miocene sediment cores obtained from International Ocean Discovery Program Expedition 359 (IODP Exp. 359) was analysed using tetramethylammonium hydroxide (TMAH) thermochemolysis, a method used to determine concentrations of lignin phenols, fatty acids, sterols and hopanoids. These early Miocene sediment cores contain thick sequences (∼100 m) of interbedded light (organic-poor) and dark (organic-rich) sedimentary layers. A total of 49 samples from cores between 704.4 mbsf to 803.95 mbsf (∼20–23 Ma) at Site U1466 (518 m water depth) was selected with particular emphasis placed on the dark layers, although the light layers were also sampled. While organic compounds were below detection limits in the lighter (white) coloured sedimentary layers, the dark layers, with concentrations of organic carbon up to 20%, contained varying concentrations of all analysed compounds. Although sterols were present, hopanoids were more abundant in the dark layers. The contribution of prokaryotic organisms, estimated from the ratio ([hopanols and hopanoic acids]/[sterols + hopanols and hopanoic acids]), indicates that it was prokaryotic organisms rather than eukaryotic organisms that contributed the majority (∼90%) of the organic matter in the dark layers. In conjunction with this record, more negative δ15N values in the darker layers suggest that the prokaryotic organisms were nitrogen-fixing microorganisms (presumably cyanobacteria). The long-to-short fatty acid ratios, together with sterol distributions and the concentration of lignin phenols, suggest the input of terrestrial plant material occurred during a high total organic carbon (TOC) interval (∼770–810 mbsf), which coincides with more negative carbon isotope ratios. Additionally, the long-to-short fatty acid ratio in this interval was significantly higher in the dark layers than the light layers, suggesting that terrestrial environments suitable for vegetation growth expanded during the interval represented by the dark layers. Furthermore, the ratio of cinnamyl lignin phenols to vanillyl lignin phenols (C/V) is relatively high during these intervals. From these biomarker tracers, we speculate that the terrestrial vegetation was dominated by herbaceous plants adapted to frequent sea-level changes.