Glacial-interglacial contrasts revealed by n-alkanes in sediments of the Equatorial Indian Ocean during the last 300,000 years

- Intense bacterial degradation takes place in the EIO sediments.
- Phytane-total alkane (Talk) relationship is a proxy of G-IG red-ox changes.
- The C37:C38 alkane ratio can capture coccolith contribution in phytoplankton.
- Land input during Interglacials is behind enhanced Corg and coccolith production.
- Alkanes reveal a hitherto unknown feature of MIS 7 centering ∼200 kyr BP.

A sediment core aged up to ∼300 kyr from the Equatorial Indian Ocean (EIO) was examined for its inorganic and organic geochemistry, the objective being to identify coherent markers of glacial-interglacial changes and hydrocarbons as biomarkers with additional unique abilities in the deciphering of the oceanic environment. CaCO3 and biogenic silica (BSi) were dominant constituents of sediments but with no clear glacial-interglacial pattern. The Al:Ti ratio was lowest during the second half of marine isotope stage (MIS) 7 and at the MIS 3/2 transition attributed to higher energy terrestrial (river) input during these interglacials. Concurrently total alkanes (Talk, the sum of C8C38 normal alkanes) were high (761 and 446 ppb respectively), and they were dominated by the longer chain (C23C34) alkanes (LCA). Coccoliths were a major contributor to phytoplankton then as indicated by their proxy the C37:C38 alkane ratio. The carbon preference index of LCA (CPI23-34, 2.39 ± 1.01), its linear increase against Talk, and the predominance of C29 and C31 alkanes indicated that terrestrial source was a dominant input throughout the core. At the same time, the CPI of shorter chain alkanes CPI15-22 of the core sediments was low (0.40 ± 0.22) and indicated significant microbial reworking on (phytoplankton originated) organic matter. Newly parameterized as even-over-odd preference (EOP15-22 = 1/CPI15-22), this bacterial re-working was linearly related to terrestrial organic matter (Talk) and inferred that either interglacial epochs were more productive or during glacial epochs the bacterial re-working was limited by bottom water sub-oxicity. The latter was supported by a new phytane-Talk linear relationship. In glacial sediments, Talk was lower and phytane increased more steeply against it than in the interglacial sediments. This relationship supports higher terrestrial flux during the interglacials, and indicated that during glacial epochs, suboxic condition prevailed at sediment-water interface agreeing with the established sluggishness of the Antarctic Bottom Water flow.