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

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 (T_alk, 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 (CPI_23–34, 2.39 ± 1.01), its linear increase against T_alk, 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 CPI_15–22 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 (EOP_15–22 = 1/CPI_15–22), this bacterial re-working was linearly related to terrestrial organic matter (T_alk) 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–T_alk linear relationship. In glacial sediments, T_alk 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.