An unusual 4-methyl sterol present in abyssal sediments from the north-east Atlantic Ocean

Abstract

Abstract Four undisturbed sediment cores were collected from the Porcupine Abyssal Plain, an area in the north-east Atlantic ocean (48°N,16°W) characterised by seasonal surface phytoplankton blooms which lead to the pulsed input of organic matter to the benthos of which holothurians are considered to be the dominant biomass. In order to investigate the geochemistry of sedimentary organic matter in the deep ocean, these sediments were analysed for sterols by gas chromatography and mass spectrophotometry. Ten sterols were detected in the surficial sediments (Z-cholesta-5,22-dien-3β-ol, E-cholesta-5,22-dien-3β-ol, cholest-5-en-3β-ol, 5α(H)-cholestan-3β-ol, 24-methylcholesta-5,22-dien-3β-ol, 4α-methyl-5α(H)-cholestan-3β-ol, 24-ethylcholesta-5,22-dien-3β-ol, 24-ethylcholest-5-en-3β-ol, 5α(H)-24-ethyl-cholestan-3β-ol, 4α,23,24-trimethyl-5α-cholest-22-en-3β-ol) which decreased rapidly in concentration with increasing sediment depth. The prominent sterol of almost all surficial samples was 4α-methyl-5α-(H)-cholestan-3β-ol, the predominant sterol in two out of four samples. The exact reason for the abundance of this unusual compound in marine sediments is still not clear. However, two possible explanations for its high concentration are: (i) a contribution from one or several species of dinoflagellate (or possibly other algae) that were present in the phytoplankton bloom, which occurred in surface waters some two months prior to sampling; and (ii) a contribution to the sediment by benthic invertebrates (from faecal material), specifically holothurians. The total sterol concentration rapidly decreased with depth in the sediments and in the deeper sections of all of the cores, only cholest-5-en-3β-ol and 24-ethyl-cholest-5-en-3β-ol were detected.