摘要：AbstractThe surface sediments collected from the southern Mariana Trench at water depths between ca. 4900 m and 7068 m were studied using lipid biomarker analyses to reveal the origin and distribution of organic matters. For all samples, an unresolved complex mixture (UCM) was present in the hydrocarbon fractions, wherein resistant component tricyclic terpanes were detected but C27–C29regular steranes and hopanes indicative of a higher molecular weight range of petroleum were almost absent. This biomarker distribution patterns suggested that the UCM and tricyclic terpanes may be introduced by contamination of diesel fuels or shipping activities and oil seepage elsewhere. The well-developed faults and strike-slip faults in the Mariana subduction zone may serve as passages for the petroleum hydrocarbons. In addition, the relative high contents of evenn-alkanes and low Carbon Preference Indices indicated that then-alkanes were mainly derived from bacteria or algae. For GDGTs, the predominance of GDGT-0 and crenarchaeol, together with low GDGT-0/Crenarchaeol ratios (ranging from 0.86 to 1.64), suggests that the GDGTs in samples from the southern Mariana Trench were mainly derived from planktic Thaumarchaeota. However, the high GDGT-0/crenarchaeol ratio (10.5) in sample BC07 suggests that the GDGTs probably were introduced by methanogens in a more anoxic environment. Furthermore, then-alkanes C19–C22and then-fatty acids C20:0–C22:0were depleted in13C by 3‰ compared ton-alkanes C16–C18and then-fatty acids C14:0–C18:0, respectively, which was interpreted to result from the preferential reaction of fatty acid fragments with carbon “lighter” terminal carboxyl groups during carbon chain elongation from the precursors to products. The abundance of total alkanes, carboxylic acids, alcohols and total lipids were generally increased along the down-going seaward plate, suggesting the lateral organic matter inputs play an important role in organic matter accumulation in hadal trenches. The extremely high contents of biomarkers in sample BC11 were most likely related to trench topography and current dynamics, since the lower steepness caused by graben texture and proximity to the trench axis may result in higher sedimentation rate. This paper, for the first time, showed the biomarker patterns in surface sediments of the Mariana Trench and shed light on biogeochemistry of the hardly reached trench environment.Graphical abstractDisplay OmittedHighlights•The even-carbon alkanes were mainly synthesized by bacteria or algae.•The C12–C22fatty acids and the dominatedn-C18alcohol was from algae and bacteria.•The GDGTs were derived from planktic Thaumarchaeota and methanogens, respectively.•Theδ13C offsets between C16–C18and C20–C22were caused by carbon chain elongation.•The contents of total and specific classes with water depth is generally increased.
