Lipid biomarker composition in surface sediments from the Carlsberg Ridge near the Tianxiu Hydrothermal Field

Hydrothermal venting has a profound effect on the chemical and biological properties of local and distal seawater and sediments. In this study, lipid biomarkers were analyzed to examine the potential influence of hydrothermal activity on the fate of organic matter(OM) in surface sediments around Tianxiu Hydrothermal Field in the Carlsberg Ridge(CR), Northwest Indian Ocean. By comparing the biomarker distributions of the samples with that of other typical hydrothermal sediments in the mid ocean ridge, it is shown that the location of the samples is not affected by the hydrothermal activity. The relatively low abundances of terrestrial n-alkyl lipids and riverine1,15-C_(32) diol suggested a minor contribution of terrigenous OM to the study area. The bacteria contributed predominantly to sedimentary marine OM;however, other marine source organisms, e.g., eukaryotes(i.e.,phytoplankton and fungi) could not be completely neglected. The marine-originated biomarkers showed significantly variable distributions between the two sediments, suggesting different dynamic physical and biogeochemical processes controlling the fate of marine OM. This study identified various diagnostic biomarkers(5,5-diethyl alkanes, diols and β-OH FAs), which may have significant environmental implications for future works in this region.

Precipitation of Calcite Veins in Serpentinized Harzburgite at Tianxiu Hydrothermal Field on Carlsberg Ridge(3.67°N),Northwest Indian Ocean:Implications for Fluid Circulation

Serpentinization and calcite precipitation of mantle peridotites exhumed along detachment faults at the slow-to ultraslow-spreading centers can provide important clues to the hydrothermal alteration processes.The Tianxiu hydrothermal field is a new-found active and ultramafichosted hydrothermal vent site along the Carlsberg Ridge,Northwest Indian Ocean.Two types of calcite veins are recognized in serpentinized harzburgite samples collected from the seafloor at the water depth of 3 500 m(3.67°N/63.83°E) and 400 m north of Tianxiu hydrothermal field.Calcite veins Ⅰ occur in the fractures that cut through mesh texture in the highly serpentinized harzburgite,while calcite veins Ⅱ precipitate within the mesh texture in the relatively weaker serpentinized harzburgite.Both veins show similar δ13CPDB(+0.54‰ and +0.58‰) but different δ18OPDB(-16.67‰ and +4.46‰) values,suggesting that they were derived from the same carbon source but precipitated at different temperatures.Taking the deep seawater temperature of 2℃as the precipitation temperature of the calcite veins I,the equilibrium δ18OV-SMOW of calcite-precipitating fluid was calculated to be 1.78‰,which is close to the average δ18OV-SMOW value(1.74‰) of vent fluid samples from the ultramafic-hosted hydrothermal systems worldwide.The formation temperature of calcite veins Ⅱ is inferred to be approximately 134℃,based on the calculated δ18OV-SMOW above.The temperature differences of calcite precipitation probably resulted from the fluid cooling conductively and mixing with seawater along the presumed fractures during slow upflow.The low-temperature calcite postdates the mesh texture,while the high-temperature calcite may precipitate under relatively low water/rock ratios,alkaline and reduced conditions among the mesh texture,which is revealed by the geochemical models.Therefore,it is suggested that they both have been influenced by hydrothermal fluids and the sampling site is near the discharge zone of hydrothermal circulation.