Abstract
Many cold seeps and gas hydrate areas have not been discovered beside the Beikan basin in the southern South China Sea (SCS), and their characteristics and histories also remain poorly known. Here we describe authigenic minerals and the carbon and oxygen isotopic composition of planktonic foraminifera Globigerinoides ruber from sediment core 2PC, recovered from the gas hydrate zone of the Nansha Trough, southern SCS, to elucidate its history of dynamic cold seepage. We infer that the occurrence of authigenic gypsum crystals and pyrite concretions, and anomalously negative δ13C values of Globigerinoides ruber, reflect paleo-methane seepage. Two major methane release events were identified, based on remarkable excursions in foraminifera δ13C at depths of 150–250 cm and 350–370 cm. Euhedral gypsum crystals and tubular pyrite concretions co-occur with extremely negative planktonic foraminifera δ13C values, indicating a shift in the sulfate methane transition zone and a change in the methane flux. Our data suggest that authigenic mineral assemblages and δ13C values of planktonic foraminifera provide a valuable tool in elucidating the characteristics of dynamic methane seepage in a marine environment.
Highlights
The seepage of fluids containing methane associated with gas hydrate dissociation and free gas release is widely observed on the seafloor at a global scale [1,2,3,4,5,6,7]
Pyrite usually formed in sulfidic environments, which indicates hydrogen sulfide formed by anerobic oxidation of methane (AOM), and the precipitation of authigenic gypsum at cold seep is caused by an enrichment in sulfate derived from pyrite re-oxidation after cold seepage ceased [12,17,29,37]
The gypsums are transparent, dark-yellow or dull, with etched surfaces and commonly alone, but gypsum can only occur with pyrite in the cold seeps [33]
Summary
The seepage of fluids containing methane associated with gas hydrate dissociation and free gas release is widely observed on the seafloor at a global scale [1,2,3,4,5,6,7]. The organic matter (CH2 O) within sediments can be oxidized by pore-water sulfate via microbial sulfate reduction: the so-called organiclastic sulfate reduction (OSR, Equation (2): 2(CH2 O) +. SO4 2− →H2 S + 2HCO3 − ) [13,14,15,16,17] These processes can induce the precipitation of authigenic carbonate concretions, gypsum (CaSO4 ·2H2 O), and pyrite (FeS2 ) within sediment [13,14,15,16]. Pyrite usually formed in sulfidic environments, which indicates hydrogen sulfide formed by AOM, and the precipitation of authigenic gypsum at cold seep is caused by an enrichment in sulfate derived from pyrite re-oxidation after cold seepage ceased [12,17,29,37]
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