Geochemical characteristics of sediments in the Haima Cold Seep area: indication for methane seepage and sediment source

Seabed methane seepage can have a significant impact on the marine sedimentary environment, causing differences in mineralogy, geochemistry, and other aspects between sediments in methane seepage areas and those in normal sea areas. Therefore, cold seep sediments are important carriers for recording methane seepage events on the seabed. A 400-cm sediment core was analyzed that located at Station ZZY7 in the newly discovered "Haima" Cold Seep area in the Qiongdongnan Basin, South China Sea. The mineral compositions and geochemical characteristics of major elements, trace and rare earth elements of the core sediments were studied with which the Bayon model was combined. Result show that terrigenous sediments were accounted for 75% of the total sediments, consisting of mainly quartz, feldspar, clay minerals etc; and biological and authigenic mineral deposits took 25%, and composed of mainly aragonite, calcite, and high magnesium calcite. The sediment minerals at the site are numerous and have diverse origins. An indication system was established using indices of ∑REE, LREE/HREE, δCe, CaO, Mo_EF, U_EF, and carbonate mineral percentages, with which methane seepage could be detected. Four sulfate methane transition zones (SMTZ) were recognized, from which four methane seepage events (MRE) in the cold seep area at 210～300 cmbsf (MRE1), 170～190 cmbsf (MRE2), 80 cmbsf (MRE3), and 10～50 cmbsf (MRE4) were inferred. Combining the sediment weathering index and methane seepage history in the study area, it was found that the potassium content of the early Cenozoic strata was high due to potassium metasomatism, while the sediments at the station were not affected by diagenesis. This discovery has certain guiding significance for using the sediment-weathering index of the Haima cold seepage area to study the history and causes of methane seepage.