The Impact of Methane Release on Early Diagenetic Alteration of Planktonic and Benthic Foraminifera in Mineralogy and Geochemistry

Methane release is common in a gas hydrate geological system. Upward diffusing methane will be consumed in sediments and water columns by a series of biogeochemical reactions. The process not only affects living foraminifera but also leads to early diagenetic alteration of buried foraminiferal tests. To systematically explore the impact of methane release process on early diagenetic alteration in foraminifera, we conducted mineralogical examinations of the test wall microstructure and geochemical investigations of stable carbon isotope and trace elements composition of planktonic and benthic foraminifera from gas hydrate-bearing sediments of the northern South China Sea. Based on the fidelity of tests identified under optical microscope, we divided the foraminiferal tests into three categories (pristine test, minor alteration and major alteration) with different degrees of diagenetic alteration and calculated the vertical distribution characteristics of the contents for each kind of foraminiferal tests in the sediment core. Meanwhile, stable carbon isotope and trace elements composition of the tests with different alteration degrees were analyzed respectively. The results show that the proportion of early diagenetic altered tests in the sediment core is not related to the burial depth. Based on the isotopical and elmental signatures, the 13C values and Mg/Ca ratios of tests are susceptible to early diagenetic alteration. There is a negative correlation between 13C values and the degree of early diagenetic alteration, but a positive correlation between Mg/Ca ratios. This might be related to the relative contribution of methane-derived authigenic high-Mg-calcite to the bulk foraminiferal carbonate. In addition, the V/Ca, Mo/Ca and U/Ca ratios also increase with the degree of early diagenetic alteration, although not as much as Mg/Ca ratios. The enrichment of these redox sensitive elements may indicate the reduction environment caused by methane release. Keywords: foraminifera, methane release, early diagenetic alteration