Deep-Sea Carbonate Lithification Influenced By Bioturbation on the Southwest India Ridge

Abstract:

The deep-sea carbonate rock is an important type of sedimentary rocks due to its effect on the composition of upper oceanic crust and contribution to the deep-sea geochemical cycles. However, the process and impact factor of deep-sea carbonate lithification at sea floor has remained a mystery for many years. Some deep-sea carbonate rocks characterized by active bioturbation and thrifty benthic faunas were collected from a large lithified area in 2008 approximately 50km west of the firstly discovered hydrothermal vent, blanketed on the ultraslow spreading Southwest Indian ridge (SWIR). Multidisciplinary approaches, including optical microscopy, scanning electronic microscopy and computerized X-ray tomography etc., were used to link the mineralization and activity of (micro)organisms that occur in this deep-sea carbonate rocks and to illuminate the geological and biological importance of deep-sea carbonate rocks on Mid-Ocean Ridges. Evidences are provided, for the first time, that deep-sea carbonate lithification is strongly influenced by bioturbation in this area. Active bioturbation provides a feasible pathway for the dissolution of original foraminiferal tests or coccolith plates and subsequently reprecipitation of calcite cements. Phylogenetic analysis by Li et al., (2014) revealed many sequence are closely related to the microorganisms involved in the N and S cycles. Such uncommon habitats for microbial communities may reflect the influence of direct volatile discharge (mainly composed of CO₂, H₂S, H₂ and SO₂) from underlying basaltic rocks. The active bioturbation and volatile discharge may trigger the dissolution of the original calcite above the saturation horizon, and thus drive deep-sea carbonate lithification on Mid-Ocean Ridges. Our results provide a novel mechanism for deep-sea carbonate lithification at deep-sea seafloor, and also suggest a special type of biospheres and geologic bodied on Mid-Ocean ridge systems.