Fluid expulsion from the Dvurechenskii mud volcano (Black Sea): Part I. Fluid sources and relevance to Li, B, Sr, I and dissolved inorganic nitrogen cycles

Abstract

Significant sediment–ocean chemical fluxes are produced by the expulsion of sedimentary fluids at continental margins. Although such fluxes could play a role in global geochemical cycles, few quantitative estimates of their global, or even regional, significance exist. We carried out a pore water geochemical study of fluids expelled from the Dvurechenskii mud volcano (DMV) in the Black Sea, with the aim of understanding the role played by mud volcanoes in Black Sea geochemical cycles. The DMV is presently expelling highly saline fluids particularly enriched in geochemically important species such as Li + (1.5 mM), B (2.17 mM), Ba 2+ (0.57 mM), Sr 2+ (0.79 mM), I (0.4 mM) and dissolved inorganic nitrogen (DIN) (22 mM). A combination of geochemical indicators shows that this geochemical signature was acquired via organic matter and silicate alteration processes in the subsurface down to 3-km depth and near-surface gas hydrate formation. We used a simple transport model to estimate the benthic fluxes of these solutes at the DMV. Our results show that the DMV is expelling fluids at a rather low seepage rate (8–25 cm year −1) resulting in a total water flux of 9.4×10 −5 km 3 year −1. This gentle regime of fluid expulsion results in Li +, B, Sr 2+, I and DIN fluxes between 3.8×10 4 and 2.1×10 6 mol year −1. Surface biogeochemical processes affect the benthic fluxes of Ba 2+ such that the deep Ba 2+ flux is completely consumed through the precipitation of authigenic barite (BaSO 4) in surface sediments. The Black Sea I cycle is likely to be affected by mud volcanism, if the 50 known Black Sea mud volcanoes share the rather sluggish activity of the DMV. Mud volcano fluxes of Li, B, Sr and DIN, instead, are too small to affect Black Sea geochemical cycles. On a global scale, mud volcanism could play a role in the marine cycles of Li, B, Sr, I and DIN if current estimates of mud volcano abundance are correct.