Sedimentary Mo isotope record across the Holocene fresh–brackish water transition of the Black Sea

Abstract

Mo isotope data on Black Sea sediments spanning the transition from Pleistocene oxic–limnic conditions to the prevailing anoxic marine conditions are presented. Samples were taken from a gravity core collected at a water depth of 396 m. Samples deposited under oxic bottom water conditions range from δ 98/95Mo MOMO − 2.2‰ to − 1.95‰ (MOMO = Mean Ocean Molybdenum) while samples deposited under anoxic bottom water conditions range from δ 98/95Mo MOMO − 1‰ to − 0.54‰. The change of sedimentary environment is also recorded in the Mo contents increasing from oxic to anoxic sediments. The Mo isotopic composition and invariably low Mo content of the oxic sediments deposited under oxic bottom water conditions are compatible with a pure detrital origin of the Mo, irrespective of whether the deposits are of limnic or brackish origin. Mo content and isotopic compositions are identical above and below a sulfidisation front, which originates from the diffusion of sulfur species and in-situ microbial activity after establishment of brackish bottom water conditions. Further, no signal of the overlaying sapropels is seen in the underlying sediments. Thus, transport of sulfur species has not mobilised Mo during diagenesis. The δ 98/95Mo MOMO values of anoxic samples indicate seawater as the dominant source of Mo. However, even the heaviest Mo value of the anoxic period recorded in this core is δ 98/95Mo MOMO = − 0.5‰, with an average of − 0.7‰ for all anoxic sediments, i.e. 0.7‰ lighter than seawater. All samples can be explained qualitatively as three component mixtures of detrital, dissolved riverine and marine Mo. For the lower units a mass balance model can be successfully applied. For the youngest unit mixing models do not yield satisfactory results given present day water fluxes. It is therefore likely that additional Mo isotope fractionation effects are involved. First order modelling suggests that the lighter δ 98/95Mo MOMO values of the most recent samples reflect the presence of some Mo remaining dissolved as MoO 4 2− in a larger part of water column above the core depth, thus allowing for a preservation of a net fractionation between MoO 4 2− and MoS 4 2−. This hypothesis is supported by the fact that the H 2S concentration critical for the MoO 4 2− ↔ MoS 4 2− chemical switch is found at about 400 m water depth in the present Black Sea, close to the depth at the sampling site. At greater depth, increased H 2S concentrations lead to almost complete Mo removal, erasing the fractionation signal. Difference between Unit I samples from this study, and those from earlier publications (with samples taken at greater depth) may thus merely reflect the fraction of Mo scavenged at different depths. The degree of Mo scavenging in fossil black shales and the continental Mo contribution are both difficult to constrain. Therefore, black shales from restricted semi-enclosed basins may not document the Mo isotopic composition of coeval ocean waters. However, oceanic Mo is a dominant Mo source in these basins, and anoxic sediments give reliable minimum values for coeval ocean water Mo isotopic compositions.