Diagenetic barium cycling in Black Sea sediments – A case study for anoxic marine environments

Abstract

High-resolution sedimentary records of major and minor elements (Al, Ba, Ca, Sr, Ti), total organic carbon (TOC), and profiles of pore water constituents (SO 2À 4 , CH 4 , Ca 2+ , Ba 2+ , Mg 2+ , alkalinity) were obtained for two gravity cores (core 755, 501 m water depth and core 214, 1686 m water depth) from the northwestern Black Sea. The records were examined in order to gain insight into the cycling of Ba in anoxic marine sediments characterized by a shallow sulfate-methane transition (SMT) as well as the applicability of barite as a primary productivity proxy in such a setting. The Ba records are strongly overprinted by diagenetic barite (BaSO 4 ) precipitation and remobilization; authigenic Ba enrichments were found at both sites at and slightly above the current SMT. Transport reaction modeling was applied to simulate the migration of the SMT during the changing geochemical conditions after the Holocene seawater intrusion into the Black Sea. Based on this, sediment intervals affected by diagenetic Ba redistribution were identified. Results reveal that the intense overprint of Ba and Ba xs (Ba excess above detrital average) strongly limits its correlation to primary productivity. These findings have implications for other modern and ancient anoxic basins, such as sections covering the Oceanic Anoxic Events which Ba is frequently used as a primary productivity indicator. Our study also demonstrates the limitations concerning the use of Ba xs as a tracer for downward migrations of the SMT: due to high sedimentation rates at the investigated sites, diagenetic barite fronts are buried below the SMT within a relatively short period. Thus, 'relict' barite fronts would only be preserved for a few thousands of years, if at all.

Hydrogeochemical models locating sulfate-methane transition zone in marine sediments overlying black shales: A new tool to locate biogenic methane?

“…Similar observations have been made by Henkel et al (2012) with simulations for anoxic Black Sea sediments. Penetration depth of sulfate increases with increasing bottom water sulfate concentrations.…”

“…However, recently, diagenetic barite has been found to be an indicator of this reactive zone (e.g. Arndt et al, 2006;Riedinger et al, 2006;Henkel et al, 2012;Kasten et al, 2012).…”

“…1; Eq. (3)) in a distinct zone slightly above the SMT, where the presence of sulfate allows the precipitation of barite (e.g. Gingele and Dahmke, 1994;Torres et al, 1996a;Dickens, 2001;Aloisi et al, 2004;Arndt et al, 2006;Riedinger et al, 2006;Henkel et al, 2012;Kasten et al, 2012).…”

Chemostratigraphy of the early Pliocene diatomite interval from MIS AND-1B core (Antarctica): Palaeoenvironment implications

“…Nevertheless, the organic matter decay or the Ba desorption in the deeper water could lead to supersaturation and precipitation of barite, this being a more refractive species with respect to organic carbon, carbonate and opal. For this reason barium is often used as a primary productivity proxy (e.g., Dehairs et al, 1980Dehairs et al, , 1991Dymond et al, 1992;François et al, 1995;McManus et al, 1998aMcManus et al, ,b, 1999Ganeshram et al, 2003;Cardinal et al, 2005;Sanchez-Vidal et al, 2005;Henkel et al, 2012). In the AND-1B diatomite sequence, the Ba/Al depth profile is characterized by few pronounced positive peaks, (i) at~437 mbsf just above the hiatus, (ii) at~420 and~425 mbsf, and (iii) at~363 mbsf at the top of diatomite interval (Fig.…”

“…The link between enhanced surface productivity and increased amount of barium in the sediment is thought to be associated with decay of phytoplankton Global and Planetary Change 102 (2013) [20][21][22][23][24][25][26][27][28][29][30][31][32] and inorganic precipitation of barite in water column or in supersaturated microenvironments at the seafloor (Dehairs et al, 1980;Bishop, 1988;Dehairs et al, 1991;von Breymann et al, 1992;Gingele and Dahmke, 1994;Paytan et al, 1996;Gingele et al, 1999;Ganeshram et al, 2003). On the other hand, under conditions of sulphate depletion in pore waters, which is usually encountered below oxygen-depleted bottom waters, the barite is dissolved and barium is released into the pore water (Brumsack and Gieskes, 1983;von Breymann et al, 1992;Gingele et al, 1999;McManus et al, 1999;Castellini et al, 2006;Scopelliti et al, 2006;Hendy, 2010;Henkel et al, 2012). In these conditions, increase of pH and carbonate alkalinity encourages the formation of dolomite and other authigenic carbonate (Vasconcelos and McKenzie, 1997;Warthmann et al, 2000;van Lith et al, 2003;Raiswell and Fisher, 2004;Scopelliti et al, 2009).…”