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 (SO42+, CH4, Ca2+, Ba2+, Mg2+, 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 (BaSO4) 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 Baxs (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 Baxs 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. (C) 2012
Elsevier Ltd. All rights reserved.
