Abstract
Organic- and sulfide-rich sediments have formed in oxygen-depleted environments throughout Earth’s history. The fact that they are generally enriched in redox-sensitive elements reflects the sedimentary environment at the time of deposition. Although the modern ocean is well oxidized, oxygen depletion occurs in certain areas such as restricted basins and high-productivity zones. We measured bulk chemical compositions (major and trace elements, total organic carbon, and total sulfur) of organic- and sulfide-rich sediments collected from eight areas having oxygen-depleted water to discuss relationships between geochemical features and sedimentary environments. Major elemental compositions generally show mixtures of terrigenous detritus and biogenic carbonate. Some redox-sensitive elements might be controlled by organic matter content, whereas others could be contained in sulfide minerals in sediments. In particular, Mo and U show a characteristic trend; areas with higher Mo and U—at least partially owing to a depositional process called the “particulate shuttle”—generally correspond to regions influenced by the open ocean. In contrast, areas with lower Mo and U are more restricted marine environments. This suggests that the degree of Mo and U enrichment reflects the geography in terms of proximity to the open ocean, or the degree of the supply of these elements from the open ocean.
Highlights
Organic-rich and laminated sediments are formed under oxygen-depleted conditions where the decomposition of organic matter and activity of benthic organisms decrease
The data distributions show linear trends as a whole toward the compositional range of terrigenous detrital materials potentially relating to the studied samples, including Svecofennian granite, central Sweden [59] for Baltic Sea, Cariaco Shelf sediment of terrigenous origin [60] for Cariaco Basin, North American shale composite (NASC) [61] for Gulf of California and Saanich Inlet, and the average upper continental crust (UCC)
We examined the bulk geochemistry of modern sediments from eight oxygen-depleted environments
Summary
Organic-rich and laminated sediments are formed under oxygen-depleted conditions where the decomposition of organic matter and activity of benthic organisms decrease. The Black Sea [37], Cariaco Basin [38], Saanich Inlet [39] and Framvaren Fjord [40] reportedly have blackish, organic- and/or sulfide-rich sediments, which reflects the development of oxygen-depleted conditions in the deeper part of the water column. These sediments should have characteristic chemical compositions reflecting their depositional environments (e.g., variations in redox conditions of water/sediment columns, geographical conditions, or differences in lithofacies of hinterlands). Based on our analytical results, including the behaviors of redox-sensitive elements, we discuss the relationships between their geochemical features and their depositional environments
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