Quantifying rates and chemical impact of marine sediment dissolution using sedimentary 230Th measurements

Description

Dissolution of marine sediment at the seafloor takes place in all ocean basins. It is an important process controlling the release of chemicals from sediment back to seawater, and in setting the composition of buried sediment. Assessing the rate and extent of this dissolution is critical for understanding internal cycling of many chemical species in the ocean, and preservation of sedimentary records used in the reconstruction of the past ocean environment. In this study, we test the use of 230Th measurements in surface sediment to assess sediment dissolution. High-resolution 230Th measurements were obtained from intact sediment samples spanning a range of water depths on the slope of the Cape Basin and the South Atlantic Mid-Ocean Ridge. Scavenged 230Th concentrations corrected for radioactive decay since deposition (230Thxs0) increase with depth in the upper 2.5 - 8 cm of sediment and are constant below. These observations can be explained as a result of sediment dissolution in the upper centimetres of the sediment, and are strong evidence to support the use of 230Thxs0 concentrations in sediments to quantify the rate of sediment dissolution at the seafloor. Observed dissolution rates within the sediment cores studied vary from 0.082 to 1.44 g/m2/yr, increasing from the shallow sediment core at 2609 m on the continental slope of the Cape Basin to 4887 m before decreasing to the lowest value in the deepest pelagic core at 5269 m in the middle of the Cape Basin. Combining 230Thxs0 concentrations with sediment composition measurements provides assessment of the constituents dissolving from the sediment. In the core situated on the continental slope bathed in water above the modern carbonate saturation horizon, there is surprisingly more dissolution of carbonate than the core situated below the carbonate saturation horizon, and can be attributed to metabolically driven dissolution. Results also suggest dissolution of the detrital silicates in two deep cores, indicating that 230Th approach may be most useful to assess the rate of dissolution of detrital silicates which have been previously been speculated to dissolve on the seafloor, but at a rate very hard to constrain. Meaurements of 231Pa on the same sediments were also conducted and show that at the sediment depths where dissolution is identified by 230Thxs0 , 231Pa/230Th ratios remain constant with depth for most cores. This result suggests that in these cores sediment dissolution does not alter 231Pa/230Th ratios, and is reassuring for the use of sedimentary 231Pa/230Th records to reconstruct paleo-ocean environment.

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