Abstract
We present pore fluid geochemistry, including major ion and trace metal concentrations and the isotopic composition of pore fluid calcium and sulfate, from the uppermost meter of sediments from the Gulf of Aqaba (Northeast Red Sea) and the Iberian Margin (North Atlantic Ocean). In both the locations, we observe strong correlations among calcium, magnesium, strontium, and sulfate concentrations as well as the sulfur isotopic composition of sulfate and alkalinity, suggestive of active changes in the redox state and pH that should lead to carbonate mineral precipitation and dissolution. The calcium isotope composition of pore fluid calcium (δ44Ca) is, however, relatively invariant in our measured profiles, suggesting that carbonate mineral precipitation is not occurring within the boundary layer at these sites. We explore several reasons why the pore fluid δ44Ca might not be changing in the studied profiles, despite changes in other major ions and their isotopic composition, including mixing between the surface and deep precipitation of carbonate minerals below the boundary layer, the possibility that active iron and manganese cycling inhibits carbonate mineral precipitation, and that mineral precipitation may be slow enough to preclude calcium isotope fractionation during carbonate mineral precipitation. Our results suggest that active carbonate dissolution and precipitation, particularly in the diffusive boundary layer, may elicit a more complex response in the pore fluid δ44Ca than previously thought.
Highlights
In modern marine settings, the layer of the uppermost sediment near the sediment–water interface marks the physical and geochemical transition between the overlying oxidizing water column and the reducing conditions of the deeper sediment column; hereafter, we will call this the boundary layer (Sayles, 1979; Sayles, 1981; Klinkhammer et al, 1982; Rudnicki et al, 2001)
We observe no minimum inflection points in any of the above species’ profiles; if these pore fluid data decrease due to some consumption reaction in the sediment, the lowest concentrations likely lie outside the sampled depth range in every analyzed core
Data from this study demonstrated the concept of this proposed onshore-tooffshore switch from upper sedimentary carbonate mineral precipitation to upper sedimentary carbonate mineral dissolution (Supplementary Figure S3)
Summary
The layer of the uppermost sediment near the sediment–water interface marks the physical and geochemical transition between the overlying oxidizing water column and the reducing conditions of the deeper sediment column; hereafter, we will call this the boundary layer (Sayles, 1979; Sayles, 1981; Klinkhammer et al, 1982; Rudnicki et al, 2001) This boundary layer is thought to be the most active part of the sediment in terms of changes in geochemical conditions, and related mineral dissolution and precipitation (Sayles, 1979; Sayles, 1981). The saturation state of carbonate minerals (Ωcc) is given by the following equation: Ωcc
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