Investigation of the controls on the cadmium isotope composition of modern marine sediments

Cadmium (Cd) isotopes have provided insights into the biogeochemical cycling of Cd in the modern oceans, and may be used to provide information about past ocean chemistry. Cadmium is removed from seawater into marine sediments, but the isotopic composition of this output ﬂux, and the processes that control the isotope composition of marine sediments, are not well understood. This lack of understanding is partly due to the currently unknown inﬂuence of Cd-sulphide formation on the isotopic composition of sediments forming in low oxygen settings, which limits the application of Cd-isotopes to palaeoenvironmental reconstruction of past ocean Cd cycling, and potentially as a tracer of past ocean conditions.
It is important to know if the biogeochemically important processes that are recorded in the modern cycling of Cd are robustly recorded in ocean sediments after remineralization and diagenesis. This comparison can be achieved through a systematic investigation of the Cd-isotope variations found in modern marine sedimentary environments and overlying waters. To address this unknown, this research involved measuring and interpreting the Cd isotopic composition and concentration of waters, sinking particles, and surface sediments accumulating under a range of primary productivity and redox conditions in the South Atlantic at 40◦S, Saanich Inlet, and Black Sea. The sample sites are settings characterised by differing levels of dissolved oxygen, in order to test the inﬂuence of Cd-sulphide formation on the bulk sediment isotopic composition. The South Atlantic at 40◦S represents an open ocean settings with high primary productivity within an ocean basin known for low concentrations of critical micronutrients (Chapter 4). The Black Sea depicts a restricted, permanently anoxic basin with a sharp chemocline and signiﬁcantly lower trace metal concentrations than the open ocean (Chapter 5). Saanich inlet is a seasonally anoxic basin characterized by vertical variability in the chemocline depth and high trace metal concentrations in the basin seawater (Chapter 6) These ﬁeld studies are supported by experiments to precipitate CdS under controlled laboratory conditions. The laboratory experiments were conducted to quantify the mechanism and magnitude of Cd isotope fractionation during precipitation of cadmium sulphides (CdS) using synthetic seawater in an anoxic environment (Chapter 3).
The ﬁndings support a link between the redox state at the water-sediment interface and the degree of Cd isotope fractionation of the core-top sediments and pore-waters relative to the overlying waters. The Cd concentration and isotope composition of near-surface sediments with anoxic deep-waters and sub-oxic conditions in the near-surface sediment porewaters indicate near-quantitative removal of Cd compared to sites with oxygenated deep-waters and pore-waters that report a preferential removal of the light Cd isotopes, as seen in the South Atlantic (Δ114/110Cdcore–top-deep-waters ranges from -0.36 ± 0.08 per mil to 0.10 ± 0.07 per mil), seasonally anoxic basin, Saanich Inlet (average Δ114/110Cdcore–top-sed–surface-waters of -0.38 ± 0.07 per mil), and the oxygenated sites deposited above the chemocline (i.e. shelf and coast) in the Black Sea (average Δ114/110Cdcore–top-sed–surface-waters = -0.30 ± 0.10 per mil). This light sink of Cd likely represents precipitation and non-quantitative removal of cadmium with trace H2S to form cadmium sulphide (CdS) complexes, an interpretation supported by the low Cd concentrations in these sediments. New experimental results conﬁrm the preferential removal of isotopically light Cd during the precipitation of insoluble CdS complexes. The experiments displayed an enrichment of light Cd isotopes during precipitation of insoluble CdS species, reﬂecting Rayleigh behaviour with a fractionation factor of 1.00011. These observations indicate the important role of redox cycling in controlling Cd isotopes in marine sediments. Likewise, the experiments suggest that the most signiﬁcant variability in Cd concentration and isotope compositions in the geological record likely coincide with redox transitions. These ﬁndings greatly improve our understanding of an important light sink of Cd in our global oceans, and how the signal of important biological processes in the surface-waters are recorded within the near-surface sediments in modern marine environments.

Keywords: modern marine sediments
Cadmium sulphides
Cd biogeochemical cycling
Cadmium concentration
Argentine continental margin
Trace metal isotopes
Black Sea
trace metal concentrations
South Atlantic Ocean
Cadmium isotopes
Saanich Inlet
Advisor: Henderson, G
Porcelli, D
Dickson, A
van den Boorn, S