Sedimentary molybdenum and uranium sequestration in silled fjords on the Swedish West coast: implications for trace-metal based paleo redox proxies

Sedimentary molybdenum (Mo) and uranium (U) enrichments are often used as redox proxies to reconstruct bottom water redox changes. However, the reliability of these redox proxies may be compromised by secondary depositional environmental factors, such as the depth of the sulfide front in porewaters. Fjords vary greatly in their depositional environments due to their unique bathymetry and hydrography, and they are highly vulnerable to anthropogenic and climatic pressures. Currently, it is unknown how Mo and U sequestration is affected by variable depositional conditions in fjords. Here, we aim to improve the reliability of Mo and U redox proxies in such systems by comparing two silled fjords on the Swedish West coast with contrasting depositional environments and bottom water redox conditions. We use a sequential extraction method designed for sedimentary trace metals and pore water data, to improve the understanding of Mo and U enrichment pathways in fjord sediments. Our data suggest that sedimentary authigenic Mo and U pools differ between the two fjords. In the seasonally hypoxic Gullmar Fjord, Mo largely binds to manganese (Mn) oxides and to a lesser extent to iron (Fe) oxides; Mo sulfides do not play a major role due to low sulfate reduction rates. U largely resides in labile carbonates and residual phases. Overall enrichment factors (EF) of both elements (relative to upper continental crustal values, UCC) are close to 1, implying minimal authigenic enrichment despite low-oxygen conditions. In the seasonally euxinic Koljö Fjord, Mo is significantly enriched relative to UCC (EF: 20.2-78.5) due to binding with more refractory organic matter complexes, thiomolybdates, and to a lesser extent to pyrites. U is also moderately enriched (EF: 1.9-5.4) and largely resides in refractory carbonates and organic matter complexes. Our data demonstrate that the pore water redox zonation (i.e., the sulfide front), and the rate of shuttling of carrier oxide phases, control the efficiency of Mo and U sequestration in seasonally hypoxic and euxinic fjords to such an extent that enrichments do not systematically record bottom water redox conditions. These results may help to explain the large variability in trace metal enrichments observed across sites of similar bottom water redox conditions.