High enrichment of U, Cr, As and V in fish scales from the Namibian shelf of the Benguela Upwelling System.

Enrichment patterns of redox- and biosensitive trace elements (TEs) are powerful tools to reconstruct depositional conditions during sedimentation in continental marine sediments. For several TEs (e.g., U, Cr, As and V), the mechanisms that lead to their accumulation in the sediment are not fully understood yet. To complement the discussion, we analyzed several major elements and TEs (P, Ca, Sr, U, V, As and Cr) in fish scales from two short cores (30 cm sediment depth) from the central and southern shelf of the Namibian anoxic-euxinic continental margin (from 67 and 100 m water depth). We found both fresh, young scales in the upper sediments and clearly altered scales, in deeper sections of the cores. The P, Ca and Sr values appear to be good indicators of the changing condition of the respective fish scales, as Ca, Sr and P decrease with the sediment depth from which the fish scales were taken. The lower the Ca, Sr and P values, the more altered the fish scales and the higher the TE enrichment. These more altered fish scales have high values of U (up to 88 ppm), Cr (up to 97 ppm) and V (up to 130 ppm), raising the importance of fish scales as a sink for these elements in shelf sediments, leading to extreme enrichments that are not primarily related to redox conditions. In contrast to the high TE values in altered scales, high values of As (up to 140 ppm) were found in fresh fish scales in core top sections and a decrease with sediment depth. Previous studies showed that high bulk U content in central Namibian shelf sediments is due to high anthropogenic U input by mining activities (in addition to U enrichment during authigenic apatite formation in the sediments). This U-mining activity in the central Namibian hinterland is further reflected in higher U values in the core from the central shelf compared to the core from the southern shelf. The increase of all TEs in scales with increasing sediment depth indicates authigenic enrichment over time within the sediment. Therefore, we suggest that fish scales are an important authigenic sink of several TEs which may be substantial for the element budget calculations in continental margins.