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Abstract:
Trace metals involved in biological cycling (e.g. Cd, Cu, Ni, Zn) typically accumulate in upwelling sediments due to a high productivity-related particle flux and an enhanced preservation at depth. However, poor constraint on the contribution of lithogenic metal fraction, early diagenetic transformation processes and anthropogenic metal inputs may complicate sediment metal signatures. The identification of source and accumulation mechanisms is essential to the validation of these metals as productivity proxies.
Here we combine data from various short cores (upper 50 cm) and two longer cores of organic-rich upwelling sediments (Peru, Namibia, Chile and Gulf of California), which suggest a highly significant, linear and uniform relationship between Ni and total organic carbon (TOC). The overall high Ni enrichment may be explained by the occurrence of diatoms, which dominate productivity in these systems. The Peru surface sediments (upper 2 cm) show a less pronounced Ni–TOC relationship and support a transition between lower Ni/TOC ratio of East Pacific water column particles and the higher Ni/TOC ratio observed in deeper sediments. In Peru surface sediments, the process is confirmed as a stoichiometric relation between Ni and total chlorins (the immediate degradation products of chlorophyll pigments), which is not observed for Cu or Zn.
Our data strongly support previous findings that Ni is a clear (if not the best) indicator of the organic sinking flux. This is also due to the fact that Ni signatures undergo less alteration associated with sulfur and manganese cycling and low contribution from anthropogenic sources. The apparently exclusive Ni–chlorin stoichiometry suggests that Ni may be associated with enzymes that are involved in photoautotrophic production, which underlines the previous finding from laboratory experiments and field work that diatoms have a dominant role in marine Ni cycling. The Ni/chlorin ratio increases with increasing sediment depth suggesting that chlorins are effected by on-going diagenesis. Therefore, Ni may serve as a reliable indicator of the original chlorophyll flux rather than chlorins. The very good correlation between Ni and TOC and the preferential preservation of Ni over TOC justify previous (paleo)productivity estimates based on Ni accumulation.
