Re-assessing copper and nickel enrichments as paleo-productivity proxies

Abstract

Copper (Cu) and nickel (Ni) are elements frequently enriched in sedimentary deposits rich in organic matter (OM). In the marine environment, they are mainly supplied to the sediments in association with sedimentary OM (organo-metal complexes). In modern environments, a good correlation between the intensity of phytoplankton productivity and the quantities of Cu and Ni transferred to sediments made it possible to establish paleo-productivity calculations based on the contents of ancient sediments in these two metals. The present study is a re-evaluation of the significance that can be attributed to these two metals as paleo-productivity proxies. The approach adopted here is based on the examination of a large database already available in the scientific literature. The choice was made to favor the examination of a large amount of data by simple means: comparisons of total organic carbon (TOC) content, enrichment in Cu and Ni (or even other trace metals), and value of the Fe:Al ratio that makes it possible to assess the availability of reactive iron. The basic idea is that the examination of a large number of geological formations makes it possible to encompass all kinds of paleo-environmental settings, thus comprising an extreme range of the factors conventionally involved in the mechanisms of accumulation of OM. The aim is to identify strong trends, valid in a large number of paleo-situations, which will have to be carefully taken into account in future detailed paleo-environmental reconstructions. It emerges from this study that, in many cases, Cu and Ni cannot be considered as faithfully reflecting the quantity of OM initially deposited. Several factors acting on the loss of Cu and Ni can be identified, among them, (1) a rapid loss linked to the decomposition of the OM before the conditions conducive to sulfate-reduction set in; (2) a low abundance of reactive iron which limits the quantity of pyrite liable to form, which significantly hampers Cu and Ni fixation in sediments. If Cu and Ni are not reliably retained in the sediments, that is, proportional to the quantity of OM supplied to the sediment, the paleo-environmental reconstitutions involving the concentrations of these metals may provide underestimated values of paleoproductivity. An interesting clue is the Fe:Al ratio that makes it possible to quickly know whether the values of the Cu and Ni enrichments are likely to be “abnormally” low.