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. In current environments, a good correlation between the intensity of
 phytoplankton productivity and the quantities of Cu & 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 value 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 & 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 thanks to the large database makes it possible to encompass
 all kinds of paleo-environmental settings, thus comprising an extreme variety 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 retained, and 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 & Ni fixation in sediments. If Cu & Ni are not reliably retained
 in the sediments, that is, proportional to the quantity of OM supplied to the sediment, the paleoenvironmental
 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 & Ni enrichments are likely to be "abnormally" low.

Reduction in Marine Primary Productivity in the Early Cambrian Nanhua Basin, South China

“…As micronutrient elements, Cu and Ni are usually applied to evaluate primary productivity. , Cu and Ni are absorbed by organisms to yield organic matter along with the photosynthesis, and they have a good relationship with organic matter in the surface ocean . However, Cu and Ni are also profoundly affected by the water-column redox conditions.…”

Paleoenvironmental Reconstruction and Organic Matter Accumulation of the Lower Cambrian Qiongzhusi Formation in the Sichuan Basin, South China

“…Copper (Cu) is an important nutrient for the biota as it can be incorporated into organic complexes and form essential metalloenzymes . The commonly observed correlation between Cu abundance and the intensity of plankton productivity in modern and ancient environments made it possible to assess the paleoproductivity. − Therefore, it can be used to indicate primary productivity through being normalized to Al for the removal of the influence of terrestrial materials. , The Cu/Al ratios mainly exceed 10 × 10 –4 in the LQS and are between 2 × 10 –4 and 10 × 10 –4 in the MQS–UQS, suggesting a higher primary productivity in the LQS (Figure B,D). In the MQS, the Cu/Al ratios in the north (5–9 × 10 –4 ) are relatively higher than in the south (2–6 × 10 –4 ), indicating that the northern basin had a higher productivity.…”

Sedimentation in the adjacencies of a southwestern Atlantic giant carbonate ridge

“…EM3 (finer fraction) presents affinity with copper, an indicator of organic matter input (Tribovillard et al, 2006;Tribovillard, 2021), and manganese, usually associated with the terrigenous fraction (Wagreich and Koukal, 2020). It is worth noting that the terrigenous fraction is an essential constituent in carbonate mounds (Richter et al, 2006;Pirlet et al, 2011;Terhzaz et al, 2018) and may represent over 50% of the mounds' framework (Titschack et al, 2009).…”