Impact of Chondrites on trace metal distribution in the sapropel S7 (ODP Site 966): Implications for paleoenvironmental and paleoceanographic reconstructions

Abstract

Organic-rich deposits are valuable paleo-archives, recording significant paleoceanographic changes linked to past climate variations and marine deoxygenation events. The deposition of organic-rich sediments stops when bottom-water reventilation/oxygenation occurs. This impedes organic matter preservation, enabling macro and micro burrowing-organisms to recover and bioturbate the seafloor. In this sense, highly bioturbated sediments require particular attention when sampling is done for geochemical analyses, as sediments can be considerably mixed. While avoiding trace fossils during sampling for geochemical analyses is easy for large traces (e.g. Planolites, Thallasinoides, and Zoophycos), it becomes challenging for smaller and complex traces (< 0.5 cm), as is the case with Chondrites. Chondrites-producers have an opportunistic behavior, being one of the first organisms to colonize the seafloor after an anoxic event. As a result, Chondrites are commonly and sometimes abundantly found in the upper layers of organic-rich sediments, including sapropels and black shales. This study focuses on sapropel S7, which was deposited between ~198.5 and 191.9 ka at the top of Eratosthenes Seamount and exhibits abundant Chondrites. For this sapropel, we demonstrated that Chondrites-producers can bioturbate significant volumes of organic-rich sediments (over 35%), introducing oxic/dysoxic material downward into anoxic sediments enriched in redox-sensitive trace metals and organic matter. This process can dilute up to 18% the original concentration of redox-sensitive trace metals (e.g. Mo, U and V). This dilution is especially important to consider when calculating trace metals and organic matter burial flux from bulk sediment data. Therefore, Chondrites-producers can play a crucial role in altering the sediment texture and distribution of minerals and organic matter at sub-cm scale, and may thus impact paleoenvironmental and paleoceanographic reconstructions based on geochemical signals. Furthermore, the subsequent redistribution of organic matter and redox-sensitive trace metals (including oligoelements) within the sediment can affect carbon and nutrient cycling, microbial activity, organic matter degradation in deep-marine sediments, and the overall sedimentary biogeochemistry.