CNRS

The inorganic-organic bridge: carbon-sulfur-metal interactions in Black Sea sediments

The sequestration of organic matter in marine sediments plays a fundamental role in regulating the climate on geological timescales. Its burial constitutes a crucial sink in the carbon cycle, influencing long-term atmospheric concentrations of gases such as O₂ and CO₂. Recent studies suggest that interactions between organic matter and reduced sulfur, or with intermediate sulfur species, may have promoted the preservation of sedimentary carbon. However, few studies focus on modern analogs that offer the opportunity to decipher the mechanisms involved and distinguish those inherited from the water column from those related to post-depositional transformations (i.e., within the sediment). Conversely, the final stage of organic matter degradation in sediments leads to methane production, a potent greenhouse gas. Methane fluxes, like those of organic matter, drive complex sedimentary biogeochemical processes, whose interactions with inorganic compounds, particularly sulfur and transition metals (e.g., Fe, Mo, Ni, V), are still poorly constrained, especially in modern anoxic marine basins . The Black Sea, the largest modern anoxic basin, serves as a natural reference laboratory for studying sedimentary mechanisms, with direct implications for understanding ancient oceans.

This PhD project addresses the quantification of interactions between organic matter, sulfur, and metals in methane-rich sediments of the Black Sea. The research will be based on a set of surface sediment samples (~40 cm) and gravity cores (~3 m), with interstitial fluids and sediments collected and preserved at different depths during the EU DOORS project.

To achieve this, three main objectives will be pursued:

1- Analyzing trace element concentration variations in interstitial fluids to identify active biogeochemical reactions and their potential imprint on sediments:
Preliminary analyses of methane concentrations and major ions (sulfate, Cl, Mg, Ca, etc.) in interstitial fluids have revealed varying intensities of methane fluxes in sediments. To expand on these results, we will analyze the concentrations of organo-reactive metals (Fe, Mo, Ni, V) using ICP-MS, including an isotopic dilution approach. These analyses will quantify the dynamics of these elements in relation to modern biogeochemical processes, particularly their potential transfer into organic/inorganic reservoirs within the sediments.

2- Investigating associations between sulfur, metals, organic matter, and diagenetic minerals by characterizing their speciation and isotopic compositions:
Biogeochemical reactions leave a direct imprint on sediments, promoting the formation of diagenetic minerals (e.g., pyrite, carbonates), which compete with organic matter interactions (e.g., sulfurization). To quantify the proportions of different “pools” and their evolution with depth, sulfur and metal concentrations and speciation in mineral and organic phases will be determined through a combination of chemical extraction techniques (acid attacks, CRS, etc.) and spectroscopic analyses (XANES). To refine our understanding of the biogeochemical processes involved, isotopic analyses (S, Fe, Ni) will also be conducted on these minerals and the organic matter.

3- Quantifying the kinetics of these processes using transport-reaction models to better understand their dynamics and implications in anoxic sedimentary environments:
A transport-reaction modeling approach will be implemented to quantify the intensity of the biogeochemical processes involved, as well as associated isotopic fractionations. The objective is not only to evaluate these modern interactions but also to reconstruct their evolution over time, particularly in response to external forcing such as methane flux variations and basin evolution.

To apply, please use this link: https://amethis.doctorat.org/amethis-client/prd/consulter/offre/1094