Evidence of postdepositional remobilization of redox-sensitive metals across sapropel boundaries: New insights from LA-ICP-MS and EDX mapping analyses

Abstract

Deoxygenation in marine systems is globally increasing due to anthropogenic pressures and climate change impacts, leading to significant ecological and environmental consequences. Understanding past deoxygenation events is crucial for assessing the response of ocean stressors to climate variability and for predicting and managing the impacts of current and future changes. Deep-marine basins with organic-rich deposits serve as valuable palaeo-archives, preserving evidence of profound palaeoceanographic changes involving deoxygenation events. Such events have been broadly studied and reconstructed using redox-sensitive trace metals (RSTMs) as proxies for oxygenation conditions. Assessing these proxies is therefore crucial for further understanding deoxygenation events. To do so, this study focuses on RSTMs distribution across the most recent sapropel, S1, which is a well-preserved organic-rich sediment layer that represents the most recent large-scale deoxygenation event in the Eastern Mediterranean. High-resolution geochemical microanalyses, including Laser Ablation ICP-MS and SEM-EDX, were conducted to investigate sub-mm scale geochemical signals and post-depositional remobilization of RSTMs within S1 and surrounding sediments. Statistical analyses have also helped to identify the geochemical fractions within S1, including detritic, carbonate, and authigenic, enriched in RSTMs and organic matter. The study revealed distinct specific geochemical intervals, such as the marker bed, oxidized interval, unoxidized interval, and synsapropel interval. These intervals provided insights into the diagenetic processes and post-depositional reactions occurring within and across sapropel S1 boundaries. The enrichment of RSTMs within pyrite aggregates in the synsapropel interval was primarily attributed to post-depositional processes rather than the initial syndepositional conditions of S1. Large pyrite surfaces below S1 and the precipitation of Mn-oxyhydroxides over S1 played a significant role in the fixation and absorption of dissolved RSTMs derived from postdepositional oxidation. Furthermore, the sub-mm scale analysis revealed spatial heterogeneities in mineralogical composition and elemental distribution, providing valuable information on the complexity of post-depositional reactions and processes. The findings highlight the importance of microscale geochemical analyses for understanding the dynamics of RSTM fixation and enrichment processes in marine sediments. By differentiating between syndepositional and post-depositional geochemical signals, this study contributes to the interpretation of palaeoenvironmental conditions and enhances our understanding of the spatial complexity of deoxygenation processes. Such insights are crucial for accurately assessing the impacts of deoxygenation on biogeochemical cycling and ecosystem functioning in marine environments.