Redox geochemical signatures in Mediterranean sapropels: Implications to constrain deoxygenation dynamics in deep-marine settings

Abstract

Global warming and anthropogenic activity are boosting marine deoxygenation in many regions around the globe. Deoxygenation is a critical ocean stressor with profound implications for marine ecosystems and biogeochemical cycles. Understanding the dynamics and evolution of past deoxygenation events can enhance our knowledge of present-day and future impacts of climate change and anthropogenic pressure on marine environments. Many studies have reconstructed the evolution redox conditions of past deoxygenation events using geochemical proxies. In this regard, the present work focuses on understanding the paleoenvironmental significance of geochemical redox signals derived from the onset, evolution and termination of regional-scale deoxygenations in deep-marine settings, with a specific focus on sapropels in the Eastern Mediterranean (EM). Sapropels, rhythmic organic-rich sediments deposited in EM, offer a unique opportunity to investigate recent deoxygenation events linked to past climate changes. Sapropels serve as paleo-archives of past deoxygenation events and can provide insights into the potential impacts of ongoing climate change on marine ecosystems and biogeochemical cycles. By integrating previous sapropel geochemical studies with a detailed analysis of new geochemical data from five Quaternary sapropels (S1, S5, S6, S7 and S8) in three different EM deep-marine settings, this study enhances our understanding of the paleoenvironmental significance of geochemical redox signals produced by deoxygenation dynamics and postdepositional processes in different deep-marine settings. The study supports that certain trace elements, such as Mo, V, U, Co, and Ni, are identified as more reliable redox proxies compared to Cr, Cu, Pb, and Zn. Four recurrent geochemical intervals attributed to specific redox conditions and postdepositional processes have been identified. Moreover, internal calibration of redox proxies thresholds has been performed and demonstrates how local environmental conditions (e.g., productivity rate) and hydrogeographic features (e.g., water-depth, particulate-shuttling intensity, deep-water renewal and fluvial input) play crucial roles in controlling the authigenic uptake rates of redox-sensitive trace metals, and subsequently, redox thresholds values of geochemical redox proxies. The results also emphasize the importance of postdepositional processes to accurately interpret geochemical signals in paleoenvironmental studies. This research enhances our overall understanding of geochemical signals associated with regional-scale deoxygenation events in deep-marine settings, offering new insights into predicting biogeochemical changes in marine environments undergoing a transition towards anoxia. By comprehending the dynamics of past and present deoxygenation, we acquire valuable knowledge regarding the potential effects of climate variability in marine ecosystems.