Anthropogenic Shadows in the Earth's Deepest Environments: Insights into Hadal Zone Pollution

The hadal trenches, with depths exceeding 6,000 meters, are critical yet understudied sinks for anthropogenic pollutants. However, advances in deep-sea technology and growing interest in ocean health drive the need for new tools to study deep-sea sediments. This study explores the prevalence and impact of organic and inorganic contaminants in the Japan Trench hadal sediments, focusing on their origins, pathways, preservation and potential ecological consequences. Sediment samples were collected from water depths of 7,000–7,800 meters in the hadal zones during IODP Exp. 386. The samples were analyzed through a multidisciplinary approach, incorporating oceanography, analytical biogeochemistry, and statistical analysis. Gas Chromatography-Mass Spectrometry (GC-MS) analysis revealed concentrations of persistent organic pollutants (POPs) ranging from µg to ng/g, including polycyclic aromatic hydrocarbons (PAHs), dichlorodiphenyltrichloroethane (DDT), and its metabolites (DDX). Sequential extraction and microwave-assisted digestion techniques quantified heavy metals and metalloids bound to sediment matrices, emphasizing the accumulation of inorganic pollutants, such as Pb, Zn, Te, As, Cd, Ni. Total Organic Carbon (TOC) and biomarkers analyses (e.g., n-alkanes) were performed to determine the primary sediment transport pathways delivering pollutants into the trench. The results, interpreted through statistical analyses of correlations between pollutant concentrations, biomarkers, biogeochemical factors (e.g., TAR, CPI ratios), and sediment accumulation rates, reveal a complex interplay between terrestrial and marine sediment sources. Oceanic processes and seismic events contribute to pollutant transport and deposition in deep-sea trenches, which act as global contaminant sinks, with pollutants transported via marine snow, seismic-induced sediment remobilization, and tsunami backwash, posing risks to fragile hadal ecosystems. By identifying biomarkers and pollutant assemblages, this study quantifies transport processes and offers valuable insights into sedimentary pollution history, its impact on biogeochemical cycles, and its consequences for marine biodiversity and ecosystem functions. The findings highlight the significant influence of both human activities and natural processes on the ocean's deepest regions, stressing the need for interdisciplinary strategies to better understand sediment-associated transport processes, as well as the fate and toxicological potential of persistent pollutants in the least studied environments on Earth – the hadal deep-sea basins.