Metal Preservation and Mobilization in Sediments at the TAG Hydrothermal Field, Mid‐Atlantic Ridge.

At the Trans‐Atlantic Geotraverse hydrothermal field, metalliferous sediments cover extinct hydrothermal mounds and the surrounding seafloor. Here, we report the morphological, mineralogical and geochemical processes that deposit these sediments, remobilize their metals, and affect their preservation. We found that the initial sediment metal tenor is controlled by physical transport of hydrothermal material from its source, followed by diagenetic redistribution and potentially diffuse fluid flow after high‐temperature hydrothermal activity has ceased. We distinguished three different environments: (a) proximal metalliferous sediments on top of extinct mounds are mainly derived from oxidative weathering of primary sulfide structures and are predominantly composed of Fe oxyhydroxides with low contents of Cu, Co, and Zn; metal enrichments in specific layers are likely related to upward flow of low‐temperature hydrothermal fluids; (b) medial distant metalliferous sediments found at the base of the mounds, deposited by mass transport, contain cm‐thick layers of unsorted sulfide sands with high base metal contents (e.g., up to 28% Cu); these buried sulfides continue to undergo dissolution, resulting in metal release into porewaters; (c) distal metalliferous sediments, found in depositional basins a few hundreds of meters from the extinct mounds, include fining‐upwards sequences of thin sulfide sand layers with Fe oxyhydroxides and were deposited by recurrent turbiditic flows. Dissolved metals (e.g., Cu2+ and Mn2+) diffuse upwards under reducing conditions and precipitate within the sediment. Hence, when using hydrothermal sediments to construct reliable geochronological records of hydrothermal activity, distance from source, local seafloor morphology, mass‐transport and depositional, and diagenetic modification should all be considered. Plain Language Summary: The seafloor massive sulfide mounds at the TAG hydrothermal field (Mid‐Atlantic Ridge) are surrounded by metalliferous sediments. These sediments can be used to understand the history of the hydrothermal activity; however, the evolution of these sediments can be complex. The aim of this study is to understand the processes that lead to the accumulation and alteration of these sediments. For this, we identified the mineralogical, geochemical and physical processes that affect the sediments. Our results show that sediments found on mound tops are mainly composed of iron and manganese oxides and hydroxides, and are low in other metal contents compared to other environments. Medial distant sediments at the base of the mounds have thick beds of sulfide‐rich sediments that contain the highest concentrations of metals and are currently undergoing dissolution. In depositional channels, several hundreds of meters distal from the mounds, metalliferous sediment have accumulated in several meters thick layers that contain high concentrations of dissolved metals. This study reveals that the metal content of hydrothermal sediments is strongly controlled by their physicochemical environment and processes after deposition, and we suggest these processes should be considered when using metalliferous sediments as records of the evolution and activity of hydrothermal systems. Key Points: TAG hydrothermal field sediments have high metal contents that vary distinctly between different seafloor depositional environmentsLowest metal contents are found in sediments on top and highest in those at the base of extinct hydrothermal moundsDeposition, remobilization, and preservation of metals in hydrothermal sediments lead to distinctively different sediment characteristics [ABSTRACT FROM AUTHOR]