Origin and composition of hydrothermally-influenced sediments at Aurora Vent Field, southwestern Gakkel Ridge (82.9°N)

Abstract

Slow–ultraslow spreading ridges represent half of the total length of the global mid-ocean ridge system and have been recognized to host hydrothermal activity. Their role as a source versus sink of elements in the oceans remains poorly constrained. Here, we present the first sediment data from the Aurora Vent Field (~3900 m depth) on the ultraslow spreading Gakkel mid-ocean ridge and the northernmost active vent field (82.9°N) explored to date. We interpret the composition of the cores at various distances from the vents based on physical parameters (P-wave velocity, gamma density, magnetic susceptibility, acoustic impedance and fractional porosity), mineralogy and (in)organic geochemistry (elemental analysis via X-Ray fluorescence and carbon–nitrogen systematics). Metalliferous sediments in two blade cores at <10 m from a black smoker are dominated by Fe-hydroxides (goethite) and Cu-Fe-bearing sulphide minerals (mainly chalcopyrite) that are rich in Co and Zn. These sediments form a friable crust associated with yellow microbial mats visible on the seafloor with the remotely operated vehicle imagery. Near-field plume deposits form an ~50 cm thick unit at the top of gravity cores close to the vents (<500 m). This unit transitions at further distance into laminated plume fallout deposits marked by high magnetic susceptibility and elevated ln(Fe/Ti), ln(Cu/Ti) and ln(Zn/Ti) values in the XRF profiles. Sedimentary organic matter in the investigated cores is dominated by a marine source, which is overprinted by microbial mats in the surface sediments of the blade cores. In one blade core, we found evidence of thermal alteration of the organic matter, typified by elevated C/N ratios and heavier δ¹³C_TOC. Radiocarbon dating of foraminifera in a gravity core yielded an age of 27 230 cal bp, evidence for protracted vent activity preceding the Last Glacial Maximum 23 to 19 ka. Finally, we propose a sedimentological model for the Aurora hydrothermally influenced system that can be potentially exported to analogues worldwide.