Sediment core studies on seafloor hydrothermal systems at the Mid-Atlantic Ridge (13°N)

Abstract

Seafloor hydrothermal systems contribute to global heat fluxes and regulate global biogeochemical cycles. High-temperature hydrothermal activity also leads to the formation of seafloor massive sulphide (SMS) deposits, which are rich in metals needed for the net-zero transition. Given their scientific significance and societal impact, it is important to investigate what controls the formation of SMS deposits, how they are preserved, and how they can be located. This thesis presents multi-disciplinary studies on deep-sea sediment cores to understand the formation, preservation and distribution of hydrothermal systems and their associated SMS mineralisation. Sediment cores were collected from the Semenov hydrothermal field, located at an oceanic core complex (OCC) at the Mid-Atlantic Ridge (MAR) (13°30’N), using gravity corers, multi-corers, and Remotely Operated Vehicle (ROV) push cores. Integrated sedimentological and geochemical analyses, along with multi-proxy geochronological dating (oxygen isotope stratigraphy, radiocarbon dating, and palaeomagnetism), of the sediment cores reveal contrasting histories and drivers of hydrothermal activity across the OCC. Near-continuous high-temperature discharge is recorded within a tectonically-stranded hanging wall, likely driven by sustained hydrothermal fluid flow along the oceanic detachment fault, results in the formation of large SMS deposits. In contrast, intermittent high-temperature hydrothermal activity at the ultramafic footwall is likely controlled by sporadic emplacement of mafic intrusions and episodic faulting, resulting in discontinuous and focussed discharge that generates smaller SMS hydrothermal mounds. Complementary geochemical analyses on the solid phase and porewaters, and mineralogical studies of sediment cores within the ultramafic footwall suggest favourable conditions for sub-seafloor sulphide precipitation, accumulation and preservation, which includes secondary copper mineralisation due to supergene enrichment. Sulphide preservation is facilitated by the development of gossan caps (highly weathered and oxidised hydrothermal sediments) and carbonate pelagic sediment cover, preventing metal loss to the water column via seawater oxidation and acid leaching. The diversity of mineralisation styles across the OCC is reflected in the spatial variations of the geochemistry of sediments proximal and distal to the SMS deposits. A new exploration technique, which leverages these variations, was developed for geochemical vectoring towards SMS deposits using rapid, non-destructive X-ray fluorescence (XRF) analyses of sediments. This technique shows potential to allow triangulation towards SMS deposits, extending out to up to seven times the diameter of a recently inactive mound. This thesis demonstrates how sediments can be used to understand seafloor hydrothermal activity and associated SMS mineralisation, and advocates for their integration and further development in on-going and future marine scientific research and deep-sea exploration.

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Identifiers

ORCID for Acer Jian Tulauan Figueroa: orcid.org/0000-0001-9359-9160

ORCID for Rachael James: orcid.org/0000-0001-7402-2315

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