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Hydrothermal sediment geochemistry south of the Antarctic Polar Front

Hydrothermal sediment geochemistry south of the Antarctic Polar Front
Hydrothermal sediment geochemistry south of the Antarctic Polar Front
This thesis uses a novel, combined mineralogical, geochemical (including stable S isotopes), and microbiological approach to semi-quantitatively determine Scotia Sea sediment formation processes. The factors that control the localisation of chemosynthetic, microbial consortia in metalliferous sediment beneath Southern Ocean vent fields is investigated along with the impact of hydrothermal venting on sediment composition. Circum-Antarctic ridges represent nearly 40 % of the Earth’s ~58,000 km ridge crest, but remain severely understudied. In the austral summer of 2009–2010, the Royal Research Ship James Cook expedition JC42 explored the northernmost (E2) and southernmost (E9) bare-rock segments of the East Scotia Ridge, and the sedimented Kemp Caldera (a southern feature of the South Sandwich Arc), and collected >20 co-registered vent fluid, chimney sulfide and hydrothermally-influenced sediment samples using the ISIS remotely-operated vehicle. The hydrothermal materials from E9 and the Kemp Caldera are the focus of this thesis. E9 sediment composition is controlled by the simple mixing of >90 % local basalt that is affected by subduction-related and enriched mantle components, <10 % particulate plume fallout, which is dominated by an Fe-, Cu-, Zn-, Ba-, and Pb-rich, near-plume phase, and <1 % collapsed chimney material. The major, minor, trace, and rare earth element sediment content at E9 is largely determined by proximity to active venting. The thin sediment cover throughout E9, indicates an early stage of sediment formation and the recent onset of venting at this site. Kemp Caldera sediment components include 55–60 % phreatomagmatic shards of local basalt that were most likely deposited by a recent, volcanic event, 30–45 % crystalline elemental S derived from the magmatic disproportionation of SO2 (identified by a δ34S signature of +4.8 ‰ to +5.9 ‰), and 0–10 % buoyant plume particles (rich in P, K, Mn, Fe, and the rare earth elements). Biogeochemical Fe, Mn, and S cycling is investigated in two very different sediment systems of the Kemp Caldera: Toxic Castle and Tubeworm Field. Toxic Castle sediments are compiled from the episodic deposition of magmatic and hydrothermal components, while pore fluid composition is strongly influenced by diffuse, upwelling hydrothermal fluid. The original magmatic-hydrothermal signature is diagenetically altered in the solid phase Tubeworm Field sediments, likely initiated by dissimilatory sulfate reduction. Pore fluid Fe and Mn redox zonation in the surface sediments at Tubeworm Field is typical of biogeochemical cycling in stratified marine sediments. Microbial cell counts (identified by 4’,6-diamidino-2-phenylindole staining and fluorescent in situ hybridisation microscopy) are relatively consistent across the major Tubeworm Field redox boundaries, although there is a significant downcore shift in the microbial community structure. A dominant presence of δ-, ε-, and γ-proteobacteria (which host known S metabolisers), confirms active, microbial S cycling in the deeper Tubeworm Field sediments. Sediment descriptions from modern hydrothermal (particularly back-arc basin-associated) systems are relatively scarce, in comparison to those of vent fluid and chimney material, which is surprising given the potential economic importance of hydrothermally-derived, metalliferous, rare earth-enriched sediments. This thesis increases our knowledge of sediment formation processes and subsequent biogeochemical cycling, in a range of back-arc-associated bare-rock and sedimented systems, along poorly-surveyed circum-Antarctic ridges, and accentuates the requirement for continued, interdisciplinary hydrothermal surveys of the global, submarine ridge system. We must fully understand the complex interaction of geological, chemical, and biological components that constitute the complete hydrothermal system, before we allow the commercial exploitation of unique ecosystems that have forever changed our perception of life in the deep sea.
Hepburn, Laura
d202d545-ffa3-4b7c-8cde-46c11cdc61dc
Hepburn, Laura
d202d545-ffa3-4b7c-8cde-46c11cdc61dc
Mills, Rachel
a664f299-1a34-4b63-9988-1e599b756706

Hepburn, Laura (2015) Hydrothermal sediment geochemistry south of the Antarctic Polar Front. University of Southampton, Ocean & Earth Science, Doctoral Thesis, 484pp.

Record type: Thesis (Doctoral)

Abstract

This thesis uses a novel, combined mineralogical, geochemical (including stable S isotopes), and microbiological approach to semi-quantitatively determine Scotia Sea sediment formation processes. The factors that control the localisation of chemosynthetic, microbial consortia in metalliferous sediment beneath Southern Ocean vent fields is investigated along with the impact of hydrothermal venting on sediment composition. Circum-Antarctic ridges represent nearly 40 % of the Earth’s ~58,000 km ridge crest, but remain severely understudied. In the austral summer of 2009–2010, the Royal Research Ship James Cook expedition JC42 explored the northernmost (E2) and southernmost (E9) bare-rock segments of the East Scotia Ridge, and the sedimented Kemp Caldera (a southern feature of the South Sandwich Arc), and collected >20 co-registered vent fluid, chimney sulfide and hydrothermally-influenced sediment samples using the ISIS remotely-operated vehicle. The hydrothermal materials from E9 and the Kemp Caldera are the focus of this thesis. E9 sediment composition is controlled by the simple mixing of >90 % local basalt that is affected by subduction-related and enriched mantle components, <10 % particulate plume fallout, which is dominated by an Fe-, Cu-, Zn-, Ba-, and Pb-rich, near-plume phase, and <1 % collapsed chimney material. The major, minor, trace, and rare earth element sediment content at E9 is largely determined by proximity to active venting. The thin sediment cover throughout E9, indicates an early stage of sediment formation and the recent onset of venting at this site. Kemp Caldera sediment components include 55–60 % phreatomagmatic shards of local basalt that were most likely deposited by a recent, volcanic event, 30–45 % crystalline elemental S derived from the magmatic disproportionation of SO2 (identified by a δ34S signature of +4.8 ‰ to +5.9 ‰), and 0–10 % buoyant plume particles (rich in P, K, Mn, Fe, and the rare earth elements). Biogeochemical Fe, Mn, and S cycling is investigated in two very different sediment systems of the Kemp Caldera: Toxic Castle and Tubeworm Field. Toxic Castle sediments are compiled from the episodic deposition of magmatic and hydrothermal components, while pore fluid composition is strongly influenced by diffuse, upwelling hydrothermal fluid. The original magmatic-hydrothermal signature is diagenetically altered in the solid phase Tubeworm Field sediments, likely initiated by dissimilatory sulfate reduction. Pore fluid Fe and Mn redox zonation in the surface sediments at Tubeworm Field is typical of biogeochemical cycling in stratified marine sediments. Microbial cell counts (identified by 4’,6-diamidino-2-phenylindole staining and fluorescent in situ hybridisation microscopy) are relatively consistent across the major Tubeworm Field redox boundaries, although there is a significant downcore shift in the microbial community structure. A dominant presence of δ-, ε-, and γ-proteobacteria (which host known S metabolisers), confirms active, microbial S cycling in the deeper Tubeworm Field sediments. Sediment descriptions from modern hydrothermal (particularly back-arc basin-associated) systems are relatively scarce, in comparison to those of vent fluid and chimney material, which is surprising given the potential economic importance of hydrothermally-derived, metalliferous, rare earth-enriched sediments. This thesis increases our knowledge of sediment formation processes and subsequent biogeochemical cycling, in a range of back-arc-associated bare-rock and sedimented systems, along poorly-surveyed circum-Antarctic ridges, and accentuates the requirement for continued, interdisciplinary hydrothermal surveys of the global, submarine ridge system. We must fully understand the complex interaction of geological, chemical, and biological components that constitute the complete hydrothermal system, before we allow the commercial exploitation of unique ecosystems that have forever changed our perception of life in the deep sea.

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Published date: 16 November 2015
Organisations: University of Southampton, Geochemistry

Identifiers

Local EPrints ID: 384574
URI: http://eprints.soton.ac.uk/id/eprint/384574
PURE UUID: ffcb76cb-53b3-49ba-9af6-b5ea7cde6488
ORCID for Rachel Mills: ORCID iD orcid.org/0000-0002-9811-246X

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Date deposited: 08 Dec 2015 13:53
Last modified: 14 Jun 2018 00:36

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Contributors

Author: Laura Hepburn
Thesis advisor: Rachel Mills ORCID iD

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