Trace metal chemistry of hydrothermal plumes

• Main Author: Lough, Alastair Jason Mackenzie
• Other Authors: Mills, Rachel
• Published: University of Southampton 2016
• Subjects: 551.46
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.698444

This thesis examines the nature of trace metal cycling in hydrothermal plumes, which have only recently been recognized as a significant source of Fe to the oceans. To study the influence of hydrothermal vents and their plumes on global trace metal cycles, two “black smoker” type vents and a previously unrecognized type of off-axis venting are examined. The trace metal chemistry of previously uncharacterized vents from the Von Damm vent field (VDVF) and the Beebe vent field (BVF) on the Mid-Cayman Rise (MCR) are described along with the processes of colloid formation in plumes over the Mid-Cayman Rise. Also Fe isotope analyses of a hydrothermal plume in the Southern Ocean reveals distinct isotope signatures to deep-waters dependent on plume chemistry. The role of soluble, colloidal and particulate partitioning of trace metals is understood to mediate the entire inventory of hydrothermal trace metals entering the ocean. In plumes over the MCR colloids are found to dominate dissolved iron (dFe) (48 to 87 % at Beebe and 14 to 81 % at Von Damm) in hydrothermal plumes. At Beebe soluble Fe (sFe) remains stable throughout plume dispersion, while particulate Fe is enriched (~25 %) by aggregating colloids. In the Von Damm plume colloidal Fe (cFe) and sFe maxima appear in the most dispersed regions of the plume where particulate Fe (pFe) is low. Plume processing of cFe and pFe will control the flux of dFe to the deep ocean from hydrothermal systems. This study shows that in order to accurately model the flux of dFe from vents, the behaviour of cFe needs to be incorporated into models of hydrothermal vent dFe fluxes, which at present do not consider these processes. Iron isotopes provide a means to measure the impact of hydrothermal venting on the oceanic Fe inventory, but no studies have examined the mechanism producing hydrothermal dFe isotope compositions. This study demonstrates that ?56Fe values of dFe (?56dFe) within the hydrothermal plume change dramatically during plume dispersal, ranging from -2.39 ± 0.05 ‰ to -0.13 ± 0.06 ‰ (2 SD). The isotopic composition of total dissolvable Fe (?56TDFe) was consistently heavier than dFe consistent with Fe oxyhydroxide precipitation as the plume ages. It is estimated that stable dFe exported from the plume will have a ?56dFe of -0.28 ‰, and this provides the first highly resolved constraint on hydrothermal plumes as a source of dFe isotopes to the ocean interior. This suggests that this distinctive isotope signature can be used to trace plume dFe inputs to the deep ocean. This will help constrain the impact of hydrothermal Fe on ocean biogeochemistry.