Behaviour of radium, barium and the rare Earth elements in mid-Atlantic Ridge hydrothermal plumes

• Main Authors: Selzer, S, https://orcid.org/orcid:0000-0002-8322-1401
• Other Authors: Homoky, W, Hsieh, Y, Annett, A, Henderson, G
• Format: Thesis
• Language: English
• Published: 2023
• Subjects: Oceanography; Radioisotopes in geology; Hydrothermal vents; Geochemistry; Stable isotope tracers; Rare earth metals; North Atlantic Ocean; Mid-Atlantic Ridge; North Atlantic
• Online Access: https://doi.org/10.5287/ora-2zknqq2aw; https://ora.ox.ac.uk/objects/uuid:e8128e7f-8848-46b7-80ef-edceba3c8be9

Hydrothermal activity influences the distributions of trace elements and isotopes (TEIs) in the ocean, which in turn impacts our understanding of oceanic TEI-cycling and the utility of TEIs as geochemical tools to study a variety of fundamental ocean processes. New data- sets have emerged in recent years that demonstrate importance of hydrothermal plumes for mediating seawater chemistry. Such influences are driven in part by vent fluid emissions, as well as the scavenging effects that mineral precipitates impose on deep waters circulating over ridge systems. Ridges themselves may also be important for driving changes to the physical mixing of deep ocean waters and the dispersion of hydrothermal TEI signatures. Herein, this thesis explores the behaviours of three potentially powerful but hitherto poorly understood marine tracers in hydrothermal plumes: short-lived radio-isotopes radium-223 and radium- 224, the Eu anomaly (Eu*), and stable-isotopes barium-134 and barium-138. The data sets gathered for these studies derive from the sampling efforts along the Mid-Atlantic Ridge of GEOTRACES research cruise GA13 (JC156). A validated Python program is presented to expedite the correction of raw data generated by Radium Delayed Coincidence Counting (RaDeCC) instruments and propagate related uncertainties. The utility of radium-223 and -224 as tracers of hydrothermal neutrally buoyant plumes (NBPs) along the Mid-Atlantic Ridge (MAR) is investigated at Rainbow and TAG vent sites. Benthic inputs, likely to be hydrothermal sulphides, are common and constitute the dominant source of radium-223, and to a lesser extent radium-224, in the MAR axial valley. Elevated vertical diffusivities are calculated within the MAR axial valley from radium-223 and -224 water-column activity profiles. The first analyses of dissolved Eu concentration ([Eu]) and Eu anomaly (Eu*) in NBPs at three vent sites show that hydrothermal Eu inputs are not expected to have an observable impact on seawater Eu*. Compiled Eu* values from global oceans show ...