Behaviour of radium, barium and the rare Earth elements in mid-Atlantic Ridge hydrothermal plumes
Keywords: Mid-Atlantic Ridge
rare Earth elements
MAR
REEs
Atlantic
thorium
U-series
hydrothermal plumes
ocean chemistry
hydrothermal
radium
barium
isotopes
radio-isotopes
actinium
mid-ocean ridge
geochemistry

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 no significant vari- ation in Eu* between ocean basins or with depth. Mean Eu* from compiled seawaters of 0.97 (±0.07, 2σ, PAAS-normalised) supports Post-Archaean Australian Shale (PAAS) and North American Shale Composite (NASC) as estimates of UCC Eu* inputs to the ocean.
The first coupled measurements of dissolved Ba concentration and δ138/134Ba in the hy- drothermal NBP is presented. NBP waters at Rainbow and TAG display higher dissolved Ba concentrations and heavier δ138/134Ba values than surrounding ambient seawaters. Small hydrothermal Ba fractions calculated in NBP samples indicate that hydrothermal Ba inputs are unlikely to have an observable impact on the ocean Ba cycle. The extent of dilution oc- curring before an effective hydrothermal Ba-isotope composition is established is calculated to be 100 times smaller than previously assumed.
The results of this thesis evidence the utility of Ra- and Ba-isotopes as well as REEs for enhancing our increasingly complex understanding of hydrothermal plume processes at slow-spreading margins.