Hydrothermal Scavenging

Hydrothermal Scavenging

Hydrothermal vents emit metal-rich fluids into the deep ocean. These metals (e.g. Fe, Mn) react with seawater to form particulate (oxyhydr-)oxides with highly-reactive surfaces capable of adsorbing or co-precipitating other dissolved ions out of solution and carrying them to the seafloor. I used dissolved and particulate thorium and protactinium isotopes measured in a hydrothermal plume extending over 4000km west of the East Pacific Rise at ~15ºS to study these scavenging conditions.

Variable depletion signatures of three different radiogenic thorium isotopes (A-C), and sitemap of study sites from GP16 section, with proposed flowpath of hydrothermal plume shown as a dashed white arrow. Modified from Pavia et al. 2019

In Pavia et al. 2018 we described the anomalous 230Th and 231Pa depletions in the plume, attributed the removal to direct adsorption onto surface sites of metalliferous particles, and developed a model for the time evolution of 230Th and 231Pa in a hydrothermal plume. In Pavia et al. 2019 we included measurements of two other radiogenic Th isotopes (234Th and 228Th) with shorter half-lives to model the kinetics of hydrothermal scavenging. We were able to determine absolute or relative changes in rate constants for adsorption, desorption, and particle aggregation. We also found that much of the 230Th depletion observed off-axis must have occurred upstream as waters flowed northward along the EPR ridge axis towards our study side.

Recent work has found that hydrothermal metal deposition varies on glacial-interglacial timescales. Given the anomalous water column 230Th scavenging we previously observed, changes in hydrothermal activity can disturb the 1-d mass balance between local burial and production by U decay that belies the use of 230Th as a constant flux proxy in paleoceanography. With David Lund (UCONN), we generated a downcore record of 230Th fluxes by normalization to a second constant flux proxy, extraterrestrial 3He, in a core near the EPR, finding 230Th burial rates that exceed the water column 230Th production rate by up to a factor of 4 during the deglacial peak in hydrothermal activity (Lund et al. 2019). Using new sedimentary records of 3He and 230Th from other ridge sites, I am currently working with Jenny Middleton (LDEO) and Kassandra Costa (WHOI) to develop a comprehensive framework for where and when there are likely to be hydrothermally-driven anomalies in 230Th burial.