Thorium-230 as a Particulate Flux Gauge

Thorium-230 as a Particulate Flux Gauge

Studies of particle flux in the ocean typically utilize sediment trap profiles with deployments at a few (~5-6) depths throughout the entire water column to diagnose the flux and regeneration of particulate constituents like particulate organic carbon (POC), calcium carbonate (CaCO3), lithogenic material, and trace metals. Sediment trap profiles are costly and typically cannot achieve sufficient vertical, spatial, or temporal resolution to study variations in flux and regeneration across biogeochemical gradients.

I am working on a method to adapt the paleoceanographic 230Th-normalization method to determine high-resolution particulate fluxes. Thorium’s residence time is sufficiently short to limit net lateral redistribution by advection and eddy diffusion. Thus, at steady-state in 1-d, the downward flux of 230Th on sinking particles balances its depth-integrated production by 234U decay. Since the production rate of 230Th is well-known, its flux through any depth horizon can be easily predicted. By measuring the ratio of any particulate constituent to 230Th in particulate matter, the fluxes of those constituents, averaged on ~1-3 year timescales, can thus be computed.

US GEOTRACES cruises collect in-situ pumped particles at 16-24 depths throughout the water column – allowing for the highest resolution profiles of particulate flux ever generated. This is important for studies of POC flux and regeneration depths, which have been hampered by limited data availability and discrepant estimates of the spatial variation in POC regeneration depths across different methods. Hayes et al. 2019 found that POC fluxes from 230Th-normalization agreed well with both other radiochemical methods and sediment traps in the North Atlantic, indicating that 230Th-normalization can be used as an internally-consistent tool to study particulate fluxes.

POC fluxes determined by particulate 230Th-normalization at selected stations from the GEOTRACES GP16 section

I am currently using 230Th-normalization to study POC flux and regeneration in the Eastern Tropical South Pacific, using samples from the GEOTRACES GP16 section. POC regeneration length scales vary strongly between the highly productive waters overlying the Peru Oxygen Minimum Zone, and the oligotrophic waters of the South Pacific Gyre (Pavia et al. 2019). I am also working on adapting 230Th-normalization to study other particulate constituent cycles, including CaCO3 dissolution and mesopelagic iron residence times.