Seasonal variability in carbon:234thorium ratios of suspended and sinking particles in coastal Antarctic waters: Field data and modeling synthesis

U-238-Th-234 disequilibrium is a powerful tool for investigating particle cycling and carbon export associated with the ocean's biological carbon pump. However, the interpretation of this method is complicated by multiple processes that can modify carbon:thorium ratios over small spatial scales. We investigated seasonal variability in the thorium and carbon cycles at a coastal site in the Western Antarctic Peninsula. Throughout the ice-free summer season, we quantified carbon and Th-234 vertical flux, total water column Th-234, particulate Th-234, and the C:Th-234 ratios of sinking material and bulk suspended material. Simultaneous identification and separation of fecal pellets from sinking material showed that fecal pellets (primarily from krill) contributed 56% of carbon flux and that as a result of lower C:Th-234 ratios than suspended particles, these fecal pellets were primary drivers of variability in the C:Th-234 ratios of sinking material. Bulk suspended particles had highly variable C:Th-234 ratios and were consistently elevated in the euphotic zone relative to deeper waters. The fraction of Th-234 adsorbed onto particles was positively correlated with chlorophyll and particulate organic carbon (POC) concentrations. The C:Th-234 ratios of suspended particles were positively correlated with POC, although during the spring diatom bloom C:Th-234 ratios were lower than would have been predicted based on POC concentrations alone. We hypothesize that diatom production of transparent exopolymers may have led to enhanced rates of thorium adsorption during the bloom, thus decreasing the C:Th-234 ratios. We used a Bayesian model selection approach to develop and parameterize mechanistic models to simulate thorium sorption dynamics. The best model incorporated one slowly-sinking POC pool and rapidly-sinking fecal pellets, with second-order sorption kinetics. The model accurately simulated temporal patterns in the C:Th-234 ratios of sinking and suspended particles and the fraction of Th-234 adsorbed to particles. However, it slightly over-estimated C:Th-234 ratios during the spring (diatom-dominated) bloom and underestimated C:Th-234 ratios during the fall (mixed-assemblage) bloom. Optimized model parameters for thorium sorption and desorption were 0.0047 +/-& nbsp;0.0002 m3 mmol C-1 d(-1) and 0.017 +/- 0.008 d(-1), respectively. Our results highlight the important role that specific taxa can play in modifying the C:Th-234 ratio of sinking and suspended particles and provide guidance for future studies that use Th-234 measurements to investigate the functional relationships driving the efficiency of the biological pump.

Automated summary

The study investigated seasonal variability in thorium and carbon cycles at a coastal site in the Western Antarctic Peninsula using U-238-Th-234 disequilibrium method. Results showed that fecal pellets (mainly from krill) were the primary contributors to carbon flux, and their lower C:Th-234 ratios influenced variability in sinking materials. Additionally, suspended particles had highly variable C:Th-234 ratios and were consistently elevated in the euphotic zone.