Evaluation of excess 234Th activity in sediments as an indicator of food quality for deep-sea deposit feeders

Deep-sea deposit feeders selectively ingest large volumes of sediment. Knowledge of the nature of this selectivity will help to elucidate the limiting nutritional requirements and geochemical impacts of these abundant animals. Shallow-water and theoretical studies suggest that deep-sea deposit feeders should select particles rich in protein, bacterial biomass, and/or chloropigment concentrations. Recent studies indicate that deep-sea megafaunal deposit feeders exhibit strong gut enrichment of excess (xs) Th-234 activity, even though Th-234(xs) lacks nutritional value. To explore the significance of selective ingestion of Th-234(xs) activity, we evaluated the correlations between Th-234(xs) activity and three potential tracers of deposit feeder food quality: chlorophyll a (chl a), enzymatically hydrolyzable amino acids (EHAA), and adenosine triphosphate (ATP). Surface sediments from three quiescent bathyal basins off Southern California (San Nicolas, Santa Catalina, and San Clemente) were collected by a multiple corer and analyzed for Th-234(xs) activity, chl a, EHAA, ATP, and total organic carbon and nitrogen. Th-234(xs) activity was positively correlated with chl a and phaeopigment concentrations and negatively correlated with EHAA concentrations. Excess Th-234 was not linearly correlated with concentrations of ATP, organic carbon, or total nitrogen. The results suggest that deep-sea deposit feeders select sediments with high Th-234(xs) activity because it is associated with recently settled phytodetrital material. There is no evidence that this Th-234(xs)-rich material has particularly high concentrations of labile amino acids or microbial biomass. Phytodetrital material may be an important source of some other limiting nutrient to deep-sea deposit feeders, e.g., polyunsaturated fatty acids, labile organic carbon and/or vitamins.