Collaborative Proposal: Using Radiocarbon Measurements of Benthic Megafauna as a Tool for Assessing Bentho-Pelagic Coupling in the Marine Organic Carbon Cycle

University Of Hawaii, Honolulu

Investigators

Abstract

This project examines the seasonal variation in the nature of the organic carbon deposited on continental margins. The flux of organic carbon reaching the seabed can be quantified by using particle traps or by modeling seabed burial and regeneration rates, but the quality (i.e., its lability) has been difficult to assess on continental margins because of co-deposition with old refractory carbon from marine and terrestrial systems. Bomb Carbon-14 (C14) is a useful tracer of recently produced marine organic matter, but this signal is masked in continental margin particle?trap samples and surface sediments by the older and refractory carbon that is re-suspended and reworked from shallower marine deposits. Surface deposit feeders in continental margin systems have been shown to sequester the labile fraction of the total organic flux reaching the seabed as a result of their selective ingestive and digestive processes. Therefore, the body tissues and the gut sediments of these benthic fauna can provide a record of the labile material reaching the continental margin seafloor. Initial C14 data indicate that the body tissues of surface depositing feeders are enriched in this radioisotope by 100 to 300 per mil relative to the surface sediments in which they live and feed. Based on mass?balance C14 calculations, the labile component may comprise only a few tenths of a percent of the total organic matter in the surface seabed, yet the surface deposit feeders exhibit C14 signatures in their body tissues (+20 to +80 per mil) dominated by the local bomb?produced signal . Thorium (Th) data from continental margins suggest that as much as 50% of the recently deposited organic matter on the continental margin sea floor may pass through the guts of deposit feeders prior to microbial degradation or burial. Therefore, C14 measurements on benthic fauna can provide a unique tool for assessing the lability of organic matter reaching the seafloor and for tracking the fate of this material in the benthic food web. Many existing diagenetic models treat organic matter regeneration by microbes and deposit feeders as a batch process with a single carbon degradation constant for the bulk organic matter. The C14 data clearly show that ingestion of organic matter and digestion of this material are very selective processes. The C14 analyses from the California Borderland and the Antarctic continental margin have shown the utility of this approach for documenting variations in the organic carbon lability of surface sediments. Drs. DeMaster and Smith will expand the initial studies to enable assessment of seasonal variations in the quality of organic carbon deposited on the seabed. Particle?trap samples, benthic?faunal tissues, gut samples and surface sediments have been collected during 6 cruises to the California Borderland and 5 cruises to the Antarctic continental margin covering the major seasonal variations in organic carbon supply. Funds are requested for making 150 C14 analyses (and complementary C13, N15, and C/N measurements) on these samples so that a time series quantifying the nature and flux of organic matter reaching the seabed can be determined. They will assess not only seasonal variations in the nature of particle selection and digestive selection processes for organic matter, but also the changes in these processes as a result of feeding strategy (epibenthic surface deposit feeders, vs. subsurface deposit feeders vs. head?down subsurface deposit feeders) on the seafloor.