Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
  • BCO-DMO Processing Description
• Data Files
• Related Publications
• Related Datasets
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

Total 234-Thorium (Th) sampling and analyses are similar to those described in Buesseler et al. (2020). Briefly, 2 liters (L) of seawater was collected from 5 - 2500 meters (m) depth. Samples were acidified to pH ~1 and a 230Th yield tracer added. After samples were allowed to equilibrate for 6-8 hours, the pH was adjusted to ~8-9, and reagents were added to form a Mn-oxide precipitate that scavenges Th. Samples were allowed to stand for ~8 hours; after which the precipitate was filtered onto a 25-millimeter (mm) quartz microfiber filter (QMA) and dried at 60 degrees Celsius (˚C) before counting on a low level RISO beta counter using a helium/1%butane gas mixture. Samples were recounted after six months to account for any residual background activity stemming from other naturally occurring radionuclides that co-precipitate with the Mn-oxide. 230Th sample recoveries were measured using ICP-MS after acid digestion and spiking with 229Th according to the methods described in Pike et al. (2005) and Umhau et al. (2019) to account for any loss of sample material during processing.

All data are decay corrected to the mid-point of sample collection. Calibration was confirmed using deep waters (> 1000 m) with overall efficiencies determined via minimizing the difference between 238U (determined from salinity) and 234Th at depth. U-238 (not reported) was calculated using the equation described in Owens et al. (2011): U-238 (dpm/L) = (Salinity * 0.0786) – 0.315

- Imported original file "Total Th Data BCO-DMO.csv" into the BCO-DMO system.
- Converted the "Date" column to YYYY-MM format.
- Renamed fields to comply with BCO-DMO naming conventions.
- Saved the final file as "923028_v1_total_th.csv"

NSF Award Abstract:
The abyssal plains of the oceans cover roughly half of the earth's surface, host enormous reservoirs of biodiversity and mineral resources, and play important roles in nutrient recycling and carbon sequestration. The most important process controlling the structure and function of these ecosystems is the quantity and quality of food (mostly sinking organic particles) that reaches the deep-sea floor. However, we do not fully understand the processes provisioning this vast ecosystem. We propose to evaluate the relative importance of small and larger "marine snow" particles that sink to deep-sea benthic communities by using the stable isotope signature of amino acids within various food sources and trace their consumption by fauna on the seafloor. This project compares ecosystems from the productive waters off California with the nutrient poor central Pacific, north of Hawaii. This project provides novel insights into how surface ocean processes are coupled to food-webs at the deep ocean seafloor and how changes in food sources potentially impact deep-sea communities. This project also provides excellent training opportunities for graduate students, a postdoctoral researcher, and undergraduates at UH and USC, particularly underrepresented minorities who pursue majors in the geosciences. The project will sponsor an annual G6-12 teacher workshop to inform Hawaii educators about the deep sea and broadly disseminate knowledge to the community. All results are communicated broadly to inform the public as concerns regarding abyssal ecosystems are rising due to interests in deep-sea mining.

The most important process controlling the structure and function of abyssal ecosystems is the quantity and quality of organic material that ultimately reaches the deep-sea floor. Despite the strong relationship between euphotic zone export flux and benthic ecology, studies of abyssal ecosystems have observed a deficit between food supply and benthic community demand. Additional work is therefore needed, particularly with regards to understanding the sources of nutrition to the deep-sea benthos. Recent evidence suggests that small particles may be significant contributors to carbon export, increasing in relative importance with depth in the mesopelagic and reaching the abyssal seafloor. This project is to evaluate the relative importance of small and larger "marine snow" particles to deep-sea benthic communities using a combination of particle flux measurements and state of the art compound specific stable isotope analysis of amino acids (AA-CSIA) at two abyssal locations that contrast in overlying productivity, seasonality, and export magnitude. Time series measurements at these locations (Sta. M off California and Sta. Aloha off Hawaii) provide a rich context for the work. In the mesopelagic central North Pacific larger particles (>53 um) can be resolved from microbially reworked, smaller (0.7-53 um) particles using AA-CSIA. This project is characterizing the isotopic compositions of key individual compounds in a continuum of particle sizes (< 1.0 um suspended particles to large sinking particles >53 um) collected using in situ filtration near the seafloor and bottom-moored sediment traps, thereby defining source-specific isotopic signatures that can be traced into benthic fauna and sediments (that are collected by ROVs and epibenthic sleds). This research to understand pelagic-benthic coupling from particles to megafauna using isotopic measurements at the compound-level will yield novel insights into the importance of small microbially reworked particles to deep-sea benthic food webs. This will more precisely couple surface ocean processes to food-webs at the deep ocean seafloor with implications for understanding climate change effects and the efficiency of energy transfer to higher trophic levels. Furthermore, isotopic measurements can also be used to further parameterize ecosystem models by quantifying trophic position across size classes and thus estimate predator-prey mass ratios in relation to variation in body size spectra, functional type, and ultimately to carbon flux and remineralization. Finally, the results will help refine interpretations of deep-sea paleorecords of past nitrogen dynamics by calibrating potential changes in organic matter isotope values between the surface and seafloor archives.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.