Table of Contents

• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

Sinking particulate nitrogen (PNsink) mass and d15N analysis: Sinking particulate material was collected using surface-tethered floating particle-interceptor traps (PIT) equipped with 12 polycarbonate cylinders, deployed for up to 70 hours. Floating sediment traps were deployed at 200 m at Stations 3 and 5 on the 2010 cruise; for all other stations, traps were deployed at 100 m. Sediment trap samples were collected into a brine solution and then filtered and split into replicate samples. In most cases, two splits were collected from each sediment trap, one of which was acidified to remove inorganic carbon. In 2010 filtration removed the contribution of “swimmers” from sediment trap material. On the 2011 cruise, “swimmers” were identified by dissecting microscopy and hand-picked from sediment trap samples using sterilized micropipettors and forceps to remove their isotopic contribution to the sinking flux. Mass and isotopic fluxes were determined by dividing the mass of sinking particulate material captured in each trap by the trap surface area and by the duration (hrs) of trap deployment. Subsequently, the total mass flux was multiplied by the %N determined by combustion analysis of trap samples at the UC Davis Stable Isotope Facility to calculate PNsink mass fluxes; the d15N of trap samples was determined at the same time. The limit of detection for combustion analysis is 1.5 µmols N and the precision of the d15N measurement is + 0.3‰. No systematic difference was observed in the d15N of acidified vs. non-acidified trap samples.

References:

Soutar, A., Kling, S. A., Crill, P. A. & Duffrin, E. MONITORING MARINE-ENVIRONMENT THROUGH SEDIMENTATION. Nature 266, 136-139, doi:10.1038/266136a0 (1977).

Knauer, G. A., Martin, J. H. & Bruland, K. W. Fluxes of particulate carbon, nitrogen, and phosphorus in the upper water column of the northeast Pacific. Deep-Sea Research 26, 97-108, doi:10.1016/0198-0149(79)90089-X (1979).

BCO-DMO Processing:

- added conventional header with dataset name, PI name, version date
- renamed parameters to BCO-DMO standard
- added cruise id column
- replaced blank cells with nd

Description from NSF award abstract:
Several independent lines of geochemical and remote sensing evidence suggest that dinitrogen (N2) fixation may be associated with surface waters downstream of major oxygen minimum zones (OMZs) and in particular in the Eastern Tropical South Pacific (ETSP). However, little direct evidence supports these inferences. Besides substantiating these indirect assessments, documenting significant N2 fixation in the ETSP would provide insight into two longstanding controversies: Is the marine N budget balanced, as implied by modeling and paleoceanographic data, and if so, how are the processes that add and remove N spatially, and thus temporally coupled?

In this project researchers at the University of Southern California and the University of Miami will test the hypothesis that fixation occurs in the ETSP at areal rates that equal or exceed those previously documented in more well-studied regions such as the oligotrophic waters of the sub/tropical North Atlantic. If scaled to the surface area of ETSP waters, this could add an additional 10-50 Tg N per year of inputs to the global marine N budget. They will undertake two cruises in the ETSP during early and late summer in two consecutive years to assess the quantitative significance of N2 fixation as a source of new N to surface waters using complementary biological and geochemical tools. N2 fixation rates will be evaluated on two temporal/spatial scales: daily/local (bottle 15N2 incubations and floating sediment traps); and seasonal/regional (d15N budget using moored sediment traps and water column TDN d15N). These estimates provide detailed observations of potential N2 fixation during station occupation in two summer seasons, when rates are expected to be greatest, as well as prolonged observation over lower expected N2 fixation periods. A combination of these different estimates will aim to determine if N2 fixation in this region can help balance the marine N budget. If all goes as planned, this study will determine the quantitative importance of N2 fixation in the ETSP, and whether these previously undocumented rates can help resolve the marine N budget. Implications include the ability of the marine N cycle to maintain homeostasis, and thus the global C cycle on glacial/interglacial time scales.