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Research Spending & Results

Award Detail

Awardee:TRUSTEES OF PRINCETON UNIVERSITY, THE
Doing Business As Name:Princeton University
PD/PI:
  • Daniel M Sigman
  • (609) 258-2194
  • sigman@princeton.edu
Award Date:03/23/2010
Estimated Total Award Amount: $ 270,643
Funds Obligated to Date: $ 270,643
  • FY 2010=$270,643
Start Date:05/01/2010
End Date:04/30/2013
Transaction Type:Grant
Agency:NSF
Awarding Agency Code:4900
Funding Agency Code:4900
CFDA Number:47.050
Primary Program Source:040100 NSF RESEARCH & RELATED ACTIVIT
Award Title or Description:Collaborative Research: GEOTRACES Atlantic Section Nitrate Isotope Measurements
Federal Award ID Number:0960802
DUNS ID:002484665
Parent DUNS ID:002484665
Program:CHEMICAL OCEANOGRAPHY
Program Officer:
  • Donald L. Rice
  • (703) 292-8582
  • drice@nsf.gov

Awardee Location

Street:Off. of Research & Proj. Admin.
City:Princeton
State:NJ
ZIP:08544-2020
County:Princeton
Country:US
Awardee Cong. District:12

Primary Place of Performance

Organization Name:Princeton University
Street:Off. of Research & Proj. Admin.
City:Princeton
State:NJ
ZIP:08544-2020
County:Princeton
Country:US
Cong. District:12

Abstract at Time of Award

Nitrogen is one of the two major nutrients required universally by plankton in the ocean, and its availability can affect the ocean's ecology, productivity, and carbon cycle. While the cycling of fixed N in the ocean is in one sense emblematic of other nutrient cycles, it is also unique in that its largest input (N fixation) and output (denitrification) are biologically mediated, which renders the ocean N budget susceptible to complex biological feedbacks. It thus provides a platform for asking one of the core questions of global biogeochemical cycles: How is it that the actions of individual organisms and groups conspire with physicochemical conditions to produce a global Earth surface environment that has been continuously habitable for billions of years? The dominant terms in the oceanic fixed N input/output budget are poorly characterized, and we focus our attention here on N fixation. Developing robust estimates of the global rate and distribution of N fixation from "direct" shipboard measurements of N fixing activity is complicated by the inherent spatial and temporal variability of this biologically mediated flux. Thus, geochemical approaches for estimating N fixation inputs have come to the forefront. Currently, nitrate stable isotope measurements, which could provide an integrative estimate of N fixation on a regional or basin scale, are sparse in the Atlantic, being focused primarily in the Sargasso Sea. The GEOTRACES program provides a platform to put these data into a broader context through the illumination of basin-scale patterns. In this project researchers from Princeton University, Brown University, and the Woods Hole Oceanographic Institution will measure the d15N of nitrate in seawater and atmospheric samples collected as part of the GEOTRACES North Atlantic Section. Nitrate d15N is a GEOTRACES "core parameter" that will complement other measurements and will by itself provide important constraints on the oceanographic processes, including N fixation, lateral nitrate transport, low latitude N cycling, the effect of the North African upwelling regions on nutrient fluxes across the basin, and the exchange of fixed N with the Mediterranean. In addition to yielding such specific process-related insights, this work will provide one of the first cross-basin views of nitrate isotopes in the interior and will thus help to simply characterize the isotope signals of different interior water masses, including the Mode Waters, Antarctic Intermediate Water, Mediterranean Intermediate Water, Lower and Upper North Atlantic Deep Water, and Antarctic Bottom Water. Finally, the isotopic characterization of atmospheric nitrate deposition will inform our understanding of the N isotope budget and isotopic gradients of the North Atlantic. Combined, these measurements will yield insight into modern biogeochemical processes and will also provide first order background information for both modern physical oceanographic and paleoceanographic applications. As an example of the latter, studies of Atlantic sediments seek to reconstruct past changes in the rate of N fixation, based on the modern finding that N fixation appears to lower the d15N of thermocline nitrate in the Sargasso Sea. Progress in this paleoceanographic work relies on a more complete picture of nitrate d15N in the modern Atlantic. Broader impacts: The broader impacts of the proposed study include the mentoring of a postdoctoral investigator and the inclusion of undergraduates in state-of-the-art research. The project will also provide a high-quality nitrate isotope data set for the North Atlantic for use by the broader community.


Project Outcomes Report

Disclaimer

This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.

Nitrate (NO3-) is the dominant form of biological available N in the ocean, a key nutrient required in large quantities by all organisms. The availability of nitrate to phytoplankton growing in the surface waters is a key determinant of marine productivity, which feeds fisheries and takes up carbon dioxide from the atmosphere.

 

The two elements in nitrate, nitrogen (N) and oxygen (O), each naturally occur in more than one stable “isotope,” which refers to the number of neutrons in the nucleus. The two isotopes of nitrogen are nitrogen-14 (14N) and nitrogen-15 (15N), and the two dominant isotopes of oxygen are oxygen-16 (16O) and oxygen-18 (18O). The 15N/14N and 18O/16O ratios in a given sample of nitrate are its “isotopic composition,” which is quantified in the terms d15N and d18O, respectively. The isotopic composition nitrate helps to identify where and how ocean nitrate is added, removed, transported, and biologically cycled. In addition, nitrogen is trapped in fossils, and its isotopic composition records changes in the ocean nitrogen cycle back through time; measurements in the modern ocean are required to ground-truth and calibrate this use of fossils to reconstruct of past changes.

 

The GEOTRACES Atlantic cruise was the US contribution to an international effort to to develop a comprehensive picture of trace elements and natural isotopes across the North Atlantic Ocean. As part of this effort, we measured the isotopic composition of nitrate, generating a picture of these properties that runs from the surface to the bottom of the ocean and from North America to Europe (Figures 1 and 2 for the nitrogen isotopes and oxygen isotopes, respectively). Several key observations are readily apparent in these “depth sections.”

 

First, there is low d15N nitrate in the shallow waters of the western North Atlantic. This is very likely the result of “nitrogen fixation,” in which certain marine phytoplankton add new biologically available nitrogen to the ocean. It will be possible to characterize the rate and spatial distribution of nitrogen fixation when these data are compiled with other data and simulated with a computer model.

 

Second, there is a relatively high d15N at roughly 1000 meters depth and strongest to the East. This feature is in water that is flowing northward from the Southern Ocean, the ocean surrounding Antarctica. The high d15N of this water is due to incomplete uptake of surface nitrate by phytoplankton in the high latitude waters of the Southern Ocean, with the remaining (high d15N nitrate) being forced into the ocean’s interior and flowing northward as “Antarctic Intermediate Water.” The measured d15N signal is actually much weaker than was originally emplaced the Southern Ocean, and its d18O signal has been erased (Figure 2). This observation can be used to quantify the cycling within the Atlantic that works to erase the Southern Ocean-derived isotopic signal.

 

Third, below ~2500 meters depth, there is a weak but measurable east-to-west d15N change, with slightly higher d15N on the western side of the basin (Figure 1). This higher d15N signal is in the water known as North Atlantic Deep Water, which is surface water that sinks into the deep ocean in high latitude North Atlantic and then flows southward. Paradoxically, this high d15N water may well derive from the Southern Ocean. The high d15N water at 1000 m described above is carried northward in Antarctic Intermediate Water all the way to Greenland, where its high d15N signal is incorporated into North Atlan...

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