Project: GEOTRACES Atlantic Section: Measurement of Helium Isotopes and Tritium

NSF Award Abstract:
This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).

The guiding mission of the GEOTRACES program is to identify processes and quantify fluxes that control the distributions of key trace elements and isotopes in the ocean. The key observational strategy for GEOTRACES is an internationally-coordinated global-scale ocean survey of key trace elements and isotopes (TEIs), and the first U.S. section as part of that survey is in the North Atlantic. Knowing rates and fluxes is a vital step in the development of mechanistic and predictive models of ocean biogeochemical cycles of TEIs, particularly within the framework of global change (both past and future). Much of what we have learned about large scale oceanic rates and fluxes has been inferred from the observation and modeling of tracer distributions, both radioactive and transient. Measurement of appropriate transient tracers alongside of core TEIs would be an effective strategy for achieving GEOTRACES goals.

In this project, researchers at the Woods Hole Oceanographic Institution will make helium isotope and tritium measurements to provide useful biogeochemical rate information for the more centrally important TEI measurements made on the first U.S. GEOTRACES global survey section. The primary contributions that tritium and 3He measurements can make to the program include: (1) Quantifying transit timescales and TEI dilution in the MOC: 3H and 3He are useful tracers for determining deep western boundary current tracer transport rates and interior mixing dilution scales, an important issue for many TEIs; (2) A shallow water chronometer: Using the tritium-3He clock, the time elapsed since fluid parcels have been subducted on timescales ranging from 6 months to several decades can be determined; (3) A TEI thermocline reflux gauge: 3He is a unique "nutrient-like" transient tracer that can be used as a "flux gauge" to determine the rates at which thermocline-remineralized TEIs are returned to the upper ocean; and (4) Gauging TEI hydrothermal dilution scales: Volcanic 3He injected during hydrothermal activity is a powerful conservative tracer of dilution in these plumes, allowing diagnosis of nonconservative behavior in some TEIs, and permitting flux estimates associated with hydrothermal activity on basin and global scale.

Broader Impacts: The proposed work is in support of the GEOTRACES program, and as such contributes to the broader societal goals and intellectual objectives espoused by that program. The primary issues related to this are pertinent to understanding the carbon cycle and predicting/mitigating climate change, as well as the marine food web and anthropogenic impacts on the oceans.