GEOTRACES Atlantic Section: Measurement of Helium Isotopes and Tritium

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

This Grant for Rapid Response Research (RAPID) will provide funding to a research team at the Woods Hole Oceanographic Institution to continue their collection of tritium and helium isotope samples on the interrupted GEOTRACES North Atlantic Zonal Section originally funded through an NSF ARRA award. Due to the mechanical failure of the research vessel KNORR's propulsion system, the original cruise was cut short about one third of the way through, and most of the scientific party was obliged to disembark in Cape Verde Islands. Extra effort is required to refurbish sampling vessels and re-stage the sampling effort for the second cruise set for October, 2011. The research team will make helium isotope and tritium measurements to provide useful biogeochemical rate information for the more centrally important TEI measurements made on the continuation of 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) As a shallow water chronometer: using the tritium-3He clock, we can determine the time elapsed since fluid parcels have been subducted on timescales ranging from 6 months to several decades. We can also combine 3H - 3He ages and evolving 3H and 3He distributions with geostrophy, T,S, O2, nutrient, and TEI distributions to estimate transformation rates. (3) A TEI thermocline reflux gauge: 3He e is a unique ?nutrient-like? transient tracer that can be used as a so-called flux gauge to determine the rates at which thermocline-remineralized TEIs are returned to the upper ocean. This is important for biogeochemical models. (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 non-conservative behavior in some TEIs, and permitting flux estimates associated with hydrothermal activity on basin and global scale. The P.I. will be responsible for the acquisition, measurement, interpretation and modeling of the tritium and 3He to achieve the above objectives. Broader Impacts: The proposed work is in support of the GEOTRACES program, 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.