Project: Measurement of Helium Isotopes on the U.S. GEOTRACES Alaska-Tahiti Section (GP15)

NSF Award Abstract:
The goal of the international GEOTRACES program is to understand the distributions of trace chemical elements and their isotopes in the oceans. An essential part of this effort is determining the sources of trace elements to the oceans. One important such source comes from seafloor hydrothermal venting. This project will support the measurement of helium isotopes on the 2018 U.S. GEOTRACES expedition in the Pacific Ocean, running south from Alaska to Tahiti. Helium isotopes provide important information on the presence and dispersion of hydrothermal plumes. On a larger scale, they can also be used to calculate global-scale inputs of hydrothermally-sourced trace elements and isotopes. Thus, the proposed work is closely tied to many other projects associated with this expedition. The measurement of helium isotopes will also allow enable the investigation of physical mixing rates within the upper ocean to help interpret the fate of other trace elements. At the northern end of the section, this approach will also be used to investigate the rate at which other trace elements and isotopes from seafloor hydrothermal venting are delivered to the more biologically productive surface ocean. In addition to supporting the GEOTRACES effort and participating in community-scale outreach efforts associated with the expedition, the investigators will also work with a visual artist to communicate their science to the public in new ways.

The U.S. GEOTRACES Pacific Meridional Transect (56°N to 20°S, along 152°W) in late 2018 will intercept: strong margin fluxes, sub-Arctic high-nutrient, low-chlorophyll waters, the oldest deep waters in the world's oceans, the distal ends of multiple hydrothermal plumes, oxygen minimum zones, subpolar and equatorial upwelling, and, in the South Pacific near 20°S, some of the most oligotrophic waters known. This section, together with the US GEOTRACES East Pacific Zonal Transect (EPZT) conducted in 2013, will also close off a large volume of the Pacific Ocean. Consequently, this work will facilitate new, improved evaluations of regional-scale fluxes of hydrothermally-sourced trace elements and isotopes (TEIs) to the oceans. Because the 2018 expedition will intercept multiple hydrothermal plumes, this project will also allow the investigation of three key questions emerging from the larger U.S. and international GEOTRACES programs: 1) Is there a significant source of hydrothermally-sourced Fe and other TEIs to the euphotic zone in the North Pacific, through upwelling, as was demonstrated for the Southern Ocean from the 2013 EPZT results? 2) How do TEI:He-3 ratios in hydrothermal plumes vary with different geologic settings of their vent-sources? 3) How do the TEI:He-3 ratios imparted to hydrothermal plumes vary along the thermohaline conveyor as a function of varying water column chemistry? Throughout the section the investigators will also combine upper-water column helium-3 measurements with water column tritium concentrations (extrapolated from data collected on past expeditions) to estimate water column ventilation time scales that can be used to quantify rates of TEI transformation across the wide range of biogeochemical regimes to be intercepted (tropical, subtropical, subpolar). In regions characterized by upwelling, precise measurements of helium isotopes can be combined with canonical gas-exchange rate estimates to make flux-gauge determinations of upwelling rates for some key TEIs.

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.