Project: Low temperature hydrothermal vent fluxes as traced by radium isotopes

Iron is one of the most abundant elements in fluids released from mid-ocean ridges, yet it has been thought to be mostly removed from seawater near to its hydrothermal vent source. Recent evidence has emerged showing this hydrothermally-derived iron can be transported great distances in the ocean, including toward the sunlit surface layer where it is an essential nutrient for phytoplankton growth. This study will use naturally-occurring radium isotopes, which are also enriched in hydrothermal fluids but are non-reactive compared to iron, as tracers of hydrothermal iron in the deep ocean. With a range of half-lives from days to centuries, radium isotopes can be used as "clocks" to measure the precise rates at which hydrothermal iron is carried by ocean currents toward the ocean interior. Such information can be incorporated into ocean models designed to determine the role of hydrothermal iron on ocean productivity, which has societal relevance due to its role in regulating the carbon dioxide concentrations of Earth's atmosphere. This project will support research opportunities for two students through the Woods Hole Partnership in Education Program, which seeks to increase diversity in ocean and environmental science by inviting college juniors and seniors to gain practical experience through a summer of classroom study and hands-on research activities.

A scientist from Woods Hole Oceanographic Institution will quantify rates of iron (Fe) transport above a major ocean spreading center to evaluate the role of hydrothermal venting in supplying this essential micronutrient to the surface ocean. Special attention will be paid to the role of low-temperature hydrothermal Fe fluxes, which are hypothesized to arrive at the seafloor in a stable form that mitigates significant removal during transport. If true, low-temperature fluids may supply a disproportionate amount of the "transportable" dissolved Fe that has not been observed in large-scale deep ocean plumes. Assessment of process rates will be accomplished through concurrent measurements of Fe concentration and speciation and the radium (Ra) "quartet" (224Ra, 223Ra, 228Ra, 226Ra) during an expedition along the southern East Pacific Rise (EPR; 15-18 degrees South). Autonomous underwater vehicle and towed-CTD surveys will be used to identify locations of low- and high-temperature discharge. At selected target areas, the investigators will test the hypotheses that low-temperature hydrothermal inputs have a distinct radium isotopic ratio fingerprint, and that the relative rate of Fe scavenging removal is lower in low-temperature sources. Given their wide ranging half-lives, Ra isotopes have the unique ability to integrate over biogeochemically-relevant time scales of Fe transport in the deep ocean. The combination of concurrent Ra isotope and total dissolved Fe measurements will allow the investigators to quantify Fe residence time for any Fe phase that is transported on large horizontal scales, as has been observed previously along the EPR. Together, Fe loss and transport from Ra measurements will lead to improvements in the predictive capabilities of ocean circulation and biogeochemistry models that assess the importance of hydrothermal Fe in supporting primary productivity and carbon drawdown in overlying surface waters.

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.