Project: Collaborative Research: Are Low-Temperature Hydrothermal Vents an Important but Overlooked Source of Stabilized Dissolved Iron to the Ocean?

NSF Award Abstract:
Hydrothermal vents are hot springs on the seafloor. They are often thousands of meters below the surface of the ocean along the large volcanic mountain ranges called the mid-ocean ridges. Relatively little is known about these hydrothermal vents because they exist so far from the ocean surface. Instead of lava, these vents emit warm to hot fluids (from several ºC up to 350 ºC) into the ocean, and contain high concentrations of a wide range of dissolved chemicals. One of these chemicals, iron, has been studied extensively by ocean scientists because of its importance as an essential nutrient for the microscopic photosynthetic algae called phytoplankton that grow in surface ocean waters. Scientists once thought that most of the dissolved iron coming out of hydrothermal vent fluids would quickly precipitate (form a solid and sink) near the vents, as a result of chemical reactions between the hot vent fluid and cold seawater, and therefore only small amounts of dissolved iron could be transported from the deep sea mid-ocean ridges to surface waters where phytoplankton grow. However, on a recent scientific expedition over one of the most active mid-ocean ridges called the southern East Pacific Rise (SEPR), scientists discovered that much of the dissolved iron discharged from the SEPR hydrothermal vents was in fact transported thousands of kilometers across the deep ocean. In addition, computer simulations suggest that a substantial amount of this 'hydrothermal iron' is transported as far as the surface waters of the Southern Ocean around Antarctica, where it could support the growth of phytoplankton.

Scientists from the University of Washington, Woods Hole Oceanographic Institution and Old Dominion University will return to the SEPR aboard a research ship and make additional measurements to better understand how hydrothermal vents add dissolved iron to the deep ocean. They will use specialized sampling equipment, including unmanned miniature submarines called autonomous underwater vehicles (AUVs), to map the distribution of hydrothermal vents in this region, and collect water samples from both high temperature (250-350 ºC) and low temperature (< 150 ºC) vents. These samples will be analyzed for chemicals that are enriched in the hydrothermal vent fluids, such as iron, manganese and helium-3, which will provide information on the chemical changes that occur as the hydrothermal vent fluids mix with surrounding seawater and are carried away from the SEPR. The scientists will use their observations and chemical analyses to address the following major questions: 1) are there more hydrothermal vents along mid ocean ridges than previously thought? and (2) are low temperature hydrothermal vents especially favorable for stabilizing dissolved iron, and ultimately transporting it to the surface ocean where it supports phytoplankton growth?

The project will contribute to the education and training of a graduate student and two senior undergraduate students, and outcomes will be incorporated into high school STEM curricula. The research activities will be communicated via public lectures and media releases, and project results will be disseminated to the scientific community by conference presentations, publications in peer-reviewed scientific journals, and freely available on-line archived data.

Along thousands of kilometers of the seafloor volcanic chains known as the mid-ocean ridges, hydrothermal vents discharge hot, acidic, metal-rich fluids into the deep ocean. These submarine hot springs are a major gateway for the exchange of heat and chemicals between the solid earth and the deep ocean, and have received intensive scientific study during the last 40 years. This research has overwhelmingly focused on high temperature (~200-350°C) hydrothermal vents. Lower temperature (<150°C) hydrothermal venting has received relatively little attention, although results from recent observational and modeling studies point to the greater abundance of low temperature discharge along the mid-ocean ridges, and its potential importance for the input of elements and chemical compounds into the deep ocean. Among the elements that are enriched in hydrothermal vent fluids, iron has received attention because of its role as an essential nutrient for primary production in the surface ocean. It has long been thought that most of the dissolved iron discharged by hydrothermal vents is lost from solution close to mid-ocean ridge sources, and thus of limited importance for ocean chemistry. But this view has been challenged by recent studies which suggest that chemical stabilization of hydrothermal dissolved iron may facilitate its long-range transport in the deep ocean. In particular, results from the US GEOTRACES program have revealed the lateral transport of a plume of hydrothermal dissolved iron over several thousand kilometers westward from its source region on the southern East Pacific Rise (SEPR).

In this project, investigators from the University of Washington, Woods Hole Oceanographic Institution and Old Dominion University will carry out a field research program on the SEPR to test the overarching hypothesis that diffuse low-temperature hydrothermal venting is a major a source of chemically-stabilized dissolved iron to the deep ocean, and hence plays an important but previously overlooked role in the ocean iron cycle. The ship-based field program will use an autonomous underwater vehicle equipped with in-situ optical and chemical sensors to identify plumes of both diffuse (low-temperature) and discrete (high-temperature) hydrothermal discharge along the SEPR. Emissions from multiple sites of both low and high temperature hydrothermal venting will be sampled for shipboard and post-cruise analyses of iron and other trace metals, and the inert hydrothermal "tracer" helium-3. The resulting data will be used to test two specific hypotheses: (1) the population of active hydrothermal discharge sites along mid ocean ridges is greater than previously estimated, (2) low temperature hydrothermal venting is conducive to the chemical stabilization of dissolved iron, thus facilitating its export to the ocean interior and ultimately to surface ocean where it supports primary production by phytoplankton. The information obtained in this research will facilitate inclusion of iron and other trace elements in numerical models of ocean biology and biogeochemistry, which will improve the ability to predict how the ocean will respond to and modulate future climatic and environmental changes.

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.