Award: OCE-1546695

Volcanic eruptions on the seafloor are common, yet most of these eruptions are undetected and completely unnoticed, so the most common type of deep sea eruption has never been directly observed*. Scientists and the public need to understand how our planet works, how the oceans affect the global planetary support system, and how unseen volcanoes affect the chemistry and biology of the oceans. When instruments connected to the Ocean Observatories Initiative cabled array detected an eruption at Axial Seamount in late April of 2015, we organized a group to study the eruption and its effects on the ocean. We wanted to know exactly where the eruption happened, how much lava was erupted onto the seafloor, and how this eruption affected the flow of hot water out of the seafloor into the overlying ocean. Seawater penetrates into the porous volcanic seafloor, is heated by and reacts with hot rock. The resulting flow of hot water provides chemical energy for chemosynthetic microbes that grow beneath the seafloor and are flushed out into the overlying ocean. For this project, we prepared specialized instruments to collect chemical and microbiological data to help us understand how deep-sea eruptions affect sub-seafloor chemosynthetic communities and the overlying ocean. We collected sensor data and water samples in hydrothermal plumes above the lava flows. We located and sampled warm springs on the new 2015 lava flows (as shown in accompanying image). With sensors, we measured temperature and oxygen content, and from recovered samples we measured dissolved gases, metals, and nutrients, beginning with shipboard analysis and continuing in shore-based laboratories. We collected microbes from the warm springs and plumes, and colleagues used DNA sequencing to characterize the microbes. Based on extensive earlier work at Axial Seamount, the most recent eruptions have all been centered south of the summit on the South Rift Zone. The 2015 eruption was surprisingly on the North Rift Zone, where no known recent eruptions have occurred and no previous hot springs were known to exist. We found warm spring sites on the thickest lava flows extending 15km north of the summit caldera. The warm spring samples we collected varied in temperature (5-35°C) and composition, but they were generally very enriched in iron and methane (more enriched than any of the many previously known vents at Axial), but not enriched in hydrogen sulfide, an important chemosynthetic energy source. Analysis done after the research cruise showed that the water column plume extended north beyond the warm springs that we were able to sample on the seafloor with limited time, and one plume property (dissolved hydrogen gas) indicated the existence of a higher temperature water source at the northern end of the lava flows. The microbial communities in the plume also varied, indicating more than one sort of sub-seafloor habitat, which generally means different temperature and chemistry. Even though most of the microbes in the water column are from normal deep ocean water, we can trace the contribution of different sub-seafloor microbial communities and link that to the warm spring chemical signal in the water column. The first results are in press for publication in Oceanography. An online cruise report is available to the public (https://www.pmel.noaa.gov/eoi/axial/2015/Axial2015-Cruise-Report-no-logs.pdf). We deployed and recovered a time-series sampler at a warm spring and found that the spring quit venting a few months after our 2015 expedition. One year of temperature and particle data from a fixed mooring also showed that the water column plume greatly diminished by the end of 2015. We will continue to work with the data from the 2015 expedition to understand how the warm spring chemistry relates to the way the volcano erupted and to understand how the high-temperature vents (which we sampled with a separate project in 2017) developed so quickly on this new lava flow. The findings of this study have contributed to our understanding of how deep-sea eruptions affect the ocean. *Two volcanoes have been directly observed erupting on volcanic arcs in the western Pacific, but none on a mid-ocean ridge. Last Modified: 01/17/2018 Submitted by: David Butterfield