Although hydrothermal vent systems have been intensively studied at mid-ocean ridges where ocean crust is being created, few such studies have been carried out in back arc basin spreading systems where crust is being subducted back into the mantle. This research assesses the chemical evolution of hydrothermal activity and its linkages to associated vent biology in one such back arc basin spreading center in the East Lau Basin. A unique series of time series fluid samples that were collected in 2009 will be analyzed for a variety of geochemical and isotopic species, including magmatic volatile species. These data will be combined with temperature measurements to examine the effect of acid volatiles on the evolution of hydrothermal systems overlying silicic magmas and how this influences the mobilization of metals. It will also be used to examine the role of seawater circulation in the ocean crust and its impact on the removal of CO2 via mineral precipitation. Broader impacts of the work include the support of an early career scientist, involvement of faculty and undergraduates from a college that serves economically struggling urban centers in southeastern Massachusetts, and engaging undergraduates involved in local summer sea-going programs. Hands-on learning modules for K-8 students will be created.
This subaward provided funds to investigate the radiocarbon content of hydrothermal vent volatiles from archived samples collected at hydrothermal vent sites at Lau Basin, the Mid Atlantic Ridge, and 9N EPR--all Ridge 2000 Study Sites. The overarching goal of the project was to measure the radiocarbon content of hydrothermal fulids, an analytical approach very rarely applied to hydrothermal vent samples. However, by carefully selecting a subset of samples that likely contained measureable radiocarbon, this measurement had potential utility to illuminate subsurface processes. Typically, the volatile component of hydrothermal fluids is dominated by CO2 that is magmatic in origin, with concentrations in the 10-100s of mmol/kg. This CO2 is "primordial" (the origin of the carbon either the mantle or has been recycled--on timelines of 100s of millions of years-- through the crustal conveyer belt), and thus has a radiocarbon content that is indistinguishable from "blank" samples. Radiocarbon measurements can only resolve carbon "ages" to ~40,000 years, so the utility of making these measurements on hydrothermal samples dominated by primordial carbon is not readily apparent. However, hydrothermal fluids begin as bottom seawater with a seawater bicarbonate concentration of ~2.3mmol/kg, and a radiocarbon content approximated by the "age" of the seawater—e.g. the last time that seawater was in equilibrium with "modern" atmospheric CO2. Radiocarbon data are especially tricky to interpret when investigating samples with near background levels of 14C, as the measurement error increases dramatically as the 14C content decreases. Evidence from our radiocarbon analyses shows clear removal of the 70-80% of "original" seawater bicarbonate from hydrothermal fluids during typical mid-ocean ridge hydrothermal circulation. We hypothesize that this bicarbonate is removed as precipitated carbonate minerals in the low-temperature recharge limb, supported by numerous carbonate minerals observed in off-axis sedimented bore holes. When extrapolated to include all hydrothermal circulation, and over geologic time horizons, this seawater bicarbonate removal may have important implications to the global carbon cycle and budget.
