Radiocarbon Inquiry of Hydrothermal CO2 at Lau Basin: A unique insight into the carbon cycle in the shallow ocean crust


DOEI Funded Research: 2008


Oceanic crust is formed by active volcanism at mid-ocean ridges and returned to the mantle at subduction zones.  For example, Pacific Plate oceanic crust is erupted to the seafloor at the East Pacific Rise, and ~200 million years later is subducted at the trenches along the western margin of the Pacific.  During the millions of years that the oceanic crust is emplaced on the seafloor it is cooled and chemically altered by shallowly circulating seawater, a process that increases the density of the rock.  When these old, cold rocks are forced up against less dense continental or more recently formed oceanic crust, the old oceanic crust subsides and plunges back into the earth.  The down going oceanic crust, called “the slab”, can be visualized seismically for more than 400km before it is completely melted and is indistinguishable from the mantle.  As the slab descends, water that has been stored in the rocks helps to partially melt the slab, and this melt rises through the overriding plate to form volcanic arc islands (e.g.  the Tongan Islands) and back-arc spreading centers (e.g.  the Lau Basin).  This proposal requests funds to study hydrothermal fluids from the Lau Basin, a back-arc spreading center, that will provide unique insight into how carbon is removed from seawater during hydrothermal circulation and how much carbon is recycled back into the hydrosphere from the down going oceanic slab.  We propose to use radiocarbon analysis of CO2 from hydrothermal fluids, a novel approach in unsedimented hydrothermal systems.  These radiocarbon analyses, along with the measurement of compositional chemistry and stable isotope chemistry, will provide a powerful tool in the deduction of the source of CO2 as being from seawater, mid-ocean ridge basalt, or the down going slab. 

Radiocarbon (14C), the radioactive isotope of carbon, is formed in the atmosphere and radioactively decays to nitrogen with a half-life of 5730 years.  Radiocarbon is typically used to as a dating tool, because the decay process begins as soon as there is no exchange of CO2 with the atmosphere (e.g.  the plant dies, or a surface oceanic water is forced below the mixing zone).  In this study we are using radiocarbon as a tracer of seawater bicarbonate (CO2 dissolved in seawater).  Because radiocarbon decays past measurable limits in ~40,000 years, radiocarbon measurements in unsedimented hydrothermal systems where the bulk of the CO2 is derived from degassing magmas and has never been exposed to the atmosphere (e.g.  contains no radiocarbon) have not been particularly effective at constraining the fate of seawater bicarbonate in hydrothermal systems.  Preliminary measurements suggest that because the Lau Basin fluids have very low concentrations of magmatic CO2, radiocarbon analyses do provide useful information about the amount of seawater bicarbonate preserved during hydrothermal circulation. 

We propose to use DOEI funds to participate in a cruise returning to the Lau Basin in early 2009.  Our aim is to collect samples specifically for radiocarbon analysis, but also for general chemistry, from the Ridge2000 ISS ABE vent site, a newly discovered site just north of ABE, and previously visited sites in order establish a time series at Lau Basin.