In an ongoing National Science Foundation-funded project, DCO researchers are investigating the flux of volatile compounds, like water, carbon dioxide and nitrogen gas, into and out of the mantle along the Hikurangi Margin, an active subduction zone off the east coast of New Zealand’s North Island.
DCO researchers selected this area because it is a well-studied subduction zone where the oceanic plate, the Hikurangi Plateau, is sinking beneath the continental crust of the Indo-Australian Plate. The researchers are sampling key volatiles, including noble gases, and measuring inputs from the subducting oceanic plate and outputs from nearby volcanoes and hot springs. DCO Reservoirs and Fluxes Community member Gray Bebout (Lehigh University, USA) and a founding member of the DECADE project, Bruce Christenson (GNS Science, New Zealand) are collaborating with Hirochika Sumino (University of Tokyo, Japan) and Ikuko Wada (University of Minnesota, USA), along with Lehigh University Ph.D. student Gabe Epstein. The group plans to use thermal and thermodynamic modeling to map the movement of volatiles out of the subducting slab, investigating major factors affecting this recycling process. The work will yield more accurate estimates of the volatiles moving in and out of the subsurface at subduction zones.
Large quantities of volatile compounds enter the mantle through subduction, but some fraction returns to the surface through arc volcanoes lining the subduction zone. Volatiles also escape in the forearc region between the subducting crust and the volcano, and the backarc region on the other side. This recycling is an important factor that regulates global climate, the evolution of the atmosphere, and the creation of new crust. But despite its importance to numerous global cycles, a lot of uncertainty exists surrounding the magnitude of the flux of volatiles at subduction zones.
Bebout and Christenson started this project with Dave Hilton (Scripps Institution of Oceanography, University of California San Diego, USA) in September 2016, with funding from the National Science Foundation’s Petrology and Geochemistry Program. Sadly, Hilton passed away in January 2018. Sumino then stepped in to continue the noble gas analysis. The project builds on previous work in the Hikurangi margin region by scientists associated with the DCO and GeoPRISMS, an NSF-funded program to investigate geological processes that impact continental margins. Also, it builds on and benefits from the longer-term observations made at this margin by Christenson and other researchers at GNS Science.
To determine the volatiles entering the subduction zone, the researchers have sediment samples from the subducting ocean plate, collected during International Ocean Discovery Program Expedition 375. For outputs, they are taking measurements at about 70 strategically placed sites throughout New Zealand’s North Island, covering the arc volcanoes and the forearc and backarc regions on either side. They also are analyzing representative North Island crustal rock types to assess possible contributions of volatiles from the wall rocks, which line the margin. The researchers plan to complete their sampling by the end of spring in 2020.
Once the researchers arrive at their estimations of volatile recycling at the Hikurangi margin, they can expand those estimates to calculate fluxes at subduction zones worldwide. There are multiple factors, however, that can complicate these estimates. Bebout notes, “The ‘devil is in the details’ when considering single-margin to global extrapolation of the ways in which volatiles cycle during subduction.”
The work is ongoing, but the group looks forward to sharing their results with the DCO community. “DCO has provided a fantastic platform for discussion of all aspects of carbon flux on Earth and beyond, and certainly has provided an active forum for our research that we greatly appreciate,” said Bebout.
Main image: White Island is New Zealand’s most active volcano, with carbon dioxide emissions ranging between 500 and 3000 tonnes per day. Credit: Julian Thompson, GNS Science