Project: Time-Series Measurements of the 13C/12C of Dissolved Inorganic Carbon

NSF Award Abstract:
The 13C/12C isotopic ratio of dissolved inorganic carbon (DIC) has been shown to be a very useful tracer of anthropogenic CO2 uptake in the ocean (Quay et al., 1992; Heimann and Maier-Reimer, 1996; Sonnerup et al., 2000; Quay et al., 2003). Seasonal changes in the d13C of DIC, coupled with corresponding changes in DIC concentration and pCO2, have been used to close the surface ocean.s carbon budget (Zhang and Quay, 1997; Gruber et al., 1998, Quay and Stutsman, in press). Time- series measurements of d13C, DIC and pCO2, therefore, allow one to separate biological from physical causes for interannual variations in the rate of oceanic CO2 uptake as Gruber et al. (2002) recently demonstrated at BATS. Despite these advantages, there are only two sites in the subtropical N. Atlantic (BATS) and N. Pacific (HOT) oceans where such records exist. This lack of ocean time series records has severely limited our ability to understand the causes of interannual variations in the ocean uptake of anthropogenic CO2 (Quay, 2002). In stark contrast, continuous records of atmospheric CO2 and d13C are being measured at over 100 sites.

In this project, researchers at the University of Washington will initiate monthly d13C measurements at a third time-series site (ESTOC) in the eastern subtropical N. Atlantic. The d13C record at ESTOC, which will complement on-going measurements of DIC, pCO2 and alkalinity at the site, offers a very useful comparison to the BATS d13C record. Gruber et al. (2002) concluded that interannual variations in CO2 uptake at Bermuda correlated strongly with sea surface temperature (SST) and the North Atlantic Oscillation (NAO) index of atmospheric circulation. They used their d13C record at BATS to conclude that interannual variations in the rate of net community production (NCP) correlated with NAO. The researchers of this project intend to use the proposed d13C measurements at ESTOC to calculate NCP and determine whether interannual variations in the eastern subtropical N. Atlantic correlate with variations at Bermuda.

The research team will also continue its program of d13C measurements at HOT. Our decade-long d13C record at HOT shows that the d13C decrease rate in the surface ocean has doubled since 1995. The DIC increase rate has tripled since 1995. However, this apparent acceleration of anthropogenic CO2 uptake and d13C decrease occurred during a period (post 1998) when salinity is the highest ever measured at HOT and summertime SST has decreased significantly. These dramatic changes at HOT correlate with a shift in the Pacific Decadal Oscillation (PDO) climate index in 1998 from positive (since the late 1970s) to negative. This correlation suggests that changes in physical forcing (e.g., thermocline depth, mixed layer depth, gyre circulation rates) in the N. Pacific may have changed the subtropical ocean.s carbon budget. If so, the situation at HOT may be similar to that found by Gruber et al. (2002) at Bermuda. The team intends to use d13C measurements at HOT to determine whether the accelerated DIC increase is a result of changes in the NCP rate at HOT.

The proposed research addresses a major societal issue, that is, how natural variability affects the ocean.s uptake of anthropogenically produced CO2. The largest single human-controlled factor in future climate change is the production of CO2 from fossil fuel combustion and deforestation. The research is expected to yield an ocean d13C data set that will be made available to the broad scientific community and serve as a useful validation test for models predicting future atmospheric CO2 concentrations. The proposed research addresses one of the specific goals of the US Carbon Cycle Science Plan (1999), that is, to better quantify and understand the uptake of anthropogenic CO2 in the oceans. The proposed work enhances infrastructure for research and education in two ways. It establishes collaboration with scientists at the Universidad de Las Palmas in the Grand Canary Islands studying the ocean's carbon cycle. It tests equipment that could be broadly used by the oceanographic community to remotely collect seawater samples for carbon analysis.