UC San Diego

The 13C/12C ratio of stable carbon isotopes in dissolved inorganic carbon (DIC) can provide insight into carbon cycle variability and trends in the surface ocean. Measurements of δ13C-DIC when combined with DIC can be used to estimate anthropogenic carbon uptake and marine productivity and can contribute to our understanding of the role that the ocean plays in the global carbon cycle. This dissertation describes surface ocean time-series measurements near Bermuda (S-BATS) and Hawaii (HOT) of δ13C-DIC, DIC, and alkalinity (ALK) as a part of the Scripps Seawater Program, focusing on δ13C-DIC. Prior to this study, there was a hiatus in seawater δ13C-DIC measurements at Scripps, while samples continued to be collected and archived for future analysis. This dissertation details the resumption of these measurements along with the calibration and methodology used for Scripps measurements of δ13C-DIC. We quantify the calibration contributions to uncertainty in the context of consistency between newer and older measurements. In addition, we introduce and document the stability of three new seawater secondary standards, including a method based on CO2 in compressed N2 gas.

In these time-series records of nearly three decades, we show that the long-term trends and seasonal cycles in sDIC and δ13C-DIC at both S-BATS and HOT are consistent with earlier studies and independent time-series records. From the full record we find no long-term changes in the seasonal cycle of sDIC, δ13C-DIC, or computed pCO2. Consistent to some earlier studies we find significant correlations at S-BATS with the North Atlantic Oscillation and at HOT with the Pacific Decadal Oscillation (PDO). We also find correlations at HOT that were not previously noted including the PDO with sea surface temperature and mixed layer depth (MLD), as well as El Nino Southern Oscillation (NINO3.4) with sDIC and MLD. Lastly, we use a combination of observations and CESM hindcast simulations to explore upper ocean carbon variability in subtropical gyres. This allows us to examine the time-series in a broader geographic context, showing coherent patterns of variability across the North Atlantic and North Pacific subtropical gyres and illustrating key differences in the controls of variability at BATS and HOT.