Time Series of Surface Water Dissolved Inorganic Carbon Isotopes from the Southern California Bight

Abstract

Dissolved inorganic carbon (DIC) in ocean water is a major sink of fossil fuel derived CO₂. Carbon isotopes in DIC serve as tracers for oceanic water masses, biogeochemical processes, and air-sea gas exchange. We present a timeseries of surface DIC δ¹³C and Δ¹⁴C values from 2011 to 2022 from Newport Beach, California. This is a continuation of previous timeseries (Hinger et al. 2010; Santos et al. 2011) that together provide an 18-year record. These data show that DIC Δ¹⁴C values have declined by 42‰ and that DIC δ¹³C values have declined by 0.4‰ since 2004. By 2020, DIC Δ¹⁴C values were within analytical error of nearby clean atmospheric CO₂ Δ¹⁴C values. These long-term trends are likely the result of significant fossil fuel derived CO₂ in surface DIC from air-sea gas exchange. Seasonally, Δ¹⁴C values varied by 3.4‰ between 2011 and 2022, where seasonal δ¹³C values varied by 0.7‰. The seasonal variation in Δ¹⁴C values is likely driven by variations in upwelling, surface eddies, and mixed layer depth. The variation in δ¹³C values appears to be driven by isotopic fractionation from marine primary producers. The DIC δ¹³C and Δ¹⁴C values record the influence of the drought that began in 2012, and a major upwelling event in 2016.

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