A regression modeling approach for studying carbonate system variability in the northern Gulf of Alaska

The northern Gulf of Alaska (GOA) shelf experiences carbonate system variability on seasonal and annual time scales, but little information exists to resolve higher frequency variability in this region. To resolve this variability using platforms-of-opportunity, we present multiple linear regression (MLR) models constructed from hydrographic data collected along the Northeast Pacific Global Ocean Ecosystems Dynamics (GLOBEC) Seward Line. The empirical algorithms predict dissolved inorganic carbon (DIC) and total alkalinity (TA) using observations of nitrate (NO₃⁻), temperature, salinity and pressure from the surface to 500 m, with R²s > 0.97 and RMSE values of 11 µmol kg⁻¹ for DIC and 9 µmol kg⁻¹ for TA. We applied these relationships to high-resolution NO₃⁻ data sets collected during a novel 20 h glider flight and a GLOBEC mesoscale SeaSoar survey. Results from the glider flight demonstrated time/space along-isopycnal variability of aragonite saturations (Ω_arag) associated with a dicothermal layer (a cold near-surface layer found in high latitude oceans) that rivaled changes seen vertically through the thermocline. The SeaSoar survey captured the uplift to <100 m of dense, high-pCO₂ waters at the shelf break that had been forced by the passage of a Yakutat eddy. During this event, the aragonite saturation horizon (depth where Ω_arag = 1) shoaled to a previously unseen depth in the northern GOA. This work is similar to recent studies aimed at predicting the carbonate system in continental margin settings, albeit demonstrates that a NO₃⁻-based approach can be applied to high-latitude data collected from platforms capable of high-frequency measurements.

Incorporation of uranium in benthic foraminiferal calcite reflects seawater carbonate ion concentration

The chemical and isotopic composition of foraminiferal shells (so-called proxies) reflects the physicochemical properties of the seawater. In current day paleoclimate research, the reconstruction of past seawater carbonate system to infer atmospheric CO₂ concentrations is one of the most pressing challenges, and a variety of proxies have been investigated, such as foraminiferal U/Ca. Since in natural seawater and traditional CO₂ perturbation experiments the carbonate system parameters covary, it is not possible to determine the parameter of the carbonate system causing, e.g., changes in U/Ca, complicating the use of the latter as a carbonate system proxy. We overcome this problem by culturing the benthic foraminifer Ammonia sp. at a range of carbonate chemistry manipulation treatments. Shell U/Ca values were determined to test sensitivity of U incorporation to various parameters of the carbonate system. We argue that [CO32-] is the parameter affecting the U/Ca ratio and consequently, the partitioning coefficient for U in Ammonia sp., D_U. We can confirm the strong potential of foraminiferal U/Ca as a [CO32-] proxy.