Mixed‐layer carbon cycling at the Kuroshio Extension Observatory

Abstract

Seven years of data from the NOAA Kuroshio Extension Observatory (KEO) surface mooring, located in the North Pacific Ocean carbon sink region, were used to evaluate drivers of mixed‐layer carbon cycling. A time‐dependent mass balance approach relying on two carbon tracers was used to diagnostically evaluate how surface ocean processes influence mixed‐layer carbon concentrations over the annual cycle. Results indicate that the annual physical carbon input is predominantly balanced by biological carbon uptake during the intense spring bloom. Net annual gas exchange that adds carbon to the mixed layer and the opposing influence of net precipitation that dilutes carbon concentrations make up smaller contributions to the annual mixed‐layer carbon budget. Decomposing the biological term into annual net community production (aNCP) and calcium carbonate production (aCaCO3) yields 7 ± 3 mol C m−2 yr−1 aNCP and 0.5 ± 0.3 mol C m−2 yr−1 aCaCO3, giving an annually integrated particulate inorganic carbon to particulate organic carbon production ratio of 0.07 ± 0.05, as a lower limit. Although we find that vertical physical processes dominate carbon input to the mixed layer at KEO, it remains unclear how horizontal features, such as eddies, influence carbon production and export by altering nutrient supply as well as the depth of winter ventilation. Further research evaluating linkages between Kuroshio Extension jet instabilities, eddy activity, and nutrient supply mechanisms is needed to adequately characterize the drivers and sensitivities of carbon cycling near KEO.

Variability and trends in surface seawater pCO2 and CO2 flux in the Pacific Ocean

“…Seasonal patterns of DIC and SSS are similar at KEO, highlighting the influence of seasonal evaporation and precipitation on sea surface DIC in this region (Figure a). A mixed layer budget developed at KEO also reveals these seasonal patterns [ Fassbender et al , ]. Although wind speed at KEO follows the same seasonal patterns as WHOTS and Stratus, wind speeds are higher and more variable through the entire year compared to the other subtropical sites (Figure S1).…”

“…Seawater p CO 2 , SST, and SSS measurements are also used to calculate dissolved inorganic carbon (DIC) and salinity normalized DIC (nDIC). First, total alkalinity (TA) is calculated based on the relationships with SST and/or SSS developed by Lee et al [] for WHOTS and Stratus (uncertainty ±8 μmol kg −1 ), by Fassbender et al [] for Papa (uncertainty ±3 μmol kg −1 ), and by Fassbender et al [] for KEO (uncertainty ±6 μmol kg −1 ). Seawater p CO 2 and TA are then used to calculate DIC in the MATLAB version (v1.1) of CO2SYS [ van Heuven et al , ] with the carbonic acid dissociation constants of Lueker et al [], sulfate dissociation constants of Dickson [], and borate‐to‐salinity ratio of Lee et al [].…”

Observing Changes in Ocean Carbonate Chemistry: Our Autonomous Future

“…Empirical algorithms relating TA to commonly measured surface variables have allowed the use of single carbonate system parameters such as pCO 2 to yield new understanding of surface carbonate chemistry. For example, in a pair of papers, Fassbender et al [19,20] used mooring observations from Ocean Station Papa (Gulf of Alaska) and the Kuroshio Extension to decompose surface biological production into its organic and inorganic components. Similarly, the equatorial pCO 2 observations used to characterize the relationship between pCO 2 and El Niño/La Niña were combined with an algorithm estimate for TA to determine that pH in the region was more variable and changing faster than expected [35].…”

“…For example, pCO 2 is required to study the air-sea CO 2 flux, as the difference between air and seawater pCO 2 describes the thermodynamic potential for CO 2 to go in or out of the water [18]. DIC is particularly useful for studying production and respiration dynamics in the open ocean [19,20]. TA can be used to study calcification and dissolution processes.…”

Seasonal Asymmetry in the Evolution of Surface Ocean pCO2 and pH Thermodynamic Drivers and the Influence on Sea‐Air CO2 Flux

“…This analysis also relies on climatologies developed from long‐term observations made at select time series sites, including the Kuroshio Extension Observatory (KEO 32.3°N, 144.5°E; https://www.nodc.noaa.gov/ocads/oceans/Moorings/KEO.html; Fassbender, Sabine, Cronin, & Sutton, ), Ocean Station Papa (OSP 50°N, 145°W; https://www.nodc.noaa.gov/ocads/oceans/Moorings/Papa_145W_50N.html; Fassbender et al, ), Bermuda Atlantic Time Series Study (BATS 32°N, 64°W; http://bats.bios.edu/), Hawaii Ocean Time Series (HOT 22.75°N, 158°W; http://www.soest.hawaii.edu/HOT_WOCE/index.php), Irminger Sea (64.3°N, 28°W; https://www.nodc.noaa.gov/ocads/oceans/Moorings/Irminger_Sea.html), Iceland Sea (68°N, 12.66°W; http://cdiac.ornl.gov/ftp/oceans/CARINA/IcelandSea//IcelandSea_V2/), and two regions in the Southern Ocean that lie north and south of the Antarctic Polar Front in Drake Passage (DP N 57°S, 64°W; DP S 61.5°S, 62°W; https://www.nodc.noaa.gov/ocads/oceans/VOS_Program/LM_gould.html; Munro et al, ). Data from these sites are incorporated due to their high‐quality and because some of the associated 3° × 3° SOCAT‐v4 grids do not have fully resolved 12‐month climatologies due to data scarcity, particularly in the high latitudes (e.g., Irminger and Iceland seas).…”