Carbonate Dissolution in the North Pacific Ocean: Results of C-DisK-IV; a Cruise Between Hawaii and Alaska to Characterize Carbonate Producers, Export and Dissolution Kinetics in an Acidifying Ocean

We set out on a N. Pacific research cruise to establish the rate at which inorganic calcite, aragonite and coccolithophore calcite dissolve. Along the Hawaii to Alaska transect we visited 7 stations and made >70 dissolution rate measurements in situ, with a novel Niskin-Incubator device hung on a floating wire and also on the ship’s hydrowire; these determinations were made at depths from 50-2000 m. Dissolution rates for calcite (3e-5 to 1.3e-3 g/g day), aragonite (6e-4 to 7e-3) and coccolithophores (9e-4 to 4e-3) show non-linear rate vs. 1-omega relationships. This non-linear behavior confirms observations obtained on the lab-bench (Subhas et al. 2015). The highest dissolution rates occurred at lowest omega values: 0.55 (aragonite), 0.7 (calcite). Undersaturation caused by two processes (1) oxygen respiration and (2) pressure (Ksp) do not appear to drive different dissolution responses. The slow dissolution rate (0.7%/day) for an aragonite grain formed in the lab (with a high surface area) does not help explain the excess alkalinity observation of Feely et al (2002). Carbonic anhydrase associated with settling and respiring material was measured and shown to be present throughout the water column; dissolution enhanced by this enzyme is proposed as an important contributor to ocean alkalinity. The abundance of pteropods, coccolithophores and forams was measured in surface and subsurface samples. Pteropods make up a significant portion of the upper ocean carbonate inventory, at all stations. Sediment traps captured flux at 100 and 200 m to help constrain the flux of and the aragonite/calcite ratio of sinking material. Surface water analyses of O2/Ar and laser-based particle-type analyses highlight long and short-scale spatial variability in productivity but with a clear transition occurring between 30-40°N. Sediment core solid-phase chemistry and pore water analyses of dissolved Si and nitrate help record the translation of particle production and sinking to a record of accumulation and benthic diagenesis. At many locations, pteropods are found to reach the sediment. A hydrographic survey included measurements of DIC, Alk, pH, del13DIC, oxygen and nutrients. These data suggest a benthic source of alkalinity