Opal and calcium carbonate are thought to regulate the biological pump’s transfer of organic carbon to the deep ocean. A global sediment trap database exhibits large regional variations in the organic carbon flux associated with opal flux. These variations are well-explained by upper ocean silica concentrations, with high opal \textquoteleft ballasting’ in the silica-deplete tropical Atlantic Ocean, and low ballasting in the silica-rich Southern Ocean. A plausible, testable hypothesis is that opal ballasting is due to mineral protection, and varies because diatoms grow thicker frustules where silica concentrations are higher, protecting less organic carbon per unit opal. These patterns do not emerge in an advanced ocean biogeochemical model when opal ballasting is represented using a single global parameterization for diatoms, indicating the need for additional parameterization of the dependence of diatoms traits on silica concentration to capture the links between elemental cycles and future changes in the biological pump.

We assess the Southern Ocean CO2 uptake (1985-2018) using data sets gathered in the REgional Carbon Cycle Assessment and Processes Project phase 2 (RECCAP2). The Southern Ocean acted as a sink for CO2 with close agreement between simulation results from global ocean biogeochemistry models (GOBMs, 0.75±0.28 PgCyr-1) and pCO2-observation-based products (0.73±0.07 PgCyr-1). This sink is only half that reported by RECCAP1. The present-day net uptake is to first order a response to rising atmospheric CO2, driving large amounts of anthropogenic CO2 (Cant) into the ocean, thereby overcompensating the loss of natural CO2 to the atmosphere. An apparent knowledge gap is the increase of the sink since 2000, with pCO2-products suggesting a growth that is more than twice as strong and uncertain as that of GOBMs (0.26±0.06 and 0.11±0.03 PgCyr-1 decade-1 respectively). This is despite nearly identical pCO2 trends in GOBMs and pCO2-products when both products are compared only at the locations where pCO2 was measured. Seasonal analyses revealed agreement in driving processes in winter with uncertainty in the magnitude of outgassing, whereas discrepancies are more fundamental in summer, when GOBMs exhibit difficulties in simulating the effects of the non-thermal processes of biology and mixing/circulation. Ocean interior accumulation of Cant points to an underestimate of Cant uptake and storage in GOBMs. Future work needs to link surface fluxes and interior ocean transport, build long overdue systematic observation networks and push towards better process understanding of drivers of the carbon cycle.

The coastal ocean contributes to regulating atmospheric greenhouse gas concentrations by taking up carbon dioxide (CO2) and releasing nitrous oxide (N2O) and methane (CH4). Major advances have improved our understanding of the coastal air-sea exchanges of these three gasses since the first phase of the Regional Carbon Cycle Assessment and Processes (RECCAP in 2013), but a comprehensive view that integrates the three gasses at the global scale is still lacking. In this second phase (RECCAP2), we quantify global coastal ocean fluxes of CO2, N2O and CH4 using an ensemble of global gap-filled observation-based products and ocean biogeochemical models. The global coastal ocean is a net sink of CO2 in both observational products and models, but the magnitude of the median net global coastal uptake is ~60% larger in models (-0.72 vs. -0.44 PgC/yr, 1998-2018, coastal ocean area of 77 million km2). We attribute most of this model-product difference to the seasonality in sea surface CO2 partial pressure at mid- and high-latitudes, where models simulate stronger winter CO2 uptake. The global coastal ocean is a major source of N2O (+0.70 PgCO2-e /yr in observational product and +0.54 PgCO2-e /yr in model median) and of CH4 (+0.21 PgCO2-e /yr in observational product), which offsets a substantial proportion of the net radiative effect of coastal \co uptake (35-58% in CO2-equivalents). Data products and models need improvement to better resolve the spatio-temporal variability and long term trends in CO2, N2O and CH4 in the global coastal ocean.

The Southern Ocean plays a critical role in the uptake, transport and storage of carbon by the global oceans. It is the ocean’s largest sink of CO2, yet it is also among the regions with the lowest storage of anthropogenic carbon. This behaviour results from the unique combination of high winds driving the upwelling of deep waters and the subduction and northwards transport of surface carbon. Here we identify the indirect effect of climate-related changes in ocean conditions relative to the direct effect of anthropogenic changes in atmospheric CO2 on the reorganisation of carbon in the Southern Ocean using a combination of modelling and observations. We show that the effect of climate variability and climate change on the storage of carbon in the Southern Ocean is nearly as large as the effect of anthropogenic CO2 during the period 1998-2018 compared with a climatology around the year 1995. We identify a distinct climate fingerprint in dissolved inorganic carbon (DIC), with elevated DIC concentration in the surface ocean that reinforces the anthropogenic CO2 signal, and reduced DIC concentration in the subsurface ocean that offsets the anthropogenic CO2 signal. The fingerprint is strongest at lower latitudes (30°S-55°S). This fingerprint could serve to monitor the highly uncertain evolution of carbon within this critical ocean basin, and better identify its drivers.