Uncertain response of ocean biological carbon export in a changing world

Abstract

The transfer of organic carbon from the upper to the deep ocean by particulate export flux is the starting point for the long term storage of photosynthetically-fixed carbon. This "biological carbon pump" is a critical component of the global carbon cycle, reducing atmospheric CO2 levels by ~ 200 ppm relative to a world without export flux. This carbon flux also fuels the productivity of the mesopelagic zone, including significant fisheries. Here we show that, despite its importance for understanding future ocean carbon cycling, that Earth System Models disagree on the projected response of the global export flux to climate change, with estimates ranging from -41% to +1.8%. Fundamental constraints to understanding export flux arise because a myriad of interconnected processes make the biological carbon pump challenging to both observe and model. Our synthesis prioritises the processes likely to be most important to include in modern-day estimates (particle fragmentation and zooplankton vertical migration) and future projections (phytoplankton and particle size spectra, and temperature-dependent remineralization) of export. We also identify the observations required to achieve more robust characterisation, and hence improved model parameterization, of export flux, and thus reduce uncertainties in current and future estimates in the overall cycling of carbon in the ocean. Main text:Biological activity in the upper ocean takes up 50-60 GtC from the atmosphere annually, of which ~ 10% sinks out of the surface ocean 1 . This 'exported' carbon fuels the biological carbon pump and hence plays a central role in storing carbon in the ocean on climatically-relevant timescales 2 . Because of the complexity of the

Revisiting five decades of <sup>234</sup>Th data: a comprehensive global oceanic compilation

“…7a shows at a glance the number of data points in the compilation that could be used to evaluate processes in the upper ocean such as export flux and export efficiency (e.g., Buesseler et al, 2020b), scavenging rates of trace metals (e.g., Black et al, 2018;Lemaitre et al, 2020), or particle sinking velocities (e.g., by using "deficit" ratios, as well as those that could be used to study processes such as particle rem-E. Ceballos-Romero et al: Revisiting five decades of 234 Th data ineralizations (e.g., Usbeck et al, 2002) by using the "excess" ratios. Recent studies have highlighted the role that disaggregation and fragmentation could play in setting the magnitude of flux attenuation (Baker et al, 2017;Briggs et al, 2020;Cavan et al, 2017) and pointed to it as the most important currently unaccounted for process for improving modern-day export flux simulations (Henson et al, 2021). 234 Th excess could be useful to study such mechanisms by detecting low-sinking (i.e., < 20 m d −1 ) POC flux below the mixed layer depth (Baker et al, 2017) by high vertical resolution sampling of 234 Th concentrations and POC: 234 Th ratios.…”