Temperature‐dependent remineralization in a warming ocean increases surface pCO₂ through changes in marine ecosystem composition

Abstract

[1] Temperature-dependent remineralization of organic matter is, in general, not included in marine biogeochemistry models currently used for Coupled Model Intercomparison Project Phase 5 (CMIP5) climate projections. Associated feedbacks have therefore not been quantified. In this study we aim at investigating how temperature-dependent remineralization rates (Q 10 = 2) in a warming ocean impact on the marine carbon cycle, and if this may weaken the oceanic sink for anthropogenic CO 2 . We perturb an Earth system model used for CMIP5 with temperature-dependent remineralization rates of organic matter using representative concentration pathway (RCP)8.5-derived temperature anomalies for 2100. The result is a modest change of organic carbon export but also derived effects associated with feedback processes between changed nutrient concentrations and ecosystem structure. As more nutrients are recycled in the euphotic layer, increased primary production causes a depletion of silicate in the surface layer as opal is exported to depth more efficiently than particulate organic carbon. Shifts in the ecosystem occur as diatoms find less favorable conditions. Export production of calcite shells increases causing a decrease in alkalinity and higher surface pCO 2 . With regard to future climate projections, the results indicate a reduction of oceanic uptake of anthropogenic CO 2 of about 0.2 PgC yr À1 toward the end of the 21st century in addition to reductions caused by already identified climate-carbon cycle feedbacks. Similar shifts in the ecosystem as identified here, but driven by external forcing, have been proposed to drive glacial/interglacial changes in atmospheric pCO 2 . We propose a similar positive feedback between climate perturbations and the global carbon cycle but driven solely by internal biogeochemical processes.Citation: Segschneider, J., and J. Bendtsen (2013), Temperature-dependent remineralization in a warming ocean increases surface pCO 2 through changes in marine ecosystem composition, Global Biogeochem.

Burial-nutrient feedbacks amplify the sensitivity of atmospheric carbon dioxide to changes in organic matter remineralisation

“…Our model does not include a comprehensive ecosystem model, and may underestimate the shift in ecosystem structures due to the nutrient drawdown as induced by the slowed POM remineralisation. For example, Segschneider and Bendtsen (2013) find an opposite effect of nutrient (i.e. nitrate) depletion on surface Alk using a more complex biogeochemistry model.…”

“…Kwon et al (2009) simulate a CO 2 drawdown by 10 and 27 ppm for an increase in the e-folding depth of POM remineralisation of 24 m in their nutrientrestoring and constant export-production model setups. Recently, Segschneider and Bendtsen (2013) estimated a reduction of anthropogenic CO 2 uptake rates of 0.2 gigatons of carbon (Gt C) per year by AD 2100 in future emission scenarios. Taucher et al (2014) isolate the influence of viscosity changes in a global warming simulation and found that ocean carbon uptake is 17 % higher when considering temperaturedriven viscosity changes on particle sinking speed compared to a control.…”

“…Taucher et al (2014) isolate the influence of viscosity changes in a global warming simulation and found that ocean carbon uptake is 17 % higher when considering temperaturedriven viscosity changes on particle sinking speed compared to a control. As a caveat, these studies either focus on the decadal-to century-scale response (Segschneider and Bendtsen, 2013) or neglect ocean-sediment interactions and the weathering-burial cycle.…”

Simulation of climate, ice sheets and CO<sub>2</sub> evolution during the last four glacial cycles with an Earth system model of intermediate complexity

“…, where remineralisation depth z r is held constant equal to 100 m. To take into account the dependence of remineralisation rate on ambient temperature, following Segschneider and Bendtsen (2013), we now use the dependence of z r on the thermocline temperature (300 m) T :…”

Ideas and perspectives: climate-relevant marine biologically driven mechanisms in Earth system models

“…A rise in temperature and a drop in pH stimulates bacterial turnover rates (Pomeroy and Deibel, 1986;Piontek et al, 2010). With enhanced remineralization, the efficiency of the organic carbon pump will be reduced, altering the ocean's carbon uptake capacity (Segschneider and Bendtsen, 2013). On the other hand, bacterial decomposition rates may be affected through a decrease in oxygen concentrations with an expansion of oxygen minimum zones (Stramma et al, 2008).…”