Response of export production and dissolved oxygen concentrations in oxygen minimum zones to <i>p</i>CO<sub>2</sub> and temperature stabilization scenarios in the biogeochemical model HAMOCC 2.0

Abstract

Abstract. Dissolved oxygen (DO) concentration in the ocean is an important component of marine biogeochemical cycles and will be greatly altered as climate change persists. In this study a global oceanic carbon cycle model (HAMOCC 2.0) is used to address how mechanisms of oxygen minimum zone (OMZ) expansion respond to changes in CO 2 radiative forcing. Atmospheric pCO 2 is increased at a rate of 1 % annually and the model is stabilized at 2 ×, 4 ×, 6 ×, and 8 × preindustrial pCO 2 levels. With an increase in CO 2 radiative forcing, the OMZ in the Pacific Ocean is controlled largely by changes in particulate organic carbon (POC) export, resulting in increased remineralization and thus expanding the OMZs within the tropical Pacific Ocean. A potential decline in primary producers in the future as a result of environmental stress due to ocean warming and acidification could lead to a substantial reduction in POC export production, vertical POC flux, and thus increased DO concentration particularly in the Pacific Ocean at a depth of 600-800 m. In contrast, the vertical expansion of the OMZs within the Atlantic is linked to increases POC flux as well as changes in oxygen solubility with increasing seawater temperature. Changes in total organic carbon and increase sea surface temperature (SST) also lead to the formation of a new OMZ in the western subtropical Pacific Ocean. The development of the new OMZ results in dissolved oxygen concentration of ≤ 50 µmol kg −1 throughout the equatorial Pacific Ocean at 4 times preindustrial pCO 2 . Total ocean volume with dissolved oxygen concentrations of ≤ 50 µmol kg −1 increases by 2.4, 5.0, and 10.5 % for the 2 ×, 4 ×, and 8 × CO 2 simulations, respectively.

Enhanced Organic Carbon Burial in Sediments of Oxygen Minimum Zones Upon Ocean Deoxygenation

“…Results of several studies with general circulation models including marine biogeochemical processes suggest a significant expansion of OMZs in response to changes in either CO 2 -driven changes in phosphorus inputs from weathering, ocean circulation or oceanic temperatures on time scales of 10 to 100 kyrs. However, these models do not show evidence for full scale oceanic anoxia (Palastanga et al, 2011(Palastanga et al, , 2013Beaty et al, 2017;Niemeyer et al, 2017;Kemena et al, 2019).…”

“…In our model scenarios, we specifically focused on changes in oxygen demand in intermediate and bottom waters due to nutrient-driven increased export production. Recently, Beaty et al (2017) used the same model (PFe-model) to assess changes in oxygen supply over 10 kyrs due to increased radiative CO 2 forcing (2-8x pre-industrial pCO 2 levels), in scenarios where the associated temperature rise affected oxygen solubility, while keeping POC export at pre-industrial values. Comparison of our results for the first 10 kyrs of the scenario with a doubling of nutrients (2xPFe) with these scenarios from Beaty et al (2017) shows that only their most extreme scenario (8xCO2; i.e., an atmospheric pCO 2 of 2238.24 ppmv and change in sea water temperature of 11.5 C • ) results in a less-well oxygenated ocean than in our scenario (Figures 4A-D).…”

“…Recently, Beaty et al (2017) used the same model (PFe-model) to assess changes in oxygen supply over 10 kyrs due to increased radiative CO 2 forcing (2-8x pre-industrial pCO 2 levels), in scenarios where the associated temperature rise affected oxygen solubility, while keeping POC export at pre-industrial values. Comparison of our results for the first 10 kyrs of the scenario with a doubling of nutrients (2xPFe) with these scenarios from Beaty et al (2017) shows that only their most extreme scenario (8xCO2; i.e., an atmospheric pCO 2 of 2238.24 ppmv and change in sea water temperature of 11.5 C • ) results in a less-well oxygenated ocean than in our scenario (Figures 4A-D). This illustrates that while a reduced solubility due to warming of surface waters is a key driver of ocean deoxygenation (Bopp et al, 2002;Plattner et al, 2002;Keeling et al, 2009), changes in nutrient supply have the potential to become equally important on long time scales.…”

“…This suggests that bottom water concentrations in the ocean are particularly vulnerable to changes in oxygen supply. In a set of additional scenarios, FIGURE 4 | POC export production (A-C) and dissolved oxygen concentrations (D-F) after 10 kyrs for our scenario with a doubling of nutrients (2xPFe) and 2 scenarios of Beaty et al (2017): one with a 8x increase in CO 2 radiative forcing (8xCO2) and one with a reduction of 25% in the ocean ventilation (25%vent). Difference in oxygen concentrations (G,H) between our scenario and those of Beaty et al (2017).…”

“…In a set of additional scenarios, FIGURE 4 | POC export production (A-C) and dissolved oxygen concentrations (D-F) after 10 kyrs for our scenario with a doubling of nutrients (2xPFe) and 2 scenarios of Beaty et al (2017): one with a 8x increase in CO 2 radiative forcing (8xCO2) and one with a reduction of 25% in the ocean ventilation (25%vent). Difference in oxygen concentrations (G,H) between our scenario and those of Beaty et al (2017). Positive values indicate that changes in nutrient-driven oxygen demand are smaller than those in temperature-or ventilation-driven oxygen supply, hence allowing higher concentrations of oxygen to remain.…”