Global climate model simulations indicate that the buildup of greenhouse gases i

Question

Global climate model simulations indicate that the buildup of greenhouse gases in the atmosphere will warm the surface of the ocean and increase its density stratification, slowing the rate at which surface waters are circulated into deeper levels (and vice versa).

(a) What is the direct effect of warming on the oxygen concentration and the pre-formed O2 component of the ocean? (1 Point)

(b) Assuming that NPP (Net Primary Production) and organic matter export would not change, how would AOU (Apparent Oxygen Utilization) change in response to the decrease in circulation rates? (1 Point)

(c) Now suppose that the biological export production decreased as a result of this circulation slowdown, due to less nutrient supply to the sunlit surface zone. How would this change affect AOU relative to your answer to (b)? (1 Point)

Explanation / Answer

ANSWER:

A) While mortality is a direct impact of oxygen loss, marine organisms can also be indirectly impacted leading to changes in behavior, reproduction, growth and behavior. Together, direct and indirect species-level impacts can lead to significant ecosystem-level consequences such as decreased resilience, stability, and resistance to other anthropogenic stressors such as fishing and pollution. Coastal waters are experiencing significant reductions in oxygen due to nutrient pollution, but these impacts are also exacerbated by climate change. Dead zones, areas where most organisms cannot live due to oxygen limitation, are now reported for more than 479 systems, and since the 1960s, their numbers have doubled approximately every decade (Diaz and Rosenberg 2008). Currently, hypoxia and anoxia are among the most widespread deleterious anthropogenic influences on estuarine and marine environments. As surface waters warm, these low oxygen coastal areas, and the animals that live within them, experience even more oxygen stress. These changes impact the people that depend on these waters for resources.

B) the corresponding large changes in AOU suggest that decadal changes in ocean circulation in the North Atlantic and/or changes in export production and remineralization of organic matter along the flow path are the primary causes of these anomalous DIC changes. The simultaneous measurements of DOC are particularly useful as the study of the oxidation of the exported DOC, its contribution to AOU and oxygen changes in concert with nutrient observations will provide information about the driving mechanisms. As we continue to process the physical and biogeochemical data from these cruises, we should be able to tease out the causes of the changes in DIC using TA, DOC and AOU measurements. The changes in pCO2 from the NOAA group (Feely, Wanninkhof) will also be useful in examining the changes in the carbon system in the Atlantic Ocean.

C) The cause of the oxygen inventory change is most likely the change in remineralization that occurs in the upper few hundred meters. We see from the upper 500 m of the phosphate remineralization rate profile in Figure 5.5.2c, that the low DOP simulation has more remineralization than the high DOP simulation. The high DOP simulation will thus have more oxygen in the main thermocline, and consequently, less of a vertical gradient to draw oxygen in from the surface ocean. The reduced oxygen input from the surface ocean is why the high DOP simulation has the highest AOU. The same arguments apply in reverse to the low DOP/low AOU simulations.

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