Ocean ventilation and deoxygenation in a warming world

For over 50 years ocean scientists have oddly represented ocean oxygen consumption rates as a function of depth but not temperature. This unique tradition or tactic now extends across such a wide range of oceanic biogeochemical processes that it inhibits useful discussion of climate change impacts where specific and fundamental temperature dependent terms are required. Depth, but not temperature, dependent functions as of now form the basis for the most widely used climate-biogeochemical models. Tracer based determinations of oxygen consumption rates, and thus CO2, PO4 etc production rates, in the deep sea are near universally reported as a function of depth in spite of their well-known microbial basis. In recent work we have shown that a carefully determined profile of oxygen consumption rates in the Sargasso Sea can be well represented by a classical Arrhenius function with and activation energy of 86.5 kJ mol-1 leading to a Q10 of 3.63. This indicates that for 2°C warming we will have a 29% increase in ocean oxygen consumption rates, and for 3°C warming a 50% increase, leading to large scale ocean hypoxia. Here we show that the same principles apply to a world-wide collation of tracer based oxygen consumption rate data and that some 95% of ocean oxygen consumption is driven by temperature, not depth, and thus has a strong climate dependence. The Arrhenius/Eyring equations are no simple panacea and they require a non-equilibrium steady state to exist. Where transient events are in progress this stricture is not obeyed and we show one such example. This rapid injection of fresh organic material and its associated microbial population is still clearly in non-steady state and revealed as such in the observed oxygen consumption rate data.

Dissolved oxygen concentration is a major determinant of the microbially mediated processes responsible for the production and turnover of climatically relevant gases in the ocean. Ocean models predict declines of 1 to 7% in the global ocean O2 inventory over the next century due to lower solubility of oxygen at warmer temperatures, and increased stratification preventing equilibration of the ocean interior with the atmosphere. Additionally, eutrophication continues to lead to hypoxic or anoxic conditions in the coastal zone.

Due to microbial respiration, water separated from equilibration with the atmosphere by increased stratification will contain both low O2 and high CO2 concentrations. Low oxygen environments enable the production of nitrous oxide as a by-product of nitrification and denitrification and methane through anaerobic methanogenesis, with most O2 deficient systems acting as net sources of CO2, N2O and CH4 to the atmosphere.

This presentation will provide an overview of the effect of ocean deoxygenation on microbial community structure and the cycling of climatically important gases including CO2, N2O and CH4.

The major biogeochemical cycles which keep the present-day Earth habitable are linked by a network of feedbacks which has led to a broadly stable chemical composition of the oceans and atmosphere over hundreds of millions of years. This includes the processes which control both the atmospheric and oceanic concentrations of oxygen. However, one notable exception to the generally well-behaved dynamics of this system is the propensity for episodes of ocean anoxia to occur and to persist for 105 – 106 years, these OAEs (Ocean Anoxic Events) being particularly associated with warm “greenhouse” climates.  OAEs are, we believe, amplified by positive feedbacks on the nutrient content of the ocean: low oxygen promotes the release of phosphorus from ocean sediments, which increases ocean productivity and drives more anoxia in the subsurface water, leading to a potentially self-sustaining condition of deoxygenation.  The rapidly increasing degree of ocean deoxygenation occurring today as a result of the warming climate could conceivably result in such a very long-lasting and unpleasant consequence for the Earth’s biogeochemical cycles which underpin the “life support” system of the biosphere.

Major deoxygenation commonly takes place in ocean regions that feature particularly intense coastal upwelling circulations. Prominent examples discussed herein include the seasonal Somali Current upwelling, which during the southwest monsoon becomes the most intense coastal upwelling cell existing in the world’s oceans, and the upwelling that occurs on a more continuous basis in the ocean off Lüderitz, Namibia, constituting the most intense of the world’s “classical” eastern ocean boundary upwelling systems. Concerns about enhanced ocean deoxygenation arise in view of the arguable likelihood that coastal upwelling systems around the world may further intensify as anthropogenic climate change proceeds.

Another suggestive example is the sudden seasonal appearance, just a decade and a half ago of an oxygen deficient “dead zone” that has since become quite a reliable annually recurring phenomena off the U.S. State of Oregon. This apparent “transience” in the causal dynamics is somewhat mirrored in an observed interannual–scale intermittence in the situation off Namibia.

A mechanistic scheme that may draw these examples into a common context is presented. The strength of horizontal surface flow divergence that drives the upwelling circulation, the pattern of horizontal flow vorticity and the potentially manageable abundance of strongly swimming small pelagic fish appear in this scheme as plausible controlling variables.

Secular decreases in dissolved oxygen concentration have been observed within the tropical Oxygen Minimum Zones (OMZs) and at mid- to high latitudes over the last ~ 50 years.  Whilst a pervasive feature of recent Earth System Model (ESM) projections is a reduction in the ocean’s oxygen inventory in response to anthropogenic warming, current models are unable to consistently reproduce observed trends and variability, particularly within the OMZs.  Here we conduct a series of targeted hindcast experiments using a state-of-the-art global ecosystem model (PlankTOM10-NEMO) including idealised model experiments forced with observationally derived oxygen trends (1960 – 2010) in order to explore and review biases in model distributions of oceanic oxygen.  Using a model representation of marine nitrous oxide (N2O) cycling we investigate the implications of simulated changes in oxygen for historical emissions of N2O.  Despite occupying less than 10 % of the ocean by volume, hypoxic and suboxic waters account for ~ 25–50 % of open ocean N2O production as a consequence of both enhanced denitrification and improved N2O yields from nitrification under low-O2 conditions.  We show that the inability of models to reproduce historical shoaling and expansion of established tropical OMZs causes major discrepancies in estimates of oceanic N2O production.  Using historical oxygen fields from a range of Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations we also assess recent past and projected changes in marine N2O from current generation ESMs.

From an Earth system (ES) perspective, ocean feedbacks on the global carbon cycle (and, thereby, climate system) are of particular interest. While the physical ocean feedbacks in the climate system are relatively well constrained, the biologically mediated feedbacks remain poorly understood. Here, the current understanding of these biological feedbacks is examined with a focus on identifying areas in urgent need of further study. The temperature sensitivity of bacterial respiration in the surface layer of the ocean is reasonably well quantified and substantial changes in the distributions of oxygen and nutrient turnover in the ocean are projected in response to changes in bacterial activity in a warming world, leading to significant changes in the surface ocean-atmosphere CO2 flux. Biologically-mediated surface to deep ocean carbon flux in the form of particulate organic carbon (POC), i.e., the “biological pump”, is also critically important for the global carbon cycle. It is often assumed on the basis of model studies that the magnitude of POC produced in surface waters will decrease in a warmer ocean as increased stratification will reduce the delivery of nutrients to the surface layer and, thereby, reduce phytoplankton photosynthesis, i.e., the primary source of POC production. This assumption warrants re-examination for several reasons. 1) increased bacterial activity in the surface layer will increase the regeneration of inorganic nutrients in this layer; 2) our limited understanding of the vertical distribution of ocean photosynthesis and 3) limited understanding of the relationship between different plankton ecosystem types and the efficiency of the biological pump.