Deep ocean oxygen concentrations and efficiency of the biological pump

Oxygen forms a major component of our atmosphere (20%) and is produced by plant photosynthesis. Photosynthesis uses carbon dioxide, water and energy from sunlight to make organic carbon. Respiration and the decay of organic carbon consumes oxygen. The amount of dissolved oxygen in seawater is much smaller than the amount of oxygen in the atmosphere, and is related to temperature-controlled solubility and respiration of organic carbon. The distribution of dissolved oxygen in the oceans is not uniform. In deep waters (deeper than 1.5 km) the remineralization of organic carbon through respiration causes a decrease in dissolved oxygen concentrations from the North Atlantic Ocean (where deep waters are formed) en route to the Pacific Ocean. Warmer seawater temperatures, increased sinking of organic carbon produced in surface waters and respiration of this organic carbon, as well as a reduction in deep water formation, are all mechanisms that lead to a reduction in deep ocean dissolved oxygen concentrations. Several recent studies have suggested that global seawater dissolved oxygen concentrations will decline by 30% by the year 3000 as a result of global warming. Knowledge of natural variations of dissolved oxygen concentrations is therefore crucial to advance future predictions and an important research question. During ice ages (glacial times) there is mounting evidence that the oceans sequestered about 30% of the carbon dioxide that is (naturally) present during warm climates (interglacials) in our atmosphere. This carbon sequestration was caused by a variety of biological (respiration), physical (temperature, ocean circulation) and chemical (carbonate buffering through preservation and dissolution of calcium carbonate microfossils) mechanisms. The contribution of each of these mechanisms is poorly quantified, and we do not fully know where in the oceans the atmospheric carbon was stored. Production of organic material by photosynthesis, and respiration deeper down the water column will isolate carbon dioxide from the atmosphere for some time. An increase in organic material production and export and remineralization at depth (e.g. stronger biological pump) is one mechanism that is proposed to have acted as a carbon dioxide sequestration mechanism. As oxygen solubility is mainly temperature controlled, we can calculate the amount of total consumed oxygen (through remineralization of organic material) of a water mass by subtracting the initial saturated dissolved oxygen concentration from the measured dissolved oxygen concentration. From this we can then calculate the amount of sequestered carbon in that water mass. I have developed a novel technique to measure deep and intermediate ocean oxygen concentrations using benthic organisms (microfossils) obtained from deep sea sediment samples. For this fellowship I propose to generate historical (glacial-interglacial) records of deep and intermediate ocean oxygen concentration and test whether changes in atmospheric carbon dioxide are directly related to biological carbon sequestration. I am then going to use earth system models to test several hypothesis of why there was more biologically sequestered carbon in the glacial ocean.