Ocean carbon dynamics associated with natural and anthropogenic climate change

Abstract

The ocean dissolves a large amount of the anthropogenic CO2 that is emitted by combustion of fossil fuel, and may thereby moderate future climate change. However, acidification caused by dissolved CO2 affects the carbonate system and significant effects marine biogeochemical cycles and ecosystems. Several modeling studies have assessed possible change in ocean chemistry under various CO2 concentration and emission scenarios. To be accurate, projections of oceanic carbonate chemistry should be made with understanding of natural variability in the oceanic carbonate system. Limited observations indicate that the carbonate chemistry of the surface ocean is significantly influenced by global and regional climate changes, but the effect has not been well constrained. Because of complex interplay among physical and biogeochemical processes, the ocean carbonate system varies substantially among environments and on various time scales. Understanding of how the ocean carbonate system will react to continued increase in anthropogenic CO2 requires research on the influence of climate change on each process. The goal of this thesis is to improve our understanding of climate variability and how it affects the oceanic carbonate system. I conducted research that disclosed a link between carbonate chemistry and large-scale climatic predictors including the El Niño Southern Oscillation (ENSO) and Southern Annular Mode in two different locations: the Chuuk coral reef in the western Pacific (Chapter 2) and the Southern Ocean (Chapter 3). In coastal waters, human inputs of nutrients can lead to excessive production of algae (eutrophication). During photosynthesis, phytoplankton release organic acids that react with protons during seawater titration, and thereby contribute to total alkalinity. The third study of the thesis work concentrates on evaluating a non-conservative variation of total alkalinity caused by organic acid accumulation (Chapter 4). As the amount of anthropogenic CO2 in the ocean increase, adverse effects of ocean acidification on coral reefs will increase in future oceanic environments. The carbonate system in coral reef environments is controlled by complex interactions of internal processes, including net community calcification (calcification minus dissolution) and net community production (organic matter production minus respiration), and external processes, such as the intrusion of surrounding seawater and air-sea CO2 exchange. The 6-year weekly records of carbonate chemistry conditions in Chuuk lagoon, located in the coral-rich western Pacific Ocean, showed that during weak intrusion of ambient seawater from the surrounding open ocean two internal biological processes (calcification and respiration) reinforced each other and collectively lowered the pH of the reef water for extended periods, of a few to several months. Our study indicates that periods of low wind speed weaken the intrusion of ambient water, thereby increasing the residence time of reef water; as a result, respiration and calcification increase its acidification. This finding is not an isolated phenomenon, but may be widespread in the coral-rich western Pacific Ocean, which hosts 50% of global coral reefs and in which the degree of ambient water intrusion is closely associated with the wind speed change associated with the ENSO. To estimate the effect of climate variability on the Southern Ocean’s capacity to uptake anthropogenic CO2, I investigated hydrographic and carbon parameters in the eastern South Pacific Ocean along the 110°W meridian from 67°S to 21°N. I used seawater δ13C datasets collected in 1994 and 2008 to determine the change in the anthropogenic CO2 inventory during this period. The δ13C change in each of the latitudinal bands was primarily caused by inputs of anthropogenic CO2 by transport and air-sea exchange. More than 50% of the total anthropogenic CO2 was added to the subpolar band by the northward movement of Antarctic Intermediate Water (AAIW) from the south; and air-sea exchange. I also calculated the ratio of the temporal change in δ13C to the change in dissolved inorganic carbon, which is a measure of the efficiency of oceanic uptake of anthropogenic CO2. The change in preformed δ13C and DIC suggests that the ratio for AAIW in 1994 (−0.017‰ (μmol kg−1) −1) was greater than that in 2008 (−0.010‰ (μmol kg−1) −1); this result means that the efficiency of CO2 uptake by the Southern Ocean was lower in 2008 relative than in 1994. AAIW remained at the surface for a shorter period in 2008 than in 1994, and thus would have taken up less atmospheric CO2 prior to subduction. The projected reduction in this ratio indicates a relative weakening of CO2 uptake by the Southern Ocean in the period. The assessment of the carbonate system builds on precise measurement of four measureable properties of the carbon dioxide system; total alkalinity, total inorganic carbon, CO2 fugacity (fCO2), and pH. In seawater dissolved organic acids produced by phytoplankton dissociate into equal amounts of hydrogen ions and conjugated bases, which maintain the seawater alkalinity (AT) unchanged. However, the resulting conjugate bases react with protons during seawater titration and thereby contribute to the alkalinity (AT−ORG) whereas the contributions of other species (e.g. HCO3−, CO32−, B(OH)4−) to AT are proportionally lowered. Production of such dissolved organic acids was confirmed in each of six phytoplankton cultures and in a coastal environment. In phytoplankton monocultures with initial concentrations of ~70 µM nitrate and ~5 µM phosphate, the contribution of organic acids to seawater alkalinity (AT−ORG) was found to be 15−40 μmol kg−1 AT−ORG upon the complete consumption of added nutrients; the magnitude of AT−ORG depended on the phytoplankton species involved. In the coastal environment as high as 15 μmol kg−1 of AT−ORG was found. If the effect of organic acids on seawater AT is not accounted for, the concentrations of inorganic carbon components calculated from pairs of carbon parameters, including AT, is inaccurate in culture studies and in studies of productive coastal environments.