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Venti, A, Kadko D, Andersson AJ, Langdon C, Bates NR.  2012.  A multi-tracer model approach to estimate reef water residence times. Limnology and Oceanography-Methods. 10:1078-1095.   10.4319/lom.2012.10.1078   AbstractWebsite

We present a new method for obtaining the residence time of coral reef waters and demonstrate the successful application of this method by estimating rates of net ecosystem calcification (NEC) at four locations across the Bermuda platform and showing that the rates thus obtained are in reasonable agreement with independent estimates based on different methodologies. The contrast in Be-7 activity between reef and offshore waters can be related to the residence time of the waters over the reef through a time-dependent model that takes into account the rainwater flux of Be-7, the radioactive half-life of Be-7, and the rate of removal of Be-7 on particles estimated from Th-234. Sampling for Be-7 and Th-234 was conducted during the late fall and winter between 2008 and 2010. Model results yielded residence times ranging from 1.4 (+/- 0.7) days at the rim reef to 12 (+/- 4.0) days closer to shore. When combined with measurements of salinity-normalized total alkalinity anomalies, these residence times yielded platform-average NEC rates ranging from a maximum of 20.3 (+/- 7.0) mmolCaCO(3) m(-2) d(-1) in Nov 2008 to a minimum of 2.5 (+/- 0.8) mmolCaCO(3) m(-2) d(-1) in Feb 2009. The advantage of this new approach is that the rates of NEC obtained are temporally and spatially averaged. This novel approach for estimating NEC rates may be applicable to other coral reef ecosystems, providing an opportunity to assess how these rates may change in the context of ocean acidification.

Andersson, AJ, Kuffner IB, Mackenzie FT, Jokiel PL, Rodgers KS, Tan A.  2009.  Net Loss of CaCO(3) from a subtropical calcifying community due to seawater acidification: mesocosm-scale experimental evidence. Biogeosciences. 6:1811-1823. AbstractWebsite

Acidification of seawater owing to oceanic uptake of atmospheric CO(2) originating from human activities such as burning of fossil fuels and land-use changes has raised serious concerns regarding its adverse effects on corals and calcifying communities. Here we demonstrate a net loss of calcium carbonate (CaCO(3)) material as a result of decreased calcification and increased carbonate dissolution from replicated subtropical coral reef communities (n=3) incubated in continuous-flow mesocosms subject to future seawater conditions. The calcifying community was dominated by the coral Montipora capitata. Daily average community calcification or Net Ecosystem Calcification (NEC=CaCO(3) production - dissolution) was positive at 3.3 mmol CaCO(3) m(-2) h(-1) under ambient seawater pCO(2) conditions as opposed to negative at -0.04 mmol CaCO(3) m(-2) h(-1) under seawater conditions of double the ambient pCO(2). These experimental results provide support for the conclusion that some net calcifying communities could become subject to net dissolution in response to anthropogenic ocean acidification within this century. Nevertheless, individual corals remained healthy, actively calcified (albeit slower than at present rates), and deposited significant amounts of CaCO(3) under the prevailing experimental seawater conditions of elevated pCO(2).

Andersson, AJ, Mackenzie FT, Lerman A.  2005.  Coastal ocean and carbonate systems in the high CO(2) world of the anthropocene. American Journal of Science. 305:875-918.   10.2475/ajs.305.9.875   AbstractWebsite

The behavior of the ocean carbon cycle has been, and will continue to be, modified by the increase in atmospheric CO(2) due to fossil fuel combustion and land-use emissions of this gas. The consequences of a high-CO(2) world and increasing riverine transport of organic matter and nutrients arising from human activities were investigated by means of two biogeochemical box models. Model numerical simulations ranging from the year 1700 to 2300 show that the global coastal ocean changes from a net source to a net sink of atmospheric CO(2) over time; in the 18th and 19th centuries, the direction of the CO(2) flux was from coastal surface waters to the atmosphere, whereas at present or in the near future the net CO(2) flux is into coastal surface waters. These results agree well with recent syntheses of measurements of air-sea CO(2) exchange fluxes from various coastal ocean environments. The model calculations also show that coastal ocean surface water carbonate saturation state would decrease 46 percent by the year 2100 and 73 percent by 2300. Observational evidence from the Pacific and Atlantic Oceans shows that die carbonate saturation state of surface ocean waters has already declined during recent decades. For atolls and other semi-enclosed carbonate systems, the rate of decline depends strongly on the residence time of the water in the system. Based on the experimentally observed positive relationship between saturation state and calcification rate for many calcifying organisms, biogenic production of CaCO(3) may decrease by 42 percent by the year 2100 and by 85 to 90 percent by 2300 relative to its value of about 24 x 10(12) moles C/yr in the year 2000. If the predicted change in carbonate production were to occur along with rising temperatures, it would make it difficult for coral reef and other carbonate systems, to exist as we know them now into future centuries. Because high-latitude, cold-water carbonates presently occur in waters closer to saturation with respect to carbonate minerals than the more strongly supersaturated waters of the lower latitudes, it might be anticipated that the cool-water carbonate systems might feel the effects of rising atmospheric CO(2) (and temperature) before those at lower latitudes. In addition, modeling results show that the carbonate saturation state of coastal sediment pore water will decrease in the future owing to a decreasing pore water pH and increasing CO(2) concentrations attributable to greater deposition and remineralization of land-derived and in situ produced organic matter in sediments. The lowered carbonate saturation state drives selective dissolution of metastable carbonate minerals while a metastable equilibrium is maintained between the pore water and the most soluble carbonate phase present in the sediments. In the future, the average composition of carbonate sediments and cements may change as the more soluble Mg-calcites and aragonite are preferentially dissolved and phases of lower solubility, such as calcites with lower magnesium content, increase in percentage abundance in the sediments.

Andersson, AJ, Mackenzie FT, Ver LM.  2003.  Solution of shallow-water carbonates: An insignificant buffer against rising atmospheric CO2. Geology. 31:513-516.   10.1130/0091-7613(2003)031<0513:soscai>;2   AbstractWebsite

Model predictions suggest that the saturation state of surface ocean waters with respect to carbonate minerals will decline during the twenty-first century owing to increased invasion of atmospheric CO2. As a result, calcareous organisms may have difficulty calcifying, leading to production of weaker skeletons and greater vulnerability to erosion. Alternatively, it has been suggested that there will be no significant impact on coral reef ecosystems because any changes in saturation state and pH will be restored by dissolution of metastable carbonate minerals. To resolve this controversy, we employ a physical-biogeochemical box model representative of the shallow-water ocean environment. Numerical simulations demonstrate that the carbonate saturation state of surface waters could significantly decrease and hamper the biogenic production of CaCO3 during the twenty-first century. Similarly, the average saturation state of marine pore waters could decline significantly, inducing dissolution of metastable carbonate phases within the pore-water-sediment system. Such dissolution could buffer the carbon chemistry of the pore waters, but overlying surface waters of reefs and other shallow-water carbonate environments will not accumulate sufficient alkalinity to buffer pH or carbonate saturation state changes owing to invasion of atmospheric CO2.