Global Biogeochemical Cycles
Páginas: 46 (11445 palabras)
Publicado: 5 de noviembre de 2012
GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 25, GB3003, doi:10.1029/2010GB003859, 2011
An assessment of ocean margin anaerobic processes on oceanic alkalinity budget
Xinping Hu1 and Wei‐Jun Cai1
Received 7 May 2010; revised 14 February 2011; accepted 31 March 2011; published 8 July 2011.
accompanying “ocean acidification” has prompted discussions on the magnitude of ocean margin alkalinityproduction via anaerobic processes. However, available estimates are largely based on gross reaction rates or misconceptions regarding reaction stoichiometry. In this paper, we argue that net alkalinity gain does not result from the internal cycling of nitrogen and sulfur species or from the reduction of metal oxides. Instead, only the processes that involve permanent loss of anaerobic remineralizationproducts, i.e., nitrogen gas from net denitrification and reduced sulfur (i.e., pyrite burial) from net sulfate reduction, could contribute to this anaerobic alkalinity production. Our revised estimate of net alkalinity production from anaerobic processes is on the order of 4–5 Tmol yr−1 in global ocean margins that include both continental shelves and oxygen minimum zones, significantly smallerthan the previously estimated rate of 16–31 Tmol yr−1. In addition, pyrite burial in coastal habitats (salt marshes, mangroves, and seagrass meadows) may contribute another 0.1–1.1 Tmol yr−1, although their long‐term effect is not yet clear under current changing climate conditions and rising sea levels. Finally, we propose that these alkalinity production reactions can be viewed as “chargetransfer” processes, in which negative charges of nitrate and sulfate ions are converted to those of bicarbonate along with a net loss of these oxidative anions.
Citation: Hu, X., and W.‐J. Cai (2011), An assessment of ocean margin anaerobic processes on oceanic alkalinity budget, Global Biogeochem. Cycles, 25, GB3003, doi:10.1029/2010GB003859.
[1] Recent interest in the ocean’s capacity to absorbatmospheric CO2 and buffer the
1. Introduction
[2] Alkalinity mass balance can be translated into carbonate mass balance because carbonate dissolution and precipitation are the dominant processes that control oceanic alkalinity inventory [e.g., Berelson et al., 2007; Chung et al., 2003]. Thus, the global ocean alkalinity cycle is an important subject in studying both the oceanic carbonate massbalance [Broecker and Peng, 1982; Milliman and Droxler, 1996; Morse and Mackenzie, 1990] and the buffering capacity of seawater in the event of increasing atmospheric CO2 [e.g., Feely et al., 2004; Frankignoulle, 1994]. However, despite many years of study, whether the global carbonate cycle is at steady state and the causes of apparent nonsteady state still remain contentious questions [Berelson etal., 2007; Iglesias‐Rodriguez et al., 2002; Milliman, 1993; Milliman and Droxler, 1996]. [3] The study by Berner et al. [1970] was one of the first to examine the issue of elevated carbonate alkalinity in anoxic sediment pore waters, where most of the alkalinity was produced through sulfate reduction. This type of alkalinity production does not involve carbonate dissolution as
1 Department ofMarine Sciences, University of Georgia, Athens, Georgia, USA.
Copyright 2011 by the American Geophysical Union. 0886‐6236/11/2010GB003859
has been observed in both shallow and deep sea calcareous sediments [Archer et al., 1989; Berelson et al., 1996; Jahnke et al., 1994, 1997; Martin and Sayles, 1996; Morse et al., 1985; Walter and Burton, 1990]. A series of studies on the anaerobic alkalinityproduction was conducted in the early 1990s in Tomales Bay, a narrow estuary of the California coast [Chambers et al., 1994; Dollar et al., 1991; Smith and Hollibaugh, 1993]. In this area, net sulfate reduction or burial of reduced sulfur compounds (mostly pyrite) was found to account for almost all of the total alkalinity export. Similarly, nonconservative mixing lines of alkalinity versus...
An assessment of ocean margin anaerobic processes on oceanic alkalinity budget
Xinping Hu1 and Wei‐Jun Cai1
Received 7 May 2010; revised 14 February 2011; accepted 31 March 2011; published 8 July 2011.
accompanying “ocean acidification” has prompted discussions on the magnitude of ocean margin alkalinityproduction via anaerobic processes. However, available estimates are largely based on gross reaction rates or misconceptions regarding reaction stoichiometry. In this paper, we argue that net alkalinity gain does not result from the internal cycling of nitrogen and sulfur species or from the reduction of metal oxides. Instead, only the processes that involve permanent loss of anaerobic remineralizationproducts, i.e., nitrogen gas from net denitrification and reduced sulfur (i.e., pyrite burial) from net sulfate reduction, could contribute to this anaerobic alkalinity production. Our revised estimate of net alkalinity production from anaerobic processes is on the order of 4–5 Tmol yr−1 in global ocean margins that include both continental shelves and oxygen minimum zones, significantly smallerthan the previously estimated rate of 16–31 Tmol yr−1. In addition, pyrite burial in coastal habitats (salt marshes, mangroves, and seagrass meadows) may contribute another 0.1–1.1 Tmol yr−1, although their long‐term effect is not yet clear under current changing climate conditions and rising sea levels. Finally, we propose that these alkalinity production reactions can be viewed as “chargetransfer” processes, in which negative charges of nitrate and sulfate ions are converted to those of bicarbonate along with a net loss of these oxidative anions.
Citation: Hu, X., and W.‐J. Cai (2011), An assessment of ocean margin anaerobic processes on oceanic alkalinity budget, Global Biogeochem. Cycles, 25, GB3003, doi:10.1029/2010GB003859.
[1] Recent interest in the ocean’s capacity to absorbatmospheric CO2 and buffer the
1. Introduction
[2] Alkalinity mass balance can be translated into carbonate mass balance because carbonate dissolution and precipitation are the dominant processes that control oceanic alkalinity inventory [e.g., Berelson et al., 2007; Chung et al., 2003]. Thus, the global ocean alkalinity cycle is an important subject in studying both the oceanic carbonate massbalance [Broecker and Peng, 1982; Milliman and Droxler, 1996; Morse and Mackenzie, 1990] and the buffering capacity of seawater in the event of increasing atmospheric CO2 [e.g., Feely et al., 2004; Frankignoulle, 1994]. However, despite many years of study, whether the global carbonate cycle is at steady state and the causes of apparent nonsteady state still remain contentious questions [Berelson etal., 2007; Iglesias‐Rodriguez et al., 2002; Milliman, 1993; Milliman and Droxler, 1996]. [3] The study by Berner et al. [1970] was one of the first to examine the issue of elevated carbonate alkalinity in anoxic sediment pore waters, where most of the alkalinity was produced through sulfate reduction. This type of alkalinity production does not involve carbonate dissolution as
1 Department ofMarine Sciences, University of Georgia, Athens, Georgia, USA.
Copyright 2011 by the American Geophysical Union. 0886‐6236/11/2010GB003859
has been observed in both shallow and deep sea calcareous sediments [Archer et al., 1989; Berelson et al., 1996; Jahnke et al., 1994, 1997; Martin and Sayles, 1996; Morse et al., 1985; Walter and Burton, 1990]. A series of studies on the anaerobic alkalinityproduction was conducted in the early 1990s in Tomales Bay, a narrow estuary of the California coast [Chambers et al., 1994; Dollar et al., 1991; Smith and Hollibaugh, 1993]. In this area, net sulfate reduction or burial of reduced sulfur compounds (mostly pyrite) was found to account for almost all of the total alkalinity export. Similarly, nonconservative mixing lines of alkalinity versus...
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