Rates of Carbonate Dissolution in Permeable Sediments Estimated From Porewater Profiles: The Role of Sea Grasses

In this study we estimate sediment carbonate dissolution rates for sandy sea grass sediments on the Bahamas Bank using an inverse pore-water advection/diffusion/reaction model constrained by field observations. This model accounts for sea grass O2 input to these sediments, and also parameterizes pore-water advection through these permeable sediments as a nonlocal exchange process. The resulting rates of carbonate dissolution are positively correlated with sea grass density, and are comparable with previous rate estimates for Florida Bay sediments. In contrast, the advective uptake of O2 by these sediments decreased with increasing sea grass density. This suggests that the competing interplay between bottom-water flow, near-seabed pressure gradients, and the presence of a sea grass canopy is important in controlling this type of sediment oxygen uptake. When the carbonate dissolution rates estimated here are examined in the context of carbonate budgets for shallow-water carbonate platforms systems, they suggest that carbonate dissolution may be a significant loss term in these budgets. Sea grass-mediated carbonate dissolution may also exert a negative feedback on rising atmospheric CO2, although the magnitude of this effect remains to be quantified. In shallow water environments such as the Bahamas Bank the production of carbonate sediments (both sands and muds) occurs by biogenic and inorganic precipitation (Macintyre and Reid 1992; Milliman 1993; Broecker et al. 2001; and others). Once formed, this material can be altered by reactions in the water column (e.g., during resuspension), on the sediment surface, and during burial (also see Winland and Matthews 1974; Melim et al. 2002; Morse 2003). The ultimate fate of this carbonate (i.e., net accumulation vs. export or dissolution) is, however, not well constrained. In particular, carbonate dissolution is a poorly quantified component of shallow-water bank and shelf carbonate budgets (Milliman 1993), although studies conducted in Florida Bay (U.S.A.) suggest that carbonate dissolution may be comparable in magnitude with the assumed offshore export of carbonate from such shallowwater environments (Walter and Burton 1990; Ku et al. 1999; Yates and Halley 2006). The role of shallow-water sediment carbonate dissolution as a sink for rising levels of atmospheric CO2 has also previously been examined by Andersson et al. (2003), although these authors concluded that this process represents an insignificant buffer to this CO2 increase. Past studies have described the occurrence of carbonate dissolution in shallow-water sediments using chemical, isotopic, and mineralogical techniques (e.g., Berner 1966; Moulin et al. 1985; Morse et al. 1987). However, rates of carbonate dissolution in the context of other sediment biogeochemical processes are less well quantified (also see Morse et al. 2003 for a review). Furthermore, previous attempts to generate carbonate dissolution budgets for shallow-water carbonate sediments have generally not been closed with respect to the observed amount of dissolution and the required amount of acid production from either sediment organic matter remineralization, i.e., aqueous CO2 production, or oxidation of reduced iron and sulfur species, i.e., H+ production (Walter and Burton 1990; Walter et al. 1993). Ku et al. (1999) proposed that enhanced oxygen transport into these sediments through the roots and rhizomes of sea grasses (sea grass ‘‘pumping’’) might resolve this mass balance problem. In support of this suggestion, our past work (Burdige and Zimmerman 2002) showed that the belowground input of photosynthetically produced O2 by sea grasses could be an order-of-magnitude larger than O2 inputs via diffusion across the sediment– water interface. In addition to providing the oxygen needed to drive sediment carbonate dissolution, sea grass productivity can be an important source of sediment organic matter, the respiration of which produces the acid needed for carbonate dissolution (Hu and Burdige 2007). The overlying leaf canopy also dissipates the energy of local currents and promotes the deposition of organic and inorganic suspended material from the water column to the underlying sediments, particularly in areas dominated by strong tidal currents (see Koch et al. 2006 for a recent summary). 1 Corresponding author ([email protected]). 2 Current address: Department of Marine Sciences, University of Georgia, Athens, Georgia 30602. Acknowledgments We thank the following people for assistance in the field and in the lab: Kim Krecek, Michelle McElvaine, Scott Kline, Jean-Paul Simjouw, Kip Gardner, Lisa Drake, Laura Bodensteiner, and Peter Sandhei. We also thank Heidi Dierssen for providing us with unpublished results from her remote sensing analyses of images of the Bahamas Bank. Finally, we thank two anonymous reviewers for useful comments on the first version of this manuscript. The staff at the Caribbean Marine Research Center (CMRC) provided us with superb logistic support and made our stays at Lee Stocking Island extremely productive. Funding for this work was provided by NSF (Chemical Oceanography Program) and CMRC (through the NOAA-NURP program). Limnol. Oceanogr., 53(2), 2008, 549–565 E 2008, by the American Society of Limnology and Oceanography, Inc.