Rates of carbonate dissolution in permeable sediments estimated from pore‐water profiles: The role of sea grasses

Abstract

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 O 2 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 O 2 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 CO 2 , 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;. 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 CO 2 has also previously been examined by Andersson et al. (2003), although these authors concluded that this process represents an insignificant buffer to this CO 2 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 o...

The role of organic acid exudates in liberating phosphorus from seagrass‐vegetated carbonate sediments

“…The consumption of OA by microbes also likely contributes to carbonate sediment dissolution through the production of aqueous CO 2 and H + . The aerobic respiration of sediment organic matter (which may include OA) results in the production of aqueous CO 2 , which lowers the saturation state of carbonate minerals in sediment pore waters (Burdige and Zimmerman 2002;Hu and Burdige 2007). Organic acids may also be consumed through fermentation and sulfate reduction, by which CO 2 and end products such as ethanol, acetic, and lactic acids are produced.…”

“…Organic acids may also be consumed through fermentation and sulfate reduction, by which CO 2 and end products such as ethanol, acetic, and lactic acids are produced. Sulfate reduction results in the net production of acidity due to the tight coupling between sulfide oxidation and sulfate reduction, which is balanced by diel fluctuations in seagrass O 2 release (Ku et al 1999;Burdige and Zimmerman 2002;Hu and Burdige 2007).…”

“…A number of studies have investigated total sediment dissolution rates and seagrass uptake rates and found that P i concentrations provided by rates of bulk sediment dissolution are not sufficient to relieve P-limitation (Jensen et al 1998;Gras et al 2003;Nielsen et al 2006). Recent evidence suggests that there are increased rates of sediment dissolution in marine rhizosphere sediments, and that high rates of oxidation caused by seagrass O 2 release may provide the required amount of acid to explain enhanced rates of dissolution (Ku et al 1999;Burdige and Zimmerman 2002;Burdige et al 2008). Organic acid exudation by seagrasses may also help to explain increases in sediment dissolution in the rhizosphere, either directly by reacting with carbonate minerals, or indirectly by providing the necessary substrates for the bacterially mediated oxidative processes.…”

“…The seagrass Thalassia testudinum has as much as 85% of its biomass in below-ground structures (Fourqurean and Zieman 1991) and has been shown to modify sediment characteristics such as O 2 concentrations and pH (Burdige and Zimmerman 2002). Recently, Burdige et al (2008) modeled carbonate dissolution rates in seagrass sediments based on oxygen release from seagrass roots and showed that rates of dissolution were positively correlated with seagrass density.…”

“…Recently, Burdige et al (2008) modeled carbonate dissolution rates in seagrass sediments based on oxygen release from seagrass roots and showed that rates of dissolution were positively correlated with seagrass density. Seagrasses are known release O 2 from root tips (Pedersen et al 1998;Frederiksen and Glud 2006) and the root tips are expected to be the dominant region of OA exudation, which may cause pockets of high rates of dissolution and reduced P i adsorption to occur around root tips.…”