Dissolution of Carbonate Sediments Under Rising pCO₂ and Ocean Acidification: Observations from Devil’s Hole, Bermuda

Abstract

Rising atmospheric pCO₂ and ocean acidification originating from human activities could result in increased dissolution of metastable carbonate minerals in shallow-water marine sediments. In the present study, in situ dissolution of carbonate sedimentary particles in Devil’s Hole, Bermuda, was observed during summer when thermally driven density stratification restricted mixing between the bottom water and the surface mixed layer and microbial decomposition of organic matter in the subthermocline layer produced pCO₂ levels similar to or higher than those levels anticipated by the end of the 21st century. Trends in both seawater chemistry and the composition of sediments in Devil’s Hole indicate that Mg-calcite minerals are subject to selective dissolution under conditions of elevated pCO₂. The derived rates of dissolution based on observed changes in excess alkalinity and estimates of vertical eddy diffusion ranged from 0.2 mmol to 0.8 mmol CaCO₃ m⁻² h⁻¹. On a yearly basis, this range corresponds to 175–701 g CaCO₃ m⁻² year⁻¹; the latter rate is close to 50% of the estimate of the current average global coral reef calcification rate of about 1,500 g CaCO₃ m⁻² year⁻¹. Considering a reduction in marine calcification of 40% by the year 2100, or 90% by 2300, as a result of surface ocean acidification, the combination of high rates of carbonate dissolution and reduced rates of calcification implies that coral reefs and other carbonate sediment environments within the 21st and following centuries could be subject to a net loss in carbonate material as a result of increasing pCO₂ arising from burning of fossil fuels.

Appendix

The appendix demonstrates the calculations of excess TA (ΔTA) based on linear regressions of normalized TA profiles (S = 36) for each sampling time in Devil’s Hole in 2004 and 2005 (Fig. 7). Excess TA was calculated by subtracting the pre-formed TA (TA⁰) from the total depth integrated TA (ΣTA) over the depth range considered. The pre-formed TA was assumed to equal the average mixed layer TA during the first day of sampling for each year. In the following equations, variables z and x represent depth and TA, respectively (see Fig. 7 for details on the linear regressions and associated variables).

1.1 Excess alkalinity 08-24-04

$$ \begin{aligned} \Updelta {\rm TA}&=\Upsigma {\rm TA}-{\rm TA}^{0} \\ &= \int\limits_{z_1 }^{z_3 } {\left[ {\Upsigma {\rm TA}} \right]{\rm d}z-} \int\limits_{z_1 }^{z_3 } {\left[ {\rm TA}^{0} \right]{\rm d}z}\\ &= \int\limits_{z_1 }^{z_2 } {\left[ {x_2 } \right]{\rm d}z+} \int\limits_{z_2 }^{z_3 } {\left[ {x_1 } \right]{\rm d}z} -\int\limits_{z_1 }^{z_3 } {\left[ {\rm TA}^{0} \right]{\rm d}z} \\ &= \int\limits_{13.5}^{19.905} {\left[ {\frac{Z+424.44}{191.86}} \right]{\rm d}z+} \int\limits_{19.906}^{23.5} {\left[ {\frac{Z+63.204}{35.885}} \right]{\rm d}z} -\int\limits_{13.5}^{23.5} {\left[ {2.2826} \right]{\rm d}z}\\ &= 0.405\,\hbox{mmol kg}^{-1}\hbox{ m} = 415\,\hbox{mmol m}^{-2} \end{aligned} $$

1.2 Excess alkalinity 09-16-04

$$ \begin{aligned} \Updelta {\rm TA}&=\Upsigma {\rm TA}-{\rm TA}^{0}\\ &= \int\limits_{z_1 }^{z_4 } {\left[ {\Upsigma {\rm TA}} \right]{\rm d}z-} \int\limits_{z_1 }^{z_4 } {\left[ {\rm TA}^{0} \right]{\rm d}z}\\ &= \int\limits_{z_1 }^{z_2 } {\left[ {x_3 } \right]{\rm d}z+} \int\limits_{z_2 }^{z_3 } {\left[ {x_2 } \right]{\rm d}z} +\int\limits_{z_3 }^{z_4 } {\left[ {x_1 } \right]{\rm d}z} -\int\limits_{z_1}^{z_4 } {\left[ {\rm TA}^{0} \right]{\rm d}z}\\ &= \int\limits_{18.8}^{20.957} {\left[ {\frac{Z+34.352}{23.409}} \right]{\rm d}z+} \int\limits_{20.958}^{21.995} {\left[ {\frac{Z+173.98}{82.505}} \right]{\rm d}z} +\int\limits_{21.996}^{23.5} {\left[ {\frac{Z-5.9582}{6.7518}} \right]{\rm d}z} -\int\limits_{18.8}^{23.5} {\left[ {2.2826} \right]{\rm d}z}\\ &= 0.466\,\hbox{mmol kg}^{-1}\hbox{ m} = 477\,\hbox{mmol m}^{-2} \end{aligned} $$

1.3 Excess alkalinity 08-02-05

$$ \begin{aligned} \Updelta {\rm TA}&=\Upsigma {\rm TA}-{\rm TA}^{0}\\ &= \int\limits_{z_1 }^{z_4 } {\left[ {\Upsigma {\rm TA}} \right]{\rm d}z-} \int\limits_{z_1 }^{z_4 } {\left[ {\rm TA}^{0} \right]{\rm d}z}\\ &= \int\limits_{z_1 }^{z_2 } {\left[ {x_3 } \right]{\rm d}z+} \int\limits_{z_2 }^{z_3 } {\left[ {x_2 } \right]{\rm d}z} +\int\limits_{z_3 }^{z_4 } {\left[ {x_1 } \right]{\rm d}z} -\int\limits_{z_1 }^{z_4 } {\left[ {\rm TA}^{0} \right]{\rm d}z}\\ &= \int\limits_{16}^{18.992} {\left[ {\frac{Z+83.367}{43.247}} \right]{\rm d}z+} \int\limits_{18.993}^{22.141} {\left[ {\frac{Z+354.64}{157.86}} \right]{\rm d}z} +\int\limits_{22.142}^{23.5} {\left[ {\frac{Z+106.24}{53.788}} \right]{\rm d}z} -\int\limits_{16}^{23.5} {\left[ {2.2976} \right]{\rm d}z} \\ &= 0.487\,\hbox{mmol kg}^{-1}\hbox{ m} = 499\,\hbox{mmol m}^{-2} \end{aligned} $$

1.4 Excess alkalinity 08-16-05

$$ \begin{aligned} \Updelta {\rm TA}&=\Upsigma {\rm TA}-{\rm TA}^{0}\\ &= \int\limits_{z_1 }^{z_4 } {\left[ {\Upsigma {\rm TA}} \right]{\rm d}z-} \int\limits_{z_1 }^{z_4 } {\left[ {\rm TA}^{0} \right]{\rm d}z}\\ &= \int\limits_{z_1 }^{z_2 } {\left[ {x_3 } \right]{\rm d}z+} \int\limits_{z_2 }^{z_3 } {\left[ {x_2 } \right]{\rm d}z} +\int\limits_{z_3 }^{z_4 } {\left[ {x_1 } \right]{\rm d}z} -\int\limits_{z_1 }^{z_4 } {\left[ {\rm TA}^{0} \right]{\rm d}z}\\ &= \int\limits_{16.68}^{20.744} {\left[ {\frac{Z+56.108}{31.892}} \right]{\rm d}z+} \int\limits_{20.745}^{22.999} {\left[ {\frac{Z+597.46}{256.54}} \right]{\rm d}z} +\int\limits_{23}^{23.5} {\left[ {\frac{Z+22.665}{18.881}} \right]{\rm d}z} -\int\limits_{16.68}^{23.5} {\left[ {2.2976} \right]{\rm d}z}\\ &= 0.520\,\hbox{mmol kg}^{-1}\hbox{ m} = 532\,\hbox{mmol m}^{-2} \end{aligned} $$

1.5 Excess alkalinity 09-06-05

$$ \begin{aligned} \Updelta {\rm TA}&=\Upsigma {\rm TA}-{\rm TA}^{0}\\ &= \int\limits_{z_1 }^{z_4 } {\left[ {\Upsigma {\rm TA}} \right]{\rm d}z-} \int\limits_{z_1 }^{z_4 } {\left[ {\rm TA}^{0} \right]{\rm d}z}\\ &= \int\limits_{z_1 }^{z_2 } {\left[ {x_3 } \right]{\rm d}z+} \int\limits_{z_2 }^{z_3 } {\left[ {x_2 } \right]{\rm d}z} +\int\limits_{z_3 }^{z_4 } {\left[ {x_1 } \right]{\rm d}z} -\int\limits_{z_1 }^{z_4 } {\left[ {\rm TA}^{0} \right]{\rm d}z}\\ &= \int\limits_{17.75}^{20.039} {\left[ {\frac{Z+9.5278}{12.092}} \right]{\rm d}z+} \int\limits_{20.040}^{23.008} {\left[ {\frac{Z+325.45}{141.29}} \right]{\rm d}z} +\int\limits_{23.009}^{23.5} {\left[ {\frac{Z+4.1585}{7.6435}} \right]{\rm d}z} -\int\limits_{17.75}^{23.5} {\left[ {2.2976} \right]{\rm d}z}\\ &= 0.684\,\hbox{mmol kg}^{-1}\hbox{ m} = 700\,\hbox{mmol m}^{-2} \end{aligned} $$