Dissolution Rates of Biogenic Carbonates in Natural Seawater at Different pCO2 Conditions: A Laboratory Study

Abstract

The bulk dissolution rates of six biogenic carbonates (goose barnacle, benthic foraminifera, bryozoan, sea urchin, and two types of coralline algae) and a sample of mixed sediment from the Bermuda carbonate platform were measured in natural seawater at pCO 2 values ranging from approximately 3000 to 5500 latm. This range of pCO 2 values encompassed values regularly observed in porewaters at a depth of a few cm in carbonate sediments at shallow water depths (\15 m) on the Bermuda carbonate platform. The biogenic carbonates included calcites of varying Mg content (2-17 mol%) and a range of specific surface areas (0.01-2.7 m 2 g -1 ) as determined by BET gas adsorption. Measured rates of dissolution increased with increasing pCO 2 treatment for all substrates and ranged from 2.5 to 18 lmol g -1 h -1 . The highest rates of dissolution were observed for the bryozoans and the lowest rates for the goose barnacles. The relative ranking in dissolution rates between different substrates was consistent at all pCO 2 levels, indicating that substrates dissolve sequentially and that some substrates will be more vulnerable than others to rising CO 2 and ocean acidification. Furthermore, dissolution rates were found to increase with increasing Mg content, though the relative dissolution rates were observed to be a function of both Mg content and microstructure (surface area).

Impacts of seawater saturation state (ΩA= 0.4–4.6) and temperature (10, 25 °C) on the dissolution kinetics of whole-shell biogenic carbonates

“…6 mole-% Mg) had higher dissolution rates than the more Mg-rich coralline red algae (> 20 mole-% Mg). Pickett and Andersson (2015) also observed that crushed specimens of bryozoa had faster dissolution rates than crushed specimens of coralline red algae, despite the bryozoa's lower Mg-content. They attributed this trend to the bryozoan skeletons possessing proportionally more microstructural surface area than the coralline algae.…”

“…A smaller body of work has investigated the dissolution kinetics of biogenic CaCO 3 minerals. The majority of these studies have focused on the dissolution kinetics of bulk CaCO 3 sediments, often with the aim of constraining reactions involved in the evolution of sedimentary porewater, early-to-late stage sedimentary diagenesis, lithification, and sedimentary buffering of the seawater carbonate system (Andersson et al, 2007;Andersson et al, 2009;Archer et al, 1989;Balzer and Wefer, 1981;Berger, 1967;Dreybrodt, 1985a, 1985b;Burdige et al, 2008Burdige et al, , 2010Burdige and Zimmerman, 2002;Boucher et al, 1998;Friedman, 1964;Gehlen et al, 2005aGehlen et al, , 2005bHales and Emerson, 1996;Hales et al, 1994;Honjo and Erez, 1978;Jahnke and Jahnke, 2004;Kinsey, 1985;Langdon et al, 2000;Leclercq et al, 2002;Milliman, 1978;Morse, 1978;Morse et al, 2006;Peterson, 1966;Pickett and Andersson, 2015;Rude and Aller, 1991;Silverman et al, 2007aSilverman et al, , 2007bSchmalz, 1965;Tynan and Opdyke, 2011;Walter et al, 1993;Walter and Burton, 1990;Yates and Halley, 2006). These studies generally investigate bulk mixtures of biogenic CaCO 3 minerals that, in some cases, have been substantially altered 196 themselves through neomorphism, diagenetic conversion, laboratory cleaning, and/or natural loss of organic impurities and protective organic layers.…”

“…Although studies investigating mechanically and/or chemically treated shells and skeletons provide important insight into the dissolution kinetics of 206 sedimentary biogenic carbonates, they are not ideal for constraining the whole-shell dissolution 207 kinetics that are relevant to predicting organismal responses to CO 2 -induced ocean acidification and warming. Pickett and Andersson (2015) have published the most recent and comprehensive investigation of dissolution kinetics of biogenic carbonates. In their seminal study, they evaluated the impact of pCO 2 ranging from 3000 to 5500 µatm on the dissolution rates of 212 pulverized shells of 5 species of marine invertebrates that produce calcite of varying a barnacle, a foraminifera, a bryozoan, an urchin, and two species of coralline algae.…”

“…Gledhill and Morse, 2004, 2006a, 2006bMorse and Arvidson, 2002). Akaike's Information Criterion (AIC) was used to investigate various forms of avoided problems associated with bi-mineralic shells and with assigning solubility products to Mg-calcites of undetermined solubilities (see Pickett and Andersson, 2015). Should the reader prefer, the y-axes of the dissolution curves can be easily transformed to the appropriate polymorph since saturation states of calcium carbonate polymorphs are related by constant proportions.…”

“…However, it is unlikely to have arisen from microbial-induced dissolution (e.g., Tribollet et al, 2009;Glover and Kidwell, 1993), as the experimental seawaters were poisoned with HgCl 2 for the duration of the experiment. Regardless of the precise mechanism that is responsible for the gross dissolution of biogenic carbonates in the oversaturated treatments, these results suggest that applying stoichiometric solubility products derived for inorganic carbonates to model gross dissolution of biogenic carbonates may substantially underestimate the impacts of ocean acidification on net calcification (gross calcification minus gross dissolution) of calcifying systems ranging in scale from individual organisms to entire ecosystems [as discussed in Andersson and Gledhill (2013) and Pickett and Andersson (2015) in the context of 'net ecosystem calcification'].…”