Shelf sediment response to the current and past episodes of ocean acidification

Projet collaboratif du Geotop

2018-2020

Ocean acidification (OA) is seen as the other carbon dioxide (CO2) problem (Andersson and Mackenzie, 2012). Since the industrial revolution, the oceans have absorbed up to ~30% of the anthropogenic CO2 emitted to the atmosphere, mitigating its accumulation in the atmosphere and the associated climate change (Sutton et al., 2016). On the other hand, oceanic uptake of CO2 has modified the ocean’s chemistry, decreasing pH, the carbonate ion concentration and the saturation state (Ω) of waters with respect to carbonate minerals (Andersson and Mackenzie, 2012; Sutton et al., 2016). These changes in seawater chemistry constitute a potential threat to the health of marine ecosystems, particularly to calcifying organisms whose ability to secrete calcium carbonate (CaCO3) skeletons and tests might be hindered by a decrease in pH and Ω. Calcite and aragonite are the dominant CaCO3 polymorphs in marine sediments, most of which have a biogenic origin. According to the IPCC “business-as-usual” emission scenario and general circulation models, atmospheric CO2 levels may reach 800 ppm by 2100 which would cause pH to drop by an additional 0.3-0.4 unit, at which point most of the surface oceans will be undersaturated with respect to aragonite (Leclercq et al., 2000).

The dissolution of carbonate-rich sediments at the seafloor is the ultimate sink for this anthropogenic CO2 (Montenegro et al., 2007), but shallow-water carbonates whose mineralogy is dominated by aragonite and magnesian calcites (Mg-calcites) may be the “first responder” to the decreasing saturation state of the surface ocean (Morse et al., 2006). Modern carbonate sediments usually found in shallow-water platforms are mostly of biogenic origin (Tucker and Wright, 1990). They are composed of the skeletons of reef-building organisms (e.g., corals) as well as the skeletons/shells of pelagic (e.g., forams, pteropods) and benthic (echinoids, molluscs, green and red algae) organisms (Wilson, 1975). These biogenic carbonates have varied carbonate mineralogies: aragonite, low Mg-calcite, and high Mg-calcite. High Mg-calcites (> 12 mole %) are more soluble than aragonite, which itself is ~50% more soluble than low Mg-calcites in seawater (Mucci et al., 1983; Bischoff et al., 1987). Consequently, high Mg-calcites and aragonite are more vulnerable to OA and dissolution. OA will hinder the proliferation of organisms that precipitate high Mg-calcites (e.g., sea urchins, red algae) and aragonite (corals, green algae) and favor the preservation of low Mg-calcites in carbonate platforms, as predicted by the thermodynamic model of Morse et al. (2006) and Andersson et al. (2006). These models, however, do not account for the differential kinetics of the dissolution reaction of the various minerals and precipitation of more stable phases.

The objective of this project is to experimentally simulate the progressive impact of OA on the mineralogy of modern platform carbonate sediments and compare our results with the mineralogy of ancient platform carbonates from key greenhouse periods (i.e., high CO2 partial pressures and low seawater pH), such as those deposited spanning the Paleocene-Eocene Thermal Maximum (PETM) (Spain) and Cretaceous-Paleogene boundary (Croatia). Their sedimentology and geochemistry will be analysed to better understand their formation, depositional environment, and possible alteration through ocean acidification. Their mineralogy as well as elemental (Ca, Mg, Sr, B, REE) and isotopic (18O, 13C, Sr) compositions will be compared to modern carbonate deposits (Australia, Bahamas and/or Red Sea) on the basis of their sedimentology, stratigraphy, and dissolution patterns. In doing so, ancient carbonate deposits could provide us with insights on how much post-depositional dissolution they have been subjected to during an acidification event and a more realistic representation of the mineralogical evolution of these sediments (e.g., selective dissolution) based not only on the thermodynamic stability of the various constituent carbonate phases but their dissolution kinetics as well.