Life on the margin: implications of ocean acidification on Mg-calcite, high latitude and cold-water marine calcifiers

Abstract

Future anthropogenic emissions of CO 2 and the resulting ocean acidification may have severe consequences for marine calcifying organisms and ecosystems. Marine calcifiers depositing calcitic hard parts that contain significant concentrations of magnesium, i.e. Mg-calcite, and calcifying organisms living in high latitude and/or cold-water environments are at immediate risk to ocean acidification and decreasing seawater carbonate saturation because they are currently immersed in seawater that is just slightly supersaturated with respect to the carbonate phases they secrete. Under the present rate of CO 2 emissions, model calculations show that high latitude ocean waters could reach undersaturation with respect to aragonite in just a few decades. Thus, before this happens these waters will be undersaturated with respect to Mg-calcite minerals of higher solubility than that of aragonite. Similarly, tropical surface seawater could become undersaturated with respect to Mgcalcite minerals containing ≥12 mole percent (mol%) MgCO 3 during this century. As a result of these changes in surface seawater chemistry and further penetration of anthropogenic CO 2 into the ocean interior, we suggest that (1) the magnesium content of calcitic hard parts will decrease in many ocean environments, (2) the relative proportion of calcifiers depositing stable carbonate minerals, such as calcite and low Mg-calcite, will increase and (3) the average magnesium content of carbonate sediments will decrease. Furthermore, the highest latitude and deepest depth at which cold-water corals and other calcifiers currently exist will move towards lower latitudes and shallower depth, respectively. These changes suggest that anthropogenic emissions of CO 2 may be currently pushing the oceans towards an episode characteristic of a 'calcite sea.' KEY WORDS: Ocean acidification · Calcification · Carbonate dissolution · Mg-calcite · High latitude · Aragonite · Saturation state · Calcite sea Resale or republication not permitted without written consent of the publisher Contribution to the Theme Section 'Effects of ocean acidification on marine ecosystems'OPEN PEN ACCESS CCESS

Growth and Dissolution of Calcite in the Presence of Adsorbed Stearic Acid

“…This faster calcite dissolution along acute (a) steps contradicts the usual trend reported for calcite dissolution 26,56 , but similar observations have been reported 23 in the presence of Mg 2+ , even at very low concentrations. 32 Other ions present in the brine can affect the surface restructuring dynamics with K + increasing the spreading rate of both acute and obtuse steps 28 and SO 4 2-inducing an elongation of the etch pits 57 , but Mg 2+ ions dominate the evolution of the crystal morphology with the usual rounding of the (o)/(o) etch pits corner 23,32 (blue box in Figure 5B), and determine the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 14 preferred direction of dissolution. The pinning effect of the adsorbed organic patches further complicates the dissolution process: when a step edge reaches an SA patch, it needs to dissolve laterally in order to avoid the organic layer protecting the edge (blue arrows in Figure 5).…”

“…1 It is also the main constituent of the shells of marine organisms, and its relative abundance is largely linked to biomineralization. 2,3 Calcite can grow or dissolve rapidly depending on its environment 4 , and plays a fundamental role in preserving the biosphere through its ability to regulate ocean acidification. 5 The dynamics of the physico-chemical transformations occurring at its surface is also key to countless industrial processes, for example in the polymer industry, 6 cement manufacturing, 7 nuclear waste storage, 8,9 waste water treatment 10 , and in the petroleum industry, given its natural abundance in oil reservoirs 11 .…”

Patterns of magnesium content in Arctic bryozoan skeletons along a depth gradient

“…With respect to carbonate minerals, seawater saturation state is lower in polar than in temperate and tropical latitudes (e.g., Andersson et al 2008) because calcium carbonate is more soluble in cold water. It is also recognized worldwide that pH and CO 2 levels change with water depth (e.g., Bathurst 1975;Palmer et al 1998;Palmer 2009, fig.…”

Trace element accumulation in the shell of the Arctic cirriped Balanus balanus

“…Declines in seawater pH (ocean acidification), occurring due to increased emissions of atmospheric CO 2 , are currently one of the most important ecological problems, especially in the Arctic (e.g. Andersson et al, 2008;Feely et al, 2008). In laboratory conditions, growing barnacles enhanced CaCO 3 production in seawater at a pH of 7.4, although their shells appeared to be significantly weaker compared to specimens reared at a pH of 8.2 (McDonald et al, 2009).…”

“…It is particularly important for Arctic species, because Arctic ecosystems become rapidly unfavourable for calcifying species due to progressing hypercapnia and declining carbonate saturation (e.g. Andersson et al, 2008).…”