Carbonate production by coralline algae and the global change

The Intergovernmental Panel on Climate Change (IPCC, 2001) predicted that the atmospheric partial pressure of carbon dioxide (pCO2) will be double that of pre-industrial levels by 2100 and will be considerably higher than at any time during the past few million years. Between 2100 and 2200, the atmospheric CO2 concentration is expected to increase to the range of 1500 to 2100 ppm that is 100 times greater than the natural fluctuations seen over recent millennia, in contrast to the stability of the past 24 million years during which time levels have remained below 500 ppm (Pearson & Palmer, 2000). As CO2 dissolves in the surface ocean it reacts with water to form ‘protons’ (H+) and dissolved inorganic carbon (DIC), which is the sum of the concentrations of carbonic acid (H2CO3), bicarbonate (HCO3–), and carbonate ions (CO32–)stored in the ocean. With increasing atmospheric pCO2, DIC will increase and the equilibrium of the carbonate system will shift to higher CO2 and HCO3– levels, while CO32– concentration and pH will decrease. These changes in carbonate chemistry, often referred to as ‘ocean acidification’, are already occurring. Current models predict that the pH of surface seawater will drop from 8.0 to 7.8 by the year 2100 (Royal Society, 2005). This dramatic change in seawater chemistry is likely to have a large impact on marine life and biogeochemical processes such as reduced biogenic marine carbonate mineral production and carbonate mineral dissolution. Assessing the impact of decreasing pH on coastal ecosystems is priority and the focus here is on calcareous algae, and on coralline red algae in particular, as carbonate component of the sediment. The precipitation of calcium carbonate is a source of CO2, whereas carbonate dissolution results to lower it (Frankignoulle et al, 1994). Therefore, carbonate dissolution is expected to buffer ocean acidification and to play an important role in the global change, though it is likely that the natural process of carbonate dissolution will be too slow to neutralize the fossil fuel CO2 (Broecker & Takahashi, 1977). Calcium carbonate occurs in natural biogenic sediments as aragonite or calcite. Magnesium (Mg2+) can substitute some of the Ca2+ in the lattice and the calcite containing >4% wt of MgCO3 is conventionally defined high-Mg calcite or Mg-calcite. The extent to which a biogenic carbonate particle is prone to dissolution in seawater depends from several factors, among which the leading ones are mineralogy and the calcium carbonate saturation state , which is dependent from the [CO32–]. The solubility of CaCO3 increases with depth. Presently, surface ocean waters are supersaturated with respect to both aragonite and calcite, but the aragonite and calcite saturation horizons of the world’s oceans are moving to shallower depths at a rate of 1-2 m per year (Guinotte & Fabry, 2008) due to the rapid influx of anthropogenic CO2 to the oceans. Aragonite is more soluble than calcite in the same seawater, but natural Mg-calcites containing > 8- 12 mol% MgCO3 are more soluble than aragonite,being the most sensitive responder to a lowering pH (Morse et al, 2006; Kuffner et al, 2008). The recruitment rate and growth of crustose coralline algae is severely inhibited under elevated pCO2 (Hall-Spencer et al, 2008). Corallinales, which are a major framework builder in temperate and cold water, are vulnerable to ocean acidification due to the solubility of their high magnesium calcite cell-walls (Kuffner et al,2008). Cool-water carbonate-rich sediments of the world shelf, often dominated by calcareous red algae, are a major CaCO3 reservoir, that can react by dissolution to a decrease of the saturation state of the seawater (Morse et al, 2006). However,despite the occurrence of a suite of local data-base on various sediment characteristics, the algal carbonates have been not characterized and quantified on a large scale, therefore preventing any hypothesis on their role as buffer in a future scenario of acidified oceans. Shelf sedimentary facies of the temperate and cold zones are enriched in calcite and Mg-calcite (from mollusks, forams and red algae). By contrast, aragonite dominates the sediments of the tropical oceans (from corals and green algae). Existing deposits of highly soluble, Mg-calcite and aragonite sediments require quantification to assess their importance as a possible buffer to acidification. What is the contribution of algae to this carbonate budget? The scientific literature started to re-evaluate the geologic role of coralline algae in the seventies (Adey & McIntyre, 1973) since for many years the common occurrence of algal carbonates in non-tropical sediments was far to be a shared concept among geologists. More recently, some comprehensive books, reviews and a number of scientific papers on cool-water carbonates pointed out the importance of calcareous red algae as carbonate producer (among others: James & Clarke, 1997; Foster, 2001; Pedley & Carannante, 2006). In the nineties biologists started to quantify the growth of calcareous red algae by staining or other techniques, thus improving our knowledge at the organism level. However, modelling the response of the oceans to the global change require a quantification of the total carbonate and its mineralogy on regional scale. Large scale quantification of coralline algae in shelf carbonates is a challenge to the exploration of new methods of seafloor mapping, combined with analyses of sediment composition and mineralogy (Bracchi et al, 2009; Savini et al, 2009). References Adey W.H., McIntyre I.G. (1973). Crustose coralline algae: a re-evaluation in the geological sciences. - Geol. Soc. Am. Bull., 84: 883-904. Bracchi V., Caragnano A., Galimberti L., Basso D.(2009). Calcareous Algae and other Biogenic Carbonates in Selected Acoustic Facies of the Continental Shelf: Pontian Islands, Tyrrhenian Sea, Italy. – 6th Regional Symposium of the IFAA, Milan, 1-5 july 2009, Abstract book: 16. Broecker W.S., Takahashi T. (1977). Neutralization of fossil fuel CO2 by marine calcium carbonate. In: Andersen N. R., Malahoff A. (Eds.) The Fate of Fossil Fuel CO2 in the Oceans, Plenum, New York: 213– 248. Foster M. (2001). Rhodoliths: between rocks and soft places. - J. Phycol., 37: 659-667. Frankignoulle M., Canon C., Gattuso J.-P. (1994).Marine calcification as a source of carbon dioxide:positive feedback of increasing atmospheric CO2. - Limnol. Oceanogr., 39: 158–462. Guinotte J.M, Fabry V.J. (2008). Ocean acidification and its potential effects on marine ecosystems. In:Ostfeld R.S., Schlesinger W.H. (Eds.) The Year in Ecology and Conservation Biology 2008, Annals of the New York Academy of Sciences: 320–342. Hall-Spencer J.M., Rodolfo-Metalpa R., Martin S.,Ransome E., Fine M., Suzanne M., Turner S.M.,Sonia J., Rowley S.J., Tedesco D., Buia M.-C.(2008). Volcanic carbon dioxide vents show ecosystem effects of ocean acidification. - Nature 454: 96-99. James N., Clarke J.A.D. (1997). Cool water carbonates.- SEPM Special Publication 56, 432 pp. Kuffner I. B., Andersson A. J. et al. (2008). Decreased abundance of crustose coralline algae due to ocean acidification. - Nature Geoscience, 1(2): 114– 117. Morse J.W., Andreas J. Andersson A.J., Fred T. Mackenzie F.T. (2006). Initial responses of carbonate-rich shelf sediments to rising atmospheric pCO2 and ”ocean acidification”: role of high Mg-calcites. - Geochimica et Cosmochimica Acta, 70: 5814–5830. Pearson P.N., Palmer M.R. (2000). Atmospheric carbon dioxide concentrations over the past 60 million years. – Nature, 406: 695-699. Pedley H.M., Carannante G. (2006). Cool-Water Carbonates: Depositional Systems and Palaeoenvironmental Controls. - Geological Society, Special Publication 255, 384 pp. Royal Society (2005). Ocean acidification due to increasing atmospheric carbon dioxide. – The Royal Society Policy document 12/05, London, ISBN0854036172, Clyvedon Press Ltd, Cardiff,60 pp. Savini A., Bracchi V., Basso D., Corselli C., Pennetta M. (2009). Maërl facies distribution offshore Cilento Peninsula (Tyrrhenian Sea, Italy). - 6th Regional Symposium of the IFAA, Milan, 1-5 July 2009, Abstract book: 54.