Mineralogy of Antarctic modern biogenic carbonates 1 Mazzoli C., 2 Montagna P., 3 Anderson J.B., 2,4 Taviani M., 1 Zorzi F. 1 Department of Geosciences, University of Padova, Via Gradenigo 6, Padova, Italy 2 Institute of Marine Sciences (ISMAR)-CNR, U.O.S. Bologna, via Gobetti 101, 40129, Bologna, Italy 3 Department of Earth Science, Rice University, 6100 Main Street, Houston, TX 77005, USA 4 Biology Department, Woods Hole Oceanographic Institution, 266 Woods Hole Road, Woods Hole, MA 02543, USA We are grateful to members of the XXIX Antarctic expedition (austral summer ) and in particular to Navy Marshals Francesco Reale, Daniele Risina, and Gianluca Giannotti who supported all the ship operations. The GEOSMART project (GEOchemical Signatures in the Antarctic MARine carbonate sysTem: present, past and future implications; PI: Paolo Montagna ISMAR-CNR) is funded by the Italian Antarctic Research Program (PNRA) The increase of atmospheric CO 2 partial pressure (pCO 2 ) connected to human activities, causes an increase of dissolved inorganic carbon and decrease of pH in surface seawater, potentially frightening calcifying organisms. The response of these organisms to ocean acidification depends on their capacity to up-regulate pH in the calcifying sites and on the specific calcium carbonate mineral secreted, determining stress levels that could be fatal. Moreover, the lowering of the saturation state with respect to calcite or aragonite increases the carbonate dissolution, potentially shifting the balance toward a state of net CaCO 3 loss. Surface seawater in the Southern Ocean is already characterized by low carbonate saturation conditions, and represents an ideal target for the study of the effects of ocean acidification on the calcifying organisms. Furthermore, because subfreezing temperatures prevent precipitation of inorganic cements, modern marine carbonates in Antarctica are biogenic in origin resulting from calcifying organisms. Such carbonates formed under the extreme polar conditions only to be found in Antarctica, are important sources of oceanographic and climatic information. A necessary prerequisite for their use as proxies in geochemical studies and for a better understanding of their potential response to ocean acidification is the precise assessment of the skeletal original mineralogy. We have, therefore, started a complete screening of constituents of Antarctic biogenic carbonates in the frame of the Geosmart project, funded by the Italian Antarctic Research Program (PNRA). A variety of modern skeletal parts belonging to dominant benthic and planktic calcifiers (molluscs, scleractinians, octocorals, serpulids, brachiopods, bryozoans, barnacles, echinoderms, foraminifers, ostracods, coralline algae) has been tested at species-level using X-ray powder diffraction (XRPD), m-Raman, optical microscopy (OM) and cold cathodoluminescence (CL) to recognize the carbonate phases (calcite, low-Mg calcite, high-Mg calcite, aragonite, vaterite) and identify the distribution of major elements. When different carbonates are detected (e.g. calcite and aragonite), their structural location and weight fractions were determined. The systematic determination of the mineralogy of the calcifying organisms along with the knowledge of their relative role in the ecosystems, is the first step towards the understanding of the species specific and ecosystemic vulnerability to increasing CO 2 levels. ANT-1 (Adamussium colbecki) ANT-16 ANT-17 OMCL M-Raman XRPDumbo region SampleCodeLocalityClassSpeciesDescriptionMain Carbonate%Other phases% ANT-1XXIX-150Tethys BayBivalviaAdamussium colbeckiDR3 ANT-1a umbo regionCalcite93.5Aragonite6.5 ANT-1b marginal regionCalcite ANT-2XXIX-691Adelie CoveBivalviaLaternula ellipticaDR9 ANT-2a umbo regionAragonite98.5Mg-calcite1.5 ANT-2b marginal regionAragonite98.5Calcite1.5 ANT-3XXIX-601 BivalviaLimatula hodgsoniDR7 ANT-3a umbo regionCalcite70.2Aragonite29.8 ANT-3b marginal regionCalcite97.3Aragonite2.7 ANT-4XXIX-616Adelie CoveBivalviaYoldia eightsiDR9Aragonite98.2Calcite1.8 ANT-5 McMurdo SoundBivalviaThracia meridionalis Aragonite98.0Calcite2.0 ANT-6XXIX-143Mt. CrummerBivalviaLimopsis marionensis ANT-6a umbo regionAragonite98.6Calcite1.4 ANT-6b marginal regionAragonite98.7Calcite1.3 ANT-7XXIX-146Tethys BayGastropodaNeobuccinum eatoniDR2Aragonite98.8Calcite1.2 ANT-8XXIX-560Adelie CoveGastropodaAmauropsis rossianaDR9Aragonite98.6Calcite1.4 ANT-9XXIX-177 BrachiopodaTerebratulidaeDR6Calcite ANT-10XXIX-555 BryozoaReteporella parvaROV2 - Type 2Mg-calcite ANT-11XXIX-175 PolychaetaSerpula narconensisDR6Mg-calcite ANT-12NBP grab CirripedBathylasma corolliforme Calcite ANT-13CARB1-st-29 HydrozoaErrina antarctica Mg-calcite95.8Aragonite4.2 ANT-14XXIX-176 AnthozoaBamboo coralDR6Mg-calcite ANT-15XXIX-018Cape Russell (T3)AnthozoaGardineria antarctica Aragonite91.4Mg-calcite8.6 ANT-16 Tethys BayBryozoa Encrusting bryozoan on AdamussiumMg-calcite ANT-17 Tethys BayPolychaetaSpirorbisSerpulid on AdamussiumMg-calcite ANT-18 Powder scratched from Adamussium surfaceCalcite91.4Mg-calcite8.6 ANT-19 Tethys Bay Surface of ANT-13, with organic matterMg-calcite Weddellite< 5% Carotenoids + Calcite Calcite Aragonite Carotenoids + Calcite marginal region Aragonite Calcite ANT-3 (Limatula hodgsoni) umbo region marginal region XRPD OM CL M-Raman Calcite Calcite + Aragonite ANT-13 (Errina antarctica) Aragonite Calcite Aragonite Calcite Aragonite Mg-Calcite OM CL XRPD M-Raman Mg-Calcite Carotenoids + Mg-Calcite ANT-7 (Neobuccinum eatoni) XRPD Aragonite Calcite pinkwhite 3, 4 6 CL OM M-Raman Aragonite Carotenoids + Aragonite Aragonite