11 Mediterranean Cold-Water Corals as Paleoclimate Archives

Abstract

Scleractinian cold-water corals preserve in their aragonite skeleton information on the past changes of the physico-chemical properties of the seawater in which they grew. Such information is stored as geochemical signals, such as changes in trace elements concentration (B/Ca, Li/Mg, P/Ca, Sr/Ca, Ba/Ca, U/Ca) or stable and radiogenic isotopes composition (δ11B, δ13C, δ18O, 14C, εNd), that are usually converted into environmental parameters using empirical calibration equations. The aragonite skeleton of cold-water corals is sufficiently uranium-rich to be suitable for U-series dating, providing precise and accurate ages for the last 600–700 kyrs. This opens the possibility to obtain reconstructions of key oceanographic parameters for the intermediate and deep water masses at sub-decadal scale resolution for climatically-relevant time windows in the past. However, part of the geochemical signal incorporated into the coral skeleton is modulated by the physiology of the coral, which complicates the interpretation of the geochemical proxies. This “vital effect” needs to be taken into account and corrected for to obtain reliable reconstructions of past changes in seawater temperature, pH and nutrient content. On the other hand, these biologically-induced geochemical signals can be used to investigate the processes controlling coral biomineralisation and better understand the resilience of cold-water corals to environmental and climate changes. In the recent years, Mediterranean cold-water corals have been targeted for geochemically-oriented studies and their trace elements and isotopes composition has contributed significantly to developing and understanding new and established coral proxies. Living in an environment characterised by relatively warm seawater temperatures (13–14 °C) and high pH (8.1), the Mediterranean cold-water corals provide the end-member geochemical composition useful to derive empirical calibration equations. In particular, the analysis of several specimens of the cold-water corals species Lophelia pertusa, Madrepora oculata and Desmophyllum dianthus live-collected in the western, central and eastern Mediterranean Sea, has contributed to the development of the Li/Mg thermometer, boron isotopes pH proxy and P/Ca nutrient proxy, as well as a better understanding of the neodymium isotopic composition of cold-water corals as a water mass tracer. A multi-proxy approach has been recently applied to precisely U/Th-dated cold-water corals fragments from coral-bearing sediment cores retrieved in the western and central Mediterranean Sea, showing large changes in the dynamics of the intermediate waters during the Holocene. Further investigations of fossil cold-water corals specimens from different Mediterranean locations will open new perspectives on the reconstruction of past changes in the physico-chemical properties of sub-surface waters and their potential role in modifying the Mediterranean climate.