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MEDITERRANEAN CLIMATE EVOLUTION AND CONNECTION WITH THE ATLANTIC OCEAN: INFERENCES FROM HIGH-RESOLUTION MARINE ARCHIVES

Final Report Summary - MEDAT-ARCHIVES (Mediterranean Climate Evolution and Connection with the Atlantic Ocean: Inferences from High-resolution Marine Archives)

Paleoceanographic reconstructions require the development of geochemical proxies and tracers that can eventually be applied to marine archives to obtain reliable information on the past variations of key environmental parameters, such as temperature, salinity, pH, nutrients and water mass circulation. Scleractinian corals are uniquely suited to provide multi-century, sub-annual resolution paleoceanographic records since they can be precisely dated by 230Th/U technique and they systematically incorporate isotopic and chemical tracers that reflect the environmental conditions of the ambient seawater (Mitsuguchi et al., 1996; McCulloch et al., 1999; Gagan et al., 2000; Pelejero et al., 2005; Montagna et al., 2006; 2007). However, recent data have shown that most of the geochemical signals in corals, especially trace elements and stable isotopes, are biologically mediated and various models have been proposed to explain the role of the coral physiology in controlling the isotopic and elemental uptake (for a review see Montagna, 2010).

The MEDAT-ARCHIVES project had two closely-linked scientific objects:

1) investigate in detail the geochemical signals encoded in Mediterranean shallow- and deep-water corals to better understand the biological component imprinted in the geochemical variability;
2) reconstruct some of the most important physical and chemical marine parameters using classical and recently developed geochemical proxies, with a focus on the variability of the seawater temperature, pH and water mass circulation.

In order to fully achieve these objectives the fellow applied a geochemical multi-proxy approach (minor/trace elements, U/Th, 14C, Nd and B isotopes) to a suite of live-collected and fossil corals (Cladocora caespitosa, Lophelia pertusa, Madrepora oculata, Desmophyllum dianthus) retrieved at different water depth, from shallow to bathyal environments. All the samples listed in the project were analysed using the cutting-edge analytical facilities hosted at Lamont-Doherty Earth Observatory at Columbia University and at the Laboratoire des Sciences du Climat et de l'Environnement in Gif-sur-Yvette, in collaboration with the MC mentors (Prof. Steve Goldstein and Dr. Norbert Frank), several other researchers, technicians and PhD students.

The results of the MEDAT-ARCHIVES project largely improved the knowledge of the coral geochemistry with important ramifications on the understanding of the climate signals encoded in the coral skeleton. The fellow demonstrated that it is possible to obtain a pure temperature signal from corals by normalising the Li/Ca to the Mg/Ca ratio (i.e. Li/Mg). By doing so, the growth rate dependence of the two ratios is cancelled out and the seawater temperature variations can be properly reconstructed. This new approach was then applied to precisely dated coral fragments to reconstruct the temperature variation for climatically-relevant time-windows in the past. Deep-water coral skeletons (Madrepora oculata) embedded in two sediment cores from the Siculo-Tunisian Strait were analysed for U/Th, Li/Mg and boron isotopes. The fellow produced a record of seawater temperature and pH over the last 150 000 yrs. Although discontinuous, this time-series represents the first record of sub-surface temperature and pH for the Levantine Intermediate Water, which constitutes up to 80 % of the Mediterranean outflow. The reconstruction of key chemico-physical parameters for this water mass is crucial to the evaluation of the impact of the Mediterranean thermohaline circulation on the Mediterranean outflow and eventually to the pre-conditioning of the North Atlantic Deep Water.

The fellow produced the first high-resolution time series of pre- and post-bomb radiocarbon concentration for the Mediterranean Sea through the analysis of a long-lived specimen (53 years) of the shallow-water coral Cladocora caespitosa from the northern Tyrrhenian Sea. The sample was further analysed for Li/Mg ratios to reconstruct the sea surface temperature.

Most of the living and fossil coral specimens were analysed for neodymium isotopes to prove the ability of corals to serve as water mass circulation archives and to reconstruct the past Mediterranean water mass dynamics. This geochemical exercise was also conducted on other calcifying organisms (gorgonian corals, bivalves, barnacles) with similar important results: all the biogenic carbonates reliably record the Nd isotopic composition of the ambient seawater without biological fractionation. Therefore, most calcifiers are potentially good archives to trace the water mass circulation in the past.

Finally, another key result of the MEDAT-ARCHIVES project was the understanding of the calcification mechanisms of zooxanthellate and azooxathellate corals in relation to ocean acidification and global warming. This was achieved using a newly-developed approach that applies boron isotopes to calculate the 'internal' pH of the coral. These findings have profound consequences as they enable to predict the calcification rates of different coral species under different pCO2 and temperature scenarios.

Over the last three years, the fellow acquired the expertise to fully independently conduct isotopic measurements for most of the isotopic systems used in paleoceanography, including U, Th, Sr, B and Nd isotopes, which strongly differ in their analytical setup and data treatment procedures. These skills are highly portable and they can be transferred to other laboratories. During the project, the fellow established fruitful collaborations with several researchers in United States of America (USA) and Europe, which already led to jointed projects and requests for oceanographic cruises.