Estimating Mediterranean Oligocene-Miocene sea-surface temperature:an approach based on coral taxonomic richness

During the Oligocene and Miocene, shallow-water carbonates of the Mediterranean region were rich in scleractinian coralsthriving within various depositional settings, including different reef types. Their diversity patterns, although related to a complexinterplay between a suite of environmental factors and palaeobiogeography, are considered to be strongly controlled by climatevariability and changes in sea-surface water temperature.By using the quantitative relationship between present-day coral taxonomic richness and prevailing sea-water temperature,underlined by the so-called “energy hypothesis”, we test zooxanthellate-coral generic richness values from a selection of 102Oligocene–Miocene localities of the Mediterranean region as a proxy for relative palaeotemperatures.For each Oligocene–Miocene stage, generic richness values per z-coral site are firstly examined, together with variations of theMediterranean z-coral generic pool. For better testing the method and assessing its potential application, patterns of generic richnessand inferred palaeotemperatures are then compared with global palaeoclimatic curves based on marine oxygen stable isotopes data orother climate proxies, such as palaeoclimatic records from European continental floras and from fossil coral linear extension rate.Results clearly show that fluctuations of coral richness-derived palaeotemperatures correspond relatively well with globalchanges of sea-water temperature especially for the entire Oligocene, the Chattian–Aquitanian boundary and the Late Miocene.The well known Mid-Miocene Climatic Optimum, however, is not recorded, suggesting that regional factors, acting together withimportant palaeogeographical changes, exerted a strong control on the generic richness of Mediterranean z-coral communities.A remarkable decline of taxonomic richness is recorded after the Burdigalian, togetherwith a gradual decrease of palaeotemperaturesin the region. From theMiddleMiocene onwards to theMessinian, however, an increase in the temperature range of z-coral localities isclearly visible, indicating that z-coral communities were able to thrive and adapt to a wider temperature range, as theMediterranean wasgradually migrating northwards, outside the tropical belt.The “energy hypothesis”, if used at global or regional scale, can be considered a promising and reliable method for estimatingCenozoic palaeotemperatures, from coral or other suitable fossil assemblages of shallow-water carbonates.