Estimating Mediterranean Oligocene – Miocene sea-surface temperatures: an approach based on coral taxonomic richness
Résumé
During the Oligocene and Miocene, shallow-water carbonates of the Mediterranean region were rich in scleractinian corals thriving within various depositional settings, including different reef types. Their diversity patterns, although related to a complex interplay between a suite of environmental factors and palaeobiogeography, are considered to be strongly controlled by climate variability 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 102 Oligocene–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 the Mediterranean z-coral generic pool. For better testing the method and assessing its potential application, patterns of generic richness and inferred palaeotemperatures are then compared with global palaeoclimatic curves based on marine oxygen stable isotopes data or other 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 global changes 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 with important 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, together with a gradual decrease of palaeotemperatures in the region. From theMiddleMiocene onwards to theMessinian, however, an increase in the temperature range of z-coral localities is clearly visible, indicating that z-coral communities were able to thrive and adapt to a wider temperature range, as the Mediterranean was gradually 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 estimating Cenozoic palaeotemperatures, from coral or other suitable fossil assemblages of shallow-water carbonates.