Upwelling history of the Mediterranean Sea revealed by stunted growth in the planktic foraminifera Orbulina universa (early Messinian, Crete, Greece)

Microfossil and stable isotope data (δ13C, δ18O) from deep-water sediments of Late Miocene age in the Mediterranean region have revealed a stepwise restriction of the Mediterranean prior to the Messinian Salinity Crisis which was modulated by a cyclicity responding to orbital precession. Very little is known with regard to the effects of these changes on shallow water environments of carbonate platforms and ramps. This work is based on a geological section on Crete (Greece) exposing sediments of early Messinian age, which documents the coeval response of pelagic and neritic systems to oceanographic changes. We discuss a dataset of planktic and benthic foraminifera assemblages in conjunction with size measurements (n = 6,777) and isotope analyses (δ18O, δ13C) of the planktic foraminifera Orbulina universa with regard to reef growth patterns. The planktic foraminifera fauna displays pronounced cyclical abundance changes of warm, oligotrophic (O. universa) and cold, meso- to eu-trophic taxa (globigerinids, neogloboquadrinids). This cyclicity corresponds to lithological changes from laminated to homogeneous marls on the precessional frequency band. In beds rich in mesotrophic foraminifera, O. universa is rare and the test is ~50 % smaller than average. Growth and size of O. universa is affected by various environmental factors, including temperature, trophic resources, illumination and O2 content of ambient water. Explaining size variability by water temperature implies a change of ~11 °C over one precessional cycle. This estimate is in disagreement with growth patterns and stable isotope data of displaced reef corals (Porites, Tarbellastraea) documenting almost constant sea-surface temperatures, but substantial evaporation. Therefore, moderate temperature changes in a context of intense evaporation (high salinity) and water column destabilization associated with upward mixing of colder water masses with higher nutrient concentrations and low O2 content better fits size variation of O. universa. From the presence of Halimeda bioherms intercalated with the sapropel cycles, we infer cyclic photic zone anoxia and at least some sections with sapropel couplets to have formed in the neritic zone. We also suggest further testing of stunted growth of O. universa as a proxy for enhanced nutrient availability in future palaeoceanographic studies.