Marta Pérez-Folgado

Western Mediterranean planktonic foraminifera events and millennial climatic variability during the last 70 kyr

Abstract Detailed study of associations of planktonic foraminifera in cores MD95-2043 and ODP 977, located in the Alboran Sea (Mediterranean Sea), has allowed the identification of 29 new faunal events, defined by abrupt changes in the abundances of Neogloboquadrina pachyderma (right and left coiling), Turborotalita quinqueloba, Globorotalia scitula, Globorotalia inflata, Globigerina bulloides and Globigerinoides ruber (white and pink varieties). The age model for ODP 977 was based on that of MD95-2043 [Cacho et al. (1999), Paleoceanogr. 14, 698–705], on the isotopic stratigraphy, and on two AMS 14C measurements. Sea Surface Temperatures (SSTs) were estimated for the last 54 kyr using the Modern Analog Technique (MAT) and were compared with the SSTs provided by the Uk′37 method. The Uk′37 record is very similar to the MAT annual mean temperature record for the last 8 kyr. However, for older times alkenone-derived temperatures are consistently higher than the annual MAT temperatures. This offset may be due to an underestimation of the SST provided by the planktonic foraminiferal method for glacial times, to an overestimation of the Uk′37 record, or to changes in the seasonal production of alkenones. Most of the variability in the fauna is related to the millennial variability of Heinrich and Dansgaard–Oeschger (D–O) events. During Heinrich events (HEs) and most of the other D–O stadials, G. bulloides, T. quinqueloba and G. scitula increased, while N. pachyderma (right coiling), G. inflata and G. ruber decreased. By contrast, N. pachyderma (left coiling) was only abundant in the HEs. The main component of the associations – N. pachyderma (right coiling) – follows a general trend similar to that of sea-level and δ18O. This species reached its highest abundance during the Last Glacial Maximum, when sea-level was at a lower position. The occurrence of a shallower nutricline owing to a shallowing of the interface between Atlantic inflowing and Mediterranean outflowing waters could have favoured the development of neogloboquadrinids in the vicinity of the Strait of Gibraltar.

A sediment–nutrient–oxygen feedback responsible for productivity variations in Late Miocene sapropel sequences of the western Mediterranean

Abstract Cyclic sediments observed throughout the Mediterranean during the Late Miocene have been related to precessional forcing of ocean stratification. Individual couplets, typically 2-m-thick sequences of sapropels and diatom-rich marls, can be reliably traced from western Spain to Crete, and were formed in restricted marginal basins. Micropaleontological evidence indicates paradoxically that the organic carbon-rich sapropels were formed under low productivity conditions marked by surface water stratification and deep anoxia, whereas the diatom-rich marls were formed under high productivity conditions marked by upwelling. Here we present geochemical evidence, mainly from detailed phosphorus determinations and paleo-redox proxies, indicating that a sediment–nutrient–oxygen feedback (herein dubbed the SNO Effect) is in part responsible for driving the observed productivity variations. During stratification, anoxic conditions in these basins cause the release of the limiting nutrient phosphorus from reducible oxide phases in the sediments. Basin stagnation causes the buildup of phosphorus below the photic zone. Subsequent overturn driven by precessionally-driven winds injects phosphorus-rich bottom waters into the photic zone. The biotic response to this overturn is high productivity in marls directly overlying the sapropels culminating in diatom mat formation. Exhaustion of the stored excess phosphorus results in lower productivity marls, which grade back into sapropels due to lower wind stresses and the return of stagnant basin conditions. These findings indicate that the SNO Effect may in part be responsible for precessional-scale productivity variations observed in parts of the Mediterranean, and perhaps present in other restricted ocean basins, like the Miocene Monterey Formation and the modern Santa Barbara Basin.

Western versus eastern Mediterranean paleoceanographic response to astronomical forcing: a high-resolution microplankton study of precession-controlled sedimentary cycles during the Messinian

Abstract Planktic foraminifera and stable isotopes were analyzed in samples from four precession-controlled sedimentary cycles from southeastern Spain and the island of Gavdos (Greece) to reconstruct the paleobiologic and paleoceanographic response of the western and eastern Mediterranean to astronomically driven climate variability during the Messinian. Although the lithological succession in the western Mediterranean cycles (sapropel–homogeneous marl–diatomite–homogeneous marl) is different from that in the eastern Mediterranean (sapropel–diatomite–homogeneous marl), the pelagic biological succession is similar. Four stages were recognized in both environments, characterized by the dominance of warm-oligotrophic foraminifera (stage 1), Neogloboquadrinids and Globorotaliids (stage 2), Globigerina bulloides (stage 3) and Globigerinita glutinata (stage 4). The first stage occurs during deposition of the upper part of the sapropels in Spain, and the sapropels and part of the diatomites on Gavdos. This stage is inferred to relate to high summer insolation and strong surface water stratification with winter temperatures not high enough to allow eutrophic foraminifera growth. Stage 2 occurs during the deposition of the lower homogeneous marls in Spain and the middle–upper part of the diatomites in Greece, most likely indicating stratification, with colder winter temperatures and a nutricline above the euphotic layer. The end of surface water stratification is indicated during stage 3 by the replacement of the Neogloboquadrinids by G. bulloides, which occurs at the base of the diatomites in Spain and near the top of the diatomites in Greece. We link this event to the appearance of dryer climates in the Mediterranean at times of relatively low summer insolation in the Northern Hemisphere which would result in higher surface salinities, deep water convection, and deep water ventilation. Stage 4, defined by the dominance of G. glutinata and low concentrations of planktic foraminifera, appears linked to the base of the sapropels both in the western and eastern Mediterranean. The onset of sapropel deposition occurred at a time of relatively high surface water salinities, as suggested by the heavy δ18O in Orbulina universa, and prior to the development of surface water stratification evident in the subsequent increase in warm-oligotrophic foraminifera. Gavdos diatomites were deposited under anoxic conditions, whereas Sorbas diatomites were formed in an oxygenated environment. The difference between western and eastern Mediterranean diatomites would be due to differences in hydrographic conditions between the two studied settings.