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 U k′ 37 method. The U k′ 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 U k′ 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 δ 18O. 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.