Ostracodes: witnesses of the Plio-Quaternary boundary in the North Eastern Atlantic, Preliminary results

Abstract

Key words: Ostracodes; Morphological variability; Seasonal contrasts; Climatic change; Plio-Pleistocene. The 2,4 MY crisis is an important event for the upper Neogene of North Atlantic. A new climatic system occur and widespread during the Quaternary and is characterized by glacial and interglacial alternance. To follow the different steps of this crisis, we have choosen a borehole situated in the Cotentin peninsula (Normandy, France) and used ostracodes such as markers. The interpretation of quantitative and qualitative data from the ostracode fauna analysis shows that bathymetry was around 40 and 60 meters with slight fluctuations, excepted during Praetiglian event where the zone was out of water. The morphological variability analysis of four species (the best represented in the core), show a low variability between the basement and 80 meters. Beyond this limit and after 10 meters, characterized by very low values, the variability increases to very high values to the top of the core. Present data; Abe et Choe, 1988; Peypouquet et al., 1988; Carbonel et al., 1990 a, 1990 b; show that there is a close link between seasonal contrast intensity and ostracode intraspecifical morphological variations. The carbonates balance at the interface water / sediment, were ostracodes live and moult, is related and reflects the intensity of the seasonal contrast. After the Praetiglian event, an increase of the seasonal contrast occurs during the Tiglian A and the lower Tiglian B.