CONSTRAINING NUTRIENT EFFECTS IN MONSOON-DRIVEN PRODUCTIVITY SHIFTS DURING THE MEDITERRANEAN SAPROPEL EVENTS

Organic rich sapropel layers deposited in the Eastern Mediterranean during the Quaternary correspond to enhanced monsoon penetration into North Africa during precessional summer insolation maxima. Sapropels are believed to reflect enhanced productivity. Coeval organic rich sapropels are not found in the Western Mediterranean. Some explanations for elevated Eastern Mediterranean productivity include increased riverine nutrients or shallowed nutricline with productivity increase confined to the deep photic zone. Sr/Ca ratios of coccoliths, generally correlated with productivity in recent sediments, were measured in an Eastern Mediterranean core (BC07) and Western Mediterranean core (274G) covering the last 10,000 years. Sr/Ca was determined in distinct size fractions of coccoliths by inductively coupled mass spectrometry, and on individually picked coccoliths of several species by secondary ion mass spectrometry (ion probe). This technique allows us to profile the productivity response of individual species in different ecological niches. Surface-dwelling species from the Eastern Mediterranean samples experience a large increase in Sr/Ca ratios, and hence productivity, whereas in the Western Mediterranean, there is no major productivity increase. In order to establish the relationship between a monsoonal event and altered productivity and calcification rates of the coccolithophores, we investigate the environmental-biological stimuli responsible for the Sr/Ca response. To this end, four separate species (Emiliania huxleyi, Gephyrocapsa oceanica, Calcidiscus leptoporus, Coccolithus pelagicus ) were grown in continuous culture and exposed to limited nutrient conditions. The chemostats were performed with the use of a peristaltic pump, with dilution rates controlled at .33, .67, and 1 dilution per day. Daily measurements were taken to determine Sr/Ca ratios, POC and PIC, cell density, and changes in cell morphology. The results of these chemostats can elucidate species-specific responses to variations in nutrient supply, the background information needed to accurately establish the reasons for the occurrence and the absence of a sapropel layer in the Eastern and Western Mediterranean cores.