FERTILITY TRACERS AND MONSOON FORCING AT AN EQUATORIAL SITE OF THE SOMALI BASIN (NORTHWEST INDIAN-OCEAN)

Temporal changes in biological and isotopic tracers have successfully been used to monitor monsoon variability in the Arabian Sea. Convincin g evidence that fertility indicators, such as the relative abundance o f the planktic foraminifer Globigerina bulloides, and SiO2 and Ba flux es, can monitor monsoon changes, has also been published. Time series of the C-13 content of the thermocline foraminifer Neogloboquadrina du tertrei, and of an Upwelling Radiolarian Index (URI), have been used t o reconstruct upwelling changes in the Somali Basin at 5 degrees N for the last 160 kyr. In order to establish a reference site for comparis on with other upwelling and/or fertility records from the same area, a time series of 8 climatic and fertility proxies (C-13 content of N. d utertrei and Globorotalia menardii, percentages of G. bulloides and of a thermocline foraminiferal group, minimal delta(18)O values of Globi gerinoides sacculifer, N. dutertrei and G. menardii) are investigated in the spectral domain for the last 360 kyr, at a site located at the equator and outside the Somali upwelling centers (Core MD 85668). Chro nostratigraphies for these records are developed by correlation of oxy gen isotope record of G. sacculifer to the Martinson standard stacked benthic oxygen isotope record. The temporal resolution of the data, is between 2 and 5 kyr. Cross spectral comparisons with ETP (Eccentricit y + Tilt + Precession-composite signal) were used to estimate coherenc es in conjunction with phase relationships and to quantify relationshi ps between fertility and climatic indicators. The timing of fertility indicator cycles at the equator shows little (obliquity band) or no (o ther orbital bands) coincidence with monsoon proxies of the Arabian Se a. At the equator, fertility responses are dissociated, and present im portant leads or lags with ETP signal at the three orbital bands (with the exception of the thermocline foraminifera, in phase with maximum eccentricity). Due to significant leads with ETP in the timing of C-13 minima and URI cycles, no relationships with the Northern Hemisphere radiation can be deduced. At the obliquity and precessional frequency bands, thermocline foraminifera cycles display significant lag with ET P and ice volume minima, which suggests that radiation is not the sole forcing mechanism. Global climatic and ice volume changes may be an i mportant forcing mechanism for these fertility proxies. The G. bulloid es record is more complicated with cycles in phase with Arabian Sea mo nsoon proxies at the obliquity band, and O-18 minima at the precession frequency. It appears that, at the equator, the southwest monsoon win ds are not the main force driving radiolarian and foraminiferal produc tivity.