Primary production response to orbitally induced variations of the southern oscillation in the equatorial Indian Ocean
Résumé
In surface samples from the Indian Ocean, assemblages of Coccolithophorids (one of the most common oceanic phytoplankton groups) are clearly linked to surface primary production (PP). In oligotrophic regions nannofloras are dominated by Florisphaera profunda and small (2-3 mu m) Gephyrocapsa, in regions of high productivity by Gephyrocapsa oceanica and Emiliania huxleyi. We calibrated the spatial distribution of two nannofossil indices to annual mean PP as estimated from Coastal Zone Colour Scanner satellite images. The first index is based on the relative abundance of F. profunda in the total nannoflora, and the second one on the relative abundance of G. oceanica in the Gephyrocapsa assemblage. We applied this method to samples from cores MD900963 (5 degrees 03N-73 degrees 53E at a water depth of 2,446 m) and MD900949 (2 degrees 05N-76 degrees 07E, at a water depth of 3,700 m). Continuous records over the last 910 kyr and 420 kyr respectively show strong fluctuations in primary productivity, which are coherent and in phase with the February insolation at the equator. Primary productivity slight leads oxygen isotope values of planktonic forminifera in the precession band, but in the eccentricity band PP and delta(18)O are in opposite phase. These data imply that the PP in the equatorial Indian Ocean is controlled directly by insolation, and is thus independent of fluctuations in global ice volume. We suggest that this control is exerted through the intensity of the Indian Equatorial Westerly winds (IEW), which show strong interannual variability as a result of Southern Oscillation, and that precessional forcing of the Southern Oscillation thus drives the PP fluctuations. We speculate that the Walker circulation varies independent From global ice volume variations, and significantly influences PP.