Variations of upwelling intensity recorded in varved sediment from the Gulf of California during the past 3,000 years

Abstract

Circulation in the Pacific Ocean is periodically affected by El Niño events which are a direct response to intensity changes in trade winds^1,2. The appearance of anomalously warm surface water in the eastern equatorial Pacific Ocean is one of the most spectacular manifestations of this event. Warm surface-water anomalies are also observed in the Northern Hemisphere along the Californian coast and in the Gulf of California. In the Gulf, strengthened trade winds induce upwelling during the winter, whereas in the summer, prevailing southerly winds provoke an input of warm water from the south³. We have sampled laminated sediments from the slopes of Guaymas Basin to reconstruct, with high resolution, variations of the hydrology of surface waters in the central part of the Gulf of California (sedimentation rate is of the order of 4 mm yr⁻¹ at the top and 1–2 mm yr⁻¹ down core). By analysing the oxygen isotope ratio of biogenic silica of diatoms⁴, we have recorded the surface-water temperature variations in the Guaymas Basin. The comparison of these fluctuations and variations of silicoflagellate abundance has allowed us to reconstruct changes in upwelling intensity during the past three thousand years. The present conditions of low upwelling activity are seen to be abnormal in comparison with those prevailing in the past.