ND ISOTOPE EVIDENCE FOR THE EVOLUTION OF THE PALEOCURRENTS IN THE ATLANTIC AND TETHYS OCEANS DURING THE PAST 180 MA

The Nd isotopic composition of Atlantic and Tethys seawater, as deduce d from marine phosphorites, varied considerably during the past 180 Ma . The early Tethys and Central Atlantic seawater from 180 to 160 Ma ag o (Early-Middle Jurassic) had a Nd isotopic signature identical to tha t of the Pacific (epsilon(Nd)approximate to-6) suggesting that Pacific seawater entered the newly forming Tethys basin. However, with time c ontinental runoff draining into the young basin became more important and led to a decrease in the Nd isotopic composition and, finally, a d ecoupling from the Pacific Ocean. During the late Early Cretaceous (12 0-90 Ma ago) Atlantic and Tethys seawater reached continental crust-li ke Nd isotopic composition values (epsilon(Nd)approximate to-10) which could reflect high weathering rates induced by the warm and humid cli mate at that time. In the time span between 80 and 50 Ma (Late Cretace ous-Early Tertiary) the Nd isotopic composition of both Tethys and Atl antic seawater strongly increased towards Pacific seawater values. In the case of the Atlantic Ocean, this change has been correlated with t he opening of the South Atlantic, which enabled the more radiogenic Pa cific seawater to travel westwards around the southern edge of Africa into the South and finally the North Atlantic. In the case of Tethys s eawater we have to assume that, during this period, large masses of Pa cific seawater entered the Tethys again directly through the Indian-Te thys seaway. The renewed importance of this seaway might be related to the Late Cretaceous-Early Tertiary first-order, global sea level high stand. The most abrupt change towards lower, more crust-like values oc curred in the early Miocene (25-17 Ma ago) in the South and North Atla ntic, as well as in the Tethys. During this period, the Nd-143/Nd-144 isotope ratios dropped from approximate to 0.5124 (epsilon(Nd)approxim ate to-4.2) down to approximate to 0.5122 (epsilon(Nd)approximate to-8 ). This change indicates a diminishing influx of Pacific seawater into the Atlantic and Tethys Oceans and can be correlated with the breakdo wn of the circum-equatorial circulation patterns of the world oceans, due to plate tectonics. From 20 to 17 Ma ago (Miocene) the Nd isotopic composition of the Atlantic Ocean increased again and reached values as high as 0.5123 (epsilon(Nd)approximate to-6.6), whereas Tethys seaw ater remained at low values. This decoupling can be correlated with th e incipient complete isolation of the Tethys; the Nd isotopic composit ion was henceforth controlled by the continental runoff from surroundi ng land masses. The major increase in the Nd isotopic composition obse rved in the Atlantic can be related to the opening of the Drake Passag e and establishment of the circum-Antarctic current system. This allow ed Pacific seawater to enter directly the South Atlantic around Antarc tica. A renewed drop in the Nd isotopic composition of Atlantic seawat er during the late Miocene can be related to the onset of North Atlant ic Bottom Water formation and its subsequent upwelling.