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

Abstract

The Nd isotopic composition of Atlantic and Tethys seawater, as deduced from marine phosphorites, varied considerably during the past 180 Ma. The early Tethys and Central Atlantic seawater from 180 to 160 Ma ago (Early-Middle Jurassic) had a Nd isotopic signature identical to that of the Pacific (εNd # -6) suggesting that Pacific seawater entered the newly forming Tethys basin. However, with time continental runoff draining into the young basin became more important and led to a decrease in the Nd isotopic composition and, finally, a decoupling from the Pacific Ocean. During the late Early Cretaceous (120-90 Ma ago) Atlantic and Tethys seawater reached continental crust-like Nd isotopic composition values (εNd # - 10) which could reflect high weathering rates induced by the warm and humid climate at that time. In the time span between 80 and 50 Ma (Late Cretaceous-Early Tertiary) the Nd isotopic composition of both Tethys and Atlantic seawater strongly increased towards Pacific seawater values. In the case of the Atlantic Ocean, this change has been correlated with the opening of the South Atlantic, which enabled the more radiogenic Pacific seawater to travel westwards around the southern edge of Africa into the South and finally the North Atlantic. In the case of Tethys seawater we have to assume that, during this period, large masses of Pacific seawater entered the Tethys again directly through the Indian-Tethys seaway. The renewed importance of this seaway might be related to the Late Cretaceous-Early Tertiary first-order, global sea level highstand. The most abrupt change towards lower, more crust-like values occurred in the early Miocene (25-17 Ma ago) in the South and North Atlantic, as well as in the Tethys. During this period, the 143Nd/144Nd isotope ratios dropped from # 0.5124 (εNd # -4.2) down to # 0.5122 (εNd # -8). This change indicates a diminishing influx of Pacific seawater into the Atlantic and Tethys Oceans and can be correlated with the breakdown 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 (εNd # -6.6), whereas Tethys seawater remained at low values. This decoupling can be correlated with the incipient complete isolation of the Tethys; the Nd isotopic composition was henceforth controlled by the continental runoff from surrounding land masses. The major increase in the Nd isotopic composition observed in the Atlantic can be related to the opening of the Drake Passage and establishment of the circum-Antarctic current system. This allowed Pacific seawater to enter directly the South Atlantic around Antarctica. A renewed drop in the Nd isotopic composition of Atlantic seawater during the late Miocene can be related to the onset of North Atlantic Bottom Water formation and its subsequent upwelling.