Dinoflagellate proxies for surface water property changes in the Miocene Atlantic Ocean

Abstract

Oceanographic and environmental changes had a strong impact on the climate progression during the middle and late Miocene. The climatic cooling during this time interval, involving the glaciation of Antarctica and the development of the Antarctic Circumpolar Current, influenced the initiation of oceanographic features of the South Atlantic Ocean, such as the Benguela upwelling in the eastern South Atlantic Ocean. The initiation of the Benguela upwelling, together with the associated Benguela Current, specifically had a strong impact on the global heat budget. It represented an important pathway for heat transport from the South Atlantic across the equator into the North Atlantic. This heat transport, combined with the return path via the North Atlantic Deep Water, encompasses a significant part of the thermohaline circulation. Another important development during the Miocene that affected thermohaline circulation was the closure of the Central American Seaway. However, it is still a matter of intense debate if thermohaline circulation, particularly the portion regarding equatorial heat transport via the North Brazil Current as well as the production of North Atlantic Deep Water, was possible at times of an open seaway between the Atlantic and Pacific. Samples from ODP Site 1085 were investigated for changes in productivity as well as water quality with data derived from calcareous dinoflagellate cyst associations. Two distinct increases in productivity and corresponding decreases in upper water temperatures reflect upwelling pulses off Namibia in relation to the Miocene cooling events, Mi-5 and Mi-6. At about 11.1 Ma, productivity increased and the polar species Caracomia arctica was found for the first time. The occurrence of C. arctica implies a fundamental change in the water quality, which was likely related to the influence of subantarctic water and allowed upwelling to fully develop into modern conditions. C. arctica is constantly found in the association from about 10.4 Ma on, which indicates that the Benguela upwelling regime was firmly established from this time forward. Investigations into changes in the accumulation rates as well as in the association of calcareous dinoflagellate cysts were undertaken on samples from ODP Site 926. Better calcite preservation based on the cyst accumulation rate was reconstructed from about 12.4-11.8 Ma, as well as at about 11.5, 10.1 and 9.7 Ma, pointing to the influence of North Atlantic Deep Water, which was forced to flow into the South Atlantic after a first uplift of the Panama Sill. From about 11.2 Ma, the dinoflagellate cyst diversity increased. The first appearance and subsequent increase of the cyst species Leonella granifera at this time implies an influence of river discharge waters, which can be related to the developing Amazon River. Amazon River waters reached Site 926 as a result of a southward reversal of the North Brazil Current. After about 10.5 Ma, the cyst association reflects a decrease in Amazon influence, probably related to a reduction in the inflow of Pacific waters into the Atlantic and a reversal of the North Brazil Current to its modern pathway. Furthermore, samples from Site 926 were analyzed for the isotopic composition of the calcareous dinoflagellates cyst Thoracosphaera heimii. The isotope trends reflect an influence of freshwater, derived from the developing Amazon River, and delivered by the southward-flowing North Brazil Current at about 10.9 Ma. After a strong decrease in isotope values, probably related to a major sea level low stand, the isotopes increases again. This could be the result of a decrease in Amazon freshwater influence. Amazon waters were, at this time, likely transported away from Site 926 by the North Brazil Current, which changed direction again to its modern flow pattern. This shift in the current direction was probably due to a decrease in Pacific inflow through the Central American Seaway.