MIOCENE HISTORY OF PRODUCTIVITY, CARBONATE BURIAL AND TERRIGENOUS INPUT IN THE SOUTHERN INDIAN OCEAN

Miocene deep-sea deposits off SW Australia recovered by IODP Expedition 369 reveal the history of carbonate and biosiliceous deposition at southern mid-latitudes during Earth’s warmest period of the last 20 Ma. Middle Miocene sedimentation at 369 Sites (paleolatitudes of ~40°S) recorded the signals of paleoenvironmental changes in the SE Indian Ocean, including the latitudinal migration of the Westerly Winds and its potential effects on Earth’s carbon cycle. We present a multi-proxy dataset from Site U1514 to evaluate the timing and magnitude of depositional changes during the Middle Miocene Climatic Optimum (~17-15 Ma) and their potential links to ocean circulation and to the shifting Westerlies. Shipboard sedimentology and biostratigraphy show that biosiliceous deposition occurred throughout the early and middle Miocene, intensifying between ~18.6-17.3 Ma. Terrigenous deposition increased first at ~18.6 Ma Ma and again between 17.3-16 Ma, as revealed by the presence of clay minerals in these intervals. In addition, high-resolution x-ray fluorescence data reveal a “crash” in carbonate deposition between ~18.6-17.3 Ma, followed by an increase in oxic conditions and terrigenous input between ~17.3-16.3 Ma. Subsequently, proxy values for ocean productivity and organic export increased between 16.7-16 Ma. Previous studies have suggested that periods of strengthened southern Westerlies result in increased ocean ventilation and carbon degassing in the Southern Ocean via intensified overturning circulation. In addition, intense Westerlies favor terrigenous input and ocean productivity off SW Australia via intensified precipitation and upwelling. We hypothesize the increase in ocean productivity and organic export observed at ~16.3 Ma may reflect strengthened southern Westerly Winds during the early phase of the Middle Miocene Climatic Optimum, which would enhance ocean degassing at southern mid-latitudes prior to warming maxima.