CENOZOIC INDO-PACIFIC GATEWAY CLOSURE

Changing land/sea distributions and water depths have influenced biogeography, climate and oceanic circulation in SE Asia and the West Pacific. Today the Indonesian Throughflow moves through the only low latitude oceanic passage and plays an important role in Indo-Pacific thermo-haline flow. The Indonesian gateway is likely to have been of considerable significance in the past. When did it close?

Tectonic reconstructions provide a guide but there is insufficient knowledge to be certain. Plate reorganisation in the Early Miocene was followed by rapid changes in tectonics, topography and land/sea distributions, but not in a simple unidirectional manner; as a deep-water barrier was eliminated and mountains rose, deep basins formed. Continent- and arc-continent collisions increased land and shelf areas, subduction maintained discontinuously emergent volcanic islands, but subduction also drove subsidence. Rapid Neogene extension in east Indonesia formed the deep Banda basins. The gateway was open before 25 Ma and significantly restricted by 5 Ma; 10 Ma seems the most likely time of closure although the complex path of the present throughflow shows why this estimate is uncertain.

There are numerous linked effects. There may have been a change from warm South Pacific to colder North Pacific waters passing through the gateway. In Borneo mountains rose from about 20 Ma. In New Guinea elevation began at around 10 Ma, but mountains probably rose to present heights since 5 Ma. Sulawesi, parts of the Banda Arc, Moluccas, Java and Sumatra were elevated above sea level since 5 Ma, whereas the Sunda shelf subsided below sea level in the late Miocene increasing the area of shallow seas. There would have been complex changes in atmospheric as well as oceanic circulation. Rainfall and erosion rates must have changed. Was there a Pacific Warm Pool before the Pliocene? How are these related to the Asian monsoon? The changes occurred within a context of long-term cooling and sea level fall, with extreme variations during the Quaternary.