Neogene to present changes in CO2 sources and sinks due to the growth and tectonic evolution of Indonesia

Following the Mid-Miocene Climatic Optimum (MMCO), global cooling accelerated at 14 Ma and again over the past 5 Ma. The geological drivers of this long-term cooling trend are unknown, but the multi-million year timescale suggests that it is related to changes in volcanic outgassing, silicate weathering, or the sensitivity of the feedback between the two. Low-latitude arc-continent collisions may cause not only a decrease in outgassing by terminating arc volcanism, but also an increase in global weatherability by exhuming soluble mafic and ultramafic rocks within the tropical rain belt. Here we propose that the post-MMCO cooling trend is related to arc-continent collisions in Indonesia and New Guinea. Importantly, the East Sulawesi and Irian Jaya-New Guinea ophiolites, which are two of the three largest preserved ophiolites on Earth, are both situated in high relief regions located firmly within the tropics, and have been uplifted and subaerially exposed over the past 15 Myrs. To test this hypothesis, we compiled geological and paleoshoreline data to quantitatively estimate the changes in the area of subaerially exposed volcanic and ultramafic rocks in SE Asia over the past 20 Myrs. We then incorporate these estimated exposure areas into a paleogeographic model to determine the position of mafic and ultramafic units relative to the warm wet tropics. We also use estimates of modern outgassing as well as tectonic reconstructions of Indonesia to approximate contemporaneous changes in arc-length and CO2outgassing. We conclude that although arc-length has not changed significantly, the area of exposed volcanic and ultramafic rocks in the tropics has increased by 3.5 times in the past 15 Myrs. Thus, if Indonesia is responsible for cooling from the Miocene to present, it is likely due to an increase in global weatherability.