Guest Seminar - Francis A. MacDonald

It is widely accepted that climate is controlled primarily by CO2. On million-year timescales, CO2 is sourced predominantly from volcanic outgassing and is consumed by silicate weathering, but the relative importance of CO2 sources and sinks as the driver of transitions between non-glacial and glacial climate regimes is a matter of debate and proxies for both come with caveats. Because the amount and variability in volcanic outgassing in the modern is very poorly constrained, it is difficult to estimate how volcanic CO2 sources may have changed in the past. In contrast, CO2 sinks are relatively well characterized from river discharge data, and are the product of variables such as lithology, paleolatitude, and uplift rates, which can be constrained with geological data. Recently we have proposed that changes paleogeography, particularly the uplift and exhumation of highly soluble mafic and ultramafic rocks in the tropics, drove Neoproterozoic, Ordovician, and Cretaceous to Cenozoic cooling trends. Here we examine the uniqueness of this hypothesis and the most recent long-term cooling trend.
After the Mid-Miocene Climatic Optimum, cooling accelerated at ~14 Ma and again over the past 5 Ma. Due to tropical temperatures and precipitation rates, rugged topography, and a predominance of highly soluble mafic and ultramafic rocks, SE Asia has the highest rates of silicate weathering in the world accounting for ~9% of the modern global carbon sink. We estimate that the Indonesia region (including the Philippines and New Guinea) increased in area by over 50% over the last 5 Myrs and ~75% over the last 10 Myrs. With simple area scaling, a proportional change in runoff from Indonesia would suggest a change from ~9% of the modern global carbon sink to ~5.5% at 5 Ma. Although this area scaling is suggestive that the recent growth of Indonesia may have been important for global climate, what is critical for the carbon sink is the volume of highly soluble Mg- and Ca-rich mafic and ultramafic rock that has been congruently weathered and delivered to the ocean in its dissolved load. Intriguingly, the East Sulawesi and Papua New Guinea ophiolites, which are two of the three largest preserved ophiolites on Earth, are both situated firmly in the tropical weathering belt in high relief region, and were uplifted and subaerially exposed recently. Rivers draining these ophiolites have high Mg/Ca ratios and unradiogenic Sr isotopes, which can account for the increase in seawater Mg/Ca and the downward deflection of seawater Sr isotopes over the past 15 Ma.