Evaluating Ophiolite Exhumation in New Guinea as a Trigger for Miocene to Present Global Climate Cooling

Abstract:

Earth’s climate is controlled by the concentration of CO₂ in the ocean-atmosphere system, which is set by sources and sinks of carbon and the silicate weathering feedback. On long timescales, the main source of CO₂ is volcanism, while the main sink of CO₂ is the chemical weathering of silicate rocks, with ultramafic and mafic lithologies being especially efficient sinks. A long-term change in either CO₂ outgassing or weatherability therefore has the potential to shift the climate to a warmer or cooler equilibrium. Increased weatherability due to changes in paleogeography in the warm, wet tropics has been proposed as a driver for Phanerozoic glaciation (Macdonald et al., 2019). Particularly, the exhumation of ultramafic and mafic rocks associated with arc-continent collisions in the tropics could trigger long-term global cooling.

Cenozoic climate reconstructions show long-term cooling trends from ~14-12 Ma, ~7.5-5.5 Ma, and ~3.5 Ma to present. We investigate if these climate trends coincide with ophiolite exhumation in Earth’s largest, most recent, arc-continent collision, exposed on the tropical island of New Guinea. Collision between the Melanesian island arc and continental Australia obducted the Irian Jaya ophiolite, which is now sandwiched between metamorphic and igneous terranes (Cloos et al., 2005). We acquired new apatite and zircon (U-Th)/He (AHe, ZHe) data from samples along three transects that cross the ophiolite and adjacent blocks to complement unpublished apatite and zircon fission-track (AFT, ZFT) results and to refine the regional exhumation history. Samples from the western and eastern transects yield error weighted mean dates of 4.8±0.6 Ma (AHe, n=11 analyses) and 10.3±0.5 Ma (ZHe, n=10), and 6.2±0.3 Ma (AHe, n=12) and 14.3±0.6 Ma (ZHe, n=6), respectively. These data indicate spatial variability in exhumation. Thermal history modeling will more fully decipher the details of exhumation by simulating AHe, ZHe, AFT, and ZFT data from the same samples, with additional data collection focused on understanding the differences in exhumation histories across fault blocks. The results will improve constraints on the timing and volume of exhumation during arc-continent collision in the tropics and facilitate comparisons with global climate records.