Reconstructing the Australasian monsoon over the last 40,000 years using speleothems and palaeoclimate modelling

Abstract

Deep atmospheric convection over the western equatorial Pacific occurs at the junction of the rising limbs of the meridional Hadley cells and the Pacific Walker circulation, making it one of the most atmospherically dynamic regions on Earth. Here, interactions between the Australasian monsoon and atmospheric convection result in highly variable regional precipitation patterns across different latitudes. The dynamics of the Australasian monsoon over the past ~40,000 years are relatively well understood at its northern limit (the East Asian Summer Monsoon), but less well known for its southern limit (the Indo-Australian Summer Monsoon). In the equatorial region however, even less is known about the past behaviour and dynamics of this major system. Here we present a new, continuous, absolutely dated speleothem record from southwest Sulawesi, Indonesia that spans the past 40,000 years. Isotopic ratios of oxygen (δ18O) and carbon (δ13C) in the speleothem calcite were analysed at ~50-yr resolution to reconstruct rainfall amount and vegetation productivity. The records show that the strength of regional deep atmospheric convection is primarily controlled by sea level via the exposure and inundation of the Sunda Shelf. This sea-level control results in a relatively dry last glacial period that was terminated by the onset of deglaciation and the inundation of the Sunda Shelf, which abruptly increased the intensity of deep atmospheric convection. The Sulawesi speleothem δ18O record does not capture millennial-scale variability in response to North Atlantic Heinrich events, in contrast to nearby speleothem records from Borneo and Flores. To explore this observation, the climatic impact of Heinrich events in the western equatorial Pacific region was simulated using idealised North Atlantic freshwater hosing experiments performed with the HadCM3 and CSIRO Mk3L general circulation models. Precessional forcing is shown to influence the manifestation of Heinrich events, particularly across the Southern Hemisphere via the varying response of the Intertropical Convergence Zone. Additionally, high atmospheric carbon dioxide levels increase the duration of the Heinrich climate anomaly, compared to pre-industrial levels. Sulawesi speleothem δ13C is interpreted as a record of changing vegetation productivity. Comparison of the speleothem carbon isotopes with ice core atmospheric methane concentrations reveals a significant relationship during the glacial and early-deglacial intervals. It is hypothesised that changing vegetation productivity as recorded by Sulawesi speleothems is indicative of broader tropical methane emissions, which are thought dominate the glacial methane budget. This idea is explored using the Sheffield Dynamic Global Vegetation Model to simulate global climate and methane emissions over the past 40,000 years. The data-model comparisons confirm that temporal changes in the Sulawesi δ13C record are in good agreement with modelled methane emissions over much of the tropics, lending weight to the likelihood that the tropics dominated total methane emissions during the glacial period when boreal sites were perennially frozen. Together this work demonstrates the importance of the western equatorial Pacific in influencing regional climate and global climate signals. It is vital therefore, to continue to explore the past dynamics of this region as a potential driver of global climate changes. Show more