Earth’s climate experienced a long-term cooling trend from the Cretaceous “Greenhouse” towards the Pleistocene “Icehouse”. This long-term trend was punctuated by relatively abrupt transitions that gave rise to new modes of variability, lasting up to several millions of years (e.g., the development of permanent continent-wide ice shields in Antarctica during the Middle Miocene or onset of northern hemisphere glaciation during the Late Pliocene). One of these periods of fundamental climate change is the Middle Pleistocene Transition (MPT) between ~1.2 and 0.8 million years (Ma) ago.

During the MPT, the fundamental “beat” of the global climate changed from a predominant ~41 kyr rhythm (a cycle of cold and warm climate lasting 41,000 years) to one of ~100 kyr. This major climate shift occurred within a wider context of ocean-atmosphere circulation changes, atmospheric pCO₂ variations, and ice volume fluctuations. The lack of substantial changes in external orbitally-driven insolation forcing implies that the climate transition was predominantly caused by internal feedbacks within the coupled ocean-atmosphere system.

Previous research has focused on reconstructing this fundamental climate transition with the aim of better constraining its timing and causes, however, a comprehensive understanding of climate evolution through the MPT is still elusive. In particular, the repercussions of the transition on tropical climate, including monsoonal systems, and the evolving relationships of high- and low-latitude climate change remain largely unknown.

Evolution of the Australian monsoon over the past 1.6 million years

Monsoons play a fundamental role in the atmospheric transport of energy and water vapor and are a central component of the global climate system. Changes in monsoon intensity or timing have the potential to affect the habitability of entire regions and impact their economies in major ways. Yet, the response of monsoons to anthropogenic CO₂ increase and to global climate change is not well understood and model predictions provide a wide range of alternative climate scenarios. In fact, we are still far from achieving a comprehensive understanding of the variability and underlying driving mechanisms of these large-scale climate systems.

The Australian monsoon represents the Southern Hemisphere component of the Austral-Asian monsoon system, which is the largest monsoon system on Earth. It is characterized by two distinct seasonal phases: during austral summer, intense and prolonged rainfall (Australian summer monsoon) increases the sediment load of local rivers, driving enhanced terrigenous deposition off NW Australia. By contrast, dry winds coming from the interior of the Australian continent during Austral winter (Australian winter monsoon) carry increased amounts of dust towards the eastern Indian Ocean. These Trade winds promote seasonal increases in primary productivity due to convective mixing within the upper water column.

A complete hemipelagic sedimentary archive spanning the past 2 million years was recovered in 2016 at Site U1483 off NW Australia during the International Ocean Discovery Program (IODP) Expedition 363. This well-preserved sequence offered the unique opportunity to reconstruct the evolution of the Australian monsoon in relation to changing climate boundary conditions including polar ice volume and atmospheric pCO₂ during the MPT. For this, we used benthic foraminiferal δ¹⁸O, X-ray fluorescence (XRF) core scanning-derived proxy records, spectral natural gamma ray, and visible light reflectance spectroscopy data to monitor secular variations in terrigenous river discharge, primary productivity, and bottom water oxygenation.

Our combined records show that during the warmer climate phase prior to ~0.95 Ma monsoonal rainfall responded primarily to changes in insolation forcing linked to ~20 kyr precessional variations of the Earth’s orbit. However, the wet monsoon became strongly influenced by ice volume fluctuations and pCO₂-related feedbacks following the MPT, when glacial-interglacial cycles lengthened and intensified. Primary productivity off NW Australia, which is closely coupled to the winter monsoon, also showed a major change from ~41 to ~100 kyr periodicity following the inception of intensified glacial-interglacial cycles after ~0.95 Ma. Both monsoonal phases of the Australian monsoon, therefore, underwent major reorganizations across the MPT with extratropical feedback processes gaining increasing influence.

A major implication of the study is that global climate variability over the last 1.6 million years exerted a stronger influence on the strength of NW Australian monsoonal winds and rainfall than changes in local insolation over the Australian continent. Our results also show that the Australian monsoon is highly sensitive to changes in polar ice volume and global greenhouse gas concentrations, implying that future global warming may lead to a substantial strengthening of monsoonal rainfall and a decline in marine productivity off NW Australia and the southern part of the Indonesian archipelago.