Monsoon driven ecosystem and landscape change in the 'Top End' of Australia during the past 35 kyr

Abstract

The Indo-Australian Summer Monsoon (IASM) is the dominant climate feature of northern Australia, affecting rainfall/runoff patterns over a large portion of the continent and exerting a major control on the ecosystems of Australia's Top End, including the viability of wetland ecosystems and the structure of the woody savanna. We examined the behaviour the IASM from 35 kyr using proxy data preserved in the sediments of Table Top Swamp, a small seasonal swamp in northern Australia. Elemental data, stable C and N isotopes, pollen and sedimentary data were combined to develop a picture of monsoon activity and landscape and ecosystem response. Results demonstrated that between 35 and 25 ka conditions were drier and more stable than present, with a more grass dominated savanna and limited wetland development, implying reduced IASM activity. After ~25 ka, there is evidence of increased moisture at the study site, but also increased IASM variability. However, despite evidence of at least periodic increases in moisture, including periods of wetland establishment, the IASM displayed a subdued response to peak precession insolation forcing by comparison to the other global monsoon systems. Instead, the greatest change occurred from ~10 ka when the continental shelf flooded, increasing moisture advection to the study site and resulting in establishment of a quasi-permeant wetland. Whereas the early Holocene was marked by both the onset of pollen preservation and a wetter vegetation mosaic, indicative of a consistently active IASM, the mid-late Holocene was marked by drier vegetation, increased fire, but also increased C 3 vegetation and runoff, implying increased IASM variability. Holocene changes in ecosystem dynamics occur coincident with an expansion in human population, which likely also influenced vegetation and landscape response at the study site. • Wetlands were likely limited across the Top End and northern Australia before 25 ka with savanna more grass dominated. • The IASM became more effective from 9 ka, with ecosystem developing their current configuration after this time. • There is evidence of human resource use impacting vegetation assemblages in the Top End from the early to mid-Holocene. • There is evidence of both increasing moisture and aridity in the mid- to late-Holocene in the Top End. • Sea level appears to be the major control of IASM strength in the Top End over the past 35 kyr.