Introduction: Southern Hemisphere records of Late Quaternary climate change, people and dust

This Special Issue of Journal of Quaternary Science is in honour of a stalwart of the Australasian Quaternary community, Dr Lynda Petherick, who died unexpectedly, aged 40, early in 2022. Lynda was a driving force behind research into late Quaternary climate change and human-environment interactions in the Southern Hemisphere and undertook pioneering work on using dust records for reconstructing atmospheric circulation. She was the leader of the INQUA-funded Southern Hemisphere Last Glacial Maximum (SHeMax) project, which combined paleodata and modelling studies from across the southern latitudes, and from both terrestrial and marine environments, in order to reconstruct environmental change and human-environment interactions during MIS 3 and 2 (Petherick et al., 2017, 2022). Lynda was also a champion for early-career researchers and women in Science. As President of the Australasian Quaternary Association (AQUA) up until the time of her death, Lynda was actively engaged in promoting stronger demographic and gender balance within the Australasian Quaternary community. In recognition of Lynda's outstanding contributions and tragic loss, an oral conference session in her honour took place at the 2022 AQUA Conference in Adelaide. Another conference session in Lynda's honour was held at the 2023 INQUA Congress in Rome (Session 21). This Special Issue can now be added to these two sessions as a tangible and lasting tribute to Lynda's outstanding career contribution to Quaternary Science, encapsulated in the title Southern Hemisphere records of Late Quaternary climate change, people and dust. Geographically, the eleven contributing papers and their authors in this Special Issue span the continents and islands of the Southern Ocean, and all relate to those key themes of past climate change, people and dust (Fig. 1). Individually, the papers mostly have a regional focus, drawn from Australia, South America, Southern Africa and the subantarctic Marion Island. An exception is the pan-hemispheric study by Grab and Knight, which considers the origins and environmental significance of mountain block deposits (blockfields, blockstreams) across various continental and island locations of the Southern Hemisphere. Using previously proposed landscape evolutionary models, they suggest that in the southern hemisphere, these features are products of a range of weathering and material mobilisation processes over long climate cycles, both Quaternary and pre-Quaternary. Although most show some association with periglacial processes, they are not exclusively periglacial in origin. They discuss field geomorphic, sedimentary and ecological methods that can be used to further understanding of these iconic landforms, including their sensitivity to ongoing climate change. Nel et al. consider another iconic cool-climate feature of the Southern Hemisphere in their study of peatland initiation in the Subantarctic Islands. They challenge the prevailing hypothesis that peat formation was directly linked to the retreat of glaciers at the start of MIS 2, through a comparison of new and previously published basal peat ages from Subantarctic Marion Island with cosmogenic isotope dates for local deglaciation. Instead, they find that peat initiation occurred after the Antarctic cold reversal (ACR) and through the early Holocene—in common with other Subantarctic islands. Before that time, the authors argue, low temperatures, extensive sea ice conditions and lower sea levels could have maintained local conditions unfavourable for peat initiation. These results reveal new insights into the relationships between extensive peat development and environmental change in the Subantarctic Islands. They also provide a caution against the uncritical use of peat basal ages as a proxy for the minimum age of regional deglaciation. The paper from southern Africa, by Fitchett and McKay, presents a multiproxy hydroclimatological investigation of a short late Holocene diatomite outcrop in the eastern Lesotho highlands, a key region for present-day water supply in southern Africa. They apply diatom analysis, an array of geochemical proxies and digital elevation modelling with the primary aim of clarifying the existence of a palaeolake, and its subsequent disappearance, at the study site. The authors hypothesise the transition from lake to present-day wetland to be the result of post-industrial warming, possibly compounded by the migration of livestock into the eastern Lesotho Highlands. These findings offer relevant historical context for understanding the water cycle of the region, which has become more critical in the modern era of accelerated climate change. Turning to the contributions from South America, the paper by Giachetti et al. focuses on marine bioerosion traces along the Atlantic coast of central Argentina, where marine molluscan biodiversity and dating using amino acid racemization and radiocarbon methods provide insight into the timing and processes affecting marine terrace deposits in the region. Different ichnotaxa and types of bioerosion are characterised. Based on the dates obtained, the terraces formed during MIS 5 and 1, and the different ichnotaxa and types of bioerosion identified reflect marine conditions and coastal processes during these interglacial periods. A second contribution from South America by Jeanneret et al. uses a combination of cosmogenic and luminescence age determination to reconstruct LGM ice dynamics based on geomorphological mapping of terraced outwash deposits in the Andes of western Argentina. Nested and downstream-stepping dated terraces suggest two late Pleistocene glacial phases, with traces of an older glacial advance (of MIS 5 d) also preserved. This provides new evidence for climatically controlled glacier fluctuations on the eastern (drier) side of the Andes. The six papers from Australia collectively span much of the fascinating diversity of climate and environment across the continent that has supported people for >60 000 years. This work highlights the multi-millennial connection between vegetation composition, climate change, nutrients and fire occurrence across these distinctive environments. The collection comprises two papers that concern the wet-tropical Kimberley region of northwestern Australia, three drawn from subtropical southeastern Queensland, and one from the Willandra Lakes World Heritage area located in the semi-arid zone of the southern Murray–Darling Basin (MDB) in southeastern Australia. Dixon et al. draw our focus to the wet tropics of Australia, and specifically the eastern Kimberley region, thus far largely understudied due to the twin challenges of access and suitable sediment archives. The Bullo River floodplain nevertheless provides an excellent deglacial through to Holocene record of hydroclimatic change, derived from elemental geochemistry and sedimentology, pollen and microcharcoal, placed within a combined luminescence and 210Pb chronological framework. This study indicates variable monsoon activity during the deglacial phase to c. 12.8 ka, followed by wet monsoonal conditions through to c. 5 ka (including a 7–5 ka marine transgression), and a relatively arid phase c. 5–1 ka. As such, this record provides a valuable terrestrial counterpart to offshore and speleothem records and complements the shorter and discontinuous archives published from other sites in the Kimberley. Also working in the Kimberley, Rudd et al. seek to determine the influence of environment, and in particular hydrological variability, on the rich archaeological and rock art records of the region—to aid in their interpretation. They present a ~19 ka-year record of monsoon rainforest variability, inferred from pollen, charcoal and major element geochemical analysis in the context of broad-scale hydroclimate variability inferred from previous studies in the region. They conclude that a positive moisture balance persisted at least locally throughout the record, and that fire activity broadly corresponds to periods of less abundant rainforest communities; fire activity was greatest over the last ~6000 years. Moving to southeastern Australia, Fitzsimmons et al. provide an investigation of 60 ky of hydroclimate in the Willandra Lakes of semi-arid southeastern Australia. They investigate lake level history and long-term hydrological connectivity across the lakes and their wider Murray–Darling Basin catchment using lunette sedimentology, stratigraphy and OSL geochronology, integrated with multi-basin hydrologic and paleoclimatic modelling. This novel methodology has enabled quantification of hydrologic connectivity across the basins and of the volume of water required to fill and sustain the lake system. Further analysis of paleoclimate parameters for the region points to certain hydroclimatic scenarios, which explain past episodes of lake filling and drying, notably during the LGM. This work not only gives important context for recent catastrophic events across Australia at both ends of the hydroclimatic spectrum, but also more generally provides a blueprint for future investigations into past surface hydroclimate in regions such as semi-arid Australia that are increasingly subject to hydroclimatic extremes. The iconic sand dune systems of Minjerribah (North Stradbroke Island) and K'gari (Fraser Island), in subtropical south-eastern Queensland, Australia, host numerous lakes and swamps containing organic-rich sediment deposits that record changes in climate, fire and surrounding vegetation as illustrated by three papers in this Issue. Two of these are centred on Minjerribah, where Lynda Petherick spent formative years developing her expertise in paleoenvironment reconstruction. Cadd et al. present a synthesis of palynological and charcoal records from six Minjerribah wetlands to determine island-wide patterns of vegetation change in response to long-term regional changes in climate, nutrients, fire and dust. Coherency of vegetation change is demonstrated using a novel approach (multitiered Monte Carlo empirical orthogonal function analysis) that allows island-wide patterns of vegetation change to be identified from a multisite compilation of pollen records. They observe first-order changes indicating a long-term shift in vegetation composition across Minjerribah from closed forests towards woodland and heath vegetation over the past 7000 years. Lynda Petherick is included as a posthumous co-author in recognition of her foundational work at Minjerribah (Petherick et al., 2008, 2017). By contrast, Marshall et al. investigate multiple cores and proxies from a single Minjerribah site, Eighteen Mile Swamp. These authors reconstruct the recent evolution of this wetland over the past two millennia, which transitioned from an oscillating coastal lagoon into a stable freshwater swamp. A range of proxies are used, including acid sulphate soil assays, particle size analysis, diatom and pollen assemblages, photosynthetic pigment content and Itrax XRF spectrometry, alongside ground-penetrating radar cross-sections. The resulting detailed ecological and sedimentological framework for the wetland's evolution was dated using radiocarbon, Pu and 210Pb analyses. This truly multiproxy study reveals the complex response of a subtropical wetland system to environmental change, with pronounced shifts in sedimentary conditions recorded in the last ca. 1000 years. These results demonstrate the value of understanding site-specific historical responses of wetlands to environmental change, which helps inform their current and future management, conservation and restoration efforts, particularly in the context of climate change and human impacts. Working further north at K'gari, Tibby et al. consider the widely held notion that eastern Australia dried during the mid-to late Holocene in response to the intensification of the El Niño phase of the ENSO cycle. Evidence from K'gari was formative to the development of this hypothesis. This study examines published and new stratigraphic, radiocarbon and sediment µXRF data from three ‘perched’ K'gari lakes that, contrary to prevailing views, demonstrate a strong drying event during the mid-Holocene, followed by a wetter late-Holocene climate. These results call for a re-evaluation of the notion that ENSO intensification drove late-Holocene drying on K'gari, and perhaps further afield. It also indicates that even large, deep lakes on K'gari—iconic landmarks with substantial cultural, ecological and economic values, and one of the few Australian locations (beyond Tasmania) with a present-day positive moisture balance—are nevertheless vulnerable to drying. Macalalad et al. provide estimates of LGM and deglacial palaeotemperatures from GDGT analysis of lipids of sediments from Broutha Waterhole. This wetland is located in the Cooloola sand mass, a mainland site, situated between K'gari and Minjerribah, and provides another rare quantitative climate estimate of the late Quaternary for eastern Australia. A temperature minimum of 17°C (~4.3°C maximum cooling) was recorded for the LGM, which is consistent with other Australian-wide cooling estimates. This is followed by a progressive warming trend during the deglacial, which is interrupted by a 0.8°C to 1.6°C cooling that coincides with the ACR. Warm, stable Holocene temperatures follow, and there are possibilities of different hiatus periods within the Cooloola sand mass from K'gari and Minjerribah, which may suggest different deposition patterns compared to these two island sand masses. In summary, each of these papers, spanning from the very core to the wider periphery of Lynda's geographical scope, touches in distinctive ways on themes she embraced during her tragically short yet influential research career. Three years on from her passing, they demonstrate that her influence lives on, embodied not just in ongoing interest in the advancement of knowledge, but also in the broader principles of teamwork, collaboration and camaraderie that she embraced to the full. It is an honour for us to present this Special Issue in commemoration of a special researcher, on behalf of our various communities, and we thank the Journal of Quaternary Science and Wiley Publishing Group for their support.