Changes in Holocene climate and the intensity of Southern Hemisphere Westerly Winds based on a high-resolution palynological record from sub-Antarctic South Georgia

Sub-Antarctic South Georgia is a key region for studying climate variability in the Southern Hemisphere, because of its position at the core of the Southern Hemisphere Westerly Wind belt and between the Antarctic Circumpolar Current and the Polar Frontal Zone. Here, we present a 5.8-m long high-resolution pollen record from Fan Lake on Annenkov Island dominated by local sub-polar vegetation, with Acaena and Poaceae being present throughout the last 7000 years. Palynological and sedimentological analyses revealed a warm late Holocene ‘climate optimum’ between 3790 and 2750 cal. yr BP, which was followed by a gradual transition to cool and wet conditions. This cooling was interrupted by slightly warmer environmental conditions between 1670 and 710 cal. yr BP that partly overlap with the Northern Hemisphere ‘Medieval Climate Anomaly’. Increases in non-native and long-distance pollen grains transported from South America (e.g. Nothofagus, Podocarpus) indicate that stronger Southern Hemisphere Westerly Winds over South Georgia possibly occurred during some ‘colder’ phases of the late Holocene, most notably between c. 2210 and 1670 cal. yr BP and after 710 cal. yr BP

Stable Southern Hemisphere westerly winds throughout the Holocene until intensification in the last two millennia

The Southern Hemisphere westerly winds sustain the Southern Ocean’s role as one of Earth’s main carbon sinks, and have helped sequester nearly half the anthropogenic CO2 stored in the ocean. Observations show shifts in the vigor of this climate regulator, but models disagree how future change impacts carbon storage due to scarce baseline data. Here, we use the hydrogen isotope ratios of sedimentary lipids to resolve Holocene changes in Southern Hemisphere westerly wind strength. Our reconstruction reveals stable values until ~2150 years ago when aquatic compounds became more 2H-enriched. We attribute this isotope excursion to wind-driven lake water evaporation, and regional paleoclimate evidence shows it marks a trend towards a negative Southern Annular Mode – the Southern Ocean’s main mode of atmospheric variability. Because this shift is unmatched in the past 7000 years, our findings suggest that previously published millennium-long Southern Annular Mode indices used to benchmark future change may not capture the full range of natural variability.publishedVersio

Late Quaternary Changes in the Westerly Winds over the Southern Ocean: the sub-Antarctic peatland record and response

The Southern Ocean plays a crucial role in modulating global atmospheric CO2 concentrations by acting as a large carbon sink. Upwelling driven by surface winds brings sequestered carbon from deep-water reservoirs to the ocean surface where it can saturate the potential sink and de-gas directly to the atmosphere. Specifically, the intensity and latitudinal position of the Southern Hemisphere Westerly Winds (SHWW) alters the magnitude of upwelling, and influences whether the Southern Ocean acts as a net sink or source of atmospheric CO2. In recent decades the SHWW have intensified and migrated further toward Antarctica, weakening the carbon sink. Over longer time-scales, the same mechanism has been implicated to explain the increase in atmospheric CO2 during the last deglaciation. Unfortunately, our ability to predict future changes in the SHWW is limited. Most existing knowledge of past wind-behaviour is based on archives located outside the core wind-belt, and hence spatial resolution in many areas of the Southern Ocean is poor. Consequently, more palaeo-data from inside the core wind-belt is needed to provide boundary conditions for earth system models. One such opportunity is provided by peatland ecosystems on the sub-Antarctic islands. Peatlands on exposed west-facing coasts receive oceanic base cations in concentrations proportional to the strength of the prevailing westerly winds. Reconstruction of bog-surface salinity could provide a direct proxy for past changes in wind behaviour, but methods to suitably extract the signal are lacking. In this thesis, we propose and test the idea that testate amoebae could record bog-surface salinity conditions. Using a land-sea transect on Marion Island (sub-Antarctica) we examine the ecology of these microorganisms and test their response to the contemporary conductivity gradient. We find that conductivity explained the most variance in community data, and show the potential of our novel proxy for palaeowind reconstruction in the wider sub-Antarctic region

Modern and Paleoecological Perspectives on a Terrestrial-Marine Linkage in the Falkland Islands

The Falkland Islands are a biodiversity hotspot in the South Atlantic Ocean, with some of the world’s most important populations of seabirds and seals. The impact of climate change on these marine populations and their coastal breeding habitat in the Falklands is unknown. Coastal grasslands of Poa flabellata (tussac grass) form critical breeding habitat for wildlife, but have been heavily degraded by the introduction of livestock in the 17th century. This dissertation investigates the impacts of global change (climate change, grazing) on P. flabellata and its sensitivity to the fecal nutrient subsidies provided by marine wildlife. Chapter 1 provides the first multi-proxy reconstruction of past environmental change to understand how marine animals breeding in the Falklands responded to climate change over the past 14,000 years. At ~5,000 years ago, seabird and/or seal populations reached higher levels than the previous 9,000 years at Surf Bay, East Falkland Island. Fires were present throughout the past 14,000 years, but fire activity was highest when marine-derived nutrient input into the coastal grasslands from seabirds and/or seals increased. The increase in marine-derived nutrients and fire coincided with an increase in grasses, as well as the onset of neoglaciation. This reconstruction suggests that the Falklands are a refuge for seabirds and/or seals during cold periods in the past, and that marine-derived nutrients are important for improving coastal grasslands. It remains unknown whether tussac grass or other grasses species responded to changes in marine-derived nutrient input in our paleoecological reconstruction. Thus, in Chapter 2, I found that phytoliths and not pollen of modern native grass species are useful in distinguishing tussac grass from other species, supporting future investigations of the response of tussac grasslands to global change. Chapter 3 examines the potential for P. flabellata, which forms extensive peat records, as a new paleoclimate proxy in the South Atlantic where other archives for paleoclimate reconstructions (i.e. tree ring records and ice cores) are absent. Through a year-long modern calibration study, I found that tussac grass tissues record inter-seasonal environmental changes in temperature and humidity, though not precipitation source. Thus, this study warrants the use of tussac grass peat records to fill in a significant gap in our knowledge of paleoclimate in the South Atlantic Ocean. Chapter 4 provides an assessment of the impact of modern and historic grazing on soils and plants in the ecologically important coastal tussac grasslands. After release from grazing, tussac grasslands recovered rapidly, and marine-derived nutrient subsidies may be critical to improve restoration efforts. Overall, this work suggests that the terrestrial-marine linkage in the Falkland Island is sensitive to both climate change and land use change

Holocene dynamics of the Southern Hemisphere westerly winds and possible links to CO2 outgassing

The Southern Hemisphere westerly winds (SHW) play an important role in regulating the capacity of the Southern Ocean carbon sink. They modulate upwelling of carbon-rich deep water and, with sea ice, determine the ocean surface area available for air–sea gas exchange. Some models indicate that the current strengthening and poleward shift of these winds will weaken the carbon sink. If correct, centennial- to millennial-scale reconstructions of the SHW intensity should be linked with past changes in atmospheric CO2, temperature and sea ice. Here we present a 12,300-year reconstruction of wind strength based on three independent proxies that track inputs of sea-salt aerosols and minerogenic particles accumulating in lake sediments on sub-Antarctic Macquarie Island. Between about 12.1 thousand years ago (ka) and 11.2 ka, and since about 7 ka, the wind intensities were above their long-term mean and corresponded with increasing atmospheric CO2. Conversely, from about 11.2 to 7.2 ka, the wind intensities were below their long-term mean and corresponded with decreasing atmospheric CO2. These observations are consistent with model inferences of enhanced SHW contributing to the long-term outgassing of CO2 from the Southern Ocean

Evidence for increased expression of the Amundsen Sea Low over the South Atlantic during the late Holocene

The Amundsen Sea Low (ASL) plays a major role in the climate and environment of Antarctica and the Southern Ocean, including surface air temperature and sea ice concentration changes. Unfortunately, a relative dearth of observational data across the Amundsen and Bellingshausen seas prior to the satellite era (post-1979) limits our understanding of the past behaviour and impact of the ASL. The limited proxy evidence for changes in the ASL are primarily restricted to the Antarctic where ice core evidence suggests a deepening of the atmospheric pressure system during the late Holocene. However, no data have previously been reported from the northern side of the ASL. Here we report a high-resolution, multi-proxy study of a 5000-year-long peat record from the Falkland Islands, a location sensitive to contemporary ASL dynamics which modulates northerly and westerly airflow across the southwestern South Atlantic sector of the Southern Ocean. In combination with climate reanalysis, we find a marked period of wetter, colder conditions most likely the result of enhanced southerly airflow between 5000 and 2500 years ago, suggesting limited ASL influence over the region. After 2500 years ago, drier and warmer conditions were established, implying more westerly airflow and the increased projection of the ASL onto the South Atlantic. The possible role of the equatorial Pacific via atmospheric teleconnections in driving this change is discussed. Our results are in agreement with Antarctic ice core records and fjord sediments from the southern South American coast, and suggest that the Falkland Islands provide a valuable location for reconstructing high southern latitude atmospheric circulation changes on multi-decadal to millennial timescales.</p

Reconstructing mid-late Holocene climate and environmental change in Antarctica using Glycerol Dialkyl Glycerol Tetraethers (GDGTs) and pigments in lake sediments

Phd ThesisAssessing the impact of past phases of natural warming on climatically sensitive areas, such as Antarctic and sub-Antarctic regions, will help us better understand the impact that climate warming may have in the future. In recent decades, the Antarctic Peninsula has been one of the fastest warming regions on Earth, warming at a rate of 3.4 °C per century, five times the global mean. Several phases of marked environmental change have occurred on the Antarctic Peninsula during the mid-late Holocene nonetheless, to date quantitative temperature reconstructions of terrestrial climates are not possible. GDGTs are one of few existing quantitative temperature proxies and recent developments of their use in lakes indicates their potential as a temperature proxy. Prior to application the relationship between GDGT-composition and several environmental factors, such as temperature, pH and conductivity, in Antarctic and sub-Antarctic lakes was assessed. Temperature explained a statistically significant independent control on the composition of branched GDGTs in the lakes and a new regional Antarctic and sub-Antarctic GDGT-temperature calibration was developed. Within this calibration, GDGT-IIIb was a statistically significant component indicating its importance in these environments. Applying the new GDGT-temperature calibration to sediment cores from Fan Lake, South Georgia and Yanou Lake, South Shetland Islands allowed the quantitative reconstruction of past temperatures during the Mid to Late-Holocene. The reconstructions showed varied temperatures throughout the records, evidencing the mid-Holocene warm period, and potentially the Medieval Climate Anomaly and Little Ice Age. Neither record, however, reconstructed the recent rapid warming seen in the glacial meltwater and instrumental records. A comparison of Fan Lake and Yanou Lake with other records from Antarctica and Chile showed periods of coherence between records alongside periods of inconsistency. This comparison also enabled potential changes in the PFZ to be considered, suggesting a more poleward position of the PFZ during warmer climates

South Georgia marine productivity over the past 15 ka and implications for glacial evolution

The subantarctic islands of South Georgia are located in the Southern Ocean, and they may be sensitive to future climate warming. However, due to a lack of well-dated subantarctic palaeoclimate archives, there is still uncertainty about South Georgia’s response to past climate change. Here, we reconstruct primary productivity changes and infer Holocene glacial evolution by analysing two marine gravity cores: one near Cumberland Bay on the inner South Georgia shelf (GC673: ca. 9.5 to 0.3cal.kyrBP) and one offshore of Royal Bay on the mid-shelf (GC666: ca. 15.2cal.kyrBP to present). We identify three distinct benthic foraminiferal assemblages characterised by the dominance of Miliammina earlandi, Fursenkoina fusiformis, and Cassidulinoides parkerianus that are considered alongside foraminiferal stable isotopes and the organic carbon and biogenic silica accumulation rates of the host sediment. The M. earlandi assemblage is prevalent during intervals of dissolution in GC666 and reduced productivity in GC673. The F. fusiformis assemblage coincides with enhanced productivity in both cores. Our multiproxy analysis provides evidence that the latest Pleistocene to earliest Holocene (ca. 15.2 to 10.5cal.kyrBP) was a period of high productivity associated with increased glacial meltwater discharge. The mid–late Holocene (ca. 8 to 1cal.kyrBP), coinciding with a fall in sedimentation rates and lower productivity, was likely a period of reduced glacial extent but with several short-lived episodes of increased productivity from minor glacial readvances. The latest Holocene (from ca. 1cal.kyrBP) saw an increase in productivity and glacial advance associated with cooling temperatures and increased precipitation which may have been influenced by changes in the southwesterly winds over South Georgia. We interpret the elevated relative abundance of F. fusiformis as a proxy for increased primary productivity which, at proximal site GC673, was forced by terrestrial runoff associated with the spring–summer melting of glaciers in Cumberland Bay. Our study refines the glacial history of South Georgia and provides a more complete record of mid–late Holocene glacial readvances with robust chronology. Our results suggest that South Georgia glaciers were sensitive to modest climate changes within the Holocene