Bella Duncan’s research while affiliated with Victoria University of Wellington and other places

Marine glycerol dialkyl glycerol tetraethers (GDGTs) are used in various proxies (such as TEX86) to reconstruct past ocean temperatures. Over 20 years of improvements in GDGT sample processing, analytical techniques, data interpretation and our understanding of proxy functioning have led to the collective development of a set of best practices in all these areas. Further, the importance of Open Science in research has increased the emphasis on the systematic documentation of data generation, reporting and archiving processes for optimal reusability of data. In this paper, we provide protocols and best practices for obtaining, interpreting and presenting GDGT data (with a focus on marine GDGTs), from sampling to data archiving. The purpose of this paper is to optimize inter-laboratory comparability of GDGT data, and to ensure published data follows modern open access principles.

Antarctica’s terrestrial ecosystems are at risk from a rapidly changing climate. Investigating how Antarctica’s vascular plants responded to major climatic variations in the geological past, especially under atmospheric CO2 values similar to modern and future projections, may provide insight into how organisms could migrate across the continent as conditions change. Here, we investigate vegetation trends across the Oligocene/Miocene Transition (OMT, ~23 Myr), one of the largest transient glaciations of the Cenozoic. Despite extensive ice sheet expansion, Antarctic vegetation survived throughout this glacial episode. We use compound specific isotope trends (δ13C and δ2H) of plant waxes in an Antarctic proximal sediment core from the Ross Sea (Deep Sea Drilling Project site 270) to investigate the response and survival mechanisms of Antarctic vegetation during this event. We detect the first observation of a marked negative n-alkane δ13C excursion over the OMT, coupled with a shift to more positive n-alkane δ2H. We interpret this as plants sacrificing water use efficiency to maintain photosynthesis and carbon uptake during increasing glacial conditions, as atmospheric CO2 decreased and orbital configurations favoured shorter, colder growing seasons with lower light intensity. We consider further drivers of these isotopic trends to be enhanced aridity, and a shift to a stunted, low elevation vegetation. These findings establish the adaptability of ancient Antarctic vegetation under atmospheric CO2 conditions comparable to modern, and mechanisms that allowed vegetation to keep a foothold on the continent despite prolonged hostile conditions.

Drill cores from the Antarctic continental shelf are essential for directly constraining changes in past Antarctic Ice Sheet extent. Here, we provide a sedimentary facies analysis of drill cores from International Ocean Discovery Program (IODP) Site U1521 in the Ross Sea, which reveals a unique, detailed snapshot of Antarctic Ice Sheet evolution between ca. 18 Ma and 13 Ma. We identify distinct depositional packages, each of which contains facies successions that are reflective of past baseline shifts in the presence or absence of marine-terminating ice sheets on the outermost Ross Sea continental shelf. The oldest depositional package (>18 Ma) contains massive diamictites stacked through aggradation and deposited in a deep, actively subsiding basin that restricted marine ice sheet expansion on the outer continental shelf. A slowdown in tectonic subsidence after 17.8 Ma led to the deposition of progradational massive diamictites with thin mudstone beds/laminae, as several large marine-based ice sheet advances expanded onto the mid- to outer continental shelf between 17.8 Ma and 17.4 Ma. Between 17.2 Ma and 15.95 Ma, packages of interbedded diamictite and diatom-rich mudstone were deposited during a phase of highly variable Antarctic Ice Sheet extent and volume. This included periods of Antarctic Ice Sheet advance near the outer shelf during the early Miocene Climate Optimum (MCO)—despite this being a well-known period of peak global warmth between ca. 17.0 Ma and 14.6 Ma. Conversely, there were periods of peak warmth within the MCO during which diatom-rich mudstones with little to no ice-rafted debris were deposited, which indicates that the Antarctic Ice Sheet was greatly reduced in extent and had retreated to a smaller terrestrial-terminating ice sheet, most notably between 16.3 Ma and 15.95 Ma. Post-14.2 Ma, diamictites and diatomites contain unambiguous evidence of subglacial shearing in the core and provide the first direct, well-dated evidence of highly erosive marine ice sheets on the outermost continental shelf during the onset of the Middle Miocene Climate Transition (MMCT; 14.2−13.6 Ma). Although global climate forcings and feedbacks influenced Antarctic Ice Sheet advances and retreats during the MCO and MMCT, we propose that this response was nonlinear and heavily influenced by regional feedbacks related to the shoaling of the continental shelf due to reduced subsidence, sediment infilling, and local sea-level changes that directly influenced oceanic influences on melting at the Antarctic Ice Sheet margin. Although intervals of diatom-rich muds and diatomite indicating open-marine interglacial conditions still occurred during (and following) the MMCT, repeated advances of marine-based ice sheets since that time have resulted in widespread erosion and overdeepening in the inner Ross Sea, which has greatly enhanced sensitivity to marine ice sheet instability since 14.2 Ma.

Ratios of glycerol dialkyl glycerol tetraethers (GDGT), which are membrane lipids of bacteria and archaea, are at the base of several paleoenvironmental proxies. They are frequently applied to soils as well as lake‐ and marine sediments to generate records of past temperature and soil pH. To derive meaningful environmental information from these reconstructions, high analytical reproducibility is required. Based on submitted results by 39 laboratories from across the world, which employ a diverse range of analytical and quantification methods, we explored the reproducibility of brGDGT‐based proxies (MBT′5ME, IR, and #ringstetra) measured on four soil samples and four soil lipid extracts. Correct identification and integration of 5‐ and 6‐methyl brGDGTs is a prerequisite for the robust calculation of proxy values, but this can be challenging as indicated by the large inter‐interlaboratory variation. The exclusion of statistical outliers improves the reproducibility, where the remaining uncertainty translates into a temperature offset from median proxy values of 0.3–0.9°C and a pH offset of 0.05–0.3. There is no apparent systematic impact of the extraction method and sample preparation steps on the brGDGT ratios. Although reported GDGT concentrations are generally consistent within laboratories, they vary greatly between laboratories. This large variability in brGDGT quantification may relate to variations in ionization efficiency or specific mass spectrometer settings possibly impacting the response of brGDGTs masses relative to that of the internal standard used. While ratio values of GDGT are generally comparable, quantities can currently not be compared between laboratories.

Cenozoic evolution of the Antarctic ice sheets is thought to be driven primarily by long-term changes in radiative forcing, but the tectonic evolution of Antarctica may also have played a substantive role. While deep-sea foraminiferal oxygen isotope records provide a combined measure of global continental ice volume and ocean temperature, they do not provide direct insights into non-radiative influences on Antarctic Ice Sheet dynamics. Here we present an Antarctic compilation of Cenozoic upper-ocean temperature for the Ross Sea and offshore Wilkes Land, generated by membrane lipid distributions from archaea. We find trends of ocean temperature, atmospheric carbon dioxide and oxygen isotopes largely co-vary. However, this relationship is less clear for the late Oligocene, when high-latitude cooling occurred despite interpretation of oxygen isotopes suggesting global warming and ice-volume loss. We propose this retreat of the West Antarctic Ice Sheet occurred in response to a tectonically driven marine transgression, with warm surface waters precluding marine-based ice-sheet growth. Marine ice-sheet expansion occurred only when ocean temperatures further cooled during the Oligocene–Miocene transition, with cold orbital conditions and low atmospheric carbon dioxide. Our results support a threshold response to atmospheric carbon dioxide, below which Antarctica’s marine ice sheets grow, and above which ocean warming exacerbates their retreat.

The past three decades have seen a sustained and coordinated effort to refine the seismic stratigraphic framework of the Antarctic margin that has underpinned the development of numerous geological drilling expeditions from the continental shelf and beyond. Integration of these offshore drilling datasets covering the Cenozoic era with Antarctic inland datasets, provides important constraints that allow us to understand the role of Antarctic tectonics, the Southern Ocean biosphere, and Cenozoic ice sheet dynamics and ice sheet–ocean interactions on global climate as a whole. These constraints are critical for improving the accuracy and precision of future projections of Antarctic ice sheet behaviour and changes in Southern Ocean circulation. Many of the recent advances in this field can be attributed to the community-driven approach of the Scientific Committee on Antarctic Research (SCAR) Past Antarctic Ice Sheet Dynamics (PAIS) research programme and its two key subcommittees: Paleoclimate Records from the Antarctic Margin and Southern Ocean (PRAMSO) and Palaeotopographic-Palaeobathymetric Reconstructions. Since 2012, these two PAIS subcommittees provided the forum to initiate, promote, coordinate and study scientific research drilling around the Antarctic margin and the Southern Ocean. Here we review the seismic stratigraphic margin architecture, climatic and glacial history of the Antarctic continent following the break-up of Gondwanaland in the Cretaceous, with a focus on records obtained since the implementation of PRAMSO. We also provide a forward-looking approach for future drilling proposals in frontier locations critically relevant for assessing future Antarctic ice sheet, climatic and oceanic change.

The Oligocene-Miocene transition (OMT) is one of the most enigmatic periods (~23.3–22.9 Ma) in Earth’s Cenozoic climate history. It is characterised in deep-sea benthic foraminiferal (δ¹⁸O) records by an increase of up to +1‰ spanning a 200–300 kyr interval associated with global cooling and growth in Antarctic ice volume to as much as 120% of present day (Mi-1 glaciation). The Mi-1 glaciation was then terminated by a −1.2‰, δ¹⁸O decrease that occurred in less than 100 kyrs, implying rapid Northern Hemisphere-style ice retreat, and potentially continental-scale deglaciation of Antarctica. Antarctic margin ocean drill core records and seismic reflection profiles display evidence of ice-proximal glacimarine deposition or glacimarine erosion, and imply continent-wide advance of the ice sheet terminus into the marine realm during the Mi-1 glaciation. This major transient glaciation occurred within a ~400 kyr-duration eccentricity cycle and appears to be coupled with an orbitally-paced perturbation of the carbon cycle. Atmospheric CO2 reconstructed from geological proxy records imply a long-term decrease during the Oligocene from about 500 to less than 300 ppm. Atmospheric CO2 declining below a threshold (~400 ppm), together with an extreme cold orbital configuration, enabled widespread seasonal sea-ice formation and the development of extensive marine-based ice margins around Antarctica. Atmospheric CO2 concentration appears to rebound rapidly following the Mi-1 glaciation, with some proxy estimates as high as 1000 ppm by the earliest Miocene. The OMT challenges our current understanding of orbitally-paced, ocean-atmosphere carbon exchange and associated feedbacks in the climate system. Prior to the OMT, between ~27 and 24 Ma, a trend towards lower δ¹⁸O values suggested an extensive period of global warmth, polar ice volume decrease and global sea-level rise. This is in contrast with widespread evidence from Ross Sea drill cores that show cooling of near surface ocean and land temperatures, and glacial advance calving ice bergs at the coastline. However, on the Wilkes Land margin cooling begins later, after ~25 Ma. Here, we summarise new evidence of the relative influences of tectonics, atmospheric carbon dioxide, ocean dynamics and orbital forcing on the evolution of the Antarctic Ice Sheet (AIS) during the Late Oligocene and across the OMT. We revisit the longstanding conundrums and provide some insights for the future (in)stability of the Antarctic Ice Sheet.

Antarctic continental ice masses fluctuated considerably during the Oligocene “coolhouse”, at elevated atmospheric CO2 concentrations of ∼600–800 ppm. To assess the role of the ocean in the Oligocene ice sheet variability, reconstruction of past ocean conditions in the proximity of the Antarctic margin is needed. While relatively warm ocean conditions have been reconstructed for the Oligocene offshore of Wilkes Land, the geographical extent of that warmth is unknown. In this study, we reconstruct past surface ocean conditions from glaciomarine sediments recovered from Deep Sea Drilling Project (DSDP) Site 274 offshore of the Ross Sea continental margin. This site, located offshore of Cape Adare is ideally situated to characterise Oligocene regional surface ocean conditions, as it is situated between the colder, higher-latitude Ross Sea continental shelf and the warm-temperate Wilkes Land margin in the Oligocene. We first improve the age model of DSDP Site 274 using integrated bio- and magnetostratigraphy. Subsequently, we analyse organic walled dinoflagellate cyst assemblages and lipid biomarkers (TEX86, TetraEther indeX of 86 carbon atoms) to reconstruct surface palaeoceanographic conditions during the Oligocene (33.7–24.4 Ma). Both TEX86-based sea surface temperature (SST) and microplankton results show temperate (10–17 ∘C ± 5.2 ∘C) surface ocean conditions at Site 274 throughout the Oligocene. Oceanographic conditions between the offshore Wilkes Land margin and Cape Adare became increasingly similar towards the late Oligocene (26.5–24.4 Ma); this is inferred to be the consequence of the widening of the Tasmanian Gateway, which resulted in more interconnected ocean basins and frontal systems. Maintaining marine terminations of terrestrial ice sheets in a proto-Ross Sea with offshore SSTs that are as warm as those suggested by our data requires a strong ice flux fed by intensive precipitation in the Antarctic hinterland during colder orbital states but with extensive surface melt of terrestrial ice during warmer orbital states.

Antarctic continental ice masses fluctuated considerably in size during the elevated atmospheric CO2 concentrations (~ 600–800 ppm) of the Oligocene “coolhouse”. To evaluate the role of ocean conditions to the Oligocene ice sheet variability requires understanding of past ocean conditions around the ice sheet. While warm ocean conditions have been reconstructed for the Oligocene Wilkes Land region, questions arise on the geographical extent of that warmth. Currently, we lack data on surface ocean conditions from circum-Antarctic locations, and ice-proximal to ice-distal temperature gradients are poorly documented. In this study, we reconstruct past surface ocean conditions from glaciomarine sediments recovered from the Deep Sea Drilling Project (DSDP) Site 274, offshore the Ross Sea continental margin. This site offshore Cape Adare is ideally located to characterise the Oligocene regional surface ocean conditions, as it is situated between the colder, ice-proximal Ross Sea continental shelf, and the warm-temperate Wilkes Land Margin in the Oligocene. We improve the existing age model of DSDP Site 274 using integrated bio- and magnetostratigraphy. Subsequently, we analyse dinoflagellate cyst assemblages and lipid biomarkers (TEX86) to reconstruct surface paleoceanographic conditions during the Oligocene (33.7–25.4 Ma). Both TEX86-based sea surface temperature (SST) and microplankton results show temperate (10–17 °C ± 5.2 °C) surface ocean conditions at Site 274 throughout the Oligocene. Increasingly similar oceanographic conditions between offshore Wilkes Land margin and Cape Adare developed towards the late Oligocene (26.5–25.4 Ma), likely in consequence of the widening of the Tasmanian Gateway, which resulted in more interconnected ocean basins and frontal systems. To maintain marine terminations of terrestrial ice sheets in a proto-Ross Sea with as warm offshore SST as our data suggests, requires a strong ice flux fed by intensive precipitation during colder orbital states in the Antarctic hinterland, but with extensive surface melt of terrestrial ice during warmer orbital states.