School of Geography, Earth and Atmospheric Sciences - Research Publications
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ItemGlobal Changes in Terrestrial Vegetation and Continental Climate During the Paleocene-Eocene Thermal Maximum(AMER GEOPHYSICAL UNION, 2022-04)Abstract Most studies of the response of terrestrial vegetation to climate change during the Paleocene‐Eocene Thermal Maximum (PETM) have focused on individual sites and sections. To get a broader perspective we compiled published records of terrestrial pollen and spores across the Paleocene‐Eocene transition at 38 sites around the globe. For the 10 sites with quantitative data PETM palynofloras were largely distinct in composition from those in the latest Paleocene or post‐PETM early Eocene. We also inferred paleoclimatic conditions at each site from the distributions of nearest living relatives (NLRs) of fossil pollen taxa among present‐day Köppen climate types. The NLRs of Paleocene high‐paleolatitude palynotaxa are most diverse in cooler climates, whereas the NLRs of PETM taxa are more diverse in warmer, wetter climates. At middle‐paleolatitudes NLRs of Paleocene palynotaxa are most diverse in warm, wet climates, whereas NLRs of PETM palynotaxa are most diverse in warm, seasonally dry climates. In the tropics there is little change from Paleocene to PETM in the climate distributions of NLRs. We compared changes in paleoclimate reconstructed from the Köppen distributions of the NLRs with those simulated from the Community Earth System Model (version CESM1.2). Paleoclimatic changes during the PETM inferred from palynological proxies are mostly consistent with modeled climate changes, including the expansion of temperate climates at the expense of cold climate types at high‐paleolatitudes and the expansion of temperate and tropical climates in middle‐paleolatitudes. Despite this concordance, modeled winter temperatures in continental interiors and high‐paleolatitudes remain colder than those reconstructed from NLR distributions.
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ItemThe Origin of Floral Lagerstatten in Coals(Society for Sedimentary Geology (SEPM), 2020-01-01)Floral Lagerstätten deposits (i.e., fossil sites with exceptional preservation and diversity) are preserved within the Miocene brown coals of the Latrobe Group, Gippsland Basin, Australia. Three independent mechanisms are conducive to their accumulation. Throughout the coal seams the conversion of plant material into charcoal (fusain) and its accumulation in a subaqueous setting provides one means of near-perfect preservation. A second and more uncommon example occurs in the form of a 20 cm thick leaf-litter horizon that extends for over two kilometers. In this case, flooding of freshwater tributaries and lakes during the early stages of low-gradient peat development resulted in an extensive, shallow, acidic and water-filled depression that subsequently accumulated and preserved the surrounding plant material. The third and most common form results from the deposition of plant material into small, isolated pools that formed as depressions on the ombrogenous (i.e., rain-fed) and domed surface of the peatlands. In all of these settings an essential component allowing detailed floral preservation is the delivery of plant material directly to the anaerobic catotelm (i.e., below the water table), hence avoiding the physical and chemical processes of degradation that typically occur in the surficial aerobic acrotelm (i.e., above the water table). Leaf litter that falls into low-energy acidic and anoxic water-filled depressions that lie below the acrotelm will likely be well-preserved.
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ItemThe rise of flowering plants in the high southern latitudes of Australia(Elsevier, 2020-01-01)The Early Cretaceous high-paleolatitude palynofloras from the Otway and Gippsland basins of southeastern Australia contain diverse angiosperm assemblages not described previously. Clavatipollenites hughesii recovered from the early Aptian Upper Cyclosporites hughesii subzone in the Gippsland Basin represents the first record of angiosperm pollen in Australia, coeval to records recovered from the Great Artesian and North West Shelf basins of northern Australia. Tricolpate pollen including Tricolpites variabilis, Rousea georgensis and Striatopollis spp., are first recorded in the late Albian Upper Coptospora paradoxa subzone in southeastern Australia. This represents the second oldest occurrence of tricolpate pollen in Australia, the first occurring in the older middle Albian Lower Coptospora paradoxa subzone in the northern Great Artesian Basin. By the latest Albian Phimopollenites pannosus Zone angiosperms had diversified rapidly in southeastern Australia. The delayed appearance, rise in abundance and diversification of eudicot angiosperms in the high-latitude southern basins of Australia, relative to low- and mid-latitude settings, supports a latitudinally diachronous pattern of angiosperm range expansion from warmer paleoequatorial regions to relatively cooler high-latitude settings. Increasing mean annual temperatures globally in the late Albian likely facilitated the expansion of angiosperms into high-latitude settings in the Southern and Northern hemispheres.
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ItemEocene to Oligocene terrestrial Southern Hemisphere cooling caused by declining pCO2(NATURE PORTFOLIO, 2021-09)The greenhouse-to-icehouse climate transition from the Eocene into the Oligocene is well documented by sea surface temperature records from the southwest Pacific and Antarctic margin, which show evidence of pronounced long-term cooling. However, identification of a driving mechanism depends on a better understanding of whether this cooling was also present in terrestrial settings. Here, we present a semi-continuous terrestrial temperature record spanning from the middle Eocene to the early Oligocene (~41–33 million years ago), using bacterial molecular fossils (biomarkers) preserved in a sequence of southeast Australian lignites. Our results show that mean annual temperatures in southeast Australia gradually declined from ~27 °C (±4.7 °C) during the middle Eocene to ~22–24 °C (±4.7 °C) during the late Eocene, followed by a ~2.4 °C-step cooling across the Eocene/Oligocene boundary. This trend is comparable to other temperature records in the Southern Hemisphere, suggesting a common driving mechanism, likely pCO 2. We corroborate these results with a suite of climate model simulations demonstrating that only simulations including a decline in pCO 2 lead to a cooling in southeast Australia consistent with our proxy record. Our data form an important benchmark for testing climate model performance, sea–land interaction and climatic forcings at the onset of a major Antarctic glaciation.
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ItemFossil evidence for the evolution of the Casuarinaceae in response to low soil nutrients and a drying climate in Cenozoic Australia(CSIRO Publishing, 2020)The Southern Hemisphere family Casuarinaceae has a long fossil record, both macrofossils and pollen, none of which provides any evidence about the morphology of the precursor to the family. However, it has long been considered, from both molecular phylogenies and morphological data, that the extant genus Gymnostoma retains key ancestral states and the highly reduced leaf area is a result of a scleromorphic response to low soil nutrients. Gymnostoma has by far the earliest, most extensive and best preserved macrofossil record, beginning in the Late Paleocene. Modification of the stomatal location from superficial in Gymnostoma to encrypted in furrows in the other genera assisted in water conservation as species evolved. We conclude that the morphology of the living and fossil vegetative branchlets provides evidence that low soil nutrients (especially phosphorus) and high water availability in a relatively light limited environment were the original drivers for evolution in the Casuarinaceae. Reducing water availability (xeromorphy) in progressively higher light environments were the major drivers of post-Eocene evolution in this unique plant family.
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ItemRecognition of peat depositional environments in coal: A review(Elsevier, 2020-02)Peat depositional environments, the sites where and conditions under which peat accumulates, significantly influence a resultant coal's physical properties, chemical composition, and coal utilization behavior. Recognition of peat depositional environments for coal is a challenging endeavor because coal's observed compositional properties not only result from a variety of geological processes operating during peat accumulation, but also reflect the influence of adjoining or external depositional sedimentary environments and alteration during later diagenesis and/or epigenesis. The maceral or microlithotype composition of any one layer of peat can be the product of years or decades of plant growth, death, decay, and post-burial infiltration by roots in addition to the symbiotic, mutualistic, parasitic, and saprophytic relationships with non-plant biota, such as arthropods, fungi, and bacteria. The overprint of increasing thermal maturation and fluid migration through time on the resulting coal can make these relationships difficult to recognize. Therefore, published models based on maceral composition alone must be used with great caution. Lipid compositions, even from lipid-poor low-rank coals, can provide important information about depositional environments and paleoclimate, especially if combined with the results of organic petrography and paleontological studies. Just as sulfur derived from seawater provides environmental clues, the ratios of two particularly relevant trace elements rather than a single trace element can be used to interpret peat depositional environments. Epigenetic minerals, as well as their corresponding chemical compositions should not be used for such a purpose; similarly, resistant terrigenous minerals deposited during peat accumulation in many cases should be used with considerable caution. The interactions of the biota present in the peat-forming ecosystem, often determined using palynological and geochemical proxies, and their interpretation in the context of geography and paleoclimate are important means for deciphering peat depositional environments. Overall, a combination of evidence from geochemistry, mineralogy, palynology, and petrology of coal and from stratigraphy, sedimentology, and sedimentary facies of related rocks is necessary for accurate and comprehensive determination of depositional environments. The need for interdisciplinary studies is underscored by peat compositional properties, which have been greatly affected by various processes during the syngenetic, diagenetic or epigenetic stages of coal formation.