Papers by Stephen Gallagher
Global and Planetary Change, 2025
The tropical North West Shelf of Australia hosts a diverse range of modern reefs. Six shelf edge ... more The tropical North West Shelf of Australia hosts a diverse range of modern reefs. Six shelf edge isolated atolls are present north of 18◦S including: Ashmore Reef, Scott Reef and Seringapatam Reef, and three Rowley Shoals. The Ningaloo Reef is a fringing reef around the North West Cape at 22◦S. All of these reefs are the remnants of a vast 2000 km long barrier reef that drowned during the Late Miocene (~10 Ma). Despite extensive hydrocarbon
exploration in the region, the history of these isolated reefs is not well known. Seismic analyses combined with stratigraphic analyses of International Ocean Discovery Site U1464 near the Rowley Shoals has revealed that
these modern isolated atolls have a complex evolution related to climate and tectonism as they managed to survive on their Miocene barrier reef foundation.
The first Miocene reefs (~17 Ma) near the Rowley Shoals were isolated small, mound-shaped features. These evolved into a barrier reef by the Middle Miocene (~16 Ma). However, by the Late Miocene (~10 Ma) this
barrier reef backstepped landward, evolved into isolated mounds/atolls and drowned prior to the Miocene-Pliocene boundary (~6 Ma) largely due regional tectonic subsidence. Early Pliocene reef expansion (~4.6
Ma) led to the growth of four isolated atolls (the Rowley Shoals) related to local faulting and Early Pliocene warmth. Subsequently a second Pliocene reef growth phase occurred from ~3.5 to 3 Ma when eastern Indian
Ocean sea surface temperatures cooled by ~4 ◦C due to Indonesian Gateway constriction and a reduced Leeuwin Current. By the Pleistocene (~2.4 Ma) one the four Rowley Shoals had drowned. Strong sea level variability,
together with Indonesian Throughflow constriction and reduction in intensity of the Leeuwin Current after 2.4 Ma may have led to enhanced cooling and regional upwelling. These factors may have been sufficient to cause
local drowning of the southerly fourth Rowley Shoal while the more northerly three Shoals survived until present.
Marine Micropaleontology, 2025
Benthic foraminiferal assemblages from Deep Sea Drilling Project (DSDP) Site 548 on the Goban Spu... more Benthic foraminiferal assemblages from Deep Sea Drilling Project (DSDP) Site 548 on the Goban Spur off southwestern Ireland shed light on the changes in bottom water oxygenation and organic matter flux to the sea floor during the late Quaternary. Correlations of benthic foraminiferal δ18O values, the relative abundance of Neogloboquadrina pachyderma (%NP), and Ice Rafted Debris (IRD) concentration to global and regional and North Atlantic datasets suggest the upper 60 m of DSDP Site 548 extend to 420,000 years. Downcore variations of >63 μm benthic foraminifera assemblages, abundance, and diversity reveal changes in dissolved oxygen concentration and organic fluxes to the seafloor related to glacial-interglacial cyclicity. Cassidulina laevigata and low dissolved oxygen indicator taxa such as Bolivina spp. and Globobulimina spp. characterised colder climates associated with lighter benthic δ13C values, suggesting minimal organic flux and/or weaker bottom water ventilation. In contrast, warmer interglacials are typified by heavier benthic δ13C, increased %CaCO3, common high dissolved oxygen indicator taxa such as Globocassidulina subglobosa and phytodetritus sensitive taxa such as Alabaminella weddellensis and Epistominella exigua, suggest a more ventilated bottom water and increased organic fluxes to the seafloor, possibly associated with the invigoration of the Mediterranean Outflow Water (MOW).
Progress in Earth and Planetary Science, 2024
The Australian monsoon is part of the global monsoon and often included as a component of the Asi... more The Australian monsoon is part of the global monsoon and often included as a component of the Asian Monsoon system although they operate out of phase. Due to their hemispheric positions, the dry (wet) Australian winter (summer) monsoon coincides with the wet summer Asian monsoon and vice versa. The Australian monsoon controls rainfall distribution in northern tropical Australia where over 80% of the median annular rainfall occurs from December to March, the summer wet season. Three types of the Australian monsoon are distinguished based on distinct atmospheric circulation and heating patterns: a northwest Pseudo-Monsoon, a northeast Quasi-Monsoon and an Australian Monsoon (sensu stricto) north of Australia. While the modern climatology of the Australian monsoon has been extensively documented, its paleohistory is poorly constrained, especially in Australia’s continental interior where harsh arid climatic conditions have degraded almost all physical evidence of monsoonal activity. However, reassessment of northern and central Australian terrestrial and marine sequences reveals a fairly robust Cenozoic history of this monsoon, especially for the Neogene, which we synthesize for the first time here. Evidence for a Paleogene Australian paleomonsoon is equivocal due to the small number of sites, their limited age control, and the poor preservation of flora with ambiguous affinities. Modeling and tectonic evidence suggest the northern part of the Australian Plate migrated to the (sub)tropical region (north of 30°S) creating “modern” boundary conditions for monsoonal onset by ~10 Ma. Cores off northwest Australia reveal arid late Miocene and humid Pliocene conditions were followed by the Pseudo-Monsoon at ~3.5 Ma when northern hemisphere glacial expansion “forced” the ITCZ (Inter Tropical Convergent Zone) south. Subsequently, variable humid and arid periods typify Quaternary high-amplitude glacio-eustatic cycles until ~1 Ma, when arid conditions expanded across Australia. Glacial/interglacial cyclicity and obliquity/precession insolation during terminations modulated Pseudo-Monsoon intensity when the ITCZ migrated northward (during glacial) and southward (during interglacial periods) from ~1 Ma to present. From ~1.6 to 1 Ma, precession paced Pseudo-Monsoon variability. Mega-lake expansion in central Australia and fluvial intensification generally correspond to wetter interglacial periods. Lake Eyre monsoonal shorelines may have been influenced by abrupt millennial events. Monsoonal conditions re-established near the base of Holocene as the ITCZ migrated across northern Australia. The Australian Monsoon (sensu stricto) and Quasi-Monsoon (a) initiated from 12.5 to 11 ka; (b) intensifying from 9 to 2 ka; then (c) weakened, possibly due to the onset of ENSO intensification. The Pseudo-Monsoon was established at ~14.5 ka off northwest Australia intensifying from 11.5 to 7 ka. It weakened after ~7 ka north of 15°S and ~5 ka to the south. In the absence of a topographic influence, insolation (precession/obliquity), abrupt millennial events and/or ITCZ variability across northern Australia were important controls on Quaternary Australian monsoon intensity. Further investigations of deeper time pre-Quaternary records off northwest and northeast Australia will reveal the paleohistory of this important domain of the Global Monsoon.
Paleoceanography and Paleoclimatology,, 2024
Paleoclimate proxy records from regions sensitive to humidity/aridity extremes provide crucial in... more Paleoclimate proxy records from regions sensitive to humidity/aridity extremes provide crucial insights into the natural forcing mechanisms underlying long-term climate variability in broader regions. One such area is Northwest Australia, where the Australian monsoon impacts its northernmost fringes, which are bordered by the Great Sandy Desert inland. Marine sediments from the Australian Northwest Shelf record fluvial runoff and eolian dust input during the wet and dry seasons, respectively. The location is therefore ideal for investigating long-term variability in the Australian monsoon and Northwest Australian dust flux over orbital timescales. However, there are few continuous, high-resolution paleoclimate records from this region spanning the early Pleistocene, when strong ice-climate feedbacks of the late Pleistocene did not yet dominate global climate. Here, we present geochemical and environmental magnetic proxy records that reveal %CaCO 3 and dust-flux variability between 2.9 and 1.6 million years (Myr) ago from International Ocean Discovery Program Expedition 356 Site U1464 on the Australian Northwest Shelf. We establish a new, orbitally-tuned chronology for Site U1464, primarily based on ∼400 thousand year (kyr) eccentricity cyclicity in %CaCO 3 , and observe strong obliquity variability (41 and 54 kyr periodicities) but almost no precession signal in the dust-flux records. We propose that the 41 kyr cycle in Northwest Australian dust flux could be a linear response to East Asian winter monsoon intensity and/or the summer inter-tropical insolation gradient (SITIG), whereas the 54 kyr cyclicity might be a non-linear response to obliquity amplitude modulation via the SITIG effect on the cross-equatorial atmospheric circulations.
Annual Review of Earth and Planetary Sciences, 2024
■ Long term Indonesian Gateway history is best determined from ocean drilling cores on the Indian... more ■ Long term Indonesian Gateway history is best determined from ocean drilling cores on the Indian and Pacific sides of the Indonesian Gateway, as records from within it are typically disrupted by tectonics. ■ Model simulations show the global impact of the Indonesian Gateway. Further modeling with ocean drilling/tectonic research will enhance our understanding of Cenozoic Indonesian Gateway history.
Borias, 2023
Deep Sea Drilling Program (DSDP) Site 548 was cored in 1984 at awater depth of 1256 m on the Goba... more Deep Sea Drilling Program (DSDP) Site 548 was cored in 1984 at awater depth of 1256 m on the Goban Spur, offshore southwest Ireland. Coring retrieved a ∼100-m-thick Pleistocene contourite sequence. This study uses planktonic foraminiferal assemblage and benthic foraminiferal oxygen isotope analyses to establish an age model for the upper 40 m of this core. This site's multidisciplinary analyses of planktonic foraminiferal assemblages, lithic grains, facies and calcium carbonate concentration reveal a 250 000-year record of the North Atlantic polar front variability and British-Irish Ice Sheet (BIIS) history. The sequence is characterized by alternations of ice rafted debris (IRD) laden pelagic mud facies with calcium carbonate-rich silty sand contourite facies that track glacial/interglacial cycles. The polar front migrated southward across the area several times during glacial maxima and stadial periods, while warmer Mediterranean Outflow Water (MOW) flowed northward across the region during interglacial and interstadial periods depositing contourites. Lithic analyses reveal a complex history of IRD deposition associated with iceberg calving from the Laurentide Ice Sheet and northwest European ice sheets, mainly the BIIS. Comparison between the Goban Spur (DSDP Site 548) and the Celtic Margin (MD03-2692) and central North Atlantic Integrated Ocean Drilling Program (IODP) Site U1308 suggests differences between the 'non-Laurentide Ice Sheet' Heinrich Events (HE) 6 and 3 at the Goban Spur, with IRD from the BIIS being prominent during HE 6 and IRD from other European ice sheets north of the BIIS likely being more dominant during HE 3. The nature of lithics in IRD-rich horizons during Terminations 3, 3A, 2 and 1 suggests significant iceberg calving episodes preceding BIIS retreat during the onset of interstadial intervals.
SP49 — IAS Special Publication 49, 2022
Understanding of global sea-level changes and coral reef development is poorly constrained during... more Understanding of global sea-level changes and coral reef development is poorly constrained during Marine Isotope Stage 3 (MIS 3; ~ 60 to 30 ka). Australia's North West Shelf (NWS), at depths of ~ 50 to 120 m below present sea-level (mbsl), represents an ideal natural laboratory to address these knowledge gaps. In this study, the authors investigate a unique suite of sea-bed rock drill (PROD) cores recovered as part of a geotechnical survey from the NWS ~ 150 km southeast of Ashmore Reef. Twenty cores, penetrating to 28 m below sea floor, were collected from the top of the now drowned platform complex in similar water depths (74.8 to 81.6 mbsl), forming two transects ~ 17 km apart. High-resolution 3D seismic and multibeam bathymetry data reveal three distinct, multigenerational platforms that are rimmed by smaller reef terraces and bisected by deeper channels, placing the core transects into a robust, regional geomorphic context that includes a succession of linear palaeo-shorelines and tidal-estuarine channel systems on the adjacent shelf between ~ 90 to 110 mbsl. The authors have completed detailed logging, highspatial resolution hyperspectral scanning, petrologic, mineralogic and sedimentary facies analysis of these cores, including a precise palaeoenvironmental reconstruction based on coral, algal and larger benthic foraminifera assemblages; and extensive radiometric dating. The authors have observed a complex suite of
Scientific Drilling, 2022
The International Ocean Discovery Program (IODP) conducted a series of expeditions between 2013 a... more The International Ocean Discovery Program (IODP) conducted a series of expeditions between 2013 and 2016 that were designed to address the development of monsoon climate systems in Asia and Australia. Significant progress was made in recovering Neogene sections spanning the region from the Arabian Sea to the Sea of Japan and southward to western Australia. High recovery by advanced piston corer (APC) has provided a host of semi-continuous sections that have been used to examine monsoonal evolution. Use of the half-length APC was successful in sampling sand-rich sediment in Indian Ocean submarine fans. The records show that humidity and seasonality developed diachronously across the region, although most regions show drying since the middle Miocene and especially since ∼ 4 Ma, likely linked to global cooling. A transition from C 3 to C 4 vegetation often accompanied the drying but may be more linked to global cooling. Western Australia and possibly southern China diverge from the general trend in becoming wetter during the late Miocene, with the Australian monsoon being more affected by the Indonesian Throughflow, while the Asian monsoon is tied more to the rising Himalaya in South Asia and to the Tibetan Plateau in East Asia. The monsoon shows sensitivity to orbital forcing, with many regions having a weaker summer monsoon during times of northern hemispheric Glaciation. Stronger monsoons are associated with faster continental erosion but not weathering intensity, which either shows no trend or a decreasing strength since the middle Miocene in Asia. Marine productivity proxies and terrestrial chemi-Published by Copernicus Publications on behalf of the IODP and the ICDP.
Australian Journal of Earth Sciences, 2022
A fully cored sequence of Hesse Clay, Port Campbell Limestone and uppermost Gellibrand Marl in th... more A fully cored sequence of Hesse Clay, Port Campbell Limestone and uppermost Gellibrand Marl in the onshore Otway Basin, southeastern Australia, offers new insight into the evolution of the mid-dle Miocene Port Campbell Limestone. The Port Campbell Limestone comprises grey unconsoli-dated to semi-consolidated and rarely lithified bioclastic muddy carbonate sands in a stack of thin repetitive cycles within cycles of predominantly shoaling-upward character. A glauconitic band with a distinctive mollusc–echinoderm–bryozoan fauna provides a distinctive marker interval in the sequence. In mineralogy, the Port Campbell Limestone is predominantly calcite with traces of rem-nant aragonite in muddier low-permeability sands, and with dolomitic zones in permeable inter-vals. The small non-carbonate component of the Port Campbell Limestone is between 3 and 15 wt% and comprises quartz silt with minor clay, feldspar and mica. Dissolution overprints are prominent throughout the carbonate sequence. Three distinct geochemical signatures of proven-ance are evident in the Port Campbell Limestone sequence, including possible volcanogenic contri-butions with felsic sources. Foraminifera are common and generally well preserved. Foraminiferal data suggest a depositional transition from outer shelf conditions in the Gellibrand Marl at ca 15 Ma to middle shelf environments in the lower part of the Port Campbell Limestone during the Middle Miocene Climatic Optimum (MMCO) at ca 14.24 Ma. Shallowing after 14 Ma indicates vari-able paleodepths of <70 m during and following the end of the Middle Miocene Climatic Transition (MMCT) at ca 13.2 Ma when the sequence was emergent for a brief but undetermined period, corresponding with sharp changes in geochemical ratios. Observed cyclicity in these mid-shelfal, cold-water carbonates is strongly correlated with orbital forcings—eccentricity and obli-quity. Sedimentation rates determined from cyclostratigraphic analysis indicate 4–6 cm/kyr at the end of the MMCO, diminishing to 1.5–3 cm/kyr during the MMCT and the subsequent accumula-tion of the Port Campbell Limestone.
Paleoceanography and Paleoclimatology, 2021
The East China Sea (ECS) seasonally receives a high organic input due to the terrestrial organic ... more The East China Sea (ECS) seasonally receives a high organic input due to the terrestrial organic matter influx, which is controlled by the East Asian Summer Monsoon (EASM), and the increased productivity
driven by upwelling of the subsurface Kuroshio Current (KC). Changes in benthic foraminiferal assemblage
composition in combination with paleoceanographic proxy data (CaCO3 (%), TOC (%), δ13Cpf, and δ18Obf) are used to reconstruct bottom water oxygenation and organic export flux variability over the last 400 kyr in the ECS. Multivariate analyses of benthic foraminiferal census data identified six biofacies characteristic of varying environmental conditions. These results suggest that enhanced EASM precipitation and KC upwelling directly influenced organic export flux and bottom water oxygen content in the ECS. The ECS bottom water was suboxic during Marine Isotope Stage (MIS) 11 to 8; suboxic to dysoxic between MIS 7 and 6, strongly dysoxic between mid-MIS 5 and 4, and exhibited high variability between MIS 3 and 1. Spectral analysis of relative abundances of representative genera Quinqueloculina (oxic), Bulimina (suboxic), and Globobulimina (dysoxic) reveals a robust 23 kyr signal, which we attribute to precessionally-paced changes in surface productivity and bottom water oxygenation related to KC variability over the past 400 kyr.
Victorian Gas Program Technical Report 8. Geological Survey of Victoria. Department of Jobs, Precincts and Regions. Melbourne, Victoria, 2019
New micropalaeontology results have been obtained from selected legacy core samples from subsurfa... more New micropalaeontology results have been obtained from selected legacy core samples from subsurface rock formations in the Victorian onshore Otway Basin. The results of these analyses fill data gaps in the Geological Survey of Victoria (GSV) historical micropalaeontological dataset. This study was only possible due to the large number of legacy core samples available at the Geological Survey of Victoria Drill Core Library, Werribee and represents one of the most comprehensive, basin-scale biostratigraphic projects to be undertaken in Australia.
Sedimentology, 2022
The continental shelf of Northwest Australia hosts an extensive tropical carbonate ramp that form... more The continental shelf of Northwest Australia hosts an extensive tropical carbonate ramp that forms an important template for the interpretation of similar systems in the sedimentary record. Yet, little is known about its development from the middle to late Quaternary, a period of high frequency glacioeustatic changes in sea-level and climate. This research describes core and seismic-reflection data from a mid-ramp to outer ramp transect at the Northwest Shelf. Core material includes the upper 70 m of International Ocean Discovery Program Site U1461 (Expedition 356), which covers the last 500 kyr. During arid glacials, sedimentation is characterized by inorganically precipitated carbonates, including aragonite-needle mud and ooids. Ooids developed under shallow marine conditions on small-scale flat-topped platforms. Seismic and sedimentological evidence indicates that these platforms developed locally on top of the present-day mid-ramp and were typically only active during a single glacial period. Aragonite-needle mud precipitated (inorganically) in shallow-water areas. Much of these fine sediments were subsequently exported into deeper water where they mixed with pelagic carbonates. Humid interglacials are generally characterized by reduced sedimentation across the Northwest Shelf of Australia, resulting in the amalgamation of glacial lowstand deposits. Yet, substantial amounts of skeletal carbonates were deposited at the studied location during the Holocene and Marine Isotope Stage 11. These sediment accumulations are interpreted as a local feature caused by a decline in current energy. The study presented highlights a ramp system where climate is as important as sea-level in controlling carbonate deposition.
Geology, 2021
Current knowledge of terrestrial ecosystem response to the Paleocene-Eocene Thermal Maximum (PETM... more Current knowledge of terrestrial ecosystem response to the Paleocene-Eocene Thermal Maximum (PETM; ca. 56 Ma) is largely based on the midlatitudes of the Northern Hemisphere. To more fully reconstruct global terrestrial ecosystem response to the PETM, we generated vegetation and biomarker proxy records from an outcrop section on the southern coast of Australia (∼60°S paleolatitude). We documented a rapid, massive, and sustained vegetation turnover as a response to regional PETM warming of ∼1-4 °C, abruptly transitioning from a warm temperate to a meso-megathermal rain forest similar to that of presentday northeastern Queensland, Australia. The onset of this vegetation change preceded the characteristic PETM carbon-isotope excursion (CIE) by several thousand years. The reconstructed ecosystem change is much stronger than in other Southern Hemisphere records, highlighting the need for consideration of regional paleoceanographic, paleogeographic, and biogeographic characteristics to fully understand the global terrestrial ecosystem response to PETM climate forcing.
Australian Journal of Earth Scicences, 2021
In this paper, we describe porosity variations in Miocene to Holocene carbonates off the west coa... more In this paper, we describe porosity variations in Miocene to Holocene carbonates off the west coast of Australia and assess their compaction trends. The porosity values were measured using discrete samples of Sites U1459–U1464 obtained by the International Ocean Discovery Program Expedition 356. The carbonate deposits have been influenced by a range of textures and diagen-etic conditions throughout a nearly continuous sequence of geological ages from the Miocene to Holocene and at core depths from 0 to 1100 m below the seafloor. The collected samples were mostly grainstone, packstone, wackestone and mudstone textures. Dolostones and dolomitic car-bonates were described at the Miocene intervals. Compaction trends were estimated exponentially and linearly based on cored sites, carbonate textures and dominant mineralogies (dolomite, cal-cite/aragonite). At all six sites, porosity distribution and reduction were generally depth-dependent. The porosity converged to about 30% between 750 and 1100 m, which suggests that the carbo-nates were close to the densest packing by mechanical compaction at a burial depth of 750 m. The porosity deviations are associated with textures and dominant mineralogies. Increasing mud content from grainstone to mudstone is a controlling factor for initial porosity and porosity reduc-tion rate. Dolomitisation, dolomitic cementation, aragonite needle-rich mud and non-skeletal grains cause deviations from the depth-dependent compaction trends. Reflux-related cementation gener-ally decreases porosity in Miocene dolomitic intervals. Higher porosity values of the Quaternary wackestone and mudstone at Site U1461 resulted from the presence of aragonite needle-rich mud hosting abundant micropores and from a high sedimentation rate. The occurrence of non-skeletal grains, such as ooids and peloids, as well as occasional meteoric exposure led to porosity inver-sion, occluding interparticle permeability and the creation of moldic pores.
Geochemistry, Geophysics, Geosystems, 2021
Accurate dating of marine sediments is essential to reconstruct past changes in oceanography and ... more Accurate dating of marine sediments is essential to reconstruct past changes in oceanography and climate. Benthic foraminiferal oxygen isotope series from such sediments record long-term changes in global ice volume and deep-water temperature. They are commonly used in the Plio-Pleistocene to correlate deep ocean records and to construct age models. However, continental margin settings often display much higher sedimentation rates due to variations in regional depositional setting and local input of sediment. Here, it is necessary to create a regional multi-site framework to allow precise dating of strata. We create such a high-resolution regional framework to determine the ages of events for the Northwest Shelf (NWS) of Australia, which was cored by International Ocean Discovery Program
(IODP) Expedition 356. We employ benthic foraminiferal oxygen and carbon isotopes to construct an astronomically-tuned age model for IODP Site U1463 (5.16–1.69 Ma). The age model is applied to the IODP Site U1463 downhole-logging natural gamma radiation (NGR) depth-series, which was then correlated to NGR depth-series of several IODP sites and industry wells in the area. This approach allows assigning ages to regional seismic reflectors and the timing of key climate-related siliciclastic phases in a predominantly carbonate-rich sequence, like the late Miocene-Pliocene Bare Formation. This age model is also used to chronologically calibrate planktonic foraminiferal biostratigraphic datums showing that the Indonesian Throughflow (ITF) had shoaled enough in the early Pliocene to act as biogeographical barrier between the Pacific and Indian Ocean.
Alcheringa, 2020
A continental shelf to upper continental slope ostracod fauna is documented from the late Cretace... more A continental shelf to upper continental slope ostracod fauna is documented from the late Cretaceous (late Turonian to Santonian) Belfast Mudstone in Voluta-1 of the Otway Basin, southeastern Australia. The fauna has palaeobiogeographical affinities with mid-late Cretaceous ostracod faunas of Western Australia, New Zealand, the Antarctic Peninsula and the Falklands Plateau. This distribution pattern probably reflects dispersal influenced by the opening of the Australo-Antarctic Gulf, and by clockwise gyre currents in the proto Southern Ocean. The presence of Philoneptunus sp. in this fauna suggests that the Australo-Antarctic Gulf was an important locus for deep sea colonization by Gondwanan neritic ostracod clades.
Tectonics, 2020
Cratons are ancient regions of relatively stable continental fragments considered to have attaine... more Cratons are ancient regions of relatively stable continental fragments considered to have attained long-term tectonic and geomorphic stability. Low-temperature thermochronology data, however, suggest that some cratons have experienced discrete Phanerozoic heating and cooling episodes. We report apatite fission track, and apatite and zircon (U-Th)/He low-temperature thermochronology data from the Archean Pilbara craton and adjacent Paleoproterozoic basement, NW Australia. Inverse thermal history simulations of this spatially extensive data set reveal that the region has experienced~50-70°C cooling, which is interpreted as a response to the unroofing of erodible strata overlying basement. The timing of cooling onset is variable, mainly~420-350 Ma in the southern and central Pilbara-eastern Hamersley Basin and~350-300 Ma in the northern Pilbara, while the westernmost Pilbara-central Hamersley Basin does not record a significant Paleozoic cooling event. These differences are attributed to variations in sedimentary thickness and proximity to adjacent rift basins, which lack Archean age zircons in their Paleozoic strata. The onset of Paleozoic cooling coincides with the timing of the episodic intraplate late Ordovician-Carboniferous Alice Springs Orogeny. This orogeny is thought to have resulted from far-field plate margin stresses, which in turn caused the opening of the adjacent Canning Basin, to the north and east of the craton. We propose that basin development triggered a change of base level, resulting in denudation and the crustal cooling event reported here. Our results provide further evidence for the transmission of far-field forces to cratons over hundreds of kilometers and support the view that cratons have experienced geomorphic changes during the Phanerozoic.
Climate of the Past, 2020
Global climate cooled from the early Eocene hothouse (∼ 52-50 Ma) to the latest Eocene (∼ 34 Ma).... more Global climate cooled from the early Eocene hothouse (∼ 52-50 Ma) to the latest Eocene (∼ 34 Ma). At the same time, the tectonic evolution of the Southern Ocean was characterized by the opening and deepening of circum-Antarctic gateways, which affected both surface-and deep-ocean circulation. The Tasmanian Gateway played a key role in regulating ocean throughflow between Australia and Antarctica. Southern Ocean surface currents through and around the Tasmanian Gateway have left recognizable tracers in the spatiotemporal distribution of plankton fossils, including organic-walled dinoflagellate cysts. This spatiotemporal distribution depends on both the physicochemical properties of the water masses and the path of surface-ocean currents. The extent to which climate and tectonics have influenced the distribution and composition of surface currents and thus fossil assemblages has, however, remained unclear. In particular , the contribution of climate change to oceanographic changes, superimposed on long-term and gradual changes induced by tectonics, is still poorly understood.
Alcheringa: An Australasian Journal of Palaeontology, 2020
Palynology is used to provide new age controls for the coastal exposures yielding Early Cretaceou... more Palynology is used to provide new age controls for the coastal exposures yielding Early Cretaceous vertebrate faunas in the Otway and Gippsland basins, southeast Australia. This work shows all vertebrate-bearing deposits in the Otway Basin are Albian in age. On the eastern margin of Cape Otway, vertebrate-bearing deposits are attributed to the early Albian Crybelosporites striatus Zone, except for one in the younger Coptospora paradoxa Zone. To the west of Cape Otway, the sampled deposits range from the oldest zone in the Albian, the C. striatus Zone, to the C. paradoxa and Phimopollenites pannosus zones, providing a complete section of the Albian stage. In the Gippsland Basin, the structural setting of vertebrate deposits is more complicated and large portions of the coastal exposures consist of repeated sections controlled by small-scale faults. Sites in this region lie below or above the Barremian/Aptian boundary. However, owing to the paucity of age indicator species, some sampled deposits remain undated. In this basin, the ranges of certain key palynomorph species (e.g., Pilosisporites notensis and Pilosisporites parvispinosus)dif-fered from those outlined in some previously published biostratigraphic schemes. The discovery of the angiosperm pollen Clavatipollenites hughesii in the lowermost Aptian deposits of the Gippsland Basin is consistent with its first occurrence elsewhere in Australia and shows it is a reliable indicator for the Aptian stage. This study shows that there is currently a collection bias towards vertebrate assemblages that are latest Barremian and early Albian in age.
Global and Planetary Change, 2020
Multiple stable isotope investigations from upper Eocene to lower Oligocene deep-water marine seq... more Multiple stable isotope investigations from upper Eocene to lower Oligocene deep-water marine sequences record the transition from global greenhouse to the icehouse conditions (Oi-1 glacial). While Southern Ocean high latitude deep sea records of this transition are well known, their shallow marine equivalents are rare and have the potential to record the eustatic and oceanic consequences of Paleogene glacial variability. The well-known high paleolatitude (~55°S) neritic carbonate sequence at Browns Creek and Castle Cove in the Otway Basin in southeast Australia spans the Eocene-Oligocene boundary. During this time the area lay on the northeastern margin of the Australo-Antarctic Gulf facing the evolving Southern Ocean. The importance of this record has been hampered by a lack of a consistent stratigraphy and contradictory microfossil interpretations. To reconcile these issues we combine new bio-, chemo-and lithostratigraphic analyses of the outcrops and a new core (Colac-2) with pre-existing data to revise the stratigraphy. This confirms the middle/upper Eocene boundary is near the base of the section. The overlying upper Eocene siliciclastic strata are truncated by an unconformity (of~0.8 Ma in duration) and overlain by glauconitic sand (the Notrostrea greensand) deposited after~35.9 Ma. Subsequently deepening to middle to outer neritic depths deposited cyclic carbonates. Shallowing after~35 Ma deposited laterally variable calcareous siliciclastic facies. These strata were tilted and eroded prior to 34 Ma leading to shallow water facies that may have been subaerially exposed during uplift. Brachiopod strontium isotope dates and an 0.5‰ carbon isotope excursion above this unconformity suggests the top of the Browns Creek and the base of the Castle Cove section correlate to Eocene-Oligocene transition (EOT-1) at~34 Ma. The subsequent persistence of positive C/O isotope values above this level records the transition to the Oi-1 glaciation at 33.7 Ma. Strong cyclicity in the inner shelf Castle Cove limestone is interpreted to record the commencement of obliquity dominated glacio-eustacy during the Oi-1 glacial phase. The shallowing from outer to inner shelf palaeodepths from the late Eocene to the early Oligocene is likely related to the onset of cryosphere expansion, however, palaeodepth estimates are complicated by the onset of regional compressional tectonism at the Eocene/Oligocene boundary that caused localized tilting and an unconformity with possible antisiphoning effects in this near-field site.
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Papers by Stephen Gallagher
exploration in the region, the history of these isolated reefs is not well known. Seismic analyses combined with stratigraphic analyses of International Ocean Discovery Site U1464 near the Rowley Shoals has revealed that
these modern isolated atolls have a complex evolution related to climate and tectonism as they managed to survive on their Miocene barrier reef foundation.
The first Miocene reefs (~17 Ma) near the Rowley Shoals were isolated small, mound-shaped features. These evolved into a barrier reef by the Middle Miocene (~16 Ma). However, by the Late Miocene (~10 Ma) this
barrier reef backstepped landward, evolved into isolated mounds/atolls and drowned prior to the Miocene-Pliocene boundary (~6 Ma) largely due regional tectonic subsidence. Early Pliocene reef expansion (~4.6
Ma) led to the growth of four isolated atolls (the Rowley Shoals) related to local faulting and Early Pliocene warmth. Subsequently a second Pliocene reef growth phase occurred from ~3.5 to 3 Ma when eastern Indian
Ocean sea surface temperatures cooled by ~4 ◦C due to Indonesian Gateway constriction and a reduced Leeuwin Current. By the Pleistocene (~2.4 Ma) one the four Rowley Shoals had drowned. Strong sea level variability,
together with Indonesian Throughflow constriction and reduction in intensity of the Leeuwin Current after 2.4 Ma may have led to enhanced cooling and regional upwelling. These factors may have been sufficient to cause
local drowning of the southerly fourth Rowley Shoal while the more northerly three Shoals survived until present.
driven by upwelling of the subsurface Kuroshio Current (KC). Changes in benthic foraminiferal assemblage
composition in combination with paleoceanographic proxy data (CaCO3 (%), TOC (%), δ13Cpf, and δ18Obf) are used to reconstruct bottom water oxygenation and organic export flux variability over the last 400 kyr in the ECS. Multivariate analyses of benthic foraminiferal census data identified six biofacies characteristic of varying environmental conditions. These results suggest that enhanced EASM precipitation and KC upwelling directly influenced organic export flux and bottom water oxygen content in the ECS. The ECS bottom water was suboxic during Marine Isotope Stage (MIS) 11 to 8; suboxic to dysoxic between MIS 7 and 6, strongly dysoxic between mid-MIS 5 and 4, and exhibited high variability between MIS 3 and 1. Spectral analysis of relative abundances of representative genera Quinqueloculina (oxic), Bulimina (suboxic), and Globobulimina (dysoxic) reveals a robust 23 kyr signal, which we attribute to precessionally-paced changes in surface productivity and bottom water oxygenation related to KC variability over the past 400 kyr.
(IODP) Expedition 356. We employ benthic foraminiferal oxygen and carbon isotopes to construct an astronomically-tuned age model for IODP Site U1463 (5.16–1.69 Ma). The age model is applied to the IODP Site U1463 downhole-logging natural gamma radiation (NGR) depth-series, which was then correlated to NGR depth-series of several IODP sites and industry wells in the area. This approach allows assigning ages to regional seismic reflectors and the timing of key climate-related siliciclastic phases in a predominantly carbonate-rich sequence, like the late Miocene-Pliocene Bare Formation. This age model is also used to chronologically calibrate planktonic foraminiferal biostratigraphic datums showing that the Indonesian Throughflow (ITF) had shoaled enough in the early Pliocene to act as biogeographical barrier between the Pacific and Indian Ocean.