Archive for the category: SESSION 2.3 | CRUST, SURFACE & COSMOS

White, Susan¹, Lipar, Matej ², Szymczak, Piotr ³,  Webb, John ¹

¹Environmental Geoscience, Department of Ecology, Environment and Evolution, La Trobe University, Melbourne, Bundoora, Victoria, 3086, Australia, ²Anton Melik Geographical Institute, Research Centre of the Slovenian Academy of Sciences and Arts, Gosposka ulica 13, SI-1000 Ljubljana, Slovenia, ³ Institute of Theoretical Physics, Faculty of Physics, University of Warsaw, Pasteura 5, 02-093 Warszawa, Poland

Focused vertical flow is one of the distinctive features of karst processes within porous calcareous media with matrix porosity, especially in calcarenites, and forms distinctive features termed solution pipes. These are vertical cylindrical voids of variable diameter and depth predominantly in the vadose (epikarstic) zone. Although dissolutional in origin, the initial trigger that focuses the water flow in the host rock matrix is poorly understood, although they appear to be related to vegetation and rock heterogeneities.

Measuring of the morphology of pipes e.g. pipe diameter, depth, density and shape,  is common but has not resolved the problem and neither has a 2D statistical data based on 5 x 5 m area in Australia which included inside and outside diameter of the pipes, their elongation, the thickness of cemented rim, and distance and direction to nearest neighbour. The stratigraphy, mineralogy, fabric and geochemistry of the surrounding rock and pipe rims and fills provide insights into pipe formation but do not address the issue of focussed flow. Thin section and X-ray diffraction has been used to distinguish between rim and host rock fabric. Stable isotope analysis partly correlated with host rock age indicates that the dissolutional process is under vadose conditions. Dating of the rim cement has not been successful due to the minute size of the carbonate cement samples. And the rock clasts of the host rock are obviously older.

Triggers to flow include lithological irregularities e.g. cracks in calcrete, surface geomorphology e.g. potholes and vegetation e.g. stem flow, buried tree trunks. If these are not present the instability of the wetting front infiltrating into the unsaturated zone breaks advances into unsaturated host rock as self organised fingers. This fingered flow, a non-uniform accelerated transport of sinking water, through the porous material due to an unstable wetting front, is particularly favoured if the media is water repellent, which causes uneven infiltration.

The roles of dynamic instabilities and self-organisation in the emergence of focusing patterns and the various challenges, especially in the vadose zone, can probably be only resolved with modelling, conjunction with field data. From the numerical point of view, modelling of solution pipe formation requires solving the coupled equations for the groundwater flow, chemical transport and porosity evolution. The ground-water system may be either saturated, or partly or completely unsaturated. Morphologically similar pipes seem to develop under both conditions. Pipes formed in deglaciated Miocene sediments in Poland  can be modelled using Darcian flow equations. However the pipes found in unglaciated areas e.g. Cape Bridgewater, Victoria, were clearly formed by dissolution in the unsaturated (vadose) so this model is inappropriate. However the Richards equation applies a  continuity requirement resulting in a general partial differential equation describing water movement in unsaturated non-swelling media. This appears to be the best option for modelling  water flow in the vadose zone

As most solution pipes form under vadose conditions and show potential as palaeoclimatic indicators, further understanding of their formation and behaviour is important.

Biography

A karst geomorphologist  who has worked on features and processes in high matrix porsity  Neogene and  Pleistocene carbonates for many years.

McNeil, Mardi¹; Nothdurft, Luke¹; Erler, Dirk²; Hua, Quan³; Webster, Jody M.⁴

¹Queensland University of Technology, Brisbane, Australia, ²Southern Cross University, Lismore, Australia, ³ANSTO, Lucas Heights, Australia, ⁴The University of Sydney, Australia

The northern Great Barrier Reef (GBR) Halimeda bioherms have accumulated on the outer continental shelf from calcium carbonate algal sediments over the past ~10,000 years and cover >6000 km² of shelf area. As such, Halimeda bioherms play a key role in the shallow marine carbon cycle over millennial timescales. The main source of nitrogen (N) to these bioherms is thought to be westward transport of upwelled NO₃^–-rich water from the Coral Sea. However, the primary N source has not been traced geochemically, and we have no understanding of any temporal variation. Here, we reconstruct patterns of N supply to Halimeda bioherms in the GBR since the mid-Holocene using the ¹⁵N/¹⁴N ratio of skeletal-bound organic N (δ¹⁵N-SOM) in modern and fossil Halimeda sediment cores.

Average Halimeda skeletal δ¹⁵N-SOM was 6.28 ± 0.26‰, consistent with δ¹⁵N-NO₃– from western tropical South Pacific (WTSP) thermocline waters. Thus geochemically validating shelf-break upwelling of an oceanic N source that appears to regulate the Halimeda bioherm spatial distribution. Halimeda δ¹⁵N-SOM decreased by 1-2 ‰ from 5000 to 2000 cal. yr BP, reaching a minima of 5.5‰ that persisted for almost 1000 years. The Halimeda δ¹⁵N-SOM variation reflects mid- to late Holocene changes in regional climate and intensified El Niño activity that likely facilitated elevated N₂ fixation in the WTSP, thereby lowering thermocline δ¹⁵N-NO₃^–. Thus, Halimeda skeletal material provides a valuable high-resolution geochemical archive of past oceanographic and climatic processes over centennial to millennial timescales, complementing existing paleoclimate proxy records.

Biography

Mardi’s interest is in cross-disciplinary research in Marine Geoscience, specifically biogenic carbonate sediments and structures at various scales from calcareous epiphytes on seagrass, to carbonate bioherms and reefs; their physical, chemical, and biological processes and their application in understanding past environments and current environmental change.

Yuexiao Shao^1,2, Zara Woolston¹, Juraj Farkaš^1,2, Briony Chamberlayne¹, John Tibby³ Deborah Haynes¹, Jonathan Tyler¹

¹Department of Earth Sciences, School of Physical Sciences, University of Adelaide, Australia; ²Metal Isotope Group (MIG), University of Adelaide, Australia; ³Department of Geography, Environment and Population, School of Social Sciences, University of Adelaide, Australia

The Coorong lagoon, as part of the wetland system at the terminus of the River Murray, is recognised not only for its ecological importance but also for its unique geomorphology and salinity gradient that ranges from fresh/brackish (< 35 PSU) in the North Lagoon to hypersaline (> 70 PSU) in the South Lagoon. The lagoon hydrology is controlled by seawater-continental water mixing processes that are traceable via the radiogenic Strontium (Sr) isotopes (⁸⁷Sr/⁸⁶Sr ratios). The hypersaline South Lagoon, being more geomorphologically restricted, is known for high degree of evaporation, which leads to ongoing calcium carbonate precipitation, which also acts as a sink for dissolved inorganic carbon (DIC) [1]. These processes involving carbonate formation and a local inorganic carbon cycling are traceable via the novel stable Sr isotope (δ^88/86Sr), which is particularly sensitive to mass-dependent isotope fractionation linked to carbonate precipitation / dissolution. Importantly, the South Lagoon has seen dramatic hydrological and ecological changes over the last ~200 years (since the European settlement), which is evident from geochemical and diatom records of the Coorong sediment cores [2], and has implications for water resource management and future strategies for the recovery of local ecosystem to more natural conditions[3]. In order to reconstruct paleo-hydrology of the Coorong before and after the European settlement, this study calibrated the δ^88/86Sr fractionation between recent aragonitic bivalve shells (Arthritica helmsi species) and local water in the modern Coorong lagoon, and a constant difference of δ^88/86Sr between the shells and the local water Δ^88/86Sr (δ^88/86Sr_solid – δ^88/86Sr_water) = -0.92‰ was discovered. Such calibrations, coupled with ⁸⁷Sr/⁸⁶Sr and δ^88/86Sr analyses of fossil A. helmsi shells from a sediment core in the South Lagoon, are complemented by radiocarbon dating and elemental concentration data, to better constrain (i) variability in the mixing of water sources in modern Coorong and over the last ~2500 years; and (ii) to reconstruct palaeo-salinity changes and associated carbonate precipitation/dissolution processes. Primary results based on ⁸⁷Sr/⁸⁶Sr of shells indicated the source of water in the South Lagoon were never purely marine; however, according to δ^88/86Sr of the shells, the the South Lagoon in the past ~2500 years was probably less evaporated than it has been in recent times (i.e., post European settlement).

[1] Shao et al (2018), Geochimica et Cosmochimica Acta 239, 90-108.

[2] McKirdy et al. (2010) Organic Geochemistry 41, 96-110.

[3] Brookes et al. (2018). Goyder Institute for Water Research Technical Report Series No. 18/04, Adelaide, South Australia. ISSN: 1839-2725

Biography

3rd year PhD student in University of Adelaide, mainly interested in alkaline metal isotopes in coastal systems as tracers of water source mixing, carbonate dynamics, and redox states, and further application of these tracers in reconstructing the paleo-climate of the local coastal system.

Salas-Saavedra, Dr Marcos¹, Webb, Professor Gregory¹, Sanborn, Dr Kelsey², Zhao, Professor Jian-xin¹, Webster, Professor Jody ², Nothdurft, Dr Luke³, Nguyen, Dr Ai¹

¹School of Earth and Environmental Sciences, The University of Queensland, Brisbane, Australia, ²Geocoastal Research Group, School of Geoscience, University of Sydney, Sydney, Australia, ³School of Earth and Atmospheric Sciences, Queensland University of Technology, Brisbane, Australia

Anthropocene climate change and water quality degradation represent unprecedented challenges to modern coral reef ecology. Projected trends for the Great Barrier Reef (GBR) suggest continuing declines in reef health. Although declining reef health after European colonization is well documented around the world and increased terrigenous sediment flux is known to have terminated deglacial reefs in the GBR, longer-term patterns of water quality are poorly understood. Without historical data, it is difficult to disentangle natural and anthropogenic reef behaviour to better model anthropogenic effects. Here we present the first proxy-based long-term Holocene water quality reconstruction for any reef. The geochronological framework provided by rotary coring on Heron and One Tree reefs (offshore, southern GBR) allowed reconstruction of offshore water quality from 8,200 to 1,800 years before present (BP) using centennial resolution microbialite-based geochemical proxies. Microbialites, which form part of growing reef framework, contain a robust proxy record of water quality through incorporation of trace elements (e.g., rare earth elements-REEs, Zr, Th, etc.) from ambient seawater. Trace elements associated with terrigenous flux were measured in dated microbialites as well as poorly consolidated Pleistocene limestone and palaeosol formed at the Pleistocene-Holocene unconformity at Heron and One Tree reefs. Paleosol samples have REE patterns similar to the Queensland based shale proxy Mud of Queensland (MuQ) consistent with local soil formed on exposed reefal limestone. Framboidal pyrite within the palaeosol suggests anoxic soil conditions during initial inundation. Younger microbialite-hosted REE and yttrium (REY) normalised to MuQ (subscript _SN) have seawater-like patterns (e.g., light REE – LREE – depletion and high Y/Ho ratios) but with a well-defined, non-linear trend of changing water quality through the ensuing Holocene.

Immediately following reef initiation (>8,300 yrs BP) data suggest increasing terrigenous influence to 8,000 yrs BP on the basis of coordinated, more mud-like microbialite proxies, including reduced LREE depletion (e.g., (Nd/Yb)_SN > 0.4) and lower Y/Ho ratios (< 53) with higher concentrations of lithofile elements. Proximal seawater became ‘cleaner’ from ~7,000 years ago, with opposing REY trends reflecting seawater with less terrigenous influence but showed marked mid-Holocene variability related to changing regional climatic factors. The strong fluctuation between intervals of high and low relative terrigenous sediment influence correlates well with particular regional and more global climate records, such as, the Indian-Australian Summer Monsoon (IASM) strength, high turbidity periods at 7.0, 5.4, and 2.7 ka BP with dampened El Niño Southern Oscillation (ENSO) frequency and fluctuations in local relative sea level. Water quality improved significantly after 3,200 yrs BP.

The new microbialite geochemical record provides a fully independent new water quality proxy and a means to interpret reef growth dynamics in relation to changing water quality associated with climate evolution at centennial to millennial scales. Such records provide valuable context for predictions of modern reef behavior in a changing world where coastal water quality is more likely to decline than to improve.

Biography

Dr. Marcos Salas Saavedra obtained his BSC in Biological Sciences at the Austral University of Chile in 2010 and he finished his PhD in Geochemistry and Geochronology at the University of Queensland in 2019. His research interest are in Biogeochemistry, Isotope Geochemistry, Carbonate Reefs Geology, U-series dating and Palaeoenvironment.

Warne, Mark¹, McDonald, Abbey¹

¹School of Life and Environmental Sciences, Deakin University, Melbourne, Australia

Cenozoic marine strata along the southeast Australian coastline contain a record of Southern Ocean evolution over geological time.  A significant aspect of this oceanic evolution, is the development and interplay of surface currents such as the East Australian Current, Leeuwin Current and Antarctic Circumpolar Current. These, and other ocean currents, have substantially shaped marine biological diversity along the southern Australian continental margin, and have left distinctive stratigraphic markers within the marine sedimentary rock record.

Traditionally, proxy records of past surface ocean currents have been derived from fossils of marine microplankton preserved in deep ocean and continental shelf sediments.  However, in life, zooplankton often exhibit variation in depth distribution, which complicates relationships with surface ocean currents.  In contrast, and paradoxically, shallow marine benthic microfossils associated with floating macroalgae, such as epiphytal ostracods and foraminifera, can provide more direct evidence of changes in global patterns of surface ocean circulation, because dispersal only occurs at the sea surface. The dispersal mechanism for these benthic micro-organisms is via attachment to seaweed, ripped up from shallow marine environments by coastal storms, and sent drifting vast distances across oceans on surface currents, until colonization occurs in new, distant shallow marine realms.

Key Neogene rafting-related ostracod migration and extinction events apparent in carbonate and siliciclastic marine strata along the southern Victorian coastline, and which defined broad phases in the oceanographic history of Bass Strait, are as follows:

Strong warm plumes of East Australian Current waters entering eastern Bass Strait (around 16.4, 5.8 and 3.2 million years ago), as evidenced by fossil occurrences of warm water, western Pacific Neohornibrookella species.  Increases in the influence of East Australian Current waters in the Bass Strait region around 3 Ma, also likely facilitated the expansion of Pacific Ambostracon spp (pumila group species) into SE Australian coastal shallow and marginal marine realms.

Sporadic incursions of Antarctic Circumpolar Current waters entering western Bass Strait between approximately 9 and 5 million years ago, as evidenced by the influx of mid to high latitude Tasmanocypris (dartnalli group) species.

The inception of warm Leeuwin Current (aka Zeehan Current) waters entering western Bass Strait (4.4 million years ago), which created a confluence with East Australian Current waters.  This is evidenced by an east-west biogeographic differentiation of shallow marine ostracod faunas across this seaway. Notable is the widespread disappearance of Neohornibrookella and Tasmanocypris (dartnalli group) species from warm shallow marine ostracod faunas west of Cape Otway, during the early Pliocene.

Widespread extinction of warm SW Pacific derived marine ostracod taxa across the entirety of Bass Strait (e.g. Neohornibrookella species) due to the inception of the cold winter Bass Cascade current (1.8 to 2.2 million years ago), which led to winter water temperature minima too cold for these taxa.  The inception of the Bass Cascade was associated with an early Quaternary northward shift in the position of the southern hemisphere, mid latitude westerly wind belt.

Biography

Mark Warne’s main work is university teaching in the disciplines of geology, physical geography, environmental science and palaeobiology. He is currently the course director for the Bachelor of Science at Deakin University. He has research interests in the fields of micropalaeontology (principally fossil Ostracoda), marine stratigraphy, palaeoecology and palaeo-oceanography.