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Wetland sediments are valuable archives of environmental change but can be challenging to date. Terrestrial macrofossils are often sparse, resulting in radiocarbon (14C) dating of less desirable organic fractions. An alternative approach for capturing changes in atmospheric 14C is the use of terrestrial microfossils. We 14C date pollen microfossils from two Australian wetland sediment sequences and compare these to ages from other sediment fractions (n = 56). For the Holocene Lake Werri Berri record, pollen 14C ages are consistent with 14C ages on bulk sediment and humic acids (n = 14), whilst Stable Polycyclic Aromatic Carbon (SPAC) 14C ages (n = 4) are significantly younger. For Welsby Lagoon, pollen concentrate 14C ages (n = 21) provide a stratigraphically coherent sequence back to 50 ka BP. 14C ages from humic acid and >100 µm fractions (n = 13) are inconsistent, and often substantially younger than pollen ages. Our comparison of Bayesian age-depth models, developed in Oxcal, Bacon and Undatable, highlight the strengths and weaknesses of the different programs for straightforward and more complex chrono-stratigraphic records. All models display broad similarities but differences in modeled age-uncertainty, particularly when age constraints are sparse. Intensive dating of wetland sequences improves the identification of outliers and generation of robust age models, regardless of program used.

This study describes a multivariate statistical model (derived using partial least squares regression, PLS-R) that derives charring intensity (reaction temperature and duration) from the attenuated total reflectance (ATR) Fourier Transform Infrared (FTIR) spectra of charcoal. Data for the model was obtained from a library of charcoal samples produced under laboratory conditions at charring intensities (CI) relevant to wildfires and a series of feedstocks representing common tree species collected from Australia. The PLS-R model developed reveals the potential of FTIR to determine the charring intensity of charcoal. Though limited by the differences between laboratory-produced charcoal and the more heterogeneous and less-structured charcoal produced in a wildfire, the method was tested against fossil charcoal from a well-dated sediment core collected from Thirlmere Lakes National Park, Australia and showed a distinct change in CI that can be related to other climatic and environmental proxies. We suggest that the method has the potential to offer insights into the conditions under which natural charcoal is formed including the modelling of charring intensities of fossil charcoal samples isolated from sediments, archaeological applications or characterisation of contemporary fire events from charcoal in soils.

The Chronos 14Carbon-Cycle Facility is a new radiocarbon laboratory at the University of New South Wales, Australia. Built around an Ionplus 200 kV MIni-CArbon DAting System (MICADAS) Accelerator Mass Spectrometer (AMS) installed in October 2019, the facility was established to address major challenges in the Earth, Environmental and Archaeological sciences. Here we report an overview of the Chronos facility, the pretreatment methods currently employed (bones, carbonates, peat, pollen, charcoal, and wood) and results of radiocarbon and stable isotope measurements undertaken on a wide range of sample types. Measurements on international standards, known-age and blank samples demonstrate the facility is capable of measuring 14C samples from the Anthropocene back to nearly 50,000 years ago. Future work will focus on improving our understanding of the Earth system and managing resources in a future warmer world.

Carbon plays a crucial role for all biological systems on Earth and for regulating the atmospheric heat budget. For instance, in the subsurface organic carbon is the ultimate electron donor for microbially mediated redox-process. Despite the size of the global groundwater store the processes controlling the natural dissolved organic carbon (DOC) in groundwater are poorly understood. This ARC Discovery project aims to answer the question: under what conditions groundwater DOC is a source or sink for carbon? and quantify processes that control concentration and character of DOC. The question will be answered via sampling of various subsurface environments and laboratory experiments of biodegradation and sorption. Samples from field, sorption and biodegradation experiments were analysed by Liquid Chromatography-Organic Carbon Detection (LC-OCD), a chromatography technique separating DOC into five fractions based on mass, plus a hydrophobic fraction. The results show that groundwater DOC concentrations are significantly lower than terrestrial (soil, sediment and river) concentrations indicating that DOC fluxes towards the subsurface are attenuated by sorption to minerals and/or biologically processed. The experiments show that the humics fraction is predominately adsorbing onto mineral surfaces and low-molecular weight neutrals (LMW-N) are predominantly being biodegraded. For field samples the humics fraction is more abundant in the total DOC than LMW-N. We therefore infer that sorption is more dominant than biological degradation. Changing environmental conditions could lead to a release of adsorbed carbon. The results will provide better understanding of the role of DOC for subsurface processes and inform guidelines for the management of groundwater resources.

There has been extensive research on terrestrial carbon and the different conditions where it is a source or sink. In constrast, research on natural groundwater organic matter (OM) is limited. Biological processing is one of the factors governing the characteristics of groundwater OM. Here, we compare the microbial degradation rate of groundwater biodegradable dissolved organic carbon (BDOC) from a variety of groundwater environments (in NSW, Australia) such as coastal sands, alluvial sediments and fractured-rock aquifers. Each site also has potential recharge from an adjacent surface water body (streams and wetlands). 0.22 µm filtered groundwater was mixed with 10 mL of unfiltered sample (as innoculum) and incubated in the dark in the presence of O2 at 20°C. Subsamples were collected after filtration (0.22 µm) on Day 0, 3, 7, 14, 21 and 28 and analysed using size-exclusion chromatography-organic carbon detection (LC-OCD), which allows the quantification of DOC concentration and five molecular weight fractions including biopolymers (BP >>20,000 g/mol), humic substances (HS ~1000 g/mol), building blocks (BB 300-500 g/mol), low molecular weight neutrals (LMWN <350 g/mol) and low molecular weight acids (LMWA 350 g/mol) as well as providing measures of humic substance aromaticity and relative molecular weight. The overall results showed that HS fraction was the dominant fraction of DOC pool ranking from (13-65%), in which greater fraction was observed in surface water and lower in alluvial and facture rock aquifer. After 28 days of incubation, the biological processing of DOC ranged from 1-59%. LMWN was the only size fraction that was significantly biodegraded over the incubation period (10-86%) in the samples from alluvial and fractured-rock aquifer while same trend was not observed in sandy aquifer. Slightly lower LMWN reduction in surface water was observed compared to that in alluvial and fractured-rock aquifer.

Title: The role of dissolved organic matter and groundwater biogeophysical processes in the carbon budget.Phetdala Oudone , , Christopher E. Marjo , Karina Meredith , Liza McDonough1,2, Helen Rutlidge2, , Martin S Andersen2,5, Denis O`Carroll2,5, Andy Baker1,2. Atmospheric CO2 concentration is acknowledged to play an important role in climate change. However, quantifying more accurate predictions requires a sound understanding of the cycle and process of carbon especially in the environment. There has been extensive research on terrestrial carbon and the different conditions where it is a source or sink. However, the knowledge on whether groundwater organic matter is a carbon source or sink is limited. This work will explore the dynamic of groundwater organic matter including both its concentration and its rate and extent of biological processing and sorption. The UNSW Wellington Research Station was selected for groundwater sampling as it represents a fractured rock aquifer and alluvial aquifer for groundwater property and interaction investigation. Samples were collected from a transect of boreholes perpendicular from the river. Literature procedures were adopted for determining biological dissolved organic carbon and investigation of organic matter sorption on pure minerals (quartz sand, iron-coated quartz sand and calcium carbonate). For each sample collected total dissolved organic concentration was measured and the organic matter present was characterized by fluorescence spectroscopy and size-exclusion chromatographic technique, LC-OCD. There was greater sorption of organic matter with calcium carbonate, followed by iron-coated sand, while minimum sorption was observed with sand. This research in conjunction with similar studies in different environments will allow conclusions to be drawn groundwater organic matter and whether it is a carbon source or sink and thereby the findings can eventually have some policy application which will enable the management of

Stalagmites play an increasingly important role in the paleoclimatic reconstruction from seasonal to orbital timescales. One of the important reasons is that 230Th-dating can provide an absolute age enabling more accurate knowledge of the stalagmite growth. Additionally, annual trace element and optical layers can provide complementary method for determining a precise age and seasonal resolution. The trace elements of a stalagmite (XMG) in Beijing Shihua Cave, which is located in the East Asian monsoon region, were analyzed by laser ablation ICP-MS and compared with stalagmite laminae. The results show that: (1) the polished section of the topmost 4 mm of stalagmite XMG has obvious bi-optical layers (fluorescence and visible light) under a conventional transmission microscope. In the rest of the sample laminae are not observed using this method. (2) The variations of P/Ca, Sr/Ca, Ba/Ca, U/Ca and Mg/Ca show seasonal cycles throughout the sample. Sr/Ca is inversely correlated to P/Ca, and its peaks correspond with the (non-fluorescing) white layers, which deposit in late winter and spring when the climate is dry. The peaks of P/Ca match closely with the (fluorescing) opaque layers, because P is a soil-derived element which increases in the high rainfall monsoon period. (3) The PCA of the five trace elements showed that the cycles of PC1 could represent the annual cycle. This stalagmite was deposited over 148 ± 4 years through peak counting and the cycles of PC1 correspond well with the annual layers. Trace element cyclicity as shown by PC1 can increase the accuracy of stalagmite dating, especially in the absence of obvious laminae. The trace elements can be used as the marker of seasonal changes in a strongly contrasting wet-dry monsoon climate regime.

© Raina et al.Phytoplankton-bacteria interactions drive the surface ocean sulfur cycle and local climatic processes through the production and exchange of a key compound: dimethylsulfoniopropionate (DMSP). Despite their large-scale implications, these interactions remain unquantified at the cellular-scale. Here we use secondary-ion mass spectrometry to provide the first visualization of DMSP at sub-cellular levels, tracking the fate of a stable sulfur isotope (34S) from its incorporation by microalgae as inorganic sulfate to its biosynthesis and exudation as DMSP, and finally its uptake and degradation by bacteria. Our results identify for the first time the storage locations of DMSP in microalgae, with high enrichments present in vacuoles, cytoplasm and chloroplasts. In addition, we quantify DMSP incorporation at the single-cell level, with DMSPdegrading bacteria containing seven times more 34S than the control strain. This study provides an unprecedented methodology to label, retain, and image small diffusible molecules, which can be transposable to other symbiotic systems.

© 2016 Elsevier Ltd. Measurement of glycerol dialkyl glycerol tetraethers (GDGTs) preserved in speleothems offers a potential proxy for past temperature but, in general, their origin is unknown. To understand the source of speleothem GDGTs, we undertook an irrigation experiment to activate drip sites within a hydrogeochemically well characterised cave. The cave drip water was analysed for GDGTs, inorganic elements (major ions and trace elements), stable isotopes and dissolved organic matter concentration and character. Published speleothem GDGT records from the site have been observed to be dominated by isoprenoid GDGTs and interpreted as deriving from in situ microbial communities within the cave or vadose zone. The drip water in our irrigation experiment had a GDGT distribution distinct from that of soil and speleothem samples, providing direct evidence that the distinctive GDGT signature in speleothems is derived from a subsurface source. Analysis of GDGTs in this context allowed further elucidation of their source and transport in cave systems, enhancing our understanding of how they might be used as a temperature proxy.

Cave monitoring provides a crucial link between hydrological and climate processes and how they may in turn effect speleothem δ18O calcite composition. This is particularly important in arid environments where potential evapotranspiration (PET) exceeds precipitation (P) and may dominate the δ18O signature. In this study we investigate climate-karst-cave interactions driving the δ18O variability in modern drip water samples from Cathedral Cave in semi-arid SE Australia and use this information to interpret a modern sub-annually resolved speleothem record. Over a two-year monitoring campaign we established that mean composition of δ18O in modern drip waters was enriched by up to 2.9‰, relative to the weighted mean annual rainfall. Furthermore, fluctuations in δ18O over time were controlled by two processes (1) evaporation in the unsaturated zone, resulting in heavier values, and (2) recharge during infiltration events, resulting in isotopically lighter values. Large soil moisture deficits in the soil zone must therefore be surpassed to initiate infiltration, resulting in relatively infrequent (0-3 per year) infiltration events. A series of infiltration experiments were conducted to elucidate water flow delivery mechanisms and water residence times. We found that the hydrological pathways and resulting mixing processes that occurred in the unsaturated zone determined the isotopic variability in the drip water δ18O between individual drip sites, and that potential storage in the unsaturated zone is generally greater than 6 months. We establish that the δ18O record in speleothems from this cave do not exclusively record rainfall composition or amount but are overlaid with an evaporative signal that is strongly affected by the time since the last infiltration event, local hydrogeology, cave climate and disequilibrium during calcite deposition. This hypothesis was tested by reconstructing the δ18O proxy record in a speleothem growing for ~50 years in the instrumental era.