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Radiocarbon dates on multiple individual charcoal fragments floating together down the Macdonald River, New South Wales, Australia, have calibrated ages spanning >1700 years. Partial explanations of this range of inherited ages can be attributed to the inbuilt age of living biomass, charcoalisation conditions, hillslope transport and storage and/or valley floor (fluvial) transport and storage, but the contribution of each of these components can be constrained only rarely. These results caution against using radiocarbon dating of charcoal as the sole dating technique to interpret Late-Holocene sedimentary histories. These findings also show that it is unlikely that deposit age has a dependable relationship to charcoal age. [ABSTRACT FROM AUTHOR]

The Murray Darling Basin contains 40% of Australia's farms and is subject to multi-year droughts that put severe pressure on southeast Australia's freshwater resources. Yet the long-term frequency, timing and potential severity of these droughts is unknown, as there are few high-resolution paleoclimate records from the basin that extend past the instrumental era. In this study, we investigate the potential of stalagmites from Careys Cave, Wee Jasper, in the Murray-Darling Basin to record past droughts. We use a multiproxy approach of stalagmite stable isotopes, trace element data, and climate reanalysis. We show that (a) stalagmite δ18O at this site likely records either local or regional precipitation amount and (b) stalagmite δ18O shows reasonable coherence with decadal-scale wet and dry changes in regional rainfall over the last 150 years, including the Federation Drought (1895–1902). Therefore, stalagmites from Wee Jasper can be used to draw regional inferences about past rainfall and have potential to extend the record of past droughts in the Murray Darling Basin beyond the limits of historical data. Extracting such a record will enable a better understanding of the causes of multi-year droughts in the region and consequently better planning, mitigation, and resilience in the basin. [ABSTRACT FROM AUTHOR]

Highlights • XRD, SEM and C isotope data were used to investigate carbonate mineral and hydrotalcite formation within mine tailings. • CO 2 availability controls carbonate with depth in the tailings at Woodsreef. • Pyroaurite and coalingite are sequestering atmospheric CO 2 at the Woodsreef chrysotile mine. • Isotope fractionation in hydrotalcites depends on carbon availability. • Detailed carbon accounting requires quantitative XRD and elemental C analysis. Abstract Carbon mineralisation of ultramafic mine tailings can reduce net emissions of anthropogenic carbon dioxide by reacting Mg-silicate and hydroxide minerals with atmospheric CO 2 to produce carbonate minerals. We investigate the controls on carbonate mineral formation at the derelict Woodsreef chrysotile mine (New South Wales, Australia). Quantitative XRD was used to understand how mineralogy changes with depth into the tailings pile, and shows that hydromagnesite [Mg 5 (CO 3) 4 (OH) 2 ·4H 2 O], is present in shallow tailings material (<40 cm), while coalingite [Mg 10 Fe3+ 2 (CO 3)(OH) 24 ·2H 2 O] and pyroaurite [Mg 6 Fe3+ 2 (CO 3)(OH) 16 ·4H 2 O] are forming deeper in the tailings material. This indicates that there may be two geochemical environments within the upper ∼1 m of the tailings, with hydromagnesite forming within the shallow tailings via carbonation of brucite in CO 2 -rich conditions, and pyroaurite and coalingite forming under more carbon limited conditions at depth. Radiogenic isotope results indicate hydromagnesite and pyroaurite have a modern (F14C > 0.8) atmospheric CO 2 source. Laboratory-based anion exchange experiments, conducted to explore stable C isotope fractionation in pyroaurite, shows that pyroaurite δ13C values change with carbon availability, and 13C-depleted signatures are typical of hydrotalcites in C-limited environments, such as the deep tailings at Woodsreef. Quantitative XRD and elemental C data estimates that Woodsreef absorbs between of 229.0–405.1 g CO 2 m−2 y−1. [ABSTRACT FROM AUTHOR]