Volcanic ash (tephra) provides an important chronostratigraphic framework for correlating Quaternary records. In ultra-distal (1500 km) settings such as ice cores and marine sediments, deposits are typically preserved as cryptotephra composed of glass shards m. This poses challenges for accurate geochemical characterization as conventional EPMA-WDS methods require beam diameters of m to limit alkali migration, restricting their application to extremely fine-grained tephra. Here, we develop and evaluate a quantitative m/1 nA EPMA-WDS method for cryptotephra analysis. Analyses of basaltic to rhyolitic glass standards demonstrate that accurate results are achieved when beam-sensitive elements are measured early, spectrometer configuration is optimized, and count times are systematically determined by element. Under these conditions, major-element concentrations deviate by 3% for oxides 1 wt.% and 10% for oxides 1 wt.%, with improved precision relative to existing small-beam approaches. We validate the m/1 nA method by analyzing cryptotephra from the 1831 CE Zavaritskii eruption (NGRIP1 ice core), producing results indistinguishable from conventional m EPMA-WDS analyses conducted across multiple laboratories. This m EPMA-WDS method extends major-element analysis to the smallest measurable cryptotephra shards, enabling robust correlation of ultra-distal archives with their proximal volcanic sources and strengthening the global tephrochronological framework.
Central Australia's ngarrawa, or 'mound springs', are unique hydrological systems that serve as some of the only near-permanent sources of water and continuous sediment deposition in Australia's arid interior. As such, they hold significant potential as palaeo-climate and -environmental archives in a region where many aspects of past climate remain uncertain. To date, few studies have investigated the potential of these sediments, including their stratigraphic integrity or if they can be radiocarbon-dated. In this study, Itrax X-ray fluorescence (XRF) scanning was performed on a series of sediment cores taken at Maka (Warburton Springs) to determine sediment integrity at mm scale for sediments that do not clearly show visual structures. Radiocarbon dating was also attempted using different materials, including plant macrofossils, charcoal, one bone remain and an insect carapace. The different materials returned a wide range of potential ages, with the plant macrofossils, which we tentatively consider to be the most robust indicator of sediment age, suggesting that the sediment records extend back to similar to 700 cal. yr BP at Maka. Highly variable XRF-inferred elemental profiles indicate that sediments at the ngarrawa are heterogeneous, not heavily mixed, with a trend towards increasingly organic sediments up-core. The elements Fe, K, Si and Ti are interpreted to reflect different detrital sources throughout the sediments with increasing Fe concentrations, suggesting increased dust mobilisation during recent times. These findings suggest that the XRF data from ngarrawa have potential as palaeoenvironmental archives to support future research into the climate and ecological history of these important desert oases, but still require more robust chronological constraints.
Stable isotope analyses of freshwater mollusc carbonate can record environmental conditions at the time of mineralisation, yet this technique remains underused in British Middle Pleistocene fluvial sequences, despite the abundance of fossil shell material in these deposits. We present new delta 18O and delta 13C measurements from calcitic Bithynia tentaculata opercula and aragonitic Valvata piscinalis shells from fluvial sediments at Swanscombe, in the Lower Thames Valley, correlated with the Hoxnian Interglacial (MIS 11c). We assess whether isotope values show sustained change through units correlated with the MIS 11c thermal maximum, and whether inter-taxon differences provide evidence for seasonal differences in carbonate mineralisation. Across the dataset (n = 173), delta 18O and delta 13C values fall within the range expected for a groundwater-influenced temperate lowland river system. Sample medians show little systematic stratigraphic change, indicating limited variation in environmental conditions during the sampled intervals of fluvial deposition. After accounting for mineralogy, the inter-taxon delta 18O offset is consistent with differences in growth season, whereas the delta 13C offset likely reflects species-specific differences in microhabitat and carbon incorporation. The Swanscombe isotope record therefore supports fully temperate MIS 11c conditions, with little evidence for sustained long-term thermal change during the sampled intervals. Instead, the paired-taxon record suggests that seasonal signals may be preserved even where mean environmental conditions remained relatively stable. These results demonstrate the value of paired-taxon stable isotope analysis in fluvial archives for reconstructing climate stability and identifying seasonal signals during Middle Pleistocene interglacials.
ABSTRACT Glacial/interglacial climate cycles since marine isotope stage (MIS) 12 influenced the discontinuous and 73 m‐long sediment record from the volcanic crater of Rodderberg in the East Eifel Volcanic Field, Germany. High‐resolution grain size and geochemical data have been valued by numerical dating techniques (radiocarbon and luminescence dating, tephrochronology), providing an environmental reconstruction since the Middle Pleistocene (439 ka). The observed hydroclimatic variability, along with silting‐up of the crater basin, significantly influenced depositional processes. This results in the formation of laminated, sand‐rich, and organic‐rich sediments during interglacial periods, as well as layered, silt‐rich minerogenic loess derivatives during glacial periods. According to chronology, the crater basin desiccated during MIS 8 and MIS 5, causing erosion of the interglacials related to MIS 9 and MIS 5e. The combined evidence over five glacial/interglacial cycles suggests a depositional evolution from a deep crater lake during MIS 12–10, desiccation during MIS 8 with erosion of MIS 9, a shallow lake for MIS 7, and a desiccating wetland during MIS 6, followed by deposition of loess (MIS 6‐2) and a Holocene soil (MIS 1).
ABSTRACT The Mesoamerican Preclassic period offers a laboratory for understanding how interactions between climate and early human occupation have shaped tropical ecosystems. Although the environmental history of the Maya lowlands during this period is relatively well known, mid‐elevation regions remain poorly understood. Here, we present a multiproxy sedimentary analysis from Lake Nahá, tropical mountains of southeastern Mexico, to elucidate climate–human interactions. Our results reveal three environmental stages that broadly align with the cultural–historical phases of the period. The first stage (~3200–2680 cal a BP) features low lake levels, high aquatic productivity, a progressively drier climate, and sparse human occupation. The second stage (~2680–2170 cal a BP) shows rising lake levels that expanded aquatic habitats, coinciding with a pause in the drying trend and greater anthropogenic impact. The third stage (~2170–1600 cal a BP) records renewed drying, lower lake levels, and moderate primary productivity, alongside a decline in fire activity and a shift toward more negative δ 13 C, coincident with the large‐scale Preclassic abandonment. These results demonstrate that Preclassic environmental trajectories in the tropical mountains were shaped by the combined influence of hydroclimate variability, coherent with that reported for the Maya lowlands, and spatially heterogeneous human occupation.
