Investigating the palaeoenvironmental context of Late Pleistocene human dispersals into Southeast Asia: a review of stable isotope applications

We review palaeoenvironmental applications of stable isotope analysis to Late Pleistocene archaeological sites across Southeast Asia (SEA), a region critical to understanding the evolution of Homo sapiens and other co-existing Late Pleistocene (124–11.7 ka) hominins. Stable isotope techniques applied to archaeological deposits offer the potential to develop robust palaeoenvironmental reconstructions, to contextualise the occupational and non-occupational history of a site. By evaluating the published research in this field, we show that sediments, guano, tooth enamel, speleothem and biomolecular material such as leaf waxes have great potential to provide site-specific palaeoenvironmental records and local and catchment-scale landscape context to hominin dispersal in the region. However, stable isotope techniques used in these contexts are in their infancy in SEA, and the diagenetic controls associated with hot and humid environments that typify the region are not yet fully understood. Additionally, availability of sources of stable isotopes varies between sites. Nonetheless, even the limited research currently available shows that stable isotope analyses can aid in developing a better understanding of the role of the environment on the nature and timing of dispersals of our species eastwards into SEA and beyond.Meghan S. McAllister, Mike W. Morley, Jonathan J. Tyler, Francesca A. McInerney, Alison J. Blyt

Plant families exhibit unique geographic trends in C4 richness and cover in Australia

Numerous studies have analysed the relationship between C4 plant cover and climate. However, few have examined how different C4 taxa vary in their response to climate, or how environmental factors alter C4:C3 abundance. Here we investigate (a) how proportional C4 plant cover and richness (relative to C3) responds to changes in climate and local environmental factors, and (b) if this response is consistent among families. Proportional cover and richness of C4 species were determined at 541 one-hectare plots across Australia for 14 families. C4 cover and richness of the most common and abundant families were regressed against climate and local parameters. C4 richness and cover in the monocot families Poaceae and Cyperaceae increased with latitude and were strongly positively correlated with January temperatures, however C4 Cyperaceae occupied a more restricted temperature range. Seasonal rainfall, soil pH, soil texture, and tree cover modified proportional C4 cover in both families. Eudicot families displayed considerable variation in C4 distribution patterns. Proportional C4 Euphorbiaceae richness and cover were negatively correlated with increased moisture availability (i.e. high rainfall and low aridity), indicating they were more common in dry environments. Proportional C4 Chenopodiaceae richness and cover were weakly correlated with climate and local environmental factors, including soil texture. However, the explanatory power of C4 Chenopodiaceae models were poor, suggesting none of the factors considered in this study strongly influenced Chenopodiaceae distribution. Proportional C4 richness and cover in Aizoaceae, Amaranthaceae, and Portulacaceae increased with latitude, suggesting C4 cover and richness in these families increased with temperature and summer rainfall, but sample size was insufficient for regression analysis. Results demonstrate the unique relationships between different C4 taxa and climate, and the significant modifying effects of environmental factors on C4 distribution. Our work also revealed C4 families will not exhibit similar responses to local perturbations or climate.Samantha E. M. MunroeID, Francesca A. McInerney, Greg R. Guerin, Jake W. Andrae, Nina WeltiID, Stefan Caddy-Retalic, Rachel Atkins, Ben Sparro

The IFITM5 mutation c.-14C > T results in an elongated transcript expressed in human bone; and causes varying phenotypic severity of osteogenesis imperfecta type V

Background The genetic mutation resulting in osteogenesis imperfecta (OI) type V was recently characterised as a single point mutation (c.-14C > T) in the 5’ untranslated region (UTR) of IFITM5, a gene encoding a transmembrane protein with expression restricted to skeletal tissue. This mutation creates an alternative start codon and has been shown in a eukaryotic cell line to result in a longer variant of IFITM5, but its expression has not previously been demonstrated in bone from a patient with OI type V. Methods Sanger sequencing of the IFITM5 5’ UTR was performed in our cohort of subjects with a clinical diagnosis of OI type V. Clinical data was collated from referring clinicians. RNA was extracted from a bone sample from one patient and Sanger sequenced to determine expression of wild-type and mutant IFITM5. Results All nine subjects with OI type V were heterozygous for the c.-14C > T IFITM5 mutation. Clinically, there was heterogeneity in phenotype, particularly in the manifestation of bone fragility amongst subjects. Both wild-type and mutant IFITM5 mRNA transcripts were present in bone. Conclusions The c.-14C > T IFITM5 mutation does not result in an RNA-null allele but is expressed in bone. Individuals with identical mutations in IFITM5 have highly variable phenotypic expression, even within the same family

Carbon isotope systematics of leaf wax n-alkanes in a temperate lacustrine depositional environment

The carbon isotope ratio (δ13C) of plant-derived organic carbon preserved in geological archives can be a valuable proxy for the relative abundance of terrestrial plants using C3 and C4 photosynthesis. In certain sedimentary archives, however, mixing of terrestrial C3- and aquatic macrophyte-sourced carbon will result in sedimentary organic matter (OM) δ13C signatures that could be misinterpreted as shifts in the abundance of C3 and C4 vegetation. There is potential for this problem to be mitigated using leaf wax n-alkane compound-specific δ13C measurements because n-alkane production differs between terrestrial vegetation and aquatic macrophytes. This approach requires an increased understanding of how mixing of terrestrial plant and aquatic macrophyte n-alkane inputs to lacustrine sedimentary archives manifests in the δ13C values of different n-alkane homologues in diverse environmental settings. This study examines a Pleistocene lacustrine sequence in southeastern Australia in which the inputs from terrestrial and aquatic macrophytes vary naturally through time, enabling the characterization of the mixing dynamics for different n-alkane homologues. Relative contributions of terrestrial vegetation and aquatic macrophytes were estimated using the relative abundance of mid-chain to long-chain n-alkanes and compared to the δ13C values of discrete n-alkane homologues. We find that δ13C values of mid- and some long-chain n-alkanes (C23–C29) are strongly impacted by mixing between C3 terrestrial- and non-emergent aquatic macrophyte-derived n-alkanes. In contrast, δ13C values of very long chain (C31–C35) n-alkanes integrated in sediments are the least affected by isotopic mixing. These results indicate that aquatic macrophyte inputs can significantly influence C29 isotopic signatures and thus the δ13C values of the very long chain n-alkanes ( C31) will provide the most robust quantification of n-alkane inputs from terrestrial plants and will be most useful for reconstructing the abundance C3 and C4 vegetation from temperate lake sediments.Jake W. Andrae, Francesca A. McInerney, J.M. Kale Sniderma

Isotopic and morphologic proxies for reconstructing light environment and leaf function of fossil leaves: a modern calibration in the Daintree Rainforest, Australia

Premise: Within closed‐canopy forests, vertical gradients of light and atmospheric CO2 drive variations in leaf carbon isotope ratios, leaf mass per area (LMA), and the micromorphology of leaf epidermal cells. Variations in traits observed in preserved or fossilized leaves could enable inferences of past forest canopy closure and leaf function and thereby habitat of individual taxa. However, as yet no calibration study has examined how isotopic, micro‐ and macromorphological traits, in combination, reflect position within a modern closed‐canopy forest or how these could be applied to the fossil record. Methods: Leaves were sampled from throughout the vertical profile of the tropical forest canopy using the 48.5 m crane at the Daintree Rainforest Observatory, Queensland, Australia. Carbon isotope ratios, LMA, petiole metric (i.e., petiole‐width2/leaf area, a proposed proxy for LMA that can be measured from fossil leaves), and leaf micromorphology (i.e., undulation index and cell area) were compared within species across a range of canopy positions, as quantified by leaf area index (LAI). Results: Individually, cell area, δ13C, and petiole metric all correlated with both LAI and LMA, but the use of a combined model provided significantly greater predictive power. Conclusions: Using the observed relationships with leaf carbon isotope ratio and morphology to estimate the range of LAI in fossil floras can provide a measure of canopy closure in ancient forests. Similarly, estimates of LAI and LMA for individual taxa can provide comparative measures of light environment and growth strategy of fossil taxa from within a flora.Alexander W. Cheesman, Heather Duff, Kathryn Hill, Lucas A. Cernusak, Francesca A. McInerne

A vegetation carbon isoscape for Australia built by combining continental-scale field surveys with remote sensing

Published online: 5 July 2022Context: Maps of C3 and C4 plant abundance and stable carbon isotope values (δ13C) across terrestrial landscapes are valuable tools in ecology to investigate species distribution and carbon exchange. Australia has a predominance of C4- plants, thus monitoring change in C3: C4 cover and δ13C is essential to national management priorities. Objectives: We applied a novel combination of field surveys and remote sensing data to create maps of C3 and C4 abundance in Australia, and a vegetation δ13C isoscape for the continent. Methods: We used vegetation and land-use rasters to categorize grid-cells (1 ha) into woody ( C3), native herbaceous, and herbaceous cropland ( C3 and C4) cover. Field surveys and environmental factors were regressed to predict native C4 herbaceous cover. These layers were combined and a δ13C mixing model was used to calculate site-averaged δ13C values. Results: Seasonal rainfall, maximum summer temperature, and soil pH were the best predictors of C4 herbaceous cover. Comparisons between predicted and observed values at field sites indicated our approach reliably predicted generalised C3: C4 abundance. Southern Australia, which has cooler temperatures and winter rainfall, was dominated by C3 vegetation and low δ13C values. C4- dominated areas included northern savannahs and grasslands. Conclusions: Our isoscape approach is distinct because it incorporates remote sensing products that calculate cover beneath the canopy, the influence of local factors, and extensive validation, all of which are critical to accurate predictions. Our models can be used to predict C3: C4 abundance under climate change, which is expected to substantially alter current C3: C4 abundance patterns.Samantha E. M. Munroe, Greg R. Guerin, Francesca A. McInerney, Irene Martín, Forés, Nina Welti, Mark Farrell, Rachel Atkins, Ben Sparro