Papers by Christopher Wurster
Scientific Reports
Fire is an essential component of tropical savannas, driving key ecological feedbacks and functio... more Fire is an essential component of tropical savannas, driving key ecological feedbacks and functions. Indigenous manipulation of fire has been practiced for tens of millennia in Australian savannas, and there is a renewed interest in understanding the effects of anthropogenic burning on savanna systems. However, separating the impacts of natural and human fire regimes on millennial timescales remains difficult. Here we show using palynological and isotope geochemical proxy records from a rare permanent water body in Northern Australia that vegetation, climate, and fire dynamics were intimately linked over the early to mid-Holocene. As the El Niño/Southern Oscillation (ENSO) intensified during the late Holocene, a decoupling occurred between fire intensity and frequency, landscape vegetation, and the source of vegetation burnt. We infer from this decoupling, that indigenous fire management began or intensified at around 3 cal kyr BP, possibly as a response to ENSO related climate vari...
Stable isotopes of faeces contain information related to the animals feeding ecology. The use of ... more Stable isotopes of faeces contain information related to the animals feeding ecology. The use of stable isotope values from subfossil faeces as a palaeoenvironmental indicator depends on how faithfully the animal records their local environment. Here we present insectivorous bat guano dC and dN values from a precipitation gradient across the southern United States and northern Mexico to compare with local vegetation and climate. We find dC values to be an excellent predictor of expected C4/CAM vegetation, indicating that the bats are non-selective in their diet. Moreover, we find bat guano d C values to be strongly correlated with summer precipitation amount and winter precipitation ratio. We also find evidence for a significant relationship with mean annual temperature. In general, we do not find dN values to be related to any parameters along the climatic gradient we examined. Additionally, we measured dC and dN values of bulk guano deposited annually from 1968 to 1987 in a varved...
Scientific Reports
Equatorial Southeast Asia is a key region for global climate change. Here, the Indo-Pacific Warm ... more Equatorial Southeast Asia is a key region for global climate change. Here, the Indo-Pacific Warm Pool (IPWP) is a critical driver of atmospheric convection that plays a dominant role in global atmospheric circulation. However, fluctuating sea-levels during the Pleistocene produced the most drastic land-sea area changes on Earth, with the now-drowned continent of Sundaland being exposed as a contiguous landmass for most of the past 2 million years. How vegetation responded to changes in rainfall that resulted from changing shelf exposure and glacial boundary conditions in Sundaland remains poorly understood. Here we use the stable carbon isotope composition (δ 13 C) of bat guano and High Molecular Weight n-alkanes, from Saleh Cave in southern Borneo to demonstrate that open vegetation existed during much the past 40,000 yrs BP. This location is at the southern equatorial end of a hypothesized 'savanna corridor' and the results provide the strongest evidence yet for its existence. The corridor would have operated as a barrier to east-west dispersal of rainforest species, and a conduit for north-south dispersal of savanna species at times of lowered sea level, explaining many modern biogeographic patterns. The Saleh Cave record also exhibits a strong correspondence with insolation and sea surface temperatures of the IPWP, suggesting a strong sensitivity of vegetation to tropical climate change on glacial/interglacial timeframes. The Indo-Pacific Warm Pool is the largest reservoir of warm water on Earth 1. Sea surface temperatures (SSTs) exceed 28 °C, and the IPWP plays a critical role in global climate 1,2. Seasonally, climate is governed by the migration of the Inter-Tropical Convergence Zone (ITCZ), with both the East Asian monsoon (EAM) and the Australian-Indonesian monsoon (AIM) resulting in deep atmospheric convection and high rainfall across the central and northern parts of the region 3,4. This climate leads to generally everwet conditions and lowland dipterocarp rainforest across most of Borneo today 5 with tropical grass relative abundance encompassing less than 0.1 for the vast majority of the island 6. However, abrupt and strong inter-annual variability in SSTs and salinities in the IPWP are associated with ENSO (El Niño-Southern Oscillation) variability that can bring periods of drought 5,7. During the Last Glacial Period (LGP, c. 110-11.7 kyr ago), reduced global sea level exposed the continental shelf from south of Thailand to Sumatra, Java, and Borneo, revealing the contiguous continent of Sundaland that reached its maximum land area extent during the Last Glacial Maximum (LGM, 26.5-19 kyr ago 8) (Fig. 1). Such a large change in land/sea area severely impacted the IPWP by reducing its size 9 while SSTs 2-4 °C lower than today in the IPWP region would have also served to reduce atmospheric convection 10,11. It has been proposed that a reduction of rainfall would lead to the development of a 'savanna corridor' running north to south/southeast from what is today Peninsula Malaysia across to southern Borneo through the Java Sea 12-14 (Fig. 1). Whether or not open vegetation existed on Sundaland, especially in Borneo, is the subject of intense debate, with significant implications for its biogeography, conservation, LGP carbon storage, and the understanding of early human dispersals through the region 14,15. Vegetation models in general simulate tropical rainforest across Sundaland during the LGM, particularly in central Sundaland 16-18. Species distribution models also suggested that rich dipterocarp forests were maintained during the LGM in central Sundaland 19 , although a few vegetation models have simulated a possible savanna corridor 18,20. Vegetation simulations are driven ultimately by climate models that simulate a range of 'dry' to 'wet'
We inferred climate change through the Pleistocene-Holocene transition from δ 13 C and δD values ... more We inferred climate change through the Pleistocene-Holocene transition from δ 13 C and δD values of bat guano deposited from 14.5 to 6.5 ka (calendar ka) in Bat Cave, Grand Canyon, Arizona. The δ 13 C and δD values generally covaried, indicating that regional late Pleistocene climate was relatively cool and wet, and early Holocene climate gradually became warmer with increased summer precipitation until ca. 9 ka, at which time the onset of modern North American Monsoon-like conditions occurred. During the Younger Dryas event, δ 13 C values decreased, whereas δD values increased, indicating a cool and possibly drier period. We also observed a distinct isotopic anomaly during the 8.2 ka event, at which time both δ 13 C and δD values decreased. The δ 13 C values abruptly increased at 8.0 ka, suggesting a rapid change in atmospheric circulation and greater infl uence from convective storms originating from the south. Deposits of bat guano represent a largely untapped source of paleoenvironmental information that can provide continuous and long-term continental archives of environmental change.
Holocene savanna dynamics in the seasonal tropics of northern Australia
Review of Palaeobotany and Palynology
Abstract An environmental history is presented from Girraween Lagoon, Darwin region of the Northe... more Abstract An environmental history is presented from Girraween Lagoon, Darwin region of the Northern Territory, Australia. Pollen and charcoal analysis of a 5-meter sediment core provides a record of vegetation change, fire history and climate spanning 12,700 cal BP to the present day. This study focusses on tree-grass vegetation dynamics, eucalypt to non-eucalypt plant interactions, and climate–fire–human relationships in an area where few long-term savanna records exist. The dataset suggests wetlands experienced alternating episodes of ephemeral waterlogging and seasonal inundation due to post-glacial monsoon variability up until permanent inundation from approximately 6000 cal BP. The surrounding catchment transformed from a terminal Pleistocene–early Holocene wooded-savanna to a later Holocene open forest. This increase in woody cover was a prominent site feature, primarily driven by climate–moisture availability. In turn, the extent of fire and fire impact, is a function of climate–vegetation feedbacks. Such interplay between fire history, climate change and vegetation pattern was also influenced by more intense human management of the area, in the last 4000 years of the record. It is proposed Girraween may have become a much-socialized and managed human landscape in this late Holocene phase. Results provide essential baseline data describing savanna dynamics linked to contemporary ecological observation, understanding and management goals, and serves as an important resource for the Quaternary sciences and archeology of northern Australia.
Understanding the main factors driving fire regimes in grasslands and savannas is critical to bet... more Understanding the main factors driving fire regimes in grasslands and savannas is critical to better manage their biodiversity and functions. Moreover, improving our knowledge on pyrogenic carbon (PyC) dynamics, including formation, transport and deposition, is fundamental to better understand a significant slow-cycling component of the global carbon cycle, particularly as these ecosystems account for a substantial proportion of the area globally burnt. However, a thorough assessment of past fire regimes in grass-dominated ecosystems is problematic due to challenges in interpreting the charcoal record of sediments. It is therefore critical to adopt appropriate sampling and analytical methods to allow the acquisition of reliable data and information on savanna fire dynamics. This study uses hydrogen pyrolysis (HyPy) to quantify PyC abundance and stable isotope composition (δ 13 C) in recent sediments across 38 micro-catchments covering a wide range of mixed C 3 /C 4 vegetation in nor...
Ecology and Evolution
This is an open access article under the terms of the Creative Commons Attribution License, which... more This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Scientific Reports, 2017
Pyrogenic carbon (PyC) and n-alkane data from sediments in the northern South China Sea reveal va... more Pyrogenic carbon (PyC) and n-alkane data from sediments in the northern South China Sea reveal variations in material from C 4 plants in East Asia over the last ~19 Ma. These data indicate the likely presence of C 4 taxa during the earliest part of the record analysed, with C 4 species also prominent during the mid and late Miocene and especially the mid Quaternary. Notably the two records diverge after the mid Quaternary, when PyC data indicate a reduced contribution of C 4 taxa to biomass burning, whereas plant-derived n-alkanes indicate a greater abundance of C 4 plants. This divergence likely reflects differences in the predominant source areas of organic materials accumulating at the coring site, with PyC representing a larger source area that includes material transported in the atmosphere from more temperate (relatively cooler and drier) parts of East Asia. Variations in the relative abundances of C 3 and C 4 taxa appear to be linked to a combination of environmental factors that have varied temporally and geographically and that are unique to East Asia. A major expansion of C 4 biomass in warmer subtropical parts of eastern Asia from ~1 Ma and particularly from ~0.4 Ma is later than other parts of the world. The Calvin-Benson cycle, the process through which plants convert inorganic carbon (C) and water to three C (C 3) sugar molecules, originated when atmospheric composition was very different from present 1, 2. One modification to the cycle, leading to reduced photorespiration effects and improved photosynthetic efficiency under certain conditions (e.g. moisture stress and relatively low pCO 2), involves production of four C (C 4) oxaloacetate as the first-formed product of photosynthesis. Uncertainty surrounds the exact date of origin of this modification. That said, there is no widely accepted evidence of C 4 taxa pre-dating the Oligocene 3 , with molecular studies indicating that the C 4 photosynthetic pathway first appeared between 35 and 30 Ma 4 , a period that includes a major decline in pCO 2 5
Advances in the reconstruction of temperature history, physiology and paleoenvironmental change : evidence from light stable isotope chemistry
University of Saskatchewan Library Electronic Theses & Dissertations. ...
Journal of Earth Science and Engineering, 2015
Fire is an important ecological factor and what constitutes appropriate fire management is much d... more Fire is an important ecological factor and what constitutes appropriate fire management is much debated in the US and elsewhere. The role of fire as a source of greenhouse gases has been intensively investigated, but less is known about the production rate of the solid black carbon residue from fires. Black carbon accumulates in soil as it has longer turnover times than plant residues. To understand the significance of black carbon production during wildfire, we quantified black carbon using hydropyrolysis in O and A horizons before and after a prescribed burn at four sites in the New Jersey Pine Barrens forest in the NorthEastern US. Black carbon was found in both O-and A-horizons at all investigated sites, stocks in the range of 61.31-168.15 g m-2 in the O-horizon and 169.59-425.25 g m-2 in the A-horizon. Total black carbon stocks did not increase following the fire suggesting that either black carbon production in fires may be small compared to the variability, or that equivalent amounts of black carbon formed in previous fires may have been consumed in the fire. The study raises questions about how black carbon production and consumption in fires can be quantified separately.
Analysis of δ18O and δD values of environmental waters at high temporal and spatial resolution by continuous diffusion sampling cavity ring-down spectrometry
ABSTRACT A novel sampling device utilises diffusion through porous PTFE tubing to deliver water v... more ABSTRACT A novel sampling device utilises diffusion through porous PTFE tubing to deliver water vapour continuously from a liquid water source for analysis of δ18O and δD values by Cavity Ring-Down Spectrometry (CRDS). Comparison of isotopic data for a range of water samples analysed by Diffusion Sampling-CRDS (DS-CRDS) and Isotope Ratio Mass Spectrometry (IRMS) shows significant linear correlations between the two methods allowing for accurate standardisation of DS-CRDS data. The internal precision for an integration period of 3 min (standard deviation = 0.1 ‰ and 0.3 ‰ for δ18O and δD values, respectively) is similar to analysis of water by injection/evaporation CRDS of discrete water samples. The isotopic effects of variable air and water temperature, water vapour concentration and water pumping rate were found to be either negligible or correctable by analysis of water standards. Separation of the analysed water vapour from non-volatile dissolved and particulate contaminants in the liquid sample minimises interferences associated with CRDS analyses of many aqueous samples. Coupling of the DS-CRDS instrument to an auto sampler enables rapid analysis (10 min) of discrete water samples. The DS-CRDS system was used in the first continuous shipboard measurement of δ18O and δD of water. Combined with continuous salinity recordings, a data set of nearly 6,000 isotope measurements was made at 30-s intervals during a 3-day voyage through the Great Barrier Reef Lagoon. Precise identification of river plumes within the Great Barrier Reef Lagoon was possible because unique δ18O/δD-salinity relationships of individual plumes were measured at high spatial and temporal resolution. Continuous shipboard measurement of δ18O/δD values by DS-CRDS provides additional discriminatory power for assessing water mass formation processes and histories at a small fraction of the cost of traditional isotope analysis of discrete samples. In a second application of DS-CRDS, continuous real-time analysis, at 30-s intervals, of precipitation at an Australian tropical location revealed extreme and rapidly changing δ18O and δD values related to variations in moisture source areas, transport paths and precipitation histories. The range of δ18O (-19.6 ‰ to +2.6 ‰) and δD (-140 ‰ to +13 ‰) values from almost 6,000 measurements of nine rain events over 15 days during an 8-month period at a single location was comparable with the range measured in 1532 monthly samples from all seven Australian Global Network of Isotopes in Precipitation stations from 1962 to 2002. Extreme variations in δ18O (-8.7 ‰ to -19.6 ‰) and δD (-54 ‰ to -140 ‰) were recorded within a single 4-h period. Real-time stable isotope monitoring of environmental waters at high temporal and spatial resolution enables new and powerful tracer applications in climatology, hydrology, eco-physiology and palaeo-climatology.
Proceedings of the National Academy of Sciences of the United States of America, 2021
Stable carbon and nitrogen isotope analyses are widely used to infer diet and mobility in ancient... more Stable carbon and nitrogen isotope analyses are widely used to infer diet and mobility in ancient and modern human populations, potentially providing a means to situate humans in global food webs. We collated 13,666 globally distributed analyses of ancient and modern human collagen and keratin samples. We converted all data to a common “Modern Diet Equivalent” reference frame to enable direct comparison among modern human diets, human diets prior to the advent of industrial agriculture, and the natural environment. This approach reveals a broad diet prior to industrialized agriculture and continued in modern subsistence populations, consistent with the human ability to consume opportunistically as extreme omnivores within complex natural food webs and across multiple trophic levels in every terrestrial and many marine ecosystems on the planet. In stark contrast, isotope dietary breadth across modern nonsubsistence populations has compressed by two-thirds as a result of the rise of industrialized agriculture and animal husbandry practices and the globalization of food distribution networks.
High-Resolution Scleroisotope Analysis: Preliminary Stable Isotope Values from the South Wellesley Islands, Gulf of Carpentaria
ABSTRACT Detailed understandings of local environmental conditions and their impact on growth and... more ABSTRACT Detailed understandings of local environmental conditions and their impact on growth and isotopic fractionation in constituent molluscan taxa are required to accurately contextualise archaeological results. Molluscs have long been recognised as sensitive recorders of local environmental conditions. Aspects of the environment, including temperature and water constitution, imprint a chemical signal within the shell matrix. Archaeologists use stable oxygen and carbon isotope profiles derived from archaeological shellfish assemblages to address questions related to palaeoenvironments, including extending instrumental records, characterising subannual environmental oscillations, and determining the timing and periodicity of resource collection and siteuse. This paper reports preliminary findings of research characterising subannual environmental cycles and their effects on extant shellfish taxa from Bentinck Island, southern Gulf of Carpentaria using subannual stable isotope profiles. Distinct shifts in local environmental conditions owing to the Australian Monsoonal Cycle are reflected within observed environmental data and molluscan isotope profiles, suggesting similar patterns will be present within archaeological assemblages from this region. These results form the basis for modern analogues through which archaeological data can be accurately contextualised.
Complexities in the palaeoenvironmental and archaeological interpretation of isotopic analyses of the Mud Shell Geloina erosa (Lightfoot, 1786)
Isotope signals derived from molluscan shell carbonates allow researchers to investigate palaeoen... more Isotope signals derived from molluscan shell carbonates allow researchers to investigate palaeoenvironments and the timing and periodicity of depositional events. However, it cannot be assumed that all molluscan taxa provide equally useful data owing to species-specific biological and ecological traits. The Mud Shell, Geloina erosa (Lightfoot, 1786) (syn. Polymesoda coaxans, syn. Polymesoda erosa), an infaunal mangrove bivalve, is a common component of archaeological deposits along Australia's tropical north coast and throughout the Indo-West Pacific. The ubiquity of G. erosa has led to numerous researchers incorporating this taxon into interpretations of associated deposits, particularly in the generation of radiocarbon chronologies and as a palaeoenvironmental proxy. Despite this, concerns have been expressed regarding the impact of G. erosa physiology and ecology on associated geochemical signals. Adaptations allowing the survival of this species within its highly changeable mangrove environment may introduce complexities into radiocarbon and environmental data archived within its shell. This study combines local environmental and hydrological data with isotopic analysis (δ18O, δ13C, and 14C) of live-collected specimens to explore the interpretability of geochemical proxies derived from G. erosa. Results suggest a number of factors may impact geochemical markers in unpredictable ways, eroding the usefulness of associated interpretations.
Continuous analysis of $delta$18O and $delta$D values of water by diffusion sampling cavity ring-down spectrometry: a novel sampling device for unattended field monitoring of precipitation, ground and surface waters
Rapid Communications in Mass Spectrometry, 2011
Biogeosciences, 2015
Widespread burning of mixed tree-grass ecosystems represents the major natural locus of pyrogenic... more Widespread burning of mixed tree-grass ecosystems represents the major natural locus of pyrogenic carbon (PyC) production. PyC is a significant, pervasive and yet poorly understood "slow-cycling" form of carbon present in the atmosphere, hydrosphere, soils and sediments. We conducted 16 experimental burns on a rainfall transect through northern Australian savannas with C 4 grasses ranging from 35 to 99 % of total biomass. Residues from each fire were partitioned into PyC and further into recalcitrant (HyPyC) components, with each of these fluxes also partitioned into proximal components (> 125 µm), likely to remain close to the site of burning, and distal components (< 125 µm), likely to be transported from the site of burning. The median (range) PyC production across all burns was 16.0 (11.5) % of total carbon exposed (TCE), with HyPyC accounting for 2.5 (4.9) % of TCE. Both PyC and HyPyC were dominantly partitioned into the proximal flux. Production of HyPyC was strongly related to fire residence time, with shorter duration fires resulting in higher HyPyC yields. The carbon isotope (δ 13 C) compositions of PyC and HyPyC were generally lower by 1-3 ‰ relative to the original biomass, with marked depletion up to 7 ‰ for grasslands dominated by C 4 biomass. δ 13 C values of CO 2 produced by combustion were computed by mass balance and ranged from ∼ 0.4 to 1.3 ‰. The depletion of 13 C in PyC and HyPyC relative to the original biomass has significant implications for the interpretation of δ 13 C values of savanna soil organic carbon and of ancient PyC preserved in the geologic record, as well as for global 13 C isotopic disequilibria calculations.
Marine Ecology Progress Series
Bat guano: record of climate change
Explaining the imbalance in $delta$13C between soil and biomass in fire-prone tropical savannas
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Papers by Christopher Wurster