Savanna in equatorial Borneo during the late Pleistocene

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 (δ13C) 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

A global carbon and nitrogen isotope perspective on modern and ancient human diet

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

Amazon Basin forest pyrogenic carbon stocks: First estimate of deep storage

Amazon Basin forest soils contain considerable soil organic carbon stocks; however, the contribution of soil pyrogenic carbon (PyC) to the total is unknown. PyC is derived from local fires (historical and modern) and external inputs via aeolian deposition. To establish an initial estimate of PyC stocks in non-terra preta forest with no known history of fire, to assess site and vertical variability, as well as to determine optimal sampling design, we sampled 37 one hectare forest plots in the Amazon Basin and analysed PyC via hydrogen pyrolysis of three individual samples per plot and of bulked samples to 200 cm depth. Using our data and published total organic carbon stocks, we present the first field-based estimate of total PyC stock for the Amazon Basin of 1.10 Pg over 0–30 cm soil depth, and 2.76 Pg over 0–100 cm soil depth. This is up to 20 times higher than previously assumed. Three individual samples per 1 ha are sufficient to capture the site variability of PyC in our plots. PyC showed significant, large-scale variability among plots. To capture 50% of the PyC in 200 cm soil profiles, soil must be sampled to a depth of at least 71 cm. PyC represents a significant (11%) portion of total organic carbon in soil profiles 0–200 cm depth. This finding highlights the potentially important role that historical fire has played in modifying soil C stocks. Our data suggest that PyC is an important carbon pool for long-term storage, involved in millennial scale biogeochemical cycling, particularly in the subsurface soil

Investigating long-term, high-resolution records of climate and extreme events in the southeast Indian and southwest Pacific oceans using the δ¹⁸O of stalagmites

There has been much debate in recent years over the impact of rising global temperatures on the climatology of tropical storms. Various disciplines employ a range of differing techniques to assess the likely impact of enhanced greenhouse conditions on the frequency and severity of natural hazards such as severe tropical storms and tropical cyclones. Often, highly sophisticated statistical and climate models and geological proxy records are used to obtain a picture of historical and prehistorical climatology. Geological proxy records are often of low temporal resolution (decades to centuries) while statistical and climate models are based on instrumental records of tropical cyclones (covering only the last 50 years) which are not of sufficient length to decipher natural variability from human induced change. This complicates the issue of identifying long-term trends in tropical cyclone activity and the associated causes and may explain to a certain extent the disparity between current trend estimates. This thesis investigates long-term tropical cyclone activity over the last 1500 years using the stable isotope composition of seasonal growth layers in stalagmites from Eastern and Western Australia (Chillagoe, Queensland and Cape Range Western Australia respectively). Specifically does tropical cyclone activity within the instrumental record reflect natural variability, are there any discernible trends in activity and do these have climatic control/s, and lastly, what impact have these changes had (if any) on the natural environment. The approach that this thesis takes to addressing this issue is three fold. The first step, and thus first aim of the thesis was to develop a new palaeo-tropical cyclone activity index thus extending the current tropical cyclone record 1500 years back in time (Chapter 4) in order to compare the present to the past, comparing the new record with long-term climatic indices (Chapter 5) and lastly, investigating the potential environmental impacts of a changing tropical cyclone regime on both regions (Chapter 6). This thesis effectively bridges the gap between the high-temporal resolution instrumental tropical cyclone record and geological proxy record by generating one seamless, quantifiable record spanning the last 1.5 millennia at two locations in Eastern and Western Australia (Chapter 4). This involved the development of a new tropical cyclone index (CAI) by linking isotope dynamics within tropical cyclones with their meteorological characteristics, and applying these principles to long-standing, annually resolved records of rainfall δ¹⁸O from terrestrial carbonate deposits (stalagmites). The components of CAI were tested against measured records of tropical cyclone rainfall δ¹⁸O. The high-resolution, long-term isotope record was then calibrated against the instrumental tropical cyclone record after de-trending the influence of the monsoon from the carbonate δ¹⁸O. The second aim of this thesis was to examine the tropical cyclone climatology in these two regions at annual, decadal and centennial scales in an attempt to decipher natural variability from anthropogenically induced change. The third aim was to take steps towards applying this new index, by comparing the long-term CAI against other climate indices in an attempt to identify potential drivers of change within the geological record and thus shortlist potential inputs for long-range tropical cyclone forecast models (Chapter 5). In Chapter 6 we assess the potential impacts (if any) on the surrounding environment by applying a duel isotope approach thus, investigating changes in δ¹³C within the stalagmite carbonate in conjunction with the observed enrichment in δ¹⁸O over the most recent 100 years (the period corresponding to the reduction in tropical cyclone activity noted in Chapter 4.) The results of this thesis indicate that tropical cyclone activity has been highly variable over the past 1,500 yr and wavelet analysis indicates the presence of decadal, interdecadal, centennial and inter-centennial scale oscillations at both sites. Trend analysis indicates that tropical cyclone activity in the Australian region has been significantly less in recent years when compared to the last 550-1,500 yr. In fact, tropical cyclone activity on the West coast of Australia is at an all time low. Analysis of the CAI using classical statistical techniques indicate that high-frequency oscillations in CAI may be driven by other climate phenomena such as the Northern Oscillation Index, highlighting the influence of meridional atmospheric patterns in addition to zonal atmospheric patterns (e.g. Southern Oscillation Index), and cross basin teleconnections between CAI in Western Australia and the ENSO Precipitation Index, Northern Oscillation Index, Trade-wind Index and between CAI in Eastern Australia and Atlantic Meridional Mode which have not been previously identified (with the exception of Trade-wind Index). In addition, cross wavelet analysis reveals a link between tropical cyclone activity in Western and Eastern Australia and solar cycles at decadal, and centennial scales, within both the instrumental record (Sun Spot Number) and over geological time scales through the comparison with cosmogenic isotope data i.e. southern hemisphere tree ring δ¹⁴C data and NGRIP Greenland. Be concentration data. The application of a dual isotope approach (δ : δ defined later in Chapter 6) indicates that the reduction in tropical cyclone activity noted in both regions has an alternate effect, resulting in a reduction in vegetation cover in Western Australia (the C4 community) which we believe is owing largely to the dominance of grasses at the site and the large contribution of seasonal tropical cyclone activity to annual rainfall totals at the site and conversely, an increase in vegetation cover in the C3 community (the Eastern Australian site) due to a reduction in large scale disturbances within the dry tropical woodland

Solar forcing over the last 1500 years and Australian tropical cyclone activity

Accurate seasonal and decadal predictions of tropical cyclone activity are essential for the development of mitigation strategies for the 2.7 billion residents living within cyclone prone regions. The traditional indices (Southern Oscillation Index and various sea surface temperature indices) have fallen short in recent years as seasonal predictors within the Australian region. The short length of these records (i.e., <50 years) has meant that our current knowledge of larger-scale drivers at interdecadal, centennial, and millennial scales is limited. The development of a new tropical cyclone activity index spanning the last 1500 years has enabled the examination of tropical cyclone climatology at higher temporal resolution than was previously possible. Here we show that in addition to other well-known climate indices, solar forcing largely drives decadal, interdecadal, and centennial cycles within the tropical cyclone record

Australian tropical cyclone activity lower than at any time over the past 550-1,500 years

The assessment of changes in tropical cyclone activity within the context of anthropogenically influenced climate change has been limited by the short temporal resolution of the instrumental tropical cyclone record(1,2) (less than 50 years). Furthermore, controversy exists regarding the robustness of the observational record, especially before 1990(3-5). Here we show, on the basis of a new tropical cyclone activity index (CAI), that the present low levels of storm activity on the mid west and northeast coasts of Australia are unprecedented over the past 550 to 1,500 years. The CAI allows for a direct comparison between the modern instrumental record and long-term palaeotempest (prehistoric tropical cyclone) records derived from the O-18/O-16 ratio of seasonally accreting carbonate layers of actively growing stalagmites. Our results reveal a repeated multicentennial cycle of tropical cyclone activity, the most recent of which commenced around AD 1700. The present cycle includes a sharp decrease in activity after 1960 in Western Australia. This is in contrast to the increasing frequency and destructiveness of Northern Hemisphere tropical cyclones since 1970 in the Atlantic Ocean(6-8) and the western North Pacific Ocean(6,7). Other studies project a decrease in the frequency of tropical cyclones towards the end of the twenty-first century in the southwest Pacific(7,9), southern Indian(9,10) and Australian(11) regions. Our results, although based on a limited record, suggest that this may be occurring much earlier than expected

Greater frequency variability of landfalling tropical cyclones at centennial compared to seasonal and decadal scales

Assessing risk from tropical cyclones and predicting the impact of this hazard under a human altered climate is based\ud exclusively upon the behaviour of these events over the past 50–100 yr and often less. Critical to these determinations is an understanding of the full extent of the natural variability of this hazard. The coarse resolution of millennial scale sedimentary records, brevity of the instrumental register, imprecision of longer historical accounts and lack of any long-term, high resolution records has led to the assumption that the total variability of tropical cyclone behaviour is encompassed within the seasonal to multi-decadal oscillations observed to date. We present a near 800 yr long, annual resolution isotope record of tropical cyclones in northeast Australia which displays marked centennial scale regimes. Our record demonstrates that the frequency variability of intense landfalling cyclones is greatest at centennial scale compared to seasonal and decadal oscillations. Switching between centennial scale regimes in this record occurred rapidly (10–20 yr) highlighting the importance of accounting for this phenomenon in coastal planning and risk assessment. Our study highlights the importance of obtaining high resolution multi-century records of tropical cyclone activity in order to more accurately assess future impacts of this hazard to human society

A carbon and nitrogen isotope perspective on ancient human diet in the British Isles

The stable carbon (δ13C) and nitrogen (δ15N) isotope composition of human bone collagen is increasingly used to investigate past mobility and subsistence strategies. This study presents a compilation of 1298 carbon and nitrogen isotope analyses of archaeological human bone collagen from the British Isles spanning much of the Holocene, along with a compilation of 4148 analyses of modern and ancient isotope analyses from the major marine and terrestrial dietary resources from the same region. We convert ancient human stable isotope data to modern diet equivalent (MDE) values for humans, and convert the isotope composition of ancient dietary items to modern tissue equivalent (MTE) isotope values. These conversions enable a direct comparison of ancient and modern datasets. Results for food groups (plants, grain, herbivores, omnivores, shellfish, freshwater fish and marine fish) show a remarkably broad range of δ13CMTE values from ∼-36 to −7‰ and δ15NMTE values from ∼-2 to +21‰ and we provide estimates for each food type that can be used in dietary reconstruction in the absence of site-specific data. We further show that there is no significant change in terrestrial stable isotope baseline values over the Holocene, with observed variability in baseline values due to local eco-physiological, edaphic and microclimatic factors. The range of values expressed in the human sample set from the beginning of the Iron Age is relatively tightly clustered with 50% of all human modern diet equivalent results falling within a ∼2‰ range in δ13CMDE values (−25.5 to −27.5‰) and a ∼3.5‰ range in δ15NMDE values from (+4‰ to +8‰). From the Iron Age to post-medieval times there is a consistent progressive shift to higher δ13CMDE and δ15NMDE values at the population level. This shift likely reflects a combination of successive innovations associated with food production, preservation and transport that enabled a broader cross-section of the population of the British Isles to incorporate a higher proportion of animal, and particularly marine protein, into their diets

A rapid throughput technique to isolate pyrogenic carbon by hydrogen pyrolysis for stable isotope and radiocarbon analysis

Rationale: Rapid, reliable isolation of pyrogenic carbon (PyC; also known as char, soot, black carbon, or biochar) for the determination of stable carbon isotope (δ13C) composition and radiocarbon (14C) dating is needed across multiple fields of research in geoscience, environmental science and archaeology. Many current techniques do not provide reliable isolation from contaminating organics and/or are relatively time‐consuming. Hydrogen pyrolysis (HyPy) does provide reliable isolation of PyC, but the current methodology is time consuming. Methods: We explored the potential for subjecting multiple samples to HyPy analysis by placing up to nine individual samples in custom‐designed borosilicate sample vessels in a single reactor run. We tested for cross‐contamination between samples in the same run using materials with highly divergent radiocarbon activities (~0.04–116.3 pMC), δ13C values (−11.9 to −26.5‰) and labile carbon content. We determined 14C/13C using accelerator mass spectrometry and δ13C values using an elemental analyser coupled to a continuous flow isotope ratio mass spectrometer. Results: Very small but measurable transfer between samples of highly divergent isotope composition was detectable. For samples having a similar composition, this cross‐contamination is considered negligible with respect to measurement uncertainty. For samples having divergent composition, we found that placing a sample vessel loaded with silica mesh adsorbent between samples eliminated measurable cross‐contamination in all cases for both 14C/13C and δ13C values. Conclusions: It is possible to subject up to seven samples to HyPy in the same reactor run for the determination of radiocarbon content and δ13C value without diminishing the precision or accuracy of the results. This approach enables an increase in sample throughput of 300–600%. HyPy process background values are consistently lower than the nominal laboratory process background for quartz tube combustion in the NERC Radiocarbon Laboratory, indicating that HyPy may also be advantageous as a relatively ‘clean’ radiocarbon pre‐treatment method

Stable isotope composition of cave guano from eastern Borneo reveals tropical environments over the past 15,000 cal yr BP

Insular southeast Asia is a key driver for global atmospheric and oceanic circulation, is a hotspot for biodiversity and conservation, and is likely to have played a unique and important role in early human dispersals. Despite this, partially due to its vast size and remote tropical location, very few continuous palaeoenvironmental records exist, especially in eastern Borneo. Therefore, we investigated δ13C and δ15N values, and geochemistry of two cave guano deposits to reconstruct palaeoenvironments in eastern Borneo. Firstly, a profile was recovered from Gomantong caves, Sabah, reflecting a continuous deposit over ~15 cal kyr BP. Secondly, a profile was recovered from Bau Bau cave, East Kalimantan, that ranged from ~15–5 cal kyr BP. The geochemical signature of each deposit confirmed the material to be ancient guano. δ13C values revealed that a continuous dense rainforest persisted over at least the last 15 cal kyr BP around the Gomantong site that was relatively insensitive to regional climate change. By contrast, δ13C values at Bau Bau indicate that, although rainforest remained dominant in the record, a significant drying occurred between 7.7 and 6.3 cal kyr BP, with up to 25% grasses present. Although most regional models suggest that sea-level rise and increased Holocene insolation led to an increase in monsoonal moisture, we find some evidence of more regional variability, and that a reduction in monsoonal precipitation could have occurred. However, we cannot discount the implementation of an anthropogenic fire regime that opened the canopy allowing more grasses to occur