Late Holocene environmental change on the NE Tibetan Plateau: A paleolimnological study of Lake Qinghai and Lake Gahai, China, based on stable isotopes.

Climate driven hydrologic variability has direct socio-economic impacts on local, regional and global scales. Particularly vulnerable is the region that lies within the boundaries of the East Asian monsoon, which is one of the most pronounced and influential phenomena of Earth's climate system. People within the heavily populated Asian countries have adapted many aspects of their society to the subtleties of monsoon rains, and are thus highly susceptible to small changes in the timing and intensity of monsoon precipitation. This study aims to reconstruct high-resolution late Holocene climatic and hydrologic variability from Lake Qinghai, NE Tibetan Plateau, using stable isotope techniques. The thesis also examines the modern isotope systematics of Lake Qinghai through mass balance methods to understand the modern hydrology of the lake in order to determine which factors are important in influencing lake water isotope composition. The isotopic compositions of Lake Qinghai's modern waters indicate that they have been modified by evaporation, as they plot below the Global Meteoric Water Line (GMWL). Isotope mass balance models quantify this, and demonstrate that 29 to 35% of the waters entering the lake are from direct precipitation and surface runoff, while evaporation from the lake's surface accounts for 44 to 54% of the lakes water loss. This suggests that the balance of precipitation to evaporation (P/E) is an important control on the composition of lake waters and therefore carbonate incorporated into the sediment record will reflect this. Four cores were used in this study and they provide a palaeohydrological history displaying distinct changes in 518Oauth and 513Cauth over the past 1500 years. These are interpreted in terms of effective moisture and one particularly pronounced event, between 1600 and 1850 AD has been attributed to decreases in evaporation. This event, when compared to other regional palaeoenvironmental archives is coincident with cold temperatures, synonymous with the Northern Hemisphere Little Ice Age, tentatively suggested to be a result of solar variability

The stable isotope composition of organic and inorganic fossils in lake sediment records: current understanding, challenges, and future directions

This paper provides an overview of stable isotope analysis (H, C, N, O, Si) of the macro and microscopic remains from aquatic organisms found in lake sediment records and their application in (palaeo)environmental science. Aquatic organisms, including diatoms, macrophytes, invertebrates, and fish, can produce sufficiently robust remains that preserve well as fossils and can be identified in lake sediment records. Stable isotope analyses of these remains can then provide valuable insights into habitat-specific biogeochemistry, feeding ecology, but also on climatic and hydrological changes in and around lakes. Since these analyses focus on the remains of known and identified organisms, they can provide more specific and detailed information on past ecosystem, food web and environmental changes affecting different compartments of lake ecosystems than analyses on bulk sedimentary organic matter or carbonate samples. We review applications of these types of analyses in palaeoclimatology, palaeohydrology, and palaeoecology. Interpretation of the environmental ‘signal’ provided by taxon-specific stable isotope analysis requires a thorough understanding of the ecology and phenology of the organism groups involved. Growth, metabolism, diet, feeding strategy, migration, taphonomy and several other processes can lead to isotope fractionation or otherwise influence the stable isotope signatures of the remains from aquatic organisms. This paper includes a review of the (modern) calibration, culturing and modeling studies used to quantify the extent to which these factors influence stable isotope values and provides an outlook for future research and methodological developments for the different examined fossil groups

Holocene El Niño–Southern Oscillation variability reflected in subtropical Australian precipitation

The La Niña and El Niño phases of the El Niño-Southern Oscillation (ENSO) have major impacts on regional rainfall patterns around the globe, with substantial environmental, societal and economic implications. Long-term perspectives on ENSO behaviour, under changing background conditions, are essential to anticipating how ENSO phases may respond under future climate scenarios. Here, we derive a 7700-year, quantitative precipitation record using carbon isotope ratios from a single species of leaf preserved in lake sediments from subtropical eastern Australia. We find a generally wet (more La Niña-like) mid-Holocene that shifted towards drier and more variable climates after 3200 cal. yr BP, primarily driven by increasing frequency and strength of the El Niño phase. Climate model simulations implicate a progressive orbitally-driven weakening of the Pacific Walker Circulation as contributing to this change. At centennial scales, high rainfall characterised the Little Ice Age (~1450–1850 CE) in subtropical eastern Australia, contrasting with oceanic proxies that suggest El Niño-like conditions prevail during this period. Our data provide a new western Pacific perspective on Holocene ENSO variability and highlight the need to address ENSO reconstruction with a geographically diverse network of sites to characterise how both ENSO, and its impacts, vary in a changing climate

Palaeolimnological evidence for environmental change over the past millennium from Lake Qinghai sediments: a review and future research prospective

Lake Qinghai, on the NE Tibetan Plateau, is China's largest natural lake that lies at a triple junction of major climatic influences, making it sensitive to global climate change. As such, the sediments of Lake Qinghai have been the focus of numerous palaeoenvironmental studies spanning a range of timescales. However, as a result of uncertain age controls, uncertainties over interpretation of the proxies, the relative dearth of proxy calibration and lack of understanding of the modern lake system a coherent picture of climate over the NE Tibetan Plateau has yet to emerge from Lake Qinghai's sediment record. We review the state of knowledge for this important site, focusing on the last millennium. A comparison of the major proxy records show significant variability with a general pattern of change over the last 1000 years, notably those linked to the onset of the Little Ice Age, but due to poor chronological constraints a detailed picture of climate change cannot be established. Further, some of the proxy records produced from Lake Qinghai's sediments are open to alternative explanations. This compounds the sediment record as a palaeoenvironmental archive. To fully realise the potential of Lake Qinghai, future research must concentrate on defining a reliable old carbon effect for the lake, calibrating proxy records with climatic processes and understanding spatial variability of proxy records within this large lake

Late Cenozoic Climate Changes in China’s Western Interior: A Review of Research on Lake Qinghai and Comparison with Other Records

We review Late Cenozoic climate and environment changes in the western interior of China with an emphasis on lacustrine records from Lake Qinghai. Widespread deposition of red clay in the marginal basins of the Tibetan Plateau indicates that the Asian monsoon system was initially established by ~8 Ma, when the plateau reached a threshold altitude. Subsequent strengthening of the winter monsoon, along with the establishment of the Northern Hemisphere ice sheets, reflects a long-term trend of global cooling. The few cores from the Tibetan Plateau that reach back a million years suggest that they record the mid-Pleistocene transition from glacial cycles dominated by 41 ka cycles to those dominated by 100 ka cycles. During Terminations I and II, strengthening of the summer monsoon in China’s interior was delayed compared with sea level and insolation records, and it did not reach the western Tibetan Plateau and the Tarim Basin. Lacustrine carbonate δ18O records reveal no climatic anomaly during MIS3, so that high terraces interpreted as evidence for extremely high lake levels during MIS3 remain an enigma. Following the Last Glacial Maximum (LSM), several lines of evidence from Lake Qinghai and elsewhere point to an initial warming of regional climate about 14 500 cal yr BP, which was followed by a brief cold reversal, possibly corresponding to the Younger Dryas event in the North Atlantic region. Maximum warming occurred about 10 000 cal yr BP, accompanied by increased monsoon precipitation in the eastern Tibetan Plateau. Superimposed on this general pattern are small-amplitude, centennial-scale oscillations during the Holocene. Warmer than present climate conditions terminated about 4000 cal yr BP. Progressive lowering of the water level in Lake Qinghai during the last half century is mainly a result of negative precipitation–evaporation balance within the context of global warming

Palaeoglaciation in the low latitude, low elevation tropical Andes, northern Peru

Characterising glaciological change within the tropical Andes is important because tropical glaciers are sensitive to climate change. Our understanding of glacier dynamics and how tropical glaciers respond to global climate perturbations is poorly constrained. Studies of past glaciation in the tropical Andes have focused on locations where glaciers are still present or recently vacated cirques at high elevations. Few studies focused on lower elevation localities because it was assumed glaciers did not exist or were not as extensive. We present the first geomorphological evidence for past glaciations of the Lagunas de Las Huaringas, northern Peru, at elevations of 3,900–2,600 m a.s.l. Mapping was conducted using remotely-sensed optical imagery and a newly created high-resolution (∼2.5 m) digital elevation model (DEM). The area has abundant evidence for glaciation, including moraines, glacial cirques, hummocky terrain, glacial lineations and ice-sculpted bedrock. Two potential models for glaciation are hypothesised: 1) plateau-fed ice cap, or 2) valley glaciation. Assuming glaciers reached their maximum extent during the Local Last Glacial Maximum (LLGM), between 23.5 ± 0.5 and 21.2 ± 0.8 ka, the maximum reconstructed glacial area was 75.6 km2. A mean equilibrium line altitude (ELA) of 3,422 ± 30 m was calculated, indicating an ELA change of −1,178 ± 10 m compared to modern snowline elevation. There is an east to west ELA elevation gradient, lower in the east and higher in the west, in-line with modern day transfer of moisture. Applying lapse rates between 5.5 and 7.5°C/km provides a LLGM temperature cooling of between 6.5–8.8°C compared to present. These values are comparable to upper estimates from other studies within the northern tropical Andes and from ice-core reconstructions. The mapping of glacial geomorphology within the Lagunas de las Huaringas, evidences, for the first time, extensive glaciation in a low elevation region of northern Peru, with implications for our understanding of past climate in the sub-tropics. Observations and reconstructions support a valley, rather than ice cap glaciation. Further work is required to constrain the timing of glaciations, with evidence of moraines younger than the LLGM up-valley of maximum glacier extents. Numerical modelling will also enable an understanding of the controls of glaciation within the region

Compound specific &#948;D values of long chain <i>n</i>-alkanes derived from terrestrial higher plants are indicative of the &#948;D of meteoric waters: evidence from surface soils in eastern China

The stable carbon and hydrogen isotope composition of higher plant-derived long chain n-alkanes (&#948;13Cn-alkanes and &#948;Dn-alkanes) from 45 surface soil samples (within well characterized vegetation zones) from eastern China (18°N–50°N) are reported. The weighted average &#948;Dn-alkanes value for n-C27, n-C29 and n-C31 in the samples and the annual average &#948;D of meteoric water recorded at 12 weather stations proximal to the sampling sites show similar spatial variations. The &#948;D of n-alkanes shows a gradual depletion in value with increasing latitude. The results demonstrate that, on a large spatial scale, the &#948;D values of long chain n-alkanes derived from higher plants have the potential to record the δD of meteoric water, although many other factors can also influence the isotope values. There appears to be no apparent relationship between the &#948;D of the n-alkanes extracted from the surface soil and the overlying vegetation type (i.e. forest/grassland or C3/C4 composition). Therefore, palaeoenvironmental studies utilizing &#948;Dn-alkanes from higher plant-derived material in geological samples have the potential to provide additional information with regard to the past hydrological cycle

Variation in leaf wax n-alkane characteristics with climate in the broad-leaved paperbark (Melaleuca quinquenervia)

In higher plants, leaf waxes provide a barrier to non-stomatal water loss, and their composition varies both between and within species. Characteristics of n-alkanes, a suite of ubiquitous compounds in these waxes, are thought to be influenced by the availability of water and the temperature in a plant’s growing environment. Longer n-alkane distributions with less variability in chain length are hypothesised to confer greater resistance to non-stomatal water loss and thus are expected in higher abundance in desiccating environments. Relationships between the distribution of n-alkanes and both precipitation and temperature have previously been observed. Despite this, it is unclear whether n-alkane chain length distributions vary plastically in response to climate, or whether they are fixed within populations in different climate settings. To better understand this, we examine the relationship between n-alkane characteristics of Melaleuca quinquenervia and both spatial and temporal climate variation. Across eastern Australia, we find that n-alkane homolog concentrations and distributions in leaves of M. quinquenervia do not vary with climate where samples are proximate, even when climate shows significant variability. However, the concentration and distribution of n-alkane homologs do differ considerably between geographically separated populations in very different climate regimes. These results suggest n-alkane characteristics are not a plastic response to climate variability, and instead are likely fixed and could be driven by genetic differences between populations. This has important implications for the use of n-alkane characteristics as palaeoenvironmental proxies.Jake W. Andrae, Francesca A. McInerney, John Tibby, Andrew C.G. Henderson, P. Anthony Hall, Jonathan C. Marshall, Glenn B. McGregor, Cameron Barr, Margaret Greenwa

Tropical Asian mega-delta ponds:Important and threatened socio-ecological systems

This paper uses multimedia to showcase the narratives and lived experiences of those who live and work in tropical Asian mega-deltas, and as such is the first journal article of its kind in the field of Regional Geography. Using videos, photography and audio this paper describes the characteristics of ponds and their place in the intrinsically connected human-environmental fabric of these delta regions. The aim is to bring to life descriptive inventories and provide greater weight in support of our conclusion that tropical Asian mega-delta ponds are important and threatened systems. River deltas comprise just 1% of land cover worldwide but support the livelihoods of more than 500 million people. Delta research has historically focused on the major river channels and the socio-ecological role of ponds has been overlooked despite their large number and surface area. Ponds are intrinsically linked to daily life (potable water, sanitation, bathing, washing), industry (aquaculture, agriculture) and the natural-cultural heritage (religion, folklore) of deltas. In contrast to the larger river channels, ponds are likely to be significant stores and processors of nutrients, including carbon, and pollutants at annual to decadal scales, on account of their heavy anthropogenic use and smaller individual sizes. Consequently, they are severely polluted water sources and pose significant public health risks. In this review, we use case studies from three Asian mega-deltas (the Red River Delta and the Mekong River Delta, Vietnam, and the Ganges-Brahmaputra-Meghna Delta, India and Bangladesh) to highlight the importance of Asian mega-delta ponds as important socio-ecological systems in their own right. We discuss future environmental challenges, knowledge gaps on the ecological function and biodiversity of these habitats, management and policy practices, and the capacity of ponds to achieve the UN Sustainable Development Goals. The information, practices and views in this article are those of the author(s) and do not necessarily reflect the opinion of the Royal Geographical Society (with IBG)