The Environmental Changes in the Quaternary Environment of Australian Drylands - Case Study Example

hе Quаtеrnаry Environment of Аustrаliаn Drylаnds Student Name: Student Number Unit: University: Date of Submission: 1. Introduction The drylands of Australia comprise one of the biggest and most unique arid regions in the Southern Hemisphere. Australia is, without a doubt, the driest inhabited continent in the world and her drylands constitute one of the most immense of the continent’s biomes (Smith, 2013). For the longest time, very little was known about the history of Australia’s dryland. This has however begun to change due to the concerted efforts of archaeologists, whose work has resulted in a treasure trove of in depth, new data on the environmental history and archaeology of the drylands that has been unearthed during the last ten years, revealing a more active and deeper history than was hitherto thought. The following discussion will provide a background into the environmental changes that have occurred in Australia’s Dryland in the past as well as present and future times and thereafter, an analysis into the process behind reconstructing Quaternary environments will be given for a comprehensive overview of this most important topic. 1.1 Environmental Changes of the Australian Dryland: Past, Present, Future According to Fitzsimmons, et.al (2013), several palaeoenvironmental changes have occurred in Australian drylands, particularly in the period between 40-0 ka. The main variable determining environmental change that took place was in relation to moisture levels in the drylands. Typically, this region is affected more by changes in precipitation than changes in temperature. Depending on where they are located, drylands generally respond to climate regimes that have either been influenced by the temperate latitude westerly systems, tropical monsoons, or both. The timing and magnitude of relatively humid and arid phases differ throughout the Australian landscape especially between the westerly wind-controlled temperate latitudes, and the north and interior which are affected by tropically sourced rainfall. The period from 40 ka to the Last Glacial Maximum (LGM) experienced extensive aridity and a colder climate and there were widespread falls in the lake level, desert dune activity, dust transport and decreased but occasional fluvial activity. In the deglacial period, the climate was spatially divergent whereby southern Australia was characterised by a short humid phase at w17-15 ka and thereafter a spike in dune activity between w14-10 ka. The post-LGM period, however, was characterised by continual arid conditions in the north that was linked to a lapsed monsoon and thereafter, there was a growth in precipitation and an escalation of the monsoon from w14 ka (Fitzsimmons, et.al, 2013). In the Last glacial to early Holocene period, there was an increase in the high-pressure belt and displacement of moisture-laden westerlies southwards which led to a reduction in moisture levels in coastal areas (Harrison, 1993). Climatic conditions were more even and there was less contrast in moisture balance between the environments in the interior and the coast. Overall, conditions were drier and there was a major decrease in the temperatures in the interior (Hope et al., 2004). The early to mid-Holocene conditions, however, were fairly humid as was evidenced by increases in the lake levels, speleothem growth and source-bordering dune activity. In the late Holocene period, arid conditions then increased especially in the central arid zone (Fitzsimmons, et.al, 2013). The Southern Hemisphere experienced a growth in seasonality and warmth during the summer but a decreased gradient between temperate and tropical regions as a result of the reduction in the westerly airflow (Dodson, 1998). At 5.5–5.0 cal ka BP, the moisture optimum began to decrease and there was an increase in climatic coupling between the northern and southern regions and related centennial-scale changes occurred from 5 cal ka BP (Luly, 1993). Additional changes took place around 3.0 cal a BP whereby conditions became a bit drier and more uneven. As this time, fire occurrences began to take place at an alarming rate throughout eastern Australia which has been attributed to Aboriginal practice among other environmental factors (Bickford and Gell, 2005). Modern palynological records reveal that there was a gradual rise in ENSO activity between 5 and 3 cal ka BP (Donders et al., 2005). The influence of the ENSO activity was in relation to moisture anomalies that occurred especially in the wet season in the south during winter and spring which corresponded with the dry season in the north (Magee et al., 2004). As a result, increased ENSO-related dry events had an impact on the temperate to subtropical regions where ENSO has continued to affect the moisture supply. After 6 ka BP there was an increase in December insolation at the equator which reached approximately present-day levels at _3ka BP. El Niño warm events emerged from the Indo-Pacific Warm Pool (IPWP) in the austral summer and ENSO intensification which has caused warm events to take place regularly (Markgraf et al., 1992). Human activities, globalization and global warming have affected the environment of Australia’s drylands in the modern times. Salinization is the major environmental change that characterises present day Australian drylands. Salt has accumulated in the soil over several thousands of years a condition exacerbated by an imbalance in the hydrological cycle or human activities such as irrigation and land clearing. Before the British settlement in 1788, there was a balance in groundwater levels due to seasonal recharge and the use of ground water by vegetation throughout the year (Clarke et.al, 2002). In the last three centuries, land clearing in favour of agriculture has caused a loss of the initial vegetation which has led to a lack of equilibrium in the hydrological cycle and consequently, widespread salinity. Over the years, this process has resulted in the irreversible salinity of the top-soil layers. By 1999, approximately 2.5 million hectares of land was salinized mainly as a result of the use of European farming methods. Presently, approximately 5.7 million hectares of land is in danger of salinization by 2050, this is expected to increase to 17 million hectares (Murphy, 1999). If this salinization continues unabated, the future of the drylands is bleak as the land will be uncultivable and aridity will increase and spread. 1.2 Reconstructing Quaternary Environments Very little information is available concerning the history of the Australian Dryland. This is despite the fact that palaeo-environmental reconstructions are very important in understanding the history of any dryland or quaternary environment. The importance of reconstructing quaternary climate changes and dynamics is widely recognised. According to Aldughairi (2013), drylands are excellent arsenals of palaeoclimatic data because landscapes in drylands are a function of processes taking place today and during past environments. Analysing desert landscapes allows us to understand past environmental responses to changing conditions and even foretell future responses. Reconstructions of quaternary environments are crucial for making model-data comparisons and in addition, they are significant for future research into environmental dynamics, all of which requires a greater integration of well-documented and accurate records. There are several methods used in the reconstruction of quaternary environments. The use of Geomorphology, sedimentology and stratigraphy, for instance, are effective methods. Responses to change, particularly with respect to moisture availability, can be recognised from landform morphology and stratigraphy linked to every environmental period. Nevertheless, before an analysis can be done, it must be recognised that these landscape response records are plagued with weaknesses. The reconstruction of arid zone landforms, especially dunes in areas that have insufficient sediments usually hampers the preservation of continuous deposits (Fitzsimmons and Telfer, 2008). Landscape change in drylands is oftentimes a response to several causative factors that have to be separated to facilitate palaeoclimatic review. Such an analysis additionally relies upon accurate and correct chronologies. Due to these inherent weaknesses, only a sweeping approach can be done to reconstruct palaeoenvironments in the interior on the premise of the overall synchronisation of proxy information. In addition, geochemical and palaeoecological proxies can be undertaken as they provide more precise palaeoclimatic data than geomorphic proxies. Organic material, like microbial lipids or charcoal fragments, give palaeoecological records (Luly, 2001). In several instances, however, this information is patchy as a result of poor preservation in the sites. Charcoal fragments, for instance, can reveal climatic change but such records are uncommon and not properly preserved. Speleothems, on their part, are more useful in palaeoenvironmental reconstruction as they allow the evaluation of rainfall variation in the past through their oxygen isotope ratios (Treble et al., 2005). Geochemical methods are also used in quaternary environment reconstructions. This method reconstructs through the analysis of palaeodiet records and the measurement of rates of amino acid racemisation which reveal ecosystem change according to the changes of carbon isotopes in fossil eggshells (Johnson et al., 1999). Furthermore, stable isotope analysis of carbon and nitrogen linked to palaeodiet, rainfall and palaeovegetation can be applied (Fraser et al., 2008). The analysis of stable isotopes can also be applied to soil carbonates which can generate palaeohydrological information (De Deckker et al., 2011). These methods are beneficial since they give palaeoclimatic data that is more precise than geomorphic records though they are limited due to insufficient preservation and accessibility. Geochronology is yet another method and in fact, our understanding of the palaeoenvironmental change in the Australian interior has been mainly reliant upon the use of geochronological techniques. The initial theories concerning the chronology of aridification in Australia were done before the prevalence of absolute dating. One hypothesis provided by Gill (1955) posited that the formation of the current Australian arid landscape occurred in the mid-Holocene period. Radiocarbon dating undertaken later, however, revealed that this formation actually took place prior to that in the LGM (Callen, 1984). Luminescence methods are perhaps the most reliable methods for quaternary environment reconstruction. In fact, the considerable improvement in the accuracy of ascertaining the ages of Australian dryland sediments is as a result of the extensive use of luminescence methods. Luminescence dating helps to establish the specific timing of sediment deposition according to the latest exposure to sunlight (Aitken, 1998). It is a very appropriate method for fining out the timing of landscape response to environmental change in drylands because of the greater possibility of sunlight exposure (Lancaster, 2008). The first luminescence method employed was thermoluminescence (TL) which was then followed by optically stimulated luminescence (OSL) (Lomax et al., 2003). OSL has, to a great extent, taken over from TL as the being best dating method for quartz-rich sediments, especially Australian quartz because of its effective response to measurement practice (Fitzsimmons et al., 2010). OSL dating is nevertheless susceptible to challenges especially when used to analyse discontinuous records. It only dates the latest depositional occurrences and as such, the accurate measurements of the length of phases are not always possible. This challenge does not, however, affect areas such as arid zone floodplains because they are usually very wide. Additionally, it is possible for the luminescence signal to be reset after deposition by exposure and bioturbation can cause a mixing of the age populations (Bateman et al., 2003). Fortunately, this challenge can be identified and corrected by opting for single grain as opposed to single aliquot measurements (Duller, 2008). Another challenge which affects the accuracy of age determination is with respect to dose rate heterogeneity age estimate precision and lastly, OSL has a 5-10% rate of uncertainty and this cannot be compared to the accuracy of radiocarbon. This compromises with the findings of palaeoenvironmental reconstruction (Lomax et al., 2007). Conclusion The Australian drylands comprise a very significant part of the continents landscape. Despite this fact, not much was known about the drylands for the longest time until relatively recently when concerted reconstruction efforts of the quaternary environment began. The discussion has documented changes in the environment over the last 40 ka, evidence of a complex response across a continent encompassing a broad range of climatic zones. The variances in precipitation and temperature that have been documented as having taken place throughout Australia during the last 40 ka reveal the fact that there were major shifts in the circulation systems and climate zones (Moros et al., 2009). There are several methods of quaternary reconstruction of environments including geochemical and geochronological methods. The most accurate, however, are luminescence methods such as OSL which has been used extensively to reconstruct the quaternary environments of Australian drylands. 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