Studies on vegetation-, fire-, climate- and human history in the mid- to late Holocene - a contribution to protection and management of the forest-steppe-biome in the Mongolian Altai

In this thesis, several sedimentological archives from Altai Tavan Bogd National Park are studied to reconstruct the vegetation-, fire-, climate- and human history of the forest-steppe biome in the Mongolian Altai. The research is carried out to improve the understanding of the dynamics and variability of this sensitive ecosystem and its unique biodiversity. Previous palynological or palaeoecological work from the Mongolian Altai is sparse, yet important for an implementation of sustainable land use as well as protection and management of the species-rich vegetation in the Altai region. The main goals of this research are to reconstruct past vegetation and to investigate the role and extent of climate, fire and anthropogenic impact on environmental change. A multi-site approach of five environmental archives (lacustrine and peat) from different locations and elevations within the forest-steppe biome (below, within and above the forest belt) is applied to obtain as much information as possible. Multi-proxy analyses including palynological and sedimentological proxies (pollen, NPPs, charcoal, diatoms and XRF-scanning) were used on the radiocarbon dated sediment archives. During the mid- and the beginning of the late Holocene (4,300 to 1,000 (2,000) cal yr BP) the vegetation in the area was characterized by open coniferous forest and high-mountain steppe indicating rather warm and humid conditions. In the further course of the late Holocene, steppe communities expanded noticeably favoring a colder and more arid climate. During the last approx. 70 years an increase in tree and shrub vegetation indicates a warming climate and a higher availability of water due to permafrost and glacier degradation in the high mountains. Regarding the human history in the Mongolian Altai, the period from 2,000 to 1,000 cal yr BP represents a transition phase from hunters and gatherers to a nomadic herding lifestyle. Coprophilous fungi reconstructions show that grazing intensified around 1,000 cal yr BP, possibly also favoring the expansion of steppe. High-resolution data show that changes in human occupation due to political shifts and changing Mongolian settlements had an impact on the vegetation in the area, especially during Mongol Empire (744 to 582 cal yr BP). Regardless of specific settlement periods, short-term changes in climatic conditions favored shifts in grazing activities. In the Mongolian Altai, fires play a tangential role. However, at around 1,000 cal yr BP the fire frequency increased in accord with growing anthropogenic impact and climate aridity. An episode of low fire activity persists since around 150 cal yr BP. Major local variances occurred regarding the time frame and extent of steppe expansion and grazing activities in the soil archive within the forest belt and with respect to the fire frequency in the peat archive above the upper forest line. The applied multi-proxy approach highlights the value of the reconstruction of several independent proxies to examine various aspects of an ecosystem in the same archive, despite of that the interpretation of results is challenging. Additionally, the multi-site study offers the best possibility to distinguish between local environmental signals and regional trends

Climate reconstructions based on GDGT and pollen surface datasets from Mongolia and Baikal area: calibrations and applicability to extremely cold–dry environments over the Late Holocene

International audienceAbstract. Our understanding of climate and vegetation changes throughout the Holocene is hampered by representativeness in sedimentary archives. Potential biases such as production and preservation of the markers are identified by comparing these proxies with modern environments. It is important to conduct multi-proxy studies and robust calibrations on each terrestrial biome. These calibrations use large databases dominated by forest samples. Therefore, including data from steppe and desert–steppe sites becomes necessary to better calibrate arid environments. The Mongolian Plateau, ranging from the Baikal area to the Gobi desert, is especially characterized by low annual precipitation and continental annual air temperature. The characterization of the climate system of this area is crucial for the understanding of Holocene monsoon oscillations. This study focuses on the calibration of proxy–climate relationships for pollen and glycerol dialkyl glycerol tetraethers (GDGTs) by comparing large Eurasian calibrations with a set of 49 new surface samples (moss polster, soil and mud from temporary dry ponds). These calibrations are then cross-validated by an independent dataset of top-core samples and applied to four Late Holocene paleosequences (two brGDGT and two pollen records) surrounding the Mongolian Plateau: in the Altai mountains, the Baikal area and the Qaidam basin, to test the accuracy of local and global calibrations. We show that (1) preserved pollen assemblages are clearly imprinted on the extremities of the ecosystem range but mitigated and unclear on the ecotones; (2) for both proxies, inferred relationships depend on the geographical range covered by the calibration database as well as on the nature of samples; (3) even if local calibrations suffer from reduced amplitude of climatic parameters due to local homogeneity, they better reflect actual climate than the global ones by reducing the limits for saturation impact; (4) a bias in climatic reconstructions is induced by the over-parameterization of the models by the addition of artificial correlation; and (5) paleoclimate values reconstructed here are consistent with Mongolia–China Late Holocene climate trends and validate the application of local calibrations for both pollen and GDGTs (closest fit to actual values and realistic paleoclimate amplitude). We encourage the application of this surface calibration method to reconstruct paleoclimate and especially consolidate our understanding of the Holocene climate and environment variations in arid central Asia

Mongolian dryland paleoenvironment and paleoclimate issues : calibrations and applicability of GDGT and pollen reconstructions over the Late Holocene.

International audience<p>Our understanding of climate changes throughout the Holocene is hampered by representativeness in sedimentary archives. Potential production and preservation biases of the markers are identified by comparing these proxies with modern environments. It is important to conduct robust calibrations on each biome. These calibrations use large database dominated by forest samples. The Mongolian plateau is especially characterized by low annual precipitation and continental annual air temperature. The characterization of the climate system of this area is crucial for the understanding of Holocene Monsoon Oscillations. This study focuses on the calibration of proxy-climate relationships for pollen and glycerol dialkyl glycerol tetraethers (GDGTs) by comparing large published Eurasian calibrations with a set of 49 new surface samples (moss polster, soil and mud from temporary dry pond). These calibrations are then cross-validated by an independent dataset of top-core samples and applied to two Late Holocene paleosequences in the Altai mountains and the Qaidam basin. We show that: (1) preserved pollen assemblages are clearly imprinted on the extremities of the ecosystem range but mitigated and unclear on the ecotones; (2) for both proxies, inferred relationships depend on the geographical range covered by the calibration database as well as on the nature of samples; (3) even if local calibrations suffer from reduced amplitude of climatic parameter due to local homogeneity, they better reflect actual climate than the global ones by reducing the limits for saturation impact, (4) a bias in climatic reconstructions is induced by the over-parameterization of the models and (5) paleoclimate values reconstructed here are consistent with Mongolia-China Late Holocene climate trends, and validate the application of local calibrations for both pollen and GDGTs. We encourage the application of this surface calibration method to reconstruct paleoclimate and especially consolidate our understanding of the Holocene climate and environment variations in Arid Central Asia.</p&gt

LATE HOLOCENE MONGOLIAN CLIMATE RECONSTRUCTIONS FROM LOCALLY CALIBRATED GDGT AND POLLEN TRANSFER FUNCTIONS FOR LAKE AYRAG.

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Multi-proxy and multi-method Mongolian late Holocene climate and environment reconstructions from Lake Ayrag.

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brGDGTs and isotopic records for Lake Ayrag: implications for climate, vegetation and human impact history of the Mongolian Plateau.

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Late Holocene Mongolian climate and environment reconstructions from brGDGTs, NPPs and pollen transfer functions for Lake Ayrag: Paleoclimate implications for Arid Central Asia

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