Paleoclimatic implications from late Quaternary terrestrial archives in the Gobi Desert : examples from the Ejina Basin and Orog Nuur Basin

  • Paläoklimatische Implikationen von spätquartären terrestrischen Archiven in der Wüste Gobi : Beispiele aus dem Ejina Becken und dem Orog Nuur Becken

Yu, Kaifeng; Lehmkuhl, Frank (Thesis advisor); Diekmann, Bernhard (Thesis advisor)

Aachen (2016, 2017)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2016

Abstract

Considerable efforts have been devoted to disentangle the late Quaternary moisture and thermal evolution of arid central Asia. However, an array of paramount aspects has inhibited our complete understanding of the broad pattern of Quaternary moisture and thermal history of arid central Asia and underlying mechanisms.The Ejina Basin, with its suite of different sediment archives, is known as one of the main sources for the loess accumulation on the Chinese Loess Plateau. In order to understand mechanisms along this supra-regional sediment cascade (aeolian, fluvial and alluvial sediments), it is crucial to decipher the archive characteristics and formation processes. Five sediment archives from three lithologic units exhibit geochemical characteristics as follows: (i) aeolian sands have high contents of Zirconium and Hafnium, whereas only Hafnium can be regarded as a valuable indicator to discriminate the coarse sand proportion; (ii) sandy loess has high Calcium and Strontium contents which both exhibit broad correlations with the medium to coarse silt proportions; (iii) lacustrine clays have high contents of felsic, ferromagnesian and mica source elements e.g., Potassium, Iron, Titanium, Vanadium, and Nickel; (iv) fluvial sands have high contents of Magnesium, Chlorine and Sodium which may be enriched in evaporite minerals; (v) alluvial gravels have high contents of Chromium which may originate from nearby Cr-rich bedrock. Temporal variations can be illustrated by four robust factors: weathering intensity, silicate-bearing mineral abundance, saline / alkaline magnitude and quasi-constant aeolian input. In summary, the bulk-composition of the late Quaternary sediments in this arid context is governed by the nature of the source terrain, weak chemical weathering, authigenic minerals, aeolian sand input, whereas pedogenesis and diagenesis exert only limited influences. Hence, here demonstrates a practical geochemical strategy to discriminate sediment archives and thereafter enhance our ability to offer more intriguing information about the sedimentary processes in the arid central Asia.On the other hand, two parallel cores (ONW I, 6.00 m; ONW II, 13.35 m) were retrieved from lake Orog Nuur, in the Gobi Desert of Mongolia. Ample evidences reveal a marked moisture pulse during the Marine Isotope Stage 3 (~36-~24 ka) which might have induced the maximum last glacial expansion in the high elevated Khangai Mountains. A sharp transition of Termination I (~11 ka) is illuminated by geochemical, palynological, and ostracod data. Lower area of the Orog Nuur catchment was dominated by Artemisia steppe community in the late Pleistocene and altered gradually to Chenopodiaceae desert steppe in the Holocene. The early Holocene is also characterized by relatively humid environment, albeit discordant downcore variability of moisture and thermal signals can be derived between palynological and bulk-geochemical signals. Water body in the Holocene appears to be a distinct alkaline environment which was subjected to frequent allogenic input and disturbance of the late Pleistocene anoxic states. These two humid pulses may be the trait of a larger scale of arid central Asia that would be documented in a suite of archives. Considering kinetics, a coupled atmospheric component comprising both East Asian Summer Monsoon and strengthened Westerlies moisture supply seems to be the driving mechanism, and this coupled mode might have been modulated broadly by boreal insolation variances. On the other hand, four major harsh climatic phases were documented in the core at ~47 ka, ~37 ka, ~19 ka (gLGM) and ~13-~11 ka (Younger Dryas) as playa phases. Reduced conveying of the Westerlies moisture along with the retreated East Asian Summer Monsoon might have contributed to these playa phases in the Gobi Desert.Continuous, high-resolution elemental abundances at a 1 cm interval were examined on core ONW II. Due to the predominant clay or silty-clay fraction in the sediments, Aluminium and Silicon display broadly identical pattern. Calcium behaviors may be ascribed to the authigenic calcite variance. Manganese and Cobalt act as sound indicator for the redox condition. Owing to the short burial time and weak diagenetic influence, Iron is not an indicator to the diagenetic processes. Likewise, Zirconium may be associated to fluvial clastic in relation to the lithotypes but not necessarily linked to the aeolian sands relative to that in the pelagic realm. Furthermore, Sulfur in lake sediments may denote the redox condition and Potassium is more likely linked to the K-feldspar which is associated with the allogenic fluvial inputs. The two sand layers corresponding to LGM and YD event were dominated by coarse sands. In the Gobi Desert of Mongolia, these two laminations provide an opportunity to be regarded as potential chronological benchmarks. As inferred from the biplot between Ca and Al/Si, disparate source lithotypes may exist before and after Termination I. The Holocene appears to be a distinct alkaline environment compared with the late Pleistocene. This may be ascribed to strengthened fluvial morphodynamics of the riverine inflows and intensified erosion of source rocks in the upper catchment of the Orog Nuur. In exceptionally arid realm, sands may exert significant influence on bulk-geochemistry of the lake sediments. In summary, considering the bulk-geochemistry of lacustrine sediments, paleoclimate would not be the solely governing factor, the coupled alteration of precursor rocks and decomposition intensity may also exert pivotal influences.