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University of California Santa Barbara (2019)

Hydroclimate Reconstruction of Semi-Arid Central Asia : Insights from a Kyrgyz Speleothem

Letts, Abby Whitmarsh

Titre : Hydroclimate Reconstruction of Semi-Arid Central Asia : Insights from a Kyrgyz Speleothem

Auteur : Letts, Abby Whitmarsh

Université de soutenance : University of California Santa Barbara

Grade : Master of Science (MS) in Earth Science 2019

Semi-arid Central Asia has been identified as a particularly vulnerable region to global climate change. However, model projections for the region have relatively high uncertainty and there is a lack of absolutely dated records to establish the region’s climatic history. In an effort to better understand Central Asia’s hydroclimate response to changes in radiative and freshwater forcings, this study focuses on a Kyrgyz speleothem (Central Asia) that spans late Marine Isotope Stage (MIS) 10 (343–358 kyr before present). Trace element, fluorescence, and calcite fabric analysis are employed to provide insights into orbital- to decadal-scale Central Asian climate history. We find that fluorescent banding is exclusively present in columnar microcrystalline calcite and interpret these intervals to be wetter than average. Furthermore, changes in calcite fabrics correspond to shifts in trace element concentrations and correlation coefficients. The correlation between Mg and Sr varies from positive to negative depending on fabric type, whereas Mg and P concentrations are negatively correlated throughout the investigated stalagmite. These results are consistent with the concept of prior calcite precipitation (PCP), which predicts high (low) Mg and low (high) P concentrations during relatively dry (wet) periods. Following the concept of PCP, we interpret Mg and P oscillations as shifts in hydroclimate and find that MIS 10 is characterized by precipitation variability at the multiannual, decadal, centennial, and millennial scale. The short-term oscillations are superimposed on an orbital-scale aridification trend observed in the Mg signal. Comparing our records with those from northern high latitude (north of our study area) and Asian monsoon regions, a consistent picture emerges ; an episode of relatively strong Siberian High (cold high latitude) from mid to late MIS 10 most likely caused a southward shift of the Westerlies and intertropical convergence zone (ITCZ), resulting in decreasing precipitation over the cave site and a weak Asian Monsoon respectively. In contrast, millennial-scale episodes of ice rafted debris (IRD) deposition in the North Atlantic correspond with wet phases in Central Asia. In conclusion, a strengthening of the Siberian High and resulting southward shift of the Westerlies and ITCZ provides a consistent and viable mechanism that reconciles northern high latitude, mid latitude (this study), and low latitude (Asian Monsoon) records. The forcings of the documented millennial-scale changes in Central Asian hydroclimate require further investigation but may be associated with the effects of North Atlantic freshwater perturbations on atmospheric circulation. On the other hand, multidecadal-scale oscillations in Central Asian precipitation may be linked to climate mode variability analogous to the modern North Atlantic Oscillation (NAO) and El Niño Southern Oscillation (ENSO).

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