Mongolia, located in Central Asia, experiences the most continental climate on Earth. Although detailed paleoclimate data are abundant for selected areas throughout Asia, datasets from within Mongolia are relatively scarce and demonstrate significant spatial and temporal variability. In older paleoclimatic reconstructions Mongolian climate was thought to be controlled by the East Asian monsoon system in a similar manner to China to the south. However in recent decades, additional paleoclimate records throughout central Asia have indicated a climate asynchronous with China, dominated by the Westerlies rather than the East Asian monsoon. The objective of this thesis study is to supplement existing paleoclimate proxy records in order to better understand Holocene climate variability in Mongolia and to assess the role of the Westerlies in producing that variability.
This study produces new bulk-carbonate isotopic and mineralogic records for north-central Mongolia from a 7,110 year Lake Telmen and a 5,100 year Lake Ugiy sediment core. Lake Telmen is a saline (4 g/l), closed-basin lake, ideal for amplifying hydrologic variability into large sedimentological responses and is thus well-suited for paleoclimate study. The relatively heavy δ18O carbonate stable isotopes (averaging -0.5 ‰) and abundant ankerite, quartz and phillipsite minerals suggest an arid climate dominated prior to approximately 4,500 yr B.P. Since 4,500 yr B.P., the Lake Telmen sediment data suggests an increase in humidity evidenced by lighter δ18O values (averaging -1.2 ‰) with a corresponding mineralogical shift to abundant monohydrocalcite and calcite. These new records are in agreement with previous Lake Telmen paleoclimate interpretations based upon pollen, diatoms and lithology, suggesting the East Asian monsoon does not control the Holocene climate in the Telmen region. The Lake Ugiy sediment core mineralogy supports this interpretation, revealing abundant terrigenous minerals, a shallow lake, and more arid conditions prior to approximately 4,000 yr. B.P. A more humid climate since 4,000 yr B.P. is based on lacustrine mud with calcite and Mg calcite inferred to represent deeper water conditions. Brief arid intervals occur during the otherwise humid late Holocene as inferred from heavy isotope values and ankerite, quartz and phillipsite abundance in Lake Telmen, and abundant Mg calcite in Lake Ugiy at about 1,300-1,600, 2,000-2,200 and 2,800-3,100 yr B.P.
The records from this thesis study support published interpretations that suggest central Asian Holocene climate is dominated by the Westerlies and is asynchronous with Chinese climate dominated by the East Asian monsoon. A North Atlantic high pressure system associated with cold sea surface temperature (SST) redirected the Westerly track causing the movement of dry air into Mongolia during the early Holocene. However, during the early Holocene increased boreal summer insolation produced a stronger East Asian summer monsoon system and increased moisture in China. By the late Holocene the disappearance of the Laurentide ice sheet allowed for increased North Atlantic SST and the development of a low pressure system. The more zonal Westerlies now carried warm moist air across Europe and into Mongolia thereby increasing the moisture balance in the late Holocene. During the late Holocene, North Atlantic SST periodically cooled due to ice rafting. The cold North Atlantic and altered atmospheric pressure gradient may have diverted the path of the Westerlies so that cold dry air flowed into Mongolia. These cold episodes are known as Bond cycles and included events dated at 1,400 and 2,800 yr B.P. These Bond cycles are similar in age to brief arid events within Mongolia and suggest hemispheric teleconnnection and a possible role for North Atlantic SST to influence Mongolian climate.