5:15 PM - 7:15 PM
[MIS14-P09] Paleoenvironment change in the mid-latitudes of East Asia during the Holocene based on elemental and mineral variations of Lake Sangiim Dalai, northwestarn Mongolia.
Keywords:varves, Enviroment change, Paleoclimate
The Holocene has a stable climate with smaller temperature changes than the Last Glacial Maximum, climatic changes have been reported, such as Bond event (climatic change every 1500 years) associated with the Greenland Ice Sheet iceberg melting, and paleoenvironmental changes in Europe (Wanner et al., 2014) associated with the North Atlantic Oscillation (NAO) and westerly wind meandering. Bond event is also reported in East Asia. Paleoenvironmental variability due to monsoon and westerly winds synchronized with Bond event has also been reported in East Asia (Zhao and Yu, 2012). However, few studies have considered the effects of westerly winds and permafrost, which are important for the environment in the mid-latitudes of East Asia. In this study, we attempted to reconstruct paleoenvironmental changes in the mid-latitudes of East Asia during the Holocene based on elemental and mineral variations in lake sediment cores from Lake Sangiin Dalai in northwestern Mongolia.
The samples used in this study were borehole cores collected in March 2019 in Lake Sangiin Dalai (N49,15'42"; E98,55' 23"; elevation 1885 m; depth 25 m) ( 19SD01~05; total length ca. 20 m) and a gravity core (19SD-G4); the 19SD core consists of alternating layers of brown calcareous mud with distinct laminae and dark brown mud with indistinct laminae that develop about every 10 cm. The age model was constructed from 14C ages of soil TOC. Elemental composition variation at 0.5 mm intervals using an XRF core scanner (Itrax), mineral composition analysis of samples separated by 1 cm using an X-ray diffractometer (XRD), resin fixation by sampling from several lamina developmental sections, and micro-area elemental and mineral composition analysis using uXRF and uXRD were also conducted.
Comparing the elemental and mineral compositional characteristics throughout the Holocene with the European record of environmental variability (Wanner et al., 2014), we find that Si/Ti was generally high from 10.0 to 7.0 ka, and the shallow lake environment was rich in clastic particles. Three stratigraphic levels with high Ca/Ti were observed, and high Mg calcite was predominant. At 7.0 to 5.0 ka, lamina development was observed about every 350 years, corresponding to a high Mn/Fe ratio. At 5.0 to 2.8 ka, laminae develop about every 700 years and tend to have high Mg calcite and high S/Ti ratios. This period corresponds to the warm and humid environment of Europe, and is interpreted as the inflow of sulfur into lake Sangiin Dalai due to the thawing of permafrost. 2.8 to 1.0 ka, laminae developed about every 1000 years, and high Mg calcite was predominant, corresponding to the cooler, drier period of Europe. From 1.0 ka to the present, laminae developed about every 200 years and MHC predominated.
Next, based on the results of microdomain elemental and mineral composition analyses and fluorescence micrographs, we interpreted that the dark layer, which is rich in organic matter, and quartz, which is an inflow material, and the light layer precipitated carbonate minerals such as MHC and high Mg calcite, which correspond to the wet season in the dark layer and the dry season in the light layer. The average sedimentation rate of 0.32 mm/yr based on the age model and the average thickness of the lamina of 0.35 mm are in close agreement, and the mineral composition of the lamina shows seasonal variations between the dry and rainy seasons, indicating that the lamina development part of Lake Sangiin Dalai is varve. The thickness change of the dry season layer of the varve shows a periodicity of about 11 years. The 11-year periodicity is considered to be the sunspot variation and 200, 350, and 1000 year periodicity to be the long-term cycle of solar activity ( David.,2015;Raimund.,2007;Mercedes and Michael.,2009), it is suggested that the climatic influence of solar activity is also involved in the formation of varve in Lake Sangiin Dalai.
The samples used in this study were borehole cores collected in March 2019 in Lake Sangiin Dalai (N49,15'42"; E98,55' 23"; elevation 1885 m; depth 25 m) ( 19SD01~05; total length ca. 20 m) and a gravity core (19SD-G4); the 19SD core consists of alternating layers of brown calcareous mud with distinct laminae and dark brown mud with indistinct laminae that develop about every 10 cm. The age model was constructed from 14C ages of soil TOC. Elemental composition variation at 0.5 mm intervals using an XRF core scanner (Itrax), mineral composition analysis of samples separated by 1 cm using an X-ray diffractometer (XRD), resin fixation by sampling from several lamina developmental sections, and micro-area elemental and mineral composition analysis using uXRF and uXRD were also conducted.
Comparing the elemental and mineral compositional characteristics throughout the Holocene with the European record of environmental variability (Wanner et al., 2014), we find that Si/Ti was generally high from 10.0 to 7.0 ka, and the shallow lake environment was rich in clastic particles. Three stratigraphic levels with high Ca/Ti were observed, and high Mg calcite was predominant. At 7.0 to 5.0 ka, lamina development was observed about every 350 years, corresponding to a high Mn/Fe ratio. At 5.0 to 2.8 ka, laminae develop about every 700 years and tend to have high Mg calcite and high S/Ti ratios. This period corresponds to the warm and humid environment of Europe, and is interpreted as the inflow of sulfur into lake Sangiin Dalai due to the thawing of permafrost. 2.8 to 1.0 ka, laminae developed about every 1000 years, and high Mg calcite was predominant, corresponding to the cooler, drier period of Europe. From 1.0 ka to the present, laminae developed about every 200 years and MHC predominated.
Next, based on the results of microdomain elemental and mineral composition analyses and fluorescence micrographs, we interpreted that the dark layer, which is rich in organic matter, and quartz, which is an inflow material, and the light layer precipitated carbonate minerals such as MHC and high Mg calcite, which correspond to the wet season in the dark layer and the dry season in the light layer. The average sedimentation rate of 0.32 mm/yr based on the age model and the average thickness of the lamina of 0.35 mm are in close agreement, and the mineral composition of the lamina shows seasonal variations between the dry and rainy seasons, indicating that the lamina development part of Lake Sangiin Dalai is varve. The thickness change of the dry season layer of the varve shows a periodicity of about 11 years. The 11-year periodicity is considered to be the sunspot variation and 200, 350, and 1000 year periodicity to be the long-term cycle of solar activity ( David.,2015;Raimund.,2007;Mercedes and Michael.,2009), it is suggested that the climatic influence of solar activity is also involved in the formation of varve in Lake Sangiin Dalai.