5:15 PM - 7:15 PM
[HTT16-P03] Paleoenvironmental reconstruction in southwest Mongolian Plateau since the last glacial period based on stable isotope compositions of Lake Olgoi sediment
Keywords:lacustrine sediment, stable isotope ratio, Mongolian Plateau
The Mongolian Plateau has experienced at warming over the past 100 years, approximately four times faster than the global average (0.71°C/100 years; IPCC, 2007). Lake Olgoi is a hydrologically closed-saline lake without inflow or outflow at the southern foot of the Khangai Mountains. This region is situated in discontinuous permafrost zones (50–90%) at the southern fringe of the Siberian permafrost in Mongolia. Recently warming is afraid to result in permafrost thawing, followed by environmental degradation in this region. To understand the environmental dynamics in response to permafrost thawing, this study conducted stable isotope ratio measurements and chemical analyses of carbon, nitrogen, and sulfur in Lake Olgoi sediment cores, recorded from the Last Glacial Period to the Holocene.
A 7.5 m-length sediment core (17OLGOI02) was retrieved from the frozen lake using a drilling core sampler in 2017. The sediment samples were sliced every 4 cm, followed by freeze-drying and grinding. Biogenic silica (BioSi) content was determined by ICP–AES after extraction of BioSi with acid and alkali solutions. Contents and stable isotope ratio (δ13CTOC, δ15NTN, δ34STS) of total organic carbon (TOC), total nitrogen (TN), and total sulfur (TS) were measured by elemental analyzer and isotope ratio mass spectrometer, respectively. For the TOC analyses, the samples were treated with 1 M HCl for 6 h to remove inorganic carbon.
The TOC contents of Lake Olgoi sediment core vary between 0.01 and 5.47% (average 1.25 ± 1.45%), with higher values at core depths of 650-580 cm, 160-130 cm, and 120-0 cm, and lower values at depths of 750-650 cm, 580-160 cm, and 130-120 cm. Their comparison to the Greenland ice core δ18O (NGRIP, 2004) suggests that the TOC content variation approximately corresponds to Northern Hemisphere climate changes such as the Marine Isotope Stage 3 (MIS3; 5.7 to 3 ka), Last Glacial Maximum (LGM; 2.1 ka), Bølling/Allerød (BA; 1.2 ka) and Younger Dryas (YD; 1.2 ka). Therefore, our sediment core has 70-kyr geochemical records from the last glacial period to the present. The TN and BioSi contents of Lake Olgoi sediment core ranged from 0.01 to 0.36% (average 0.09 ± 0.10%) and from 0.01 to 9.74% (average 1.70 ± 1.73%), respectively, showing similar variation to TOC content. This means that biological productivity increased in the catchment and lake during the warm period, while it decreased during the cold period. The C/N molar ratio (average 13.7 ± 5.8 for cold period, 23.7 ± 2.0 for MIS3, 16.8 ± 1.8 for Holocene) and δ13CTOC values (average -17.3 ± 2.2‰ for cold period, -10.4 ± 0.4‰ for MIS3, -19.2 ± 0.7‰ for Holocene) indicate that the sedimentary organic matter was predominantly originated from phytoplankton during cold period, and C4 plants during MIS3, and a mixture of C3 plants and phytoplankton during Holocene. The TS content and δ34STS range from 0.01 to 1.08% (average 0.22 ± 0.20%) and from -11.8 to 5.7‰ (average -1.8 ± 4.5‰), respectively. The δ34STS value of surficial sediment was 2.8‰ and the δ34S value of SO42- in lake water was 16.2‰. This difference of 13‰ is attributed to isotopic fractionation by sulfate reduction in porewater. Positive δ34STS values were observed in four horizons which is corresponded to warm periods such as the Holocene and BA. If we assume that those warm periods have the same degree of isotopic fractionation as modern environments, the SO42- should have >10‰ higher than the δ34STS values. It is suggested that 34S-enriched SO42- input into lake resulted from enhanced sulfate reduction in the frozen soils in the watershed due to warming on the plateau.
A 7.5 m-length sediment core (17OLGOI02) was retrieved from the frozen lake using a drilling core sampler in 2017. The sediment samples were sliced every 4 cm, followed by freeze-drying and grinding. Biogenic silica (BioSi) content was determined by ICP–AES after extraction of BioSi with acid and alkali solutions. Contents and stable isotope ratio (δ13CTOC, δ15NTN, δ34STS) of total organic carbon (TOC), total nitrogen (TN), and total sulfur (TS) were measured by elemental analyzer and isotope ratio mass spectrometer, respectively. For the TOC analyses, the samples were treated with 1 M HCl for 6 h to remove inorganic carbon.
The TOC contents of Lake Olgoi sediment core vary between 0.01 and 5.47% (average 1.25 ± 1.45%), with higher values at core depths of 650-580 cm, 160-130 cm, and 120-0 cm, and lower values at depths of 750-650 cm, 580-160 cm, and 130-120 cm. Their comparison to the Greenland ice core δ18O (NGRIP, 2004) suggests that the TOC content variation approximately corresponds to Northern Hemisphere climate changes such as the Marine Isotope Stage 3 (MIS3; 5.7 to 3 ka), Last Glacial Maximum (LGM; 2.1 ka), Bølling/Allerød (BA; 1.2 ka) and Younger Dryas (YD; 1.2 ka). Therefore, our sediment core has 70-kyr geochemical records from the last glacial period to the present. The TN and BioSi contents of Lake Olgoi sediment core ranged from 0.01 to 0.36% (average 0.09 ± 0.10%) and from 0.01 to 9.74% (average 1.70 ± 1.73%), respectively, showing similar variation to TOC content. This means that biological productivity increased in the catchment and lake during the warm period, while it decreased during the cold period. The C/N molar ratio (average 13.7 ± 5.8 for cold period, 23.7 ± 2.0 for MIS3, 16.8 ± 1.8 for Holocene) and δ13CTOC values (average -17.3 ± 2.2‰ for cold period, -10.4 ± 0.4‰ for MIS3, -19.2 ± 0.7‰ for Holocene) indicate that the sedimentary organic matter was predominantly originated from phytoplankton during cold period, and C4 plants during MIS3, and a mixture of C3 plants and phytoplankton during Holocene. The TS content and δ34STS range from 0.01 to 1.08% (average 0.22 ± 0.20%) and from -11.8 to 5.7‰ (average -1.8 ± 4.5‰), respectively. The δ34STS value of surficial sediment was 2.8‰ and the δ34S value of SO42- in lake water was 16.2‰. This difference of 13‰ is attributed to isotopic fractionation by sulfate reduction in porewater. Positive δ34STS values were observed in four horizons which is corresponded to warm periods such as the Holocene and BA. If we assume that those warm periods have the same degree of isotopic fractionation as modern environments, the SO42- should have >10‰ higher than the δ34STS values. It is suggested that 34S-enriched SO42- input into lake resulted from enhanced sulfate reduction in the frozen soils in the watershed due to warming on the plateau.