Holocene environments and climate in the Mongolian Altai reconstructed from the Hoton-Nur pollen and diatom records: a step towards better understanding climate dynamics in Central Asia

This study presents the results of the palynological and diatom analyses of the sediment core recovered in Hoton-Nur Lake (48°37′18″N, 88°20′45″E, 2083 m) in 2004. Quantitative reconstruction of the Holocene vegetation and climate dynamics in the semiarid Mongolian Altai suggests that boreal woodland replaced the primarily open landscape of northwestern Mongolia at about 10 kyr BP (1 kyr = 1000 cal yr) in response to a noticeable increase in precipitation from 200–250 mm/yr to 450–550 mm/yr. A decline of the forest vegetation and a return to a predominance of open vegetation types occurred after 5 kyr BP when precipitation sums decreased to 250–300 mm/yr. Prior to 11.5 kyr BP diatom concentrations are relatively low and the lake is dominated by planktonic Cyclotella and small Fragilariaceae, suggesting the existence of a relatively deep and oligotrophic/mesotrophic lake. The great abundance of Staurosirella pinnata from the beginning of the record until 10.7 kyr BP might imply intensified erosion processes in the catchment and this is fully consistent with the presence of scarce and dry vegetation and the generally arid climate during this period. From about 10.7 kyr BP, more planktonic diatom taxa appeared and increased in abundance, indicating that the lake became more productive as diatom concentration increased. This change correlates well with the development of boreal woodland in the catchment. Decrease in precipitation and changes in the vegetation towards steppe are reflected by the rapid increase in Aulacoseira distans from about 5 kyr BP. The Holocene pollen and diatom records do not indicate soil and vegetation cover disturbances by the anthropogenic activities, implying that the main transformations of the regional vegetation occurred as a result of the natural climate change. Our reconstruction is in agreement with the paleomonsoon records from China, demonstrating an abrupt strengthening of the summer monsoon at 12 kyr BP and an associated increase in precipitation and in lake levels between 11 and 8 kyr BP, followed by the stepwise attenuation of the monsoon circulation and climate aridization towards the modern level. The records from the neighboring areas of Kazakhstan and Russia, situated west and north of Hoton-Nur, demonstrate spatially and temporally different Holocene vegetation and climate histories, indicating that the Altai Mountains as a climate boundary are of pivotal importance for the Holocene environmental and, possibly, habitation history of Central Asia.     

Fractures and Breakouts Analysis from Borehole Image Logs: Preliminary Results for Interpretation of Stress History and In Situ Stress State

Fractures and borehole breakouts from image data acquired from Logging-While-Drilling (LWD) were identified and analyzed using GMI Imager software. Conductive (e.g., dark-colored images) and resistive fractures (e.g., light-colored images) were identified on the images. Breakouts occurring along the borehole wall in the direction of the minimum horizontal stress were also investigated. For fracture analysis, we investigated dip and direction of fractures on the resistivity images acquired from two sites in the Ulleung Basin, East Sea. Dip angles at two sites are 42° and 62.5° on average, respectively. Dip direction shows preferred orientation northerly. From fracture analysis, the maximum horizontal stress direction may be the NW-SE direction. This pattern likely reflects regional stress regime of this area. We also analyzed borehole breakouts on the LWD image data (borehole radius and density data) acquired from Site U1378, IODP Exp. 334 off Costa Rica. We estimated present-day in situ stress orientation from borehole breakouts. Breakout orientation of slope sediments at Site U1378 indicates that maximum horizontal principal stress direction is oriented northwest-southeast. This direction is probably related to plate motion in this area. This study presents preliminary results in order to interpret not only stress history of the Ulleung Basin but also in situ stress state of continental slope off Costa Rica in near future.     

Estimating fractional green vegetation cover of Mongolian grasslands using digital camera images and MODIS satellite vegetation indices

ABSTRACT

Fractional green vegetation cover (FVC) is a useful indicator for monitoring grassland status. Satellite imagery with coarse spatial but high temporal resolutions has been preferred to monitor seasonal and inter-annual FVC dynamics in wide geographic area such as Mongolian steppe. However, the coarse spatial resolution can cause a certain uncertainty in the satellite-based FVC estimation, which calls attention to develop a robust statistical test for the relationship between field FVC and satellite-derived vegetation indices. In the arid and semi-arid Mongolian steppe, nadir pointing digital camera images (DCI) were collected and used to produce a FVC dataset to support the evaluation of satellite-based FVC retrievals. An optimal DCI processing method was determined with respect to three color spaces (RGB, HIS, L*a*b*) and six green pixel classification algorithms, from which a country-wide dataset of DCI-FVC was produced and used for evaluating the accuracy of satellite-based FVC estimates from MODIS vegetation indices. We applied three empirical and three semi-empirical MODIS-FVC retrieval models. DCI data were collected from 96 sites across the Mongolian steppe from 2012 to 2014. The histogram algorithm using the hue (H) value of the HIS color space was the optimal DCI method (r² = 0.94, percent root-mean-square-error (RMSE) = 7.1%). For MODIS-FVC retrievals, semi-empirical Baret model was the best-performing model with the highest r² (0.69) and the lowest RMSE (49.7%), while the lowest MB (+1.1%) was found for the regression model with normalized difference vegetation index (NDVI). The high RMSE (>50% or so) is an issue requiring further enhancement of satellite-based FVC retrievals accounting for key plant and soil parameters relevant to the Mongolian steppe and for scale mismatch between sampling and MODIS data.     

蒙古全新世气候变化研究进展

简要地回顾了蒙古全新世气候变化研究。蒙古全新世气候变化研究侧重于对树轮、风成沉积和湖泊沉积物的研究。蒙古的树轮研究恢复了杭爱山（Hangai）地区过去1 783年以来的温度变化及蒙古中东部地区过去340多年以来的降水和径流量变化。蒙古全新世风成沉积主要分布在蒙古北部的走廊地带,全新世古土壤的层数可以和中国北方地区的古土壤层数对比。蒙古的湖泊研究显示蒙古全新世气候演化存在2种模式,即冷干—暖湿型和冷湿—暖干型,这2种模式以杭爱山为分界线。这说明蒙古全新世气候变化的复杂性,其水汽来源和水热组合存在差异。迄今为止,蒙古全新世气候变化的研究仍较薄弱,有待进一步加强研究。     

Fracture characteristics and stress directions in the quaternary sediments of the Ulleung Basin,East Sea: Based on resistivity image log data

Abstract

Geophysical evidence indicating the presence of gas hydrate has been found in the Ulleung Basin, which lies off the east coast of the Korean Peninsula; however, hydrate distribution in the basin is not well understood. Logging-while-drilling data for 13 sites in the Ulleung Basin, East Sea, were obtained to investigate the distribution pattern of gas hydrate. Most of the sites yielded log data indicating the presence of gas hydrate. Prominent fractures (both resistive and conductive fractures) were clearly identified on the resistivity borehole images, particularly at seismic chimney sites. Resistive fractures, which contain large amounts of gas hydrate, are prominent in the seismic chimney sites. The strike and dip of each fracture was calculated and displayed on a stereographic plot and rosette diagram. From the fracture orientations on the stereographic plots, the maximum horizontal stress is NW–SE, reflecting the regional stress regime around the Ulleung Basin, although the fracture orientations are broadly distributed, indicating that the fracture pattern is not well-ordered on the rosette diagram. The fracture dips are between 36.46° and 63.66°; the range of dip azimuths is 0.94°–359°, and exhibit little change with depth. The dip azimuths are generally westerly to southwesterly.     

Petrogenesis and Tectonic Significance of CarboniferousGranites on the North Side of the Solonker Suture,CentralSouth Mongolia

The central part of South Mongolia, located to the north of the Solonker Suture, is a key region for studying the late Paleozoic tectonic evolution of the Central Asian Orogenic Belt(CAOB). Voluminous late Paleozoic granitic rocks,especially of Carboniferous age, were intruded in this area. However, these granitoids have not been well studied and there is a lack of precise ages and isotopic data. This has hampered our understanding of the tectonic evolution of southeastern Mongolia, and even the entire CAOB. In this paper, we provide new U-Pb isotopic ages and geochemical analyses for these Carboniferous granites. One granite from the Ulaanbadrakh pluton yielded a zircon U-Pb age of 326 Ma, which indicates emplacement in the Early Carboniferous, and three other granites from the Khatanbulag region gave zircon U-Pb ages of316 Ma, 315 Ma, and 311 Ma, which indicate emplacement in the Late Carboniferous. The Early Carboniferous granite has SiO2 contents of 70.04–70.39 wt% and K2 O + Na2 O contents of 6.48–6.63 wt%, whereas the Late Carboniferous granites have more variable compositions(SiO2 = 65.29–77.91 wt% and K2 O + Na2 O = 5.30–7.27 wt%). All the granites are weakly-peraluminous I-types that are relatively enriched in U, Th, K, Zr, Hf, and LREEs. The whole rock Sr-Nd and zircon in situ Lu-Hf isotope analyses for the Early Carboniferous granite gave positive values of εNd(t)(2.87) and εHf(t)(4.31–12.37) with young Nd(TDM = 860 Ma) and Hf(TDMc = 1367–637 Ma) two-stage model ages, indicating derivation from juvenile crustal material. In contrast, the Late Carboniferous granites had more diverse values of εNd(t)(–4.03 to 2.18) and εHf(t)(–12.69 to5.04) with old Nd(TDM = 1358–1225 Ma) and Hf(TDMc = 2881–1294 Ma) depleted mantle two-stage model ages,suggesting derivation from remelting of Precambrian basement. Based on the existing results, the tectonic setting of the Late Carboniferous granites in the central part of South Mongolia is known for its diversity, and this paper believes that the tectonic background of the carboniferous granite records the tectonic transition from a continental-margin-arc to a postcollisional extensional setting during the Late Carboniferous–Permian.