Ruilin Wen

Holocene weak monsoon intervals indicated by low lake levels at Hulun Lake in the monsoonal margin region of northeastern Inner Mongolia, China

A 170 cm long sediment core spanning the last 10 000 years was recovered from Hulun Lake in the northeastern margin of the East Asian summer monsoon. The sediment core was analyzed at 1 cm intervals for grain-size distribution and sedimentary component partition. These data provide a proxy record of the monsoon variability on millennial to centennial scales during the Holocene. We used a lognormal distribution function fitting method to partition three to six components from fine to coarse modes within the individual polymodal distributions into overlapping lognormal distributions. Three coarse components representing nearshore suspension, saltation and traction, together with the sand-fraction percentage and the median grain size of bulk samples, indicate the lake levels that fluctuated in response to the intensity of the monsoonal precipitation. Higher percentages of the nearshore components accompanied by more sand-fraction proportions and coarser median grain sizes reflect lower lake stands resulting from weaker monsoon circulations. The results show low levels at Hulun Lake c. 8000—7850, 6400—6050, 5150—4900, 4500—3800, 3050—2800, 1650—1400, 1150—900, 700—600, and 400—350 cal. yr BP, indicating the weakened East Asian summer monsoon during these intervals. We suggest that these weak monsoon events would not only result from the reduced ocean—atmosphere interacting processes in the western tropical Pacific, but could also be related to cold climatic conditions in the Northern Hemisphere high latitudes.

Spatial heterogeneity of the population age structure of the ostracode Limnocythere inopinata in Hulun Lake, Inner Mongolia and its implications

Most existing studies of lacustrine ostracodes have been focused on species composition, and little attention has been paid to population structure. In this study, 39 surface-sediment samples from Hulun Lake, Inner Mongolia, were analyzed for ostracode species composition and 17 of them also for population structure of Limnocythere inopinata, which is the dominant ostracode in the lake. A total of nine ostracode species were recovered, most of which show high abundances at sites with rich plant detritus, implying the possible control of food supply on ostracode species occurrence in the lake. On the other hand, both the subfossil and the living populations of L. inopinata have greater numbers of early instars in deeper waters, while middle and late instars occur mostly in shallower waters, which is related to the grainsize composition of the substrate, denoting a hydraulic control on population structure. Pre- and post-mortem transport of ostracodes may be responsible for the observed within-lake changes in the population structure, although other factors such as migration, intrinsic population dynamics, and life cycle could also be involved. Our data provide insights into the interpretation of lake conditions when using ostracodes in paleolimnological research of lakes with similar natural settings.

Differential transport and preservation of the instars of Limnocythere inopinata (Crustacea, Ostracoda) in three large brackish lakes in northern China

Population age structure, preservation, and carapace abundance of the valve remains of Limnocythere inopinata in 88 surface-sediment samples from Lakes Daihai, Dali, and Hulun were examined to better understand taphonomy of limnic ostracods. The spatial changes in population structure of L. inopinata match the within-lake hydraulic gradients, with the older instars preferentially deposited in the littoral zone while the younger instars being more abundant in deeper waters. We propose that molt and mortem remains of ostracods, especially those of the small juveniles, experience downslope transport. The magnitude of transport in a lake is controlled by wind-driven hydrodynamics. There is no chemical dissolution of ostracod valves in these alkaline lakes, and degradation is mainly represented by physical breakage. The preservation of L. inopinata is related to sedimentation rate and hydraulic stability, and the spatial pattern of preservation varies between lakes. Spatial and seasonal changes in sedimentation rate and hydraulic condition, and the ostracod life cycle may have contributed to this complexity. Adult and A-7 carapaces are the most frequent among the instars. We suggest that the preferential preservation of juvenile carapaces is either related to high mortality rate or to the good preservation in deep waters.

The manifestation of the Younger Dryas event in the East Asian summer monsoon margin: New evidence from carbonate geochemistry of the Dali Lake sediments in northern China:

The processes and mechanisms of the Younger Dryas (YD) event in the modern northern margin of the East Asian summer monsoon (EASM) are still heatedly debated. This study presents new high-resolution (~25 years) records of elements and stable isotopes of <38-µm calcites from a sediment core from Dali Lake in order to investigate the climatic change in the EASM margin at the last glacial–interglacial transition. The <38-µm calcites in the Dali Lake sediments are cubical or rhombohedral, implying that they are predominated by endogenic calcites precipitated within the water body of the lake. High values of Ca and Mg concentrations of the endogenic calcites are interpreted as strong evaporation and low dissolved CO2 concentration of the lake water related to high regional temperature. Concurrent increases in δ13C and δ18O values of the endogenic calcites are interpreted as the result of intensified evaporation associated with high temperature or low precipitation in the region. These data indicate that the climate in the Dali Lake region was relatively warm and wet from 15,500 to 12,800 and from 11,550 to 10,000 cal. yr BP, and cold and dry from 12,800 to 11,550 cal. yr BP, which was generally supported by the evidence from the data of sedimentary organic matter from the same sediment core. In addition, the abruptness of the temperature change in the Dali Lake region from 12,800 to 11,550 cal. yr BP could be corresponded, within age uncertainties, to the YD cold reversal occurring over northern high latitudes. The atmospheric coupling between the North Atlantic region and the EASM margin was proposed as the dominant pattern influencing the climatic change in the Dali Lake region during the YD event.

Spatial patterns of vegetation and climate in the North China Plain during the Last Glacial Maximum and Holocene climatic optimum

Reconstructing the spatial patterns of regional climate and vegetation during specific intervals in the past is important for assessing the possible responses of the ecological environment under future global warming scenarios. In this study, we reconstructed the history of regional vegetation and climate based on six radiocarbon-dated pollen records from the North China Plain. Combining the results with existing pollen records, we reconstruct the paleoenvironment of the North China Plain during the Last Glacial Maximum (LGM) and the Holocene Climatic Optimum (HCO). The results show that changes in the regional vegetation since the LGM were primarily determined by climatic conditions, the geomorphic landscape and by human activity. During the LGM, the climate was cold and dry; mixed broadleaf-coniferous forest and deciduous-evergreen broadleaf forest developed in the southern mountains, and cold-resistant coniferous forest and mixed broadleaf-coniferous forest were present in the northern mountains. The forest cover was relatively low, with mesophytic and hygrophilous meadow occupying the southern part of the plain, and temperate grassland and desert steppe were distributed in the north; Chenopodiaceae-dominated halophytes grew on the exposed continental shelf of the Bohai Sea and Yellow Sea. During the HCO, the climate was warm and wet; deciduous broadleaf forest and deciduous-evergreen broadleaf forest, with subtropical species, developed in the southern mountains, and deciduous broadleaf forest with thermophilic species was present in northern mountains. Although the degree of forest cover was greater than during the LGM, the vegetation of the plain area was still dominated by herbs, while halophytes had migrated inland due to sea level rise. In addition, the expansion of human activities, especially the intensification of cultivation, had a significant influence on the natural vegetation. Our results provide data and a scientific basis for paleoclimate modelling and regional carbon cycle assessment in north China, with implications for predicting changes in the ecological environment under future global warming scenarios.