Dayou Zhai

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.

On some Recent non-marine ostracods from northern China, with description of one new species

Preliminary investigations of urban waters in Beijing City and small water-bodies in Inner Mongolia yielded a total of 12 living ostracod species. Five species are new records for China: Candona quasiakaina Karanovic & Lee, 2012, Fabaeformiscandona myllaina Smith & Kamiya, 2007, Heterocypris auricularis sp. nov. (previously known as Heterocypris sanukiensis Okubo, 2004 nomen nudum), Ilyocypris mongolica Martens, 1991 and Plesiocypridopsis newtoni (Brady & Robertson, 1870). Bradleycypris vittata (Sars, 1903), Ilyocypris angulata Sars, 1903, Limnocythere stationis Vavra, 1891 and Physocypria kraepelini G. W. Muller, 1903 are the first records for Palaearctic China. Cypridopsis vidua (O. F. Muller, 1776), Heterocypris incongruens (Ramdohr, 1808) and Limnocythere inopinata (Baird, 1843) are also reported in this paper, although they had already been found in China before. Of the above species, B. vittata, I. angulata and I. mongolica are re-described. The present study expands our knowledge on the taxonomy and geographical distribution of ostracod species.

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.

Cluster analyses of Ostracoda based on dimensions of body structures: implications for taxonomic classification

We measured selected podomeres, setae and claws in different ostracods and calculated the between-specimen morphological difference, which is expressed as a Canberra dissimilarity index. Our data indicate that morphological differences between ostracods increase with their taxonomic distance. Cluster analyses of ostracod specimens based on Canberra dissimilarity are able to discriminate different species and concur with existing classifications. We suggest that the dimensions of body structures are taxonomically valuable, and that ostracod species identification can be assisted based on the dimensional data of body structures. Species discrimination with such a method does not rely on explicit morphological hiatuses, such as the presence/absence of particular setae, but instead utilizes measurable morphological differences. Our numerical methods also show good potential for studying phenotypic diversity. Analyses on ostracod populations from isolated temporary pools and those from permanent but geographically distant habitats indicate that dispersal improbability is responsible for the observed morphological differentiation.