Hongchun Li

δ13C–δ18C covariance as a paleohydrological indicator for closed-basin lakes

The relationship between δ18O and δ13C in a closed-basin lake is a function of hydrological change, vapor exchange, lake productivity, and total CO2 (or carbonate alkalinity) concentration. Covariance of δ18O and δ13C usually occurs in lake sediments under the condition of hydrological closure for time periods of the order of 5000 yr or longer. On shorter time scales, however, certain subtleties in the use of the δ13C–δ18O covariance as a hydrology history indicator must be recognized. The covariant trend may not be found in hyper-alkaline lakes because of the insensitivity of δ13C to lake volume changes. For less alkaline closed-basin lakes, covariance of δ13C–δ18O will result from relatively rapid increase or decrease of lake volume, with the rapid-decrease condition giving rise to heavy and narrow-ranged isotopic values for both δ13C and δ18O due to the coupled evaporation-productivity effect. When the lake volume remains stable (as is the case for an over-flowing open lake), poor δ13C–δ18O covariance will ensue, due to the effect of vapor exchange with the atmosphere. n nWe have used the measured δ13C–δ18O covariance in carbonate sediments of Mono Lake, California, to deduce paleo-hydrological and lake-water alkalinity (and salinity) variations and found the results in consonance with Holocene lake-level history derived from the δ18O and other information.

Evidence from central Mexico supporting the Younger Dryas extraterrestrial impact hypothesis

We report the discovery in Lake Cuitzeo in central Mexico of a black, carbon-rich, lacustrine layer, containing nanodiamonds, microspherules, and other unusual materials that date to the early Younger Dryas and are interpreted to result from an extraterrestrial impact. These proxies were found in a 27-m-long core as part of an interdisciplinary effort to extract a paleoclimate record back through the previous interglacial. Our attention focused early on an anomalous, 10-cm-thick, carbon-rich layer at a depth of 2.8 m that dates to 12.9 ka and coincides with a suite of anomalous coeval environmental and biotic changes independently recognized in other regional lake sequences. Collectively, these changes have produced the most distinctive boundary layer in the late Quaternary record. This layer contains a diverse, abundant assemblage of impact-related markers, including nanodiamonds, carbon spherules, and magnetic spherules with rapid melting/quenching textures, all reaching synchronous peaks immediately beneath a layer containing the largest peak of charcoal in the core. Analyses by multiple methods demonstrate the presence of three allotropes of nanodiamond: n-diamond, i-carbon, and hexagonal nanodiamond (lonsdaleite), in order of estimated relative abundance. This nanodiamond-rich layer is consistent with the Younger Dryas boundary layer found at numerous sites across North America, Greenland, and Western Europe. We have examined multiple hypotheses to account for these observations and find the evidence cannot be explained by any known terrestrial mechanism. It is, however, consistent with the Younger Dryas boundary impact hypothesis postulating a major extraterrestrial impact involving multiple airburst(s) and and/or ground impact(s) at 12.9 ka.

Climate and hydrology of the Last Interglaciation (MIS 5) in Owens Basin, California: isotopic and geochemical evidence from core OL-92

Abstract δ 18 O, δ 13 C, total organic carbon, total inorganic carbon, and acid-leachable Li, Mg and Sr concentrations on 443 samples from 32 to 83xa0m depth in Owens Lake core OL-92 were analyzed to study the climatic and hydrological conditions between 60 and 155xa0ka with a resolution of ∼200xa0a. The multi-proxy data show that Owens Lake overflowed during wet/cold conditions of marine isotope stages (MIS) 4, 5b and 6, and was closed during the dry/warm conditions of MIS 5a, c and e. The lake partially overflowed during MIS 5d. Our age model places the MIS 4/5 boundary at ca 72.5xa0ka and the MIS 5/6 boundary (Termination II) at ca 140xa0ka, agreeing with the Devils Hole chronology. The diametrical precipitation intensities between the Great Basin (cold/wet) and eastern China (cold/dry) on Milankovitch time scales imply a climatic teleconnection across the Pacific. It also probably reflects the effect of high-latitude ice sheets on the southward shifts of both the summer monsoon frontal zone in eastern Asia and the polar jet stream in western North America during glacial periods.

PRELIMINARY STUDY ON CLIMATIC SIGNALS OF STABLE ISOTOPES FROM HOLOCENE SPELEOTHEMS UNDER MONSOON CONDITION

Within the East Asian monsoon zone the change in oxygen isotope of Holocene stalagmite is mainly related to monsoon rain rather than to temperature. And carbon isotope can be taken as a comprehensive signal which contains the natural information and the information of anthropogenic activity. Isotopic analysis is also compared with the annual layer records.

Paleotemperature changes over the past 3000 years in eastern Beijing, China: A reconstraction based on Mg/Sr records in a stalagmite

The Mg/Sr ratio in a stalagmite, which is directly proportional toDMg/ca, the solid-liquid distribution coefficient of Mg, is proposed as a geochemical thermometer to estimate paleotemperature changes. The paleotemperature change in eastern Beijing over the past 3000 years has been reconstructed by using this thermometer in the stalagmite ZFFS-1 from Jingdong Cave. The records can be divided into two periods: 3000–2000 aBP, cool and wet, the air temperature averaging 9.8°C; and 2000 aBP-present, hot and dry, the air temperature being 1°C higher than the mean value (11.7°C) of the past 3000 years. During 500–200 aBP, the air temperature was about 1.2°C lower than that of the present, corresponding to the Little Ice Age in Europe. Like δ18O and δ13C, Mg/Sr (orDMg/Ca) reflects an obviously warming trend of the past 200 years. These results are supported by historic records.

δ13C, δ18O and deposition rate of tufa in Xiangshui River, SW China: implications for land-cover change caused by climate and human impact during the late Holocene

Abstract Measurements of δ13C and δ18O values of the riverine tufa samples dated by the accelerating mass spectrometry-14C method have been used for discussion of land-cover change in the catchment area of the Xiangshui River, SW China during the late Holocene. The results show that tufa was deposited in this short river only between c. 4280 and c. 110 years BP. Based on the characteristics of δ13C values of the complete tufa profile in the river, three stages of land-cover conditions in the headwater area could be identified. The earliest, Stage I, contained the most extensive vegetation cover with mainly C3 plants, as shown by the lightest δ13C value, whereas the latest, Stage III, had the least land cover, reflected by the heaviest δ13C value. By comparison of speleothem and historical records, it was found that the land cover in Stage I was controlled mainly by climate change, whereas the land-cover changes in the later two stages were most probably related to major human disturbance (land use), especially since the Qin Dynasty and Ming Dynasty, in the headwater area of the river.

Age dating of lacustrine sediments in Lake Tianshuihai of the Tibetan Plateau by using U-series isochron method and its significance

The age of the clastic-clay sediments with varied content of carbonate in core TS95 taken from the Lake Tianshuihai in the northwest of the Tibetan Plateau was determined successfully by using the total sample dissolution U-series isochron method for the first time. With this method, the serial absolute timescale (isotope time scale) covered the last 240 ka in the main body of the Tibetan Plateau with a sea level up to 4000 m is also established for the first time, thereby laying the foundation for the research on the paleoclimate and palaeoenvironment in the Lake Tianshuihai region as well as the relationship between evolution of the Tibetan Plateau and global changes. In addition to a brief introduction of the basic principles, the procedures of experiments, and the results, this note also presents the adaptability and superiority of this method as well as its possible problems.