CAS scientists have made progress in investigating the past atmospheric changes by retrieving oxalate records from an ice core of Tianshan glaciers. An analysis shows that the variation of the organic acid mirrors the history of the regional economic development as well as environment protection in the west China over the past 40 years.
The work was conducted by Prof. Lee Xinqing from the State Key Laboratory of Environmental Geochemistry at the CAS Institute of Geochemistry in Guiyang in collaboration with Prof. Qin Dahe from the Laboratory of Ice Core and Cold Regions Environment at the CAS Cold and Arid Regions Environmental and Engineering Research Institute.
Oxalic acid is the simplest dicarboxylate in the troposphere. Despite low concentration, it acts as cloud condensation nucleus as well as other forms of organic acids in the atmosphere. Oxalic acid exists in the air mainly in the form of aerosols, according to Prof. Lee. It can be scavenged by precipitation and accumulated year after year on ice sheets or glaciers, which, due to their low temperature, preserve the organic compound from being decomposed by microbes. Assuming the knowledge between the atmosphere and ice, changes of the organic acid in the atmosphere can be reconstructed by analyzing the ice sequences formed in hundreds of thousands of years.
The atmospheric oxalate can come directly from forest fires, automobile exhausts and soil emissions; or indirectly from the photochemical reactions of unsaturated hydrocarbons, to which human activities such as production and livelihood and the growth of vegetations are both principal contributors. These inputs leave their fingerprints in organic acids in snow and ice in high resolution.
The study on organic acid records is at the cutting-edge of ice core-related geochemistry in the world. Although the past two decades have witnessed some research efforts on the acids in ice and snow of both Antarctica and Greenland, few studies were undertaken on ice cores of mid- and low-latitude mountain glaciers.
With the support of the National Natural Science Foundation of China and CAS, Prof. Lee and colleagues made a two-year study into the chemical parameters of an ice core from Glacier 1, a mid-latitude alpine glacier at the headwater of the Urumqi River in the Tianshan Mountains in northwest China. Their analyses reveal that the general characteristic of the oxalate record of the ice core over the past 43 years features some spikes against a background value. The background is found to be about 1 ng/g and even lower than the detection limits in some cases. On the other hand, the majority of the spike values reaches or surpasses 10 ng/g. The average content of oxalate is 3.6 ± 9.2 ng/g, more than 70% of which consist of spike values. The late 1950s was a period of low values, being slightly higher than the background level, while by the 1960s and 1970s, the value reached its climax, averaging 5.0 ng/g. During the 1980s, the average value once again went down to the level of the late 1950s. By 1990s, it rebounded to the background.
A study on the economic development landscape shows that a steel mill and a cement plant were put in operation 50 km away in the valley of the Urumqi River in 1958, whereas 105 km away lies the metropolitan Urumqi, the capital city of the Xinjiang Uyger Autonomous Region. The anthropogenic atmospheric pollutants can be readily carried to the No.1 glacier under favorable wind conditions. Since the 1990s, a concerted development between the economic growth and environmental protection was seen in the vicinity, leading to reduction of the atmospheric pollutants. The general trend of the organic acids in the past 43 years turns out to be consistent with the itinerary of the dual development in the regional industrial growth and environmental protection. Also, this consistency is verified by the data from the local bureau of environmental protection.
Retrieval of oxalate records from ice cores can shed light on its biogeochemical cycles, and is helpful to understanding of past environment changes, notes Prof. Lee. Due to its proximity to the sources, the mid- and low-latitude glaciers are higher than the polar ice sheets in concentration of the organic species. The advantage attracted more and more attention from the polar ice sheets to the alpine glaciers in recent years. In the ALPCLIM program focusing on Alps glaciers and jointly funded by EU countries from 1998, for example, retrieval of organic acid records constitutes an important part in its research priorities.
