Spatial variability of 14C reservoir effects in Tibetan Plateau lakes
Introduction
The significance of late glacial and Holocene climate inferences from lake sediment analyses largely depends on the reliability of the chronological data. Environmental and climate reconstructions based on lake records from relatively dry and cold regions such as the Tibetan Plateau are commonly hampered by the sparse terrestrial vegetation in the catchments and the resulting lack of terrestrial plant remains in lake sediments suitable for radiocarbon dating. As a consequence of lacking terrestrial plant matter and an alternative standard dating method in addition to radiocarbon dating, the establishment of age–depth relationships for sediment cores from Tibetan Plateau lakes is often based on the assessment of the present-day LRE (Henderson and Holmes, 2009; Watanabe et al., 2010; Mischke et al., 2010a). Hou et al. (2012) summarized LRE data for 21 lakes on the Tibetan Plateau and discussed the spatial and temporal pattern of LREs for this region. They showed that the LRE is mostly in the range from 1000 to 3000 14C years and cannot be neglected in studies which more and more address comparisons of regional sites, correlations with distant climate archives such as Greenland ice core records and marine sediment cores, and the leads and lags in the climate and environmental systems. Hou et al. (2012) concluded that the catchment geology, the residence time of lake water and the presence of peat or wetlands in the catchment are the most important parameters which control the order of the LRE in a specific lake setting. However, it remains unclear whether the LRE varies significantly between different sites within individual lakes and if so, whether a specific spatial pattern for the distribution of LREs in a lake can be expected.
This paper discusses and compares newly obtained radiocarbon data from Lake Donggi Cona with earlier published results of the LRE assessment from the same lake and five other lakes on the Tibetan Plateau (Fig. 1). The research questions of the study are: 1) are there significant differences for LREs within different individual lakes; 2) if so, is there a common spatial pattern; 3) what are the major controls of LREs on Tibetan Plateau lakes; and 4) what are appropriate approaches for the determination of present day and past LREs?
Section snippets
Study area
Lake Donggi Cona (35.3°N, 98.5°E, 4090 m) is an open-basin lake in the northeastern part of the Tibetan Plateau. Peak elevations of nearby mountains reach 5000 m altitude. Permian and Triassic limestones, sandstones and siltstones dominate in the catchment and are exposed along the northern and southern lake shores (Wang and Yang, 2004). The lake basin represents a pull-apart structure of the Kunlun Fault which is one of the large strike–slip faults of the Tibetan Plateau (Van der Woerd et al.,
Materials and methods
Water samples from springs, streams and Lake Donggi Cona were collected in October 2011 (Table 1). Water samples were collected close to lake shores or stream banks from 0.1 to 0.4 m depth below the water level apart from a lake sample collected from bottom waters at 15 m depth with a water sampler. Samples for cation and anion analysis were filtered through 0.45 μm cellulose acetate. Three drops of concentrated nitric acid were added to the cation samples. Water samples were stored in 60 ml
Results
The results of the water chemistry analyses are presented in Table 1 and as Electronic Supplementary Materials, S1 and S2. The samples 3–5 from the lake and its outflow show a low variability of all chemical parameters apart from nitrate (Electronic Supplementary Material, S1). The conductivity of the major inflow, the two lake-water samples and the outflow ranges from 627 to 802 μS cm−1. The minor inflow and the two springs at the northern shore have conductivities of 1225 and ca. 2000 μS cm−1
Water chemistry and LREs of Lake Donggi Cona
The low variability of the water chemistry data for samples from the lake (samples 3–4) and the outflow (sample 5) shows that Lake Donggi Cona was a well-mixed, open basin lake during the time of sampling. Ca/Na and Mg/Na ratios > 1 for water from the major inflow suggest that carbonate weathering plays a stronger role in its catchment in comparison to that of the minor inflow (Gaillardet et al., 1997). This result reflects differences in catchment geology since the catchment of the major
Conclusions
Compilation and discussion of present-day LREs for sites within individual lake and catchment areas on the Tibetan Plateau showed that:
- 1)
the LREs can display large differences of up to 19,000 14C years among different sites within individual lakes,
- 2)
the LREs are apparently lower in the central parts of the lakes and increase towards its shores,
- 3)
the lower LREs in the lake centers result from more prolonged exchange of DIC of lake water with atmospheric CO2 whilst the highest LREs at spring sites
Acknowledgements
Catharina Clewing and Parm von Oheimb kindly provided three Radix shells from Lake Donggi Cona. We thank Paula Reimer and two anonymous reviewers for very helpful comments. Funding was provided by the German Research Foundation (DFG).
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