Spatial variability of 14C reservoir effects in Tibetan Plateau lakes

doi:10.1016/j.quaint.2013.01.030

Quaternary International

Volumes 313–314, 5 November 2013, Pages 147-155

https://doi.org/10.1016/j.quaint.2013.01.030 Get rights and content

Abstract

Radiocarbon dating of lake sediments is often hampered by the presence of a lake reservoir effect (LRE, also ‘dead carbon’ or ‘old carbon’ effect) especially in dry and cold regions with a sparse plant cover in the catchment. The Tibetan Plateau became a hotspot of the palaeoenvironmental and climate research community in recent years and the assessment of present-day LREs is a crucial prerequisite for the establishment of reliable radiocarbon age–depth relationships for lake sediment cores. This paper examines the spatial variability of LREs within individual lakes, through a discussion of new data for Lake Donggi Cona and a compilation of previously published data for five additional lakes where LRE data are available for different sites. Lake reservoir effects for Lake Donggi Cona on the northeastern Tibetan Plateau were determined for shells of aquatic snails collected alive close to the lake's shore. The largest determined LRE of 20,000 ¹⁴C years is significantly larger than previously reported LREs from the central part of the lake, and larger than any previously published LRE for the Tibetan Plateau. Relatively low LREs in the central regions of lakes, higher LREs towards the margins, and high LREs in tributaries and spring waters are apparently a common pattern of Tibetan Plateau lakes. The differences in LREs within individual lakes or catchment areas are attributed to the more prolonged exchange of the lake water's dissolved inorganic carbon with the atmospheric CO₂ in central lake regions on the one hand and the increasing influence of ¹⁴C free or poor stream and groundwater due to the dissolution of carbonaceous basement rocks towards its margins. Generally higher LREs were recorded in the three tectonically active lake regions of the six examined catchments, and it is speculated that rising crustal CO₂ further contributes to the LREs in these catchments. In addition to these observations and inferences, elevated ¹⁴C levels of the atmosphere as a result of nuclear bomb testing are often ignored if LREs for modern materials are reported by convention relative to the atmospheric ¹⁴C activity of the year 1950. LRE data reported in this way represent unrealistic minimum estimates.

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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Spatial variability of 14C reservoir effects in Tibetan Plateau lakes

Abstract

Introduction

Section snippets

Study area

Materials and methods

Results

Water chemistry and LREs of Lake Donggi Cona

Conclusions

Acknowledgements

Quaternary International

Palaeogeography, Palaeoclimatology, Palaeoecology

Quaternary Science Reviews

Chemical Geology

Quaternary International

Quaternary Science Reviews

Quaternary Science Reviews

Journal of Archaeological Science

Science of the Total Environment

Quaternary International

Palaeogeography, Palaeoclimatology, Palaeoecology

Quaternary Geochronology

Chemical Geology

Palaeogeography, Palaeoclimatology, Palaeoecology

Palaeogeography, Palaeoclimatology, Palaeoecology

Quaternary Research

Palaeogeography, Palaeoclimatology, Palaeoecology

Chemical Geology

Quaternary Science Reviews

Quaternary Geochronology

Tracing the carbon cycle

Catchment instability and Asian summer monsoon variability during the early Holocene in southwestern China

Boreas

Spatial variability of ¹⁴C reservoir effects in Tibetan Plateau lakes