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Exposure age dating of Chinese tiankengs by 36Cl-AMS
Nuclear Inst. and Methods in Physics Research B 459 (2019) 29–35
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Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Exposure age dating of Chinese tiankengs by a,b,c,d,⁎
b
36
Cl-AMS
a,c,d
b
b
Hongtao Shen , Kimikazu Sasa , Qi Meng , Masumi Matsumura , Tetsuya Matsunaka , Seiji Hosoyab, Tsutomu Takahashib, Maki Hondab, Keisuke Suekib, Ming Hed, Baojian Huange, Huijin Lua,c, Lisha Chena,c, Yongfu Qina,c, Jiahao Lia,c, Haihui Lana,c, Zhaomei Lia,c, Zhenchi Zhaoa,c, Mingji Liua,c, Siyu Weia,c, Mingli Qia,c, Qingzhang Zhaod, Kejun Dongd, Yongjin Guanf, Xiangdong Ruanf, Shan Jiangd
T
a
College of Physics, Guangxi Normal University, Guilin 541004, China University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan Guangxi Key Laboratory of Nuclear Physics and Nuclear Technology, Guangxi Normal University, Guilin 541004, China d China Institute of Atomic Energy, P.O. Box 275(50), Beijing 102413, China e Institute of Karst Geology, Chinese Academy of Geological Sciences, Guilin 541004, China f College of Physics, Guangxi University, Nanning 530004, China b c
ARTICLE INFO
ABSTRACT
Keywords: Tiankeng 36 Cl Exposure age Erosion rate AMS
The Guangxi Zhuang Autonomous Region in southern China is one of the most typical karst relief areas with various forms, and tiankeng is one of the most typical karst relief forms of the Quaternary Period. In this work, accurate measurements of tiankeng exposure ages were made. The exposure ages of carbonate rocks from the five largest tiankengs in the world, the DaShiwei, HuangJing, ChuanDong, DaTuo, and DaCao tiankengs, were determined by accelerator mass spectrometry to be at least 20–400 thousand years, which provides significant proof for activity of new tectonic movement of the Eurasian continent in the Quaternary.
1. Introduction A tiankeng is a collapse doline that is more than 100 m deep and wide and has a steep profile with vertical cliffs surrounding its perimeter. The Guangxi Zhuang Autonomous Region of southern China, one of the most typical karst relief areas of the Late Quaternary Period, plays an important role in researching the surface shape and evolution of karst relief [1,2]. At present, the time of formation and evolutionary process of tiankeng are very poorly known and require further investigation to clarify the dispute between evolution theory and catastrophe theory [3,4]. The concentration of cosmogenic nuclides produced in situ in surface rock is dependent on the exposure time and erosion rate of the rock, which can be measured by long half-life radionuclides at a timescale from thousands of years to millions of years. Chlorine-36, a long-lived radionuclide found in the lithosphere and hydrosphere, has been used widely in different geological applications, such as the study of glacier activity, geomorphological processes, volcanism, meteorite impacts, flooding, and debris flow [5]. For example, Houmark et al. [6] used 36Cl to determine the exposed stage of ice erosion in Denmark, Matsushi et al. [7] used it to determine the rate of erosion in some areas of Japan, Zerathe et al. [8] measured the time ⁎
of landslides in the southern Alps and established a time series of landslides, and Wang et al. [9] determined the limestone erosion rate in the Beijing area. In situ 36Cl is now a reliable nuclide widely used to quantify surface processes in the geosciences. In the tiankeng area, the main component of the rock of karst relief is calcium carbonate, and calcium is the target element of a cosmic ray reaction that produces 36Cl, which has been shown to be a reasonable radionuclide for measuring the exposure time of limestone, because of its high production rate and rapid accumulation. In this study, 36Cl exposure dating was applied to karst tiankeng carbonate from DaShiwei Tiankeng Group National Geopark, Leye County, China. The purpose of the work was to provide basic data and establish research methods for investigating the formation, evolution, and local tectonic uplift rates of the DaShiwei Geological Area and hence for further research of the Chinese uplift belt, the collisions between the South Asian Subcontinent and the Eurasian Plate, and even the formation of the Himalayas and the Tibetan Plateau. In this work, concentrations of 36Cl in rock samples from the five of the largest tiankengs in the world, the DaShiwei, HuangJing, ChuanDong, DaTuo, and DaCao tiankengs, were measured at the AMS facility at the University of Tsukuba in an effort to estimate the time of formation of this unique karst relief.
Corresponding author at: College of Physics, Guangxi Normal University, Guilin 541004, China. E-mail address: [email protected] (H. Shen).
https://doi.org/10.1016/j.nimb.2019.07.006 Received 31 January 2018; Received in revised form 28 June 2019; Accepted 4 July 2019 Available online 03 September 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
Nuclear Inst. and Methods in Physics Research B 459 (2019) 29–35
H. Shen, et al.
Fig. 1. The four super tiankengs in China.
2. Materials and methods
avoid any coverage by rainwater or snow. Additionally, a blank sample was collected from a tunnel ~100 m below the surface of the mountain to estimate the background 36Cl.
2.1. Sites and sampling The study area is located in southwest China, 15 km west of the town of Guilin (Guangxi Zhuang Autonomous Region), at the first front of the tectonic uplift of China (southeastern external Tibetan Plateau). Since the early Miocene, this area with abundant groundwater resources has undergone several tectonic phases linked to Himalayan orogenesis [10,11]. The main resulting structures show that the tectonic uplift has caused underground rivers to cut down several hundred meters, forming large underground cave passages, which eventually collapse to forms this type of giant tiankeng, as shown in Fig. 1. In this study, rock samples were taken from the vertical inner walls of five of the largest tiankengs in the world, the DaShiwei, HuangJing, ChuanDong, DaTuo, and DaCao tiankengs, located in Leye County, Guangxi Zhuang Autonomous Region, China, as shown in Table 1. For each site, a rock core with a length of approximately 30 cm and diameter of approximately 10 cm was taken from a wall at a position ~100 m below the top of the cliff, as shown in Fig. 2. The outer 5 cm of the outer end of the rock core was taken for 36Cl-AMS analysis. The sampling sites for the sample collection were selected at locations where the rock had good structure and exposure, without any coverage of vegetation or gravel. The sampling sites were also high enough to
2.2. Sample preparation and
Latitude (°N)
Longitude (°E)
Altitude (m)
Weight (g)
9# Tunnel blank 20# HuangJing-TK 22# DaCao-TK 24# ChuanDong-TK 26# DaTuo-TK 32# DSW-E02
24°46′20.60″ 24°47′42.64″ 24°47′33.51″ 24°48′29.23″ 24°48′37.68″ 24°48′28.75″
106°32′44.22″ 106°31′15.19″ 106°30′37.38″ 106°29′20.70″ 106°28′33.34″ 106°26′47.99″
1008 1117 1171 1296 1246 1486
836.78 878.3 856.75 580.63 801.36 526.94
± ± ± ± ± ±
3 3 3 3 3 3
Cl extraction
The outer 5 cm of the rock column was detached for the chemical sample pretreatment. The rock pieces were crushed and sieved to a size range of 0.25–1 mm for AMS preparation (the < 0.25 mm fraction was used for whole-rock ICPMS analyses). Each sample was rinsed and sonicated until no fine dust could be washed from it. Afterward, it was rinsed several times in MQ water and then dried overnight in an oven at 70 °C. Approximately 35 g of dry sample and 100 mL of MQ water were transferred to a 400 mL glass conical beaker and swirled to wet the sample thoroughly. Two rounds of an 8 mL 4 mol/L HNO3 leaching procedure were performed to eliminate meteoric 36Cl and other potential contaminants, removing the outermost 10% of the grain surfaces. The acid was mixed in slowly to prevent foaming. Afterward, the grains were rinsed and sonicated in MQ water to ensure complete removal of Ca(NO3)2 and then dried overnight in an oven at 120 °C. Each etched sample (~20 g) was transferred to a 500 mL polypropylene bottle, spiked with Cl carrier solution (35Cl purity: 99 atom% mg; the highly enriched 35Cl carrier was used for the isotope dilution AMS technique, which enabled us to conduct a simultaneous measurement of 36Cl and Cl with a single sample preparation for AMS), soaked with ~50 mL of ultrapure water, and then dissolved gradually with ~110 mL of 4 mol/L HNO3 by separatory funnel. To avoid foaming, the acid was added in aliquots of ~10 mL every 30 min, and the bottle was not shaken or swirled until dissolution was almost complete. The sample bottle was kept loosely capped between additions of acid. After complete dissolution, the dark sample solution was decanted into a 175 mL centrifuge bottle through a 0.45 µm PTFE membrane vacuum filter. Then, ~250 µL of 0.3 M AgNO3 was added to the sample solution to precipitate AgCl. After sitting in the dark, the sample solution was
Table 1 Description of the tiankeng samples. Sample ID
36
30
Nuclear Inst. and Methods in Physics Research B 459 (2019) 29–35
H. Shen, et al.
Fig. 2. Core samples from the tiankengs. 36
Cl, 41Ca, and 129I) [12,13]. The details of the 36Cl-AMS measurements were as follows. The AgCl sample powders were pressed into a large Cu sample holder (inner diameter of 6 mm) with a compact AgBr backing in a 20-sample copper target wheel, by which the isobar 36S within the target could be further suppressed [14]. The Cl− anions were extracted, selected by an injection magnet, and injected into the accelerator. At the low-energy side, the ion injector provided good separation of Cl isotopes due to the combination of energy and momentum analysis, which efficiently removed energy tails from the sputtering process in the negative ion source. The vacuum chamber of the injection magnet is insulated and connected to acceleration gaps on the entrance and exit sides. This allows for a fast sequential injection of different isotopes into the tandem by applying high-voltage pulses to the magnet chamber. In this way, the energy of ions with different mass can be adjusted, resulting in the same magnetic rigidity inside the magnet. The associated power supply is capable of providing three independent voltage pulses with durations from 100 μs up to several seconds and allows for simultaneous measurement of three different isotopes. In the case of chlorine, isotopes of 35Cl, 36Cl, and 37Cl are usually injected with 0.5, 2.3, and 4.1 kV (Reg0, Reg1, and Reg2) pulses, respectively, on the magnet chamber. The beam pulse sequence Reg0–Reg1–Reg2 was set as 100 μs–100 ms–100 μs and was repeated with a frequency of 10 Hz to
centrifuged at 3500 rpm for 15 min. Without losing the AgCl, the supernatant was decanted off as much as possible. Then, 1 mL of 3 M NH4OH was added until the AgCl was dissolved, and the mixture was left for ~30 min to precipitate M(OH)x, where M is a metal element, such as Be, Mg, Ca, Fe, Cu, and other possible elements in limestone). Next, the solution was filtered with a 0.20 μm membrane syringe and transferred to a 10 mL PP centrifuge tube. The AgCl was reprecipitated by gradual addition of 2 mL of 4 M HNO3. After decanting the supernatant, the AgCl was re-dissolved by addition of 2 mL of 3 M NH4OH. Then, 0.5 mL of saturated Ba(NO3)2 and 3 mL of MQ water were added, the mixture was shaken vigorously, and allowed to sit overnight to remove sulfur as BaSO4 precipitate. Next, the solution was filtered with a 0.20 μm membrane syringe, 1 mL of saturated Na2CO3 was added, and the mixture was allowed to sit overnight to remove the excess barium as BaCO3 precipitate. After membrane filtration, the AgCl was re-precipitated again by addition of 2 mL of 4 M HNO3, washed and dried, and finally pressed into cathode cones for AMS. 2.3.
36
Cl AMS measurements at UTTAC
The 6 MV AMS facility at the University of Tsukuba (UTTAC), has been used for routine analyses of multiple nuclides (10Be, 14C, 26Al,
Fig. 3. The two-dimensional spectra (⊿E1 + E2 − ⊿E4 + E5) of the standard (A), blank (B), and tiankeng sample (C). 31
Nuclear Inst. and Methods in Physics Research B 459 (2019) 29–35
< 5% 6.16 × 10−07 4.20 × 10−07 4.26 × 10−07 1.02 × 10−06 5.02 × 10−07 3.53 × 10−07
< 10% 2.43 × 10−06 2.13 × 10−06 2.22 × 10−06 2.57 × 10−06 6.77 × 10−07 8.28 × 10−07
< 5% 4.52 × 10−06 6.10 × 10−06 4.49 × 10−06 4.65 × 10−06 3.31 × 10−06 2.94 × 10−06
allow for a fast sequential injection of the three isotopes into the tandem. A terminal voltage of 6.0 MV was used for acceleration, and 36 Cl ions with 7 + charge state and 47.3 MeV energy were selected by analyzing the magnet after carbon foil stripping and passing them through two switching magnets and a 22.5° electrostatic analyzer. Then, they were finally recorded by a five-anode ionization chamber filled with 23 torr isobutane gas. Differential energy loss signals could be measured with the anodes to give ⊿E1, ⊿E2, ⊿E3, ⊿E4, and ⊿E5. Total energy (Et) could be obtained from the cathode. The two-dimensional spectra of ⊿E1 + ⊿E2 and ⊿E4 + ⊿E5 were used for ion identification, as shown in Fig. 3. With these settings, the background level was obtained as 36Cl/Cl ~ 3 × 10−15 with a commercial blank sample prepared from ACS-grade NaCl (Fisher Scientific, USA). The simulation transport procedure of 36Cl was as follows. Firstly, a commercial AgCl sample was used, and 37Cl− ions were extracted from the ion source to simulate the 36Cl7+ beam transport of a sample containing 36Cl. In the simulation transport, the voltages applied to the magnet chamber and the terminal were set as 2.3 kV and 6.0 MV to make the 37Cl7+ ions have the same energy as 36Cl7+ ions. The electric and magnetic parameters of the ion optics system were tuned for the optimum state of beam transport. Then, all magnets were scaled back from 37 to 36. Finally, sample containing 36Cl was used, and 36Cl− ions were extracted from the ion source and recorded by the multi-anode ionization chamber after passing through the injection magnet, accelerator, analyzing magnet, and electrostatic analyzer. The transmission efficiency for 36Cl was about 20% for charge state 7+. The relatively high efficiency of measurement of 36Cl enabled measurement of more than 50 samples per day. The 36Cl/Cl values were determined by simultaneous measurements 35 of Cl and 37Cl current by the offset Faraday cups at the high-energy side and the count rate of 36Cl in the multi-anode ionization chamber. The mean value of the 36Cl/Cl ratio of each sample was determined by normalizing the measured value against the values of a series of standard samples (1.00 × 10−11, 1.60 × 10−12, 5.00 × 10−13) prepared by K. Nishiizumi [15]. Nine 5-min runs were performed for each sample. The abundance values were based on the averages, and the uncertainties were based on the relative standard deviations of the nine runs and the counting uncertainty.
< 10% 3.65 × 10−06 3.58 × 10−06 2.70 × 10−06 6.77 × 10−06 1.57 × 10−06 1.15 × 10−06 < 5% 2.14 × 10−04 2.13 × 10−04 1.80 × 10−04 1.79 × 10−04 4.51 × 10−04 1.64 × 10−04 < 5% 1.43 × 10−04 1.39 × 10−04 1.22 × 10−04 1.73 × 10−04 1.14 × 10−04 3.45 × 10−04 Uncertainty 9# Tunnel blank 20# HuangJing-TK 22# DaCao-TK 24# ChuanDong-TK 26# DaTuo-TK 32# DSW-E02
< 5% 4.61 × 10−05 4.21 × 10−05 3.11 × 10−05 3.76 × 10−05 1.27 × 10−05 1.08 × 10−05
< 5% 5.80 × 10−05 6.15 × 10−05 3.10 × 10−05 5.39 × 10−05 2.80 × 10−05 3.41 × 10−05
[U] (g/g) [Zr] (g/g) [Cu] (g/g) [Ti] (g/g) [Si] (g/g) [Al] (g/g) Sample ID
[K] (g/g)
[Fe] (g/g)
< 1% 1.30 × 10−03 3.82 × 10−03 2.65 × 10−03 1.15 × 10−01 1.68 × 10−03 1.67 × 10−02 < 5% 6.68 × 10−07 3.33 × 10−07 4.00 × 10−07 3.31 × 10−07 3.47 × 10−07 1.92 × 10−07 < 5% 3.81 × 10−07 2.88 × 10−07 2.99 × 10−07 3.20 × 10−07 1.23 × 10−07 1.07 × 10−07 < 10% 5.75 × 10−07 2.84 × 10−07 3.24 × 10−07 2.84 × 10−07 1.50 × 10−07 6.97 × 10−08 < 5% 6.30 × 10−07 2.80 × 10−07 3.44 × 10−07 2.71 × 10−07 2.76 × 10−07 1.58 × 10−07 < 15% 1.40 × 10−05 7.94 × 10−06 3.84 × 10−06 1.47 × 10−05 6.90 × 10−06 5.91 × 10−06 < 1% 3.97 × 10−05 2.03 × 10−05 1.39 × 10−05 3.51 × 10−05 6.65 × 10−06 1.19 × 10−05 < 1% 0.373 0.395 0.384 0.249 0.384 0.387
[Gd] (g/g)
Uncertainty 9# Tunnel blank 20# HuangJing-TK 22# DaCao-TK 24# ChuanDong-TK 26# DaTuo-TK 32# DSW-E02
Table 2 Chemical composition of the tiankeng samples.
[Cl] (g/g) [Ca] (g/g) Sample ID
[B] (g/g)
[Sm] (g/g)
[Eu] (g/g)
[Dy] (g/g)
[Mg] (g/g)
H. Shen, et al.
3. Exposure age calculations of tiankengs 3.1.
36
Cl production rate
To reconstruct the target chemistry of the carbonate samples, concentrations of major elements (e.g., Ca, K, Fe, Mg, Al, Si, and Ti) and trace elements that function as neutron absorbers (Sm, Gd, B, Gd, and Dy) were measured by inductively coupled plasma mass spectrometry (ICP-MS) in aliquots of the AMS sample solutions. Chlorine content was measured using the isotope dilution method with AMS [7]. The results of the chemical analyses are summarized in Table 2. Chlorine-36 production rates in the limestone samples depend on their calcium, potassium, and chlorine contents:
P36Cl = Pca [Ca] + PK [K ] +
n
35 N35/
i Ni i
(1)
where Pca and PK are the production rates for reactions on calcium and potassium, respectively, [Ca] and [K] are the element concentrations, and n is the thermal neutron capture rate (in units of neutrons absorbed g−1 a−1). The ( 35 N35/ i i Ni ) term gives the fraction of stopped neutrons captured by 35Cl, determined by the effective cross sections ( i ) [16] of the different target elements (Ca, B, Gd, Sm, etc., as listed in Table 2) and their corresponding atoms (Ni ). The value of Pca, which includes production by both calcium spallation and muon capture on 40Ca, at the tiankeng site was estimated to be 86 ± 5 atoms g Ca−1 a−1, as derived by scaling the production rate at sea level and high latitude (54 ± 3 atoms g Ca−1 a−1) [17]. 32
Nuclear Inst. and Methods in Physics Research B 459 (2019) 29–35
H. Shen, et al.
Table 3 Production rates for Sample ID
20# 22# 24# 26# 32#
36
Cl of tiankeng samples. [Ca] (g/g)
HuangJing-TK DaCao-TK ChuanDong-TK DaTuo-TK DSW-E02
[Cl] (g/g)
[K] (g/g)
P36Cl (atoms g−1 a−1)
i i Ni
Meas. val.
1 S.D.
Meas. val.
1 S.D.
Meas. val.
1 S.D.
Meas. val.
1 S.D.
Meas. val.
1 S.D.
0.395 0.384 0.249 0.384 0.387
0.003 0.007 0.001 0.004 0.001
2.032 × 10−05 1.386 × 10−05 3.510 × 10−05 6.647 × 10−06 1.188 × 10−05
1.432 × 10−07 9.402 × 10−08 2.478 × 10−07 5.574 × 10−08 8.520 × 10−08
4.205 × 10−05 3.110 × 10−05 3.756 × 10−05 1.266 × 10−05 1.079 × 10−05
1.038 × 10−06 1.492 × 10−06 5.286 × 10−07 1.125 × 10−06 4.732 × 10−07
1.70 × 10−03 1.64 × 10−03 2.21 × 10−03 6.07 × 10−04 1.17 × 10−03
1.03 × 10−04 8.77 × 10−05 2.41 × 10−04 4.94 × 10−05 1.01 × 10−04
10.4 10.2 6.8 10.0 10.2
0.7 0.7 0.5 0.7 0.7
The Pk value at the tiankeng site was taken as 300 ± 22 atoms g K−1 a−1, as derived by scaling the potassium rate at sea level and high latitude (190 ± 14 atoms g K−1 a−1), [18] and the production rate by thermal neutron capture was n = 490 ± 38 n g−1 a−1, as obtained by scaling the sea level/ high latitude value of n = 307 ± 24 n g−1 a−1 [19]. Due to the low uranium and thorium contents of the tiankeng samples, production from the spontaneous fission of these elements contributes little and is generally negligible for near-surface samples. The 36Cl production rates also depend on the shielding of cosmic rays by mountains, sloped surfaces, and local rock formations. When a large part of the sky is blocked, this correction is large and requires consideration. In this case, the topographic shielding factor S was considered to be ~0.30, according to formula (2) with the same method as Dunne [20], where is the local slope and z is the sampling depth. Sample locations and calculated production rates for 36Cl are presented in Table 3.
as the brown varnish of the limestone outcrops evidencing long duration of exposure and the lack of granular disintegration of the sampled surfaces, we deemed that a correction for denudation was unnecessary because the effect of denudation would be negligible over the timescale investigated here. Thus, the exposure age t was calculated from the measured 36Cl concentration (Ctot.) as follows:
t=
3.6 × 10
6 2.64 ) e
( z ) 1 + 5000
1
(Ctot .
· ln 1
CBG ) P36cl
,
(3)
36
−6
−1
where is the Cl decay constant (2.30 × 10 yr ), P36 is the total production rate given in Eq. (1) in Section 3.1, Ctot is the total 36Cl concentration (atoms (g rock)−1) measured by AMS at UTTA, and CBG is the background of 36Cl concentration derived from the tunnel sample. The calculated exposure ages vary from several tens of thousands of years to several hundreds of thousands of years, as summarized in Table 4. The relatively large uncertainties in the exposure ages arise mainly from the 36Cl production rate (approximately 6%) and the analytical method. The erosion rate was ignored in the calculation. The youngest exposure age, for the DaShiwei tiankeng, is 27.8 ± 2.4 ka, which is consistent with the value of 20.5 ± 10.0 ka [21] obtained from long-term denudation rates in Chinese karst areas. The other exposure ages of 200–400 ka for the HuangJing, DaCao, and DaTuo tiankengs are consistent with the value of 200–300 ka estimated for ages of the large tiankengs in the Nakanai karst in New Britain [22], where the meteorological conditions are quite similar to those in the Chinese Karst area.
2
S(z, ) = (1
35 N35/
(2)
3.2. Exposure ages It is important to note that prior to the formation of the tiankengs, despite the vertical inner walls of the tiankengs being buried and therefore not exposed at the surface as they are today, the samples were nevertheless exposed to cosmic rays passing through the bedrock (especially to muon particles). This implies that they may have accumulated a certain amount of cosmogenic nuclides at depth before being exhumed by the collapse event. Considering that all collected samples were located at depths of ~100 m from the top of the cliffs, the inherited 36Cl component was estimated from a tunnel blank (Table 4) collected in a tunnel 100 m below the mountain surface located close to the study area and was determined to be ~1.18 × 105 atoms g−1. Additionally, the studied samples brought to the surface by the collapse event have been affected by different denudation rates. Considering the almost vertical inner wall inclination angle and the shielding effect of the sinkholes against external wind and rain, as well
4. Results and discussion In this work, a methodology for accurate analysis of 36Cl at very low concentrations in geological applications, including transmission and detection of 36Cl, identification of isobars, and quantitative calibration of the 6 MV AMS system at UTTAC, was established. The background level obtained with a commercial blank sample prepared from ACSgrade NaCl was 36Cl/Cl ~ 3 × 10−15 (Fisher Scientific, USA). In addition, an isotope dilution-accelerator mass spectrometry (ID-AMS) technique was used to measure the natural Cl content in tiankeng
Table 4 Data sheet for tiankeng exposure dating at UTTAC. Sample ID
9# Tunnel blank 20# HuangJing-TK 22# DaCao TK 24# ChuanDong-TK 26# DaTuo-TK 32# DSW-E02
Calcite wt. for AMS (g)*1
Cl carrier wt. (μg)*2
35
Cl/37Cl*3
Meas. val.
1 S.D.
Meas. val.
1 S.D.
Meas. val.
1 S.D.
Meas. val.
20.004 19.999 20.016 20.071 20.071 20.015
0.05% 0.05% 0.05% 0.05% 0.05% 0.05%
1482 1484 1485 1482 1487 1480
0.001% 0.674% 0.673% 0.675% 0.672% 0.676%
10.85 18.10 24.92 11.81 47.47 28.38
0.11% 0.20% 0.06% 0.20% 0.50% 0.23%
6.1 × 10−14 7.7 × 10−13 4.9 × 10−13 1.6 × 10−12 9.7 × 10−13 4.0 × 10−13
*1 Net weight used for AMS without insoluble materials in the sample solution. *2 Total spiked wt. of 99.4% 35-enriched Cl carrier from Aldrich Co. 33
36
[36Cl] (atoms g−1)
Minimum exposure age (yr)
1 S.D.
Meas. val.
1 S.D.
Meas. val.
1 S.D
4.16% 4.50% 5.18% 4.40% 4.50% 5.27%
1.18 × 105 1.25 × 106 7.44 × 105 3.02 × 106 1.34 × 106 5.86 × 105
4.16% 4.61% 5.27% 4.51% 4.63% 5.36%
/ 9.05 × 104 4.34 × 104 4.02 × 105 1.04 × 105 2.78 × 104
/ 8.37% 8.91% 8.22% 8.45% 8.80%
Cl/Cl
Nuclear Inst. and Methods in Physics Research B 459 (2019) 29–35
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Fig. 4.
36
Cl exposure ages and corresponding geologic era.
limestone. The measurement of the 35Cl/37Cl ratio allows an accurate determination of chlorine content in the sample and enables simultaneous measurement of 36Cl and Cl with a single AMS sample preparation and without any elaborate or time-consuming procedures. Furthermore, the sample size required is reduced by the addition of the spike, which plays the role of carrier and provides an accurate analysis of chlorine content, even at the very low concentrations in geological applications. To obtain reliable exposure ages of the largest tiankengs in the world, the atoms of different target elements producing 36Cl in tiankeng samples were measured. Additionally, the cosmic ray shielding effect caused by mountains and sloped surface was considered. The exposure ages of the samples from the five super tiankengs in South China were finally determined to vary from several tens of thousands of years to several hundreds of thousands of years, all within the Quaternary and almost entirely within the upper Pleistocene, as shown in Fig. 4. Most tiankengs in China are relatively youthful features, as they truncate the topography of solution dolines and conical hills within fengcong karst. However, such evidence only dates the surface collapse that created the open tiankeng. Development of the underground cavern must have taken longer. The sheer size of many tiankengs suggests a very long history that could allow time for the erosional removal of the huge volumes of missing rock. We obtained the initial ages of different Chinese tiankengs, which provide significant evidence for activity associated with new tectonic movement of the Eurasian continent in the Quaternary.
Acknowledgments The authors gratefully acknowledge the technical staff at UTTAC for smooth accelerator operation. This work was supported by the National Natural Science Foundations of China (NSFC) under grant Nos. 11565008, 11775057, 11765004, 11675269, 11575296, 11705287, and 11665006, the Guangxi Natural Science Foundation under grant Nos. 2017GXNSFFA198016 and 2018JJA110037, the Guangxi Excellence Scholar Program, the Fundamental Research Funds from the Institute of Karst Geology, CAGS, grant No. 200716, and KAKENHI grant Nos. 24110006, 26600138, and 15H02340 from JSPS. References [1] Zhu Xuewen, Tony Waltham, Tiankeng: definition and description, Speleogenesis Evol. Karst Aquifers 4 (1) (2006) 1–8. [2] T. Waltham, Cave and Karst Sci. 32 (2–3) (2005) 51. [3] Zhu Xuewen, Chen Weihai, Tiankengs in the Karst of China, Carsologica 25 (1) (2006) 7–23. [4] Zhang Ji’an, Liu Jingrong, Discussion on the formation of Dashiwei Tiankeng and related problems, Land Resour. Southern China 2 (2008) 32–34. [5] H.W. Bentley, F.M. Phillips, S.N. Davis, et al., 36Cl in the terrestrial environment, Handbook Environ. Isotope Geochem. 2 (1) (1986) 427–475. [6] M. Houmark-Nielsen, H. Linge, D. Fabel, et al., Cosmogenic surface exposure dating the last deglaciation in Denmark: discrepancies with independent age constraints suggest delayed periglacial landform stabilization, Quaternary Geochronol. 13 (6) (2012) 1–17. [7] Y. Matsushi, K. Sasa, T. Takahashi, et al., Denudation rates of carbonate pinnacles in Japanese Karst areas: estimates from cosmogenic 36Cl in calcite, Nucl. Instr. Meth. Phys. Res. 268 (7–8) (2010) 1205–1208. [8] S. Zerathe, T. Lebourg, R. Braucher, et al., Mid-Holocene cluster of large-scale landslides revealed in the Southwestern Alps by 36Cl dating. Insight on an Alpinescale landslide activity, Quaternary Sci. Rev. 90 (1) (2014) 106–127. [9] Wang Yue, Y. Nagashima, et al., Determining limestone erosion rate with 36Cl AMS measurement, Phys. Exp. 25 (3) (2005) 11–14. [10] Y. Sun, The study of lower crust flow and structure of the upper mantle beneath SE Tibetan plateau, Central South University, 2013 (Dissertation, Ph.d. thesis, in Chinese). [11] Z. Song, The geomorphology of the typical rivers in the eastern tibet and their active tectonic implications, China Earthquake Administration, 2014 (Dissertation, Master thesis, in Chinese). [12] K. Sasa, et al., Nucl. Instr. Meth. Phys. Res. Sect. B 361 (2015), https://doi.org/10. 1016/j.nimb.2015.04.028. [13] S. Hosoya, et al., Nucl. Instr. Meth. Phys. Res. B 406 (2017) 268–271. [14] S. Hosoya, et al., Nucl. Instr. Meth. Phys. Res. B 438 (2019) 131–135. [15] P. Sharma, P.W. Kubik, U. Fehn, H.E. Gove, K. Nishiizumi, D. Elmore, Development of 36Cl standards for AMS, Nucl. Instr. Meth. Phys. Res. B 52 (1990) 410–415. [16] R.B. Firestone, Table of Isotopes, 8th ed., John Wiley & Sons, New York, 1998. [17] J.O. Stone, G.L. Allan, L.K. Fifield, R.G. Cresswell, Geochim. Cosmochim. Acta 60 (4) (1996) 679–692. [18] M.G. Zreda, Development and calibration of the cosmogenic “Cl surface exposure dating method and its application to the chronology of late Quatemary glaciations” (Ph.D. dissertation), Institute of Mining and Technology Socorro, New Mexico, 1994. [19] M.G. Zreda, F.M. Phillips, D. Elmore, P.W. Kubik, P. Sharma, Cosmogenic chlorine36 production rates in terrestrial rocks, Earth Planet. Sci. Lett. 105 (1991) 94–109.
5. Conclusion This study applied 36Cl measurements with isotope dilution for quantitative exposure dating of carbonate rocks from karst tiankeng. Exposure ages of 20–400 ka were determined for the five super tiankengs in Guangxi Province, southern China. These ages provide significant evidence for new tectonic movement of the Eurasian continent in the Quaternary, especially in the upper Pleistocene. We intend to follow up this study with 10Be and 41Ca determinations in carbonate rocks. Measurements of these nuclides may be accompanied by some difficulties [23,24], especially for 41Ca [25], in the present sample preparation strategy. However, this multi-nuclide coupling will provide significant clues to test the exposure dates in the Pleistocene–Holocene epochs and to produce a more detailed modeling of Chinese tiankeng landscape evolution in karst areas. In addition, to study the potential weathering effects on determined exposure ages, more than one sample should be taken from different areas of the same tiankeng, as the oldest date among the spread should in principle be closest to the true exposure age. 34
Nuclear Inst. and Methods in Physics Research B 459 (2019) 29–35
H. Shen, et al. [20] J. Dunne, et al., Scaling factor for the rates of production of cosmogonic nuclides for geometric shielding and attenuation at depth on sloped surfaces, Geomorphology 27 (1999) 3–11. [21] Dong, et al., Nucl Instr. Meth. Phys. Res. B 294 (2013) 611–615. [22] R. Maire, Giant shafts and underground rivers of the Nakanai Mountains (New
Britain), Spelunca Suppl. 2 (1981) 8–30. [23] R. Brancer, et al., Geochim. Cosmochim. Acta 69 (6) (2005) 1473–1478. [24] S. Merchel, R. Braucher, L. Benedetti, O. Grauby, D.L. Bourlès, Quat. Geochronol. 3 (2008) 299. [25] W. Henning, et al., Science 236 (4802) (1987) 725–727.
35
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Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Exposure age dating of Chinese tiankengs by a,b,c,d,⁎
b
36
Cl-AMS
a,c,d
b
b
Hongtao Shen , Kimikazu Sasa , Qi Meng , Masumi Matsumura , Tetsuya Matsunaka , Seiji Hosoyab, Tsutomu Takahashib, Maki Hondab, Keisuke Suekib, Ming Hed, Baojian Huange, Huijin Lua,c, Lisha Chena,c, Yongfu Qina,c, Jiahao Lia,c, Haihui Lana,c, Zhaomei Lia,c, Zhenchi Zhaoa,c, Mingji Liua,c, Siyu Weia,c, Mingli Qia,c, Qingzhang Zhaod, Kejun Dongd, Yongjin Guanf, Xiangdong Ruanf, Shan Jiangd
T
a
College of Physics, Guangxi Normal University, Guilin 541004, China University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan Guangxi Key Laboratory of Nuclear Physics and Nuclear Technology, Guangxi Normal University, Guilin 541004, China d China Institute of Atomic Energy, P.O. Box 275(50), Beijing 102413, China e Institute of Karst Geology, Chinese Academy of Geological Sciences, Guilin 541004, China f College of Physics, Guangxi University, Nanning 530004, China b c
ARTICLE INFO
ABSTRACT
Keywords: Tiankeng 36 Cl Exposure age Erosion rate AMS
The Guangxi Zhuang Autonomous Region in southern China is one of the most typical karst relief areas with various forms, and tiankeng is one of the most typical karst relief forms of the Quaternary Period. In this work, accurate measurements of tiankeng exposure ages were made. The exposure ages of carbonate rocks from the five largest tiankengs in the world, the DaShiwei, HuangJing, ChuanDong, DaTuo, and DaCao tiankengs, were determined by accelerator mass spectrometry to be at least 20–400 thousand years, which provides significant proof for activity of new tectonic movement of the Eurasian continent in the Quaternary.
1. Introduction A tiankeng is a collapse doline that is more than 100 m deep and wide and has a steep profile with vertical cliffs surrounding its perimeter. The Guangxi Zhuang Autonomous Region of southern China, one of the most typical karst relief areas of the Late Quaternary Period, plays an important role in researching the surface shape and evolution of karst relief [1,2]. At present, the time of formation and evolutionary process of tiankeng are very poorly known and require further investigation to clarify the dispute between evolution theory and catastrophe theory [3,4]. The concentration of cosmogenic nuclides produced in situ in surface rock is dependent on the exposure time and erosion rate of the rock, which can be measured by long half-life radionuclides at a timescale from thousands of years to millions of years. Chlorine-36, a long-lived radionuclide found in the lithosphere and hydrosphere, has been used widely in different geological applications, such as the study of glacier activity, geomorphological processes, volcanism, meteorite impacts, flooding, and debris flow [5]. For example, Houmark et al. [6] used 36Cl to determine the exposed stage of ice erosion in Denmark, Matsushi et al. [7] used it to determine the rate of erosion in some areas of Japan, Zerathe et al. [8] measured the time ⁎
of landslides in the southern Alps and established a time series of landslides, and Wang et al. [9] determined the limestone erosion rate in the Beijing area. In situ 36Cl is now a reliable nuclide widely used to quantify surface processes in the geosciences. In the tiankeng area, the main component of the rock of karst relief is calcium carbonate, and calcium is the target element of a cosmic ray reaction that produces 36Cl, which has been shown to be a reasonable radionuclide for measuring the exposure time of limestone, because of its high production rate and rapid accumulation. In this study, 36Cl exposure dating was applied to karst tiankeng carbonate from DaShiwei Tiankeng Group National Geopark, Leye County, China. The purpose of the work was to provide basic data and establish research methods for investigating the formation, evolution, and local tectonic uplift rates of the DaShiwei Geological Area and hence for further research of the Chinese uplift belt, the collisions between the South Asian Subcontinent and the Eurasian Plate, and even the formation of the Himalayas and the Tibetan Plateau. In this work, concentrations of 36Cl in rock samples from the five of the largest tiankengs in the world, the DaShiwei, HuangJing, ChuanDong, DaTuo, and DaCao tiankengs, were measured at the AMS facility at the University of Tsukuba in an effort to estimate the time of formation of this unique karst relief.
Corresponding author at: College of Physics, Guangxi Normal University, Guilin 541004, China. E-mail address: [email protected] (H. Shen).
https://doi.org/10.1016/j.nimb.2019.07.006 Received 31 January 2018; Received in revised form 28 June 2019; Accepted 4 July 2019 Available online 03 September 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
Nuclear Inst. and Methods in Physics Research B 459 (2019) 29–35
H. Shen, et al.
Fig. 1. The four super tiankengs in China.
2. Materials and methods
avoid any coverage by rainwater or snow. Additionally, a blank sample was collected from a tunnel ~100 m below the surface of the mountain to estimate the background 36Cl.
2.1. Sites and sampling The study area is located in southwest China, 15 km west of the town of Guilin (Guangxi Zhuang Autonomous Region), at the first front of the tectonic uplift of China (southeastern external Tibetan Plateau). Since the early Miocene, this area with abundant groundwater resources has undergone several tectonic phases linked to Himalayan orogenesis [10,11]. The main resulting structures show that the tectonic uplift has caused underground rivers to cut down several hundred meters, forming large underground cave passages, which eventually collapse to forms this type of giant tiankeng, as shown in Fig. 1. In this study, rock samples were taken from the vertical inner walls of five of the largest tiankengs in the world, the DaShiwei, HuangJing, ChuanDong, DaTuo, and DaCao tiankengs, located in Leye County, Guangxi Zhuang Autonomous Region, China, as shown in Table 1. For each site, a rock core with a length of approximately 30 cm and diameter of approximately 10 cm was taken from a wall at a position ~100 m below the top of the cliff, as shown in Fig. 2. The outer 5 cm of the outer end of the rock core was taken for 36Cl-AMS analysis. The sampling sites for the sample collection were selected at locations where the rock had good structure and exposure, without any coverage of vegetation or gravel. The sampling sites were also high enough to
2.2. Sample preparation and
Latitude (°N)
Longitude (°E)
Altitude (m)
Weight (g)
9# Tunnel blank 20# HuangJing-TK 22# DaCao-TK 24# ChuanDong-TK 26# DaTuo-TK 32# DSW-E02
24°46′20.60″ 24°47′42.64″ 24°47′33.51″ 24°48′29.23″ 24°48′37.68″ 24°48′28.75″
106°32′44.22″ 106°31′15.19″ 106°30′37.38″ 106°29′20.70″ 106°28′33.34″ 106°26′47.99″
1008 1117 1171 1296 1246 1486
836.78 878.3 856.75 580.63 801.36 526.94
± ± ± ± ± ±
3 3 3 3 3 3
Cl extraction
The outer 5 cm of the rock column was detached for the chemical sample pretreatment. The rock pieces were crushed and sieved to a size range of 0.25–1 mm for AMS preparation (the < 0.25 mm fraction was used for whole-rock ICPMS analyses). Each sample was rinsed and sonicated until no fine dust could be washed from it. Afterward, it was rinsed several times in MQ water and then dried overnight in an oven at 70 °C. Approximately 35 g of dry sample and 100 mL of MQ water were transferred to a 400 mL glass conical beaker and swirled to wet the sample thoroughly. Two rounds of an 8 mL 4 mol/L HNO3 leaching procedure were performed to eliminate meteoric 36Cl and other potential contaminants, removing the outermost 10% of the grain surfaces. The acid was mixed in slowly to prevent foaming. Afterward, the grains were rinsed and sonicated in MQ water to ensure complete removal of Ca(NO3)2 and then dried overnight in an oven at 120 °C. Each etched sample (~20 g) was transferred to a 500 mL polypropylene bottle, spiked with Cl carrier solution (35Cl purity: 99 atom% mg; the highly enriched 35Cl carrier was used for the isotope dilution AMS technique, which enabled us to conduct a simultaneous measurement of 36Cl and Cl with a single sample preparation for AMS), soaked with ~50 mL of ultrapure water, and then dissolved gradually with ~110 mL of 4 mol/L HNO3 by separatory funnel. To avoid foaming, the acid was added in aliquots of ~10 mL every 30 min, and the bottle was not shaken or swirled until dissolution was almost complete. The sample bottle was kept loosely capped between additions of acid. After complete dissolution, the dark sample solution was decanted into a 175 mL centrifuge bottle through a 0.45 µm PTFE membrane vacuum filter. Then, ~250 µL of 0.3 M AgNO3 was added to the sample solution to precipitate AgCl. After sitting in the dark, the sample solution was
Table 1 Description of the tiankeng samples. Sample ID
36
30
Nuclear Inst. and Methods in Physics Research B 459 (2019) 29–35
H. Shen, et al.
Fig. 2. Core samples from the tiankengs. 36
Cl, 41Ca, and 129I) [12,13]. The details of the 36Cl-AMS measurements were as follows. The AgCl sample powders were pressed into a large Cu sample holder (inner diameter of 6 mm) with a compact AgBr backing in a 20-sample copper target wheel, by which the isobar 36S within the target could be further suppressed [14]. The Cl− anions were extracted, selected by an injection magnet, and injected into the accelerator. At the low-energy side, the ion injector provided good separation of Cl isotopes due to the combination of energy and momentum analysis, which efficiently removed energy tails from the sputtering process in the negative ion source. The vacuum chamber of the injection magnet is insulated and connected to acceleration gaps on the entrance and exit sides. This allows for a fast sequential injection of different isotopes into the tandem by applying high-voltage pulses to the magnet chamber. In this way, the energy of ions with different mass can be adjusted, resulting in the same magnetic rigidity inside the magnet. The associated power supply is capable of providing three independent voltage pulses with durations from 100 μs up to several seconds and allows for simultaneous measurement of three different isotopes. In the case of chlorine, isotopes of 35Cl, 36Cl, and 37Cl are usually injected with 0.5, 2.3, and 4.1 kV (Reg0, Reg1, and Reg2) pulses, respectively, on the magnet chamber. The beam pulse sequence Reg0–Reg1–Reg2 was set as 100 μs–100 ms–100 μs and was repeated with a frequency of 10 Hz to
centrifuged at 3500 rpm for 15 min. Without losing the AgCl, the supernatant was decanted off as much as possible. Then, 1 mL of 3 M NH4OH was added until the AgCl was dissolved, and the mixture was left for ~30 min to precipitate M(OH)x, where M is a metal element, such as Be, Mg, Ca, Fe, Cu, and other possible elements in limestone). Next, the solution was filtered with a 0.20 μm membrane syringe and transferred to a 10 mL PP centrifuge tube. The AgCl was reprecipitated by gradual addition of 2 mL of 4 M HNO3. After decanting the supernatant, the AgCl was re-dissolved by addition of 2 mL of 3 M NH4OH. Then, 0.5 mL of saturated Ba(NO3)2 and 3 mL of MQ water were added, the mixture was shaken vigorously, and allowed to sit overnight to remove sulfur as BaSO4 precipitate. Next, the solution was filtered with a 0.20 μm membrane syringe, 1 mL of saturated Na2CO3 was added, and the mixture was allowed to sit overnight to remove the excess barium as BaCO3 precipitate. After membrane filtration, the AgCl was re-precipitated again by addition of 2 mL of 4 M HNO3, washed and dried, and finally pressed into cathode cones for AMS. 2.3.
36
Cl AMS measurements at UTTAC
The 6 MV AMS facility at the University of Tsukuba (UTTAC), has been used for routine analyses of multiple nuclides (10Be, 14C, 26Al,
Fig. 3. The two-dimensional spectra (⊿E1 + E2 − ⊿E4 + E5) of the standard (A), blank (B), and tiankeng sample (C). 31
Nuclear Inst. and Methods in Physics Research B 459 (2019) 29–35
< 5% 6.16 × 10−07 4.20 × 10−07 4.26 × 10−07 1.02 × 10−06 5.02 × 10−07 3.53 × 10−07
< 10% 2.43 × 10−06 2.13 × 10−06 2.22 × 10−06 2.57 × 10−06 6.77 × 10−07 8.28 × 10−07
< 5% 4.52 × 10−06 6.10 × 10−06 4.49 × 10−06 4.65 × 10−06 3.31 × 10−06 2.94 × 10−06
allow for a fast sequential injection of the three isotopes into the tandem. A terminal voltage of 6.0 MV was used for acceleration, and 36 Cl ions with 7 + charge state and 47.3 MeV energy were selected by analyzing the magnet after carbon foil stripping and passing them through two switching magnets and a 22.5° electrostatic analyzer. Then, they were finally recorded by a five-anode ionization chamber filled with 23 torr isobutane gas. Differential energy loss signals could be measured with the anodes to give ⊿E1, ⊿E2, ⊿E3, ⊿E4, and ⊿E5. Total energy (Et) could be obtained from the cathode. The two-dimensional spectra of ⊿E1 + ⊿E2 and ⊿E4 + ⊿E5 were used for ion identification, as shown in Fig. 3. With these settings, the background level was obtained as 36Cl/Cl ~ 3 × 10−15 with a commercial blank sample prepared from ACS-grade NaCl (Fisher Scientific, USA). The simulation transport procedure of 36Cl was as follows. Firstly, a commercial AgCl sample was used, and 37Cl− ions were extracted from the ion source to simulate the 36Cl7+ beam transport of a sample containing 36Cl. In the simulation transport, the voltages applied to the magnet chamber and the terminal were set as 2.3 kV and 6.0 MV to make the 37Cl7+ ions have the same energy as 36Cl7+ ions. The electric and magnetic parameters of the ion optics system were tuned for the optimum state of beam transport. Then, all magnets were scaled back from 37 to 36. Finally, sample containing 36Cl was used, and 36Cl− ions were extracted from the ion source and recorded by the multi-anode ionization chamber after passing through the injection magnet, accelerator, analyzing magnet, and electrostatic analyzer. The transmission efficiency for 36Cl was about 20% for charge state 7+. The relatively high efficiency of measurement of 36Cl enabled measurement of more than 50 samples per day. The 36Cl/Cl values were determined by simultaneous measurements 35 of Cl and 37Cl current by the offset Faraday cups at the high-energy side and the count rate of 36Cl in the multi-anode ionization chamber. The mean value of the 36Cl/Cl ratio of each sample was determined by normalizing the measured value against the values of a series of standard samples (1.00 × 10−11, 1.60 × 10−12, 5.00 × 10−13) prepared by K. Nishiizumi [15]. Nine 5-min runs were performed for each sample. The abundance values were based on the averages, and the uncertainties were based on the relative standard deviations of the nine runs and the counting uncertainty.
< 10% 3.65 × 10−06 3.58 × 10−06 2.70 × 10−06 6.77 × 10−06 1.57 × 10−06 1.15 × 10−06 < 5% 2.14 × 10−04 2.13 × 10−04 1.80 × 10−04 1.79 × 10−04 4.51 × 10−04 1.64 × 10−04 < 5% 1.43 × 10−04 1.39 × 10−04 1.22 × 10−04 1.73 × 10−04 1.14 × 10−04 3.45 × 10−04 Uncertainty 9# Tunnel blank 20# HuangJing-TK 22# DaCao-TK 24# ChuanDong-TK 26# DaTuo-TK 32# DSW-E02
< 5% 4.61 × 10−05 4.21 × 10−05 3.11 × 10−05 3.76 × 10−05 1.27 × 10−05 1.08 × 10−05
< 5% 5.80 × 10−05 6.15 × 10−05 3.10 × 10−05 5.39 × 10−05 2.80 × 10−05 3.41 × 10−05
[U] (g/g) [Zr] (g/g) [Cu] (g/g) [Ti] (g/g) [Si] (g/g) [Al] (g/g) Sample ID
[K] (g/g)
[Fe] (g/g)
< 1% 1.30 × 10−03 3.82 × 10−03 2.65 × 10−03 1.15 × 10−01 1.68 × 10−03 1.67 × 10−02 < 5% 6.68 × 10−07 3.33 × 10−07 4.00 × 10−07 3.31 × 10−07 3.47 × 10−07 1.92 × 10−07 < 5% 3.81 × 10−07 2.88 × 10−07 2.99 × 10−07 3.20 × 10−07 1.23 × 10−07 1.07 × 10−07 < 10% 5.75 × 10−07 2.84 × 10−07 3.24 × 10−07 2.84 × 10−07 1.50 × 10−07 6.97 × 10−08 < 5% 6.30 × 10−07 2.80 × 10−07 3.44 × 10−07 2.71 × 10−07 2.76 × 10−07 1.58 × 10−07 < 15% 1.40 × 10−05 7.94 × 10−06 3.84 × 10−06 1.47 × 10−05 6.90 × 10−06 5.91 × 10−06 < 1% 3.97 × 10−05 2.03 × 10−05 1.39 × 10−05 3.51 × 10−05 6.65 × 10−06 1.19 × 10−05 < 1% 0.373 0.395 0.384 0.249 0.384 0.387
[Gd] (g/g)
Uncertainty 9# Tunnel blank 20# HuangJing-TK 22# DaCao-TK 24# ChuanDong-TK 26# DaTuo-TK 32# DSW-E02
Table 2 Chemical composition of the tiankeng samples.
[Cl] (g/g) [Ca] (g/g) Sample ID
[B] (g/g)
[Sm] (g/g)
[Eu] (g/g)
[Dy] (g/g)
[Mg] (g/g)
H. Shen, et al.
3. Exposure age calculations of tiankengs 3.1.
36
Cl production rate
To reconstruct the target chemistry of the carbonate samples, concentrations of major elements (e.g., Ca, K, Fe, Mg, Al, Si, and Ti) and trace elements that function as neutron absorbers (Sm, Gd, B, Gd, and Dy) were measured by inductively coupled plasma mass spectrometry (ICP-MS) in aliquots of the AMS sample solutions. Chlorine content was measured using the isotope dilution method with AMS [7]. The results of the chemical analyses are summarized in Table 2. Chlorine-36 production rates in the limestone samples depend on their calcium, potassium, and chlorine contents:
P36Cl = Pca [Ca] + PK [K ] +
n
35 N35/
i Ni i
(1)
where Pca and PK are the production rates for reactions on calcium and potassium, respectively, [Ca] and [K] are the element concentrations, and n is the thermal neutron capture rate (in units of neutrons absorbed g−1 a−1). The ( 35 N35/ i i Ni ) term gives the fraction of stopped neutrons captured by 35Cl, determined by the effective cross sections ( i ) [16] of the different target elements (Ca, B, Gd, Sm, etc., as listed in Table 2) and their corresponding atoms (Ni ). The value of Pca, which includes production by both calcium spallation and muon capture on 40Ca, at the tiankeng site was estimated to be 86 ± 5 atoms g Ca−1 a−1, as derived by scaling the production rate at sea level and high latitude (54 ± 3 atoms g Ca−1 a−1) [17]. 32
Nuclear Inst. and Methods in Physics Research B 459 (2019) 29–35
H. Shen, et al.
Table 3 Production rates for Sample ID
20# 22# 24# 26# 32#
36
Cl of tiankeng samples. [Ca] (g/g)
HuangJing-TK DaCao-TK ChuanDong-TK DaTuo-TK DSW-E02
[Cl] (g/g)
[K] (g/g)
P36Cl (atoms g−1 a−1)
i i Ni
Meas. val.
1 S.D.
Meas. val.
1 S.D.
Meas. val.
1 S.D.
Meas. val.
1 S.D.
Meas. val.
1 S.D.
0.395 0.384 0.249 0.384 0.387
0.003 0.007 0.001 0.004 0.001
2.032 × 10−05 1.386 × 10−05 3.510 × 10−05 6.647 × 10−06 1.188 × 10−05
1.432 × 10−07 9.402 × 10−08 2.478 × 10−07 5.574 × 10−08 8.520 × 10−08
4.205 × 10−05 3.110 × 10−05 3.756 × 10−05 1.266 × 10−05 1.079 × 10−05
1.038 × 10−06 1.492 × 10−06 5.286 × 10−07 1.125 × 10−06 4.732 × 10−07
1.70 × 10−03 1.64 × 10−03 2.21 × 10−03 6.07 × 10−04 1.17 × 10−03
1.03 × 10−04 8.77 × 10−05 2.41 × 10−04 4.94 × 10−05 1.01 × 10−04
10.4 10.2 6.8 10.0 10.2
0.7 0.7 0.5 0.7 0.7
The Pk value at the tiankeng site was taken as 300 ± 22 atoms g K−1 a−1, as derived by scaling the potassium rate at sea level and high latitude (190 ± 14 atoms g K−1 a−1), [18] and the production rate by thermal neutron capture was n = 490 ± 38 n g−1 a−1, as obtained by scaling the sea level/ high latitude value of n = 307 ± 24 n g−1 a−1 [19]. Due to the low uranium and thorium contents of the tiankeng samples, production from the spontaneous fission of these elements contributes little and is generally negligible for near-surface samples. The 36Cl production rates also depend on the shielding of cosmic rays by mountains, sloped surfaces, and local rock formations. When a large part of the sky is blocked, this correction is large and requires consideration. In this case, the topographic shielding factor S was considered to be ~0.30, according to formula (2) with the same method as Dunne [20], where is the local slope and z is the sampling depth. Sample locations and calculated production rates for 36Cl are presented in Table 3.
as the brown varnish of the limestone outcrops evidencing long duration of exposure and the lack of granular disintegration of the sampled surfaces, we deemed that a correction for denudation was unnecessary because the effect of denudation would be negligible over the timescale investigated here. Thus, the exposure age t was calculated from the measured 36Cl concentration (Ctot.) as follows:
t=
3.6 × 10
6 2.64 ) e
( z ) 1 + 5000
1
(Ctot .
· ln 1
CBG ) P36cl
,
(3)
36
−6
−1
where is the Cl decay constant (2.30 × 10 yr ), P36 is the total production rate given in Eq. (1) in Section 3.1, Ctot is the total 36Cl concentration (atoms (g rock)−1) measured by AMS at UTTA, and CBG is the background of 36Cl concentration derived from the tunnel sample. The calculated exposure ages vary from several tens of thousands of years to several hundreds of thousands of years, as summarized in Table 4. The relatively large uncertainties in the exposure ages arise mainly from the 36Cl production rate (approximately 6%) and the analytical method. The erosion rate was ignored in the calculation. The youngest exposure age, for the DaShiwei tiankeng, is 27.8 ± 2.4 ka, which is consistent with the value of 20.5 ± 10.0 ka [21] obtained from long-term denudation rates in Chinese karst areas. The other exposure ages of 200–400 ka for the HuangJing, DaCao, and DaTuo tiankengs are consistent with the value of 200–300 ka estimated for ages of the large tiankengs in the Nakanai karst in New Britain [22], where the meteorological conditions are quite similar to those in the Chinese Karst area.
2
S(z, ) = (1
35 N35/
(2)
3.2. Exposure ages It is important to note that prior to the formation of the tiankengs, despite the vertical inner walls of the tiankengs being buried and therefore not exposed at the surface as they are today, the samples were nevertheless exposed to cosmic rays passing through the bedrock (especially to muon particles). This implies that they may have accumulated a certain amount of cosmogenic nuclides at depth before being exhumed by the collapse event. Considering that all collected samples were located at depths of ~100 m from the top of the cliffs, the inherited 36Cl component was estimated from a tunnel blank (Table 4) collected in a tunnel 100 m below the mountain surface located close to the study area and was determined to be ~1.18 × 105 atoms g−1. Additionally, the studied samples brought to the surface by the collapse event have been affected by different denudation rates. Considering the almost vertical inner wall inclination angle and the shielding effect of the sinkholes against external wind and rain, as well
4. Results and discussion In this work, a methodology for accurate analysis of 36Cl at very low concentrations in geological applications, including transmission and detection of 36Cl, identification of isobars, and quantitative calibration of the 6 MV AMS system at UTTAC, was established. The background level obtained with a commercial blank sample prepared from ACSgrade NaCl was 36Cl/Cl ~ 3 × 10−15 (Fisher Scientific, USA). In addition, an isotope dilution-accelerator mass spectrometry (ID-AMS) technique was used to measure the natural Cl content in tiankeng
Table 4 Data sheet for tiankeng exposure dating at UTTAC. Sample ID
9# Tunnel blank 20# HuangJing-TK 22# DaCao TK 24# ChuanDong-TK 26# DaTuo-TK 32# DSW-E02
Calcite wt. for AMS (g)*1
Cl carrier wt. (μg)*2
35
Cl/37Cl*3
Meas. val.
1 S.D.
Meas. val.
1 S.D.
Meas. val.
1 S.D.
Meas. val.
20.004 19.999 20.016 20.071 20.071 20.015
0.05% 0.05% 0.05% 0.05% 0.05% 0.05%
1482 1484 1485 1482 1487 1480
0.001% 0.674% 0.673% 0.675% 0.672% 0.676%
10.85 18.10 24.92 11.81 47.47 28.38
0.11% 0.20% 0.06% 0.20% 0.50% 0.23%
6.1 × 10−14 7.7 × 10−13 4.9 × 10−13 1.6 × 10−12 9.7 × 10−13 4.0 × 10−13
*1 Net weight used for AMS without insoluble materials in the sample solution. *2 Total spiked wt. of 99.4% 35-enriched Cl carrier from Aldrich Co. 33
36
[36Cl] (atoms g−1)
Minimum exposure age (yr)
1 S.D.
Meas. val.
1 S.D.
Meas. val.
1 S.D
4.16% 4.50% 5.18% 4.40% 4.50% 5.27%
1.18 × 105 1.25 × 106 7.44 × 105 3.02 × 106 1.34 × 106 5.86 × 105
4.16% 4.61% 5.27% 4.51% 4.63% 5.36%
/ 9.05 × 104 4.34 × 104 4.02 × 105 1.04 × 105 2.78 × 104
/ 8.37% 8.91% 8.22% 8.45% 8.80%
Cl/Cl
Nuclear Inst. and Methods in Physics Research B 459 (2019) 29–35
H. Shen, et al.
Fig. 4.
36
Cl exposure ages and corresponding geologic era.
limestone. The measurement of the 35Cl/37Cl ratio allows an accurate determination of chlorine content in the sample and enables simultaneous measurement of 36Cl and Cl with a single AMS sample preparation and without any elaborate or time-consuming procedures. Furthermore, the sample size required is reduced by the addition of the spike, which plays the role of carrier and provides an accurate analysis of chlorine content, even at the very low concentrations in geological applications. To obtain reliable exposure ages of the largest tiankengs in the world, the atoms of different target elements producing 36Cl in tiankeng samples were measured. Additionally, the cosmic ray shielding effect caused by mountains and sloped surface was considered. The exposure ages of the samples from the five super tiankengs in South China were finally determined to vary from several tens of thousands of years to several hundreds of thousands of years, all within the Quaternary and almost entirely within the upper Pleistocene, as shown in Fig. 4. Most tiankengs in China are relatively youthful features, as they truncate the topography of solution dolines and conical hills within fengcong karst. However, such evidence only dates the surface collapse that created the open tiankeng. Development of the underground cavern must have taken longer. The sheer size of many tiankengs suggests a very long history that could allow time for the erosional removal of the huge volumes of missing rock. We obtained the initial ages of different Chinese tiankengs, which provide significant evidence for activity associated with new tectonic movement of the Eurasian continent in the Quaternary.
Acknowledgments The authors gratefully acknowledge the technical staff at UTTAC for smooth accelerator operation. This work was supported by the National Natural Science Foundations of China (NSFC) under grant Nos. 11565008, 11775057, 11765004, 11675269, 11575296, 11705287, and 11665006, the Guangxi Natural Science Foundation under grant Nos. 2017GXNSFFA198016 and 2018JJA110037, the Guangxi Excellence Scholar Program, the Fundamental Research Funds from the Institute of Karst Geology, CAGS, grant No. 200716, and KAKENHI grant Nos. 24110006, 26600138, and 15H02340 from JSPS. References [1] Zhu Xuewen, Tony Waltham, Tiankeng: definition and description, Speleogenesis Evol. Karst Aquifers 4 (1) (2006) 1–8. [2] T. Waltham, Cave and Karst Sci. 32 (2–3) (2005) 51. [3] Zhu Xuewen, Chen Weihai, Tiankengs in the Karst of China, Carsologica 25 (1) (2006) 7–23. [4] Zhang Ji’an, Liu Jingrong, Discussion on the formation of Dashiwei Tiankeng and related problems, Land Resour. Southern China 2 (2008) 32–34. [5] H.W. Bentley, F.M. Phillips, S.N. Davis, et al., 36Cl in the terrestrial environment, Handbook Environ. Isotope Geochem. 2 (1) (1986) 427–475. [6] M. Houmark-Nielsen, H. Linge, D. Fabel, et al., Cosmogenic surface exposure dating the last deglaciation in Denmark: discrepancies with independent age constraints suggest delayed periglacial landform stabilization, Quaternary Geochronol. 13 (6) (2012) 1–17. [7] Y. Matsushi, K. Sasa, T. Takahashi, et al., Denudation rates of carbonate pinnacles in Japanese Karst areas: estimates from cosmogenic 36Cl in calcite, Nucl. Instr. Meth. Phys. Res. 268 (7–8) (2010) 1205–1208. [8] S. Zerathe, T. Lebourg, R. Braucher, et al., Mid-Holocene cluster of large-scale landslides revealed in the Southwestern Alps by 36Cl dating. Insight on an Alpinescale landslide activity, Quaternary Sci. Rev. 90 (1) (2014) 106–127. [9] Wang Yue, Y. Nagashima, et al., Determining limestone erosion rate with 36Cl AMS measurement, Phys. Exp. 25 (3) (2005) 11–14. [10] Y. Sun, The study of lower crust flow and structure of the upper mantle beneath SE Tibetan plateau, Central South University, 2013 (Dissertation, Ph.d. thesis, in Chinese). [11] Z. Song, The geomorphology of the typical rivers in the eastern tibet and their active tectonic implications, China Earthquake Administration, 2014 (Dissertation, Master thesis, in Chinese). [12] K. Sasa, et al., Nucl. Instr. Meth. Phys. Res. Sect. B 361 (2015), https://doi.org/10. 1016/j.nimb.2015.04.028. [13] S. Hosoya, et al., Nucl. Instr. Meth. Phys. Res. B 406 (2017) 268–271. [14] S. Hosoya, et al., Nucl. Instr. Meth. Phys. Res. B 438 (2019) 131–135. [15] P. Sharma, P.W. Kubik, U. Fehn, H.E. Gove, K. Nishiizumi, D. Elmore, Development of 36Cl standards for AMS, Nucl. Instr. Meth. Phys. Res. B 52 (1990) 410–415. [16] R.B. Firestone, Table of Isotopes, 8th ed., John Wiley & Sons, New York, 1998. [17] J.O. Stone, G.L. Allan, L.K. Fifield, R.G. Cresswell, Geochim. Cosmochim. Acta 60 (4) (1996) 679–692. [18] M.G. Zreda, Development and calibration of the cosmogenic “Cl surface exposure dating method and its application to the chronology of late Quatemary glaciations” (Ph.D. dissertation), Institute of Mining and Technology Socorro, New Mexico, 1994. [19] M.G. Zreda, F.M. Phillips, D. Elmore, P.W. Kubik, P. Sharma, Cosmogenic chlorine36 production rates in terrestrial rocks, Earth Planet. Sci. Lett. 105 (1991) 94–109.
5. Conclusion This study applied 36Cl measurements with isotope dilution for quantitative exposure dating of carbonate rocks from karst tiankeng. Exposure ages of 20–400 ka were determined for the five super tiankengs in Guangxi Province, southern China. These ages provide significant evidence for new tectonic movement of the Eurasian continent in the Quaternary, especially in the upper Pleistocene. We intend to follow up this study with 10Be and 41Ca determinations in carbonate rocks. Measurements of these nuclides may be accompanied by some difficulties [23,24], especially for 41Ca [25], in the present sample preparation strategy. However, this multi-nuclide coupling will provide significant clues to test the exposure dates in the Pleistocene–Holocene epochs and to produce a more detailed modeling of Chinese tiankeng landscape evolution in karst areas. In addition, to study the potential weathering effects on determined exposure ages, more than one sample should be taken from different areas of the same tiankeng, as the oldest date among the spread should in principle be closest to the true exposure age. 34
Nuclear Inst. and Methods in Physics Research B 459 (2019) 29–35
H. Shen, et al. [20] J. Dunne, et al., Scaling factor for the rates of production of cosmogonic nuclides for geometric shielding and attenuation at depth on sloped surfaces, Geomorphology 27 (1999) 3–11. [21] Dong, et al., Nucl Instr. Meth. Phys. Res. B 294 (2013) 611–615. [22] R. Maire, Giant shafts and underground rivers of the Nakanai Mountains (New
Britain), Spelunca Suppl. 2 (1981) 8–30. [23] R. Brancer, et al., Geochim. Cosmochim. Acta 69 (6) (2005) 1473–1478. [24] S. Merchel, R. Braucher, L. Benedetti, O. Grauby, D.L. Bourlès, Quat. Geochronol. 3 (2008) 299. [25] W. Henning, et al., Science 236 (4802) (1987) 725–727.
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