Geophysical Research Letters
Supporting Information for
Incision of the Yangtze River Gorge at the First Bend determined by three-nuclide
burial dating
Devin McPhillips*1, Gregory D. Hoke1, Jing Liu-Zeng2, Paul R. Bierman3, Dylan H. Rood4,
Samuel Niedermann5
1
Department of Earth Science, Syracuse University, Syracuse, NY 13244 USA.
2
State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake
Administration, Beijing 100029, P. R. China.
3
Department of Geology, University of Vermont, Burlington, NY 05405 USA.
4
Department of Earth Science and Engineering, Imperial College London, South Kensington
Campus, London SW7 2AZ, UK & Scottish Universities Environmental Research Centre, East
Kilbride G75 0QF, UK.
5
GFZ German Research Center for Geosciences, Telegrafenberg, 14473 Potsdam, Germany.
*Corresponding author: devin.mcphillips@gmail.com
CONTENTS OF THIS FILE
Text S1 to S4
Figures S1 to S8
ADDITIONAL SUPPORTING INFORMATION (FILES UPLOADED SEPARATELY)
Tables S1 to S6
INTRODUCTION
This material has four main section: (1) Field descriptions of the caves and samples,
including four photos; (2) Cosmogenic nuclide data, including supplementary text/figure
regarding measurements and complete data tables (as a separate Excel file); (3)
Description of the modeling procedure used for burial dating, including one equation, two
figures, and several data tables (as a separate Excel file); and (4) Details of the
implementation of the Qtqt thermochronologic model, including a plot of the best fit model
relative to the data.
TEXT S1. DESCRIPTIONS OF CAVES AND SAMPLES
Y12 – C1. Cave C1 travels a considerable distance upstream from its opening in a minor
tributary near the Yangtze River at the First Bend. Constrictions prevented exploration
along its full length. At the time of sampling in 2012, as well as upon returning to the cave
in 2013, the floor was covered by flowing water and coarse, sandy sediment (see Figure S1
for a example from a different cave, also at river level). The samples were collected from
the thalweg of the stream approximately 20 m from the entrance.
Y13 – C2. Cave C2 is located approximately 30 m above the floor of a steep debris flow
channel in marble cliffs (Figure S2). The cave is shallow, less than 15 m deep, and 6 to 8 m
in diameter. The rear of the cave terminated in sparry calcite. The cave floor was dry. We
estimated that the cave was 40 to 50 m below the level of the relict landscape at the top of
the cliff. We sampled ~1 cm quartz pebbles extracted from bedded sandstones with mud
drapes (Figure S3). Scours were evident throughout the deposit, which filled a depression
in the cave floor. There is evidence of a minor travertine cap, but it has been worn away by
human traffic.
Y13 – C3. Cave C3 is located approximately 20 m above the floor of a steep debris flow
channel in marble cliffs. Another cave opening is visible at the same elevation on the
opposite side of the valley, although that cave is very difficult to access on a vertical face.
Cave C3 is ~70 m below the top of the cliffs. The cave is very deep. We explored several
hundred meters beyond the entrance in tall, narrow chambers that rise at a shallow
gradient from the opening. The floor was formed on marble breccia, which we interpret be
the product of roof collapse in the past. We sampled within 5 m of the cave opening, where
we could access quartz pebble conglomerate beneath the marble breccia. Both the floor of
the cave and the conglomerate surface below were dry.
Y13 – C4 Cave C4 is located on a steep hillslope, 90 m above a tributary channel. The cave is
30 m deep and ~10 m in diameter, with an oblate cross-section. The rear of the cave
appeared to terminate in sparry calcite but travertine obscured some sections. We sampled
quartz pebbles from interbedded sandstones and mudstones located beneath a thick
travertine cap, approximately 10 m inside the cave. As in all cases, the pebbles were ~1 cm
in diameter and well-rounded (Figure S4). The floor of the cave was dry. Given the steep—
but not vertical—angle of the hillslope, the sample had tens of meters of overburden above
it.
Figure S1. Stream flow in an active cave at river level, analogous to Y12-C1.
Figure S2. Descending from the cave entrance at Y13-C2.
Figure S3. Bedded sandstone from which C2 was sampled.
Figure S4. Rounded quartz pebbles from C4.
TEXT S2. COSMOGENIC NUCLIDE DATA
Details of Be-10 and Al-26 measurements
The quartz was purified at Syracuse University. The process involved crushing and sieving to
0.25 to 0.8 mm, leaching in concentrated HCl-HNO3 and 1 to 5% by volume HF-HNO3,
followed by magnetic separation. Beryllium and aluminum were extracted from quartz at the
University of Vermont Cosmogenic Laboratory. Prior to quartz dissolution, Be-9 and Al carrier
solutions were added for isotope dilution. The Be-9 carrier was made from beryl and calibrated
to SPEX 1000 ppm standard. The Al carrier was SPEX 1000 ppm. Following dissolution in hot
HF, Be and Al were separated by ion-exchange chromatography, precipitated as hydroxide gels,
and ignited to form oxides. Accelerator mass spectrometer (AMS) measurements of 26Al were
made at Scottish Universities Environmental Research Centre [Xu et al., 2015]. 26Al/27Al ratios
were normalized to standard Z92-0222 with an assumed 26Al/27Al ratio of 4.11×10-11 [Nishiizumi,
2004]. AMS measurements of 10Be were made at Lawrence Livermore National Laboratory
[Rood et al., 2010], using standards normalized to NIST standard material with a reported 10Be
/9Be ratio of 2.79 × 10-11 and a 10Be half-life of 1.387 Myr [Nishiizumi et al., 2007; Chmeleff et
al., 2010; Korschinek et al., 2010]. Because samples were split between multiple AMS facilities,
blank measurements were not available for all samples; thus, we use long-term average values
for the fume hoods at the University of Vermont laboratory instead. Precise replication of
laboratory standard quartz supports this approach (Tables S1 and S2).
Details of Ne-21 Measurements
The noble gas analysis was carried out at GFZ Potsdam. Prior to the analysis, quartz
samples were ground to <100 μm in an agate mill in order to open part of the fluid
inclusions and thereby reduce the contribution of trapped atmosphere-like Ne. Samples
were then wrapped in Al foil and loaded into the sample carrousel above the extraction
furnace, which was baked at 100°C for about one week. Noble gases were extracted by
stepwise heating (at 400, 600, 800, and 1200° C) for 20 minutes each. In addition, aliquots
of all samples were crushed in vacuo to determine the isotopic composition of Ne trapped
in fluid inclusions. After gas extraction by either heating or crushing, chemically active
gases were removed in two Ti sponge and two SAES (ZrAl) getters, and He, Ne, and Ar-KrXe were separated from each other by trapping in a cryogenic adsorber at 11 K and
subsequent sequential release. Noble gas concentrations and isotopic compositions were
determined in a VG5400 sector field mass spectrometer, and were corrected for isobaric
interferences, instrumental mass fractionation, and analytical blanks. Further details about
the analytical procedures can be found in Niedermann et al. (1997).
The 21Ne/20Ne ratios determined in the crushing extractions were between 0.00302 and
0.00323 (Table S3, Fig. S5), not quite overlapping each other within 2 error limits.
Therefore, we used the individual values to calculate 21Ne excesses in each heating step
from the measured 21Ne/20Ne ratios and 20Ne concentrations. Excess 21Ne may be either
cosmogenic or nucleogenic.
In the three-isotope plot (Fig. S5), the 400-800° C data generally align along the “spallation
line”, i.e. the mixing line between atmospheric and cosmogenic Ne (Niedermann et al.,
1993), or along a parallel line through the crusher data. Most 1200° C data, however, plot
considerably above the line. Because cosmogenic Ne is quantitatively degassed from quartz
at 800° C (Niedermann, 2002), the 21Ne excesses observed in the 1200° C steps (15% of the
total at most) must be of nucleogenic origin. A significant contribution of nucleogenic 21Ne
in the lower heating steps is not indicated, and the total cosmogenic 21Ne has thus been
estimated for each sample by summing up the 21Ne excesses in the 400-800° C steps (Table
S3).
Table S1. Be-10 results (see Supplementary Excel File, DataSet S1)
Table S2. Al-26 results (see Supplementary Excel File, DataSet S1)
Table S3. Ne-21 results (see Supplementary Excel File, DataSet S1)
Figure S5. Neon three-isotope diagram for stepwise heating extractions of quartz samples.
Symbol colors indicate extraction temperatures (white – 400°C, gray – 600°C, red – 800°C,
black – 1200°C) as well as crushing extractions (green). “Spallation line” denotes the
mixing line of atmospheric and cosmogenic Ne (Niedermann et al., 1993); mfl is the mass
fractionation line. 2 uncertainties are shown.
0.112
mfl
0.110
20
Ne/ Ne
0.108
22
0.106
0.104
Spallation line
Y12-C1
Y13-C2
Y13-C3
Y13-C4
0.102
AIR
0.100
0.003
0.004
0.005
21
0.006
20
Ne/ Ne
TEXT S3. BURIAL AGE MODEL AND CALCULATIONS
0.007
0.008
In order to explain our observation, particularly the unusual combination of high (>6.2) Al26/Be-10 ratios, high (>>4) Ne-21/Be-10 ratios, and low radionuclide concentrations
(relative to the modern river sediment), we developed a model of in situ muogenic nuclide
production. As described in the main text, this model assumes that TCN production by
neutron spallation ceased upon burial, but production by fast and negative muons
continued (albeit at potentially insignificantly low rates, initially). Steady erosion of the
overburden above the roof of the caves caused a progressive increase in the rate of
muogenic production. Figure S6 details the steps required to calculate burial ages using
this model.
Figure S6. Workflow diagram describing steps used for burial age calculations, assuming in
situ muogenic production.
Table S4. Muon attenuation length approximations (see Supplementary Excel File, DataSet
S1)
Table S5. Erosion rates derived from initial nuclide concentrations (see Supplementary
Excel File, DataSet S1)
Table S6. Burial age results (see Supplementary Excel File, DataSet S1)
THERMOCHRONOLOGIC MODELING AND RESIDUALS
Qtqt uses a Markov Chain Monte Carlo approach to find best-estimate parameters and
generate confidence intervals (Gallagher, 2012). The approach is robust, given a sufficiently
large set of forward model iterations. We use 30,000 “burn-in” iterations and 60,000 “postburn-in”. This is sufficient, as indicated by a plot of log likelihood against model iteration
number, which shows little structure (Figure S7). The best-fit parameters closely
reproduce the observed ages, as illustrated by this plot of observed and predicted age
versus elevation (Figure S8). The scatter in the observed ages at any single elevation
reflects a typical level of reproducibility for (U-Th)/He in apatite.
Figure S7. Plot of iteration number and log-likelihood value. Little structure indicates
sufficiently sample set.
Figure S8. Age-elevation plot presenting observed and best-estimate predicted (U-Th)/He
ages, as output from the Qtqt model.
REFERENCES
Bierman, P., and E. J. Steig (1996), Estimating rates of denudation using cosmogenic isotope
abundances in sediment, Earth Surf. Process. Landforms, 21, 125–139.
Chmeleff, J., F. von Blanckenburg, K. Kossert, and D. Jakob (2010), Determination of the
10Be half-life by multicollector ICP-MS and liquid scintillation counting, Nuclear
Instruments and Methods in Physics Research Section B: Beam Interactions with
Materials and Atoms, 268(2), 192–199, doi:10.1016/j.nimb.2009.09.012.
Gallagher, K. (2012), Transdimensional inverse thermal history modeling for quantitative
thermochronology, J. Geophys. Res., 117(B2), B02408–16, doi:10.1029/2011JB008825.
Granger, D. E., and P. Muzikar (2001), Dating sediment burial with in situ-produced
cosmogenic nuclides: theory, techniques, and limitations, Earth and Planetary Science
Letters, 118, 269–281.
Korschinek, G. et al. (2010), A new value for the half-life of 10Be by Heavy-Ion Elastic Recoil
Detection and liquid scintillation counting, Nuclear Instruments and Methods in Physics
Research Section B: Beam Interactions with Materials and Atoms, 268(2), 187–191,
doi:10.1016/j.nimb.2009.09.020.
Niedermann, S., T. Graf, J. S. Kim, C. P. Kohl, K. Marti, and K. Nishiizumi (1994), Cosmic-rayproduced ^2^1Ne in terrestrial quartz: the neon inventory of Sierra Nevada quartz
separates, 125, 341–355.
Niedermann, S., W. Bach, and J. Erzinger (1997), Noble gas evidence for a lower mantle
component in MORBs from the southern East Pacific Rise: Decoupling of helium and
neon isotope systematics, Geochimica et Cosmochimica Acta, 61(13), 2697–2715,
doi:10.1016/S0016-7037(97)00102-6.
Nishiizumi, K. (2004), Preparation of 26 Al AMS standards, Nuclear Instruments and
Methods in Physics Research …, 223-224, 388–392, doi:10.1016/j.nimb.2004.04.075.
Nishiizumi, K., M. Imamura, and M. W. Caffee (2007), Absolute calibration of 10 Be AMS
standards, Nuclear Instruments and …, 258(2), 403–413,
doi:10.1016/j.nimb.2007.01.297.