A clear understanding of erosion processes is fundamental in order to comprehend the
evolution of actively deforming mountain ranges. However, the relative contributions
of tectonic and climatic factors and their feedbacks remain highly debated. In order to contribute to the debate, we quantify basin-wide
denudation rates from cosmogenic
The rapid evolution of active mountain chains
depends on complex interactions between tectonically sustained rock uplift
and climate-driven processes
Variations in erosion were found to correlate with climate fluctuations that
govern glacial processes
In this study, we aim at determining the magnitude of denudation in the Pamir
on the timescale of 10
The Pamir is located at the northwestern end of the India–Asia collision
zone. The series of sutures, magmatic belts and crustal blocks is assumed to
consist of along-strike equivalents of the Tibetan Plateau that accreted to
the Eurasian plate during the Paleozoic to Mesozoic
Regional setting of the Panj River system and sample locations (CN:
cosmogenic nuclide; a.s.l.: above sea level). Panel (
The active frontal range of the Pamir curves nearly 180
The position of the Pamir at the transition between the Westerlies and the
ISM makes the region highly sensitive to variations in atmospheric
circulation patterns. The Tropical Rainfall Measurement Mission (TRMM)
spatial product 3B42 V7
Past fluctuations of glacial extents may affect erosion and sediment yields,
when estimates integrate over longer time spans or when erosion products
still reside in the basins. Glacial remnants on the Pamir Plateau attest to
significant climatic variations during the late Quaternary. Their
successively less extensive advances correspond to an increasing aridity in
central Asia
Beryllium-10 concentrations in modern fluvial sediments are proportional to
the time of rock surface exposure to interactions with cosmic rays and,
consequently, inversely proportional to the rate of landscape lowering by
weathering and physical erosion
The inverse proportionality of
We sampled 11 locations of the Panj River network
(Fig.
We chose locations before confluences as far as possible from upstream tributaries. This ensured a complete mixing of sediment grains that represent all upstream source areas. Locations where slope failure or fan sedimentation from minor tributaries indicated local perturbations have been avoided. We sampled the uppermost 1–3 cm of the sediment in the active river channel. All samples consisted of predominantly sand-sized, polymineral material.
Thus, we collected 3–5 kg of fluvial sediment per sample to retrieve
sufficient quantities of quartz for Be extraction and subsequent
The polymineral sediment samples required the enrichment of quartz before
starting chemical cleaning and Be extraction because of relatively low quartz
but high feldspar contents (up to 50 %) as well as mineral coatings.
Quartz enrichment included wet sieving (with a focus on
250–500
Atmospheric
Adding Nb (6 times the dry oxide weight) finalized the target preparation.
AMS measurements were conducted at DREAMS (Helmholtz-Zentrum
Dresden-Rossendorf, 6 MV, Cu cathode) using the in-house standard SMD-Be-12
We calculated
Variations in permanent snow and ice cover during the last millennia are
unknown. Palaeorecords only give information on glacier extents on longer
timescales and do not include the distribution of snow cover nor do MODIS
data distinguish between snow and ice. Hence, the MODIS data from 2010 may
underestimate the snow and ice cover during the last millennium.
Nevertheless, we assume only low variations because glacier extents have
already largely retreated since the Last Glacial Maximum, corresponding to an
increasing aridity
We corrected production rate estimates by topographic shielding. Shielding
factors for each GDEM raster cell refer to the horizon line within a 10 km
distance according to the method of
The raster-cell-resolved production rates and shielding factors show
non-normal distributions skewed to polymodal according to the basin
topography. We used the arithmetic mean and calculated uncertainties based on
the standard deviation to account for the high value variability within
basins. The uncertainties of denudation rates represent the sum of the AMS
measurement error of the
Basin-wide denudation rates have been found to correlate with altitude,
slope, relief, precipitation and glaciated area
The median and 0.25 and 0.75 quartiles of each parameter were used for
(multiple) linear regression analyses. We thus expect to discriminate between
the potential topographic and climatic parameters that can explain the
variance in denudation. The use of quartiles allows us to test the relevance
of the general condition within a basin and to evaluate the influence of low
or high values of individual parameters. Linear regression analyses represent
a standard tool for a simple, straightforward evaluation of basin
characteristics. Sensu stricto, correlation analyses based on linear
regression are designed for normally distributed data. We used the robust
linear regression model available in the R environment
The Panj River network is strongly asymmetric. The trunk reach connects
tributary outlets from south to north close to the western drainage divide.
Basins of the southern Panj and of the major Panj tributaries show
preferential east–west lengthened shapes (Fig.
The median basin altitudes slightly decrease from 4800 to 4200 m a.s.l.
along the course of the Panj (cf. Fig.
Frequency distributions of altitude
Details on sampling sites and related upstream drainage area (sample
basin). Ice: permanent ice and snow cover, based on the year 2010 from MODIS
MCD12Q1
The relative proportions of slopes within basins correspond to the respective
altitude distributions. Highly variable slopes display strongly bimodal
distributions, where east–west elongated basins range over plateau and
marginal basin sections (Fig.
Areas of permanent ice and snow cover reflect the predominant moisture supply
from the northwest and south, while the central-eastern parts of the Pamir
remain arid (Fig.
The
Estimated production rates of
Parameters and results of denudation rate calculation. AMS
measurements were performed at DREAMS, Helmholtz-Zentrum Dresden-Rossendorf.
The
The two largest basins (TA23A and TA08B) reveal a high average denudation for
the entire Pamir, with rates of
The basin-wide rates of the major Panj tributaries (Gunt, Bartang, Vanj)
reveal strong contrasts in denudation across the Pamir, ranging from
0.05
Basin-wide denudation rates of along Panj and major tributary
samples (CN: cosmogenic nuclide; colour code represents magnitude of
denudation with green for low and red for high rates). Calculations based
upon AMS measurements of
We estimate the denudation rates of the lower sections of the Gunt (GUNT) and
the Bartang (BARlow) based on the measured data we have for the entire basin
and its upstream basin section. We scaled the denudation rates proportionally
to their relative area using a simple approximation. Accordingly, the
denudation rate of the entire basin (
The area factor
Approximated denudation rates of sub-basins using weighting factors
that account for basin area (
The absence of any trend with increasing basin size suggests no significant
nuclide acquisition during grain transit through the basin. Results reveal a
primary role of topographic basin parameters in variations in denudation
rates (Fig.
The relationship applies to measures of altitude differences (relief) within
a given area. The goodness of fit strongly depends on scale, i.e. the
reference area used to quantify the relief. The basin relief (BR,
Fig.
Robust linear regression analyses for denudation rates and the basin
parameters relief
The only slight variations in mean annual precipitation between 270 and
380 mm (based on TRMM data) cannot explain the pattern of denudation
(
We performed a multiple linear regression analysis with two predictors for
denudation to test for additive effects. Other regression models are possible
but inappropriate for the small data set and less comparable to any linear
relations found in the one predictor linear regression analysis. The multiple
linear regression addresses the question of whether any predictor requires
preconditions to become effective. Including more components resulted in
multicollinearity and insignificant effects on the goodness of correlation.
The best results were obtained by combining the 0.75 quartiles of slope and
TRMM precipitation data. The
For a robust interpretation of the Pamir denudation rates, it is important to consider the scales of averaging in terms of time and space.
The denudation rates in the Pamir are averaged over timescales of 10
However, the estimated
Another point to consider in terms of timescales is the nuclide build-up
during the transport of grains from the source rock to the sampled site.
Robust cosmogenic nuclide-derived denudation rates require short grain travel
times through the sampled basin when compared to
Centennial- to millennial-scale
On the 10
The basin-wide denudation rates represent average estimates of the upstream
areas. They may be biased in tectonically active landscapes, in which certain
basin sections deliver disproportionally high amounts of sediments to the
river channels, for example in the form of landslides
In particular, the sediment delivery from snow- and ice-covered areas
requires attention. There is currently no ideal way to deal with such
entirely shielded areas of a basin. We excluded areas based on the permanent
snow and ice cover (MODIS MCD12Q1,
However, the snow- and ice-covered areas deliver sediments that likely were
completely shielded before (glacio-)fluvial transport. Their negligible
Grains carried by glaciofluvial streams dilute the CN (cosmogenic nuclide)
concentration with zero-CN grains that mimic instant erosion independent of
the actual rate below the glacier. The effect on the total basin-wide
denudation rate depends on the efficiency of glaciers to erode. But the
efficiency of glacial processes and their influence on denudation is highly
debated
The relative abundance of glaciers seems to cause no systematic effects in the basins we studied as the percentage of snow- and ice-covered areas did not correlate with denudation rates.
The denudation rates of basins along the Panj vary only slightly from the
Pamir average of
Overall, the
Both process rates have lower values in southern Panj basins (denudation:
0.58
The slope-weighted estimates (ISHs and ISHn) highlight the local change in
denudation from 0.46
Variation in basin-averaged denudation rates and fluvial incision
along the Panj (CN: cosmogenic nuclide; OSL: optically stimulated
luminescence). The along-Panj samples (filled circles) represent
The magnitude of denudation is comparable with rates determined across the
steep escarpment of the Himalaya
Of all the predictors we have looked at, steep slopes (0.75 quartiles) are
also the primary factor controlling denudation in the Pamir (
The basin-wide denudation rates of
The prevalent steep slopes, in particular, affect denudation rates in the
Pamir. The 0.75 quartiles of basin slope steepness explain about 80 % of
the variance in denudation rates (
The magnitude of denudation is similar to rates determined across the south
Himalayan escarpment and Tibetan Plateau
We would like to thank the Department of Meteorology and Hydrology of
Tajikistan for the support and organization during fieldwork in 2011. We are
also grateful for the support of the DREAMS operator team facilitating AMS
beam time, with special thanks to Shavkat Akhmadaliev, Stefan Pavetich and
René Ziegenrücker. Furthermore, we thank the DFG for funding our
research associated with the TIPAGE project (GI362/4-1). We used GMT
(P. Wessel and W. H. F. Smith, New, improved version of the Generic Mapping
Tools released, EOS Trans. AGU, 79, 579, 1998), QGIS (