Weathering Intensity in Lowland River Basins: From the Andes to the Amazon Mouth

Bouchez, Julien; Gaillardet, Jérôme; Blanckenburg, Friedhelm von

doi:10.1016/j.proeps.2014.08.063

Cited by 30 publications

(18 citation statements)

References 28 publications

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“…According to the negative correlation between δ 41 K and WI (Fig. 3A), rivers draining mountainous regions are expected to have higher dissolved δ 41 K values because silicate WI is low in regions of higher denudation rates (40), whereas lower δ 41 K are expected in rivers draining tectonically stable cratons where denudation rates are low and primary silicate minerals are intensely weathered (30,41). B A Fig.…”

Section: Relationship Between δ 41 K Values Of Riverine Dissolved Loamentioning

confidence: 99%

K isotopes as a tracer for continental weathering and geological K cycling

Beard

et al. 2019

Proc. Natl. Acad. Sci. U.S.A.

114

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The causal effects among uplift, climate, and continental weathering cannot be fully addressed using presently available geochemical proxies. However, stable potassium (K) isotopes can potentially overcome the limitations of existing isotopic proxies. Here we report on a systematic investigation of K isotopes in dissolved load and sediments from major rivers and their tributaries in China, which have drainage basins with varied climate, lithology, and topography. Our results show that during silicate weathering, heavy K isotopes are preferentially partitioned into aqueous solutions. Moreover, δ41K values of riverine dissolved load vary remarkably and correlate negatively with the chemical weathering intensity of the drainage basin. This correlation allows an estimate of the average K isotope composition of global riverine runoff (δ41K = −0.22‰), as well as modeling of the global K cycle based on mass balance calculations. Modeling incorporating K isotope mass balance better constrains estimated K fluxes for modern global K cycling, and the results show that the δ41K value of seawater is sensitive to continental weathering intensity changes. Thus, it is possible to use the δ41K record of paleo-seawater to infer continental weathering intensity through Earth’s history.

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Section: Relationship Between δ 41 K Values Of Riverine Dissolved Loamentioning

confidence: 99%

K isotopes as a tracer for continental weathering and geological K cycling

Beard

et al. 2019

Proc. Natl. Acad. Sci. U.S.A.

114

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“…This conclusion is in line with Bouchez et al . [, ] who found only minor increases in weathering during transfer of sediment through the floodplains.…”

Section: Weathering Intensities Erosion Rates and Denudation Ratesmentioning

confidence: 99%

“…Evidence for this explanation is reported in Bouchez et al . [], who find (a) an overall low degree of weathering in rivers draining Andean sediments, an observation that these authors attribute to the fact that Andean source rocks are recycled, preweathered metasedimentary rocks, and (b) that the dissolved weathering export only increases slightly across the lowlands [ Bouchez et al ., ]. Estimates of D will be too low if a value for [ 9 Be] parent is used that exceeds the true value. We assume a [ 9 Be] parent of 2.5 × 10 −6 g/g for all three geomorphic parts of the Amazon basin throughout.…”

Section: Weathering Intensities Erosion Rates and Denudation Ratesmentioning

confidence: 99%

A test of the cosmogenic¹⁰Be(meteoric)/⁹Be proxy for simultaneously determining basin-wide erosion rates, denudation rates, and the degree of weathering in the Amazon basin

Wittmann

Blanckenburg

Dannhaus

et al. 2015

J. Geophys. Res. Earth Surf.

113

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We present an extensive investigation of a new erosion and weathering proxy derived from the 10 Be(meteoric)/ 9 Be(stable) ratio in the Amazon River basin. This new proxy combines a radioactive atmospheric flux tracer, meteoric cosmogenic 10 Be, with 9 Be, a trace metal released by weathering. Results show that meteoric 10 Be concentrations ([ 10 Be]) and 10 Be/ 9 Be ratios increase by >30% from the Andes to the lowlands. We can calculate floodplain transfer times of 2-30 kyr from this increase. Intriguingly however, the riverine exported flux of meteoric 10 Be shows a deficit with respect to the atmospheric depositional 10 Be flux. Most likely, the actual area from which the 10 Be flux is being delivered into the mainstream is smaller than the basin-wide one. Despite this imbalance, denudation rates calculated from 10 Be/ 9 Be ratios from bed load, suspended sediment, and water samples from Amazon Rivers agree within a factor of 2 with published in situ 10 Be denudation rates. Erosion rates calculated from meteoric [ 10 Be], measured from depth-integrated suspended sediment samples, agree with denudation rates, suggesting that grain size-induced variations in [ 10 Be] are minimized when using such sampling material instead of bed load. In addition, the agreement between erosion and denudation rates implies minor chemical weathering intensity in most Amazon tributaries. Indeed, the Be-specific weathering intensity, calculated from mobilized 9 Be comprising reactive and dissolved fractions that are released during weathering, is constant at approximately 40% of the total denudation from the Andes across the lowlands to the Amazon mouth. Therefore, weathering in the Amazon floodplain is not detected. (2015), A test of the cosmogenic 10 Be(meteoric)/ 9 Be proxy for simultaneously determining basin-wide erosion rates, denudation rates, and the degree of weathering in the Amazon basin,

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“…One explanation for this stability in the 1021 two regimes is that sediments contained in the Amazon lowlands are pre-weathered 1022 in the Andes and potentially depleted of their 9 Be (Wittmann et al, 2015). Other 1023 studies Bouchez et al, 2014) we first briefly repeat the types of rate information derived from both types, so that 1031 true geologic causes can be distinguished from differences introduced by possible 1032 methodological artefacts. Such comparison is possible for the Amazon basin, where 1033 both methods have been applied.…”

Section: Be/mentioning

confidence: 99%

The geological significance of cosmogenic nuclides in large lowland river basins

Wittmann

Blanckenburg

2016

Earth-Science Reviews

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12Measured from a bag of sand taken in large rivers, the concentration of cosmogenic 13 nuclides such as in situ-produced 10 Be, 26 Al, and 14 C can be used to constrain the 14 mean sediment flux of the headwaters and assess the duration of sediment storage 15 from source to sink. We revisit these principles, with examples from the Amazon and 16 Ganga basins. 17We identify two end member cases controlling the concentration of cosmogenic 18 nuclides in lowland river sediment: 1) if the time scale of floodplain sediment storage 19 is short compared to the half-life of the nuclide, in situ cosmogenic nuclide 20 concentrations are not significantly altered in lowland basins. In this case the 21 concentration of e.g. in situ-produced 10 Be in the sediment taken in the lowland 22 2 basin equals in most cases that exported from the sediment source area, but the 23 variability in nuclide concentrations between headwater streams is significantly 24 averaged-out. Thus to convert the measured river sediments' in situ cosmogenic 25 nuclide concentration into a catchment-wide denudation rate, production rates are 26 scaled to include those of the sediment-producing mountainous areas only, rather 27 than the entire catchment area. Nuclide production in the lowlands, where no 28 sediment is being produced, is hence excluded. This correction is termed "floodplain 29 Be ratio on Be adsorbed to particles provides the denudation 51 rate. Both are similar to denudation rates from in situ 10 Be. We show that this 52 system responds more sensitively to sediment storage than the in situ system, and 53 both accumulation and decay of meteoric concentrations may thus be used to 54 determine sediment residence time. 55 56

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Weathering Intensity in Lowland River Basins: From the Andes to the Amazon Mouth

Cited by 30 publications

References 28 publications

K isotopes as a tracer for continental weathering and geological K cycling

K isotopes as a tracer for continental weathering and geological K cycling

A test of the cosmogenic¹⁰Be(meteoric)/⁹Be proxy for simultaneously determining basin-wide erosion rates, denudation rates, and the degree of weathering in the Amazon basin

The geological significance of cosmogenic nuclides in large lowland river basins

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Weathering Intensity in Lowland River Basins: From the Andes to the Amazon Mouth

Cited by 30 publications

References 28 publications

K isotopes as a tracer for continental weathering and geological K cycling

K isotopes as a tracer for continental weathering and geological K cycling

A test of the cosmogenic10Be(meteoric)/9Be proxy for simultaneously determining basin-wide erosion rates, denudation rates, and the degree of weathering in the Amazon basin

The geological significance of cosmogenic nuclides in large lowland river basins

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A test of the cosmogenic¹⁰Be(meteoric)/⁹Be proxy for simultaneously determining basin-wide erosion rates, denudation rates, and the degree of weathering in the Amazon basin