1 Introduction

2 Method and limitations

3 Results from the Tian-Shan

4 Results from the Himalayas

5 A High resolution10Be variability?

6 Conclusions

CRPGPoster n°: EP53E-1076

time

Dep

osi

t

10 B

eco

ncen

trat

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Determined by magnetostratigraphy

Analysis

1

2

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Hillslopeerosion

Progressive shieldingand decay

Exh

um

atio

n

and

rive

r in

cisi

on

1

2

3

P1,C1 P2,C2

P3,C3

Pmean, Cmean,→ Emean

time

0.5-10 Ma

Secondary cosmic flux

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4

2

δ18O

SM

OW (‰

)

0

1

2

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4

0 1 2 3 4 5 6 7 8 9 10

Den

udat

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(mm

.yr-1

)

Age (Ma)

KuitunYahaJingu

Surai

}Tian-Shan

Himalayas

0 1 2 3 4 5 6 7 8 9

Den

udat

ion

(mm

.yr-1

)

0

0.2

0.4

0.6

0.8

1

Age (Ma)

No shieldingcorrection

1σ envelop

Yaha 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 1 2 3 4 5 6 7 8

Age (Ma)

Den

udat

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(mm

.yr-1

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No shieldingcorrection

1σ envelop

Jingu

0

0.5

1

1.5

2

2.5

3

3.5

4

0 1 2 3 4 5 6 7 8 9

Age (Ma)

Den

udat

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(mm

.yr-1

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Charreau et al. 2011New data

No shieldingcorrection

1σ envelop

Kuitun

0

20

40

60

80

100120

140

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200220

240

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300320

340

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400380

cm Argi

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ltsFi

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Med

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oars

e sa

ndG

rave

lPe

bble

sB

ould

ers

Stratigraphicprofile

0 1 20.5 1.5 2.5

×104 at.g-1

10Be concentration

0

1

2

3

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5

6

1 2 3 4 5 6 7

Den

udat

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(mm

.yr-1

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Age (Ma)

No shieldingcorrection

1σ envelop

Po basin

Scotian basin

Ganga basin

Indus fan

Bengal fan

Peri-Alpineregion

West CoastSouth IslandNorthern Australia

Voring plateau

Northern Apennineforedeep

Linxia

Turfan

Hetian

Manas

Annanba

Anxi

North Sea

Williston basin

Mississippi basin

Jiuquan

Mergui Sumatra

Qingdongnan

Yinggehai

Bounty fan

Depression of Great LakesChu

Kashgar

Valley of Lakes

Baicheng

Kuitun

85°E

85°E

80°E

80° E

45°N 45°N

40°N 40°N

0 100 200 300 40050

Kilometres

N

Jingu

Yaha

Tarim Basin

Jungaar Basin

Urumqi

Theoretical evolution of the 10Be in a quartz grain from bed rock until analysis

Picture of the Jingu river section

→ The abrupt spike in the Kuitun section re-ported in Charreau et al. (2011) is not found in the nearby Jingu section: the spike is thus a local feature and does not imply a global clima-tic change.→ The Southern piedmont have denudation rates comparable to those of the Northern pied-mont, despite a dryer climate.

→ All the records are noisy but do not display a clear synchronous rise in denudation rates at the onset of quaternary glaciations.

Geographic location of the studied sections with the present day corresponding watersheds

Picture from the Surai Khola section.

Location of the section and present day watershed of the West Rapti river

Ebi Lake

Sediment accumulutaion rates since 65Ma, normalised to their highest value (Figure from Zhang et al., 2001)

Kuitun

Benthic foraminifera δ18O record(Zachos et al., 2001)

Age (Ma)

δ18 O

SMO

W (‰

)

Paleo-denudation rates from in-situ produced cosmogenic isotopes:Method and new results from the Tian-Shan and the Himalayas

1Puchol, N.; 1Blard; P.-H.; 1Charreau, J.; 1Pik, R. 1Lavé, J.; 2Bourles, D.; 3Dominguez S.1CRPG - Université de Lorraine, Vandoeuvre-lès-Nancy, France; 2CEREGE - Université de Provence, Aix-en-Provence, France; 3Geosciences Montpellier, Montpellier, France

Contact: puchol@crpg.cnrs-nancy.fr, 15 rue Notre Dame des Pauvres, 54500 Vandoeuvre-lès-Nancy, France

(1) Cosmogenic nuclides are produced in rock surfaces all over the basin at a rate Pi: the concentration in the sand at the outlet Cmean is inversely proportional to the average denudation rate Emean → Drainage basin must be stable over time or changes in its mean altitude must be quantified.

• Paleo-denudation rates (i.e. physical + chemical weathering) at geological time scales are usually derived from accumulation rates of terrigenous sediments in continental and off-shore deposits.This method however suffers from severe flaws and drawbacks: migrations of depocenters, difficulty and expense of determining 3D basin geometries, higher probability of a gap in a longer sedimentary record...

• The Plio-Pleistocene transition is a particular matter of debate: Worldwide sediment accumulation rates seem to have dramatically increased since ~5Ma (Zhang et al. 2001). This would be due to an intensification of denudation rates caused by the increased variabi-lity of the quaternary glaciations cycles.

• In-situ cosmogenic isotopes are a particularly powerful tool for determining reliable estimations of denudation rates from whole drainage basins, and are widely used in present day rivers.We extended this method to ancient sediments (0-10Ma) in well dated sections. Despite methodological limita-tions, this method represents a very direct means of determining paleo-denudation rates, at a high temporal resolu-tion and independantly from basin and deposition geometries.

(2) The sediment is deposited and progressively buried. It accumulates cosmogenic nuclides until completely shielded from the cosmic flux. In the case of a radioactive isotope (10Be in this case), the concentration simultanously decreases with time.→ Accumulation rates at the time of deposition must be precisely known and the sedi-ment must be accurately dated. → For 10Be, after ~10Ma, analyses become very difficult, especially for high denuda-tion rates (low initial concentration.)

(3) The deposits are tectonised and incised by rivers. Such recent exposition can lead to accumulation of a small amount of cosmogenic isotopes. This is not a pro-blem when exhumation results from human construction or fore core-drilling.→ The modern exposure to cosmic rays must be estimated.→ Recent exposure or canibalism of old sediment can be tested by independent da-tation using cosmogenic isotope couples (such as 26Al/10Be) and comparison with results from a reliable dating technique (magnetostratigraphy.)

• A key area for the Plio-Pleistocene transition debate: - Endorheic setting, disconnected from sea-level va-riations.- Accomodates ~40% of India-Asia convergence.

• Very favorable setting:- Dry climate, tectonically active. Limited glacial cover.- Continuous sediment records on the piedmonts, well dated by magnetostratigraphy.- Stable drainage basins (known from δ13C and δ18O analysis.)- Very fast river incision (known from terraces ages.)

N

• The Surai Khola section in the Siwaliks hills:- In the active piedmont of the Himalayas.- Exposed by a roadcut: no recent exposure.- Well dated by several magnetostratigraphic and pa-leontological studies.

• Is the noise in the records due to sedi-ment deposition processes and hete-rogenous shielding durations?

→ Test in a fluvial sequence (Kuitun section) shows limi-ted variations and no correlation to sedi-mentary facies.

• Is the noise due to a glacial/interglacial erosion variability?→ In Tian-Shan, terraces deposited during last glacial maximum display initial 10Be concentrations similar to those of modern river sediments.→ Published data in the literature show no or limited short-term variability in river sand cosmogenic concentrations between gla-cial and interglacial material.

→ Preliminary results show a transient increase in denudation rates, not synchronous to any known climatic event.→ This could be due to changes in the supplying drainage basin.→ Future Sr and Nd isotopic analysis and Raman spectroscopy on graphites will help constrain sediment provenance.

→ Cosmogenic nuclides in magnetostratigraphically dated section are a new direct way to derive high reso-lution paleo-denudation rates.They can be used in old sedimentary sections or cores, provided good estimations exist of paleo-accumulation rates, and paleo topography of the drainage basin, and recent exposure was limited.

- No synchronous abrupt increase in denudation rates, that would be caused by a global climatic event. Variations recorded in denudation rates seem to be local features, and the onset of highly variable climate and glacial cycles at 2-4 Ma does not seem to have significantly impacted long term denudation rates in these regions.- However, denudation may have, on average, increased slightly but progressively since 5-6 Ma. In the Northern Tian-Shan piedmont, variability seems to have increased, but we cannot conclude yet if this is provoked by tectonic or by climatic forcing.

→ In progress: - Analysis of a core drilled in Ebi Lake sediments (Northern Tian Shan): 0-4.5 Ma- Analysis of two high resolution profiles in the Jingu section (1.5-1 Ma and 6-5.3 Ma) to check the response of denudation to long orbital cycles before and after the onset of quaternary glaciations.- Complementary analyses in the Surai Khola section to provide a more detailed record and constrain evolution of sediment prove-nance .

→ Paleo-denudation rates in three sections in the Tian-Shan mountain piedmont and one section in the Siwakiks hils (Himalayan piedmont) show:

References: Zhang P., et al., Nature, 2001; Zachos et al. Science, 2001; Charreau, J., et al, Earth and Planetary Science Letters, 2011.

This theory has since been challenged by 3D sediment records, new dating re-sults, and chemical alteration proxies.