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Quantifying erosion in mountainous landscapes Mikaël ATTAL Marsyandi valley, Himalayas, Nepal Acknowledgements: Jérôme Lavé, Peter van der Beek and other.

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Presentation on theme: "Quantifying erosion in mountainous landscapes Mikaël ATTAL Marsyandi valley, Himalayas, Nepal Acknowledgements: Jérôme Lavé, Peter van der Beek and other."— Presentation transcript:

1 Quantifying erosion in mountainous landscapes Mikaël ATTAL Marsyandi valley, Himalayas, Nepal Acknowledgements: Jérôme Lavé, Peter van der Beek and other scientists from LGCA (Grenoble) and CRPG (Nancy) Eroding landscapes: fluvial processes National Student Satisfaction survey (4 th year)

2 Lecture overview I. Bedrock erosion processes II. Quantifying fluvial (and landscape) erosion on the long-term III. Quantifying fluvial erosion on the short-term

3 I. Bedrock erosion processes Abrasion (bedload impact) Abrasion (suspended load) Plucking (Ukak River, Alaska, Whipple et al., 2000) Cavitation (www.irrigationcraft.com)

4 Abrasion (bedload impact) Amount of abrasion is a function of: kinetic energy = 0.5mv 2 ; angle of impact; difference in rock resistance between projectile and target I. Bedrock erosion processes

5 Plucking BEDLOAD EXERTS A KEY ROLE Amount of erosion is a function of: joint density; stream power; kinetic energy of impacts = 0.5mv 2 ; angle of impact. Whipple et al., 2000 (Ukak River, Alaska) I. Bedrock erosion processes

6 Requires turbulence (eddies) affects mostly obstacles protruding in the channel (e.g. boulders) Abrasion by suspended load Whipple et al., 2000 I. Bedrock erosion processes

7 Most of the time, sediment is resting on the bed and protects it from erosion bedrock erosion (abrasion by bedload impacts + plucking) happens during floods V D I. Bedrock erosion processes

8 V D Consider 1 point in the channel, at a given time, during 1 flow event Because sediments in river include a wide range of grain sizes, some particles will move while some others (larger) will rest on the river bed TOOLS & COVER Transport capacity: Q c = k(τ – τ c ) 3/2 where k and τ c are constants [Meyer-Peter-Mueller, 1948] Fluvial shear stress: τ 0 = ρ g R S Stream power per unit length: Ω = ρ g Q S

9 Whipple et al., 2000: process-based theoretical analysis within the frame of the SPL e = KA m S n = kτ a where n = 2a/3 (and m is adjusted to obtain m/n = 0.5) Remark: if n = 1, m = 0.5 and a = 3/2 Incision Specific Stream power (law 2). if n = 2/3, m = 1/3 and a = 1 Incision basal shear stress (law 3). Abrasion (bedload) Not analyzed Abrasion (suspension) n = 5/3 a = 5/2 Plucking n = 2/3 1 a = 1 3/2 Cavitation n up to 7/3 a up to 7/2 I. Bedrock erosion processes

10 II. Quantifying fluvial (and landscape) erosion on the long-term ( years) 1) Fluvial erosion rates using terrace dating 2) Catchment-wide erosion rates using the fluvial network as an age homogenizer

11 Long-term fluvial erosion rates ( years): fluvial terraces Courtesy J. Lavé STRATH TERRACES Note: rivers can erode and form terraces even without uplift

12 Strath terraces: thin (or no) alluvium cover, contact alluvium- bedrock relatively flat Siwaliks hills, Himalayas (J. Lavé) Central Range, Taiwan

13 Siwaliks hills, Himalayas (J. Lavé) Strath terraces: thin (or no) alluvium cover, contact alluvium- bedrock relatively flat

14 Fluvial incision rates using strath terrace dating Age = 0 yr Age = n yr h Incision rate = h/n Dating methods: - 14 C, - Optically stimulated luminescence (OSL), - Cosmogenic nuclides.

15 Fluvial incision rates using strath terrace dating Bagmati River, Himalayas (Lavé & Avouac, 2000, 2001)

16 Fluvial incision rates using strath terrace dating Terraces are correlated in the field + using remote sensing Bagmati River, Himalayas (Lavé & Avouac, 2000, 2001)

17 Fluvial incision rates using strath terrace dating Bagmati River, Himalayas (Lavé & Avouac, 2000, 2001) Relatively constant incision rates since the end of the Pleistocene (PL 3 is ~ 22ky old) Reminder: the Quaternary Period includes the following epochs: Pleistocene (1.8 Ma ~12 ka) and Holocene (~12ka present)

18 FILL TERRACES: usually the result of landslides damming the valley (or large alluviation events filling narrow valleys) Tal, Marsyandi valley, Himalayas Upstream of the dam !

19 FILL TERRACES: usually the result of landslides damming the valley (or large alluviation events filling narrow valleys) ! Chame, Marsyandi valley, Himalayas Thick alluvium, up to hundreds of meters, contact alluvium- bedrock highly irregular. Local effect must not be used to determine long-term erosion rates. image28.webshots.com

20 The events that lead to the formation of fill terraces are relatively frequent in actively eroding landscapes Amount of erosion at a given point along the river Time (x 10 5 years) Models, long-term measurements Reality FILL TERRACES: usually the result of landslides damming the valley (or large alluviation events filling narrow valleys) !

21 Terrace dating methods a) 14 C on organic debris in alluvium (up to ~40 ka). 3 carbon isotopes: 12 C (natural abundance %), 13 C (n.a %) and 14 C (n.a. 1 part / trillion). [ 14 C] in the atmosphere is ~ constant (equilibrium between rate of production and decay) and is ~ to [ 14 C] in living organisms. 14 C formed in the atmosphere (interaction between cosmic rays and N molecules): 14 N + n 14 C + p When organism dies no more exchange with atmosphere the number of 14 C atoms decreases due to radioactive decay. 14 C (or radiocarbon) is a radioactive isotope which decays with a half-period of 5730 years. Age of terraces can be estimated by counting the number of 14 C atoms in organic fragments (assuming that the time between the organisms death and its incorporation into the alluvium is negligible). 14 C organism / 14 C atm (%)

22 Terrace dating methods b) Optically stimulated luminescence: burial ages of quartz or feldspar crystals, ages from 100 yrs to yrs. Radioactive isotopes + cosmic rays charge carriers (e.g., electrons e-, electron holes h+) travelling in crystals Charge carriers Charge carriers can become trapped in lattice defects. They progressively accumulate in these traps over geological timescales.

23 Terrace dating methods b) Optically stimulated luminescence: burial ages of quartz or feldspar crystals, ages from 100 yrs to yrs. Radioactive isotopes + cosmic rays charge carriers (e.g., electrons e-, electron holes h+) travelling in crystals Charge carriers Charge carriers can become trapped in lattice defects. They progressively accumulate in these traps over geological timescales. Exposure to light, heat, or high pressures can release charge carriers from trapping sites reset the system The release process is associated with a photon release. Number of photons released = f (number of trapped charge carriers released).

24 Terrace dating methods b) Optically stimulated luminescence: burial ages of quartz or feldspar crystals, ages from 100 yrs to yrs. Sunlight releases trapped charge carriers. If a crystal gets buried, charge carriers are going to accumulate in trapping sites. The longer the burial, the larger the number of trapped charge carriers. Optical stimulation (light) release of charge carriers release of photons light emission The older the terrace, the longer the burial, the higher the number of trapped charge carriers the larger the number of photons released with the charge carriers the higher the intensity of the light emitted!

25 Terrace dating methods c) Cosmogenic Nuclides: exposure ages. Cosmic rays interact with atoms in the atmosphere and in the rocks exposed at the surface of the Earth nuclear reactions cosmogenic nuclides. Examples: 3 He, 10 Be, 14 C, 21 Ne, 26 Al, 36 Cl. Cosmogenic nuclides accumulate in minerals in the 1-2 m thick layer at the top of the Earth. Stable T 1/2 = 1.5 Ma T 1/2 = 5730 a Stable T 1/2 = 0.73 Ma T 1/2 = 0.3 Ma The longer the rock exposure, the higher the amount of cosmogenic nuclides Depth Cosmogenic nuclide production rate 1-2 m Concentration in cosmogenic nuclides in minerals = f (EXPOSURE TIME, latitude, altitude, topography, type of mineral, type of cosmogenic nuclide).

26 Terrace dating methods c) Cosmogenic Nuclides: exposure ages. Beryllium: 9 Be = stable isotope; 10 Be = cosmogenic isotope formed by interactions between cosmic rays and O, N, Si, Mg, Fe. Beryllium in Quartz frequently used in geomorphology to date objects up to millions of years old. Chlorine: 35 Cl and 37 Cl = stable isotopes; 36 Cl = cosmogenic isotope formed by interactions between cosmic rays and Ar, Fe, K, Ca, Cl. Chlorine in calcite is a method which begins to be reliable to date objects up to millions of years old. Boulders on terraces Bedrock strath terrace © Scott T. Smith/CORBIS

27 II. Quantifying fluvial (and landscape) erosion on the long-term ( years) 1) Fluvial erosion rates using terrace dating 2) Catchment-wide erosion rates using the fluvial network as an age homogenizer Detrital methods Assumption: time spent in the fluvial network is negligible Photo Eric Gayer

28 Catchment-wide erosion rates a) Cosmogenic ages on fluvial sands (Q + Grt). Courtesy Eric Gayer

29 Catchment-wide erosion rates Main limitation: assumption that landscape is eroding at a constant rate through time Uplift Erosion 1-2 m a) Cosmogenic ages on fluvial sands (Q + Grt).

30 Catchment-wide erosion rates Main limitation: assumption that landscape is eroding at a constant rate through time Uplift Erosion 1-2 m a) Cosmogenic ages on fluvial sands (Q + Grt).

31 Catchment-wide erosion rates Main limitation: assumption that landscape is eroding at a constant rate through time Uplift Erosion 1-2 m a) Cosmogenic ages on fluvial sands (Q + Grt).

32 Catchment-wide erosion rates Main limitation: assumption that landscape is eroding at a constant rate through time Uplift Erosion 1-2 m a) Cosmogenic ages on fluvial sands (Q + Grt).

33 Catchment-wide erosion rates Main limitation: assumption that landscape is eroding at a constant rate through time Uplift Erosion 1-2 m a) Cosmogenic ages on fluvial sands (Q + Grt).

34 Catchment-wide erosion rates Main limitation: assumption that landscape is eroding at a constant rate through time Uplift Erosion 1-2 m Landslide gives the impression that the catchment includes zones with low, moderate and extremely high erosion rates! a) Cosmogenic ages on fluvial sands (Q + Grt).

35 Catchment-wide erosion rates b) Detrital termochronology: fission tracks Bernet & Garver, 2005 pangea.stanford.edu Fission tracks in zircon or apatite

36 Catchment-wide erosion rates b) Detrital termochronology: fission tracks Bernet & Garver, 2005 If erosion rate is constant, lag time is constant. Example: lag-time = 20 Ma Deposition age (age of sediment td) (Ma) FT age (tc) Ma tc = 50 Ma td = 30 Ma 0

37 Catchment-wide erosion rates b) Detrital termochronology: fission tracks Bernet & Garver, 2005 If erosion rate is constant, lag time is constant. Example: lag-time = 20 Ma Deposition age (age of sediment td) (Ma) FT age (tc) Ma tc = 40 Ma td = 20 Ma 0

38 Catchment-wide erosion rates b) Detrital termochronology: fission tracks Bernet & Garver, 2005 If erosion rate is constant, lag time is constant. Example: lag-time = 20 Ma Deposition age (age of sediment td) (Ma) FT age (tc) Ma tc = 30 Ma td = 10 Ma 0

39 Catchment-wide erosion rates b) Detrital termochronology: fission tracks Bernet & Garver, 2005 If erosion rate is constant, lag time is constant. Example: lag-time = 20 Ma Deposition age (age of sediment td) (Ma) FT age (tc) Ma tc = 20 Ma td = 0 Ma 0 Slope 1:1 Short lag-time Long lag-time

40 Catchment-wide erosion rates b) Detrital termochronology: fission tracks Bernet & Garver, 2005 If erosion rate is constant, lag time is constant. Example: lag-time = 20 Ma Deposition age (age of sediment td) (Ma) FT age (tc) Ma tc = 30 Ma td = 15 Ma 0 Lets imagine that erosion rate increases at 30 Ma Lag-time = 15 Ma

41 Catchment-wide erosion rates b) Detrital termochronology: fission tracks Bernet & Garver, 2005 If erosion rate is constant, lag time is constant. Example: lag-time = 20 Ma Deposition age (age of sediment td) (Ma) FT age (tc) Ma tc = 20 Ma td = 5 Ma 0 Lets imagine that erosion rate increases at 30 Ma Lag-time = 15 Ma

42 Catchment-wide erosion rates b) Detrital termochronology: Ar/Ar or K/Ar methods (very simplified here) 39 K is stable. 40 K decays into 40 Ar (gas) with a half-life of 1.25 billion years. Degassed at high temperature, accumulates in minerals at temperatures < closure temperature Isotherm corresponding to closure temperature T > closure temperature: 40 Ar degassed T = closure temperature clock starts 40 Ar accumulates in mineral. Amount of 40 Ar = f (time since crossing the isotherm) Biotite: 300 ºC Muscovite: 400 ºC Hornblende: 550 ºC

43 Catchment-wide erosion rates b) Detrital termochronology: Ar/Ar or K/Ar methods (very simplified here) Central Himalayas, Nepal (Wobus et al., 2005) Ar/Ar ages on detrital muscovite Isotherm 400 ºC Migration of the MCT?


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