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Earth and Planetary Science Letters 444 (2016) 101–115
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Earth and Planetary Science Letters
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Multi-approach quantification of denudation rates in the Gulf of Lion
source-to-sink system (SE France)
S. Molliex a,b,∗, M. Rabineau a, E. Leroux a,b, D.L. Bourlès c, C. Authemayou a, D. Aslanian b, F. Chauvet c, F. Civet a,d, G. Jouët b
a Laboratoire Domaines Océaniques UMR CNRS 6538, Institut Universitaire Européen de la Mer, Université de Bretagne Occidentale, place N. Copernic, F-29280 Plouzané, Franceb IFREMER, Département Geosciences Marines, Z.I. pointe du Diable, BP70, F-29280 Plouzané, Francec Aix-Marseille University, CNRS-IRD-Collège de France, UM34 CEREGE, BP 80, F-13545 Aix-en-Provence Cedex 4, Franced LPGNantes, UMR CNRS 6112, Université de Nantes, 2 rue de la Houssinière, F-44322, Nantes, France
a r t i c l e i n f o a b s t r a c t
Article history:Received 12 October 2015Received in revised form 25 March 2016Accepted 28 March 2016Available online xxxxEditor: A. Yin
Keywords:denudation rates10BequaternaryGulf of Lionsource-to-sink
During the Pliocene and the Quaternary, the Gulf of Lion, the northern passive margin of the Liguro-Provençal basin in the western Mediterranean Sea, received sediments from a 120 000 km2 drainage area constituted by several structural domains. The denudation of mountainous areas, source of this sedimentary supply, results from complex interactions between tectonics, climate, morphology, and rock erodibility. In this study, denudation rates from the present-day and ranging back to the Quaternary and the Pliocene are quantified using four independent methods allowing an investigation over different time scales: 1) compilation of present-day measured sediment fluxes, 2) determination of catchment-scale cosmogenic denudation rates through measurements of in situ-produced 10Be concentrations in sands sampled at the outlet of present-day rivers, 3) estimation of eroded volumes within catchments using a DEM to quantify long-term averaged Quaternary denudation rates, and 4) quantification of sediment volumes deposited within the marine realm of the Gulf of Lion. The results obtained by these four methods are in agreement within the range of uncertainties. The internal part of the Alps exhibits significantly higher denudation rates (∼700 mm ka−1) than those estimated in the other structural domains: 150–250 mm ka−1 in the foreland Alps, ∼100 mm ka−1 in the Pyrenees, and 55–75 mm ka−1 in the Massif Central. The Alpine domain provides at least 80% of the total eroded volume supplied towards the Gulf of Lion. A quantitative geomorphological approach shows that denudation rates are controlled at the first order by catchment morphologies (slope, relief) over different time scales, suggesting glacial conditioning to be the main driver on denudation from the Quaternary to present-day. Throughout the Pliocene–Quaternary, a doubling of denudation rates related to the mid-Pleistocene Revolution (∼0.9 Ma) is highlighted.
© 2016 Elsevier B.V. All rights reserved.
0. Introduction
Relief results from the interaction between tectonics, erosion, and climate (e.g. Whipple, 2009). One of the traditional ways to estimate denudation rates is the use of sediment fluxes measured directly in the river (e.g. Holeman, 1968; Milliman and Meade, 1983) or inferred from the volume deposited in sedimentary basins (e.g. Hinderer, 2001; Kuhlemann et al., 2002). The recent devel-
* Corresponding author at: Laboratoire Domaines Océaniques UMR CNRS 6538, Institut Universitaire Européen de la Mer, Université de Bretagne Occidentale, place N. Copernic, F-29280 Plouzané, France.
E-mail address: [email protected] (S. Molliex).
http://dx.doi.org/10.1016/j.epsl.2016.03.0430012-821X/© 2016 Elsevier B.V. All rights reserved.
opment of quantitative geomorphological methods (e.g. Anderson and Anderson, 2010), including the use of the cosmogenic nuclides (e.g. Dunai, 2010), allows us to quantify continental denudation processes at different time scales (102 to 106 years). Continental margins are the place where sediments are deposited, the geome-try being controlled by accommodation space (sea level fluctuation + subsidence which depends on thermal, tectonic, or isostasic pro-cesses) and sedimentary fluxes. Surface processes (sedimentation, denudation) and lithosphere dynamics are thus intimately linked (e.g. Cloetingh et al., 2008). Due to the numerous data acquired offshore and onshore over the past few years, the Gulf of Lion source-to-sink system can be considered as an ideal laboratory to study feedbacks between denudation, sedimentation, and associ-ated vertical displacements.
102 S. Molliex et al. / Earth and Planetary Science Letters 444 (2016) 101–115
Fig. 1. Morphologic context of the Gulf of Lion margin and studied catchments. Max-imum extension of glaciers after Delmas et al. (2009) for Pyrenees and Coutterand(2010) for the Alps. Names of catchments: 1) Rhône; 2) Léman; 3) Arve; 4) Isère; 5) Durance; 6) Ain; 7) Fier; 8) Bourget; 9) Guiers; 10) Drôme; 11) Aygues; 12) Ou-vèze; 13) Eyrieux; 14) Ardèche; 15) Cèze; 16) Gard; 17) Hérault; 18) Orb; 19) Agly; 20) Têt; 21) Tech; 22) Saône; 23) Bourbre; 24) Oron; 25) Aude.
In source-to-sink systems, the integration time scale is a crit-ical parameter because of its influence on the signal generation, propagation, preservation and analysis (Romans et al., 2016). In-deed, some processes occur over different time scales and can also be distorted along source-to-sink pathways. Knowledge of both the geometry of Pliocene–Quaternary sediments in the Gulf of Lion (Aloïsi, 1986; Rabineau, 2001; Leroux, 2012) and the chronology of strata recovered by drillings (Cravatte et al., 1974; Suc et al., 1995;Bassetti et al., 2008; Sierro et al., 2009) allow the calculation of sediment volumes and sedimentary fluxes during the Pliocene–Quaternary. Onshore, sediment fluxes are often inferred by direct measurements over decadal time scales (>30 yr). On the other hand, the catchment is constituted by several structural units in which quartz-bearing rocks are present allowing for the calculation of catchment-scale denudation rates by cosmogenic nuclide 10Be analyses. In this study, denudation rates were determined over dif-ferent time scales using 4 different independent direct and indirect approaches: (i) measurement of modern river sedimentary fluxes (∼10–102 yr), (ii) measurements of in situ-produced 10Be concen-trations in river sediments (∼102–104 yr), (iii) continental eroded volumes estimates from Digital Elevation Models (∼105–106 yr), (iv) deposited sediment volumes estimates from sedimentary body geometries (106–107 yr). These results associated with a quanti-tative geomorphologic approach allow us to better understand the relationship between denudation and sedimentation and their trig-gering processes during the Quaternary.
Fig. 2. Geological context of Gulf of Lion catchments and distribution of surface type of rocks. Geology after Chantraine et al. (1996). Maximum extension of glaciers after Delmas et al. (2009) for Pyrenees and Coutterand (2010) for the Alps. The structural areas are: A) Massif Central; B) Pyrenees; C) Alpine foreland; D) Inner part of the Alps; E) Plain.
1. Geological framework
The Gulf of Lion (GOL) represent the northern margin of the so-called “Liguro-Provençal” basin located in the western Mediter-ranean sea (Fig. 1). It is a particularly well-suited area to study in detail the formation and evolution of passive margins. The basin opened recently, at the end of the Oligocene, which allows an ac-curate interpretation and understanding of sediments architecture (e.g. Seranne, 1999). Quaternary sedimentation is well constrained through numerous studies and analyses of cored boreholes that considerably improved our understanding of 100 ka-climatic cy-cles (Rabineau et al., 2005; Bassetti et al., 2008; Sierro et al., 2009). Studies also focused on Pliocene–Quaternary (Lofi et al., 2003; Rabineau et al., 2014) and Miocene-Messinian time scales (Bache et al., 2009, 2010). All these works led to improved spa-tial and temporal correlation of sedimentary reflectors from shelf to deep basin enabling calculation of volumes and terrigenous sed-iment yields (Leroux et al., in press).
Sedimentation in the GOL results from the denudation of sev-eral catchments distributed in different structural domains cover-ing a total drainage area of ∼120,000 km2 (Fig. 1). Sedimenta-tion is mainly controlled by the Rhône River (95,000 km2) which currently provides 90% of sedimentary fluxes (Aloisi, 1986). The remaining inputs come from Languedocian rivers (Herault, Orb, Aude, Agly, Tet and Tech) (Fig. 2). The area drained is mainly composed of (i) crystalline rocks, located in the Massif Central, Inner Pyrenees, and Alps; (ii) Mesozoic carbonate rocks, located in the external Pyrenees and Alps and in the Jura fold-and-thrust belt; (iii) marls and conglomerates, located in plains and infill-
S. Molliex et al. / Earth and Planetary Science Letters 444 (2016) 101–115 103
Fig. 3. Change in drainage pattern of the Gulf of Lion catchment between 2.4 Ma and present-day. The drainage area was 160,000 km2 before the capture of Aar and Upper Rhine rivers by the lower Rhine (2.4 Ma). The drainage area is currently 120,000 km2.
ing Oligocene grabens and Pliocene sub-aerial canyons (Fig. 2). Until 2.4 Ma ago, the upper Rhine and Aar Rivers were flowing into the Rhône River through the Doubs valley, before being cap-tured by the Rhine River flowing to the north (Bonvalot, 1974;Petit et al., 1996) (Fig. 3). As a result, the size of the catchment changed from 160,000 km2 before 2.4 Ma to 120,000 km2 after that time. Since 3 Ma, the convergence between the African and Eurasian plates is considered to be mostly accommodated south of Sardinia, with only residual shortening and very low defor-mation rates in southern France, even in the Alps (Nocquet andCalais, 2003; D’Agostino et al., 2008; Larroque et al., 2009). Cur-rently, the Alps are the most active structure in the studied area and undergo an extension perpendicular to the belt (e.g. Sue et al., 2007) and a quaternary rock uplift which reached 1.5 mm yr−1
over the last century (Kahle et al., 1997; Schlatter et al., 2005). Tectonic exhumation driven by deep processes such as slab detach-ment coupled to quaternary enhanced denudation due to glacial processes are cited as the main causes of Quaternary rock uplift of the Alps (e.g. Vernon et al., 2008; Champagnac et al., 2009;Fox et al., 2015), while extension is better explained by denuda-tion processes induced by isostatic rebound rather than by gravita-tional collapse (Vernant et al., 2013). Although climate may influ-ence denudation rates in the Alps, especially since the beginning of the Quaternary glacio-eustatic cycles (Champagnac et al., 2009;Herman et al., 2013), some studies suggest that denudation rates correlate more satisfactorily with tectonic uplift (e.g. Wittmann et al., 2007).
2. Materials and method
Since the GOL catchment includes many structural domains, we tried to infer denudation rates by studying individual main river catchments. Twenty-five catchments were studied, with at least three catchments in each structural domain (inner part of the Alps, Alpine foreland, Massif Central, Pyrenees and plain) (Figs. 1 and 2).
Denudation rates were estimated using three different onshore approaches:
– The compilation of modern sediment flux (MSF) data in the main rivers from the literature.
– The measurements of in situ-produced cosmogenic nuclide concentrations (CNC) in river sediments.
– The estimation of continental eroded volume (CEV) from the 90 m SRTM DEM (Farr et al., 2007).
The obtained values are also compared to deposited sediment volumes (DSV) quantification on the offshore margin over the Pliocene–Quaternary (Leroux et al., in press).
2.1. Modern sediment flux (MSF) data in the rivers
In the studied area more than 30 yrs of MSF measurements were available in the literature. Data include measurements or esti-mates of the sum of suspended, dissolved, and bed load sediments, which allow denudation rates estimated on the basis of an eroded rock with 2.5 t m−3 mean density. Several parameters are partic-ularly difficult to measure, such as bed load or dissolved charge. These parameters are not always quantified because it consumes a lot of time, with values therefore exhibiting large uncertainties. In the Alpine foreland, dissolvable Mesozoic carbonated rocks repre-sent 30 to 40% of the catchment surface (Fig. 2). When uncertainty values were lacking in published data, we assigned an arbitrary value of ±20%, which also corresponds to mean measured uncer-tainties.
2.2. Cosmogenic nuclides concentration (CNC) in modern river sand
Quartz-bearing rocks exposed to cosmic rays accumulate 10Be cosmogenic nuclides whose concentrations depend on the produc-tion and denudation rates (e.g. Dunai, 2010). When the rock is exposed sufficiently, 10Be gains due to production are equal to losses due to denudation and radioactive decay (e.g. von Blanck-enburg, 2005). The measure of 10Be concentrations on sediment sampled at a catchment outlet enables us to estimate a mean catchment denudation rate averaging concentrations from all parts of the catchment (Brown et al., 1995; Bierman and Steig, 1996;Granger et al., 1996).
We sampled rivers sands near catchment outlets in May 2011. We sampled close to the main active riverbed, except for the Rhône where we sampled directly the main riverbed, just up-stream of the delta (Table 1; Fig. 2). 10Be measurements were per-formed only for catchments draining quartz-bearing areas (14 sam-ples). Nevertheless, the sampling concerned all structural zones.
The chemical treatment and 10Be/9Be measurements were respectively carried out at the Laboratoire National des Nu-cléides Cosmogéniques (LN2C) and the French National Acceler-ator Mass Spectrometry facility (ASTER) in the Centre Européen de Recherche et d’Enseignement des Géosciences de l’Environ-nement (CEREGE), Aix-en-Provence. Samples were prepared for
104 S. Molliex et al. / Earth and Planetary Science Letters 444 (2016) 101–115
Tabl
e1
10Be
conc
entr
atio
ns in
pre
sent
-day
rive
r san
ds sa
mpl
ed at
catc
hmen
t out
lets an
d de
rive
d de
nuda
tion
rate
s.
Catc
h.N
ame
Lati
tude
(◦
N)
Long
itud
e (◦
E)M
ean
elev
atio
n (m
)M
ass o
f qua
rtz
(g)
9Be
carr
ier
(μg)
10Be
/9Be
(×
10−1
4)
10Be
at
g(Q
z)−1
Prod
ucti
on
rate
at
g(Q
z)−1
yr−1
Den
sity
(g
cm−3
)A
ppar
ent a
ge
(ka)
Den
udat
ion
rate
(m
mka
−1)
% Q
z roc
ks in
th
e cat
ch.
1Rh
ône
43.9
324.
745
786.
218
.725
305
5.57
±0.
5655
569±6
182
7.82
±0.
392.
57.
2±
1.2
118.
2±
19.0
473
Arv
e46
.173
6.24
413
6512
.894
306
1.3
±0.
3791
59±4
829
13.3
4±
0.67
2.5
1.2
±0.
686
9.5
±39
7.5
334
Isèr
e45
.018
4.93
914
52.3
12.1
762
306
1.33
±0.
3510
820±5
080
13.8
3±
0.69
2.5
1.3
±0.
578
8.4
±32
2.9
535
Dur
ance
43.6
635.
562
1200
.514
.430
42.
1±
0.19
2455
7±2
256
9.08
±0.
452.
52.
7±
0.4
310.
0±
43.8
307
Fier
45.8
865.
925
954 .
0711
.923
430
51.
59±
0.29
1611
0±5
333
6.19
±0.
312.
53.
6±
1.1
255.
1±
78.8
339
Gui
ers
45.5
925.
636
801 .
511
.476
330
72.
03±
0.40
2281
2±6
825
5.65
±0.
282.
55.
5±
1.7
165.
0±
49.9
4813
Eyri
eux
44.8
264.
795
758 .
928
.89
307
10.7
8±
1.41
6322
4±8
772
6.9
±0.
352.
510
.8±
2.0
79.6
±15
.098
14A
rdèc
he44
.284
4.59
857
934
.21
308
15.1
0±
1.83
7750
0±9
402
7.07
±0.
352.
512
.6±
2.2
67.4
±11
.555
15Ce
ze44
.146
4.68
732
0 .1
32.0
130
410
.31
±1.
4752
745±8
369
5.74
±0.
292.
511
.1±
2.3
78.0
±16
.219
16G
ard
43.9
294.
568
287 .
924
.79
304
8.08
±0.
8055
101±6
637
5.22
±0.
262.
512
.2±
2.1
70.0
±11
.948
17H
erau
lt43
.483
3.45
736
7 .3
33.1
530
410
.94
±0.
9957
620±5
977
4.96
±0.
252.
513
.1±
2.0
64.6
±9.
949
19A
gly
42.7
542.
963
439 .
428
.44
304
10.5
8±
1.60
6088
3±9
729
5.16
±0.
262.
514
.2±
3.0
60.7
±21
.736
20Te
t42
.714
2.98
510
36.9
3.53
303
1.55
±0.
2443
075±1
3498
8.91
±0.
452.
57.
2±
2.6
132.
1±
47.4
9921
Tech
42.5
833.
016
751 .
633
.60
303
9.42
±1.
7144
075±8
139
6.52
±0.
332.
58.
4±
2.0
104.
0±
24.3
94
10Be concentration measurements following the chemical proce-dures of Brown et al. (1991) and Merchel and Herpers (1999). 10Be/9Be ratios were corrected for procedural blanks and cali-brated against the National Institute of Standards and Technology standard reference material 4325 by using an assigned value of 2.79 ± 0.03 × 10−11 and a 10Be half-life of 1.387 ± 0.012 × 106 yrs(Korschinek et al., 2010; Chmeleff et al., 2010). Analytical uncer-tainties (reported as 1σ ) include uncertainties associated with AMS counting statistics, chemical blank measurements, and AMS internal error (0.5%). Long-term AMS measurements of procedural blanks yield a background 10Be/9Be ratio of 2.4 ± 1.5 × 10−15 at ASTER (ASTER Team, pers. comm., 2016). A sea-level, high-latitude (SLHL) spallation production of 4.03 ± 0.18 at g−1 yr−1 was used (Molliex et al., 2013 for references) and scaled for latitude and elevation (Stone, 2000). The contribution of muons to the produc-tion rate was calculated using the physical parameters evaluated by Braucher et al. (2011). The production rate is calculated for each cell of the 90 m SRTM DEM and the mean catchment produc-tion rate is computed following the method described by Dunne et al. (1999) and Delunel et al. (2010) and using the script of Balco(2001) for the calculation of the topographic shielding factor. Ar-eas without quartz-bearing rocks and not linked to the stream network (with low slope (<3◦) or located upstream of lakes) were excluded from calculation. No corrections were performed for ice cover since it only represents a very small part of our studied area.
2.3. Quantification of continental eroded volume (CEV)
Long-term denudation rates estimates from CEV quantification were performed using the 90 m SRTM DEM. The method used is based on the reconstruction of an envelope surface using present day high points (Small and Anderson, 1998; Brocklehurst and Whipple, 2002). The difference between present-day elevation and this envelope corresponds to the minimum CEV, since the crests certainly undergo denudation. Here, we reconstructed the envelope surface by automatic extraction of high points and main interfluves using ArcGis software with Spatial Analyst extension. We used a flow accumulation grid computed from an inversed DEM to ex-tract main interfluves and highpoints. Nevertheless, the resulting volume highly depends on the “tension” of the surface envelope, i.e. the number of high points chosen (Ahnert, 1984; Lucazeau and Hurtrez, 1997; Champagnac et al., 2012). If they are too sparse, the interpolated envelope can be lower than the present-day topogra-phy and some important reliefs may not be taken into account. On the contrary, when too many high points are kept, the interpo-lated envelope is too close to present-day topography which leads to a non-representative long-term eroded volume. For CEV estima-tion, we used 2 envelope surfaces with different “tension” using 2 boundary values of flow accumulation (over 200 and 400 cells) which correspond to the range of representative values. We then assumed that the volume obtained globally represents the CEV since the beginning of the incision caused by the onset of Qua-ternary glacio-eustatic cycles. Indeed, river incision is enhanced by successive falls of sea level during glacial periods and by the long-term geological uplift related to isostasic compensation re-sulting from glacier retreat and denudation (Schlatter et al., 2005; Champagnac et al., 2007, 2009). The glacio-eustatic cycles started 2.6 Ma ago (Gibbard et al., 2010), but the main phase of incision only occurred after 0.9 Ma in the Alps (Haeuselmann et al., 2007;Valla et al., 2011). To calculate the denudation rates we assumed that the incision started 0.9 Ma ago in glacially conditioned en-vironments, whereas it started 2.6 Ma ago in the remaining ar-eas. Glacially conditioned areas are defined as areas covered by ice during the most extensive glacial maxima, the “Riss” glacial phase (see Fig. 2 for “Riss” glacial extension and Table 2 for per-cent of ice-covered area in each catchment). Due to assumptions
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andPlanetary
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444(2016)
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includes bedload, suspended load, and dissolved load. n and Piegay (1999); 9) Bourrin et al. (2006); 10) Serrat
Max. present ice cover (%)
Denud. rate from MSF (mm kyr−1)
Denud. rate from CNC (mm kyr−1)
Denud. rate from CEV (mm kyr−1)
1.8 93.2 ± 44.81,2,3,4 118.2 ± 19.0 137.8 ± 27.7
14.5 539.6 ± 108.05 319.3 ± 53.810.0 526.4 ± 274.61 869.5 ± 397.5 324.9 ± 63.53.0 161.0 ± 32.21 788.4 ± 322.9 324.8 ± 64.80.5 194.0 ± 38.86 310.0 ± 43.8 177.6 ± 32.8
0.0 51.8 ± 8.21 – 127.4 ± 23.90.0 187.7 ± 37.61 255.1 ± 78.8 262.8 ± 50.50.0 111.3 ± 22.27 – 219.6 ± 61.60.0 91.7 ± 8.31 165.0 ± 49.9 256.7 ± 64.70.0 58.4 ± 11.78 – 97.0 ± 21.90.0 – – 73.9 ± 14.30.0 42.2 ± 14.11 – 41.1 ± 10.5
0.0 42.2 ± 8.41 79.6 ± 15.0 67.8 ± 10.80.0 36.0 ± 13.41 67.4 ± 11.5 58.5 ± 12.50.0 38.2 ± 7.61 78.0 ± 16.2 40.6 ± 7.50.0 32.6 ± 10.91 70.0 ± 11.9 42.8 ± 8.70.0 13.5 ± 2.79 64.6 ± 9.9 47.6 ± 10.60.0 28.7 ± 5.79 – 56.5 ± 10.9
0.0 71.0 ± 14.210 60.7 ± 12.7 66.5 ± 12.30.0 15.4 ± 3.19 132.1 ± 47.4 85.9 ± 13.80.0 17.6 ± 3.511,12 104.0 ± 24.3 74.8 ± 14.3
0.0 30.1 ± 10.01 – 37.1 ± 9.40.0 57.1 ± 11.41 – 80.1 ± 13.60.0 – – 30.2 ± 6.20.0 15.6 ± 3.19 – 44.2 ± 8.7
Table 2Synthesis of main morphological parameters of Gulf of Lion catchments and their denudation rates determined by 3 independent methods. The modern sediment fluxReferences: 1) IRS (2000); 2) Miliman and Syvintsky (1992); 3) Pont et al. (2002); 4) Delmas et al. (2012); 5) Touchard (1993); 6) Alary (1998); 7) Chapron (1999); 8) Lando(1999); 11) Serrat et al. (2001); 12) Kettner and Syvintsky (2009).
Name Catchment Area (km2)
Diam-eter (km)
Shape fac-tor
Drainage density (km km2)
Long. out-let (◦E)
Lat. out-let (◦N)
Alt. out-let (m)
Strah-ler order
Mean eleva-tion (m)
Hypso-metric integral
Mean slope (◦)
Geophys-ical relief(m)
Ksn River lenght (km)
River slope
Dis-charge (m3 s−1)
Max. LGM ice cover (%)
1 Rhone 96443 524.7 0.592 4.630 43.629 1 10 786.2 0.169 11.19 151.7 144.03 796.05 0.296 1780.0 36.7
INNER ALPS2 Leman 7983 188.0 0.475 2.009 6.179 46.250 392 8 1648.4 0.335 19.86 282.8 323.61 271.97 0.679 252.0 75.13 Arve 1891 74.6 0.583 2.015 6.178 46.178 403 7 1365 0.227 19.36 288.9 209.95 102.80 0.838 74.4 82.54 Isere 11732 187.2 0.579 1.855 4.875 44.998 112 8 1452.3 0.351 20.51 298.8 291.7 308.27 0.816 333.0 66.05 Durance 13169 204.2 0.562 1.871 4.795 43.916 18 8 1200.5 0.300 15.92 224.5 147.15 341.66 0.736 176.0 33.8
ALPINE FORELAND6 Ain 3726 134.3 0.455 1.986 5.220 45.820 196 7 669.4 0.326 8.47 105.9 44.359 184.50 0.362 123.0 91.27 Fier 1449 58.3 0.653 1.838 5.834 45.935 252 7 954.07 0.294 16.24 224.7 168.34 66.77 2.277 41.2 85.98 Bourget 596 43.0 0.568 1.891 5.805 45.820 249 6 628.2 0.256 12.46 158.7 56.304 45.54 1.818 6.3 88.09 Guiers 604 37.7 0.652 1.907 5.642 45.580 221 6 801.5 0.313 15.16 192.4 138.82 44.52 3.648 16.0 80.610 Drome 1648 74.8 0.543 1.777 4.783 44.754 91 6 786.1 0.358 16.63 233.0 109.68 105.58 0.876 17.7 0.011 Aygues 1064 77.4 0.421 1.878 4.773 44.107 28 6 635.8 0.355 13.53 195.3 86.08 97.68 1.089 6.2 0.012 Ouveze 2937 74.7 0.726 2.087 4.845 43.965 19 7 506 0.259 8.06 97.9 63.085 95.12 0.802 6.1 0.0
MASSIF CENTRAL13 Eyrieux 853 48.7 0.600 1.783 4.789 44.826 97 7 758.9 0.403 14.99 166.3 117 73.25 1.354 15.6 0.014 Ardeche 2368 80.0 0.608 2.271 4.639 44.278 41 7 579 0.328 12.62 140.5 102 108.55 1.126 65.0 0.015 Ceze 1170 71.6 0.477 2.421 4.674 44.146 35 7 320.1 0.181 8.69 105.8 41.179 105.21 0.629 18.0 0.016 Gard 2103 94.9 0.483 2.045 4.645 43.817 8 7 287.9 0.211 8.89 102.6 47.767 121.77 0.466 32.7 0.017 Herault 2608 91.9 0.556 1.970 3.447 43.312 1 7 367.3 0.235 9.03 110.7 82.496 137.12 1.018 43.7 0.018 Orb 1534 65.5 0.598 1.908 3.281 43.290 5 6 395.6 0.348 11.49 137.9 93.289 121.40 0.502 23.7 0.0
PYRENEES19 Agly 1082 57.4 0.574 2.393 2.959 42.752 8 7 439.4 0.236 11.60 166.3 113.66 87.61 1.719 6.3 0.020 Tet 1397 89.4 0.418 2.212 3.008 42.714 5 6 1036.9 0.363 15.73 169.1 168.33 104.68 1.918 11.0 11.121 Tech 726 60.2 0.447 1.790 3.009 42.605 4 6 751.6 0.276 15.91 166.9 148.76 75.57 1.925 9.6 3.0
PLAIN22 Saone 29429 317.6 0.540 2.117 4.829 45.796 171 9 373.4 0.159 4.52 55.2 20.281 415.87 0.041 473.0 15.523 Bourbre 646 39.6 0.642 2.198 5.185 45.769 195 6 347.6 0.296 4.43 61.8 28.932 56.88 0.369 7.7 99.024 Oron 659 54.6 0.470 2.382 4.831 45.299 152 6 431.5 0.441 4.22 38.8 22.695 67.08 0.769 48.725 Aude 4970 118.4 0.596 2.476 2.970 43.251 10 8 453.8 0.160 8.42 97.2 76.026 186.21 1.139 43.7 2.7
106 S. Molliex et al. / Earth and Planetary Science Letters 444 (2016) 101–115
Fig. 4. Estimation of eroded volumes by reconstitution of the envelope surface before the Quaternary incision, by extraction of high points and main interfluves. A) Envelope surface reconstructed. B) Incision map resulting from the subtraction between envelope surface and present-day elevations. C) Distribution of the eroded volumes with respect to structural areas.
regarding the initial morphology and the inferred age for the be-ginning of the incision, this method, although only allowing a rough estimate of long-term denudation rates, enables to give an insight on their spatial distribution (Small and Anderson, 1998;Brocklehurst and Whipple, 2002) (Fig. 4).
2.4. Sediment volume deposited (SVD) in the Gulf of Lion margin and catchment
Compilation of a large dataset (seismic data and wells) enabled Leroux et al. (in press) to calculate offshore SVD for 5 different periods within the Pliocene–Quaternary (after time-depth correc-tions and decompaction). Following Hinderer et al. (2013), we con-verted these decompacted SVD into denudation rates by changing the density from 1.8 t m−3 for uncompacted sediments (in accor-dance with wells data) to 2.5 t m−3 for bedrock. Since Leroux et al.’s data do not integrate the whole Rhône deep-sea fan, these de-nudation rates must be considered as minimum. Moreover, a part of sediments deposited in the GOL might source from surround-ing catchments (south–eastern margin for example) or may have been transported and deposited by oceanic currents. Kuhlemann(2000) estimated a total SVD of offshore Pliocene–Quaternary sed-iments for the whole GOL (151,700 km3). This value can thus be considered as a maximum. We assumed that the variations within Pliocene and Quaternary follow the same trends as those described by Leroux et al. (in press).
In order to add the volume of Quaternary sediments trapped in the onshore domain, we used the mapping of Chantraine et al. (1996) and the French open-access borehole database
(http://infoterre.brgm.fr). Onshore, Quaternary deposits are mainly composed of alluvial terraces, moraines, alluvial fans and colluvi-ums. The thickness of alluvial deposits is often lower than 30 m, except in some traps such as lakes or in the Rhône delta where it reaches about 120 m. Other deposits such as moraines or colluvi-ums are not widely spread over the area and their thickness rarely exceeds 50 m.
2.5. Morphometric indexes
Several morphometric parameters were computed for each catchment from the 90 m STRM DEM using Rivertools (http://rivix.com/) and ArcGis softwares (Table 2).
Mean elevation and mean slope were computed by calculating the average value of each pixel from original and derivative DEM, respectively. The diameter is the maximum width of the catchment perpendicular to the river. The shape factor corresponds to the ratio between area and diameter of the catchment. The drainage density is the ratio between the total length of the catchment drains and its area. The Strahler order (Strahler, 1952) is defined as follows: a drain is of order 1 if it has no tributary. A drain formed by the confluence of two drains of order X is of order X +1. A drain formed by the confluence of two drains of different order is of the order of the highest order drain.
The hypsometric integral (Hi) is a non-dimensional parameter representing after normalization the distribution of the drainage area as a function of the main catchment stream elevations (Strahler, 1952). The value of Hi expresses the volume that has not yet been eroded and thus aims to quantitatively express the evo-
S. Molliex et al. / Earth and Planetary Science Letters 444 (2016) 101–115 107
Fig. 5. Comparison of denudation rates obtained with 4 different methods. A) Comparison of denudation rates from Gulf of Lion catchments deduced from 3 independent methods (MSF data in blue, CEV in red, and CNC in green. B) Mean value of denudation rate deduced from each method for each structural domain. Caption is the same than A). C) Evolution of denudation rates since 5.3 Ma. In this graph, data from DSV in the Gulf of Lion comes from Kuhlemann (2000) and Leroux et al. (in press), are represented in grey and are compared with denudation rates deduced from the other methods used. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
lution of a catchment (Strahler, 1952). A high integral value (>0.6) will characterize a weakly eroded catchment, while a low inte-gral value (<0.4) will characterize a strongly eroded catchment. This parameter is highly dependent on the erodibility of the rocks (Hurtrez and Lucazeau, 1999) but also on the size of the catchment (Walcott and Summerfield, 2008).
The local relief is defined as the difference in elevation between the interfluve and the lowest point in the erosive channel (Ahnert, 1984). It therefore characterizes the incision efficiency, the relief is high if incision is strong (e.g. Champagnac et al., 2014). The geo-physical relief is another more convenient parameter, calculated as the difference between a smooth surface connecting the highest points in the current landscape and the current topography.
A hydrographic network presents a relationship between its slope and its drainage area (Hack, 1957; Flint, 1974; Howard and Kerby, 1983), which can be written as the slope-drainage area re-lationship:
S = ks.A − θ (1)
where S is the slope (mm−1), A is the drainage area (m2), ksand θ are the steepness and the concavity of the studied stream, respectively. The concavity θ only depends on erosion processes and is a constant ranging from 0 and 1, whereas the steepness index ks is a constant which depends on erodability and rock up-lift (Howard et al., 1994; Whipple and Tucker, 1999). Since these two parameters are independent, we can normalize ks, using the same reference concavity for all watersheds (Snyder et al., 2000;Duvall et al., 2004). This new parameter (ksn) is often used to ac-count for rock uplift in geomorphological studies (e.g. Kirby and Whipple, 2012). We used a reference concavity of 0.5 to calculate a weighted average ksn from the drains of each catchment.
3. Results
3.1. Quantification of short to medium term denudation rates
3.1.1. From Modern Sediment Flux (MSF)The results are compiled in Table 2 and Fig. 5. At present,
the MSF of the Rhône River provides 90% of the sediment in-puts in the GOL (Aloisi, 1986). The denudation rates deduced from MSF data are ∼100 mm ka−1 for the Rhône catchment (Miliman and Syvitski, 1992; Pont et al., 2002; Delmas et al., 2012). Val-ues of the same order of magnitude are generally associated to catchments from the same structural area (Table 2; Fig. 5). In the Inner Alps, the deduced denudation rates range between 150 and 800 mm ka−1 (IRS, 2000; Touchard, 1993; Alary, 1998); be-tween 50 and 200 mm ka−1 in the Alpine foreland (IRS, 2000;Chapron, 1999; Landon and Piegay, 1999) and are only about 35 mm ka−1 in the eastern Massif Central (IRS, 2000; Bourrin et al., 2006). The mean denudation rates deduced for Pyrenean catchments, calculated from experimental measurements (Bourrin et al., 2006) or obtained by modeling (Delmas et al., 2012), are ∼20 mm ka−1, except for Agly River (∼70 mm ka−1; Serrat, 1999).
3.1.2. From Comogenic Nuclide 10Be Concentration (CNC)Results are compiled in Table 1 and Fig. 5. The integration time
periods vary from 1 to 15 ka with respect to denudation rates. Catchments located in the Inner Alps present the highest denuda-tion rates (∼300–900 mm ka−1; sample # 3, 4, 5), while they range between 150 and 250 mm ka−1 in the Alpine foreland (sample # 7, 9). These values are of the same order of magnitude as those previously determined for other parts of the Alps (Wittmann et al., 2007; Delunel et al., 2010; Norton et al., 2011). In the Pyre-
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nees (sample # 19–21), denudation rates are ∼100 mm ka−1, and range between 55 and 75 mm ka−1 in the Massif Central (sample # 13–17). The sample 1 (located at the outlet of the Rhône) gives a mean denudation rate of about 120 mm ka−1. The presence of present-day glaciers in some catchments of the Inner Alps violates the steady state assumption inherent to this method and may lead to an overestimation of denudation rates (e.g. Vance et al., 2003;Portenga et al., 2015). LGM, Older and Younger Dryas glaciations should have minor impact on denudation rates since integration time are less than 8 ka in the concerned catchments. Glacial prod-ucts such as moraine material inherited from these glacial phases should be close to cosmogenic steady state by now (Wittmann et al., 2007). Nevertheless, present-day glacial cover is about 10% for the Arve (sample # 3) and less than 3% for the others (samples # 1, 4, 5), that should lead to only a slight overestimation of denuda-tion rates.
3.2. Quantification of long-term denudation rates
3.2.1. From Continental Eroded Volumes (CEV)The minimum CEV for the GOL catchment is 19,000 ±
4,000 km3. This volume corresponds to a mean incision of 152 ± 32 m, which leads to a denudation rate of 118 ± 24 mm ka−1
over the last 0.9 Ma. Detailed denudation rates deduced for each catchment are displayed in Table 2 and Fig. 5. Fig. 4C presents the percentage of CEV for each structural area. The Inner Alps domain exhibits the highest mean denudation rate (292 ± 55 mm ka−1). This rate is 153 ± 27 mm ka−1 in the Alpine foreland. Catchments located in the Alpine foreland show large variability ranging from 41 ± 10 mm ka−1 (Ouvèze, #12) to 263 ± 51 mm ka−1 (Fier, #7) (Table 2; Fig. 5A). The whole Alpine domain (Inner Alps and Alpine foreland) is responsible for 81% (49 and 32%, respectively) of the total CEV from the GOL catchment (Fig. 4C), although it represents only 50% of the drainage area. The Massif Central domain yields a mean denudation rate of 51 ± 10 mm ka−1, representing about 5% of the total CEV, while the mean denudation rate of the Pyre-nees is 77 ± 13 mm ka−1, representing less than 4% of the GOL catchment total CEV. No significant variations of denudation rates between catchments are noticed in these two structural domains. Denudation of the plain domain is only 39 ± 9 mm ka−1.
3.2.2. From Deposited Sediment Volume (DSV)3.2.2.1. At the late-glacial period and Holocene time scale An off-shore sediment budget for the last late glacial period and the Holocene (Aloisi, 1986; Jouet, 2007) suggest a denudation rate of ∼96 mm ka−1. Because onshore sediment traps are not considered, this value is underestimated for the Holocene and present-day. The largest sediment trap is the Geneva Lake with a sediment seques-tration of 2.33 × 1011 t over the last 16 ka (Hinderer, 2001). Over the same time period, sediment inputs in the GOL are estimated at 3.84 ×1011 t (Jouet, 2007). Taking into account sediments from the Geneva Lake, the corrected Holocene denudation rate derived from DSV, is ∼154 mm ka−1 and the present-day one is ∼45 mm ka−1. However, these values still need to be considered as minima be-cause many other sediment traps exist (e.g. Lake Bourget and the Lake Annecy), even if their volume is much lower (e.g. Preusser et al., 2010).
3.2.2.2. At the Pliocene and Quaternary scale The mean denudation rate deduced from offshore decompacted DSV in the GOL is about 117 ± 40 mm ka−1 integrated over the last 5.3 Ma (Kuhlemann, 2000; Leroux et al., in press). If a mean thickness of 60 ± 20 m for onshore Quaternary deposits is assumed, a volume of about 1500 ± 500 km3 for continental sediments is reached, which only represents 4 to 8% of the offshore volume. This value is within
the range of uncertainties of offshore estimates and thus has no significant impact on deduced denudation rates.
Mean denudation rates were calculated from DSV for five pe-riods (Fig. 5C): from 500 ka to present-day, 168 ± 56 mm ka−1; from 1 Ma to 500 ka, 169 ± 56 mm ka−1; from 1.6 Ma to 1 Ma, 91 ± 30 mm ka−1; from 2.6 Ma to 1.6 Ma, 75 ± 25 mm ka−1; from 5.3 Ma to 2.6 Ma, 118 ± 39 mm ka−1.
3.3. Relationships between denudation rates and morphometric indices
Morphometric parameters computed are presented in Table 2. The integral hypsometric values from GOL catchments range from 0.16 to 0.44 with an average value of 0.29, suggesting that the hydrographic network is mature (e.g. Strahler, 1952). The high-est values for mean elevation (Z = 1200–1650 m), mean slopes (S = 16–21◦), mean geophysical relief (GR = 225–299 m), K sn(147–323) are from catchments of the internal zone of the Alps. Catchments from The Alpine foreland present values similar to those of the Pyrennees (Z = 440–1050 m; S = 8–17◦; GR =98–233 m; K sn = 49–170), while morphometric values of Mas-sif Central are globally lower than any other moutainous areas (Z = 287–759 m; S = 8–15◦; GR = 102–167 m; K sn = 41–117). Other morphometric indexes do not show significative variations with respect to structural zones.
In order to define the morphological characteristics of each structural zone, we investigated potential interdependencies of morphometric variables, denudation rates inferred from MSF, CEV, and 10Be CNC by computing a Pearson’s correlation matrix, (Ta-ble 3). A good correlation (R > 0.7) between mean slope, geo-physical relief, K sn and mean elevation can be noticed (Table 3; Fig. 6). No other relevant correlations could be highlighted. How-ever, it should be noted that neither the value of the hypsometric integral, drainage area, shape factor, nor drainage density corre-late with other parameters, suggesting a high heterogeneity among the different zones as well as the independence and uniqueness of each catchment. Moreover, whatever the method used to estimate the denudation rates, a global positive exponential correlation ex-ists between denudation rates and relevant morphometric indexes (mean slope, mean elevation, geophysical relief or K sn) (Table 3). To further discuss the role of morphology on denudation rates, we used mean elevation as a representative parameter for global mor-phology of the catchment since it correlates with main parameters characterizing the catchment morphology (mean slope, geophysical relief and K sn). Fig. 7A shows the correlation between mean eleva-tion and denudation rates deduced from each inferred method. In detail, it is possible to decipher some particularities related to each method. Denudation rates estimated from MSF (excluding catch-ments #17, #20, #21 and #25, see explanation in discussion, Sec-tion 4.1.1) present an exponential increase with elevation similar to those derived from CNC, but are lower by about 25%, according to the trend curve equations (Fig. 7A). Denudation rates deduced from CEV exhibit a less constrained exponential increase with ele-vation, different from that obtained by CNC. Indeed, above a mean elevation of about 1000 m the values of CEV do not increase as much as those from the CNC. This point will be further discussed in Section 4.1.2.
4. Interpretations and discussions
4.1. Methods comparison
4.1.1. Short-term denudation: comparisons between CNC and MSF denudation rates
Denudation rates deduced from CNC are about twice as high as those deduced from MSF (Fig. 5B), for all structural units ex-cept the Pyrenees. Several explanations are possible for these lower
S.Molliex
etal./Earth
andPlanetary
ScienceLetters
444(2016)
101–115109
in bold italic. Some important geomorphic parameters rdless of the method used.
er pe
Max. LGM ice cover
Denud. Rate from MSF
Denud. Rate from CNC
Denud. Rate from CEV
326 0.019 0.009 0.043 −0.037449 0.019 0.099 0.109 0.065166 0.166 −0.020 0.113 0.219182 −0.188 −0.181 −0.432 −0.289
022 0.712 0.621 0.654 0.525050 0.781 0.550 0.608 0.589099 0.807 0.774 0.739 0.677438 −0.012 0.198 0.149 0.169147 0.441 0.780 0.825 0.823
016 0.260 0.706 0.665 0.700190 0.132 0.078 0.128 0.103
388 0.171 0.612 0.692 0.771279 0.269 0.727 0.770 0.897
212 0.273 0.712 0.731 0.806325 0.078 0.073 0.126 0.017455 0.015 0.109 0.127 0.093
0.146 −0.053 0.249 −0.1711 0.537 0.777 0.696
1 0.771 0.841
1 0.848
1
Table 3Pearson’s correlation matrix for geomorphic parameters and denudation rates deduced from the 3 independent methods. Values higher than 0.6 (good correlation) arecorrelated each other such as mean elevation, mean slope, geophysical relief or normalized steepness index. These morphometric indexes also correlate with each other rega
Parameters Area Diameter Shape Factor
Drainage density
Long. outlet
Lat. outlet
Alt. outlet
Strahler order
Mean elevation
Max. elevation
Hypsometric integral
Mean slope
Geophysical relief
Ksn Discharge River lenght
Rivslo
Area 1 0.939 0.093 −0.047 0.030 −0.079 −0.143 0.774 0.084 0.454 −0.398 −0.078 −0.027 0.099 0.993 0.938 −0.Diameter 1 −0.025 −0.047 0.052 −0.018 −0.088 0.883 0.200 0.519 −0.404 −0.037 0.030 0.184 0.938 0.993 −0.Shape Factor 1 −0.062 0.204 0.172 0.093 0.155 −0.067 0.007 −0.179 −0.045 0.010 −0.010 0.077 0.006 0.Drainage
density1 −0.310 −0.205 −0.176 0.095 −0.419 −0.243 −0.310 −0.561 −0.537 −0.360 −0.074 −0.059 −0.
Long. outlet 1 0.908 0.806 0.083 0.417 0.221 0.126 0.218 0.334 0.209 0.077 0.029 −0.Lat. outlet 1 0.897 0.032 0.327 0.061 0.172 0.079 0.194 0.121 −0.031 −0.052 −0.Alt. outlet 1 −0.024 0.509 0.276 0.122 0.289 0.380 0.357 −0.083 −0.119 0.Strahler order 1 0.219 0.548 −0.539 −0.019 0.064 0.238 0.777 0.885 −0.Mean
elevation1 0.839 0.293 0.857 0.878 0.918 0.156 0.226 0.
Max elevation 1 −0.169 0.703 0.740 0.812 0.499 0.550 0.Hypsometric
integral1 0.301 0.234 0.216 −0.359 −0.383 0.
Mean slope 1 0.966 0.881 −0.013 −0.001 0.Geophysical
relief1 0.883 0.041 0.069 0.
Ksn 1 0.180 0.220 0.Discharge 1 0.939 −0.River Lenght 1 −0.River Slope 1Max. LGM ice
coverDenud. Rate
from MSFDenud. Rate
from CEVDenud. Rate
from CNC
110 S. Molliex et al. / Earth and Planetary Science Letters 444 (2016) 101–115
Fig. 6. Correlations between morphological parameters. A) Mean elevation vs Slope; B) Mean elevation vs Geophysical relief; C) Mean elevation vs K sn; D) Mean elevation vs Hypsometric integral; E) Mean elevation vs Drainage area; F) Hypsometric integral vs Drainage area. The mean elevation correlates with slope, geophysical relief, and ksn, but does not correlate with hypsometric integrals and drainage area, suggesting that mean elevation is an independent representative parameter of catchment morphology.
estimates from MSF: (i) direct measurements of MSF were of-ten obtained over the last 30 yrs only. Extreme events (such as millennial or centennial floods) were thus not necessarily taken into account though they must have a great impact on fluxes (e.g. Serrat et al., 2001), in particular for bedload sediments which are especially difficult to quantify in low-to-moderate erosion do-mains; (ii) the intensive damming of the Rhône valley would imply the removal of gravels from the riverbed during the last century and a decreased MSF measured in the Rhône River and tributaries (Van Straaten, 1957); (iii) a real difference exists between denuda-tion rates integrated over the last hundreds to thousands years (CNC rate) and present-day denudation rates. This would imply a change in denudation processes and/or climate conditions within the Holocene.
For some catchments (Hérault #17, Têt #20, Tech #21 and Aude #25; Fig. 5A), denudation rates derived from MSF are sig-nificantly lower than those estimated from CNC. These catchments are located in hydrological regime characterized by sudden ex-treme floods significantly contributing to sediment flux. Serrat et al. (2001) showed that 78% of the sediment load of the Têt River in the Pyrenees was transported during only 0.7% of the studied period (50 days in 20 yrs). This observation favors the first hy-pothesis (extreme events are not taken into account) to explain the underestimation of MSF-derived denudation rates. Note that the estimated denudation rates used for Agly catchment were derived from modeled sediment fluxes (Serrat, 1999), and are more accu-
rate than those from surrounding catchments (Bourrin et al., 2006;Delmas et al., 2012).
4.1.2. Long-term denudation: comparison between CEV and DSVThe difference between CNC and CEV-derived denudation rates
in the exponential increase relationship with elevation in Fig. 7A can be explained if we assume that CNC-derived denudation rates are slightly overestimated by the presence of present-day glaciers at high elevations. On the other hand, denudation rates from CEV are most likely underestimated due to the fact that interfluves and high points are also undergoing denudation, especially in high de-nudation domains. No data are available in the GOL catchment to quantify this underestimation but estimation of long-term CNC de-nudation rates of interfluves (8 to 20 mm ka−1) were proposed by Kuhlemann et al. (2007) for Corsica granites, which are petrograph-ically similar to those constituting the Inner Alps. Moreover, this value is consistent with similar studies worldwide in Alpine ranges under temperate climate (e.g. Small et al., 1997).
The main phase of continental incision started either at 2.6 Ma (Gibbard et al., 2010) or at 0.9 Ma as suggested for the Alps (Haeuselmann et al., 2007; Valla et al., 2011). Without correc-tion of interfluves denudation, a total eroded volume of 19,000 ± 4,000 km3 was calculated from topographic reconstruction that corresponds to a volume of 26,400 ± 5500 km3 for uncom-pacted sediments. By applying the maximum value for denuda-tion of interfluves and maximum age for the beginning of inci-
S. Molliex et al. / Earth and Planetary Science Letters 444 (2016) 101–115 111
Fig. 7. A) Distribution of denudation rates with respect to mean elevation. An exponential trend between denudation rates and mean elevations is highlighted for each method used. B) Same plot but only using glacially impacted catchments. C) Same plot but only using non-glacially impacted catchments. The exponential trend is driven by glacially conditioning catchments, since no trend is highlighted in other catchments.
sion (2.6 Ma), a maximum volume of uncompacted sediment of about 35,000 km3 is obtained from CEV. This value is still signifi-cantly lower than the 49,000 km3 estimated from DSV over the last 2.6 Ma (Leroux et al., in press). This difference may be explained by the overestimation of DSV, since offshore sediments are pos-sibly not provided exclusively by the catchment, but may also be transported by marine currents from other oceanic sources. Nev-ertheless, the difference of volume seems to be too high to match such an hypothesis.
If the incision started 0.9 Ma ago, the maximum uncompacted sediment volume deduced from CEV corrected for denudation of interfluves is 29,000 km3, which is close to the 25,300 km3 of DSV over the last 0.9 Ma (Leroux et al., in press). The CEV could be larger than the DSV since dissolved carbonates are not necessarily directly precipitated in the basin. Moreover, carbonates represent a large part of the catchment surface. Our results favor an incision phase beginning 0.9 Ma ago, at least in the Alps.
4.1.3. Short-term versus long-term denudation: comparisons between CEV and CNC derived denudation rates
Although almost systematically lower, denudation rates de-duced from CEV are in good agreement with those deduced from CNC (Fig. 5A; 5B). In rapidly eroded terrains (Alps), denudation rates deduced from CEV seem to be more significantly underesti-mated (Fig. 5A; 5B) than in slowly denuding terrains (Massif Cen-tral, Pyrenees). In such slowly eroding zones, the underestimation of CEV is probably lower and the deduced denudation rates are closer to effective rates. In high denudation rate zones, high points and interfluves probably undergo a stronger denudation which leads to a larger underestimation of the CEV. CNC deduced de-nudation rates are integrated over the last hundreds or thousands of years and representative of short-term denudation, whereas CEV derived rates are integrated over the last 0.9 Ma. The consistency of denudation rates obtained using both methods suggests that de-nudation rates remained relatively constant through time despite climate variability (glacial and interglacial periods).
4.2. Pliocene–Quaternary evolution of denudation rates
The mean Pliocene–Quaternary denudation rate deduced from DSV is 117 ± 40 mm ka−1, but it varies through time (Fig. 5C). The Pliocene (5.3 to 2.6 Ma) denudation rate is 118 ± 39 mm ka−1 and decreases to 75 ± 25 mm ka−1 during the early Pleistocene (2.6 to 1.8 Ma). Lower early Pleistocene denudation rates may result from the modification of the hydrographic network due to the capture of the Aar-Doubs River (Fig. 3). Indeed, based on eroded volumes, this capture would cause a decrease of sediment flux of 37 ± 5%. This value exactly matches the observed decrease in DSV within the GOL. Other factors might explain a decrease of denudation rates at the beginning of Quaternary: (1) global climatic changes with a drop in temperature and precipitation at 2.6 Ma (Suc et al., 1995;Popescu et al., 2010) (2) a decrease of the tectonic uplift of the Alps and surrounding reliefs. Indeed, some authors consider that an uplift pulse may have occurred in the Alps after the Messinian Salinity Crisis (post-5.3 Ma) (Willett et al., 2006), although most of the Alpine deformation was completed before the crisis (Clauzon et al., 2011).
During the middle Pleistocene a significant increase of de-nudation rates from 91 ± 30 mm ka−1 (1.8 to 0.9 Ma) to 169 ±56 mm ka−1 occurred after 0.9 Ma (Fig. 5C). It can be interpreted as the result of the onset of the 100-ka glacial cycles. This is corroborated by the enhanced valley incision in the Alps which began at that time (Haeuselmann et al., 2007; Valla et al., 2011). Moreover, denudation rates inferred from CEV and from DSV give similar values when the enhanced incision is assumed to begin 0.9 Ma ago. Such an increase in denudation rates at that time has been documented in different mountain ranges of the world (in the Alps: Glotzbatch et al., 2011; in the Himalayas: Charreau et al., 2011). It suggests a global climatic control, linked to the change of dominant climatic cycles from 41 ka to 100 ka also related to a doubling in sea-level amplitudes (from ∼50 to ∼100 m) and an enhancement of glaciers advances in these mountain ranges (e.g. Head and Gibbard, 2005).
112 S. Molliex et al. / Earth and Planetary Science Letters 444 (2016) 101–115
4.3. Relationship between denudation rates and geomorphology
4.3.1. Are glacially impacted morphologies controlling denudation?The sparse presence of glaciers and moraines in high-elevation
catchments may lead to a slight overestimation of the 10Be-derived denudation rates (e.g. Dunai, 2010; Ward and Anderson, 2011) but the exponential relationship between morphometric parame-ters and denudation rates inferred by three independent methods suggests that high denudation rates are controlled by catchments morphologies, especially for high-elevation catchments which were glaciated during glacial maxima (Fig. 7B and 7C). Indeed, the location of glacier-covered areas seems to be a major parame-ter controlling denudation rates, since non-glaciated catchments erode overall at much lower rates and do not exhibit a correla-tion with elevation (Fig. 7B), but rather show uniform denuda-tion rates (50 to 100 mm ka−1) with elevation (Fig. 7C). The satisfactory agreement between MSF, CEV, and CNC derived de-nudation rates suggests furthermore that the processes responsible for denudation are the same for the short-term (present-day to millennial-scale) and the long-term (Quaternary-scale). Thus, the morphology resulting from the glacial conditioning since 0.9 Ma seems to be the main controlling factor on denudation rates, even at present-day, as recently suggested for the Alps (Schlunegger and Hinderer, 2003; Norton et al., 2010; Glotzbach et al., 2013;Champagnac et al., 2014), since our data do not show any ev-idence for a difference in denudation rates between interglacial and glacial stages. This morphological control on denudation may be dependent on some specific characteristics of glacially im-pacted valleys such as higher slopes of valley flanks, higher re-lief, and greater weathering of rocks due to the intensive rock breakdown by frost action during glacial periods. Moreover glacial denudation also leads to the formation of moraines which are preserved during glacial periods because of lower precipitation rates and the lower transport capacity due to water discharge. This kind of residual material is partly stored in the catchment, and could thus constitute a significant volume of sediment that can be easily remobilized during interglacial periods when fluvial processes become more efficient (Harbor and Warburton, 1993;Hinderer, 2012).
4.3.2. Is denudation controlling rock uplift?At present, and probably since 3 Ma, the Alps are experienc-
ing an extension perpendicular to the belt (e.g. Sue et al., 2007) interpreted as resulting from a gravitational collapse. The other structural domains can be considered as tectonically stable since this period. This collapse is associated to an efficient exhumation of the Inner Alps (Cederbom et al., 2004; Vernon et al., 2008) and a present-day uplift that can locally reach more than 1500 mm ka−1
(Schlatter et al., 2005). Several processes can account for the present-day uplift of the Alps including an uplift linked to the pas-sive unloading due to denudation (Schlunegger and Hinderer, 2001, 2003; Cederbom et al., 2004; Wittman et al., 2007; Champagnac et al., 2009; van der Beek and Bourbon, 2008; Willett, 2010;Vernant et al., 2013). For example ongoing convergence between the Adriatic microplate and Europe (Persaud and Pfiffner, 2004;Lardeaux et al., 2006), rebound after removal of the glacial loads at the end of the LGM (Gudmundsson, 1994), the mantle dynamics linked to a “slab tearing” (Lippitsch et al., 2003) are processes that may drive denudation in the Alps. However, we suggest that the correlation between denudation rates and catchment morphologies and the significant increase of these rates in glaciated areas during the LGM (regardless of the method and time scale used) high-lights a strong effect of glacial processes on short and long-term denudation rates, which is consistent with the denudation-driven uplift of the Alps, even if long-term denudation alone cannot ac-
count for modern rock uplift in the Alps (Champagnac et al., 2009;Fox et al., 2015).
5. Conclusion
The use of four different methods gave consistent denudation rates at different time scales in a large Mediterranean catchment. Offshore Sediment archives (DSV) allow estimations of paleo-denudation denudation rates over large time scales, but some pro-cesses remain difficult to quantify such as sedimentary hiatuses, amount of remobilization, and transport by currents. The geomor-phologic approach, with estimation of continental eroded volumes (CEV), is a simple and efficient approach, but it significantly un-derestimates denudation rates especially in high-elevation zones (glaciated areas). The modern sediment flux measurements (MSF) often underestimate long-term denudation rates and are not effi-cient in some specific hydrologic regime systems (sudden extreme flood systems, torrential rivers). Finally, the cosmogenic nuclide approach (CNC) seems to be the most accurate method but it is dependent on catchment lithologies (need of Quartz for 10Be deter-mination). Nevertheless, the results obtained from the four meth-ods are consistent and the combination of different approaches allowed a better understanding of the relationships between de-nudation, sedimentation, geomorphology, and rock uplift in the GOL source-to-sink system. The Alpine domains provide the largest portion of the sediments with denudation rates that could reach more than 1000 mm ka−1.
We observe a positive exponential correlation between de-nudation rates and morphometric parameters representing catch-ment morphologies such as mean catchment elevation. This cor-relation exists for all methods (and thus all time scales) con-sidered. While in non-glacially impacted catchments denudation rates always range between 50 and 100 mm ka−1, denudation rates are significantly higher and show a correlation with elevation in glacially-impacted catchment, suggesting that glacially impacted morphologies play a major role in the Quaternary distribution and values of denudation rates. Doubling of denudation rates since the Mid-Pleistocene revolution (0.9 Ma) infers a correlation with global climatic processes related to the increase (doubling) of sea-level amplitudes and change from 41 ka to 100 ka duration of glacial-interglacial cycles.
Acknowledgements
S. Molliex benefited from post-doc fellowships granted by the Conseil Général du Finistère (CG29), LabexMER, and IFREMER. This work further benefited from a State Grant from the French “Agence Nationale de la Recherche (ANR)” in the Program “Investissements d’Avenir” (ANR-10-LABX-19-01, Labex Mer). The French CNRS INSU SYSTER and Actions Marges Programs financed sampling and cos-mogenic nuclide concentration measurements. M. Arnold, G. Au-maître and K. Keddadouche are thanked for their valuable as-sistance during 10Be measurements at the ASTER AMS national facility (CEREGE, Aix en Provence), which is supported by the INSU/CNRS, the ANR through the “Projets thématiques d’excel-lence” program for the “Equipements d’excellence” ASTER-CEREGE action, IRD, and CEA. C. Vella is thanked for providing a sand sam-ple of Rhône River. J.-P. Suc and P.H. Blard are thanked for fruitful discussions. We are also grateful to J.D. Champagnac and an anony-mous reviewer for their thorough and in-depth reading and com-ments which contributed to improve the manuscript. The authors are grateful to K. Kovacs for post-editing the English style.
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