basin research 1–18, doi: 10.1111/bre.12038 ... indusbasin p. d. clift*,† and l. giosan ... the...

Sediment fluxesand buffering in thepost-glacialIndus BasinP. D. Clift*,† and L. Giosan‡

*Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA, USA†South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China‡Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA, USA

ABSTRACT

The Indus drainage has experienced major variations in climate since the Last Glacial Maximum

(LGM) that have affected the volumes and compositions of the sediment reaching the ocean since

that time. We here present a comprehensive first-order source-to-sink budget spanning the time

since the LGM.We show that buffering of sediment in the floodplain accounts for ca. 20–25% of

the mass flux. Sedimentation rates have varied greatly and must have been on average three times

the recent, predamming rates. Much of the sediment was released by incision of fluvial terraces con-

structed behind landslide dams within the mountains, and especially along the major river valleys.

New bedrock erosion is estimated to supply around 45% of the sedimentation. Around 50% of

deposited sediment lies under the southern floodplains, with 50% offshore in large shelf clinoforms.

Provenance indicators show a change of erosional focus during the Early Holocene, but no change in

the Mid–Late Holocene because of further reworking from the floodplains. While suspended loads

travel rapidly from source-to-sink, zircon grains in the bedload show travel times of 7–14 kyr. The

largest lag times are anticipated in the Indus submarine fan where sedimentation lags erosion by at

least 10 kyr.

INTRODUCTION

Marine sediments represent the longest and most com-

plete archives of continental environmental evolution,

and they have been used to reconstruct past patterns, rates

of erosion and changing environments at the time of

deposition. These in turn can be used to assess the influ-

ence that climate or tectonics have over continental envi-

ronments. However, if the marine record is to be utilized

to its full potential, we must first constrain the transport

of clastic sedimentary particles from continental sources

to the marine sink.

To what extent are sediments deposited in the deep

ocean representative of the erosion in the source

regions at the time of their sedimentation? Clastic sedi-

ments do not relocate instantly into the sea after ero-

sion in mountains, but their transport to the ocean

could vary in efficiency and speed, spanning from days

or weeks, to >105–106 years, because of storage and

reactivation en route. Indeed, it has been argued that

many Asian river systems have maintained a relatively

constant mass flux to the ocean over at least the past

2 Ma because of sediment buffering in the floodplain

(M�etivier & Gaudemer, 1999). While this has been dis-

puted over timescales >106 years (Clift, 2006), there is

presently little control on how effective this buffering

process might be on shorter timescales.

In this study, we present a new sediment budget for the

post-glacial Indus River system in order to quantify the

degree of buffering in a large drainage system and to see

how the deep-sea sediment record might be used to

understand erosion in the mountain sources. We use sedi-

mentary provenance data to assess the speed of sediment

transport from the source ranges to the ocean and further

to the deep-sea. We focus on the post-Last Glacial Maxi-

mum (LGM) period because the sediment stored on the

shelf clearly sits on a glacial unconformity, thus allowing

ready estimation of the total deposited volume. This is also

an appropriate time period for estimating eroded volumes

onshore because a number of incised fluvial terraces in the

floodplains (Giosan et al., 2012a) and mountains are

known to date from ca. 10 ka (Bookhagen et al., 2006; He-

witt, 2009). It has been recognized that the Early Holocene

was a period of rapid sediment flux to South Asian deltas

(Goodbred & Kuehl, 2000a; Giosan et al., 2006a), but it isnot clear to what extent this sediment was freshly eroded

bedrock rather than older fluvial sediments reworked from

the floodplains and terraces in the mountains. While some

sediment was released by retreating glaciers after the

LGM, this affected the central and eastern Himalaya more

than the Indus watershed because of the relatively dry cli-

mate of this western region (Owen et al., 2008). Hedrick

Correspondence: P. D. Clift, Department of Geology andGeophysics, Louisiana State University, E235 Howe-Russell,Baton Rouge, LA 70803, USA. E-mail: [email protected] Giosan, Department of Geology and Geophysics, WoodsHole Oceanographic Institution, Woods Hole, MA 20543, USAE-mail: [email protected]

© 2013 The AuthorsBasin Research © 2013 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 1

Basin Research (2013) 25, 1–18, doi: 10.1111/bre.12038

EAGE

et al. (2011) noted that while those regions north of and inthe rain shadow of the Greater Himalaya experienced only

moderate glacial advance at the LGM, there was signifi-

cant glacial advance in the more monsoonal regions within

and south of the Greater Himalaya. Retreat of those gla-

ciers would have released significant sediment quantities

that could then be transported to the ocean.

The Indus River basin has a strongly erosive mon-

soonal climate over parts of the drainage, as well as

steep topography and well-defined areas of rapid rock

uplift within the source mountains [e.g. Nanga Parbat,

south Karakoram metamorphic domes (Zeitler et al.,1993; Maheo et al., 2004)]. In addition, there is a strong

rainfall gradient across the mountains from wet in the

south to the dry regions of Ladakh, Karakoram and

Tibet in the north (Bookhagen & Burbank, 2006)

(Fig. 1). The sediment sources in the Himalaya and Ka-

rakoram are very diverse in composition, and thermo-

chronologic ages that make large-scale sediment

provenance quantification simpler than in many drain-

ages (Clift et al., 2004; Garzanti et al., 2005). Crucially,it is known that the origin of the sediment reaching the

ocean has changed sharply since the LGM (Clift et al.,2008). The Indus is a sediment-productive river, which

used to supply at least 250 Mt year�1 to the Arabian

Sea prior to modern damming (Milliman & Syvitski,

1992), although other estimates emphasize a larger range

from 100 to 675 Mt year�1 (Ali & De Boer, 2008).

Modern and relict wide floodplains flank the Indus and

stretch >1300 km from the mountain front to the delta,

providing ample potential opportunity for storage and

reworking.

ERODEDVOLUMES

Eroded and incised fluvial terraces are recognized across

the northern half of the Indus floodplains, as well as in

Indus

Kabul

66˚ 68˚ 70˚ 72˚ 74˚ 76˚ 78˚ 80˚ 82˚

24˚

26˚

28˚

30˚

32˚

34˚

36˚

Thar Desert

SutlejChenabRavi

BeasJhellum

Figure 3

Gilgit

Lesser Himalaya

Greater HimalayaPunjab

Thatta

Karakoram

Ladakh

Hindu Kush

Sul

aim

an R

ange

s

Kir

thar

Ran

ges

KetiBandar

Cholistan

Rann of Kutch

Figure 12

Harappa

Jati Gul

KL

A B

Sukkur Figure 2

Sindh

Moenjodaro

MatliKarachi

Fig. 1. Shaded topographic map of the Indus River basin showing the divisions between the major tributaries. Trunk streams are

marked in white, with the drainage divides depicted with black lines. The map also shows the major geographical regions and moun-

tain ranges mentioned in the text. Image is from NASAWorldWind (http://worldwind.arc.nasa.gov/). Our borehole locations (Gio-

san et al., 2006b) are shown in the lower Indus plain and delta (i.e. Keti Bandar, Jati, Thatta, Gul, Matli) together with transects A-B

and K-L (Fig. 11) using our data and published borehole info from Kazmi (1984). The dotted black lines along the Indus indicate the

presumed boundaries of the incised valley (Kazmi, 1984).

© 2013 The AuthorsBasin Research © 2013 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists2

P.D. Clift and L. Giosan

the mountains, and constitute a potentially significant

source of sediment for the river. Reworking of sediment

stored in these terraces is likely a major source of sedi-

ment to the Indus since the LGM (Fig. 1). Giosan et al.(2012a) noted that the alluvial plain in Punjab shows inci-

sion by the modern Indus and its tributary rivers forming

valleys only 10–20 m deep, but tens of kilometres wide

(Fig. 2). The valleys are separated from each other by

elevated plateau regions or interfluves. Optically stimu-

lated luminescence (OSL) dating of the sediments at the

top of the interfluves has revealed that sedimentation was

ongoing until ca. 10 ka and that the valleys have thus

been cut after that time (Giosan et al., 2012a). A high-

resolution digital elevation model of the region reveals

that the incision is deepest closest to the mountain front

and decreases southward, becoming insignificant close to

the final confluence of the Indus and the Sutlej Rivers

(Figs 1 and 2). By assuming that the northern floodplains

were accretionary and had a smooth topography prior to

incision, we can estimate how much material has been

excavated from the valleys. Using maximum and mini-

mum estimates of the total area of the incised valleys

(Fig. 2) and assuming a linear reduction in depth from

north to south, we estimate that the amount of sediment

reworked further south since 10 ka is ca. 925 km3 if we

use the volume under the upper presumed interfluve sur-

face in Fig. 2. A minimum estimate can be derived using

the lower interfluve surface, which implies erosion of ca.370 km3 (Table 1). The upper figure for the eroded vol-

ume assumes that relief on the interfluves is largely the

result of fluvial erosion, whereas the lower figure esti-

mates only the main valley erosion of the Indus and its

Himalayan tributaries assuming that the interfluve relief

is caused by aeolian remodelling. These two estimates

also give a measure of the uncertainty for this source of

sediment, although we do expect that the larger value is

closer to reality because active dunes are mostly limited

to the Thal Desert (the Indus–Jhelum–Chenab inter-

29˚

30˚

31˚

33˚

Multan

Thar Desert

29˚

30˚

29

Indu

s

Jhel

lum

Chena

bRavi

Sutlej

X

Y

Z

71˚ 72˚ 73˚ 74˚ 75˚

W V

180

190

200

210

220

230

0 50 100 150 200 250 300

ESWN

(X)

100

110

120

130

140

150

0 50 100 150 200 250 300

(Z)

sand dunes

Distance (km)

180

200

220

240

260

280

300

Interfluve (W)

(V)River valley

0 50 100 150 200140

150

160

170

180

190

200

0 50 100 150 200 250 300 350 400

(Y)

Ele

vatio

n ab

ove

seal

evel

(m

)

Distance (km)

Fig. 2. Shaded topographic map of the northern half of the Indus River floodplains showing the modern courses of the major rivers

and the ‘interfluve’ terraces between them. Optically stimulated luminescence dating has shown that these terraces formed up to 10 ka

and has been incised. Transects across the floodplain show that the river now occupy incised valleys. We calculate the maximum and

minimum limits for eroded sediment as the regions under the dotted lines interpreted as a smooth surfaces at 10 ka. Topography is

from NASA’s Shuttle Radar Topography Mission via GeoMapAppTM. Stars show the age control points from Giosan et al.(2012b).


Sediment flux in the Indus Basin

fluve) and in the Thar Desert, which is largely south of

our study area (the interfluve south of the Sutlej–Induscourses; Fig. 2).

Assessing the volumes eroded from the mountains is

more complex and several sectors need to be considered

separately if a realistic figure of remobilized sediment is to

be achieved for this region. We divide the mountains into

four sectors based on climate and geology. One sector

covers the Greater and Lesser Himalayan ranges that are

influenced by heavy summer monsoon rains. These are

expected to be more sediment productive and potentially

more tectonically active than those areas to the north that

lie effectively on the western edge of the Tibetan Plateau

and that experienced major uplift earlier in the Palaeogene

(Harris, 2006). The main tributaries that join the Indus

from the east are largely sourcing their sediment in this

Himalayan regime (Fig. 3). The trunk Indus, the Zanskar

and the northern parts of the Sutlej River drain a separate

region lying in the rain shadow of the Himalaya. We infer

similar conditions for the western parts of the Kabul

River, and those streams draining the Karakoram and

Hindu Kush (i.e. the Shyok, Hunza, Gilgit, Swat and

Chitral Rivers) because these regions are also not heavily

monsoonal in character. The third major division lies

around the Nanga Parbat massif (Fig. 3) where it has

been recognized that especially strong landsliding has

caused large-scale damming of the Indus in the past

20 kyr (Korup et al., 2010). We follow the study of

Hewitt (2009) in defining this region to be 100 km around

the peak of Nanga Parbat itself, including the major rivers

passing around the peak to the north and east, as well as

the Shigar River (Fig. 3). A separate estimate was made

for a fourth sector in which sediment is stored behind

landslide rock dams in the Karakoram. Hewitt (1998)

mapped a series of large dams and the extent of lake ter-

races behind them in Baltistan, east of the Hunza River

(Fig. 3). Not all the lakes were completely filled with sedi-

ment at the point that the dam was breached, but we use

the estimates of the breached height to calculate how

much material may have been reworked, at least from

those dams defined by Hewitt (1998) (Table 1). The

Table 1. Estimates of the eroded volumes in different sectors

of the Indus floodplains and Himalayan source regions since the

Last Glacial Maximum

Erosion sources

Area of

incision (km2)

Volume eroded (km3)

Maximum Minimum

Floodplains 92 545 925 370

Rumbak drainage 162 0.077 0.051

Volumes eroded

outside main valleys

510 609 242 161

Volumes in Kirthar

and Sulaiman

Ranges

195 402 93 62

Length (km) Maximum Minimum

Indus in Ladakh 70 7 4

Indus upstream

of Gilgit

670 71 38

Indus around

Nanga Parbat

220 601 323

Trunk

Indus –Himalaya

realm

425 53 15

Shyok River 270 33 11

Hunza River 225 27 9

Shigar River 93 254 137

Gilgit River 160 20 7

Zanskar River 180 22 7

Swat Valley 280 34 11

Chitral Valley 236 29 10

Kohistan River 98 12 4

Nanga Parbat River 165 450 243

Trunk

Sutlej –Himalaya

realm

290 36 10

Trunk Sutlej – Tibetanrealm

230 29 8

Trunk Jhelum 213 27 8

Jhelum tributary 169 21 6

Trunk Chenab 365 46 13

Chenab Tributary 155 19 5

Trunk Ravi 217 27 8

Trunk Beas 176 22 6

Trunk Kabul – EastBranch

300 38 11

Trunk Kabul –West

Branch

400 51 14

Total 1929 908

Volume (km3) Percentage

Maximum Minimum Maximum Minimum

Total

Himalayan

rivers

252 71 6 4

Total

Karakoram,

Kohistan

and Hindu

Kush

1122 374 27 21

(continued)

Table 1 (continued)

Volume (km3) Percentage

Maximum Minimum Maximum Minimum

Total

Floodplains

925 370 23 21

Total around

Nanga

Parbat

1305 703 32 40

Total from

rain

shadow

137 39 3 2

Total from

side valleys

363 195 9 11

Grand total 4104 1752



uncertainties on these numbers are high if poorly defined,

but it is clear that damming and rock slides are important

sediment buffers in the Karakoram, and that major vol-

umes of sediment have been stored in the mountain val-

leys. We use an uncertainty of �50% for the eroded

volumes in these rock-dammed valleys, following the

logic for the large Himalayan rivers outlined below.

In contrast to the sediment stored behind dams in the

major river valleys, we recognize that sediment is also

stored within the tributary valleys to these large trunk

rivers, as well as on the wider landscape in the upper

parts of the drainage. In an attempt to quantify how

much sediment may be released from these areas, we

consider the drainage that covers ca. 160 km2 located

SW of Leh and south of the Indus River in Ladakh

(Figs 3 and 4). We do not choose the Rumbak drainage

for any particular reason and indeed aim to use it as a

typical example of a catchment in the rain shadow of

the Greater Himalaya. It was selected for study based

on its moderate size that allowed it to be surveyed across

most of its extent and because it is relatively accessible.

Comparison of the Rumbak with other valleys in

Ladakh suggests that it is typical in its form and

36˚

30˚

31˚

32˚

33˚

34˚

35˚

37˚

70˚ 71˚ 72˚ 73˚ 74˚ 75˚ 76˚ 77˚ 78˚ 79˚ 80˚ 81˚

Lahore

Attock

Indus

Jhellum

ChenabRavi

Sutlej

Beas

Kabul

Islamabad

Gilgit

NangaParbat

Leh

Rumbak

Shyok

Hunza

Shigar

Gilgit R.

Zanskar

Chitral

Swat

Kohistan

Pang

KhalsiKyber Pass

Fig. 3. Shaded topographic map of western Himalaya, Karakoram and Hindu Kush showing the major rivers feeding the Indus. Dot-

ted lines show divisions between major region of contrasting climate and geomorphology. North of the line on Sutlej, we assume La-

dakh-type dry sediment storage, while between the lines along the main Indus around Nanga Parbat large-scale landslide damming is

assumed.

Indus

5 km

34˚6’

34˚8’

34˚0’

34˚2’

34˚4’

77˚20’

77˚28’

77˚22’

77˚24’

77˚26’

Rumbak

1

2

3

4

5

6

7

9

10

11

12

13

8

Fig. 4. Satellite image of the Rumbak drainage near Leh

showing the main tributaries considered in this study and

whose sediment volumes are provided in Table 2. See Fig. 3

for location.



location. In the future, surveying of wider areas will test

whether the Rumbak is indeed representative as we

argue. The wider region is made up of a series of smal-

ler catchments of this type, and this is therefore a rea-

sonable starting point for making a budget in this part

of the Indus basin. Surveying of this drainage revealed

the presence of an incised terrace system running all the

way from the confluence with the Indus to the upper

reaches (Fig. 5a). While the incision is greatest towards

the base of the drainage, this reduces to zero in the

upper reaches. We used field measurements of the inci-

sion to calculate the volume of sediment removed since

the incision of the terrace, which is presumed to be

post-glacial, based on comparison with other Himalaya

terraces (Table 2).

Figure 6a shows the erosion calculation approach. A

terrace dating to ca. 10 ka or older is identified, and a

profile across the valley is measured from maps or from

NASA Shuttle Radar Topography Mission (SRTM)

data. Sediment is presumed to have filled the valley to

the level of the terrace top, so that the sediment volume

eroded can be estimated by projecting the terrace level

across the valley and calculating the area of sediment

missing. A volume can then be derived by multiplying

that area by the length of the stream. In the case of the

Rumbak drainage where the terraces are known to thin

up the valley to zero, we can estimate the volume by

assuming a linear decrease from the downstream end of

the valley where the maximum incision is noted and the

top of the valley where zero erosion has occurred

(Fig. 6b).

We estimated 0.064 km3 (64.1 9 106 m3) of erosion

over the 160 km2 of the Rumbak Valley, and we used this

value to extrapolate how much material might be held in

similar systems across the entire mountainous parts of the

Indus catchment, including the Kirthar and Sulaiman

Table 2. Estimates of the sediment volumes eroded from the individual side valleys within the Rumbak drainage used to determine

sediment storage within the upper part of the Indus source regions. See Fig. 4 for location

Streams

Length of segment

(km)

Distance from

confluence (km)

Maximum incision

(km)

Maximum channel width

(km)

Volume eroded

(km3)

1 15.20 0.00 0.0500 0.0500 0.0190

2 5.30 9.30 0.0194 0.0388 0.0020

3 3.40 10.90 0.0141 0.0283 0.0007

4 4.20 5.10 0.0332 0.0664 0.0046

5 9.60 1.00 0.0467 0.0934 0.0209

6 6.10 7.90 0.0240 0.0480 0.0035

7 5.20 5.60 0.0316 0.0632 0.0052

8 5.85 8.70 0.0214 0.0428 0.0027

9 2.20 12.50 0.0089 0.0178 0.0002

10 2.20 11.00 0.0138 0.0276 0.0004

11 1.90 7.20 0.0263 0.0526 0.0013

12 1.70 8.50 0.0220 0.0441 0.0008

13 2.45 5.10 0.0332 0.0664 0.0027

Total 0.0641

(a) (b)

Fig. 5. Field photographs showing examples of terracing in the Indus basin (a) in the Rumbak drainage (see Figs 3 and 4) where the

terrace is around 20 m high, and (b) the Pang gorge, south of the more plains in Ladakh (location on Fig. 3). Terrace here shows ca.250 m of relief.



Ranges in the west. These latter ranges are not part of the

Himalaya, and they experience a drier climate than the

monsoonal southern face of the Himalaya. Until they can

be surveyed themselves, we use the sediment yield from

the Rumbak Valley because this is located in the Himalaya

rain shadow and is also subject to a more arid environ-

ment. The amount of material derived from such settings

appears to be much smaller than other sources and is esti-

mated here at 279 km3, although clearly the uncertainties

in such an estimates must be significant if difficult to

quantify at the present time (Table 1). While we recog-

nize that in some locations much greater thicknesses have

been stored and released, for example the Pang gorge

south of the more plains in Ladakh (Fig. 5b), such areas

are of generally limited areal extent and are not believed

to make a large difference to the basin-wide budget. Simi-

larly, we recognize that in some locations, such as the Nu-

bra–Shyok Valley in Ladakh, glacial advance resulted in

formation of a large lake system, but sedimentary facies

analysis has shown that this was largely a water-filled

basin and did not release large volumes of sediment as the

glaciers retreated (Dortch et al., 2010).To estimate the amount of sediment released from the

Himalayan valleys, we use the terrace mapping and dating

work of Bookhagen et al. (2006) in the Sutlej Valley as a

type example. This study estimated that the Sutlej has

lost 77.1 m3 km�1 of stream in that study area. To under-

stand the potential uncertainties in extrapolating this stor-

age capacity along the length of the river, we measured

five sections across the Sutlej upstream and downstream

of the Bookhagen et al. study area and estimated the

possible amount of material that would have been eroded

had the valley been filled up the level of the terrace dated

at 10 ka by Bookhagen et al. (2006). Figure 7 shows that

the valley is much narrower over significant lengths com-

pared with where the terraces are well developed. None-

theless, the average eroded estimate from these five

sections was still 80.4 m3 km�1 of stream, but with a

standard deviation equivalent to �56%. Other significant

sources of uncertainty to the eroded volumes in other

parts of the Himalaya, which have not been the subject of

detailed study, are the height of the terraces above the

river and the age of the terraces, as the length and width

of the river valleys are reasonably easily measured. Based

on the existing study of terraces, we estimate height

uncertainties of not more than 20% and a similar value

for the age, although terraces dating at 8–10 ka are found

where studies have been performed in the monsoonally

affected Himalaya (Bookhagen et al., 2006; Dortch et al.,2008).

We apply a value of 80.4 m3 km�1 of incision to the

entire length of the Beas, Ravi, Chenab and Jhelum Riv-

ers, as well as their major tributaries in order to estimate

the possible released volumes. Because incision of terraces

is known to be high immediately into the frontal Siwalik

Hills in Nepal (Lav�e & Avouac, 2001), there is no reason

to consider a reduced incision adjacent to the floodplains.

We further apply this rate to the Himalayan parts of the

Sutlej (i.e. those south of the Himalayan rain shadow), the

Indus downstream of the Nanga Parbat region and the

eastern parts of the Kabul River that lie in the wetter

regions east of the Khyber Pass (Fig. 3). Unlike the Rum-

bak drainage, we assume a constant terrace incision along

the length of the valley between the range front and the

northern side of the Greater Himalaya. We follow the ear-

lier studies in considering that this terracing has resulted

from landsliding and damming of the river followed by

back-filling (Bookhagen et al., 2006). The common occur-

rence of landsliding in the Himalaya (Dortch et al., 2008)and the record of localized lake sediment (Phartiyal et al.,2005) testifies to this being an active and important pro-

cess in controlling sediment flux in that part of the river

basin.

We estimate sediment storage along the trunk Indus in

Ladakh using a different data set because of the contrast-

ing drier climate and less tectonically active geological set-

ting here that might result in different conditions

compared with the monsoonal Sutlej. Figure 8 shows a

transect along the Indus gorge between Leh and Khalsi

(Fig. 3) with a number of terrace levels identified at a

range of heights above the Indus stream. We note that

because no single terrace extends all the way along the

70 km survey, we infer that the valley has been filled

locally by landslides damming the river, especially in

Terrace dated ~10 kaEroded material

Modernriver

Z

Y

(a)

(b)

Fig. 6. Schematic diagram showing how the volumes of eroded

sediment are calculated. (a) In areas where a 10 ka or other post-

glacial terrace are known the area of sediment removed since the

start of incision can be used to calculated volumes along a river.

(b) In a side valley, such as the Rumbak drainage, the depth of

incision is normally at a maximum where the stream reaches the

trunk river (at point Z) and reduces to zero at the head of the val-

ley (point Y).



regions where the gorge is narrow and prone to blocking.

Although landslides are common, they are usually rapidly

breached and destroyed, but when large rock landslides

block the river, then the possibility exists for the forma-

tion of long-lived lakes and their subsequent infilling by

sediment (Costa & Schuster, 1988). That such events

have occurred along the upper Indus is testified by the

presence of laminated lake deposits interbedded with flu-

vial and alluvial fan sediments in the Indus Valley around

Leh (Phartiyal & Sharma, 2009). 14C dating of these sedi-

ments shows that they roughly predate the LGM and

were formed between 31 and 51 ka, during a period of

stronger monsoon. We thus know that Terrace B (Fig. 8)

was likely incised prior to the LGM. Terrace A, B and C

are 30 ka or older, allowing us to assume that the lower

terraces (i.e. D–I) post-date the glacial maximum and

provide an estimate of the amount of sediment released

from the trunk Indus in Ladakh (Table 3). Although bet-

ter age control would be required for a more precise esti-

mate, our approach here will provide a first-order

approximation for the amount of sediment reworked dur-

ing the Holocene. We estimate that volume to be

5.69 km3 over the 70-km transect, allowing us to calculate

how much material could have been stored in the Indus

Valley upstream of the Nanga Parbat region. Although

the region of study is reasonably well defined in terms of

77˚ 27’ 77˚ 30’ 77˚ 33’ 77˚ 36’ 77˚ 39’

31˚20’

31˚22’

31˚24’

31˚26’

31˚28’

5 km

AB

C

D

E

3.02.01.00

(a)

800

1000

1000

1200

1000

1200

1000

1400

1200

1000

1200

3.02.01.00

Elev

atio

n (m

asl)

Elev

atio

n (m

asl)

3.02.01.00

10 ka

10 ka

10 ka

10 ka

10 ka

120 m3/km

104 m3/km 111 m3/km

45 m3/km

19 m3/km

(b)

(c) (d)

(e)

Fig. 7. Shaded topographic map of a section of the Sutlej River showing the variation in storage capacity along a stretch of the main-

stream. Sections show that depending on the valley geometry significant differences in storage capacity exist. Section D corresponds

to the study area of Bookhagen et al. (2006).



volume and to a lesser extent in age, we assign an uncer-

tainty of �30% to the total estimate, reflecting along

strike possible variability in storage capacity. This value is

less than that assigned to the Sutlej because the surveyed

region of the Indus spans ca. 70 km, rather than being

focused in a restricted zone as was the case with the

Sutlej. This is considerably less than the sediment storage

capacity of the Himalayan Sutlej; 0.081 km3 km�1 com-

pared with 0.771 km3 km�1 (Bookhagen et al., 2006). We

also apply this ‘rain shadow’ rate to those lengths of the

Kabul River west of the Khyber Pass, as well as to the

Shyok, Hunza, Gilgit, Swat, Chitral, Kohistan and Zans-

kar Rivers because they all lie north of the Himalayan

range crest and thus are generally much drier (Bookhagen

& Burbank, 2006).

The total eroded volume is estimated at 1752–4104 km3 (Table 1). The regional breakdown shows that

the largest single source of sediment is the peri-Nanga

Parbat region (32–40%), with other major contributions

distributed between the Karakoram–Hindu Kush (21–27%) and the northern floodplains (21–23%) (Fig. 9).

Overall, uncertainties are estimated at �40% reflecting

the relatively well-defined volumes in the floodplains, but

the less well-known values from the large river valleys, or

eroded from the high-mountain topography.

DEPOSITEDVOLUMES

The Indus alluvial plain gradually becomes depositional

south of the confluence with the Punjabi tributaries and

shows maximum aggradation near the modern channel

belt (Giosan et al., 2012a). The resulting landscape is a

unique fluvial mega-ridge of subdued relief extending

from the lower Punjab to the Arabian Sea. Under the

deposits of the mega-ridge, post-glacial sediments fill up

the Indus’ Pleistocene incised-valley system (Kazmi,

1984; Giosan et al., 2006a).

2900

A

2900

3000

3100

3200

3300

3400

Alti

tude

(m

)

76˚50’ 77˚30’77˚20’77˚10’77˚00’

Distance (km)

0 10 20 30 40 50 60 70

Khalsi

Basgo

Nimu

Spitok

Saspo

l

Nurla

Ule Tok

po

Alchi

West East

3000

3100

3200

3300

3400

Phey

Zansk

ar

Conflu

ence

?

CB

D

A

F

AG

AHAI

E

Terrace sediment

Lacustrine sediment

Fig. 8. Section along the Indus River in Ladakh showing the terraces and their extent along the trunk river. The lacustrine sediments

at Spitok are roughly dated at 33–51 ka (Phartiyal et al., 2005) so that the next lowest terrace is presumed to be the post-glacial.

Table 3. Estimates of the sediment volumes eroded from under mapped fluvial terraces in Ladakh along the trunk Indus River

divided into sectors based on morphology of the valley and extent of terraces

Sector

Volume (km3)

Elevation (m) 3320 m 3285 m 3250 m 3180 m 3160 m 3120 m 3070 m 3000 m 2975 m

Length (km)

Terrace

A

Terrace

B

Terrace

C

Terrace

D

Terrace

E

Terrace

F

Terrace

G

Terrace

H Terrace I

Leh Valley 40 0.00 12.83 6.95 0.00 0.00 0.00 0.00 0.00 0.00

Zanskar

confluence

5 0.39 0.00 0.15 0.00 0.03 0.02 0.00 0.00 0.00

Basgo–Nimu

Valley

8 3.30 0.00 1.39 0.00 0.41 0.19 0.00 0.00 0.00

Basgo–Alchi 12 3.39 0.00 1.89 0.00 0.65 0.30 0.00 0.00 0.00

Ule Tokpo 14 0.00 0.00 0.00 0.78 0.00 0.42 0.11 0.00 0.00

Nurla 8 0.00 0.00 0.00 1.09 0.00 0.00 0.25 0.00 0.00

Khalsi 10 0.00 0.00 0.00 1.24 0.00 0.00 0.00 0.14 0.06

Total 97 7.08 12.83 10.39 3.11 1.08 0.93 0.36 0.14 0.06



We estimated the volumes stored by the incised valley

and the mega-ridge as irregular prisms (Fig. 10) with

average widths of 100 and 150 km for the Indus alluvial

plain and the delta, respectively. We employed average

gradients for the modern alluvial plain (7 cm km�1)

(Giosan et al., 2012a) and the Pleistocene incised-valley

profile (25 cm km�1) (Giosan et al., 2006a) starting froma deposit thickness of 90 m at the coast. The depth

assumption is supported by interceptions of the valley

bottom by our drill sites at the coast (Figs 1 and 11) at

Keti Bandar as well as inland (Jati and Gul at ca. 70 km

from the coast and Matli at ca. 170 km from the coast).

These values should provide a minimum estimate of the

valley fill volume with the incised valley extending ca.250 km from the present coast. Then, using Kazmi’s

interpretation of an incised-valley extending over 700 km

upstream, we obtained a maximum volume estimate.

Based on our drill core data, we assume Kazmi’s view

of an incised valley that is very wide and deep with a low

gradient that allows the valley to reach far inland. This

view is supported by available data so far but will need to

be confirmed by future chronologies of the infill in the

Sindh plain. However, the Indus incised valley as such

envisioned is well within the empirical range for other

large palaeovalley dimensions (Blum et al., 2013).

Although the proposed flanking of the incised valley by

patches of previous highstand deposits (Kazmi, 1984) is

in line with some existing models for palaeovalleys (see

e.g. Blum et al., 2013), this is not supported by our Matli

site where Holocene sediments overlie Eocene limestone

at ca. 60 m depth rather than being entirely Pleistocene

(Fig. 11) (Giosan et al., 2006b). Nevertheless, to allow

for a first-order uncertainty estimate, we calculated mini-

mum and maximum stored volumes in the alluvial plain

by assuming rectangular and triangular cross sections for

the incised-valley system, respectively (Table 4;

Fig. 10a). Within these parameters, together, the alluvial

plain and the delta have stored between ca. 2100 and

3600 km3 of post-glacial sediment.

In front of the Indus delta, post-glacial Indus sedi-

ments have been stored within the Indus canyon and as

mid-shelf clinoforms on both sides of the canyon

(Fig. 12) (Giosan et al., 2006b; Limmer et al., 2012). Athird clinoform has developed along the Kutch coast with

most sediment probably advected eastward alongshore

from the Indus delta and redeposited on the mid-shelf by

the offshore-directed tidal jet at the Gulf of Kutch mouth

(Giosan et al., 2006a). However, it is possible that the

Kutch clinoform may only be a surficial feature as the

incised valley of the Indus may have not extended so far

east. We calculated the maximum volumes stored within

the clinoforms below the modern shelf bathymetry down

to �90 m water depth assuming a lowstand erosion of the

shelf to that depth (Table 4). This assumption is sup-

ported by the depth and width of the incised valley at the

coast (Fig. 11) as well as by available seismic profiles

(Giosan et al., 2006b; Limmer et al., 2012).Our recent survey of the Indus canyon shows a transi-

tion at ca. 800 m water depth from an active depositional

region in the upper canyon to a meandering channel mor-

phology indicative of intermittent turbidity flow (Clift

et al., 2013). Sediments recovered from the upper canyon

document extremely high rates of sedimentation, while

the lower Indus canyon shows instead that turbidite

sedimentation ended shortly after 6.9 ka (Clift et al.,2013). Previous coring in the canyon dated a cessation of

turbidite sedimentation after 11 ka ca. 75 km from the

canyon head (Prins et al., 2000). We calculated the

Indus Holocene Depocenters

Sindhalluvial plain

Deltaplain

Easternclinoform

Westernclinoform

Kutchclinoform

Erosion sources Indus

Himalayanrivers

KarakoramKohistanHindu Kush

FloodPlains

NangaParbat

Ladakh and western Tibet

Sidevalleys

Fig. 9. Pie diagrams showing the origin and fate of sediment in the post-glacial Indus River system. Note equal division between on

and offshore sedimentation and the moderate contribution from reworking of the floodplains.



volume of the sediment prism accumulated in the upper

Indus canyon using a 4 km average width, the present

average slope of the canyon to 800 m water depth and a

predepositional lowstand profile extending from 800 m

water depth to �90 m subsurface at the coast (Fig. 10c).

Overall, the maximum total volume of sediment stored

offshore in the canyon and clinoforms is ca. 2800 or

1900 km3 when we do not consider the Kutch clinoform,

values that are of the same order as the Indus inland stor-

age (Table 4). First-order uncertainties vary between

55% for the inland part of the system and 35% offshore.

MINERALTRANSPORT TIMES

The transport times of mineral grains need to be con-

strained for the Indus system if different phases are to be

used to reconstruct erosion patterns in the mountains that

can be correlated with changes in climate. Available

U-series dating of sediment in the Ganges River, for

example, suggests long transport times of ca. 100 kyr for

sediments travelling between the range front and the

confluence with the Brahmaputra near the delta (ca.1650 km) (Chabaux et al., 2006; Granet et al., 2007),

although residence times were estimated to be shorter ca.25 kyr for finer-grained sediment (Granet et al., 2010)and most recently 10Be cosmogenic dating argues for sedi-

ment transfer times of ca. 1400 years in the Ganges (Lup-

ker et al., 2012) consistent with the similar short

residence times deduced in the Andean foreland (Witt-

mann et al., 2011). Furthermore, different mineral phases

will have different lag times depending on their density,

shape and grain size. Radiometric dating of mica by39Ar-40Ar methods and zircons by U-Pb methods in the

(a)

(b)

(c)

desert

older Indusdeposits

post-glacial Indus

Mega-ridge deposits

Shelf clinoforms Delta Alluvial plainStarvedshelf

Post-glacial Indusdeposits

Bedrock or older indus deposits

Upper canyon Delta Alluvial plain

Post-glacial Indusdeposits

Bedrock or older indus deposits

90 m

800 m

90 m

100 km

BedrockBedrock

Fig. 10. Schematic figures showing how

the volumes of sediments stored in the

various parts of the southern floodplains

and delta are estimated. (a) Diagram

showing a cross section through the

major depocentres in the Sindh flood-

plains showing the maximum (solid line)

and minimum (dashed line) eroded vol-

umes, (b) diagram showing the longitudi-

nal cross section through the Sindh

floodplains, delta plain and offshore shelf

regions, (c) diagram of the stored sedi-

ment in the region of the Indus canyon.

Thicknesses of post-Last Glacial Maxi-

mum sediment in the canyon is

unknown, but coring in the mid-canyon

has shown no sediment post-dating ca.11 ka at 75 km from the head of the can-

yon (Prins et al., 2000).



Red River has highlighted how different mineral groups

may indicate quite different provenances for the same

river sediment samples (Hoang et al., 2009, 2010). In the

Red River example, the discrepancy was explained as

reflecting the slower passage of zircon grains through the

river compared with the mica or clay fractions. Zircon is

documented to be concentrated in the bedload of large

Himalayan rivers (Garzanti et al., 2010; Lupker et al.,2011).

Suspended sediments tend to be transported quickly

from source to the ocean, but the grains of the river

bedload would be expected to move more slowly and may

undergo significant transport only during the strongest

flood events (Williams, 1989; Syvitski et al., 2000).

Bedload not only moves more slower than suspended

load, but stepwise with potentially very significant storage

intervals in fluvial bars and floodplain.

Here we compare the evolution in bulk sediment Nd

isotopes with the changing zircon age populations in the

same sediments cored at Keti Bandar at the river mouth.

Nd is a useful sediment proxy because it is controlled by

the average age of the crust from, which it is eroded and is

generally not affected by weathering processes (Goldstein

et al., 1984). A shift in eNd to more negative values

MatliThatta

12.4 ka

~7.5 ka

BadinMirpurBatoro NabisarDigh

–40 m

–80 m

–120 m

–40 m

–80 m

–120 m

~100 km

Keti Bandar

~130 km~14 ka

~9 ka~12 ka

Jati Gul

Transect K-L

Transect A-BD

epth below sealevel (m

)

Fig. 11. Simplified sedimentary logs for two transect of boreholes in the Indus delta and lower Indus alluvial plain (location for tran-

sects in Fig. 1). Mud (brown) and sand (yellow) fill a wide incised valley floored by gravel or Indurated rock (orange). Available data

suggest that the fill is latest Deglacial to Holocene in age.

Table 4. Estimates of the deposited volumes in different sec-

tors of the Indus floodplains, delta plain and marine regions

since the Last Glacial Maximum

Depositional environment Volume (km3)

Sindh alluvial plain 725–2500Delta plain 1410

Shelf delta 2790

Eastern clinoform 1260

Western clinoform 630

Kutch clinoform 900

Canyon 25

Deposition 4950–6725Deposition without Kutch 4050–5825



between 14 and 9 ka was interpreted by Clift et al. (2008)as reflecting stronger erosion of older, more continental

rocks compared with younger more primitive ones during

the Early Holocene (Fig. 13). Such a trend would indicate

a shift of erosion away from arc-like rocks in the Indus

suture zone and towards the range front of the Himalaya,

as these are composed of deformed but very old Indian

plate rocks. Today, the strongest monsoon rains are in the

Lesser Himalaya, and the change in eNd values is consis-

tent with a climatic trigger for that shift, not least because

the speed of the change precludes tectonic explanations

and drainage capture can be ruled out in this case. Nd

isotopes are dominated by clay minerals in the fine-

grained fraction (Goldstein et al., 1984) and because theseusually travel rapidly as suspended load it is not surprising

that the change in eNd values is synchronous with the gen-

eral increase in summer monsoon strength over the same

time period (Fleitmann et al., 2003; Herzschuh, 2006).

What is more difficult to explain is why the subsequent

reduction in summer monsoon rains is not paralleled by a

matching rise in eNd values as the erosion of the Lesser

Himalaya weakened. The lack of a response suggests a

buffer in the system. We suggest here that the stability of

eNd values analysed at the Indus delta since 7 ka reflects a

dominant reworking of sediment since that time, and that

although the discharge from the Indus tributaries now

66˚ 67˚ 68˚ 69˚ 70˚

23˚

25˚

24˚

Karachi

western

clinoform

eastern

clinoformKutchclinoform

IndusCa

nyon

Delta Plain

20

406080

100

Arabian Sea

Rann of Kutch

Jati Gul

KL

AB

Matli

KetiBandar

Fig. 12. Shaded bathymetric and topo-

graphic map of the Indus delta area

showing the major depositional areas esti-

mated in our budget. Map is contoured

between 200 m water depth and 200 m

elevation above sea level at 20-m inter-

vals. Topography is from NASA’s Shut-

tle Radar Topography Mission via

GeoMapApp.

–15–13–110

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Age

(ka

)

εNd

–42 –38 –34

GISP icecoreδ18O

Monsoon strengthδ18O (VPDB)

–8–6–5

Wetter

–7

Qunf

Timta

(a) (c)(b)

Younger Dryas

Drier

20

Indus delta

Indus canyon

Proportion of totalsediment (%)

0 10 20 30 40 50

warmercolder

ContinentalPrimitive

Maximum change in Nd

Maximum change in zircons

(d)

Nanga ParbatTranshimalaya-KarakoramGreater HimalayaLesser Himalaya

Fig. 13. Diagram showing changing

sediment provenance compared with

evolving continental climate in SW Asia.

(a) The GISP2 ice core climate record

(Stuiver & Grootes, 2000), (b) the inten-

sity of the SWmonsoon traced by speleo-

them records from Qunf and Timta

Caves (Fleitmann et al., 2003; Sinhaet al., 2005) in Oman and by pollen

(Herzschuh, 2006) from across Asia

(black line), (c) Nd isotopic evolution of

sediments from the Indus delta and Indus

canyon (Clift et al., 2008) and (d) chang-ing zircon age populations from the delta

at Keti Bandar (Clift et al., 2010).



favours a stronger erosion from the Karakoram and Koh-

istan compared with that in the Early Holocene (Alizai

et al., 2011), this signal has yet to be communicated to the

delta, at least in terms of Nd isotopes (Fig. 14). Indeed,

the most recent change in eNd values near the delta is a

decrease from �15 to �16, the opposite of the change

expected from the water discharge (Fig. 13). We interpret

this most recent change to be linked partly to reworking

in the floodplain and also to the effect of synthetic dam-

ming on the Indus since the 1930s (Inam et al., 2007).U-Pb dating of zircon sand grains can be used to assess

the provenance of the bedload dominated fluvial fraction,

and it has been shown that this method is particularly

suitable to the Indus because of the diversity of ages in the

potential source terrains and the documented variation

the age spectra of zircons in the tributaries (Alizai et al.,2011). We note that while the zircon populations seen at

the delta change with time in a way that is consistent with

the variation in Nd isotopes (i.e. with more Karakoram–Trans-himalayan grains (<300 Ma) at the LGM

compared with the Holocene), these changes are not syn-

chronous for the two proxies. Figure 13 highlights the

fact that while eNd values had fallen to a minimum by 9 ka

and stabilized at a new low value of �13 to �14 after

7 ka, the zircon populations remain relatively constant

over that entire time period. The most obvious change in

zircon ages occurs sometimes after 7 ka, when the propor-

tion of Lesser Himalayan zircons (1500–2300 Ma) rose

from 22% to 40%, at the same time that Karakoram–Trans-himalayan grains (<300 Ma) dropped from 37% to

22%.

We propose that the two provenance proxies are out

of phase with one another because they represent differ-

ent parts of the sediment load. While the bulk sediment

Nd signature is dominated by sediment transported in

the suspended load, especially clays and mica, the zir-

cons are necessarily part of the bedload because of the

high density (ca. 4.6 g cm�3 compared with 2.75 for

illite or 2.68 g cm�3 for plagioclase) and coarse grain

size (>100 lm required for the laser dating method).

The platiness of phyllosilicates including micas means

that these are preferentially carried in the suspended

load compared with the more rounded aspect of zircon

grains. We suggest that the time difference between the

change in Nd and that seen in zircons reflects the trans-

port time of the zircon in the bedload, that is, ca. 7–14 kyr. This number is significantly less than the

100 kyr estimated by Granet et al. (2007), although the

course of the Ganges and the Indus have comparable

lengths: ca. 1300 km from the end of the Sutlej gorge to

the Indus delta and ca. 1650 km from the end of the

Ganges gorge to the confluence with the Brahmaputra.

Our estimate is longer than the 10Be-derived rates of

Lupker et al. (2012), although because that 1400 year

estimate is based on a quartz-dominated mineralogy, it

might be expected to be shorter than the zircon travel

time.

DISCUSSION

The sediment budget components presented above are

clearly subject to significant uncertainties, but it is

nonetheless noteworthy that the total sediment volumes

deposited, 4050–6725 km3, are only moderately greater

than the volumes eroded 1752–4104 km3. This means

that much (26–99%, average 54%) of the sediment depos-

ited in the Indus delta, on the Indus shelf and within the

Indus canyon since the LGM can be explained in terms of

reworking from pre-existing sediments stored in the allu-

vial plains, rather than from newly generated sediment

from bedrock erosion. Because it seems unlikely that ero-

sion would simply cease for any significant length of time,

our result implies that the products of new erosion may be

stored in terraces or landslides and that erosion is much

slower in the absence of extensive glaciers. Our budget

reconstruction shows that the floodplains are one of the

most important depocentres and sediment sources, but

Modern sediment

at thatta

Modern discharge from himalaya

based on water budget

(b)(a)

0-300 Ma

300-750 Ma750-1250 Ma

1500-2300 Ma

Fig. 14. Pie diagram showing (a) the

estimated zircon age population of Indus

River sands now being discharged from

the mountains based on water discharge

and the known composition of the major

tributaries (Alizai et al., 2011), and (b)the observed zircon age populations from

close to the delta at Thatta (Clift et al.,2004).



the volumes stored and released are just 21–23% of the

total transport. The concept of sediment flux to the ocean

being buffered by the floodplains alone clearly does not

apply to the Indus system (M�etivier & Gaudemer, 1999).

The 4050–6725 km3 of sediment deposited since ca.14 ka (the age of the base of the post-glacial delta as dated

at Keti Bandar; Clift et al., 2008) corresponds to an aver-

age rate of 506–841 Mt year�1, assuming an average den-

sity of 1.75 g cm�3. Considering that the 725–2500 km3

stored under the Sindh alluvial plain post-dates 14 ka, the

average rate of mass delivery would be 325–1120 Mt year�1. We note that this is considerably greater

than the 250 Mt year�1 estimated as predamming flux by

Milliman & Syvitski (1992) but lies in the range from 100

to 675 Mt year�1 estimated by Ali & De Boer (2008).

Even in a more conservative estimate with 20 ka as the

lower limiting age for the budget, the average rate stays

high, at 420 Mt year�1. This implies a large degree of

temporal variation in sediment flux: low rates in more

recent times would have to be balanced by higher rates in

the early Holocene, and this would be consistent with the

observation of the delta prograding seawards from 14 to

8 ka despite the rapidly rising sea level of that time

(Giosan et al., 2006a). Such a pulse mirrors the observa-

tion of such an event in the Bengal delta (Goodbred &

Kuehl, 2000b) and further argues against major sediment

buffering in the net flux to the ocean.

The eroded material derived from the Nanga Parbat

region is one sediment flux anomaly that emerges from

our budget: while sediment eroded from that region

accounts for 32–40% of the total erosional side of the

budget, only 3% of zircons can be clearly tied to Nanga

Parbat at the delta (Clift et al., 2010). This apparent con-tradiction is related to the definition of Nanga Parbat.

The very young (<12 Ma) U-Pb ages of zircons that are

considered characteristic of the massif are found within

the gneisses and granites in the core of the uplift (Zeitler

& Chamberlain, 1991; Zeitler et al., 1993). However, the

region of heaviest landsliding and sediment storage is

more widely defined within 100 km of the peak (Hewitt,

2009) and thus encompasses areas of the Kohistan Arc

and the southern Karakoram that are experiencing some

additional rock uplift centred around the peak. These

areas may not be experiencing the degree of uplift and

exhumation seen at Nanga Parbat itself, but they are also

clearly more strongly uplifted and subject to seismic shak-

ing that other regions in the mountain source regions. If

this wider definition of the Nanga Parbat syntaxis is used,

then the apparent discrepancy with the eastern Namche

Barwa syntaxis is significantly reduced. Petrology and

geochemical indicators suggest that ca. 35% of the Brah-

maputra sediment downstream of Namche Barwa is

eroded from that massif (Garzanti et al., 2004), far more

than the values normally assigned for the western syntaxis

at 3% based on zircons (Clift et al., 2010) and 6% based

on heavy minerals (Garzanti et al., 2005).We note that there is a reasonable agreement between

the estimated sources of the eroded sediments and the

zircon populations seen at Thatta and Keti Bandar. Our

eroded budget suggests that 21–27% of the total sediment

load is being derived from the Karakoram/Hindu Kush

and another 32–40% from ‘greater Nanga Parbat’

(including the Kohistan Arc), a total of 53–67%. This

proportion is significantly higher than the 24% of zircons

with U-Pb ages less than 300 Ma seen at Thatta (Clift

et al., 2004), but closer to the 44% of grains dated at

<300 Ma from the LGM sediments at Keti Bandar (Clift

et al., 2010). Zircons with this <300 Ma age range are

considered typical of erosion from Karakoram, Kohistan

and Nanga Parbat sources. Given the zircon transport

time, we would expect those sediments eroded from ter-

races in the mountains since 10 ka to have now reached

the delta and be represented by the modern sample from

Thatta. However, Alizai et al. (2011) noted that the trunk

Indus is much less rich in Zr, and thus zircon, compared

with most of the Himalayan tributaries. Indus sands

yielded only 18 ppm in Zr compared with as much as

63 ppm in the Jhelum. These concentrations are pre-

sumed to be representative of the zircon load in the mod-

ern rivers, yet we note that the Zr concentrations are

much less than the 162 � 47 ppm reported from the

delta by Clift et al. (2010). The analysed sands from the

delta may have been enriched in zircon as a result of

hydraulic sorting concentrating these heavy minerals in

the samples taken, although the concentration is within

the range of 152–238 ppm reported by Hu & Gao (2008)

for the continental upper crust. Alternatively, hydraulic

sorting may have caused dilution of heavy minerals in the

samples analysed by Alizai et al. (2011). In the absence of

alternative ways of correcting for zircon abundance in the

different streams feeding the delta, we use the ratios of Zr

in the different streams as a proxy for weighting their con-

centrations.

Using the modern water discharge values of the main

eastern Himalaya tributaries as a proxy for relative dis-

charge, we can calculate a weighted average Zr concentra-

tion for sediment from these rivers of 37 ppm. This in

turn allows us to calculate the predicted proportion of

<300 Ma vs. Himalayan zircons in the mixed sands. If

59–61% of the sediment since the LGM were from the

trunk Indus, then we calculate that ca. 30% of the zircons

would be <300 Ma in the mixed sediment. This is closer

to the 24% of such zircons recorded by Clift et al. (2004)at Thatta and suggests that our eroded sediment budget

lies within error of the predicted budget based on zircon

ages. If the zircon concentration correction is in error,

then there may be a significant mismatch between our

sediment budget and the provenance of Holocene delta

sands.

On millennial timescales, sediment flux rates and

sources vary significantly, indicating that several buffers

are active in the Indus sediment transport system.

Although the flux must have been much higher earlier

in the Holocene than in the pre-industrial recent, the

composition of sediment varies in ways that makes simple

linkage between erosion and climate records difficult to



make using marine sediments. This is particularly true if

deep-sea fan sediments are considered because the flux of

sediment to the final depocentre ceased during the Holo-

cene as a normal consequence of rising sea level and

following the classical sequence stratigraphic models of

Vail et al. (1977). Prins et al. (2000) dated the youngest

turbidite on the upper fan at 11 ka, while our coring of

the Indus canyon itself shows that turbidite sedimentation

in the lower parts ended shortly after 6.9 ka (unpublished

data), during a period of rapid sea level rise and when can-

yon sedimentation would normally have finished. Con-

versely, sedimentation has been rapid both on the

clinoforms of the Indus shelf and likely offshore the Rann

of Kutch, as well as at the head of the Indus canyon where

the most rapid recent sedimentation has been occurring

(Giosan et al., 2006a). Presumably, if the sea level

remains stable and sediment delivery continued at a high

rate, then sediment flux to the deep canyon and submar-

ine fan would be re-established in the near future (Bur-

gess & Hovius, 1998). However, in most glacial cycles, sea

level has not been as stable for as long as during the Holo-

cene but instead fell again shortly after reaching a maxi-

mum (Stuiver & Grootes, 2000).

This scenario suggests that the sediment flux to the

deep-sea would have been re-established on the fan after

a moderate nondepositional break, but the source of the

sediment reaching the fan is primarily from reworking.

Since 7 ka, sediment transported to the delta appears to

be dominated by sediment reworked from terraces in

intramountain tributary valleys with a subordinate but

significant reworking of the lower alluvial plains. As sea

level fell, erosion of the shelf clinoforms and incision of

the neighbouring delta plain would have resulted in a

mixed array of sources and ages for the sedimentary

grains delivered to the submarine fan during the regres-

sion phase. It thus seems unlikely that detailed prove-

nance work on fan turbidite sands could be correlated in

any meaningful way with the monsoon climate record on

millennial timescales or shorter because the initial erosion

of these grains may have occurred during the LGM. An

initial reworking phase from mountain terraces occurred

in the Early Holocene. This material might then be

deposited in the northern floodplains from where it may

be reworked again in the latter parts of the Holocene. A

lag of at least 10 kyr would seem to be implied for sedi-

mentary particles travelling from the peaks of the Karak-

oram to the bottom of the Arabian Sea. This is not

significant if the record is to be used to look at erosion on

tectonic timescales (106 year). If the goal is to look at ero-

sion on millennial timescales, records on the shelf front-

ing the delta may be of greater use because the delta

stratigraphy does show a clear change in provenance dur-

ing the 14–9 ka period synchronous with the climate

change, at least in the fast-travelling suspended sediment

load. However, even in these deposits, further reworking

of the lower alluvial plain masks any erosional change dri-

ven by a weakening monsoon in the upper parts of the

drainage since 8 ka.

CONCLUSIONS

The sediment flux to the Arabian Sea from the Indus

River has experienced significant variability after the

LGM. We estimate that 4050–5675 km3 of sediment has

been deposited in the incised valley, subaerial delta, shelf

clinoforms and the upper Indus canyon. About half of the

stored sediment lies offshore on the shelf and in the can-

yon. While ca. 21–23% of the total depositional volume

comes from incision of the upper alluvial plain after

10 ka, the bulk of the sediment is derived from the deep

gorges around the Nanga Parbat syntaxis that account for

32–40% of the sediment flux despite comprising only ca.5% of the area of the mountain sources in the Himalaya–Karakoram–Hindu Kush region. The Karakoram is also

an important source, accounting for ca. 21–27% of the

total sediment released. Sediment buffering in the moun-

tains appears to be largely controlled by landsliding,

which is in part controlled by climate (Bookhagen et al.,2005). Only 5% is eroded from terraces in major river val-

leys within the monsoonal regions of the Lesser and

Greater Himalaya.

Our budget implies that primary weathering of bedrock

supplied ca. 46% of the sediment reaching the delta since

the LGM. Although climate seems to control the source

and rate of sediment supply in the Early Holocene, strong

reworking from terraces and floodplains during the Mid–Late Holocene means that changes in erosion patterns

after ca. 8 ka are not recorded in the changing composi-

tion of delta sediments. We estimate that zircon grains in

the bedload take between 7 and 14 kyr to travel to the

delta compared with shorter travel times for the sus-

pended load. This further inhibits our ability to relate

changes in provenance to climatic events as the bedload

and suspended load travel times are decoupled and lag the

initial climatic trigger.

ACKNOWLEDGEMENTS

PC thanks the Leverhulme Trust, the University of Aber-

deen and NERC for support for the science project

underlying this study. LG acknowledges support from

the National Science Foundation and Woods Hole

Oceanographic Institution. The authors thank colleagues

at National Institute of Oceanography, Karachi for their

help over many years of work on and offshore Pakistan

and Bodo Bookhagen for advice on sediment storage in

the Sutlej Valley. We thank editor Peter van der Beek and

reviewers Eduardo Garzanti and Steven Goodbred for

their helpful advice in improving the initial submission.

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Manuscript received 08 October 2012; In revised form04 May 2013; Manuscript accepted 08 July 2013.



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