Sediment Fluxes and Buffering in the Postglacial Indus Basin
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EAGE Basin Research (2014) 26, 369–386, doi: 10.1111/bre.12038 Sediment fluxes and buffering in the post-glacial Indus Basin P. 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 (Metivier & Gaudemer, 1999). While this has been dis- puted over timescales >106 years (Clift, 2006), there is Marine sediments represent the longest and most com- presently little control on how effective this buffering plete archives of continental environmental evolution, process might be on shorter timescales. and they have been used to reconstruct past patterns, rates In this study, we present a new sediment budget for the of erosion and changing environments at the time of post-glacial Indus River system in order to quantify the deposition. These in turn can be used to assess the influ- degree of buffering in a large drainage system and to see ence that climate or tectonics have over continental envi- how the deep-sea sediment record might be used to ronments. However, if the marine record is to be utilized understand erosion in the mountain sources. We use sedi- to its full potential, we must first constrain the transport mentary provenance data to assess the speed of sediment of clastic sedimentary particles from continental sources transport from the source ranges to the ocean and further to the marine sink. to the deep-sea. We focus on the post-Last Glacial Maxi- To what extent are sediments deposited in the deep mum (LGM) period because the sediment stored on the ocean representative of the erosion in the source shelf clearly sits on a glacial unconformity, thus allowing regions at the time of their sedimentation? Clastic sedi- ready estimation of the total deposited volume. This is also ments do not relocate instantly into the sea after ero- an appropriate time period for estimating eroded volumes sion in mountains, but their transport to the ocean onshore because a number of incised fluvial terraces in the could vary in efficiency and speed, spanning from days floodplains (Giosan et al., 2012a) and mountains are or weeks, to >105–106 years, because of storage and known to date from ca. 10 ka (Bookhagen et al., 2006; He- reactivation en route. Indeed, it has been argued that witt, 2009). It has been recognized that the Early Holocene many Asian river systems have maintained a relatively was a period of rapid sediment flux to South Asian deltas constant mass flux to the ocean over at least the past (Goodbred & Kuehl, 2000a; Giosan et al., 2006a), but it is 2 Ma because of sediment buffering in the floodplain not clear to what extent this sediment was freshly eroded bedrock rather than older fluvial sediments reworked from Correspondence: P. D. Clift, Department of Geology and the floodplains and terraces in the mountains. While some Geophysics, Louisiana State University, E235 Howe-Russell, sediment was released by retreating glaciers after the Baton Rouge, LA 70803, USA. E-mail: [email protected] Liviu Giosan, Department of Geology and Geophysics, Woods LGM, this affected the central and eastern Himalaya more Hole Oceanographic Institution, Woods Hole, MA 20543, USA than the Indus watershed because of the relatively dry cli- E-mail: [email protected] mate of this western region (Owen et al., 2008). Hedrick © 2013 The Authors Basin Research © 2013 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 369 P.D. Clift and L. Giosan et al. (2011) noted that while those regions north of and in provenance quantification simpler than in many drain- the rain shadow of the Greater Himalaya experienced only ages (Clift et al., 2004; Garzanti et al., 2005). Crucially, moderate glacial advance at the LGM, there was signifi- it is known that the origin of the sediment reaching the cant glacial advance in the more monsoonal regions within ocean has changed sharply since the LGM (Clift et al., and south of the Greater Himalaya. Retreat of those gla- 2008). The Indus is a sediment-productive river, which ciers would have released significant sediment quantities used to supply at least 250 Mt yearÀ1 to the Arabian that could then be transported to the ocean. Sea prior to modern damming (Milliman & Syvitski, The Indus River basin has a strongly erosive mon- 1992), although other estimates emphasize a larger range soonal climate over parts of the drainage, as well as from 100 to 675 Mt yearÀ1 (Ali & De Boer, 2008). steep topography and well-defined areas of rapid rock Modern and relict wide floodplains flank the Indus and uplift within the source mountains [e.g. Nanga Parbat, stretch >1300 km from the mountain front to the delta, south Karakoram metamorphic domes (Zeitler et al., providing ample potential opportunity for storage and 1993; Maheo et al., 2004)]. In addition, there is a strong reworking. 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) ERODED VOLUMES (Fig. 1). The sediment sources in the Himalaya and Ka- rakoram are very diverse in composition, and thermo- Eroded and incised fluvial terraces are recognized across chronologic ages that make large-scale sediment the northern half of the Indus floodplains, as well as in Figure 3 Karakoram 36˚ Gilgit Hindu Kush Kabul Ladakh 34˚ Indus Ravi Chenab 32˚ Jhellum Beas Sutlej Greater Himalaya Lesser Himalaya Harappa Punjab 30˚ Sulaiman Ranges Cholistan Sukkur Figure 2 Thar Desert 28˚ Moenjodaro Sindh Kirthar Ranges 26˚ K Matli Figure 12 Karachi Thatta L A B Jati Keti Gul Bandar Rann of Kutch 24˚ 66˚ 68˚ 70˚ 72˚ 74˚ 76˚ 78˚ 80˚ 82˚ 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 NASA WorldWind (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 Authors 370 Basin Research © 2013 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists Sediment flux in the Indus Basin NW SE 230 33˚ X 220 W 210 V 200 Y Ravi 190 (X) Jhellum Chenab 180 Indus 0 50 100 150 200 250 300 150 31˚ sand dunes Z 140 130 Multan 120 Sutlej 30˚ 110 (Z) 100 0 50 100 150 200 250 300 29˚29 300 Thar Desert 280 71˚ 72˚ 73˚ 74˚ 75˚ 260 200 190 240 Interfluve (W) 180 220 170 (V) 160 200 River valley 150 (Y) 180 140 0 50 100 150 200 250 300 350 400 Elevation above sealevel (m) sealevel above Elevation 0 50 100 150 200 Distance (km) 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). the mountains, and constitute a potentially significant incision, we can estimate how much material has been source of sediment for the river. Reworking of sediment excavated from the valleys. Using maximum and mini- stored in these terraces is likely a major source of sedi- mum estimates of the total area of the incised valleys ment to the Indus since the LGM (Fig. 1). Giosan et al. (Fig. 2) and assuming a linear reduction in depth from (2012a) noted that the alluvial plain in Punjab shows inci- north to south, we estimate that the amount of sediment sion by the modern Indus and its tributary rivers forming reworked further south since 10 ka is ca.