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Soil development on Late Quaternary river terraces in a high montane valley in Bhutan, Eastern Himalayas

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Soil development on Late Quaternary river terraces in a high montane valley in Bhutan, Eastern Himalayas
  Soil development on Late Quaternary river terraces in a high montane valley inBhutan, Eastern Himalayas Tshering Dorji a,e , T. Caspari b,c , R. Bäumler b,d , A. Veldkamp e , A. Jongmans e , Kado Tshering a ,Tsheten Dorji a , I. Baillie a,f , ⁎ a National Soil Services Centre, Ministry of Agriculture, Bhutan b Institute of Soil Science, Technical University of Munich, Germany c Institute of Soil Science and Forest Nutrition, University of Freiburg, Germany d Institute of Geography, University of Erlangen-Nurnberg, Germany e Land Dynamics Group, Landscape Centre, Wageningen University, The Netherlands f  National Soil Resources Institute, Cran  fi eld University, UK  a b s t r a c ta r t i c l e i n f o  Article history: Received 17 June 2008Received in revised form 12 February 2009Accepted 16 February 2009 Keywords: LoessChemical weatheringPedogenesisSoil chronosequenceCyclothemMicromorphology We examined the geochemistryand micromorphology of the soils on a suite of morphologically well-de fi nedand visually distinct  fl uvial terraces, up to 40 m elevation above the current riverbed, at Thangbi in the upperBumthang Valley, Bhutan. The alluvia forming each of the terraces are lithologically and structurally similar,with shallow or moderately deep, clast-free sandy loam overbank deposits capping deep clast-supportedbeds of rounded boulders and interstitial sand. The topsoils on the 40 m terrace have more silt than those onthe lower terraces. The soils are interpreted mainly as a monoclinal post-incisive chronosequence. Featuresthat indicate progressive pedogenesis with increasing elevation include subsoil rubefaction, crystallinity of free Fe sesquioxides, and weathering of susceptible primary minerals, such as biotite and hornblende.However other soil attributes show no systematic trends and the overall impression is of limitedpedogenesis, even in the soils on the higher terraces. We examine possibilities that the immaturity of thesoils is due to pedogenic rejuvenation by post-incision additions to the soil parent materials.© 2009 Elsevier B.V. All rights reserved. 1. Introduction Alluvial deposition by rivers in high mountain ranges on activeorogens is limited because downcutting is rapid, beds are steep, anddischarge energies are high (Lave and Avouac, 2001; Whipple, 2004). However, uplift may be spasmodic (Fort, 1987) and, together withnon-tectonic factors like log and landslip dams, this may giveintermittent hiatuses in local base level depression and possibilitiesfor alluvial deposition. The consequent alluvia are usually rudcaeousandweaklygraded.Basalbedsofmixedclastsoftencomprisethebulkof each cyclothem, commonly with only moderate or thin overbankcapsofnon-stony fi nes.Asmanythalwegsarealignedalongfaultsandsimilar geostructural features (Colombo et al., 2000), they tend to be narrow and linear. Although channels are laterally mobile wherepossible, the narrow thalwegs constrain them tolinearorcompressedsinuous courses (Kolla, 2007). Point bars, cover plains and other dynamic deposition/erosion features that characterise the lateralspatial distributions of alluvia in lowland  fl oodplains (Blum andTornquist, 2000) are therefore not well differentiated (Ferguson andBrierley, 1999; Pratt-Sitaula et al., 2004). Montane alluvial sequencesarealsolikelytobemixedwithextraneousmaterials.Mountainslopesare adjacent, long, steep and recently or currently active. Variablequantities of hillslope materials can be intermixed or imbricated intothealluvium(Schneideretal.,2007),especiallyalongtoeslope/thalwegmargins (Grant and Swanson, 1995). There may also be admixtureofnon-mainstreamalluviawheretributarystreamdebouchanddepositfans of poorly sorted and locally-derived debris.Giventhelimitationsonalluvialdeposition,itisnotsurprisingthatriver terraces are not extensive in high montane valleys (Lave andAvouac, 2001), particularlyatlatitudes and altitudes where Pleistoceneglaciations and periglacial processes have substantially removed,buried or otherwise disrupted pre-Holocene deposits. However, whereterrace remnants have survived, they are often clearly demarcatedbecause rapid uplift and incision create substantial differences inelevation.Remnants of river terraces occur at altitudes of 1000 – 3000 m the basin-like inner valleys of the mountainous Eastern Himalayankingdom of Bhutan, mainly upstream of major knick-points on thepro fi les of the high order N – S rivers (Baillie et al., 2004). As these rivers differ in catchment geology, dissection history, longitudinalpro fi le, and valley form (Baillie and Norbu, 2004), the terrace sediments vary between catchments in provenance, mineralogy,granulometry and spatial juxtaposition. Although inextensive, the Catena 78 (2009) 48 – 59 ⁎  Corresponding author. National Soil Resources Institute, Cran fi eld University, UK. E-mail address: (I. Baillie).0341-8162/$  –  see front matter © 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.catena.2009.02.018 Contents lists available at ScienceDirect Catena  journal homepage:  terraces are economically and socially important because they form asubstantial proportion of Bhutan's very limited arable land. Theirgentle gradients make them especially important for padi rice.The terraces in the high altitude inner valley of Chamkhar Chhu( Chhu =riverinDzongkha)inBumthangdistrictinEastCentralBhutanare the most comprehensive and best preserved in the country. Patchysuites of terraces stretch for 15 – 20 km along the  fl anks of the valleybetween2500and2900ma.s.l.Asindicatedbytheextremeroundingof competent clasts, gradients and tectonic context, the valley is a veryhigh-energysedimentaryenvironment,andthe,extentandpreservation Fig.1.  Location of study area and sites.49 T. Dorji et al. / Catena 78 (2009) 48 – 59  of the terraces are unexpected. The terraces up to 60 m elevation haveclear morphological de fi nitions. There are more cryptic and degradedremnants of higher terrace up to elevations of 300 m. Gurung (2001,2005)designatedtheChamkharterracesasglacio fl uvial.Hedatedwoodfragments from the 40 m terrace near Jakar town at 27300±180 yearsBP, and materialin the65 m terrace at29940±220 BP, indicatingrapiduplift during the late Quaternary.The area is climaticallymarginal for rice, evenwith recentlyissuedhigh altitude varieties, and for many other crops. However, the gentletopography of the terraces makes the valley a nationally importantcentre for cool temperate crops, such as potatoes and buckwheat, andfor dairy products.Because of their clear delineation and temporal juxtaposition,suites of river terraces are much favoured for the examination of soilchronosequences (Vreeken 1975; Huggett, 1998). They can indicate relative and sometimes absolute ages of soils and rates of pedogenicprocesses(Tsaietal.,2006),andcancontributetothecharacterisation of palaeoenvironments (Shaw et al., 2003). Caspari (2007) and Caspari et a. (2004) examined pedogenesis on the remnants of 28 terraces up to 280 m elevation at Thangbi in thevalley of Chamkhar Chhu, about 12 km upstream of Gurung's studyarea. They concluded that the higher terraces at Thangbi predate theLast Glacial Maximum (LGM) and that late Pleistocene glaciations didnot penetrate below3000 m in this area.Theyfoundthat silt contentsincrease upwards, both in elevation and withinpro fi les, and very siltytopsoilsareprominentfeaturesin the soilsonterracesatelevationsof 40 m and above. Silty epipedons are common in soils elsewhere inBhutan (Baillie et al., 2004) and complicate the interpretation of  textural indicators of pedogenetic development. Indicators showingprogressive pedogenesis with terrace elevation at Thangbi included:increased speci fi c surface area; increased crystallinity of free ironsesquioxides; transformation of micas to 2:1:1 secondary minerals;and increased values for the Parker indices of weathering. Thesetrends are quite noisy, partly because  14 C dating suggests thatpedogenesis was slow in the periglacial conditions of the LatePleistocene, and that signi fi cant soil development only began at theend of the Younger Dryas, ca 10 – 11 ka BP. Effective soil ages thereforedo not fully re fl ect the age range of the sediments.The pedogenetic trends were particularly fuzzy in the data fromthe soils on the lower terraces. As these are important for localagriculture and livelihoods, we re-examined soils on these terraces,especially their micromorphologies, to further characterise theirpedogenic status and the implications for the development of regoliths and landscape.We use  ‘ elevation ’  for the relative height above the orthogonallydownslope main riverbed and  ‘ altitude ’  for height above sea level. 2. Materials and methods  2.1. Study area This study concentrates on the terraces at elevations of 40 m andbelow(T1 – T7of Casparietal.,2004)nearThangbi(27°37 ′ N,90°42 ′ E)in Bumthang district, East Central Bhutan (Figs. 1 and 2). The altitudeof the main riverbed falls from 2710 to 2660 m a.s.l. in the study area,andthesummitsof theadjacentmountainsexceed4000 m a.s.l.(Dorji,2003).Schists of the Thimphu group underlie the study area (Konayaga,2005), butthealluviaarelithologicallyheterogeneous.Theyconsistof coarse clasts of granite, quartzite and gneiss derived from Tertiaryleucogranites, Tethyan metasediments and Thimphu group meta-morphics upstream (Golani and Singh, 1993; Bhargava, 1995;Konayaga, 2005), with leucogranite predominant (Gurung, 2005). Thealluviaaretypicalofveryhigh-energydischarges.Theymainlyconsist of clast-supported beds of boulders, stones and gravel, mostlylessthan50cm,butwithoccasionalsubroundedbouldersupto5mindiameter. The main interstitial material is medium and coarse sand.Silt is scarce and there is virtually no clay. The clasts are extremelyrounded, with most granites and quartzites completely spherical andthebandedgneissesarespheriovoid(Figs.3 – 5).Thealluviaarepoorlygraded and unlayered, with a few lenses of clast-free sand less than50 cm thick and pinching out within dekametres. The alluvia arelaterally homogenous and are not differentiated into coarser texturedbarand fi ner texturedcover plain deposits. All but fl oodplaindepositsare capped by moderate ( b 1 m deep) overbank deposits of stone-freesand and silt.The terraces are clearly de fi ned and well preserved, withhorizontal treads and steep risers (Fig. 2). The fronts of the treadsare  fl at, and there are no ramparts inherited from depositional bars.There has been some progradation since terrace incision. This is Fig. 2.  Thangbi terraces. Settlement and prayer  fl ags are on the 40 m terrace, with forested colluvial slope behind.50  T. Dorji et al. / Catena 78 (2009) 48 – 59  apparent as intermittent, low ( b 1 m) and narrow ( b 10 m) benches of roundedstonesandbouldersatthebackofthetreads,derivedfrombyclasts rolling down from supradjacent risers. Hill wash, sometimes inthe form of small, very low angle fans, overlies the alluvium at rear of the40mterraceinthoseplaceswhereitabutsdirectlyontomountainslopes (Dorji, 2003).The most extensive and continuous of the Thangbi terraces arethose at elevations of 17 m and 40 m, with lesser treads at 9, 27 and34m.Thetreadaremostly30 – 100mwide.However,the40mterraceis up to 250 m wide near Thangbi, where it abuts the slope on whichCaspari et al. (2004) identi fi ed remnants of higher terraces (Fig. 1).Further north, the 40 m terrace narrows and backs directly on to thecolluvialmountainslopes.Belowthe9mterracetherearenarrowanddiscontinuous benches at elevations of 2, 3, 4, 5, and 7 m. The lowestbank height is 2 m, and appears to be overtopped by 10 – 100 year fl oods. The terraces do not have soli fl uction or periglacial morpholo-gical features, such as those seen on the inclined treads and roundedrisers on the terrace remnants at higher elevations (Caspari et al.,2004). We have not dated the terrace deposits in this study, butGurung gave the 40 m terrace downstream an age of about 27 ka B.P.,i.e. LGM. Caspari et al. (2004) obtained Holocene datings for organiccarboninsomesoilsontheThangbiterracesatelevationsabove40m, Fig. 4.  Brownish grey sandy loam cap over bouldery alluvium on 17 m terrace. Fig. 3.  Road cutting through clast-supported basal alluvium of 27 m terrace.51 T. Dorji et al. / Catena 78 (2009) 48 – 59  but these refer to pedogenic C sequestration, not the age of thedeposits. Moreover, their interpretations are confounded by thelability of soil organic matter. From these  fi ndings and the lack of glacial disruption, we conclude that the 40 m terrace at Thangbi is of LGM – Dryas age, and that the lower terraces and benches areHolocene.The riverbed has an irregular pro fi le with an overall gradient of 1 – 2%, and alternates between stretches of exposed bedrock andtransient deposits of boulders and coarse stones. Although thisstretch of Chamkhar Chhu is a bedrock river, road cuttings in, andlandslip scars near, the study area show that the terrace alluvia aredeep, with no signs of bedrock  fl oors that typify strath deposits.Therearenometeorologicaldataforthestudyareabutithasacooltemperate climate. Mean monthly temperatures at Jakar,12 km tothesouth and 150 m lower in altitude, range from 3 – 4 ° C in December –  January to 17 – 18 °C in June – August. The mean annual rainfall is about800 mm, of which about 75% falls in the monsoon months of June – September (Bhutan Soil Survey,1998). The natural vegetation is bluepine( Pinuswallichiana )forestwithanunderstoryofyoungpines,rose( Rosa sericea ) and a few poplar ( Populus  sp.). In wetter patches therearedensestandsof bamboo(mainly Yushania microphylla ), oftenwithsedges (  Juncus  spp . ) on the imperfectly drained fringes. However,little forest remains on these terraces, as they are mostly used formixed smallholder farming. The main crops are rainfed potatoes,barley and some wheat. The soils are moderately fertilised, mainlywith organic materials, although the use of inorganic fertilisers isincreasing. The organic fertilisers are cattle manure bulked up withpine needles collected from nearby forests. The cattle are grazed onpost harvest residues, uncultivated risers, improved and roughpastures, common land adjacent to Thangbi village, and in themountain forests.  2.2. Field methods Altitudes and elevations were derived from consensus betweenthree GPS instruments and three aneroid altimeters, all calibratedmorning and afternoon at a Survey of Bhutan spot height near riverlevel. Individual GPS instruments gave spot altitudes that  fl uctuatedby up to 30 m within one day. This variability is attributed to thelocally high horizon angles and narrow cones of satellite capture.Rapidly changeable pre-monsoon barometric pressures during  fi eld-work in May 2006 gave diurnal altitude  fl uctuations of similarmagnitude for the individual aneroid instruments.The soils were augered along four transects running orthogonallyup from the river (Fig.1) at the front, centre and back of all treads, atintermediate points on the wider treads, and at midslope on somerisers. The augered soils were characterised to depths of 1 m, wherestone contents permitted, for colour, stones, hand texture, andmoisture content. The soils were more fully described using standardmethodsandterminology(SoilSurveyStaff,1993;BhutanSoil Survey,2002; Dorji, 2003) in at least one pro fi le each on the 2 m and 4 mbenches and on the 9 m,17 m, 27 m, 34 m and 40 m terraces (Fig.1).Thepro fi lesweredescribedcollaborativelytominimiseinter-operatorvariability (Reheis et al.,1989).Disturbed samples were collected from all horizons of a pro fi le oneach of the 9, 17, 27, 34 and 40 m terraces. Core samples for bulkdensity (BD) were collected from non-stony horizons with 100 cm 3 metal rings, with light spray-moistening of coarse textured materialwhen necessary. Micromorphological samples were collected in100×100×10 mm boxes from selected non-stony horizons in the17, 27, 34 and 40 m terrace soils.  2.3. Laboratory methods The undisturbed bulk densities were measured at the Soil andPlant Analytical Laboratory, National Soil Services Centre, Semtokha,Bhutan. The air-dried  b 2 mm disturbed samples were analysed at theInstitute of Soil Science, Technical University of Munich, Germany bythe following methods: organic matter by combustion (Elementarvario EL); pH in deionised water and in 0.1 M KCl at a soil – solutionratio of 1:2.5; cation exchange capacity (CEC) by leaching withunbuffered 0.5 M NH 4 Cl at a soil – solution ratio of 1:20 (Trüby andAldinger, 1989); exchangeable cations by induction coupled plasma(ICP) spectrometry of the leachate; particle size analysis by pre-treatment with H 2 O 2 , dispersion with sodium pyrophosphate, sandfractionation by wet sieving, and silt and clay determination byautomated X-ray attenuation (XRA) (Micrometrics Sedigraph 5100).Trace elements were measured by neutron activation analysis (NAA)at the University of Missouri, USA.Micromorphological thin sections (100×100 mm) were preparedin the Laboratoryof Soil Science and Geology, Wageningen University,Netherlands (Fitzpatrick, 1970) and described using the terminologyof  Bullock et al. (1985). The grain sizeswere quanti fi ed bygrid counts,as were the proportions of the main minerals and rock types( Jongmans et al.,1991). Frequencies of multi-mineral rock fragments,single mineral grains, clay with iron and organic matter, openchannels, and in fi lled voids were categorised as absent ( b 2%), few(2 – 5%), common (5 – 15%) and many ( N 15%). The degrees of weath-ering of multi-mineral clasts and single mineral grains were visuallyassessed as fresh, initial, moderate or intensive; microtunnels refer to Fig. 5.  Reddish brownish silty loam cap over bouldery alluvium on 40 m terrace.52  T. Dorji et al. / Catena 78 (2009) 48 – 59
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