Geomorphological indicators of large-scale climatic changes in the Eastern Bolivian lowlands

Does a monsoon climate Abstract: Geomorphological indicators of large-scale climatic changes in the Eastern Bolivian lowlands This study provides an inventory of geomorphologi¬ cal landforms in Eastern Bolivia at different spatial scales. Landforms and associated processes are interpreted and discussed regarding landscape evolution and paleoclimatic significance. Thereby. preliminary conclusions about past climate changes and the geo¬ morphic evolution in Eastern Bolivia can be


Geomorphological indicators of large-scale climatic changes in the Eastern Bolivian lowlands
Jan-Hendrik May, Berne 1 Introduction Late Quaternary tropical paleoclimatic research has become successively more important for our under¬ standing of the global climate System. However, the major part of paleoclimatic data in tropical South America still originates from the Andean highlands. Data from the tropical lowlands are relatively scarce and in some cases contradictory. Without additional records from the tropical lowlands, any conclusion regarding the paleoclimatic history of tropical South America will thus be biased (Coltrinari 1993). Situated at the transition zone between the tropical-humid and the subtropical semi-arid climatic regimes, the Eastern Bolivian lowlands (EBL) are therefore par¬ ticularly suited for the detection of large-scale climate changes.
Up to now, only few studies have been concerned with the geomorphology, landscape evolution and paleocli¬ matic history of the EBL. A general geomorphological overview is provided by Werding (1977) and Iriondo (1993). Servant et al. (1981) conducted first studies of paleosol-sediment-sequences, and only recently Mayle et al. (2000) and Burbridge et al. (2004) pub¬ lished results of Vegetation and climate reconstructions deduced from pollen analysis in Northeastern Bolivia. Another pollen record comes from a bog within the Andean cloud forest (Mourguiart & Ledru 2003).
This study provides a revised inventory of geomorpho¬ logical landforms in Eastern Bolivia at various spatial scales, benefiting from the increased availability and simplified use of remote sensing data (University of Maryland 2005). In combination with field studies remote sensing turned out to be a powerful tool for area-wide geomorphological mapping. Landforms and landform associations are interpreted and discussed regarding landscape evolution and paleoclimatic his¬ tory. This is the base for further and more detailed paleoclimatic research currently conducted in Eastern Bolivia.

Study area
Mountain building and Andean deformation associ¬ ated with the subduction of the Nazca plate has pro¬ gressive^migrated eastward throughout the Cenozoic (Gubbels et al. 1993;Isacks 1988). leading to the thickening and horizontal shortening of the Continental crusl and the formation of the Eastern Cordillera and the Subandean zone ( Fig. 1). At least since Miocene times, the erosional products of active deformation in the Subandean zone have been transported into the foreland of Eastern Bolivia (Gubbels et al. 1993). Therefore. the Eastern Bolivian lowlands are a retroarc foreland basin System with the Chaco Piain (-18°-24°S) being one of several active depocenters adjacent to the Central Andes (Horton & DeCelles 1997).
The Subandean foothills can be regarded as the topographic transition from the Subandean zone into the foreland basin (Fig. l).To the east, active deformation structures are buried beneath Cenozoic Sediments (Hinsch et al. 2002;Horton & DeCelles 1997), which thin out towards the east and onlap the pre-Andean basement. The northeastern margin ofthe Chaco piain is delimited by the outcrop of the Precambrian Brazil¬ ian shield.The transition between the Chaco piain and the Brazilian shield is interrupted by the Chiquitana ranges consisting mainly of sedimentary rocks of the Palaeozoic Chaco basin. Palaeozoic rocks are unconformably overlain by a thin cover of Mesozoic rocks (Welsink et al. 1995) and extend onto a topographic and structural high to the south (Izozog arch) repre¬ senting the flexural forebulge of the Andean deforma¬ tion (Horton & DeCelles 1997 (Berri & Inzunza 1993). The SALLJ is a strong N-S wind System east of the Andes active throughout the year (Saulo et al. 2004). However. due to the lack of topographic barriers, cold air incursions from mid-latitude South America periodically penetrate deep into the Amazon basin (Garreaud 2000) 1980). The deduetion of landscape evolution (genetic geomorphology) results from comparison of past land¬ forms and present processes. This requires spatially and temporally diverse information. Visual interpre¬ tation incorporates colour. density and texture of the imagery. but also deduces information from elevation. Vegetation and land-use patterns (Verstappen 1977).
Apart from the remote sensing data, this study integrates information collected during several months of field work conducted over three years from 2003-2005.

Results
The EBL can be outlined following structural and top¬ ographic criteria. In contrast to the adjacent Andes to the west and the Brazilian shield to the east. the gener¬ ally low relief is due to active Sedimentation processes throughout the recent geological past. The visualization and analysis of digital elevation data allows the subdivision of the lowlands into three distinct geomor¬ phological units ( Fig. 1 Unites geomorphologiques de la zone d'etude ä grande echelle, issues eles donnees nutneriques d'altitude (les rectangles noirs indiquenl la localisalion des images) Source: SRTM 90-m data, Global Land Cover Facility http://www.landcover.org. (Baker 1986;Horton & DeCelles 1997). On shorter timescales focussing on late Quaternary landscape and climate history, present processes typical for each of these three landscape elements can be compared to relict landform generations. Thus it is possible to detect changes in geomorphic processes over time, which in turn serves as indicator for changes in the Controlling parameters of landscape evolution, namely tectonics, climate and humans (Schumm 1999;Summerfield 2000).

Piedmont
The Andean piedmont forms the transition zone between the Subandean ranges and the fluvial Systems of the large rivers of Eastern Bolivia. Morphologically, it is a gently eastward inclined slope. Low slope angles of -0.35-0.55% and the apparent lack of alluvial fan morphology indicate that confined stream flow is the main process of piedmont construction (Smith 2000). With the piedmont being a typical alluvial slope. pale¬ oclimatic implications may be derived from its sedimentological architecture and the reconstruction of paleohydrology.
In the study area, the piedmont can be subdivided into a northern and a southern part (AI and A2) separated by the Rio Grande megafan. Both parts are situated in climatically different environments, with the northern part being characterized by higher total annual pre¬ eipitation and less pronounced seasonality. Therefore differences in type and intensity of the dominant geo¬ morphic processes can be expected. Paläodünen (1) und grösstenteils inaktive Gerinnebellen (weisse gestrichelte Linien, 2) sind die charakteristischen Formen auf dem Piedmont zwischen Erosionsterrasse des Rio Grande (3) und den gehobenen, zerschnittenen Subandinen Vorbergen (4) (Pfeil Paläo-Windrichumg). Paleodunes (1) el cheneiux de drainage en grande partie inactifs (lignes discontinues, 2) sont des elements distinctifs du piemont situe entre Tescarpment erosifdu Rio Grande (3) et les collines elevees et decoupees subandines (4) (la fliehe indique la direction du paleovent). Source: Landsat TM 230-73, Band combination 5-4-3, Global Land Cover Facility http://www.landcover.org. sharp thrust-fault, they are characterized by a highly integrated drainage network at an advanced stage of dissection. Today the entire area is covered by dense forest and drainage Channels within the foothills are largely inactive. Floodplains and Channel beds do not show evidence of aclive sediment transport on Ihe piedmont. In some cases Valleys several hundred meters wide are presently not oecupied by any recognizable stream.Therefore the dissection of the foothills must have occurred under past climatic conditions dif¬ ferent from today. The drainage network on the piedmont follows the inclination of the piedmont slope. No drainage Chan¬ nels presently reach the Rio Grande or Rfo Parapeti.
Active floodplains and significant sediment transport on the piedmont have only been observed in the northern part of the piedmont (AI) in the vicinity of Santa Cruz and along the southern part of the pied¬ mont (A2). In both areas the drainage Channels have incised into the proximal part ofthe piedmont surface.
In between these areas, the drainage Channels are essentially inactive and do not show any evidence for either sediment transport or incision (Fig. 2).
Along the southern piedmont the drainage Channels deposit their bed-load when emerging from the incised reach of the Channel (Fig. 3). This process causes a delta-shaped lobe of coarse fluvial Sediments referred to as floodout (Tooth 2000). These floodouts seem to have been located significantly further downslope in the past as evident from large areas of reduced density of forest cover. The shift of the floodouts to the proxi¬ mal parts of the piedmont probably indicates reduced intensities of the discharge events.

Paleodunes
The most characteristic geomorphological features along the northern part of the piedmont are several paleodune fields (Fig. 2). They all consisl of NW-SE to N-S trending parabolic dune forms, corresponding to the dominant wind direction (Agrotecnologica Amazonica 2005). The parabolic dune forms indicate a uniform wind regime and the presence of a Vegeta¬ tion cover dense enough to fix the lateral limbs of the dunes during dune migration (McKee 1979). In some cases the limbs are several kilometres long, implying a high movement rate and/or a relatively long time period of dune activity. Paleodune Systems have only been observed along the southern margins of drainage Channels. Apparently, past dune formation was closely tied to sufficient sediment supply. Both. the fluvial transport of sediment out of the Subandean catch¬ ments onto the piedmont, and the subsequent aeolian reworking most likely indicate generally drier climatic conditions with reduced forest cover, intensified dis¬ charge events and a prolonged dry season.
A large dune field (Lomas de Guanacos) exists on the southern part of the piedmont (Fig. 4, 6). It is largely inactive today. Its size (-2,250 km2) as well as its position along the southern border of the Rio Grande megafan point to the Rio Grande as the source of the aeolian sands that build up the Lomas de Guanacos. In contrast to the smaller paleodune formations to the north, the Lomas de Guanacos exhibit a complex internal struc¬ ture. Three dune generations can be distinguished.The Paläodünenfeld Lomas de Guanacos mit ältester Dünengeneration (I), jüngeren Formationen (2) und aktiven Dünen (3) (Pfeil Paläo-Windrichtung) Champ de paleodunes de Lomas de Guanacos montrant la generation la plus ancienne (1), les formations plus jeunes (2)  ple illustrates that dune formation does not require desert like conditions, but sufficient sediment supply, strong winds and a pronounced dry season in order to transfer the material out of the floodplains. Based on the extent and occurrence of paleo-and active dunes, areas prone to present deflation and dune formation are apparently more restricted (Fig. 5). Increased dis¬ charge intensity and sediment supply may also have played a role during the initial evolution of the Lomas de Arena.

Megafans
Three fluvial megafans have been formed by the three major rivers in Eastern Bolivia. The Rio Piray megafan northeast of Santa Cruz de la Sierra (-4.300 km-) is the smallest one: the Rio Grande megafan (-37,500 km2) and the Rio Parapeti megafan (-15,000 km2) are con¬ siderably larger. Megafans are characteristic for large subtropical rivers (Leier et al. 2005) and show a downstream zonation depending on the hydrological and geomorphological characteristics of each megafan (Shukla et al. 2001 At about 19°S the Rio Parapeti deposits most of its coarse sediment load within a highly migrational inland delta, the wetlands of the Banados de Izozog. A large paleodune field of mainly parabolic and longitudinal morphology exists along the southeastern margin of the Banados de Izozog in the Kaa' Iya National Park (Fig. 6). Based on the large size of this paleodune field (800 km2) it may be assumed that a former Channel of the Rio Grande was the source of the aeolian sands.
Near the proximal part of the megafan, the Rio Parapeti has formed two distinct terrace levels cor¬ responding to enhanced incision, probably indicating decreasing sediment loads at the transition to wetter conditions and increased humidity.
The present course of the Rio Grande (B2) is confined to the northwestern margin of the megafan (Fig. 6). In analogy to the Parapeti megafan, several paleochan¬ nels exist due to a northward shift of the river Chan¬ nel from a formerly W-E direction towards the present SW-NE-NW direction. At the eastern border of the Rio Grande megafan the Rio Parapeti cuts through the Chiquitana ranges at the Quimome gap (Fig. 6). This gap is probably an antecedent gorge resulting from stable discharge conditions over a long period of time. The antecedence is assumed to be inherited from the formerly W-E flowing Rio Grande under conditions wetter than today, because the present Rio Parapeti' rarely produces discharge events powerful enough to reach the gap.
Although parabolic paleodunes (3-10 km2) occur at several places along the southern margins of the Rio Grande paleochannels. they do not match the paleo¬ dunes along the Rio Parapeti' paleochannels in size.
Most likely this can be explained with the N-S climatic gradient. Due to a shorter dry season the Rio Grande was less prone to sand deflation than the Rio Para¬ peti.
Along the proximal part of its megafan, the Rio Grande has laterally eroded the piedmont, forming pronounced erosional scarps along the western and southern margin (Fig. 2. 6). Meander-like curvature of the scarps possibly implies a phase of enhanced meandering under wetter conditions (Fig. 2). In addition to lateral erosion. incision has occurred postdating the major river migration. possibly at the transition to wetter conditions. In contrast to the Parapeti' fan, small episodic second¬ ary drainage Channels (cahadas) have formed on the Rio Grande megafan within several of the paleochan¬ nels as well as in the areas between the paleochannels.
The orientation of the caiiadas and their coupling to the paleochannels indicate the existence of topographically elevated fluvial ridges (Brierley 1997). These ridges mark the former courses of the Rio Grande. Thus, the reconstruction of the paleochannels in the northern part of the megafan can be inferred from the network of caiiadas (Fig. 6), whereas in the southern part it is based predominantly on the interpretation of Vegetation patterns. In general, caiiadas become more frequent towards the northern part of the Rfo Grande megafan owing to climatic conditions characterized by increased humidity. In addition, the patterns of caiiadas on the Rio Grande megafan indicate that the avulsion pointthe location where the river abandons its Channel to occupy a new one -has significantly moved downstream through time (Fig. 6). While this might reflect the natural process of propagating fluvial deposition into actively aggrading sedimentary basins (Hanagarth 1993), the large-scale shift of the avulsion point might also document a change towards more constant discharge and sediment supply, and gradual construction of fluvial ridges under wetter climatic conditions (Bristow et al. 1999  Überblick über den Rio Grande «megafan» und die damit assoziierten Paläodiinenfelder (1), Paläoflussläufe (schattiert, 2), der antezedente Quimome Flussdurchbruch (3), die kartierten «canadas» (schwarze gestrichelte Linien), Dammufer (FR-I bis FR-V) auf der Basis eines topographischen Transektes (4) und die Verlagerung des «avulsion point» (weisser gestrichelter Pfeil, A-I to A-IV) (Pfeil Paläo-Windrichlung) Vue d'ensemble du megaeventail du Rio Grande et des champs de paleodunes qui lui sont associes (1), paleochenaux (ombre, 2), fosse ancien de Quimome (3), «canadas» cartographiees (lignes noires discontinues), levees fluviatiles (FR-I ä FR-V) basees sur le transect topographique (4) et propagation septentrionale du «point d'avulsion» (fliehe discontinue blanche, A-I a A-IV) (la fliehe indique la direction du paleovent) Source: SRTM 90-m data. Global Land Cover Facility http://www.landcover.org.
(2001) suggested stream capture as responsible for Channel abandonment and migration without speeifying the causes and mechanisms of this process. Werding (1977) proposed that the successive northward Channel displacement could be explained by enhanced aeolian accumulation along the southern Channel margin under dry climatic conditions. The relatively small number and extent of paleodunes along the Rio Grande paleochannels in comparison to the Parapeti paleochannels east doubt on this mechanism being responsible for large-scale shifts in both megafans. In addition the northward displacement of the avulsion point is not explained by enhanced dune accumulation alone. Hanagarth (1993) points out that increased Sedimentation rates could accelerate the process of Channel migration on the Rio Grande megafan. Within the context of the geomorphological framework and the manifold indicators of climatic change, increased Cuesta (1), mesa (2) el zone hypothetique de paleodunes (pointilles, 3); bassins paleolacustres (hachure) et lacs sales actuels (en blanc) avec paleodunes et reseaux incises de drainage entre les bassins (lignes discontinues) (la fliehe indique la direction du paleovent) Source: SRTM 90-m data, Global Land Cover Facility http://www.landcover.org. Sedimentation rates resulting from climatic and paleohydrological changes could have substantially altered the fluvial regime of the megafan rivers and are likely responsible for the large-scale Channel shifts.

Uplands (structural high)
In contrast to the Subandean zone or the Brazilian shield, the structural high corresponding to the Andean forebulge does not have a well-developed drainage network.The most striking features are the W-E orientated cuesta (escarpment) of Mesozoic and Palaeozoic rocks and the isolated mesa of the Cerro San Miguel, representing geomorphological evidence for the long erosional history of these upland areas (Fig. 7).
The dissection of the cuesta essentially follows a NW-SE direction. To the south. an elongated, ramplike and topographically elevated area extends in NW-SE direction to Paraguay (Fig. 7). Several fields of parabolic paleodunes are superimposed on top of this ramp. These paleodunes can be interpreted as the product of past deflation from the Rio Grande mega¬ fan, aeolian transport over the uplands and deposition below the southern rim of the cuesta. In this context the ramp-like feature is assumed to represent a multiphase sand-ramp from repeated phases of aeolian deposition, giving evidence for the importance of aeo¬ lian processes in long-term landscape evolution of the entire EBL.  Fig. 8: Sequential landscape evolution as observed for the geomorphological units of the study area and the tentative correlation to paleoclimatic phases (grey shades and black dots) Sequentielle Landschaftsgeschichle für die geomorphologischen Grosseinheiten im Untersuchungsgebiet und die vorläufige Korrelation mit Paläoklimaphasen (grau schattiert und gepunktet) Evolution sequentielle des unites geomorphologiques de la zone d'etude et correlation possible avec les phases paleoclimatiques (ombre grise et pointilles) A series of small lake basins occurs in the area between the cuesta and the Paraguayan border. Most of the basins are presently covered by forest; few of them contain seasonally inundated saline lakes and sah flats (Fig. 7). The basins do not seem to be integrated into an active drainage network. However, several incised valleys/gorges characterize the former drain¬ age network within the lake catchment areas (Fig. 7). It is assumed that higher lake levels, overflowing and incision prevailed under substantially wetter climatic conditions. Along the south-eastern rim of the basins, ridges of up to 10 meters height have been detected and interpreted as parabolic paleodunes (lunette dunes). All of these paleodunes are presently inactive and covered by forest. Their formation most likely documents increased aeolian activity during dry con¬ ditions pre-dating modern conditions, which favour forest growth. 5 Discussion absolute age datings are available so far. Nevertheless, a careful interpretation is attempted in order to correlate events and distinguish phases of landscape evo¬ lution.
The concept of landscape stability and activity (Rohdenburg 1970) uses the intensity and spatial distribu¬ tion of geomorphic processes (activity and stability) as an indicator for paleoecological conditions. Due to the complexity of feedbacks within the geomorphic System and the difficulties to define thresholds, the effects of climate changes do not only depend on the direction of the change (e.g. from dry to wet) but also on the cli¬ matic and geoecological conditions before the change (e.g. total preeipitation, seasonality) (Thomas 2004;Wolman & Gerson 1978).Therefore, the discussion of landscape evolution and climate history is restricted to the identification of regional sequences of events and landforms, providing a large-scale paleogeoecological frame rather than quantitative paleoclimatic data for the Eastern Bolivian lowlands.
The inventory of landforms presented in this study provides manifold evidence for changing geomorphic processes in Eastern Bolivia during the late Quater¬ nary. Figure 8 Fig. 2).
At some places the geomorphological evidence points to increased humidity in the study area, preceding the highly active interval of sediment transport and dune formation. This is the case for the markedly smoothed morphology of the older dune generations, which must have undergone a time of erosion and reshaping, and the paleolake basins, which document even wetter conditions than today, possibly because of a significant increase of preeipitation during the dry season. Despite of the limited number of observations, these findings corroborate previous studies reporting increased mois¬ ture availability in the Paraguayan Chaco (Barboza et al. 2000;Kruck 1996) Werding (1977) notes the existence of coarse fluvial gravel through¬ out the Rio Grande megafan. An overall much more torrential fluvial regime has been reported from sev¬ eral tropical rivers in lowland South America during marine isotope stage 3 (MIS 3) (Latrubesse 2003 Fluvial and aeolian processes are presently restricted to a few locations in the study area. A much more active landscape has been inferred from large-scale Channel shifts and extensive paleodune Systems.
Mobilization. transport and deposition of Sediments are thought to be the result of climatic conditions drier than today. However. there are also indications of formerly wetter conditions such as fluvial erosion and paleolake basins. In conclusion, the documentation and interpretation of the manifold landforms has shown to contain a considerable amount of paleoecological information, which might serve as the base for further paleoclimatic research in the central part of