ARTICLE IN PRESS GEOMOR-02923; No of Pages 15 Geomorphology xxx (2009) xxx–xxx Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph Late Quaternary Paleohydrology of the Madre de Dios River, southwestern Amazon Basin, Peru Catherine A. Rigsby a,⁎, Erin M. Hemric a, Paul A. Baker b a Department of Geological Sciences, East Carolina University, Greenville, NC 27858 USA b Division of Earth & Ocean Sciences, Nicholas School of the Environment, Duke University, Durham, NC 27705 USA article info abstract Article history: Late Quaternary climatic and hydrologic variability triggered changes in fluvial deposition and erosion along Received 3 December 2007 the course of the Madre de Dios River, Peru, the largest tributary basin of the Madeira basin, itself the largest Accepted 11 November 2008 tributary basin of the Amazon. Three laterally extensive, Quaternary-age, terrace tracts are present within the Available online xxxx Madre de Dios basin. Analysis of sedimentary facies, present in the modern cut banks and terraced sequences, along with radiocarbon dates on fossil wood and leaf material preserved in the terraced strata, Keywords: allow reconstruction of the Late Quaternary depositional history of the sedimentary sequences, including Quaternary Peru determination of the approximate timing of aggradation and downcutting episodes and its relationship to the Amazon timing of past climate change in this portion of the Amazon basin and beyond. River The Quaternary sediments underlying the terraces most often recorded deposition in a coarse-grained Paleohydrology meandering fluvial system. The T3 terrace, the highest terrace, is underlain by the Miocene (?) Ipururi Formation, which is unconformably overlain by the late Miocene–Pleistocene (?) (N48,000 cal yrs BP) Madre de Dios Formation, a multistory coarse-sandy to gravelly channel and point bar complex. The latter was downcut before 29,850±100 cal yrs BP. This downcut landscape was infilled by meandering fluvial strata characterized by gravelly channel deposits in a sequence dominated by floodplain and lateral accretion deposits. These strata were in turn downcut to form the T2 terrace before 11,970±100 cal yrs BP. A third episode of aggradation resulted in the deposition of a sand-dominated meandering channel complex that infilled the T2 valley and was subsequently downcut after 3780±50 cal yrs BP. This most recent terrace is infilled by the modern fluvial sediment, which has been actively aggrading since at least 870±50 cal yrs BP. Importantly, the Madre de Dios fluvial system actively aggraded between 30,000 and 25,000 cal yrs BP, (and likely much younger, as dated samples were, thus far, only found near the base of the T2 sequence). This observation implies that some combination of (1) increased precipitation and decreased temperature, (2) decreased evapotranspiration and increased runoff, (3) increased Andean glacial erosion and increased sediment supply, and (4) decreased atmospheric CO2 (hence decreased rain-forest primary productivity and altered rain-forest physiology/ecology), entering the last glacial maximum period brought about increased floodplain deposition in the southwestern Amazon. Elsewhere in the Amazon basin few, if any, fluvial sediments of this age range have been observed. The start of the next major phase of aggradation coincided with the Younger Dryas and suggested that floodplain sedimentation in the lowlands was again related to cold and wet conditions in the adjacent highlands (and perhaps in the lowlands as well) and that Madre de Dios history was also tied to large-scale global climate. This aggradation may have continued throughout the early and mid-Holocene, until at least 3,780 cal yr BP. If so (and this is uncertain), this episode of sedimentation took place during a dry period. © 2009 Elsevier B.V. All rights reserved. 1. Introduction biodiversity that is undergoing rapid changes as a result of direct human endeavor and climate change. Despite growing recognition of this The Amazon River drains approximately 38% of the area of transformation, few studies exist of the paleohydrology of the Amazon continental South America and accounts for over 18% of the total Basin designed to determine the range and mechanisms of past changes freshwater input to the oceans. From the Andes to the Atlantic, the river of fluvial processes as well as to elucidate the future of this fluvial runs through the largest tropical rainforest, a region of unsurpassed environment (e.g., Latrubesse, 2003; Latrubesse et al., 2005). And, paleohydrologic information is largely absent from most paleoenviron- mental discussions of the Amazon region, despite the Amazon landscape fl ⁎ Corresponding author. being dominantly of uvial origin. Understanding the relatively sparse E-mail address: [email protected] (C.A. Rigsby). and geographically restricted Quaternary vegetation history of the 0169-555X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2008.11.017 Please cite this article as: Rigsby, C.A., et al., Late Quaternary Paleohydrology of the Madre de Dios River, southwestern Amazon Basin, Peru, Geomorphology (2009), doi:10.1016/j.geomorph.2008.11.017 ARTICLE IN PRESS 2 C.A. Rigsby et al. / Geomorphology xxx (2009) xxx–xxx Amazon (e.g., Colinvaux et al., 2000; Mayle et al., 2007; Anhuf et al., abundant. And opinions differ widely about the nature of Amazon 2006)isdifficult, at best, without independent information about climate, particularly prior to the Holocene. Quaternary climate change and attendant fluvial evolution in the region. In this paper, we document fluvial history in the southwestern Moreover, the paucity of long-lived and well-preserved natural archives Amazon basin by examining the Late Quaternary sedimentary of past climate and hydrology in the Amazon lowlands (e.g., Ledru et al., sequences exposed in terraced strata along a portion of the Madre 1998; Bush et al., 2004; Anhuf et al., 2006) has largely precluded de Dios River (Peru) and we compare these sequences with records of definitive conclusions regarding the history of Amazon precipitation and climate and fluvial history elsewhere in tropical South America. Our runoff. As a result, we do not know for sure if Amazon climate has shifted reconstruction of the history of the Madre de Dios fluvial system in lockstep with climate in the neighboring highlands, where paleocli- provides landscape level evidence of past hydrologic change that mate and paleohydrologic records of the late Quaternary are more extends back to about 30,000 cal yrs BP and is an important step Fig. 1. Map showing the study area in the Madre de Dios Department in southeastern Peru, southwestern Amazon Basin. Triangles indicate measured section locations. Refer to Table 1 for site abbreviations. Please cite this article as: Rigsby, C.A., et al., Late Quaternary Paleohydrology of the Madre de Dios River, southwestern Amazon Basin, Peru, Geomorphology (2009), doi:10.1016/j.geomorph.2008.11.017 ARTICLE IN PRESS C.A. Rigsby et al. / Geomorphology xxx (2009) xxx–xxx 3 toward achieving a broader paleohydrologic history of the entire Amazon basin. 2. Environmental setting 2.1. Geological setting The Madeira River is the largest tributary of the Amazon in area and discharge. Its watershed has an area of 1,380,000 km2, 20.1% of the total area of the Amazon watershed, and an average discharge of about 30,000 m3 s− 1, approximately 15% of the total Amazon discharge (Goulding et al., 2003), twice the average annual discharge of the Mississippi. The Madeira drains the entire southwestern Amazon region and is itself comprised of several large tributary basins including the Guaporé–Iténéz with headwaters in the Brazilian Shield, the Mamore draining much of lowland eastern and central Bolivia, and the Beni basin of northwestern Bolivia and southeastern Peru. The Fig. 3. Longitudinal profile of the Madre de Dios River from the Upstream of Diamante Madre de Dios River is the largest tributary to the Beni; the drainage section on the Alto Madre de Dios River to the confluence of the Madre de Dios and basin of the Madre de Dios roughly comprises the western two-thirds Tambopata Rivers at Puerto Maldonado. Locations of confluences with tributary rivers of the Beni basin. Some of the tributaries of the Madre de Dios are indicated by vertical dotted lines. All profile points (including measured section originate in the glaciated Cordillera Oriental of the Andes whereas locations) are plotted as triangles (see Fig. 1 for locations of measured sections; Table 1 other tributaries arise in the lowland rain forests of Peru and Bolivia. for site abbreviations). The anomaly in elevation upstream of the Inambari tributary is – – the result of a rapid and short-lived increase in river level after precipitation the day The 650-km long reach of the Madre de Dios River (12 13° S, 69 before the measurement was taken. 71° W) discussed in the present study (Fig. 1) stretches from upstream of the confluence of the Alto Madre de Dios and Manu Rivers at Boca de Manu, Peru, downstream to the confluence of the Madre de Dios and Tambopata Rivers at Puerto Maldonado, Peru (including as well a portion of the lower Tambopata River). Throughout this region the 2.2. Climatic setting course of the Madre de Dios is eroded into thick Tertiary and Quaternary sedimentary sequences of the Andean foreland basin (e.g., Baby et al., The modern climate of the lowland portions of the southwestern 1997; Galloso et al.,1996; Guyot et al.,1999; Horton and DeCelles,1997). Amazon basin is humid tropical with mean monthly temperatures The strata examined in this study are exposed in cut-banks and terraced varying just a few degrees throughout the year. The diurnal reaches of the modern river valley. The strata are inset within deposits of temperature range is several times the seasonal cycle. The mean the late Miocene–Pleistocene (age uncertain) Madre de Dios Formation, monthly low temperature of 16.8 °C occurs in July and the mean which is composed of fluvial gravel, sand and mud deposits.