The role of Late Pleistocene-Holocene tectono-sedimentary history on the origin of patches of savanna vegetation in the middle Madeira River, southwest of the Amazonian lowlands

Palaeogeography, Palaeoclimatology, Palaeoecology 526 (2019) 136–156

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The role of Late Pleistocene-Holocene tectono-sedimentary history on the origin of patches of savanna vegetation in the middle Madeira River, southwest of the Amazonian lowlands

T

Dilce de Fátima Rossettia, , Rogério Gribelb, Marcelo Cancela Lisboa Cohenc, Márcio de Morisson Valerianoa, Sonia Hatsue Tatumid, Marcio Yeed ⁎

a

Brazilian National Institute for Space Research-INPE, Rua dos Astronautas 1758, São José dos Campos, SP 12245–970, Brazil Coordination of Biodiversity, Brazilian National Institute of Amazonian Research-INPA, Av. André Araújo, 2936, Manaus, AM 69067-375, Brazil Universidade Federal do Pará-UFPA, Av. Augusto Correa 1, Belém 66075-900, Pará, Brazil d Federal University of São Paulo, Santos, SP 11070-100, Brazil b c

ARTICLE INFO

ABSTRACT

Keywords: Sedimentary processes Late Pleistocene-Holocene Environmental control Savanna vegetation Amazonian lowlands

Savanna patches are features of the Amazonian landscape that have been long under intense debate, but there are still questions about the main factors that have determined their establishment and evolution within the rainforest matrix. In particular, their geological substrates were poorly documented. The aim of this work is to reconstruct the tectono-sedimentary history of four savanna patches in the middle Madeira River, southwest Amazonia, and discuss its potential control over the development of the savanna communities. The approach consisted in the integration of geomorphological, sedimentological, chronological and floristic data. The results revealed that the savanna substrates are sandy deposits aged between 118.9 and 35.6 ky, overlain by late Pleistocene to Holocene muddier successions < 25,700 cal yr BP. Most of the geologically stable and topographically higher terrains of the geomorphological unit T1 is dominated by forests. These intermingle with open habitats formed by renewed sedimentation that were colonized by tree and shrub species of the Amazonian biome. By contrast, the ground of unit T2 subsided to a level that allowed the meandering of the main river. This process resulted in the replacement of the rainforest by open habitats dominated by savannas with lower richness and diversity than in the T1 unit, which were dominated by species with wide distribution in the cerrado biome of Central Brasil. The cerrado species might have expanded into this region during Pleistocene drier climatic episodes, but environmental filters shaped by geological processes determined the floristic contrasts between units T1 and T2. The lack of competition with Amazonian species probably constituted an important factor for the preferential colonization of arboreal and shrubby species from neighboring cerrados on meandering scroll bars, marginal levees and crevasse splays of unit T2.

1. Introduction The Amazonian lowlands contain numerous savanna patches, which correspond to discontinuous areas covered by vegetation ranging from grasslands, shrublands and woodlands in sharp contact with surrounding rainforest. These patches vary in size in several orders of magnitude, from a few to a thousand square kilometres. The existence of such features within the modern tropical forest has been a subject of great interest. This is because many Amazonian savanna areas are considered sites of endemism that may have played a key role in increasing the tropical biodiversity (Fine et al., 2010). The most common hypotheses to explain the existence of savanna ⁎

patches in the Amazonian rainforest are the complex interplay between biological interactions and environmental filtering (Fine and Baraloto, 2016; Guevara et al., 2016), the latter generally related to past episodes of arid climate (Freitas et al., 2001; Pessenda et al., 2005), soil gradients (Richards, 1941; Pires and Prance, 1985; ter Steege et al., 1993; Tuomisto et al., 2003; Villacorta et al., 2003; Vicentini, 2004; Cochrane and Cochrane, 2010; Baraloto et al., 2011; Zanchi et al., 2011) or anthropogenic fires (Hammond and ter Steege, 1998; Flores et al., 2017; Rull et al., 2017). However, past geological processes may have played a central role in controlling changes in the sedimentary environments of the Amazonian lowlands, with major impacts on species interactions and ecological functions (Wittmann and Householder, 2017). The

Corresponding author. E-mail addresses: [email protected] (D.d.F. Rossetti), [email protected] (M.d.M. Valeriano), [email protected] (S.H. Tatumi).

https://doi.org/10.1016/j.palaeo.2019.04.017 Received 22 August 2018; Received in revised form 5 April 2019; Accepted 14 April 2019 Available online 17 April 2019 0031-0182/ © 2019 Elsevier B.V. All rights reserved.

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evolution of the Andean mountains in the Miocene and Pliocene has been commonly claimed as the main geological event to increase the Amazonian biodiversity (e.g., Hoorn et al., 2010, 2017; Wittmann and Householder, 2017). The Andean uplift created a highly seasonal monsoon flood regime, which reorganized river courses (Kaandorp et al., 2005; Irion and Kalliola, 2010) and increased sedimentation rates in the lowlands (Wittmann and Householder, 2017). The high dynamics of past alluvial environments would have created terrains with new geological characteristics, as well as different distributions of dry (terra firme) and flooded lands. As a result, selective pressure on species composition may have increased, with the consequent creation of numerous habitat specialists (Wittmann et al., 2013). Although the Miocene-Pliocene geological events related to the Andean evolution have modified the sedimentary environments of the Amazonian lowlands, they are not able to explain the savanna patches within the forest matrix. This is because several savanna substrates from the Amazonian lowlands were dated as Late Pleistocene/Holocene (Rossetti et al., 2005, 2015, 2016; Rossetti, 2014; Gonçalvez Jr. et al., 2016). The patches of savanna vegetation, which are confined to paleolandforms resulting from the abandonment of fluvial and megafan depositional systems during the Late Pleistocene-Holocene, support a connection to sedimentary processes (e.g., Cordeiro and Rossetti, 2015; Cordeiro et al., 2016; Rossetti et al., 2017a). Changes in river systems in the Late Pleistocene/Holocene were presumably influenced by global sea-level fluctuations and increased seasonality due to climatic glaciations (Irion et al., 2010; Wittmann and Householder, 2017). They were also strongly affected by neotectonic reactivations (see several references in the review article of Rossetti, 2014), evidenced even in most recent times (Rossetti et al., 2017b, 2018). Therefore, changes in river dynamics during recent geological times should be investigated preferentially to discuss the controls on the establishment of savanna patches in the Amazonian lowlands. A vast area of the middle Madeira River has a large number of savanna patches confined to channel paleolandforms, as suggested based on remote sensing data (Hayakawa et al., 2010, Hayakawa and Rossetti, 2015; Fig. 1). However, there is no sedimentary information focusing the substrates of these savannas. An exception was the publication of Bertani et al. (2015), who related one savanna substrate to a fluvial ria in reference to the paleolandform similar to many enlarged river mouths of the Amazonian landscape (cf. first defined by Gourou, 1949, and followed by Sternberg, 1950, 1955; Dumont, 1993; Sioli, 1984; Tricart, 1977; Dumont and Fournier, 1994; Bezerra, 2003; Irion and Kalliola, 2010; but see also the larger use of coastal rias as reviewed by Goudie, 2018). As a result, the sedimentary history of most savanna patches in this region remains unrecorded, as well as the characterization of the associated vegetation communities. The aim of this work is to reconstruct the paleoenvironments and the depositional history of four savanna substrates of the western margin of the middle Madeira River, and then to analyze the role of sedimentary dynamics as the driver of changes in savanna composition. The research was based on geomorphological, sedimentological, and chronological data. In addition, the floristic inventories of the savanna patches allowed detecting changes in species composition and species richness comparing the four savanna patches. The integration of all these data was useful to discuss the influence of geomorphological, tectonic and/or depositional characteristics of individual savanna substrates during plant colonization. Although local, this multidisciplinary investigation is interesting due to the potential assessment of many other patches of savanna vegetation within the rainforest matrix that typifies the Amazonian wetlands.

(Radambrasil, 1978). Geologically, the study area is located in the Paleozoic Solimões Basin (Fig. 1A, B), which is an intracratonic Paleozoic structure with a foreland phase in the Cretaceous and Cenozoic due to Andean uplifting (Tassinari and Macambira, 1999). The sedimentary fill of the Solimões Basin includes a 3.8-km thick succession, but the surface of the study area consists only of fluviatile deposits mapped as the Içá Formation. Although the age of this unit is commonly inferred as Plio-Pleistocene (cf. Maia et al., 1977), radiocarbon and optically stimulated luminescence dating of deposits from the margin of the Madeira River, where the study area is located, recorded only middle-late Pleistocene and Holocene ages (Rossetti et al., 2014, 2015). In addition to autochthonous processes linked to sedimentary dynamics, the history of deposition and erosion in the study area was also controlled by neotectonic reactivations along the Madre de DiosItaquatiara Fault Zone (Souza-Filho et al., 1999; Hayakawa et al., 2010; Rossetti et al., 2014; Fig. 1C). This NE-trending structural zone includes strike slip and normal faults that parallel the lower reaches of the Madeira River and many other structural lineaments of the Amazonian lowlands. E-W and NW- and NNW-trending structures are also present in this region, and are related to strike slip and normal faults along preexisting transpressive and transtensive zones (Costa and Hasuy, 1997; Bezerra, 2003) reactivated during the Neogene and even the Quaternary periods (Franzinelli and Latrubesse, 1993; Costa et al., 1996, 2001; Franzinelli et al., 1999). The studied landscape is topographically low, with altitudes in general < 100 m, and consists of three geomorphological units located between 100–65 m (T1), 85–50 m (T2) and 65–45 m (T3) (Fig. 1D, E). The highest T1 unit, located in interfluvial areas distant from the course of the Madeira River, recorded radiocarbon ages ranging from > 43,500 cal yr BP to 31,696–32,913 cal yr BP (Rossetti et al., 2014; Fig. 1F) and OSL ages between 65.4 ± 16.9 and 346.6 ± 48.6 ky (Rossetti et al., 2015). The T2 unit with intermediate elevations had ages between 25,338–26,056 and 14,129–14,967 cal yr BP, whereas the lowest T3 unit recorded only ages younger than 12,881–13,245 cal yr BP (Rossetti et al., 2014). Despite this overall geomorphological and chronological partition, the three geomorphological units may also have deposits younger than average ages due to renewed sedimentation caused by locally superposed drainage or overflows during the high stages of the river. The three geomorphological units record the successive lowering of terrace and sediment aggradation, as the Madeira River responded to the reactivation of faults during the Late Pleistocene-Holocene (Rossetti et al., 2014). According to these authors, these units were formed by varying rates of subsidence promoted by alternating episodes of fault activity and tectonic quiescence. This river dynamics would have changed the flow and the load of sediments, modifying the river's capacity to erode and transport sediments. 3. Material and methods The morphological and topographic characterization of savanna patches was achieved using the Topodata digital elevation model (DEM) with a resolution of 1 arc-second (30 m) (Brasil, 2008). The Topodata is a Brazilian full-coverage geomorphometric database constructed from the Shuttle Radar Topography Mission (SRTM). The Topodata data were refined from the first 3 arc-second SRTM version released to South America (Valeriano and Rossetti, 2012), and downloaded from http://edc.usgs.gov/srtm/data/obtainingdata.html. Processing theses data included creating custom shading schemes and palettes to reveal features of interest. For comparisons of SRTM elevations between savanna and forest areas, canopy effect was corrected for forest areas. Since savanna areas were observed below forested areas, the canopy effect was overcompensated for the latter by fully subtracting average tree heights estimated in the field. The volumetric interaction prevailing in C-band forest backscatter (Le Toan et al., 1992) means that a relative penetration does happen inside the canopy.

2. Physiography and regional geological context The middle Madeira River occurs in a region of tropical climate, with an average annual temperature of 28 °C, precipitation ranging from 2500 to 3000 mm/year, and vegetation dominated by dense ombrophilous rainforest having numerous patches of savanna vegetation 137

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56ºW

68ºW

South Atlantic C 70o W Ocean

B Guyana Shield Fig. C

60o W Venezuela

Guiana

1

0º

Colombia

0o

Manaus 2 SOLIMÕES

Belém

8ºS

BASIN

70o

Porto Velho Brazilian Shield

1

2

Manaus

3 4

50o

A o

0

B

5

6

Brazil

7**

10 S

Figure D 40o 1 Iquitos Arch 2 Purus Arch Precambrian/Paleozoic basement

Fault Zone River

o

07o30’S

63 30’W T1

Madeira River

I T2

T1

7

8

Humaitá T3 9

10

3

PV5

E T1 I

T2

80

II

T1

8 300 km

**

1-Branco 4-Solimões 5-Juruá 7-Madeira 3-Japurá 6-Purus 8-Tapajós

** Madre de Dios-Itacoatiara 2-Negro ( altitude in m above msl )

D

Tectonic lineament

* **

Geomorphological units T3 T2 T1

Madeira R.

16ºS

o

II

T3

T1

40 0

50

F

(<43,500 to 31,196-32,913) T1 (25,338-26,056 to 14,129-14,967) T2 (12,881-13,245 T3 to 3,158-3,367)

(km) 100 150 Abandoned drainage on T2 (12,867-13,200 to 3,365-3,490) Abandoned drainage on T1 (20,085-20,544 to 5,928-6,124)

08o30’S

1

M

4

11 11

13

Porto Velho

Precambrian rocks (Brazil-Central Shield) Paleochannel

ad

ei

ra

Upper unit

Ri

ve

r Intermediate unit

30 km

Lower unit

Fig. 1. Location, geological context, and geomorphological characterization of the study area. A, B) Location in the southeastern Solimões Basin, Brazilian Amazonian lowlands. C) Main structural trends, including the location of the Itacoatiara Fault Zone, parallel to the Madeira River. D) Geological and geomorhological context, illustrating the three geomorphological units that constitute the late Quaternary sedimentary history of the middle Madeira River. E) DEM-SRTM topography along the transect I-II, transverse to the Madeira valley (see D for location). F) Summary of the geomorphogical units and their chronology (based on Rossetti et al., 2014), with all ages in cal yr BP.

from the surrounding rainforest. High resolution optical images from Digital Globe and WebGLEarth (webglearth.com) have completed this research. The spatial analysis was combined with geological and chronological information from 3 sediment cores from previous publications (i.e., Rossetti et al., 2014, 2015, 2016), but we also added new data from 23 other sediment cores (see geographic coordinates of all cores in Supplement 1). The cores were collected using a soil sampling hand auger with a 20 cm sampler. The sedimentary facies were described considering features such as lithology, texture, sedimentary structure and type of facies contact, and these data were recorded in measured lithostratigraphic sections. Sedimentation rates were calculated for several stratigraphic intervals, but considering only the ages of the deposits between discontinuity surfaces to avoid errors through unconformities or hiatuses. These values allowed to compare the amount

This was confirmed in the equations of Kellndorfer et al. (2004) relating canopy and SRTM elevations, who calculated that about 92% to 89% of average canopy variation is reflected on phase center height variations. Therefore, subtracting the canopy height, while not modifying elevations in the savanna areas, was an overcompensation considered enough to assure the remaining difference to be due to ground differences. In fact, this is the main justification for the recommended procedure for handling upscaling errors. Other remote sensing data included optical images, such as Landsat–5/TM Thematic Mapper and Landsat–7/ETM+ Enhanced Thematic Mapper Plus images, Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) images Landsat–5/TM Thematic Mapper and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). Basic processing applying R (red) G (green) B (blue) color image compositions helped to distinguish savanna patches 138

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Table 1 AMS 14C dating of samples from sedimentary deposits of the patches of savanna vegetation studied in the middle Madeira River. All samples are from organic sedimentary deposits. Terrain/savanna

Locality/Lab number

Coordinates

T1/S1 T1/S1 T1/S1 T1/S1 T1/S1 T1/S1 T1/S1 T2/S3A T2/S3A T2/S3A T2/S3A T2/S3A T2/S3B T2S3B T2/S3B T2/S3B T2S3B T2/S3B T2/S3B T2/S3A T2/S3A T2/S3A T2/S3A T2/S3A T2S3A T1/S2 T1/S2 T1/S2 T1/S2 T1/S2 T1/S2 T1/S2 T1/S2 T1/S1 T1/S1 T1/S1 T1/S1 T1/S1 T2/S3A T2/S3A

PV3 (406680)b PV3 (296244)a,b PV3 (309797)a,b PV3 (288716)a,b PV4 (406683)b PV4 (406684)b PV4 (406685)b PV5 (288718)a,b PV5 (288719)a,b PV5 (285261)a,b PV5 (304791) PV5 (288720)a,b PV10 (304793)a PV10 (304794)a PV12 (304797)a PV12 (309801)a PV50A (397678) PV50A (406686) PV50A (397679) PV51A (406687) PV51A (406688) PV52A (397681) PV52A (406691) PV53A (397683) PV53A (397685) PV60A (397695) PV60A (406698) PV60A (397696) PV67A (454618) PV67A (454623) PV67A (476311) PV67A (545623) PV68A (545622) PV107A (476318) PV109 (476320) PV109 (476321) PV110A (476322) PV111A (476323) PV114A (476325) PV114A (476326)

7°42′43″S/63°05′32″W 7°42′43″S/63°05′32″W 7°42′43″S/63°05′32″W 7°42′43″S/63°05′32″W 8°06′02″S/63°45′31″W 8°06′02″S/63°45′31″W 8°06′02″S/63°45′31″W 7°55′26″S/63°04′60″W 7°55′26″S/63°04′60″W 7°55′26″S/63°04′60″W 7°55′26″S/63°04′60″W 7°55′26″S/63°04′60″W 7°41′59″S/63°07′16″W 7°41′59″S/63°07′16″W 6°42′28″S/61°42′28″W 6°42′28″S/61°42′28″W 8°49′44″S/63°10′10″W 8°49′44″S/63°10′10″W 8°49′44″S/63°10′10″W 7°56′13″S/63°05′47″W 7°56′13″S/63°05′47″W 7°56′25″S/63°04′36″W 7°56′25″S/63°04′36″W 7°56′27″S/63°05′02″W 7°56′27″S/63°05′02″W 7°54′30″S/63°14′54″W 7°54′30″S/63°14′54″W 7°54′30″S/63°14′54″W 7°56′38″S/63°13′21″W 7°56′38″S/63°13′21″W 7°56′38″S/63°13′21″W 7°56′38″S/63°13′21″W 7°57′34″S/63°10′43″W 8°07′27″S/63°47′54″W 7°55′18″S/63°50′51″W 7°55′18″S/63°50′51″W 8°04′09″S/63°48′54″W 7°55′57″S/63°14′04″W 7°57′03″S/63°02′47″W 7°57′03″S/63°02′47″W

a b

Depth (m)

0.4 0.8 2.6 8.3 0.6 2.6 3.8 0.8 2.7 4.7 5.0 6.8 0.8 4.6 0.9 4.3 0.6 1.5 2.9 0.2 1.0 1.3 2.7 0.2 4.8 0.4 0.5 2.2 0.1 0.3 0.6 1.3 1.5 2.0 0.6 2.5 1.5 1.3 0.3 1.4

14

C yr BP conventional

3990 ( ± 30) 4730 ( ± 30) 18,090 ( ± 70) 21,460( ± 130) 2380 ( ± 30) 18,590 ( ± 70) 18,430 ( ± 60) 6450 ( ± 40) 13,770 ( ± 60) 30,770 ( ± 170) 34,000 ( ± 200) 38,110 ( ± 360) 13,150 ( ± 60) 15,060 ( ± 60) 12,440 ( ± 50) 15,630 ( ± 60) 2700 ( ± 30) 9870 ( ± 30) 16,000 ( ± 60) 580 ( ± 30) 1600 ( ± 30) 5460 ( ± 50) 14,530 ( ± 40) 1190 ( ± 63) 15,170 ( ± 50) 1750 ( ± 30) 4170 ( ± 30) 17,400 ( ± 60) 102,3 ( ± 0.4) 820 ( ± 30) 4350 ( ± 30) 7600 ( ± 30) 8810 ( ± 30) 14,820 ( ± 40) 2780 ( ± 30) 11,340 ( ± 30) 6230 ( ± 30) 14,540 ( ± 40) 1120 ( ± 30) 5680 ( ± 30)

Cal yr BP 2-sigma calibration-AMS (median probability) 4285–4450 (4367) 5314–5484 (5399) 21,607–22,124 (21,865) 25,467–25,982 (25,724) 2308–2485 (2396) 22,315–22,628 (22,471) 22,443–22,007 (22,225) 7265–7422 (7345) 16,316–16,863 (16,574) 34,272–34,999 (34,657) 41,770–42,768 (42,271) 42,033–43,168 (42,600) 15,275–16,532 (15,903) 18,027–18,398 (18,212) 14,129–14,967 (14,548) 18,626–18,914 (18,770) 2742–2844 (2772) 11,190–11,275 (11,233) 19,028–19,482 (19,252) 510–561 (535) 1374–1527 (1450) 6029–6303 (6266) 17,485–17,865 (17,658) 963–1094 (1027) 18,287–19,597 (18,440) 1558–1702 (1630) 4529–4729 (4629) 20,700–21,182 (20,941) 225–253 (239) 686–784 (735) 4850–4974 (4912) 8322–8421 (8371) 9598–9913 (9755) 17,865–18,181 (18,023) 2758–2892 (2825) 13,068–13,238 (13,153) 6968–7178 (7073) 17,497–17,874 (17,685) 928–999 (963) 6318–6490 (6404)

Age from Rossetti et al. (2014). Age from Rossetti et al. (2016).

isotope ratio mass spectrometer (δ13C precision of ± 0.3‰). Acceptance was defined as the total laboratory error known to be within 2sigma. Radiocarbon ages are reported in years before to 1950 CE (yr BP) normalized to δ13C of −25‰ VPDB. The software CALIB - Radiocarbon Calibration version 7.1 html was used for age calibration (reported as cal yr BP) according to SHCal13 (Reimer et al., 2013). The OSL measurements were obtained at the Laboratory of Dating and Dosimetry of the Federal University of São Paulo (UNIFESP) using a Risø OSL/TL reader models DA-20 and DA–15, blue LEDs, Hoya U-340 filters, and built-in 90Sr/90Y beta sources with dose rate of 0.089 Gy/s. The chemical separation of the coarse quartz grains began with H2O2 (30%) for 24 h to remove the organic matter. The dried samples were sieved to select grain sizes between 100 and 150 μm, which were etched using HF (20%) for 4 h to eliminate the carbonate and remove part of the grain surface (25 μm), in order to avoid the ionization of alpha particles. Then, the grains were treated with 20% HCl for 2 h to remove the fluoride crystals eventually formed during the HF etching. Finally, the quartz grains were separated using sodium polytungstate (SPT). No feldspar grains were found in the samples after this conventional chemical treatment, and a test was done stimulating the samples with infrared light. We used the single dose aliquot regeneration protocol (SAR) to measure the equivalent doses (De) (Murray and Wintle, 2003; Duller, 2004) in quartz grains, with preheating at 220 °C for 10 s, rate

of sediment accumulated during the stratigraphic intervals of equivalent time along the various geological sections. A total of 36 samples were collected from the lithostratigraphic sections to improve the chronological framework, with 24 samples of organic sediments being dated by 14C (Table 1), and 16 samples of sandy sediments being dated by optically stimulated luminescence (OSL) (Tables 2 and 3). We integrated these results with 16 14C and 16 OSL ages already available in the literature. The 14C ages were acquired from bulk organic material. The pretreatment consisted in dispersing the sediment samples in deionized water, followed by sieving through a 180 micron sieve to recover the plant material after removal of inorganic particles. In order to avoid natural contamination by shell fragments, roots and/or seeds, the sediment samples were physically cleaned under a stereomicroscope. This procedure was able to remove fulvic and/or humic acids, which were potential sources of young carbon contaminants for the samples, and provided ages that generally decreased upward along sections (with some exceptions commented on the results). The treatment consisted of extracting residual material with 2% HCl at 70 °C during 2 h. The samples were rinsed to neutral and dried at 100 °C for 12 h before following the standard chemical treatment at the Beta Analytic Radiocarbon Dating Laboratory, Florida, USA. The ages were obtained using a 250 kV NEC single stage particle accelerator (AMS precision of ± 0.001–0.004) and a Thermo Delta-Plus 139

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Table 2 Summary of radioisotope concentrations in samples dated by OSL. Sample

Coordinate lat/long

PV4a PV4a PV4a PV5 PV5 PV12a PV12a PV12a PV50A PV50A PV52A PV52A PV53A PV60A PV60A PV111A

7°42′03″S/63°05′32″W 7°42′03″S/63°05′32″W 7°42′03″S/63°05′32″W 7°55′26″S/63°04′60″W 7°55′26″S/63°04′60″W 7°37′38″S/63°04′87″W 7°37′38″S/63°04′87″W 7°37′38″S/63°04′87″W 8°49′44″S/63°10′10″W 8°49′44″S/63°10′10″W 7°56′25″S/63°04′36″W 7°56′25″S/63°04′36″W 7°56′27″S/63°05′02″W 7°54′30″S/63°14′54″W 7°54′30″S/63°14′54″W 7°55′57″S/63°14′04″W

a

Depth m

U ppm

4.8 6.3 7.7 8.6 9.7 5.6 6.8 7.7 4.8 8.7 4.3 4.9 4.9 4.3 5.7 0.4

2.70 1.93 3.16 4.49 5.00 3.20 1.84 1.95 3.11 2.95 3.01 2.98 2.88 3.08 3.22 3.27

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.22 0.20 0.44 0.07 0.08 0.56 0.42 0.50 0.41 0.42 0.32 0.32 0.36 0.36 0.45 0.07

Th ppm

K %

10.5 ± 0.41 6.28 ± 0.20 8.12 ± 0.22 13.20 ± 0.15 17.57 ± 0.17 9.75 ± 0.60 7.05 ± 0.43 9.96 ± 0.61 10.12 ± 0.55 12.20 ± 0.65 11.25 ± 0.65 10.11 ± 0.35 10.24 ± 0.38 12.03 ± 0.61 10.04 ± 0.55 7.64 ± 0.40

1.32 0.52 0.52 1.86 2.15 1.55 1.10 1.16 1.45 1.21 1.32 1.47 1.23 1.36 1.65 0.06

Water (%) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.11 0.08 0.08 0.01 0.02 0.53 0.44 0.68 0.30 0.72 0.12 0.25 0.52 0.15 0.51 0.01

30 29 31 28 29 27 30 40 22 30 24 23 17 28 30 07

Age from Rossetti et al. (2015).

heating at 5 °C/s, stimulation with blue LED stimulation at 125 °C for 20 s, and heat cut at 200 °C for 10 s. A dose recovery test was performed with doses given between 65 and 175 Gy. Aliquots with recycling test ratios not exceeding 10% unit deviation and < 5% recuperation test of the natural signal were used for the final De calculations. The De values were statistically analyzed using the numOSL package-R software (Peng et al., 2013). The central age model (CAM) (Galbraith and Roberts, 2012), the minimum age model (MAM) (Galbraith and Laslett, 1993) and the finite mixture age model (FMM) (Galbraith and Green, 1990) were used, depending on the values of overdispersion (O.D.) (Figs. 2 and 3). Thus, CAM was used to O.D. < 30% and MAM to O.D. > 30%. The values of De obtained with the MAM generally agreed with those obtained with the FMM. Although the FMM results generally provide more detail on the percentage of grains that are not well bleached, we used MAM for final age calculations because all samples were fluvial sediments. Calibration of the SAR growth curves was performed using the intensity integral of channels 1 to 5, subtracting the BG intensities of channels 200–250. Annual dose rates (AD) were calculated based on natural concentrations of U, Th and K-40 radionuclides. These were determined by gamma spectroscopy using high purity germanium detector (HPGe) on a Canberra Inc. ultralow bottom shield. Sample spectra were compared to those emitted by standard JR-1, JB-3, JG1a

and JG-3 soil samples. The conversion table of Adamiec and Aitken (1998) was used for the conversion of dose rates. Cosmic radiations were theoretically calculated with the equations of Prescott and Stephan (1982). Sedimentation rates provided for several dated depth intervals assumed continuous sediment deposition. However, deposits with large contrasting ages separated by discontinuity surfaces evidenced episodes of discontinuous sedimentation, which were also taken into account in the present analysis. Floristic inventories of trees and large shrubs with stem circumferences 20 cm above the ground ≥10 cm were performed in a total of 14 plots of 100 × 40 m (0.4 ha) placed in areas covered by savanna vegetation. Species of trees were identified in the field based on previous knowledge of the local flora. For each morphospecies, samples were collected for additional comparisons with botanical material from the herbarium of the Brazilian National Institute of Amazonian Research (INPA). The floristic analysis was based on the richness of tree and large shrub species (i.e., on the number of species recorded), and on the diversity of species found in the plots (alpha diversity) and between the plots (beta diversity). The diversity index of Shannon (Shannon, 1948) was used as an estimative of alpha diversity, while beta diversity was calculated as proposed by Whittaker (1972). The complete floristic database used in this work is included in Supplement 2. Richness and

Table 3 Samples dated by the OSL method, with corresponding depths, recycling rates (RC), recovery rates (RP), overdispersion (O.D.), equivalent doses (De), total annual dose rates (AD), number of aliquots used in the evaluation of De per sample (N), and ages. Sample PV4a PV4a PV4a PV5 PV5 PV12a PV12a PV12a PV50A PV50A PV52A PV52A PV53A PV60Aa PV60Aa PV111Aa

Depth (m) 4.8 6.3 7.7 8.6 9.7 5.6 6.8 7.7 4.8 8.7 4.3 4.9 4.9 4.3 5.7 0.4

RC rate

RP rate

O.D. (%)

De (Gy)

AD (Gy/ky)

1.020 0.970 1.050 0.971 0.981 0.973 1.040 1.010 0.922 0.945 0.890 0.977 0.989 0.934 0.944 0.823

0.330 0.250 0.200 0.245 0.239 0.106 0.100 0.098 0.245 0.234 0.244 0.256 0.345 0.256 0.345 0.382

20.0 19.8 19.2 19.1 18.9 18.2 16.4 10.2 9.2 7.5 14.6 11.8 10.3 9.2 10.2 49.3

213 ± 28 184 ± 16 259 ± 34 261 ± 25 284 ± 15 154 ± 33 146 ± 18 195 ± 19 123 ± 08 221 ± 10 112 ± 07 133 ± 08 105 ± 08 105 ± 06 108 ± 05 1.65 ± 0.06

3.94 2.17 2.77 3.37 3.46 2.36 1.57 1.64 2.86 2.66 2.78 2.84 2.01 2.87 3.01 1.73

N = number of measured aliquot. a Age from Rossetti et al. (2015). 140

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

2.25 0.94 1.00 0.69 0.13 0.35 0.28 0.39 0.25 0.57 0.12 0.21 0.31 0.14 0.41 0.34

N

Age (ky)

16 16 16 26 25 16 11 9 26 33 26 27 26 29 26 26

54.0 ± 7.6 84.9 ± 8.3 93.5 ± 12.7 77.5 ± 7.6 82.1 ± 5.3 65.4 ± 16.9 92.9 ± 20.2 118.9 ± 30.7 43.0 ± 4.7 83.9 ± 18.3 40.2 ± 3.0 46.8 ± 4.4 52.2 ± 9.0 36.6 ± 2.7 35.9 ± 5.1 1.1 ± 0.04

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A

B

1,100 Natural 50ß

900 800 700

Lx/Tx

OSL (cts per 0.08 s)

1,000

600 500 400 300 200 100 0 0

2

4

6

8

10 12 Time (s)

14

16

26 24 22 20 18 16 14 12 10 8 6 4 2 0 0

18

50

100 150 Dose (s)

200

250

SAMPLE PV111A

C

CAM De=2.35 ± 0.19 Gy O.D.= 37.7% BIC= 33.00259 Maxlik= -13.2432

Standardised Estimate

4

3

2 0 -2

2

0

20

10

5

10

Relative Error (%) 6.7 5 15 Precision

20

4

3.3

25

30

D

MAM De=1.65 ± 0.06 Gy BIC= 25.03116 Maxlik= -7.62844

Standardised Estimate

4

3

2 0 -2

2

0

20

10

5

10

Relative Error (%) 6.7 5 15 Precision

20

4

3.3

25

30

E

FMAM P 57% 30% 12% BIC=32.64094

3

2

De (Gy)

Standardised Estimate

4

0

De 1.69± 0.05Gy 3.11±0.07 4.92±0.25 Maxlik= -8.17523

-2

2

0

20

10

5

10

Relative Error (%) 6.7 5 15 20 Precision

4

3.3

25

30

Fig. 2. OSL data provided by a single aliquot of pure coarse grained quartz for the youngest sample (PV11A) of the study area. A) Shine-down curve. B) OSL growth curve. C–E) Radial plots of the distribution of De using the central age model (CAM) (C), minimum age model (MAM) (D) and finite mixture age model (FMM) (E).

141

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diversity estimates of Shannon as well as beta diversity are presented in Supplements 3 and 4.

Savanna 1, Savanna 2 occurred at an averaged SRTM topographic level at least 5 m lower (Fig. 4). The morphology of Savanna 2 was also dissimilar, comprising a single and essentially flat, WNW-trending plateau (Fig. 5D) of homogeneous texture under the SRTM-DEM view. A notable feature of Savanna 2 was its shape, defined by orthogonal NEand NW-trending rectilinear morphostructural lineaments, some extending into surrounding areas of the rainforest (see dashed lines in Fig. 5D). The lineaments were particularly observed in the western sector of this savanna, where they were places for the entrenchment of some modern short rivers that drain of this plateau. The Savanna 2 substrate also comprised sands and muds, but with a greater proportion of the latter. These strata internally contained episodic, coarsening/thickening upward successions (e.g., PV68A, PV68A1, PV111A, and PV113A; Fig. 8A) delimited by discontinuity surfaces on top of massive and hardened deposits with root marks and rootlets, which were attributed to paleosols. Similar to Savanna 1, the basal interval consisted of a fining upward sandy succession with a sharp base (PV60A; Fig. 8A), but mud deposits were also present (PV111A and PV113A). The ages of the sandy lithologies were younger than those recorded in PV4, i.e., 36.6 ± 2.7 to 35.9 ± 5.1 ky (Tables 2 and 3). The intermediate and upper intervals were 20,900 to 9700 cal yr BP old and < 4600 cal yr BP, respectively. It is interesting to note the lowering of the three stratigraphic units from east to west in the Savanna 2. A point of interest with respect to the Savanna 2 substrate was the abundance of microfractures with slickensides (Fig. 8B–C), often associated with contorted and undulating layers (Fig. 8D). These features were particularly observed in sections PV60A and PV111A, where morphostructural lineaments were also more frequent. Savanna 2 presented intervals with similar but slightly more variable sedimentation rates than the Savanna 1, ranging from 0.10 to 0.40 mm/yr (Table 4).

4. Results 4.1. Faciological and morphological descriptions The four savanna patches occurred on Late Pleistocene-Holocene substrates, with Savannas 1 and 2 being located in the T1 unit and Savannas 3 and 4 in the T2 unit (Fig. 4). The sedimentary deposits consisted of sand and mud lithologies that formed three stratigraphic intervals with ages ranging from ~119 ky to 34,7 cal yr BP, 25,724 to 9755 cal yr BP, and < 73,435 cal yr BP (Fig. 4B). In spite of the lithological similarities, the savanna substrates presented some sedimentological and morphological peculiarities of relevance to analyze their evolution and potential influence on the growth of the associated savanna vegetation, as discussed in the following. 4.1.1. Savanna 1 Savanna 1 (Figs. 5A–C, 6) of unit T1 consisted of a north-south, elongated and sinuous, open vegetation belt in sharp contact with adjacent dense forest (Fig. 6A). The savanna belt was approximately 120 km in length and over 10 km in width, and formed an overall dendritic network of branches converging to the south. In addition, its southern edge ended abruptly 20 to 30 km northwest of the Madeira River. The analysis of high resolution WebGLEarth images revealed that the Savanna 1 nucleus was supported by a sinuous to slightly meandering morphology, also with branches converging to the south, which resembles the shape of many fluvial channels in plain view (Fig. 6B, C). DEM-SRTM topographic profiles transverse to these features revealed concave reliefs with lower topographic values in relation to their surroundings (profiles i-i′, ii-ii′, iv-iv′ of Fig. 6D); however, profiles crossing the southern sector of this savanna patch, where the channels were no longer perceptible, had an overall convex morphology (profile iii-iii′ of Fig. 6D). Another marked observation was a number of discontinuous and sharp bounded terrains in the west sector of Savanna 1, which were slightly higher, more dissected and imprinted by a higher number of drainage channels, compared to other areas within this patch (Fig. 6E, F). The Savanna 1 substrate (Fig. 7) consisted of a lower 4-m thick, fining upward succession of fine to coarse grained sands with contrasting older ages between 93.5 ± 12.7 and 54.0 ± 7.6 ky (Figs. 4 and 7; Tables 2 and 3). These deposits were overlain by the intermediate and upper intervals that consisted of packages up to 5-m thick of parallel laminated or massive muds, heterolithic bedded sand/mud deposits and massive muddy sands. These deposits, which may show plant fragments, formed coarsening/thickening upward successions defined by tops locally hardened and with abundant roots and/or root marks. Sharp based, fining upward, medium to very fine-grained sands (up to 1 m thick) occurred in the upper interval of thetop of two profiles (e.g., PV65A1 and PV65A2 in Fig. 7). An interval < 1 m thick of fining upward sandy to heterolithic deposits was also recorded in the intermediate interval of PV4, being overlapped by an essentially massive mud package of 4.5-m thick. All ages in the intermediate and upper intervals of Savanna 1 substrate decreased consistently upward. An exception was PV4, which recorded a mean age of 22,200 cal yr BP below a mean age of 22,500 cal yr BP. The age variation in the intermediate interval ranged from 22,500 to 13,100 cal yr BP, while the upper interval recorded only ages younger than 2800 cal yr BP (Fig. 4). However, the variation for the first age between 22,000 and 22,400 cal yr BP was within the error range of the latter age, between 22,300 and 22,600 cal yr BP. Sedimentation rates calculated from these deposits ranged from 0.10 to 0.26 mm/yr.

4.1.3. Savannas 3 and 4 These savannas occurred in the same geomorphological unit T2, located closer to the Madeira River in relation to the T1 unit (Fig. 5A). Savanna 3 (south) was approximately equidimensional, but Savanna 4 (north) trended in the N-S direction. Most strikingly, both savannas were found at an average topographic level of 12 m lower than Savanna 2 (Fig. 4). Although disconnected, they developed on substrates with sets of parallel semicircular growth lines (Fig. 5D–F). It is interesting to note that these features were also perceptible in the forested area adjacent to Savannas 3 and 4 (see rectangle in Fig. 5D). Savanna areas with growth lines were in sharp contact with savanna areas exhibiting a morphologically homogeneous surface in the remote sensing products (Fig. 5E–F). Savannas 3 and 4 had comparable distribution of sand and mud deposits. Thicker (i.e., up to 2 m thick) sand beds were locally sharp based and arranged in fining upward successions, while mud intervals contained numerous coarsening/thickening upward sandy layers (Figs. 4 and 8). The latter were often topped by hardened and massive beds with root marks related to paleosols. The lower and intermediate portions of the geological sections of Savannas 3 and 4 consisted of sandy successions with ages ranging from 82.1 ± 5.3 to 40.2 ± 3.0 ky and 118.9 ± 30.9–43.0 ± 4.7 ky, respectively (Figs. 4B, 9 and 10; Tables 2 and 3); exceptionally, PV5 basal sands graded upward into a mud interval with ages of 42.6 and 34.7 cal yr BP. These ancient strata were unconformably covered by two intervals of fining or coarsening upward interbedded sands, muds and heterolithic bedded deposits, which were separated by a discontinuity surface. In Savanna 3, this surface had a relief up to 3 m and separated a lower, 2.5-m thick interval with ages between 18,400 and 16,600 cal yr BP from an overlying 3.5-m thick package < 7300 cal yr BP. Sedimentation rates in these packages ranged from 1.23 (lower) to 0.11–0.55 (upper) mm/yr. Savanna 4 had an intermediate stratigraphic interval up to 7.6 m thick, ages between 25,700 and 11,200 cal yr BP, and even higher sedimentation rates (0.81 to 1.65 mm/yr) than correlated deposits of Savanna 3. It should be noted that the thick deposits of Savanna 4

4.1.2. Savanna 2 Although located in the same geomorphological unit (T1) as 142

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A

B

1,100 Natural 50ß

900

PV11A

800 700

Lx/Tx

OSL (cts per 0.08 s)

1,000

600 500 400 300 200 100 0 0

2

4

6

8

10 12 Time (s)

14

16

18

C

0

A

Standardised Estimate

4

3

2

26 24 22 20 18 16 14 12 10 8 6 4 2 0

0 -2

50

100 150 Dose (s)

200

250

CAM De=2.35 ± 0.19 Gy O.D.= 37.7% BIC= 33.00259 Maxlik= -13.2432

2

0

20

10

5

10

Relative Error (%) 6.7 5 15 Precision

20

4

3.3

25

30

C

MAM De=1.65 ± 0.06 Gy BIC= 25.03116 Maxlik= -7.62844

D Standardised Estimate

4

3

2 0 -2

2

0

20

10

5

10

Relative Error (%) 6.7 5 15 Precision

20

4

3.3

25

30

D

FMM

E

P 57% 30% 12% BIC=32.64094

3

2

De (Gy)

Standardised Estimate

4

0

De 1.69± 0.05Gy 3.11±0.07 4.92±0.25 Maxlik= -8.17523

-2

2

0

20

10

5

10

Relative Error (%) 6.7 5 15 20 Precision

4

3.3

25

30

E

Fig. 3. OSL data supplied by a single aliquot of pure, coarse grained quartz for the oldest sample (PV50A) of the study area. A) Shine-down curve. B) OSL growth curve. C–E) Radial plots of De distribution using central age model (CAM) (C), minimum age model (MAM) (D) and finite mixture age model (FMAM) (E).

coincided with the lowering of the basal discontinuity surface (Fig. 10). In contrast, the upper interval < 5300 cal yr BP in age was < 2 m thick and had sedimentation rates ≤0.4 mm/yr. It is interesting to note that the intermediate and upper intervals were thicker in savannas 4 and 3, respectively (Fig. 4B).

4.2. Floristic data The savannas were floristically distinct when comparing the two geomorphological units, with the most abundant species in T1 consisting of Bonyunia antoniifolia Progel, Ruizterania retusa (Spruce ex 143

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A a´ GEOMORPHOLOGICAL UNIT T1

GEOMORPHOLOGICAL UNIT T2 SAVANNA 3

80

PV 12

SAVANNA 4 PV 3

PV 10

PV 113A PV 82A PV 83A PV 84A PV 51A PV 53A PV 52A PV 5 PV 69A

PV 68A PV 68A1

90

Hs1 Hs2

70 60

Hs3/4

20

10

0

Forest

30

40

50

Grassland/shrubland

60

13,153 84.9±8.3 93.5±12.7

1630 4629 20,941

?

17,685

8371

?

PV

68

735 239 4912

PV

A

8A

67

1.1± 0.04

Stratigraphy

PV6

PV

111 A

A

PV

60

22,471

PV

2396

11 3

A1

A

4 PV

A

7A 18,023

22,225

?

130 km

120

10 PV

2 PV 11 0 7073

54.0±7.6

70

110

100

SAVANNA 2

65A

1 2825

PV

66A PV 65A

PV

109 A PV

75

90

Hs1/4= estimated SRTM height of savanna areas discounting the height of tree canopy

Deforestation

SAVANNA 1

80

80

70

B

< 7345 cal yr BP 25,724 - 9755 cal yr BP 118.9±30.7 ky - 34,657 cal yr BP

9755

?

36.6 ±2.7 35.9 ±5.1

65

SAVANNA 4

SAVANNA 3

50

1450

1027

6066

6404

11,233 16,574

PV 11 9

12 Pv

Pv 4367

2772

7345

963

10

3

50

PV

PV

52 A PV 11 4A PV 71 PV 5A

PV

3A

A 535

PV 5

84 A PV 51

A

infered fault msl mean sea level OSL age (ky) C14 age (cal yr BP)

PV

A 83

55

A

GEOMORPHOLOGIC UNIT T1 60 PV

altitude above msl (m)

PV 67A

PV 60A PV 111A

SAVANNA 2

PV 50A

SAVANNA 1 Paleoria PV 4

altitude above msl (m)

a

5399

?

21,865

19,232 14,548

18,440 17,658 52.2 40.2±3.0 ±9.0 46.8±4.4

15,903

34,657 43.0 ±4.7

18,770

42,600 18,212

77.5±7.6 82.1±5.3

25,724 83.9 ±18.3

65.4± 16.9

92.9±20.2 118.9±30.7

GEOMORPHOLOGIC UNIT T2 Fig. 4. General topography and stratigraphy of the four savanna patches, with the location of the geological sections (PV) along the a′-a″ transect (see location in Fig. 7A). A) The topographic data are from the SRTM-DEM, thus they represent combination of terrain topography and tree heights. Horizontal dashed lines indicate the mean terrain topography, estimated by measuring elevations in non-forested areas and also extracting the average tree heights based on field observations. B) Proposed stratigraphic correlation of the geological sections (horizontal distances between geological sections are not on the scale). For more detailed lithostratigraphic information, refer to Fig. 6.

Warm.) Marc.-Berti, Caraipa savannarum Kubitzki, Simarouba amara Aubl., Byrsonima crassifolia (L.) Kunth, Bellucia acutata Pilg., Byrsonima linguifera Cuatrec., Lacistema polystachyum Schnizl., and Mauritiella armata (Mart.) Burret. (Supplement 2). In contrast, the savannas in the T2 unit exhibited continuous grass cover associated with tortuous, coriaceous trees, often with thick, corky bark. The trees were sparsely distributed or occurred as clumps. The most abundant savanna tree species of this unit were Byrsonima crassifolia (L.) Kunth, Curatella americana L., Byrsonima linguifera Cuatrec., Vochysia haenkeana Mart., Myrcia eximia DC., Guatteria procera R.E.Fr., Hancornia speciosa Gomes, Alibertia edulis (Rich.) A. Rich. ex DC., and Caraipa savannarum Kubitzki. Pioneer forest species, such as Tapirira guianensis Aubl., Simarouba amara Aubl. and Vismia cayennensis (Jacq.) Pers., were common to all savannas patches, apparently being able to invade non-

forest formations under the current environmental conditions. We also found some contrasting floristic parameters comparing the diversity of the four savannas and the two fluvial terraces (Supplement 3). Savannas of the T1 unit, especially Savanna S1, had plots with greater level of differentiation (beta diversity) when compared to the savannas in the T2 unit (Supplement 4). Although the variation of the Shannon index (alpha diversity) was low, the total richness was very different when comparing the savannas of the T2 and T1 units (S = 21 and 28 species in Savannas 3 and 4, respectively, versus S = 61 and 43 in Savannas 1 and 2, respectively). As a whole, the nine floristic plots of Savanna 1 were very different from each other, exhibiting much greater diversity (beta diversity) than those observed in the other inventoried areas (Fig. 11). The tree and shrub flora of Savanna 2 seems to be a subsample of the richer and more heterogeneous vegetation types of 144

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07oS

D.d.F. Rossetti, et al.

A Fig. B 4 1

s ru Pu

ve

Ri r

ve

09oS

i aR

d Ma

3

2

r

eir

50 km

Rainforest Woodland savanna Grassland/shrubland savanna 1-4 Studied savannas o

o

63 W

65 W

B

Fig. E

N

C

Savanna 4 1

2

3

Fig. D r Rive a r i e 20 km ad

r Rive a r i e 20 km Mad

M

D

PV60A PV111A

5 km

E

F

Fp Fp Ch Fig. F 10 km

10 km 145

Fig. 5. Morphological characterization of the patches of savanna studied (1–4) in the eastern margin of the Madeira River. A) General view of the savannas, with the inside box locating the study area. Observe the dendritic pattern of Savanna 1, as well as of other savannas non-studied to the west, formed by branches that converge southward (Modified from Bertani et al., 2015). B, C) General aspect of the four studied savannas visualized in a Landsat-TM image with composite colours (magenta = grassland/shrubland; green = dense rainforest) (B) and SRTM/ DEM in gray scale (C). D) Detail of B, illustrating Savannas 2 and 3. Note the location of PV111A in a central area of Savanna 2, where there are several rectilinear and orthogonal morphostructural lineaments (dashed lines) possibly resulting from tectonic reactivation (rectangle indicates numerous meander growth lines running north from Savanna 3 to Savanna 4). E) Detail of B, illustrating Savanna 4. F) Detail of Savanna 4, with several broad semicircular meander growth lines related to channel paleolandforms (Ch) in sharp contact (dashed line) with amorphous areas of abandoned floodplains (Fp). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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A

C

B i

i´

Fig B ii´

ii

Fig E

iii´ iii

D i altitude above msl (m)

i´

i´

paleoria

70

E

F

60 0

3

6

ii 75 70 65 0

paleoria

9

iv´

ii` iv higher terrain

3

iii

paleoria

6

9

iii´ i´

80 70 60 0

10

iv

older higher terrain

78 76 74 0

20

30

paleoria

iv´

2 (distance - km) 4

Fig. 6. Characterization of the Savanna 1 area. A) General view of the patch (light color) of savanna vegetation, illustrating the geometry of an elongated, dendritic belt formed by branches that converge southward in contrast to the rainforest matrix (darker color). B, C) Detail of a belt segment (see location in A), where a sinuous central channel is present (dashed lines in C highlight the channel). D) DEM-SRTM topographic profiles along four transects transverse to the savanna patch (see location of i-i′, ii-ii′ and iii-iii′ in A and of iv-iv′ in F). E, F) Detail of a belt segment (see location in A) illustrating the morphology of the terrain located at a relatively high topographic position on the western margin of the patch. (A–C and E, F are high resolution optical images from WebGLEarth™).

Savanna 1 (Fig. 11). Savannas 3 and 4 harbored communities of trees and large shrubs very similar to each other (i.e., with low bet -diversity) and considerably differentiated from the inventoried savannas in the T1 unit.

environments. The sedimentological and morphological characteristics of the savanna substrates studied allowed proposing different origins. The concave topographic profiles in the central and northern sectors of Savanna 1 (profiles i-ii, ii-ii′ and iv-iv′ in Fig. 6D) attest to a residual channel system. The thin, erosion-based, fining upward sandy succession superimposed by a thicker muddy interval, as recorded in PV4, is consistent with the deactivation of a short-lived channel, followed by a long episode of mud deposition. However, mud settling occurred during low energy flows in a channel environment, rather than along floodplains, as indicated by the mud deposits related to the south trending, dendritic paleolandform disconnected from the Madeira River. A previous publication (i.e., Bertani et al., 2015) related this feature to a fluvial ria, which is an interpretation consistent with the morphology of many ria valleys that typify the Amazonian wetlands. Similar to the Savanna 1 substrate, numerous fluvial rias in this region consist of complex depositional systems, which are also characterized by fluvial valleys with central channels separated from other subaqueous environments by marginal levees (Fig. 12A). As sandy deposition ends, the central channels are filled by mud that were either transported along the river from distant areas or scoured from numerous creeks of

5. Discussion 5.1. Sedimentary environments The integration of sedimentological and geomorphological data allowed the construction of the past environmental conditions that led to the origin of the fluvial deposits studied in the middle Madeira River. Thus, the thickening upward sandy successions at the base of the sections point to deposition by relatively high energy flows. The various stratigraphic intervals of massive and hardened beds, markedly overlain by discontinuous surfaces with abundant roots and/or root marks, were related to frequent subaerial exposure and soil formation. These characteristics are consistent with mud deposition from suspensions in floodplain environments, with possible sand inflows through crevasse splays. The interbedded finning/thinning upward sandy successions were attributed to deposits shaped by decreasing flows in channel 146

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A

PV119A

a`` Pv12

B Pv10

0

PV50A

2

200 100

Depth (m)

4

0

Altitude (SRTM) m

PV109A

PV113A PV60A

PV66A PV65A1

PV 03 PV71A PV 5 PV114A

PV52A PV53A PV51A PV84A PV67A PV83A PV68A1 PV68A

PV111A

PV65A2 PV 04

a´

PV107A

PV110A

r ive

aR

PV110A

Ma

75 masl

ir de

PV65A1

30 km

PV 4

PV65A2

2396

7073 Mud Sand

6

PV107A

2

18,023

PV109A 2825

Mud Sand

22,471

4 Mud Sand

8

PV66A

Mud Sand

13,153 54.0 ±7.6

Mud Sand

10

22,225

84.9 ±8.3

Mud Sand

93.5

ud

M

Sa

nd

PV78A2 ±12.7 Massive gravel Lenticular/streaky heterolithic deposit Cross-laminated or cross-stratified sand Massive sand Mud Sand Massive/motled mud Muddy sand Rootlet Parallel-laminated mud Plant remain (disperse) Paleosol locally with concretions Luminescence age (ky; *saturated sample) of iron oxides/hydroxides Radiocarbon AMS age (cal yr BP) Microfracture with slickenside Discontinuity surface Distorted bed masl Meters above modern mean sea level Coal fragment Ferruginous concretion Grain size Clasts of ferruginous concretion

Fig. 7. Location and chronostratigraphic characterization of the geological sections along the studied savanna patches. A) Location of the geological sections plotted on SRTM-DEM background. B) Geological sections with the sedimentological characteristics and representative ages of the Savanna 1 substrate.

adjacent floodplains. The development of ria valleys may produce deposits sedimentologically and morphologically similar to those of Savanna 1 substrate. The mud succession superimposed on PV4 channel deposits was

related to the time when a tributary system of the Madeira River disconnected from the stem stream. This process would have disrupted the river flow and promoted flooding along the valley, which evolved into a low-energy, mud dominated ria lake system, an event already 147

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PV68A1 Mud Sand

A 0

PV111A

PV60A 0

1630

PV67A

1.1±0.04

4912 8371

17,685

Fe Fe

Mud Sand

20,941

Depth (m)

72 masl

9755

?

2

PV68A

239 735

4629

2

PV113A

Mud Sand

B 4

4

36.6 ±2.7 Mud Sand

6

35.9 ±5.1

2 cm

C

6 Mud Sand

Mud Sand

2 cm

D

2 cm Fig. 8. Characteristics of the Savanna 2 substrate. A) Geological sections with sedimentological characteristics and ages (see Fig. 7A for location and Fig. 7B for legend). B–C) Sedimentological details of the deposits, illustrating muddy successions with a polished slickenside plane that form lineations (double black arrow) oblique to bedding (double white arrow). C) Plane marked by two sets of grooved striae nearly orthogonal (see arrows). D) Contorted (undulating) bedding (dashed lines).

established in ~22,000 cal yr BP. The sharp bounded, topographically higher terrain in the west sector of Savanna 1 (Fig. 6E, F) marks the boundary of the ria valley where sedimentation probably ended earlier. The convex topography with corresponding coarsening upward deposits of the southern sector of the ria paleolandform (profile iv-iv′ of Fig. 6D) was related to small deltas. The modern rias of the Amazon basin may present upstream flows with high sediment loads during flooding, which favors river deltas (Fig. 12B). The discharge of high energy flows into the low-energy ria environment caused the rapid decrease in flux, compared to many deltaic deposits around the world (e.g., Galloway and Hobday, 1996). This process may have naturally formed the convex deposits internally marked by coarsening upward successions of the Savanna 1 substrate. We therefore interpret that part of the ria valley was also filled with sediments sourced from the Madeira River through local inlets, a process that formed internal deltas. The Savanna 2 substrate records a distinct depositional

environment. The prevalence of mud deposits marked on the top by paleosols suggests sedimentation from suspensions in a shallow basin, where subaerial exposure was frequent, as is typical of floodplains. Considering this environmental context, coarsening/thickening upward successions were probably formed by the episodic entry of silts and sands during flooding, thus recording the deposition of sediments along marginal levees or crevasse splays. The interval with fining upward sands from the base of the PV60A is compatible with flow decrease within the confinement of a channel environment. The fining upward sand successions locally with abrupt bases and frequently alternating muddier deposits with paleosols of Savannas 3 and 4 were related to a site where the deposition of sand within channels alternated more frequently with mud settling along floodplains than in Savanna 2. However, the floodplain environment of Savannas 3 ad 4 was also marked by episodic sand inflows, which formed the numerous coarsening/thickening upward cycles related to 148

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Table 4 Sedimentation rates calculated for individual depth intervals dated of the studied deposits between discontinuity surfaces. Depositional environment

Measured time range and sedimentation rates (mm/yr) per depth interval (m) in individual cores Savanna 1

Savanna 2 a

Savanna 3 a

High-energy sandy channel

93,500–54,000 (0.38) 6.3–4.8 PV4 (0.16) 7.7–6.3 PV4

36,600–35,600

Low energy, muddy abandoned channel and floodplain complex

22,471–2396 (0.10) 2.6–0.6 (0.25) 0.6–0.0 (0.11) 2.0–0.0 (0.18) 2.5–0.6 (0.26) 0.6–0.0 (0.21) 1.5–0.0

20,941–239 (0.10) 2.2–0.7 (0.24) 0.4–0.0 (0.20) 1.3–0.6 (0.40) 0.3–0.1 (0.15) 1.5–0.0 (0.40) 0.4–0.0

PV4 PV4 PV107A PV109A PV109A PV110A

Savanna 4 a

PV60A PV60A PV67A PV67A PV68A PV111A

82,100–34,600 (0.09) 4.9–4.3 PV52A (0.23) 9.7–8.6 PV5 (0.06) 8.6–6.8 PV5 (0.27) 6.8–4.7 PV5 18,400–535 (0.11) 0.8–0.0 PV5 (0.55) 1.0–0.2 PV51A (0.37) 0.2–0.0 PV51A (0.21) 1.3–0.0 PV52A (1.23) 4.8–3.8 PV53Ab (0.20) 0.2–0.0 PV53A (0.20) 1.4–0.3 PV114A (0.31) 0.3–0.0 PV114A

118,900–43,000a (0.09) 8.7–4.8 PV50A (0.03) 7.7–6.8 PV12 (0.04) 6.8–5.6 PV12 25,724–2772 (1.48) 8.3–2.6 (0.40) 0.8–0.4 (0.09) 0.4–0.0 (1.65) 4.6–0.8 (0.05) 0.8–0.0 (0.81) 4.3–0.9 (0.06) 0.9–0.0 (0.17) 2.9–1.5 (0.11) 1.5–0.6 (0.22) 0.6–0.0

PV3 PV3 PV3 PV10 PV10 PV12 PV12 PV50A PV50A PV50A

Ages = 14C cal yr BP. a OSL (yr BP). b based on correlation of an age at 2.7 depth of PV52A, located < 800 m apart.

many deltaic environments around the world (e.g., Galloway and Hobday, 1996).

marginal levees and/or crevasse splays. The sets of parallel semicircular growth lines on the surface of Savannas 3 and 4 were related to ridges and swales of scroll bars from depositional sides of meandering belts. In summary, an important aspect of the study area is the sandy unit of the base of most geological sections formed between 118,900 ky and 34,600 cal yr BP, when high-energy channels prevailed. In general, these deposits contrast with superimposed Late Pleistocene-Holocene muddy strata, formed after 25,700 cal yr BP, indicating deposition in progressively lower energy environments. Also relevant are the ria lake deposits of Savanna 1, related to the time when a tributary system of the Madeira River disconnected from this stream. This process would have disrupted the flow of the river, with the consequent flooding of the valley that evolved into the low-energy, mud-dominated ria lake system, an event already initiated ~22,000 cal yr BP. Part of the ria valley may also have been filled with sediments of the Madeira River through local inlets, as evidenced by the delta and crevasse splay deposits. High energy flows that enter low energy ria environments rapidly decrease to form depositional lobes similar to those that typify 0

5.2. Tectonics as a potential control of sedimentary dynamics The previous discussion suggests that the area adjacent to the middle Madeira River was subject to dynamic fluvial processes during the Late Pleistocene-Holocene, with frequent changes in depositional environments over time. The most impressive was the shift from a coarser-grained, high-energy fluvial system between 118.9 ky and 34,700 cal yr BP to a progressively muddier, lower-energy river after 25,700 cal yr BP. This fluvial pattern indicates a marked paleohydrological change, characteristic also reported in other areas of Amazonia and related to climatic fluctuations (e.g., Latrubesse and Ramonell, 1994; Latrubesse, 2002). However, an earlier publication suggested that coarse grained river deposits from different Amazonian locations are not equivalent in time (Rossetti et al., 2014). Based on the integration of detailed analysis of sedimentary facies and radiocarbon

58 masl

PV71A PV53A

PV51A

PV52A

535

PV84A

PV114A

1027

7345*

963

1450

2

PV 5

Mud Sand Fe

6066

6404

16,574

Fe

Depth (m)

PV83A Mud Sand

4

Fe Fe

Mud Sand

17,658

Mud Sand

6

18,440 52.2 ±9.0

Mud Sand

8

40.2 ±3.0 46.8 ±4.4 Mud Sand

Mud Sand

34,657

42,600

77.5 ±7.6 82.1 ±5.3 Mud Sand

Fig. 9. Geological sections with the sedimentological characteristics and chronology of the Savanna 3 substrate (see Fig. 7A for location and Fig. 7B for legend). 149

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PV 3

PV50A

PV119A 58 masl

0

4367 5399

2772

? FAB

11,233

2

PV12

21,865

PV10 19,232

4

Depth (m)

Fe

14,548 15,903

43.0 ±4.7

Fe

Fe

6 18,770

25,724 Mud Sand

8

65.4 ±16.9 18,212

Mud Sand

83.9 ±18.3

Mud Sand

92.9 ±20.2 118.9 ±30.7

10 Mud Sand Mud Sand

12 Fig. 10. Geological sections with the sedimentological characteristics and chronology of the Savanna 4 substrate (see Fig. 7A for location and Fig. 7B for legend).

evolution of the middle Madeira terraces. The stratigraphic and morphological characteristics described here also lead us to invoke a tectonic influence to the origin of the savanna substrates. The most striking evidence of tectonic displacement is provided by the topographic location of the Savannas 3 and 4 substrates with respect to the others. The occurrence of Savannas 3 and 4 in a geomorphological unit that stands 17 m and 12 m below Savannas 1 and 2, respectively (Fig. 4B), cannot be related simply to terrace downcutting due to sedimentary processes. In this case, younger ages would be expected in the basal sandy unit in Savannas 3 and 4. Instead, the fact that the lower stratigraphic interval in these savannas record same range of age variation than these strata in Savannas 1 and 2 is more consistent with the tectonic displacement of a sedimentary substrate previously deposited throughout the area (Figs. 4B and 13A, B). This interpretation supports the model of Rossetti et al. (2014), which theproposed that evolution of the terraces along the middle Madeira River was due to tectonic reactivations, as previously presented in Section 2. It is also in agreement with the neotectonic framework proposed for the Amazonian lowlands (Costa et al., 2001; Bezerra, 2003; Silva et al., 2007; Hayakawa et al., 2010), expressed in the middle Madeira River by tectonic reactivations along the NE-SW trending, Madre de Dios-Itacoatiara Transcurrent Zone (Souza-Filho et al., 1999). There are many other evidence of neotectonic reactivations throughout the Amazonian lowlands (see a review in Rossetti, 2014), including a seismite in the middle Madeira basin region, dating to ~1300 cal yr BP (Rossetti, 2017). The creation of new space for sediment accumulation through tectonics is consistent with the higher sedimentation rates (i.e., up to 1.65 mm/yr; Table 4) recorded between 25,700 and 14,500 cal yr BP in Savannas 3 and 4, particularly in the latter case where the relief of the basal discontinuity surface is lower (i.e., PV3, PV10 and PV12). The progressively smaller thicknesses of the deposits formed within this time frame in Savanna 3 (i.e., between PV5 and PV53A; Fig. 9) are attributed to erosion by channel scouring, a process that is attested by

α-diversity β-diversity 4 43 28

Diversity

3

62

21

81 36

2

1

S4

0 T1 ) T2 (953/9 ) /5 3 (81

S2

S1

T1 ) T1 (596/6 ) T2 /3 7 (35 T2 82/2) (2 ) (531/3

S3

plots) al unit ologic /number of h p r o Geom s sampled of tree r e (numb

Fig. 11. Mean alpha (black bars) and beta diversity (white bars), and total richness (numbers above bars) of trees and large shrubs on the four savannas (S1, S2, S3 and S4) inventoried. These data are presented individually for savannas S1, S2, S3, and S4 (four graphs to the right) and for the total data acquired in the T1 and T2 geomorphological units (two graphs to the left). Information about the T1 or T2 unit where each savanna occurs is also included in this graph. The geomorphological terrace on which each savanna occurs is also indicated.

chronology, these authors also showed that erosion and depositional episodes in the instance of the Madeira terraces can not be explained only by fluctuations in base level due to climatic fluctuations, and they. As a result, they provided evidence to claim a tectonic influence in the 150

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A

5 km

B

Manacapuru

Solimões River

5 km Fig. 12. Modern fluvial rias of the Amazonian lowlands. A) A fluvial ria complex showing an embedded inner channel marked by marginal levees and ana abandoned channel segment (arrows). B) The mouth of a fluvial ria locally connected to the Solimões River by a narrow channel to the north, near the town of Manacapuru. Sediments of the Solimões River passed through this inlet to form several fan-shaped deposits within the fluvial ria (area inside the square). See text for more explanation.

the superimposed sharp-based, fining upward sand units. The sedimentary record at the top of all profiles in this savanna, and also in Savanna 4 (Figs. 9 and 10), reveals low sedimentation rates in standing water environments, such as abandoned channels and floodplain complexes, during the last ~7000 yr BP. Although savannas 3 and 4 form discontinuous patches, their comparable semicircular meander growth lines (Fig. 5D–F) suggest that their substrates were formed by similar depositional processes. The presence of such morphologies also in the forested area between the two savannas evidences the deposition by fluvial meanders that extended beyond the savanna areas. On the other hand, the morphologically homogeneous surface of these savannas where the scroll bar morphologies are missing (Fig. 5E–F) are probably related to adjacent floodplains. Thus, the most likely is that the uppermost deposits and associated morphologies on the surface of these savannas were formed when the Madeira River was still scouring west of its modern location (Fig. 13B). Although the substrate of Savanna 2 could simply record a partlyfilled palaeo-valley, a hypothesis that cannot be rule out, the foregoing discussion leads us to present the alternative interpretation that the substrate of Savanna 2 might have been formed in a low energy basin of the T1 unit, probably due to local active tectonics (Fig. 13B, C). The tectonic influence invoked here is consistent with the orthogonal shape

of Savanna 2, defined by rectilinear and lineaments, which are features related to tectonically-controlled morphostructural anomalies (e.g., Howard, 1967; Ouchi, 1985; Doornkamp, 1986; Deffontaines and Chorowicz, 1991). The fact that the lineaments in Savanna 2 parallel main NE- and NW-trending regional tectonic structures that typify the Amazonian lowlands (e.g., Souza-Filho et al., 1999; Bezerra, 2003; Costa et al., 2001) reinforces this hypothesis in the instance of the study area. The various rectilinear morphostructural lineaments in surface sediments as young as a few hundred years, as recorded in the top of sections PV60A, PV67A and PV111A (Fig. 9A), are consistent with the proposed tectonic influence. A tectonic origin could also explain the microfractures locally with slickensides described in the sedimentary substrate of Savanna 2. In fact, similar features can also be formed by pedogenesis, and the abundance of paleosols in the study area could be used to suggest such an origin. However, the slickensides of this instance could not be related to pedogenesis because they were found in stratigraphic intervals where paleosols were not present (e.g., see profiles PV60A and PV119A in Figs. 8 and 10, respectively), and they were absent in the intervals where paleosols were described. On the contrary, the restriction of fractures and slickensides to deposits with distorted bedding, added to their occurrence in areas with abundant morphostructural lineaments parallel to regional tectonic trends, are

151

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A

Madeira River

Purus River

Madeira River

B

Purus River

S1

S2

S3 S4

Madeira River

C

Purus River

Pre Late Pleistocene fluvial deposits (118,900-35,600 yr BP) Late Pleistocene fluvial deposits (25,724-~22,000 cal yr BP) Late Pleistocene fluvial and ria deposits (<22,000 cal yr BP) Regional tilting Savanna patch Paleochannel Water Meander lines

Amazonian rainforest tree Open species sourced from Amazonian biome Open species sourced from cerrado biome

Fig. 13. Schematic diagram illustrating the proposed tectono-sedimentary evolution of the studied area in the western margin of the Madeira River (S1 to S4 = Savannas 1 to 4; see text for further explanation).

characteristics that reinforce the tectonic influence in the study area. Taking into account the invoked tectonic influence, then the 5 m variation between mean elevations within Savannas 1 and 2, as suggested by the SRTM data (Fig. 4), might not be due to errors, as would be natural for local data. Although this value is derived from SRTM

data, which are generally described as presenting errors > 5 m (e.g., Rodriguez et al., 2006), the local sources of C-band disturbances (e.g., canopy topography) tend to be constant when considering a broader extension (Valeriano and Rossetti, 2017). Thus, the lower location of Savanna 2 might suggest an area of subsidence. However, this 152

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interpretation is inconsistent with the low sedimentation rates recorded in Savanna 2, as subsiding areas are expected to have higher rates of sediment accommodation. In addition, the fact that this terrain functions as the head of modern drainage (Fig. 5D) is most suitable with a tectonic high. Alternatively, Savanna 2 may record a flat plateau preferably dissected at the edges by the entrenchment of channels. Therefore, the sedimentological and chronological data, integrated with the morphological context based on the analysis of remote sensing products, revealed that the four savannas substrates show lithologically similar sedimentary successions deposited within the same chronological range. However, as discussed above, they differ somewhat in terms of depositional environment or topographic location in relation to the modern stem stream, the latter being most likely controlled by river dynamics under the influence of tectonic reactivations.

biome (Cohen et al., 2014). Thus, the hypothesis that the cerrado species of the T2 unit were transported from existing neighboring areas of cerrado vegetation into the Amazonian biome over time, instead of being a residual vegetation formed by local colder/drier climate episodes, should be also accounted for. Regardless of whether the cerrado species were transported from neighboring areas or record in situ colonization favored by drier climates, their restriction only to the T2 unit reveals the influence of some ecological process. We argue that the cerrado species preferentially occupied the T2 unit as a response of geomorphological and/or hydrological characteristics resulting from the tectono-sedimentary evolution of individual savanna patches. Despite representing a terra firme area in the modern environment, the T2 unit constituted a surface where the Madeira River meandered before its eastward shift to the present position (Fig. 13B, C). The numerous paleomeanders described in the T2 unit evidence this past dynamics of the river, also demonstrated by geomorphological data collected downstream of the study area (Hayakawa et al., 2010). Over the geological time, this scenario would have maintained the T2 unit as an open area within the matrix of the rainforest. As the ecosystem stabilized and sedimentation ceased, many new environments were available for the eventual colonization of some cerrado species (Fig. 13B, C). However, the soils in the T2 unit are still under hydrological stress due to exceptional floods, such as the floods of 1997 and 2014, which kept submerged areas larger than 20 km from the main river for several weeks or even a few months (cf. http://maisro.com.br/sipam-garante-que-nova-enchente-recorde-domadeira-so-daqui-a-180-anos/ and http://www.ceped.ufsc.br/2014cheia-do-rio-madeira-afeta-rondonia-acre-e-amazonas/). Since this environmental disturbance may have occurred in the past, the T2 unit has been a place not suitable for the maintenance of Amazonian rainforest species. The lack of competition with these species is likely to be an important factor in allowing relatively more flood-protected sites, such as scrollbars of meander belts, marginal levees and crevasse splays, to be colonized by tree and shrub species from neighboring cerrados. However, only the species with the greatest adaptability to such soil conditions would have been introduced, which explains the general low richness and low beta diversity of T2 savannas. In contrast, the T1 unit corresponds to more elevated terrains also located well above the base level of the main river in relation to T2. The overall undisturbed environments of the T1 unit contain hydrologically more stable soils that allowed the Amazonian rainforest to remain practically untouched over time (Fig. 13A). Exceptions were open areas of Savanna 1 that are confined to the ria lake paleolandform and the small tectonically controlled fluvial basin of Savanna 2. The concave geometry of the main paleochannel of Savanna 1 nucleus and sedimentary succession younger near the surface of Savanna S2 indicate terrains that remained submerged until recently, and which are still flooded during wet seasons. This perhaps could explain why these savannas are colonized by grasses and shrubs mixed with tree species from the neighboring Amazonian biome that are adapted to open seasonally flooded areas. An earlier publication recorded that tree species from many open areas of the Amazonian plain originated from adjacent mainland forests (e.g., Wittmann and Householder, 2017). Depositional heterogeneities in wetland environments of the Amazonian lowlands and their past geological history, as recorded in this work, have been increasingly recognized as determinants of changes in soil topography and hydrology that shaped floristic patterns (e.g., Pennington and Lavin, 2016; Tuomisto et al., 2016). These studies, added to the results of the present work, lead us to invoke whether the high floristic diversity of the Amazonian biome could have been affected by the numerous sedimentary changes related to the high fluvial dynamics of watersheds through the geological time. This hypothesis, previously raised by many other authors (e.g., Rossetti et al.,

5.3. Tectono-sedimentary history regulating savanna communities The patterns of floristic composition comparing the assemblages of savanna trees of the T1 and T2 geomorphological units are intriguing. This is because, in general, the species found in the savannas of the T1 unit are recorded in open habitats in the Amazon basin and in the Guyana Shield, being rare or absent in the Brazil Central cerrados. In contrast, T2 savannas are physiognomically similar to those known as cerrado stricto sensu of Brazil Central (Eiten, 1972), and some, such as Curatella americana and Byrsonima crassifolia, have a broad neotropical distribution. More studies are needed to identify the possible historical or ecological factors that determined the floristic dissimilarity between the savannas colonizing nearby terraces so close to each other, and the data presented here allow us to discuss some plausible hypotheses. One possibility is that typical cerrados species occur in the savanna patches of the T2 unit as relics of an expanded vegetation in this area during drier paleoclimatic episodes. The influence of past climates on the expansion of savannas over the Amazonia rainforest has been claimed in several works focused mainly on palynological data (Absy et al., 1991; van der Hammen and Absy, 1994; Mayle et al., 2000; Sifeddine et al., 2001). In general, changes in climate have been related to the late Pleistocene glacial/interglacial fluctuations (van der Hammen and Hooghiemstra, 2000; Cohen et al., 2014) and, during the Holocene, to the El Niño Southern Oscillation (Martin et al., 1997) and/or the displacement of the Inter Tropical Convergence Zone (Weng et al., 2002). The origin of the savanna patches of the middle Madeira River was previously assessed using δ13C from the soils, being related to dry climates during the early to mid-Holocene (e.g., Pessenda et al., 2001). However, a more recent publication based on a much larger number of δ13C analyses, combined with C/N, sedimentological and morphological data, revealed that the onset of the savanna patches in this area are neither coeval in time nor synchronous with Holocene dry episodes previously documented in the Amazonian lowlands (Rossetti et al., 2016). This fact, coupled with the confinement of the savanna vegetation to fluvial paleolandforms, led these authors to propose a model of species colonization by topographically controlled hydrological gradients, triggered by sedimentary dynamics. An alternative hypothesis is that the savanna species of the T2 unit were brought into the middle Madeira region from neighboring cerrado areas. This is suggested because the studied savanna patches occur within a sector of the Amazonian rainforest marginal to the northwest expansion of the cerrados of Brazil Central (e.g., IBGE, 2004). These cerrado formations already existed at the end of the Quaternary (Ledru, 2002). In the study area, tropical forest species appear to have coexisted with savannas throughout the Holocene, when cold-adapted, Late Pleistocene plant species, such as Alnus, Hedyosmum, Weinmannia, Podocarpus, Ilex and Drymis, disappeared from the Amazonian forest

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2005; Higgins et al., 2011; Misiewicz and Fine, 2014; Pennington and Lavin, 2016; Tuomisto et al., 2016; Cárdenas et al., 2017), is worthy of further discussion.

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6. Final remarks Despite the great proximity, the patches of savanna vegetation studied in the west margin of the middle Madeira River had communities with marked floristic composition and level of richness. The geological context and sedimentary evolution of their substrates also varied. This work led to propose that hydrological contrasts imposed by the geological history and sedimentary processes explain the establishment of the savanna patches within the rainforest matrix in this area of the Amazonian lowland. It may also justify changes in floristic composition and species diversity when comparing savanna patches of different geomorphological units and perhaps even within individual units. However, the model proposed needs to be tested in the light of further studies aimed at improving the reconstruction of the sedimentary history of individual savanna patches, as well as of their current and past floristic variations. In particular, other studies focusing the characterization of modern and past plant species, the latter based on pollen analysis integrated with δ13C and C/N data, should expanded to better define the extent of geological processes on the origin of the patches of savanna vegetation in the studied region. This approach may also contribute to the discussion of whether the arrival of cerrado species in the savannas of the T2 unit was favored by drier climatic episodes. Despite the open debate, the present work motivates the application of a similar methodological strategy to investigate other areas of savanna vegetation of the Amazonian lowlands. This approach may serve to the purpose of verifying if environmental changes triggered by the complex evolution of the Amazon basin over the geological time have also contributed to increase the diversity of the Amazonian rainforest. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.palaeo.2019.04.017. Acknowledgments The Research Funding Institute of the State of São Paulo-FAPESP (#13/50475-5, #09/02069-2 and 2009/02069-2) provided the financial support to carry out this work. The Brazilian Council for Scientific and Technological Development-CNPq is recognized for providing research grants to DFR, RG, MCLC and SHT. The Geological Survey of Brazil-CPRM collaborated with the researchers providing logistic support during the fieldwork. The technicians Luiz de Souza Coelho and José Ferreira Ramos, both from the Brazilian National Institute of Amazonian Research (INPA), assisted the field inventories. We appreciate the careful review of two anonymous reviewers and of Dr. Paul Hesse, which helped us to improve singificantly the early version of the manuscript. References Absy, M.L., Cleef, A., Fournier, M., Martin, L., Servant, M., Sifeddine, A., Silva, F., Soubié, F., Suguio, K., Turcq, B., van der Hammen, T., 1991. Mise en évidence de quatre phases d´ ouverture de la forêt dense dans le sud-est de L'Amazonie au tours des 60.000 dernières années. Première comparaison avec d'autres régions tropicales. Comptes Rendus Acad. Sci. (Series II) 312, 673–678. Adamiec, G., Aitken, M.J., 1998. Dose-rate conversion factors: update. Ancient TL 16, 37–50. Baraloto, C., Anaud, S., Molto, Q., Blanc, L., Fortunel, C., Herault, B., Davila, N., Mesones, I., Rios, M., Valderrama, E., 2011. Disentangling stand and environmental correlates of aboveground biomass in Amazonian forests. Glob. Change Biol. 17, 2677–2688. Bertani, T.C., Rossetti, D.F., Hayakawa, E.H., Cohen, M.C.L., 2015. Understanding Amazonian fluvial rias based on a Late Pleistocene-Holocene analog. Earth Surf. Proc.

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