Holocene Environmental Changes from the Rio Curuá Record in the Caxiuanã Region, Eastern Amazon Basin

Quaternary Research 53, 369 –377 (2000) doi:10.1006/qres.1999.2117, available online at http://www.idealibrary.com on

Holocene Environmental Changes from the Rio Curua´ Record in the Caxiuana˜ Region, Eastern Amazon Basin Hermann Behling Center for Tropical Marine Ecology, Fahrenheitstrasse 1, 28359 Bremen, Germany E-mail: [email protected]

and Marcondes Lima da Costa Department of Geochemistry and Petrology, Geosciences Center—Federal University of Para´, Campus of Guama´, P.O. Box 1611, 66075-110 Bele´m/Para´, Brazil E-mail: [email protected] Received February 23, 1999

of the Amazon rain forest to biodiversity, the global carbon cycle, and past global changes. The presence of humans in the Amazon Basin during the Holocene and their influence on the Amazon ecosystem is little known. For all of Amazonia, which is larger than the area of Europe, only a few Late Quaternary pollen records have been published (Absy, 1979; Absy et al., 1991; Behling, 1996; Behling et al., 1999; Bush and Colinvaux, 1988; Bush et al., 1989; Colinvaux et al., 1988, 1996; Frost, 1988; Liu and Colinvaux, 1988; Urrego, 1997). Studies on mineralogy and geochemistry of lacustine sediments are available from, e.g., Caraja´s (Soubie´s et al., 1991) and the central Amazon (Irion et al., 1995). To study Holocene vegetation dynamics, climate change, mineralogical and geochemical changes of sediments, and early human settlement and its impact in Amazonia, we selected the Caxiuana˜ region, located between Marajo´ Island and the upper Xingu River in the eastern Amazon Basin. We focused our studies on this region because of the recently (1989) created research station “Estac¸a˜o Cientifica Ferreira Penna” (ECFPn) in the National Forest Reserve Caxiuana˜, where paleoenvironmental studies will provide background information for ongoing multidisciplinary research projects. Furthermore, the Caxiuana˜ region, with an inland bay and blocked rivers, is an example of modern environmental situations that occur in many other regions of the Amazon lowland.

Holocene environments have been reconstructed by multiproxy studies of an 850-cm-long core from Rio Curua´ dating to >8000 14 C yr B.P. The low-energy river lies in the eastern Amazon rain forest in the Caxiuana˜ National Forest Reserve, 350 km west of Bele´m in northern Brazil. Sedimentological, mineralogical, and geochemical dates demonstrate that the deposits correspond to two different environments, sediments of an active river before 8000 14C yr B.P. and later a passive river system. The pollen analytical results indicate four different local and regional Holocene paleoenvironmental periods: (1) a transition to a passive fluvial system and a well-drained terra firme (unflooded upland) Amazon rain forest with very limited development of inundated forests (va´rzea and igapo´) (>7990 –7030 14C yr B.P.); (2) a sluggish river with a local Mauritia palm-swamp and similar regional vegetation, as before (7030 –5970 14C yr B.P.); (3) a passive river, forming shallow lake conditions and with still-abundant terra firme forest in the study region (5970 –2470 14C yr B.P.); and (4) a blocked river with high water levels and marked increase of inundated forests during the last 2470 14C yr B.P. Increased charcoal during this last period suggests the first strong presence of humans in this region. The Atlantic sea level rise was probably the major factor in paleoenvironmental changes, but high water stands might also be due to greater annual rainfall during the late Holocene. © 2000 University of Washington. Key Words: Brazil; Amazon rain forest; Holocene; paleoenvironment; pollen; charcoal; multielement geochemistry; vegetation dynamics; fire history; pre-Columbian settlement; sea level; Holocene transgression.

INTRODUCTION

Holocene paleoenvironmental changes in the immense Amazon region have been poorly studied, despite the significance 369

STUDY AREA

The Caxiuana˜ region (Fig. 1) is characterized by a warm, humid tropical climate without marked dry periods (Nimer, 1989). The climate station Breves is about 100 km east of the study area. Here, the mean annual temperature is about 26°C, the highest measured temperature is 32.9°C, and the lowest is 0033-5894/00 $35.00 Copyright © 2000 by the University of Washington. All rights of reproduction in any form reserved.

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FIG. 1.

Satellite image showing location of the study region of Caxiuana˜. The map shows the location of the study site Rio Curua´.

22.2°C (Moraes et al., 1997). Mean annual precipitation is about 2500 mm (1969 –1980). The Caxiuana˜ National Forest Reserve is located 350 km west of Bele´m (Para´ State) in the lower Amazon region of northern Brazil (Fig. 1). The study region is a relatively flat plain, only a few meters above sea level, which is covered with Amazonian rain forest. The region includes the Baı´a de Caxiuana˜, part of the former Anapu River, which drains the southern region of Para´ State. The Baı´a is an inland bay, about 40 km long and 8 –15 km wide, which is drained by one principal, though relatively small, east-flowing stream. Water of the Baı´a

de Caxiuana˜ flows about 400 km via Baı´a de Portel, Baı´a de Melgac¸o, and Rio Para´ to the Atlantic Ocean. No modern connection exists between Baı´a de Caxiuana˜ and the Amazon River, which lies about 50 km northwest of the study site. A number of small rivers flow into the Baı´a, one of which, Rio Curua´, was chosen for this study (Fig. 1). Baı´a de Caxiuana˜ is mostly shallow, with water depths between 2 and 5 m. The water level is ⬍3 m above sea level. Therefore the Rio Curua´ is a low-energy river, controlled by the sediments of the Baı´a de Caxiuana˜. The studied core from Rio Curua´ was taken about 200 m

HOLOCENE ENVIRONMENTAL CHANGES, EASTERN AMAZON BASIN

upstream from the entrance of the research station ECFPn of the Caxiuana˜ National Forest Reserve (1°44⬘07⬙ S, 51°27⬘47⬙ W). Here, the river is about 40 m wide; during coring in June 1995, it was 5 m deep. Fluctuations between high and low water levels at the Caxiuana˜ station, as recorded by Hida et al. (1997), average 33 cm (December 1995–April 1996). At this location, there is still a tidal influence; the range between low and high tides is approximately 17–21 cm. Highly diverse terra firme (unflooded upland), va´rzea, and igapo´ (seasonally inundated) Amazon forest covers most of the Caxiuana˜ region. About 85% of the forest area is covered by terra firme forest and about 10% by va´rzea and igapo´ forests. On studied plots (13 hectares) at the ECFPn, 2452 plant species with a diameter at breast height of ⬎10 cm have been recorded (Lisboa et al., 1997; Ferreira et al., 1997). The most common tree species of the terra firme forest are Rinorea guianensis (Violaceae), Tetragastris panamensis (Burseraceae), Lecythis idatimon, Eschweilera coriacea and E. grandiflora (Lecythidaceae), and Vouacapua americana (Caesalpiniaceae). Characteristic species of inundated forests (va´rzea) are Virola surinamensis (Myristicaceae), Pachira aquatica (Bombacaceae), and Euterpe oleracea and Mauritia flexuosa (Arecaceae). Small areas of savanna-like vegetation with a dominance of herbaceous plants (Poaceae), formed by inundation or human activity, are also found in this region. The region is very sparsely populated, and secondary forests are restricted to small patches, mainly along the eastern part of the Baı´a de Caxiuana˜ and along the rivers. That humans have lived in this region since pre-Columbian times is evident from several archeological sites with black soils (terra preta), along the shore of the Baı´a de Caxiuana˜ (Kern and Costa, 1997). Thermoluminescence-dated ceramic fragments are no older than 720 yr B.P. (Kern, 1996). METHODS

Fieldwork The sediment core was collected in the center of the river from a wooden platform supported by two inflatable rubber boats by using a modified Livingstone piston corer. The 850cm-long core was taken in 1-m sections of 5 cm diameter. For the collection of modern pollen-rain data, 25 plastic funnel pollen traps (Bush, 1992) were installed in one line (20 m between traps) in the plot of the research station ECFPn. The distance of the pollen traps in the terra firme forest to the core site is about 1 km. The collecting period was from June 1994 to June 1995. Physical–Chemical, Mineralogical, and Geochemical Analysis Eight sediment samples (10 cm thick) were taken from different levels of the core. Measurements include pH and Eh of wet and dry samples, as well as organic matter and NaCl

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contents. The main minerals and accessory minerals were identified by X-ray diffractometry (XRD). The major elements were determined by the classic wet-chemical technique (SiO 2, Al 2O 3, Fe 2O 3, TiO 2, P 2O 5) and by atomic absorption spectrometry (Na 2O, K 2O MgO, CaO). Trace elements were determined by optical spectrography (B, Ba, Be, Ga, Mo, Sc, Sn, Sr, V, Y, Zr), atomic absorption spectrometry (Co, Cr, Cu, Mn, Ni, Pb, Zn), and hydride generation (As and Hg). Pollen and Charcoal Analysis For pollen analysis, 1-cm 3 samples were taken at 20-cm intervals along the core. Prior to processing, one tablet of exotic Lycopodium spores was added to each sample for calculation of pollen concentration (grains/cm 3) and pollen accumulation rate (grains/cm 2/yr). All samples were prepared using standard pollen analytical techniques and acetolysis (Faegri and Iversen, 1989). Sample residues were mounted in a glycerin gelatin medium. The processing of the pollen trap samples followed Behling et al. (1997). Identification of pollen grains and spores was carried out by pollen morphological descriptions published by Behling (1993), Herrera and Urrego (1996), Roubik and Moreno (1991), and Behling’s own reference collection. A minimum of 300 pollen grains (except for a few difficult samples) were counted for the total pollen sum of each sample. This total pollen sum excludes aquatic taxa, fern spores, and the algae Botryococcus. The sample at 640 cm depth and samples below 700 cm depth contain too little (or no) pollen to obtain reliable statistics. Pollen and spore data are presented in pollen diagrams as percentages of the total pollen sum. Carbonized particles (5–200 ␮m) were counted on pollen slides to calculate concentration (particles/cm 3) and accumulation rate (particles/cm 2/yr). Pollen taxa were grouped into broad ecological categories including palms, trees, shrubs, climbers and epiphytes, herbs, aquatics, and ferns. The software TILIAGRAPH was used to plot the pollen diagrams. TILIA was used for calculations and CONISS was used for the cluster analysis of terrestrial pollen taxa (Grimm, 1987). RESULTS

Stratigraphy The 850-cm-long sediment core from Rio Curua´ consists mainly of silty sand (850 – 830 cm depth), sandy silt (830 –700 cm), clayey silt (700 –595 cm), and fine detrital mud (595– 0 cm) (Table 1). Radiocarbon Data Five of the bulk samples were dated by AMS at the Van der Graaff Laboratory of the University of Utrecht, and one was dated by the conventional method (Beta Analytic) (Table 2). The dates suggest a relatively uniform sedimentation rate for the past 8000 yr (Fig. 4, bottom). The lowermost dated sample at 840 cm core-depth is 1500 yr younger than the sample at

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TABLE 1 Stratigraphy of the Rio Curua´ Core Depth (cm)

Description

0–40

Brown fine detrital mud (FDM), very soft, rich in organic matter, some leaves Brown FDM, relatively soft, some leaves Brown FDM, medium compact, some leaves Transition to light-brown FDM Light-brown FDM; below 580 cm, coarse plant remains (wood and leaves) Light-gray clayey silt, many wood and leaf fragments Light-grayish-brown clayey silt; below 680 cm, a few wood and leaf fragments; below 690 cm, more wood and leaf fragments Light-gray fine sandy silt, with a few roots and rootlets, other plant remains are very rare Silty sand with rare small roots and rootlets

40–300 300–540 540–560 560–595 595–680 680–700

700–830 830–850

681– 690 cm depth. The reason might be contamination by small roots and rootlets, which were observed in the lower core section. Physical Chemistry, Mineralogy, and Geochemistry Data on pH, Eh, amount of organic matter, and mineralogical and chemical composition of the core samples of Rio Curua´ are shown in Figure 2. The measured pH values (wet and dry) are between pH 5 and 6 for the silty sand deposits (core base, below 700 cm depth) and between pH 4 and 5 in the sections of clayey silt and fine detrital mud (above 700 cm). Values of Eh are low for the core base (between 65 and 87 mV) and high (between 100 and 150 mV) for the upper sediments. The organic content is very low at the core base and increases markedly above 700 cm depth. A gradual increase in the organic component of the upper core section may reflect reduced decomposition. The main mineralogical components of the core are quartz and kaolinite (Fig. 2), which originate from laterites, kaolin, and latosols of the river margins. The measured quartz values include SiO 2 of diatoms and the spiny “cauixi” (SiO 2 ⫻ 6 H 2O). The high quartz peak at 430 cm is probably caused by a diatom layer in the analyzed sample. Feldspars, anatase, and illite/white mica occur in small amounts. Iron minerals are too low in concentration for detection by XRD; they probably include siderite, pyrite, and oxi-hydroxides. The base of the column below 700 cm depth is characterized by a high input of inorganic material from the river margins. The marked increase of organic matter with less influx of inorganic components, starts at 700 cm depth and suggests low-energy conditions in the river system. The sediment samples from the core base contain relatively high amounts of SiO 2 and TiO 2 , and low amounts of Al 2 O 3 (Fig. 2). Values of SiO 2 and TiO 2 decrease in the upper core sections. The Al 2 O 3 content is higher in the

clayey silt. Ca, Mg, Na, K, and P are very low in all samples. Ba, Ga, B, Sc, Sn, Sr, V, and Zr were noted, but Co, Cr, Cu, Mn, Ni, Zn, Pb, Mo, As, and Hg are at or below the limit of detection. Modern Pollen Data Most pollen traps were destroyed during the year of installation. Data from the remaining five pollen traps provide some insight on the modern pollen rain from the terra firme forest. The most frequent pollen and spore taxa and the pollen accumulation rate are shown Figure 3. Pollen accumulation rate ranges between 3000 and 20,000 grains/cm2/yr. The assemblages that characterize the modern terra firme forest have relatively high percentages of Moraceae/Urticaceae, Melastomataceae/Combretaceae, Sapotaceae, Protium, Cecropia, and Dinizia excelsa. Virola, Tapiriratype, palm trees, herbs, and ferns are infrequent in the primary terra firme forest. Fossil Pollen Data The pollen diagram displays the most frequent fossil pollen and spore taxa of the 176 different types identified (Fig. 4, top and bottom). About 35 pollen types remain unknown. Based on CONISS cluster analysis, four pollen zones were recognized, zone CAX-I (700 – 610 cm, 7990 –7030 14C yr B.P., 4 samples), zone CAX-II (610 –510 cm, 7030 –5970 14C yr B.P., 6 samples), zone CAX-III (510 –250 cm, 5970 –2470 14C yr B.P., 12 samples), and zone CAX-IV (250 – 0 cm, 2470 14C yr B.P.–modern, 12 samples). The pollen record is characterized throughout by the predominance of arboreal tropical Amazon rain forest pollen taxa (Fig. 4, top and bottom) and the rare presence of herb pollen (Poaceae, Asteraceae, Alternanthera) and fern spores, indicating a diverse and dense tropical rain forest during the Holocene. Most taxa are found in all four pollen zones. Some taxa such as Moraceae/ Urticaceae, Melastomataceae/Combretaceae, Sapotaceae, Myrtaceae, Hymenaea-type, and Fabaceae II are relatively frequent. Others are rare, including Amanoa, Cecropia, Malpighiaceae, Protium, Fabaceae V and VI, Cassia-type, Tapirira-type, Alchornea, Mimosa I–III, and Piper. Mauritia-type pollen is abundant in zone CAX-II; Fabaceae IV, Dinizia excelsa, Ilex are abundant in TABLE 2 AMS and Conventional Radiocarbon Dates of Rio Curua´ Core Samples Lab. number UtC-4971 UtC-5406 UtC-5507 Beta-87869 UtC-4972

Depth (cm)

Age 14C yr B.P.

100 285 485

1288 ⫾ 44 2750 ⫾ 60 5700 ⫾ 80

⫺32.2 ⫺32.0 ⫺34.5

681–690 840

7870 ⫾ 70 6343 ⫾ 33

⫺31.2 ⫺29.8

13

C/ 12C

Calibrated age (cal yr) A.D. 647–781 928–822 B.C. 4676–4640, 4618– 4459 B.C. 5288–5259 B.C.

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FIG. 2.

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Selected results of the physical– chemical, mineralogical, and geochemical analysis of the Rio Curua´ core.

zone CAX-III; Euterpe/Geonoma-type, Virola, Fabaceae I, Banara/Xylosma-type, Didymopanax, Sloanea, Macrolobiumtype, Casearia sylvestris-type are abundant in zone CAX-I. Pachira aquatica is more frequent in the upper two zones than in the lower two zones. The aquatic pollen taxa Cyperaceae and Sagittaria are rare or absent in zone CAX-I and frequent in zone CAX-II and in the lower part of zone CAX-III, but they are less abundant in the upper core. Two single Manihot pollen grains were found in zone CAX-IV.

Charcoal Data The concentration and influx rate of charcoal particles in the studied sediments are low during the early and mid-Holocene intervals (zone CAX-I to zone CAX-III) until about 2500 14C yr B.P. (Fig. 4, bottom). A marked increase of carbonized particles is documented in the late Holocene (zone CAX-IV), which suggests the beginning of pre-Columbian settlement with slash and burn activity. Fires may have occurred at the

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FIG. 3. Percentage pollen diagram of the most frequent pollen and spore taxa from pollen traps in the terra firme forest area of Estac¸a˜o Cientifica Ferreira Penna (ECFPn).

margin of Rio Curua´ or, more likely, at the margins of Baı´a de Caxiuana˜, which is relatively close to the study site. INTERPRETATION AND DISCUSSION

Reconstruction of the Holocene Paleoenvironments The physical– chemical parameters and the mineralogical and chemical composition of the core sediments demonstrate two main sedimentary environments. Before 8000 14C yr B.P., sandy silt (below 700 cm depth) accumulated under conditions of low acidity and low redox potential. The relatively coarse grain size of the sediment suggests an active fluvial system. Much finer organic-rich clayey silt and brown fine detrital mud above 700 cm depth accumulated under conditions of lower acidity and higher redox potential, reflecting a low-energy river system with lake-like conditions. The rarity or absence of pollen and spores below 700 cm depth corresponds to an actively flowing Rio Curua´. The deposition of pollen and spores under an inferred passive river regime began after 8000 14C yr B.P. Above the transition (clayey silt deposits in zone CAX-I) Mauritia pollen increase markedly. This change indicates formation of a local Mauritia palm swamp on the surrounding floodplains ca. 7000 14C yr B.P. (base of zone CAX-II). Small roots and rootlets below this swamp may belong to the Mauritia, in which case Mauritia grew on the riverbed. The young radiocarbon date at 840 cm depth may reflect root contamination. By 7000 14C yr B.P., the water surface of the swamp was about 11 m below modern river level. The surrounding upland was covered by dense and diverse well-drained terra firme Amazon rain forest. Frequent pollen taxa from the traps in the terra firme forest, such as Moraceae/Urticaceae and Melastomataceae/Combretaceae, are

also frequent in the sediments. The area of va´rzea and igapo´ along the rivers was small. The strong increase of the aquatic Cyperaceae, and especially the shallow-water indicator Sagittaria, in the upper part of the zone CAX-II (Mauritia decreases here), suggests a rise in water level. The local swamp in the riverbed changed to a shallow lake-like environment. The water level apparently was shallow at the beginning of zone CAX-III, but it rose continuously to high levels at the end of this zone, as implied by the decrease of the aquatic taxa. During zone CAX-III (5970 –2470 14C yr B.P.), terra firme rain forest dominated the surrounding catchment. A good indicator is the ⬎40-m-tall Dinizia excelsa (Ferreira et al., 1997), the pollen of which reaches its highest values in this zone. Areas of inundated forest along the river were small, but the presence of Pachira aquatica suggests a slight increase of inundated areas. Ilex, which occurs in the zone CAX-III, has not been found in the study plots, but there is one species in the herbarium of the Emı¨lio Goeldi museum in Bele´m from the va´rzea forest of Rio Caxiuana˜, a neighboring river. Perhaps Ilex was frequent in the va´rzea forest of Rio Curua´ during that time. Vegetational changes are inferred at the end of zone CAXIII and at the beginning of zone CAX-IV (ca. 2500 14C yr B.P.). The increase of Virola (probably Virola surinamensis), Euterpe/Geonoma-type (probably the palm Euterpe oleracea), and Macrolobium-type, trees which are common in inundated forests (Ferreira et al., 1997), suggest a marked expansion of va´rzea and igapo´. Those taxa are rare or missing in the modern pollen samples of the terra firme forest. The terra firme tree Dinizia excelsa is much less abundant in zone CAX-IV. These vegetational changes are inferred to represent higher water in

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FIG. 4. (top) Percentage pollen diagram of the most frequent pollen and spore taxa of the Rio Curua´ core. (bottom) Summary pollen diagram showing vegetation groups, concentration and accumulation rates of pollen and carbonized particles, and CONISS cluster analysis of the Rio Curua´ core.

Rio Curua´, forming environmental conditions similar to present ones. The abundance of Banara/Xylosma (secondary vegetation) and Didymopanax (in vegetation with an open canopy) may suggest human impact on the vegetation by slash and burn agricultural activities (evidence of two Manihot pollen grains) during the last 2500 14C yr. The marked increase of carbonized particles supports this assumption. The abundance of Euterpe/Geonoma might also be related to the use of palm fruits from Euterpe oleracea (Ac¸ai) as food by local people. Factors of Environmental Changes The different Holocene environmental intervals identified are thought to be primarily related to Rio Curua´ water-level

rises. Changes of the river system may be caused by several factors, such as Atlantic sea level changes, blocking of the river system, neotectonic activity, and climate change. It seems most likely that the sea level rise indirectly changed the river from an active to a passive system ca. 8000 14C yr B.P. The riverbed at that time was more than 9 m deeper than the modern sea level. Sea level rise decreased the river gradient so that sediment was deposited. Sea level rise also formed large estuaries. Baı´a de Caxiuana˜ is one such estuary; sediments within the bay are at least 12 m thick (Costa et al., 1997). Sea level rise during the early Holocene was probably also the major reason for the change from local palm swamp at 7000 14C yr B.P. to a shallow lake-like environment at 6000 14C

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yr B.P. Dates related to sea level oscillations on the northeastern Brazilian coast (Suguio et al., 1985) show that sea level ca. 7000 –5500 yr B.P. was about 1–2 m higher than present. The maximum Holocene transgression occurred between 5500 and 5000 yr B.P. when sea level was 4 –5 m higher than today. However, the early Holocene and strong mid-Holocene transgression is not evident from the Rio Curua´ record. The reconstructed paleoenvironment indicates a continuous rise of the water level at the study site between 6000 and 2500 14C yr B.P. About 2500 14C yr B.P. water reached the modern level. This conflicts with data from the northeastern Brazilian coast. However, interpretations of the Brazilian sea level data are under discussion (Angulo and Lessa, 1997). Quaternary tectonic activity was also an important factor in the western Amazon region (Bemerguy, 1997), but the extent to which the Holocene record was influenced by tectonic activity is unclear. The study region may also be sensitive to Holocene climate changes. Although present annual precipitation is high (2500 mm at Breves), during four months rainfall is reduced (Moraes et al., 1997). A change from drier early Holocene conditions to wetter late Holocene conditions have been interpreted from several pollen records. The record at 700 – 800 m on Carajas Mountain in southeastern Para´ State (Absy et al., 1991) suggests extension of edaphic savanna during the early Holocene until ca. 3000 14C yr B.P., reflecting drier conditions. During the late Holocene, the tropical rain forest expanded, implying wetter conditions. Drier early Holocene and wetter late Holocene conditions are also reported from the central Colombian Amazon (Behling et al., 1998) and the Llanos Orientales in Colombia (Behling and Hooghiemstra, 1998a, 1998b). In the Llanos Orientales, the climate after 6000 14C yr B.P. was characterized by wetter periods and shorter dry seasons. The wettest period occurred during the last ca. 3800 14C yr. Dry periods, characterized by lower precipitation in the Amazon Basin, are inferred to have occurred between 4000 and 3500 yr B.P. and between ca. 2100 yr B.P. and ca. 700 yr B.P. (Absy, 1979). However, these drier conditions are not apparent in the Rio Curua´ record. Middle- and late-Holocene shifts to wetter conditions, as reported in the northwestern Amazon, may also have occurred in the eastern regions. At Rio Curua´, the highest recorded water level stands occurred during the late Holocene. Although these wet episodes might be attributed to high annual precipitation, the principal factor seems to be related to rising late-Holocene sea level. Human impact near the research site since 2500 14C yr B.P. is suggested by the marked increase in charcoal. This evidence is about 1800 yr earlier than dated ceramic fragments from archeological sites of this region (Kern, 1996). The first Amerindians likely arrived in the western Amazon during lateglacial time (Roosevelt et al., 1996; Behling, 1996). The formation of the inland bay and possible population pressure during late-Holocene time may have led to settlement of the Caxiuana˜ region. Anthropogenic influence on the environment

during the last 2500 yr was minimal. There is no clear change in the geochemical composition of the sediments, and vegetational changes were minor and likely were restricted to the rivers and bays. CONCLUSION

The relatively high-energy Rio Curua´ changed to a lowenergy river ca. 8000 14C yr B.P. Continuous pollen and spore deposition under swamp and subsequent lake-like conditions document Holocene Amazon rain forest since that time. The period prior to 7030 14C yr B.P. was characterized by the transition to a passive fluvial system and a well-drained highly diverse terra firme (unflooded upland) Amazon rain forest with a narrow margin of inundated forests (va´rzea and igapo´). Subsequently, Mauritia palm-swamp developed in the present location of the riverbed. After 5970 14C yr B.P., the river changed to shallow lake-like conditions. Abundant terra firme rain forest still occupied well-drained areas. About 2500 14C yr B.P., the Curua´ river reached a level similar to that of present, and the vegetation reflected a change to seasonally inundated forest areas (va´rzea and igapo´). High water stands reduced the inundated terra firme forest area, which was less-well drained than during earlier periods. In addition, the climate may have been wetter, as it was in the western Amazon. Human settlement in the late Holocene occurred along the rivers and inland bay. Slash and burn activity during the last 2500 yr may have contributed to environmental change. Rising sea level, which reached approximately its modern level ca. 7000 14C yr B.P., was the major factor in environmental change. However, evidence of early and mid-Holocene sea level stands at ca. ⫹2 m and ⫹4 –5 m on the eastern Brazilian coast was not found in the Rio Curua´ record. Huge areas of terra firme Amazon rain forest likely have been replaced by va´rzea and igapo´ forests since ca. 2500 14C yr B.P. ACKNOWLEDGMENTS We gratefully acknowledge Valdenira F. Santos for help in fieldwork and Natalino Valente de Siqueira and Elias Lea˜o Moraes for supporting the chemical analysis. Museu Paraense Emı¨lio Goeldi provided infrastructure assistance. Fieldwork (1995 and 1996) was done with a post-doctoral fellowship at the Smithsonian Tropical Research Institute under the supervision of Dr. Paul A. Colinvaux. We thank Dr. Rob Marchant and Dr. Stephen C. Porter for reading the English text and Dr. Mark B. Bush and Dr. Kam-biu Liu for providing constructive reviews. Fieldwork and one radiocarbon date were funded by the National Science Foundation (BSR-9007019). This study received financial support from the Brazilian Council for Sciences and Technology through CNPq (Grant 520.041/95). The first author thanks Deutsche Forschungsgemeinschaft (DFG) for the scholarship support.

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