Holocene environmental dynamics in coastal , eastern and central Amazonia and the role of the Atlantic sea-level change

2drainage area which is larger than Europe. Because of its huge area, Amazonia plays a significant role in the establishment of global climate, water and carbon budget (e.g. Saint-Paul et al. 1999). Amazonia is mostly covered by rainforest and some savanna ecosystems. The Amazon Basin is low lying with >1,000,000 km 2 at <100 m elevation. For much of its course, the Amazon has a grade of 1 in 100,000. The water level in Manaus, which lies 1200 km inland is only 14 m a.s.l. (Grabert 1991). As a consequence, in the eastern half of the basin the development of rivers and lakes is strongly influenced by Atlantic sea-level fluctuations. The amount of annual rainfall as well as the sea-level dynamics influence the occurrence of seasonally inundated forests, which form an area of up to 700,000 km 2 (Pavia 1995). Also Amazonian coastal ecosystems such as mangroves and salt marshes are a result of sea-level fluctuations. In order to understand past environmental changes in Amazonia such as the natural amplitude of Amazonian ecosystem dynamics, including coastal ecosystems such as mangroves, as well as Amazonian river and climate changes, palaeoecological and palaeoenvironmental studies are important. Several lacustrine and mangrove sediment deposits have been studied by pollen analysis. The aim of this paper is to compare and summarise recently published pollen records and to give an overview of natural environmental changes in the coastal and Amazon Basin region, related to the impact of the Holocene sealevel changes. Tectonic movements/subsidence may have played a certain role as well, but will be not considered in this study.


Hermann Behling, Göttingen 1 Introduction
The Amazon river system occupies a 7,900,000 km 2 drainage area which is larger than Europe.Because of its huge area, Amazonia plays a significant role in the establishment of global climate, water and carbon budget (e.g.Saint-Paul et al. 1999).Amazonia is mostly covered by rainforest and some savanna ecosystems.The Amazon Basin is low lying with >1,000,000 km 2 at <100 m elevation.For much of its course, the Amazon has a grade of 1 in 100,000.The water level in Manaus, which lies 1200 km inland is only 14 m a.s.l.(Grabert 1991).As a consequence, in the eastern half of the basin the development of rivers and lakes is strongly influenced by Atlantic sea-level fluctuations.The amount of annual rainfall as well as the sea-level dynamics influence the occurrence of seasonally inundated forests, which form an area of up to 700,000 km 2 (Pavia 1995).Also Amazonian coastal ecosystems such as mangroves and salt marshes are a result of sea-level fluctuations.
In order to understand past environmental changes in Amazonia such as the natural amplitude of Amazonian ecosystem dynamics, including coastal ecosystems such as mangroves, as well as Amazonian river and climate changes, palaeoecological and palaeoenvironmental studies are important.Several lacustrine and mangrove sediment deposits have been studied by pollen analysis.The aim of this paper is to compare and summarise recently published pollen records and to give an overview of natural environmental changes in the coastal and Amazon Basin region, related to the impact of the Holocene sealevel changes.Tectonic movements/subsidence may have played a certain role as well, but will be not considered in this study.
The atmospheric circulation of the Amazon region is controlled by the position of the Intertropical Convergence Zone (ITCZ), shifting from 7-9°N in July to 10-20°S in January.Consequently two main climate types are represented in Amazonia: a climate without dry season at equatorial latitudes, and a climate with a marked dry season north and south of the equatorial latitudes.South America north of the equator is influenced by the northeasterly trade winds blowing from the Caribbean, whereas the region south of the equator is influenced by the Atlantic southeasterlies, which originate from the subtropical high pressure cell over the south Atlantic Ocean.For most of the Amazon regions the annual precipitation ranges between 1750 and 3500 mm, and the average annual temperature is between 20 and 26 °C (Nimer 1989;Snow 1976;Weischet 1996).The Amazon coastal region receives between 2000 and over 3250 mm precipitation per year.The climate station of Belém documents a mean annual precipitation of 2277 mm (Walter & Lieth 1967).The average annual temperature is 25.9 °C.Measured maximum and minimum temperatures are 31.7 °C and 18.0 °C, respectively.

Sites used in the comparison
For the Amazon lowland regions only a few pollen records are available, reflecting the impact of Holocene sea-level changes (Fig. 1).For the coastal region of Amazonia there is Lagoa da Curuça (Behling 1996(Behling , 2001)), Lago Crispim (Behling & Costa 2001), three records from the Braganca Peninsula and Lago Aquiri (Behling & Costa 1997).For the Amazon Basin region there is Rio Curuá (Behling & Costa 2000;Costa et al. 1997) and Lago Calado (Behling et al. 2001).

Lagoa da Curuça
Holocene environmental dynamics in coastal, eastern and central Amazonia and the role of the Atlantic sea-level change 100 km northeast of the city of Belém in northern Brazil (Fig. 1).The lake is circular shaped, covers an area of ca. 15 hectares and is mostly about 2 m deep.Lagoa da Curuça is isolated from rivers and lies in a relatively plain landscape at 35 m elevation, covered with mostly secondary Amazon rainforest vegetation.The lake is about 15 km from the Atlantic Ocean, but mangrove vegetation grows along small rivers within a distant of about 1.5 km to the lake.
Lago Crispim (0°46' S, 47°51' W) also lies in the coastal region of northeastern Pará, about 130 km northeast of Belém (Fig. 1).The lake is located near the village Crispim, at the west side of the Baiá do Maruda, which is formed by the Rio Marapanim.The isolated lake is 1-2 m above sea-level and only 500 m from the modern shore of the bay.The more or less circular lake has a diameter of about 100 m and a water depth of about 1 m.The lake seems to be part of a former inter-dune valley or channel in a relatively flat coastal area.Modern vegetation near the lake includes coastal vegetation (mangrove, restinga) and, further inland, strongly disturbed Amazon rainforest and edaphic white sand vegetation.
Bragança Peninsula is located between the mouth of the rivers Maiaú and Caeté in the coastal region of northeastern Pará (Fig. 1).The area is near Bragança City, about 200 km east of Belém.The 30 km long and up to 15 km wide peninsula is mostly covered by mangroves.Pollen records are available from three different areas.The first site, Bosque de Avicennia (00°55'65" S, 46°40'09" W, 2.4 m a.s.l), is located on the relatively high central southern part of the peninsula.Only Avicennia trees make up the mangrove forest here.The second site, Campo Salgado (00°54'46" S, 46°40'63" W, 2.7 m a.s.l), is in Cyperaceae-dominated open salt marsh of the central part of the peninsula.The third site, Furo do Chato (00°52'25" S, 46°39'00, 1.9 m a.s.l), is in the northern part of the study area at a lower elevation than the two other sites.Here Rhizophora trees dominate the mangrove, but Avicennia trees occur close to the site.
Lago do Aquiri (3°10' S, 44°59' W, 10 m a.s.l.) is located 3 km north from the village Viana, about 120 km southwest of São Luis, capital of Maranhão State, and 450 km southeast of Belém (Fig. 1).The lake lies in a soft rolling landscape with elevated areas between 20-40 m a.s.l. and flood plain areas between 3-10 m a.s.l.. To the west, the study area is influenced by the river systems of the Rio Mearim.The shortest distance between Lago do Aquiri and Rio Mearim is 20 km, but sometimes a connection forms during the rainy season.During the wet season the fresh water lake is about 11 km long by 1-3 km wide, while the water depth is 3 m.During the dry season the lake contracts to a small basin 1-2 km in diameter.The present-day vegetation to the west of the lake is anthropogenic palm forest/savanna.Mangroves were not observed in the Lago do Aquiri region.
Rio Curuá (1°44'07'' S, 51°27'47'' W) is an inactive river within the Caxiuanã National Forest Reserve, located 350 km west of Belém (Pará State) in eastern Amazonia (Fig. 1).The surrounding area is relatively flat, only a few meters above sea-level, and covered with Amazonian rainforest.Rio Curuá is connected to the Baía de Caxiuanã which is about 40 km long and 8-15 km wide.Water of the inland bay flows about 400 km to the Atlantic Ocean.Baía de Caxiuanã is mostly shallow, with water depths between 2 and 5 m.The water level is < 3 m above sea-level.Thus, the Rio Curuá is a low energy river controlled by the amount of water in the Baía de Caxiuanã.
Lago Calado (3°16' S, 60°35' W, 23 m a.s.l.) is located on the northern bank of the Rio Solimões near the village Manacapuru, about 80 km upstream from the confluence of the Rio Negro and the Rio Solimões (Fig. 1).The terra firme lake has an outflow to the Rio Solimões that connects the water bodies during times of high water levels.A network of small rainforest streams from the Amazon terra firme forest flow into the lake.Controlled by the Solimões water level, the water depth of the lake fluctuates on an annual cycle between about 1-2 m in November and roughly 12 m in June.During the same period the lake surface area changes from 2 to 8 km 2 .Today várzea/igapó forests cover about 5-10% of the total forest area of the Lago Calado region.

Coastal region of Amazonia
Vegetational changes which reflect sea-level fluctuations are evident in pollen records from the coastal region of northern Brazil.In the Lagoa da Curuça sediment core, Rhizophora pollen grains were already present during the Lateglacial/Holocene transition.These pollen grains were probably transported by wind over some distance into the lake.Mangrove apparently developed along the rivers near the lake between 8080 and 6380 calibrated radiocarbon years before present (cal BP).The subsequent retreat of mangroves from these rivers reflects lower relative sea-level stands between ca.6380 and 3320 cal BP.Mangrove was replaced by successional stages of palms, first Mauritia, then Arecaceae II and Mauritiella, suggesting a somewhat lower groundwater table in the Lagoa da Curuça area.Mangrove expanded again along the rivers near the lake after 3350 cal BP, indicating the return of relatively high relative sea-levels.
Based on the Lago do Crispim record, mangroves first developed along the river close to the core site between 8370 and 7520 cal BP.There is evidence that areas, originally covered by dense, tall coastal Amazon rainforest, were partly replaced by mangrove and some restinga vegetation during the early Holocene.Decreasing Rhizophora pollen abundances document a retreat of mangroves, reflecting sea-level regression starting at around 7850 cal BP.The marked reduction of mangroves near the lake indicates a lower relative sea-level between around 7520 and 3960 cal BP.During this period a local Mauritia/Mauritiella palm swamp formed.That palm trees are sensitive to the local ground table changes is well known (Henderson 1995).Marked coastal environmental changes occurred at around 3960 cal BP driven by sea-level transgression.Mangroves expanded again close to the site.The local palm swamp was replaced by a Cype-raceae swamp.Rainforest and restinga vegetation adjacent to the swamp were replaced by salt marshes as sea-level rose.The Atlantic Ocean was close to the core site, but the site, which is only 1-2 m above modern sea-level, was apparently neveraffected by marine incursions during the Holocene.Reduced mangrove vegetation since ca.1780 cal BP may be due to a slightly lower relative sealevel or to human impact due to higher occurrence of fire (Behling & Costa 2001).
The Lago Aquiri record, far inland from the modern ocean, shows the formation of mangrove between 8280 and 7570 cal BP.Due to a sedimentary gap, only the last century is recorded.For this period pollen data indicate the present-day environment, seasonally inundated swamp savanna and secondary forests on somewhat higher elevated areas.Mangroves were not found.
Sea-level changes also play an important role in the development and dynamic of mangrove ecosystems on the Bragança Peninsula.The radiocarbon dates indicate that the development of mangroves started at the three sites at different times: Campo Salgado at around 5850 cal BP, Bosque de Avicennia at 2210 cal BP and Furo do Chato at 1340 cal BP.The development of mangrove during the early Holocene, as documented from the other sites, is so far not registered on the peninsula.
The presence of mangrove at Campo Salgado, the highest elevated site on the peninsula which today is a salt marsh, suggests relatively high sea-levels since the mid-Holocene.The highest amount of non-mangrove shrub and tree pollen in the basal samples, suggests that mangroves here also replaced an earlier coastal forest ecosystem prior to 5850 cal BP.Compared with other sites from northern Brazil, it is suggested that the relative sea-level during the mid-Holocene was only slightly higher than today.Results from the site Lago do Crispim, which is located next to the modern coastline and only 1-2 m a.s.l., indicate that it was never flooded during the Holocene (Behling & Costa 2001).Poor pollen preservation between 700 and 450 cal BP, indicate that mangrove deposits were exposed and the area of the Campo Salgado site was relatively dry.The frequency of inundation must have been lower in response to lower sea-levels.Pollen assemblages indicate that an open Poaceae-dominated salt marsh with Avicennia shrubs developed after 450 cal BP.The change from a Poaceae-to a Cyperaceae-dominated modern salt marsh during the last 160 cal BP, may be related to a lower relative sea-level.The high Avicennia pollen concentrations in the sediments from Bosque de Avicennia during the last 150 cal BP, also suggest a regression of sea-level.Studies by Cohen et al. (2005), show that there has been a recent relative sea-level rise on the Bragança Peninsula during the last three decades.

Eastern and central region of Amazon lowland
Sedimentological data from the Rio Curuá core in the Caxiuanã region of eastern Amazonia show that the relatively high energy river changed to a low energy river at ca. 8900 cal BP.The period prior to 7880 cal BP was characterised by the transition to a passive fluvial (lake-like) system.A well-drained highly diverse terra firme Amazon rainforest was found at that time.Later, a Mauritia palm-swamp developed along the margins of the river between 7880 and 6810 cal BP.The area of inundated várzea and igapó forest along rivers were at that time relatively small.Subsequently, the river changed to shallow lake-like conditions.Abundant terra firme rainforest still occupied the well-drained areas.At 3340 cal BP, the increase of Virola, Euterpe/ Geonoma and Macrolobium, trees which are common in inundated forests (Ferreira et al. 1997), suggest a first expansion of inundated várzea and igapó forest area.After about 2560 cal BP, the pollen data reflect the largest extension of seasonally inundated forests and Rio Curuá reached a water level similar to that of present.
Sedimentological data indicate that the deposits of the core base from Lago Calado in central Amazonia correspond to a fluviatile palaeoenvironment in the Lago Calado valley prior to 9290 cal BP.Pollen analytical results show the formation of a local Mauritia palmswamp along the river margin between 9290 and 8490 cal BP.At that time, highly diverse terra firme Amazon rainforest and poorly developed várzea/igapó forests along the river characterised the vegetation of the study region.The small expansion of várzea/igapó forests and a strong presence of aquatic plants started at 8490 cal BP, reflecting the increase of the Amazonian water level and the formation of the Lago Calado.The occurrence of abundant Poaceae and Cyperaceae pollen in the lake sediments since 8490 cal BP, suggest both the local colonisation of unflooded mud banks around the lake margin and the formation of flooding meadows.Open exposed unflooded mud areas, periodically colonised by herbs, are related to cyclic annual Amazonian high and low water stands.In respect to the abundance of Poaceae pollen during the early and mid-Holocene, these open areas were large, suggesting short annual high and long annual low Amazonian water stands.After 4610 cal BP the area of várzea/igapó forests increased.Herbs were less frequent, probably due to the smaller area of unflooded mud banks.The Amazonian water levels must have been higher than before and the period of the annual high water stands was probably longer.Since 2060 cal BP, the largest proportion of várzea/igapo forests is recorded, reflecting the highest Amazonian water level.Herbs were rare, suggesting that open unflooded mud areas around the lake were relatively small.This may perhaps also indicate that the cyclic annual period of the Amazonian high water level was the longest since 2060 cal BP.

Discussion and conclusions
During the Last Glacial Maximum (around 21,000 cal BP), the Atlantic sea-level was around 120 m lower than today (e.g.Shackleton & Opdyke 1973).During the Lateglacial/early Holocene sea-level rise, huge areas of the exposed coastal shelf were inundated by the Atlantic Ocean.The exposed area along the north Brazilian coast was a belt mostly about 150-200 km wide.Nothing is known about these former ecosystems, but this zone could have been partly covered by Amazon rainforest, savanna, mangrove and other coastal vegetation types.
The compared and summarised pollen record from the Amazon coastal and basin region document remarkable vegetational and environmental changes, related to the Atlantic sea-level rise during the Holocene (Fig. 2, Table 1).The coastline shifted inland during the Lateglacial/early Holocene sea-level rise.Ancient low elevated coastal ecosystems were destroyed and new ecosystems, such as mangroves developed on intertidal, now higher elevated areas, replacing the former Amazon rainforest.First occurrence of mangrove pollen (Rhizophora) in the sediment deposits reflects the early Holocene sea-level rise close to the modern sea-level.Mangrove developed near Lagoa da Curuça between 8080 and 6380 cal BP, at Lago do Aquiri between 8280 and 7570 cal BP and near Lago Crispim between 8370 and 7520 cal BP.The occurrence of some Rhizophora pollen grains in the Lagoa da Curuça record, already at the beginning of the Holocene (at 10660 cal BP or earlier), is probably related to wind transported pollen over somewhat longer distances.Evidence of a mangrove environment in the Aquiri region, about 120 km inland from the modern coastline, suggests a significant early Holocene transgression near the Rio Mearim.
Due to the very low elevational gradient of the central and eastern Amazonian rivers, the levels of the Amazonian water depend also on Atlantic sea-level.During last glacial low sea-level stages, when the sea-level was over 100 m lower than today, the gradient was steeper resulting in erosional processes in the Amazon Basin.The Amazon and its tributaries incised and the riverbeds were several tens of meters deeper than today (Irion 1982(Irion , 1984;;Irion et al. 1995Irion et al. , 1997;;Sioli 1957).With the post-glacial rise of the Atlantic sea-level, the waters in the Amazon Basin rose as well.
The influence of the early Holocene sea-level rise in the Amazon Basin is found by the change of active high erosive to a passive river (lake-like) system of the Rio Curuá record in eastern Amazonia and of the Lago Calado record in central Amazonia at ca. 8900 and at 9290 cal BP, respectively.These events occurred about 500-1000 years earlier than the first mangrove formation near the modern coastline.In both records a local Mauritia palm swamp stage is found, in Rio Curuá in eastern Amazonia between 7880 and 6810 cal BP and in Lago Calado in central Amazonia between

Zusammenstellung der Vegetationsveränderungen an der Küste und im Innern des Amazonasgebietes im Zusammenhang mit holozänen Meeresspiegelschwankungen Graphique synthétisant les changements de la végétation de la région côtière et des basses terres de l'Amazonie en relation avec les changements du niveau marin
Graphics: H. Behling 9290 and 8490 cal BP.The timing and length of this successional period differs, which may be related to the different location and topography of the two rivers.These palm swamp stages occurred before or during the period of the mangrove development in coastal Amazonia.It is supposed that the Mauritia palmswamp stage is a regional phenomenon for low-lying areas in the Amazon Basin, related to the sea-level rise during the early Holocene.First várzea and igapó forests developed at that time as well, but the area of seasonally inundated forests was small.
In the coastal region a retreat of the mangroves is found, reflecting a lower relative sea-level: in Aquiri since 7770 cal BP, in Lago Crispim between 7520 and 3960 cal BP and in Lagoa da Curuça between about 6380 and 4410 cal BP.The interpolated age of 6380 cal BP for the Lagoa record can be older, due to the poor radiocarbon dating of the core.During this mid-Holocene sea-level regression, in lowland Amazonia shallower water levels than today are recorded, between 6810 and 3340 cal BP in Rio Curuá and between 8490 and 4610 cal BP in Lago Calado.
The second major period of mangrove formation at the modern coastline occurred during the late Holocene, at the Lagoa da Curuça record since around 4410 cal BP and at the Lago Crispim record since 3960 cal BP, reflecting the highest Holocene sea-level stands.The first development of mangroves on the Bragança Peninsula is found in the Campo Salgado area at around 5850 cal BP.It is suggested that the radiocarbon date form the Campo Salgado core base is too old in comparison with events of the Lago do Crispim, Lagoa da Curuça and the two Amazon Basin record.The beginning of the mangrove development at the Campo Salgado site is probably not older than 4500 cal BP.The formation of mangroves at the Bosque de Avicennia site started at around 2210 cal BP and at the Furo do Chato since around 1340 cal BP.
In the eastern Amazon Basin a marked increase of várzea/igapó forests is documented in the Rio Curuá record since 3340 cal BP and especially since 2560 cal BP.In the central Amazonia a marked increase of seasonally inundated forest is found since 4610 cal BP and especially since 2060 cal BP.The marked increase of inundated areas of várzea/igapó forests is apparently related to the late Holocene sea-level rise and almost the complete low lying Amazon Basin seem to have experienced this change.Huge areas of terra firme Amazon rainforest were replaced by várzea and igapó forests during the late Holocene.The development of large seasonally inundated areas must have had an important influence on the Amazonian water and carbon cycle and the regional climate in Amazonia.
The development and the modern extension of mangrove forests in the Amazon coastal region and the large extended areas of modern seasonally inundated várzea/igapó forests in the lower Amazon Basin, consequently, are relatively young in age.
The Atlantic sea-level rise was probably the major factor in palaeoenvironmental changes, but high water stands might also partly be related to climate change involving greater annual rainfall rates in Amazonia and in the Andes.There is evidence of Amazon rain forest expansion, both north and south of the equator, reflecting a change to wetter climatic conditions (higher precipitation rates and longer wet periods) since mid and especially during the late Holocene (Behling & Hooghiemstra 2001;Behling et al. 2010).In the northwestern Amazon rainforest/savanna transition zone (Llanos Orientales in Colombia) rainforest expansion is documented since 6850 cal BP and was further intensified after 3200 cal BP (Behling & Hooghiemstra 2000).In southwestern Amazonia, there is evidence of Amazon rain forest expansion (Laguna Bella Vista and Chaplin, Bolivia) during the late Holocene at least since 3200 cal BP (Mayle 2011;Mayle et al. 2000).The expansion of the Amazon rain forest after 6260 cal BP is also reported for the southeastern Amazon region (Behling 2002).
In western Amazonia (Colombia) the lower terrace of Rio Caquetá was poorly drained after 4480 cal BP, as a result of higher river levels, probably due to higher precipitation rates (Behling et al. 1999).In this context in the Lago Calado record, possibly relatively short annual high water levels during the early Holocene and relatively long annual high water levels since 4610 and especially after 2060 cal BP, can be interpreted as indicators of climate change to wetter late Holocene periods (Behling et al. 2001).Somewhat cooler climatic conditions and therefore decreasing evaporation may also have played a certain role.
Thus, although the Atlantic sea-level rise was probably the major factor in palaeoenvironmental changes, high water stands might also be due to greater annual rainfall during the late Holocene in western, central and eastern Amazonia.
mostly about 4500 cal BP or somewhat later at the present-day coastline.First impact of early Holocene sealevel rise in the Amazon Basin is found by the change of an active to a passive (lake-like) river system about 8900 cal BP.Later, local Mauritia palm swamps developed along river margins in central and eastern Amazonia.This stage is probably a regional phenomenon for the whole low lying Amazon Basin.First várzea and igapó forests (seasonally inundated forests) developed at that time as well, but their size was still small.In eastern and central Amazonia, marked increase of várzea/igapó forests is documented since mid-to late Holocene.The development of huge seasonally inundated areas must have had an important influence on the Amazonian water and carbon cycle and the regional climate in Amazonia.

Fig. 2 :
Fig.2: Graph summarising vegetational changes in the coastal and Amazon lowland region related to Holocene sea-level changes Zusammenstellung der Vegetationsveränderungen an der Küste und im Innern des Amazonasgebietes im Zusammenhang mit holozänen Meeresspiegelschwankungen Graphique synthétisant les changements de la végétation de la région côtière et des basses terres de l'Amazonie en relation avec les changements du niveau marin Graphics: H. Behling List of the summarised and compared pollen records highlighting two major events of environmental changes.The table includes data on the current elevation (m above mean modern sea-level) of the coring site, as well as elevation values at the time of the environmental changes.Liste der verglichenen Pollenprofile mit Altersstellung zweier besonderer Phasen der Umweltveränderungen.In der Tabelle sind auch die Angaben zur heutigen und vorzeitlichen Meereshöhe (m ü.M.) an den Standorten dargestellt.Liste synthétique et comparative des relevés polliniques montrant les deux principaux changements environnementaux.Le tableau présente l'altitude du site de carottage (m au-dessus du niveau moyen marin actuel) et les altitudes durant les changements environnementaux.