NINETEEN The origin of the modern Amazon rainforest: implications of the palynological and palaeobotanical record Carlos Jaramillo1, Carina Hoorn2, Silane A.F. Silva3, Fatima Leite4, Fabiany Herrera1, Luis Quiroz5, Rodolfo Dino6 and Luzia Antonioli7 1Smithsonian Tropical Research Institute, Balboa, Republic of Panama 2University of Amsterdam, The Netherlands 3Instituto Nacional de Pesquisas da Amazonia-INPA, Manaus, Brazil 4University of Brasília, Brazil 5Smithsonian Tropical Research Institute, Balboa, Republic of Panama, and University of Saskatchewan, Canada 6Cidade Universitária – Ilha do Fundão, Rio de Janeiro, Brazil 7Universidade Estadual do Rio de Janeiro (UERJ), Rio de Janeiro, Brazil Abstract Northern South America harbours a highly diversifi ed forest vegetation. However, it is not clear when this remarkable diversity was attained and how it was produced. Is the high diversity the product of a positive speciation–extinction balance that accumulated species over long time periods, or is it the product of high origination rates over short time periods, or both? Middle Cretaceous fl oras, although very poorly studied, are dominated by non-angiosperm taxa. By the Paleocene, pollen and macrobotanical fossils suggest that the basic phylogenetic composition and fl oral physiognomy of Neotropical rainforests were already present. Hence there was a profound change in Amazonian fl ora during the Late Cretaceous, that still needs to be documented. Levels of Paleocene diversity are much lower than those of modern tropical rainforests. By the Early Eocene, however, pollen diversity was very high, exceeding values of modern rainforests. At the Eocene- Oligocene a major drop in diversity coincided with an episode of global cooling. The palynological and palaeobotanical records of Amazonia suggest that high levels of diversity existed during the Miocene, a period when the boundary conditions for sustaining a rainforest (e.g. low seasonality, high precipitation, edaphic het- erogeneous substrate) were met. The predecessor of the present rainforest was formed during the Paleogene and Neogene when the western Amazon lowlands were affected by Andean tectonism, which radically changed drainage systems and promoted wetland development. An overall global cooling during the Neogene also may have affected the rainforest, decreasing its area and expanding adjacent savanna belts. Recent events like the Quaternary ice ages also played a role in the forest dynamics and composition, although it seems to have been minor. In this chapter we will review the main characteristics of the Neogene palynological and palaeobotani- cal records in Amazonia, and we will make some comparisons with pre- and post-Neogene records. The data indicate that the Amazonian rainforest is more likely to be a product of a dynamic geological history stretching back over the past 25 million years rather than the last few hundred thousand years. Amazonia, Landscape and Species Evolution: A Look into the Past, 1st edition. Edited by C. Hoorn and F.P. Wesselingh. © 2010 Blackwell Publishing HHoorn_ch19_Final.inddoorn_ch19_Final.indd 331717 110/24/20090/24/2009 11:56:59:56:59 SShobhahobha 318 C. Jaramillo et al. Introduction Palynology The Cretaceous and Cenozoic history of the Neotropical rain- Cretaceous Amazonia forest is still not well understood. Very few studies of Cretaceous Amazonian fl oras have been done. Most of the Cretaceous stud- Cretaceous sequences of intracratonic Brazilian basins are mostly ies have been carried out in the eastern margin of South America characterized by terrestrial siliciclastic rocks (see Chapters 3 & 7), (e.g. Herngreen 1973, 1975; Regali et al. 1974; De Lima 1979), and which often give a poor yield of palynomorphs. The Cretaceous most of them have focused on palynology. Alter do Chão Formation forms the basal unit of the Javari Group, Paleogene records, mainly deriving from northern South which represents the beginning of the fi nal sedimentation episode America, show that a rainforest with family-level fl oristic com- in the Amazonas and Solimões Basins. Fossils are rare in the pre- position and leaf physiognomy similar to modern Neotropical dominantly fl uvial Alter do Chão Formation and limited to single rainforests already existed by the Middle Paleocene (Wing fi ndings. Price (1960) found a terapode tooth in the upper part et al. 2004; Doria et al. 2008; Herrera et al. 2008a). However, of the formation in the 1-NO-1-AM well in the Amazonas Basin. its diversity was much less than modern lowland Neotropical Daemon & Contreiras (1971) dated the formation as Cenomanian rainforests (Wing et al. 2004; Jaramillo et al. 2007a). The be- to Maastrichtian, based on the correlation with the K-400-K-600 ginning of the Eocene shows a very rapid increase in diversity palynozones defi ned in the Barreirinhas Basin by Lima (1971). and the radiation of several Neotropical plant families. Levels They also mentioned the occurrence of teeth and fragments of of diversity by the Middle Eocene were greater than those of vertebrates in the upper part of the formation. modern Amazonian forests (Jaramillo et al. 2006). Eocene paly- Daemon (1975) analysed the palynology of two wells that drilled nofl oras contain a large number of pollen taxa that range into the formation (1-NO-1-AM and 1-AC-1-AM), and esta blished an the Neogene and are more similar to each other than to the early Albian to early Cenomanian age for the lower part of the for- Paleocene palynofl oras. At the Eocene-Oligocene boundary a mation, and a late Cenomanian to Turonian age for the middle part. marked decrease in diversity occurred, and the number of pol- The upper part remained undated. The age was given by correlation len taxa fell below modern levels. This drop correlates with a with the palynostratigraphic scheme of Lima (1971) and Herngreen major global cooling and the beginning of the Antarctic glacia- (1973) for the Barreirinhas Basin. Dino et al. (1999) studied 43 core tion (Jaramillo et al. 2006). samples from the Alter do Chão Formation in 1-NO-1-AM and The Neogene was a period characterized by a changing climate, 9-FZ-28-AM wells (Fig. 19.1). They described two sequences in the fl uctuating sea levels and tectonic instability (Zachos et al. 2001). formation. The predominantly sandy lower sedimentary sequence These three phenomena all left their mark in the Amazonian land- was formed during the late Aptian-Albian from terrigenous infl uxes scape and its vegetation development (see Chapter 26). Although fed by cycles of anastomosing fl uvial systems with secondary aeo- the Neogene sedimentary record is incomplete, outcrops along lian reworking. At the base, unconformably overlying the Andirá the rivers and well data obtained through mineral exploration Formation, there are meandering deposits with abandoned channels together have provided us with an insight into the vegetational fi lled with clay. Those clays are rich in vegetal, amber fragments, root history. prints, fi sh remains, freshwater ostracods and conchostracan frag- The record of plant diversity in the Amazons is still incomplete. ments. The upper sequence accumulated during the Cenomanian. Nevertheless, palynological and palaeobotanical data reveal that It is almost entirely composed of fi ne-grained sediments that are during the Neogene Amazonia already was covered by a highly interpreted to represent fl uvial-deltaic-lacustrine settings. diversifi ed and multistratifi ed forest that varied in composition Dino et al. (1999) identifi ed two distinct palynofl oras (see and distribution over time under the infl uence of the major Fig. 19.1). Characteristic pollen and spores from the late Aptian- events (Hoorn 1993, 1994a, 1994b, 2006). The potential effect Albian palynofl ora (from the lower sequence) and the Cenomanian on Amazonian forests of global cooling and possible associated fl ora from the upper sequence are listed in Tables 19.1& 19.2. changing precipitation patterns over the last 5 million years is The Cretaceous vegetation was completely dominated by non- unclear. Preliminary evidence suggests a major reduction in angiosperm taxa (ferns and gymnosperms), with very few angio- area from that formerly covered by rainforest. Areas in northern sperms, unlike modern tropical forests, which are populated chiefl y Venezuela (e.g. Urumaco in Falcon Dept.) that were fl oristically by angiosperms (Gentry 1982).The presence of large numbers of similar to Amazonia during the Late Miocene, became isolated by spores, pollen grains and woody fragments of terrestrial origin, as the rise of the Andes and subsequently underwent a transforma- well as the absence of marine elements, suggests a strong continen- tion to dry vegetation. There was also an extensive development tal infl uence during the deposition of the Cretaceous Alter do Chão of tropical savannas, that further encroached on the Amazonian Formation. The low frequency of palynomorphs produced by plants rainforest. The overall effect of this reduction in forested area on better adapted to dry climates (e.g. Classopollis, Equisetosporites and Amazonian vegetation is unclear, but it might have caused a loss Gnetaceaepollenites) suggests that the Alter do Chão Formation was in diversity. However, it is now evident that the Quaternary gla- not deposited under arid climatic conditions. cial cycles did not signifi cantly affect diversity in Amazonia (Bush 1994; Rull 2008; see also Chaper 20). Amazonian Holocene cores do not show a signifi cant change in diversity or fl oristic compo- Paleogene northern South America sition. Furthermore, most of the species dated using molecular techniques indicate origination ages older than 2 million years Tropical Paleogene palynology of tropical South America has been ago (Rull 2008). widely researched since the 1950s (Van der Hammen 1954, 1956a, HHoorn_ch19_Final.inddoorn_ch19_Final.indd 331818 110/24/20090/24/2009 11:56:59:56:59 SShobhahobha Origin of the modern Amazon rainforest 319 63º 30' 57º 30' 51º 30' 1º 00' 1º 00' 1-AC-1-PA North Platform 9-FZ-28-AM 1-NO-1-AM Central Trough 3º 00' 3º 00' 2-MD-1-AM os SOUTH HINGE South Platform 5º 00' 5º 00' 63º 30' 57º 30' 51º 30' A B C Fig.