Carina Hoorn

Amazonia Through Time: Andean Uplift, Climate Change, Landscape Evolution, and Biodiversity

The Making of Amazonian Diversity The biodiversity of the Amazon Basin is legendary, but the processes by which it has been generated have been debated. In the late 20th century the prevalent view was that the engine of diversity was repeated contraction and expansion of forest refugia during the past 3 million years or so. Hoorn et al. (p. 927) analyze findings from a diverse range of disciplines, including molecular phylogeny, ecology, sedimentology, structural geology, and palaeontology, to offer an overview of the entire history of this region during the Cenozoic era (66 million years ago). The uplift of the Andes was a pivotal event in the evolution of Amazonian landscapes because it continually altered river drainage patterns, which in turn put a variety of pressures on organisms to adapt to changing conditions in a multiplicity of ways. Hence, the diversity of the modern biota of the Amazon has more ancient origins than previously thought. The Amazonian rainforest is arguably the most species-rich terrestrial ecosystem in the world, yet the timing of the origin and evolutionary causes of this diversity are a matter of debate. We review the geologic and phylogenetic evidence from Amazonia and compare it with uplift records from the Andes. This uplift and its effect on regional climate fundamentally changed the Amazonian landscape by reconfiguring drainage patterns and creating a vast influx of sediments into the basin. On this “Andean” substrate, a region-wide edaphic mosaic developed that became extremely rich in species, particularly in Western Amazonia. We show that Andean uplift was crucial for the evolution of Amazonian landscapes and ecosystems, and that current biodiversity patterns are rooted deep in the pre-Quaternary.

Andean tectonics as a cause for changing drainage patterns in Miocene northern South America

NewdatafromNeogenestratainnorthernSouthAmericasuggestthatMiocenetectonisminthenortheasternAndeswasresponsible for the genesis of the Amazon River and changes in the drainage patterns of other major rivers such as the Magdalena and the Orinoco. Here we present a new model for the paleogeographic evolution of northern South America during the Miocene. In the early Miocene, a large part of the drainage of northwest Amazonia was directed northward along the paleo‐Orinoco river system to a deltainLakeMaracaibo.UpliftoftheEasternCordillerainthelate middle Miocene caused thefirst development of the Amazon River; however, no connection with the Atlantic was established, and the Amazon fed the paleo‐Orinoco river system, which drained toward the Caribbean. Substantial Andean uplift in the late Miocene resultedinmajorchangesinpaleogeography:theOrinocochangedits course, the Amazon established a connection to the Atlantic, causing the drowning of carbonate platforms, and the Amazon-Caribbeanconnectionwasclosed.Thusthedrainageandpaleogeography ofnorthernSouthAmericaintheMiocenewerestronglycontrolled by tectonic movements in the northeastern Andes.

Marine incursions and the influence of Andean tectonics on the Miocene depositional history of northwestern Amazonia: results of a palynostratigraphic study

Abstract New palynological and sedimentological data permit the age of Neogene sediments in northwestern Amazonia to be established more precisely, and indicate that major environmental changes occurred in the area during this time. Based on a study of borehole samples the age of the Solimoes Formation (Solimoes Basin, northwestern Brazil) is determined as Miocene and five pollen zones are distinguished which are correlated with existing zonations for northern South America. In addition, 30 new sporomorphs are described which belong to the following 12 genera: Psilamonocolpites, Retimonocolpites, Retitricolpites, Retibrevitricolpites, Psilatriporites (nov. gen.), Bombacacidites, Psilatricolporites, Retitricolporites, Rugutricolporites, Psilastephanoporites (nov. gen.), Psilastephanocolporoties and Heterocolpites. A correlation is made between the Brazilian wells and some of the studied outcrops in Colombian and Peruvian Amazonia based on palynological marker species. The presence of coastal elements such as mangroves (Zonocostites), indicates that during the Miocene this area was influenced by marginal marine conditions caused by several marine incursions. These incursions may be related to global sea level fluctuations. Miocene marine phases also are known from sites elsewhere in northern South America which at present, like northwestern Amazonia, are entirely ruled by continental conditions. Sediment composition shows that during the Early Miocene the Guyana Shield was the major source area of sediment input in the basins of northwestern Amazonia. In the interval from Early to Middle Miocene the Andes became the major sediment source area. The change in provenance is related to the uplift of the Eastern Cordillera. This event caused a major change in palaeoenvironment and palaeogeography in northwestern Amazonia which was characterized by the reverse of a northwestward directed fluvial system into an eastward directed fluvial-lacustrine system with an estuarine character.

An environmental reconstruction of the palaeo-Amazon river system (Middle to late Miocene, N.W. Amazonia

Abstract New sedimentological and palynological data from the Tertiary sediments in the Upper Amazon River area suggest that these sediments are fluvio-lacustrine deposits of Middle to Late Miocene age. They were generated as a result of the uplift of the Eastern Cordillera (Andes) and constitute possibly the oldest relics of the Amazon River system. The palaeoenvironment in which these sediments were deposited is characterized by extensive wetlands environments formed by swamps, shallow lakes, crevasse splay channels and crevasse-delta lakes where the channel environment is poorly represented. The palaeovegetation was dominated by palms (e.g. Mauritia and Grimsdalea), riverine taxa (e.g. Bombacaceae, Amanoa and Alchornea), ferns and fern allies (e.g. Polypodiaceae and Selaginellaceae), floating meadows (Gramineae) and aquatic taxa (Ceratopteris, Botryococcus and Azolla). The relative abundance of Gramineae and the occurrence of Andean-type pollen taxa is related to the Andean origin of the fluvial system. The varzeas of the present Upper Amazon River flood-basin are probably the best analogue for the Middle to Late Miocene environment. Intervals rich in marine palynomorphs, mangrove pollen, brackish tolerant molluscs and ostracods, and ichnofossils of the Thalassinoides-Teichichnus association suggest that the palaeoenvironment was characterized by brackish conditions and marine influence. These marine incursions are possibly related to the Langhian and the Serravallian global sea-level rise. Although in the Middle Miocene a global cooling is known to have occurred, no indicators of a cooler climate have been observed in the Miocene palynoflora of the Upper Amazon River area. Finally, four new sporomorph species are described belonging to the form-genera Psilatriletes, Clavainaperturites and Psilaperiporites.

Tibetan uplift prior to the Eocene-Oligocene climate transition: Evidence from pollen analysis of the Xining Basin

Uplift of the Tibetan Plateau and the Himalayas since the onset of the Indo-Asia collision is held responsible for Asian aridifi cation and monsoon intensifi cation, but may also have gradually cooled global climate, leading to the 34 Ma Eocene-Oligocene transition. To unravel the interplay between Tibetan uplift and global climate, proxy records of Asian paleoenvironments constrained by accurate age models are needed for the Paleogene Period. Here we report the 38 Ma appearance of high-altitude vegetation recovered from palynological assemblages in precisely dated lacustrine sediments from the Xining Basin of the northeastern Tibetan Plateau region. This result confi rms previous evidence for important regional uplift in the central and northern Tibetan Plateau regions during the early stage of the Indo-Asia collision. This is consistent with the idea that the associated increase in rock weathering and erosion contributed to lowering of atmospheric CO 2 , leading to the Eocene-Oligocene transition.

Fluvial palaeoenvironments in the intracratonic Amazonas Basin (Early Miocene-early Middle Miocene, Colombia)

Abstract Based on a detailed study of Early Miocene to early Middle Miocene sediments in the Caqueta River area (Amazonas Basin, Colombia), a reconstruction of the palaeoenvironmental and palaeogeographical history is made. Sediment composition, sedimentary sequences and transport directions indicate that the succession was deposited by a low sinuosity fluvial system with an anastomosing character that originated from the Guyana Shield. Basin subsidence, Precambrian and Palaeozoic palaeorelief, and a changing base level controlled deposition in this fluvial system. Sedimentary environments such as channels, abandoned channels, crevasse splays, backswamps and palaeosoils characterized this system where fluvial flooding was common. The Early Miocene to early Middle Miocene sequences have been truncated by a more recent fluvial system, probably of Pliocene to Pleistocene age, representing the precursor of the present Caqueta River. Palynological analysis suggest that the sediments are part of the Retitricolporites and the Psiladiporites-Crototricolpites pollen zones. Fifteen new species are described which belong to the genera: Psilatricolpites, Retitricolpites, Retitricolporites, Bombacacidites, Syncolporites and Psilastephanocolporites. The palynological assemblages indicate that the area was characterized by palm swamps, riparian vegetation and a rather diverse tropical forest. Intervals rich in marine palynomorphs accompanied by high concentrations of mangrove pollen grains (Zonocostites) suggest periods of marine influences and mangrove development. It is thought that the marine intervals are related to the Late Burdigalian transgression.

Amazonia: landscape and species evolution: a look into the past

The book focuses on geological history as the critical factor in determining the present biodiversity and landscapes of Amazonia. The different driving mechanisms for landscape evolution are explored by reviewing the history of the Amazonian Craton, the associated sedimentary basins, and the role of mountain uplift and climate change. This book provdes an insight into the Meso- and Cenozoic record of Amazonia that was characterized by fluvial and long-lived lake systems and a highly diverse flora and fauna. This fauna includes giants such as the ca. 12 m long caiman Purussaurus, but also a varied fish fauna and fragile molluscs, whilst fossil pollen and spores form relics of ancestral swamps and rainforests. Finally, a review the molecular datasets of the modern Amazonian rainforest and aquatic ecosystem, discussing the possible relations between the origin of Amazonian species diversity and the palaeogeographic, palaeoclimatic and palaeoenvironmental evolution of northern South America. The multidisciplinary approach in evaluating the history of Amazonia has resulted in a comprehensive volume that provides novel insights into the evolution of this region.

Mangrove Forests and Marine Incursions in Neogene Amazonia (Lower Apaporis River, Colombia)

Abstract The lower Apaporis River area (Colombian Amazonia) is characterized by fluvial and coastal sediments of Middle to Late Miocene age. These sediments, here informally called Apaporis sand unit, are in nonconformable contact with the Precambrian basement, and were deposited in a low-sinuosity fluvial system with an anastomosing character that originated in the Guyana Shield. The predominantly sandy unit has organic-rich clay intervals that contain a palynological assemblage dominated by the mangrove Zonocostites ramonae (Rhizophora) and the palm Mauritiidites franciscoi (Mauritia). Abundant Zonocostites (25–85%), together with the occurrence of marine palynomorphs (dinoflagellates and foraminiferal inner-wall linings), indicate the presence of well-developed coastal mangrove forests and marine incursions. Occasional decrease of Zonocostites in favor of Mauritiidites suggests that the coastline fluctuated and the mangroves were replaced by palm vegetation. The Middle to Late Miocene age of these sediments is based on presence of the palynological marker species Grimsdalea magnaclavata and absence of the older biostratigraphic marker Crassoretitriletes vanraadshoovenii and the younger Asteraceae. This makes the unit equivalent in age to the upper Pebas/Solimões Formation. The marine ingression and coastal conditions in the heart of Amazonia possibly are related to a combination of global sea-level rise (Serravallian?) and subsidence in the periphery of the Guyana Shield. Modern analogues of the Miocene Apaporis fluvial/ coastal interface are present-day fluvial and coastal environments in Surinam and the Guyanas.

Pollen morphology of Ephedra (Gnetales) and its evolutionary implications

Abstract The Ephedra lineage can be traced at least to the Early Cretaceous. Its characteristically polyplicate pollen is well-represented in the fossil record and is frequently used as an indicator of paleoclimate. However, despite previous efforts, knowledge about variation and evolution of ephedroid pollen traits is poor. Here, we document pollen morphology of nearly all extant species of Ephedra, using a combination of scanning electron microscopy (SEM) and light microscopy (LM), and reconstruct ancestral states of key pollen traits. Our results indicate that the ancestral Ephedra pollen type has numerous plicae interspaced by unbranched pseudosulci, while the derived pollen type has branched pseudosulci and (generally) fewer plicae. The derived type is inferred to have evolved independently twice, once along the North American stem branch and once along the Asian stem branch. Pollen of the ancestral type is common in Mesozoic fossil records, especially from the Early Cretaceous, but it is less commonly reported from the Cenozoic. The earliest documentation of the derived pollen type is from the latest Cretaceous, after which it increases strongly in abundance during the Paleogene. The results of the present study have implications for the age of crown group Ephedra as well as for understanding evolution of pollination syndromes in the genus.

An early start for the Panama land bridge

The land bridge between North and South America formed 10 million years earlier than previously thought [Also see Report by Montes et al.] The birth of the Panama land bridge, which connects the Americas, has been associated with one of the biggest biological exchanges in Earth history as numerous species migrated from one continent to the other (1). Nevertheless, the timing of formation of the land bridge is still much debated (2). On page 226 of this issue, Montes et al. (3) propose that the Central American Seaway, which separated South and North America, closed about 15 to 13 million years ago, more than 10 million years earlier than previously thought (4), with important implications for ocean circulation, climate, and biotic exchange.