H. Hooghiemstra

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.

Comparison of terrestrial and marine records of changing climate of the last 500,000 years

A broad correspondence between long pollen sequences and the deep-sea oxygen isotope record has been noted for some time, but there has been little effort to explore just how similar the two types of evidence are in terms of their overall structure on glacial-interglacial timescales and also how they may differ. These questions have profound importance both for how we view the stratigraphic record of changing climate in different regions and for our understanding of the climate system. Here we link the four longest European pollen records and derive a terrestrial sequence of vegetation events and a coherent stratigraphic scheme for the last 500,000 years. Comparison of the terrestrial and marine records shows good agreement, but it also reveals that the pollen sequences contain a higher degree of climate sensitivity than the oxygen isotope record. In addition, it suggests that neither an oxygen isotope record nor a Milankovitch-forced ice volume model may provide an appropriate template for fine-tuning the terrestrial record and that better chronologies will depend on an improved understanding of controls on sedimentation rates in individual sedimentary basins

A multi-proxy study of Holocene environmental change in the Maya Lowlands of Peten, Guatemala

We used multiple variables in a sediment core from Lake Peten-Itza, Peten, Guatemala, to infer Holocene climate change and human influence on the regional environment. Multiple proxies including pollen, stable isotope geochemistry, elemental composition, and magnetic susceptibility in samples from the same core allow differentiation of natural versus anthropogenic environmental changes. Core chronology is based on AMS 14C measurement of terrestrial wood and charcoal and thus avoids the vagaries of hard-water-lake error. During the earliest Holocene, prior to ∼9000 14C yr BP, the coring site was not covered by water and all proxies suggest that climatic conditions were relatively dry. Water covered the coring site by ∼9000 14C yr BP, coinciding with filling of other lakes in Peten and farther north on the Yucatan Peninsula. During the early Holocene (∼9000 to ∼6800 14C yr BP), pollen data suggest moist conditions, but high δ 18O values are indicative of relatively high E/P. This apparent discrepancy may be due to a greater fractional loss of the lakes water budget to evaporation during the early stages of lake filling. Nonetheless, conditions were moist enough to support semi-deciduous lowland forest. Decrease in δ 18O values and associated change in ostracod species at ∼6800 14C yr BP suggest a transition to even moister conditions. Decline in lowland forest taxa beginning ∼5780 14C yr BP may indicate early human disturbance. By ∼2800 14C yr BP, Maya impact on the environment is documented by accelerated forest clearance and associated soil erosion. Multiple proxies indicate forest recovery and soil stabilization beginning ∼1100 to 1000 14C yr BP, following the collapse of Classic Maya civilization.

A Holocene vegetation history from lowland Guatemala

A 5.45-m core from Lake Peten-Itza, lowland Guatemala, contains a near-complete record of Holo cene sedimentation. The age-depth relationship for the core is based on AMS 14C dating of terrestrial wood fragments and provides a reliable chronology in this karst region where hard-water lake error has typically confounded sediment geochronology. In the basal part of the sequence, pollen of the Moraceae-Urticaceae group dominate, indicating the presence of widespread tropical forest during the early Holocene (c. 8600-5600 yr BP). Relative abundance of pollen of high forest taxa declined beginning as early as 5600 yr BP, indicating climatic drying or perhaps initial land clearance. Deforestation by prehistoric Mayan inhabitants is documented clearly in the pollen record beginning about 2000 yr BP (106 cal BC-122 cal AD, 95.4%, 2 sigmas) by the appearance of disturbance taxa (e.g. Ambrosia and Poaceae) and presence of Zea. Forest regrowth occurred following the Classic Maya collapse, c. AD 900, as reflected by a relative increase in Moraceae-Urticaceae pollen.

Making sense of palaeoclimate sensitivity

Many palaeoclimate studies have quantified pre-anthropogenic climate change to calculate climate sensitivity (equilibrium temperature change in response to radiative forcing change), but a lack of consistent methodologies produces a wide range of estimates and hinders comparability of results. Here we present a stricter approach, to improve intercomparison of palaeoclimate sensitivity estimates in a manner compatible with equilibrium projections for future climate change. Over the past 65 million years, this reveals a climate sensitivity (in K W−1 m2) of 0.3–1.9 or 0.6–1.3 at 95% or 68% probability, respectively. The latter implies a warming of 2.2–4.8 K per doubling of atmospheric CO2, which agrees with IPCC estimates.

Distribution and Ecology of Parent Taxa of Pollen Lodged Within the Latin American Pollen Database.

The cornerstone of palaeoecological research, concerned with vegetation dynamics over the recent geological past, is a good understanding of the present-day ecology and distribution of the taxa. This is particularly necessary in areas of high floral diversity such as Latin America. Vegetation reconstructions, based on numerous pollen records, now exist with respect to all major vegetation associations from Latin America. With this ever-increasing number of sedimentary records becoming available, there is a need to collate this information and to provide information concerning ecology and distribution of the taxa concerned. The existing Latin American Pollen Database (LAPD) meets the first of these needs. Information concerning the ecology and distribution of the parent taxa responsible for producing the pollen, presently lodged within the LAPD, is the focus of this paper. The ‘dictionary’ describes the ecology and distribution of the parent taxa responsible for producing pollen identified within sedimentary records. These descriptions are based on a wide range of literature and extensive discussions with members of the palaeoecological community working in different parts of Latin America investigating a range of different vegetation types.

Establishing a terrestrial chronological framework as a basis for biostratigraphical comparisons.

The palynological signature of interglacial deposits in the fragmentary European terrestrial record has often been used as the basis for determining their chronostratigraphical position and ultimately their age. This has placed emphasis on the presence/absence and abundance of certain characteristic taxa, but given the lack of continuous stratigraphies and independent chronologies, it has been difficult to assess the extent to which this strategy has produced reliable schemes. Here, an alternative approach is adopted whereby a chronological framework is developed for long and continuous pollen sequences from southern Europe. This in turn allows the emergence of a complete stratigraphical scheme of major vegetation events for the last 430 thousand years (ka) and the evaluation of the stage record of different taxa and their potential diagnostic value for biostratigraphical correlation. The comparison shows distinct similarities among some temperate stages of the terrestrial equivalent complexes of Marine Isotope Stages (MIS) 5 and 7 and also of MIS 9 and 11, but examination of combined records of taxa provides a possibility to differentiate between individual stages. A numerically-derived dichotomous key for the terrestrial stages based on the palynological records of 10 taxa is presented. Carpinus, Fagus, Abies, Pterocarya and Buxus emerge as the best ‘indicator pollen types’ because of their variable behaviour from one stage to the next, possibly a result of their late expansion within a temperate stage or reduced genetic variability. The analysis shows that the palynological signature of a temperate deposit can constrain the range of chronostratigraphical possibilities, but vegetation and palynological variability arising from local factors could result in difficulties in making a definite assignment at individual sites.

Comparison of changes in vegetation in northeast Greece with records of climate variability on orbital and suborbital frequencies over the last 450 000 years

Abstract A new direct pollen–orbital tuning procedure, based on a correspondence between changes in certain vegetation elements and March and June perihelion configurations, is applied to the Tenaghi Philippon record, northeast Greece. The development of a refined chronological scheme allows comparisons to be made with records of climate variability from the North Atlantic as well as of global sea level/ice volume and atmospheric CO2 content. On orbital frequencies, the comparison reveals a close correspondence between relative ice volume extent and tree population size during glacial intervals. During interglacial and interstadial periods the degree of forest development is more closely associated with high-latitude insolation and related climate regimes rather than extent of residual ice volume. On suborbital frequencies, the Greek record shows similar repeat times in peaks of steppe vegetation with North Atlantic ice-rafting events, but the amplitude of this variability in the two records is not always proportional. Overall, what emerges is that the major shifts in the relative abundance of forest v. open vegetation communities at Tenaghi Philippon on orbital and suborbital frequencies over the last 450 000 years are coherent with high-latitude changes affecting atmospheric and oceanic circulation. Glacial decreases in atmospheric CO2 content contributed to the elimination of tree populations by exacerbating water stress conditions during arid intervals, but do not appear to be the primary driver of the observed vegetation changes at this latitude. Variations in CO2 levels (in the order of 40 ppmv) between different interglacial/interstadials periods with adequate moisture availability appear to have had a limited effect on the nature and size of tree populations.

Holocene Amazon rainforest-savanna dynamics and climatic implications: high-resolution pollen record from Laguna Loma Linda in eastern Colombia

We present a high-resolution pollen record of a 695-cm-long sediment core from Laguna Loma Linda, located at an altitude of 310 m in the transitional zone between the savannas of the Llanos Orientales and the Amazonian rainforest, about 100 km from the Eastern Cordillera. Based on eight AMS 14 C ages, the record represents the last 8700 14 C yr BP. During the period from 8700 to 6000 14 C yr BP the vegetation was dominated by grass savanna with only a few woody taxa, such as Curatella and Byrsonima, present in low abundance. Gallery forest along the drainage system apparently was poorly developed. Compared with today, precipitation must have been significantly lower and seasonality stronger. During the period from 6000 to 3600 14 C yr BP, rainforest taxa increased markedly, reflecting an increase in precipitation. Rainforest and gallery forest taxa such as Moraceae/Urticaceae, Melastomataceae, Alchornea, Cecropia and Acalypha, were abundant, whereas Poaceae were reduced in frequency. From 3600 to 2300 14 C yr BP rainforest taxa continued to increase; Moraceae/Urticaceae became very frequent, and Myrtaceae and Myrsine became common. Savanna vegetation decreased continuously. We infer that precipitation was still increasing, and that the length of the annual dry period possibly shortened. From 2300 14 C yr BP onwards, grass savanna (mainly represented by Poaceae) expanded and Mauritia palms became frequent. This reflects increased human impact on the vegetation. Copyright

Climatic forcing of asymmetric orogenic evolution in the Eastern Cordillera of Colombia

New apatite fission-track data, paleoelevation estimates from paleobotany, and recently acquired geological data from the Eastern Cordillera of Colombia document the onset of increased exhumation rates in the northeastern Andes at ca. 3 Ma. The Eastern Cordillera forms an efficient orographic barrier that intercepts moisture-laden winds sourced in the Amazon lowlands, leading to high rainfall and erosion gradients across the eastern flank of the range. In contrast, the drier leeward western flank is characterized by lower rates of deformation and exhumation. In light of the geological evolution of the Eastern Cordillera, the combination of these data sets suggests that the orographic barrier reached a critical elevation between ca. 6 and ca. 3 Ma, which ultimately led to protracted, yet more focused erosion along the eastern flank. Sequentially restored structural cross sections across the eastern flank of the Eastern Cordillera indicate that shortening rates also have increased during the past 3 Ma. From fission-track and structural cross-section balancing, we infer that accelerated exhumation led to increasing tectonic rates on the eastern flank, creating a pronounced topographic and structural asymmetry in the Eastern Cordillera. The tectonic and climatic evolution of this orogen thus makes it a prime example of the importance of climatic forcing on tectonic processes.