Valentí Rull

A palynological record of a secondary succession after fire in the Gran Sabana, Venezuela

Fire has been considered one of the most important factors in the expansion of savannas in the Gran Sabana region. In Urue, an important fire event that occurred before 1.6 kyr BP led to the replacement of ‘primary’ forests by savannas and morichales (monospecific communities of the palm Mauritia). In the present work, the secondary succession after fire is reconstructed by palynological analysis of a previously dated clay core, and the results are compared with those from studies based on present-day ecosystems. Charcoal analysis is used to infer fire incidence and surface samples are used as modern analogues. The secondary succession is subdivided into seven seral stages: open secondary forests, helechal or dense fern community, transitional savanna, wet savanna with morichales, treeless savanna, wet savanna with morichales, and treeless savanna. Fires were common at the beginning, but climate constituted the main successional control from the transitional savanna stage onwards. The process is characterised by a continuous impoverishment of taxa, but there was a steady increase in Mauritia, due to its ability to colonise new habitats created by disturbances. The conclusions of this palynological reconstruction show good correspondence with present-day studies on fire ecology. Copyright

Palaeofloristic and palaeovegetational changes across the Paleocene/Eocene boundary in northern South America

Abstract A floral change occurring in northern South America at the Paleocene/Eocene boundary is analysed using palynological data. The sequence studied is an outcrop from the Venezuelan Maracaibo basin, deposited in shallow marine to coastal environments without apparent stratigraphic breaks. Significant pollen and spore counts from 237 samples were studied stratigraphically and statistically in order to compare Paleocene and Eocene palaeofloras and palaeoecological trends. The Late Paleocene/Early Eocene transition is the boundary between two floras which differ both qualitatively and quantitatively. However, the change is not sudden but stepped and gradual. Paleocene taxa seem to be of pantropical distribution, whereas Eocene assemblages are more restricted to the Neotropics. The global warming well documented elsewhere is proposed as the major cause for these changes. Trends in sporomorph diversity in this record appear to track changes in temperature documented in isotopic records from temperate regions. The extinct parent plant of Echitriporites trianguliformis is tentatively proposed to be intolerant to high temperatures, because of its absence during the Early Eocene warm phase. Palaeoecologically, although marsh and back-mangrove swamps dominated both Late Paleocene and Early Eocene assemblages, their taxonomic compositions were different, especially in the inland marsh forests. Mangrove components are scarce or absent through the whole sequence studied, suggesting the absence of these communities during the time-interval analysed. A palaeoecological subdivision into assemblage zones was not possible; instead, a recurrent pattern suggesting palynological cycles was observed. However, palynocycles could not be studied in detail due to the lack of knowledge of botanical affinities for many of the taxa involved and the apparent absence of mangrove assemblages.

Middle Eocene mangroves and vegetation changes in the Maracaibo Basin, Venezuela

As a part of a general project that aims to reconstruct the paleosuccession of Paleogene mangroves of the Maracaibo Basin, this paper deals with the quantitative reconstruction of Middle Eocene mangrove communities, and their relation to potential forcing factors. Four palynological assemblages were found. These represent, respectively, inland forests (A1), back-mangrove herbaceous swamps (B1), mangroves (B2), and an unknown plant community dominated by the extinct Echitriporites trianguliformis. Mangroves were dominated by Pelliciera and Nypa; Brevitricolpites variabilis, which has been considered the dominant taxon of the early and middle Eocene mangroves in nearby areas, has not been found in this study. The succession of coastal vegetation, linked to sea-level changes, could be reconstructed from these assemblages. The trends constitute a palyno-cycle which began and ended with a low sea-level plant community dominated by unknown stands represented by E. trianguliformis and interpreted low paleosalinities; intermediate high sea-level vegetation is represented by mangroves and interpreted high paleosalinities. This cycle is correlated chronologically with the global eustatic cycle TEJAS A 3.4, extending from 44 to 42.5 Ma (Lutetian). The floristic composition of middle Eocene mangroves was very different from those of the Oligocene to Recent. An important, probably worldwide, evolutionary change occurred during the late middle Eocene and the late Eocene in these communities. Pollen taxa botanically related to known and extant mangrove elements seem scarce for this time span.

Ecostratigraphic Study of Paleocene and Early Eocene Palynological Cyclicity in Northern South America

Abstract A quantitative palynological study of the Paleocene/Eocene transition in western Venezuela was undertaken to detect and analyze possible cyclic patterns. Two different methodologies were used, palynocycles and ecologs, and their results are compared. A total of 237 outcrop samples from three formations deposited in continental to coastal environments were analyzed for pollen and fern spores. Several palynological cycles are recorded and correlated with third-order global eustatic cycles. A high-frequency cyclicity of ca. 220,000-year period also was found. Both methodologies recorded the same cyclic patterns and can be considered complementary. Ecologs are easier to use, but have less interpretative potential. Palynocycles are more complex, but also more descriptive and help detect small hiatuses. Diversity values reach maxima at cycle boundaries and their minima in the middle of cycles. This distribution has been interpreted in terms of different palynomorph sources under conditions of high and low sea level. During the Paleocene/Eocene transition, diversity shows a constant ascending trend, probably due to a long-term, worldwide climatic warming.

Surface palynology of a small coastal basin from Venezuela and potential paleoecological applications

The palynomorph content of surface samples from a sea-land transect in a small coastal basin was analysed, in order to characterise palynologically the sedimentary subenvironments and vegetation types. These results should provide the basis for the interpretation of further palaeoecological analyses on stratigraphical samples from the same site. The main vegetation types are mangroves, swamps, Cocos plantations and abandoned crops, arranged in a zonal pattern. All the palynomorph types found were recorded and counted, including pollen, fern and allied spores, fungi spores and unknown remains (probably algal material and animal parts). Both general and individual trends of these palynomorphs followed the main vegetation patterns, and both zonal sea-land arrangement of plant communities and their disturbance patterns were reproduced by them. Furthermore, particular types were found to be direct or indirect indicators of given plant zones and local disturbance. Detrended correspondence analysis allowed definition of the sedimentary attraction domains for the involved communities and ordination of both samples and palynomorphs with respect to them. The results are encouraging for palaeoecological studies, since the main key environmental and vegetational patterns are reflected by the palynomorphs of surface sediments. Limitations are also discussed.

A QUANTITATIVE PALYNOLOGICAL RECORD FROM THE EARLY MIOCENE OF WESTERN VENEZUELA, WITH EMPHASIS ON MANGROVES

The quantitative reconstruction presented in this paper documents paleoecological trends of the northern Maracaibo basin (western Venezuela), during the Early Miocene, through pollen analysis of drill core samples from the La Rosa (shallow marine) and Lagunillas (coastal plain) formations. Palynological assemblages were grouped statistically into three assemblages representing mangroves, herbaceous back‐mangrove swamps, and inland palm/ fern swamps. Mangroves were the local vegetation during the deposition of La Rosa Formation and, after a regressive event, were replaced by herbaceous back‐mangroves during the sedimentation of Lagunillas Formation. The palynological assemblage representing palm/fern swamps dominates the entire sequence and is interpreted as a large background signal reflecting transport by rivers. Mangrove communities had few species, and represented a transitional phase in the mangrove community evolution, after the terminal Eocene biotic crisis.

High-impact palynology in petroleum geology: Applications from Venezuela (northern South America)

This article documents the application of high-impact palynology (HIP) in the Maracaibo Basin of Venezuela and its influence on such exploration and production aspects as regional planning and strategies, risk reduction, optimal drilling decisions and investment, petroleum-system modeling, new discoveries, and secondary recovery by fluid injection, among others. High-impact palynology has been defined as the coupling of high-resolution sequence biostratigraphy, multidisciplinary work, and the alignment of palynology with the attainment of business goals. The first part of this article explains the high-resolution ecostratigraphic methods used and the concept of integrated work applied. The second part of the article shows the results obtained in selected case studies, which illustrate the advantages of HIP. Among the most relevant studies are high-resolution ecostratigraphic frames at a basin level, timing between structural trap formation and oil migration, differentiation of petroleum systems in adjacent reservoirs, the concept of palynoblocks in structurally complex areas to estimate missing sections, stratigraphical models for exploratory wells with better predictions of target horizons, fine-scale reservoir correlations, and discovery of new reservoirs. The use of HIP in other areas is recommended, with palynology as a common in-house practice within multidisciplinary teams formed especially for each specific task.

Chrysophycean stomatocysts in a Caribbean mangrove

A chrysophyte cyst assemblage from surface sediments of a tropical, brackish-water environment in the southern Caribbean coast is described and depicted. All the cysts are unornamented and relatively large. The taxonomic diversity is low, due probably to human disturbance or salinity stress. It is assumed that the chrysophyte taxa involved are salinity-tolerant, freshwater algae. This is the first record of chrysophyte cysts in mangrove environments, and extends the range of habitats in which they are commonly found.

Palaeoecological significance of chrysophycean stomatocysts: a statistical approach

The statistical relationship between chrysophycean cyst abundances and ecologically known factors, derived from multivariate analyses, is proposed as a useful way to derive palaeoecological information of non-identified cysts. The method is applied to a case-study from the Spanish Pyrenees, and encouraging results are obtained for some of the morphotypes found.

Climatic and ecological history of Pantepui and surrounding areas with biogeographical and evolutionary considerations

This chapter reviews the available paleoecological information on Pantepui since the Last Glacial Maximum (LGM), in order to reconstruct the ecological dynamics that have led to the present configuration of plant communities and to unravel the potential environmental drivers involved, with emphasis on regional climate changes and fire. To date, no LGM sediments have been retrieved atop the tepuis, so the vegetation of these summits during the last glaciation remains unknown. Some lowland records suggest that cold LGM climates favored downward migration of temperature-sensitive tepuian species, which drove changes in the taxonomic composition of lowland forests. The available paleoecological record of Pantepui ranges from the early Holocene to the present. These records show two contrasting situations. Some tepui summits exhibit a long vegetation constancy extending back to the mid Holocene, whereas others document significant changes in sensitive species that have been associated with regional climatic events such as the Holocene Thermal Maximum (HTM), the latitudinal migration of the Intertropical Convergence Zone (ITCZ), the El Nino-Southern Oscillation (ENSO) variability, or recent climatic shifts such as the Medieval Climatic Anomaly (MCA) or the Little Ice Age (LIA). This has been interpreted in terms of site’s sensitivity and it has been recommended to obtain past records preferably on elevational ecotones, where vertical migrations of species are more easily detected. During the last millennium, fire, most probably of anthropogenic origin and likely originated on the surrounding uplands, has been a major driver of vegetation change on some tepuis. These studies are useful both to understand the biodiversity and composition of present Pantepui plant communities and to test classical biogeographical and evolutionary hypotheses on the origin of biodiversity and endemism patterns.