Variation In Pollen Assemblages Research Articles

Pollen and spore records constrained by millennial prodelta evolution: a case study of the Huanghe (Yellow River) delta

Pollen and spore records in prodeltaic sediments hold significant potential for reconstructing paleoecologic and paleoclimatic evolution. However, uncertainties in these reconstructions arise from millennial-scale prodelta evolution, which dominates stratigraphic development and consequently influences sedimentary processes and/or pollen provenance. Here we explore the intricate relationship between pollen/spore records and prodelta stratigraphic evolution, using established seismic stratigraphy and ten sediment cores (five new, five from literature) within both the proximal and distal (mud belt) parts of the Huanghe (Yellow River) prodelta. In the proximal region, dominant lobate stratigraphic development, accompanied by shifts in river mouth and depocenter, leads to variations in pollen assemblages and contents within individual cores and differences in vertical pollen distribution across core sites. Transport distance appears to be a key factor, with arboreal pollens, particularly saccate ones (e.g., Pinus), positively correlating with the distance from the river mouth in their percentages within a single delta lobe, while non-arboreal and non-saccate arboreal pollens show higher percentages within shorter transport distances, despite longer distances leading to decreased total pollen concentrations. Likely due to the total pollen concentration after extended long-distance transport, this pattern is not observable in the distal mud belt. Subsurface stratigraphy in this mud belt reveals a complex pollen provenance characterized by Artemisia-Ulmus-Chenopodiaceae-Pinus, with non-arboreal pollens in dominance. Therein, non-arboreal pollens are not consistent with deposition from long-distance transport, and Ulmus pollens are uncommon in the western Bohai Sea. Interestingly, surface sediments in the mud belt display a different assemblage, characterized by Pinus-Artemisia-Quercus, consistent with the nearby Luanhe River prodelta, suggesting recent pollen supply from nearby sources, possibly due to the recent abandonment of the mud belt. Additionally, an energetic longshore transport/erosional regime reduces pollen content at the mud-belt margins and create pollen sinks (with the highest concentration) in the mud patch (accumulation area) within the erosion-dominated region adjacent to the mud belt. Our findings confirm that stratigraphic evolution, alongside hydrodynamic conditions and pollen provenance, governs pollen assemblages in deltaic/prodeltaic sediments. They can provide insights for palynological and pollen-based paleoclimatic and paleoecologic studies in other deltas.

Analysis of pollen across the surface sediments of Lake Imbradas, Lithuania

AbstractIn conventional pollen analysis, usually one sediment core per basin is analyzed to reconstruct past environmental conditions. This approach does not consider spatial heterogeneity of pollen assemblages, and assumes that one analyzed location is representative of the whole basin. To improve the spatial resolution of fossil pollen studies, further knowledge of the factors influencing variations in pollen assemblages throughout a basin is needed. We examined the spatial heterogeneity of pollen assemblages from 45 lacustrine surface samples from a lake with relatively simple hydrology and compared this dense network of surface pollen samples with the Lithuanian State Forest Service arboreal vegetation database. Calculations of pollen productivity at different locations across the lake revealed variations in the behavior of a pollen-vegetation relationship model in different parts of the basin. Our findings suggest that the model underestimated pollen contributions from the lakeshore vegetation. We demonstrate that detailed investigations of surface pollen as a step prior to fossil pollen investigations can provide useful insights, including understanding the influence of sedimentation rate on modelling results and spatial variations in pollen composition, thus providing guidance for site selection for fossil pollen studies.

Late Pleistocene and Holocene vegetation changes and anthropogenic impacts in the Cauvery delta plains, southern India

A 5 m long core from Uttarangudi in the distal plain of Cauvery delta in South India provides evidences of vegetation, climate and anthropogenic activities during the late Pleistocene and Holocene. The pollen catchment area for this region is the localised sources, including the adjoining Tertiary upland plateau. The recorded variation in pollen assemblages and sediments indicates that the environmental conditions have changed over time from aerial oxidising to lacustrine or ponded environment. The lack of pollen records from 17,802 cal BP until 11,535 cal BP suggests severe drier conditions and an extremely patchy, species-poor Tropical Dry Evergreen Forest (TDEF) vegetation community associated with an arid climate. Between 11,535 and 8487 cal BP, partial climatic amelioration appears to have occurred, and the trend become humid until 7034 cal BP characterised by the expansion of the TDEF vegetation and strengthening of the monsoon precipitation. The period from 7034–3553 cal yr BP recorded a regressive transition of the forest taxa influenced by a weakened monsoon precipitation and enhanced anthropogenic activity. The subsequent increase of disturbance signals post 3553 cal yr BP is in agreement with independent archaeological evidences obtained from this region.

Exploring the prevalence and diversity of pollen carried by four species of migratory Old World warbler (Sylvioidea) on arrival in the UK

Capsule Pollen encrusted around the bill of migrating warblers can reveal marked differences in foraging ecology between bird species.Aims To examine patterns of the prevalence and diversity of pollen in four species of warbler, and explore the potential of pollen to act as an indicator of recent foraging behaviour.Methods By isolating pollen from bill encrustations using laboratory palynological techniques and identification by light microscopy, we examined variation in the prevalence of the five most common pollen taxa, and variation in pollen assemblages in four species of warbler arriving on the south coast of England.Results All samples contained abundant pollen, with 19 floral taxa identified. Sylvia warblers tended to carry Prunus and Citrus pollen, while Phylloscopus warblers mainly carried Eucalyptus pollen. Pollen assemblages varied markedly between bird species.Conclusion Commercial and garden flowering trees are an important resource for migrating warblers. Pollen may be such a valuable resource that flowering plants might be included in the conservation management of stopover sites. The use of pollen to resolve migratory routes may be problematic however, requiring detailed knowledge of both the distribution and flowering phenology of plants en route.

Quantitative reconstruction of precipitation changes on the NE Tibetan Plateau since the Last Glacial Maximum – extending the concept of pollen source area to pollen-based climate reconstructions from large lakes

Abstract. Pollen records from large lakes have been used for quantitative palaeoclimate reconstruction, but the influences that lake size (as a result of species-specific variations in pollen dispersal patterns that smaller pollen grains are more easily transported to lake centre) and taphonomy have on these climatic signals have not previously been systematically investigated. We introduce the concept of pollen source area to pollen-based climate calibration using the north-eastern Tibetan Plateau as our study area. We present a pollen data set collected from large lakes in the arid to semi-arid region of central Asia. The influences that lake size and the inferred pollen source areas have on pollen compositions have been investigated through comparisons with pollen assemblages in neighbouring lakes of various sizes. Modern pollen samples collected from different parts of Lake Donggi Cona (in the north-eastern part of the Tibetan Plateau) reveal variations in pollen assemblages within this large lake, which are interpreted in terms of the species-specific dispersal and depositional patterns for different types of pollen, and in terms of fluvial input components. We have estimated the pollen source area for each lake individually and used this information to infer modern climate data with which to then develop a modern calibration data set, using both the multivariate regression tree (MRT) and weighted-averaging partial least squares (WA-PLS) approaches. Fossil pollen data from Lake Donggi Cona have been used to reconstruct the climate history of the north-eastern part of the Tibetan Plateau since the Last Glacial Maximum (LGM). The mean annual precipitation was quantitatively reconstructed using WA-PLS: extremely dry conditions are found to have dominated the LGM, with annual precipitation of around 100 mm, which is only 32% of present-day precipitation. A gradually increasing trend in moisture conditions during the Late Glacial is terminated by an abrupt reversion to a dry phase that lasts for about 1000 yr and coincides with "Heinrich event 1" in the North Atlantic region. Subsequent periods corresponding to the Bølling/Allerød interstadial, with annual precipitation (Pann) of about 350 mm, and the Younger Dryas event (about 270 mm Pann) are followed by moist conditions in the early Holocene, with annual precipitation of up to 400 mm. A drier trend after 9 cal. ka BP is followed by a second wet phase in the middle Holocene, lasting until 4.5 cal. ka BP. Relatively steady conditions with only slight fluctuations then dominate the late Holocene, resulting in the present climatic conditions. The climate changes since the LGM have been primarily driven by deglaciation and fluctuations in the intensity of the Asian summer monsoon that resulted from changes in the Northern Hemisphere summer solar insolation, as well as from changes in the North Atlantic climate through variations in the circulation patterns and intensity of the westerlies.

Declining moisture availability on the Antarctic Peninsula during the Late Eocene

Paleobotanical data have indicated that the Antarctic landscape shifted from a beech (Nothofagus)-dominated forest to a more sparsely vegetated taiga-like woodland and tundra during the Late Eocene, coincident with progressive cooling and glacial growth. Reduced moisture availability may have contributed to this vegetation change, but there is limited evidence for assessing the Late Eocene hydrologic regime. We evaluated the relationship between Nothofagus δ13C and moisture availability by determining δ13C of modern Nothofagus pollen, sporopollenin, and leaves and comparing these results to precipitation data. To assess plant moisture availability and vegetation composition just prior to the Eocene–Oligocene boundary, we measured δ13C of fossil Nothofagus sporopollenin (Nothofagidites) from the SHALDRIL 3C cores (which date to ~35.9My) and evaluated these results in the context of temporal variation in pollen assemblages from the same sediments. Values of carbon isotope discrimination (∆) for modern Nothofagus sporopollenin range between 18.1 and 22.4‰. These values are positively correlated with precipitation amount, as well as pollen and leaf ∆, which suggests that fossil sporopollenin ∆ records the level of plant moisture availability. ∆ values obtained from Nothofagidites sporopollenin from the SHALDRIL 3C sediments range between 17.9 and 20.2‰ and generally decline through time. These results suggest a decrease in plant moisture availability on the Antarctic Peninsula during the Late Eocene, perhaps as a result of declining precipitation and/or soil moisture. Therefore, moisture stress experienced by Nothofagus likely contributed to the shift to a more sparsely vegetated Late Eocene landscape. Our results show that carbon isotopic analysis of pollen from C3 plants may aid understanding how variations in moisture availability contribute to shifts in plant community composition in the paleorecord.

A North European pollen–climate calibration set: analysing the climatic responses of a biological proxy using novel regression tree methods

We present a new pollen–climate calibration data-set from northern Europe consisting of 583 modern pollen samples and high-resolution, GIS-based modern climate data. The pollen data are characterised by high taxonomic resolution (167 taxa) and homogenous taphonomy (all samples are from small-to-medium-sized lakes). To assess the potential of this calibration set for the reconstruction of different climatic parameters, we use novel regression tree methods to analyse the effect on pollen composition and variability of four parameters: summer temperature, winter temperature, water balance, and continentality index. We use multivariate regression trees to analyse the variation in pollen assemblages in modern climate space, while boosted regression trees are used to analyse the relative influence of different climatic parameters on each taxon. We find taxon responses to be relatively individualistic. While most taxa (65%) are most responsive to summer temperature, other parameters are either primary determinants or significant secondary determinants for many taxa. At the assemblage level, significant variation is found in assemblages from similar summer temperature regimes, with distinct clusters of assemblages also identified along the continentality gradient. As a multivariate method, we consider boosted regression trees highly effective in describing and modelling modern climate–taxon relationships.

Mid- to Late Holocene pollen and alluvial record of the arid Andean piedmont between 33° and 34°S, Mendoza, Argentina: Inferences about floodplain evolution

In arid and semiarid regions of Central West Argentina, paleoenvironmental conditions and history of the Holocene vegetation is still scanty. This paper presents the analysis of palynological records of two Mid- to Late Holocene alluvial sequences (La Escala -LES- and Brazo Abandonado -BA-) in the fluvial basin of the Arroyo La Estacada (33°27′S and 69°03′W), placed in the Cordillera Frontal central piedmont of Mendoza province. Variations in pollen assemblages of the two sequences related to the environments developed in the floodplains of the arroyo allowed us to infer the evolution of local plant communities. Between the ca. 4000 and 3000 14C years BP and between ca. 500 and 400 14C years BP, a larger representation of hydrophytic communities related to environments with local water availability was inferred; and between ca. 3000 and 500 14C years BP an increase in shrubby xerophytic communities related to environments with lower water availability was detected. These results led us to propose that the Holocene pollen spectra fluctuations observed in the alluvial sedimentary sequences prove mainly changes in plant communities of the floodplain environment in response to the fluvial dynamics of the arroyo.

Palaeoenvironmental reconstructions using the modern analogue technique: the effects of sample size and decision rules

Palaeoenvironmental information is often extracted from pollen records using the modem analogue technique (MAT). It is largely unknown how sensitive the MAT is to subsampling-induced variation in pollen assemblages, or to the decision rules used to reconstruct climate and vegetation. We examine these issues in a Monte Carlo framework in which simulated pollen assemblages at eight count sizes were created from four fossil pollen assemblages. Simulated and fossil assemblages are compared with modern data sets at squared chord distance cut-off values between 0.05 and 0.55 to: (1) quantify count-size-induced variation in pollen assemblages, and (2) determine how count-size-induced variation affects analogue selection and climate reconstruction. The effects of sample size and decision rules on vegetation reconstructions are examined using two analogue selection schemes and two levels of minimum required analogues. The results show that decision rules have as much impact on reconstruction precision and accuracy as do large differences in count size. At counts of 150 grains, the best-performing cut-off values yield annual precipitation estimates within 75 mm of those produced by the reference fossil assemblages, and January and July temperature estimates within 0.75C. Counts as low as 150 grains yield vegetation reconstruction accuracies of?90% using the best-case reconstruction rules; the worst-case reconstruction rules may not achieve this accuracy at 1000 grain counts. Although larger count sizes do lead to greater reconstruction precision and accuracy, the results indicate that analyses of the kind presented here can inform count-size decisions and allow significant re-allocation of analytical effort.

Use of plots to define pollen–vegetation relationships in densely forested ecosystems of Tropical Africa

Modern soil samples from South Congo were analyzed for pollen content and compared to forest inventories to define modern pollen–vegetation relationships. A correspondence analysis (CA) was applied independently to botanical and pollen data and a hierarchical cluster analysis to pollen data only. Subsequently, a CA using a presence–absence approach has been made to directly compare the two types of data. Results show that the pollen rain and floristic composition of the sampled sites are not directly linked to altitudinal or precipitation gradients, but clear evidence of variation in relation to hygromorphy and soil type is detected. The forests occurring in swampy environments are well differentiated from the forests developed on well-drained soils by pollen and floristic data. Among forests on well-drained soils, a good distinction can be made between those growing on sandy soils and those growing on ferralitic soils. The comparison between pollen spectra and vegetation shows site-to-site variations in pollen assemblages in relation to the floristic heterogeneity of forests, and it appears that few taxa show a good correlation between plant cover and pollen abundance.

Recent lake level variations in Lake Haubi, central Tanzania, interpreted from pollen and sediment studies

Variations in pollen assemblages and in physical and chemical composition of a dated sediment record from the small Lake Haubi in north central Tanzania, reveal lake level fluctuations since the late 19th century. Lake Haubi changed from a seasonally inundated swamp to a lake in the beginning of the 20th century. With the exception of 1942-44, when it dried out completely, it remained water filled until 1994 when it again turned into a swamp. The lake level fluctuations in Lake Haubi are largely in phase with fluctuations of the larger East African lakes levels during the 20th century, and are therefore interpreted as being mainly controlled by regional climatic fluctuations. However, the initial formation of Lake Haubi at the turn of the century was likely due to local catchment specific causes, e.g. changes in land use, as the rapid increase in the water level at this time does not correspond to other lake level records from the region.

Pollen preservation and quaternary environmental history in the Southeastern United States

Abstract Reconstructions of Quaternary environmental history based upon modern pollen/vegetation/climate calibrations are more tenable if the factors responsible for variation in pollen assemblages are evaluated. Examination of the state of preservation of Quaternary palynomorphs provides quantitative data concerning the degree of information loss due to alteration of pollen assemblages by syndepositional and post‐depositional deterioration. The percentage, concentration, and influx values for total indeterminable pollen are useful criteria in providing an objective and quantitative basis for evaluating the comparability of pollen spectra within and between sites. Supporting data concerning sediment particle‐size distribution, organic matter content, and concentration, influx, and taxonomic composition of both determinable pollen and plant macrofossils aid in reconstructing past depositional environments. The potential is high for deterioration of pollen in sediments from the southeastern United States, although considerable variation is found in both kind and degree of deterioration between lacustrine and alluvial sites of different ages and in different latitudes. Modern analogs are a basis for late Quaternary environmental reconstructions when pollen deterioration has not significantly biased the information content of fossil pollen assemblages.