Modern Phytolith Assemblages Research Articles

An analysis of phytolith assemblages for the quantitative reconstruction of late Quaternary environments of the Lower Taieri Plain, Otago, South Island, New Zealand I. Modern assemblages and transfer functions

An analysis of modern phytolith assemblages is presented. Phytolith assemblages were studied in modern surface soils and sediments of 28 sites from east Otago, New Zealand, within a range of vegetation types and microclimates. No simple distinction could be made between vegetation types on the basis of phytolith assemblage composition. A Principal Components Analysis (PCA) of the phytolith data set revealed that festucoid, chloridoid and spherical phytolith morphotypes formed strong associations with sites from wetland, grassland, and forest vegetation types, respectively. More importantly, a comparison of sample replicates from each field site using Squared Chord Distance (SCD) assemblage analysis showed that wetland and grassland sites tended to produce more internally consistent phytolith assemblages than forest sites. Environmental variables including pH, conductivity, altitude, precipitation and temperature were also gathered for each site. The ability of each environmental variable to reflect variance in the entire phytolith data set was estimated by a series of Redundancy Analyses (RDA) with Monte Carlo permutation tests of statistical significance. After a forward selection process, transfer functions were generated using Partial Least Squares (PLS) regression and calibration with jack-knife validation. The final transfer functions have root mean squared errors of prediction for pH (0.47), log conductivity (0.38 mS cm), average annual precipitation (63 mm), and average annual (0.28 8C), spring (0.38 8C) and autumn temperature (0.41 8C); the smallest group of environmental variables explaining the most variance in the modern phytolith data set. The most useful transfer functions for application to fossil phytolith data and paleoenvironmen- tal interpretation are pH, log conductivity and annual precipitation. The relationship between changes in pH and annual precipitation and phytolith assemblage composition found in this study presents a prima facie relationship with the potential to provide direct proxies for soil weathering and indirectly for paleoenvironmental reconstruc- tion.

Phytoliths as paleoenvironmental indicators, West Side Middle Awash Valley, Ethiopia

Phytolith assemblage analysis offers the potential to refine our knowledge of paleoecosystems where grasses and sedges predominate. In this work, Holocene and Pleistocene sediments from an arid tropical region in Ethiopia have been analyzed for their phytolith content, presented as detailed counts and diagrams according to the Twiss classification. The aim is to test the usefulness of phytolith assemblages to indicate paleoenvironments at Middle Awash, where fossil pollen grains are poorly preserved in sediments that yielded abundant archaeological remains. The vegetation in the Middle Awash subdesertic valley is currently a shrub steppe dominated by C4 grasses adapted to arid conditions, with a narrow riparian forest limited to the Awash River. Our results show that modern surface samples, Holocene and Pleistocene sediments contain well-preserved and different phytolith assemblages, and therefore that no translocation processes from modern soil to geological strata seem to occur. Fossil records and modern assemblages are interpreted using phytolith ratios to estimate the density of the tree cover, the aridity and the proportion of C3 versus C4 grasses, as applied to phytolith assemblages from North America and West Africa. The phytolith assemblages from modern soil samples correctly reflect the proportion of trees and shrubs versus grasses, different in the riparian vegetation and the shrub steppe. Modern phytolith assemblages appear to be a mixed signature of local and regional vegetation. Phytolith analysis of the Holocene sample suggests a grassland, where the grass community is constituted by the Chloridoideae subfamily, adapted to warm and dry conditions and where C3-Pooideae cover the highlands. Phytolith analysis of the Pleistocene sample evidences grassland formation with scattered woody elements, where C4-Panicoideae grasses, adapted to warm and humid conditions dominate the grass cover. However, these conclusions need to be confirmed by more complete study on phytolith assemblages from modern vegetation from Ethiopia.

Calibrating grass phytolith assemblages in climatic terms: Application to late Pleistocene assemblages from Kansas and Nebraska

Abstract The use of modern phytolith assemblages for the interpretation of fossil phytolith assemblages demonstrates the utility of phytolith analysis in reconstructing grassland vegetation and climate in the Great Plains of North America. The modern data presented are the typical (average) assemblages for 34 modern localities taken throughout the Great Plains. Modern assemblage variability across the region is substantial. This variability shows a coherent geographical pattern consistent with modern grassland composition. These modern phytolith assemblages are calibrated in terms of modern climate data: specifically July mean temperature. Seven fossil assemblages from three late Pleistocene localities in the central Great Plains are presented. Two alternative phytolith-temperature regression models are applied to these fossil assemblages to estimate paleo-temperature. Resulting estimates are consistent with other efforts to calculate late Pleistocene temperature departures in the region (ca. −8.0 to −7.0°C). The fossil data also show rapid change in grassland composition and climate during the Pleistocene-Holocene transition. These results demonstrate how phytolith analysis can complement, or substitute for, pollen analysis in the Great Plains of North America.

Phytoliths: indicators of grassland dynamics during the late Holocene in intertropical Africa

The reconstruction of African tropical grassland history during the late Holocene can be carried out using phytolith analysis. Fossil phytolith assemblages from Lake Guiers, in the Sahelian region of Senegal, and from Lake Sinnda, in the Guineo-Congolian region of Congo were investigated. The results are interpreted on the basis of modern phytolith assemblages from the same regions and compared to pollen data previously obtained. Tall or short grass associations are discriminated by their phytolith index Iph(%) = saddle/(cross + dumbel + saddle), while the density of shrubs and trees is indicated by relative proportions of the dicotyledon phytoliths. The phytolith data emphasize that, in the Guineo-Congolian region around Lake Sinnda, the driest phase of the late Holocene occurred between 4000 and 1200 yr B.P., commencing with the opening of the dense forest and its replacement by a short grass savanna. From ca. 1000 yr B.P., wetter climatic conditions developed, as represented by the setting up of a tall grass savanna woodland. The modern shrub and tall grass savanna was developed ca. 700 yr B.P. In the Sahelian region around Lake Guiers, the driest phase occurred after about 2000 yr B.P. and has not been followed by moister conditions. A tall grass savanna woodland was gradually replaced by a shrub and short grass savanna which still occurs. A short period of development of swampy vegetation, which can be related to a lake level change, interrupted the semi-arid adaptation of the vegetation, between about 2000 yr B.P. and the present.

Modern Phytolith Assemblages from the North American Great Plains

The analysis of fifty modem phytolith assemblages demonstrates the practicality of opal-phytolith analysis as a surrogate for pollen analysis in the North American Great Plains. The fifteen native grassland sites used in the study span both the east-west moisture and north-south temperature gradients of the central North American grasslands. Multiple soil samples analysed from each grassland site measure within-site variability and provide evidence of how assemblages are formed. Soil assemblages strongly reflect regional grassland composition rather than local vegetation. Five factors contribute to assemblage formation: decay-in-place, fire, eolian transport, herbivory and fluvial/colluvial deposition. Soil assemblages should be interpreted as the end result of these processes integrated over long time periods. The correspondence of assemblages to local conditions controlled by landscape position (e.g. upland v. lowland) is less important than the regional correspondence but is detectable and predictable. Data presented here will provide a basis for interpretation of fossil assemblages from late Quatemary paleosols in the North American Great