Analysis Of Phytolith Assemblages Research Articles

First record of phytolith assemblages from the Salto Ander Egg Formation (Late Pleistocene). Palaeoenvironment inferences in the Last Interglacial Stage in southwestern Mesopotamia (Entre Ríos, Argentina)

This contribution aims to describe the first palaeobotanical record of the Late Pleistocene in the Salto Ander Egg Formation (SAEF) through the analysis of phytolith assemblages. The microremains association allowed us to characterise the palaeoenvironmental conditions for these deposits and to complete the palaeobotanical record for the stratigraphic sequence of the Late Pleistocene in southwestern Entre Ríos province, Argentina. The Salto Ander Egg Formation is composed of fluvial deposits with a well-defined floodplain and channel facies dated at ages ranging from 120 to 60 ky. The phytolith content of the unit was analysed along two stratigraphic sections in outcrops exposed on the cliffs of the fluvial valleys located in the drainage basin of La Ensenada creek. The vegetation composition was interpreted principally from the proportions of very frequent and frequent morphotypes in the phytolith assemblages and was grouped in plant functional group affinities: “non-GSSCPs” (diagnostic grass phytoliths); “GSSCPs” (grass silica short cell phytoliths); “Woody dicots” and “Palms”. The phytolith assemblages describe a palaeoflora composed of a continuous herbaceous stratum of C3/C4 grasses, although C3 might have been more significant accompanied by isolated trees and palms in an open savanna physiognomy, probably with a temperate/warm climate with low rainfall and average to high temperatures.

Phytolith Assemblages as a Promising Tool for Quantitative Canopy Coverage Reconstruction in Subtropical Forests, China.

This study investigates the reliability of phytolith assemblage analysis for characterizing subtropical vegetation and explores the potential for using these modern phytolith–vegetation relationships for paleoenvironmental interpretation in southeastern China. The samples were collected from five common subtropical vegetation communities in the Daiyun Mountains, southeastern China, with the above-ground vegetation recorded at each plot. Constrained ordination analysis was used to determine the most important factor governing the variations in phytolith assemblages that could be quantitatively reconstructed with weighted averaging partial least squares regression (WAPLS). The relationship between modern phytolith assemblages and the parent vegetation, as well as production, dispersal, and taphonomic processes, was discussed. Results demonstrated that the main subtropical biomes in southeastern China could be well distinguished by soil phytolith assemblages. In particular, the overall amount of tree coverage was well represented by topsoil phytolith assemblages. Grass silica short cell phytoliths (GSSCP) tended to occur in higher proportions in open habitats (shrub-meadow) at higher elevations, whereas non-grass phytolith morphotypes attained higher frequencies under mixed and broadleaf forests at lower elevations. Human-induced deforestation might increase the frequency of GSSCP within the bulk phytolith assemblage. Our results constitute the primary phytolith reference data for the subtropical zone in southeastern Asia where vegetation change during the Holocene period, particularly forest shifts, anthropogenic deforestation, and early agriculture are poorly documented.

Phytolith analysis of the main species of Distichlis sp. (Chloridoideae: Poaceae) distributed in South America

The Saltgrasses are pioneer plants in the early stages of succession in saline soils and constitute particular zonal communities of low species variability. Phytolith assemblage analysis of these species allows a community type to be characterized that consists mainly of species with well-defined environmental adaptations, which are significant because of their paleoecological importance. The aim of the present study is to characterize phytolith assemblages of Distichlis species in South America, emphasizing their leaf morphotypes. Four Distichlis species, mainly of South American distribution, and four chloridoids species from different clades of the subfamily (Lycurus, Eragrostis, Muhlenbergia and Sporobolus) were selected for this study. The phytoliths were obtained following Labouriau's calcination technique. Quantitative analysis of the morphotype abundance in the phytolith assemblages studied allowed the three main groups to be distinguished, with the common presence of multicellular phytoliths with long, short, bulliform and subepidermal silicified cells, as well as unicellular phytoliths, such as short narrow saddle, prickles and hairs, polyhedral and long elongate types. Conversely, the exclusive presence of horned towers, short truncated cones, circular/ovoid, papillae trichome, silicified microhairs, and isolated and articulated irregular epidermal cell phytoliths in Monanthochloinae clade species distinguishes them from the Muhlenbergiinae clade species with square/rectangular and subrectangular saddles and acute dumbbells. Eragrostis and Sporobolus species have shown some particular morphotypes (such as hairs with cushions, long truncate cones, dumbbell and dumbbell/saddle). The results not only allowed us to identify the Distichlis species, but also to distinguish them from other grasses, including those in their subfamily; and represent a potential tool to identified halophytes environments outside New World saltgrasses.

A 13,000-year peatland palaeohydrological response to the ENSO-related Asian monsoon precipitation changes in the middle Yangtze Valley

Quantitative reconstructions of the depth to water table (DWT) of ombrotrophic (rain-fed) peatlands are important for understanding the palaeohydrological responses of peatlands to past climate changes. This understanding can provide insights into projecting peatlands future variability and evolution. However, the postglacial DWT reconstruction of peatlands in China is challenging due to complications of the atmospheric circulation system, the scarcity of hydrological proxies, and the site-specific nature of hydrological signals. Here we present a postglacial quantitative DWT reconstruction based on the analysis of fossil phytoliths from the Dajiuhu Peatland, central China. The reconstructions were based on a phytolith-DWT calibration model using a weighted averaging partial least-squares regression analysis of peatland topsoil calibration datasets. Three shallow DWT (wet) periods at 13,000–11,500 cal yr BP, 9,600–7,500 cal yr BP, and 3,000 cal yr BP-present, and two extended deep DWT (dry) periods at 11,500–9600 cal yr BP and at 7,500–3,000 cal yr BP are found based on the cluster analysis of phytolith assemblages and reconstructed DWT changes. These five documented hydrological periods are consistent with regional precipitation reconstructions from independent records in the middle Yangtze Valley (MYV). We interpret these changes as mostly reflecting changes in ENSO at various timescales. An amplified ENSO forced a southward Western Pacific Subtropical High and caused the persistence of the Meiyu Front in the mid-lower Yangtze Valley, consistent with the intense rainfall periods in our study region. Our results indicate that phytolith records are a reliable and sensitive proxy for the calibration of water table models and the quantitatively palaeo-DWT reconstruction of peatlands and reveal a remarkable link between the local hydrological variations and the coupled atmospheric-oceanic circulation, which is significant for the prediction of future hydrological changes in the Asian monsoon region under the background of global warming.

Climatic controls on peat swamp formation and evolution since 1300 year BP as recorded by phytoliths in the Xishan Mountains, Jiangxi Province, China

Over the past millennium, there have been several precipitation–temperature cycles characterized by instabilities in the eastern monsoon region in China, but the processes, factors, and anthropogenic activities potentially responsible remain poorly understood. In this study, we present an analysis of phytoliths from borehole core drilled through an ombrotrophic peat mire in Jiangxi Province, China. Our results record three climatic episodes over the past 1300 year: a warmer interval, c. 800–1305 CE, similar to the Medieval Warm Period (MWP); a cooler interval, c. 1305–1860 CE, similar to the Little Ice Age (LIA); and another warmer interval, as the climate entered the Present Warm Period (PWP) after c. 1860 CE. Quantitative analysis of phytolith assemblages demonstrates that the MWP comprised two intervals: the early MWP, c. 800–1140 CE, was progressively wetter in a warm–dry setting and late MWP, c. 1140–1305 CE, was warm and humid. The LIA also comprised two intervals: the early LIA, c. 1305–1610 CE, was cool and dry, and the late LIA, c. 1610–1860 CE was cool and humid. Some abrupt climate events occurred at: (1) c. 1050, 1110–1130, and 1780–1845 CE (wet events); (2) c. 1980–1990 CE (dry events); (3) c. 920 and 1770 CE (warm events); (4) c. 980 and 1050 CE (cold events). The transition from the MWP to the LIA, as indicated by phytoliths, was a gradual process that took c. 100 years, and exhibited frequent temperature fluctuations. Correlations between the phytolith assemblages and the solar activity, East Asia Summer Monsoon, El Niño, and La Niña are evident. Solar maxima and La Niña-like conditions are related to warmer and humid conditions that led to clay–sand accumulation during the MWP. Solar minima and El Niño-like conditions were associated with a cold and wet climate that led to peat accumulation during the LIA. These observations provide important insights into paleoclimatic change in the eastern monsoonal region of China, and provide a basis on which to understand the response of the Xishan Mountains in SE China to the MWP and LIA, and explore centennial-scale climate fluctuations and their driving mechanisms.

Phytolith assemblage analysis for the identification of rice paddy

The rice arable system is of importance to both society and the environment. The emergence of rice paddies was a crucial step in the transition from pre-domestic cultivation to systematic land use and management. However, many aspects of the formation of rice farming systems remain unclear. An important reason is the lack of reliable methods for identifying early rice paddies. One possible means of remedying this knowledge deficit is through analysis of phytolith assemblages, which are closely related to their parent plant communities. In this study, phytolith assemblages from 27 surface soil samples from wild rice fields, 91 surface soil samples from modern rice paddies, and 50 soil samples from non-rice fields were analysed to establish a discriminant function. This discriminant function enabled classification of 89.3% of the samples into appropriate groups. Further, the results suggested that phytolith assemblages can be used to identify rice fields and differentiate between wild rice fields and domesticated rice fields. The method was demonstrated to be an effective way of utilising the large amounts of unidentifiable phytoliths discovered at archaeological sites to provide a modern analogue that may be a valuable key to unlocking the past.

Vegetation response during the lead-up to the middle Miocene warming event in the Northern Rocky Mountains, USA

The mid-Miocene climatic optimum (MMCO; initial warming beginning ca. 18Ma and with peak warming ca. 17–14.75Ma) constitutes the Earth's most recent greenhouse episode, characterized by a transient shift to higher global atmospheric CO2 levels and warmer, possibly wetter, climatic conditions. Combined with the spread of grass-dominated habitats, it has been suggested that the MMCO lead to significant turnover and modernization of fauna and flora. However, records documenting local vegetation change during the MMCO are rare, hence this hypothesis remains largely untested. Herein, we combine phytolith assemblages and δ13C records from paleosol organic matter from the Railroad Canyon section (RCS), eastern Idaho, to provide the first direct, detailed, and continuous, long-term record of vegetation composition and structure in a single basin during the lead-up to the MMCO (22.9–15.2Ma; late Arikareean–early Barstovian). Phytolith assemblage analysis indicates that grasses, primarily C3 pooids, dominated early–middle Miocene vegetation. Potential C4 PACMAD grasses were present by ca. 21.6–20.7Ma (late Arikareean) and vary in abundance (0.4–17.2% of diagnostic phytoliths) through the section. Although relatively rare overall, PACMAD grasses decrease significantly through time, in parallel with declining diatom abundances and an increase in pooid grasses. In contrast, paleosol δ13Corg indicates 0–29.1% C4 vegetation but no consistent temporal change. Where both phytolith- and isotope-based estimates are made from the same paleosol, they are within error most (82%) of the time; thus, the apparent discrepancy in temporal trends could either be due to differences in the resolution of the two records, or could be explained if many PACMADs were not C4 grasses, but instead water-loving C3 species. Overall, these data suggest that open-habitat mosaic vegetation with grass-dominated grasslands and open woodlands occurred in the North Rocky Mountains, USA, by at least the early Miocene, more comparable to the timing for the spread of grass-dominated habitats in the Great Plains than previously thought. Additionally, phytolith data suggest that overall vegetation structure remained relatively stable throughout the RCS, inconsistent with published paleosol morphology data. This points to regionally unique floral patterns decoupled from global climate change leading into the MMCO.

The missing crop: investigating the use of grasses at Els Trocs, a Neolithic cave site in the Pyrenees (1564 m asl)

The issue of resource exploitation, both plants and animals, by Neolithic communities has always attracted vast interest. In particular, resource exploitation at mountain cave sites is still being widely discussed. This paper explores the use of grass resources at the archaeological site of Els Trocs (Aragón, Spain), a Neolithic mountain site in the Pyrenees. The analysis of phytolith samples suggests that grasses growing in the surrounding of the site were widely used. The morphological assemblages identified, and their spatial distribution, indicate that wild grasses were probably used as floor spread. The integrated approach used in this study, combining phytolith, spherulite and micromorphological analyses, confirms this hypothesis. Furthermore, the analysis of phytolith assemblages and micromorphological traits indicate the seasonal occupation of the site, placing human frequentation at this location during late spring/early summer. Several studies have highlighted the presence of charred seeds of domesticated cereals in the archaeological record of mountain cave sites however, in many instances, whether these crops were cultivated near the sites or whether the grains were transported to the cave from the valley bottom remains under debate. This paper also contributes to this debate by showing that no crop-processing activities were taking place at the site.

Opal phytolith extraction in oxisols

The analysis of phytolith assemblages is an important method in studies that uses the soil as an environmental record in order to reconstruct the paleoclimatic conditions. Despite being underused in soil science, this technique is used complementary to the analysis of pollen and other microfossils, as well as to evaluate silicon reserves in the soil. Currently, there are several concentration methods for phytolith extraction from sediments, soils, and paleosols. Most were developed and applied to materials of temperate zones or hydromorphic environments. Few have been conducted in tropical soils where oxides and hydroxides of iron and aluminum, as well as organic matter that often blankets the soil matrix, are common and hamper the extraction, the observation and identification of phytoliths, compromising the morphological analysis and quantification of phytolith assemblage in soils. In this paper, three methods of pre-treatment for the removal of the coating of silt and sand particles were applied to samples of an Oxisol (Humic Hapludox) in order to compare the cleaning efficiency, integrity and number of phytoliths. The first method consisted of the oxidation of soil organic matter and of an acid hydrolysis for the removal of carbonates and oxides. In the second method, the dominant process was the reduction of iron using a combination Dithionite–Citrate–Sodium Bicarbonate. In the third method, only acetate and sodium dithionite dissolved in water was used. Overall, Method 1 was the most aggressive to phytoliths and proved less efficient and more selective in phytolith extraction. Methods 2 and 3 were similar in the pre-treatment of samples. Method 2 allows the conservation of greater variety and number of phytoliths, and smaller quantities of other particles. Method 3 is relatively inexpensive and faster because it uses fewer chemicals and centrifugation procedures.

The Neogene transition from C3to C4grasslands in North America: stable carbon isotope ratios of fossil phytoliths

C4grasses form the foundation of warm-climate grasslands and savannas and provide important food crops such as corn, but their Neogene rise to dominance is still not fully understood. Carbon isotope ratios of tooth enamel, soil carbonate, carbonate cements, and plant lipids indicate a late Miocene-Pliocene (8–2 Ma) transition from C3vegetation to dominantly C4grasses at many sites around the world. However, these isotopic proxies cannot identify whether the C4grasses replaced woody vegetation (trees and shrubs) or C3grasses. Here we propose a method for reconstructing the carbon isotope ratio of Neogene grasses using the carbon isotope ratio of organic matter trapped in plant silica bodies (phytoliths). Although a wide range of plants produce phytoliths, we hypothesize that in grass-dominated ecosystems the majority of phytoliths will be derived from grasses, and will yield a grass carbon isotope signature. Phytolith extracts can be contaminated by non-phytolith silica (e.g., volcanic ash). To test the feasibility of the method given these potential problems, we examined sample purity (phytolith versus non-phytolith silica), abundance of grass versus non-grass phytoliths, and carbon isotope ratios of phytolith extracts from late Miocene-Pliocene paleosols of the central Great Plains. Isotope results from the purest samples are compared with phytolith assemblage analysis of these same extracts. The dual record spans the interval of focus (ca. 12–2 Ma), allowing us, for the first time, to investigate how isotopic shifts correlate with floral change.We found that many samples contained high abundances of non-biogenic silica; therefore, only a small subset of “pure” samples (>50% of phytoliths by volume) with good preservation were considered to provide reliable carbon isotope ratios. All phytolith assemblages contained high proportions (on average 85%) of grass phytoliths, supporting our hypothesis for grass-dominated communities. Therefore, the carbon isotope ratio of pure, well-preserved samples that are dominated by grass biosilica is considered a reliable measure of the proportion of C3and C4grasses in the Neogene.The carbon isotope ratios of the pure fossil phytolith samples indicate a transition from predominantly C3grasses to mixed C3-C4grasses by 5.5 Ma and then a shift to more than 80% C4grasses by 3–2 Ma. With the exception of the Pliocene sample, these isotopic data are broadly concordant with phytolith assemblages that show a general increase in C4grasses in the late Miocene. However, phytolith assemblage analysis indicates lower relative abundance of C4grasses in overall vegetation than do the carbon isotopes from the same phytolith assemblages. The discrepancy may relate to either (1) incomplete identification of (C4) PACMAD phytoliths, (2) higher production of non-diagnostic phytoliths in C4grasses compared to C3grasses, or (3) biases in the isotope record toward grasses rather than overall vegetation. The impact of potential incomplete characterization of (C4) PACMAD phytoliths on assemblage estimates of proportion of C4, though important, cannot reconcile discrepancies between the methods. We explore hypothesis (2) by analyzing a previously published data set of silica content in grasses and a small data set of modern grass leaf assemblage composition using analysis of variance, independent contrasts, and sign tests. These tests suggest that C4grasses do not have more silica than C3grasses; there is also no difference with regard to production of non-diagnostic phytoliths. Thus, it is most likely that the discrepancy between phytolith assemblages and isotope ratios is a consequence of hypothesis (3), that the isotope signature is influenced by the contribution of non-diagnostic grass phytoliths, whereas the assemblage composition is not. Assemblage-based estimates of % C4within grasses, rather than overall vegetation, are in considerably better agreement with the isotope-based estimates. These results support the idea that, in grass-dominated assemblages, the phytolith carbon isotope method predominantly records shifts in dominant photosynthetic pathways among grasses, whereas phytolith assemblage analysis detects changes in overall vegetation. Carbon isotope ratios of fossil phytoliths in conjunction with phytolith assemblage analysis suggest that the late Neogene expansion of C4grasses was largely at the expense of C3grasses rather than C3shrubs/trees. Stable isotopic analysis of phytoliths can therefore provide unique information about grass community changes during the Neogene, as well as help test how grass phytolith morphology relates to photosynthetic pathway.

Phytoliths of East African grasses: An assessment of their environmental and taxonomic significance based on floristic data

Relations between phytolith occurrences, taxonomy, and habitat are assessed for 184 East African grass species through the re-analysis of two qualitative surveys of phytolith types in leaf epidermis. This is done in conjunction with data on grass subfamily, photosynthetic pathway, and requirement for light and moisture compiled from floras and the literature. This survey includes ca 79% of the grass genera listed in the flora of tropical East Africa. It aims to further investigate the potential for grass short cell phytoliths to characterize the environment, and therefore improve reconstructions of past vegetation and climate in Africa. In this analysis, we identified ca 60 phytolith types (within the main categories Rondel, Trapeziform Short Cell, Bilobate, Cross, Polylobate, Saddle, and Trapeziform Sinuate) reported to occur in 10 grass subfamilies (Pharoideae, Bambusoideae, Ehrhartoideae, Pooideae, Danthonioideae, Arundinoideae, Chloridoideae, Centothecoideae, Panicoideae, and Incertae Sedis Streptogyna). These subfamilies include hydrophytic, helophytic, mesophytic and xerophytic species, with C3 or C4 photosynthetic pathways, and with affinities for shade, open, or semi-shade habitats. Analysis of phytolith occurrences shows that few morphotypes are restricted to some species only. However, there are morphological variations (of size and number of lobes) within the main phytolith categories Rondel, Bilobate, and Cross, which could additionally be considered to improve environmental and taxonomical interpretation of phytolith assemblages. The Rondel phytolith with a base diameter > 15 μm was only reported in C3-Pooideae, while the Rondel with a base diameter of < 15 μm occurs in several grass subfamilies (including Pooideae). Bilobates with long shanks between the two lobes are most frequently reported in xerophytic species, while Bilobates with short shanks are most frequently reported in mesophytic grass species. Finally, three-lobed crosses are reported only in Panicoideae and Chloridoideae, all being C4, light-loving species. A correspondence analysis confirms already known relationships between 1) Saddle forms, C4 pathway, open and xeric habitats, 2) Bilobates, Crosses, Polylobates, shaded and hydric habitats, 3) Trapeziform Sinuates and Pooideae. Of major implication for palaeoenvironmental reconstructions in East Africa, we found that the Trapeziform Sinuate phytoliths mark the presence of C3-grasses in the Afromontane zone, whereas the Rondels alone do not because they also occur in many C4 species of the Chloridoideae subfamily. We also establish that collapsed saddles are not diagnostic for Bambusoideae closed-habitat grasses since they occur in xerophytic species of the Chloridoideae, characteristic of several open habitats. In conclusion, this study contributes to better characterize Afromontane vegetation and better discriminate mesic and xeric vegetation types in East Africa. It also brings caution for future phytolith studies that rely on the presence of diagnostic types instead of phytolith assemblage analysis to trace the presence of particular taxa and/or environments in East Africa.

Use of space in a Neolithic village in Greece (Makri): phytolith analysis and comparison of phytolith assemblages from an ethnographic setting in the same area

Phytolith analyses were conducted in a Pottery Neolithic village (Makri) of Northern Greece in order to reconstruct aspects of past human activities as a function of both space and time. The analyses of phytolith assemblages were based on a reference collection of modern plant phytoliths ( Tsartsidou et al., 2007), as well as an ethnographic study in an agropastoral community (Sarakini) in the same area that showed that many phytolith assemblages are characteristic of the activities carried out in different locations within and around the village ( Tsartsidou et al., 2008). The same approach was used for studying the phytolith assemblages in the Neolithic village of Makri, namely measuring phytolith concentrations, diversities of phytolith assemblages relative to control samples collected from samples outside the village and detailed analysis of various phytolith morphotypes. At Makri samples from floors and various constructions (i.e. pit, platforms) were analysed, as well as sediments from an open area inside the village. The results show that Neolithic Makri was a society with a mixed agricultural and pastoral economy. Wheat and barley were cultivated for food and fodder and free-range animals were raised in a village inhabited year round. Indoor areas were not clearly differentiated from outdoor areas inside the village. The phytolith assemblages in only one series of floors produced at a specific location over an extended period of time reflected the use of that space for cereal storage or food processing. The phytolith assemblages from all the other floors examined did not reflect the local activities, but rather the constructional materials used for producing the floors.

The use of multiple discriminant analysis in classifying prehistoric phytolith assemblages recovered from cooking residues

In the investigation of the dispersal of maize ( Zea mays ssp. mays) from south-central Mexico to the northern and southern limits of agriculture in the Western Hemisphere archaeologists and paleoethnobotanists are increasingly turning to the microbotanical record. Recent analysis of phytolith assemblages from charred cooking residues on pottery sherds in central New York recovered using 209 rondel phytolith variables has identified maize as early as 2270 ± 35 B.P. In this article we use discriminant analysis to re-classify these rondel phytolith assemblages resulting in only seven variables. The results are consistent with those achieved earlier using many more variables and a less formal statistical approach in terms of classification and in similarity of the original and reduced data matrix as seen by the Mantel test and cluster analyses.

Reply to comment on “Methodological concerns for analysis of phytolith assemblages: Does count size matter?” (A. Alexandre and L. Brémond)

Reply to comment on “Methodological concerns for analysis of phytolith assemblages: Does count size matter?” (A. Alexandre and L. Brémond)

Comment on the paper in Quaternary International: “Methodological concerns for analysis of phytolith assemblages: Does count size matter?” (C.A.E. Strömberg)

For the last three decades, fossil phytolith assemblages from soils, buried soils and lake sediments have been used, in addition to other vegetation indicators, to reconstruct paleovegetation changes. However, in the objective of strengthening further accuracy for these reconstructions, and in addition to comparison with reference plant collections, recent effort has been made to provide quantitative relationships between modern phytolith assemblages from soils, vegetation and bio-climatic parameters (e.g., Alexandre et al., 1997; Fredlund and Tieszen, 1997; Bremond et al., 2004, 2005a, b, 2008; Blinnikov, 2005; Albert et al., 2006; Lu et al., 2006; Barboni et al., 2007). Along this line, Stromberg (Quaternary International, this volume) presented a paper entitled ‘‘Methodological concerns for analysis of phytolith assemblages: Does count size matter?’’, articulated on (1) calculation of confidence intervals related to count size and (2) investigation on how addition of these confidence intervals may affect vegetation interpretations. The first point is a relevant contribution for increasing accuracy of phytolith data. We comment here on the second point. For the purpose of her demonstration, the author used published modern phytolith data from West Africa (Bremond et al., 2005a, b) for which she misinterpreted the vegetation significance. The phytolith index D/P was first presented by Alexandre et al. (1997) and later further defined by Bremond et al. (2005a) as the ratio of ligneous dicotyledon phytolith type (globular granulate) characteristic of trees in tropical areas of low elevation, over the sum

Methodological concerns for analysis of phytolith assemblages: Does count size matter?

In quantitative phytolith analysis, chance error associated with insufficient counts can affect the robustness of the interpretation, whether it is vegetation reconstruction or taxonomic differentiation. It is therefore vital to choose a count size that will ensure statistically reliable results, while minimizing the time expended. Numerical statistical methods (bootstrapping) that have become available over the past few decades have made it possible to model even complex phytolith assemblages with relative ease. This study used bootstrapping as well as analytic statistical formulas to evaluate the influence of count size on vegetation reconstruction by means of two commonly used indices, D/ P (tree cover index) and I ph (aridity index). The analysis indicates that the count size needed to ensure statistical precision depends on the question as well as the observed assemblage composition. Importantly, it is the count of specimens relevant to a specific ratio or other index (“index-specific” count) that matters, whereas the total count is less important. Based on these results, some general guidelines for choice of count size and for the use of statistics in phytolith analysis are suggested.

Extending the Phytolith Evidence for Early Maize (Zea mays ssp. mays) and Squash (Cucurbita sp.) in Central New York

The timing of the adoptions of maize and squash across eastern North America has been a topic of long-standing interest among archaeologists and paleoethnobotanists. The use of flotation for macrobotanical remains beginning in the 1960s and 1970s coupled with the application of accelerator mass spectrometry dating beginning in the 1980s has led to substantial revisions of knowledge about the history of these crops in the region. A complementary source of evidence for the crops' histories in the eastern North America comes from opal phytoliths. Analysis of phytolith assemblages recovered from charred food residues has shown that maize and squash were being used in central New York well before the macrobotanical record indicates. In combination with previously analyzed samples, 16 additional residue assemblages help to clarify the history of maize and squash in central New York. The results indicate that maize and squash were being used in New York by 2270 B.P. and 2945 B.P., respectively.

Using phytolith assemblages to reconstruct the origin and spread of grass-dominated habitats in the great plains of North America during the late Eocene to early Miocene

This paper outlines a general approach for analyzing siliceous microfossils from plants (phytoliths) in geologic sediments and uses this method to reconstruct the evolution of grasslands in the central Great Plains during the late Tertiary. Phytolith assemblage analysis is widely employed for reconstructing vegetation types in Pleistocene–Holocene paleoecology and archaeology, but no standardized methodology is currently available. Published phytolith studies are often system-specific and tailored after specific modern analogues. As a result, they vary in extraction technique, classification scheme, and method of inference. For ancient phytolith assemblages whose modern analogues are not well understood, a more general analytic approach is necessary. This analytic approach includes studying all size fractions (2–250 mm), using a classification scheme based on the available literature and a comprehensive modern phytolith reference collection, and comparing relative frequencies through time of all diagnostic phytoliths to interpret changes in habitat structure. This general method was applied to late Eocene to early Miocene sediment samples from northwestern Nebraska to deduce vegetation history and the results were compared to those obtained using a more specific technique [Strömberg, Paleogeogr. Paleoclimatol. Paleoecol. 177 (2002) 59]. In both studies, phytoliths were extracted by means of heavy liquid separation from the following lithostratigraphic units: (1) the late Eocene Chadron Formation and the early Oligocene Brule Formation (White River Group); (2) the late Oligocene–early Miocene Monroe Creek Formation, the early Miocene Harrison Formation and Anderson Ranch Formation (Arikaree Group); and (3) the early Miocene Runningwater Formation and Box Butte Formation (Ogallala Group). The two techniques, general vs. specific, produced results that differ markedly. Data from the general approach indicate the presence of relatively closed habitats in late Eocene to early Oligocene, typified by the presence of phytoliths from bambusoid grasses, woody and herbaceous dicotyledons, and palms. The introduction and spread of modern, open-habitat grasses, such as pooids, arundinoids, and panicoids, occurred between the late Oligocene and early Miocene. This resulted in a savanna or woodland type habitat by the early Miocene. In contrast, analysis using the specific method suggested the presence of open habitats in northwestern Nebraska from the late Eocene onward. The difference in interpretation based on the two approaches is principally due to inclusion of phytoliths >50 mm and the use of an expanded reference collection in the current study (general analytic approach). These results point to the importance of methodological choices in phytolith analysis and emphasize the usefulness of phytoliths as paleoecological indicators in the fossil record.

Phytolith assemblages as a promising tool for reconstructing Mediterranean Holocene vegetation

The reliability of phytolith assemblage analysis for characterizing Mediterranean vegetation is investigated in this study. Phytolith assemblages are extracted from modern and buried Holocene soils from the middle Rhône valley (France). The relation between modern phytolith assemblages and the surrounding vegetation, as well as between fossil assemblages and contemporaneous vegetation, already reconstructed through other proxies, is discussed. We demonstrate that the main northwestern Mediterranean biomes are well distinguished by soil phytolith assemblage analysis. In particular, the density of pine and nonconiferous trees (densities expressed relatively to the grass cover) and the overall degree of opening of the vegetation appear well recorded by three phytolith indexes. North Mediterranean vegetation changes during the Holocene period, mainly tree line shifts, pine wood development and deforestation are poorly documented, due to the scarcity of proxy-preserving sites. Phytolith assemblage analysis of soils, buried soils, and sediments appears to be a promising technique to fill this gap.

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.