tufA Sequences Research Articles

Molluscan and ostracod biostrati graphy of Holocene tufa in the Test valley at Bossington, Hampshire, UK

Holocene tufa deposits in the Test valley, Hampshire, UK, have been examined as part of a wider project into prehistoric activity and landscape change in the central southern chalklands. Molluscan and ostracod analysis on three tufa sequences shows that the tufa began developing around 9300 BP in open marsh/open woodland environment. Although one of the sequences (Boss 10) demonstrates a wooded environment throughout, another sequence (Boss 360n/60e) indicates that the initial woodland is followed by reversion to more open conditions. Both profiles then show a short 'drier' woodland phase. Subsequent to this, one of the profiles then shows wetter woodland conditions developing (Boss 10), the other (Boss 360n/60e) more open wetland. Deposition rates are comparable to tufa deposits elsewhere and indicate that the short-lived 'drier' woodland phase perhaps lasted only a few hundreds of years. There are also indications of woodland clearance, associated with a charcoal peak, although no causal mechanism can be proposed. The formation of substantial portions of Holocene tufa deposits within open or only A lightly wooded landscapes has not previously been documented for southern Britain, where generally tufa HOLOCENE has been shown to have formed in densely wooded conditions. Through the tufa deposits the ostracod and RESEARCH molluscan evidence are in accord and provide detailed hydrological and environmental information respectively. Fine sampling (1 cm units) allowed a temporal resolution of perhaps around 20 years per sample. It is clear that the tufas studied here formed as groundwater-fed paludal deposits within the valley bottom.

Climatic change recorded by stable isotopes and trace elements in a British Holocene tufa

AbstractCombined stable isotope (δ18O and δ13C) and trace element (Mg, Sr) geochemistry from bulk tufa calcite and ostracod shell calcite from an early Holocene British tufa reveal clear records of Holocene palaeoclimatic change. Variation in δ18O is caused principally by change in the isotopic composition of Holocene rainfall (recharge), itself caused mainly by change in air temperature. The δ13C variability through much of the deposit reflects increasing influence of soil‐zone CO2, owing to progressive woodland soil development. Bulk tufa Mg/Ca and Sr/Ca are controlled by their concentrations in the spring water. Importantly, Mg/Ca ratios are not related to δ18O values and thus show no temperature dependence. First‐order sympathetic relationships between δ13C values and Mg/Ca and Sr/Ca are controlled by aquifer processes (residence times, CO2 degassing and calcite dissolution/reprecipitation) and probably record intensity of palaeorainfall (recharge) effects. Stable isotope records from ostracod shells show evidence of vital effects relative to bulk tufa data. The ostracod isotopic records are markedly ‘spiky’ because the ostracods record ‘snapshots’ of relatively short duration (years), whereas the bulk tufa samples record averages of longer time periods, probably decades.The δ18O record appears to show early Holocene warming, a thermal maximum at ca. 8900 cal. yr BP and the global 8200 yr BP cold event. Combined δ13C, Mg/Ca and Sr/Ca data suggest that early Holocene warming was accompanied by decreasing rainfall intensity. The Mg/Ca data suggest that the 8200 yr BP cold event was also dry. Warmer and wetter conditions were re‐established after the 8200 yr BP cold event until the top of the preserved tufa sequence at ca. 7100 cal. yr BP. Copyright © 2004 John Wiley & Sons, Ltd.

Larval caddis-fly nets and retreats: a unique biosedimentary paleocurrent indicator for fossil tufa deposits

Caddis-fly larvae (Order: Trichoptera; Family: Hydropsychidae; Genus: Cheumatopsyche) build distinctive nets and retreats on the surface of tufa deposits in spring-fed rivers of the Barkly karst, northern Australia. Their activities are confined specifically to swiftly flowing reaches, where the nets and retreats form linear arrays arranged almost perpendicular to stream flow. The arrays become encrusted by calcium carbonate and, when high-velocity conditions are maintained, are succeeded by the next generation of caddis flies. The arrays are well preserved in many fossil tufa deposits, where their alignment and long-section morphology indicate paleoflow direction at the time of caddis activity. This can assist in the paleoenvironmental reconstruction of fossil tufa sequences.

Molluscan successions from two Holocene tufas near Northampton, English Midlands

AimTo investigate the molluscan successions from two Holocene tufa sequences in the English Midlands to establish whether similar patterns of colonization occur and to document how they relate to other successions reported from southern Britain and elsewhere.LocationCourteenhall and Weston Favell near Northampton, English East Midlands.MethodsClose serial sampling and quantitative analyses have allowed detailed faunal successions to be reconstructed for each site. Radiocarbon dating of Cepaea shells by AMS has provided a chronology for each site.ResultsRemarkable similarities were found to exist between the two sites. The record from Courteenhall extended back to the end of the Devensian late‐glacial and continued until about 6000 yr BP. The sequence from Weston Favell was shorter, commencing at the beginning of the Holocene and ending about 7500 yr BP. Both successions show a progressive replacement of open country assemblages by those of woodland. At Courteenhall arctic‐alpine species, such as Columella columella (Martens) and Vertigo genesii (Gredler), were present in these early open‐ground communities. Both sites supported extremely rich (> 30 species) land snail assemblages during the mid Holocene. This richness cannot be matched at any single site in Britain today. These mid Holocene assemblages include species such as Spermodea lamellata (Jeffreys), Leiostyla anglica (Wood) and Vertigo alpestris Alder that are rare or absent today in southern Britain. Discus ruderatus (Férussac), which has a modern boreo–montane range, occurred at Courteenhall between about 9700 and 8750 yr BP but was also present in two consecutive samples just below a level dated at about 7000 yr BP, its latest known occurrence in Britain. These assemblages have no modern analogue because the ranges of several species (e.g. Discus ruderatus and Leiostyla anglica) never overlap today.General conclusionsDuring the Holocene land snails appear to have colonized southern Britain in a relatively ordered sequence. The history of some species is unpredictable and they may occur at different sites at different times. For others, the general pattern shows greater consistency, enabling the recognition of a series of mollusc zones applicable over large areas of southern Britain. These Northamptonshire sites also contribute to the debate surrounding the notion of a ‘late Holocene tufa decline’. Tufa formation began unusually early (c. 10,280 yr BP) at Courteenhall but it had ceased to form at both sites by 6000 year (maybe by 7500 yr BP at Weston Favell). There is no evidence for late Holocene tufa formation at either site, a pattern consistent with many other British sites. Rates of tufa growth from 2.7 to 7.0 cm 100 years–1, calculated from the radiocarbon dates from Courteenhall, are comparable with accumulation rates established from other Holocene tufas.

A covariotide model explains apparent phylogenetic structure of oxygenic photosynthetic lineages.

The aims of the work were (1) to develop statistical tests to identify whether substitution takes place under a covariotide model in sequences used for phylogenetic inference and (2) to determine the influence of covariotide substitution on phylogenetic trees inferred for photosynthetic and other organisms. (Covariotide and covarion models are ones in which sites that are variable in some parts of the underlying tree are invariable in others and vice versa.) Two tests were developed. The first was a contingency test, and the second was an inequality test comparing the expected number of variable sites in two groups with the observed number. Application of these tests to 16S rDNA and tufA sequences from a range of nonphotosynthetic prokaryotes and oxygenic photosynthetic prokaryotes and eukaryotes suggests the occurrence of a covariotide mechanism. The degree of support for partitioning of taxa in reconstructed trees involving these organisms was determined in the presence or absence of sites showing particular substitution patterns. This analysis showed that the support for splits between (1) photosynthetic eukaryotes and prokaryotes and (2) photosynthetic and nonphotosynthetic organisms could be accounted for by patterns arising from covariotide substitution. We show that the additional problem of compositional bias in sequence data needs to be considered in the context of patterns of covariotide/covarion substitution. We argue that while covariotide or covarion substitution may give rise to phylogenetically informative patterns in sequence data, this may not always be so.

Phylogenetic Analysis of tufA Sequences Indicates a Cyanobacterial Origin of All Plastids

DNA sequences of the gene tufA, encoding elongation factor Tu, were determined from five cyanobacteria and 21 plastids. Three were full-length (ca. 1230 bp) sequences from cloned DNA, and 23 were partial (ca. 740 bp) sequences from PCR fragments. These sequences were aligned with sequences available from the literature, creating a data set of 56 tufA sequences of eubacterial or plastid origin. Phylogenetic analysis was performed on inferred amino acid sequences with parsimony and neighbor joining techniques, and on first and second position nucleotide sequences with maximum likelihood, and bootstrapping was performed with each method. Trees determined by the three methods were highly congruent with respect to well supported nodes. All examined plastids, including those of green and red algae, chromophytes, and Cyanophora paradoxa, cluster strongly with the cyanobacteria in all analyses. A cyanobacterial origin of all plastids confirms phylogenetic analyses of 16S rRNA and atpB sequences, but conflicts with those of rbcL and rbcS sequences. This discrepancy may be attributable to an ancient gene transfer of the rubisco operon in an ancestor of red algae and chromophytes. Maximum likelihood analysis also provides some support for a monophyletic origin of all plastids, while neighbor joining and parsimony analyses showed cyanobacteria and red, brown, and green plastid lineages as an unresolved polytomy. These tufA analyses also provide a broad perspective on eubacterial evolution and, in conjunction with published rRNA trees, point to at least two major radiations within eubacteria and their descendants: one of many eubacterial phyla, a second of cyanobacteria, and possibly a third radiation early in plastid evolution.

Special Paper: Comparison of Post-Glacial Molluscan and Vegetational Successions from a Radiocarbon-Dated Tufa Sequence in Oxfordshire

Pollen, molluscan, and sedimentological analyses have been undertaken from a tufa, which forms part of a near-complete Post-glacial succession from Sidlings Copse near Oxford. Local pollen and mollusc zones are defined and the times of arrival of critical species compared. It is shown that some pollen and mollusc zone boundaries coincide pre- cisely but that others do not, with some faunal changes occurring during periods of vegetational stability. A detailed chronology is provided by radiocarbon dating using acceler- ator mass spectrometry. Tufa formation began at about 9300 yr BP as hazel (Corylus avellana) woodland began to fill the previously open land- scape. An unshaded marsh initially developed on the surface of the tufa, which supported snails such as Vertigo angustior and Discus ruderatus (a boreo-alpine species now extinct in Britain). Hazel woodland gradually encroached and caused the replacement of the early marsh communities by woodland mollusc assemblages. Discus ruderatus was replaced by D. rotundatus at 8990 ? 90 yr BP, and subsequently Leiostyla anglica and then Spermodea lamellata appeared. At 7180 ? 85 yr BP lime (Tilia cordata), slightly preceded by alder (Alnus glutinosa), spread into the area. Oxychilus cellar- ius first appeared at this time. Lime and elm (Ulmus) declined at 5250 t 75 yr BP, and this was accompanied by changes in the molluscan fauna and vegetation suggesting a reduction of shading. The land snail fauna at this time was composed of an unusual assemblage dominated by Leiostyla anglica and Oxychilus cellarius. Tufa formation ceased shortly after 5065 ? 120 yr BP, perhaps due to hydrological changes initiated by human activity. Rates of tufa growth from 1.3 to 5.4 cm/100 years have been calculated from the radiocarbon dates. These accumulation rates are similar to those derived from the overlying organic sediments. The upper sediments are highly organic and contain very few molluscs, but pollen continues to occur, enabling reconstruction of vegetational history up to the present. The biostratigraphy of Sidlings Copse is compared with neighbouring sites and other sequences from British tufas.