Holocene Tufa Research Articles

Sunspot cycles recorded in Mesoproterozoic carbonate biolaminites

Well-preserved carbonate biolaminites from the early Mesoproterozoic Wumishan Formation (ca. 1.5–1.45Ga) of North China show submillimeter-scale yearly couplets that consist of alternating dark and light laminae, and are texturally similar to those reported from Holocene tufas. Power spectrum and wavelet transform analyses of laminar couplet thickness variations and geochemical series (Ca, Fe, Co/Ti, Cr/Ti, and Br) reveal a prominent periodicity at 9.0–11.7 and a less prominent periodicity at 19.7–21.4 couplets. These coherent periodic modes match well with the 11-yr Schwabe sunspot cycle and 22-yr solar Hale cycle. The observed layering pattern is thus interpreted as recording solar induced climate changes that may have modulated microbial growth rate and biomass production in restricted subtidal environments on a broad epicontinental platform. The documented example represents the first reported solar signature in Mesoproterozoic marine carbonates and implies the sensitivity of microbial life to environmental changes prior to metazoan evolution.

Molluscan assemblages in Late Holocene tufa cones in the Pieniny Mountains (southern Poland)

Limestone rock walls of various inclination form a favourable substrate for the deposition of calcareous tufas that form cone-shaped structures located at the foot of slopes or fill niches within them. Such carbonate deposits usually contain rich and diversified molluscan assemblages, whose composition and structure closely correspond to the ambient environmental conditions in the immediate vicinity. The study of the Pieniny Mountains area revealed 10 sites with such deposits. The identified malacofouna was subjected to detailed analysis. It enabled identification of three types of faunistic assemblages ( Bythinella austriaca assemblage, Pyramidula pusilla assemblage, and a shade-loving species assemblage), which are related to the conditions prevailing during the deposition. Tufa cones found in the Pieniny Mountains represent the historic period (Late Holocene). The deposits provide a record of two climatic phases: warmer and colder. The former (Medieval Warm Period) involved the intensified deposition of carbonate material and the rapid growth of tufa cones. The latter – the Little Ice Age – entailed the slow degradation of cones and slower sedimentation of tufas, while the intensified physical weathering led to the increase in the clast material proportion (sharp-edged limestone fragments). Tufas accumulated in the zone unaffected by high anthropogenic pressure and thus represent a record of natural environmental changes during the historic period.

Holocene tufa in the Slovak Karst: facies, sedimentary environments and depositional history

Several tufa complexes are known in the Slovak Karst which is a typical karst area of a temperate climate. This area is built of Mesozoic carbonates, mainly Triassic in age. The karst systems drain carbonate plateaux and lead water to resurgences located in valleys which are up to 300 m deep. Below the resurgences there are Holocene fossil tufa deposits that exceed 12 m in thickness. The tufas include stromatolite, moss, phytoclastic, oncoidal, and intraclastic facies. Extensive barrages which once dammed the upper reaches of the streams were formed in narrow valleys. They are composed predominantly of moss facies and stromatolites, with subordinate oncoidal and phytoclastic facies. Phytoclastic, oncoidal and intraclastic facies are dominant in dammed segments of streams, and include gastropod shells and charcoal fragments. Some small moss cushions are also developed. Barrages and dammed areas formed in a longitudinal fluvial depositional system. Conversely, below resurgences located on plateau slopes tufas of a perched springline depositional system were formed. These comprise deposits of prograding cascades constructed by moss, phytoclastic and stromatolitic facies. Presently, the tufas analysed are inactive. They stopped growing in the Late Holocene time, after which there was abrupt incision of the streams. This caused downcutting into Holocene tufas, in some places reaching Mesozoic bedrock. At present tufa is being precipitated from streams in all the sites studied.

Sedimentology and depositional architecture of tufas deposited in stepped fluvial systems of changing slope: Lessons from the Quaternary Añamaza valley (Iberian Range, Spain)

AbstractThe Pleistocene and Holocene tufas of the Añamaza valley (stepped build‐ups, up to 70 m thick, along the valley) consist of several depositional stages separated by erosional surfaces. Eight associations of tufa and related carbonate facies, plus minor polygenic detrital facies, represent the processes that occurred in different fluvial and related environments. The bedrock lithology and structure controlled the location of the knickpoints along the valley and allowed separation of two stepped stretches with distinct conceptual facies models. The moderate‐slope model includes extensive standing‐water areas dammed by barrage‐cascades. In the lakes, bioclastic silts, sands and limestones along with phytoclastic and marly, at places peaty, sediments formed. Abundant stem phytoherms account for extensive palustrine areas. The high‐slope model consists of smaller dammed areas between close‐up cascades and barrage‐cascades, which were composed primarily of moss phytoherms and phytoclastic tufas. An outstanding feature is the extensive steep reach with phytoclastic and polygenic detrital sediments, and stepped cascades consisting of stromatolitic and moss phytoherms. There, the steep slope limited the preservation of stem phytoherms and favoured erosion. The geometry and thickness of the sedimentary fill (wedge‐shaped units composed of cascade and barrage‐cascade deposits downstream, and dammed and gentle‐sloped channel deposits upstream) are therefore different for each model. Multi‐storey wedges are a distinctive feature of the high‐slope model. The initial knickpoint geometry and the tufa aggradation/progradation ratio on such steep surfaces (for example, related to changes in discharge) controlled the growth style of the cascades or barrage‐cascades and, hence, the extent, thickness and vertical evolution of the upstream deposits. The sedimentological attributes and stable‐isotope composition of the carbonate facies suggest a higher and more variable precipitation/evaporation ratio during the Pleistocene than during the Holocene, consistent with an overall decrease in the river discharge. This evolution was coupled with warm conditions, which prevailed during the stages of tufa formation. These results may help to assess architectural patterns in interpreting other basins, and underscore the significance of tufas as records of past hydrology and climate.

El sistema de barreras tobáceas holocenas de Las Parras de Martín (Cordillera Ibérica, Teruel)

A well-developed Holocene tufa system occurs in the headwaters of Las Parras River, Iberian Range (Teruel province). The longitudinal profile of the river shows two knickpoints downstream of two gorges known as Hocino de las Palomas and Hocino del Pajazo. The largest tufa build-ups in this area (49 m and 26 m thick) were found just downstream these two gorges and associated waterfalls. Barrage and cascade structures and facies suggest a rapid down-stream and vertical development of the tufa system. Between the knickpoints and upstream, the river valley displays a low-gradient (2%) and tufa deposits occur as laterally extensive terraces, with variable thickness (up to 8.2 m in El Batán) and fluvio-lacustrine tufa sequences associated to dammed areas. The Las Parras Tufa system represents a depositional environment including stepped fluvial systems with barrage-cascade and associated dammed areas separated by low gradient fluvial stretches. According to the chronological data, the development of Las Parras Holocene tufa occurred between 9.9 ky and 3.1 ky, with the highest growth rate from 5.5 to 3.1 ky, and a significant reduction of tufa activity during the last 2000 years. This evolution reflects the water availability in the area, and is coherent with changes in summer insolation during the Holocene.

The Holocene tufa at Daours (Somme Valley, northern France): Malacological succession and palaeohydrological implications

One of the thickest and most extensive tufa deposits in northern France occurs at Daours, about 8 km upstream of Amiens at the confluence of the River Somme and its tributary the Hallue. It covers an area of about 80 ha and reaches 10 m above the level of the present valley, probably owing to the construction of a tufa barrage, which caused periodic damming of the valley. The molluscan succession from a 4‐m section through the Daours tufa provides an unusually detailed record of environmental change during the Holocene. The faunal record, supported by four radiocarbon dates, indicates that the tufa sequence at this location does not extend back to the early Holocene but began c. 7340 cal. a BP and ceased to form shortly after c. 4938 cal. a BP, consistent with the notion of a NW European ‘late Holocene tufa decline’. The molluscan record is divisible into six zones that define three aquatic phases separated by three terrestrial episodes, when the tufa surface dried out sufficiently to allow colonization by dry‐ground species, including some shade‐demanding elements. These events are also reflected by differences in the lithology of the tufa. The earliest aquatic phase at the base of the sequence represents the most stable and permanent water‐body. The two subsequent aquatic phases appear to represent smaller, more ephemeral, water‐bodies surrounded by marsh. The land snail assemblages show ecological changes within each terrestrial episode, as well as faunal differences between them. These events, which reflect changes in the local hydrology, were short‐lived, each lasting for only a few hundred years. The tufa at Daours has also yielded flint artefacts and pottery, but it seems unlikely that the dynamic environmental record of the site results from anthropogenic activity.

Early to Middle Holocene glacier contraction, tufa deposition and forest rise in Western Alps (Valle d'Aosta, Italy)

Early to Middle Holocene glacier contraction, tufa deposition and forest rise in Western Alps (Valle d'Aosta, Italy)

Mid-Holocene erosive episodes in tufa deposits from Trabaque Canyon, central Spain, as a result of abrupt arid climate transitions

During the Holocene a series of tufa sediments were deposited from a karstic spring along the Trabaque Canyon, central Spain. A long-term lowering of the water table caused the location of the spring to be displaced from the upper sector of the canyon during the early Holocene to the lower sector in the late Holocene. This progressive shift was in response to decreased precipitation in the region. The depositional environments and their geomorphologic relationships were characterized. Thus, five morphosedimentary units (MSUs) were identified within the Holocene tufa deposits. Erosive episodes were recognized between MSUs and, as most of the MSUs partially overlap, the older ones are perched in relation to the younger ones. The chronology of the MSUs is based on 14C dates, and stable isotopes and total organic carbon content provided environmental information during the period of tufa sedimentation. The successive dissection episodes in the Trabaque Canyon tufa deposits took place during transitioning climate conditions from the relatively wet early Holocene to the more arid late Holocene. During this transitional period in the mid-Holocene, wet phases alternated with more arid conditions, causing the largest hydrological regime gradients in the Holocene. Thus, the four erosive episodes causing tufa dissection in Trabaque during the Holocene are interpreted as the result of abrupt transitional periods towards arid conditions.

Preliminary palaeomagnetic investigation of riverine tufa: is riverine tufa a reliable recorder of the geomagnetic field direction?

SUMMARY Riverine tufas are freshwater carbonate deposits that precipitate in springs and streams distributed in limestone areas. They are noteworthy for their annual laminations that contain both dense and porous laminae and their fast precipitation rate. These features indicate that riverine tufa may be an excellent material for recording the secular variation of the geomagnetic field. Part of recently deposited tufas in Nagaya in Okayama Prefecture, Japan have a stable natural remanent magnetization (NRM), which is a reliable record of the average geomagnetic field direction. However, only the dense laminae in the recent tufa, however, carry a faithful record of the geomagnetic field direction; the porous laminae do not. The NRM of the porous laminae seems to be affected considerably by the dip direction of the slope on which the tufa is deposited. Based on the results of isothermal remanent magnetization acquisition experiments and microscopic observations, the dominant magnetic mineral in the recent tufa was determined to be magnetite. A Holocene palaeotufa, which was formed 2000–2800 years ago, was found to have no stable remanent magnetization. This is possibly due to the absence of dense layers where magnetic particles are tightly fixed. Since it is very important in palaeomagnetic secular variation studies to be able to reconstruct much older, continuous records of the variation of the geomagnetic field, tufa deposits are not ideal materials and their applicability for this purpose is rather low, although the further investigation is needed.

Effects of climatic change and bog development on Holocene tufa formation in the Lorze Valley (central Switzerland)

A massive tufa complex in the lower Lorze Valley (550 m a.s.l., north-central Switzerland) formed between 9000 and 5000 cal. yr BP. In the plain 4 km southeast of the complex lies Neugrundmoor (830 m a.s.l.), a mire in the hydrologic recharge area of the springs responsible for the tufa formation. Here we address an earlier, untested hypothesis that the transition of Neugrundmoor from fen to raised bog caused the end of the tufa formation, by means of pollen and testate amoebae analyses on a peat section. The chronology of the section is based on ten AMS 14C-dates of terrestrial plant macrofossils. Our results suggest that until 7100 cal. yr BP the plain had shallow ponds, which gradually developed into fens. Starting 5350 cal. yr BP, large raised bogs developed in the fens in a few centuries. This agrees with the hypothesis that the end of tufa formation coincided with the transition from fen to raised bog. It also coincided with the end of the mid-Holocene temperature optimum, when climate became cooler and wetter. We suggest that the main factor causing the end of tufa formation at 5500—5000 cal. yr BP was declining air temperatures, which on the one hand led to diminished calcium precipitation, on the other to enhanced bog formation. Peat has a low hydraulic conductivity, so we suggest that less water became available for tufa formation because of increased thickness of peat layers, exacerbating the direct effect of cooling on calcium precipitation. Noticeable human influence in the surroundings did presumably not start before c. 3500 cal. yr BP and was then very weak, so this was not responsible for the end of the tufa formation. Extrapolation of our results suggests that climatic cooling alone is sufficient to explain the widespread termination of tufa formation after the mid Holocene.

Three detailed, radiocarbon-dated, Holocene tufa and alluvial fan mollusc successions from southern Ontario: The first in northeastern North America

Fossil mollusc remains, particularly those of land snails, provide a wealth of detailed information concerning local Quaternary paleoenvironments and regional paleoclimatic conditions. However, this important resource is generally overlooked in North America and consequently quantitative, well-dated terrestrial mollusc successions are extremely rare here. In an attempt to redress this imbalance, a project examining the Holocene molluscan faunal history of northeastern North America has been initiated. Thus far, detailed mollusc successions and associated radiocarbon age determinations have been obtained from three sites in southern Ontario; two alluvial fans (Chine Drive Fan and Cudia Park Fan, both from the Scarborough Bluffs east of Toronto) and a tufa (West River Road, near Cambridge). The observed land snail successions to provide an initial view of the molluscan faunal history of this region during the period from the late glacial to the mid Holocene, between approximately 10,500 and 5000 radiocarbon years BP (yr BP). The molluscan data have also been used to reconstruct the paleoenvironmental history of southern Ontario during this time, documenting the progression from exposed, open habitats characterised by species such as Vertigo modesta, Vallonia albula, Columella columella and Vitrina angelicae limpida, to closed woodland biotopes indicated by the presence of taxa including Carychium nannodes, Nesovitrea binneyana, Paravitrea multidenta and Helicodiscus parallelus. This study has already demonstrated the potential of this technique in the study of Quaternary climatic and environmental change in eastern North America and represents a strong foundation for extending the analysis of fossil land snail successions to other sites throughout this region. It has also provided new and important insights into the establishment of the modern molluscan fauna, which are essential to the understanding of present day distributional patterns and environmental associations.

Late-glacial and Early Holocene climate and environment from stable isotopes in Welsh tufa

The Caerwys tufa in North Wales (UK) contains basal deposits thought to represent carbonate precipitation during the Late-glacial interstadial. These deposits are used to test whether stable isotope data record all or part of a warming-cooling-warming trend through the transition of the Late-glacial interstadial, Younger Dryas stadial and early Holocene. The d18O values and molluscan abundance data suggest that deposition of the Late-glacial tufa occurred mainly during a relatively warm period (? GL-1c or 1e) followed by cooling, the latter likely to be the transition into the Younger Dryas stadial. Tufa deposition is not recorded in the coldest part of the stadial: it is replaced by a sandy horizon. d13C values around -8.5‰ in the basal Late-glacial interstadial tufas show there was a significant influence from isotopically light soil-zone CO2, consistent with development of birch scrub and woodland further south in North Wales. During climatic cooling into the Younger Dryas stadial a 1‰ decrease in tufa d13C is interpreted to represent decreasing phytoplankton photosynthetic activity in pools causing less isotopic enrichment of the tufa d13C. In the (pre-9000 cal. years BP) early Holocene tufas at Caerwys, palaeo-water temperatures calculated from d18O in tufa calcite and from shell carbonate of Lymnaea peregra agree well and suggest summer water temperatures in the range 13 to 16.5°C. d13C data from these early Holocene pool-micrite tufas demonstrate that phytoplanktonic photosynthetic activity within the water column resumed under the warmer conditions causing isotopic enrichment of the tufa d13C values.

Palaeoclimatic records from stable isotopes in riverine tufas: Synthesis and review

It has been known for some time that viable palaeoclimatic information should be recorded in groundwater-fed riverine tufas, particularly barrage, paludal or spring-line deposits. Stable isotope variation in active tufa-depositing systems is well understood. Variability in δ 18O is driven mainly by changes in water temperature and the δ 18O value of the recharging water. Variability in δ 13C reflects principally the relative contribution of isotopically light CO 2 from soil organic matter, and isotopically heavier carbon derived from the dissolution of the aquifer limestone, modulated by equilibration of the aquifer, spring and stream water with atmospheric CO 2 (degassing) and by in-aquifer or in-stream calcite precipitation. In fast-flowing riverine systems the effects of evaporation and in-stream biological activity are negligible. Disequilibrium isotope effects away from actively degassing springs are probably very small, although this can be difficult to demonstrate, especially for δ 13C. Highly resolved sub-sampling of annual layers in active/sub-recent and Holocene tufas show conclusively that seasonal records of temperature ( δ 18O) and relative recharge intensity ( δ 13C) are recorded. At decadal-scale sampling resolution tufa deposits record (mostly) variation in δ 18O of meteoric recharge, which depending on locality may reflect source or amount effects (particularly continentality), or temperature change. In Holocene tufas from NW Europe, centennial-scale climatic cooling, culminating in a sharp 8.2 ka minimum is recorded. At one site a detailed δ 13C record suggests progressive dryness accompanying coldness at 8.2 ka. Although cementation of void space, micritization and aggrading neomorphism all occur in sub-recent and older tufas, there is no unequivocal evidence that diagenesis is either pervasive or that it seriously changes depositional stable isotope values. As tufa deposits accumulate rapidly, where annually laminated they offer an excellent opportunity to study seasonality in Quaternary palaeoclimates. Tufa stable isotope records should be considered complementary to the longer-term palaeoclimatic data provided by speleothems, particularly as these terrestrial carbonates are often closely associated geographically.

Seasonal records of climatic change in annually laminated tufas: short review and future prospects

AbstractMany Recent and fossil freshwater tufa stromatolites contain millimetre‐scale, alternating laminae of dense micrite and more porous or sparry crystalline calcites. These alternating laminae have been interpreted to represent seasonally controlled differences in the biotic activity of microbes, and/or seasonally controlled changes in the rate of calcification. Either way, couplets of these microbially mediated alternating calcified laminae are generally agreed to represent annual seasonality. Combined stable isotope (δ18O and δ13C) and trace element (Mg, Sr, Ba) geochemistry from Recent tufa stromatolites show that seasonal climatic information is available from these calcites. Variability in δ18O (and in one case Mg concentration) has been shown to be controlled primarily by stream temperature change, usually driven by solar insolation. In arid climates, seasonal evaporation can also cause δ18O enrichment by at least 1‰. Variability in δ13C results potentially from: (1) seasonal change in plant uptake of 12C‐enriched CO2; (2) seasonal change in degassing of 12C‐enriched CO2 in the aquifer system; and (3) precipitation of calcite along the aquifer or river flow path, a process that increases δ13C of dissolved inorganic carbon (DIC) in the remaining water. Mechanisms 2 and 3 are linked because calcite precipitates in aquifers where degassing occurs, e.g. air pockets. The latter mechanism for δ13C enrichment has also been shown to cause sympathetic variation between trace element/Ca ratios and δ13C because trace elements with partition coefficients much greater than 1 (e.g. Sr, Ba) remain preferentially in solution. Since degassing in air pockets will be enhanced during decreased recharge when water saturation of the aquifer is lowest, sympathetic variation in trace element/Ca ratios and δ13C is a possible index of recharge and therefore precipitation intensity.High‐resolution geochemical data from well‐dated tufa stromatolites have great potential for Quaternary palaeoclimate reconstructions, possibly allowing recovery of annual seasonal climatic information including water temperature variation and change in rainfall intensity. However, careful consideration of diagenetic effects, particularly aggrading neomorphism, needs to be the next step. Copyright © 2005 John Wiley & Sons, Ltd.

Sedimentology of holocene tufa carbonates in Orissa State, India

Holocene tufas of Orissa State, India, are formed in stream/fluvial environments. Sparry and micritic laminae in stromatolitic tufa represent seasonal deposition and contain cyanobacteria and diatoms. Micropores in spar crystals suggest bacterial activity. Aragonitic laminae show radial, bushy, and spherulitic fabrics which may be microbially controlled. Phytohermal (moss-rich) tufas show micritic to microsparry encrustations and reveal epiphytic microorganisms. CO2 degassing due to agitation from waters supersaturated with respect to calcite, is the dominant process of precipitation. Aragonite precipitation is possibly controlled by Sr concentrations in the waters. Pisoids have been formed by alternate disposition of micritic and microsparry laminae and sediment-rich zones around intraclastic tufa. Vadoids show development of fine cortical laminae around spherulites and radial needles. Microfabric of speleothem-like crusts suggest precipitation of finer crystallites and their subsequent transformation to a coarser fabric. Rhombic, needle and spike-like cements and neomorphic spar (inverted from aragonite) are developed in a vadose setting. Aragonite to calcite inversion is supported by texture-chemistry variance. The tufas have been formed in three depositional environments (1) waterfall environment, (2) slope environment and (3) river channel margin environment.

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.

230Th/ 234U dating of Holocene tufas: possibilities and problems

High-resolution chronologies in continental carbonate deposits such as tufas are required for detailed palaeoclimatic and environmental studies. This work set out to establish if high-resolution U-series dating of detritus-rich Holocene tufas is routinely possible. The study centres on a paludal Holocene tufa from southern England that already has an existing Holocene chronology, based on 14C and supported by biostratigraphy, against which to compare U-series dates. The results show that significant detrital contamination combined with low initial U concentrations, and short time for ingrowth of radiogenic 230Th make high-resolution U-series dating of Holocene tufa very difficult. Moreover, a single ( 230Th/ 232Th) initial value to correct for the presence of detrital 230Th is not appropriate at the study site, a finding that may apply to most Holocene tufas. Total sample dissolution of coeval samples demonstrates considerable variability in the isotopic composition of the detritus. The total sample dissolution data are too scattered to constrain chronologies at the required resolution and may indicate the incorporation of a 230Th-rich component in the detritus.

Molluscan successions from the Holocene tufa of St Germain‐le‐Vasson, Normandy (France) and their biogeographical significance

AbstractA detailed molluscan succession from a 10 m thick deposit of Holocene tufa at St Germain‐le‐Vasson, Normandy, provides the most complete record from northern France and has shed new light on the historical biogeography of several species of land snail. The succession has been reconstructed from four profiles and a chronology provided by accelerator mass spectrometry radiocarbon dating of charcoal, wood and shell. The onset of tufa formation occurred after 9700±90 yr BP and persisted until 4213±77 yr BP. The tufa appears to have accumulated at a remarkably constant rate (14.4–16.5 cm 100 yr−1), except for the upper levels, where the rate increases fourfold. The succession has been divided into six local molluscan zones. An early assemblage consisting of ecologically tolerant species and those indicative of marshy grassland is replaced by a sequence of shade‐demanding taxa, reflecting the encroachment of woodland. Shaded conditions persist until the end of the sequence but the most hygrophilous elements decline after 5422±60 yr BP (zone 5), a change also reflected in the tufa lithology by the development of silty grey horizons. Notable species recovered from the tufa include Acicula fusca, Vertigo substriata, V. alpestris, V. moulinsiana (all rare or unknown living in northern France). Hygromia limbata, a twentieth century introduction to Britain, was previously thought to be a relatively recent arrival in northern France, but its record at St Germain shows that it has been present in Normandy since 6500 yr BP. Azeca goodalli, another shade‐demanding species, appeared at St Germain much later, just after 4420±65 yr BP. Several other species present in the tufa, such as Pomatias elegans, no longer live on the site, adding to the evidence for a distributional decline in Normandy and elsewhere. Perhaps the most noteworthy record is that of Leiostyla anglica, between about 8500 yr BP and 5000 yr BP, because this constitutes its only Holocene occurrence from mainland Europe. Following this discovery it seems likely that further Holocene sites with L. anglica may be found along the western seaboard of Europe connecting populations in Iberia with those of the British Isles. Copyright © 2004 John Wiley & Sons, Ltd.

Differences in the 14C age, δ 13C and δ 18O of Holocene tufa and speleothem in the Dinaric Karst

We studied Holocene speleothems and tufa samples collected in numerous caves and rivers in the Dinaric Karst of Croatia, Slovenia, Bosnia and Herzegovina, as well as Serbia and Montenegro. Differences in the formation process of tufa and speleothems are discussed in the context of their isotopic composition ( 14C, 13C and 18O), as well as the chemistry of surface water (rivers, lakes) and drip water (in caves). The physical and chemical parameters monitored in the surface water (tufa precipitation) and drip water (speleothem precipitation) show that more stable conditions accompany speleothem rather than tufa formation. This is particularly obvious in the water temperature variations (2–22°C in surface water and 7–12°C in drip water) and in saturation index variation (3–11 in surface water and 1–6 in drip water). The range of 14C ages recorded by Holocene speleothems (∼12 000 yr) is wider by several thousands years than that of Holocene tufa samples (∼6000 yr). δ 13C values for tufa samples range from −12‰ to −6‰ and for speleothem samples from −12‰ to +3‰ reflecting higher soil carbon and/or vegetation impact on the process of tufa than on speleothem formation. The differences in δ 18O values of tufa and speleothem samples from different areas reflect different temperature conditions and differing isotopic composition in the water. The study shows that speleothems from the Dinaric Karst can be used as global palaeoclimatic records, whereas tufa records changes in the local palaeoenvironment.