Volcanic Ash Horizons Research Articles

An integrated proximal-distal radiocarbon dating approach provides improved age constraints for a key Holocene tephra isochron

Volcanic ash (tephra) horizons are an essential tool for the spatial and temporal correlation of many natural archives. However, tephrochronology is accompanied by age uncertainties that often limit its capacity to effectively constrain age models. This is particularly problematic for paleoseismology in complex tectonic settings such as New Zealand, where the accuracy and precision of earthquake ages is important for interpreting past fault interactions and earthquake sequences. We use the widespread Waimihia tephra, erupted at ∼3.4 ka from Taupō volcano, as a case study for generating improved age constraints for Holocene tephra isochrons. Our dating strategy minimizes uncertainties by (i) conducting detailed assessments to identify the most reliable dateable material, (ii) using precise AMS dating techniques, and (iii) constructing Bayesian age models. By applying consistent procedures to date tephra deposits at proximal, onshore distal and offshore distal locations, we also facilitate the detection of any geographic radiocarbon bias. Based on high agreement between the independently derived tephra ages from each location, we demonstrate no offsets in radiocarbon results between proximal and distal deposits, precluding bias from near-vent magmatic degassing. Our integrated age model exploits the benefits of each individual dating approach and uses the full suite of 42 new radiocarbon dates to yield an age of 3574–3478 cal yr BP (highest 95% range) for the Waimihia eruption. Compared to previous estimates, this older and more precise age has implications for refining existing paleoearthquake correlations and other chronologies that rely on the Waimihia isochron to provide age control. We recommend applying this multi-location approach to existing tephrochronologies where possible, to achieve more accurate and precise eruption age estimates that allow for the detection of radiocarbon bias.

Rapid Tephra Identification in Geological Archives With Computed Tomography: Experimental Results and Natural Applications

Volcanic ash (tephra) horizons represent powerful chronological and stratigraphic markers: rapid and widespread deposition allows for correlation of geological records in time and space. Recent analytical advances enable identification of invisible ash (cryptotephra) up to thousands of kilometers from its volcanic source. This momentum has greatly expanded the reach and potential of tephrochronology: some deposits can now be traced across continents and oceans. However, the laborious laboratory procedures required to identify tephra horizons in geological archives hold back the pace of progress. By allowing the rapid visualization of ash at micrometer (µm) scales, computed tomography (CT) holds great promise to overcome these restrictions. In this study, we further demonstrate the potential of this tool for the tephra community with experimental results and applications on conventionally analyzed archives. A custom-made scanner helps us strike a balance between the convenience of whole-core medical scanners and the µm-resolution of micro-CT systems. Using basic image processing tools that can be readily mastered by tephrochronologists, we identified invisible horizons down to ∼500 shards in synthetic cores. In addition, procedures for the removal of image artifacts can be used to visualize other paleoenvironmental indicators such as bioturbation burrows, ice rafted debris or mineral dust. When applied on segments of manually counted natural archives, our approach captures cryptic glass shard maxima down to ∼300 shards/cm3. We also highlight the value of CT to help optimize sampling strategies by identifying micrometer-scale ash horizons that were not detected in shard count profiles. In conclusion, this work helps broaden the applicability of CT as a promising frontier in tephrochronology that can advance the field by optimizing the efficiency and accuracy of isochron detection.

Mineralogy and origin of the alkaline Nsungwe Formation tuffs of the Rukwa Rift Basin, southwestern Tanzania

The incipient phases of continental rift development remain enigmatic, as early rift sequences are rarely exposed, or poorly preserved in the geologic record. Here, we present petrographic and geochemical analyses of heavy mineral separates from pervasively weathered volcanic horizons intercalated within syn-rift sedimentary deposits from the Rukwa Rift Basin, southwestern Tanzania. These Oligocene (25–26 Ma) ashfalls represent the first magmatic products associated with the initiation of rifting in the western branch of the East African Rift System (EARS). In this work, we combine detailed single crystal petrography with in-situ geochemistry to characterise an alkaline volcanic mineral assemblage consisting of titanite, phlogopite, fluorapatite, clinopyroxene, Ti-rich andradite, sanidine, pyrochlore, nepheline, cancrinite, Fe-sulphide (pyrrhotite), calcite, Na-REE‑carbonate and peralkaline silicate melt. These minerals are significantly enriched in trace elements, particularly Ba, Sr and rare earth elements (REE), with REE + Y contents consistent with derivation from peralkaline phonolitic melts. Placed in context of EARS development, these tuffs display the hallmarks of explosive alkaline‑carbonatite volcanism in their resistant minerals and inclusions; the nature and occurrence of which is consistent with early rift progression elsewhere in the EARS. Importantly, we showcase the depth of extractable petrographic information preserved in extensively weathered volcanic ash horizons deposited into an active continental rift basins. Such an approach may have significant implications for recognising early volcanogenic sequences in continental rift zones worldwide, but particularly in other segments of the western branch of the EARS.

Palaeoenvironmental evolution of the late Miocene palaeolake at Zahle (Bekaa Valley, Lebanon)

Neogene non-marine sedimentary rocks of the Bekaa Valley (Lebanon) are microfossil rich and show minimal lateral facies changes and post-depositional diagenetic alteration across continuous exposures. In this paper, we analyze the sedimentary facies and microfossils of three closely-spaced stratigraphic sections near the town of Zahle. The sedimentary sequence is divided, from base to top, into three intervals: (1) a massive conglomerate unit interpreted as alluvial fan deposits, (2) fossiliferous yellowish marls and limestone interpreted as perennial lacustrine deposits, and (3) an organic-rich marlstone intercalated with volcanic ash horizons interpreted as a palustrine interval representing an overall shallowing sequence. Intervals 2 and 3 contain a diverse microfossil assemblage comprising charophytes, terrestrial molluscs, a freshwater bivalve, ostracods as well as fish and mammal remains. The palaeoenvironmental characteristics and evolution of these lacustrine deposits are inferred through facies analysis and through comparison of microfossil tolerances with the ecological requirements of their nearest living relatives. Results suggest that during the Miocene the Bekaa Valley was occupied by a relatively shallow, stable, oligotrophic freshwater or slightly oligohaline lake that evolved to a very shallow eutrophic lake with a dense palustrine vegetation belt. The palaeolake ultimately regressed to the south due to climatic changes and tectonic stresses leaving the lacustrine deposits exposed in the area of Zahle as an erosional surface.

High-precision U–Pb ages in the early Tithonian to early Berriasian and implications for the numerical age of the Jurassic–Cretaceous boundary

Abstract. The numerical age of the Jurassic–Cretaceous boundary has been controversial and difficult to determine. In this study, we present high-precision U–Pb geochronological data around the Jurassic–Cretaceous boundary in two distinct sections from different sedimentary basins: the Las Loicas, Neuquén Basin, Argentina, and the Mazatepec, Sierra Madre Oriental, Mexico. These two sections contain primary and secondary fossiliferous markers for the boundary as well as interbedded volcanic ash horizons, allowing researchers to obtain new radioisotopic dates in the late Tithonian and early Berriasian. We also present the first age determinations in the early Tithonian and tentatively propose a minimum duration for the stage as a cross-check for our ages in the early Berriasian. Given our radioisotopic ages in the early Tithonian to early Berriasian, we discuss implications for the numerical age of the boundary.

The age of volcanic tuffs from the Upper Freshwater Molasse (North Alpine Foreland Basin) and their possible use for tephrostratigraphic correlations across Europe for the Middle Miocene

The Middle Miocene Upper Freshwater Molasse sediments represent the last cycle of clastic sedimentation during the evolution of the North Alpine Foreland Basin. They are characterized by small-scale lateral and temporal facies changes that make intra-basin stratigraphic correlations at regional scale difficult. This study provides new U–Pb zircon ages as well as revised 40Ar/39Ar data of volcanic ash horizons in the Upper Freshwater Molasse sediments from southern Germany and Switzerland. In a first and preliminary attempt, we propose their possible correlation to other European tephra deposits. The U–Pb zircon data of one Swiss (Bischofszell) and seven southern German (Zahling, Hachelstuhl, Laimering, Unterneul, Krumbad, Ponholz) tuff horizons indicate eruption ages between roughly 13.0 and 15.5 Ma. The stratigraphic position of the Unterneul and Laimering tuffs, bracketing the ejecta of the Ries impact (Brockhorizon), suggests that the Ries impact occurred between 14.93 and 15.00 Ma, thus assigning the event to the reversed chron C5Bn1r (15.032–14.870 Ma) which is in accordance with paleomagnetic evidence. We combine our data with published ages of tuff horizons from Italy, Switzerland, Bavaria, Styria, Hungary, and Romania to derive a preliminary tephrochronological scheme for the Middle Miocene in Central Europe in the age window from 13.2 to 15.5 Ma. The scheme is based on the current state of knowledge that the Carpathian–Pannonian volcanic field was the only area in the region producing explosive calc-alkaline felsic volcanism. This preliminary scheme will require verification by more high-quality ages complemented by isotopic, geochemical and paleomagnetic data.

Glass compositions and tempo of post-17 ka eruptions from the Afar Triangle recorded in sediments from lakes Ashenge and Hayk, Ethiopia

Numerous volcanoes in the Afar Triangle and adjacent Ethiopian Rift Valley have erupted during the Quaternary, depositing volcanic ash (tephra) horizons that have provided crucial chronology for archaeological sites in eastern Africa. However, late Pleistocene and Holocene tephras have hitherto been largely unstudied and the more recent volcanic history of Ethiopia remains poorly constrained. Here, we use sediments from lakes Ashenge and Hayk (Ethiopian Highlands) to construct the first <17 cal ka BP tephrostratigraphy for the Afar Triangle. The tephra record reveals 21 visible and crypto-tephra layers, and our new database of major and trace element glass compositions will aid the future identification of these tephra layers from proximal to distal locations. Tephra compositions include comendites, pantellerites and minor peraluminous and metaluminous rhyolites. Variable and distinct glass compositions of the tephra layers indicate they may have been erupted from as many as seven volcanoes, most likely located in the Afar Triangle. Between 15.3−1.6 cal. ka BP, explosive eruptions occurred at a return period of <1000 years. The majority of tephras are dated at 7.5−1.6 cal. ka BP, possibly reflecting a peak in regional volcanic activity. These findings demonstrate the potential and necessity for further study to construct a comprehensive tephra framework. Such tephrostratigraphic work will support the understanding of volcanic hazards in this rapidly developing region.

Reprint of Glass compositions and tempo of post-17 ka eruptions from the Afar Triangle recorded in sediments from lakes Ashenge and Hayk, Ethiopia

Numerous volcanoes in the Afar Triangle and adjacent Ethiopian Rift Valley have erupted during the Quaternary, depositing volcanic ash (tephra) horizons that have provided crucial chronology for archaeological sites in eastern Africa. However, late Pleistocene and Holocene tephras have hitherto been largely unstudied and the more recent volcanic history of Ethiopia remains poorly constrained. Here, we use sediments from lakes Ashenge and Hayk (Ethiopian Highlands) to construct the first <17 cal ka BP tephrostratigraphy for the Afar Triangle. The tephra record reveals 21 visible and crypto-tephra layers, and our new database of major and trace element glass compositions will aid the future identification of these tephra layers from proximal to distal locations. Tephra compositions include comendites, pantellerites and minor peraluminous and metaluminous rhyolites. Variable and distinct glass compositions of the tephra layers indicate they may have been erupted from as many as seven volcanoes, most likely located in the Afar Triangle. Between 15.3−1.6 cal. ka BP, explosive eruptions occurred at a return period of <1000 years. The majority of tephras are dated at 7.5−1.6 cal. ka BP, possibly reflecting a peak in regional volcanic activity. These findings demonstrate the potential and necessity for further study to construct a comprehensive tephra framework. Such tephrostratigraphic work will support the understanding of volcanic hazards in this rapidly developing region.

First discovery of Holocene cryptotephra in Amazonia.

The use of volcanic ash layers for dating and correlation (tephrochronology) is widely applied in the study of past environmental changes. We describe the first cryptotephra (non-visible volcanic ash horizon) to be identified in the Amazon basin, which is tentatively attributed to a source in the Ecuadorian Eastern Cordillera (0–1°S, 78-79°W), some 500-600 km away from our field site in the Peruvian Amazon. Our discovery 1) indicates that the Amazon basin has been subject to volcanic ash fallout during the recent past; 2) highlights the opportunities for using cryptotephras to date palaeoenvironmental records in the Amazon basin and 3) indicates that cryptotephra layers are preserved in a dynamic Amazonian peatland, suggesting that similar layers are likely to be present in other peat sequences that are important for palaeoenvironmental reconstruction. The discovery of cryptotephra in an Amazonian peatland provides a baseline for further investigation of Amazonian tephrochronology and the potential impacts of volcanism on vegetation.

Environmental significance of Upper Miocene phosphorites at hominid sites in the Lukeino Formation (Tugen Hills, Kenya)

The Lukeino Formation contains an important sedimentary and fossiliferous record of the late Miocene (6.09–5.68Ma), which has in particular yielded the fossil remains of the oldest East African bipedal hominid called Orrorin tugenensis. This fluvio-lacustrine sedimentary succession crops out in the Kenyan part of the East African Rift. It is mainly composed of clay to sandy clay deposits intercalated with volcanic ash horizons, and localized layers of carbonates and diatomites. A detailed sedimentological and mineralogical study of the Lukeino Formation was conducted to throw light on the environmental conditions in which the hominids lived.Several centimetric, relatively continuous and indurated phosphatic horizons, of sedimentary origin, were identified at two sites (Sunbarua and Kapcheberek). Mineralogical (XRD) and geochemical analyses as well as observations by SEM, which was coupled with an energy dispersive spectroscopy (EDS) microprobe, indicate that the autochthonous phosphate layers are composed of a micritic matrix of francolite (38–93%), with incorporation of silicates in variable proportions from one layer to another. The phosphate matrix contains very well preserved and abundant diatom frustules in the basal phosphate layer. These diatoms are identified as Aulacoseira granulata, implying a pH of 7.8–8.2 for freshwaters of the Palaeolake Lukeino. Calcitic tubular structures, linked to a possible bacterial origin, are also observed locally.Phosphate layers occur abruptly within a thick clay–sandy series, associated with an intense runoff phase during the deposition of this interval of the Lukeino Formation. The massive and cyclic input of phosphorus to the lake promoted productivity to the stage where it caused a diatom bloom. The establishment of several phosphate horizons testifies to successive phases of eutrophication of Palaeolake Lukeino. The diatom cells provided some of the organic matter, which was decomposed by bacterial activity at the bottom of the lake in suboxic conditions, but in insufficient quantities to fully form the phosphatic materials. The rest of the organic matter needed for phosphogenesis came from terrigenous supply (plant debris), suggesting the presence of dense vegetation in the catchment of the Palaeolake Lukeino, during this well-drained interval of deposition of the Lukeino Formation.

A chronological framework for a long and persistent archaeological record: Melka Kunture, Ethiopia

New 40Ar/ 39Ar geochronological data for several volcanic ash horizons from Melka Kunture, Ethiopia, allow for significantly more precise age constraints to be placed upon the lithostratigraphy, archaeology and paleontology from this long record. Ashes from the Melka Kunture Formation at Gombore yielded the most reliable age constraints, from 1.393 ± 0.162 Ma 2 2 Uncertainties are provided at 1σ, here and throughout. (millions of years ago) near the base of the section to 0.709 ± 0.013 Ma near the top. Dating the Garba section proved more problematic, but the base of the section, which contains numerous Oldowan obsidian artifacts, may be >1.719 ± 0.199 Ma, while the top is securely dated to 0.869 ± 0.020 Ma. The large ignimbrite from the Kella Formation at Kella and Melka Garba is dated to 1.262 ± 0.034 Ma and pre-dates Acheulean artifacts in the area. The Gombore II site, which has yielded two Homo skull fragments, ‘twisted bifaces,’ and a preserved butchery site, is now constrained between 0.875 ± 0.010 Ma and 0.709 ± 0.013 Ma. Additional ashes from these and other sites further constrain the timing of deposition throughout the section. Integration with previously published magnetostratigraphy has allowed for the first time a relatively complete, reliable timeline for the deposition of sediments, environmental changes, archaeology, and paleontology at Melka Kunture.

New 40Ar- 39Ar and detrital zircon U-Pb ages for the Upper Cretaceous Wahweap and Kaiparowits formations on the Kaiparowits Plateau, Utah: implications for regional correlation, provenance, and biostratigraphy

In order to better constrain the age and provenance of the Upper Cretaceous Wahweap and Kaiparowits formations in southern Utah, U-Pb SHRIMP ages were obtained for detrital zircons from three sandstone samples, in addition to the first 40Ar- 39Ar age for the Wahweap Formation, obtained from a devitrified volcanic ash horizon (bentonite). The ash horizon, located ∼40 m above the base of the Wahweap Formation, yields an age of 80.1 ± 0.3 Ma. The new radiometric data improve upon previous biostratigraphic age estimates for the Wahweap Formation and indicate that the formation was deposited between approximately 81 and 76 Ma. The youngest population of detrital zircons from the base of the Wahweap Formation clusters around 83–82 Ma, while the youngest population in the capping sandstone near the top of the formation is between 77–81 Ma, consistent with the 40Ar- 39Ar age. Detrital zircons from the base of the overlying Kaiparowits Formation include a younger population clustering around 77–76 Ma, but are otherwise broadly similar to those in the lower Wahweap. Detrital zircon assemblages suggest the lower Wahweap and Kaiparowits sandstones were primarily deposited by longitudinal stream systems sourced in the Cordilleran magmatic arc in southern California or western Nevada, along with Mesozoic volcanics in southern Arizona. The capping sandstone contains detrital zircons that suggest it was proximately sourced from transverse stream systems that drained eastward out of uplifted Mesozoic quartzose sandstones in the Sevier thrust belt to the west. Revised correlations between the Wahweap Formation and coeval strata and faunas across the Western Interior Basin show that the Wahweap Formation is coeval with Judithian age localities including the type-Judithian Judith River Formation. This suggests that the Aquilan and Judithian North American Land Mammal “ages” are in need of recalibration based on recent acquisition of this and other new radiometric data, as well as new faunal data. Moreover, this study provides critical temporal constraint for important mammalian and dinosaurian faunas of the Wahweap Formation.

Impact of differential tectonic subsidence on isolated carbonate-platform evolution: Triassic of the Nanpanjiang Basin, south China

The Nanpanjiang Basin of south China contains four exceptionally well-exposed, isolated Triassic carbonate platforms. Detailed mapping of two-dimensional transects and description of stratigraphic sections allow the reconstruction of facies architecture, sequence stratigraphy, and evolution of the platforms. Biostratigraphy, magnetic-susceptibility profiles, and volcanic-ash horizons allow chronostratigraphic correlation and, thus, a basinwide evaluation of mechanisms controlling platform evolution. A comparison of platform architecture demonstrates that southerly platforms have substantially greater thickness, backstepping geometry, pinnacle development, and earlier drowning that resulted from greater tectonic subsidence proximal to a probable convergent margin along the southern perimeter of the basin. Felsic volcanics thicken southward and contributed to the termination of the southernmost platform, indicating the development of a volcanic arc along the southern margin of the South China tectonic block. The northernmost isolated platform had greater longevity and lesser accumulation and lacks backstepping and pinnacle phases of development. Basin-margin intertonguing relationships, or lack thereof, demonstrate that earlier siliciclastic influx into the basin to the south and concurrent starved-basin conditions to the north impacted the evolution of platform-margin geometries. Comparative analysis of platform evolution shows that the timing and rates of tectonic subsidence controlled the timing of platform termination by drowning, backstep geometries, pinnacle development, and overall platform thickness. The timing of siliciclastic basin fill dictated differences in platform-margin geometries such as slope angle, relief above basin floor, and the presence or absence of basinward platform progradation. Despite the dramatic differences in platform architecture, eustatic sea level fluctuations imparted a basinwide sequence-stratigraphic signal.

Early Upper Paleolithic in Eastern Europe and Implications for the Dispersal of Modern Humans

Radiocarbon and optically stimulated luminescence dating and magnetic stratigraphy indicate Upper Paleolithic occupation-probably representing modern humans-at archaeological sites on the Don River in Russia 45,000 to 42,000 years ago. The oldest levels at Kostenki underlie a volcanic ash horizon identified as the Campanian Ignimbrite Y5 tephra that is dated elsewhere to about 40,000 years ago. The occupation layers contain bone and ivory artifacts, including possible figurative art, and fossil shells imported more than 500 kilometers. Thus, modern humans appeared on the central plain of Eastern Europe as early as anywhere else in northern Eurasia.

Mass mortality following disturbance in Holocene coral reefs from Papua New Guinea

The frequency and intensity of disturbance on living coral reefs have been accelerating for the past few decades, resulting in a changed seascape. What is unclear but vital for management is whether this acceleration is natural or coincident only with recent human impact. We surveyed nine uplifted early to mid-Holocene (11,000-3700 calendar [cal] yr B.P.) fringing and barrier reefs along similar to 27 km at the Huon Peninsula, Papua New Guinea. We found evidence for several episodes of coral mass mortality, but frequency was 16 km along the ancient coastline, occurred ca. 9100-9400 cal yr B.P., and is associated with a volcanic ash horizon. Recolonization of the reef surface and resumption of vertical reef accretion was rapid (< 100 yr), but the post-disturbance reef communities contrasted with their pre-disturbance counterparts. Assessing the frequency, nature, and long-term ecological consequences of mass-mortality events in fossil coral reefs may provide important insights to guide management of modern reefs in this time of environmental degradation and change.

Holocene climatic and environmental change from pollen records of lakes Zempoala and Quila, central Mexican highlands

Pollen records of a 520-cm long core from Lake Zempoala (2800 m altitude) and a 884-cm long composite core from Lake Quila (3010 m altitude), both located 65 km SW of Mexico City, show changes in vegetation and climate. The Zempoala record covers the last c. 6320 cal yr BP, while the Quila record spans the last c. 11,610 cal yr BP. These lakes, with submerged aquatics and hydroseral shore vegetation, lie today in the Abies religiosa-dominated forest belt. From c. 6320 to 2530 cal yr BP Zempoala shows a mixed forest with mesophyllous taxa, indicating warmer and more humid conditions than at present. From c. 2530 to 160 cal yr BP Abies forest and Pinus hartwegii forest dominated, indicating a temperate climate. During the first part of this period, from c. 2530 to 1150 cal yr BP abundant Pinus forest is indicative of drier climate conditions. During the last c. 310 cal yr of this period Abies-dominated forest prevailed, with a floral composition resembling the modern Abies forests. This is indicative of temperate and humid climatic conditions. During the last c. 160 cal yr BP the presence of Zea mays and Plantago reflect human impact. From c. 11,610 to 10,650 cal yr BP Quila shows Alnus- and Pinus-dominated forest, reflecting humid climatic conditions. From 10,650 to 5000 cal yr BP Pinus and Arceuthobium are indicative of a Pinus hartwegii forest, suggesting slightly lower temperatures than at present. A 25-cm thick volcanic ash horizon at 380–355 cm core depth is the well-known ‘yellow ash’ marker with a known age of c. 4200–4800 14C yr BP. This age is supported by our bracketing dates of 5620 and 4650 14C yr BP. From 4630 to 1000 cal yr BP mixed forest, including Pinus, Quercus, Carpinus, Hedyosmum, Juglans and Artemisia, is indicative of mesophytic conditions. From 4630 to 1010 cal yr BP Abies forest was abundant, reflecting colder and more humid conditions. From c. 1010 to 20 cal yr BP Pinus forest was very abundant suggesting, as in site Zempoala, dry climatic conditions. The pollen record from Quila documents four different types of forest: Pinus/Alnus forest (possibly Alnus jorullensis in zone QUI-I), Pinus forest (most likely Pinus hartwegii forest in zone QUI-II), mixed forest in zone QUI-III), and Abies forest ( Abies religiosa forest in zone QUI-IV). The last c. 1000 years show a decline of mixed forest and increasing Abies forest, reflecting cooler and more humid conditions. Quila also shows, during the most recent past, human impact by crop cultivation (Zea mays) and vegetation disturbance ( Plantago) leading to erosion. Other microfossils, among which coprophilous fungi, were recorded and illustrated.

Towards a Holocene tephrochronology for Sweden: geochemistry and correlation with the North Atlantic tephra stratigraphy

AbstractNorthern Europe has been affected by fallout of tephra from volcanic eruptions in Iceland throughout at least the Late‐glacial and Holocene. Microscopic volcanic ash horizons found in the British Isles, northern Germany and Scandinavia (e.g. Hekla 4) now provide important regional isochrons. Over recent years analytical techniques such as electron probe microanalysis (EPMA) have helped identify these distal tephra layers by glass geochemistry. Early tephra work in Sweden used refractive indices and bracketing radiocarbon dates for identification, resulting in tentative correlation. This paper presents geochemical data for Icelandic Holocene tephra in Sweden. Several sites in central Sweden have four distinct mid‐Holocene tephra layers. Correlation with the Icelandic record indicates that the following tephra layers are present: Hekla 4 (ca. 3830 yr BP), Kebister tephra (ca. 3600 yr BP), Hekla 3 (ca. 2880 yr BP) and Askja ad 1875. Preliminary analyses suggest that more, previously unidentified, tephra layers are present in low concentrations in the region, indicating potential for expanding the Swedish tephrochronology for palaeoecological and palaeoenvironmental work in northern Europe. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Continuous Late Quaternary proxy climate records from loess in Beringia

Loess deposits in eastern Beringia contain continuous proxy records of the effects of past climatic change on terrestrial landscapes at high latitudes. Variations of environmental magnetism and sedimentology of high-latitude loess deposits indicate that the timing and pattern of responses to local variations in wind intensity, storminess, and pedogenesis in eastern Beringia closely resemble the pattern of global climate change during the Late Quaternary deduced from studies of marine and ice core records. The age of paleoclimatic fluctuations, permafrost features, volcanic ash horizons, buried forest layers and paleosols, and other features of the eastern Beringian loess record can be determined using a variety of Quaternary dating methods. Tephrochronologic correlations between the loess record and the glacial history of eastern Beringia indicate the Delta Glaciation occurred during marine isotope stage 6. Several other middle and Late Quaternary glaciations across eastern Beringia can be tephrochronologically tied to the loess record, and appear to have been in phase with episodes of global cooling recorded in deep-sea records.

Late and middle pleistocene climatic change and forest development in Colombia: pollen record Funza II (2–158 m core interval)

Late and middle pleistocene climatic change and forest development in Colombia: pollen record Funza II (2–158 m core interval)

The orbital-tuned marine oxygen isotope record applied to the middle and Late Pleistocene pollen record of Funza (Colombian Andes)

Assuming that orbital variations are the main external cause of the succession of Late Pleistocene ice ages, deep-sea stratigraphers developed an orbital-tuned chronology of the marine oxygen isotope record. The pollen record of Funza (Eastern Cordillera, Colombia) displays in great detail long-range altitudinal shifts of the forest line, being also an expression of long-range fluctuating global temperatures. Both records of palaeotemperature were visually correlated: 48 isotopic events are assumedly reflected in the pollen record. This enables an “orbital-tuning” of the initial time frame of the pollen record, which was based on 14 dated volcanic ash horizons, and provides a detailed record of changes in sediment accumulation in the sedimentary basin of Bogotá. Inferences for the time frame of the pollen record of Páramo de Agua Blanca from the same area are discussed.