Marker Horizons Research Articles

A high-precision age estimate of the Holocene Plinian eruption of Mount Mazama, Oregon, USA

The climactic eruption of Mount Mazama in Oregon, North America, resulted in the deposition of the most widespread Holocene tephra deposit in the conterminous United States and south-western Canada. The tephra forms an isochronous marker horizon for palaeoenvironmental, sedimentary and archaeological reconstructions, despite the current lack of a precise age estimate for the source eruption. Previous radiocarbon age estimates for the eruption have varied, and Greenland ice-core ages are in disagreement. For the Mazama tephra to be fully utilised in tephrochronology and palaeoenvironmental research, a refined (precise and accurate) age for the eruption is required. Here, we apply a meta-analysis of all previously published radiocarbon age estimations ( n = 81), and perform Bayesian statistical modelling to this data set, to provide a refined age of 7682–7584 cal. yr BP (95.4% probability range). Although the depositional histories of the published ages vary, this estimate is consistent with that estimated from the GISP2 ice-core of 7627 ± 150 yr BP (Zdanowicz et al., 1999).

Paleoenvironmental reconstruction of the coastal Monte Léon and Santa Cruz formations (Early Miocene) at Rincón del Buque, Southern Patagonia: A revisited locality

Sedimentological, ichnological and paleontological analyses of the Early Miocene uppermost Monte León Formation and the lower part of the Santa Cruz Formation were carried out in Rincón del Buque (RDB), a fossiliferous locality north of Río Coyle in Santa Cruz Province, Patagonia, Argentina. This locality is of special importance because it contains the basal contact between the Monte Léon (MLF) and the Santa Cruz (SCF) formations and because it preserves a rich fossil assemblage of marine invertebrates and marine trace fossils, and terrestrial vertebrates and plants, which has not been extensively studied. A ∼90 m-thick section of the MLF and the SCF that crops out at RDB was selected for this study. Eleven facies associations (FA) are described, which are, from base to top: subtidal–intertidal deposits with Crassotrea orbignyi and bioturbation of the Skolithos-Cruziana ichnofacies (FA1); tidal creek deposits with terrestrial fossil mammals and Ophiomorpha isp. burrows (FA2); tidal flat deposits with Glossifungites ichnofacies (FA3); deposits of tidal channels (FA4) and tidal sand flats (FA5) both with and impoverish Skolithos ichnofacies associated; marsh deposits (FA6); tidal point bar deposits recording a depauperate mixture of both the Skolithos and Cruziana ichnofacies (FA7); fluvial channel deposits (FA8); fluvial point bar deposits (FA9); floodplain deposits (FA10); and pyroclastic and volcaniclastic deposits of the floodplain where terrestrial fossil mammal remains occur (FA11).The transition of the MLF–SCF at RDB reflects a changing depositional environment from the outer part of an estuary (FA1) through the central (FA2–6) to inner part of a tide-dominated estuary (FA7). Finally a fluvial system occurs with single channels of relatively low energy and low sinuosity enclosed by a broad, low-energy floodplain dominated by partially edaphized ash-fall, sheet-flood, and overbank deposits (FA8–11). Pyroclastic and volcaniclastic materials throughout the succession must have been deposited as ash-fall distal facies in a fluvial setting and also were carried by fluvial streams and redeposited in both estuarine and fluvial settings. These materials preserve most of the analyzed terrestrial fossil mammals that characterize the Santacrucian age of the RDB's succession. Episodic sedimentation under volcanic influence, high sedimentation rates and a relatively warm and seasonal climate are inferred for the MLF and SCF section.Lateral continuity of the marker horizons at RDB serve for correlation with other coastal localities such as the lower part of the coastal SCF south of Río Coyle (∼17.6–17.4 Ma) belonging to the Estancia La Costa Member of the SCF.

Faunal epiboles in the Upper Ordovician of north-central Kentucky: Implications for high-resolution sequence and event stratigraphy and recognition of a major unconformity

Mixed carbonate-siliciclastic strata of the lower Richmondian (Upper Ordovician, Cincinnatian) in the Cincinnati Arch region represent deposition along a gentle epeiric ramp in the Taconic foreland basin. Previous stratigraphic investigation of this interval largely emphasized local facies, with little consideration of regionally persistent patterns and widespread marker horizons. The current study describes a distinctive package within the lower Rowland Member of the Drakes Formation of northern Kentucky; this interval comprises a four-part succession of units, several of which are characterized by unusually high abundance of particular taxa (proliferation epiboles). Similar (to nearly identical) successions, including epiboles in corresponding positions, are present within the undifferentiated Bull Fork Formation of central Kentucky and the Fort Ancient Member of the Waynesville Formation in southwestern Ohio. Further, progressive erosion of the widely traceable “lower Rowland” succession beneath a distinctive, prominent marker bed indicates the presence of a previously undocumented, regionally angular unconformity within the middle Rowland Member (the “mid-Richmondian unconformity” predicted by some earlier workers). Hence, a widely persistent, nearly layer-cake pattern can be documented over 150km across the Cincinnati Arch. Although the mechanism(s) responsible for such widespread epiboles is (are) enigmatic, this study demonstrates the significant value of epiboles in regional stratigraphic correlation and recognition of unconformities.

The Jaramillo Subchron, a geochronological marker horizon in the palaeoanthropological record of China

The Jaramillo Subchron is routinely recognised in Chinese loess-palaeosol sequences, as well as in fluvial-lacustrine sequences such as those in the Nihewan Basin, North China. Many of these sections extend back to and below the Olduvai Subchron, and several (especially in the Nihewan Basin) contain palaeolithic sites. Initially, the Jaramillo Subchron was used to determine whether a palaeolithic site or hominin specimen lay above or below it, and it played an important role in demonstrating that the Chinese hominin and palaeolithic record extended past 1 Ma. Its main application now is as a marker horizon in sections that also contain the Olduvai Subchron. Estimation of the age of palaeolithic material between these two marker horizons at Nihewan and elsewhere was initially based on estimates of average sedimentation rate between these two points. The dating of Majuangou III to 1.66 Ma is one example, and provides the current earliest date for hominins in North China. However, age estimates have become more sophisticated by estimating the timing and during of fluvial-lacustrine cycles, and by attempts to link these to the marine isotope record. Although this has resulted in sites such as Xiaochangliang now having a variety of possible ages, the precise age and climatic context of Early Pleistocene sites in such settings should become clearer. Overall, these studies involving the Jaramillo Subchron have been more successful in estimating the first arrival of hominins in North China than in southern China, where the first appearance of hominins remains unclear.

Consistently dated records from the Greenland GRIP, GISP2 and NGRIP ice cores for the past 104 ka reveal regional millennial-scale δ18O gradients with possible Heinrich event imprint

We present a synchronization of the NGRIP, GRIP and GISP2 ice cores onto a master chronology extending back to 104 ka before present, providing a consistent chronological framework for these three Greenland records. The synchronization aligns distinct peaks in volcanic proxy records and other impurity records (chemo-stratigraphic matching) and assumes that these layers of elevated impurity content represent the same, instantaneous event in the past at all three sites. More than 900 marker horizons between the three cores have been identified and our matching is independently confirmed by 24 new and previously identified volcanic ash (tephra) tie-points. Using the reference horizons, we transfer the widely used Greenland ice-core chronology, GICC05modelext, to the two Summit cores, GRIP and GISP2. Furthermore, we provide gas chronologies for the Summit cores that are consistent with the GICC05modelext timescale by utilizing both existing and new gas data (CH4 concentration and δ15N of N2). We infer that the accumulation contrast between the stadial and interstadial phases of the glacial period was ∼10% greater at Summit compared to at NGRIP. The δ18O temperature-proxy records from NGRIP, GRIP, and GISP2 are generally very similar and display synchronous behaviour at climate transitions. The δ18O differences between Summit and NGRIP, however, changed slowly over the Last Glacial–Interglacial cycle and also underwent abrupt millennial-to-centennial-scale variations. We suggest that this observed latitudinal δ18O gradient in Greenland during the glacial period is the result of 1) relatively higher degree of precipitation with a Pacific signature at NGRIP, 2) increased summer bias in precipitation at Summit, and 3) enhanced Rayleigh distillation due to an increased source-to-site distance and a potentially larger source-to-site temperature gradient. We propose that these processes are governed by changes in the North American Ice Sheet (NAIS) volume and North Atlantic sea-ice extent and/or sea-surface temperatures (SST) on orbital timescales, and that changing sea-ice extent and SSTs are the driving mechanisms on shorter timescales. Finally, we observe that maxima in the Summit–NGRIP δ18O difference are roughly coincident with prominent Heinrich events. This suggests that the climatic reorganization that takes place during stadials with Heinrich events, possibly driven by a southward expansion of sea ice and low SSTs in the North Atlantic, are recorded in the ice-core records.

Tephrochronology and the extended intimate (integration of ice-core, marine and terrestrial records) event stratigraphy 8–128 ka b2k

The comparison of palaeoclimate records on their own independent timescales is central to the work of the INTIMATE (INTegrating Ice core, MArine and TErrestrial records) network. For the North Atlantic region, an event stratigraphy has been established from the high-precision Greenland ice-core records and the integrated GICC05 chronology. This stratotype provides a palaeoclimate signal to which the timing and nature of palaeoenvironmental change recorded in marine and terrestrial archives can be compared. To facilitate this wider comparison, without assuming synchroneity of climatic change/proxy response, INTIMATE has also focussed on the development of tools to achieve this. In particular the use of time-parallel marker horizons e.g. tephra layers (volcanic ash). Coupled with the recent temporal extension of the Greenland stratotype, as part of this special issue, we present an updated INTIMATE event stratigraphy highlighting key tephra horizons used for correlation across Europe and the North Atlantic. We discuss the advantages of such an approach, and the key challenges for the further integration of terrestrial palaeoenvironmental records with those from ice cores and the marine realm.

Developing a robust tephrochronological framework for Late Quaternary marine records in the Southern Adriatic Sea: new data from core station SA03-11

Tephra layers are assuming an increasingly important role in the dating and correlation of Late Quaternary marine sequences. Here we demonstrate their potential by reporting a new study of the sediment sequence of marine core SA03-11, recovered from the Southern Adriatic Sea, which spans the last c. 39 ka. A total of 28 discrete tephra layers are reported from this sequence, 10 of which are visible in the core and a further 18 are non-visible cryptotephra layers. These have been analysed using more than 1400 WDS-EPMA measurements of glass chemistry and results have been compared with published chemical measurements obtained from relevant proximal and distal sites which preserve eruptive material dating to within the same time interval. The data show that a high proportion of the layers originate from the Campi Flegrei volcanic field but more distinctive layers are sourced from Vesuvius, the Aeolian Islands and Vulcano, and these provide key marker horizons. The results show that the sequence extends in time to the Campanian Ignimbrite at the base, that a number of the layers have robust age estimates that permit a better constrained age–depth model to be constructed for the sequence, and that the potential exists for importing terrestrially-based age estimates into marine contexts, thereby circumventing problems of incorporating reservoir uncertainties associated with marine radiocarbon dates. The WDS-EPMA dataset generated here also provides important new data that constrain key Late Quaternary tephra layers in the central Mediterranean region.

Siderite microconcretions in the glauconite-bearing clayey-silty rocks of the Khaipakh Formation (Middle Riphean, Olenek Uplift)

Siderite microconcretions in the glauconite-bearing clayey-silty rock member of the lower sub-formation of the Khaipakh Formation (Middle Riphean, Olenek Uplift) are scrutinized for the first time. In two Khorbusuonka River sections located with a spacing of 12 km, the microconcretions occur as lenses and interlayers. Together with glauconitites, they serve as a distinct marker horizon of this stratigraphic interval. Their structures, morphologies, diffraction characteristics, chemical compositions, and isotope data are considered. They were examined comprehensively with modern investigation methods (X-ray diffraction, scanning electron microscopy with the application of local microprobe analysis, and others). Structural-morphological types of microconcretions are identified. Calculation of the chemical composition of carbonates based on numerous microprobe analyses made it possible to reveal different degrees of mineral heterogeneity in each type in terms of the distribution of macro- and microlevel isomorphous trace-elements and to refine the character of their secondary alterations at different stages of lithogenesis. The results of oxygen and hydrogen isotopic studies are presented for algal dolomites and limestones from the Middle and Lower Riphean sections in the Olenek Uplift (Debengda, Arymass, and Kyutingda formations). They demonstrated that siderites are similar to limestones and dolomites in terms of the oxygen isotopic composition (δ18O = 17.6–24.8, δ18Oav = 20.0 ± 2.4‰), but are marked by low δ13C values (from −6.3 to −12.0‰ (δ13Cav = −8.6 ± 2.1‰), suggesting the formation of microconcretions during early diagenesis. Siderite microconcretions were formed in the clayey-silty sediment slightly after glauconite, whose grains could serve as crystallization centers and (or) be entrapped during the growth of separate microcrystals. The role of catalyzers during the formation of both glauconite and siderite was played by bacterial communities, whose poorly preserved remnants have been detected not only in the studied member of the lower Khaipakh subformation, but also above and below the section. Separate types of microconcretion could be formed during the replacement of oncolites by siderite.

Sedimentation in created freshwater riverine wetlands: 15 years of succession and contrast of methods

This study summarizes five separate sedimentation studies spanning 15 years (years 3–17 following wetland creation in 1994) of two 1ha experimental flow-through wetlands. Included are methods and analyses of the most recent (2009–2010) comparative study that attempted to quantify both erosion and bioturbation processes. Depending on techniques used, two distinct types of sedimentation rates were estimated—gross and net sedimentation. Gross sedimentation in 2004–2005 (years 11 and 12) using sediment trap bottles was 45kgm−2 for 4 months during and after spring flood pulsing conditions in 2004 and 39kgm−2 for the same 4 months during and after steady flow conditions in 2005. Annual sediment accretion using feldspar and other horizon markers was 31.7±4.4kgm−2yr−1 (4.2±0.6cmyr−1) in 1996 (year 3 after wetland creation) and 34.4±4.5kgm−2yr−1 (5.5±0.8cmyr−1) in 2009 (year 16 after wetland creation). Net sedimentation using soil cores to estimate accumulation of sediments over antecedent soil horizon layers was 4.7±0.3kgm−2yr−1 (0.9±0.07cmyr−1) in 2004 (year 11) and 6.0±0.4kgm−2yr−1 (0.9±0.06cmyr−1) in 2009 (year 16). Net sedimentation, using estimates of sedimentation and erosion with the sediment erosion table (SET) method in 2009–2010 (years 16–17) was 3.9±6.1–9.0kgm−2yr−1 (1.3±0.8–1.4cmyr−1). Bioturbation by macrofauna significantly decreased sedimentation rates during the 2009 (year 16) study. Spatial patterns, consistent among horizon marker and net sedimentation studies, showed that deep, open-water areas had higher rates of sedimentation than shallow areas with emergent vegetation, and that sedimentation rates were higher when closer to the inflow than at the outflow of these flow-through wetlands. Net sedimentation of these created riverine wetlands ranged from 1.2cm/yr to 1.4cm/yr, suggesting that these wetlands will accumulate about 30cm of sediments in a little more than two decades.

Conodonts of the Mississippian/Pennsylvanian boundary interval in Central Iran

The record of conodonts related to the Mississippian/Pennsylvanian boundary interval was investigated in four sections in Central Iran from two different structural units. Two sections from the Sanandaj–Sirjan trend zone (Asad-abad, and Darchaleh sections) and two from the East-Central Iran Microplate (Shesh-angosht and Kale-Sardar sections) exhibit a nearly complete record previously described across the Mississippian/Pennsylvanian boundary in Iran. The investigated sections can be subdivided in three formations (Ghaleh-, Absheni-, and Zaluda Formation) which belong to the Sardar Group. The mid-Carboniferous boundary was defined by the occurrence of Declinognathus noduliferus s.l.. Bio-event characteristics of the Carboniferous conodont fauna (Mississippian genera Gnathodus and Lochriea have been replaced by Pennsylvanian genera Declinognathus and Idiognathodus) as well as sedimentological changes within overall shallow water deposits were located approximately 33° S of the paleoequator and suggest sea-level changes within the framework of the Late Paleozoic Ice Age (LPIA). Furthermore, a widespread crinoid marker horizon previously described from two localities in Iran can be subdivided into three units of different ages.

Fast acoustic velocity tomography of focusing operators

A 2D velocity model was estimated by tomographic imaging of overlapping focusing operators that contain one-way traveltimes, from common-focus points to receivers in an aperture along the earth’s surface. The stability and efficiency of convergence and the quality of the resulting models were improved by a sequence of ideas. We used a hybrid parameterization that has an underlying grid, upon which is superimposed a flexible, pseudolayer model. We first solved for the low-wavenumber parts of the model (approximating it as constant-velocity pseudo layers), then we allowed intermediate wavenumbers (allowing the layers to have linear velocity gradients), and finally did unconstrained iterations to add the highest wavenumber details. Layer boundaries were implicitly defined by focus points that align along virtual marker (reflector) horizons. Each focus point sampled an area bounded by the first and last rays in the data aperture at the surface; this reduced the amount of computation and the size of the effective null space of the solution. Model updates were performed simultaneously for the velocities and the local focus point positions in two steps; local estimates were performed independently by amplitude semblance for each focusing operator within its area of dependence, followed by a tomographic weighting of the local estimates into a global solution for each grid point, subject to the constraints of the parameterization used at that iteration. The system of tomographic equations was solved by simultaneous iterative reconstruction, which is equivalent to a least-squares solution, but it does not involve a matrix inversion. The algorithm was successfully applied to synthetic data for a salt dome model using a constant-velocity starting model; after a total of 25 iterations, the velocity error was [Formula: see text] and the final mean focal point position error was [Formula: see text] wavelength.

An anthropogenic marker horizon in the future rock record

Recognition of increasing plastic debris pollution over the last several decades has led to investigations of the imminent dangers posed to marine organisms and their ecosystems, but very little is known about the preservation potential of plastics in the rock record. As anthropogenically derived materials, plastics are astonishingly abundant in oceans, seas, and lakes, where they accumulate at or near the water surface, on lake and ocean bottoms, and along shorelines. The burial potential of plastic debris is chiefly dependent on the material’s density and abundance, in addition to the depositional environment. Here, we report the appearance of a new “stone” formed through intermingling of melted plastic, beach sediment, basaltic lava fragments, and organic debris from Kamilo Beach on the island of Hawaii. The material, herein referred to as “plastiglomerate,” is divided into in situ and clastic types that were distributed over all areas of the beach. Agglutination of natural sediments to melted plastic during campfire burning has increased the overall density of plastiglomerate, which inhibits transport by wind or water, thereby increasing the potential for burial and subsequent preservation. Our results indicate that this anthropogenically influenced material has great potential to form a marker horizon of human pollution, signaling the occurrence of the informal Anthropocene epoch.

Applied stratigraphy and carbonate petrography of the Arabescato Orobico dimension stone from the Bergamasc Alps (Calcare Rosso, Italy)

The Arabescato Orobico is a decorative stone and building material from the Bergamasc Alps, Italy. From the geological standpoint it belongs to the Calcare Rosso, latest Ladinian-Early Carnian in age, which is a peritidal tepee-rich limestone strongly modified by superimposed early diagenetic processes, outcropping in Lombardy Southern Alps. In the last twenty-five years four commercial varieties have been available on the market, namely Grigio, Grigio-Rosa, Rosa, and Rosso, whose working and abandoned quarries, are located around the extraction districts of Camerata Cornello, and San Giovanni Bianco in the median Brembana Valley. Until the 1970s of the last century some different historical types, namely Rosso Antico, Rosso Venato, and Venato Finissimo, where extracted around Ardesio, in the Seriana Valley, as well. This paper deals with the stratigraphic analysis based on detailed physical correlations of several marker horizons mainly recognized along abandoned and active quarries, and their carbonate facies analysis. Stratigraphic results permit to divide the Calcare Rosso unit in three lithozones (CR1, CR2, and CR3), with a very low angle onlapping geometry at the top of the Esino Limestone. Southward (basinward) the Calcare Rosso gradually passes to dominant grey peritidal carbonates poor in tepeehorizons corresponding to the lower Breno Fm.. The petrographic characterization of representative samples from Mecca and Cadei quarries, permits to distinguish peculiar textural and compositional features of the four contemporaneous commercial types. As concerns the Grigio we identify almost five different subtypes, respectivelydenominated Grigio Laguna, composed of fossiliferous subintertidal grey to dark grey and black wackestone-packstones, Grigio Granulare, composed of inter-supratidal pisolitic grey grainstonerudstones, Nero Raggiato, very rich in “raggioni”, dark grey to black fibrous-radial early diagenetic calcite, Grigio Laminato composed of poorly to middle deformed antiformal structures, namely embryo to mature tepees, and, finally, Grigio Brecciato, a grey-colored diagenetic-pedogenetic breccia. Remaining types, ascribed to pinkishgrey, Grigio-Rosa, pinkish, Rosa, and reddish, Rosso, colored diagenetic supratidal facies, can be divided into laminated or brecciated subtypes. The former are composed of antiformal syndepositional to early diagenetic structures, i.e. embryo to mature tepees; the brecciated subtypes consist of more diagenetically modified senile tepees passing to breccias. These types present different amounts of sediment filling, including internal calcitic and dolomitic sediments, pisolites, terra-rossa paleosols and green/reddish clays, and early cementation, such as fibrous isopachous crust, “raggioni”, sparry calcite, of the primary and early diagenetic cavities. The stratigraphic organization, and the facies analysis of this carbonate succession, have been used to evaluate the areal and vertical distribution of the Arabescato Orobico commercial types; in this way the Calcare Rosso depositional model is also suitable for geological prospecting and quarrying activities.

Early Middle Pleistocene sediments at Sidestrand, northeast Norfolk, yield the most extensive preglacial cold stage beetle assemblage from Britain

Fluvial sediments (Cromer Forest-bed Formation) at Sidestrand, northeast Norfolk, have yielded the most extensive preglacial early Middle Pleistocene cold (arctic) stage beetle assemblage known from Britain. The assemblage is composed of 59 taxa indicating severely cold and continental climatic conditions. Mutual Climatic Range reconstructions suggest that the mean temperature of the warmest month (July) was between 10 °C and 13 °C and the mean temperature of the coldest months (January and February) between −17 °C and −10 °C, although the actual palaeotemperatures were probably towards the lower end of these ranges. Associated pollen and macroflora remains were poorly represented but all are known from other cold stage contexts.Excavations reveal that this freshwater arctic assemblage occurs within units between two important stratigraphic marker horizons, the Sidestrand Hall Member of the Cromer Forest-bed Formation and the first lowland glacigenic deposit (Happisburgh Till Member) in eastern England, although the ages of both remain equivocal. Recent amino-acid chronologies of molluscan faunas from the Sidestrand Hall Member indicate a MIS 13 age, with by inference a MIS 12 age for the overlying arctic units with cold beetle fauna and for the Happisburgh Till Member. However, the arctic units are separated from the two stratigraphic marker horizons by shallow marine deposits (Wroxham Crag Formation) demonstrating at least two intervening phases of marine transgression and a cold climate marine regression. The climatic significance of these marine transgressions and their chronostratigraphic implications are currently uncertain.

A joined rock magnetic and colorimetric perspective on the Late Pleistocene climate of Orlovat loess site (Northern Serbia)

This study represents the first detailed joined colorimetric and rock magnetic investigation on a loess–paleosol sequence in the Vojvodina region, Northern Serbia. The aim of this paper is to derive new information on the Late Pleistocene paleoenvironment from a high resolution multi-proxy assessment of the iron mineralogical composition at the Orlovat loess paleosol sequence. The Orlovat section is the first site from the Banat loess region analyzed for Quaternary paleoclimate information, and accordingly it is the easternmost loess section in the Serbian loess region studied. In contrast to other Serbian sites, the Orlovat site is not located in a plateau. Rock magnetic analysis and the luminescence-dating based chronology revealed active slope processes at this site during the Late Pleistocene. This is the most strikingly evidenced by the absence of a Middle Wurmian (MIS 3) interstadial pedocomplex, present as marker horizon in the Serbian plateau-loess sites.Nevertheless, important stratigraphic units and distinct pattern of the proxy records can be correlated to other key sites of the Vojvodina loess region. Hence, this section provides an opportunity to reconstruct local as well as regional environmental and climatic conditions during the Late Pleistocene. The applied multiproxy approach especially allowed valuable insight in the environmental conditions that prevailed during formation of the last interglacial pedocomplex. Accordingly, in the Eemian pronounced summer dryness prevailed. During the formation of the Eemian soil, the dry period was warmer and lasted longer or was characterized by less precipitation as during formation of the modern soil. Although the soil moisture deficit in the Eemian summers was most likely higher, the rainfall was more pronounced in the period of from spring to early summer/and or autumn as during the Holocene. These findings support previous observations from the Batajnica site, a key site of the Vojvodina loess region, and highlight the Eemian as potential past analogue to future climate warming in Serbia. This study points out the potential of a joined colorimetric or spectrometric analyses and rock magnetic analyses in paleoenvironmental investigations on loess paleosol sequences.

Late Quaternary tephrostratigraphy of southern Chile and Argentina

The Southern and Austral Volcanic Zones of the Andes comprise 74 volcanic centres with known post-glacial activity. At least 21 of these have had one or more large explosive eruptions in the late Quaternary, dispersing tephra over vast areas. These tephra layers therefore have great potential as tephrochronological marker horizons in palaeoenvironmental studies in southern Chile and Argentina, a region that is particularly useful to study climate dynamics of the southern hemisphere. However, to date tephrochronology has rarely been fully utilised in this region as a correlation and dating tool. Here we review the existing post-glacial tephrostratigraphic record of the Southern and Austral Volcanic Zones, and compile a database of known occurrences of tephra from these volcanoes in ice and lacustrine, marine, peat, and cave sediment records. We address the inconsistencies in and revisions of the tephrostratigraphies presented in prior literature, and discuss the challenges in correlating tephras and the limitations of the tephrostratigraphic record in this area. This study highlights the many gaps that still exist in our knowledge of the eruptive histories of these volcanoes, but also reveals the largely under-utilised potential of tephra as a correlation tool in this region. This is exemplified by the severe lack of adequate geochemical analysis of tephra layers preserved in many lacustrine and peat sediment sections, which are particularly important tephrostratigraphic records in southern Chile and Argentina due to the paucity of surface preservation.

Physical and geochemical controls on sedimentation along an ancient continental margin: The deep-marine Old Fort Point Formation (Ediacaran), southern Canadian Cordillera

Abstract The approximately 608 Ma Old Fort Point Formation (OFP) is a unique, mixed siliciclastic and carbonate stratigraphic marker horizon exposed locally over an area of about 35 000 km2 within the Neoproterozoic Windermere Supergroup (WSG) of the southern Canadian Cordillera. Paleogeographically, these strata were deposited by a variety of deep-marine processes in slope to basin-floor settings along the continental margin of Laurentia (ancestral North America). The OFP ranges from approximately 60 to 450 m thick and comprises three informal lithostratigraphic members, which stratigraphically upward are the Temple Lake (TLM), Geikie Siding (GSM) and Whitehorn Mountain (WMM) members. The TLM and GSM are generally uniform in thickness and lithofacies and are interpreted to reflect basin-wide, synchronous deposition. The TLM is characteristically composed of variably coloured fine-grained rocks that stratigraphically-upward grade from siltstone to rhythmically bedded limestone-siltstone couplets. These fine-grained strata represent deposition during a major, post-glacial eustatic rise that essentially terminated the supply of coarse-grained siliciclastic sediment into the deep-water part of the Windermere basin. These strata are then gradationally overlain, although over an interval only <0.5–2 m thick, by strata of the GSM that stratigraphically-upward become progressively more organic-rich. The GSM is interpreted to have accumulated during terminal transgressive and highstand conditions, likely in anoxic bottom-water below a postulated pycnocline. The youngest member of the OFP is the WMM. The base of the WMM is almost always sharp and locally marked by scours more than 100 m deep. This surface is interpreted to be the result of an abrupt fall of relative sea level related to regional tectonic uplift, which in turn caused widespread mass wasting on the slope, and locally the formation of deeply incised submarine canyons. Sediment that accumulated in the canyons and more distally on the basin floor during the ensuing lowstand and early transgression was composed of remobilized slope sediment mixed with texturally and mineralogically immature clastic sediment derived directly from the hinterland. As transgression proceeded, the shelf eventually became flooded and shut-off the hinterland sediment supply. At the same time (late transgression and/or highstand), however, new, more landward canyons formed, or existing canyon heads eroded headward and reactivated the slope-basin floor transport system. This time, however, sediment was made up of a mixture of texturally and mineralogically mature palimpsest siliciclastic and carbonate shelf sediments. Notwithstanding its unique lithological and geochemical characteristics, strata of the OFP are underlain and overlain by a thick monotonous succession of mostly siliciclastic deep-marine sandstones and mudstones, suggesting that deposition of the OFP represented only a short-term anomaly superimposed on a much longer-term history of physical and geochemical stability throughout the Neoproterozoic deep-water Windermere basin.

Temporal Calibration and Biochronology of the Centenario Fauna, Early Miocene of Panama

New excavations along the Panama Canal have yielded a growing Early Miocene assemblage of mammals referred to as the Centenario Fauna. Despite the area’s proximity to South America, the mammals of the Centenario Fauna have entirely North American affinities. The Centenario Fauna is distributed throughout a ∼115-m stratigraphic interval encompassing the uppermost Culebra and Cucaracha Formations within the Panama Canal basin. Previously published ages constrain the age of the lower limit of the Centenario Fauna to no younger than ∼19 Ma, but the upper limit has remained problematical. A fresh exposure of the Cucaracha tuff, a prominent marker horizon within our measured sections, has yielded two new radioisotopic determinations: (1) an 40Ar/39Ar age of 18.96 ± 0.90 Ma and (2) a U-Pb zircon age of 18.81 ± 0.30 Ma. In addition, magnetostratigraphic data indicate that the Centenario Fauna occurs within chron C5Er, from 18.78 to 19.05 Ma on the geomagnetic polarity timescale of Gee and Kent. These correlations further confirm the calibration of the latest Arikareean (Ar4) to early Hemingfordian (He1) transition in Nebraska, at the base of chron C5Er, at about 19.05 Ma. The Centenario Fauna occurs at the beginning of the Hemingfordian North American Land Mammal Age, i.e., He1. A broad faunal province existed during the early Hemingfordian that can be recognized across a north-south range of 5000 km throughout North America, with the southernmost limits defined by the Centenario Fauna of Panama.

Indicators of the Anthropocene: is there a case for conservation?

The Earth's atmosphere, biosphere and lithosphere are increasingly being modified by human activity. Given this anthropogenic influence on the natural environment, the case for recognizing an Anthropocene Epoch has recently been made and there is ongoing debate as to whether, and how, to formally characterize and define such an epoch. As a contribution to this debate, this article explores whether or not the landscapes, deposits, landforms and key marker horizons that may be used to characterize and define an Anthropocene Epoch could, and should, be identified and conserved in the same manner as other parts of the geological succession. Anthropogenic features pose a conservation challenge, however, as they often cross cut existing conservation frameworks which tend to focus on the natural, historic or cultural environment. Developing a coherent approach to the conservation of the indicators of an Anthropocene Epoch would, therefore, require an integrated system of describing, auditing and designating features for conservation. A major benefit of such an approach would be the opportunities for interdisciplinary collaboration between the wide range of researchers interested or involved in studying and conserving the record of anthropogenic activity.

Assessing the time of final deposition of Youngest Toba Tuff deposits in the Middle Son Valley, northern India

We present optical ages for quartz and K-feldspar grains extracted from sedimentary deposits above and below Youngest Toba Tuff (YTT) at two localities in the Middle Son Valley (MSV), Madhya Pradesh, India. These ash deposits have been the focus of past palaeoenvironmental investigations that aim to understand the effects of the ~74ka Toba super-eruption on ecosystems and human populations in northern India. Age estimates from both quartz and feldspar grains post-date the Toba eruption and single-grain age distributions suggest that YTT ash-bearing sediments in the MSV are mainly composed of a mixture of recently sun-exposed, flood-transported grains and older grains from slumped river bank deposits. This study highlights the hazards of using YTT ash as an isochronous marker horizon in the MSV, and illustrates the need to independently assess the time of final deposition of reworked volcanic ash beds in sedimentary sequences used in stratigraphic correlations and palaeoenvironmental reconstructions.