Reliable Chronology Research Articles

Sedimentary record of nutrients and sources of organic matter in the Shuanglong reservoir, Dianchi watershed, China.

In the last few decades, the eutrophication of lakes has been a serious issue in the middle and lower reaches of the Yangtze River watershed. To explore the relationship between lake systems and anthropogenic activities, sediments were collected from the Shuanglong reservoir in the Dianchi watershed in Southwest China. Total nitrogen (TN), total phosphorus (TP), total organic carbon (TOC), and the carbon isotopic ratio (δ13C) were analyzed in sediment cores to reconstruct the effects of natural succession and human activities on the past lacustrine environmental conditions. A reliable chronology of the sediment core was established by using the 210Pb dating technique, which indicated that the age span of the 70-cm sediment core is from the years 1871 to 2011. Above - 31 cm depth in the core, TN, TP, TOC, C/N, and δ13C increased significantly, indicating that eutrophication has occurred since the 1980s. By combining the indicators of δ13C and C/N, it was shown that terrestrial and lacustrine components were the main sources of organic matter (OM) in the reservoir, which was mostly controlled by terrestrial C3 plants and algae. Since the 1980s, increased sewage discharge, fish aquaculture, fertilizer application, population, and economic strength have sped up the eutrophication process, and the eutrophication was further intensified in 2001.

Morphological changes of silica aged under environmental conditions by three-dimensional nanoscale quantifications

Combining techniques, we follow surfaces, particles, and pores within aggregates and their evolution. A reliable ageing chronology was achieved on several length scales for an ultraporous nanostructured silica via 3D quantification.

First reliable chronology for the Early Khvalynian Caspian Sea transgression in the Lower Volga River valley

During the late Quaternary, dramatic changes in relative sea level (~170 m) are known to have occurred in the Caspian Sea. However, all previous attempts at resolving the uncertainty associated with the timing of these transgressive/regressive events, primarily using radiocarbon, have produced inconclusive or controversial results. Here we present the first reliable chronology for the largest known transgression (Early Khvalynian). This was derived using optically stimulated luminescence (OSL) analysis of sand‐sized quartz, with support from infrared stimulated luminescence (IRSL) from K‐rich feldspar grains, all extracted from 16 sediment samples collected along the Lower Volga River. These samples were taken from loessic sediments, marine clays (known as Chocolate Clays) and the overlying modern soils exposed at three sections (Srednyaya Akhtuba, Raygorod, Leninsk) ~200 km upstream of the present‐day estuary. The differential bleaching rates of the quartz OSL and feldspar IRSL signals were used to evaluate the degree of resetting of quartz (and feldspar) signals; it was possible to conclude that all signals, and particularly quartz OSL, were sufficiently reset at deposition to allow accurate age estimation. Our results show unambiguously that the Early Khvalynian marine Chocolate Clays present at all three sections were deposited post‐LGM, between ~17 and ~13.1 ka ago. These age estimates are further constrained by those from the overlying Kastanozem soils (9.6–0.7 ka) and underlying loess‐soil series (32–19 ka), confirming a young (17–13 ka) age of the transgressive stage of the Early Khvalynian. Relative sea level during this period must have been well above the sampling altitudes of 11.7 m a.s.l. (Srednyaya Akhtuba), 11.3 m a.s.l. (Raygorod) and 4.7 m a.s.l. (Leninsk) to explain the absence of significant alluvial sand and to allow the deposition of the fine Chocolate Clays marker horizon.

The first human settlements out africa into Europe: A chronological perspective

A reliable chronology for the oldest settlements of Europe from the Early to the beginning of the Middle Pleistocene is an important requirement for understanding human evolution and especially the routes used to reach Western Europe from Africa. The paucity of sites, plus the limited number of available samples for dating, and the many contexts which do not favor the use of particular dating methods, further complicate the picture. This contribution addresses several questions: Which dating methods are relevant for establishing the chronology of the main sites? Are the first settlements older than the Jaramillo event? Which routes were used to enter Europe? Were the earliest human occupations intermittent or continuous?

Increasing effective moisture during the Holocene in the semiarid regions of the Yili Basin, Central Asia: Evidence from loess sections

The evolution of Holocene moisture in semiarid and arid regions of Central Asia is key to understanding changes in current and future water resources, and has been a hotly debated topic in the past few decades. Central Asian loess deposits have shown potential as records of past moisture changes, particularly during the Holocene. Here, the variations in effective moisture during the Holocene in the semiarid regions of the Yili Basin, Central Asia, are obtained from three loess sections. Reliable chronologies were constructed using 26 quartz optically stimulated luminescence (OSL) ages. The strengthening pedogenesis, steady increases in magnetic susceptibility, and continuous decreases in mean grain size in the three loess sections all suggest that effective moisture continuously increased throughout the Holocene, indicating a dry early Holocene, a moderately humid middle Holocene, and a wet late Holocene. Rainfall carried by the Asian summer monsoon and the Westerlies contributes to the moisture changes in Central Asia. However, considering the dominance of evaporation over rainfall in controlling aridity-humidity shifts in this region, the continuous weakening of evaporation caused by a decrease in Northern Hemisphere summer insolation and associated atmospheric temperatures is primarily responsible for the wetting trend throughout the Holocene in the semiarid regions of the Yili Basin.

From radiometry to chronology of a marine sediment core: A 210Pb dating interlaboratory comparison exercise organised by the IAEA

Laboratories from 14 countries (with different levels of expertise in radionuclide measurements and 210Pb dating) participated in an interlaboratory comparison exercise (ILC) related to the application of 210Pb sediment dating technique within the framework of the IAEA Coordinated Research Project.The laboratories were provided with samples from a composite sediment core and were required to provide massic activities of several radionuclides and an age versus depth model from the obtained results, using the most suitable 210Pb dating model. Massic concentrations of Zn and Cu were also determined to be used for chronology validation.The ILC results indicated good analytical performances while the dating results didn't demonstrate the same degree of competence in part due to the different experience in dating of the participant laboratories. The ILC exercise enabled evaluation of the difficulties faced by laboratories implementing 210Pb dating methods and identified some limitations in providing reliable chronologies.

Cenozoic moisture fluctuations on the northeastern Tibetan Plateau and association with global climatic conditions

Abstract The Cenozoic history of aridification in the Asian interior is thought to be intimately linked to uplift of the Tibetan Plateau and long-term global cooling. How fluctuating climatic conditions influenced regional moisture levels during the Cenozoic remains largely unknown. To infer moisture changes, we present oxygen isotope values of carbonates (δ18Ocarb) and carbon isotope values of long-chain n-alkanes (δ13Calk) from a continuous geological section in the Qaidam Basin, northwestern China. The section spans most of the Cenozoic Era, from ~52 to 5 million years ago (Ma), based on conventional chronology, whereas recently a drastically revised chronology has placed its basal age at ~26 Ma. The association of moisture changes with global climate largely depends on the chosen chronology. With the revised chronology, our records are inconsistent with existing moisture records from the Asian interior and do not show clear association with global climatic conditions. However, with the conventional chronology, our δ18Ocarb record shows a long-term aridification trend over the Cenozoic, whereas the δ13Calk suggests reduced moisture levels during the Oligocene. Both records consistently show enhanced aridity, notably during the Oligocene, ~20–17 Ma and after ~13 Ma, which largely corresponded to relatively cool global periods. Our records thus suggest that global climatic conditions during the Cenozoic exerted more influence on the moisture levels in the Asian interior than previously thought, if the conventional chronology is correct. Our study also highlights the importance of constructing a reliable chronology in order to make an association of Cenozoic terrestrial records with global climate.

Downcore variations of carbon reservoir ages linked to lake level changes in northwest China

Abstract Lacustrine sediments provide an excellent opportunity to study climate change beyond the instrumental period and further to elucidate its intrinsic interactions with ecosystem. However, the radiocarbon (14C)-based chronological framework of such natural archives, on which robust interpretation of geochemical proxies primarily depends, is commonly challenged due to the difficulty in properly correcting carbon reservoir effect. Here we combine new and existing 14C dates on total organic carbon (TOC) in two sediment cores from Lake Qinghai and Manas, northwest China, to evaluate the reservoir effect (expressed by reservoir age, RA) over the past ~1500 years. Generally, these two cores characterize RA values ranging from ~2100 to 4200 and from ~350 to 2100 years respectively. Straightforward comparison with paleorecords generated from the same cores indicates a close correlation between RA changes and lake level fluctuations on multi-centennial timescales. Our results suggest that lake level changes (or hydrological variations), which are expected to control the relative input of terrigenous old carbon, merit careful consideration to reduce dating uncertainty. It is essential to select TOC samples across discrete depths, under the control of similar lake status, to allow comparable RAs and, as a result, to establish reliable chronology.

Time marker of 137Cs fallout maximum in lake sediments of Northwest China

Caesium-137 (137Cs) has no natural source and is commonly employed in establishing the chronology of recent lake sediments in the Anthropocene epoch. Because of their location in the northern mid-latitude Westerlies circulation region, downwind of the Chernobyl accident and their proximity to the Lop Nur Chinese Nuclear Testing (CNT) site, lake sediments in NW China record a137Cs fallout maximum of global atmospheric thermonuclear weapon tests (GTWT) in 1963–1964 Common Era (CE), with probably local fallout subpeaks due to the Chernobyl accident in 1986 CE and CNT in 1976 CE. This complexity means that the use of 137Cs fallout maximum in lake sediments as a time marker in NW China is thus far not well constrained, impeding assessment of recent and future changes in paleoclimate and human activities. To identify the 137Cs time marker and provide a reliable chronology of surface lake sediments for investigating paleoclimatic changes and for defining the Anthropocene epoch in NW China during the last 50–100 years, here we describe chronologies of 6 near-surface sediment cores from NW China based on 137Cs and 210Pb activities, and review the chronology of 23 lake sediments and 10 ice cores from NW China and central Asia based on 137Cs, 210Pb, 239+240Pu, 3H, 241Am, and beta-activity dating methods, and annual layer counting, as well as 240Pu/239Pu isotopic ratios. Based on the potential influence of radioactivity transport from the Chernobyl accident and CNT, and the comparison with deposition records of 26 European lake sediments and 5 Alpine ice cores, we propose that the 137Cs fallout maximum of lake sediments in NW China and central Asia is primarily attributable to GTWT and that there is no unambiguous evidence to confirm the Chernobyl- and CNT-derived 137Cs local-fallout subpeaks. Taking into account the approximately one-year delay between injection of 137Cs into the atmosphere and its incorporation in lake sediment records, we conclude that the 137Cs fallout maximum in lake sediments of NW China provides a reliable GTWT time marker for 1964 CE.

Climate History of Southern Mongolia Since 17 ka: The Ostracod, Gastropod and Charophyte Record From Lake Ulaan

Late glacial and Holocene environmental and climate change in the Gobi Desert is poorly understood due to the lack of appropriate geological archives and commonly faced difficulties in establishing reliable chronologies. Here, the relatively well-dated sediment record from the terminal Lake Ulaan is used to reconstruct the lake history and climate in the region. Most abundant calcareous fossils in the lake sediments are ostracod (micro-crustacean) valves. The ostracod assemblage is dominated by Limnocythere inopinata, and generally very shallow conditions and sodium-dominated waters are inferred for Lake Ulaan. A single period of significantly increased silt accumulation in the late glacial was recorded at ca. 16 ka, probably as a result of melting glaciers in the uppermost reaches of the lake’s main tributary, the Ongin River, and the influx of glacially-ground, fine materials. Lake Ulaan had a higher salinity afterwards during the Greenland Interstadial 1, and dry climate prevailed in the region. The lake level started to rise and the salinity decreased since ca. 12.3 ka, leading to the establishment of freshwater conditions in the early Holocene. Highest inflows and highest productivity in and around the lake, and wettest climate conditions in the region were recorded in the early Holocene. Lake conditions, most favourable for the aquatic fauna, culminated ca. 10 ka. Afterwards, Lake Ulaan experienced a step-wise lake-level decline and salinity increase at 9.4 and 6.8 ka. The further shrinkage of the lake and the approaching Ongin River mouth near the central part of the basin is recorded since 3.2 ka.

Dating of an alpine ice core from the interior of the Tibetan Plateau

Accurate archive-based paleoclimate reconstruction requires precise dating. However, the dating of mid- and low-latitude alpine ice cores is in challenge, in particular for the lower parts of ice cores. Here, we present a new dating work for a 109 m ice core collected to bedrock from the Qiangtang No. 1 glacier, central Tibetan Plateau. Annual layer counting based on seasonal indicators, in combination with absolute dating of radioactivity peaks, indicates that the upper part (0–79 m) captures a period of 678 years from 1334–2011 AD, with an uncertainty of ±20 years at 79 m. In the lower part (79–109 m), we dated by visible annual layer thickness-depth relationships and found the lower ice core captures 665 years of deposition and can be dated back to 669 ± 66 AD at 109 m. This new method reduces the ambiguity of utilizing glacier flow models that have been widely used in previous study. On the presence of observable optical annual layers, the polynomial fitting method based on annual layer thickness-depth relationships is a superior method to obtain reliable chronology in the lower parts. Furthermore, this method has the potential to improve the dating of other alpine ice cores with similar settings.

The emergence of hunter-gatherer pottery in the Urals and West Siberia: New dating and stable isotope evidence

The emergence of pottery among Stone Age hunter-gatherer societies of Eurasia constitutes one of the major open questions in Old World prehistory. Located halfway between the earliest Late Glacial cores of pottery production in East Asia, and Eastern Europe with forager ceramic starting around 6000 cal BC, the Urals and West Siberia are a key region in various scenarios currently under discussion. A lack of reliable absolute dates has been hindering an in-depth understanding of the temporal and spatial scales of the initial spread of the ceramic innovation. A Russian-German dating programme has now created a more reliable chronology of the early pottery phase, based on 28 AMS dates from across the study region. Taking freshwater reservoir effects into account, we can show that the earliest reliable evidence for pottery stems from the West Siberian forest steppes and Urals foothills, dating to the end of the 7th millennium cal BC. Over the following centuries, the innovation spread rapidly north into the taiga. Here, the early pottery horizon coincides with a unique set of innovations and intensification in the settlement system and the socio-economic sphere, including the appropriation of vast previously barely settled regions, the emergence of complex and even fortified settlements, and of ritual mounds. Pilot isotopic analyses of pottery charred crusts indicate diverse functions of the early vessels that were apparently not restricted to the processing of fish. The emerging wider picture indicates a surprisingly late, largely concurrent appearance of pottery in hunter-gatherer groups over extensive areas along the southern fringes of the taiga to both sides of the Urals at the end of the 7th millennium cal BC which is apparently not connected to the earlier, Late Pleistocene ceramic traditions in Trans-Baikalia and further East. Possible links to the 8.2 ka climatic event, other underlying triggers as well as the detailed chronology of these developments are still poorly understood and require further archaeological, biomolecular and typological studies.

A taxonomically harmonized and temporally standardized fossil pollen dataset from Siberia covering the last 40 kyr

Abstract. Pollen records from Siberia are mostly absent in global or Northern Hemisphere synthesis works. Here we present a taxonomically harmonized and temporally standardized pollen dataset that was synthesized using 173 palynological records from Siberia and adjacent areas (northeastern Asia, 42–75∘ N, 50–180∘ E). Pollen data were taxonomically harmonized, i.e. the original 437 taxa were assigned to 106 combined pollen taxa. Age–depth models for all records were revised by applying a constant Bayesian age–depth modelling routine. The pollen dataset is available as count data and percentage data in a table format (taxa vs. samples), with age information for each sample. The dataset has relatively few sites covering the last glacial period between 40 and 11.5 ka (calibrated thousands of years before 1950 CE) particularly from the central and western part of the study area. In the Holocene period, the dataset has many sites from most of the area, with the exception of the central part of Siberia. Of the 173 pollen records, 81 % of pollen counts were downloaded from open databases (GPD, EPD, PANGAEA) and 10 % were contributions by the original data gatherers, while a few were digitized from publications. Most of the pollen records originate from peatlands (48 %) and lake sediments (33 %). Most of the records (83 %) have ≥3 dates, allowing the establishment of reliable chronologies. The dataset can be used for various purposes, including pollen data mapping (example maps for Larix at selected time slices are shown) as well as quantitative climate and vegetation reconstructions. The datasets for pollen counts and pollen percentages are available at https://doi.org/10.1594/PANGAEA.898616 (Cao et al., 2019a), also including the site information, data source, original publication, dating data, and the plant functional type for each pollen taxa.

Timing and pathways of East Antarctic Ice Sheet retreat

A lack of reliable chronology often leaves Antarctic ice sheet reconstructions incomplete. Despite being the most heavily investigated region of the Antarctic continental shelf, the post-Last Glacial Maximum (LGM) record of marine-based ice sheet dynamics in the Ross Sea has remained largely elusive and at odds with fringing terrestrial records of ice sheet decay. Issues with radiocarbon dating stem primarily from poorly preserved carbonate, contamination by glacially-recycled carbon, and insufficient consideration for glacial geomorphic context and sedimentary facies. We assess nearly 70 newly acquired and more than 200 published radiocarbon ages in the context of a consistent sedimentary facies and geomorphic framework to interpret the timing of the local glacial maximum and subsequent retreat of the East Antarctic Ice Sheet (EAIS) in the Ross Sea and potential causes of retreat. Compound-specific radiocarbon ages help to provide good constraints on open marine onset, and we demonstrate that, with proper corrections and considerations, acid insoluble organic (AIO) radiocarbon ages can be reliable in open marine sediments and carbonate ages can be reliable in grounding-zone proximal sediments. We find that the ice flowing through Pennell and JOIDES troughs remained at maximum extent for approximately 10,000 years before retreating at 15.1 and 13 cal ka BP, respectively. An ice shelf covered the outer continental shelf until ∼9.5 cal ka BP, after which it collapsed in JOIDES Trough, creating deep iceberg furrows. Retreat through Pennell Trough was more episodic, marked by grounding-zone wedges that backstep upstream and onto lateral banks creating deglacial emergent ice rises. As ice retreated from ∼9 to ∼4 cal ka BP, there was considerable reorganization of drainage and at least one episode of readvance through southern JOIDES Trough that began drawdown of terrestrial ice near Ross Island at ∼7.8 cal ka BP. The ice shelf appears to have retreated to a near-present configuration by 2 cal ka BP. However, due to inconsistencies in calendar ages, grounding line position must be inferred from relative chronology based on glacial landforms, which suggests grounded ice retreated nearly to Drygalski Ice Tongue and lingered atop Crary Bank where it was maintained by ice from David Glacier. Our reconstruction further suggests that the EAIS in the western Ross Sea was not a discernible contributor to Meltwater Pulse 1A, but likely contributed to eustatic sea level rise well through the middle—perhaps even late—Holocene.

14C Dated Chronology of the Thickest and Best Resolved Loess/Paleosol Record of the LGM from SE Hungary Based on Comparing Precision and Accuracy of Age-Depth Models

ABSTRACTThe Madaras profile found at the northernmost fringe of Bácska loess plateau is one of the thickest and best-developed last glacial loess sequences of Central Europe. The 10-m profile corresponds to a period between 29 and 12 b2k. To unravel feedback to small-scale centennial climatic fluctuations at our site, recorded in the Greenland ice and North Atlantic marine cores, construction of a reliable chronology is needed. Reliability is expressed in terms of best achievable chronological precision. Accuracy however is based on choosing the model best describing the sedimentological features of our profile. Five different age-depth models had constructed and compared relying on 1514C dates using various statistical, probabilistic approaches to choose the model with the highest achievable precision. Accuracy was also evaluated using accumulation rates against stratigraphy. Models constructed using the computer program Bacon performed best in terms of achieving the best possible stratigraphic accuracy. Seven meters of the profile represents the period of the LGM. The average sedimentation time was 16.8 yr/cm with the highest confined to the period of the LGM. Calculated average sedimentation rates were 4 times higher than previously reported. The peak accumulation periods are dated to the nadir of the LGM.

Influence of climatic conditions on growth rings of Salix uva-ursi Pursh from the southeastern shore of Hudson Bay, Subarctic Canada

ABSTRACT Over the past decades, warmer air temperature and spatiotemporal changes in the amount and patterns of precipitation have been observed at high latitudes. Such interannual variability in climatic conditions has a strong influence on the dynamics of biological processes regulating terrestrial ecosystems. Dendroclimatology can improve our understanding of the impacts of climate change on vegetation. Based on ring width and frost rings of bearberry willow (Salix uva-ursi) individuals sampled on the southeastern shore of Hudson Bay, Nunavik (Subarctic Québec, Canada), we built a reliable 105-year growth chronology. A complex dendroclimatological analysis, using the standard approach (mean temperature and precipitation sums), extreme variables, and various climatological indices based on daily data, was conducted in order to precisely describe the relationship between annual variations in growth rings and climatic conditions. The growth of Low Arctic willows in the Hudson Bay region is determined by changes in air temperatures as well as precipitation during the summer. However, temperature increases and reductions in amounts of rainfall may intensify drought stress, which is unfavorable for this species. We found that analysis of daily amounts and intensity of precipitation improved our understanding of the factors conditioning the growth of the studied species in an extreme habitat.

Early Chalcolithic Pottery from Tepecik-Çiftlik and Gelveri, Central Anatolia: From Local to Supra-Regional Ceramic Style

We intend to decipher both a chronological and a cultural problematic raised by the sudden appearance of geometric incised decorations on Anatolian pottery, seemingly at the beginning of the sixth millennium cal B.C. Frequently designated as ‘Gelveri type’ in reference to the Gelveri-Güzelyurt Cappadocian pottery assemblage, the lack of a reliable chronology based on a good stratigraphy at the site made it difficult to cope with the nature of the development of such pottery style. Excavations at Tepecik-Çiftlik as well as recent archaeological work at Gelveri have provided us with new stratigraphic and decoration-related technological evidence. Hence, we have the possibility to analyse the emergence and the development of such pottery styles within a reliable archaeological context. Seen on such a large geographical scale, covering at least the entire Anatolian plateau, the nature of this pottery style development cannot be considered as a pottery phenomenon in itself only. It is necessary to investigate the cultural interactions at play, enabling stylistic patterns to be shared within different cultural areas.

Luminescence chronology and lithic technology of Tianhuadong Cave, an early Upper Pleistocene Paleolithic site in southwest China

AbstractTianhuadong is a cave site located in the northwest of Yunnan Province, China. Since 2010, several surveys and one test excavation have yielded more than 1000 stone artifacts. The lithic assemblage shows some features of Levallois and Quina technologies, similar to those found in Middle Paleolithic sites in the Western Hemisphere. In this study, we summarize the lithic industry and propose a reliable chronology for the site using optically stimulated luminescence (OSL) dating of individual quartz grains extracted from sediments. We applied the standardized growth curve method to deal with the problem associated with the saturation in natural OSL signals in quartz. Our dating results yielded ages of 90–40 ka, suggesting that the associated lithic assemblage could be assigned to Marine Oxygen Isotope Stages 5 and 4 and could potentially represent Middle Paleolithic technologies. Because the number of Middle Paleolithic sites in southwest China is small, this site provides one of the few traces of human occupation in southwest China during the early upper Pleistocene. Thus, it is important for understanding hominin evolution and dispersal in this region.

A robust geochronology of the Yangtze River Delta based on magnetostratigraphy and cyclostratigraphy of sediment core ZKA2

Studying the evolution of sea-level fluctuations, climate oscillations and tectonic movements in the Yangtze River Delta requires a reliable chronology. Here, we present the results of a comprehensive study of the magnetostratigraphy and cyclostratigraphy of a 295-m-long sediment core (ZKA2) recovered from Nantong City, Jiangsu Province, China. Anisotropy of magnetic susceptibility (AMS) studies reveal that the petrofabrics are mainly primary. Based on rock magnetic and thermal demagnetization analyses, the major magnetic minerals in the studied sediments are magnetite, hematite and greigite, but only magnetite and hematite carry the primary remanence. The magnetostratigraphy reveals seven reversed polarity intervals, nine normal polarity intervals, and one interval of mixed polarity. Variations in natural gamma-ray activity reflect variable continental weathering in the sediment source regions and are paced by Earth orbital cycles. Notably, a prominent 405-kyr long eccentricity cycle and ~100-kyr short eccentricity cycle are evident, which allow us to constrain the basal age of core ZKA2 to ~3.1 Ma. The magnetostratigraphic and cyclostratigraphic results together enable us to define the depths of key chronostratigraphic and magnetostratigraphic boundaries. They include the Late/Middle Pleistocene boundary at ~18-m depth, the Brunhes/Matuyama boundary at ~88-m depth, and the Matuyama/Gauss boundary at ~254-m depth. According the new chronology, we find that although clear long eccentricity (~405 kyr) and short eccentricity cycles (~100-kyr) cycles are evident in the magnetic susceptibility record, they are not well matched with cycles of natural gamma-ray activity. This suggests the source and post depositional process of the magnetic particles is complicated, and it should be cautious to choose magnetic properties for cyclostratigraphic analysis.

Glass geochemical compositions from widespread tephras erupted over the last 200 years from Mount St. Helens

ABSTRACTBuilding reliable chronologies from lake sediments, peat and other paleoenvironmental archives can be challenging, especially for historical times where radiocarbon is unreliable. Nineteenth‐ and 20th‐century eruptions from Mount St. Helens (MSH) provide important chronostratigraphic markers for regional paleoenvironmental studies within this time frame, but are constrained by poorly geochemically characterized tephra and/or limited published data. Here, we present glass geochemistry from the most significant eruptions from this time. This includes proximal, medial and distal deposits of the 18 May 1980 MSH eruption, layer T ( ad 1799/1800), a new tephra that we argue represents the ad 1842 eruption, and the 22 July 1980 eruption that had reported ashfall in Canada. Our results indicate that most tephras ejected during these eruptions, within a time frame of ~200 years, have distinct glass geochemical characteristics that can be used to identify distal deposits for tephrochronological studies. Layer T is on trend with analyses of the 1980 eruption but has a distinct dacitic glass population. The 1980 and ad 1842 eruptions are similar, both having rhyolitic glass compositions, but the ad 1842 event can be differentiated by a more constrained SiO2 range in the main geochemical population, and the presence of a unique SiO2 sub‐population.