Late quaternary glacial stades in the Cordillera Central,Colombia, based on glacial geomorphology,tephra–soil stratigraphy,palynology, and radiocarbon dating

Using data from glacial geomorphology, tephra–soil stratigraphy and mineralogy, palynology, and radiocarbon dating, a sequence of glacial and bioclimatic stades and interstades has been identified for the last ca. 50000 yr in the Ruiz-Tolima massif, Cordillera Central, Colombia. Six Pleistocene cold stades separated by warmer interstades occurred: before 48000, between 48000 and 33000, between 28000 and 21000, from ≥16000 to ca. 14000, ca. 13000–12400, and ca. 11000–10000 yr BP. Although these radiocarbon ages are minimum-limiting ages obtained from tephra layers on top of tills, the tills are not significantly older because most are bracketed by dated tephra sets in measured stratigraphic sections. Two minor moraine stages likely reflect glacier standstill during cold intervals ca. 7400 yr BP and slightly earlier. Finally, glaciers readvanced between the seventeenth and nineteenth centuries. In contrast to the ice-clad volcanoes of the massif, ca. 34 km² in area above an altitude of ca. 4800 m, the ice cover expanded to 1200 km² during the Last Glacial Maximum (LGM) and was still 800 km² during Late-glacial time (LGT). Glacier reconstructions based on the moraines suggest depression of the equilibrium line altitude (ELA) by ca. 1100 m during the LGM and 500–600 m during LGT relative to the modern ELA, which lies at ca. 5100 m in the Cordillera Central. Glaciers in this region apparently reached their greatest extent when the climate was cold and wet, e.g. during stades corresponding to Oxygen Isotope Stage 3; glaciers were still expanding during the LGM ca. 28000–21000 yr BP, but they shrank considerably after 21000 yr BP because of greatly reduced precipitation. © 1997 John Wiley & Sons, Ltd.     

Grounding‐line dynamics during the last deglaciation of Kveithola,W Barents Sea,as revealed by seabed geomorphology and shallow seismic stratigraphy

A marine geophysical study reveals a complex deglaciation pattern in the Kveithola trough, W Barents Sea. The data set includes multibeam swath bathymetry and sub‐bottom sediment profiler (chirp) data acquired for the whole extent of a palaeo, marine‐terminating ice stream, along with high‐resolution single‐channel seismic data from chosen profiles. The multibeam data show a geomorphic landform assemblage characteristic of ice streams. The results of a combination of seismic and chirp unit stratigraphy reveal that the seabed geomorphology is governed by a deeper‐lying reflector. The reflector dominates surface expressions of several subglacial and ice‐marginal units, each connected to a separate episode of ice‐margin stillstand/advance. Analysis of the combined data set has resulted in a conceptual model of the ice‐stream retreat. The model depicts complex deglaciation of a small, confined ice‐stream system through episodic retreat. It describes the formation of several generations of grounding‐zone systems, characterized by high meltwater discharges and the deposition of fine‐grained grounding‐line fans. The inferred style of grounding‐zone deposition in Kveithola deviates from that of other accounts, and is suggested to be intermediate in the previously described continuum between morainal banks and grounding‐line wedges. The results of this paper have implications for grounding‐zone theory and should be of interest to modellers of grounding‐line dynamics and ice‐stream retreat.     

Lateglacial and early Holocene lake sediments from the northern Apennines, Italy – pollen stratigraphy and radiocarbon dating

Radiocarbon-dated pollen profiles are presented from two basins in Prato Spilla, near Val Parma in the northern Apennines. One basin contains a complete Holocene succession, the other a full Lateglacial to mid-Holocene record. The data provide the most comprehensive Lateglacial-early Holocene pollenstratigraphic succession yet reported from the northern Apennines accompanied by an internally consistent radiocarbon chronology. They provide fresh impetus for (a) a discussion of the strength of the Younger Dryas 'signal' in pollen-stratigraphic profiles from southern Europe, (b) an assessment of the palaeovegetation of northern Italy during the last glacial-interglacial transition, and (c) the altitude of the snowline in the region during the Younger Dryas.     

The last deglaciation of Svalbard

Svalbard has been completely covered by an extensive ice sheet at least once, but not in the Late Weichselian (max. 18,000–20,000 years ago). Areas in the western and northwestern parts of Svalbard have been ice-free for more than 40,000 years. The extension and time of a Barents Shelf glaciation are questions still open for discussion. For most of the Svalbard area we do not know when the last deglaciation started, geographically and in time. The oldest datings for the interval 15,000 to 10,000 years B.P. have an age of about 12,600 years, and datings from between 11,000 and 10,000 years B.P. are rather frequent in the western and northern parts of Spitsbergen. No moraines from Younger Dryas have been found in Svalbard and the glaciers were probably less extensive 10,000 years ago than today. The maximum extension of glaciers in the Holocene took place only a few hundred years ago.     

Palaeoceanographic changes in the northern Barents Sea during the last 16 000 years – new constraints on the last deglaciation of the Svalbard–Barents Sea Ice Sheet

The sediment core NP05‐71GC, retrieved from 360 m water depth south of Kvitøya, northwestern Barents Sea, was investigated for the distribution of benthic and planktic foraminifera, stable isotopes and sedimentological parameters to reconstruct palaeoceanographic changes and the growth and retreat of the Svalbard–Barents Sea Ice Sheet during the last ～16 000 years. The purpose is to gain better insight into the timing and variability of ocean circulation, climatic changes and ice‐sheet behaviour during the deglaciation and the Holocene. The results show that glaciomarine sedimentation commenced c. 16 000 a BP, indicating that the ice sheet had retreated from its maximum position at the shelf edge around Svalbard before that time. A strong subsurface influx of Atlantic‐derived bottom water occurred from 14 600 a BP during the Bølling and Allerød interstadials and lasted until the onset of the Younger Dryas cooling. In the Younger Dryas cold interval, the sea surface was covered by near‐permanent sea ice. The early Holocene, 11 700–11 000 a BP, was influenced by meltwater, followed by a strong inflow of highly saline and chilled Atlantic Water until c. 8600 a BP. From 8600 to 7600 a BP, faunal and isotopic evidence indicates cooling and a weaker flow of the Atlantic Water followed by a stronger influence of Atlantic Water until c. 6000 a BP. Thereafter, the environment generally deteriorated. Our results imply that (i) the deglaciation occurred earlier in this area than previously thought, and (ii) the Younger Dryas ice sheet was smaller than indicated by previous reconstructions.     

Rapid climatic shifts of the northern Norwegian Sea during the last deglaciation and the Holocene

High resolution cores from the upper continental slope, northern Norwegian Sea, document rapid climatic fluctuations during the latest deglaciation and the Holocene. Based on down-core analysis of planktic and benthic foraminifera, stable oxygen and carbon isotopes, carbonate and organic carbon and radiocarbon dating, the following evolution is proposed: sea-ice cover broke up, the surface ocean warmed and an in situ benthic foraminiferal fauna was established at 12 500 BP. The Younger Dryas was characterized by reduced sedimentaion and foraminiferal production, due to surface ocean cooling. At the end of the Younger Dryas there were major shifts in both surface and bottom water conditions. The surface ocean warmed to temperatures similar to modern levels within < 100 years, reaching a maximum at about 9200 BP when foraminiferal production was high. A benthic foraminiferal assemblage indicative of bottom water conditions similar to present conditions was established at 10 000 BP. This was followed by a gradual decline in nutrients or an increase in ventilation of the bottom water throughout the Holocene. A gradual surface ocean cooling of c . 2°C ended around 6500 BP followed by a second warming that culminated at 2000 BP. The warming at the end of the Younger Dryas and the succeeding older Holocene temperature maximum correlate to a June insolation maximum in the northern hemisphere. In addition, fluctuating surface temperatures in the Holocene may be driven by variations in inflow of Atlantic Water.     

<Superscript>10</Superscript>Be surface exposure dating of the last deglaciation in the Aare Valley,Switzerland

The combined Rhone and Aare Glaciers presumably reached their last glacial maximum (LGM) extent on the Swiss Plateau prior to 24 ka. Two well-preserved, less extensive moraine stades, the Gurten and Bern Stade, document the last deglaciation of the Aare Valley, yet age constraints are very scarce. In order to establish a more robust chronology for the glacial/deglacial history of the Aare Valley, we applied ¹⁰Be surface exposure dating on eleven boulders from the Gurten and Bern Stade. Several exposure ages are of Holocene age and likely document post-depositional processes, including boulder toppling and quarrying. The remaining exposure ages, however yield oldest ages of 20.7 ± 2.2 ka for the Gurten Stade and 19.0 ± 2.0 ka for the Bern Stade. Our results are in good agreement with published chronologies from other sites in the Alps.     

Origin of methane in high-arsenic groundwater of Taiwan – Evidence from stable isotope analyses and radiocarbon dating

Groundwaters in the confined aquifers of the Chianan and Ilan coastal plains of Taiwan are rich in dissolved methane (CH₄). Serious endemic “blackfoot disease”, which occurred in the Chianan plain, especially during AD1950-1970, has been demonstrated to have arisen from drinking highly reducing groundwater with abnormal arsenic and humic substance levels. In order to explore the origin of CH₄ and its hydrological implications, stable carbon isotope ratios (δ¹³C) and radiocarbon (¹⁴C) ages of exsolved CH₄, dissolved inorganic carbon (DIC), and sedimentary biogenic sediments from a total of 34 newly completed water wells at 16 sites were determined. The main results obtained are as follows: (1) The δ¹³C_CH4 (−65‰ to −75‰) values indicate that, except for one thermogenic sample (δ¹³C_CH4=38.2‰) from the Ilan plain, all CH₄ samples analyzed were produced via microbially mediated CO₂ reduction. Many δ¹³C_DIC values are considerably greater than −10‰ and even up to 10‰ due to Rayleigh enrichment during CO₂ reduction. (2) Almost all the ¹⁴C ages of CH₄ samples from the shallow aquifer (I) (<60 m depth) are greater than the ¹⁴C ages of coexisting DIC and sediments, suggesting the presence of CH₄ from underlying aquifers. (3) The ¹⁴C ages of coexisting CH₄, DIC and sediments from aquifer (II) of the Chianan plain are essentially equal, reflecting in-situ generation of CH₄ and DIC from decomposition of sedimentary organic matter and sluggishness of the groundwater flow. On the other hand, both CH₄ and DIC from each individual well of the relatively deep aquifers (III) and (IV) in the Chianan plain are remarkably younger than the deposition of their coexisting sediments, indicating that current groundwaters entered these two aquifers much later than the deposition of aquifer sediments. (4) Each CH₄ sample collected from the Ilan plain is older than coexisting DIC, which in turn is distinctly older than the deposition of respective aquifer sediments, demonstrating the presence of much older CO₂ and CH₄ from underlying strata.     

Glacial geomorphology and chronology of deglaciation,South Georgia,sub-Antarctic

We mapped and dated the glacial geomorphology of north-east South Georgia, in the maritime sub-Antarctic. The aim was to examine the timing of deglaciation of the island in the context of inter-hemispheric phasing of climate change. Former glacier limits are restricted to the inner fjords, and our detailed mapping of them has demonstrated a consistent geomorphological pattern that is similar across several different glacier types and sizes. The pattern comprises three suites of moraines (categories “a–c”), not all of which are represented at every site because the outer suite is often overridden by younger suites. Category “a” is an outer wide, low amplitude moraine ridge, category “b” comprises 2–4 sharp-crested, bouldery moraines that are often located close to or even over-riding “a”, and category “c” is a series of lower amplitude moraines with overprinted streamlined landforms such as flutings. Analysis of in situ cosmogenic ¹⁰Be in boulders on these moraines has allowed us to determine a deglacial chronology for the older two moraine groups. The age of the inner (youngest) group has been estimated from soil development. The cosmogenic nuclide ages show that the outermost moraine was deposited ca 12.2±1.5 ka BP, but that a subsequent readvance in the mid-Holocene (ca 3.6±1.1 ka BP) reached and, in places, over-rode this earlier moraine. This latter advance coincides with the “Mid Holocene Hypsithermal”. A final Late Holocene advance reached closely similar limits to the previous two fluctuations and is estimated from soil data to have an age of ca 1.1 ka BP. We suggest that the close concordance of Late-Glacial and interglacial limits (in this case associated with warming) can be explained by a change in dominant forcing. During glacials, extensive sea-ice limits precipitation availability and so glaciers are restricted to the inner fjords. During interglacials precipitation is not limited in the same way by sea-ice cover and so during warming precipitation increases and tidewater glaciers on the island have responded by advancing. This study emphasises the importance of a clear understanding of geomorphology in order to interpret chronological information.     

Carbonates, granulometry, and color of tills on the south-central Canadian Shield and their implications for stratigraphy and radiocarbon dating

Till containing over 10% matrix carbonate extends in a belt 200–300 km wide south of the Hudson Bay Paleozoic basin source. The southern boundary is represented by the 'carbonate line', extending from Wawa to near Timmins and Cochrane in the study area. Higher silt content and lighter color are associated with the higher carbonate till. The carbonate tine corresponds approximately to the Chapleau moraine and correlative moraines to the northwest and may signify a discrete stratigraphic unit (possibly identifiable with the Matheson Till) formed by a glacial readvance. Glaciolacustrine sediments have higher carbonate content than nearby till and similarly form widespread sediment blankets even beyond that of the carbonate-rich till. Radiocarbon dating of amorphous organic sediments may be at risk because of old carbon error on such terrain and there may be some risk even in areas of much smaller carbonate content. A greater effort should be made to establish chronologies based on terrestrial plant matter, now more often possible with AMS radiocarbon dating.     

The geomorphology and stratigraphy of the Lizard Loess in south Cornwall, England

The Quaternary deposits of south Cornwall are described with an emphasis placed on the loess component. A formal lithostratigraphic name – the Lizard Loess – is proposed for the loess formation with a holostratotype designated from the Lizard Peninsula. Two environments of loess deposition are distinguished on the basis of geomorphic position and degree of solifluction activity.     

Investigating the last deglaciation of the Scandinavian Ice Sheet in southwest Sweden with 10Be exposure dating

In this study we have obtained 17 cosmogenic exposure ages from three well‐developed moraine systems – Halland Coastal Moraines (HCM), Göteborg Moraine (GM) and Levene Moraine (LM) – which were formed during the last deglaciation in southwest Sweden by the Scandinavian Ice Sheet (SIS). The inferred ages of the inner HCM, GM and LM are 16.7 ± 1.6, 16.1 ± 1.4 and 13.6 ± 1.4 ka, respectively, which is slightly older than previous estimates of the deglaciation based on the minimum limiting radiocarbon ages and pollen stratigraphy. During this short interval from 16.7 ± 1.6 to 13.6 ± 1.4 ka a large part (100–125 km) of the marine‐based sector of the SIS in southwest Sweden was deglaciated, giving an average ice margin retreat between 20 to 50 m a^?1. The inception of the deglaciation pre‐dated the Bølling/Allerød warming, the rapid sea level rise at 14.6 cal. ka BP and the first inflow of warm Atlantic waters into Skagerrak. We suggest that ice retreat in southwest Sweden is mainly a dynamical response governed by the disintegration of the Norwegian Channel Ice Stream and not primarily driven by climatic changes. Copyright © 2011 John Wiley & Sons, Ltd.     

The last deglaciation of the Franz Victoria Trough, northern Barents Sea

A study of two piston cores and a 3.5 kHz seismic profile from the Franz Victoria Trough provides new stratigraphic, stable isotopic and foraminiferal AMS ¹⁴C data that help constrain the timing of ice-sheet retreat in the northern Barents Sea and the nature of the deglacial marine environment. Silty diamicton at the base of each core, interpreted as till or ice-marginal debris flow, suggests that the Barents ice sheet was grounded at the core sites (470 m water depth). Eight AMS ¹⁴C dates on sediment overlying the diamicton indicate that the ice sheet retreated from both core sites by 12.9 ka and that postglacial sedimentation began 10 ka ago. These dates, combined with a recently published ¹⁴C date from a nearby core, suggest that the Franz Victoria Trough may not have been deglaciated until c . 13 ka, 2000 years later than modeled ice-sheet reconstructions indicate. In the trough, oxygen isotopic ratios in planktonic foraminifera ^N. pachyderma (sinistral) were 0.5–0.750, lower during deglaciation than after, probably as a result of ice-sheet and/or iceberg melting. Foraminiferal assemblages suggest that Atlantic-derived intermediate water may have begun to penetrate the trough c . 13 ka ago.     

The mode and mechanism of the last deglaciation: Oceanic evidence

Changes in ocean temperature, carbonate productivity, and ice-rafted detritus in the North Atlantic suggest that half of the Northern Hemisphere ice volume at the last glacial maximum had disappeared by 13,000 yr B.P., despite the still-extensive limits of the ice sheets. This early thinning of the ice sheets occurred during a time when summer insolation values were slowly rising but when pollen evidence south of the ice margins indicates cold, dry air masses. We infer that this rapid early ice disintegration (16,000–13,000 yr B.P.) was caused by oceanic mechanisms: (1) rising sea level, causing increased calving along ice margins; (2) the chilling of the sea-surface by icebergs and meltwater, reducing moisture extraction by the atmosphere and transport to the ice sheets; and (3) winter freezing of the low-salinity meltwater layer, suppressing local moisture extraction and the regional influx of moisture-bearing storms from lower latitudes in winter and hence starving the ice sheets. These oceanic feedback mechanisms were strongest from 16,000 to 13,000 yr B.P., and weaker but still active from that date until the end of deglaciation at 6000 yr B.P.     

Timing of the last deglaciation in Belarus

We measured ¹⁰Be concentrations in boulders collected from the Orsha and Braslav moraines, associated with the Last Glacial Maximum extent and a recessional stage of the Scandinavian Ice Sheet (SIS), respectively, providing a direct dating of the southeastern sector of the ice-sheet margin in Belarus. By combining these data with selected existing radiocarbon ages, we developed a chronology for the last deglaciation of Belarus. The northeastern part of the country remained ice free until at least 19.2±0.2 cal. kyr BP, whereas the northwestern part of the country was ice free until 22.3±1.5 cal. kyr BP. A lobate ice margin subsequently advanced to its maximum extent and deposited the Orsha Moraine. The ice margin retreated from this moraine at 17.7±2.0 ¹⁰Be kyr to a position in the northern part of the country, where it deposited the Braslav Moraine. Subsequent ice-margin retreat from that moraine at 13.1±0.5 ¹⁰Be kyr represented the final deglaciation of Belarus. Direct dating of these moraines better constrains the relation of ice-margin positions in Belarus to those in adjacent countries as well as the SIS response to climate change.     

Timing of the last deglaciation in Lithuania

Boulders from the Grūda Moraine, which is associated with the maximum extent of the Scandinavian Ice Sheet (SIS) during the last glaciation, and the Baltija (also referred to as the South Lithuanian), the Middle and North Lithuanian moraines, which are associated with recessional stages of the SIS, were sampled for surface exposure dating using ¹⁰Be. By combining these data with existing radiocarbon ages, we developed a chronology for the retreat of the SIS margin in Lithuania. Our new ¹⁰Be ages suggest that the SIS margin began to retreat from its maximum extent at 18.3 ± 0.8 ¹⁰Be kyr. Based on a probable correlation of the Baltija Moraine with the Pomeranian Moraine in Poland, we infer that the Baltija Moraine was formed following a re-advance of the SIS margin. The ice margin retreated from the Baltija position at 14.0 ± 0.4 ¹⁰Be kyr. The SIS-margin retreat paused at least two more times to form the Middle Lithuanian Moraine at 13.5 ± 0.6 ¹⁰Be kyr and the North Lithuanian Moraine (tentatively correlated to the Pajūris Moraine) at 13.3 ± 0.7 ¹⁰Be kyr. Subsequent ice-margin retreat from the North Lithuanian Moraine represented the final deglaciation of Lithuania. Direct dating of these moraines better constrains the relation of ice-margin positions in Lithuania to those in adjacent countries as well as the SIS response to climate change.     

Shallow site archaeology: Artifact dispersal,stratigraphy, and radiocarbon dating at the Barger Gulch locality B Folsom Site,Middle Park,Colorado

Shallowly buried archaeological sites are particularly susceptible to surface and subsurface disturbance processes. Yet, because cultural deposition often operates on short time scales relative to geologic deposition, vertical artifact distributions can be used to clarify questions of site formation. In particular, patterns in artifact distributions that cannot be explained by occupation histories must be explained by natural processes that have affected sites. Buried only 10–50 cm beneath the ground surface for 10,450 ¹⁴C yr, the Folsom component at Barger Gulch Locality B (Middle Park, Colorado) exhibits many signs of post‐depositional disturbance. Through examination of variation in the vertical distribution of the artifact assemblage, we are able to establish that only a Folsom component is present. Using vertical artifact distributions, stratigraphy, and radiocarbon dating, we are able to reconstruct the series of events that have impacted the site. The Folsom occupation (˜10,450 ¹⁴C yr B.P.) was likely initially buried in a late‐Pleistocene eolian silt loam. Erosion brought the artifacts to rest on a deflation surface at some time prior to 9400 ¹⁴C yr B.P. A mollic epipedon formed in sediments that accumulated between 9400 and 7000 ¹⁴C yr B.P. Some time after 5200 ¹⁴C yr B.P., this soil was partially truncated, and artifacts that had previously dispersed upward created a secondary lag at its upper contact. This surface was buried again and artifact dispersal continued. © 2005 Wiley Periodicals, Inc.     

Late Pleistocene foraminiferal stratigraphy on the Vøring Plateau, Norwegian Sea

Quantitative studies of foraminiferal faunas and isotope ratios have been done in two gravity cores from the Vøring Plateau. Core 23199 from about 2000 m water depth included sediments younger than oxygen Stage 7. Core 23205 from about 1400 m ended in sediments from Stage 5. According to the transfer temperatures, the onset of abundances of Cibicidoides wuellerstorfi and the δ¹³C values of Neogloboquadrinapachyderma , the Vøring Plateau was covered by pack-ice during Stages 6,4,2 and, to some extent, also during Stage 3. Ice-free sea-surface conditions prevailed from late Substage 5e throughout Stage 5 and during stage 1. The transition from Stage 6 to Stage 5 (Termination II) took place in two steps: Ila is defined by the δ¹⁸O signal of N. pachyderma (reduced global ice volume) and lib by the increased occurrence of C. wuellerstorfi (melting of local pack-ice). A contemporaneous development of an interglacial deep-water circulation system is indicated.     

Comparison between radiocarbon and varve dating in Lake Lampellonjärvi, south Finland

The annual nature of organic laminations in the sediment from a small lake, Lampellonjärvi (61°04'N; 25°04'E), was determined. The core was obtained using an in situ freezing method and the laminations were checked by a detailed microscopical analysis of adhesive tape preparations. A series of six radiocarbon dates were obtained for levels in the core which had previously been dated by means of varve counts to ages between A.D. 182 and AD. 1513. The radiocarbon dates were between 547 and 1525 years older. Two additional ¹⁴C dates from the lower part of the same profile (ca. 1880 B.C. and ca. 3100 B.C.), however, gave expected results. Erosion of old organic terrestrial material due to agricultural activity in the surroundings of the lake was assumed to have been the cause of abnormally old radiocarbon ages. Dates from the pre-agricultural period had a deviation from varve years similar to the difference between tree rings, historical documents and radiocarbon dating recorded in other studies.     

Conference on the last deglaciation: Timing and mechanism

For years paleoclimatologists have held the general view that the last deglaciation began around 17,000 to 15,000 yr ago, that the shape of the globally integrated deglacial curve was smoothly sigmoidal with the fastest rate of change centered around 11,000 yr ago, and that the deglaciation ended around 7000 to 5000 yr ago. Recent studies have challenged several aspects of this consensus and have suggested that the mechanisms responsible for the deglaciation are significantly different from those previously proposed. As a result, an international workshop was held at Airlie House in Virginia during May 2–6 of 1983 to evaluate a wide range of evidence relevant to this controversy. The conference results suggested that (1) the decrease in global ice volume occurred in two steps, with the dating of the earlier step still in doubt, but the later step occurring at about 10,000–7000 yr ago and (2) the most likely feedback mechanisms for accelerating the initial forcing by orbital variations are delayed bedrock rebound, marine downdraw/calving, and CO₂ heating.