Early Holocene Changes Research Articles

Role of eccentricity in early Holocene African and Asian summer monsoons

The effect of precession on paleoclimate changes depends on eccentricity. However, whether and to what degree eccentricity relates to millennial-scale monsoonal changes remain unclear. By investigating climate simulations with a fixed precession condition of 9 ka before the present, we explored the potential influence of eccentricity on early-Holocene changes in the Afro–Asian summer monsoons. Compared with the lower eccentricity of the present day, higher eccentricity in the early Holocene strengthened the continental summer monsoons, Pacific anticyclone, and Hadley circulation, particularly over the ocean. Over Africa, the eccentricity-induced “dry-gets-wetter” condition could be related to the Green Sahara, suggesting a superimposed effect of precession. Over the western Pacific, the tropical response to eccentricity may have been competitive in terms of what an extremely high obliquity may have caused. A downscaled modulation of eccentricity in relation to precession and obliquity cannot be ignored when paleomonsoon records are studied. Regarding early-Holocene monsoonal changes in South Asia, however, a high eccentricity may have had only a secondary effect on enhancing the monsoonal precipitation in the southern edge of the Tibetan Plateau, exhibiting the weak power of candle-like heating. This suggested that sizable monsoonal changes over the northern Indian Ocean and India–Pakistan region are unrelated to early-Holocene eccentricity.

Cryosphere response resolves conflicting evidence for the timing of peak Holocene warmth on Baffin Island, Arctic Canada

Holocene paleotemperature reconstructions across the western Arctic suggest strong spatial variations in air temperature despite zonally uniform Northern Hemisphere insolation forcing. Although most paleotemperature reconstructions across the Eastern Canadian Arctic place the Holocene thermal maximum >3 kyr after peak summer insolation, other records indicate maximum summer warmth was coincident with peak summer insolation. To address this discrepancy, we use radiocarbon inventories in plants and rocks recently exposed by receding ice caps to reconstruct early Holocene changes in glacier dimensions, a reliable proxy for summer air temperature. Fourteen radiocarbon ages from newly exposed plants in growth position collected at the margins of eight retreating ice caps on Baffin Island indicate ice caps expanded 9.3 ± 0.3 ka, and remained over these sites until recent summer warming, indicating peak early Holocene summer warmth occurred prior to ∼9.3 ka. We test this conclusion using in situ cosmogenic 14C (in situ14C) inventories in rocks at four sites with dated plants. In situ14C production simulations using realistic Holocene ice-cover histories constrained by plant 14C ages show that measured in situ14C concentrations are consistent with centuries to millennia of pre-9.3 ka exposure, followed by continuous ice burial. The combination of plant and in situ14C data provide firm evidence that the Eastern Canadian Arctic experienced peak Holocene summer warmth prior to ∼9.3 ka, consistent with peak summer insolation, followed by continuously colder summers until modern warming.

Triple water‐isotopologue record from WAIS Divide, Antarctica: Controls on glacial‐interglacial changes in 17Oexcess of precipitation

AbstractMeasurements of the 17Oexcess of H2O were obtained from ice cores in West and East Antarctica. Combined with previously published results from East Antarctica, the new data provide the most complete spatial and temporal view of Antarctic 17Oexcess to date. There is a steep spatial gradient of 17Oexcess in present‐day precipitation across Antarctica, with higher values in marine‐influenced regions and lower values in the East Antarctic interior. There is also a spatial pattern to the change in 17Oexcess between the Last Glacial Maximum (LGM) and Holocene periods. At coastal locations, there is no significant change in 17Oexcess. At both the West Antarctic Ice Sheet Divide site and at Vostok, East Antarctica, the LGM to Early Holocene change in 17Oexcess is about 20 per meg. Atmospheric general circulation model (GCM) experiments show that both the observed spatial gradient of 17Oexcess in modern precipitation, and the spatial pattern of LGM to Early Holocene change, can be explained by kinetic isotope effects during snow formation under supersaturated conditions, requiring a high sensitivity of supersaturation to temperature. The results suggest that fractionation during snow formation is the primary control on 17Oexcess in Antarctic precipitation. Variations in moisture source relative humidity play a negligible role in determining the glacial‐interglacial 17Oexcess changes observed in Antarctic ice cores. Additional GCM experiments show that sea ice expansion increases the area over which supersaturating conditions occur, amplifying the effect of colder temperatures. Temperature and sea ice changes alone are sufficient to explain the observed 17Oexcess glacial‐interglacial changes across Antarctica.

Late Pleistocene and early Holocene change in the Weddell Sea: a new climate record from the Patriot Hills, Ellsworth Mountains, West Antarctica

ABSTRACTThe transition from the late Pleistocene to the Holocene (30 000–5000 years ago) was a period of considerable climate variability, which has been associated with changes in deep water formation and the intensity of the Meridional Overturning Circulation. Although numerous records exist across the North Atlantic region, few Antarctic ice core records have been obtained from the south. Here we exploit the potential of upwelling ancient ice – so‐called blue ice areas (BIAs) – from the Patriot Hills in the Ellsworth Mountains to derive the first deuterium isotope record (δD) from continental Antarctica south of the Weddell Sea. Gas analysis and glaciological considerations provide a first relative chronology. Inferred temperature trends from the Patriot Hills BIA and snow pit suggest changing climate influences during the transition between the last glacial period and Holocene. Under modern conditions, the interplay between the Antarctic high‐pressure system and the Southern Annular Mode appears to play a significant role in controlling katabatic wind flow over the site while the BIA record suggests that greater sea ice extent during the last glacial period was a major control. Our results demonstrate the considerable potential of the Patriot Hills site for reconstructing past climate change in the south Atlantic region.

The mid-Holocene climatic transition in the Mediterranean: Causes and consequences

In the Mediterranean there is often no clear time gap separating an early-Holocene period of nature-dominated environmental change from a human-dominated late-Holocene one. This mid-Holocene ‘mélange’ has been the subject of debates that have often been polarised between support for climatic causation and those favouring anthropogenic explanations for changes in vegetation, river flooding, wildfire regimes, etc. One way to shed light on the causes of mid-Holocene landscape changes is to focus on natural archives, such as lake and cave isotopes, the records of which can be unambiguously attributed to climatic forcing. These primary climate proxies can then be compared and contrasted with secondary or response variables, such as pollen and microcharcoal data, which can be the product of either climate changes or human activity or both, ideally using a multiproxy approach. In addition, synthesised primary palaeoclimate data can be compared with regional-scale climate modelling simulations. Both model output and proxy data suggest an east—west division in Mediterranean climate history. They indicate that the eastern Mediterranean experienced an increase in winter-season precipitation during the early Holocene, followed by an oscillatory decline after ~6 ka BP. In western parts of the Mediterranean, early-Holocene changes in precipitation were smaller in magnitude and less coherent spatially, and maximum increases occurred during the mid Holocene, around 6—3 ka BP, before declining to present-day values. Coincident with and partly stimulated by these climatic changes, complex societies developed across the Mediterranean, particularly in eastern parts of the basin during the Bronze Age. In consequence, by the mid-first millennium BC, human-induced land cover conversion, a drier and more variable climate, and changed fire regimes combined to establish typical sclerophyllous vegetation and landscapes in the circum-Mediterranean region.

Records of the 8200 cal BP cold event reflected in the composition of subfossil Cladocera in the sediments of three lakes in Poland

The 8200 cal BP cold event was documented by analyzing cladoceran subfossil remains in sediments of three Polish lakes. To date, this cold event has primarily been identified in areas located at higher latitudes of the Northern Hemisphere. It has mainly been confirmed by results of oxygen and carbon stable isotope analyses. This survey represents an attempt to find traces of the 8200 cal BP cold event in lake sediments, based on changes in the species composition and frequency of cladoceran individuals. Both archival, accurately dated sediments (Lake Ostrowite, Lake Gościąż), and recently cored samples (Lake Charzykowskie) were used. The analysis of subfossil cladocerans was performed at a high resolution. This allowed derivation of detailed conclusions regarding Early Holocene changes within lakes, and therefore conclusions on short-term climate changes. In Lakes Ostrowite, Charzykowskie, and Gościąż, changes in the zooplankton Cladocera composition occurred at the same time as the cold event. Certain species disappeared completely, and the percentage share of the majority of zooplankters decreased significantly. Certain species ( Camptocercus lilljeborgi, Alonopsis elongata), that are typical of lakes in Northern areas, appeared during the cold event, although not in large numbers.

An Important Beginning

An Important Beginning

Pollen evidence for the transition of the Eastern Australian climate system from the post-glacial to the present-day ENSO mode

A review of Holocene climate patterns in eastern Australia is presented on the basis of a series of high-resolution pollen records across a north-to-south transect. Previously published radiocarbon data are calibrated into calendar years and fitted with an age-depth model. The resulting chronologies are used to compare past environmental changes and describe patterns of climate change on a calendar-age scale. Based on the present-day Australian climate patterns and impact of the El Niño-Southern Oscillation (ENSO), the palynological data are interpreted and the prevalent climate mode throughout the Holocene reconstructed. Results show that early Holocene changes are strongly divergent and asynchronous between sites, while middle to late Holocene conditions are characterized by more arid and variable conditions and greater coupling between northern and southern sites, which is in agreement with increasing influence of ENSO.

A multi‐proxy study of Holocene lake development, lake settlement and vegetation history in central Ireland

AbstractStratigraphical investigations, geomorphological mapping, and diatom, plant macrofossil and pollen analyses were undertaken in and around two lakes in central Ireland to establish correlations between changes in lake conditions and catchment vegetation throughout the Holocene. Similar investigations of an adjacent mire reveal early Holocene changes in lake level and area. The palaeoecological data show high correlations related to variations in lake depth and area, catchment vegetation type, organic inputs and trophic status. Catchment‐scale deforestation is gradual and occurs through the Bronze and the Iron Ages, and the construction of a crannog in the early Medieval period (seventh century AD) appears to be associated with a widespread increase in deforestation and mixed agriculture in the catchment. Both pollen and plant macrofossils suggest that one of the crannogs was used for crop storage in addition to domestic and any other activities. In the early to middle Holocene similarities in the proxy‐data appear to be climatically driven through changing lake levels and areal extent whereas the later Holocene record is clearly dominated by anthropogenic changes within the catchment and the construction of crannogs in the lakes. The advantages of combining multi‐proxy indicators of lake hydroecology with the vegetation record are illustrated. Copyright © 2005 John Wiley & Sons, Ltd.

Late‐glacial and early Holocene changes in vegetation and lake‐level at Hauterive/Rouges‐Terres, Lake Neuchâtel (Switzerland)

AbstractPalynological and sedimentological analyses of a sedimentary sequence sampled at Hauterive/Rouges‐Terres, Lake Neuchâtel (Switzerland) provide documentation of changes in vegetation and lake‐level during the Bølling, Younger Dryas and Preboreal pollen zones, and have allowed a comparison with sequences covering the same period from other sites located in the western part of the Swiss Plateau. The Juniperus–Hippophaë zone (regional pollen assemblage zone (RPAZ) CHb‐2, first part of the Bølling, ca. 14 650–14 450 cal. yr BP) was characterised by a generally low lake‐level. A weak rise occurred during this zone. The Juniperus–Hippophaë to Betula zone transition coincided with a lake‐level lowering, interrupted by a short‐lived but marked phase of higher lake‐level recorded at the neighbouring site of Hauterive‐Champréveyres, but not present at Hauterive/Rouges‐Terres owing to an erosion surface. Shortly after the beginning of the Betula zone (RPAZ CHb‐3, second part of the Bølling, ca 14 450–14 000 cal. yr BP), a marked rise in lake‐level occurred. It was composed of two successive periods of higher level, coinciding with high values of Betula, separated by a short episode of relatively lower lake‐level associated with raised values in Artemisia and other non‐arboreal pollen. The last part of RPAZ CHb‐3 saw a fall in lake‐level. The lower lake‐levels during RPAZ CHb‐2 to early RPAZ CHb‐3 can be correlated with the abrupt warming at the beginning of the Greenland Interstadial (GI) 1e thermal maximum. The successive episodes of higher lake‐level punctuating the GI 1e might be linked to the so‐called Intra‐Bølling Cold Oscillations identified from several palaeoclimatic records in the North Atlantic area, and also documented in oxygen‐isotope data sets from Swiss Plateau lakes. The Hauterive/Rouges‐Terres lake‐level record provides evidence for marked climatic drying through the second part of the Younger Dryas event (GS1), during the GS1–Preboreal (RPAZ CHb‐4b–4c) transition (except for a rise at ca. 11 450–11 400 cal. yr BP), and at the RPAZ CHb‐4c–5 (Preboreal–Boreal) transition, following the Preboreal Oscillation (after 11 150 cal. yr BP). The Preboreal Oscillation coincided with higher lake‐levels, its end being followed by a rapid expansion of Corylus, Quercus, Ulmus and Tilia. The Hauterive/Rouges‐Terres lake‐level record suggests that radiocarbon plateau at 12 600, 10 000 and 9500 14C yr BP corresponded to periods of generally lower lake‐level. This suggests that an increase in solar activity may have contributed to both climatic dryness and a decrease in atmospheric radiocarbon content. Copyright © 2002 John Wiley & Sons, Ltd.

Early Holocene change in atmospheric circulation in the Northern Great Plains: an upstream view of the 8.2 ka cold event

Elk Lake, in northwestern Minnesota, contains numerous proxy records of climatic and environmental change contained in varved sediments with annual resolution for the last 10,000 years. These proxies show that about 8200 calendar years ago (8.2 cal. ka; 7300 radiocarbon years) Elk Lake went from a well-stratified lake that was wind-protected in a boreal forest to a well-mixed lake in open prairie savanna receiving northwesterly wind-blown dust, probably from the dry floor of Lake Agassiz. This change in climate marks the initiation of the widely recognized mid-Holocene “altithermal” in central North America. The coincidence of this change with the so-called 8.2 cal. ka cold event, recognized in ice-core and other records from the circum-North Atlantic, and thought by some to be caused by catastrophic discharge of freshwater from proglacial lakes Agassiz and Ojibway, suggests that the two “events” might be related. Our interpretation of the Elk Lake proxy records, and of other records from less accurately dated sites, suggests that change in climate over North America was the result of a fundamental change in atmospheric circulation in response to marked changes in the relative proportions of land, water, and, especially, glacial ice in North America during the early Holocene. This change in circulation probably post-dates the final drainage of proglacial lakes along the southern margin of the Laurentide ice sheet, and may have produced a minor perturbation in climate over Greenland that resulted in a brief cold pulse detected in ice cores.

Historical population size change of bowhead whales inferred from DNA sequence polymorphism data.

Nucleotide sequence data from the mitochondrial control region were used from a phylogenetic context to investigate the long-term history of a population of bowhead whales (Balaena mysticetus). In addition, the coalescence time of these sequences was used to estimate the age of the inferred patterns of population size change. The results indicate that mitochondrial genetic polymorphism was not affected by a recent bottleneck that occurred near the turn of the 20th century, thereby preserving the signature of historical population size change in the mitochondrial genome. Further analysis showed that this population underwent an expansion initiated in the Middle to Late Pleistocene. As such, early Holocene changes in Arctic sea ice distribution appear to have had little influence on patterns of genetic variability in this population.

HISTORICAL POPULATION SIZE CHANGE OF BOWHEAD WHALES INFERRED FROM DNA SEQUENCE POLYMORPHISM DATA

Nucleotide sequence data from the mitochondrial control region were used from a phylogenetic context to investigate the long-term history of a population of bowhead whales (Balaena mysticetus). In addition, the coalescence time of these sequences was used to estimate the age of the inferred patterns of population size change. The results indicate that mitochondrial genetic polymorphism was not affected by a recent bottleneck that occurred near the turn of the 20th century, thereby preserving the signature of historical population size change in the mitochondrial genome. Further analysis showed that this population underwent an expansion initiated in the Middle to Late Pleistocene. As such, early Holocene changes in Arctic sea ice distribution appear to have had little influence on patterns of genetic variability in this population.Corresponding Editor: S. Karl

Emergence of the Varanger Peninsula, Arctic Norway, and climate changes since deglaciation

Fields of glacioisostatically uplifted beach ridges on the eastern Varanger Peninsula coast in northern Norway record the occurrence of major storms since deglaciation. More than 70 radiocarbon dates of palaeoshoreline indicators establish the Late Weichselian and Holocene history of beach-ridge building and relative sea-level change. Optimal ridge-building in the region requires sustained, strong easterlies driving high waves across the Barents Sea. Such winds occur when low pressure systems are forced across the North Sea and the Baltic landmass to the region of the Gulf of Bothnia and the Kola Peninsula. This situation probably frequently occurs during transition periods from milder to cooler climate in the region. The beach-ridge morphology reveals a fall of local sea level following the stormy episodes. Over the short term, this is related to the end of storm-induced set-up and the termination of mild-phase-related warm coastal currents. Over the long term, local sea-level falls with the onset of sustained calm winter-type conditions and continued glacioisostatic uplift. Pre- and early-Holocene changes in climate regime occurred with a frequency of several decades to several centuries. The mid- to late-Holocene records show increased climatic stability, transitions occurring with less frequency, and mild phases frequently lasting many hundreds of years.

Chronological potential of palaeomagnetic oscillations recorded in late quaternary sediments from Lake Tanganyika

The measurements of Depositional and Post-Depositional Remanent Magnetizations (DRM and PDRM) in two piston cores collected at water depths of 140 m (core MPU3) and 422 m (core MPU12) in the southern Lake Tanganvika (Mpulungu Sub-Basin, Zambia) provide a palaeomagnetic record for south-east Central Africa, which spans over the last 25-5 ka. Ages (on a conventional 14C time scale) for the most characteristic inclination peaks were estimated by correlating the palaeomagnetic record with the Lake Barombi-Mbo (Cameroon) and the Western Europe Secular Variation type curves. AMS 14C ages on Total Organic Matter (TOM) measured in MPU12 core appear to be 2 to 4 ka older in the lower part of the record, from ca. 25 to 15 ka BP, i.e. during the last arid episode of the Late Pleistocene. These apparent 14C ages exceed PDRM acquisition lagtimes (evaluated to be about 800 years at most), and also account for residence time of carbon in TOM prior to its sedimentation. Therefore magnetostratigraphic correlations are believed to provide a better overall chronology for the Late Pleistocene and Early Holocene changes in Lake Tanganyika level. Major drops shown by breaks of deposition in core MPU3 (at 1000 cm and 825 cm sub-bottom) are respectively dated at ca. >21.5 ka BP and between 17 and 15 ka BP.