Abrupt Climate Oscillations Research Articles

Annual proxy data from Lago Grande di Monticchio (southern Italy) between 76 and 112 ka: new chronological constraints and insights on abrupt climatic oscillations

Abstract. We present new annual sedimentological proxies and sub-annual element scanner data from the Lago Grande di Monticchio (MON) sediment record for the sequence 76–112 thousand years before present (ka). They are combined with the previously published decadal to centennial resolved pollen assemblage in order to provide a comprehensive reconstruction of six major abrupt stadial spells (MON 1–6) in the central Mediterranean during the early phase of the last glaciation. These climatic oscillations are defined by intervals of thicker varves and high Ti-counts and coincide with episodes of forest depletion interpreted as Mediterranean stadial conditions (cold winter/dry summer). Our chronology, labelled as MON-2014, has been updated for the study interval by tephrochronology and repeated and more precise varve counts and is independent from ice-core and speleothem chronologies. The high-resolution Monticchio data then have been compared in detail with the Greenland ice-core δ18O record (NorthGRIP) and the northern Alps speleothem δ18Ocalcite data (NALPS). Based on visual inspection of major changes in the proxy data, MON 2–6 are suggested to correlate with Greenland stadials (GS) 25–20. MON 1 (Woillard event), the first and shortest cooling spell in the Mediterranean after a long phase of stable interglacial conditions, has no counterpart in the Greenland ice core, but coincides with the lowest isotope values at the end of the gradual decrease in δ18Oice in NorthGRIP during the second half of the Greenland interstadial (GI) 25. MON 3 is the least pronounced cold spell and shows gradual transitions, whereas its NorthGRIP counterpart GS 24 is characterized by sharp changes in the isotope records. MON 2 and MON 4 are the longest and most pronounced oscillations in the MON sediments in good agreement with their counterparts identified in the ice and spelethem records. The length of MON 4 (correlating with GS 22) supports the duration of stadial proposed by the NALPS timescales and suggests ca. 500 year longer duration than calculated by the ice-core chronologies GICC05modelext and AICC2012. Absolute dating of the cold spells provided by the MON-2014 chronology shows good agreement among the MON-2014, the GICC05modelext and the NALPS timescales for the period between 112 and 100 ka. In contrast, the MON-2014 varve chronology dates the oscillations MON 4 to MON 6 (92–76 ka) as ca. 3500 years older than the most likely corresponding stadials GS 22 to GS 20 by the other chronologies.

231Pa/230Th evidence for a weakened but persistent Atlantic meridional overturning circulation during Heinrich Stadial 1.

The strength of Atlantic meridional overturning circulation is believed to affect the climate over glacial-interglacial and millennial timescales. The marine sedimentary (231)Pa/(230)Th ratio is a promising paleocirculation proxy, but local particle effects may bias individual reconstructions. Here we present new Atlantic sedimentary (231)Pa/(230)Th data from the Holocene, the last glacial maximum and Heinrich Stadial 1, a period of abrupt cooling ca. 17,500 years ago. We combine our results with published data from these intervals to create a spatially distributed sedimentary (231)Pa/(230)Th database. The data reveal a net (231)Pa deficit during each period, consistent with persistent (231)Pa export. In highly resolved cores, Heinrich (231)Pa/(230)Th ratios exceed glacial ratios at nearly all depths, indicating a significant reduction, although not cessation, of overturning during Heinrich Stadial 1. These results support the inference that weakened overturning was a driver of Heinrich cooling, while suggesting that abrupt climate oscillations do not necessarily require a complete shutdown of overturning.

Environment during the Middle to Late Palaeolithic transition in southern France: The archaeological sequence of Tournal Cave (Bize-Minervois, France)

Late Pleistocene deposits of Tournal Cave, southwestern France, provided several human occupations attributed to Mousterian, Aurignacian, and Magdalenian cultures. Some human remains (Homo neanderthalensis and Homo sapiens) were identified in each unit. This study presents a detailed temporal reconstruction of habitats surrounding the cave, using various proxies based on rich large mammal assemblages and related to biological activities. The levels correlated to MIS 3 and MIS 2 are characterized by alternating carnivore, Neanderthal, and Anatomically Modern Human (AMH) occupations. Faunal exploitation was mainly oriented towards Equus caballus and Rangifer tarandus, with a significant increase in the latter prey from the last Aurignacian level onwards. The results exhibit geographic and temporal variations of reindeer and horse body size. Reindeer can be used as a suitable ecological marker, as rapid changes are correlated to environmental turnover, whereas the horse presents a different pattern and is an accurate chronological estimator. The palaeoecological results indicate a major climate change between the two Aurignacian levels. The Mousterian and the first Aurignacian levels, documenting the Middle to Late Palaeolithic transition, show high indices of specific richness and diversity of large mammals in relation with a periglacial moderate cold and wet climate. The region developed a mixed landscape and displayed a non-analogue fauna with E. caballus, R. tarandus, Cervus elaphus, Megaloceros giganteus, Bison priscus, Bos primigenius, Capra caucasica praepyrenaica, Sus scrofa and Coelodonta antiquitatis for the ungulates and Ursus spelaeus, Ursus arctos, Crocuta crocuta spelaea, Panthera leo spelaea, Panthera pardus, Lynx spelaeus, Canis lupus and Vulpes vulpes for the carnivores. The breakdown occurring in the last Aurignacian level is characterized by a change in ecological settings, with a colder and drier climate and opening of the landscape, involving shifts in seasonality, plant phenology, reindeer body size and animal population densities. This impacted habitat fragmentation and geographic distribution of populations, implying various selective pressures that were reflected in human meat procurement and dispersal events. At Tournal Cave, the late Middle/early Late Palaeolithic transition is not directly marked by significant differences in terms of faunal exploitation, due to local climate and site function, but the major environmental shift recorded is delayed. The results indicate that abrupt climate oscillations during MIS 3 contributed to the decline of Neanderthal populations, notably because of habitat fragmentation. AMHs could have generated additional stresses.

Holocene glacier culminations in the Western Alps and their hemispheric relevance

The natural variability of Holocene climate defi nes the baseline to assess ongoing climate change. Greenland ice-core records indicate warming superimposed by abrupt climate oscillations in the early Holocene, followed by a general cooling trend throughout the middle and late Holocene that culminated during the Little Ice Age (LIA). Tropical precipitation changes correlate with these patterns throughout the Holocene. Here we use mountain glaciers in the European Alps to reconstruct the regional Holocene climate evolution and to test for a link between mid-latitude, North Atlantic, and tropical climate. Our precise 10 Be chronology from Tsidjiore Nouve Glacier, western Swiss Alps, indicates a glacier culmination during the earliest Holocene ~11.4 k.y. ago, likely related to the Preboreal Oscillation. Based on our data, no Holocene glacier advance of similar amplitude occurred until ~3.8 k.y. ago, when the glacier reached LIA limits. The 10 Be ages between 500 and 170 yr correspond to the LIA, while the youngest 10 Be ages overlap with the historically recorded post-LIA glacier positions. Integrating our data with existing records, we propose a hemispheric climate link between the Alps, North Atlantic temperature, and tropical precipitation patterns for the Holocene, supporting the concept of a pervasive climate driver. These fi ndings from northern mid-latitudes are consistent with the hypothesis formulated for the tropics that the Earth’s thermal equator, responding to North Atlantic temperature changes, might have migrated southward throughout the Holocene, reaching the southern turning point toward the end of the LIA.

Impact of climate variability in the western Mediterranean during the last 20,000 years: oceanic and atmospheric responses

Eolian input fluctuations and paleoceanographic conditions in the western Mediterranean have been reconstructed by using a multi-proxy approach to infer climate variability for the last 20,000 yr. A sediment core from the East Alboran Sea basin provides high-resolution geochemical, mineralogical and grain size distribution records from the Last Glacial Maximum onward. The accurate chronology obtained, closely related with the North Greenland Ice core Project, has provided a detailed paleoclimate reconstruction at the centennial–millennial time scale. Mineral and chemical compositions as well as sediment grain size distribution have been used to establish both fluvial and eolian input variations, deep-water current intensity, and ventilation and productivity conditions at the time of deposition. During the cold period Greenland Stadial-2a, the analyzed proxies evidence significant climate oscillations. Redox proxies, showing marked Mn/Al and Fe/Al peaks, as well as granulometric parameters, support major changes in thermohaline circulation, with active circulation triggered by the Western Mediterranean Deep Water formation. Ba/Al ratios and the presence of authigenic barite point to high productivity during the Last Heinrich event. At this time, Zr/Al, Si/Al and Ti/Al ratios as well as quartz and palygorskite contents also corroborate dryer conditions in the Alboran borderlands, with a major reorganization of atmospheric circulation at the beginning of the deglaciation and during the Last Heinrich event. In contrast, a decreasing trend in the Zr/Al ratio and increasing Mg/Al, K/Al and illite + chlorite/kaolinite ratios would indicate enhanced river runoff and supporting lower eolian dust input and wetter periods during the Bölling–Alleröd. Short-term and abrupt climate oscillations such as the Intra-Alleröd Cold Period are likewise reflected by minor yet conspicuous changes in eolian proxies. During the Younger Dryas, we distinguish an initial cold and dry phase, with major westerlies intensity, thermohaline reactivation, and high productivity levels, and a second wetter phase characterized by sporadic rainfalls over poorly vegetated surrounding continental areas. Then, during the Holocene, alternating phases of major humidity and aridity were recorded by major fluctuations in detrital input. These are also correlated with recurrent forest cover declines reported from pollen records, with a noteworthy early decline in the African Humid Period. During the Holocene, an initial demise of the Last Organic-Rich Layer is also evidenced, tentatively associated with aspiration forces promoted by intense thermohaline circulation through the Gibraltar Strait.

Evidence of centennial-scale drought from southeastern Massachusetts during the Pleistocene/Holocene transition

A principal method for studying past hydroclimatic change is the reconstruction of paleo-lake levels. Here, we provide high-resolution lake-level records from New Long Pond and Rocky Pond in southeastern Massachusetts, which each contain evidence for multiple, sub-centennial-to-millennial scale low stands during the transition between the Late Pleistocene (15.0 ka) and Middle Holocene (ca 7.0 ka). Data from New Long Pond also demonstrate sedimentary evidence for a drop in water levels in the early to mid AD 20th century, when long-term trends in instrumental data show lower-than-average precipitation in the northeastern United States. Local data show the most precipitous declines in precipitation and groundwater levels are concurrent with the most severe drought in the AD 1960s, which occurred during a period of low sea-surface temperatures in the western North Atlantic. Ground penetrating radar and sediment core data indicate five intervals with numerous paleo-shoreline deposits between ca 15.0 and 7.0 ka, similar to the layer deposited in the AD 1960s. Many of the intervals of low lake levels coincide with proposed meltwater release events or abrupt climate oscillations in the circum North Atlantic. For example, we document at least three low stands during the Younger Dryas (12.9–11.6 ka) and in association with the “9.2” and “8.2” ka events. The combined evidence of (1) concurrent paleo-droughts in southeastern New England with documented North Atlantic abrupt cooling events and (2) recent drought with the modern association of low sea-surface temperatures indicates that freshening and cooling of the western North Atlantic is a viable mechanism for decreasing moisture within the region. Large-scale changes in seasonality and ice sheet extent also may have increased the susceptibility of the northeast to dry conditions triggered by changes in the North Atlantic.

A 40,000-year record of environmental change from ancient Lake Ohrid (Albania and Macedonia)

Lake Ohrid is considered to be of Pliocene origin and is the oldest extant lake in Europe. A 1,075-cm-long sediment core was recovered from the southeastern part of the lake, from a water depth of 105 m. The core was investigated using geophysical, granulometric, biogeochemical, diatom, ostracod, and pollen analyses. Tephrochronology and AMS radiocarbon dating of plant macrofossils reveals that the sediment sequence spans the past ca. 39,500 years and features a hiatus between ca. 14,600 and 9,400 cal. year BP. The Pleistocene sequence indicates relatively stable and cold conditions, with steppe vegetation in the catchment, at least partial winter ice-cover of the lake, and oxygenated bottom waters at the coring site. The Holocene sequence indicates that the catchment vegetation had changed to forest dominated by pine and summer-green oak. Several of the proxies suggest the impact of abrupt climate oscillations such as the 8.2 or 4.0 ka event. The observed changes, however, cannot be related clearly to a change in temperature or humidity. Human impact started about 5,000 cal. year BP and increased significantly during the past 2,400 years. Water column mixing conditions, inflow from subaquatic springs, and human impact are the most important parameters influencing internal lake processes, notably affecting the composition and characteristics of the sediments.

The lacustrine sediment record of Oberwinkler Maar (Eifel, Germany): Chironomid and macro‐remain‐based inferences of environmental changes during Oxygen Isotope Stage 3

The lacustrine record of Oberwinkler Maar (Eifel, Germany) is the northernmost continuous record documenting the Weichselian Pleniglacial in central Europe – a period characterized by multiple abrupt climate oscillations known as the Dansgaard/Oeschger cycles. Here, the results of a high‐resolution study of chironomid remains are presented, with a focus on the earlier part of Oxygen Isotope Stage (OIS) 3 (60–50 kyr BP) covering four stadial/interstadial cycles. During the stadials, the chironomid fauna of the former lake was dominated by many cold‐stenothermic chironomid taxa, indicating a cold, oligotrophic lake. The concentrations of chironomid remains were lower during the interstadials, and featured a higher number of warm‐indicating taxa. This could have been the result of a higher summer temperature at the study site, but also of bottom‐water anoxia, an increase in trophic state or a combination of all these factors. During the stadial intervals, a taxon that is restricted to (sub‐)arctic environments is present in our record, suggesting a change in the temperature regime rather than in‐lake processes as the driving mechanism for the changes in the chironomid record. Although, consistently, there was a response of the lake ecosystem to climate changes, the amplitude of this response was not constant. This study provides unique and independent evidence of the impact of Dansgaard/Oeschger climate variability on the European continent during the earlier part of OIS‐3.

Long-term solar activity variations as a stimulator of abrupt climate change

Analysis of solar forcing of climate on long time scales has shown that it is necessaryto take into consideration the influence of long-term solar cyclicity, such as 200 and 2300–2400-year cycles, on climate. Even in the relatively warm climate of the last 10,000 years, atendency to climate cooling at deep minima of long-term solar cyclicity is observed. Alongwith this, a long-term solar forcing of climate manifests itself not only as an external factordue the influence of solar irradiance variations on the atmosphere-ocean system, but alsoas a stimulator of internal processes in the climatic system, which, in turn, can lead toabrupt climate change. Large-scale abrupt climate oscillations – warmings and subsequentcoolings (Dansgaard-Oeschger cycles) – have been revealed in cores of Greenland ice forthe interval 60,000–10,000 years BP. They are attributed to the ice-rafting events in theNorth Atlantic. A comparative analysis of the development of Dansgaard-Oeschger eventsand solar activity variations (variations in the

Paleomagnetic excursions recorded in the Yanchi Playa in middle hexi corridor, NW China since the last interglacial

Paleomagnetic determinations on lithological profiles of two paralleled long drilling cores covering the past 130 kyr B.P., GT40 and GT60, from the Yanchi Playa in the arid Northwestern China, indicate that a series of pronounced paleomagnetic excursions have been documented. By correlating our results with published regional and worldwide reports, 4 excursion events out of 10 apparent reversal signals (labeled from GT-1 to GT-10) were identified as excursion events coeval with the Mono Lake Event (28.4 kyr–25.8 kyr), Laschamp Event (43.3 kyr–40.5 kyr), Gaotai Event (82.8 kyr–72.4 kyr) and the Blake Event (127.4 kyr–113.3 kyr), respectively. GT-9 correlates with the above-mentioned Gaotai Event, GT-7 and GT-6 correspond to two stages of the Laschamp Event and GT-5 to the Mono Lake Event. It is noteworthy that the so-called Gaotai Event has not been reported as a pronounced paleomagnetic excursion in the Northwestern China. Every magnetic excursion event corresponds to paleointensity minima, anteceding those established abrupt paleoclimatic change events, such as the Younger Drays and the Heinrich Events (H1–H6). Here, we tentatively propose that these geomagnetic excursions/reversals can be viewed as precursors to climate abruptness. During the transitional stages when the earth’s magnetic field shifted between a temporal normal and a negative period, the earth’s magnetic paleointensity fell correspondingly to a pair of minima. Although more precise chronology and more convincing rock magnetic parameter determinations are essentially required for further interpretation of their intricate coupling mechanism, these results may have revealed, to some extent, that the earth’s incessantly changing magnetic field exerts an strong influence on the onset of saw-tooth shaped abrupt climate oscillations through certain feedback chains in arid Central Asia or even North Hemispheric high latitude regions.

Changes in the volume and salinity of Lake Khubsugul (Mongolia) in response to global climate changes in the upper Pleistocene and the Holocene

Two gravity cores (1.1 and 2.2 m long) of deep-water bottom sediments from Lake Khubsugul (Mongolia) were studied. The Holocene, biogenic silica and organic matter-rich part of the first core was subjected to AMS radiocarbon dating which placed the date of dramatic increase of pelagic diatoms (40 cm below sediment surface) at a calendar age of 11.5 cal ky BP. ICP-MS analysis of weak nitric acid extracts revealed that the upper Pleistocene, compared to the Holocene samples, were enriched in Ca, C inorg, Sr, Mg and depleted of U, W, Sb, V and some other elements. Transition to the Holocene resulted in an increase of total diatoms from 0 to 10 8 g −1, of BiSi from 1% to 20%, of organic matter from <1% to >6%. The Bølling–Allerød–Younger Dryas–Holocene abrupt climate oscillations manifested themselves in oscillations of geochemical proxies. A remarkable oscillation also occurred at 22 cm (ca. 5.5 ky BP). The Pleistocene section of the second, longer core was enriched in carbonate CO 2 (up to 10%) and water-extractable SO 4 2− (up to 300 times greater than that in Holocene pore waters). All this evidence is in an accord with the earlier finding of drowned paleo-deltas at ca. 170 m below the modern lake surface of the lake [Dokl. Akad. Nauk 382 (2002) 261] and suggests that, due to low (ca. 110 mm) regional precipitation at the end of the Pleistocene, Lake Khubsugul was only 100 m deep, and that its volume was ca. 10 times less than today.

High resolution hematite/goethite records from Chinese loess sequences for the last glacial‐interglacial cycle: Rapid climatic response of the East Asian Monsoon to the tropical Pacific

In order to study the variability of dry versus humid conditions resulting from the East Asian monsoon (EAM) we analyzed hematite and goethite from two high‐resolution Chinese loess sections by diffuse reflectance spectroscopy. The hematite and goethite concentrations are higher in interglacial paleosols than in glacial loess sediments, whereas the hematite/goethite (Hm/Gt) ratios are lower in paleosols than in loess. Such changes are consistent with higher precipitation and temperature and increased pedogenesis during interglaciation and lower precipitation and temperature and decreased pedogenesis during glaciation. The Hm/Gt ratio can be employed as a tool to assess short‐term variations in humidity and aridity of the EAM. The Hm/Gt ratios of the last glacial loess displays millennial‐scale abrupt climate oscillations, revealing a pattern of significantly reduced and enhanced summer monsoon moisture. Our results indicate that the EAM responded to tropical Pacific climate by millennial‐scale changes in precipitation during the last glaciation.

Speleothem U-series dating of semi-synchronous climate oscillations during the last deglaciation

Evidence for nearly synchronous climate oscillations during the last deglaciation has been found throughout the Northern Hemisphere but few records are based on independent time scales of calendar years. We present a rare uranium-series dated oxygen–carbon isotope record for a speleothem from Tangshan Cave, China, which demonstrates that abrupt deglacial climatic oscillations from 16 800 to 10 500 yr BP are semi-synchronous with those found in Greenland ice core records. Relatively rapid shifts in speleothem oxygen isotope ratios demonstrate that the intensity of the East Asian monsoon switched in parallel with the abrupt transitions separating the Bølling–Allerød, Younger Dryas, and pre-Boreal climatic reversals. However, the dated isotopic transitions appear to have lasted longer. Our results demonstrate the dominant role of atmospheric teleconnections in the rapid propagation of deglacial climatic signals on a hemispheric scale, and highlight the importance of U-series dated speleothems in the timing and characterization of abrupt climate change.

Precise dating of Dansgaard-Oeschger climate oscillations in western Europe from stalagmite data.

The signature of Dansgaard-Oeschger events--millennial-scale abrupt climate oscillations during the last glacial period--is well established in ice cores and marine records. But the effects of such events in continental settings are not as clear, and their absolute chronology is uncertain beyond the limit of (14)C dating and annual layer counting for marine records and ice cores, respectively. Here we present carbon and oxygen isotope records from a stalagmite collected in southwest France which have been precisely dated using 234U/230Th ratios. We find rapid climate oscillations coincident with the established Dansgaard-Oeschger events between 83,000 and 32,000 years ago in both isotope records. The oxygen isotope signature is similar to a record from Soreq cave, Israel, and deep-sea records, indicating the large spatial scale of the climate oscillations. The signal in the carbon isotopes gives evidence of drastic and rapid vegetation changes in western Europe, an important site in human cultural evolution. We also find evidence for a long phase of extremely cold climate in southwest France between 61.2 +/- 0.6 and 67.4 +/- 0.9 kyr ago.

A high resolution, multiproxy Late‐glacial record of climate change and intrasystem responses in northwest England

AbstractA lacustrine carbonate sequence from Hawes Water, Lancashire, UK, has been studied using stable isotopic, lithological, pollen and mineral magnetic analysis. The data reveal four abrupt climatic oscillations in the Late‐glacial Interstadial leading up to the onset of the Loch Lomond Stadial. The data also point to climatic warming relatively early within the stadial, ca. 12 500 GRIP yr, prior to the onset of the Holocene. The oxygen isotope record is taken as a signature of climate forcing against which the response of the lake‐system can be monitored. By adopting this approach it is revealed that the response of the biological system to the rapid climatic oscillations is non‐linear and primarily a function of the antecedent conditions. A significant end‐Devensian isotopic excursion (A) is matched by only minor changes in the cold‐adapted floras and faunas. During the warmer interstadial, the response of the biological ecosystem (events B–D) is clearly influenced by thresholds: major changes in the catchment vegetation associated with relatively minor oscillations in the isotopic signature. The stratigraphical patterns reveal significant lag effects between the onset of climate deterioration and resulting changes in vegetation. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Abrupt and sudden climatic transitions and fluctuations: a review

AbstractThis review paper summarizes recent research work on abrupt climatic changes and oscillations. The climatic system is viewed as a dissipative, highly non‐linear system, under non‐equilibrium conditions, and, as such, should be expected to have some unusual properties. These unusual properties include bifurcation points with marked instability just before the point, magnification of semi‐periodic oscillations around bifurcation points, and variations in the strength of teleconnections with distance from equilibrium. These properties are discussed and illustrated for the climatic system using both the historical, Holocene and glacial climatic records. It is found that there are abrupt climatic changes and oscillations on all time‐scales. The amplitudes and frequencies of climate variability and teleconnections are found to vary between different time periods. A number of persistent oscillations exist, particularly one about 1500 years, but their amplitudes vary considerably between time periods. The Holocene appears to be no more climatically benign than the similar period in the Eemian. The importance of the North Atlantic thermohaline circulation for generating abrupt climatic changes in Europe, particularly in association with sudden pulses of fresh water, is illustrated. The concept of antiphase temperature changes between the North and South Atlantic is discussed. Externally generated abrupt climatic deteriorations owing to explosive volcanic eruptions and variations in solar irradiance are also discussed. Copyright © 2001 Royal Meteorological Society

Response of interior North America to abrupt climate oscillations in the North Atlantic region during the last deglaciation

Several broad-scale climatic oscillations during the last deglaciation are well documented in regions around the North Atlantic Ocean. This paper reviews empirical evidence for these deglacial climatic oscillations from non-coastal North America and discusses implications for testing climatic simulations and for understanding the cause and transmittal mechanisms. Paleoclimatic interpretation of oxygen-isotope records from several small sites in the eastern Great Lakes region indicates a classic deglacial climatic sequence that is comparable with records from Europe and Greenland. The climatic events as interpreted from Crawford Lake oxygen isotopes include the Bølling–Allerød (BOA) warming at ∼12,700 14C BP, a warm BOA at ∼12,500–10,920 14C BP, an intra-Allerød cold period shortly before 11,000 14C BP, a cold Younger Dryas (YD) climate reversal at 10,920–10,000 14C BP, the Holocene warming at 10,000 14C BP, a brief Preboreal Oscillation (PB) at 9650 14C BP, and an early-Holocene cold event at 7500 14C BP (8200 cal BP). Some of these events were also evident from changes in upland and aquatic vegetation and sediment lithology. The pronounced YD climatic reversal has been documented from pollen records along the ecotones at this time and from glacier readvances in the Great Lakes region. Along the Rocky Mountains from Alberta to Colorado, the YD event is indicated by alpine glacier advance and/or shift in timberline vegetation. In Minnesota and upper New York, early-Holocene climatic instability is also suggested by oxygen-isotope records and/or varve thickness. The regional variations in evidence for the YD and other events in North America suggest that climatic oscillations may have different expressions in paleo-records, depending on geographic location and characteristics of a particular site. The extent and magnitude of these climatic oscillations across North America suggest that these oscillations are an expression of climatic change that was probably widespread rather than locally induced by a nearby glacier. The location of these sites implies that climatic signals were likely carried over the Northern Hemisphere through the atmosphere, as indicated by general circulation models. We hypothesize that the lack of evidence for a cold YD in interior North America west of the Great Lakes region and east of the Rocky Mountains was caused by the trapping of cold arctic air mass north of the Laurentide ice sheet and by uninterrupted northward expansion of warm Caribbean air; this strongly contrasted climate also produced non-analogous biological assemblages.

Abrupt climate oscillations during the last deglaciation in central north america

Evidence from stable isotopes and a variety of proxies from two Ontario lakes demonstrate that many of the late glacial-to-early Holocene events that are well known from the North Atlantic seaboard, such as the Gerzensee-Killarney Oscillation (also known as the Intra-Allerod Cold Period), Younger Dryas, and Preboreal Oscillation, also occurred in central North America. These results thus imply that climatic forcing acted in the same manner in both regions and that atmospheric circulation played an important role in the propagation of these events.

Successive Oceanic and Solar Forcing Indicated by Younger Dryas and Early Holocene Climatic Oscillations in the Jura

The recent extension of (1) the residual Δ 14C curve back to 11,400 cal yr B.P. and (2) the lake-level reconstruction in the Jura back to ca. 13,500 cal yr B.P. offers the opportunity of testing by proxy data the relationships between climate, atmospheric 14C, the sun, and the ocean recently suggested from the atmospheric 14C record. The climatic significance of the Jura record is supported by correlations with climatic oscillations reconstructed in the Alps from glaciers and timberline movements. Correspondence between the 14C and paleoclimatic record from the Jura suggests a working hypothesis: two intervals within the Holocene can be distinguished in the middle latitudes of western and central Europe. An early Holocene period shows abrupt climatic oscillations linked to ocean forcing. Major colder climate phases developed between ca. 9000 and 8800, and between ca. 8000 and 7000 cal yr B.C. that coincide with higher Δ 14C values. After 6000 cal yr B.C., a second period is characterized by smoother multicentury climatic oscillations linked to solar forcing.

The hierarchical structure of glacial climatic oscillations: interactions between ice-sheet dynamics and climate

Abrupt climatic oscillations around the North Atlantic have been identified recently in Greenland ice cores as well as in North Atlantic marine sediment cores. The good correlation between the ‘Dansgaard-Oeschger events’ in the ice and the ‘Heinrich events’ in the ocean suggests that climate, in the North Atlantic region, underwent several massive reorganizations in the last glacial period. A characteristic feature of these events seems to be their hierarchical structure. Every 7 to 10-thousand years, when the temperature is close to its minimum, the ice-sheet undergoes a massive iceberg discharge. This Heinrich event is then followed by an abrupt warming, then by several other oscillations, each one lasting between one and two thousand years. These secondary oscillations do not have a clear signature in marine sediments but constitute most of the ‘Dansgaard-Oeschger events’ found in the ice. Here we use a simplified model coupling an ice-sheet and an ocean basin, in order to illustrate how the interactions between these two components can lead to such a hierarchical structure. The ice-sheet model exhibits internal oscillations composed of ice-sheet growing phases and basal ice melting phases that induce massive iceberg discharges. These massive fresh water inputs in the ocean stop for a moment the thermohaline circulation, enhancing the temperature contrast between low- and high-latitudes. Just after this event, the thermohaline circulation restarts and an abrupt warming of high-latitude regions is observed. For some parameter values, these warmer temperatures have in turn some influence on the ice-sheet, inducing secondary oscillations similar to those found in paleoclimatic records. Although the mechanism presented here may be too grossly simplified, it nevertheless underlines the potential importance of the coupling between ice-sheet dynamics and oceanic thermohaline circulation on the structure of the climatic records during the last glacial period.