Dansgaard-Oeschger events

Abrupt climate changes constitute a relatively new field of research, which addresses variations occurring in a relatively short time interval of tens to a hundred years. Such time scales do not correspond to the tens or hundreds of thousands of years that the astronomical theory of climate addresses. The latter theory involves parameters that are external to the climate system and whose multi-periodic variations are reliably known and 15 almost constant for a large extent of Earth history. Abrupt changes, conversely, appear to involve fast processes that are internal to the climate system; these processes varied considerably during the past 2.6 Myr, and yielded more irregular fluctuations. In this paper, we reexamine the main climate variations determined from the U1308 North Atlantic marine record, which yields a detailed calving history of the Northern Hemisphere ice sheets over the past 3.2 Myr. The magnitude and periodicity of the ice-rafted debris (IRD) 20 events observed in the U1308 record allow one to determine the timing of several abrupt climate changes, the larger ones corresponding to the massive iceberg discharges labeled Heinrich events (HEs). In parallel, abrupt warmings, called Dansgaard-Oeschger (DO) events, have been identified in the Greenland records of the last glaciation cycle. Combining the HE and DO observations, we study a complex mechanism that may lead to the observed millennial-scale variability corresponding to the abrupt climate changes of last 0.9 Myr. This 25 mechanism relies on amended Bond cycles, which group DO events and the associated Greenland stadials into a trend of increased cooling, with IRD events embedded into every stadial, the latest of these being an HE. These Bond cycles may have occurred during the last 0.9 Ma when Northern Hemisphere ice sheets reached their maximum extent and volume, thus becoming a major player in this time interval's climate dynamics. Since the waxing and waning of ice sheets during the Quaternary period are orbitally paced, we conclude that the 30 abrupt climate changes observed during the Mid and Upper Pleistocene are therewith indirectly linked to the astronomical theory of climate.

During the last glacial period, proxy records throughout the Northern Hemisphere document a succession of rapid millennial-scale warming events, called Dansgaard-Oeschger (DO) events. A range of different mechanisms have been proposed that can produce similar warming in model experiments, however the progression and ultimate trigger of the events is still unknown. Because of their fast nature, the progression is challenging to reconstruct from paleoclimate data due to the limited temporal resolution achievable in many archives and cross-dating uncertainties between records. Here we use new high-resolution multi-proxy records of sea-salt (derived from sea spray and sea ice over the North Atlantic) and terrestrial (derived from the Central Asian deserts) aerosol concentrations over the period 10-60 ka from the Greenland NGRIP and NEEM ice cores in conjunction with local precipitation and temperature proxies from one of the cores to investigate the progression of environmental changes at the onset of the warming events at annual to multi-annual resolution. Our results show on average a small lead of the changes in both local precipitation and terrestrial dust aerosol concentrations over the change in sea-salt aerosol concentrations and local temperature of approximately one decade. This suggests that, connected to the reinvigoration of the Atlantic Meridional Overturning Circulation and the warming in the North Atlantic, both synoptic and hemispheric atmospheric circulation change at the onset of the DO warming, affecting both the moisture transport to Greenland and the Asian monsoon systems. Taken at face value, this suggests that a collapse of the sea-ice cover was not the initial trigger for the DO events. 1 Introduction In the course of the last glacial period, ice-core records from Greenland reveal millennial-scale warming episodes, called Dansgaard-Oeschger (DO) events (Dansgaard et al., 1993; NGRIP project members, 2004). During their onset, temperatures in Greenland increased rapidly by 10-15°C from cold stadial (GS, Greenland Stadial) to warmer interstadial (GI, Greenland Interstadial) conditions within a few decades (Kindler et al., 2014; Huber et al., 2006; Severinghaus, 1999), going along with an almost doubling of the local snow accumulation (Andersen et al., 2006). Moreover, aerosol records from Greenland ice cores show coinciding rapid changes in aerosol concentrations. These changes are partly caused by reduced atmospheric lifetime of Clim. Past Discuss.,

Heinrich Stadial 4 during the last glacial period was marked by severe cooling at northern high latitudes along with the attendant changes in Asian Monsoon (Chinese Stadial 4) and South American Monsoon (South American Stadial 4). Here we present improved constraints on timings of Heinrich/Chinese/South American Stadial 4 onset and termination at sub-centennial precision based on speleothem records. We show that their initial onsets were essentially synchronous (40.20 ± 0.08 thousand years ago) and led the Antarctic warming by ~300 years. The Heinrich/Chinese Stadial 4 termination commenced at 38.34 ± 0.07 thousand years ago following a centennial-scale reduction in the Amazon River runoff and a poleward shift of the Southern Westerly wind belt. These two precursor events may have contributed to a reduced Amazon Plume Region and an enhanced Agulhas salt/heat leakage that led to an abrupt resumption of the Atlantic Meridional Ocean Circulation eventually triggering the Heinrich/Chine...

Our study aims to reconstruct climate changes that occurred at Lake Ohrid (south-western Balkan Peninsula), the oldest extant lake in Europe, between 160 and 70 ka (covering part of marine isotope stage 6, MIS 6; all of MIS 5; and the beginning of MIS 4). A multi-method approach, including the "Modern Analog Technique" and the "Weighted Averaging Partial Least-Squares Regression", is applied to the high-resolution pollen sequence of the DEEP site, collected from the central part of Lake Ohrid, to provide quantitative estimates of climate and bioclimate parameters. This allows us to document climatic change during the key periods of MIS 6 and MIS 5 in southern Europe, a region where accurate climate reconstructions are still lacking for this time interval. Our results for the penultimate glacial show cold and dry conditions, while the onset of the "last interglacial" is characterized by wet and warm conditions, with temperatures higher than today (by ca. 2 • C). The Eemian also shows the well-known climatic tri-partition in the Balkans, with an initial pre-temperate phase of abrupt warming (128-121 ka), a central temperate phase with decreasing temperatures associated with wet conditions (121-118 ka), followed by a posttemperate phase of progressive change towards cold and dry conditions (118-112 ka). After the Eemian, an alternation of four warm/wet periods with cold/dry ones, likely related to the succession of Greenland stadials and cold events known from the North Atlantic, occurred. The observed pattern is also consistent with hydrological and isotopic data from the central Mediterranean. The Lake Ohrid climate reconstruction shows greater similarity with climate patterns inferred from northern European pollen records than with southern European ones, which is probably due to its intermediate position and the mountainous setting. However, this hypothesis needs further testing as very few climate reconstructions are available for southern Europe for this key time period.

Centennial-scale mineral dust peaks in last glacial Greenland ice cores match the timing of lowest Greenland temperatures, yet little is known of equivalent changes in dust-emitting regions, limiting our understanding of dust-climate interaction. Here, we present the most detailed and precise age model for European loess dust deposits to date, based on 125 accelerator mass spectrometry 14C ages from Dunaszekcső, Hungary. The record shows that variations in glacial dust deposition variability on centennial-millennial timescales in east central Europe and Greenland were synchronous within uncertainty. We suggest that precipitation and atmospheric circulation changes were likely the major influences on European glacial dust activity and propose that European dust emissions were modulated by dominant phases of the North Atlantic Oscillation, which had a major influence on vegetation and local climate of European dust source regions.

Paleoecological analysis of over 1500 mammoth remains from the famous non-archaeological Berelyokh site (~13e12 ka BP) has demonstrated that~42% show destructive changes (osteoporosis, osteolysis, osteofibrosis, osteomalacia, articular diseases and others). For the first time, non-closure of cervical vertebral foramina transversaria and loose intra-articular bodies have been recorded in mammoths. The overall pathological picture resembles that of Kashin-Beck (or Urovskaya) disease, the etiology of which is associated with mineral starvation. The alimentary (dietary/nutritional) character of the observed osteodystrophy can be explained by the strong acidification of geochemical landscapes, which is manifested in the territory of Northern Eurasia after 30 ka BP and especially clearly during the Late Glacial (~15e10 ka BP). Thus, the Berelyokh site reflects the terminal stage of the last mass extinction of large mammals.

Few studies have reported field descriptions of flower structures associated with strike-slip faults. This study describes and illustrates flower structures near Brazzaville (Republic of Congo) and explains their implication for the tectonic history of the Paleozoic Inkisi Group. Field observations show that the Inkisi Group is affected by two major strike-slip fault systems. The oldest system is dominated by north-northwest–south-southeast striking sinistral strike-slip faults and minor east–west striking dextral strike-slip faults. The youngest system consists of dominant northeast–southwest striking dextral strike-slip faults and minor northwest–southeast striking sinistral strike-slip faults. Flower structures within these major strike slip faults show four types of arrangements that likely depend on fault growth, propagation and damage zones: (i) flower structures associated with wall damage zones; (ii) flower structures associated with linking damage zones; (iii) flower struct...

Heinrich Stadial 4 during the last glacial period was marked by severe cooling at northern high latitudes along with the attendant changes in Asian Monsoon (Chinese Stadial 4) and South American Monsoon (South American Stadial 4). Here we present improved constraints on timings of Heinrich/Chinese/South American Stadial 4 onset and termination at sub-centennial precision based on speleothem records. We show that their initial onsets were essentially synchronous (40.20 ± 0.08 thousand years ago) and led the Antarctic warming by ~300 years. The Heinrich/Chinese Stadial 4 termination commenced at 38.34 ± 0.07 thousand years ago following a centennial-scale reduction in the Amazon River runoff and a poleward shift of the Southern Westerly wind belt. These two precursor events may have contributed to a reduced Amazon Plume Region and an enhanced Agulhas salt/heat leakage that led to an abrupt resumption of the Atlantic Meridional Ocean Circulation eventually triggering the Heinrich/Chine...

Several abrupt shifts from periods of extreme cold (Greenland stadials, GS) to relatively warmer conditions (Greenland interstadials, GI) called Dansgaard-Oeschger events are recorded in the Greenland ice cores. Using cryo-cell UV-laser-ablation inductively-coupled-plasma mass spectrometry (UV-LA-ICPMS), we analysed a 2.85 m NGRIP ice core section (~ 250 years; 2691.50–2688.65 m depth) across the transitions of GI-21.2, a short-lived interstadial prior to interstadial GI-21.1 (GI-21.2: 84.87–85.09 ka b2k). GI-21.2 is a ~ 100-year-long period with δ<sup>18</sup>O values 3–4 ‰ higher than the following ~ 200 years of stadial conditions (GS-21.2), which precede the major GI-21.1 warming. We report concentrations of "major" elements indicative of dust and/or sea salt (Na, Fe, Al, Ca, Mg) at a spatial resolution of ~ 200 μm, while maintaining detection limits in the low-ppb range...

Marine sediment cores that span the last 50,000 years frequently show considerable variability in sediment and biogeochemical variables. In the North Atlantic, a series of massive iceberg and meltwater events (Heinrich events) were sourced from Hudson Strait and the Laurentide Ice Sheet with a periodicity of ~7.2 ky and had significant impacts on the Atlantic meridional overturning circulation and the global hydrological and biogeochemical cycles. Some marine proxies show higher frequency, abrupt events, that may match the Dansgaard-Oeschger (D-O) oscillations. Predictable millennial-scale periodicities and associated forcings have proved elusive, and an observed ~1.5 ky cycle may be linked to stochastic resonance.

Speleothems (cave carbonate deposits) have been recognized as a multi-proxy paleoclimate archive. Variations in carbon and oxygen isotopes in speleothems can record past climate changes (e.g., temperature, rainfall and vegetation) under isotopic equilibrium conditions. However, non-climatic noises caused by in-cave processes may affect these stable isotope records under non-equilibrium isotopic conditions. The Ph.D. Thesis-M. EL-Shenawy; McMaster University-School of Geography and Earth Sciences viii Table of Contents Abstract………………………………………………………………….……………….iii Acknowledgements……………………………………………………………………....vi Table of Contents……………………………………………………….……………...viii List of Figures……………………………………………………………….…………..xii List of Tables……………………………………………………...…………………….xiv List of Equations…………………………………………………………..….…………xv Contribution of Authors……………………………………………………….………xvi Chapter 1-Introduction……………………………………………………...………….1 Chapter 2-Carbon and oxygen isotope systematics in cave environments: Lessons from an artificial cave………………………………………………………..…………18 2.1 Abstract………………………………………………………………………………19 2.2 Introduction…………………………………………………………………..………21 2.3 Experimental and analytical Methods……………………………………….……….26 2.3.1 Experimental Setup………………………………………………………..……26 2.3.2 Preparation of solutions……………………………………..…………………29 2.3.3 Stable isotope and other analyses…………………………………………..….30 2.4 Results……………………………………………………………………..…………32 2.4.1 Mineralogy and weight of CaCO 3 precipitates ………………………………..32 2.4.2 DIC concentration, pH and isotopic compositions of drip water and cave CO 2 …………………………………………………………………………………...33 2.4.3 Carbon and oxygen isotope compositions in the precipitated calcite………….35 2.4.3.1 Carbon and oxygen isotope compositions of calcite precipitated on the upper watch glasses (stalagmite-like settings)……………………...……….35 2.4.3.2 Carbon and oxygen isotope compositions of calcite precipitated on the lower watch glasses (pool-like settings)………………………………..……36 2.5 Discussion……………………………………………………………………………37 2.5.1 Temperature and flow rate effects on stalagmite growth rate…………….…..39 Ph.D. Thesis-M. EL-Shenawy; McMaster University-School of Geography and Earth Sciences ix 2.5.2 Chemical and isotopic evolution of drip water…………………………….….39 2.5.2.1 Chemical evolution in drip water………………………………………….39 2.5.2.2 Carbon isotope evolution in drip water……………………………………40 2.5.2.3 Oxygen isotope evolution in drip water……………………………..……..42 2.5.3 Equilibrium carbon and oxygen isotope fractionations during slow calcite precipitation in poollike settings……………………………………...……….44 2.5.3.1 Equilibrium carbon isotope fractionation between calcite and DIC in lower watch glasses…………………………………………………….……44 2.5.3.2 Equilibrium oxygen isotope fractionation between calcite and H 2 O in lower watch glasses…………………………………………………….……46 2.5.4 Non-equilibrium carbon and oxygen isotope fractionations during fast calcite precipitation in stalagmitelike settings………………………………….49 2.5.4.1 Flow rate and temperature effects on carbon isotope compositions of the calcite precipitated on the upper watch glasses…………………………..…50 2.5.4.2 Flow rate and temperature effects on oxygen isotope compositions of the calcite precipitated on the upper watch glasses……………………………..55 2.5.5 Insights into the debate of equilibrium oxygen isotope fractionation between calcite and water in cave environments……………………………………………..58

Fuel is a basic resource enabling energy production, and its exploitation was a major activity in Neanderthal daily life. In this work we present charcoal results obtained from the Abric Romaní site in order to evaluate fuel use among the human groups occupying this rock shelter from 40 to 70 ka BP. The Abric Romaní, a Middle Palaeolithic site, has yielded evidence of a well-preserved sequence of Neanderthal occupations. The results of this taxonomic and taphonomic study have allowed us to characterise the charcoal assemblage as mainly comprising Pinus sylvestris type. This assemblage gives us an understanding of Neanderthal fuel acquisition strategies, mobility and occupation patterns.

The early Eocene (∼ 55 Ma) was the warmest period of the Cenozoic and was most likely characterized by extremely high atmospheric CO 2 concentrations. Here, we analyze simulations of the early Eocene performed with the IPSL-CM5A2 Earth system model, set up with paleogeographic reconstructions of this period from the DeepMIP project and with different levels of atmospheric CO 2. When compared with proxy-based reconstructions, the simulations reasonably capture both the reconstructed amplitude and pattern of early Eocene sea surface temperature. A comparison with simulations of modern conditions allows us to explore the changes in ocean circulation and the resulting ocean meridional heat transport. At a CO 2 level of 840 ppm, the early Eocene simulation is characterized by a strong abyssal overturning circulation in the Southern Hemisphere (40 Sv at 60 • S), fed by deepwater formation in the three sectors of the Southern Ocean. Deep convection in the Southern Ocean is favored by the closed Drake and Tasmanian passages, which provide western boundaries for the buildup of strong subpolar gyres in the Weddell and Ross seas, in the middle of which convection develops. The strong overturning circulation, associated with subpolar gyres, sustains the poleward advection of saline subtropical water to the convective regions in the Southern Ocean, thereby maintaining deepwater formation. This salt-advection feedback mechanism is akin to that responsible for the present-day North Atlantic overturning circulation. The strong abyssal overturning circulation in the 55 Ma simulations primarily results in an enhanced poleward ocean heat transport by 0.3-0.7 PW in the Southern Hemisphere compared to modern conditions, reaching 1.7 PW southward at 20 • S, and contributes to keeping the Southern Ocean and Antarctica warm in the Eocene. Simulations with different atmospheric CO 2 levels show that ocean circulation and heat transport are relatively insensitive to CO 2 doubling.

High-resolution pollen and dinoflagellate cyst records from sediment core M72/5-25-GC1 were used to reconstruct vegetation dynamics in northern Anatolia and surface conditions of the Black Sea between 64 and 20 ka BP. During this period, the dominance of Artemisia in the pollen record indicates a steppe landscape and arid climate conditions. However, the concomitant presence of temperate arboreal pollen suggests the existence of glacial refugia in northern Anatolia. Long-term glacial vegetation dynamics reveal two major arid phases ∼ 64-55 and 40-32 ka BP, and two major humid phases ∼ 54-45 and 28-20 ka BP, correlating with higher and lower summer insolation, respectively. Dansgaard-Oeschger (D-O) cycles are clearly indicated by the 25-GC1 pollen record. Greenland interstadials are characterized by a marked increase in temperate tree pollen, indicating a spread of forests due to warm/wet conditions in northern Anatolia, whereas Greenland stadials reveal cold and arid conditions as indicated by spread of xerophytic biomes. There is evidence for a phase lag of ∼ 500 to 1500 yr between initial warming and forest expansion, possibly due to successive changes in atmospheric circulation in the North Atlantic sector. The dominance of Pyxidinopsis psilata and Spiniferites cruciformis in the dinocyst record indicates brackish Black Sea conditions during the entire glacial period. The decrease of marine indicators (marine dinocysts, acritarchs) at ∼ 54 ka BP and increase of freshwater algae (Pediastrum, Botryococcus) from 32 to 25 ka BP reveals freshening of the Black Sea surface water. This freshening is possibly related to humid phases in the region, to connection between Caspian Sea and Black Sea, to seasonal freshening by floating ice, and/or to closer position of river mouths due to low sea level. In the southern Black Sea, Greenland interstadials are clearly indicated by high dinocyst concentrations and calcium carbonate content, as a result of an increase in primary productivity. Heinrich events show a similar impact on the environment in the northern Anatolia/Black Sea region as Greenland stadials.

A number of short-lasting warm periods (interstadials) interrupted the otherwise cold climate of the last glacial period. These events are supposedly linked to the inflow of the warm Atlantic surface water to the Nordic seas. However, previous investigations of planktonic foraminifera from the Nordic seas have not been able to resolve any significant difference between the interstadials and intervening cold stadials, as the faunas are continuously dominated by the polar species Neogloboquadrina pachyderma s. Here we examine the planktonic foraminifera assemblages from a high-resolution core, LINK17, taken at 1500 m water depth off northern Scotland below the warmest part of the inflowing Atlantic water. The core comprises the time period 34-10 calibrated ka B.P., the coldest period of the last glaciation and the deglaciation. The results reveal a hitherto unknown faunistic variability indicating significant fluctuations in both surface water inflow and in summer sea surface temperatures. During the interstadials, relatively warm Atlantic surface water (4-7°C) flowed north into the eastern Norwegian Sea. During the stadials and Heinrich events the surface inflow stopped and the temperatures in the study area dropped to <2°C. The Last Glacial Maximum was nearly as warm as the interstadials, but the inflow was much more unstable. The data reveal two previously unrecognized warming events each lasting more than 1600 years and preceding Heinrich events HE3 and HE2, respectively. By destabilizing the ice sheets on the shelves the warmings may have played a crucial role for the development of Heinrich events HE2 and HE3.

The issue of diagenetic alteration of carbonate deposits in caves (speleothems) has gained increasing importance in recent years, as this process has serious consequences for speleothembased paleoclimate studies. In this study stable hydrogen and oxygen isotope data of water trapped in fluid inclusions were collected for recently forming stalagmites and flowstones in order to determine how dripwater compositions are reflected and preserved in the inclusion water compositions. Hydrogen isotope compositions were found to reflect dripwater values, whereas the oxygen isotope data were increasingly shifted from the local dripwater compositions with the time elapsed after deposition. The δ 18 O data are correlated with X-Ray diffraction full width at half maximum values (related to crystal domain size and lattice strain), suggesting that the oxygen isotope shift is related to recrystallization of calcite. Transmission electron microscope analyses detected the presence of nanocrystalline (<50 nm) calcite, whose crystallization to coarser-grained calcite crystals (>200 nm) may have induced re-equilibration between the carbonate and the trapped inclusion water. The Ostwald ripening process provides an explanation for unexpectedly low oxygen isotope compositions in the inclusion water. The detected diagenetic alteration and its isotopic effects should be taken into consideration during sampling strategies and data evaluation as speleothems containing nanocrystalline calcite during their deposition are prone to late-stage oxygen isotope water-carbonate re-equilibration, which may shift the oxygen isotope composition of the inclusion water to more depleted values while the hydrogen isotope composition remains intact.

The past 3.2 Myr have seen drastic climate changes with the development, waxing and waning of huge continental ice sheets over the Northern Hemisphere. These striking phenomena have been observed in various records from ice cores, as well as marine and terrestrial sediments. These proxy records showed periodicities associated with the three orbital parameters that affect our planet’s insolation, namely eccentricity, obliquity and precession. Until recently, these periodicities were considered as the canonical ones for the Quaternary Period and beyond. However, the improvement of the time resolution of available records has allowed one to describe climate changes occurring abruptly and with periodicities that are not related to those of the orbital parameters. In this paper, we show that, in fact, these abrupt climate changes may still be related, albeit indirectly, to the astronomical theory of climate.

High-resolution pollen and dinoflagellate cyst records from sediment core M72/5-25-GC1 were used to reconstruct vegetation dynamics in northern Anatolia and surface conditions of the Black Sea between 64 and 20 ka BP. During this period, the dominance of Artemisia in the pollen record indicates a steppe landscape and arid climate conditions. However, the concomitant presence of temperate arboreal pollen suggests the existence of glacial refugia in northern Anatolia. Long-term glacial vegetation dynamics reveal two major arid phases ∼ 64-55 and 40-32 ka BP, and two major humid phases ∼ 54-45 and 28-20 ka BP, correlating with higher and lower summer insolation, respectively. Dansgaard-Oeschger (D-O) cycles are clearly indicated by the 25-GC1 pollen record. Greenland interstadials are characterized by a marked increase in temperate tree pollen, indicating a spread of forests due to warm/wet conditions in northern Anatolia, whereas Greenland stadials reveal cold and arid conditions as indicated by spread of xerophytic biomes. There is evidence for a phase lag of ∼ 500 to 1500 yr between initial warming and forest expansion, possibly due to successive changes in atmospheric circulation in the North Atlantic sector. The dominance of Pyxidinopsis psilata and Spiniferites cruciformis in the dinocyst record indicates brackish Black Sea conditions during the entire glacial period. The decrease of marine indicators (marine dinocysts, acritarchs) at ∼ 54 ka BP and increase of freshwater algae (Pediastrum, Botryococcus) from 32 to 25 ka BP reveals freshening of the Black Sea surface water. This freshening is possibly related to humid phases in the region, to connection between Caspian Sea and Black Sea, to seasonal freshening by floating ice, and/or to closer position of river mouths due to low sea level. In the southern Black Sea, Greenland interstadials are clearly indicated by high dinocyst concentrations and calcium carbonate content, as a result of an increase in primary productivity. Heinrich events show a similar impact on the environment in the northern Anatolia/Black Sea region as Greenland stadials.

The early Eocene (∼ 55 Ma) was the warmest period of the Cenozoic and was most likely characterized by extremely high atmospheric CO 2 concentrations. Here, we analyze simulations of the early Eocene performed with the IPSL-CM5A2 Earth system model, set up with paleogeographic reconstructions of this period from the DeepMIP project and with different levels of atmospheric CO 2. When compared with proxy-based reconstructions, the simulations reasonably capture both the reconstructed amplitude and pattern of early Eocene sea surface temperature. A comparison with simulations of modern conditions allows us to explore the changes in ocean circulation and the resulting ocean meridional heat transport. At a CO 2 level of 840 ppm, the early Eocene simulation is characterized by a strong abyssal overturning circulation in the Southern Hemisphere (40 Sv at 60 • S), fed by deepwater formation in the three sectors of the Southern Ocean. Deep convection in the Southern Ocean is favored by the closed Drake and Tasmanian passages, which provide western boundaries for the buildup of strong subpolar gyres in the Weddell and Ross seas, in the middle of which convection develops. The strong overturning circulation, associated with subpolar gyres, sustains the poleward advection of saline subtropical water to the convective regions in the Southern Ocean, thereby maintaining deepwater formation. This salt-advection feedback mechanism is akin to that responsible for the present-day North Atlantic overturning circulation. The strong abyssal overturning circulation in the 55 Ma simulations primarily results in an enhanced poleward ocean heat transport by 0.3-0.7 PW in the Southern Hemisphere compared to modern conditions, reaching 1.7 PW southward at 20 • S, and contributes to keeping the Southern Ocean and Antarctica warm in the Eocene. Simulations with different atmospheric CO 2 levels show that ocean circulation and heat transport are relatively insensitive to CO 2 doubling.

Centennial-scale mineral dust peaks in last glacial Greenland ice cores match the timing of lowest Greenland temperatures, yet little is known of equivalent changes in dust-emitting regions, limiting our understanding of dust-climate interaction. Here, we present the most detailed and precise age model for European loess dust deposits to date, based on 125 accelerator mass spectrometry 14C ages from Dunaszekcső, Hungary. The record shows that variations in glacial dust deposition variability on centennial-millennial timescales in east central Europe and Greenland were synchronous within uncertainty. We suggest that precipitation and atmospheric circulation changes were likely the major influences on European glacial dust activity and propose that European dust emissions were modulated by dominant phases of the North Atlantic Oscillation, which had a major influence on vegetation and local climate of European dust source regions.

The oxygen isotope composition of speleothems is a widely used proxy for past climate change. Robust use of this proxy depends on understanding the relationship between precipitation and cave drip water δ 18 O. Here, we present the first global analysis, based on data from 163 drip sites, from 39 caves on five continents, showing that drip water δ 18 O is most similar to the amount-weighted precipitation δ 18 O where mean annual temperature (MAT) is < 10°C. By contrast, for seasonal climates with MAT > 10°C and < 16°C, drip water δ 18 O records the recharge-weighted δ 18 O. This implies that the δ 18 O of speleothems (formed in near isotopic equilibrium) are most likely to directly reflect meteoric precipitation in cool climates only. In warmer and drier environments, speleothems will have a seasonal bias toward the precipitation δ 18 O of recharge periods and, in some cases, the extent of evaporative fractionation of stored karst water.

We present a meta-analysis of data from 22 caves and 96 drip sites from 4 continents where both the cave drip water d18O and the weighted mean d18O of precipitation have been measured. Drip water d18O is similar to the weighted mean d18O of precipitation (within ± 0.3 ‰) for sites where mean annual temperature (MAT) is less than 15 °C (85% of drips where MAT &lt; 15 °C) and an aridity index (annual precipitation (P) / annual potential evapotranspiration (PET)) is greater than 0.65 (74% of drips at sites where P/PET &gt; 0.65). In contrast, at warmer locations with increased water deficit, drip water d18O deviates from the weighed mean precipitation d18O by +3 ‰ and -1.5 ‰. We argue that this is due to evaporation in the soil and shallow vadose zone (thereby increasing drip water d18O) and lower water storage in the vadose zone, leading to relatively less mixing (thereby increasing the range in drip water d18O to more closely reflect recharge water d18O). Speleothems that have formed...

Paleoecological analysis of over 1500 mammoth remains from the famous non-archaeological Berelyokh site (~13e12 ka BP) has demonstrated that~42% show destructive changes (osteoporosis, osteolysis, osteofibrosis, osteomalacia, articular diseases and others). For the first time, non-closure of cervical vertebral foramina transversaria and loose intra-articular bodies have been recorded in mammoths. The overall pathological picture resembles that of Kashin-Beck (or Urovskaya) disease, the etiology of which is associated with mineral starvation. The alimentary (dietary/nutritional) character of the observed osteodystrophy can be explained by the strong acidification of geochemical landscapes, which is manifested in the territory of Northern Eurasia after 30 ka BP and especially clearly during the Late Glacial (~15e10 ka BP). Thus, the Berelyokh site reflects the terminal stage of the last mass extinction of large mammals.

Ideal cyclone-sensitive speleothems would have the following properties: active at the time of sampling and recording the past few millenia, with fast growth rate, made of clean primary calcite, precipitated without kinetic fractionation, and fed by water with a short residence time allowing for the isotopic signal of short events like cyclones to be transferred but long enough for the drip water to be supersaturated. The screening for this type of speleothem is still in progress and the results presented here are very preliminary. U-Th dating: low U concentration (9-40ng/g; host rock is reef limestone) ; significant detrital content, involving large corrections and large final age uncertainties. Growth rate is highly variable, from ~0.12 mm/yr to ~0.03mm/yr. Tau12A: δ 18 O profile shows some quite large variations (~3‰), with several abrupt events that we will try to identify in other archives. δ 13 C signal amplitude is very large (~-4 to ~-14‰) suggesting the effect of prior calcite precipitation. The δ 18 O signal gets heavier when the δ 13 C does, which could reflect the rainfall amount in this context. Var12C: δ 18 O profile shows values between-3.1 and-6.3‰, with decadal or centennial fluctuations, superimposed on a general trend which is consistent with the one of the δ 13 C profile without being correlated, which implies that calcite precipitation could occur in conditions close to isotopic equilibrium. Once properly dated, this record should provide interesting information about past rainfall regimes on the island. Var12-A: δ 18 O and δ 13 C profiles are very similar, signalling the effect of kinetic fractionation altering the record of the climate signal. Moreover, the δ 13 C values are very high (~+2 to-9‰) compared to the other stalagmites and to what would be expected under such soil. However, if kinetic processes are linked to the variations in drip rate or to the cave hygrometry, they may inform us qualitatively about the effective rainfall through time. In such a case, we could focus on the longer term trends. Rurutu Enregistrement karstique de l'activité cyclonique à Rurutu KR7 « Climate change: The Karst record »-Melbourne, 29 September

Glacial climate was characterised by two types of abrupt events. Greenland ice cores record Dansgaard-Oeschger events, marked by abrupt warming in-between cold, stadial phases. Six of these stadials appear related to major Heinrich events (HEs), identified from ice-rafted debris (IRD) and large excursions in carbon-and oxygen-stable isotopic ratios in North Atlantic deep sea sediments, documenting major ice sheet collapse events. This finding has led to the paradigm that glacial cold events are induced by the response of the Atlantic Meridional Overturning Circulation to such massive freshwater inputs, supported by sensitivity studies conducted with climate models of various complexities. These models also simulate synchronous Greenland temperature and lower-latitude hydrological changes. To investigate the sequence of events between climate changes at low latitudes and in Greenland, we provide here the first 17 O-excess record from a Greenland ice core during Dansgaard-Oeschger events 7 to 13, encompassing H4 and H5. Combined with other ice core proxy records, our new 17 O-excess data set demonstrates that stadials are generally characterised by low 17 O-excess levels compared to interstadials. This can be interpreted as synchronous change of high-latitude temperature and lower-latitude hydrological cycle (relative humidity at the oceanic source of evaporation or change in the water mass trajectory/recharge) and/or an influence of local temperature on 17 O-excess through kinetic effect at snow formation. As an exception from this general pattern, stadial 9 consists of three phases, characterised first by Greenland cooling during 550 ± 60 years (as shown by markers of Greenland temperature δ 18 O and δ 15 N), followed by a specific lower-latitude fingerprint as identified from several proxy records (abrupt decrease in 17 O-excess, increase in CO 2 and methane mixing ratio, heavier δD-CH 4 and δ 18 O atm), lasting 740 ± 60 years, itself ending approximately 390 ± 50 years prior to abrupt Greenland warming. We hypothesise that this lower-latitude signal may be the fingerprint of Heinrich event 4 in Greenland ice cores. The proposed decoupling between stable cold Greenland temperature and low-latitude climate variability identified for stadial 9 provides new targets for benchmarking climate model simulations and testing mechanisms associated with millennial variability. 1 Introduction Glacial climate is characterised by millennial variability, recorded with specific expressions in different archives and at different latitudes (Voelker, 2002; Clement and Peterson, 2008). Greenland ice core records of ice δ 18 O, a qualitative proxy of air temperature, unveiled at high resolution the succession of cold phases (Greenland Stadials, GS) and warm phases (Greenland Interstadials, GI) forming the 25 Dansgaard-Oeschger events (DO) of the last glacial period (NGRIP members, 2004). DO events are recorded through climatic and environmental changes in other North Atlantic/European archives such as speleothems (Genty et al., 2010; Boch et al., 2011), pollen and marine bioindicators from North Atlantic marine cores (e.g. Voelker, 2002; Sánchez Goñi and Harrison, 2010), with interstadials being Published by Copernicus Publications on behalf of the European Geosciences Union.

The 2001 Intergovernmental Panel on Climate Change (IPCC) Working Group I report on the scientific basis of climate change suggested that the Atlantic meridional overturning circulation (AMOC) could weaken over the 21 st century (Houghton et al., 2001). In the following year, 2002, the US National Research Council's report Abrupt Climate Change: Inevitable Surprises? highlighted the North Atlantic circulation as at risk of abrupt change in a warming climate (NRC, 2002). In 2007, the Ocean Research Priorities Plan issued by the US Joint Subcommittee on Ocean Science and Technology (NSTC JSOST, 2007) identified improving understanding of AMOC as a key near-term priority. These reports noted the significant consequences for climate that are associated with AMOC, especially for those regions bordering the North Atlantic, but also further afield. Such reports provided the impetus for the deployment in 2004 of the AMOC observing system at 26.5°N in the Atlantic under the auspices of the joint UK-US Rapid Climate Change program (a.k.a., RAPID) and the formation in 2008 of the US AMOC Program and Science Team. The aim of this AMOC virtual special issue in AGU journals is to review and synthesize what has been learnt about the AMOC since the beginning of the RAPID and US AMOC Programs, in terms of both observations (modern and paleo) and modeling (theoretical and numerical). In addition, the special issue manuscripts identify many remaining gaps and challenges, providing guidance for future studies. The issue consists of 9 papers across three journals:

Quaternary records provide an opportunity to examine the nature of the vegetation and fire responses to rapid past climate changes comparable in velocity and magnitude to those expected in the 21st-century. The best documented examples of rapid climate change in the past are the warming events associated with the Dansgaard–Oeschger (D–O) cycles during the last glacial period, which were sufficiently large to have had a potential feedback through changes in albedo and greenhouse gas emissions on climate. Previous reconstructions of vegetation and fire changes during the D–O cycles used independently constructed age models, making it difficult to compare the changes between different sites and regions. Here, we present the ACER (Abrupt Climate Changes and Environmental Responses) global database, which includes 93 pollen records from the last glacial period (73–15 ka) with a temporal resolution better than 1000 years, 32 of which also provide charcoal records. A harmonized and consist...

Modern rainwater, cave dripwater and cave stalagmite geochemical time series from a cave in Guam (13°38′N, 144°53′E) are used to better understand how changes in cave stalagmite geochemistry relate to aboveground changes in rainfall at a tropical location. A scientific field team based in Guam collects $monthly samples from multiple sites for geochemical analyses at a cave and aboveground rainfall from a nearby location. We compute a transfer function between rainfall amount and oxygen isotopic composition (d 18 O) of a decrease (increase) of 0.94 AE 0.3 m/year for every 1‰ increase (decrease) in rainfall d 18 O, based on data extracted from the International Atomic Energy Agency (IAEA) Global Networks of Isotopes in Precipitation (GNIP) database and from data generated in this study. Dripwater d 18 O and Mg/Ca ratios show annual cyclicity at some, but not all sites, accentuating the complex nature of cave hydrology. A stalagmite d 18 O record for the last $160 years indicates the existence of droughts of decadal length, when rainfall is estimated to be $0.65 AE 0.3 m/year less than average conditions. This estimate of rainfall reduction most likely refers to wet season months, as these months preferentially contribute to groundwater recharge. The proxybased climate record at Guam provides new evidence highlighting how a rainy site in the Western Pacific Warm Pool today can experience considerable changes in rainfall on decadal timescales.

The sequence from Abric Romaní has provided several palaeoecological studies. This paper presents a synthesis of the results yielded by palynology, including the data from the entire non-excavated deposits, and the data obtained through the study of large mammals, micromammals, herpetofauna, and anthracology from the excavated layers. The palaeobotanical studies show that the sequence is mainly related to pine forests, and near or within these dominant forests there were smaller plant formations and/or secondary species with a Mediterranean and/or Atlantic character. Nevertheless, continuous forest did not dominate the entire region. The fauna occupying the territory indicate open spaces in which seasonal grasslands proliferated, dry assemblages in sunny areas and humid ones in mountain shadows. The non-excavated archaeological layers correspond to MIS 4, initially characterized by temperate and humid conditions, and thereafter cool and humid until HS 6 and MIS 3. Afterwards, at the beginning of the MIS 3, the DansgaardeOeschger cycles, w1500 yr rapid and abrupt climatic changes, occurred during the deposition of the archaeological levels O, N, M, L, K and J. Later, these events were interrupted by a cold and dry phase, between ca 49 and 47 ka BP, which affected levels Ja, I and H, culminating with HS 5. Finally, this cold episode was followed by the Hengelo interstadial or DO 12, characterized by a temperate and humid climate identified in levels E and D.

The sequence from Abric Romaní has provided several palaeoecological studies. This paper presents a synthesis of the results yielded by palynology, including the data from the entire non-excavated deposits, and the data obtained through the study of large mammals, micromammals, herpetofauna, and anthracology from the excavated layers. The palaeobotanical studies show that the sequence is mainly related to pine forests, and near or within these dominant forests there were smaller plant formations and/or secondary species with a Mediterranean and/or Atlantic character. Nevertheless, continuous forest did not dominate the entire region. The fauna occupying the territory indicate open spaces in which seasonal grasslands proliferated, dry assemblages in sunny areas and humid ones in mountain shadows. The non-excavated archaeological layers correspond to MIS 4, initially characterized by temperate and humid conditions, and thereafter cool and humid until HS 6 and MIS 3. Afterwards, at the beginning of the MIS 3, the DansgaardeOeschger cycles, w1500 yr rapid and abrupt climatic changes, occurred during the deposition of the archaeological levels O, N, M, L, K and J. Later, these events were interrupted by a cold and dry phase, between ca 49 and 47 ka BP, which affected levels Ja, I and H, culminating with HS 5. Finally, this cold episode was followed by the Hengelo interstadial or DO 12, characterized by a temperate and humid climate identified in levels E and D.

Here we present an overview of speleothem δ 18 O records from South America, which mostly are available in the Speleothem Isotopes Synthesis and Analysis (SISAL_v1) database. South American tropical and subtropical δ 18 O time series are primarily interpreted as being driven by the amount effect and, consequently show the past history of the convection intensity of convergence zones such as the Intertropical Convergence Zone and the South America Monsoon System. We investigate past hydroclimate scenarios in South America related to the South American Monsoon System in three different time scales: Late Pleistocene, Holocene and the last two millennia. The precession driven insolation is the main driver of convective variability over the continent during the last 250 kyrs, including the Holocene period. However a dipole is observed between the west and east portions of the continent. Records located in the central region of Brazil appear to be weakly affected by insolation driven variability and more susceptible to the South Atlantic Convergence Zone. Cold episodic events in Northern Hemisphere increase the activity of the South American Monsoon System on all time scales, in turn increasing rainfall amounts in South America, as was documented during Heinrich events in the late Pleistocene and Bond events in the Holocene, as well as during the Little Ice Age.

In this paper, we describe a new method for quantitative reconstruction of bioclimate using a combination of multi-scale ecological datasets of present-day distributions of species with fossil plant and bird data derived from cave deposits of known age. In the specific case of Gorham's Cave, Gibraltar, we make predictions as to the prevailing climate during the Last Glacial Maximum (LGM). Our results indicate only slight variations compared to present-day climate. The present-day climatic regime is encompassed by a wider regime during the LGM. The importance of caves as climate archives is highlighted and our results additionally give focus to the generally underestimated importance of small-scale refugia during glacials.

The oxygen isotope composition of speleothems is a widely used proxy for past climate change. Robust use of this proxy depends on understanding the relationship between precipitation and cave drip water δ 18 O. Here, we present the first global analysis, based on data from 163 drip sites, from 39 caves on five continents, showing that drip water δ 18 O is most similar to the amount-weighted precipitation δ 18 O where mean annual temperature (MAT) is < 10°C. By contrast, for seasonal climates with MAT > 10°C and < 16°C, drip water δ 18 O records the recharge-weighted δ 18 O. This implies that the δ 18 O of speleothems (formed in near isotopic equilibrium) are most likely to directly reflect meteoric precipitation in cool climates only. In warmer and drier environments, speleothems will have a seasonal bias toward the precipitation δ 18 O of recharge periods and, in some cases, the extent of evaporative fractionation of stored karst water.

Here, we establish a spatiotemporal evolution of the sea-surface temperatures in the North Atlantic over Dansgaard-Oeschger (DO) events 5-8 (approximately 30-40 kyr) using the proxy surrogate reconstruction method. Proxy data suggest a large variability in North Atlantic seasurface temperatures during the DO events of the last glacial period. However, proxy data availability is limited and cannot provide a full spatial picture of the oceanic changes. Therefore, we combine fully coupled, general circulation model simulations with planktic foraminifera based seasurface temperature reconstructions to obtain a broader spatial picture of the ocean state during DO events 5-8. The resulting spatial sea-surface temperature patterns agree over a number of different general circulation models and simulations. We find that sea-surface temperature variability over the DO events is characterized by colder conditions in the subpolar North Atlantic during stadials than during interstadials, and the variability is linked to changes in the Atlantic Meridional Overturning circulation and in the sea-ice cover. Forced simulations are needed to capture the strength of the temperature variability and to reconstruct the variability in other climatic records not directly linked to the seasurface temperature reconstructions. This is the first time the proxy surrogate reconstruction method has been applied to oceanic variability during MIS3. Our results remain robust, even when age uncertainties of proxy data, the number of available temperature reconstructions, and different climate models are considered. However, we also highlight shortcomings of the methodology that should be addressed in future implementations.

The oxygen isotope composition of speleothems is a widely used proxy for past climate change. Robust use of this proxy depends on understanding the relationship between precipitation and cave drip water δ 18 O. Here, we present the first global analysis, based on data from 163 drip sites, from 39 caves on five continents, showing that drip water δ 18 O is most similar to the amount-weighted precipitation δ 18 O where mean annual temperature (MAT) is < 10°C. By contrast, for seasonal climates with MAT > 10°C and < 16°C, drip water δ 18 O records the recharge-weighted δ 18 O. This implies that the δ 18 O of speleothems (formed in near isotopic equilibrium) are most likely to directly reflect meteoric precipitation in cool climates only. In warmer and drier environments, speleothems will have a seasonal bias toward the precipitation δ 18 O of recharge periods and, in some cases, the extent of evaporative fractionation of stored karst water.

Sampling of deposits at the Queens Hotel site, York, produced a substantial number of small terrestrial vertebrate remains from the Anglo-Scandinavian features. By studying bone surface modification, fragmentation and skeletal completeness as taphonomic indicators, it was possible to demonstrate that the assemblage had resulted from two very different modes of accumulation and deposition. Refuse pits situated within the boundaries of the tenements had acted as accumulators of the fragmented and abraded small mammal and amphibian bones that existed as a sub-surface death assemblage within the local environment. In contrast, the excellent preservation and skeletal completeness of numerous frogs recovered from the basal fill of a wooden well could be accounted for by their direct entry into the burial environment as a result of pitfall trapping. This paper also discusses the implications that the temporal and spatial variation in deposition demonstrated by the micro-faunal remains has for the reconstruction of local ecological and environmental conditions within this site, and for other such sites. Copyright

Centennial-scale mineral dust peaks in last glacial Greenland ice cores match the timing of lowest Greenland temperatures, yet little is known of equivalent changes in dust-emitting regions, limiting our understanding of dust-climate interaction. Here, we present the most detailed and precise age model for European loess dust deposits to date, based on 125 accelerator mass spectrometry 14C ages from Dunaszekcső, Hungary. The record shows that variations in glacial dust deposition variability on centennial-millennial timescales in east central Europe and Greenland were synchronous within uncertainty. We suggest that precipitation and atmospheric circulation changes were likely the major influences on European glacial dust activity and propose that European dust emissions were modulated by dominant phases of the North Atlantic Oscillation, which had a major influence on vegetation and local climate of European dust source regions.

The issue of diagenetic alteration of carbonate deposits in caves (speleothems) has gained increasing importance in recent years, as this process has serious consequences for speleothembased paleoclimate studies. In this study stable hydrogen and oxygen isotope data of water trapped in fluid inclusions were collected for recently forming stalagmites and flowstones in order to determine how dripwater compositions are reflected and preserved in the inclusion water compositions. Hydrogen isotope compositions were found to reflect dripwater values, whereas the oxygen isotope data were increasingly shifted from the local dripwater compositions with the time elapsed after deposition. The δ 18 O data are correlated with X-Ray diffraction full width at half maximum values (related to crystal domain size and lattice strain), suggesting that the oxygen isotope shift is related to recrystallization of calcite. Transmission electron microscope analyses detected the presence of nanocrystalline (<50 nm) calcite, whose crystallization to coarser-grained calcite crystals (>200 nm) may have induced re-equilibration between the carbonate and the trapped inclusion water. The Ostwald ripening process provides an explanation for unexpectedly low oxygen isotope compositions in the inclusion water. The detected diagenetic alteration and its isotopic effects should be taken into consideration during sampling strategies and data evaluation as speleothems containing nanocrystalline calcite during their deposition are prone to late-stage oxygen isotope water-carbonate re-equilibration, which may shift the oxygen isotope composition of the inclusion water to more depleted values while the hydrogen isotope composition remains intact.

Centennial-scale mineral dust peaks in last glacial Greenland ice
cores match the timing of lowest Greenland temperatures, yet
little is known of equivalent changes in dust-emitting regions,
limiting our understanding of dust−climate interaction. Here, we
present the most detailed and precise age model for European
loess dust deposits to date, based on 125 accelerator mass spectrometry
14C ages from Dunaszekcs}o, Hungary. The record shows
that variations in glacial dust deposition variability on centennial–
millennial timescales in east central Europe and Greenland were
synchronous within uncertainty. We suggest that precipitation
and atmospheric circulation changes were likely the major influences
on European glacial dust activity and propose that European
dust emissions were modulated by dominant phases of the North
Atlantic Oscillation, which had a major influence on vegetation
and local climate of European dust source regions.

The Late Gravettian site of Kraków Spadzista is important for understanding human behaviour and adaptation in cold, northern and marginal landscapes approaching the coldest part of the last glacial cycle. This paper focuses on the large assemblage of Arctic fox (Vulpes lagopus L.) remains found at the site, and presents new data on the mobility patterns and season of death of the animals killed by Gravettian hunters. Laser ablation strontium isotope analysis of teeth from five individuals indicates that each analysed fox was born and grew up in a different and isotopically distinct location, and had migrated tens or hundreds of kilometres into the vicinity of Kraków Spadzista before being killed by Gravettian hunters. Season of death data gathered from the dental cementum of at least 10 fox individuals demonstrate that the majority were killed in a window between late winter and late spring. Given the predictable nature of seasonal changes in Arctic fox hide quality and bodily fat reserves, we argue that the foxes were most likely killed at the start of this window, i.e. in late winter. The results are interesting for reconstructing the context to human hunting strategies in the Late Gravettian, revealing the choices made by hunters about where and when to procure these small prey.

The sequence from Abric Romaní has provided several palaeoecological studies. This paper presents a synthesis of the results yielded by palynology, including the data from the entire non-excavated deposits, and the data obtained through the study of large mammals, micromammals, herpetofauna, and anthracology from the excavated layers. The palaeobotanical studies show that the sequence is mainly related to pine forests, and near or within these dominant forests there were smaller plant formations and/or secondary species with a Mediterranean and/or Atlantic character. Nevertheless, continuous forest did not dominate the entire region. The fauna occupying the territory indicate open spaces in which seasonal grasslands proliferated, dry assemblages in sunny areas and humid ones in mountain shadows. The non-excavated archaeological layers correspond to MIS 4, initially characterized by temperate and humid conditions, and thereafter cool and humid until HS 6 and MIS 3. Afterwards, at the beginning of the MIS 3, the DansgaardeOeschger cycles, w1500 yr rapid and abrupt climatic changes, occurred during the deposition of the archaeological levels O, N, M, L, K and J. Later, these events were interrupted by a cold and dry phase, between ca 49 and 47 ka BP, which affected levels Ja, I and H, culminating with HS 5. Finally, this cold episode was followed by the Hengelo interstadial or DO 12, characterized by a temperate and humid climate identified in levels E and D.