Ky BP Research Articles

Crustal contamination and crystal entrapment during polybaric magma evolution at Mt. Somma–Vesuvius volcano, Italy: Geochemical and Sr isotope evidence

New major and trace element analyses and Sr-isotope determinations of rocks from Mt. Somma–Vesuvius volcano produced from 25 ky BP to 1944 AD are part of an extensive database documenting the geochemical evolution of this classic region. Volcanic rocks include silica undersaturated, potassic and ultrapotassic lavas and tephras characterized by variable mineralogy and different crystal abundance, as well as by wide ranges of trace element contents and a wide span of initial Sr-isotopic compositions. Both the degree of undersaturation in silica and the crystal content increase through time, being higher in rocks produced after the eruption at 472 AD (Pollena eruption). Compositional variations have been generally thought to reflect contributions from diverse types of mantle and crust. Magma mixing is commonly invoked as a fundamental process affecting the magmas, in addition to crystal fractionation. Our assessment of geochemical and Sr-isotopic data indicates that compositional variability also reflects the influence of crustal contamination during magma evolution during upward migration to shallow crustal levels and/or by entrapment of crystal mush generated during previous magma storage in the crust. Using a variant of the assimilation fractional crystallization model (Energy Conservation–Assimilation Fractional Crystallization; [Spera and Bohrson, 2001. Energy-constrained open-system magmatic processes I: General model and energy-constrained assimilation and fractional crystallization (EC–AFC) formulation. J. Petrol. 999–1018]; [Bohrson, W.A. and Spera, F.J., 2001. Energy-constrained open-system magmatic process II: application of energy-constrained assimilation–fractional crystallization (EC–AFC) model to magmatic systems. J. Petrol. 1019–1041]) we estimated the contributions from the crust and suggest that contamination by carbonate rocks that underlie the volcano (2 km down to 9–10 km) is a fundamental process controlling magma compositions at Mt. Somma–Vesuvius in the last 8 ky BP. Contamination in the mid- to upper crust occurred repeatedly, after the magma chamber waxed with influx of new mantle- and crustal-derived magmas and fluids, and waned as a result of magma withdrawal and production of large and energetic plinian and subplinian eruptions.

Quantitative reconstruction of the last interglacial vegetation and climate based on the pollen record from Lake Baikal, Russia

Changes in mean temperature of the coldest (T c) and warmest month (T w), annual precipitation (P ann) and moisture index (α) were reconstructed from a continuous pollen record from Lake Baikal, Russia. The pollen sequence CON01-603-2 (53°57′N, 108°54′E) was recovered from a 386 m water depth in the Continent Ridge and dated to ca. 130–114.8 ky BP. This time interval covers the complete last interglacial (LI), corresponding to MIS 5e. Results of pollen analysis and pollen-based quantitative biome reconstruction show pronounced changes in the regional vegetation throughout the record. Shrubby tundra covered the area at the beginning of MIS 5e (ca. 130–128 ky), consistent with the end of the Middle Pleistocene glaciation. The late glacial climate was characterised by low winter and summer temperatures (T c ~ −38 to −35°C and T w~11–13°C) and low annual precipitation (P ann~300 mm). However, the wide spread of tundra vegetation suggests rather moist environments associated with low temperatures and evaporation (reconstructed α~1). Tundra was replaced by boreal conifer forest (taiga) by ca. 128 ky BP, suggesting a transition to the interglacial. Taiga-dominant phase lasted until ca. 117.4 ky BP, e.g. about 10 ky. The most favourable climate conditions occurred during the first half of the LI. P ann reached 500 mm soon after 128 ky BP. However, temperature changed more gradually. Maximum values of T c ~ −20°C and T w~16–17°C are reconstructed from about 126 ky BP. Conditions became gradually colder after ca. 121 ky BP. T c dropped to ~ −27°C and T w to ~15°C by 119.5 ky BP. The reconstructed increase in continentality was accompanied by a decrease in P ann to ~400–420 mm. However, the climate was still humid enough (α~0.9) to support growth of boreal evergreen conifers. A sharp turn towards a dry climate is reconstructed after ca. 118 ky BP, causing retreat of forest and spread of cool grass-shrub communities. Cool steppe dominated the vegetation in the area between ca. 117.5 ky and 114.8 ky BP, suggesting the end of the interglacial and transition to the last glacial (MIS 5d). Shift to the new glaciation was characterised by cool and very dry conditions with T c ~ −28 to −30°C, T w~14–15°C, P ann~250 mm and α~0.5.

Characterization of Expandable Clay Minerals in Lake Baikal Sediments by Thermal Dehydration and Cation Exchange

AbstractThe sedimentary series from Academician Ridge, Lake Baikal, eastern Siberia, was examined using cation exchange capacity (CEC) to estimate the amount of expandable clay minerals (ECM) and high-temperature X-ray diffraction (HT-XRD) to determine their basic classification. The comparison of the magnetic susceptibility (MS) at sub-millennial resolution and the δl8O record of a reference Atlantic core (ODP 980) was used to create an age model. The most closely studied part of the series covered the major part of the last glacial cycle (120–20 ky BP). The HT-XRD analysis is based on monitoring the course of ECM dehydration with 5°C steps between 25 and 250°C and enabled us to improve the discrimination between ECM, chlorite and micas. The CEC obtained at millennial resolution showed that the neoformation of ECM in warmer periods of the last interglacial was either insignificant or fully compensated by their dissolution or dilution. The CEC record was correlated with the main climatic stages in the period studied. Both MS and CEC records reflected the environmental changes at about millennial resolution, including climatic instabilities between 117 and 73 ky BP (late MIS5).

The impact of precession changes on the Arctic climate during the last interglacial–glacial transition

Three sensitivity experiments using an Ocean Atmosphere General Circulation Model (OAGCM) are conducted to simulate the climate impact of precession. The relative contributions of components of the hydrological cycle including the albedo of Arctic sea ice, advection of atmospheric water vapor and sea surface temperature to the summer Arctic melt process are evaluated. Timing of the perihelion is varied in each experiment with meteorological spring (SP), winter (WP) and autumn (AP) perihelion corresponding to conditions at 110, 115 and 120 ky BP, respectively. Obliquity is unchanged at the 115 ky level which is lower than today. The experiments are assessed relative to the present day control, which has been shown to simulate current conditions based on observations. In the SP experiment, top of the atmosphere (TOA) insolation is weaker than today between the summer solstice and autumnal equinox. In the AP case representing the interglacial, it is less intense between vernal equinox and summer solstice but stronger during the remainder of the year. Although the incident solar radiation is reduced in summer in the SP experiment, increased melting of snow is found primarily as a result of feedbacks from the delayed seasonal cycle of hydrologic components. This is in contrast to both the WP and AP cases in which the perennial snow cover is simulated. At the time of the last glacial inception, 115 ky BP, the WP experiment shows lower insolation to the high northern latitudes in late spring and summer mainly as a result of lower obliquity than today. Dynamical ocean–atmosphere interactions in response to precession maintain the reduced sea ice melting in late spring, strengthen the annual equator-to-pole sea surface temperature (SST) gradient and increase atmospheric moisture convergence in glaciation-sensitive regions. In both the WP and AP experiments seasonal sea ice melting is weakened resulting in pronounced outgoing radiative flux at the locations of expanded sea ice. This leads to further cooling and increased snowfall due to the reduced atmospheric water holding capacity and increased atmospheric moisture convergence from the subtropical Atlantic. In agreement with Milankovitch theory, our results show favorable conditions for glacial inception at 115 ky BP but with obliquity unchanged, they also show perennial snow cover at 120 ky BP resulting from the reduced strength of spring insolation.

Palaeovegetational and palaeoenvironmental trends in the summit of the Guaiquinima massif (Venezuelan Guayana) during the Holocene

AbstractThe summits of the table mountains (tepuis) from the Neotropical Guayana region are remote environments suitable for palaeoecological studies with evolutionary, biogeographical and palaeoclimatic implications. Here, using palynological analyses of two radiocarbon‐dated peat bogs from a tepui summit, the Holocene palaeovegetational trends are reconstructed, and related to possible forcing factors. Because of the pristine character of the Guaiquinima summit, the recorded palaeoenvironmental changes are probably due to natural causes, which makes them valuable archives of the natural component of climatic change at a millennial time scale. The sequence begins with pioneer communities or meadows similar to present‐day ones, between about 8.4 and 4.5 ky BP. After this date, and until about 2 kyr BP the expansion of gallery forests suggests an increase in precipitation, documented also at regional (Neotropical) level. Between ca. 2 kyr BP and the last century, gallery forests are replaced by forests characteristic of the upper Guaiquinima altitudes, coinciding with a regional phase of reduced moisture. The present‐day meadows, established relatively quickly during the last century, substituted the former upland forests. In the locality studied, the main controlling factor of the vegetation during the Holocene seems to have been the moisture balance. In contrast to other tepui summits, there is no clear evidence for changes linked to temperature oscillations. This could be due to the elevation of the site, far from any characteristic ecological boundary, that makes it insensitive to this parameter. Copyright © 2005 John Wiley & Sons, Ltd.

Vegetation and climate variability during the Last Interglacial evidenced in the pollen record from Lake Baikal

A pollen record from the core sediments collected in the northern part of Lake Baikal represents the latest stage of the Taz (Saale) Glaciation, Kazantsevo (Eemian) Interglacial (namely the Last Interglacial), and the earliest stage of the Zyryanka (Weichselian) Glaciation. According to the palaeomagnetic-based age model applied to the core, the Last Interglacial in the Lake Baikal record lasted about 10.6 ky from 128 to 117.4 ky BP, being more or less synchronous with the Marine Isotope Stage 5e. The reconstructed changes in the south Siberian vegetation and climate are summarised as follows: a major spread of shrub alder ( Alnus fruticosa) and shrub birches ( Betula sect. Nanae/ Fruticosae) in the study area was a characteristic feature during the late glacial phase of the Taz Glaciation. Boreal trees e.g. spruce ( Picea obovata) and birch ( Betula sect. Albae) started to play an important role in the regional vegetation with the onset of the interglacial conditions. Optimal conditions for Abies sibirica– P. obovata taiga development occurred ca. 126.3 ky BP. The maximum spread of birch forest-steppe communities took place at the low altitudes ca. 126.5–125.5 ky BP and Pinus sylvestris started to form forests in the northern Baikal area after ca. 124.4 ky BP. Re-expansion of the steppe communities, as well as shrubby alder and willow communities and the disappearance of forest vegetation occurred at about 117.4 ky BP, suggesting the end of the interglacial succession. The changes in the pollen assemblages recorded in the sediments from northern Baikal point to a certain instability of the interglacial climate. Three phases of climate deterioration have been distinguished: 126–125.5, 121.5–120, and 119.5–119 ky BP. The penultimate cooling signal may be correlated with the cool oscillation recorded in European pollen records. However, such far distant correlation requires more careful investigation.

A diatom record of centennial resolution for the Kazantsevo Interglacial stage in Lake Baikal (Siberia)

Diatom analysis of a sediment core from the Northern basin of Lake Baikal (Siberia) has been carried out with focus on the Kazantsevo Interglacial period (equivalent to the Eemian in Europe). The age model, based on palaeomagnetism, indicates that this stage lasted about 11,000 years in the region of Lake Baikal, from ca. 128 to 117 ky BP. Diatom data (sampling resolution of 1-cm interval, ca. 130 years), including both relative percentage and biovolume accumulation rate (BVAR) data, are used to infer past limnological conditions and the climatic factors that drove them. Our record reveals that peak interglacial conditions, characterized by highest diatom production, occurred between ∼124 and 120 ky BP. During that period, the diatom record suggests stable, warm and moist conditions. This climatic optimum is then interrupted by a low amplitude cold event at ∼120 ky BP. The last part of the interglacial in Lake Baikal, between ∼120 and ∼117 ky BP, is marked by colder and probably drier climate. Low-resolution diatom analysis spanning the Kazantsevo was also carried out on another core taken further south at Academician Ridge. Diatom successions at the two sites investigated were remarkably similar suggesting that our sedimentary records are complete and representative for at least a large part of Lake Baikal.

Mid–late Holocene monsoon climate retrieved from seasonal Sr/Ca and δ18O records of Porites lutea corals at Leizhou Peninsula, northern coast of South China Sea

South China Sea (SCS) is a major moisture source region, providing summer monsoon rainfall throughout Mainland China, which accounts for more than 80% total precipitation in the region. We report seasonal to monthly resolution Sr/Ca and δ 18O data for five Holocene and one modern Porites corals, each covering a growth history of 9–13 years. The results reveal a general decreasing trend in sea surface temperature (SST) in the SCS from ∼6800 to 1500 years ago, despite shorter climatic cycles. Compared with the mean Sr/Ca–SST in the 1990s (24.8 °C), 10-year mean Sr/Ca–SSTs were 0.9–0.5 °C higher between 6.8 and 5.0 thousand years before present (ky BP), dropped to the present level by ∼2.5 ky BP, and reached a low of 22.6 °C (2.2 °C lower) by ∼1.5 ky BP. The summer Sr/Ca–SST maxima, which are more reliable due to faster summer-time growth rates and higher sampling resolution, follow the same trend, i.e. being 1–2 °C higher between 6.8 and 5.0 ky BP, dropping to the present level by ∼2.5 ky BP, and reaching a low of 28.7 °C (0.7 °C lower) by ∼1.5 ky BP. Such a decline in SST is accompanied by a similar decrease in the amount of monsoon moisture transported out of South China Sea, resulting in a general decrease in the seawater δ 18O values, reflected by offsets of mean δ 18O relative to that in the 1990s. This observation is consistent with general weakening of the East Asian summer monsoon since early Holocene, in response to a continuous decline in solar radiation, which was also found in pollen, lake-level and loess/paleosol records throughout Mainland China. The climatic conditions ∼2.5 and ∼1.5 ky ago were also recorded in Chinese history. In contrast with the general cooling trend of the monsoon climate in East Asia, SST increased dramatically in recent time, with that in the 1990s being 2.2 °C warmer than that ∼1.5 ky ago. This clearly indicates that the increase in the concentration of anthropogenic greenhouse gases played a dominant role in recent global warming, which reversed the natural climatic trend in East Asian monsoon regime.

A relic coral fauna threatened by global changes and human activities, Eastern Brazil

Coral species composition of drilled cores from emergent bank reefs, and coral cover of the surface of old and living reefs located along the coast of the state of Bahia, Eastern Brazil, revealed that there is a marked change in the occurrence of the major building coral species in different time intervals of the reef structure, as well as in the living surface of reefs located in two different geographical sites. Holocene core sections from two reef areas (12° 40′S–38° 00′W and 18° 00′S–39° 00′W) have as major reef builders, on its topmost core interval (3 to 4 ky old), the endemic coral Mussismilia braziliensis Verrill, 1868, which also dominate on the 2.5–3.5 ky old surfaces of truncated reef tops. At the base of the cores (the 2 m lower interval, older than 4 ky BP), another endemic coral Mussismilia harttii Verrill, 1868 is the dominant reef component. The relative abundance of M. braziliensis on the living surfaces of shallow reefs from both areas, shows that in the southern area, it is up to 98% on reefs located 60 km off the coast, in depths between 3 and 4 m, but do not exceed 1.3% on the surface of the northern reefs located 1–2 km off the coast in depths 4–5 m. The Holocene falling sea level that occurred along the coast of Brazil since 5.1 ky BP, causes an increasing runoff into the area of coastal reefs. This phenomenon may have affected the nearshore reef building fauna, replacing a more susceptive coral fauna with one better adapted to low light levels and higher sediment influx. The high turbidity associated with early Holocene shelf flooding, should also be responsible for the absence of M. braziliensis during the initial stages of reef buildup in Brazil. At the present time, the rapidly increasing human pressure, due to changes in land uses of the coastal zone (increasing sedimentation rate, nutrification of coastal waters, industrial pollution) and underwater practices, such as overfishing and an intense tourism, is aggravating the recovery capacity of this already naturally threatened coral community. If this situation coupled with increasing sea surface temperature persists, modern coral reef growth, in Brazil cannot be maintained and the major reef building coral species of the reefs in Bahia, a remnant endemic coral fauna will very soon appear in the list of endangered species.

Lateglacial and Holocene water-level variations along the NE German Baltic Sea coast: review and new results

A comprehensive overview is given of the current knowledge regarding the Lateglacial and Holocene water-table evolution along the West Pomeranian coast, which is related to the four evolutionary stages of the Baltic Sea: the Baltic Ice Lake, the Yoldia Sea, the Ancylus Lake and the Littorina Sea. The first three stages were characterized by intensive water-table fluctuations in the Baltic basin. The balance between glacio-isostatic uplift and water-table rise due to meltwater and river discharge produced either a temporary connection with the ocean or isolation from it. New findings are presented on the water levels reached during the different stages. Only at about 8 ky BP did the Baltic obtain a permanent connection with the ocean. The first marine sediments in the recent coastal zone, ca. 7.8 ky old, are found at −15 m and are related to the initial Littorina Sea. The maximum of the main Littorina-transgression was reached at −1.5 m to −2.0 m ca. 5.8 ky BP. The course of the sea level reveals that later only some minor oscillations took place, correlated with climatic fluctuations. These indicate a control by regional changes in the North Atlantic circulation rather than by global sea-level alterations. Isostatic movements were determined by comparing the reconstructed relative sea-level curve with curves assumed as nearly eustatic. The resulting isostatic curve shows uplift at 11.5 ky BP of about 55 m, which decreased until 7.8 ky BP to 6 m and ceased at ca. 3 ky BP. Currently, the coastal zone experiences a slight tilting with uplift in the northeastern part and inundation in the southwestern area.

Man and woolly mammoth at the Kraków Spadzista Street (B) – taphonomy of the site

Excavations at the Kraków Spadzista Street (B) site, Poland were conducted over 16 years in which approximately 150 m 2 of the site was studied. The radiocarbon dates clustered around 23–24 ky BP, placing the site in the Gravettian cultural complex. In this paper, results from a detailed taphonomic study of faunal remains are discussed in conjunction with previous analyses of stone artefacts and site structure in order to address questions regarding site formation and Gravettian occupation. Up to 1994, ∼9000 bone remains were recovered and it was possible to identify the species and skeletal element for 5860 remains. The site contained bones of seven animal species associated with Pleistocene steppe–tundra habitats. At Kraków Spadzista Street (B), 99% of the faunal remains belong to the woolly mammoth, with other large mammal taxa represented by only isolated bones and teeth. This site yielded the largest number of mammoth bones and most individuals (MNI = 86) ever found in one place in Poland. Kraków Spadzista Street (B) represents a mammoth butchering locality and probably a mammoth hunting site as well. However, it is not yet possible to distinguish the mammoths killed by Gravettian hunters from those that died naturally and then were scavenged by people.

A biomass burning record from the West Equatorial Pacific over the last 360 ky: methodological, climatic and anthropic implications

To reconstruct past fire activity in the West Pacific–East Asian region, we investigate a 360-ky pelagic sedimentary record from the Caroline Basin in the Western Pacific Warm Pool (WPWP) (core MD97-2140, 2547 m. water depth). In the lack of standard protocol and terminology for the determination of combustion-derived products in sedimentary archives, three proxies were compared with other paleoceanographic proxies: Oxidation Resistant Elemental Carbon (OREC), Black Carbon (BC), and microcharcoal. The mineral (CaCO3) and organic carbon (Corg) records of core MD97-2140 generally covary with the oxygen isotope record, suggesting that glacial/interglacial changes in ice-volume and sea-level control the preservation of CaCO3 and terrigeneous inputs of Corg in the deep-sea sediments of the Caroline Basin. The changes in OREC are primarily connected to the changes in Corg record. In contrast, the BC and charcoal records, which better reflect the input of biomass burning products, are tightly connected with changes in the precession band (23 ky), likely through low-latitude atmospheric circulation patterns. A peculiar control by the competing influence of the long-term El Niño-Southern Oscillation (ENSO)-like forcing and the glacial/interglacial cycle on the East Asian summer monsoon (30-, 19-, 11- and 6-ky periods) is suggested. In addition, large increases in BC and charcoal are observed between ca. 53–43 and 12–10 ky BP. These events strikingly correspond to the main Late Pleistocene and Early Holocene periods of human colonization in the region, likely suggesting an additional anthropic impact on the fire activity.

ESR dating at K and X band of northeastern Brazilian megafauna

The archaeological dose (AD) was measured in three tooth samples of giant mammals that belonged to Brazilian megafauna using electron spin resonance (ESR) spectroscopy at X-band (ν ∼9.5 GHz) and K-band (ν ∼24 GHz). Samples were collected in Lagoa de Dentro, Puxinanã city in Paraiba, a northeast state in Brazil and were identified as Haplomastodon waringi (Holland) (two teeth) and one tooth sample of Xenorhinotherium bahiense (Cartele and Lessa). The average AD led to an age for the Haplomastodon samples of 11.6 ky bp. For one sample ( Haplomastodon) K-band was also employed to evaluate the AD. The K-band spectrum had three components, determined using spectral simulation as follows: a wide isotropic line with g factor 2.0048, an orthorhombic line with g x = 2.0034 , g y = 2.0022 and g z = 1.9974 , and another isotropic line with g factor 2.0008. The amplitude of these three signals increase with the added dose and the average dose found was 26±5 Gy. This result is compatible with the AD determined with X-band 21±3 Gy.

A biomass burning record from the West Equatorial Pacific over the last 360 ky: methodological, climatic and anthropic implications

To reconstruct past fire activity in the West Pacific–East Asian region, we investigate a 360-ky pelagic sedimentary record from the Caroline Basin in the Western Pacific Warm Pool (WPWP) (core MD97-2140, 2547 m. water depth). In the lack of standard protocol and terminology for the determination of combustion-derived products in sedimentary archives, three proxies were compared with other paleoceanographic proxies: Oxidation Resistant Elemental Carbon (OREC), Black Carbon (BC), and microcharcoal. The mineral (CaCO 3) and organic carbon (Corg) records of core MD97-2140 generally covary with the oxygen isotope record, suggesting that glacial/interglacial changes in ice-volume and sea-level control the preservation of CaCO 3 and terrigeneous inputs of Corg in the deep-sea sediments of the Caroline Basin. The changes in OREC are primarily connected to the changes in Corg record. In contrast, the BC and charcoal records, which better reflect the input of biomass burning products, are tightly connected with changes in the precession band (23 ky), likely through low-latitude atmospheric circulation patterns. A peculiar control by the competing influence of the long-term El Niño-Southern Oscillation (ENSO)-like forcing and the glacial/interglacial cycle on the East Asian summer monsoon (30-, 19-, 11- and 6-ky periods) is suggested. In addition, large increases in BC and charcoal are observed between ca. 53–43 and 12–10 ky BP. These events strikingly correspond to the main Late Pleistocene and Early Holocene periods of human colonization in the region, likely suggesting an additional anthropic impact on the fire activity.

Tracking Holocene environmental changes in an alpine lake sediment core: application of regional diatom calibration, geochemistry, and pollen

Diatom abundances in the surface sediment samples of 41 mountain lakes in the central Austrian Alps (Niedere Tauern) were related to environmental variables using multi-variate techniques. Canonical correspondence analysis (CCA) revealed that the pH, date of autumn mixing (A mix), mean August water temperature (T Aug), dissolved organic carbon (DOC), and relative water depth (Z rel) made significant contributions to explain the diatom assemblage variation in the lakes of the training set. A weighted averaging partial least square regression and calibration model was used to establish Di-pH (R 2 boot= 0.72, RMSEPboot= 0.131), and a thermistor measurements-based PLS model for A mix (R 2 boot= 0.71, RMSEPboot= 0.006 log10 Julian days). The latter showed a better prediction than T Aug, and was used in terms of climate change. These transfer functions, together with analyses of loss on ignition (LOI), the total carbon/nitrogen (C/N)-ratios, and selected pollen, were applied to an early to mid-Holocene (11.5–4 cal. ky BP) sediment core section from an Austrian Alpine treeline lake on crystalline bedrock. Additionally, passive sample scores in the CCA of the diatom training set were used to show trends in the variables DOC and Z rel. During the early Holocene, diatoms indicative of increased pH, extended warm summers, and low water levels dominated. Between 10.2 and 7.6 cal. ky BP it was followed by diatom assemblages that indicated an increase in lake water depth and an earlier A mix. The multi-proxy data suggest that the A mix decline is the result of a series of snow-rich summer cool and wet climate fluctuations, which were divided by climate warming at ∼9 cal. ky BP. Increased A mix, LOI and DOC, and the correspondent decline in the C/N-ratios, show subsequent climate warming between 7.3 and 6 cal. ky BP. The long-term trend in Di-pH indicates the impact of catchment-related processes during the early-Holocene, that were superimposed by climate.

Quantification of rapid temperature change during DO event 12 and phasing with methane inferred from air isotopic measurements

The description of rapid climatic changes during the last glacial period at high northern latitudes has been largely documented through Greenland ice cores that are unique climatic and environmental records. However, Greenland ice isotopic records are biased temperature proxies and it is still a matter of debate whether changes in the high latitudes lead or lag rapid changes elsewhere. We focus here on the study of the mid-glacial Dansgaard Oeschger event 12 (45 ky BP) associated to a large δ 18O ice change in the GRIP (GReenland Ice core Project) ice core. We use combined measurements of CH 4, δ 15N and δ 40Ar in entrapped air associated with a recently developed firn densification and heat diffusion model to infer (i) the phasing between methane and temperature increases and (ii) the amplitude of the temperature change. Our method enables us to overcome the difficulty linked with rapid accumulation change in quantifying the temperature change. We obtain a 12±2.5 °C temperature increase at the beginning of DO event 12 thus confirming that the conventional use of water isotopes in the Greenland ice cores largely underestimates the actual amplitude of rapid temperature change in central Greenland. In agreement with previous studies, methane and temperature increase are in phase at the sampling resolution of that part of our profile (90 years).

Late Pleistocene–Holocene paleoclimate and related paleoenvironmental changes as recorded by calcareous nannofossils and planktonic foraminifera assemblages in the southern Tyrrhenian Sea (Cape Palinuro, Italy)

A quantitative analysis of calcareous nannofossils and planktonic foraminifera assemblages and geochemical studies were carried out on core C18 (40°00′55ʺN, 15°09′22ʺE), recovered near Cape Palinuro (southern Tyrrhenian Sea, Italy). Four radiocarbon ages place the core in the time interval 32–0 ky Bp. Coccolithus pelagicus, Gephyrocapsa caribbeanica and Gephyrocapsa müllerae dominated the coccolith assemblages during the glacial period, while foraminifera assemblages show high percentages of Turborotalita quinqueloba and Globorotalia scitula. Calcidiscus leptoporus and Gephyrocapsa oceanica seem to be controlled by nutrient availability; moreover, low percentages of iron seem to influence the distribution of G. oceanica. Low nutrient availability seems to overlap the climatic signal in the distribution of Syracosphaera pulchra. An increase in Umbilicosphaera sibogae and Rhabdosphaera claviger, the occurrence of tropical taxa and an increase in sub tropical and temperate species, among the planktonic foraminifera, characterise the Holocene assemblages. During this period, Globorotalia truncatulinoides reached its highest percentages, testifying to the onset of deep winter convection and vertical mixing that characterise the modern Tyrrhenian Sea.

Surface water conditions in the Northern Benguela Region (SE Atlantic) during the last 450 ky reconstructed from assemblages of planktonic foraminifera

Planktonic foraminiferal records from Site 1083 (ODP Leg 175) were used to investigate changes in surface water conditions in the Northern Benguela Region over the past 450 ky. The assemblages of planktonic foraminifera are dominated by four species: sinistral coiling Neogloboquadrina pachyderma, dextral coiling N. pachyderma, Globigerina bulloides and Globorotalia inflata. Besides, tropical species deliver a small contribution to the assemblage. The most prominent temporal variations, displayed by N. pachyderma (s+d), represent changes in the coastal upwelling and the presence of cold, nutrient rich waters over the core site. Neogloboquadrina pachyderma (s+d) shows cyclic variability in the eccentricity and, to a less extent, precession frequencies. The changes indicate increased upwelling intensity in glacial maxima and precession maxima, and correlate well with the wind-strength record of Stuut et al. (2002). During glacial maxima, steep temperature gradients over the Southern Hemisphere caused strong SE trade winds and strong upwelling. Precession maxima cause a weak monsoonal circulation, more zonal SE trade winds, strong coastal upwelling, and nutrient-rich surface waters over ODP Site 1083. Advection of Angola Current (AC) surface water into the Walvis Basin, indicated by the tropical species, occurs when the Angola Benguela Front (ABF) is positioned southward. Occasionally, this happened during glacial maxima, as can be explained with the reconstructed and predicted meridional movements of the ABF. The amount of AC water was never sufficient to suppress the marine biological production at the core site. The contribution of Benguela Current (BC) water, reflected by Globorotalia inflata, is greatly determined by the upwelling. In periods of strong upwelling, the BC influence is suppressed. In several glacial substages, the temperature of the upwelling South Atlantic Central Water (SACW) may have been increased, as suggested by the dominance of Neogloboquadrina pachyderma (d) in the upwelling record. This phenomenon may be due to intensified subduction in the central South Atlantic that induces the formation of SACW, or to larger contributions of Eastern SACW to the upwelling water. Around 250–200 ky BP, a long-term shift to higher productivity occurred that is absent in the upwelling record. It was accompanied with a transition from a precession and obliquity variability to an eccentricity dominated variability in the Globorotalia inflata (BC) record. The shift was probably connected to a long-term southward shift of the circumpolar oceanic frontal systems south of the African continent.

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.

Holocene development of the Belize Barrier Reef

Previously, knowledge of the Holocene development of the Belize Barrier Reef (BBR)—the largest reef system in the Atlantic Ocean—was limited to one location (Carrie Bow Cay). We present new data from 11 rotary drill cores taken at 9 locations and 36 radiometric ages that indicate that the BBR was established from >8.26 to 6.68 ky BP on Pleistocene reef limestones, presumably deposited during oxygen isotope stage 5. The nonsynchronous start of Holocene reef growth was a consequence of variation in elevation of antecedent topography, largely controlled by underlying NNE-trending structures. From north to south, Pleistocene elevation decreases along these structural trends, probably reflecting differential subsidence and variations in karst topography. Reef anatomy is characterized by three facies. In order of decreasing abundance, these facies are represented by corals (mainly Acropora palmata and members of the Montastraea annularis group), by unconsolidated sand and rubble, and by well-cemented coral grainstones-rudstones. Holocene reef accumulation rates average 3.25 m/ky. The degree of reef consolidation is negatively correlated with Holocene thicknesses, indicating that slowly growing reefs are better cemented than fast growing ones. We present a Holocene sea-level curve for Belize based on 36 dates from this study and 33 dates from our previous studies in the area.