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Abstract: On the 102- to 103-year timescale, internal reorganizations of the climate system result in abrupt climatic changes of great magnitude. Although a large body of data has been obtained, the physical mechanisms responsible for these changes are still poorly understood. Instrumental data are too short to fully record the variability of the climate system. Palaeoclimatic records provide estimates of the past atmospheric composition, temperature, precipitation, vegetation, extension of glaciers, and past ocean circulation. On the 104- to 106-year timescale, the astronomical theory of palaeoclimates accounts for most of the reconstructed variations. To cite this article: J.-C. Duplessy et al., C. R. Geoscience 337 (2005). [Copyright &y& Elsevier]

Abstract: The hydrological impact of the massive iceberg discharges during Heinrich events has been quantified for Heinrich 4 about 37 kyr ago. This event took place in a context of relatively stable orbital forcing and global ice volume. In this article, we propose to quantify the hydrological impact of the Heinrich event 1 which occurred at about 16 kyr BP. This time period is particularly interesting because orbital forcing was strong and it marks the onset of the last deglaciation. The results obtained here show that Heinrich event 1 has a larger isotopic amplitude than Heinrich event 4 when considering the cores located near the melting source. Concerning surface circulation, the northward progression of the North Atlantic Drift is affected only by Heinrich event 4, possibly because of the latitudinal extension of the melting area during this event. To cite this article: E. Cortijo et al., C. R. Geoscience 337 (2005). [Copyright &y& Elsevier]

Abstract: Holocene millennial climate variability is smaller than that of the last glaciation, due to the disappearance of large unstable ice sheets. Marine records show that the sea-surface temperature (SST) exhibited small variations, mainly in the high and low latitudes. They may be interpreted as a linear response to the mean annual insolation. Major changes in the hydrological cycle have been evidenced in the Asian and African monsoon area, resulting in enhanced precipitation and large river outflow in the Bay of Bengal and the Gulf of Niger. Enhanced rainfall over the Mediterranean Basin resulted in a weak circulation and sapropel formation below 800-m water depth in the eastern Mediterranean Sea. Finally small changes in the thermohaline circulation and the warm North Atlantic Drift have been detected in the Nordic Seas. The Holocene climatic variability is therefore similar to that of the Quaternary, but with small amplitude, while continents experienced large rainfall variations. To cite this article: J.-C. Duplessy et al., C. R. Geoscience 337 (2005). [Copyright &y& Elsevier]

Abstract: Changes in benthic foraminifera δ 18O and δ 13C have been followed over the last deglaciation period in seven sediment cores from different water depths in the North Atlantic, Indian and Pacific Ocean. The two periods of most rapid δ 18O changes, at about 17 and 12 ka, are recorded in all intermediate water cores simultaneously with the penetration of brines formed along the polar ice shelves, exhibiting low δ 18O and δ 13C signatures. At the opposite, exchanges between intermediate and deep waters were slow. But here again, Atlantic and Pacific deep waters were affected simultaneously. These results would give a major role to convection around Antarctica a major role in deep circulation during these periods. To cite this article: L. Labeyrie et al., C. R. Geoscience 337 (2005). [Copyright &y& Elsevier]

As a symptom of evolution, human beings are always trying to better understand current and past events so as to be able to foresee future situations. In order to do that the development of new tools and/or methods is a must. Currently, the Modern Analog Technique (MAT) is one of the most used techniques in paleoceanography and it is applied for the quantitative reconstruction of environmental conditions of the past. Here we propose a new tool, PaleoAnalogs, a method that combines the MAT technique with an interactive analysis to produce faster and more accurate down core reconstructions of (i.e.) sea surface temperature. The tool is tested here using planktonic foraminifers from Pleistocene sediments in the Mediterranean sea, where a well-build data-base is available. PaleoAnalogs is programmed with flexibility in order to enable the use of this technique for different micropaleontological groups and it is available for any operating system. [ABSTRACT FROM AUTHOR]

Flank-collapse events are now recognized as common processes of destruction of volcanoes. They may occur several times on a volcanic edifice pulling out varying volumes of material from km3 to thousands of km3. In the Lesser Antilles Arc, a large number of flank-collapse events were identified. Here, we show that some of the largest events are correlated to significant variations in erupted magma compositions and eruptive styles. On Montagne Pelée (Martinique), magma production rate has been sustained during several thousand years following a 32ka old flank-collapse event. Basic and dense magmas were emitted through open-vent eruptions that generated abundant scoria flows while significantly more acidic magmas were produced before the flank collapse. The rapid building of a new cone increased the load on magma bodies at depth and the density threshold. Magma production rate decreased and composition of the erupted products changed to more acidic compared to the preceding period of activity. These low density magma generated plinian and dome-forming eruptions up to the Present. In contrast at Soufrière Volcanic Centre of St. Lucia and at Pitons du Carbet in Martinique, the flank-collapses have an opposite effect: in both cases, the acidic magmas erupted immediately after the flank-collapses. These magmas are highly porphyritic (up to 60% phenocrysts) and much more viscous than the magmas erupted before the flank-collapses. They have been generally emplaced as voluminous and uptight lava domes (called “the Pitons”). Such magmas could not ascent without a significant decrease of the threshold effect produced by the volcanic edifice loading before the flank-collapse. [ABSTRACT FROM AUTHOR]

We report here on a multi-proxy study of the changes in the dynamics and the properties of bottom water mass in the subpolar North Altantic during Holocene. Magnetic properties coupled with sortable silt and benthic carbon isotopes are investigated for Holocene marine sedimentary sequences located in the Charlie–Gibbs fracture zone (53°N) and in central (57°N) and southern Gardar drift (59°N). All the cores are located at water depths bathed by the Iceland–Scotland Overflow Water (ISOW), mixed at the southernmost locality with southern sourced water masses. The long-term variations in measured proxies are fitted with similar polynomial curves. An early Holocene event characterized by a shutdown/shoaling of the bottom circulation at the deepest sites is most likely related to the main deglacial freshwater inputs. It is followed by a progressive strengthening/deepening of the overflow water which culminates around 6kyr, in coincidence with the Holocene thermal maximum. After 6kyr corresponding to a drastic hydrological reorganization in the North Atlantic, a general decline in the bottom flow strength is observed until about 2kyr B.P. when it reached its present day state. After detrending, several short periods of reduced bottom flow strength and sedimentary transport from the northern detrital sources are observed, with a periodicity of around 600yr with no clear relationship at this time scale between surface and deep ocean. [ABSTRACT FROM AUTHOR]

Abstract: The Last Interglacial (Marine Isotopic Stage or MIS 5e) surface ocean heat flux from the Rockall Basin (NE Atlantic) towards the Arctic Ocean was reconstructed by analysing dinoflagellate cyst (dinocyst) assemblages in four sediment cores. Together with records of stable isotopes and ice-rafted detritus, the assemblage data reflect the northward retreat of ice(berg)-laden waters and the gradual development towards interglacial conditions at the transition from the Saalian deglaciation (Termination II) into MIS 5e. At the Rockall Basin, this onset of the Last Interglacial is soon followed by the appearance of the thermophilic dinocyst species Spiniferites mirabilis, with relative abundances higher than those observed at present in the area. North of the Iceland-Scotland Ridge, however, S. mirabilis only appears in significant numbers during late MIS 5e, between ∼118 and 116.5 ka. Hence, fully marine Last Interglacial conditions with most intense Atlantic surface water influence occurred during late MIS 5e in the Nordic seas, and consequently also farther north in the Arctic Ocean, and at times when northern hemisphere summer insolation was already significantly decreased. The stratigraphic position of this Late Interglacial optimum is supported by planktic foraminifers and contrasts with the timing of the early Holocene climatic optimum in this area. We interpret the delayed northward expansion of Atlantic waters towards the polar latitudes as a result of the Saalian ice sheet deglaciation and its specific impact on the subsequent water mass evolution in this region. [Copyright &y& Elsevier]

Abstract: Measurements of the δ18O in tests of planktonic and benthic foraminifera in the Florida Straits are used to reconstruct the properties of the water column through time over the last 12ka . The isotopic composition of the foraminifera largely reflects the vertical density gradient. We use this reconstruction and δ18O measurements on G loborotalia truncatulinoides in a nearby core to track the depth habitat of this species from the last deglaciation to 1.6ka B.P. Around 9ka, G. truncatulinoides was calcifying in much shallower water than during the late Holocene. The downward migration toward its modern habitat is a regional phenomenon over the western tropical Atlantic continental slope. The cause is still unclear but we hypothesize that the shallower calcification depth may be a response to the presence of glacial melt water or to circulation changes. This study points to the value of further study of the ecology, life cycle and calcification depth for G. truncatulinoides and other planktonic foraminifera that are used to reconstruct the history of the thermocline and upper water column structure. [Copyright &y& Elsevier]

An outstanding climate anomaly 8200 years before the present (B.P.) in the North Atlantic is commonly postulated to be the result of weakened overturning circulation triggered by a freshwater outburst. New stable isotopic and sedimentological records from a northwest Atlantic sediment core reveal that the most prominent Holocene anomaly in bottom-water chemistry and flow speed in the deep Limb of the Atlantic overturning circulation begins at ∼8.38 thousand years B.P., coeval with the catastrophic drainage of Lake Agassiz. The influence of Lower North Atlantic Deep Water was strongly reduced at our site for -100 years after the outburst, confirming the ocean's sensitivity to freshwater forcing. The similarities between the timing and duration of the pronounced deep circulation changes and regional climate anomalies support a causal link. [ABSTRACT FROM AUTHOR]

Abstract: We present a conceptual framework for a new approach to environmental calibration of planktonic foraminifer census counts. This approach is based on simultaneous application of a variety of transfer function techniques, which are trained on geographically constrained calibration data sets. It serves to minimise bias associated with the presence of cryptic species of planktonic foraminifera and provides an objective tool for assessing reliability of environmental estimates in fossil samples, allowing identification of adverse effects of no-analog faunas and technique-specific bias. We have compiled new calibration data sets for the North (N=862) and South (N=321) Atlantic and the Pacific Ocean (N=1111). We show evidence that these data sets offer adequate coverage of the Sea-Surface Temperature (SST) and faunal variation range and that they are not affected by the presence of pre-Holocene samples and/or calcite dissolution. We have applied four transfer function techniques, including Artificial Neural Networks, Revised Analog Method and SIMMAX (with and without distance weighting) on faunal counts in a Last Glacial Maximum (LGM) data set for the Atlantic Ocean (748 samples in 167 cores; based on the GLAMAP-2000 compilation) and a new data set for the Pacific Ocean (265 samples in 82 cores) and show that three of these techniques provide adequate degree of independence for the advantage of a multi-technique approach to be realised. The application of our new approach to the glacial Pacific lends support to the contraction and perhaps even a cooling of the Western Pacific Warm Pool and a substantial (>3°C) cooling of the eastern equatorial Pacific and the eastern boundary currents. Our results do not provide conclusive evidence for LGM warming anywhere in the Pacific. The Atlantic reconstruction shows a number of robust patterns, including substantial cooling of eastern boundary currents with considerable advection of subpolar waters into the Benguela Current, a cooling of the equatorial Atlantic by ∼5°C, and steep SST gradients in the mid-latitude North Atlantic. The transfer function techniques generally agree that subtropical gyre areas in both hemispheres did not change significantly since the LGM, although the ANN technique produced glacial SST in the southern gyre 1–2 °C warmer than today. We have revisited the issue of sea-ice occurrence in the Nordic Seas and using the distribution of subpolar species of planktonic foraminifera in glacial samples, we conclude that the Norwegian Sea must have been ice-free during the summer. [Copyright &y& Elsevier]

The most characteristic records of the millennial variability of climate during the last glacial period may be separated into two classes of signals: Greenland and Antarctic. The Greenland type is found over the whole North American and Euro-Asian continental blocks. Continental cooling associated with the maximum ice sheet extension, and resulting surges on the marine margins have affected the whole latitude range, with a probable strong albedo feedback due to winter snow cover. At the opposite, the surface temperature of tropical oceans did not change as much, which created large seasonal meandering of the Intertropical Convergence Zone (ITCZ), and facilitated meridional atmospheric vapour transport towards cold continents, and heat transfer in particular to the southern hemisphere. The rapid growth of ice sheets facilitated periodic surges and decreases in meridional overturning circulation. We propose that the major difference in the characteristics of the ‘Greenland’ and ‘Antarctic’ classes of climate variability result mostly from the difference in continent/ocean distribution in both hemispheres. To cite this article: L. Labeyrie et al., C. R. Geoscience 336 (2004). [Copyright &y& Elsevier]

Long-term climatic changes, such as glacial–interglacial cycles, are usually explained in term of changes in solar energy received at the top of the atmosphere. In particular, daily insolation in the high Northern Hemisphere latitudes during summer is widely used in interpreting palaeoclimate records. This insolation forcing is strongly marked by changes in precession. However, some climate variations are much more imprinted by changes in obliquity. This was the case for sea surface temperature in the North Atlantic during the Eemian period, as well as for the Vostok ice core deuterium excess history over the last 250 ka. Therefore, we investigate the insolation forcing in order to identify characteristics that could explain the observed climate response. This is mainly the case for annual mean insolation variations. Simple hypotheses for how this forcing could act on climate are also suggested, these being mainly related to changes in the moisture transport induced by the annual insolation gradient between low and high latitudes. Along these lines, a simple conceptual model of annual mean temperature variations illustrates the role of annual mean insolation on climate. [Copyright &y& Elsevier]

The sedimentary sections of three cores from the Celtic margin provide high-resolution records of the terrigenous fluxes during the last glacial cycle. A total of 21 14C AMS dates allow us to define age models with a resolution better than 100 yr during critical periods such as Heinrich events 1 and 2. Maximum sedimentary fluxes occurred at the Meriadzek Terrace site during the Last Glacial Maximum (LGM). Detailed X-ray imagery of core MD95-2002 from the Meriadzek Terrace shows no sedimentary structures suggestive of either deposition from high-density turbidity currents or significant erosion. Two paroxysmal terrigenous flux episodes have been identified. The first occurred after the deposition of Heinrich event 2 Canadian ice-rafted debris (IRD) and includes IRD from European sources. We suggest that the second represents an episode of deposition from turbid plumes, which precedes IRD deposition associated with Heinrich event 1. At the end of marine isotopic stage 2 (MIS 2) and the beginning of MIS 1 the highest fluxes are recorded on the Whittard Ridge where they correspond to deposition from turbidity current overflows. Canadian icebergs have rafted debris at the Celtic margin during Heinrich events 1, 2, 4 and 5. The high-resolution records of Heinrich events 1 and 2 show that in both cases the arrival of the Canadian icebergs was preceded by a European ice rafting precursor event, which took place about 1–1.5 kyr before. Two rafting episodes of European IRD also occurred immediately after Heinrich event 2 and just before Heinrich event 1. The terrigenous fluxes recorded in core MD95-2002 during the LGM are the highest reported at hemipelagic sites from the northwestern European margin. The magnitude of the Canadian IRD fluxes at Meriadzek Terrace is similar to those from oceanic sites. [Copyright &y& Elsevier]

Reconstructions of upper ocean temperature (T) during the Holocene (10–0 ka B.P.) were established using the alkenone method from seven, high accumulation sediment cores raised from the northeast Atlantic and the Mediterranean Sea (36°N–75°N). All these paleo-T records document an apparent long-term cooling during the last 10 kyr. In records with indication of a constant trend, the apparent cooling ranges from −0.27 to −0.15°C kyr−1. Records with indication of time-variable trend show peak-to-peak amplitudes in apparent temperatures of 1.2–2.9°C. A principal component analysis shows that there is one factor which accounts for a very large fraction (67%) of the total variance in the biomarker paleo-T records and which dominates these records over other potential secondary influences. Two possible contributions are (1) a widespread surface cooling, which may be associated with the transition from the Hypsithermal interval (∼9–5.7 ka B.P.) to the Neoglaciation (∼5.7–0 ka B.P.); and (2) a change in the seasonal timing and/or duration of the growth period of alkenone producers (prymnesiophyte algae). The first contribution is consistent with many climate proxy records from the northeast Atlantic area and with climate model simulations including Milankovitch forcing. The second contribution is consistent with the divergence between biomarker and summer faunal paleo-T from early to late Holocene observed in two cores. Further work is necessary, and in particular the apparent discordance between biomarker and faunal T records for the relative stable Holocene period must be understood, to better constrain the climatic and ecological contributions to the apparent cooling observed in the former records. [Copyright &y& Elsevier]

A response by Peter U. Clark and colleagues to a letter to the editor about his article "Reduced North Atlantic deep water coeval with the glacial Lake Agassiz freshwater outburst" in the January 4, 2008 issue.