[1] To develop a better understanding of the abrupt Dansgaard-Oeschger mode of climate change, it is essential that we establish whether the ice sheets are actively involved, as trigger or amplifier, or whether they merely respond in a passive manner. This requires careful assessment of the fundamental issues of magnitude and phasing of global ice volume fluctuations within marine isotope stage 3 (MIS 3), which to date remain enigmatic.We review recent advances in observational studies pertaining to these key issues and discuss the implications for modeling studies. Our aim is to construct a robust stratigraphic framework for the MIS 3 period regarding sea level variability, using the most up-to-date arguments available by combining insights from both modeling and observational approaches.

Abstract. The EPICA (European Project for Ice Coring in Antarctica) Dome C drilling in East Antarctica has now been completed to a depth of 3260 m, at only a few meters above bedrock. Here we present the new official EDC3 chronology, which is based on the use of 1) a snow accumulation and mechanical flow model, and 2) a set of independent age markers along the core. These are obtained by pattern matching of recorded parameters to either absolutely dated paleoclimatic records, or to insolation variations. We show that this new time scale is in excellent agreement with the Dome Fuji and Vostok ice core time scales back to 100 kyr within 1 kyr. Discrepancies larger than 3 kyr arise during MIS 5.4, 5.5 and 6, which points to anomalies in either snow accumulation or mechanical flow during these time periods. We estimate that EDC3 gives accurate event durations within 20% (2σ ) back to MIS11 and accurate absolute ages with a maximum uncertainty of 6 kyr back to 800 kyr.

Abstract. Using the chronological information available in the Vostok records, we apply an inverse method to assess the quality of the Vostok glaciological timescale. The inversion procedure provides not only an optimized glaciological timescale and its confidence interval but also a reliable estimate of the duration of successive events. Our results highlight a disagreement between orbitally tuned and glaciological timescales below -2700 rn (i.e., -250 kyr B.P., thousands of years before present). This disagreement could be caused by some discontinuity in the spatial variation of accumulation upstream of Vostok. Moreover, the stratigraphic datings of central Greenland ice cores (GRIP and GISP2) appear older than our optimized timescale for the late glacial. This underlines an unconsistency between the physical assumptions used to construct the Vostok glaciological timescale and the stratigraphic datings. The inverse method allows the first assessment of the evolution of the phase between Vostok climatic records and insolation. This phase significantly varies with time which gives a measure of the nonlinear character of the climatic system and suggests that the climatic response to orbital forcing is of different nature for glacial and interglacial periods. We confirm that the last interglacial, as recorded in the Vostok deuterium record, was long (16.2 + 2 kyr, thousands of years). However, midtransition of termination II occurred at 133.4 + 2.5 kyr BP, which does not support the recent claim for an earlier deglaciation. Finally, our study suggests that temperature changes are correctly estimated when using the spatial present-day deuterium-temperature relationship to interpret the Vostok deuterium record.

Abstract. We present the first detailed and rigorous comparison of six different computational techniques used to reconstruct sea surface temperatures (SST) from planktonic foraminifer census data. These include the Imbrie-Kipp transfer functions (IKTF), the modem analog technique (MAT), the modem analog technique with similarity index (SIMMAX), the revised analog method (RAM), and, for the first time, a set of back propagation artificial neural networks (ANN) trained on a large faunal data set, including a modification where geographical information was added among the input variables (ANND). By training the techniques on an identical database, we were able to explore the differences in SST reconstructions resulting solely from the use of different mathematical methods. The comparison indicates that while the IKTF technique consistently shows the worst performance, ANN and RAM perform slightly better than MAT and that the inclusion of the geographical information into the training database (SIMMAX and ANND) further improves the accuracy of modem SST estimates. However, when applied to an independent validation data set and an additional fossil data set, the results did not conform to this ranking. The largest differences in the reconstructed SST values occurred between groups of techniques with different approaches to SST reconstruction; that is, ANN and ANND produced SST reconstructions significantly different from those produced by RAM, SIMMAX, and MAT. The application of the various techniques to the validation data set, which allowed comparison of SST reconstructions with instrumental records, suggests that artificial neural networks might provide better paleo-SST estimates than the other techniques. IntroductionOne of the most remarkable consequences of Charles Lyell's uniformitarian principle for the field of paleoceanography has been the opportunity to use the relationship between the distribution of modem faunas and floras and present-day physical conditions in the ocean to reconstruct climatic variations in the Quaternary period. In addition, if this relationship were expressed in the form of a mathematical formula, past climatic variations could be quantified in standard physical scales and units. This appealing prospect was discovered early in paleoceanographical studies, and quantitative reconstruction of Quaternary climate change by means of fossil faunas has become a standard and routinely applied procedure.Some The third approach, using artificial neural networks (ANN), a branch of artificial intelligence, relies on the sole assumption that there, indeed, is a relationship between the distribution of modem faunas and the physical properties of the environment. ANNs have the ability to overcome problems of fuzzy and nonlinear relationships between sets of input and output variables. This computer-intensive approach is based on an algorithm that has the ability of autonomous "learning" of a relationship between two groups of numbers [14/assetman, 1989; Beale and Jackson, 1990]. Once trained, the ne...

The millennial variability of the North Atlantic Current (NAC) and the deep NADW has been studied in Core CH 69-K09 in relation with the evolution of the Laurentide ice sheet, the Heinrich events (HE) and the orbitally driven changes of insolation. This core is located at the base of the Newfoundland margin, near the boundary between NAC and the Labrador current. Changes in sea surface temperature, G. bulloides and N. pachyderrna s. õ'•0 indicate that both the northward oceanic heat transport by NAC and the Polar Front oscillated at millennial frequency in parallel with the oscillations of the Northern ice sheets. The benthic foraminifera õ'3C record, used as proxy for deep North Atlantic water ventilation (4100m water depth), and thus the thermohaline conveyor belt, varied in phase with NAC. The millennial evolution exhibits analogies with the chaining of events occurring under the influence of insolation changes associated with the Earth precession variations. NAC transport of warm water to the Northern Atlantic Ocean goes on for several kyr, as high latitude polar water cool when summer solstice drift out of perihelie. Thermohaline circulation follows NAC activity as an efficient positive feedback. The large temperature gradients favored snow accumulation and fast growth of ice sheets. Cold phases initiate as ice sheets expend to the ocean and collapse pseudo-periodically, thus reducing (1) Also at D6partement de G6ologie, Universit6 d'Orsay, 78 ORBITAL AND MILLENNIAL SCALES VARIABILITY high latitude surface salinity and stopping deep water convection. Besides HE, icebergs invaded frequently the northern seas, with associated melting and salinity drop. These invasions occurred also during warm periods of ice sheets retreat and within the Holocene. Simultaneously the Polar Front shifted southward and deep Atlantic ventilation decreased. 2. MATERIAL, METHODS AND DATA The core CH 69-K09 was recovered from an area of smooth topography, at the foot of the Newfoundland margin. The 3.5 khz profiles in the area show a very regular succession of alternating transparent and soft reflectors, with no evidence of slumping or turbidites [Labeyre et al., 1996]. This site is swept by the lower North Atlantic Deep Water (NADW) contour current. The Laurentide ice sheet was not far to the north-west during the last glacial period, with icebergs flowing from the North transported by the Labrador current (Figure 1.2). This place is thus appropriate to study the temporal relationship between Laurenfide ice sheet, northward surface heat and salt transfers, and evolution of the thermohaline activity. Pastouret et al. [ 1975] initially studied core CH 69-K09 and demonstrated, by a careful sedimentological and micropaleontological study, that this 14.95 m core covers at least the last glacial-interglacial cycle. The record contains a succession of oscillations of the "Gulf Stream" (in fact the NAC [Rossby, 1996]), alternating with periods of increased input of "ice-rafted debris and relatively coarse turbidite-type beds", which corresp...