U23A-01 13:40h
Modelling Climate of the Last Million Years
Around 1 Myr BP, the power of the 100-kyr period in eccentricity starts to decrease progressively for the benefits of the 400-kyr period which dominates up to now and mainly over the next 400 kyr. This eccentricity cycle fades away at the same time that the 100-kyr cycle appears stronger and stronger in climate records. This underlines the importance of non-linear feedback mechanisms which are necessary to create the 100-kyr climate signal. It shows also that a direct response to eccentricity can not be invocated. Modeling experiments using the LLN (Louvain-la-Neuve) climate model of intermediate complexity forced with daily insolation for each latitude shows spectral features which are pretty similar to those found in proxy records. Spectral characteristics of the simulated annual mean temperature and ice volume are broadly the same, but the signal in the 100-kyr band is much weaker in temperature than in ice volume.
U23A-02 INVITED 13:55h
A High Resolution Isotopic ice Core Record Covering the Last 800 000 Years.
The two cores drilled at the Dome C site in East Antarctica (elevation 3233 m, mean annual temperature, - 54.5°C) in the framework of the European Program for Ice Coring in Antarctica, now provide a continuous deuterium profile covering the last 800 ky. Thanks to a recent extension over its last 60 meters, the 3201 m Antarctic climate record now extends back to marine stage 20. Whereas published data (EPICA Community members, 2004) are of low temporal resolution (e.g. 3000 years), we will have at the time of the meeting a much more detailed deuterium profile to present, at least for most of the core. We will first discuss the EPICA profile in view of existing East Antarctic ice core records (Vostok and Dome F) and its temperature interpretation. This comparison will also include the results from the second EPICA core drilled at Kohnen Station in the Atlantic sector of East Antarctica, available at that time. We will then focus on the spectral properties of the 800 ky climate record and on its comparison with marine records. Of particular interest are the change of pacing before and after the MBE (Mid-Brunhes Event) and the timing and duration of stadials, interstadials, and interglacials which will lead us to discuss how the EPICA timescale was derived. Particular attention will be paid to the last glacial cycle, for which the EPICA deuterium record shows counterparts for each of the successive Dansgaard/Oeschger events, both for large and small ones, with possible implications for the seesaw mechanism.
U23A-03 14:10h
PLEISTOCENE GLACIAL TERMINATIONS
Major climate cycles of the last 800 ky are well established as product of orbital forcing. In addition, 100-ky glacial cycles and in particular, their abrupt terminations (Broecker and von Donk, 1970) record abrupt ocean changes that require additional forcing and threshold mechanisms in the field of ocean thermohaline circulation (THC). In this study we compare records of circulation events with decadal-millennial-scale resolution, which characterize the glacial terminations near both the terminus and the source regions of global THC, that is in the subarctic N.W. Pacific, the South China Sea, and the S.W. Greenland Sea. In these regions glacial terminations and early peak interglacials were likewise marked by extremes in the intensity of meridional overturning circulation, extremes that apparently presented short-term recurrences of the mid-Pliocene circulation regime of the ocean, with a maximum intensity of the Denmark Strait Overflow, a maximum incursion of Upper Pacific Deepwater into the South China Sea, and a strong vertical turnover rate in the subarctic Pacific. This flushing of the deep ocean was totally different from modern THC conditions.
U23A-04 14:25h
The Last 1 Million Years at Tenaghi Philippon: Terrestrial, Marine and Ice Core Comparisons
Palaeoenvironmental archives spanning multiple glacial-interglacial cycles provide a unique perspective into the evolution of Quaternary climate variability. However, while marine and ice core records may furnish the best estimates of global climate variability, we still remain in relative ignorance as to how these changes translate to events on land, because of a shortage of suitable terrestrial sequences and inadequate chronological control. Continuous, pollen-bearing lake sediment sequences are found only in exceptional circumstances and a handful of these occur in S. Europe. Here we examine the longest of these records, from Tenaghi Philippon (TP), northeast Greece, spanning the last 1 million years. A new astronomical calibration procedure, based on a correspondence between changes in certain vegetation elements and March and June perihelion configurations, is applied to the entire section, leading to a revised timescale especially for the lower part of the sequence. The TP sequence shows a close correspondence between marine and terrestrial stages over the last million years, although individual boundaries may not be precisely isochronous. However, inspection of the record reveals a significant departure in terms of the amplitude of glacial-interglacial variability, with the onset of more extreme interglacials occurring after MIS 16, rather than MIS 12 as seen in marine and ice core records. More specifically, TP interglacials before MIS 16 are characterized by lower arboreal pollen increases, as a result of sustained presence of grasses. Part of this grass signal is derived from local semi-aquatic vegetation, suggesting increased marsh area and lower water levels. This may be a function of local factors (e.g. tectonic changes) altering the hydrological regime of the Philippi plain. Alternatively, the occurrence of a shift in interglacial amplitudes right at the MIS 16/15 transition, may suggest a climatic control. Comparisons with other records are needed in order to establish the local or regional character of such changes.
U23A-05 14:40h
MIS 11 and the mid-Brunhes Dissolution Interval
The mid-Brunhes dissolution interval (MBDI) represents a prolonged period of marine carbonate dissolution centred around Marine Isotope Stage 11 (MIS 11). The MBDI has been observed in all the major ocean basins and at all water depths. We present a 1Myr foraminiferal shell weight record from ODP site 982 in the North Atlantic (57.5N, 15.9W, 1134m) which is strongly affected by dissolution over this period, demonstrating the vulnerability of even shallow sites to the effects of dissolution. From a compilation of carbonate mass accumulation rates (MARs) we suggest that a global increase in marine carbonate production was responsible for the carbonate crash at this time. This is a variant on the basin to shelf hypothesis which argues that enhanced growth of shallow water carbonates in low latitudes during MIS 11 resulted in a global dissolution event. We model the effects of the proposed increase in carbonate production within the constraints of observed atmospheric CO2 variability.
U23A-06 14:55h
An 800 kyr record of ice core chemistry from the EPICA Dome C ice core
This abstract is written on behalf of the entire EPICA chemistry team (only 1 representative per group is listed in the author list). The Dome C ice core now reaches back about 800 kyr in time. The isotope record shows the familiar pattern of 100 kyr cycles, but with a much lower amplitude in the earlier part of the record than in the later. Ice core chemical records can add substantial additional information about important parameters such as sea ice, atmospheric transport, etc. In this abstract we will present (at low resolution) the 800 kyr record of ionic chemistry (sea salt, terrestrial elements, sulfate). We will look at the pattern, and also at the relationship between the chemical concentrations and the temperature signal, in particular to see if the relationship is the same in the early and late part of the record. We will then interpret the signals to make preliminary inferences about South American climate (based on terrestrial elements), about sea ice extent (based on a tentative interpretation of the sea salt record) and other parameters.
U23A-07 15:10h
Obliquity Pacing of the 40 and 100 kilo-year Modes of Glacial Variability
The Mid-Pleistocene Transition (MPT), $\sim$700ky ago, marks the switch from a climate with maximum variability at 40ky periods to one with maximum variability at $\sim$100ky periods. There is consensus that variations in Earth's obliquity control the 40ky variability, but there are conflicting hypotheses regarding the more recent $\sim$100ky glacial/interglacial cycles---variously attributing control to Earth orbital variations or mechanisms internal to the climate system. A formal test is made of the $\sim$100ky orbital hypotheses, distinguishing eccentricity, precessional and obliquity control of glacial cycle timing. The null hypothesis that glacial cycles are independent of obliquity is rejected at the 5% significance level. In contrast, the corresponding null-hypotheses for eccentricity and precession cannot be rejected. The simplest inference, consistent with the observations, is that the MPT represents a mode switch from ice-sheets terminating every obliquity cycle to a mode where ice-sheets terminate every second (80ky) or third (120ky) obliquity cycle (averaging $\sim$100ky). A modified version of \textit{Imbrie and Imbrie}'s [1980] simple glacial model is used to describe the obliquity pacing of the glacial cycles. The model reproduces the correct timing for each termination as well as many of the linear and nonlinear features identified in marine $\delta^{18}O$ records. Furthermore, under a wide range of parameterizations the model's response to obliquity period forcing is chaotic and has two distinct modes. One mode lies near an unstable fixed point of the model and gives successive ablation events which are relatively small, of nearly equal magnitude, and periodic at 40ky; in the other mode ice accumulates over two or three forcing cycles before rapidly ablating---in agreement with the hypothesis test results. The model chaotically switches between these 40ky and $\sim$100ky modes of glacial variability, suggesting that the MPT may be a spontaneous event independent of shifts in the background climate state.
U23A-08 INVITED 15:25h
``Pre-Vostok'' Greenhouse Gas Concentrations Reconstructed From the EPICA Dome C Ice Core
The new deep ice core recovered from Dome Concordia in the framework of EPICA, the European Project of Ice Coring in Antarctica, contains a continuous climate history of the past 740,000 years [EPICA Community Members, 2004]. We present the current status of measurements of CO$_2$, CH$_4$ and N$_2$O on air trapped in the bubbles of the Dome C ice core. CO$_2$ is measured using laser absorption spectroscopy on samples of less than 10 g of ice which are mechanically crushed or milled. CH$_4$ and N$_2$O are extracted using a melt-refreeze technique and then measured by gas chromatography. The ice core contains an uncontaminated climate record down to Marine Isotope Stage 14 (MIS 14) as verified by a consistent gas age/ice age difference determined at terminations V and VI. CO$_2$ and CH$_4$ results from MIS 11 show that the normal levels of greenhouse gases prevailed during this exceptionally long interglacial. This demonstrates that the length of the interglacial was not due to exceptionally high greenhouse gas levels. MIS 13 and earlier interglacials, however, show significantly colder interglacials. In addition, the glacials are shorter which results in a more balanced sequence of cold and warm phases. Measurements of the greenhouse gas concentrations are central in understanding the mechanisms in the climate system which cause the significant change of character of the ice age cycles at around 400 kyr BP. We will present greenhouse gas measurements covering the first of the "pre-Vostok" interglacials from MIS 11 to MIS 14 (410 to 550 kyr BP) for CO$_2$, and from MIS 11 to MIS 16 (410 to 620 kyr BP) for CH$_4$. These measurements will resolve the "EPICA Challenge" [Wolff et al., 2004] put out to modelers to predict the expected greenhouse gas levels prior to 400 kyr BP based on the knowledge of the orbital parameters, and known paleoclimatic proxies (sea level from marine sediment records, dust load and isotopic concentration of precipitation in Antarctica from the EPICA Dome C ice core). \scriptsize EPICA Community Members, {\it Nature, 429}, 623-628, 2004. Wolff, E.W., et al., {\it EOS; Trans.\ Am.\ Geophys.\ Un.}, in press, 2004.