Raimund Muscheler

High-resolution analyses of an early Holocene climate event may imply decreased solar forcing as an important climate trigger

Early Holocene lacustrine, tree-ring, ice-core, and marine records reveal that the Northern Hemisphere underwent a short cooling event at 10 300 calendar yr B.P. (9100 14 C yr B.P.). The records were compared on a common high-resolution time scale and show that the event lasted less than 200 yr, with a cooling peak of 50 yr, and the event coincides with a distinct Holocene thermohaline disturbance recognized in the North Atlantic Ocean. In spite of wellknown freshwater forcings at the time of the event, the negligible difference between the modeled D 14 C record, based on the GISP2 (Greenland Ice Sheet Project 2) 10 Be data, and the measured values, does not allow for detectable D 14 C changes related to global ocean ventilation. We can, however, show that the onset of the cooling coincides with the onset of one of the largest Holocene 10 Be flux peaks. This finding may imply that the climate system is more

Presence of the Solar de Vries Cycle (∼205 years) during the Last Ice Age

Certain characteristic periodicities in the Δ14C record from tree rings, such as the well-known 11-yr Schwabe cycle, are known to be of solar origin. The origin of longer-period cycles, such as the 205-yr de Vries cycle, in the Δ14C record was less certain, and it was possible to attribute it either to solar or climatic variability. Here, we demonstrate that the de Vries cycle is present in 10Be data from the GRIP ice core during the last ice age (25 to 50 kyr BP). Analysis of the amplitude of variation of this cycle shows it to be modulated by the geomagnetic field, indicating that the de Vries cycle is indeed of solar, rather than climatic, origin.

Chlorine-36 evidence for the Mono Lake event in the Summit GRIP ice core

Abstract A distinct peak has been discovered in the 36 Cl data from the GRIP ice core between the Dansgaard Oeschger (D–O) events 6 and 7 at approximately 32 kyr BP. This peak can be attributed to a minimum of the geomagnetic dipole field associated with the Mono Lake event. Since the 36 Cl peak reflects a higher production rate of all cosmogenic radionuclides, it has an impact on the 14 C dating of the last ice age. Furthermore, it provides an additional time marker similar to a peak found earlier corresponding to the Laschamp event at approximately 39 kyr BP.

Geomagnetic field intensity, North Atlantic Deep Water circulation and atmospheric Δ14C during the last 50 kyr

We present simulated records of past changes in the atmospheric Δ14C for the last 50 kyr due to changes in geomagnetic field intensity and in the strength of the North Atlantic Deep Water (NADW). A new geomagnetic record was used, largely based on the NAPIS-75 record [Laj et al., Phil. Trans. R. Soc. London A 358 (2000) 1009–1025] which has been extended for the 0–20 kyr interval using archeomagnetic and volcanic data. Past changes of the NADW were derived from a mineral magnetic study of the cores used in the construction of NAPIS-75. Two box models of different complexity (4 and 17 boxes) were used to simulate the carbon cycle. Calculated records of Δ14C are consistent with experimental determinations for the last 24 kyr. For older ages, the records calculated with variable oceanic circulation conditions reach values as high as 600‰ (with an average of 500‰) between 20 and 40 kyr with maxima around 21, 30 and 38 kyr (GISP2 age model), while low values are observed prior to 42 kyr. Although large inconsistencies in experimental data preclude precise comparison, the average record simulated with the 17-box model is overall consistent with the Icelandic Sea record [Voelker et al., Radiocarbon 40 (1998) 517–534; 42 (2000) 437–452], except for the extremely high peak observed in this record at 40.5 kyr. On the other hand, the results recently reported from a stalagmite recovered from a submerged cave in the Bahamas [Beck et al., Science 292 (2001) 2453–2458] are inconsistent with all our model simulations. In the 20–45 kyr interval, the improved geomagnetic record combined with the new NADW profile allows us to give a modeled evaluation of the relative contribution of these factors to changes in atmospheric Δ14C. The average simulation provides a first order modeled correction for conventional radiocarbon ages older than 25 kyr for which no calibration curve is available as yet.

Reconstruction of the paleoaccumulation rate of central Greenland during the last 75 kyr using the cosmogenic radionuclides 36Cl and 10Be and geomagnetic field intensity data

Abstract The accumulation rate is one of the most fundamental climate parameters to be derived from ice cores. In addition to its climatic importance, the accumulation rate provides a crucial constraint on depth–age scales where annual layer counting is not possible. So far, there is no consistent picture of the accumulation rate for the last ice age in central Greenland. Therefore, we have derived the accumulation rate for the time interval of about 3–75 kyr BP by applying a method based on the cosmogenic radionuclides 36Cl and 10Be in the Summit ice cores and on geomagnetic field data. The main difference between our approach and the methods applied previously is that the accumulation rate based on 36Cl and 10Be does not depend on an ice flow model estimating the original thickness of annual layers. Our new reconstruction agrees well with the accumulation rate published by Johnsen et al. [Tellus 47B (1995) 624–629] until 60 kyr BP and differs significantly from that of Cuffey and Clow [J. Geophys. Res. 102 (1997) 26383–26393].

Atmospheric radiocarbon during the Younger Dryas: production, ventilation, or both?

A new reconstruction of past atmospheric v 14 C( v 14 Catm) based on Polish lake varved sediments has suggested that previous v 14 Catm values (e.g. from the Cariaco basin record) for the beginning of the Younger Dryas cold event (YD) are overestimates and that the v 14 Catm rise at the YD onset could only be due to changes in atmospheric 14 C production (PCˇ14). This result would have profound climatic implications, for the YD is a paradigm example of abrupt climate change which is usually thought to have been triggered by a reduction in the northward heat flux by the Atlantic thermohaline circulation. Here we examine results from a large number of simulations (300) based on a zonally averaged ocean circulation model, to constrain the effect on v 14 Catm of PCˇ14 changes during the YD as reconstructed from a Greenland ice core record of 10 Be flux. Our results suggest that the scatter in the lake data set is too large to exclude the probable change in deep ocean ventilation at the onset of the YD. By contrast, the model fit to the higher v 14 Catm levels throughout the YD detected in the marine record is generally better when a substantial decrease in deep ocean ventilation is simulated. The early v 14 Catm drawdown that initiated during the first half of the YD, however, could entirely be due to production changes. If this was the case, the drawdown would not reflect an increasing formation of North Atlantic Intermediate Water or Southern Ocean water, as previously suggested. The rapid v 14 Catm rise at the YD onset documented in the marine record, however, remains unexplained. fl 2001 Elsevier Science B.V. All rights reserved.