Juerg Beer

North Atlantic palaeointensity stack since 75ka (NAPIS–75) and the duration of the Laschamp event

Six relative palaeointensity records from the north Atlantic Ocean were stacked together to produce a new record for the last 75 kyr (NAPIS–75). Five of these records have been previously correlated at millennial scale and placed on the GISP2 age scale, the sixth record was tied to the others using magnetic susceptibility. From 75 ka the field strength exhibits some oscillations, with a first minimum ca.65 ka, followed by a progressive increase to a broad maximum centred at ca.48 ka. There is then a well–marked low at 40 ka, corresponding to the directional anomaly of the Laschamp event. Another intensity low, observed at ca.34 ka, corresponds in age to the Mono Lake event. After a high at 33 ka and two lows at 30 and 24 ka with a broad maximum between, the field strength seems to slowly increase to the upper limit of the studied interval. In the 10–20 kyr interval some differences exist between individual records, and fine–scale details are not always resolved. In the 20–75 kyr interval, on the other hand, well–resolved millennial–scale features are superimposed to the broader trends. The duration of the Laschamp event, which is recorded directionally in five cores, appears to be about 1500 years, consistent with a recent suggestion on the origin of geomagnetic excursions.

High Resolution Global Paleointensity Stack Since 75 kyr (GLOPIS‐75) Calibrated to Absolute Values

We have obtained a global relative paleointensity stack using a selection of published records from marine cores with sedimentation rates in excess of 7 cm/kyr, from the North and South Atlantic Oceans, the Mediterranean and the Indian Ocean. After correlation of the cores, the results were stacked using a new approach to minimize possible local disturbances. This relative paleointensity stack extends from 75 kyr to about 10-12 kyr BE This new stack presents a larger overlap than previously published records with the archeomagnetic/volcanic absolute paleointensity determinations that are all well constrained in age for the period 0-12 kyr. This allows calibration to absolute values, but additional work is still necessary on this point. Superimposed to the long-term features, rapid fluctuations are identified indicating that GLOPIS-75 has attained high global resolution. The duration of the Mono Lake and Laschamp excursions can be estimated to about 1500 years consistently with the suggestion that excursions are shorter than reversals.

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.

Multiradionuclide evidence for the solar origin of the cosmic-ray events of ᴀᴅ 774/5 and 993/4

The origin of two large peaks in the atmospheric radiocarbon (14C) concentration at AD 774/5 and 993/4 is still debated. There is consensus, however, that these features can only be explained by an increase in the atmospheric 14C production rate due to an extraterrestrial event. Here we provide evidence that these peaks were most likely produced by extreme solar events, based on several new annually resolved 10Be measurements from both Arctic and Antarctic ice cores. Using ice core 36Cl data in pair with 10Be, we further show that these solar events were characterized by a very hard energy spectrum with high fluxes of solar protons with energy above 100 MeV. These results imply that the larger of the two events (AD 774/5) was at least five times stronger than any instrumentally recorded solar event. Our findings highlight the importance of studying the possibility of severe solar energetic particle events.

The past 5000 years history of solar modulation of cosmic radiation from 10Be and 14C studies

10Be is produced in a similar way as 14C by the interaction of cosmic radiation with the nuclei in the atmosphere. Assuming that the 10Be and 14C variation are proportional and considering the different behaviour in the Earth system, the 10Be concentrations in ice cores can be compared with the 14C variations in tree rings. A high correlation is found for the short-term variations (14C-Suess-wiggles). They reflect with a high probability production rate variations. More problematic is the interpretation of the long-term trends of 14C and 10Be. Several explanations are discussed. The reconstructed CO2 concentrations in ice cores indicate a rather constant value (280 ± 10 p.p.m. by volume) during the past few millenia. Measurements on the ice core from Byrd Station, Antarctica, during the period 9000 to 6000 years BP indicate a decrease that might be explained by the extraction of CO2 from the atmosphere-ocean system to build the terrestrial biomass pool during the climatic optimum.

Testing for Bias in the Climate Record

calculate the distribution of such an influence, trends at each of the 144 U.S. stations were simulated with an annual cycle calculated from observed daily data (the first five harmonics captured most of the variance in the annual cycle) as well as the autocorrelation and variance of the standard monthly means. An auto-regressive model (5) of order 1 (AR-1 model) was fit to the residuals of the daily temperatures minus the value of the sum of the harmonics, and the variance of each month was calculated in the same manner. The data at each station were simulated with the use of the mean appropriate for the day of the year calculated from the harmonics and the perturbation about this mean from the AR-1 model. Trends were then calculated using the same calendar offset as calculated from precession of the Earths orbit, as described above, and subtracted from the trends without the offset. The results were stratified by the magnitude of the persistence and variance term in the AR-I model. A 2 x 2 continency table was developed with class limits based on the median value of persistence and variance for all stations and all months. A significant portion of the difference in trends was found to be a result of the vagaries of weather and had nothing to do with precession of the orbit (Fig. 1). We show the result for the minimum temperature (Fig. 1), but results are similar for the maximum and mean temperature. The effect of perihelion should be most apparent in the longest daily data set available, the CET. After incorporating the perihelion dates into the CET data set, spring (March, April, and May) warmed relative to the standard trends by 0.42°C over the 219 years and autumn cooled by 0.46°C. Using standard monthly data, we found that winter warmed relative to summer over 1772 to 1990 by I.1 °C; allowing for perihelion reduced this difference to 1.0°C. For the last 100 years (1891-1990), trend differences for the CET series are within the +0.250C range, falling in the center of the U.S. distributions (Fig. 1). Inspection of the differences in trends calculated with and without the shift in calendar dates indicated that a bias was introduced during the transition seasons. It averaged between 0.050 and 0.10°C per century, with a positive bias during spring and a negative one during the autumn. This was due to shifting the calendar to later in the season. During summer and winter the slight shift of 2 days showed little bias, and even a 5-day offset in the CET time series produced only small differences. In sum, we find Thomsons concerns about the manner in which climatologists have calculated trends on monthly and annual time series to be of little consequence during the instrumental climate record.

14C dating of terrestrial moss in Tern Lake deposits, Antarctica

Accurate radiocarbon ages were obtained from terrestrial moss from two drill holes in Tern Lake deposits, Antarctica, using liquid scintillation counting (LSC) and accelerator mass spectrometry (AMS). The results show that the lake deposits have been accumulating since the end of the last glacial epoch ca. 12,600 cal BP at the rate of 0.13-1.1 mm a (super -1) . We discuss the validity of (super 14) C ages of Antarctic lake deposits, with respect to the latitude effect of (super 14) C productivity, the reservoir effect, the environment effect and the hard-water effect.