Alain Mazaud

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.

Correlation of marine 14C ages from the nordic seas with the GISP2 isotope record : Implications for 14C calibration beyond 25 ka BP

We present two new high-resolution sediment records from the southwestern Iceland and Norwegian Seas that were dated by numerous (super 14) C ages up to 54 (super 14) C ka BP. Based on various lines of evidence, the local (super 14) C reservoir effect was restricted to 400-1600 yr. The planktic stable isotope records reveal several meltwater spikes that were sampled with an average time resolution of 50 yr in PS2644 and 130 yr in core 23071 during isotope stage 3. Most of the delta (super 18) O spikes correlate peak-by-peak to the stadials and cold rebounds of the Dansgaard-Oeschger cycles in the annual-layer counted GISP2 ice core, with the major spikes reflecting the Heinrich events 1-6. This correlation indicates large fluctuations in the calibration of (super 14) C ages between 20 and 54 (super 14) C ka BP. Generally the results confirm the (super 14) C age shifts as predicted by the geomagnetic model of Laj, Mazaud and Duplessy (1996). However, the amplitude and speed of the abrupt decrease and subsequent major increase of our (super 14) C shifts after 45 (super 14) C ka BP clearly exceed the geomagnetic prediction near 40-43 and 32-34 calendar (cal) ka BP. At these times, the geomagnetic field intensity minima linked to the Laschamp and the Mono Lake excursions and confirmed by a local geomagnetic record, probably led to a sudden increase in cosmogenic (super 14) C and (super 10) Be production, giving rise to excess (super 14) C in the atmosphere of up to 1200 per mil.

High-resolution record of the Upper Olduvai transition from Po Valley (Italy) sediments: support for dipolar transition geometry?

Abstract A detailed record of the Upper Olduvai polarity transition, composed of > 100 transitional directions, has been obtained from the Crostolo section in northern Italy. A careful examination of the rock magnetic properties of the sediments using standard paleomagnetic techniques, thermomagnetic, scanning electron microscopy, microprobe, X-ray diffraction and Mossbauer analysis, shows that greigite is the main magnetic carrier of the remanence. The correlation of a shift in the magnetic record to a small-scale sedimentary feature observed in the section indicates that the magnetization was acquired at deposition, or very shortly after. The virtual geomagnetic pole (VGP) path is largely confined along a great circle over North and South America ∼ 90° west of the site and consists of three stages: first, the VGP moves to southern mid-latitudes, then, after a period of standstill, it comes back to almost true north, and, finally, the south pole is reached in a third step. Although different from a record from the southern Indian Ocean, this path is virtually identical to those obtained for the same transition from North Pacific deep-sea cores, and partly coincides with a North Atlantic record, which suggests that a dipolar component may be present in the transitional field during the Upper Olduvai reversal. A review of the recently obtained records of various transitions shows that in more than two-thirds of the cases the VGP paths are similarly confined along a meridian over the Americas or antipodal to them, irrespective of the sampling site and of the sense of the transition. Although not deterministic, this tendency suggests that a similar dipolar component might be present in the transitional fields of other reversals of different ages.

Geomagnetic intensity and 14C abundance in the atmosphere and ocean during the past 50 kyr

We have used a new sedimentary record of geomagnetic field intensity, combined with published volcanic data, to examine the extent to which geomagnetic changes have affected 14C production in the past. The production record was then converted into an atmospheric Δ14C signal using a 4-box model of past oceanic circulation. The results indicate that changes in geomagnetic field intensity account for at least 80% of the Δ14C shift documented by published U-Th data, which are of the order of 500 per mil with respect to a constant production hypothesis. Model simulations show that changes in past oceanic circulation account for only ±100 per mil of Δ14C variation, which is within the uncertainty of the geomagnetic correction. In terms of dating, these results show that the radiocarbon ages have to be shifted by 2 to 3.5 kyr towards older ages during the 20–40 ka interval.

The Blake Geomagnetic Polarity Episode recorded in Chinese loess

A geomagnetic anomaly consisting of three clearly defined periods of reverse polarity separated by two short intervals of normal polarity has been documented in a loess section near Xining, western China. On the basis of previous studies of Chinese loess stratigraphy, we correlate this geomagnetic polarity episode with the Blake event. While more research is needed to improve our understanding of the exact process of remanence acquisition of loess deposits, this study shows that loess may provide records of fast geomagnetic events and lends further support to the global nature of the Blake geomagnetic event.

Geomagnetic field control of 14C production over the last 80 Ky: Implications for the radiocarbon time‐scale

We have used a recent estimate of the geomagnetic dipole field strength over the last 80 ky to examine the extent to which changes in geomagnetic field intensity have affected the radiocarbon time scale. The magnetic record was converted into a 14C production signal which was then used to produce a synthetic atmospheric Δ14C record using a two box model for the carbon cycle. This calibration suggests that the geomagnetic field has been the main factor governing the production of cosmogenic 14C atoms over the period of time considered, and that Δ14C changes due to other causes, such as climatic changes, have been of lesser importance. The results indicate that the radiocarbon ages are 2 to 3 ky younger than true age in the interval 18–40 ky b.p., in good agreement with recent U-Th and 14C age comparisons. Moreover the results predict that a negligible difference between true and 14C ages should be observed around 45–50 ky.

The Matuyama-Brunhes and Upper Jaramillo transitions recorded in a loess section at Weinan, north-central China

Abstract The paleomagnetic records of the Matuyama-Brunhes (MB) and Upper Jaramillo (UJ) polarity reversals have been obtained from a loess section at Weinan, Shaanxi province, in China. An investigation of the rock magnetic properties using stepwise IRM acquisition, hysteresis loops, thermomagnetic analysis and X-ray diffraction on magnetic extracts, identifies pseudo-single domain magnetite as the carrier of the main fraction of the remanence, associated with maghemite and hematite. The total duration of the directional change (based upon independent loess accumulation rates), is about 5000 yr for the MB reversal and about 3200 yr for the UJ reversal. These values are consistent with previous estimates obtained from marine or lacustrine sediments. Apart from a few intermediate points observed on North America (MB) and in the Pacific (UJ), the transitional Virtual Geomagnetic Pole (VGP) paths are situated in two longitudinal bands over eastern Asia and Australia (MB) and the Americas (UJ). The bulk of the UJ record and the part of the MB record over the Americas are not consistent with the hypothesis of a record arising from smoothing of pre- and post-transitional directions [1]. A reappraisal of the statistical approach of McFadden et al. [2] shows that the longitudinal distribution of the records of reversals is consistent with the hypothesis of genuine, geographically preferred sectors for the paths, independent of site longitude, with a probability higher than 0.45 when the two Weinan records are considered. Thus, these results, obtained from wind-blown loess, sustain the hypothesis of two preferential longitudinal bands for reversing transitional VGPs suggested from the study of marine and lacustrine sediments.

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.

Relative changes of the geomagnetic field intensity during the last 280 kyear from piston cores in the Açores area

Abstract Relative changes in geomagnetic field intensity over the last 280 kyears have been recovered from the study of three marine cores from the Acores area. One core was precisely dated by oxygen isotope study and the other two records were linked to it using light reflectance analysis which allowed precise correlation. Rock magnetic analysis shows that the main magnetic mineral is magnetite with a very homogeneous grain-size distribution in the pseudo-single domain range for the three cores. Changes in the amount of magnetite do not exceed a factor of 10. Therefore, these cores appear to be suitable for relative palaeointensity determinations. Two mineral magnetic components with periodicities of 23 and 18 kyears are present in the records of bulk magnetic parameters and natural remanent magnetisation (NRM), but the power at these frequencies is not significant at the 95% level for the normalised remanence records. For each core, normalisation of the NRM using different normalising parameters yields virtually identical results. Using saturation isothermal remanent magnetisation (SIRM) as the normalising parameter, the results of the three cores were then combined into a stacked curve. Squared coherence analysis between this stacked curve and bulk mineral-magnetic parameters reveals that the 18 kyear component is still present in the record, but that the power spectrum is barely above noise level at this frequency. Some of the features of this North Atlantic record are consistent with the main characteristics already documented in other sedimentary or volcanic records. For instance, distinct periods of low intensity are observed around 40, 120, and 190 kyears, and periods of high intensity at 50 and 80 kyears. Because of the uniformity of the mineral-magnetic characteristics of the three cores, we suggest that this record may be a suitable palaeointensity reference curve for the Central North Atlantic Ocean region.

Normalised natural remanent magnetisation intensity during the last 240 000 years in piston cores from the central North Atlantic Ocean: geomagnetic field intensity or environmental signal?

We have examined in detail the stratigraphic variations in magnetic parameters of four cores from the North Atlantic Ocean in areas where the depositional environment has varied with climatic changes. Our objective is to assess whether normalisation of the natural remanent magnetisation (NRM) intensity can cancel the effect of climatically induced variations in magnetic mineral content and grain size and whether a reliable record of relative changes in geomagnetic field intensity can be obtained. After selecting the core sections which meet published criteria for paleointensity normalisation, we have attempted to obtain a record of the geomagnetic field intensity variations over the past 240 kyear, using both ARM and IRM as normalising factors for the NRM. The two methods yield overall similar results, except for the interval 20–30 kyear, where IRM normalisation yields a record more consistent with previous sedimentary and volcanic results, than does the more frequently employed ARM normalisation. The final intensity record docmments a picture of the dipole field moment which presents large similarities with profiles obtained from sediments deposited in different environmental conditions in various regions around the world. We observe a major low at about 42 kyear, which could correspond to the Laschamp event, a broad low in the interval 90–130 kyear, possibly connected to the Blake event, and another low at about 190 kyear, which could reflect the Biwa I event. The processes of acquisition of depositional and postdepositional remanent magnetisation and the physics of the normalising methods are still insufficiently understood. However, these results, obtained from a region characterised by complex environmental dynamics, confirm the potential of sedimentary deep sea cores for relative paleointensity determinations.