Laschamp Excursion Research Articles

Authigenic 10Be/ 9Be signature of the Laschamp excursion: A tool for global synchronisation of paleoclimatic archives

Authigenic 10Be and 9Be isotope concentrations have been measured in marine sediments deposited between 10 and 60 ka BP in the western equatorial Pacific and their ratio compared to the geomagnetic relative paleointensity (RPI) record obtained from the same core [C.L. Blanchet, N. Thouveny, T. de Garidel-Thoron, New evidences of geomagnetic moment lows between 30 and 45 ka BP from a sedimentary sequence of the west equatorial Pacific, Quat. Sci Rev. (in press)]. Over the studied time interval, dated by radiocarbon and oxygen isotope stratigraphy, three periods of reduced RPI occurring at ∼ 32, 37 and 45 ka, according to the constructed age model, are concomitant with three significant increases in atmospheric 10Be production. Since the most prominent 10Be/ 9Be peak necessarily results from the weakest geomagnetic moment event, i.e. that associated with the Laschamp excursion, we assign to this cosmogenic nuclide event the age of the Laschamp excursion (i.e. 40.4 ± 2.0 ka BP) [H. Guillou, B.S. Singer, C. Laj, C. Kissel, S. Scaillet, B.R. Jicha, On the age of the Laschamp geomagnetic excursion, Earth Planet. Sci. Lett. 227 (2004) 331–343]. Cosmogenic nuclide production peaks provide critical complementary evidence for the identification of geomagnetic dipole lows linked to excursions and constitute accurate markers for global intercorrelation of paleoclimatic archives.

Late Brunhes polarity excursions (Mono Lake, Laschamp, Iceland Basin and Pringle Falls) recorded at ODP Site 919 (Irminger Basin)

Component natural remanent magnetizations derived from u-channel and 1-cm 3 discrete samples from ODP Site 919 (Irminger Basin) indicate the existence of four intervals of negative inclinations in the upper Brunhes Chronozone. According to the age model based on planktic oxygen isotope data, these “excursional” intervals occur in sediments deposited during the following time intervals: 32–34 ka, 39–41 ka, 180–188 ka and 205–225 ka. These time intervals correspond to polarity excursions detected elsewhere, known as Mono Lake, Laschamp, Iceland Basin and Pringle Falls. The isotope-based age model is supported by the normalized remanence (paleointensity) record that can be correlated to other calibrated paleointensity records for the 0–500 ka interval, such as that from ODP Site 983. For the intervals associated with the Mono Lake and Laschamp excursions, virtual geomagnetic poles (VGPs) reach equatorial latitudes and mid-southerly latitudes, respectively. For intervals associated with the Iceland Basin and Pringle Falls excursions, repeated excursions of VGPs to high southerly latitudes indicate rapid directional swings rather than a single short-lived polarity reversal. The directional instability associated with polarity excursions is not often recorded, probably due to smoothing of the sedimentary record by the process of detrital remanence (DRM) acquisition.

Geomagnetic field behavior during the Iceland Basin and Laschamp geomagnetic excursions: A simple transitional field geometry?

We present four new records of the Iceland Basin Excursion (IBE) and five new records of the Laschamp Excursion (LE) obtained from rapidly deposited marine sediments in the North Atlantic Ocean, the Nordic Seas, the Gulf of Mexico, the South China Sea, and the southern Indian Ocean. Marked minima in relative paleointensity correspond with the paleomagnetic directional changes associated with all of the excursion records. The virtual geomagnetic pole (VGP) paths of the four IBE records are all similar. The VGPs move southward over Europe and Africa, reaching the southern hemisphere (three reach Antarctica), and then move to more eastern longitudes before returning northward over Australia and east Asia, describing a large counterclockwise loop. The same VGP pattern is observed in other published records. The VGP paths observed for the LE are similar to those of the IBE; however, they loop clockwise instead of counterclockwise. Despite the different sense of looping, the marked similarity among the paths for the two excursions suggests that a similar, relatively simple geometry dominated the transitional field during both the IBE and the LE. Similar dynamo mechanisms must therefore have been active in the Earth's core for both excursions. The duration of the excursions is estimated at <2,000 years, which supports the suggestion that a difference exists between the mechanisms for excursions and reversals. However, the coincidence of the longitudinal bands for VGPs associated with excursions compared to some reversal paths could also indicate an inherent link between the mechanisms for reversals and excursions.

Identifying the origin of the magnetic directional anomalies recorded in the Datong loess profile, northeastern Chinese loess plateau

SUMMARY To better understand the origin of short-term features of palaeomagnetic signals recorded by the Chinese loess, detailed palaeomagnetic and magnetic fabric studies were conducted on a loess profile at Datong, the northeastern margin of the loess plateau, which is characterized by both high sedimentation rate (∼18 cm ka−1) and variations of sedimentary environments. It appears that the magnetic fabrics, specifically the inclination of the maximum magnetic susceptibility principal axis (K1-Inc), are robust means to identify the deposition environments. Our results (<∼74 ka) record four palaeodirectional anomalies (referred as A–D). Among them, anomaly C with relatively normal fabrics can be correlated to the Laschamp (∼40 ka) excursion. In contrast, anomalies A, B and D accompanying with K1-Inc anomalies, are interpreted to be caused by post-depositional disturbances. The new results improve our understanding of the origins of the natural remanent magnetization recorded by the Chinese loess.

Evidence for multiple paleomagnetic intensity lows between 30 and 50 ka BP from a western Equatorial Pacific sedimentary sequence

A paleomagnetic study was carried out on the radiocarbon dated MD97-2134 core located in the western Equatorial Pacific (Southern Papua New Guinea margin). Rock magnetic investigations revealed changes of the magnetic mineralogy along the hemi-pelagic sedimentary sequence but the reconstruction of past direction and relative paleointensity variations of the geomagnetic field remained feasible. Four successive paleointensity drops are recorded between 30 and 50 ka BP. The largest one is associated with an abrupt swing of declination and inclination interpreted as a smoothed signature of the Laschamp excursion (∼41 ka BP). The succession of four events of weak intensity between 30 and 50 ka BP introduces a complex behaviour of the geomagnetic field in the time interval spanning over the Laschamp and the Mono Lake excursions.

Deep‐sea sediment records of the Laschamp geomagnetic field excursion (∼41,000 calendar years before present)

We have recovered two new high‐resolution paleomagnetic records of the Laschamp Excursion (∼41,000 calendar years B.P.) from deep‐sea sediments of the western North Atlantic Ocean. The records document that the Laschamp Excursion was characterized locally by (1) declination changes of ±120°, (2) inclination changes of more than 140°, (3) ∼1200‐year oscillations in both inclination and declination, (4) near 90° out‐of‐phase relationships between inclinations and declinations that produced two clockwise loops in directions and virtual geomagnetic poles (VGPs) followed by a counterclockwise loop, (5) excursional VGPs during both intervals of clockwise looping, (6) magnetic field intensities less than 10% of normal that persisted for almost 2000 years, (7) marked similarity in excursional directions over ∼5000 km spatial scale length, and (8) secular variation rates comparable to historic field behavior but persisting in sign for hundreds of years. All of these features, with the exception of anomalously large directional amplitude, are consistent with normal magnetic field secular variation. Comparison of our Laschamp Excursion paleomagnetic records with other late Quaternary excursion records suggests that there is a group of excursions, which we term class I, which have strikingly similar patterns of field behavior and likely share a common cause as part of the overall core dynamo process. Three general models of secular variation are described that can qualitatively produce class I excursions. On the basis of these observations we conclude that class I excursions, epitomized by the Laschamp Excursion, are more closely related to normal secular variation and are not necessarily a prelude to magnetic field reversal.

On the age of the Laschamp geomagnetic excursion

The Laschamp geomagnetic excursion is a critical tie-point found directly in deep-sea sediment cores and revealed in polar ice as an abrupt change in the rate of cosmogenic nuclide flux. Despite the importance of this excursion to quantifying paleoclimate proxy records archived in sediment and ice, and to providing an independent calibration of the radiocarbon calendar, its timing remains poorly known. Previous K–Ar, 40Ar/ 39Ar, and U–Th isochron determinations from lava flows at the type locality in the Massif Central, France, vary widely, are imprecise, and suggest a mean age of about 46.2±2.5 ka (±2 σ). Results of 6 new unspiked K–Ar and 13 40Ar/ 39Ar incremental heating experiments on subsamples from three sites on the Laschamp and Olby flows are concordant and give a weighted mean age of 40.4±1.1 ka (2 σ uncertainty including analytical sources only) that is 10% younger than the previous estimates. Considering that the 40K→ 40Ar decay constant is not known to a precision better than ±2.4%, the most probable radioisotopic age for the Laschamp excursion is 40.4±2.0 ka (2 σ, analytical plus decay constant uncertainties). This new age for the Laschamp excursion agrees precisely with that deduced from the NAPIS-75 deep-sea sediment paleointensity stack when calibrated against the GISP2 ice core chronology using the O isotopes in ice and the magnetic properties of the marine cores.

Paleomagnetic excursions recorded in the Yanchi Playa in middle hexi corridor, NW China since the last interglacial

Paleomagnetic determinations on lithological profiles of two paralleled long drilling cores covering the past 130 kyr B.P., GT40 and GT60, from the Yanchi Playa in the arid Northwestern China, indicate that a series of pronounced paleomagnetic excursions have been documented. By correlating our results with published regional and worldwide reports, 4 excursion events out of 10 apparent reversal signals (labeled from GT-1 to GT-10) were identified as excursion events coeval with the Mono Lake Event (28.4 kyr–25.8 kyr), Laschamp Event (43.3 kyr–40.5 kyr), Gaotai Event (82.8 kyr–72.4 kyr) and the Blake Event (127.4 kyr–113.3 kyr), respectively. GT-9 correlates with the above-mentioned Gaotai Event, GT-7 and GT-6 correspond to two stages of the Laschamp Event and GT-5 to the Mono Lake Event. It is noteworthy that the so-called Gaotai Event has not been reported as a pronounced paleomagnetic excursion in the Northwestern China. Every magnetic excursion event corresponds to paleointensity minima, anteceding those established abrupt paleoclimatic change events, such as the Younger Drays and the Heinrich Events (H1–H6). Here, we tentatively propose that these geomagnetic excursions/reversals can be viewed as precursors to climate abruptness. During the transitional stages when the earth’s magnetic field shifted between a temporal normal and a negative period, the earth’s magnetic paleointensity fell correspondingly to a pair of minima. Although more precise chronology and more convincing rock magnetic parameter determinations are essentially required for further interpretation of their intricate coupling mechanism, these results may have revealed, to some extent, that the earth’s incessantly changing magnetic field exerts an strong influence on the onset of saw-tooth shaped abrupt climate oscillations through certain feedback chains in arid Central Asia or even North Hemispheric high latitude regions.

Geomagnetic moment variation and paleomagnetic excursions since 400 kyr BP: a stacked record from sedimentary sequences of the Portuguese margin

A paleomagnetic study was performed in clayey-carbonate sedimentary sequences deposited during the last 400 kyr on the Portuguese margin (Northeast Atlantic Ocean). Declination and inclination of the stable remanent magnetization present recurrent deviations from the mean geomagnetic field direction. The normalized intensity documents a series of relative paleointensity (RPI) lows recognized in other reference records. Three directional anomalies occurring during RPI lows chronologically correspond to the Laschamp excursion (42 kyr BP), the Blake event (115–122 kyr BP) and the Icelandic basin excursion (190 kyr BP). A fourth directional anomaly recorded at 290 kyr BP during another RPI low defines the ‘Portuguese margin excursion’. Four non-excursional RPI lows are recorded at the ages of the Jamaica/Pringle Falls, Mamaku, Calabrian Ridge 1, and Levantine excursions. The RPI record is characterized by a periodicity of ∼100 kyr, paleointensity lows often coinciding with the end of interglacial stages. This record sets the basis of the construction of an authigenic 10Be/9Be record from the same sedimentary sequences [Carcaillet et al., this issue].

K-Ar ages of the Auckland geomagnetic excursions

K-Ar age determinations were made on two monogenetic volcanoes in the Auckland volcanic field, New Zealand, which have recorded the Auckland geomagnetic excursions. For the Wiri volcano with the north-down intermediate paleomagnetic direction, five samples gave a weighted mean age of 27 ± 5 (1σ) ka. For the Hampton Park volcano with the west-up intermediate direction, three samples gave a weighted mean of 55 ± 5 (1σ) ka. Since these two K-Ar ages are distinguished at 2σ level, it is inferred that at least two geomagnetic excursions can be recognized in Auckland. The age of the Hampton Park is barely distinguished from the established age range of the Laschamp excursion (39–45 ka) at 2σ level. The age of the Wiri coincides with the age of c. 30 ka in which excursions have been found from sedimentary and volcanic records. The reported excursions from volcanic rocks show a VGP cluster in the central to northern Pacific region which is distinct from the VGP paths or clusters during polarity reversals.

Further rock magnetic and chronostratigraphic results on reversal excursions during the last 50 ka as derived from northern high latitudes and discrepancies in precise AMS14C dating

A total of five sediment cores from three sites, the Arctic Ocean, the Fram Strait and the Greenland Sea, yielded evidence for geomagnetic reversal excursions and associated strong lows in relative palaeointensity during oxygen isotope stages 2 and 3. A general similarity of the obtained relative palaeointensity curves to reference data can be observed. However, in the very detail, results from this high-resolution study differ from published records in a way that the prominent Laschamp excursion is clearly characterized by a significant field recovery when reaching the steepest negative inclinations, whereas only the N–R and R–N transitions are associated with the lowest values. Two subsequent excursions also reach nearly reversed inclinations but without any field recovery at that state. A total of 41 accelerator mass spectrometry (AMS) 14C ages appeared to allow a better age determination of these three directional excursions and related relative palaeointensity variations. However, although the three sites yielded more or less consistent chronological as well as palaeomagnetic results a comparison to another site, PS2644 in the Iceland Sea, revealed significant divergences in the ages of the geomagnetic field excursions of up to 4 ka even on basis of uncalibrated AMS 14C ages. This shift to older 14C ages cannot be explained by a time-transgressive character of the excursions, because the distance between the sites is small when compared with the size of and the distance to the geodynamo in the Earth's outer core. The most likely explanation is a difference of reservoir ages and/or mixing with old 14C-depleted CO2 from glacier ice expelled from Greenland at site PS2644.

Paleomagnetism of the late Quaternary Ontake Volcano, Japan: directions, intensities, and excursions

Study of paleosecular variation was carried out to mostly andesite lavas from the late Quaternary Ontake Volcano which have detailed stratigraphy and accurate radiometric ages. Among 42 sites, some of the results from lava successions were combined due to possible equivalent ages, remaining 35 sites. After excluding two low latitude VGPs, angular dispersion of 15.6° was obtained for a period which is accurately confined between 20 ka and 90 ka. The obtained angular dispersion is not different from the typical value for the last 5 my, and this is not surprising if we consider that the global paleointensity was not necessarily low and rather oscillatory during the period of 40–100 ka which precedes the global low in 20–40 ka. Two low latitude VGPs from 48 ka lava (41.9°N, 196.2°E) and 80 ka lava (15.9°N, 183.3°E) indicate existence of excursions in Japan during the latest Pleistocene. Preliminary paleointensity experiments indicate that the 80 ka excursion is accompanied by a low paleointensity of about 5 μT while the 48 ka one is not. These two excursions are probably related to those reported previously not only from the Ontake Volcano and its proximity but also from other several sites in Japan, although they were not conclusive. The 48 ka excursion probably correlates to the Laschamp excursion. This is consistent with the fact that the VGP position from the 48 ka lava comes to the central Pacific region, similar to those from the Laschamp excursion found in New Zealand. Possible correlation of the 80 ka excursion is the one from Norwegian-Greenland Sea and Arctic Ocean, which are stratigraphically between the Laschamp and the Blake excursions.

Age of the Mono Lake excursion and associated tephra

The Mono Lake excursion (MLE) is an important time marker that has been found in lake and marine sediments across much of the Northern Hemisphere. Dating of this event at its type locality, the Mono Basin of California, has yielded controversial results with the most recent effort concluding that the MLE may actually be the Laschamp excursion (Earth Planet. Sci. Lett. 197 (2002) 151). We show that a volcanic tephra (Ash ♯15) that occurs near the midpoint of the MLE has a date (not corrected for reservoir effect) of 28,620±300 14C yr BP (∼32,400 GISP2 yr BP) in the Pyramid Lake Basin of Nevada. Given the location of Ash ♯15 and the duration of the MLE in the Mono Basin, the event occurred between 31,500 and 33,300 GISP2 yr BP, an age range consistent with the position and age of the uppermost of two paleointensity minima in the NAPIS-75 stack that has been associated with the MLE (Philos. Trans. R. Soc. London Ser. A 358 (2000) 1009). The lower paleointensity minimum in the NAPIS-75 stack is considered to be the Laschamp excursion (Philos. Trans. R. Soc. London Ser. A 358 (2000) 1009).

Laschamp Excursion at Mono Lake?

The Laschamp Geomagnetic Excursion (ca. 41 ka) and a related increase of cosmogenic nuclides provides a global tie point among sedimentary and ice core records. In the Wilson Creek Formation, Mono Lake, California, the Laschamp Excursion has not been recognized although the so-called Mono Lake excursion was found in the section with an estimated age of about 28 14C ka. However, our reevaluation of the age of the Mono Lake excursion at its type locality using new 14C dates on carbonates and 40Ar/ 39Ar sanidine dates on ash layers yields an estimate of 38–41 ka. This chronology and the absence of a second excursion in the Wilson Creek Formation suggest that the distinct paleomagnetic feature with negative inclinations at Mono Lake is correlative with the Laschamp Excursion.

Magnetostratigraphic evidence for the occurrence of pre-Brunhes (&gt;780 kyr) sediments in the northwestern part of the Kathmandu Valley, Nepal

Fine-grained silt/clay sediments constituting several sections of the fluvio-lacustrine sediments distributed in the central and northwestern parts of the Kathmandu Valley have been studied for the magnetic polarity stratigraphy. These sediments carry a stable primary remanence, residing in hematite and/or magnetite, which is isolated after alternating field demagnetization above 25- 30 mT. The sediments below the Thimi geomorphic surface show dominantly normal polarity except for a short reverse polarity event near the base probably attributable to the Laschamp event (39-45 Ka). In the northwestern part, a prominent reverse polarity magnetozone occurs within the lower parts of the sections that are exposed below the Gokarna geomorphic surfaces. This implies to the deposition of the sediments yielding the reverse polarity prior to the Brunhes Normal Polarity Chron (i.e. before 780,000 yrs). This new discovery requires a thorough chronological study of the northern part of the valley with regard to the extent and distribution of such old deposits, which may directly correlate to the Lokundol Formation occurring widely in the southern part of the Kathmandu Valley.

South Atlantic and North Atlantic geomagnetic paleointensity stacks (0–80 ka): implications for inter-hemispheric correlation

Recent coring of high-deposition-rate Atlantic sediments has led to the development of North Atlantic (NAPIS) and South Atlantic (SAPIS) geomagnetic paleointensity stacks. The SAPIS stack comprises five records from the sub-Antarctic South Atlantic (41–47°S, 6–10°E). Four are from piston cores collected during the Ocean Drilling Program (ODP) Leg 177 site survey cruise (Earth Planet. Sci. Lett. 175 (2000) 145). The other is ODP Site 1089 (Leg 177) (J. Geophys. Res., 2001, submitted for publication). All but one (4-PC03) of the SAPIS cores have oxygen isotope records which are used in conjunction with lithologic and geomagnetic variability to derive an optimized correlation to Site 1089. The Site 1089 age model is derived by mapping the benthic and planktic isotope data to those from nearby core RC11-83 which has 14 calibrated radiocarbon ages in the 11–41 ka interval. Below this level, the chronology is derived by matching the Site 1089 benthic oxygen isotope data to the SPECMAP stack. The NAPIS stack comprises six records from a wide area of the North Atlantic (Philos. Trans. R. Soc. Ser. A 358 (2000) 1009). Correlation between cores was based on marine isotopic stage boundaries, augmented by millennial-scale features (Heinrich events and fluctuations in concentration-dependent magnetic parameters). NAPIS was placed on the GISP2 chronology, using the marine to ice-core oxygen isotope correlation proposed by Voelker et al. (Radiocarbon 40 (1998) 517) for one of the NAPIS cores. In the resulting age model, the Laschamp Event, recorded in five of six cores, falls in a very narrow (<1 kyr) age range and coincides with the 10Be and 36Cl peak measured in the ice cores. Comparison of the two stacks, placed on their own independent age models, indicates that common millennial scale paleointensity features are preserved. Although more work is needed to define the “true” global content of the paleointensity record as well as the precise age of many features, it is readily apparent that paleointensity can provide a global correlation tool at a resolution unattainable from isotope data alone.

Climate-induced variations of cosmogenic beryllium-10 in the sediments of Lake Baikal of the last 150 ky from AMS, SRXRF and NAA data

A 150 ky record of 10Be found in the sediments of Lake Baikal is compared with the two geochemical stacks indicating warm and cold climates (“warm stack” and “cold stack”). The stacks were calculated from the concentrations of elements which were determined by Synchrotron Radiation X-Ray Fluorescent Analysis and Neutron Activation Analysis. The profiles of 10Be concentrations and flux suggest that the main reason of the fluctuations was the changing extents of dilution with terrigenous matter and diatom algae in layers belonging to warm and cold periods marked by the peaks of geochemical stacks. Contrary to the explanations proposed for other records, the rate of production of cosmogenic 10Be is not the principal reason of 10Be fluctuations in the sediments of Lake Baikal; however, the minor peak of 10Be flux at ca. 40 ky BP may be partly due to the global increase in the production of 10Be around the Laschamp excursion. The concentrations of 10Be in the terrigenous fraction, as well as values of the warm geochemical stack, abruptly decreased at the wake of the cold stages, suggesting an increased rate of production of clastic matter which was the source of the terrigenous fraction of the lake sediments. Hence, they are candidates for the proxy of the accumulation rate of terrigenous materials in Lake Baikal sediments.

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

A distinct peak has been discovered in the 36Cl 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 36Cl peak reflects a higher production rate of all cosmogenic radionuclides, it has an impact on the 14C 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.

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 (NAPIS75). Five of these records have been previously correlated...

Reliability of geomagnetic secular variations recorded in a loess section at Lingtai, north-central China

An investigation of the rock magnetic properties using stepwise isothermal remanence (IRM) acquisition, thermomagnetic analysis and temperature-dependent susceptibility history, identifies magnetite as the carrier of the main fraction of the remanence, associated with maghemite and hematite in Malan loess (L1), Holocene soil (S0) and last-glacial paleosol (S1). The presence of short-lived direction fluctuations indicates that no significant smoothing occurs in L1 when its remanence is locked, and thus L1 is capable of recording the geomagnetic secular variation (PSV), while the PSV has been severely smoothed or wiped out by pedogenic processes during S1 formation. It has been suggested that the Mono Lake and Laschamp excursions are two independent geomagnetic events based on this study.