Magnetostratigraphic results from the eastern Arctic Ocean: AMS 14C ages and relative palaeointensity data of the Mono Lake and Laschamp geomagnetic reversal excursions

A new high-resolution magnetostratigraphic record from the eastern Arctic Ocean has yielded further evidence for the existence of the Laschamp excursion (37–35  ka), the Mono Lake excursion (27–25.5  ka) and possibly another very short excursion (22  ka) inferred from steep negative inclinations. Ages are based on nine AMS (accelerator mass spectrometry) ¹⁴C dates, oxygen isotope stratigraphy and correlation with ODP site 983. Estimates of relative palaeointensity variations for the time interval from 80 to 10  ka have revealed that the documented geomagnetic excursions are linked to large fluctuations of the relative palaeointensity. The lowest values were obtained for the two excursions and the normal–reversed (N–R) and reversed–normal (R–N) transitions of the Laschamp polarity excursion, which itself is characterized by a slight increase of relative palaeointensity during its reversed state. The results are in general agreement with palaeointensity studies from other regions, indicating that these fluctuations could be global phenomena and that the geomagnetic field of the Brunhes Chron was very variable in amplitude as well as in geometry. The new result is one of the rare records comprising large directional as well as large relative palaeointensity variations.     

A 640 kyr geomagnetic and palaeoclimatic record from Lake Baikal sediments

Magnetic remanence vectors for 1737 samples from two ∼100 m cores of Lake Baikal sediments are reported along with complete magnetic susceptibility profiles obtained from a pass-through system. Chronological control is established by means of two independent correlations; first, by matching susceptibility variations to the oceanic oxygen isotope record and second, by matching the relative palaeointensity variations to the SINT-800 global reference curve. These both imply an average deposition rate of 15 cm kyr^–1 and a basal age of ∼640 ka. Spectral analysis reveals the presence of Milankovitch signals at ∼100 kyr (eccentricity), ∼41 kyr (obliquity) and ∼23 and ∼19 kyr (precession). Stable remanence vectors are almost all of normal polarity. The few exceptions comprise brief intervals of low and/or negative inclinations which probably represent geomagnetic excursions. However, these are far less numerous than the high sedimentation rate would lead one to expect. Furthermore, only four of them can be readily matched to the—still poorly understood—global pattern. These are the Laschamp, the Albuquerque, the Iceland Basin and perhaps the West Eifel excursions which occurred at ∼38 000, ∼146 000, at 180 000–190 000 and at 480 000–495 000 yr ago, respectively.     

Palaeomagnetism and K—Ar ages of Lower Ordovician and Upper Silurian—Lower Devonian rocks from north-west Argentina

Summary. In this paper we present palaeomagnetic data from 87 hand samples collected in a sequence of tuffs and shales (Surf Formation) of Llanvirnian age, exposed in north-western Argentina (27° 47' S, 68° 06' W). After cleaning, the majority of samples showed reversed polarity and yielded a palaeomagnetic pole at 5.9° E, 8.5° S (α₉₅= 5.9°). They also showed reversals of declination and inclination at the top of the sequence, which we have associated with geomagnetic excursions. Whole rock K—Ar age de-terminations suggest an age older than 416 ± 25 Myr for the Suri rocks. The predominant reversed stable remanence of these rocks is consistent with the reversed polarity reported for Early Llanvirnian rocks from USSR. The palaeomagnetic pole for the Suri Formation is consistent with the interpretation that Gondwana was a single unit in Early Palaeozoic times.
Palaeomagnetic data from 27 hand samples collected from 10 igneous units of Late Silurian—Early Devonian age (Ñuñorco Formation), exposed in the same area, are also given. The majority of the igneous units showed reversed polarity after cleaning. The positions of VGP's for the Ñuñorco igneous units are scattered and they are not used for geodynamic interpretations. Whole rock K—Ar age determinations suggest ages of 416 ± 25 and 360 ± 10 Myr for two igneous units of the Ñuñorco Formation.     

Palaeomagnetic directions (223-1.4 ka) recorded in the volcanites of Lipari, Aeolian Islands

b
The island of Lipari is formed by Pleistocene volcanites emplaced during four main periods of volcanic activity. A study has been made of their magnetic properties, primarily with the aim of determining changes in the palaeomagnetic directions. Titanomagnetite is always the primary ferromagnetic mineral and its magnetic characteristics are common to the various lithotypes. However, the concentration of titanomagnetite and the degree of magnetic anisotropy vary systematically and correlate with the types of magma (basalt-andesite in the first and second volcanic periods; rhyolite in the third and fourth periods). All palaeomagnetic directions are of normal polarity (Brunhes epoch). Their mean overall palaeomagnetic pole (86N, 238E; dp = 5, dm = 6) is statistically indistinguishable from the geographic pole.
The variations in declination and inclination with age, however, are marked by some evidence of a discontinuity between 150 ± 10 and 127 ± 8 ka. This age can be correlated with the beginning of the Blake event of reverse polarity. The discontinuity might therefore correspond to magnetic excursions which occurred immediately before.     

Geomagnetic excursion in the late Cretaceous

Summary. Two sedimentary cores from the western Pacific display a palaeo-magnetic record of the late Cretaceous long normal interval and the boundary reversed interval corresponding to seafloor spreading anomalies 33–34. Near the young end of this reversed interval, a systematic excursion of inclinations is observed in both cores. Samples are very stable to both alternating field and thermal demagnetization. Blocking temperatures and Curie points suggest that the remanence is carried primarily by magnetite, but with an additional contribution from hematite. Approximate sedimentation rates derived from biostratigraphy suggest that the excursion had a duration of between 46 000 and 54 000yr and occurred about 236000–303000 yr before the succeeding polarity reversal. The excursion, thus, may represent an aborted geomagnetic field reversal.     

Palaeomagnetic record in Late Pleistocene and Holocene dry lake deposits at Tlapacoya, Mexico

Summary. A palaeomagnetic investigation of Carbon-14-dated marsh and near-shore lacustrine sediments deposited between about 25000–5000 yr bp at Tlapacoya, Mexico, reveals normal polarity of the geomagnetic field in all samples measured. At one site, anomalous palaeomagnetic directions in a mud unit dated about 14500 yr bp raised the possibility of a geomagnetic excursion, but subsequent work at six additional sites in the unit revealed no abnormal directions. Thus the anomalous directions are most likely not a true reflection of geomagnetic field behaviour, although no specific alternative explanation is entirely convincing. The preliminary Tlapacoya data of anomalous directions have been cited by others as positive evidence for an excursion. We strongly recommend it no longer be considered as such.     

High-resolution magnetostratigraphy of four sediment cores from the Greenland Sea–I. Identification of the Mono Lake excursion, Laschamp and Biwa I/Jamaica geomagnetic polarity events

High-resolution magnetostratigraphic analysis of three sediment cores from the base of the volcanic seamount Vesteris Banken in the Greenland Basin and one core from the Jan Mayen Fracture Zone revealed records of three pronounced geomagnetic events within the last 200 ka. Dating by stable carbon and oxygen isotope analysis, AMS¹⁴C measurements and biostratigraphic data (foraminifera abundances) yielded ages of 28–27 ka for the Mono Lake excursion, 37–33 ka for the Laschamp event, and 189–179 ka for the Biwa I event. In at least one of the cores the Laschamp event exhibits a full reversal of the local geomagnetic field vector. The same is true of the Biwa I event, documented in one of the cores.     

11 million years of Oligocene geomagnetic field behaviour

An 11 million year long record of the Oligocene geomagnetic field has been obtained from pelagic sediments of DSDP Hole 522 An average sample spacing of 4 cm yielded approximately one specimen per 4 to 8 kyr. The rock magnetics are remarkabh consistent across the entire interval. Previous work demonstrated a magnetic mineralogy dominated by magnetically stable magnetite. The natural remanent magnetism (NRM) carries an Oligocene polarity timescale that is in excellent agreement with the Oligocene reversal record as determined from marine magnetic anomalies (MMAs), including many of the so-called 'crypto-chrons'. Normalized NRM intensities from the undisturbed portions of the record yield a time series of variations with features consistent with a number of other palaeointensity time series derived from both sedimentary and lava sequences. These features include consistent, major decreases in palaeointensity (DIPs) at reversal boundaries, and occasional DIPs between reversal boundaries that could correspond to lineated 'tiny wiggles' in the MMA records. The data set suggests that the overall field strength was 40 per cent higher in the first half of the Oligocene when the average reversal frequency was 1.6 Myr^-1 than in the second half when the reversal frequency was 4 Myr^-1. There is also a weak dependence of average field strength on length of polarity interval. Finally, in the three cores suited to spectral analysis (of coherent polarity and relative intensity independent of lithological contamination), there is a persistent ca. 30–50ka periodicity in the variations of the relative intensity, suggesting that the geomagnetic field 'pulses' at about this frequency, not only during the Brunhes (as demonstrated by Tauxe & Shackleton 1994), but in the Oligocene as well.     

东南极普里兹湾NP93－2柱样古地磁结果

（王保贵）（候红明）（汤贤赞）（袁友仁）ＰａｌｅｏｍａｇｎｅｔｉｃｒｅｓｕｌｔｓｏｆＣｏｒｅＮＰ９３－２ｆｒｏｍｔｈｅＰｒｙｄｚＢａｙ，ＥａｓｔｅｒｎＡｎｔａｒｃｔｉｃａ￥ＷａｎｇＢａｏｇｕｉ；ＨｏｕＨｏｎｇｍｉｎｇ；ＴａｎｇＸｉａｎｚａｎａｎｄＹｕ...     

Palaeomagnetism of a sequence of red beds of the Middle and Upper Sections of Pagnazo Group (Argentina) and the correlation of Upper Palaeozoic-Lower Mesozoic rocks

Palaeomagnetic data from 182 hand samples collected in a rock sequence of about 620-m of red beds of Late Palaeozoic to Early Triassic age exposed in north-western Argentina (30.3° S 67.7° W), are given.
After cleaning, the majority of the Upper Palaeozoic samples (Middle Section of Paganzo Group) show reversed polarity and yield a palaeomagnetic pole at 78° S 249° E (α₉₅= 3°). They also record a polarity transition which we have correlated with the Middle Permian Quebrada del Pimiento Normal Event. The position of the palaeomagnetic pole and the K-Ar age of a basalatic sill at the base of the sequence support this correlation.
Stable remanent magnetization has been isolated in the majority of samples from the Upper Section of the Paganzo Group; it is predominantly reversed and reveals three normal events and also three geomagnetic excursions suggesting an Illawarra Zone age (post Kiaman, Late Tatarian-Early Scythian). The palaeomagnetic pole of the reversely magnetized samples is located at 75° S 285° E(α₉₅= 13°).
The red beds involved in this study are correlated with red beds from the Corumbataí Formation (State of Paraná, Brazil) and with igneous rocks from the Quebrada del Pimiento Formation (Province of Mendoza, Argentina).
The South American Middle and Upper Permian, Upper Permian—Lower Triassic, Lower, Middle and Upper Triassic and Middle Jurassic palaeomagnetic poles reflect a quasistatic period with mean pole at 82° S 244° E, (α₉₅= 4°) which followed the South American Late Palaeozoic polar shift.     

The magnetic polarity stratigraphy of a Hauterivian/Barremian carbonate sequence from southern Italy

Remanence directions, measured at 2  cm intervals along a composite 88  m bore-core, enable mean palaeomagnetic poles to be defined at 13.6°S, 25.2°W and 13.6°N, 154.8°E. The directions of remanence vary very smoothly away from each palaeomagnetic pole, extending more than 90° from them. This raises doubts about the physical meaning of polarity definitions based on the distance between virtual and mean palaeomagnetic poles. For practical purposes, intermediate polarity is defined as directions whose virtual poles lie more than 25° from the mean pole, enabling at least five normal subchrons to be specified within the upper predominately reversed quarter of the core and 11 reversed subchrons within the lower predominantly normal three-quarters of the core. The stratigraphic thickness between these subchrons shows a very high linear correlation ( r >0.99) with the stratigraphic thickness of other terrestrial sequences and the distances between marine polarity sequences of comparable age. The analysed sequence contains wavelength spectra which, when transformed to the temporal realm, match periodicities determined for three marine magnetic anomaly profiles of similar age. These also match planetary orbital periodicities for the Cretaceous. These observations suggest that secular variations and polarity transitions are driven by common core processes whose surface expression is influenced by changes in the planetary orbits. Such detailed geomagnetic features enable far greater reliability in establishing magnetostratigraphic correlations and also enable them to be dated astronomically.     

库姆塔格沙漠东南缘BL剖面粒度记录的全新世快速气候事件

地质历史过程中全新世时期是一个温暖湿润的间冰期气候过程,中间出现过多次变冷或变干的快速气候事件。BL剖面位于库姆塔格沙漠东南缘,是一典型的风成砂黄土沉积地层,沉积厚度约350 cm。通过沉积地层光释光测年和沉积物粒度变化分析,结果显示：（1）BL剖面沉积的年代始于8.3 ka,处在全新世早期;（2）区域干旱气候条件下,反映冬季风的代用指标则以粗端组分含量为主,BL剖面沉积物平均粒度受到颗粒粗端的影响较大,对比3个不同粗颗粒组分含量所指示的古气候变化,选用>110 μm的沉积物颗粒组分含量作为冬季风研究的替代性指标,具有较好的指示意义;（3）剖面粒度对全新世中晚期的6次气候快速变化过程均有不同程度的记录,其特点是对全球性冷事件敏感性较强,对干旱事件响应较弱。对库姆塔格沙漠地区全新世气候过程的研究,可为极端干旱区域气候对全球气候变化过程的响应提供一定借鉴。     

Palaeomagnetism of Upper Cretaceous—Lower Tertiary igneous rocks from Central Argentina

Summary. From nine Upper Cretaceous—Lower Tertiary (85 ± 5–66 ± 5 Ma) volcanic hills in Central Argentina (33°S, 65°W), 26 hand samples were collected yielding a palaeomagnetic pole at 45°E 70°s ( A ₉₅ = 12.1°; k = 13.6; N = 12) after AC cleaning. Three sites show normal and nine reversed polarity. This pole is close to the pole for the late Cretaceous (69 Ma) Andacolo Series.     

Magnetostratigraphy of mid-Cretaceous limestones from the Sierra Madre of northeastern Mexico

We present the results of a palaeomagnetic study of four mid-Cretaceous limestone sections exposed in northeastern Mexico. The limestones are weakly magnetized and exhibit two- to three-component magnetizations. These magnetization components appear to be carried by both a sulphide mineral and a magnetite-titanomagnetite mineral. The sulphide mineral carries a reverse polarity overprint that often makes it difficult to isolate definitively the higher-unblocking-temperature component. The high-unblocking-temperature component is well defined in the upper portion of the Santa Rosa Canyon section and in the Cienega del Toro section and passes the fold test. The characteristic remanent magnetization (ChRM) inclinations agree well with predicted mid-Cretaceous inclinations for these sites, although the declinations differ by more than 100°. The relative rotation between these two sites probably occurred as the thrust sheets were emplaced during Laramide deformation. At two of the sections, namely Cienega del Toro and the overturned Los Chorros sections, only normal polarity directions are observed. The La Boca Canyon and Santa Rosa Canyon sections exhibit zones of both normal and reverse polarity magnetization. Correlation of these polarity zones with the geomagnetic polarity timescale provides a time framework for lithostratigraphic and palaeoceanographic studies of these sections.     

A detailed record of normal-reversed-polarity transition obtained from a thick loess sequence at Jiuzhoutai, near Lanzhou, China

A record of normal-reversed-polarity transition has been obtained from a 4 m thickness of loess exposed at a section near Lanzhou, China. Magnetostratigraphic studies suggest it may represent a reversal bounding the onset of a reversed-polarity zone within the Jaramillo Normal Subchron. The natural remanent magnetization consists of two components: a low-coercivity (≤20mT), low-unblocking-temperature (≤300°C) component of viscous origin and a high-coercivity (>20mT), high-unblocking-temperature (250–700 °C) component carrying the characteristic remanence. Mineral magnetic analyses confirmed the presence of magnetite, its low-temperature oxidation products and haematite, each contributing to the remanence properties. Grain size and concentration showed limited variations and there was little evidence for the presence of the ultrafine magnetic phase commonly associated with palaeosol formation. Pedogenic processes appeared negligible and their effects unimportant, with detrital processes dominating the mineralogy and most probably the acquisition of the characteristic remanence. The reversal record was characterized by the decay and recovery of the geocentric axial dipole term with large directional swings occurring during periods of reduced relative palaeofield intensity. The virtual geomagnetic poles traced a complex path exhibiting no particular geographical confinement. Relative palaeofield intensity determinations were insensitive to the choice of normalization parameter and showed a distinctive asymmetry. Striking similarities were observed with the Matuyama-Jaramillo reversal record, obtained from the same section (Rolph 1993), and the Steens Mountain reversal record (Prévot el al. 1985), lending further support for the existence of unusually high post-transitional field intensities     

Record of the younger part of the Pringle Falls excursion at Long Valley, California

The younger of two closely spaced palaeomagnetic excursions at Pringle Falls, Oregon, is recorded in lacustrine silts that crop out in Long Valley, California. Assigned an age of about 220 000 years, the virtual geomagnetic poles of the younger excursion form a clockwise loop that reached 35 °S latitude east of South America before returning to the northern hemisphere in the Pacific Ocean west of Central America. The poles then form a narrow band across North America while moving to high northern latitudes. This record matches extremely well feature B of the original excursion record from Pringle Falls reported by Herrero-Bervera et al. (1994) and is similar to this excursion at Summer Lake, Oregon ( Negrini et al. 1994 ), in that the pole path is confined primarily to the east–central Pacific Ocean. On the basis of an assumed sedimentation rate of 30  cm per thousand years, the younger excursion (feature B at Pringle Falls) spans an estimated 1200 years and followed by about 1000 years a larger excursion (feature A at Pringle Falls) previously discovered at the same Long Valley site. At a second Long Valley site 30  m away, the younger excursion (feature B) is only partially recorded because of a presumed small hiatus in the sedimentary section.     

A pseudo-Thellier relative palaeointensity record, and rock magnetic and geochemical parameters in relation to climate during the last 276 kyr in the Azores region

In the pseudo-Thellier method for relative palaeointensity determinations (Tauxe et al. 1995) the slope of the NRM intensity left after AF demagnetization versus ARM intensity gained at the same peak field is used as a palaeointensity measure. We tested this method on a marine core from the Azores, spanning the last 276  kyr. We compared the pseudo-Thellier palaeointensity record with the conventional record obtained earlier by Lehman et al . (1996 ), who normalized NRM by SIRM. The two records show similar features: intensity lows with deviating palaeomagnetic directions at 40–45  ka and at 180–190  ka. The first interval is associated with the Laschamps excursion, while the 180–190  ka low represents the Iceland Basin excursion (Channell et al. 1997). The pseudo-Thellier method, in combination with a jackknife resampling scheme, provides error estimates on the palaeointensity.
  Spectral analysis of the rock magnetic parameters and the palaeointensity estimates shows orbitally forced periods, particularly 23  kyr for climatic precession. This suggests that palaeointensity is still slightly contaminated by climate. Fuzzy c -means cluster analysis of rock magnetic and geochemical parameters yields a seven-cluster model of predominantly calcareous clusters and detrital clusters. The clusters show a strong correlation with climate, for example samples from detrital clusters predominantly appear during rapid warming. Although both the pseudo-Thellier palaeointensity m _a and fuzzy clusters show climatic influences, we have not been able to find an unambiguous connection between the clusters and m _a .     

Evidence for the Blake event in volcanic rocks from Lipari (Aeolian Archipelago)

Palaeomagnetic and geochronological measurements have been carried out on the late Pleistocene basaltic–andesitic unit of Monte Chirica–Costa Rasa, on the island of Lipari (Aeolian Archipelago). The lava flow sequence is about 10  m thick and has been sampled in detail. Magnetic properties are rather uniform; Curie temperatures of 540° to 580 °C, and the saturation IRM reached at applied values of 0.1  T point to titanomagnetite as the main magnetization carrier. Thermal and AF demagnetization have shown the presence of secondary magnetization components. These were removed mostly at 450°–500 °C or 20–30  mT, indicating a highly stable ChRM with directions from transitional to reverse. Where a ChRM could not be isolated by application of the demagnetization techniques, the converging remagnetization circles method gave a mean ChRM value fully comparable with that obtained from other methods. ⁴⁰Ar/³⁹Ar determinations were performed on two lava flows, in the lower and upper parts of the sequence. The former shows a transitional ChRM direction and a whole-rock age of 157±12  ka, the latter a reverse direction, a whole-rock age of 143±17  ka and a ground-mass age of 128±23  ka. The radiometric data and the reconstructed stratigraphy, which indicate ages of 150±10  ka and 104±3.5  ka, respectively, for the volcanic units at the bottom and top of the Monte Chirica–Costa Rasa unit, suggest that the reverse directions recorded in Lipari are related to the Blake event.     

会宁剖面黄土粒度记录的MIS3阶段气候变化研究

深海氧同位素第三阶段（MIS3）是末次冰期中的一段特殊时期,通过对会宁黄土剖面粒度变化特征的研究分析恢复重建了会宁MIS3阶段古气候。会宁剖面记录了较明显的5次降温事件——Heinrich事件（H2：23.22～23.51 ka,H3：30.52～31.19 ka,H4：38.3～39.93 ka,H5：43.67～44.82 ka,H6：63.54～64.41 ka）,其中的MIS3阶段根据粒度特征分为3 a（46.36～55.48 ka）较为温湿,3b（30.04～46.36 ka）干冷,3c（24.47～30.04 ka）温湿三个阶段,具明显的D-O旋回。     

Palaeomagnetism of the Proterozoic Zig-Zag Dal Basalt and the Midsommersø Dolerites, eastern North Greenland

Summary. Palaeomagnetic investigations are reported from 24 sites in the Proterozoic Zig-Zag Dal Basalt Formation and 12 sites in the Midsominersø Dolerites of eastern North Greenland. The Zig-Zag Dal Basalt is a typical tholeiitic flood basalt sequence, and dolerite intrusions in the underlying sandstones are thought to be genetically related to the basalts.
After a detailed AF demagnetization programme 19 sites in the basalts and 10 sites in the dolerites reveal one stable component of magnetization, probably of TRM and/or CRM origin residing in small single domain titano-magnetite grains. The degree of anisotropy has not affected the direction of the remanent magnetization. The maximum axis of the anisotropy ellipsoid is parallel to the flow direction of the magma, whereas the minimum axis is perpendicular to the flow plane.
Only one polarity of the geomagnetic field was found. The mean palaeomagnetic pole positions for the two rock types are not significantly different (basalt: 12.2°S, 62.8°E with A ₉₅= 3.8°; dolerites: 6.9°S, 62.0°E with A ₉₅ = 5.1°). After correction for Phanerozoic drift of Greenland the two mean poles compare closely to a relevant North American APW-curve for 1250–1350 Ma, in good agreement with Rb-Sr isochron ages of 1250 Ma obtained for the intrusives. The palaeogeographical position of Greenland was near equator with the major geographical axis orientated E-W.