A Late Holocene geomagnetic secular variation record from Erhai Lake, southwest China

A secular variation record of the geomagnetic field direction for the last 6.5  kyr has been obtained from the magnetization of sediment cores from Erhai Lake, southwest China. In order to make a comparison with this record, secular variation in east-central China was investigated by combining available magnetic field data from historical records and archaeomagnetic measurements since about 350 bc . The secular variation in Erhai Lake shows features consistent with the combined record, except for the oldest three observed declination swings in Sian from 720 to 900 ad . Many features of declination and inclination in China also occur in Japan. From 500 to 1000 ad , declination was westerly ranging from about −20° to −5° in Erhai Lake, east-central China, and Japan.     

Geomagnetic secular variations in Greece through the last 6000 years obtained from lake sediment studies

Summary. Geomagnetic declination and inclination secular variation curves have been constructed back to 6000 yr bp from palaeomagnetic measurements of sediments cored in Lakes Volvi, Trikhonis and Begoritis, Greece. They have been dated by palynological studies and by comparison with observatory and archaeomagnetic records. Radiocarbon ages are all affected by the'old carbon'factor. Cores from all three lakes contain a thin ash layer which we associate with the Pompeii eruption of Vesuvius in ad 79. Averaged curves of secular variations of declination and inclination have been computed for the whole study. The broader scale pattern of secular variations is basically similar to lake sediment records from the UK, Poland and Switzerland, though finer detail in the structure of some of the principal features is resolved.     

Geomagnetic variations recorded in Older (≫ 23 000 B.P.) and Younger Yoldia Clay (∼ 14 000 B.P.) at Nørre Lyngby, Denmark

Summary. Detailed palaeomagnetic results from a rapidly deposited 12.5-m Younger Yoldia Clay sequence of age around 14 000 B.P. at the coast cliff at Nørre Lyngby (northern Jutland, Denmark) and a 2-m Older Yoldia Clay sequence of age somewhere between 23 000 and 40 000 B.P. are presented and discussed. The Younger Yoldia Clay spans some 1000–1500 yr and shows swings in inclination and declination of about that period, and also more rapid oscillations which are particularly marked in inclination, showing that rapid secular variations as have occurred during historic times were indeed also present back in time. There exist easterly declinations of 80° to 90° in the upper half of the Younger Yoldia Clay which cause the virtual geomagnetic pole to migrate clockwise to around 50° away from the rotation pole. This we name the 'Nørre Lyngby declination excursion'.
In the Older Yoldia Clay, as well as secular variations in both declination and inclination, significant low inclination values are found, confirming the existence of the recently named 'Rubjerg low inclination excursion', with the virtual geomagnetic pole moving first in a clockwise then in an anticlockwise sense at 40° to 60° away from the rotation pole.
It is therefore inferred that models for the Earth's geomagnetic field should involve at least local rapid eastward as well as westward 'drift' of the non-dipole field components at various times in the past.     

Palaeomagnetism of Holocene lake sediments from north Poland

Summary. Detailed palaeomagnetic investigations of 17 cores of Holocene lake sediments from four lakes in Poland show a pattern of declination variations similar to those found in Lake Windermere, England and in addition a well defined pattern of inclination variations. A total of nine inclination swings are observed compared to five declination swings and their phase relationships are such that both sets of swings cannot be periodic. Exact dating of the palaeomagnetic record is not possible as radiocarbon ages are over 1000 yr older than those deduced from palynological studies. Thermomagnetic and X-ray studies on magnetic concentrates confirm the results of studies of remanence cooling, IRM acquisition and coercivity experiments which are consistent with fine grained magnetite as the main magnetic mineral in the sediments.     

Holocene palaeomagnetic secular variation at Lake Biwa, central Japan

We have conducted a palaeomagnetic study on three sediment cores obtained from a single site in Lake Biwa, central Japan. A total of 1430 discrete samples from three cores show strong and stable remanence, which is carried by pseudo-single-domain magnetite. Palaeomagnetic records from individual cores show little scatter of directions, and there is excellent correlation among the records from the site. A composite palaeomagnetic secular variation (PSV) record for the Holocene was constructed after fine-scale adjustment and stacking of the three records. The age model is based on two wide-spread tephra layers: Kawagodaira (3150 yr BP, calendar years) and Kikai-Akahoya (7250 yr BP). Our results show that the declinations and inclinations are strikingly similar to archaeomagnetic data from southwest Japan for the last 2000 years. The PSV record from Lake Biwa shows good correlation in directions with that of Beppu Bay, located about 500 km west of Lake Biwa, although the Beppu Bay record shows relatively smaller amplitudes of both inclination and declination, probably due to heavy smoothing of the initial results. When the Lake Biwa record is compared with the proposed type curve for southwest Japan, there is considerable disagreement throughout the Holocene. A re-analysis of the data sets and other information used to construct the type curve revealed that the discrepancy in the results was due to inadequate age determination for the cores used in previous studies. We conclude that the stacked PSV record from Lake Biwa meets the criteria required for a reliable curve. Moreover, our results resolve the complications found in the correlation of PSV records in southwest Japan and thereby contribute to a better understanding of Holocene PSV.     

Time series analysis of the 10 000 yr geomagnetic secular variation record from SE Australia

Summary. A 10 000 yr continuous secular variation record from intensively dated lake sediments in SE Australia has been subjected to periodogram and maximum entropy method analysis. Tests on synthetic data reveal some of the limitations of the latter method, particularly when applied to complex number series. Anticlockwise precession of the magnetic vector at a period of 5000 ± 1000 yr is tentatively ascribed to dipole precession, and clockwise precession at a period of about 2800 yr is probably due to westward drift of features of the non-dipole field.
The effect of calibrating the radiocarbon time-scale is important and results in periodicity shifts of up to 25 per cent. Even for well-dated lacustrine sequences power spectra are poorly constrained: it is thus possible that the geomagnetic secular variation on a time-scale of thousands of years is more uniform than often supposed. Mismatches between declination and inclination spectra can arise as a natural consequence of certain types of source mechanism and should not be simply attributed to figments of the analysis employed.     

A geomagnetic secular variation study (31000–19 500 bp) in Western Canada

Summary. Palaeomagnetic results from 212 horizons spread evenly through an 18 m sedimentary sequence in southern British Columbia are reported. Radiocarbon ages suggest that the sequence spans the interval from 31 200 to 19 500 yr bp. No evidence for any large geomagnetic excursions (such as the so-called Mono Lake Excursion) is found, but a distinctive pattern of 'normal' secular variation is observed with declination and inclination swings of 45° and 25° peak to peak amplitude respectively. For the most part the secular variation consists of low amplitude oscillations about the field vector of a geocentric axial dipole expected at the site latitude, but three relatively large perturbations occur at approximately 4000 yr intervals. These perturbations systematically bias the overall mean to shallow inclinations and easterly declinations in a manner reminiscent of the spatially non-isotropic secular variation model proposed by Cox. The bias involved is about 6° in declination and 3° in inclination (overall mean D = 5.8°E, I = 64.2°, α95 = 0.9°, N = 212 horizons), which leads to a pole which is both 'far-sided' and 'right-handed'. If the horizons involved in the three major perturbations are eliminated the mean direction ( D = 1.2°E, I = 67.2°, α95 = 0.8°, N = 125 horizons) does not differ from that of a geocentric axial dipole despite the small cone of confidence.     

Secular variation in North America during historical times

Summary. The secular variation of the declination, inclination and total force of the geomagnetic field has been plotted for 74 locations in North America. A comparison of the occurrences of maxima and minima in the SV curves from different stations shows little evidence of drift in North America. Although a declination maximum exhibits westward drift up to 1915, all other extrema in declination, inclination and total intensity occur almost simultaneously over a wide area. The major feature of SV in North America appears to be a 4000 nT decrease in the total field since 1850, which may be due to a decrease in the dipole moment coupled with the decay of a large non-dipole anomaly situated under the continent. Short-period changes in the rate of decrease are possibly jerks of the magnetic field. Maximum entropy spectral analysis of all three components of the field indicates periods of 102 and 53 yr.     

Long-term features of drifting and standing non-dipole fields as determined from Holocene palaeomagnetic secular variations

Geomagnetic field motions of Holocene secular variations are investigated using a separation method. The palaeomagnetic secular variations from Britain, North America and Australia have been subjected to maximum-entropy method analyses. Based on the results of spectral analyses, the secular variations are separated by band-pass filters into low-frequency components, generally including the period band 1800-3600 yr, and high-frequency components, generally including the period band 1000-1200 yr. There is an interval, from 4200 to 1700 yr BP, which shows clockwise rotational motions in the low-frequency components of all three sites. Westward drifting of geomagnetic fields may be globally dominant. Swinging or elliptical looping motions constrained to a certain direction were observed in the low-frequency components of the British data. The time duration for the persistence of the swinging motion constrained to a certain direction was 3500 years or so, which could be the lifetime of an oscillating stationary field. The duration of the transitional motion was 1000-1300 years, which may indicate the recurrence time of a stationary field.     

Summarized results of the archaeomagnetic investigation of the geomagnetic field variation for the last 8000 yr in south-eastern Europe

Summary. Measurements of the declination, inclination and intensity of the archaeomagnetic field have been made on samples collected from 175 sites in south-eastern Europe. The results reveal both long and short term fluctuations in the ancient virtual dipole moment as well as providing a record of the virtual pole position over an 8000 yr period.     

南极布兰斯菲尔德海峡沉积物的地磁场长期变与定年

对位于布兰斯菲尔德海峡东北部的沉积物岩心D1-7进行了初步环境磁学和古地磁研究,获得了连续的相对地磁场强度和方向。沉积物岩性总体为灰色软塑性粘土,中下部出现一层黑色火山灰层。结合该孔沉积物和有孔虫AMS¹⁴C测年结果（王汝建等未发表资料）,相对地磁场强度及其方向提供了12 ka以来连续的定年标尺,其中相对地磁场强度与具有相似沉积速率的南美Laguna Potrok Aike湖泊记录对比提供了六个对比点;同时特征剩磁倾角和磁偏角提供了另外六个对比点。磁化率各向异性分析揭示了全新世早、中和晚期岩心所在位置底流（南极深层水~1000 m）发生了阶段性显著变化,同时磁性矿物含量、粒度、沉积物湿密度等也发生了相应的变化。这些环境磁学和沉积学的变化主要受控于南极相应纬度处的太阳辐射量以及与辐射量相关的夏季季风降水量的变化,降水量增加导致磁性矿物粒度变细。D1-7也记录了一些千年尺度旋回变化,但是南极地区目前缺乏与之相似分辨率的气候参数记录,无法进一步探讨这些气候事件的缘起和分布范围。对比发现,地磁场长期变产生的年龄与沉积物全样有机碳AMS¹⁴C测年结果之间存在系统的差异,在6 ka以来比有机碳年龄年轻,在6 ka之前则比有机碳年龄老。南极布兰斯菲尔德海峡内各种不同水体的混合以及在全新世全球升温背景下冰川的动态变化和由此引起的中、深层水变化,都是造成水体性质复杂的原因。而地磁场强度和方向变化则不受水体性质的影响,因此可以提供更合理的年龄信息。自12 ka至今,25 cm/ka的平均沉积速率表明南极布兰斯菲尔德海峡地区整个全新世内稳定和丰富的沉积物供应。     

Palaeointensity In the Pelagic Realm: Marine Sediment Data Compared With Archaeomagnetic and Lake Sediment Records

Summary. Four box cores collected from the Ontong—Java plateau during the Eurydice expedition have been used to make relative geomagnetic palaeo-intensity measurements. Rock magnetic measurements on the sediments show that they are characterized by a uniform magnetic mineralogy, and that they are suitable for relative intensity estimates. These are obtained by normalizing the NRM by an ARM imparted in a low DC bias field. the palaeoceanographic event known as the preservation spike is used to establish a crude time-scale for the record so that it may be compared with other data from the same region, and also with global palaeointensity estimates. the marine sediment data are quite similar to Australian intensity data from lake sediments and archaeomagnetic sources, but as might be expected exhibit some obvious differences from the global record.     

Palaeomagnetism of A Core Sediment From the Inland Sea, Japan (Seto Naikai)

Palaeomagnetic measurements on a giant core sample 20 cm in diameter and 7.38 m long collected from Mizushima-Nada, the Inland Sea. Japan (Seto Naikai) provide evidence of post-depositional magnetization. the geomagnetic secular variation from about 4000 to 8000 yr BP is characterized by a long period of westerly declination before 6600 yr BP. the maximum deflection is beyond 50°W during this period. the palaeomagnetic record further demonstrates that there is a hiatus over 3000 yr in sedimentation due 10 the sea-level change around 6.85 m from sea bottom, and that the lower limit of the sea-level around 8000 yr BP is 18.5 m beneath the present sea-level.     

A late Holocene varved sediment record of environmental change from northern Ellesmere Island, Canada

A composite record of varve sedimentation is presented from high arctic meromictic Lake C2. The combination of a short runoff and sediment transport season with the strong density stratification of the lake lead to the formation of annual sediment couplets. This conclusion was confirmed by ²¹⁰Pb determinations. High intra-lake correlation of the varves allowed the construction of a composite record of varve sedimentation from overlapping segments of multiple sediment cores. Cross-dating between core segments isolated counting errors in individual cores, that could be attributed to minor sediment disturbances and vague structures. Resolving counting errors by cross-dating reduced the chronological error of the composite series to an estimated ±57 years.The Lake C2 series is the first non-ice cap, high resolution late-Holocene environmental record from the Canadian high arctic. The composite varve series compares favorably with other high resolution proxies from the arctic, in particular with the ice core records from Devon Island and Camp Century, Greenland. A general correspondence between the varve record and other North American proxies for the little Ice Age period (1400–1900 AD) suggests that the Lake C2 record is sensitive to large-scale synoptic changes.This is the tenth in a series of papers published in this issue on the Taconite Inlet Lakes Project. These papers were collected by Dr R. S. Bradley.     

Observed geomagnetic field intensity in London since 1820

Our main purpose is to collect all magnetic intensity data observed in the vicinity of London and to adjust these to a common site (Greenwich) to complement the 400-year series of declination ( D ) and inclination ( I ) data of Malin & Bullard (1981 ). The present series is necessarily shorter, since a method for the measurement of intensity in absolute units was not devised until 1832. We have also supplemented the D and I series of Malin & Bullard with recently acquired data.
We have also made observations of D , I and total intensity ( F ) at a number of the sites, partly to bring the series up to date and partly to check on the site differences. With the increasing urbanization of London it is necessary to seek data from remoter sites. It is shown that the site differences change significantly with time, but that allowance can be made for this.
We present curves of our best estimates of the variation of D , I , F and the horizontal intensity ( H ) that define the complete geomagnetic vector at Greenwich for the interval 1820–1998. Frequency analysis shows little support for a 60-year line in the power spectrum. Within the uncertainty of their determinations, there is good continuity between archaeomagnetic intensity measures and the present results. The moving eddy hypothesis of Malin & Bullard is found to be untenable.     

Paleolimnological assessment of recent environmental change in Lake Baikal: sediment chronology

Sediment cores collected from six sites in Lake Baikal as part of an extensive investigation of sediment records of recent environmental change were dated radiometrically by ²¹⁰Pb. Although the results obtained were in some respects comparable to those obtained by previous ²¹⁰Pb studies, there were some significant differences. Most notably, a core from abyssal depths in the centre of the middle basin appeared to record at least wo major episodes of rapid sedimentation, possibly caused by turbidity currents. All cores, including those from the North Basin, contained significant records of weapons test fallout ¹³⁷Cs, though these were of little chronological value due to the long residence time of ¹³⁷Cs in the water column. Models and budgets for sediment and radionuclide transport through Baikal are presented and used to help validate the ²¹⁰Pb results.     

Paleolimnology and recent environmental change in Lake Baikal: an introduction and overview of interrelated concurrent studies

Recent environmental change research in Lake Baikal is introduced together with an overview of several interrelated papers published concurrently in this issue of Journal of Paleolimnology. Five themes are tackled by analysis of recent Baikal sediment cores, dating, geochemistry, particulate pollutants, magnetism and diatoms. The concurrent papers focus on the first four themes in some detail and summary results of diatom analysis (from Mackay et al., 1998) are given here. Taken together these studies provide a time-space framework for recent environmental change in Lake Baikal not previously available.There are significant shifts in species composition of the endemic planktonic diatom assemblages in uppermost sediments collected from throughout the lake. However, these changes usually precede the sediment record of low level but widespread contamination by industrial products. The most clear sign of industrial contamination is the presence of particles from fossil fuel combustion in sediment post dating the 1930s.Although evidence for widespread biostratigraphic changes by pollution is lacking, radionuclide, diatom, lithostratigraphic and magnetic stratigraphies indicate two main features, (i) it is possible to make stratigraphic correlations within and between basins using recent sediment cores, (ii) that turbidite deposits, from several to tens of cm thick, are frequently encountered in recent sediments.Turbidite deposits occur in ²¹⁰Pb dated and pre-²¹⁰Pb sediment core sections and are undoubtedly a major macro-disturbance feature in many deep water locations in Lake Baikal. If profiles are to be used as direct proxy records of climate variability, then screening of cores for turbidites is a pre-requisite for quality assurance in future paleoenvironmental studies.On-going international research including Swiss, Russian and British joint paleoenvironmental studies on the distribution and biological formation of recent sediments will hopefully lead to better interpretation of Holocene and pre-Holocene sediment records in Lake Baikal.     

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.     

The geomagnetic field over the past 5 million years

The modern geomagnetic field is usually expressed as a spherical harmonic expansion. Although the palaeomagnetic record is very incomplete in both space and time, sufficient data are available from a span of ages to generate time-averaged spherical harmonic field models with many degrees of freedom. Here three data sets are considered: directional measurements from lavas, inclination measurements from ocean sediments, and intensity measurements from lavas. Individual data are analysed, as well as site-averages, using the same methods that have been developed for the modern field, to give models for the past 5 Myr. The normal-polarity field model has an axial-dipole intensity similar to that of the modern-day field, whilst the equatorial-dipole component is very much smaller. The field is not axisymmetric, but shows flux concentrations at the core's surface under Canada and Siberia similar to those observed in the field over historical timescales. Tests on synthetic data show that it is unlikely that these similarities result from the overprinting of the palaeomagnetic field due to inadequate cleaning of the samples. The reverse-polarity field model does not show such obvious features, but this may be due to the sparsity of the data.
The patterns observed in the normal-polarity field, with persistent features in the northern hemisphere and a smooth southern hemisphere, could be explained if the present pattern of secular variation is typical of the past several million years. This would reveal itself as large variations over time in the direction of the magnetic vector in regions of high secular variation, with relatively little change over quieter regions. However, we have been unable to find any evidence for a geographical pattern of secular variation in the data.     

Evidence for mid-Eemian cooling in continental climatic record from Lake Baikal

The discussion on climatic instability observed in Greenland ice cores during the Eemian period (substage 5e) resulted in discovery of a pronounced mid-Eemian cooling event. We report that the mid-Eemian cooling is found for the first time in the biogenic silica climatic record and microfossil abundance record of Lake Baikal. Timing of this event in Lake Baikal correlates well with timing of the European pollen records and marine sedimentary records. The presence of the mid-Eemian cooling signal in the Lake Baikal record suggests a much closer link between Asian climate influenced by strong pressure fields over the vast land masses and the climate-controlling processes in the North Atlantic during interglacial periods, than what was generally believed. Furthermore, the Lake Baikal record suggests that after the mid-Eemian cooling, the climatic conditions returned close to the warmth of the 5e optimum and thus argues that the warm conditions of the last interglacial persisted in Siberia throughout 5e, and did not end with the mid-Eemian cooling as suggested by several published marine records.