Palaeomagnetic Secular Variation Research Articles

NZPSV11k.2023 and NZPSV1k.2023: Holocene palaeomagnetic secular variation master records for New Zealand

The southwest Pacific region represents a large area of the globe that is under-represented in palaeomagnetic secular variation databases. Reliable data from the region are however crucial in constructing reliable global models and master records that can be used for palaeomagnetic and archaeomagnetic dating. NZPSV11k.2023 and NZPSV1k.2023 are the result of many years of work during which the palaeomagnetic records and age control of several sequences of lacustrine sediments have been developed and refined, and during which volcanic and archaeomagnetic records have been collected. The New Zealand Holocene record presented here, NZPSV11k, combines continuous records of declination and inclination from multiple sediment cores from each of three lakes, each sequence being independently dated by radiocarbon estimates and/or tephrochronology. The relative palaeointensity record from one of the lakes has been calibrated and added. All records are referenced to 40°S, 175°E. The resulting record differs significantly from the predictions of global models that do not incorporate data from the region. NZPSV11k shows periods of regular, but aperiodic, moderate amplitude directional swings between 11,250 and 8000 BP and between 4000 BP and the present, but lower amplitude variations during the interim 4000 years. NZPSV1k is a separate, high-resolution, full-vector record covering the past millennium. It combines data from the New Zealand geomagnetic observatory, calculations from the gufm1 model, lake sediment data and palaeointensities from archaeological hāngī (Māori earth oven) stones.It is recommended that the three individual lake sediment records are incorporated separately in future global models of the geomagnetic field, while NZPSV11k and NZPSV1k will be valuable in dating Holocene and recent materials carrying a palaeomagnetic direction and/or intensity.

New archaeomagnetic directions from Late Neolithic sites in Shandong province, China

SUMMARY Archaeomagnetism provides important constraints to help us understand the past behaviour of the geomagnetic field. For archaeologists, archaeomagnetic dating has become a potential supplement to traditional dating methods (e.g. radiocarbon dating). Although China has a long history with numerous archaeological discoveries, the collection of archaeomagnetic data remains scarce. In this paper, we provide new archaeomagnetic directions from four late Neolithic (c. 2000 BC) sites in Shandong province, China. After a careful characterization of magnetic mineralogy and a detailed alternating-field demagnetization of the oriented samples, a total of nine archaeomagnetic directions (each with both declinations and inclinations) were obtained, which fill the large gap at c. 2000 BC in the Chinese palaeomagnetic secular variation (PSV) curve. Combining these new results with previously published data, we updated the Chinese PSV curve for the last 7 kyr. We have compared the updated curve with several global geomagnetic models (e.g. pfm9k.1a, ARCH10k.1, CALS10k.1b). Comparisons show that the CALS10k.1b model does not yield a reasonable fit of the data, and the fit becomes worse for older intervals. This poor fit could be explained by the fact that the CALS10k.1b model consists of a large amount of sedimentary data. Therefore, the PSV pattern is difficult to determine due to the strong aliasing effect. On the contrary, the ARCH10k.1 model gives a much better fit than other models because its data are mostly from archaeological materials and the data are mainly from the Northern Hemisphere. The field intensity and PSV are potentially correlated, with a weak field corresponding to an enhanced PSV. However, due to the lack of data for certain time intervals, the proposed correlations need to be further tested. To explore if PSV exhibits longitudinal symmetric or latitudinal antisymmetric patterns like those of the geocentric-axial-dipole (GAD) model, we compiled and compared data from three East Asia countries (China, Korea and Japan) and from four areas (East Asia, North America, Europe and the Middle East, and Australia and New Zealand) of which the latitudes are between 30° and 40°. In the East Asia region, the PSV patterns shown in each data set are consistent because of the geographic proximity of these three countries. However, when comparing the PSV curves from the four global areas, we suggest a potential declinational minimum between 0 AD and 2000 AD. Although further confirmation and investigation are needed, this declination minimum could be diachronous, sweeping from East Asia to Australia and New Zealand, and then North America. Future studies should focus on adding more reliable and precisely dated data to better delineate the PSV trends. Archaeomagnetic dating is promising when a PSV curve can be continuously reconstructed.

Late Holocene cryptotephra and a provisional 15 000-year Bayesian age model for Cascade Lake, Alaska

Abstract. Multiple chronometers can be employed for dating Holocene palaeoenvironmental records, each with its own inherent strengths and weaknesses. Radiocarbon dating is one of the most widely used techniques for producing chronologies, but its application at high-latitude sites can sometimes be problematic. Here, cryptotephra were identified in a core from Cascade Lake, Arctic Alaska, and used to identify and resolve an age bias in Late Holocene radiocarbon dates from the top 1.42 m of the sediment sequence. Identifiable geochemical populations of cryptotephra are shown to be present in detectable concentrations in sediment from the north flank of the Brooks Range for the first time. Major-element glass geochemical correlations are demonstrated between ultra-distal cryptotephra and reference samples from the Late Holocene caldera-forming eruption of Opala, Kamchatka, as well as three eruptions in North America: the White River Ash (northern lobe), Ruppert tephra and the Late Holocene caldera-forming eruption of Aniakchak. The correlated ages of these cryptotephra provide evidence for an old-carbon effect and support preliminary palaeomagnetic secular variation (PSV) correlated ages reported for Cascade Lake. Chronological data from Cascade Lake were then combined using a Bayesian approach to generate an age–depth model that extends back through the Late Holocene and provisionally to 15 000 cal yr BP.

High resolution inclination records from the Gulf of Alaska, IODP Expedition 341 Sites U1418 and U1419

SUMMARYInternational Ocean Drilling Program (IODP) Expedition 341 recovered sediments from the south Alaska continental slope that preserves a well resolved and dated inclination record over most of the past ∼43 000 yr. The Site U1419 chronology is among the highest resolution in the world, constrained by 173 radiocarbon dates, providing the ability to study Palaeomagnetic Secular Variation (PSV) on centennial to millennial timescales. This record has an exceptionally expanded late Pleistocene sedimentary record with sedimentation rates commonly exceeding 100 cm kyr–1, while also preserving a lower resolution Holocene PSV record at the top. Natural and laboratory-induced magnetic remanences of U1419 u-channels from the 112-m-long spliced record were studied using stepwise AF demagnetization. Hysteresis loops were obtained on 95 and IRM acquisition curves on 9 discrete samples to facilitate magnetic domain state, coercivity and magnetic mineralogical determinations. Due to complexities related to lithology, magnetic mineralogy, and depositional and post-depositional processes, Site U1419 sediments are not suitable for palaeointensity studies and declination could not be robustly reconstructed. Progressive (titano-)magnetite dissolution with depth results in decreasing NRM intensity and signal-to-noise that is exacerbated at higher demagnetization steps. As a result, inclination measured after the 20 mT AF demagnetization step provides the most reliable directional record. Inclination appears to be well resolved with removal of just a few intervals influenced by depositional and/or sampling and coring deformation. The shipboard inclination stack from nearby IODP Site U1418, on a new age model developed from 19 radiocarbon dates on U1418 and 18 magnetic susceptibility-based tie-points to site survey core EW0408-87JC, verifies centennial to millennial scale variations in inclination observed in U1419. Comparisons with other independently dated records from the NE Pacific and western North America suggest that these sites likely capture regional geomagnetic variability. As such, this new high-resolution and well-dated inclination record, especially robust between 15 and 30 cal kyr BP, offers new geomagnetic insights and a regional correlation tool to explore this generally understudied part of the world.

High‐resolution chronology of 24 000‐year long cores from two lakes in the Polar Urals, Russia, correlated with palaeomagnetic inclination records with a distinct event about 20 000 years ago

ABSTRACTBased on radiocarbon dating, a tephra horizon, varve counts and palaeomagnetism, detailed age models covering the last ~24 k cal abp, have been developed for the stratigraphy in the lakes Bolshoye Shchuchye and Maloye Shchuchye in the Polar Ural Mountains, Russia. The inclination curves from these lakes show nearly identical palaeomagnetic secular variations in the studied cores from both lakes, allowing for a precise correlation between the cores. A large and very distinct inclination deviation, named the Bolshoye Shchuchye Event, was identified in all cores retrieved from both lakes. It lasted over a period of 1245 years, from 20 470 to 19 225 cal abp.The well‐dated palaeomagnetic inclination graph offers a new possibility to correlate archives in this part of the Arctic for the last ~24 k cal abp, probably also over longer distances. The sedimentation rate shows the same trend in all cores from both lakes, including high input during the Last Glacial Maximum and gradually lowering after ~18 k cal abpto lower and stable Holocene values.

A Holocene palaeomagnetic secular variation record from Lake Pupuke, New Zealand

ABSTRACT Palaeomagnetic records from three cores of lake sediment have been merged, producing a new Holocene geomagnetic secular variation record for northern New Zealand. The cores are from Lake Pupuke, a maar lake in Auckland Volcanic Field (AVF). They contain tephra from Rangitoto Volcano (530 ± 10 yr BP), Taupō (1718 ± 10 yr BP), Tūhua/Mayor Island (7637 ± 100 yr BP) and Rotomā tephra from Ōkataina Volcanic Centre (9423 ± 120 yr BP). These tephra dates are supplemented by selected radiocarbon age estimates. The cores were correlated using tephra and fine-scale variations in magnetic susceptibility. The natural remanent magnetisation is strong, stable and carried by fine-grained titanomagnetite from AVF’s basaltic volcanoes. The resulting palaeosecular variation (PSV) record spans from 10,000 to 1500 yr BP. It shows well-defined, millennial-scale swings in direction between 10,000 and 8000 yr BP and between 4000 and 1500 yr BP, while between 8000 and 4000 yr BP variations are of higher frequency and lower amplitude and centre on the geocentric axial dipole field direction. This is in excellent agreement with published PSV records from Mavora Lakes, 1000 km further south, indicating the regional nature of PSV – resulting from broad-scale changes in the circulation of conducting iron-rich fluid of Earth’s outer core.

Refined dating using palaeomagnetic secular variations on a lake sediment core from Guozha Co, northwestern Tibetan Plateau

Lake sediments are widely applied to investigate palaeoclimatic and palaeoenvironmental changes on the Tibetan Plateau (TP). Radiocarbon dating of various materials is one of the most commonly used methods to constrain the timing of lake sediment deposition, but it is problematic for most lakes on the TP due to carbon reservoir effects. Because palaeomagnetic secular variations (PSV) share similar patterns at the regional scale, PSV-stratigraphic correlation can provide an alternative approach to date lake sediments. Here, we present a PSV-depth model for a sediment core taken from Guozha Co, a glacial meltwater-fed lake on the northwestern TP. To establish reliable chronological constraints on the sediment core, we compared our PSV-depth model with the PSV stack of East Asia, PSV records of Tangra Yumco and Lake Baikal. The establishment of this new chronology at Guozha Co is a major step toward dating lake records more widely in the TP to reveal palaeoclimatic changes at the world's ‘Third Pole’.

On the relationship between palaeomagnetic secular variation and excursions—records from MIS 8–ODP Leg 172

SUMMARY Palaeomagnetic secular variation (PSV) and excursion data obtained across MIS 8 (243–300 ka) from the western North Atlantic Ocean ODP (Ocean Drilling Program) sites 1060–1063 show composite high-resolution PSV records (both directions and relative palaeointensity) developed for each site and intercompared. Two methods of chronostratigraphy allow us to date these records. First, we used published results that compared the calcium carbonate records of ODP Leg 172 sediments and tuned them with Milankovich cyclicity. We also compared our palaeointensity records with the PISO-1500 global palaeointensity record that was dated with oxygen isotope stratigraphy. We prefer the PISO-1500 record to date our cores. Two excursions are preserved in our PSV records—Excursions 8α and 9α. Our revised age estimates for both excursions are 8α (236.7–239.8 ka) and 9α (283.7–286.9 ka). We have compared shipboard measurements of the two excursions with u-channel measurements of selected excursion intervals. Excursion 8α is interpreted as a ‘Class II’ excursion (local reversal) with in-phase inclination and declination changes; Excursion 9α is a ‘Class I’ excursion with 90° out-of-phase inclination and declination changes. Averaged directions (after removal of true excursional directions) and relative palaeointensity in 3 and 9 ka overlapping intervals show significant PSV directional variability over 104 yr timescales that is regionally correlatable among the four sites. A notable pattern of angular dispersion variability involves most time spent with low (∼10°) dispersion, with three shorter intervals of high (∼25°) dispersion. The relative palaeointensity variability also shows significant variability over 104 yr timescales with three notable intervals of low palaeointensity in all four records and a direct correspondence between the three low-palaeointensity intervals and the three intervals of high angular dispersion. The two magnetic field excursions occur in two of the three low-palaeointensity/high-dispersion intervals. This suggests that the geomagnetic field operates in two states between reversals, one with regular to high palaeointensity and low directional variability and one with low palaeointensity and significantly higher directional variability and excursions.

Dynamic time warping of palaeomagnetic secular variation data

SUMMARY We present and make publicly available a dynamic programming algorithm to simultaneously align the inclination and declination vector directions of sedimentary palaeomagnetic secular variation data. This algorithm generates a library of possible alignments through the systematic variation of assumptions about the relative accumulation rate and shared temporal overlap of two or more time-series. The palaeomagnetist can then evaluate this library of reproducible and objective alignments using available geological constraints, statistical methods and expert knowledge. We apply the algorithm to align previously (visually) correlated medium to high accumulation rate northern North Atlantic Holocene deposits (101–102 cm ka–1) with strong radiocarbon control. The algorithm generates plausible alignments that largely conform with radiocarbon and magnetic acquisition process uncertainty. These alignments illustrate the strengths and limitations of this numerical approach.

A 4500 year record of palaeomagnetic secular variation and relative palaeointensity from the Tyrrhenian Sea

Abstract A marine sediment core from the western Mediterranean provides a new high-resolution 4500 year record of palaeomagnetic secular variation and relative palaeointensity. In 2013, the 7.1 m C5 core was recovered from the Tyrrhenian Sea as part of the NextData climate data project. The coring site, 15 km offshore from the Volturno river mouth, is well located to record combined marine and terrestrial palaeoclimatic influences, and the fine-grained, rapidly deposited sediments are effective palaeomagnetic recorders. We investigate the palaeomagnetic field direction and strength recorded in the core, which provide a valuable high-resolution record of Holocene geomagnetic variation in the area. Using rock magnetic techniques, we constrain the magnetic mineralogy of the studied sediments and confirm their suitability for palaeomagnetic analysis. Palaeomagnetic declination and inclination records were determined by stepwise alternating-field demagnetization, and relative palaeointensity estimates were obtained based on normalization to anhysterestic and isothermal remanent magnetization and to magnetic susceptibility. The age of the core is well constrained with a tephra and biostratigraphic age model, and its magnetic records are compared with relevant core and model data for the region, demonstrating that our record is compatible with previous results from the area. An automated curve matching approach is applied to assess the compatibility of our data with the existing secular variation path for the Mediterranean area.

Palaeomagnetism for chronologies of recent alpine lake sediments: successes and limits

Chronologies of lake-sediment records covering the last centuries to millennia are usually based on both short-lived radionuclides and radiocarbon dating. However, beyond the range of short-lived radionuclides, age model accuracy often suffers from large radiocarbon uncertainties. For high-altitude records, this issue is even more prominent as terrestrial plant fragments for radiocarbon dating are often lacking due to the sparse vegetation in such environments. In this study, we evaluate the potential of the geomagnetic field secular variations as a complementary tool to establish more robust age–depth relationships. Our palaeomagnetic study, applied to five high-altitude lakes from the western European Alps, first shows that recent unconsolidated sediments can carry stable remanent magnetization. The analysis of the magnetic parameters indicates that low-coercivity pseudo-single domain magnetite grains carry the natural magnetization. Nevertheless, the quality of palaeomagnetic secular variation records varies from one lake to another. This quality can be illustrated through the calculation of the declination/inclination maximum angular variations and their comparison to the expected value. Compared with available models, the declination variations are usually too large and the inclination too high. We discuss the validity of palaeosecular variation (PSV) of the Earth’s magnetic field regarding rock magnetism, magnetization processes and possible deformation during coring. From a magnetic point of view, the quality of data is variable, but the characteristic remanent magnetization direction is consistent at site level between neighbouring lakes and with the reference curve, suggesting that geomagnetic field secular variations are approximately recorded. Finally, we attempt to correlate the declination/inclination variations of the characteristic remanent magnetization measured in the five records to the reference geomagnetic model to provide additional chronological markers for age–depth modelling. These stratigraphic chrono-markers appear in systematic agreement with our previous chronological data and enable a reduction of dating uncertainties up to 30% when including these chrono-markers in the age–depth modelling. This agreement supports the interpretation that PSV may have been recorded more or less accurately depending on the studied lake. Therefore, coupled with a comprehensive understanding through other analysis (sedimentology, dating, geochemistry), PSV can be used to improve the age models in the more favourable cases.

Coring induced sediment fabrics at IODP Expedition 347 Sites M0061 and M0062 identified by anisotropy of magnetic susceptibility (AMS): criteria for accepting palaeomagnetic data

SUMMARY Anisotropy of magnetic susceptibility data obtained from discrete subsamples recovered from two Integrated Ocean Drilling Program sites (Expedition 347 sites M0061 and M0062 in the Baltic Sea) by an Advanced Piston Corer are compared to results obtained on subsamples recovered by replicate 6-m-long Kullenberg piston cores. Characteristic natural remanence directions were obtained from the total of 1097 subsamples using principal component analyses. The three principal anisotropy axes of subsamples taken from Advanced Piston Core liners align to the subsample axes, with the maximum axis (K1) parallel to the split core surfaces, possibly caused by outwards relaxation of the core-liners after splitting. A second anomalous anisotropy fabric is characterized by steep values of the angular difference between the inclination of the minimum anisotropy axes (K3) and that expected for horizontal bedding (90°). This fabric is confined to the upper 1–2 m of the Kullenberg cores and specific sections of the advanced piston cores, and we attribute it to conical deformation caused by either excessive penetration speeds and downwards dragging of sediment along the edge of the liner or stretching caused by undersampling. By using our data in an example, we present a protocol to accept palaeomagnetic secular variation data that uses (i) a threshold 90-K3 value of 15°, combined with a modelled, locally applicable minimum inclination of 65° and (ii) an A95 cone of confidence based on Fisher statistics applied to virtual geomagnetic pole distributions.

Palaeo- and rock magnetic investigations of Late Quaternary sediments from the Upper Congo deep-sea fan: on the difficulty in obtaining palaeomagnetic secular variation records from low latitudes

We report here on results of palaeo- and rock magnetic investigations of two sediment cores from the Upper Congo deep-sea fan. The sediments have a high organic content and contain a heterogeneous Fe-mineral assemblage with biogenic magnetite and detrital (Ti-)magnetite as the main magnetic carrier minerals. Pyrite, hematite, and Fe-oxyhydroxides were identified by comparing high-temperature magnetic susceptibility curves with those from Fe-minerals of known composition. According to AMS 14C dates, the 6.8 m-long profile spans the last 37 kyr. Sediments older than 20 ka are affected by reductive diagenesis that has led to a loss of the fine-grained magnetic mineral fraction. Sediments younger than 20 ka have stable magnetizations. Characteristic remanent magnetization records of inclination and declination were obtained for each core. There is a little agreement between these records, modelled curves, and other sediment records from Equatorial Africa, so no composite record could be established. The cores are not ideal relative palaeointensity recorders and estimates using different normalizers did not yield consistent signals from both cores. Normalization methods used for relative palaeointensity estimation were not developed for sediments that contain large amounts of ultra-fine-grained biogenic magnetite; therefore, the relative palaeointensity estimates should be considered with caution. However, in view of the incoherent picture given by the scarce available palaeointensity information from the region off South-West Africa, the GeoB6517-2 record may provide a tentative relative palaeointensity record for comparison, at least for the past 10 kyr.

Chronostratigraphy and spatial distribution of magnetic sediments in the Chukchi and Beaufort seas since the last deglaciation

Palaeomagnetic investigation of three sediment cores from the Chukchi and Beaufort Sea margins was performed to better constrain the regional chronostratigraphy and to gain insights into sediment magnetic properties at the North American Arctic margin during the Holocene and the preceding deglaciation. Palaeomagnetic analyses reveal that the sediments under study are characterized by low‐coercivity ferrimagnetic minerals (magnetite), mostly in the pseudo‐single domain grain‐size range, and by a strong, stable, well‐defined remanent magnetization (MAD <5°). Age models for these sediment cores were constrained by comparing their palaeomagnetic secular variations (inclination, declination and relative palaeointensity) with previously published and independently dated sedimentary marine records from the study area. The magnetostratigraphical age models were verified by AMS radiocarbon dating tie points, tephrochronology and 210Pb‐based sedimentation rate estimate. The analysed cores 01JPC, 03PC and 02PC span c. 6000, 10 500 and 13 500 cal. a BP, respectively. The estimated sedimentation rates were stable and relatively high since the deglaciation in cores 01JPC (60 cm ka−1) and 03PC (40–70 cm ka−1). Core 02PC shows much lower Holocene sedimentation rates with a strong decrease after the deglaciation from ~60 to 10–20 cm ka−1. Overall, this study illustrates the usefulness of palaeomagnetism to improve the dating of late Quaternary sedimentary records in the Arctic Ocean.

New and revised palaeomagnetic secular variation records from post-glacial volcanic materials in New Zealand

Global databases suffer from a paucity of palaeomagnetic secular variation (PSV) data from the SW Pacific region and are often affected by inconsistencies in data quality, or available age control of the sampled units. Here we present new PSV records obtained from post-glacial (≤15kyrsBP) volcanic materials (mainly lavas) from New Zealand. The new dataset incorporates a compilation and critical assessment of all previous discrete PSV data and their age controls, and the results of new sampling in the Taupo Volcanic Zone. The overall dataset includes 23 directional and 10 absolute palaeointensity records primarily from lavas of rhyolitic or andesitic composition. Age constraints on the rhyolitic lavas are available from correlation with distal, radiocarbon dated tephra deposits, age controls on the andesites from field relationships or from 40Ar/39Ar dating. Directional swings, ranging from 326.5° to 26.0° declination and −81.4 to −46.3° inclination and intensity variations from 37.0 to 71.6µT, fall well into the range of PSV expected for this time-frame in New Zealand and reproduce features of continuous sediment records.

The impact of changing reservoir effects on the 14C chronology of a Holocene sediment record from South Africa

A 30.5 m sediment core was recovered from the coastal lake Eilandvlei (EV13), which represents a unique high-resolution record of environmental change for southern Africa. For the establishment of a robust chronology, special emphasis was placed on the calibration of radiocarbon ( 14 C) ages obtained from the dating of different material. However, the reliability of 14 C ages can be problematic since coastal lakes interact with different source pools providing 14 C-depleted (“old”) carbon thus causing reservoir effects. The origin of old carbon affecting the EV13 samples was most likely sourced from the Indian Ocean. Two pre-bomb marine molluscan shells were therefore analysed to determine the regional marine reservoir offset (ΔR), with obtained ΔR values of 134 ± 38 and 161 ± 38 14 C yrs providing the first available data for the south coast of South Africa. However, the application of the resulting average ΔR mean = 148 ± 27 14 C yrs for the calibration of the entire EV13 record underestimates the variable reservoir effects throughout the Holocene. These were possibly caused by past changes in the connectivity between the present lake system and the ocean as well as a varying degree of upwelling in this area. To solve this problem, three sample pairs (each consisting of wood fragments and bulk organic sediment from the same core depth) were dated to calculate the variable past reservoir effects. This approach provided a median basal age of 8920 +200 / -250 cal BP. Palaeomagnetic secular variation stratigraphy was used to corroborate the chronology for the topmost 1.5 m of the record (past millennium), thus providing the first Holocene sediment based inclination and declination data from South Africa.

A new high-resolution record of Holocene geomagnetic secular variation from New Zealand

We present the first full Holocene palaeomagnetic secular variation record from New Zealand. The 11 500 year-long record, from the sediments of Mavora Lakes, comprises composite declination, inclination and relative palaeointensity logs, compiled from two six-metre long cores and the uppermost 1.5 m of another. An age model has been developed from 28 AMS radiocarbon age determinations on fragments of southern beech (Lophozonia menziesii and Fuscospora cliffortioides) leaves. The excellent between-core correlation in all three components of the field results in a high-resolution palaeosecular variation record, with precise and accurate age control. The variations change in character from high amplitude in-phase declination and inclination swings in the earliest part of the record to low amplitude variations in the middle part and declination and inclination swings that are 90° out of phase, leading to broad looping of the vector in the upper part of the record, that is consistent with westward drifting sources in the outer core. The present-day field at the site (Dec = 24.2°E, Inc=−70.7°, F=59 μT) represents a rare steep and easterly extreme direction, but close to average intensity. The palaeointensity is inferred to have varied between about 40 and 90 μT, with variations that, to some extent, mirror variations in the virtual axial geomagnetic dipole moment seen from global data, but also show some notable differences, particularly in the past few thousand years.

Post-depositional remanent magnetization lock-in depth in precisely dated varved sediments assessed by archaeomagnetic field models

Accurate and precise chronologies are needed to evaluate the existence and effect of a post-depositional remanent magnetization lock-in process on sedimentary palaeomagnetic records. Here we present lock-in modelling results of two palaeomagnetic records from varved lake sediments (lakes Kälksjön and Gyltigesjön) in Sweden by using model predictions based on archaeomagnetic data. We used the 14C wiggle-match dating technique to improve the precision of the Kälksjön varve chronology in the period between 3000 and 2000 cal BP, which is characterized by pronounced palaeomagnetic secular variation. This method allowed us to infer an age model with uncertainties of ±20 years (95.4% probability range). Furthermore, we compared the palaeomagnetic record of Kälksjön to Gyltigesjön, which has a corresponding 14C wiggle-matched chronology. The ages of palaeomagnetic features derived from the wiggle-matched varve chronologies are older than those predicted by the archaeomagnetic models. Lock-in modelling was performed with different filter functions to explain the temporal offset and the amplitude of the lake sediment palaeomagnetic data. The analyses suggest that a linear lock-in function with lock-in depths (the depth below which no more natural magnetic remanence is acquired) that range between 30 and 80 cm in Kälksjön and 50 and 160 cm in Gyltigesjön are most appropriate to explain the data. These relatively deep lock-in depths in sediments without a bioturbated ‘mixed-zone’ can be attributed to the relatively high organic contents and low density of the lake sediments, which contribute to a thick unconsolidated upper zone of the sediment sequence in which re-alignment of magnetic particles can take place.

A deglacial palaeomagnetic master curve for Fennoscandia – Providing a dating template and supporting millennial-scale geomagnetic field patterns for the past 14 ka

Reconstructions of palaeomagnetic secular variation (PSV) in sediment cores can be compared to well-dated regional PSV master curves to infer deposition age. The existing PSV master curve for Fennoscandia, “Fennostack” (Snowball et al., 2007), is limited to the past 10 ka. In this study, we construct a deglacial (for the interval 14–11 ka) PSV master curve for Fennoscandia by including data from a number of existing studies in the region, updating geochronologies where necessary. We also produce new deglacial PSV data from Baltic Sea long-core sediments. By selecting three suitable sites, one in southern Sweden and two in northwest Russia, we produce, for the first time, a deglacial PSV master curve for Fennoscandia, which will provide a useful alternative dating tool for deglacial time intervals, especially considering that deglacial sediments are often unsuitable for 14C dating. Additionally, we use the deglacial PSV master curve to assess current hypotheses regarding geomagnetic field changes. Time varying geomagnetic field models constrained by Holocene PSV data from around the globe have predicted the presence of latitudinal and longitudinal patterns in the position of the north geomagnetic pole (NGP). Specifically, a 1350 year cycle in NGP latitude has been noted, along with two preferred dominant mode longitudinal bands for NGP; in Europe and North America (Korte et al., 2011; Nilsson et al., 2011). Most PSV studies of sediment are, however, limited to the Holocene epoch. By combining our deglacial PSV master curve with ‘Fennostack’, we are able to assess general patterns in inclination for the past 14 ka, and compare these to a general prediction of regional inclination for the last 14 ka, based on an extrapolation of the latitudinal and longitudinal NGP periodicity noted by Nilsson et al. (2011). The model prediction suggests that the Fennoscandian PSV for the past 14 ka should reveal three recurring intervals of generally steeper inclination due to a dominant NGP longitudinal band in Europe. We find that the Fennoscandian PSV does indeed show these intervals of generally steeper inclination for the time periods expected, supporting the hypothesised periodic NGP variation, which may represent an inherent feature of the geodynamo.

Seasonal formation of clastic-biogenic varves: the potential for palaeoenvironmental interpretations

Clastic-biogenic varved lake sediments from Nautajärvi, Finland, covering the last ca. 10 000 years have previously been investigated with respect to varve chronology, laminae composition and thickness variations, palaeomagnetic secular variation, the pollen record and the diatom composition of selected sections. In this study, we provide detailed information on the seasonal sedimentation of different varve components and palaeolimnological indicators (diatoms, chrysophycaean cysts) using near-bottom sediment-trap monitoring. The monitoring results confirm previous interpretations of the formation of clastic-biogenic varves in Fennoscandian lakes. The results also indicate that seasonal sediment fluxes correspond with environmental changes. Higher snow storage in winter and the discharge intensity during the following spring snowmelt clearly increase the suspended sediment load, transportation and net accumulation of detrital mineral matter during spring and early summer. A prolonged autumn and subsequent freezing and thawing cycles in winter, conversely, result in an incremental but slow accumulation of assorted mineral matter and organic particles during winter, whereas the subsequent spring flooding and detrital sediment yield are diminished and thinner clastic laminae are formed. The accumulation rates of diatoms and chrysophyte cysts suggest that these algal groups are more dependent on seasonal processes (e.g. spring and autumnal overturn) than on rapid, short-lived environmental episodes such as the spring discharge peak.