Relative Paleointensity Proxies Research Articles

Wavelet-based verification of a relative paleointensity record from the North Pacific

We present a relative paleointensity (RPI) record for the last ~ 1.1 Myr estimated from a sediment core in the central North Pacific, with quality verification using wavelet analysis. Rock magnetic analysis reveals that a stable remanence is carried mainly by single-domain (SD) biogenic magnetite and pseudo-SD detrital magnetite and that concentration- and grain-size-related bulk magnetic parameters vary by a factor of 3, initially meeting a conventional standard for RPI estimation. However, a further test using wavelet spectra of RPI proxies normalized by anhysteretic remanent magnetization (ARM) or isothermal remanent magnetization (IRM) shows intermittent orbital contamination at period of 100 kyr. A part of the 100-kyr orbital frequency in ARM-normalized RPI has a coherence and physical relationship with the normalizer. For the same time interval, a prominent 100-kyr cycle in the ARM/IRM wavelet spectra is also coherent with RPI, indicating that relative changes in biogenic and detrital magnetite were not well compensated by ARM normalization. For the selected RPI proxy using IRM normalization, a significant physical relationship with the lithology of magnetic minerals was not detected in the wavelet analysis, and thus, its intermittent orbital cycles could be of climatic origin, probably induced by non-magnetic factors. Nevertheless, our RPI record is consistent with global RPI stacks and provides a successful reconstruction of paleointensity with geomagnetic field origin in the North Pacific.

Relative paleointensity (RPI) and age control in Quaternary sediment drifts off the Antarctic Peninsula

Lack of foraminiferal carbonate in marine sediments deposited at high latitudes results in traditional oxygen isotope stratigraphy not playing a central role in Quaternary age control for a large portion of the globe. This limitation has affected the interpretation of Quaternary sediment drifts off the Antarctic Peninsula in a region critical for documenting past instability of the West Antarctic Ice Sheet (WAIS) and Antarctic Peninsula Ice Sheet (APIS). Here we use piston cores recovered from these sediment drifts in 2015 during cruise JR298 of the RRS James Clark Ross to test the usefulness for age control of relative paleointensity (RPI) data augmented by scant δ18O data. Thermomagnetic and magnetic hysteresis data, as well as isothermal remanent magnetization (IRM) acquisition curves, indicate the presence of prevalent magnetite and subordinate oxidized magnetite (“maghemite”) in the cored sediments. The magnetite is likely detrital. Maghemite is an authigenic mineral, associated with surface oxidation of magnetite grains, which occurs preferentially in the oxic zone of the uppermost sediments, and buried oxic zones deposited during prior interglacial climate stages. Low concentrations of labile organic matter apparently led to arrested pore-water sulfate reduction explaining oxic zone burial and downcore survival of the reactive maghemite coatings. At some sites, maghemitization has a debilitating effect on RPI proxies whereas at other sites maghemite is less evident and RPI proxies can be adequately matched to the RPI reference template. Published RPI data at ODP Site 1101, located on Drift 4, can be adequately correlated to contemporary RPI templates, probably as a result of disappearance (dissolution) of maghemite at sediment depths >∼10 m.

Oligocene–Miocene relative (geomagnetic) paleointensity correlated from the equatorial Pacific (IODP Site U1334 and ODP Site 1218) to the South Atlantic (ODP Site 1090)

Late Oligocene to Early Miocene relative paleointensity (RPI) proxies can be correlated from the equatorial Pacific (IODP Site U1334 and ODP Site 1218) to the South Atlantic (ODP Site 1090). Age models are constrained by magnetic polarity stratigraphy through correlation to a common geomagnetic polarity timescale. The RPI records do not contain significant power at specific (orbital) frequencies, and hence there is no significant coherency between RPI proxies and the normalizers used to construct the proxies, although orbital power is present in some normalizers. There is no obvious control on RPI proxies from mean sedimentation rate within polarity chrons, magnetic grain size proxies or magnetic concentration parameters. The salient test is whether the equatorial Pacific records can be correlated one to another, and to the records from the South Atlantic. All records are dominated by RPI minima at polarity reversals, as expected, although the comparison within polarity chrons is compelling enough to conclude that the intensity of the Earthʼs axial dipole is being recorded. This is supported by the fit of RPI data from Sites U1334 and 1218 after correlation of the two sites using diverse core-scanning data, rather than polarity reversals alone. We do not see a consistent relationship between polarity-chron duration and mean RPI, and no consistent skewness (“saw-tooth” pattern) for RPI within polarity chrons. Stacks of RPI records for 17.5–26.5 Ma include long-term changes in RPI on Myr timescales that are superimposed on RPI minima associated with polarity reversals, and shorter-term variations in RPI with an apparent pacing of ∼50 kyr. The equatorial Pacific to South Atlantic correlations indicate that RPI can be used as a (global) stratigraphic tool in pre-Quaternary sediments with typical pelagic sedimentation rates of a few cm/kyr.

Relative Paleointensity Recorded in the Core from Northwestern Philippine Sea over the Last 125 ka and Its Influencing Factors

AbstractRelative paleointensity (RPI) from sediments provides constraints for geomagnetic field modeling and also the age information of deposits. The relative paleointensity in a core from the northwestern Philippine Sea was reported and its possible influencing factors including rock magnetic and sedimentological property are examined. Except for the rotation of declination in the brown clay at lower part of the core, both directions of anisotropy of magnetic susceptibility and of the geomagnetic field indicate original sedimentary nature. The magnetically uniform sediment conforms to the requirement for relative paleointensity study and the correlation of relative paleointensity proxies NRM/ARM and NRM/κ with PISO1500 and SINT800 produces depth‐age tie points. The sparse oxygen isotopes on paleointensity chronology in this core agree with the global stack LR04, confirming the correlation of paleointensity. NRM/κ is usually lower than NRM/ARM and the latter has no correlation with ARM and ARM/κ and also no astronomical period, but the former is correlated with ARM/κ and κ and a period of 13∼12 ka by spectral and coherence analysis. Therefore NRM/ARM is regarded to shun off imprint of climate. Further investigations of the intensity difference and its influence elements find that differences between NRM/ARM and NRM/κ vary with the super‐paramagnetic content and magnetic size variation, and the latter contributing 90% of the difference. Other elements like carbonate percentage and mineralogy change are not responsible for the difference. How to correct the effect of magnetic size on paleointenstiy estimation is the focus of further work.

High-resolution paleomagnetic secular variations and relative paleointensity since the Late Pleistocene in southern South America

Paleomagnetic inclination, declination and relative paleointensity were reconstructed from the sediments of Laguna Potrok Aike in the framework of the International Continental scientific Drilling Program (ICDP) Potrok Aike maar lake Sediment Archive Drilling prOject (PASADO). Here we present the u-channel-based full vector paleomagnetic field reconstruction since 51.2 ka cal BP. The relative paleointensity proxy (RPI) was built by normalising the natural remanent magnetisation with the anhysteretic remanent magnetisation using the average ratio at 4 demagnetisation steps part of the ChRM interval (NRM/ARM10–40 mT). A grain size influence on the RPI was removed using a correction based on the linear relationship between the RPI and the median destructive field of the natural remanent magnetisation (MDFNRM). The new record is compared with other lacustrine and marine records and stacks from the mid- to high-latitudes of the Southern Hemisphere, revealing consistent millennial-scale variability, the identification of the Laschamp and possibly the Mono Lake geomagnetic excursions, and a direction swing possibly associated to the Hilina Pali excursion at 20 ka cal BP. Nonetheless, a global-scale comparison with other high-resolution records located on the opposite side of the Earth and with various dipole field references hint at a different behaviour of the geomagnetic field around southern South America at 46 ka cal BP.

Paleomagnetic constraints on the Holocene stratigraphy of the Arctic Alaskan margin

Two long Holocene piston cores (HLY0501-06JPC and -08JPC; herein after referred to as 6JPC and 8JPC) were raised from high sediment accumulation areas in the Arctic Alaskan margin in order to reconstruct the millennial- to centennial-scale behavior of Earth's magnetic field and to better constrain the regional chronostratigraphy of the Western Arctic. Paleomagnetic analyses using a u-channel cryogenic magnetometer (natural, anhysteretic and isothermal remanent magnetizations: NRM, ARM and IRM) and a vibrating sample magnetometer indicate that a strong and stable single component characteristic remanent magnetization carried by low coercivity pseudo-single domain (PSD) grains such as magnetite can be isolated in the postglacial unit of both cores, where the inclination values vary around the geocentric axial dipole (GAD) for the latitude of the coring sites and where the maximum angular deviation (MAD) values are generally lower than 5°. Apart from one interval in each core, all the derived relative paleointensity proxies (NRM/k LF, NRM/ARM and NRM/IRM) yield similar results. NRM/IRM was used as the preferred proxy because, based on cross-spectral analysis, it is not coherent with its normalizer. Based on the comparison with paleomagnetic records from Western North America, the paleomagnetic and physical analyses indicate that both sedimentary sequences have recorded some of the first reliable Arctic high-resolution records of paleomagnetic secular variation (inclination and declination) and relative paleointensity during the Holocene. In addition, full vector paleomagnetic correlations (inclination, declination and relative paleointensity) were used to constrain the chronology of core 6JPC, using core 8JPC and other previously published and independently dated sedimentary and volcanic records from Western North America. The Accelerator Mass Spectroscopy (AMS) radiocarbon-based postglacial chronology of core 8JPC indicates sedimentation rates higher than 300 cm/kyr on the continental shelf near Barrow Canyon from approximately 8000 to 5000 cal BP, followed by a major decrease in sediment deposition. In contrast, the postglacial deposition on the slope at core site 6JPC is relatively constant and sedimentation rates are nearly three times lower.

Chronostratigraphy of sediment cores from the Bering Sea and the subarctic Pacific based on paleomagnetic and oxygen isotopic analyses

The chronostratigraphy of six piston cores collected from the Bering Sea and one piston core from the subarctic Pacific is reported. Age models are primarily based on oxygen isotope data from Cores BOW-8A and ES in conjuction with relative paleointensity proxy records, biogenic opal data, and a radiolarian datum from other cores whose sediments mainly consist of diatom microfossils and poorly preserved calcium carbonate. The records, the first paleointensity proxy reported from the Bering Sea, show that characteristic paleointensity lows at � 40, � 60 and � 100 ka, which have been well documented in other paleointensity records. Those features can be correlated to standard paleointensity curves with the aid of biogenic opal data and a radiolarian datum. The mean sedimentation rates of the diatom-rich cores collected in the eastern Bering Sea range from 10 to 20 cm kyr � 1 . In contrast, the mean sedimentation rate of Core BOW-8A, from the western edge of the Bowers Ridge, is only � 3 cm kyr � 1 , which is comparable to Core ES from the pelagic realm in the subarctic Pacific. These results suggest that biological productivity has been quite high during the late Pleistocene in the eastern part of the Bering Sea, but low over the western edge of the Bowers Ridge. r 2005 Elsevier Ltd. All rights reserved.

A ∼580 kyr paleomagnetic record from the sub‐Antarctic South Atlantic (Ocean Drilling Program Site 1089)

We report geomagnetic directional paleosecular variation, relative paleointensity proxies and oxygen isotope data from the upper 88 m composite depth (mcd) at South Atlantic Ocean Drilling Program (ODP) Site 1089 (40°56.2′S, 9°53.64′E, 4620 m water depth). The age model is provided by high‐resolution oxygen isotope stratigraphy, augmented by radiocarbon dates from the upper 8 mcd of nearby piston core RC11‐83. Mean sedimentation rates at Site 1089 are in the range of 15 to 20 cm/kyr. Two intervals during the Brunhes Chron, at ∼29.6 mcd (∼190 ka) and at ∼48 mcd (∼335 ka), have component magnetization directions with positive (reverse polarity) inclination; however, the excursional directions are heavily overprinted by the postexcursional field. Magnetite is the dominant carrier of magnetic remanence, and occurs in the pseudosingle‐domain (PSD) grain size. An additional higher‐coercivity magnetic carrier, characterized by low unblocking temperatures (<350°C), is assumed to be authigenic pyrrhotite. A decrease in magnetization intensity down core is mirrored by a reduction in pore water sulfate, indicating diagenetic reduction of magnetite. Despite down‐core changes in magnetic mineralogy, normalized intensity records from Site 1089 are comparable with high‐resolution paleointensity records from the North Atlantic (e.g., ODP Sites 983 and 984). Sediment properties and sedimentation patterns within the Cape (Site 1089) and Iceland (Sites 983 and 984) Basins are distinctly different at both millennial and orbital timescales and therefore preclude lithologic variability from being the source of this correlation. Variations in normalized intensity from Site 1089 therefore appear to reflect changes in global‐scale geomagnetic field intensity.

Holocene paleomagnetic records from the St. Lawrence Estuary, eastern Canada: centennial- to millennial-scale geomagnetic modulation of cosmogenic isotopes

Two long Holocene piston cores (MD99-2220 and MD99-2221) were raised from the St. Lawrence Estuary, eastern Canada because of the expanded Holocene sediment sequence this location provides. A u-channel-based paleomagnetic study, supported by an accelerator mass spectrometry (AMS) 14C chronology, rock-magnetic and sedimentological data, indicates that these sediments provide a paleomagnetic directional secular variation (PSV) and relative paleointensity (RPI) proxy records for the last ∼8500 cal BP. Sedimentation rates vary from ∼1.2 to 4.2 m/kyr. Comparison of inclination and declination features with other North American Holocene PSV records are generally consistent and temporal offsets are within chronological uncertainties. The normalized natural remanent magnetization intensity record, a RPI proxy, from the postglacial sediments of core 2220 compares favorably with North American and European Holocene RPI records at millennial and even some centennial timescales. Comparisons between core 2220 RPI proxy and the 10Be flux record from the Greenland Summit (GISP2) ice core [Finkel and Nishiizumi, J. Geophys. Res. 102 (1997) 26699–26706] and a 14C production rate record [Bond et al., Science 294 (2001) 2130–2136] suggest that geomagnetic modulation may control the millennial- and even some centennial-scale variability within cosmogenic isotope records. This implies that the core 2220 RPI record reflects changes in global-scale geomagnetic field at these timescales.

Geomagnetic paleointensity for the last 100 kyr from the sub-antarctic South Atlantic: a tool for inter-hemispheric correlation

We report relative paleointensity proxy records from four piston cores collected near the Agulhas Ridge and Meteor Rise (South Atlantic). The mean sedimentation rate of the cores varies from 24 cm/kyr to 11 cm/kyr. The two cores with mean sedimentation rates over 20 cm/kyr record positive remanence inclinations at 40–41 ka coeval with the Laschamp Event. Age models are based on oxygen isotope data from three of the cores, augmented by radiocarbon ages from nearby Core RC11-83, and by correlation of paleointensity records for the one core with no oxygen isotope data. The relative paleointensity proxy records are the first from the South Atlantic and from the high to mid-latitude southern hemisphere. Prominent paleointensity lows at ∼40 ka and ∼65 ka, as well as many other features, can be correlated to paleointensity records of comparable resolution from the northern hemisphere. The records are attributable, in large part, to the global-scale field, and therefore have potential for inter-hemispheric correlation at a resolution difficult to achieve with isotope data alone.