Yohan Guyodo

Wavelet analysis of relative geomagnetic paleointensity at ODP Site 983

Abstract We performed spectral analysis of the record of relative geomagnetic paleointensity obtained at ODP Site 983, covering the time interval 0–1.1 Ma. The results confirm the presence of significant power at frequencies corresponding to the earth orbital parameters (eccentricity, obliquity, precession). The construction of the evolutionary spectrum allowed us to establish the non-stationarity of the signal at those frequencies. The subsequent use of wavelet techniques made it possible to demarcate the time intervals over which orbital frequencies are present in the record. For those intervals, the paleointensity record shows some coherency with the isothermal remanent magnetization, normalizer of the natural remanent magnetization, and the ratio anhysteretic remanent magnetization/k. These results suggest that the orbital frequencies embedded in the paleointensity record are the expression of lithologic variations, and probably not a characteristic of the geodynamo itself. Extracting (filtering) these wavelet components from the Site 983 paleointensity record indicates, however, that this secondary overprint is of sufficiently low level that it has a minor effect on the overall character of the record.

A 13 200 year history of century to millennial-scale paleoenvironmental change magnetically recorded in the Palmer Deep, western Antarctic Peninsula

Abstract A 13u2008200-yr record of magnetic parameters from the Palmer Deep, western Antarctic Peninsula, records a sequence of five distinct shifts in glacimarine sedimentation coupled with century-scale variations in paleoproductivity. The five major shifts are manifested as abrupt, order of magnitude changes in low-field magnetic susceptibility, accompanied by changes in magnetic particle size and mineralogy. The Late Holocene (3.4–0 ka), the Early Holocene (11.5–9 ka), and the Last Glacial Maximum (prior to 13.2 ka) are intervals of strong low-field magnetic susceptibility and are characterized by multi-domain (MD) magnetite. MD magnetite is associated with zones of abundant gravel grains and is interpreted here as an indicator of material transported as ice-rafted debris. Deglaciation (13.2–11.5 ka) and the Middle Holocene (9–3.4 ka) were times of enhanced productivity. The Middle Holocene marks the onset of century-scale productivity cycles seen in high-field magnetic susceptibility, which is responding to variations in biogenic silica. Deglaciation and the Middle Holocene interval contain pseudo-single domain magnetite and titanomagnetite, respectively. These observations are surprising given the abundance of coarse MD magnetite available in local source rocks. The magnetic mineral assemblage in the Deglaciation interval, however, can be explained by density sorting in meltwater plumes. During the Middle Holocene, the magnetic mineral assemblage suggests the reduction or cessation of locally derived terrigenous sediment, and by inference, the reduction or cessation of iceberg generation. The Early Holocene–Middle Holocene shift in terrigenous sedimentation may be responding in part to sea level, which controls the position of the grounding line. The Palmer Deep Late Holocene interval coincides with the Neoglacial period, a time of glacier re-advances worldwide.

Deconvolution of u-channel paleomagnetic data near geomagnetic reversals and short events

[1]xa0The deconvolution scheme of Oda and Shibuya [1996], designed for half-core data from the pass-through magnetometer aboard the Joides Resolution, has been adapted for treatment of u-channel data. The program was tested using u-channels from Ocean Drilling Program (ODP) Site 1090. A significant increase in resolution of the u-channel data can be achieved by deconvolution, resulting in data comparable with that obtained by continuous discrete sampling. Large amplitude changes of the magnetization vector revealed by the deconvolution treatment are in good agreement with the results obtained on discrete samples extracted from the same u-channels. Application of this procedure to u-channel data from ODP Sites 983 and 984, over a short time interval around 1.255 Ma, permitted enhanced definition of the so-called Bjorn geomagnetic event.

U channel track for susceptibility measurements

[1]xa0A lightweight, transportable, and inexpensive magnetic susceptibility track for u channel samples is described. The track utilizes a Sapphire Instruments 3.3 cm square coil having a response function of shape similar to that of the 2G-Enterprises u channel magnetometer, and a half-peak width of 3 cm. The instrument has been calibrated using a u channel standard made of manganese dioxide. The software incorporates automated track control and measurement drift correction. Susceptibility measurements on this system compare favorably with the 45 mm circular Bartington loop used for u channel samples at the Laboratoire des Sciences du Climat et de lEnvironnement (Gif-sur-Yvette, France) and with data from the whole core susceptibility track aboard the Ocean Drilling Program research vessel JOIDES Resolution.

A sedimentary paleomagnetic record of the Matuyama chron from the Western Antarctic margin (ODP Site 1101)

A high-resolution paleomagnetic record for part of the Matuyama chron (0.7^2.1 Ma) is reported for Ocean Drilling Program Site 1101 (Leg 178), off the Antarctic Peninsula Pacific margin. A rock-magnetic investigation of 62 discrete samples revealed that the natural remanent magnetization (NRM) is carried by pseudo-single domain magnetite. Progressive alternating field demagnetization of 83 m of U-channels provided a polarity stratigraphy down to the Olduvai subchron. Two geomagnetic events preceding the Jaramillo subchron were identified, including the Cobb Mountain polarity interval. The bulk magnetic parameters vary by more than a factor of 20 over the entire time interval, but by less than a factor of 6 over the 0.7^1.1 Ma interval. This latter interval was selected for paleointensity determinations, which were carried out by normalizing the NRM by the anhysteretic remanent magnetization (ARM). Direct comparison of the Site 1101 paleointensity record with other curves available for the same time interval suggests a geomagnetic origin for features present in the record. A more quantitative comparison was achieved by means of a jackknife test performed on nine records of relative paleointensity over the 0.95^1.1 Ma interval. This test yielded no outlier for the period considered, confirming the geomagnetic character of the records. We have constructed a lowresolution stack revealing some of the characteristic paleointensity features of the Jaramillo subchron. fl 2001 Elsevier Science B.V. All rights reserved.

High-resolution paleomagnetic records from Holocene sediments from the Palmer Deep, Western Antartic Peninsula

Thick Holocene sedimentary sections (>45 m) cored in the Palmer Deep by the United States Antarctic Program (USAP) and during Ocean Drilling Program (ODP) Leg 178 provide the first opportunity to examine past geomagnetic field behavior at high southern latitudes. After removal of a low-coercivity drilling overprint the sediments display a stable, single-component remanent magnetization. Two short cores that recovered the uppermost 2.6 m of sediment have inclinations that fluctuate about the present day inclination (−57°) measured at Faraday Station, and several features with wavelengths of 10 to 20 cm appear to be correlative. However, shipboard measurements of inclination fluctuations on split-core samples from three holes drilled at ODP Site 1098 do not correlate well with each other, even though the intensity and susceptibility data correlate very well and the overall mean inclination for cores from each hole is consistent with the expected geocentric axial dipole (GAD) inclination. The correlation is improved dramatically by using inclinations measured on u-channels taken from the pristine center of a split core. Consequently, the anomalous directions and the resulting poor between-hole correlation of inclinations obtained from shipboard data can be attributed to coring-induced deformation, which is common on the outer edge of ODP piston cores, and/or measurement artifacts in the split-core data. Our preferred inclination record is thus derived from u-channel results. The upper ∼25 m represents continuous sedimentation over the past 9000 yr, with an average sedimentation rate exceeding 250 cm/kyr (0.25 cm/yr). Given that remanence measurements on u-channels average over an interval <7 cm long, we obtained independent measurements of the paleo-geomagnetic field that average over only ∼30 yr. This high-resolution record is characterized by an inclination that fluctuates within ±15° of the current GAD inclination.

Detection, identification and mapping of iron anomalies in brain tissue using X-ray absorption spectroscopy.

This work describes a novel method for the detection, identification and mapping of anomalous iron compounds in mammalian brain tissue using X-ray absorption spectroscopy. We have located and identified individual iron anomalies in an avian tissue model associated with ferritin, biogenic magnetite and haemoglobin with a pixel resolution of less than 5u200aμm. This technique represents a breakthrough in the study of both intra- and extra-cellular iron compounds in brain tissue. The potential for high-resolution iron mapping using microfocused X-ray beams has direct application to investigations of the location and structural form of iron compounds associated with human neurodegenerative disorders—a problem which has vexed researchers for 50 years.

North Atlantic Paleoceanography: The Last Five Million Years

In the North Atlantic, cold, relatively salty water sinks in the icy Labrador and Greenland seas, forming North Atlantic Deep Water (NADW).This circulates through the global ocean, driving ocean overturning and global heat transport and, thus, impacting global climate. As one of the most climatically sensitive regions on Earth, the North Atlantic has experienced abrupt changes to its oceanatmosphere-cryosphere system, triggered by fluctuations in meltwater delivery to source areas of NADW formation. n nFor about the past 100 thousand years, these abrupt jumps in climate state have manifested as ‘Dansgaard/Oeschger’ (D/O) oscillations (millennial-scale warm-cold oscillations) and Heinrich events in ice and marine sediment cores, respectively [e.g., Dansgaard et al., 1993; Bond and Lotti, 1995]. These Heinrich events are characterized as huge input of ice-rafted debris (IRD) and meltwater pulses, documenting episodes of sudden instability and collapse of the current Greenland ice sheets and the Laurentide ice sheet, the latter of which covered northern North America several times during the Pleistocene Epoch.

Corrigendum to ''A 13 200 year history of century- to millennial-scale paleoenvironmental change magnetically recorded in the Palmer Deep, western Antarctic Peninsula'' (Earth Planet. Sci. Lett. 194 (2002) 311^326) C

This article contained an error in Fig. 2. Thecorrect depth^age equation for the interval be-tween turbidite 2 (T2) and turbidite 3 (T3)(33.22 to 40 mcd) is:y¼ 3613:4 þ187:27x; R¼ 0:90974Fig. 2 is reprinted below with the correctedequation. The authors would like to apologizefor any inconvenience caused by this error.