Search

Paleomagnetic analysis and radiocarbon dating of an expanded Holocene deep-sea sediment sequence recovered by Integrated Ocean Drilling Program (IODP) Expedition 303 from Labrador Sea Site U1305 (Lat.: 57 degrees 28.5 N, Long.: 48 degrees 31.8 W, water depth 3459 m) provides insights into mechanisms that drive both paleomagnetic secular variation (PSV) and magnetization acquisition in deep-sea sediments. Seventeen radiocarbon dates on planktonic foraminifera define postglacial (ca. 8 ka) sedimentation rates as ranging from 35 to > 90 cm/kyr. Alternating field (AF) demagnetization of u-channel samples show that these homogeneous sediments preserve a strong, stable, and consistently well-defined component magnetization. Normalized remanence records pass reliability criteria for relative paleointensity (RPI) estimates. Assuming that the age of magnetization is most accurately defined by well dated PSV records with the highest sedimentation rates, allows us to estimate and correct for temporal offsets at Site U1305 interpreted to result from postdepositional remanence acquisition at a depth of similar to 20 cm. Comparisons indicate that the northern North Atlantic PSV and RPI records are more consistent with European than North American records, and the evolution of virtual geomagnetic poles (VGP) are temporally and longitudinally similar to global reconstructions, though with much larger latitudinal variation. The largest deviations from a geocentric axial dipole (GAD) are observed during times of the highest intensities, in contrast to the usual assumption. These observations are consistent with the idea that PSV in the North Atlantic and elsewhere during the Holocene results from temporal oscillations of high-latitude flux concentrations at a few recurrent locations.

Four piston cores collected in 1999 and 2002 from the Eirik Drift (southern Labrador Sea, off SE Greenland) provide paleomagnetic, environmental magnetic, and oxygen isotope records back to marine isotope stage 11. Age models for the cores are based on a combination of planktonic oxygen isotope data, relative geomagnetic paleointensity proxies, and the identification of geomagnetic excursions (Laschamp and Iceland Basin). Environmental magnetic data delineate two distinct detrital signals, interpreted to reflect the behavior of the surrounding ice sheets (Greenland and Laurentide) to orbital-and millennial-scale climate forcing. Broad decimeter-scale intervals of increased magnetic concentration and grain size occur during the early part of interglacial marine isotopic stages (MIS) 1, 5, 7, 9, and 11. Discrete centimeter-scale layers, recognized by magnetic concentration and grain-size sensitive parameters, gamma ray attenuation (GRA) bulk density, and carbonate content, are observed in glacial and interglacial stages as well as during terminations. On the basis of glacial reconstructions on Greenland during the last termination, the broad decimeter-scale coarser-grained intervals can be attributed to detrital influx associated with the retreat of the terrestrial-based Greenland Ice Sheet in the early Holocene. A similar magnetic signal observed within interglacial MIS 5, 7, 9, and 11 indicates similar Greenland Ice Sheet behavior during these time intervals. Two types of centimeter-scale detrital layers are also recognized back to MIS 11. Detrital carbonate (DC) layers reflect predominately ice-rafted debris (IRD) deposition, while the low detrital carbonate (LDC) layers reflect mass movement as evidenced by sharp basal contacts, graded bedding, and traction structures, likely from the Greenland slope. Some detrital layers on Eirik Drift, particularly the DC layers, can be tentatively correlated to detrital layers observed in the central North Atlantic and to those documented on the southern side of the Labrador Sea at Orphan Knoll.

An Oligocene magnetostratigraphy from ODP Sites 1218 and 1219 (Equatorial Pacific) has been obtained by measurements made on u-channel samples, augmented by about 221 discrete samples. U-channel samples were measured at 1 cm intervals and were stepwise demagnetized in alternating fields (AF) up to a maximum peak field of 80 mT. The magnetization directions were determined at 1 cm intervals by principal component analysis of demagnetization steps in the 20 to 60 mT peak field range. A similar treatment was carried out on the discrete samples, which confirmed the results obtained with u-channel measurements. Sites 1218 and 1219 were precisely correlated based on multisensor track, paleontological and shipboard magnetostratigraphic data; this correlation is substantiated by u-channel measurements. Although the magnetostratigraphy obtained from the u-channels is similar to the interpretation deduced from shipboard measurements based on blanket demagnetization at peak AF of 20 mT, the u-channel results are substantially more robust since many interpretative uncertainties are resolved by the stepwise demagnetization and higher stratigraphic resolution. The temporal resolution of u-channel-based magnetic stratigraphy in the Oligocene section of Sites 1218 and 1219 is better than 5 kyr, and it is therefore suitable for detection of brief polarity subchrons. However, in spite of the high resolution, we did not find any reversals corresponding to the numerous cryptochrons identified in this time span by Cande and Kent [S.C. Cande, D.V. Kent, Revised calibration of the geomagnetic polarity time scale for the Late Cretaceous and Cenozoic, J. Geophys. Res. 100 (1995) 6093–6095].

Pliocene and Miocene magnetostratigraphy from ODP Site 1218 (Equatorial Pacific) has been obtained by measurements made on u-channel samples, augmented by about 50 discrete samples. U-channel samples were measured at 1 cm intervals and stepwise demagnetized in alternating fields up to a maximum peak field of 80 mT. The component magnetization directions were determined by principal component analysis for demagnetization steps in the 20–60 mT peak field range. A relatively small number of discrete samples were subject to both thermal and alternating field (AF) demagnetization and gave results compatible with u-channel measurements. Magnetostratigraphy from u-channel samples are compared with shipboard data that were based on blanket demagnetization at peak AF fields of 20 mT. U-channel measurements add more detail to the magnetostratigraphic record and allow identification of thin polarity zones especially in the upper part of the section were the sedimentation rates are very low (∼2 m/Myr). The component magnetization directions determined from u-channel measurements also gave more reliable and precise estimates of inclination (paleolatitude). The magnetostratigraphy from Site 1218 can be unambiguously correlated with the reference geomagnetic polarity time scale and gives a means of dating the sedimentary sequence. Both Miocene–Pliocene and Oligocene–Miocene stage boundaries were easily identified from the magnetostratigraphic record. Although calculation of paleomagnetic poles is hindered by the low precision of the cores' azimuthal orientation, the data from both u-channel and discrete samples allow determination of the paleolatitude of the Site through time with good precision. Paleomagnetic data indicate that the paleolatitude of Site 1218 has increased form nearly equatorial latitude in the Oligocene to its present-day latitude close to 9°N. Within the precision of the paleomagnetic data, this is in agreement with current predictions of plate motion models based on fixed hotspots.