Sedimentation rates in the Makarov Basin, central Arctic Ocean: A paleomagnetic and rock magnetic approach

Three long sediment cores from the Makarov Basin have been subjected to detailed paleomagnetic and rock magnetic analyses. Investigated sediments are dominated by normal polarity including short reversal excursions, indicating that most of the sediments are of Brunhes age. In general, the recovered sediments show only low to moderate variability in concentration and grain size of the remanence-carrying minerals. Estimations of relative paleointensity variations yielded a well-documented succession of pronounced lows and highs that could be correlated to published reference curves. However, together with five accelerator mass spectrometry C-14 ages and an incomplete Be-10 record, still two different interpretations of the paleomagnetic data are possible, with long-term sedimentation rates of either 1.3 or 4 cm kyr(-1) However, both models implicate highly variable sedimentation rates of up to 10 cm kyr(-1), and abrupt changes in rock magnetic parameters might even indicate several hiatuses

Palaeomagnetism of late precambrian-cambrian volcanics and intrusives from the armorican massif, France

Received 1983 January 14 ; in original form 1982 June 25International audienceSamples of the keratophyre tuffs of Tréguier (640 ± 12 Myr) of the Upper Brioverian series, and the microgranite dykes of Loguivy (548 ± 24 Myr) have been collected from the north coast of Brittany, France. The samples were progressively demagnetized with AF and, predominantly, thermal methods. Blocking temperatures, coercivities and acquisition of Isothermal Remanent Magnetization (IRM) indicate that the keratophyre remanence is variably carried in magnetite, hematite, or occasionally in both. After removal of a soft viscous component, two stable palaeomagnetic directions were found, one in magnetite, the other in hematite. The characteristic magnetite direction (Tb= 300-580°C) is a pre-folding primary Thermoremanent Magnetization (TRM) which after structural correction has a mean declination/inclination of 217.9°/-30.4°. The younger hematite component has a mean in situ direction of 320.4°/+20.3°.The microgranite Natural Remanent Magnetization (NRM) intensity distribution is distinctly bimodal, forming high and low (≤ 1.6 × A m-1) intensity groups. Upon demagnetization, a single stable remanence (in magnetite) was removed between 300 and 580°C or 20 and 100 mT. Two characteristic directions were found to be statistically related to the intensity groups at the 99 per cent confidence level. Low-intensity samples have a mean direction of 201.9°/+2.3° and probably resulted from a Late Devonian - Early Carboniferous local magnetization. High intensity microgranites have a mean direction of 200°/+50.8° and this remanence is shown to be a primary TRM. Palaeopoles obtained from the two keratophyre magnetizations (magnetite 44.7°N, 116.4°E; hematite 39.5°N, 233.6°E), as well as the high intensity microgranite remanence (5.8°S, 339.9°E), show good agreement with similar age poles from the apparent polar wander path for the Armorican Massif. Comparison of these data to recent palaeomagnetic data from southern England and Wales gives additional evidence to link that region with the Armorica plate as proposed by Van der Voo and Hagstrum et al

Paleointensity of the earth's magnetic late quaternary volcanic sequences at (Indian ocean) field recorded by the island of La Réunion

International audienceA paleomagnetic study has been undertaken on two late Quaternary volcanic sequences from the Piton de la Fournaise volcano (island of La R6union) in order to provide absolute paleointensities of the Earth's magnetic field from a site in the southern hemisphere. The pattern of secular variation recorded by both sequences clearly indicates that the extrusion rate of the lava flows was not constant with time. A detailed investigation of the magnetic properties of the samples was carried out in parallel with paleointensity experiments which were performed with the Thellier method. Samples with minimum overprint, composed by single-domain or pseudo-single-domain magnetite showing good thermal stability and a narrow blocking temperature spectrum were the most successful during Thellier's experiments. Values of the paleofield strength from 19 to 55 mT have been obtained on 23 flows among the 30 which were sampled. Few variations (7.5-9.9 x 10$^{22}$ A m$^2$) are observed in the virtual dipole moments (VDMs) determined on the youngest sequence (5-12 ka). Those values are within the range of fluctuations of the worldwide variations of VDMs for this period. On the other hand, data from the oldest sequence (82-98 ka) are the first paleointensity values obtained in that time period, and the observed VDMs extend from 4.1 to 8.8 x 10$^{22}$ A m$^2$. These results suggest that for this time interval, the field did not depart from its average behavio

Palaeomagnetism of late precambrian-cambrian volcanics and intrusives from the armorican massif, France

Received 1983 January 14 ; in original form 1982 June 25International audienceSamples of the keratophyre tuffs of Tréguier (640 ± 12 Myr) of the Upper Brioverian series, and the microgranite dykes of Loguivy (548 ± 24 Myr) have been collected from the north coast of Brittany, France. The samples were progressively demagnetized with AF and, predominantly, thermal methods. Blocking temperatures, coercivities and acquisition of Isothermal Remanent Magnetization (IRM) indicate that the keratophyre remanence is variably carried in magnetite, hematite, or occasionally in both. After removal of a soft viscous component, two stable palaeomagnetic directions were found, one in magnetite, the other in hematite. The characteristic magnetite direction (Tb= 300-580°C) is a pre-folding primary Thermoremanent Magnetization (TRM) which after structural correction has a mean declination/inclination of 217.9°/-30.4°. The younger hematite component has a mean in situ direction of 320.4°/+20.3°.The microgranite Natural Remanent Magnetization (NRM) intensity distribution is distinctly bimodal, forming high and low (≤ 1.6 × A m-1) intensity groups. Upon demagnetization, a single stable remanence (in magnetite) was removed between 300 and 580°C or 20 and 100 mT. Two characteristic directions were found to be statistically related to the intensity groups at the 99 per cent confidence level. Low-intensity samples have a mean direction of 201.9°/+2.3° and probably resulted from a Late Devonian - Early Carboniferous local magnetization. High intensity microgranites have a mean direction of 200°/+50.8° and this remanence is shown to be a primary TRM. Palaeopoles obtained from the two keratophyre magnetizations (magnetite 44.7°N, 116.4°E; hematite 39.5°N, 233.6°E), as well as the high intensity microgranite remanence (5.8°S, 339.9°E), show good agreement with similar age poles from the apparent polar wander path for the Armorican Massif. Comparison of these data to recent palaeomagnetic data from southern England and Wales gives additional evidence to link that region with the Armorica plate as proposed by Van der Voo and Hagstrum et al

Palaeomagnetism and K-Ar dating of lower carboniferous dolerite dykes from northern Brittany

Accepted 1986 March 6. Received 1986 February 12 ; in original form 1985 August 9International audiencepas de résum