Magnetic properties of Cenozoic MORB and Cretaceous MORB from DSDP and ODP holes

The fact that the natural remanent magnetization (NRM) intensity of mid-oceanic-ridge basalt (MORB) samples shows systematic variations as a function of age has long been recognized: maximum as well as average intensities are generally high for very young samples, falling off rather rapidly to less than half the recent values in samples between 10 and 30 Ma, whereupon they slowly rise in the early Tertiary and Cretaceous to values that approach those of the very young samples. NRM intensities measured in this study follow the same trends as those observed in previous publications. In this study, we take a statistical approach and examine whether this pattern can be explained by variations in one or more of all previously proposed mechanisms: chemical composition of the magnetic minerals, abundance of these magnetization carriers, vectorial superposition of parallel or antiparallel components of magnetization, magnetic grain or domain size patterns, low-temperature oxidation to titanomaghemite, or geomagnetic field behavior. We find that the samples do not show any compositional, petrological, rock-magnetic, or paleomagnetic patterns that can explain the trends. Geomagnetic field intensity is the only effect that cannot be directly tested on the same samples, but it shows a similar pattern as our measured NRM intensities. We therefore conclude that the geomagnetic field strength was, on-average, significantly greater during the Cretaceous than during the Oligocene and Miocene.

Magnetic properties and oxide petrography of ODP Hole 111-504B

Magnetic properties measurements were performed on 47 samples drilled during Leg 111 of the Ocean Drilling Program and oxide petrography was studied in 32 samples taken at depths throughout the sheeted dike complex in Hole 504B. Integration of these data with results from previous DSDP legs shows that while natural remanent magnetization is constant with depth, magnetic susceptibility increases and median demagnetizing field and the Q ratio decrease with depth in the section. These trends appear to be a result of an increase in deuteric oxidation and a decrease in hydrothermal alteration of primary titanomagnetite with depth. A distinct change in stable magnetic inclination occurs between the extrusive basalts and the sheeted dikes and may be a result of tectonic rotation of the upper extrusive basalts.

Rock-magnetic proxies from a sediment profile of Aike Lake

The sedimentary archive from Laguna Potrok Aike is the only continuous record reaching back to the last Glacial period in continental southeastern Patagonia. Located in the path of the Southern Hemisphere westerly winds and in the source region of dust deposited in Antarctica during Glacial periods, southern Patagonia is a vantage point to reconstruct past changes in aeolian activity. Here we use high-resolution rock-magnetic and physical grain size data from site 2 of the International Continental scientific Drilling Program (ICDP) Potrok Aike maar lake Sediment Archive Drilling prOject (PASADO) in order to develop magnetic proxies of dust and wind intensity at 52°S since 51,200 cal BP. Rock-magnetic analysis indicate the magnetic mineral assemblage is dominated by detrital magnetite. Based on the estimated flux of magnetite to the lake and comparison with distal dust records from the Southern Ocean and Antarctica, kLF is interpreted as a dust indicator in the dust source of southern Patagonia at the millennial time scale, when ferrimagnetic grain size and coercivity influence is minimal. Comparison to physical grain-size data indicates that the median destructive field of isothermal remanent magnetisation (MDFIRM) mostly reflects medium to coarse magnetite bearing silts typically transported by winds for short-term suspension. Comparison with wind-intensity proxies from the Southern Hemisphere during the last Glacial period and with regional records from Patagonia since the last deglaciation including marine, lacustrine and peat bog sediments as well as speleothems reveals similar variability with MDFIRM up to the centennial time scale. MDFIRM is interpreted as a wind-intensity proxy independent of moisture changes for southeastern Patagonia, with stronger winds capable of transporting coarser magnetite bearing silts to the lake.

Cenozoic magnetostratigraphy of ODP holes 113-689B and 113-690B

A detailed paleomagnetic study was carried out on biosiliceous and calcareous sediments drilled on Maud Rise, Antarctica, during ODP Leg 113. High-quality APC sections were retrieved in the upper 220 m of Holes 689B and 690B. Average deposition rates range from 3 to 15 m/m.y. A close (25 cm) paleomagnetic sample spacing provided a medium-resolution magnetostratigraphic sequence for the Paleogene and Neogene. Paleomagnetic samples were demagnetized stepwise by alternating fields, and characteristic remanent magnetization directions were derived from detailed vector and difference vector component analysis. A magnetochronologic framework has been established for the first time for the Southern Ocean sedimentary sequences spanning Paleocene to Oligocene and middle Miocene to early Pliocene times. Biosiliceous and calcareous microfossil stratigraphies were used to constrain magnetostratigraphic age assignments. Although average sedimentation rates were rather low, nearly complete sections of the geomagnetic polarity time scale (e.g., Chrons C5 and C5A) could be correlated with the inferred polarity pattern. Miocene and Pliocene records are marked by a high number of hiatuses mainly identified by diatom biostratigraphy. Good paleomagnetic correlation between the two holes is afforded in particular in the middle to upper Miocene. Oligocene magnetostratigraphy reveals a high-quality paleomagnetic record with a mostly complete Oligocene section in Hole 689B at ~5 m/m.y. deposition rate. Hole 690B exhibits higher deposition rates (7-12 m/m.y.), although two hiatuses are present. Early and late Eocene sedimentary sequences could be analyzed in both holes, but in Hole 689B middle Eocene chrons were disrupted by hiatuses and only incomplete polarity intervals C21 and C24 were encountered. Highest resolution (14 m/m.y.) was achieved in Hole 690B in a complete early Eocene and late Paleocene sequence from Chrons C23 to C26, with a number of short polarity intervals detected within Chrons C24 and C25.

Magnetostratigraphy of ODP Leg 104 holes

Results of a detailed paleomagnetic study on largely undisturbed sedimentary sequences recovered in the Voring Plateau region of the Norwegian Sea during Ocean Drilling Program Leg 104 are presented. At each drill site an essentially continuous downhole magnetic reversal pattern could be defined to depths between 200 and 300 m below seafloor allowing correlations with a calibrated geomagnetic time scale and establishing almost complete magnetostratigraphic records for the core material analyzed. A composite section of the drill holes represents the first high-quality chronostratigraphic framework from the lower Miocene through Holocene obtained in the Norwegian Sea. It should provide a basis for first-order correlations with calcareous and siliceous microfossil events and contribute to a further elaboration of the regional paleoceanographic history. A series of major hiatuses in the upper and middle Miocene accounts for about 4 million yr of missing stratigraphic record.

Magnetic petrology of ODP Leg 121 holes

Given the importance of the inversion of seamount magnetic anomalies, particularly to the motion of the Pacific plate, it is important to gain a better understanding of the nature of the magnetic source of these features. Although different in detail, Ninetyeast Ridge is composed of submarine and subaerial igneous rocks that are similar to those found at many seamounts, making it a suitable proxy. We report here on the magnetic petrology of a collection of samples from Ninetyeast Ridge in the Indian Ocean. Our purpose is to determine the relationship between primary petrology, subsequent alteration, and magnetic properties of the recovered rocks. Such information will eventually lead to a more complete understanding of the magnetization of seamounts and presumably improvements in the accuracy of anomaly inversions. Three basement sites were drilled on Ninetyeast Ridge, with recovery of subaerial basalt flows at the first two (Sites 756 and 757) and submarine massive and pillow flows at the final one (Site 758). The three sites were distinctly different. Site 756 was dominated by ilmenite. What titanomagnetite was present had undergone deuteric alteration and secondary hematite was present in many samples. The magnetization was moderate and stable although it yielded a paleolatitude somewhat lower than expected. Site 757 was highly oxidized, presumably while above sea level. It was dominated by primary titanomagnetite, which was deuterically altered. Secondary hematite was common. Magnetization was relatively weak but quite stable. The paleolatitude for all but the lowermost flows was approximately 40° lower than expected. Site 758 was also dominated by primary titanomagnetite. There was relatively little oxidation with most primary titanomagnetite showing no evidence of high-temperature alteration. No secondary hematite was in evidence. This site had the highest magnetization of the three (although somewhat low relative to other seamounts) but was relatively unstable with significant viscous remanence in many samples. Paleolatitude was close to the expected value. It is not possible, at present, to confidently associate these rocks with specific locations in a seamount structure. A possible and highly speculative model would place rocks similar to Site 757 near the top of the edifice, Site 756 lower down but still erupted above sea level, and Site 758 underlying these units, erupted while the seamount was still below sea level.

Paleomagnetic of DSDP Leg 91 basalts and sediments

Paleomagnetic studies were carried out on 23 basalt and 74 sediment samples from Leg 91 of the Deep Sea Drilling Project, recovered from a portion of the southwestern Pacific plate (24°S, 166°W) dating back to the Early Cretaceous to perhaps Late Jurassic. The expected geocentric axial dipole inclination at this latitude is -41°. The corrected mean stable inclination of -75° for the basalts indicates a paleolatitude of 63°S for their formation and thus 39° of northward motion during the last 100 m.y. Sediment inclinations steepen rapidly below 13-m depth in the core, suggesting little northward motion of this part of the Pacific plate until about 25 m.y. ago. Examination of the opaque minerals in polished section, as well as the Curie temperatures determined for six basalt samples, reveals no evidence of high- or intermediate-temperature oxidation and thus no reheating of the basement rock since its formation.

Magnetic properties of basalts from DSDP Hole 69-504B

The NRM intensity, AF demagnetization characteristics, hysteresis parameters, initial susceptibility, and thermomagnetic characteristics of 18 basalt specimens from Deep Sea Drilling Project Hole 504B were determined. In six samples, the grain size was large enough to allow microprobe analysis. We conclude (1) that the dominant magnetic mineral is titanomagnetite/titanomaghemite; (2) that, except for the upper few meters of the core where the grains are in the stable monodomain state, the grain size of the magnetic mineral lies in the pseudo-single-domain range (< 10 µm); (3) that maghemitization (i.e., low-temperature (< 350°C) oxidation) has taken place. We discuss possible geological histories.

Paleomagnetic of igneous rocks of ODP Leg 114

A paleomagnetic study was made of 12 samples of trachytic basalt from the base of ODP Hole 698A on the Northeast Georgia Rise (southwest Atlantic) and four samples of andesitic basalt and nine samples of volcanic breccia from the base of ODP Hole 703A on the Meteor Rise (southeast Atlantic). The magnetic intensities of the Hole 703A samples are anomalously low, possibly reflecting alteration effects. The mean magnetic intensity of the Hole 698A samples is high, and compatible with the model of Bleil and Petersen (1983) for the variation of magnetic intensity with age in oceanic basalts, involving progressive low-temperature oxidation of titanomagnetite to titanomaghemite for some 20 m.y. followed by inversion to intergrowths of magnetite and other Fe-Ti oxides during the subsequent 100 m.y. These results support the interpretation of the Hole 698A basalts as true oceanic basement of Late Cretaceous age rather than a younger intrusion. Well-defined stable components of magnetization were identified from AF and thermal demagnetization of the Hole 698A basalts, and less well-defined components were identified for the Hole 703A samples. Studies of the magnetic homogeneity of the Hole 698A basalts, involving harmonic analysis of the spinner magnetometer output, indicate the presence of an unevenly distributed low-coercivity component superimposed on the more homogeneous high-coercivity characteristic magnetization. The former component is believed to reside in irregularly distributed multidomain magnetite grains formed along cracks within the basalt, whilst the latter resides in more uniformly distributed finer magnetic grains. The inclination values for the high-coercivity magnetization of five Hole 698A basalt samples form an internally consistent set with a mean value of 59° ± 5°. The corresponding Late Cretaceous paleolatitude of 40° ± 5° is shallower than expected for this site but is broadly compatible with models for the opening of the South Atlantic involving pivoting of South America away from Africa since the Early Cretaceous. The polarity of the stable characteristic magnetization of the Site 698 basalts is normal. This is consistent with their emplacement during the long Campanian to Maestrichtian normal polarity Chron C33N.

Paleomagnetic of ODP Hole 133-823A

Hole 823A covers the upper 120 m (Subunits IA and IB) of Site 823 at the bottom of the Queensland Trough. This hole contains an abundance of gravity-flow deposits, but is thought to have a monotonic age sequence. Above 32 mbsf, a strong, stable (normal) magnetic remanence having a relatively small viscous remanent magnetization (VRM) is seen. Below 32 mbsf, the sediments are subject to widespread VRM, which appears to obliterate the primary magnetization and precludes identification of the Brunhes/Matuyama boundary. Progressive alternating field (AF) demagnetization is limited to low fields (typically <400 Oe) by the weak magnetization in these sediments. As a consequence, the possibility of a high-coercivity component of primary magnetization cannot be ruled out. Lowrie-Fuller tests indicate that this VRM overprinting does not have a multidomain origin. An approximately linear relationship exists between median destructive field (MDF) and the logarithm of the natural remanent magnetization (NRM). Carbonate dilution does not appear to be a dominant factor in controlling variations in concentration-dependent magnetic parameters, such as magnetic susceptibility. The sedimentological distinction between Subunits IA and IB does not show up in the magnetic record. However, a sharp change in magnetic properties does occur at 32 mbsf, with low background magnetizations below this level and high background magnetizations above it. The boundary coincides with a change from thick (>10 cm thick) to thin (<10 cm thick) turbidite deposition, and is also near the boundary separating the sulfate-reduction zone in the upper part of the sequence from the sulfate-free zone beneath. The abrupt nature of the magnetic boundary is evidence that nannofossil subzone CN14b is not condensed, but is missing in a hiatus at 32 mbsf. Nine peaks have been identified in the susceptibility (K) record that are superimposed on ôbackgroundö signals. ARM/K ratios are uniformly low for the background sediments below 32 mbsf, intermediate for strong susceptibility peaks, and high for background sediments above 32 mbsf and weak susceptibility peaks. Comparisons with results from Site 820 suggest that (1) the background sediments above 32 mbsf and the weak susceptibility peaks carry a stable single-domain magnetization, and (2) the high susceptibility peaks are caused by the addition of a superparamagnetic contribution. Expectations are that the distinctive features of the Hole 823A magnetic record are linked to major environmental changes.

Paleomagnetic, stable oxygen isotope ratios, and U/Th activities of ODP Site 172-1062

Geomagnetic excursions are recognized as intrinsic features of the Earth's magnetic field. High-resolution records of field behaviour, captured in marine sedimentary cores, present an opportunity to determine the temporal and geometric character of the field during geomagnetic excursions and provide constraints on the mechanisms producing field variability. We present here the highest resolution record yet published of the Blake geomagnetic excursion (~125 ka) measured in three cores from Ocean Drilling Program (ODP) Site 1062 on the Blake-Bahama Outer Ridge. The Blake excursion has a controversial structure and timing but these cores have a sufficiently high sedimentation rate (~10cm/ka) to allow detailed reconstruction of the field behaviour at this site during the excursion. Palaeomagnetic measurements of the cores reveal rapid transitions (<500 yr) between the contemporary stable normal polarity and a completely reversed state of long duration which spans a stratigraphic interval of 0.7 m. We determine the duration of the reversed state during the Blake excursion using oxygen isotope stratigraphy, combined with 230Th excess measurements to assess variations in the sedimentation rates through the sections of interest. This provides an age and duration for the Blake excursion with greater accuracy and with constrained uncertainty. We date the directional excursion as falling between 129 and 122 ka with a duration for the deviation of 6.5±1.3 kyr. The long duration of this interval and the fully reversed field suggest the existence of a pseudo-stable, reversed dipole field component during the excursion and challenge the idea that excursions are always of short duration.

Magnetic properties of ODP Hole 109-670A

Magnetic fabrics of serpentinized peridotites are related to anisomorphic magnetite formed during serpentinization. In the less serpentinized facies they are, however, mainly mimetic of the high temperature deformation prior to serpentinization. In more serpentinized peridotites, the magnetic fabrics, related to magnetite veins which are more developed in this case, are superimposed on mimetic fabrics. Remanent properties, hysteresis loop parameters, and Curie temperatures were measured. Natural remanent magnetizations (NRM) have crystallization remanent magnetic (CRM) origin. Measured magnetic parameters suggest that pseudo-single domain (PSD) grains of magnetite are present in samples with low degree of serpentinization. The samples with high degree of serpentinization contain mainly multi-domain (MD) magnetite grains.

Magnetic properties of basalts and sediments of ODP Leg 135 sites

Paleomagnetic and rock-magnetic investigations of basalts from Hole 834B in the Lau backarc basin and of sediments from Holes 841A and 841B at the Tonga Ridge are reported. Three groups of blocking temperatures in the basalts suggest the presence of at least three magnetic phases: pure magnetite, a Ti-poor titanomagnetite, and a Ti-rich phase. The drill-string-induced remanence in the basalts is typically between three and six times the original normal remanent magnetization intensity, but it is mostly removed by alternating-field (AF) cleaning in 5 mT. Volume susceptibility values range from 0.04 * 10**-3 to 4 * 10**-3 cgs. The modified Q-ratio J5/sus ranges from 0.5 to 10. The drill-string-induced remanence behaves different in the two sediment cores from Holes 841A and 841B, which may be the result of differences in the sediment or caused by the different drilling equipment used. The AF-cleaned inclinations of the sediment in Holes 841A and 841B suggest a slight flattening with increasing depth (up to 6° under a load of 400 m of sediment) to be present. This flattening is likely to be caused by the differential rotation of detrital particles under compaction during diagenesis.