Influence of drilling on two records of the Matuyama/Brunhes polarity transition in marine sediment cores near Gran Canaria

Abstract

Two records of the Matuyama/Brunhes transition were obtained from the Ocean Drilling Program (ODP) at Sites 953 and 954, which were drilled in the volcanic apron of Gran Canaria in the Canary Islands. Rock magnetic analyses revealed magnetic carriers close to magnetite in composition and in the pseudo-single-domain grain-size range. The transitions were sampled with u-channels and with discrete samples that were taken on both sides of the u-channels covering the transitional region. Each core exhibits a stable record of the geomagnetic transition. Assuming a dipolar field, the virtual geomagnetic pole (VGP) starts at high southern latitudes and ends with well-established VGPs in high northern latitudes.However, there is a large difference of nearly 60° in the longitude of the VGP paths from the two sites. This difference is not the result of different field directions at the two localities, because the sites are only 48 km apart. The remanent magnetization of the discrete samples taken alongside the u-channels is demonstrably contaminated with a drilling-produced magnetic remanence with a horizontal component that is directed radially inward and hard to demagnetize. In addition, a much softer drill moment was observed that is directed steeply downward. The radial character of the magnetic overprint was found because the samples were taken on both sides of the u-channels. Even in the u-channels taken from the center of the core section, the horizontal radial moment appears to be present. The demagnetization characteristics of the radial moment are comparable to the natural remanent magnetization. The presence of the pervasive drill moments and their ability to introduce false transitional points in the reversal records suggest caution in the interpretation of ODP reversal records, which have not been adequately tested for the presence of such drilling-produced overprints.

Relative Paleointensity and Inclination Anomaly Over the Last 8 Myr Obtained From the Integrated Ocean Drilling Program Site U1335 Sediments in the Eastern Equatorial Pacific

“…The proportion of the directional data that passed the selection criteria decreases prior to ~6 Ma, which suggests the possibility of a greater influence of secondary magnetic components. The changes in Δ I s at ~6 Ma might have been caused by incomplete removal of a coring‐induced magnetic overprint of a steep downward direction, which has often been reported for ODP/IODP advanced piston corer cores (e.g., Herr et al, ), although AF demagnetization diagrams do not show any evidence that such a component remained. We did not find any downcore magnetic property change at ~6 Ma that may have caused more intensive magnetic overprints.…”

Environmental rock‐magnetism of Cenozoic red clay in the South Pacific Gyre

“…The interpretation from the onboard preliminary measurements is supported by the post cruise measurements (Figure 3). Secondary magnetization that was probably acquired during coring, which is sometimes serious for ODP/IODP APC cores [e.g., Herr et al, 1998], could generally be removed by peak fields of 5-15 mT. The stepwise AF demagnetization indicates the presence of a stable, well-defined characteristic remanent magnetization (ChRM).…”

Anisotropy of Magnetic Susceptibility (AMS) of Sediments From Holes U1480E and U1480H, IODP Expedition 362: Sedimentary or Artificial Origin and Implications for Paleomagnetic Studies

“…However, AMS of poorly lithified sediments is susceptible to “artificial” (i.e., nongeologic) deformation produced during coring and recovery (Aubourg & Oufi, ; Herr et al, ; Shimono et al, ) and subsequent sampling procedures (Copons et al, ; Gravenor et al, ). Copons et al (), for example, found in soft sediments from Lake Barrancs (Central Pyrenees, Spain) a horizontal K max axis parallel to the core‐splitting surface, although K min axes are perpendicular to bedding planes, indicating a normal sedimentary fabric.…”

“…Those data are critical to a wide variety of topics including paleocurrent estimation (e.g., Dall'olio et al, 2013;Felletti et al, 2016;Novak et al, 2014;Parés et al, 2007;Tamaki et al, 2015), (paleo)stress analysis in active accretionary prisms (Housen et al, 1996;Kanamatsu et al, 2012;Ujiie et al, 2003;Yang et al, 2013), tectonic evolution (e.g., Kitamura et al, 2010;Macrì et al, 2014), and sediment diagenesis (e.g., Kopf & Berhman, 1997;Schwehr et al, 2006). However, AMS of poorly lithified sediments is susceptible to "artificial" (i.e., nongeologic) deformation produced during coring and recovery (Aubourg & Oufi, 1999;Herr et al, 1998;Shimono et al, 2014) and subsequent sampling procedures (Copons et al, 1997;Gravenor et al, 1984). Copons et al (1997), for example, found in soft sediments from Lake Barrancs (Central Pyrenees, Spain) a horizontal K max axis parallel to the core-splitting surface, although K min axes are perpendicular to bedding planes, indicating a normal sedimentary fabric.…”