Shipboard measurements of archived advanced piston corer (APC) cores during Leg 157 revealed a preponderance of radially inward-directed magnetization. The nature of the radial magnetization was investigated with tests on a wash core. The coring-induced magnetization has higher coercivity than a simple isothermal remanent magnetization (IRM). Well-lithified samples do not have this coring-induced, high-coercivity contamination. The coring-induced moment, therefore, appears to be related to the coring or recovery process in relatively poorly lithified sediments.The magnetic fields of bottom-hole assemblies (BHAs), APC barrels, and APC cutting shoes were measured as possible source fields for the drill moments. Fields of tens of mT were found in the core barrels, with fields only 1 order of magnitude smaller at the cutting edge of the shoes. The fields associated with the BHAs were weaker and fell off rapidly on the scale length of the drill bit. The coring-induced magnetization appears to be caused by the mechanical disturbance of sediments during coring, pull-out, or passage up the drill string in the presence of the field of the APC barrel.
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.
Sequences from the volcanic apron of Gran Canaria of the Canary Islands were evaluated for continuity, steadiness of deposition rate, and age resolution to determine their fitness for study of geologic cycles through time. An age model was constructed using regression analysis of the polarity record for Site 953, which had relatively complete magnetostratigraphy from the Brunhes Chron through the middle Miocene. Local ages for biostratigraphic datum levels were estimated from the polarity record at Site 953, and were used to construct age models for the other sites, where polarity records were highly discontinuous. The regression age models were refined using biostratigraphic and sedimentologic data to delimit hiatuses, slumps, and repeated sequences, and the accuracies of the age models were checked with radiometric dates where possible.Local ages of foraminifer and calcareous nannofossil first and last occurrences were estimated from magnetostratigraphy of the sequence. Local ages of the first and last occurrences of most species do not differ significantly from current global/oceanwide ages, except those of nine species: the last occurrences of Globoquadrina dehiscens and Discoaster loeblichii and the first occurrences of Neogloboquadrina acostaensis, Globigerina nepenthes, Discoaster berggrenii, Minylitha convallis, Discoaster hamatus, Discoaster coalitus, and Discoaster kugleri. Hiatuses at the four sites generally group within four periods: the late Fataga eruptive period, the pre-Roque Nublo eruptive period, the Roque Nublo eruptive period, and the Late Pliocene and Quaternary Epochs, when changes in sea level were large and volcanic eruptions were sporadic on Gran Canaria.Bio-and magnetostratigraphic evaluations of the Canary Island sites of Leg 157 clearly show that the section cored at Site 953 is the most continuous with the steadiest rates of sedimentation and the finest age resolution. Most bio-and magnetostratigraphic zones are present, and hiatuses last <0.4 m.y. Site 954 ranks second, but is flawed by several hiatuses >1.0 m.y. The best intervals at these sites, with standard errors of the age estimate <0.1 m.y., are adequate to resolve the broad highs and lows of volcanic activity since the early middle Miocene, third-order sea-level changes, and broad environmental cycles, possibly even 0.4 m.y. Milankovitch cycles, but they are inadequate to resolve shorter cycles, like fourth-order sea-level changes, without improvement to the age models. By contrast, stratigraphic sequences at Sites 955 and 956 were generally unsuitable for time series studies requiring anything but crude age resolution over long intervals.
A thick sequence of volcaniclastic sediments drilled at site 953 during Ocean Drilling Program (ODP) Leg 157 northeast of Gran Canaria (Canary Islands) contains an almost complete magneto-stratigraphy back to the shield stage of the island 14.8 Ma ago. Onshore, a sequence of reversals has been identified and dated in 19 dominantly peralkaline rhyolitic ignimbrites, one rhyolitic, and one basaltic lava flow of the Mogán Group (13.35-13.95 Ma), which overlie basalt flows of the island's shield stage (>14 Ma). The magneto-stratigraphy of the ignimbrites onshore has been correlated with the marine magneto-stratigraphy at site 953, determined in syn-ignimbritic volcaniclastic turbidites, which were deposited practically synchronously immediately following the entry of the parent pyroclastic flows into the sea around the circumference of the island. The four polarity intervals recorded in the sequence of the Mogán Group ignimbrites correspond to C5ACr, C5ACn, C5ADr and C5ADn. Single crystal 40 Ar/ 39 Ar-age determinations of the ignimbrites bracketing the polarity changes gave the following ages and uncertainties for the reversals C5AD (t) (13.95±0.07 Ma), C5AC(o) (13.89±0.08 Ma), and C5AC(t) (13.47±0.09 Ma). The newly dated polarity changes fit and refine the Miocene age model proposed in the global polarity time scale.
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