Paleomagnetic and rock magnetic properties of ODP Leg 173 sites, west Iberia continental margin

We present detailed paleomagnetic and rock magnetic results of rock samples recovered during Leg 173. The Leg 173 cores display a multicomponent magnetization nature. Variations in magnetic properties correlate with changes in lithology that result from differences in the abundance and size of magnetic minerals. The combined investigation suggests that the magnetic properties of the "fresher" peridotite samples from Site 1070 are controlled mainly by titanomagnetite, with a strong Verwey transition in the vicinity of 110 K, and with field- and frequency-dependent susceptibility curves that resemble those of titanomagnetites. These results are in excellent agreement with thermomagnetic characteristics where titanomagnetites with Curie temperature ~580°C were identified from the "fresher" peridotites. In contrast to the magnetic properties observed from the "fresher" peridotites, the low-temperature curves for the "altered" peridotites did not show any Verwey transition. Thermomagnetic analysis using the high-temperature vibrating sample magnetometer also failed to show evidence for titanomagnetites. The remanent magnetization is carried by a thermally unstable mineral that breaks down at ~420°C, probably maghemite. The field- and frequency-dependent relationships are also directly opposite to those in the reversal zone, with no signs of titanomagnetite characteristics. Altogether, these rock magnetic data seem to be sensitive indicators of alteration and support the contention that maghemite is responsible for the magnetic signatures displayed in the altered peridotites of the upper section. The magnetic minerals of the basement rocks from Sites 1068, 1069, and 1070 are of variable particle size but fall within the pseudo-single-domain size range (0.2-14 µm). The average natural remanent magnetization (NRM) intensity of recovered serpenitinized peridotite is typically on the order of 20 mA/m for samples from Site 1068, but ~120 mA/m for samples from Site 1070. The much stronger magnetization intensity of Site 1070 is apparently in excellent agreement with the observed magnetic anomaly high. Nearly half of the NRM intensity remained after 400°C demagnetization, suggesting that the remanence can contribute significantly to the marine magnetic anomaly.

Calcareous nannofossil stratigraphy and key biohorizons of ODP Leg 173 sites

Six sites were drilled on the southern Iberia Abyssal Plain during Ocean Drilling Program (ODP) Leg 173. Three holes (1067A, 1068A, and 1069A) recovered Eocene sediments consisting of thinly bedded turbidite deposits with interbedded hemipelagic sediments (Bouma sequence Te) deposited near the calcite compensation depth. The hemipelagic sediments are barren of nannofossils, necessitating the use of the turbidite deposits to erect an Eocene biostratigraphy for these holes. Moderately preserved, diverse assemblages of nannofossils were recovered from silty clays (Bouma sequence Td) and poorly preserved, less diverse assemblages were recovered from sandy/silty clays (Bouma sequence Tc). Hole 1067A has a continuous record of sedimentation (Subzones CP9a-CP14a) and Holes 1068A and 1069A have similar continuous records (Subzones CP9a-CP12a), although all holes contain barren intervals. Holes 1067A, 1068A, 1069A, 900A (ODP Leg 149), and 398D (Deep Sea Drilling Project Leg 47B) display a similar increase in mass accumulation rates in the lowermost middle Eocene. A reliable Eocene biostratigraphy has been erected using nannofossil data from turbidite sequences, allowing for correlation between Iberia Abyssal Plain sites.