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.

Planktonic foraminiferal assemblages of DSDP Hole 75-532

Qualitative and quantitative analyses of planktonic foraminiferal assemblages from Deep Sea Drilling Project site 532 shed light on hydrographic changes over the Walvis Ridge during the past 500,000 years. From changes in distribution of foraminiferal assemblages, two major hydrographic regimes (coastal and geostrophic branches of the Benguela Current and the Angola Current) can be distinguished at site 532. It is suggested that the hydrographic situation on the northeastern Walvis Ridge was characterized by intensified upwelling and a westward expansion of the coastal upwelling cells during several global cooling pulses. During glacial stages 2-4, the middle part of stage 6, sporadically from the lower stage 8 through upper stage 10, and during stage 12, site 532 was located beneath the coastal branch of the Benguela Current because faunal distribution patterns indicate intensified upwelling. The Angola Current probably intruded the area of study during the lower stages 5, sporadically 6-8, and 11, as documented by the increased abundance of Neogloboquadrina dutertrei.

Radiolarian biostratigraphy in the central Indian Ocean

Identifiable radiolarians of stratigraphic importance were recovered at eight of the sites drilled on Leg 115. The assemblages range in age from Holocene to middle Eocene (Dictyoprora mongolfieri Zone, about 48 Ma). Faunal preservation is particularly good in two stratigraphic intervals: the Holocene through upper Miocene (0-9 Ma), and the lowermost Oligocene to middle Eocene (35-48 Ma). Fluctuating rates of silica accumulation at these drill sites during the Cenozoic reflect changing tectonic and paleoceanographic conditions. In particular, the gradual closure of the Indonesian and Tethyan seaways and the northward migration of the Indian subcontinent severely restricted zonal circulation and silica accumulation in tropical latitudes during the late Oligocene through middle Miocene. By the late Miocene the Indian subcontinent had moved sufficiently north of the equator to allow trans-Indian zonal circulation patterns to become reestablished, and biosiliceous sedimentation resumed. The composition of the radiolarian assemblages in the tropical Indian Ocean is closely comparable with that of the 'stratotype' sequences in the equatorial Pacific. However, there are some notable exceptions in Indian Ocean assemblages: (1) the scarcity of the genera Pterocanium and Spongaster in the Neogene; (2) the absence of the stratigraphically important Podocyrtis lineage, P. diamesa -> P. phyxis -> P. ampla, in the middle Eocene; and (3) the scarcity of taxa of the genus Dorcadospyris, with the exception of D. ateuchus. The succession of radiolarian events was tabulated for those stratigraphic intervals where the assemblages were well preserved. We identified 55 events in the middle Eocene to earliest Oligocene, and 31 events in the late Miocene to Holocene. The succession of events is closely comparable with that of the tropical Pacific. However, there are exceptions that appear to be real, rather than artifacts of sample preservation, mixing, and core disturbance.

(Table S1) Cation compositions of titanomaghemite in DSDP Hole 89-462A basalts

Cretaceous lava flows overlie Jurassic to Early Cretaceous oceanic crust in the Nauru Basin of the western equatorial Pacific, but their exact age and origin is controversial. In one model, they are generically related to volcanism forming the Ontong Java Plateau. However, paleomagnetic data from basalts recovered by ocean drilling in the Nauru Basin have been interpreted as recording numerous geomagnetic reversals, suggesting the Nauru Basin basalts are older than the Early Aptian flows on the Ontong Java Plateau, and the correlative volcanism seen in the western equatorial and southwestern Pacific Ocean basin. Here, we examine the magnetic fidelity of the Nauru Basin basalts through rock magnetic and paleomagnetic approaches. We find the magnetic carriers in the lavas are unlike most basaltic units recovered by oceanic drilling in that they are magnetically soft. This quality makes the rocks especially prone to the acquisition of secondary magnetic components induced during drilling. We demonstrate that the reversed polarity intervals are illusory, and instead record subtle changes in magnetic hardness that result in partial and complete overprinting by the magnetic field associated with the drill string (e.g., the core barrel, drill pipe and bit). The recognition of these magnetic overprints, the identification of only normal polarity in the Nauru Basin basalts, and a critical consideration of the available radiometric and biostratigraphic age data lead us to conclude that coeval formation of the Nauru Basin basalts and Ontong Java Plateau in Aptian times remains a viable hypothesis.

(Table 1) Oxygen isotopes in minerals at DSDP Hole 83-504B basalts

The D/H, 18O/16O and 87Sr/86Sr ratios of the basaltic basement from the Leg 83 section of DSDP Hole 504B show that in that area the oceanic crust has experienced intensive but not pervasive alteration. Isotope ratios of the basalts are very heterogeneous because of an input of oxygen, hydrogen, and strontium from seawater. The hydrogen isotopic composition of many samples displays the complete thermal history of the water-rock interactions. High-temperature mineral formations (actinolites, epidotes, and chlorites) were overgrown by a mineralization at lower temperatures (mixedlayer smectites, iddingsites, and smectites) during successive stages of cooling of the oceanic crust by cold seawater. From 87Sr/86Sr data bulk water/rock ratios up to 5:1 have been calculated. There is evidence that some primary minerals like high-An plagioclases contain oxygen from altered basalts. We have discussed the probability that there existed a seawater/crust interface, now at a depth of 620 m sub-basement, during the high-temperature water/rock interactions. This interface was covered during later magmatism by thick flows, pillow lavas, and intrusives.