Oligocene planktonic foraminifera from the Ontong Java Plateau

Ocean Drilling Program Hole 803D (Leg 130) from the western tropical Pacific (Ontong Java Plateau) and Hole 628A (Leg 101) from the western subtropical North Atlantic (Little Bahama Bank) contain rich assemblages of planktonic foraminifers. The uppermost Eocene-basal Miocene section of Hole 803D is apparently complete, whereas the Oligocene section of Hole 628A contains three unconformities based on planktonic foraminiferal evidence. Anomalous ranges are recorded for Chiloguembelina cubensis and Globigerinoides primordius. C. cubensis is found to range throughout the upper Oligocene of both sites, and G. primordius first occurs near the base of upper Oligocene Zone P22 in Hole 628A. Paleomagnetic stratigraphy provides constraints on the last occurrence (LO) of Subbotina angiporoides, the first occurrence (FO) of Globigerina angulisuturalis, the FO of Globigerinoides primordius, the FO of Paragloborotalia pseudokugleri, and the LO of Chiloguembelina cubensis. In general, taxon ranges, total diversity, and the composition of the planktonic foraminiferal assemblages from Holes 628A and 803D are similar. Differences in the composition of planktonic foraminiferal assemblages between the two sites are interpreted to be primarily the result of enhanced dissolution at Site 803 (e.g., paucity of Globigerina angulisuturalis and absence of G. ciperoensis). However, the greater abundances of Subbotina angiporoides in subtropical Hole 628A and Paragloborotalia opima in tropical Hole 803D are probably related to oceanographic differences between the two low-latitude sites. Comparison between the low and southern high latitudes illustrates some similarities in the composition of Oligocene planktonic foraminiferal assemblages as well as some important differences. Species such as Pseudohastigerina spp., Turborotalia increbescens, "Turborotalia" ampliapertura, Paragloborotalia opima, P. pseudokugleri, P. semivera/mayeri, Globigerinella obesa, Globigerina angulisuturalis, G. gortanii, G. ouachitaensis, G. sellii, G. tapuriensis, G. tripartita, G. pseudovenezuelana, Subbotina? eocaena and S.? yeguaensis are absent or have rare occurrences in the subantarctic Oligocene assemblages. Biogeographic gradients, although not as pronounced as during the late Neogene, were nonetheless significant during the Oligocene.

Calcium isotope composition of Oceanic Anoxic Events 1a and 2 sediments

Calcium-isotope ratios (d44/42Ca) were measured in carbonate-rich sedimentary sections deposited during Oceanic Anoxic Events 1a (Early Aptian) and 2 (Cenomanian-Turonian). In sections from Resolution Guyot, Mid-Pacific Mountains; Coppitella, Italy; and the English Chalk at Eastbourne and South Ferriby, UK, a negative excursion in d44/42Ca of ~0.20 per mil and ~0.10 per mil is observed for the two events. These d44/42Ca excursions occur at the same stratigraphic level as the carbon-isotope excursions that define the events, but do not correlate with evidence for carbonate dissolution or lithological changes. Diagenetic and temperature effects on the calcium-isotope ratios can be discounted, leaving changes in global seawater composition as the most probable explanation for d44/42Ca changes in four different carbonate sections. An oceanic box model with coupled strontium- and calcium-isotope systems indicates that a global weathering increase is likely to be the dominant driver of transient excursions in calcium-isotope ratios. The model suggests that contributions from hydrothermal activity and carbonate dissolution are too small and short-lived to affect the oceanic calcium reservoir measurably. A modelled increase in weathering flux, on the order of three times the modern flux, combined with increased hydrothermal activity due to formation of the Ontong-Java Plateau (OAE1a) and Caribbean Plateau (OAE2), can produce trends in both calcium and strontium isotopes that match the signals recorded in the carbonate sections. This study presents the first major-element record of a weathering response to Oceanic Anoxic Events.

Eolian deposition on the Ontong Java Plateau

The record of eolian deposition on the Ontong Java Plateau (OJP) since the Oligocene (approximately 33 Ma) has been investigated using dust grain size, dust flux, and dust mineralogy, with the goal of interpreting the paleoclimatology and paleometeorology of the western equatorial Pacific. Studies of modern dust dispersal in the Pacific have indicated that the equatorial regions receive contributions from both the Northern Hemisphere westerly winds and the equatorial easterlies; limited meteorological data suggest that low-altitude westerlies could also transport dust to OJP from proximal sources in the western Pacific. Previous studies have established the characteristics of the grain-size, flux, and mineralogy records of dust deposited in the North Pacific by the mid-latitude westerlies and in the eastern equatorial Pacific by the low-latitude easterlies since the Oligocene. By comparing the OJP records with the well-defined records of the mid-latitude westerlies and the low-latitude easterlies, the importance of multiple sources of dust to OJP can be recognized. OJP dust is composed of quartz, illite, kaolinite/chlorite, plagioclase feldspar, smectite, and heulandite. Mineral abundance profiles and principal components analysis (PCA) of the mineral abundance data have been used to identify assemblages of minerals that covary through all or part of the OJP record. Abundances of quartz, illite, and kaolinite/chlorite covary throughout the interval studied, defining a mineralogical assemblage supplied from Asia. Some plagioclase and smectite were also supplied as part of this assemblage during the late Miocene and Pliocene/Pleistocene, but other source areas have supplied significant amounts of plagioclase, smectite, and heulandite to OJP since the Oligocene. OJP dust is generally coarser than dust deposited by the Northern Hemisphere westerlies or the equatorial easterlies, and it accumulates more rapidly by 1-2 orders of magnitude. These relationships indicate the importance of the local sources on dust deposition at OJP. The grain-size and flux records of OJP dust do not exhibit most of the events observed in the corresponding records of the Northern Hemisphere westerlies or the equatorial easterlies, because these features are masked by the mixing of dust from several sources at OJP. The abundance record of the Asian dust assemblage at OJP, however, does contain most of the features characteristic of dust flux by means of the Northern Hemisphere westerlies, indicating that the paleoclimatic and paleometeorologic signal of a particular source area and wind system can be preserved in areas well beyond the region dominated by that source and those winds. Identifying such a signal requires "unmixing" the various dust assemblages, which can be accomplished by combining grain-size, flux, and mineralogic data.

Isotopic composition of Manihiki and Ontong Java basalts

Pb, Nd, and Sr isotopic results for lavas of the Cretaceous Ontong Java and Manihiki oceanic plateaus fall well within the modern-day oceanic island or hot pot field. The data provide no evidence of old continental basements but indicate a major involvement of 'Kerguelen-type' or 'EM-I'-like mantle in the sources of both plateaus, which appear to have probably been formed, at least in part, by hotspots. However, the presently active hotspots that Pacific plate reconstructions suggest might have been possible plateau sources lack Kerguelen-type isotopic compositions. Either these hotspots did not participate in the formation of the two plateaus, or if they did, Kerguelen-type material must have been volumetrically much more important early in their existence. Two hypotheses for the origins of these plateaus which involve hotspot sources are consistent with the sparse available geochemical, geochronological and geophysical data. The first holds that the plateaus formed cataclysmically in association with surfacing plume heads; the second posits a relatively steady but robust hotspot at or near a ridge crest and requires a much longer period of formation. A near-ridge origin appears to be indicated by evidence that most of the Pacific plateaus were built largely on relatively young ocean crust. However, we suggest that a near-ridge origin is also compatible with the plume head concept in that plume heads appear very likely to become associated with spreading axes through their influence on rift propagation, which should be substantially greater than for ordinary hotspots. In either case, the lack of hotspot tracks (seamount chains) attached to the two plateaus would be a consequence of ridge migration or rift propagation in a near-ridge setting.