Sedimentology, mineral magnetism, andoxygen isotope stratigraphy of ODP Hole 133-819A

To investigate late Quaternary paleoclimatic and paleoceanographic change in the sedimentary record, preserved on the Australian Continental Margin during the late Quaternary, core material was collected from Ocean Drilling Program, Leg 133, Site 819. An expanded sequence of late Quaternary, rhythmically bedded, predominantly hemipelagic sediments were recovered from Hole 819A. The foraminiferal d18O record preserved at Hole 819A suggests that the late Quaternary section is incomplete. Both benthic and planktonic d18O stratigraphies can be traced tentatively downcore to stage 6 at about 32.5 mbsf, where a major hiatus occurs. At this level, a slump detachment surface has been identified (Shipboard Scientific Party, 1991). This slump has removed marine oxygen isotope stages 7 to 13. Below 32.5 mbsf, continuous correlation can be achieved in the planktonic d18O curve, with existing deep-sea foraminiferal oxygen isotope stratigraphies from stage 14 through stage 28. The major hiatus at 32.5 mbsf marks the position of a significant change in the character of the sedimentation at Site 819. Sediments below 32.5 mbsf, relative to those above 32.5 mbsf, are characterized by less variation in mean particle size; lower percentages of carbonate content in the coarse fraction (>63 µm); a stronger relationship between the percentage of fine fraction and magnetic mineral concentration, and lower foraminiferal abundances. Above the hiatus, large fluctuations in mean particle size occurred, which have been interpreted to be the result of high foraminiferal abundances. Early highstands show high terrigenous influx in the fine fraction above the hiatus. This is the opposite of the general idea of high terrigenous influx during lowstands of sea level on siliciclastic dominated continental margins. We are far from understanding the origin of this material and further investigation will be required (see also Glenn et al., this volume). All our records, except the planktonic foraminiferal oxygen isotope record, indicate that the major hiatus marks the position of a significant change in the environment at Site 819. The planktonic foraminiferal d18O record suggests that environmental change occurred prior to the formation of the hiatus (i.e., near the Brunhes/Matuyama [B/M] boundary). The interval between the B/M boundary and the hiatus represents a transitional period between two different patterns of ocean circulation. Throughout most of the lower part of the sequence, Site 819 was at a shallow-water depth and local oceanographic conditions were dominated by sluggish Subtropical Central Water (SCW) flow. However, near the B/M boundary, ocean circulation patterns intensified, reflecting a worldwide change in paleoenvironment. Enhanced ocean circulation patterns were possibly aided by tectonic subsidence. During this period Site 819 became progressively more under the influence of Antarctic Intermediate Water (AAIW), than SCW. In the upper part of the sequence at Hole 819 A, we see a continuation of the pattern of oceanographic reorganization suggested during stages 21 through 14. Intensification of the subsurface oceanographic circulation was also accompanied by the progressive wedging southward of surface waters associated with the East Australian Current (EAC). The change in the nature of the records in the lower and upper parts of the sequence at Site 819 are thought to reflect perturbations by the orbital eccentricity cycle.

Strontium isotope dating of ODP Leg 133 sediments

The strontium-isotope dating method, based on the strontium-isotope seawater curve, was used to date stratigraphic events recognized in carbonate sediments drilled during Leg 133 on the Queensland and Marion plateaus. The strontium isotope ages of these events are used to correlate paleoceanographic changes, delineated from oxygen isotope signals, and paleoenvironmental or facies changes recorded in the lithostratigraphy. Results indicate that a strong connection exists between prevailing paleoenvironmental conditions and the developmental style of a carbonate platform. Also, the strontium-isotope ages of discrete dolomite intervals within the sequences were determined, indicating that multiple dolomitization events took place and that a hydrodynamically driven process may be currently active within the modern carbonate platform.

Paleomagnetic of ODP Leg 133 holes

Paleomagnetic and rock-magnetic analyses from discrete samples of carbonate sites on the Queensland Plateau were used to determine magnetic polarity reversal stratigraphy and the nature of magnetization in these sediments. Magnetic polarity zones were correlated with the geomagnetic polarity time scale in the upper portions of cores at Sites 812 through 814, usually back to a late Pliocene age. Loss of reliable directional data was coincidental with a major decrease in magnetic intensity, below which, no stable polarity zones could be identified. The intensity reduction is either an in-situ alteration of magnetic grains, or an input signal representing progressive increase in the magnetic component of Queensland Plateau sediments. Although not conclusive at this point, the geochemical conditions and differing age of intensity reduction support the former hypothesis. Rock-magnetic analysis of carbonate sediments suggests that ultrafine-grained magnetite or maghemite crystals is an important carrier of remanence and may be biogenic in origin. Application of a recently calibrated anhysteretic remanent magnetization test to assess configuration of single-domain crystal within a natural matrix indicates that cementation (ooze-chalk-limestone) may be important in post-depositional changes affecting magnetostatic grain interaction.