Shallow water foraminifera of ODP Hole 133-821A

Reworked shallow-water foraminifers that settled on the upper slope of the central Great Barrier Reef at Site 821 (water depth, 212.6 m) were used as indicators of the paleoclimatic and paleoenvironmental conditions that have controlled the Pleistocene evolution of the adjacent platform. Throughout the 400-m-thick sequence drilled, the nature, composition, and distribution of the shallow-water foraminiferal assemblages studied indicate that (1) all the species recorded are at present living in diverse tropical, reef-related areas of the Indo-Pacific and Atlantic provinces; (2) the composition of the microfaunal taphocoenoses is almost identical between the different stratigraphic intervals studied and the modern Great Barrier Reef environments; (3) inner-neritic, tropical environments have continued to develop since the middle Pleistocene; (4) high- to moderate-energy platform edges occurred repeatedly throughout Pleistocene time. These factors may suggest that, since the beginning of the Pleistocene, several reef-like tracts have grown successively on the central area of the northeastern Australian shelf edge. These tracts probably had a sufficiently evolved morphological zonation to act as shelters for foraminiferal biocoenoses of high species diversity.

Planktonic and benthic foraminiferal abundances in ODP Holes 133-819A and 133-821A

To reveal changes in the oceanic environment on the continental slope adjacent to the Great Barrier Reef, east of Cairns (NE Australia), planktonic and benthic foraminiferal abundances were counted and planktonic percentages (P/B ratios) were determined in sediments from two sites. Counts of planktonic and benthic specimens per gram of sediment over the last glacial/interglacial cycle at the shallowest Site 821, located in a water depth of 212 m just below the core of Subtropical Lower Water, show high abundances in the last glacial compared with the Holocene interglacial. We interpret the apparent increase in abundances during the last glacial as mainly a consequence of fluctuations in the intensity of flow of Subtropical Lower Water along the outer shelf edge and upper slope. During the lowstand in sea level, the increased flow winnowed the sediments, concentrating the foraminiferal skeletons. The P/B ratios are low throughout, with the highest values occurring during the Holocene interglacial and glacial stage 2. This suggests that some upwelling might have occurred during glacial stage 2. The relatively deeper water Site 819 is located in 565.2 m of water in a zone of mixing between Subtropical Lower Water and Antarctic Intermediate Water. The studied record at this site represents middle to upper Quaternary sediments, but it was interrupted by a hiatus just above stage 15 (Alexander et al., this volume); stages 7 through 13 are missing. Below the hiatus (isotopic stages 15 through 21), the foraminiferal abundances are low, while above the hiatus, the highest abundances occur in isotopic stage 6. In addition, a major change in the P/B ratio occurs across the unconformity. Below the hiatus, the ratios are low and resemble the values of the top of Site 821; but above it, ratios rapidly fluctuate, with a tendency for high values during glacial periods. We interpret the changes across the hiatus as having been caused by a shift in the position of the mixing zone between subsurface Subtropical Lower Water and Antarctic Intermediate Water. The mixing zone of these watermasses was farther down the slope in isotopic stages 15 through 21. This is indicated by the low P/B ratios, similar to the values found in the top of Site 821, which presently is bathed in subtropical waters. Above the hiatus, the influence of Antarctic Intermediate Water increased, as inferred from the high P/B ratios.

Particulate and phytoplankton absorption during POLARSTERN cruises ANT-XXVI/3 and ANT-XXVIII/3

The euphotic depth (Zeu) is a key parameter in modelling primary production (PP) using satellite ocean colour. However, evaluations of satellite Zeu products are scarce. The objective of this paper is to investigate existing approaches and sensors to estimate Zeu from satellite and to evaluate how different Zeu products might affect the estimation of PP in the Southern Ocean (SO). Euphotic depth was derived from MODIS and SeaWiFS products of (i) surface chlorophyll-a (Zeu-Chla) and (ii) inherent optical properties (Zeu-IOP). They were compared with in situ measurements of Zeu from different regions of the SO. Both approaches and sensors are robust to retrieve Zeu, although the best results were obtained using the IOP approach and SeaWiFS data, with an average percentage of error (E) of 25.43% and mean absolute error (MAE) of 0.10 m (log scale). Nevertheless, differences in the spatial distribution of Zeu-Chla and Zeu-IOP for both sensors were found as large as 30% over specific regions. These differences were also observed in PP. On average, PP based on Zeu-Chla was 8% higher than PP based on Zeu-IOP, but it was up to 30% higher south of 60°S. Satellite phytoplankton absorption coefficients (aph) derived by the Quasi-Analytical Algorithm at different wavelengths were also validated and the results showed that MODIS aph are generally more robust than SeaWiFS. Thus, MODIS aph should be preferred in PP models based on aph in the SO. Further, we reinforce the importance of investigating the spatial differences between satellite products, which might not be detected by the validation with in situ measurements due to the insufficient amount and uneven distribution of the data.

Nannofossil biohorizons for ODP Leg 133 sites

Paleontological studies conducted subsequent to the completion of Leg 133 led to refinements of the biostratigraphy for the Leg 133 sites. These biostratigraphic refinements bear on the calculations of sedimentation rates and on the age-depth plots prepared for the Initial Reports volume for Leg 133. To make available the revised data to anyone who may wish to make use of it, the revised biostratigraphic information is presented here in tabulated form. Revised age-depth plots also are presented for all of the sites to facilitate comparison of sedimentation rate curves and to identify intervals where significant changes have been made based on post-cruise studies. The revised age-depth plots include calcareous nannofossils only, and the revised data have been taken from thechapters contributed for this volume (Gartner et al., 1993, doi:10.2973/odp.proc.sr.133.213.1993; Wei and Gartner, 1993, doi:10.2973/odp.proc.sr.133.216.1993). Planktonic foraminifer biostratigraphy revisions became available subsequently and could not be readily incorporated. The age-depth plots for Sites 812 through 818 were made with the (ADP) program provided to ODP by Dave Lazarus.

Pleistocene sedimentology of ODP Hole 133-820A from the northeastern Australian margin

A principal objective at Site 820, situated on the outer shelf, upper slope of the northeastern Australian continental margin, was to test the relationships between changes in Pleistocene sea level and sedimentary packages produced on a mixed carbonate-siliciclastic continental margin. To this end, we have examined the downcore distribution of grain size, magnetic susceptibility, and calcium-carbonate content throughout Hole 820A and, in particular, the top 35 meters below the seafloor (mbsf). These data are compared with variations in the oxygen-isotope signal defined for the same hole and are interpreted as indicating sea-level oscillations. The distribution of sand, mud, calcium carbonate of the mud fraction and total sample, and magnetic susceptibility during the last 20,000 yr defines the position of a sea-level regression (41,000-18,000 yr B.P.), a lowstand, early (18,000-9,400 yr B.P.) and late transgressions (9400-900 yr B.P.), and a highstand (4900 yr to the present). The regression is seen first in a high-carbonate content peak. Calcium carbonate constituents mainly comprise skeletal carbonate grains, with abundant planktonic and benthic foraminifers, and lime muds. The lowstand is characterized by a maximum abundance of the sand fraction, which contains dominantly skeletal carbonate grains and a minor abundance of lithoclasts. Sand-sized terrigenous sediments are proposed to have bypassed the continental shelf during a lowstand of sea level. Sedimentation rates throughout the regression and lowstand are low (3.0 cm/k.y.). The early transgression, marked by highest values in magnetic susceptibility, displays a rapid increase in sedimentation rate that coincided with an increase in terrigenous mud. Highest sedimentation rates of 82.3 cm/k.y. occurred during the late transgression, with increasing percentages of lime-mud. A decrease in noncarbonate constituents in the mud fraction during the late transgression and highstand of sea level is thought to be the result of restricted inner-shelf sedimentation of terrigenous sediments. The same relationship is also seen in the major sea-level oscillation, which is interpreted as isotope stage 6.