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.
Drilling of the shallow fore-reef slope during Leg 133 of the Ocean Drilling Program has allowed the definition of an event history critical for understanding the initiation and evolution of the Great Barrier Reef. Within or immediately before the time period of isotope stages 8 and 9, a fundamental change in climate, driven by a switch from 19 000-yr obliquity to 100 000-yr precessional orbital cycles, led to raised sea surface temperatures and the initiation of the Great Barrier Reef. It is therefore only 300 000 yr old and an ecosystem response to environmental change. Subsequent development occurred as a series of high sea-level slices effected by four or five sea-level oscillations and growing progressively retrogressively to the west. The subreef section, although unknown, is postulated to be analogous to a mid to outer shelf coralline dominated environment comparable with that growing on the shelf to the south of the Great Barrier Reef today. The vertical and lateral (latitudinal) facies variations obey Walther's Law of Succession as a consequence of a sedimentary response to subsidence, latitude and climate change.
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.
Oxygen-isotope ratio measurements are presented for the planktonic species Globigerinoides ruber collected from shallow- water, upper-slope sediments from Holes 820A and 820B in 280 m of water, on the seaward edge of the Great Barrier Reef. Correlation of the Site 820 isotope curve with deep-sea reference curves of the Pacific Ocean (Core V28-238, Hole 677A, Hole 607A) permits the definition of isotope stages 1 to 19 in the top 145 m of Holes 820A and 820B. However, paleontological data indicate that stages 4 and 7 might be missing and that two hiatuses occur at a depth of 8.05 to 12.1 and 34.55 to 35.8 mbsf. Using deep-sea Hole 677A as a reference for ice-volume variations, we determine the difference in isotopic signature between it and Site 820. We propose that this difference is a regional signal representing a progressive 4°C increase in surface-water temperature at Site 820. The proposed temperature change was initiated at about 400 k.y. and corresponds to a change from high-to-low frequency variations in Pleistocene isotope signals. We postulate that these changes may have catalyzed the growth of the Great Barrier Reef. The shift also coincides with changes in seismic character and some physical and chemical sediment characteristics.
Palaeomagnetic and rock magnetic properties have been determined for sediment cores collected during ODP Leg 133 from the continental slope adjacent to the Great Barrier Reef. The magnetic remanence properties of the sediments are poor, with a pervasive viscous overprint that obscures most reversal boundaries. However, an inter-core magnetic susceptibility stratigraphy has been established that permits some refinement of the existing biostratigraphy. The quality of the palaeomagnetic remanence record relates to the residence time of magnetic particles in the corrosive, near-surface, sulphate-reducing zone. Magnetic susceptibility variations appear to reflect glacio-eustatic sea level cycles, and show a strong inverse correlation with both δ18O and carbonate content. For the last several glacial maxima there are marked changes In magnetic properties, largely grain size-related, with sharp peaks in susceptibility and a reversal of the δ18O-susceptibility phase relationship. One possible explanation can be found in terms of fluvio-deltaic processes and inter-reefal lagoonal reservoirs that develop during times of low sea level and become reworked during transgressions. Changes in the pattern of susceptibility variation occur at approximately 0.8 Ma and 0.4 Ma. Boundaries at these ages are seen in seismic profiles, they occur in the δ18O frequency spectrum, and there is a rise in sea surface temperature at ~0.4 Ma. We suggest that these boundaries relate through palaeoclimatic and palaeoceanographic changes to stages in evolution of the Great Barrier Reef.
