Paleoceanographic Changes and Reef Growth off the Northeastern Australian Margin: Stable Isotopic Data from ODP Leg 133 Sites 811 and 817 and DSDP Leg 21 Site 209

Abstract

Stable isotopic data obtained from planktonic and benthic foraminifers were used to study paleoceanographic changes along the northeastern Australian margin from late Miocene (10 Ma) to Holocene time, and to evaluate the influence of these changes on reef growth. The data indicate that variations in surface-water temperatures may have had an important effect on the reef complexes on the Queensland Plateau and possibly off the northeastern Australian margin. Three sites were studied: Leg 21, Site 209 on the eastern edge of the Queensland Plateau, and Leg 133, Site 811 on the western margin, and Site 817 on the lower southern slope of the plateau.Shallow-water bioclasts recovered from Holes 811A and 817A indicate extensive reef growth on the Queensland Plateau during the middle Miocene (before 12 Ma), signifying surface-water temperatures of 20°C or greater. The amount of reefal detritus produced during the late Miocene (10.0-5.2 Ma) decreased progressively, resulting in a reduction in area of the reef complexes. The isotopic data from planktonic foraminifers in these late Miocene age sediments indicate the presence of relatively cool surface waters (16°-19°C), which may have been a major factor contributing to the demise of the reefs on the Queensland Plateau. Surface waters remained cool until the middle Pleistocene (1.2-0.5 Ma), when the surface-water temperature apparently increased to approximately 25 °C, recorded both in the isotopic data and by renewed reef growth. This increase occurred simultaneously (within the error of the age model) with the initiation of the Great Barrier Reef. We propose that cooling of surface waters during the early late Miocene contributed to reef decline on the Queensland Plateau, and that subsequent warming of surface waters during the middle Pleistocene promoted the initiation of reef growth on the northeastern Australian margin. Reef development on the Queensland Plateau never recovered to the middle Miocene extent because of a combination of tectonic (accelerated subsidence of the plateau) and paleoceanographic (the cooler surface waters present from the late Miocene throughout the Pliocene) factors.Variations in seafloor δ 18 θ appear to be controlled by regional factors, as indicated by the similarity of data from Sites 811 and 817 to those from Site 590 on Lord Howe Rise. INTRODUCTIONTwo objectives of Leg 133 were to study the temporal and spatial evolution of the Queensland Plateau and associated reef systems, and to determine the effects of changing sea level, climate, and oceanic conditions on carbonate platform growth. Material recovered during Leg 133, and its correlation to seismic profiles, showed extensive reef growth on the Queensland Plateau before 12 Ma (Davies, McKenzie, Palmer-Julson, et al., 1991). Reef growth began to decline during the late middle Miocene, as shown by a decrease in the amount and type of bank-derived components in the slope sediments (Davies, McKenzie, Palmer-Julson, et al., 1991). This decline may have been the result of plate subs...

Global synchronous changes in the carbon isotopic composition of carbonate sediments unrelated to changes in the global carbon cycle

“…At one of these locations (Site 817), previously published ␦ 13 C values of two different species of foraminifera (Globigerinoides ruber and Cibididoides sp.) separated from the bulk sediment show a similar pattern to the global curve (24), thus confirming that the general oceanic characteristics at this location were similar to those at Site 525 (Fig. 4).…”

“…In this case the ␦ 13 C of the sediments would then become more negative, as the site at a particular location would be more inf luenced by oceanic carbonate. Although this hypothesis cannot be proven at all of the sites, it has been established by using seismic data that the western margin of GBB has prograded toward the west Ϸ20 km, over the past [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]. At the other locations, where studies have been less detailed, it is clear based on the sediment description that there is a decrease in the proportion of shallow water-derived sediments with increasing depth (28,30), a change consistent with a change in sea level and proximity to the shallow-water source.…”

A dynamic process for drowning carbonate reefs on the northeastern Australian margin

“…However, for the fi rst time we show that a signifi cant contribution toward long-term decline in reef pro-ductivity can be attributed to a geodynamic process. We propose that reef drowning on the Marion Plateau was caused by a combination of weakening due to long-term dynamic subsidence and shorter-term factors such as sea surface temperature fall (Betzler, 1997;Isern et al, 1993Isern et al, , 1996 and global sea-level rise. Geodynamic processes played a signifi cant role in long-term reef weakening on the northeastern plateaus.…”

Do drifts deposited adjacent to carbonate platforms record the signal of global carbon isotopic values?

“…The record from ODP Site 709 and the benthic foraminifera signal show essentially similar δ 13 C values over the past 5 Myr with only a minimal decrease in δ 13 C values (0·2 to 0·1‰). In contrast, the record from Deep Sea Drilling Project (DSDP) Site 525 and from individual planktonic foraminifera from ODP Sites 817 and 811 (Isern et al ., ) on the Queensland plateau show a decrease of between 0·5‰ and 1·0‰ over the same interval. In older sediments, all sites exhibit a gradual increase in δ 13 C values reaching a maximum value coincident with the excursion in δ 13 C values between 13 Ma and 17 Ma.…”

“…For example Fig. shows a comparison of planktonic signals from Ocean Drilling Program (ODP) Site 811 (north‐east Australia) measured on Globeriginoides ruber (Isern et al ., ) compared to a bulk record from ODP Site 525 (Shackleton & Hall, ) in the South Atlantic, Site 709 from the Indian Ocean (Baker et al ., ) and the Zachos et al . () benthic data set.…”