The wide Lacepede Shelf and narrow Bonney Shelf are contiguous parts of the south‐eastern passive continental margin of Australia. The shelves are open, generally deeper than 40 m, covered by waters cooler than 18°C and swept by oceanic swells that move sediments to depths of 140 m. The Lacepede Shelf is proximal to the ‘delta’of the River Murray and the Coorong Lagoon. Shelf and upper slope sediments are a variable mixture of Holocene and late Pleistocene quartzose terrigenous clastic and bryozoa‐dominated carbonate particles. Bryozoa grow in abundance to depths of 250 m and are conspicuous to depths of 350 m. They can be grouped into four depth‐related assemblages. Coralline algae, the only calcareous phototrophs, are important sediment producers to depths of 70 m. Active benthic carbonate sediment production occurs to depths of 350 m, but carbonate sediment accumulation is reduced on the open shelf by continuous high energy conditions. The shelf is separated into five zones. The strandline is typified by accretionary sequences of steep shoreface, beach and dune carbonate/siliciclastic sediments. Similar shoreline facies of relict bivalve/limestone cobble ridges are stranded on the open shelf. The shallow shelf, c.40–70 m deep, is a wide, extremely flat plain with only subtle local relief. It is a mosaic of grainy, quartzose, palimpsest facies which reflect the complex interaction of modern bioclastic sediment production (dominated by bryozoa and molluscs), numerous highstands of sea level over the last 80 000 years, modern mixing of sediments from relatively recent highstands and local introduction of quartz‐rich sediments during lowstands. The middle shelf, c.70–140 m deep, is a gentle incline with subtle relief where Holocene carbonates veneer seaward‐dipping bedrock clinoforms and local lowstand beach complexes. Carbonates are mostly modern, uniform, clean, coarse grained sands dominated by a diverse suite of robust to delicate bryozoa particles produced primarily in situ but swept into subaqueous dunes. The deep shelf edge, c. 140–250 m deep, is a site of diverse and active bryozoa growth. Resulting accumulations are characteristically muddy and distinguished by large numbers of delicate, branching bryozoa. The upper slope, between 250 and 350 m depth, contains the deepest platform‐related sediments, which are very muddy and contain a low diversity suite of delicate, branching cyclostome bryozoa. This study provides fundamental environmental information critical for the interpretation of Cenozoic cool water carbonates and the region is a good model for older mixed carbonate‐terrigenous clastic successions which were deposited on unrimmed shelves.
The start of the Ediacaran period is defined by one of the most severe climate change events recorded in Earth history--the recovery from the Marinoan 'snowball' ice age, approximately 635 Myr ago (ref. 1). Marinoan glacial-marine deposits occur at equatorial palaeolatitudes, and are sharply overlain by a thin interval of carbonate that preserves marine carbon and sulphur isotopic excursions of about -5 and +15 parts per thousand, respectively; these deposits are thought to record widespread oceanic carbonate precipitation during postglacial sea level rise. This abrupt transition records a climate system in profound disequilibrium and contrasts sharply with the cyclical stratigraphic signal imparted by the balanced feedbacks modulating Phanerozoic deglaciation. Hypotheses accounting for the abruptness of deglaciation include ice albedo feedback, deep-ocean out-gassing during post-glacial oceanic overturn or methane hydrate destabilization. Here we report the broadest range of oxygen isotope values yet measured in marine sediments (-25 per thousand to +12 per thousand) in methane seeps in Marinoan deglacial sediments underlying the cap carbonate. This range of values is likely to be the result of mixing between ice-sheet-derived meteoric waters and clathrate-derived fluids during the flushing and destabilization of a clathrate field by glacial meltwater. The equatorial palaeolatitude implies a highly volatile shelf permafrost pool that is an order of magnitude larger than that of the present day. A pool of this size could have provided a massive biogeochemical feedback capable of triggering deglaciation and accounting for the global postglacial marine carbon and sulphur isotopic excursions, abrupt unidirectional warming, cap carbonate deposition, and a marine oxygen crisis. Our findings suggest that methane released from low-latitude permafrost clathrates therefore acted as a trigger and/or strong positive feedback for deglaciation and warming. Methane hydrate destabilization is increasingly suspected as an important positive feedback to climate change that coincides with critical boundaries in the geological record and may represent one particularly important mechanism active during conditions of strong climate forcing.
This is a synthesis of major middle and late Cenozoic oceanographic and climatic events revealed in a diverse suite of studies by Leg 90 investigators and involving analyses of oxygen and carbon isotopes, sediment character, and accumulation rates and microfossils. The benthic δ 18 θ record in Leg 90 sites exhibits a number of large changes that reflect the sequential development of polar glaciation and cooling of bottom waters beginning in the latest Eocene-earliest Oligocene. Major climatic cooling events in the Leg 90 sequences include the Terminal Eocene Event (37 Ma); middle Oligocene cooling events clustered close to 31 Ma; the Middle Miocene Event (16.5-13.5 Ma); further temporary cooling events during the late middle Miocene (12.5 to 11.5 Ma) and the earliest late Miocene (11-9 Ma); the Terminal Miocene Event (-6.2-5.0 Ma), the Middle Pliocene Cooling Event at 3.4 Ma; the Late Pliocene Event at 2.6-2.4 Ma; and amplification of glacial-interglacial oscillations during the Quaternary at 0.9 Ma (Jaramillo Paleomagnetic Subchron). The climax of Neogene warmth occurred during the early Miocene, especially between 19.5 and 16.5 Ma. The sequences record the development during the Cenozoic of latitudinal and vertical thermal gradients in the southwest Pacific region. Major, permanent increases in the vertical temperature gradients occurred in association with the Middle Miocene Event (16.5-13.5 Ma), the Middle Pliocene Cooling Event (3.4 Ma) and the Late Pliocene Event (2.6-2.4 Ma). Deep-sea benthic foraminiferal assemblages underwent important changes near the Eocene/Oligocene boundary and during the earliest Middle Miocene, the latest Miocene, and the late Pliocene and Quaternary. Late Neogene benthic foraminiferal changes are, in part, related to changes in the organic flux rates that accompanied changes in biogenic sedimentation rates and inferred surface-water productivity. Changes in clay mineralogy, wind-blown terrigenous sediments, and opal phytoliths in the Lord Howe Rise Sites record a general expansion of Australian deserts. Important steps in aridification occurred during the Middle Miocene Event; the Terminal Miocene Event (~5 Ma), and the Middle Pliocene Cooling Event (-3.4 Ma).
The mineralogy and isotope geochemistry of carbonate minerals in the Coorong area are determined by the water chemistry of different depositional environments ranging from seawater to evaporitically modified continental water. The different isotopic compositions of coexisting calcite and dolomite suggest that each of the above two minerals was formed from water of composition and origin unique to that specific mineral. In addition, the dolomite was not formed by simple solid state cation exchange. The occurrence of two types of dolomite was shown by isotope analysis and SEM observations. The dolomite, which is isotopically light (δ13C = ‐1 to ‐2%0; δ18O=+3 to +5%0) and of fine grain size (˜ 0·5 μm) probably precipitated under the influence of evaporitically modified continental water. Coarser grained dolomite (up to 4 μm) is isotopically heavier (δ13C=+3 to +4%0; δ18O=+5 to + 6%0) contains Mg in excess of Ca and was formed in or close to equilibrium with atmospheric CO2 probably by the dolomitization of aragonite.
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