SEDIMENTOLOGICAL AND CHEMICAL ATTRIBUTES OF LATE PLEISTOCENE COOL-WATER CARBONATES FROM THE SOUTHERN AUSTRALIAN CONTINENTAL MARGIN: EARLY SEAFLOOR DIAGENESIS AND PALEOCEANOGRAPHY
Rivers, John M.
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The high-energy open shelf along the southern Australian continental margin is blanketed by heterozoan cool-water carbonates of late Pleistocene and Holocene age. Three distinct assemblages have been identified and radiocarbon dated. These include: i) Holocene grains, unaltered biofragments deposited during Marine Isotope Stage 1; ii) stranded grains, grey and buff-colored abraded biofragments and intraclasts marooned during the sea level rise associated with the latter stages of Marine Isotope Stage 2; and iii) relict grains, highly abraded, iron-stained intraclasts originally deposited during the intermediate sea-level stands of Marine Isotope Stages 3 and 4. Whereas the skeletal makeup of Holocene grains has been previously detailed, attributes of the older grains have not been elucidated. Relict-grain skeletal composition indicates that during Marine Isotope Stages 3 and 4 shallow, warm, oligotrophic, marine grassbed environments developed across the western portion of the region, whereas more heterozoan assemblages to the east imply cooler marine waters and that an overall upwelling regime was in effect. Stranded grains (Marine Isotope Stage 2) are mostly heterozoan across the whole region, reflecting deposition on narrower shelves (restricted euphotic zone) and in generally cool waters. Stable isotopic compositions of relict and stranded foraminifera indicate that the western portion of the region (the Great Australian Bight) had water of elevated salinity. Environments analogous to outer Shark Bay are interpreted to have formed across the Great Australian Bight during portions of Marine Isotope Stages 2, 3 and 4. Comparison between relatively unaltered Holocene grains and altered late Pleistocene stranded and relict grains reveals pathways of early diagenesis in this cool-water marine realm. Both calcitic and aragonitic biogenic grains display dissolution features. Dissolution of calcitic components over the past ~20,000 years is incomplete (stranded and relict sediments are predominantly Mg-calcite). Aragonitic skeletons, however, are mostly dissolved over this same time period. Contemporaneously, micritic cement also precipitates in this environment, wherein Mg-calcite (~12 mol%) infills skeletal pores of many stranded and most relict skeletons. Similar cement locally precipitates between grains, forming cemented grain aggregates. Mobilization of metals in the slightly reducing pore waters of the southern Australian margin has resulted in the formation of Fe- and Mn-oxides that discolor the stranded and relict grains. Such oxides precipitated on the surface of shells, in empty microbial borings, and in skeletal micropores, scavenge other trace metals, altering the original elemental makeup of these cool-water carbonates. Trace element analysis of Holocene carbonate grains and of living gastropod skeletons indicates that these coatings begin to precipitate during or soon after shell formation. Marine dissolution/precipitation dynamics in addition to mobilization of metals in pore water, fundamentally changes the sedimentological and chemical attributes of these cool-water carbonates on the seafloor soon after deposition.