Stable isotopic studies of early Proterozoic banded iron formations and late proterozoic carbonate platform sequences.

摘要

Carbon-isotopic studies of carbonates and organic carbon in early Proterozoic (2.5 Ga) banded iron-formations (BIFs) from Western Australia and South Africa, and in late Proterozoic (0.9 to 0.57 Ma) platform carbonate sequences suggest that oceans were often stratified during deposition of these chemical sediments. Iron-rich carbonates in the Brockman and Kuruman Iron Formations are depleted in {dollar}sp{lcub}13{rcub}{dollar}C by 5 to 15{dollar}perthous{dollar} relative to carbonates in associated limestones and dolomites. It appears possible that deep, poorly-oxygenated water, beneath which the BIF accumulated, was enriched in soluble Fe{dollar}sp{lcub}2+{rcub}{dollar} and depleted in {dollar}sp{lcub}13{rcub}{dollar}C({dollar}deltasp{lcub}13{rcub}{dollar}C {dollar}approx{dollar} -5{dollar}perthous{dollar}), while shallow, well-oxygenated water, where limestone accumulated, was depleted in Fe{dollar}sp{lcub}2+{rcub}{dollar} and enriched in {dollar}sp{lcub}13{rcub}{dollar}C({dollar}deltasp{lcub}13{rcub}{dollar}C {dollar}approx{dollar} 0{dollar}perthous{dollar}). Patterns of rare earth element abundances in samples of BIF and interbedded carbonates from the Kuruman iron Formation, determined by Klein and Beukes (1990), suggest that this stratification was likely related to significant hydrothermal inputs to the deep ocean. Further, stratification is believed to be responsible for spatial separation of BIF and limestone deposition as well as primary precipitation of siderite in the early Proterozoic ocean. Additional depletion of {dollar}sp{lcub}13{rcub}{dollar}C in BIF carbonate microbands is attributed to biological oxidation of organic carbon prior to lithification. Millimeter-scale variations in carbon- and oxygen-isotopic abundances are shown to be controlled by diagenetic mineral assemblages.; Distinctive stratigraphic patterns of secular variation and enrichment in {dollar}sp{lcub}13{rcub}{dollar}C abundances have been determined in a number of late Proterozoic successions worldwide. Prolonged periods of carbon-13 enrichment ({dollar}geq{dollar} +5{dollar}perthous{dollar}) in carbonates and organic carbon formed in near-shore, platform sequences suggest that oceanographic conditions favored high rates of organic carbon burial. Again, it is likely that oceans were stratified, with deep waters anoxic, permitting the buildup of ferrous iron from hydrothermal sources. In the late Proterozoic Damara Supergroup of Namibia, three separate glacial horizons contain carbonates depleted in {dollar}sp{lcub}13{rcub}{dollar}C. Also associated with one of these glacial episodes, represented by the Chuos tillite, is iron-formation. It is suggested that at the onset of glaciation during the late Precambrian, upwelling would have brought {dollar}sp{lcub}13{rcub}{dollar}C-depleted and iron-rich deep water onto shallow shelves where contact with cold, oxygenated surface water led to precipitation of ferric iron.