The palaeoenvironmental context of Neoproterozoic carbon-isotope excursions
Author(s)Klaebe, Robert Matthew
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AbstractThe Neoproterozoic record of sedimentary carbonates preserves > 20‰ of variability in carbon-isotope (δ¹³C) values that dwarfs the ~4‰ variability recorded in Phanerozoic marine carbonates known to record changes in the Earth’s carbon cycle through time. When interpreted as primary seawater values, large magnitude δ¹³C swings in Neoproterozoic strata are commonly interpreted to reflect significant changes to the biosphere that are causally related to the oxygenation of the Earth’s atmosphere, global-scale glaciations and the evolution of complex metazoan life. This thesis considers each of the key isotopic features used to define the δ¹³C record of Neoproterozoic seawater to identify the potential for local sedimentary controls on δ¹³C variation versus global secular change by focussing on the origin of the host carbonate phases from which the δ¹³C values are recorded. The expected bias toward shallow-water intracratonic basins preserved in the Precambrian record predicts that intervals of the record are particularly susceptible to hydrologic restriction and physical isolation from marine connections. The Bitter Springs Formation of the Amadeus Basin in central Australia records a positive-negative-positive stratigraphic δ¹³C trend in its upper Gillen Member. This δ¹³C excursion has been termed the Bitter Springs Anomaly and is routinely used as a global stratigraphic tie point at ~800 Ma. A detailed sedimentological study of the Wallara-1 and BRO5 drill cores and two field sections in the Amadeus Basin reveal that two distinctly different facies associations occur, a lower unit of cyclic-bedded microbial and grainstone marine limestones and an upper unit of red beds and dolostones interbedded with evaporites and characterised by evidence of subaerial exposure and desiccation. The abrupt decline in recorded δ¹³C values from +6‰ to -4‰ and the following recovery back to +6‰ occurs across the stratigraphic transitions between the evaporative lacustrine and shallow-marine carbonate intervals. Positive δ¹³C values occur exclusively in the evaporative lacustrine facies and were likely modified locally by evaporative processes in a highly restricted sabkha-type basin, indicated by interbedded and nodular anhydrites, halite pseudomorphs and karstic dissolution. Where local facies changes to subaqueous cyclic stromatolitic facies are preserved, δ¹³C values abruptly fall to -4‰ producing the binary isotopic shift that defines the Bitter Springs Anomaly here. The recurrence of this stratigraphic and isotopic association across the basin suggests that changes to local depositional conditions resulting from periodic hydrological restriction and evaporation explain the Bitter Springs Anomaly here rather than global isotopic change in a seawater value that should be decoupled from changes in local sediment composition. The Trezona Formation in the South Australian Flinders Ranges records a -18‰ decline in δ¹³C values stratigraphically below Marinoan-aged (~635 Ma) glacial deposits that is used to argue for a link between carbon cycle perturbations reflected in seawater DIC and global glaciation. A high resolution sedimentary study of 9 field sections across the Flinders Ranges shows that the lower Trezona Formation records evidence of deposition at or near base level, indicated by a sequence boundary at its base described by palaeosols and trough cross bedded channelised sandstones, and mud cracks and channelised siliciclastic (fluvial) deposits throughout. The upper Trezona Formation records an uninterrupted sequence of microbial and grainstone carbonates with little evidence of emergence. The vertical recovery from δ¹³C values of -9‰ towards modest values of -2‰ begins across the facies change between lower Trezona Formation interbedded mudstones and limestones and upper Trezona Formation microbial limestones in each section examined, regardless of variations in the stratigraphic thickness of each unit. Indicators of shallow water and exposure coupled with the limited aerial distribution of the Trezona Formation describes a water body that was physically isolated during sea level fall and may have periodically supported phases of non-marine deposition. Here, the dominance of meteoric fluids bearing negative δ¹³C values over seawater exchange allowed for a -9‰ carbonate phase to be precipitated that is likely primary, but is unrepresentative of coeval seawater chemistry. This value then systematically recovered to -2‰ with changing sediment composition as local basinal conditions developed along a deepening trend, inconsistent with the shoaling upward trend toward glacial lowstand proposed by previous studies and obscuring the interpreted causal link between isotope values and the onset of glaciation. The stratigraphic reproducibility of Neoproterozoic δ¹³C profiles of similar age within and between basins provides a principal line of evidence supporting their utility as global chronological tie points and monitors of whole-ocean change. The upper Andree Land Group of NE Greenland records a shift in δ¹³C values from +6‰ to variable negative values of -4‰ to -10‰, before recovering to +6‰ stratigraphically below Sturtian-aged (~720 Ma) deposits interpreted as glaciogenic in origin, and is termed the Islay Anomaly. The upper platform to slope transition that precedes the onset of diamictite deposition was studied on Ella Ø in NE Greenland and compared to a lateral section on Kap Weber that was calibrated along two regionally traceable sequence boundaries, providing time-significant surfaces along which spatially disparate δ¹³C profiles can be compared. In both sections, platform carbonates comprising dolomitised stromatolites, pisolitic limestones, and laminated microsparites exclusively record positive δ¹³C values of ~+6‰ while dolomite-bearing siltstones, mudstones and carbonate debrites associated with slope deposition record negative values of -4‰ to -10‰. On Kap Weber, a return to positive δ¹³C values follows slope-mudstone deposition coincident with a return to platform carbonate deposition that is absent on Ella Ø, which instead records negative δ¹³C values into the base of Sturtian-aged glacial diamictites with no unconformity, confirmed by interbedding and soft sediment deformation across the facies transition. This disparity in terminal δ¹³C values at the base of the glaciogenic Ulveso Formation suggests that the most negative δ¹³C values recorded in the basin (-10‰) occur broadly synchronous with values of +6‰, but are related to local sediment composition and the dominant process by which carbonate was precipitated on the platform versus the slope. Further, the onset of diamictite deposition occurs asynchronously across the basin, with a correlative conformity at the base of the Ulveso Formation on Ella Ø corresponding to erosion and subaerial exposure on Kap Weber. Elemental mapping of slope mudstones shows that the carbonate phase that records the negative δ¹³C values that define the Islay Anomaly here is a rhombic dolomite phase that is unsorted with surrounding sediment and associated with clays, organic matter and euhedral pyrite, supporting an authigenic origin associated with bacterial sulphate reduction that is not expected to constrain marine water-column δ¹³C variation. The Neoproterozoic δ¹³C record in general preserves a positive ‘background’ value of approximately +6‰ that is punctuated by negative excursions. When interpreted in the same way as Phanerozoic deep-ocean records, sustained δ¹³C values of +6‰ implies signifycantly elevated rates of global carbon burial as photosynthetic biomass, inconsistent with the low oxygen conditions during the Neoproterozoic. While Cenozoic deep-marine pelagic sediments record δ¹³C values as positive as +2.5‰, a compilation of coeval platform carbonates shows values as positive as +6‰ attributed to photosynthetic effects under hydrologically restricted conditions. Positive δ¹³C values from eight different Neoproterozoic sections from Australia, NW Canada, East Svalbard and NE Greenland, including six intervals that preserve analogous carbonate platform sedimentation as thick packages of microbial and grainstone carbonates, karstic features, desiccation cracks and dolomitisation, are compared with these modern data and are shown to largely plot within the < +6‰ range of δ¹³C variation. As the majority of Neoproterozoic carbonate successions are known to have accumulated above carbonate platforms and ramps, in inland seas, and in other cratonic and transitional-marginal environments, it is proposed that some portion of the positive background δ¹³C value reflects the inherent bias in Neoproterozoic stratigraphy towards these types of depositional conditions.
Thesis (Ph.D.) (Research by Publication) -- University of Adelaide, School of Physical Sciences, 2015.