Person:

Schrag, Daniel

The ca. 2.45–2.22 Ga Turee Creek Group, Western Australia, contains carbonate- rich horizons that postdate earliest Proterozoic iron formations, bracket both Paleoproterozoic glaciogenic beds and the onset of the Great Oxidation Event (GOE), and predate ca. 2.2–2.05 Ga Lomagundi-Jatuli C-isotopic excursion(s). As such, Turee Creek carbonate strata provide an opportunity to characterize early Paleoproterozoic carbonate sedimentation and carbon cycle dynamics in the context of significant global change. Here, we report on the stratigraphy, sedimentology, petrology, carbon isotope chemostratigraphy, and stromatolite development for carbonate-rich successions within the pre-glacial part of the Kungarra Formation and the postglacial Kazput Formation. Kungarra carbonate units largely occur as laterally discontinuous beds within a thick, predominantly siliciclastic shelf deposit. While this succession contains thin microbialite horizons, most carbonates consist of patchy calcite overgrowths within a siliciclastic matrix. C-isotopic values show marked variation along a single horizon and even within hand samples, reflecting spatially and temporally variable mixing between dissolved inorganic carbon in seawater and isotopically light inorganic carbon generated via syn- and post-depositional remineralization of organic matter. In contrast, the Kazput carbonates consist of subtidal stromatolites, grainstones, and micrites deposited on a mixed carbonate-siliciclastic shelf. These carbonates exhibit moderate δ13 C values of -2‰ to +1.5‰ and likely preserve a C-isotopic signature of seawater. Kazput carbonates, thus, provide some of the best available evidence that an interval of unexceptional C-isotopic values separates the Lomagundi-Jatuli C-isotopic excursion(s) from the initiation of the GOE as inferred from multiple sulfur isotopes (loss 4 of mass independent fractionation). The Kazput Formation also contains unusual, m-scale stromatolitic buildups, which are composed of sub-mm laminae and discontinuous, convex upward lenticular precipitates up to a few mm in maximum thickness. Laminae, interpreted as microbial mat layers, contain quartz and clay minerals as well as calcite, whereas precipitate lenses consist of interlocking calcite anhedra, sometimes showing faint mm-scale banding. These cements formed either as infillings of primary voids formed by gas emission within penecontemporaneously lithified mats, or as local seafloor precipitates that formed on, or within, surface mats. It is possible that both mechanisms interacted to form the unique Kazput stromatolites. These microbialites speak to a distinctive interaction between life and environment early in the Paleoproterozoic Era.

The fossilization of organic remains and shell material by calcium phosphate minerals provides an illuminating, but time-bounded, window into Ediacaran–Cambrian animal evolution. For reasons that remain unknown, phosphatic fossil preservation declined significantly through Cambrian Series 2. Here, we investigate the phosphorus (P) sources for phosphatic Cambrian carbonates, presenting sedimentological, petrographic, and geochemical data from the Cambrian Series 2–3 Thorntonia Limestone, Australia, some of the youngest Cambrian strata to display exceptional phosphatic preservation of small shelly fossils. We find that within Thorntonia sediments, phosphate was remobilized by organic decay and bacterial iron reduction, with subsequent reprecipitation largely as apatite within the interiors of small shelly fossils. We discuss the merits of bioclastic-derived, organic matter–bound, or iron-bound P as potential sources to these strata. Petrographic observations suggest that the dissolution of phosphatic skeletal material did not provide the P for fossil preservation. In contrast, high organic carbon contents imply significant organic fluxes of P to Thorntonia sediments. Sedimentology and iron-speciation data indicate that phosphorus enrichment occurred during times of expanded anoxic, ferruginous conditions in subsurface water masses, suggesting that phosphorus adsorption to iron minerals precipitating from the water column provided a second significant P source to Thorntonia sediments. Simple stoichiometric models suggest that, by themselves, neither organic carbon burial nor an iron shuttle can account for the observed phosphorus enrichment. Thus, we infer that both processes were necessary for the observed phosphorus enrichment and subsequent fossil preservation in the Thorntonia Limestone.

The ca. 1.8 Ga Duck Creek Formation, Western Australia, preserves 1000 m of carbonates and minor iron formation that accumulated along a late Paleoproterozoic ocean margin. Two upward-deepening stratigraphic packages are preserved, each characterized by peritidal precipitates at the base and iron formation and carbonate turbidites in its upper part. Consistent with recent studies of Neoarchean basins, carbon isotope ratios of Duck Creek carbonates show no evidence for a strong isotopic depth gradient, but carbonate minerals in iron formations can be markedly depleted in [super]13C. In contrast, oxygen isotopes covary strongly with depth; δ[super]18O values as positive as 2‰ VPDB in peritidal facies systematically decline to values of −6 to −16‰ in basinal rocks, reflecting, we posit, the timing of diagenetic closure. The Duck Creek Formation contains microfossils similar to those of the Gunflint Formation, Canada; they are restricted to early diagenetic cherts developed in basinal facies, strengthening the hypothesis that such fossils capture communities driven by iron metabolism. Indeed, X-ray diffraction data indicate that the Duck Creek basin was ferruginous throughout its history. The persistence of ferruginous waters and iron formation deposition in Western Australia for at least several tens of millions of years after the transition to sulfidic conditions in Laurentia suggests that the late Paleoproterozoic expansion of sulfidic subsurface waters was globally asynchronous.

The ∼780 to 540 Ma Windermere Supergroup of western North America records the protracted development of the western Laurentian passive margin and provides insights into the nature, timing, and kinematics of Rodinia's fragmentation. Here we present a refined tectono- and chemo-stratigraphic model for circa 780 to 720 Ma sedimentation in NW Canada through a study of the Callison Lake Formation (formalized herein) of the Mount Harper Group, spectacularly exposed in the Coal Creek and Hart River inliers of the Ogilvie Mountains of Yukon, Canada. Twenty-one stratigraphic sections are integrated with geological mapping, facies analysis, carbon and oxygen isotope chemostratigraphy, and Re-Os geochronology to provide a depositional reconstruction for the Callison Lake Formation. Mixed siliciclastic, carbonate, and evaporite sediments accumulated in marginal marine embayments formed in discrete hangingwall depocenters of a prominent Windermere extensional fault zone. Deposition of the Windermere Supergroup in NW Canada post dates the eruption of the circa 780 Ma Gunbarrel Large Igneous Province by ∼30 million years, is locally associated with compressional or transpressional tectonism, and predates the successful rift-drift transition by ∼200 million years. In order to accommodate evidence for coeval extensional and compressional tectonism, abrupt facies change, and Neoproterozoic fault geometries, we propose that NW Laurentia experienced strike-slip deformation during the ∼740 to 660 Ma early fragmentation of the supercontinent Rodinia. Sequence stratigraphic data from the Callison Lake Formation and other basal Windermere successions in the northern Canadian Cordillera delineate three distinct depositional sequences, or transgressive-regressive (T-R) cycles, that are coeval with similar stratigraphic packages in the ∼780 to 720 Ma Chuar-Uinta Mountain-Pahrump basins of the western United States. The global circa 735 Ma Islay carbon isotope excursion is consistently present in carbonate strata of the third T-R cycle and is interpreted to represent a primary perturbation to the global carbon cycle, possibly driven by the uplift and weathering of extensive shallow epicontinental seaways and evaporite basins.

High-resolution carbon isotope measurements of multiple stratigraphic sections in south China demonstrate that the pronounced carbon isotopic excursion at the Permian-Triassic boundary was not an isolated event but the first in a series of large fluctuations that continued throughout the Early Triassic before ending abruptly early in the Middle Triassic. The unusual behavior of the carbon cycle coincides with the delayed recovery from end-Permian extinction recorded by fossils, suggesting a direct relationship between Earth system function and biological rediversification in the aftermath of Earth's most devastating mass extinction.

Few existing studies illuminate the operation of the carbon cycle before the rise of atmospheric oxygen circa 2400 million years ago. Stable carbon isotopic measurements of shallow stromatolitic carbonates (~0 [parts per thousand] VPDB) and basinal carbonate minerals (-6 [parts per thousand]) in iron formation have been used to infer a strong isotopic depth gradient in Archean ocean basins. From new diamond drill cores obtained by the Agouron Drilling Project from the Griqualand West structural basin in the Northern Cape Province, South Africa, we present [delta C-13] data from carbonates and organic matter that offer fresh insights into the Late Archean carbon cycle. Three drill cores cover the development, progradation, and ultimate demise (by drowning) of the Campbellrand carbonate platform (ca. 2590-2500 Ma); one captures the platform top shallow marine and intertidal paleoenvironments, the other two run through slope and basinal sections deposited adjacent to the platform margin, increasing in water depth (likely to >1 km). Both shallow and deep-water carbonates precipitated on the seafloor consistently show [delta C-13] values around -0.5 [parts per thousand], incompatible with a strong Late Archean isotopic depth gradient. A mathematical model suggests that these isotopic data are consistent with a reduced biological pump, increased dissolved inorganic carbon in seawater due to higher atmospheric [P-CO2], or both. Certain horizons do show distinct isotopic variability. Such areas are commonly shaly, and they tend to be organic and/or iron rich. Strong C-isotopic variations occur on a cm scale and most likely stem from diagenetic remineralization of organic matter. In sediment-starved areas where iron formation developed, siderite tends to be [C-13]-depleted, sometimes by as much as -14 [parts per thousand]. These observations suggest a carbon cycle in which iron respiration played a conspicuous role. Carbon isotope ratios from organic matter in shales are commonly >1 [parts per thousand] lighter than stratigraphically contiguous carbonates, but there is no clear water depth trend in the organic carbon isotopic data. Taken as a whole, the [delta C-13] of organic matter can be explained by several non-unique sets of processes, including different autotrophic mechanisms of carbon fixation, heterotrophic recycling (including fermentation and methanotrophy), and post-depositional diagenesis. The most striking feature is the occurrence of organic [delta C-13] values <-40 [parts per thousand], a feature that appears to be commonplace in Late Archean successions. Framed in the context of carbon cycle isotopic mass balance, both organic and carbonate carbon isotopic data suggest that the proportion of carbon buried as organic matter was not radically different before the appearance of free environmental oxygen.

The glaciations of the Neoproterozoic Era (1,000 to 542 MyBP) were preceded by dramatically light C isotopic excursions preserved in preglacial deposits. Standard explanations of these excursions involve remineralization of isotopically light organic matter and imply strong enhancement of atmospheric (CO_2) greenhouse gas concentration, apparently inconsistent with the glaciations that followed. We examine a scenario in which the isotopic signal, as well as the global glaciation, result from enhanced export of organic matter from the upper ocean into anoxic subsurface waters and sediments. The organic matter undergoes anoxic remineralization at depth via either sulfate- or iron-reducing bacteria. In both cases, this can lead to changes in carbonate alkalinity and dissolved inorganic pool that efficiently lower the atmospheric (CO_2) concentration, possibly plunging Earth into an ice age. This scenario predicts enhanced deposition of calcium carbonate, the formation of siderite, and an increase in ocean pH, all of which are consistent with recent observations. Late Neoproterozoic diversification of marine eukaryotes may have facilitated the episodic enhancement of export of organic matter from the upper ocean, by causing a greater proportion of organic matter to be partitioned as particulate aggregates that can sink more efficiently, via increased cell size, biomineralization or increased C∶N of eukaryotic phytoplankton. The scenario explains isotopic excursions that are correlated or uncorrelated with snowball initiation, and suggests that increasing atmospheric oxygen concentrations and a progressive oxygenation of the subsurface ocean helped to prevent snowball glaciation on the Phanerozoic Earth.