Alan J. Kaufman

Neoproterozoic variations in the C-isotopic composition of seawater: stratigraphic and biogeochemical implications.

The recent proliferation of stratigraphic studies of delta 13C variation in carbonates and organic C in later Neoproterozoic and basal Cambrian successions (approximately 850-530 Ma) indicates a strong oscillating trend in the C-isotopic composition of surface seawater. Alone, this trend does not adequately characterize discrete intervals in Neoproterozoic time. However, integrated with the vectorial signals provided by fossils and Sr-isotopic variations, C isotope chemostratigraphy facilitates the interbasinal correlation of later Neoproterozoic successions. Results of these studies are evaluated in terms of four stratigraphic intervals: (1) the Precambrian/Cambrian boundary, (2) the post-Varanger terminal Proterozoic, (3) the late Cryogenian, and (4) the early Cryogenian. Where biostratigraphic or radiometric data constrain the age of Neoproterozoic sedimentary sequences, secular variations in C and Sr isotopes can provide a level of stratigraphic resolution exceeding that provided by fossils alone. Isotopic data place strong constraints on the chemical evolution of seawater, linking it to major tectonic and paleoclimatic events. They also provide a biogeochemical framework for the understanding of the initial radiation of macroscopic metazoans, which is associated stratigraphically, and perhaps causally, with a global increase in the burial of organic C and a concomitant rise of atmospheric O2.

A Whiff of Oxygen Before the Great Oxidation Event

High-resolution chemostratigraphy reveals an episode of enrichment of the redox-sensitive transition metals molybdenum and rhenium in the late Archean Mount McRae Shale in Western Australia. Correlations with organic carbon indicate that these metals were derived from contemporaneous seawater. Rhenium/osmium geochronology demonstrates that the enrichment is a primary sedimentary feature dating to 2501 ± 8 million years ago (Ma). Molybdenum and rhenium were probably supplied to Archean oceans by oxidative weathering of crustal sulfide minerals. These findings point to the presence of small amounts of O2 in the environment more than 50 million years before the start of the Great Oxidation Event.

Biostratigraphic and Geochronologic Constraints on Early Animal Evolution

Two distinct evolutionary pulses, represented by the Vendian Ediacaran fauna and Cambrian small shelly faunas, are generally thought to characterize the emergence of macroscopic animals at the end of Precambrian time. Biostratigraphic and uranium-lead zircon age data from Namibia indicate that most globally distributed Ediacaran fossils are no older than 549 million years old and some are as young as 543 million years old, essentially coincident with the Precambrian-Cambrian boundary. These data suggest that the most diverse assemblages of Ediacaran animals existed within 6 million years of the Precambrian-Cambrian boundary and that simple discoid animals may have appeared at least 50 million years earlier.

THE ABUNDANCE OF 13C IN MARINE ORGANIC MATTER AND ISOTOPIC FRACTIONATION IN THE GLOBAL BIOGEOCHEMICAL CYCLE OF CARBON DURING THE PAST 800 MA

Abstract New records of the abundance of 13 C in marine organic matter have been compiled for (i) the later Neoproterozoic, from 800 to 543 Ma (346 analyses), (ii) the Cambrian through the Jurassic (1616 analyses), and (iii) the Cretaceous and Cenozoic (2493 analyses). Comparison of these to existing compilations of the abundance of 13 C in sedimentary carbonates has allowed development of a record of the isotopic fractionation (≡eTOC) accompanying the production and burial of organic material. Over time, globally averaged values of eTOC have fallen in three ranges: (i) greater than 32‰ and apparently indicative of significant inputs from sulfide-oxidizing or other chemoautotrophic bacteria, notably during late Proterozoic interglacials at 752, 740–732, and 623–600 Ma; (ii) between 28 and 32‰ and indicative of maximal fractionation of carbon isotopes by phytoplanktonic producers, during the Neoproterozoic from 800 to 750 and from 685 to 625 Ma and during the Phanerozoic up to the early Oligocene; and (iii) less than 28‰, probably reflecting a reduction of primary fractionation by some combination of low levels of CO2, rapid rates of growth, and high ratios of cellular volume to surface area during Neoproterozoic glaciations (740, 720, and 575 Ma) and since the early Oligocene. Evidence of similar variations during the Ordovician and Gondwanan glaciations is absent. The decline in eTOC since the early Oligocene, from 30 to 22‰, has been nearly linear. The structure of the record of eTOC suggests that the maximal isotopic fractionation between dissolved CO2 and primary biomass has consistently been 25‰. Overall, the records provide compelling evidence that values of eTOC have varied widely and that the long-term average fractionation is roughly 30‰.

Secular variation in carbon isotope ratios from Upper Proterozoic successions of Svalbard and East Greenland

Analyses of stratigraphically continuous suites of samples from Upper Proterozoic sedimentary successions of East Greenland, Spitsbergen and Nordaustlandet (Svalbard) provide an approximation to the secular variation in carbon isotope ratios during a geologically and biologically important period of change from around 900 million years ago to the beginning of the Cambrian period. Late Riphean carbonates and organic material show a stratigraphically useful pattern of enrichment in 13C relative to Phanerozoic or earlier Proterozoic samples. Isotopic compositions of isolated samples from other localities are consistent with a worldwide extended interval of enhanced organic burial and consequent net survival of oxidized material, probably O2, just before the initial radiation of metazoans.

THE SR, C AND O ISOTOPIC EVOLUTION OF NEOPROTEROZOIC SEAWATER

Abstract Sr and C isotopic data obtained on stratigraphic suites of well-preserved marine limestone from Siberia, Namibia, Canada, Svalbard and East Greenland provide a relatively detailed first-order record of isotopic variation in seawater through the late Neoproterozoic Era. This data is used to revise the 87 Sr / 86 Sr and δ 13 C curves of this important interval, during which several discrete global ice ages occurred and the first macroscopic animals evolved. Through this time, the lowest 87 Sr / 86 Sr values (ca. 0.7056) characterize the interval between about 750–800 Ma and have been interpreted to reflect a major hydrothermal event. From 750 to 600 Ma, the Sr isotope values oscillate between highs and lows, ranging between 0.7063 and 0.7074. Between 600 Ma and the Early Cambrian (ca. 535 Ma), 87 Sr / 86 Sr values rise sharply from 0.7063 to 0.7087. This is thought to reflect enhanced continental input to the oceans associated with a Pan-African continental collision. This small subset of limestone samples (dolomites dominate the Neoproterozoic record) shows the δ 13 C curve rises from values close to 0 prior to 800 Ma to about +6‰ at 750 Ma and about +8‰ for the time between 600 and 730 Ma. During the time between 600 and 542 Ma, the highest values are about +4‰ (higher values in each interval are preserved in little-altered dolomites). Strong positive-to-negative excursions to values of −5‰ are associated with both Vendian glaciations estimated at about 575 and 590 Ma and with Sturtian glaciations estimated at about 720 and 740 Ma. In strong contrast, based on our view of least altered samples, there are no distinct changes in 87 Sr / 86 Sr across Neoproterozoic glacial intervals. The duration of these global refrigeration events is a subject of considerable debate. However, consideration of Sr residence times based on elemental partitioning, and the relationship between δ 13 C and 87 Sr / 86 Sr variations, suggest that these negative carbon isotope excursions would have lasted at least 350,000 years and no more than about one million years, assuming modern diagenetic fluxes of Sr to the oceans and total absence of continental fluxes.

Sedimentary cycling and environmental change in the Late Proterozoic: Evidence from stable and radiogenic isotopes

We report C, Sr, and O isotopic as well as selected major and trace element data from Late Proterozoic (ca. 540–900 Ma) marine carbonates in three widely separated basins. The isotopic and elemental data are used to evaluate effects of post-depositional alteration of 87Sr86Sr and δ13C. Using our present best estimates for unaltered samples, we construct a new δ13C-curve for 500–850 Ma marine carbonates using data in this paper and from literature sources. δ13C values are high (+4 to +8%) during most of the late Riphean (ca. 600–900 Ma) with brief negative excursions likely associated with glacial periods. Similarly, in the Vendian δ13C falls sharply (from late Riphean highs) to < −3% around the Varanger glaciation (ca. 600 Ma), and then returns to high values (+4 to +2%) remaining until the Precambrian-Cambrian boundary where the curve drops to a value of about − 1% in Lower Cambrian carbonates. Coupling of the Sr and C isotopic data is used to develop a simple model for evaluating organic carbon (Corg) burial in Late Proterozoic oceans. These calculations indicate that Corg burial rates were lower than present-day values during much of the late Riphean, at the same time that erosion rates were low. Excess O2 produced by the burial of Corg was likely balanced by oxidation of reduced hydrothermal fluids and weathering reactions. Near the time of the Varanger glaciation, Corg burial rates dropped but quickly recovered and reached a maximum (a factor of 2–4 greater than present day) in Vendian sediments. High Corg burial rates were probably driven by high sedimentation rates, and possibly high productivity. The high Corg burial rate likely gave rise to a large flux of O2; high values of δ34S in Late Proterozoic marine sulfates suggest that this O2 flux was not balanced by increased sulfate formation. Further, the Sr-isotopic record indicates that excess O2 was not balanced by oxidation of submarine hydrothermal fluids. Increased oxidative weathering was probably an important sink for O2; nonetheless, we conclude that a significant and rapid increase in atmospheric O2 occurred in the Vendian. These results have important implications for environmental changes during the first appearance of an Ediacaran metazoan fauna.

The Vendian Record of Sr and C Isotopic Variations in Seawater: Implications for Tectonics and Paleoclimate

New Sr and C isotopic data, both obtained on the same samples of marine carbonates, provide a relatively detailed record of isotopic variation in seawater through the latest Proterozoic and allow, for the first time, direct correlation of these isotopic changes in the Vendian (∼ 540–610 Ma). The strong isotope variations determined in this study record significant environmental and tectonic changes. Together with a fairly poorly constrained Nd isotopic record, the Sr and C isotopic records can be used to constrain rates of erosion, hydrothermal alteration and organic C burial. Further, comparison of these records with those of the Cenozoic permit investigation of the general relationship between global tectonics and continental glaciation. In particular, results of this study show a very large change in the 87Sr/86Sr of marine carbonates from low pre-Vendian ( > 610 Ma) values ( ∼ 0.7066) to high Middle Cambrian values ( ∼ 0.7090). This change is greater in magnitude than the significant increase in seawater 87Sr/86Sr through the Cenozoic. Both changes are attributed to high erosion rates associated with continent-continent collisions (Pan-African and Himalayan-Tibetan). In the latest Proterozoic these high erosion rates, probably coupled with high organic productivity and anoxic bottom-water conditions, contributed to a significant increase in the burial rate of organic C. Ice ages mark both the Neoproterozoic and Cenozoic, but different stratigraphic relationships between the Sr isotopic increase and continental glaciation indicate that uplift-driven models proposed to explain Cenozoic climatic change cannot account for the latest Proterozoic ice ages.

Pulsed oxidation and biological evolution in the Ediacaran Doushantuo Formation

Recent geochemical data from Oman, Newfoundland, and the western United States suggest that long-term oxidation of Ediacaran oceans resulted in progressive depletion of a large dissolved organic carbon (DOC) reservoir and potentially triggered the radiation of acanthomorphic acritarchs, algae, macroscopic Ediacara organisms, and, subsequently, motile bilaterian animals. However, the hypothesized coupling between ocean oxidation and evolution is contingent on the reliability of continuous geochemical and paleontological data in individual sections and of intercontinental correlations. Here we report high-resolution geochemical data from the fossil-rich Doushantuo Formation (635–551 Ma) in South China that confirm trends from other broadly equivalent sections and highlight key features that have not been observed in most sections or have received little attention. First, samples from the lower Doushantuo Formation are characterized by remarkably stable δ13Corg (carbon isotope composition of organic carbon) values but variable δ34SCAS (sulfur isotope composition of carbonate-associated sulfate) values, which are consistent with a large isotopically buffered DOC reservoir and relatively low sulfate concentrations. Second, there are three profound negative δ13Ccarb (carbon isotope composition of carbonate) excursions in the Ediacaran Period. The negative δ13Ccarb excursions in the middle and upper Doushantuo Formation record pulsed oxidation of the deep oceanic DOC reservoir. The oxidation events appear to be coupled with eukaryote diversity in the Doushantuo basin. Comparison with other early Ediacaran basins suggests spatial heterogeneity of eukaryote distribution and redox conditions. We hypothesize that the distribution of early Ediacaran eukaryotes likely tracked redox conditions and that only after ≈551 Ma (when Ediacaran oceans were pervasively oxidized) did evolution of oxygen-requiring taxa reach global distribution.

Late archean biospheric oxygenation and atmospheric evolution

High-resolution geochemical analyses of organic-rich shale and carbonate through the 2500 million-year-old Mount McRae Shale in the Hamersley Basin of northwestern Australia record changes in both the oxidation state of the surface ocean and the atmospheric composition. The Mount McRae record of sulfur isotopes captures the widespread and possibly permanent activation of the oxidative sulfur cycle for perhaps the first time in Earths history. The correlation of the time-series sulfur isotope signals in northwestern Australia with equivalent strata from South Africa suggests that changes in the exogenic sulfur cycle recorded in marine sediments were global in scope and were linked to atmospheric evolution. The data suggest that oxygenation of the surface ocean preceded pervasive and persistent atmospheric oxygenation by 50 million years or more.