1] A large (11-15%) negative shift in d 13 C is observed in shallow water carbonates directly ben... more 1] A large (11-15%) negative shift in d 13 C is observed in shallow water carbonates directly beneath Neoproterozoic glacial deposits (or correlative disconformity) in northwest Namibia ascribed to a snowball Earth. Reproducibility and stratigraphic concordance of this anomaly in 16 sections across the ancient continental shelf support a primary origin, and field relations show it predates the fall in sea level associated with the Ghaub glaciation. We crudely estimate the duration of the isotopic shift as $0.6 Â 10 6 years from a simple thermal subsidence model. Similar or larger d 13 C anomalies are found directly beneath Neoproterozoic glacial units in Australia, Canada, China, Scotland, and Svalbard. After considering conventional interpretations for negative d 13 C anomalies, we conclude that a prolonged methane release to the atmosphere is most consistent with the timescale, magnitude, and geological context of the anomaly in Namibia. Counterintuitively, an anomalous methane flux that is sustained for 100s kyr may be consistent with a snowball glaciation.
Carbonates within the Rainstorm Member in the terminal Neoproterozoic Johnnie Formation of Death ... more Carbonates within the Rainstorm Member in the terminal Neoproterozoic Johnnie Formation of Death Valley, California record a remarkable negative carbon isotope anomaly -to a nadir of near -11‰ -that accompanies a dramatic rise in trace sulfate abundance and fall of carbonate associated sulfate δ 34 S values. The carbonates, including the laterally extensive Johnnie Oolite, were deposited during marine flooding atop a sequence boundary best observed in cratonward sections. The 12 C-rich alkalinity delivered to the seawater pool was likely associated with a dramatic rise in atmospheric O 2 , which resulted in 1) the oxidation of exposed continental shelf sediments rich in fossil organic matter and sulfides, and 2) the ventilation of the oceans. The carbon isotope anomaly is recorded in broadly equivalent successions that post-date known Marinoan glacial deposits and pre-date the Precambrian-Cambrian boundary in Oman, India, China, Australia, and Namibia, supporting the view that the oxygenation event was global. Large metazoan fossils (Ediacaran animals) first appear directly above this anomaly, suggesting that a critical threshold with respect to atmospheric O 2 had been crossed at this time. A negative excursion of similar magnitude occurs in overlying strata at the Precambrian-Cambrian boundary, which likely reflects similar processes.
The minor extent of sulfur isotope fractionation preserved in many Neoarchean sedimentary success... more The minor extent of sulfur isotope fractionation preserved in many Neoarchean sedimentary successions suggests that sulfate-reducing microorganisms played an insignificant role in ancient marine environments, despite evidence that these organisms evolved much earlier. We present bulk, microdrilled, and ion probe sulfur isotope data from carbonate-associated pyrite in the ~2.5-billion-year-old Batatal Formation of Brazil, revealing large mass-dependent fractionations (approaching 50 per mil) associated with microbial sulfate reduction, as well as consistently negative Δ(33)S values (~ -2 per mil) indicative of atmospheric photochemical reactions. Persistent (33)S depletion through ~60 meters of shallow marine carbonate implies long-term stability of seawater sulfate abundance and isotope composition. In contrast, a negative Δ(33)S excursion in lower Batatal strata indicates a response time of ~40,000 to 150,000 years, suggesting Neoarchean sulfate concentrations between ~1 and 10 μM.
Siliciclastic rocks of the Ruyang Group, southern Shanxi, and the broadly equivalent Gaoshanhe Gr... more Siliciclastic rocks of the Ruyang Group, southern Shanxi, and the broadly equivalent Gaoshanhe Group, Shaanxi, contain exceptionally well-preserved, large (Ø ≈ 150 μm) acanthomorphic acritarchs recently interpreted as late Neoproterozoic (Sinian, c. 800-544 Ma) in age. This biostratigraphic interpretation is based on the presence of large acanthomorphs in Sinian successions of South China and elsewhere and the perceived absence of
Sedimentary rocks in the western Anabar region, northwestern Siberia, preserve an exceptional rec... more Sedimentary rocks in the western Anabar region, northwestern Siberia, preserve an exceptional record of evolution and biogeochemical events near the Proterozoic-Cambrian boundary. Carbon isotopic data on petrographically and geochemically screened samples collected at 1 to 2 m intervals support correlation of the lower Anabar succession (Staraya Reckha and lower Manykai Formations) with sub-Tommotian carbonates of the Ust'-Yu-doma Formation in southeastern Siberia. In contrast, the upper Manykai and most of the overlying Medvezhya Formation appear to preserve a sedimentary and paleontological record of an evolutionary important time interval represented in southeastern Siberia only by the sub-Tommotian unconformity. Correlation of the Anabar section with other northern Siberian successions that contain well-dated volcanic rocks permits the estimate that the sub-Tommotian unconformity in southeastern Siberia spans approximately 3 to 6 m.y. Diverse small shelly fossils (but not archaeocyathans) previously thought to mark the base of the Tommotian Stage evolved sequentially throughout this earlier interval.
An enduring enigma of Neoprotero- zoic Earth history is the intimate associ- ation of glacial dia... more An enduring enigma of Neoprotero- zoic Earth history is the intimate associ- ation of glacial diamictites with typical warm-water carbonates. Among the many hypothesized explanations for this paleoclimatic dichotomy are high orbital obliquity, true polar wander, reduced solar luminosity, snowball albedo, CO2 drawdown, stagnant ocean overturn, and reinterpretation of diamictites as mega-impact ejecta. The Otavi carbonate platform on the Congo
Thick, unmetamorphosed successions of siliciclastic and carbonate rocks in eastern and western Si... more Thick, unmetamorphosed successions of siliciclastic and carbonate rocks in eastern and western Siberia preserve a record of Middle Riphean to Early Upper Riphean sedimentary environments and geochemistry. Consistent with data from other continents, our studies in the Uchur-Maya region in southeastern Siberia and the Turukhansk Uplift in northwestern Siberia suggest a first-order shift in l 13 C from values near 0‰ in the early Mesoproterozoic to values near +3.5‰ after about 1300 Ma. Over this same interval, primary 87 Sr/ 86 Sr values decrease from \ 0.7060 to B 0.7053. Combining lithologic, biostratigraphic, and geochemical data sets with available geochronologic constraints, we present a refined correlation between these two key Proterozoic successions in Siberia and add this dataset to a growing body of C and Sr isotopic data from this time interval. Carbon isotope chemostratigraphy from these regions supports the occurrence and timing of a first-order, 3.5‰ positive shift ca. 1250-1300 Ma, approximately coeval with the onset of active margin activity that predates the main phase of Rodinia assembly. Sr isotopic data may also be interpreted within the context of the evolving Mesoproterozoic tectonic regime. Available data suggest that no dramatic rise in 87 Sr/ 86 Sr heralds the main phase of Rodinia assembly in the terminal Mesoproterozoic, suggesting that significant juvenile crust was involved in mountain building, that relative hydrothermal flux from mid-ocean ridges remained high throughout the assembly of Rodinia and/or that increased continental runoff related to intense erosion of Grenvillian mountain belts terminated shortly after orogeny.
The Lapa Formation is a thick carbonate sequence (∼900 m) that constitutes the upper part of the ... more The Lapa Formation is a thick carbonate sequence (∼900 m) that constitutes the upper part of the Vazante Group on the São Francisco craton, Brazil. It conformably overlies a previously unrecognized glacial diamictite unit of poorly constrained age. The sequence, above the glacial unconformity, consists predominantly of organic-rich shale, subtidal rhythmic dolomicrites and microbialaminites, and intertidal stromatolites. Four boreholes, spanning different depositional settings, were sampled at highresolution and investigated for their petrographic and chemical criteria to evaluate their degree of preservation.
The early Paleoproterozoic Snowy Pass Supergroup of the Medicine Bow Mountains and Sierra Madre, ... more The early Paleoproterozoic Snowy Pass Supergroup of the Medicine Bow Mountains and Sierra Madre, Wyoming, USA and Huronian Supergroup, Ontario, Canada were deposited along the present-day southern flank of the Wyoming and Superior cratons. Whereas three discrete levels of glacial diamictite are developed in both successions, carbonate strata are known only directly above the middle diamictite (Vagner and Bottle Creek formations in Medicine Bow Mountains and Sierra Madre, respectively, and Espanola Formation in southern Ontario) in these thick correlative siliciclastics-dominated strata. The carbonates from each succession record negative ␦ 13 C values (−4.0 to −0.8‰, V-PDB) and attenuated carbon isotopic difference between organic and inorganic phases. Oxygen in carbonates is strongly depleted in 18 O suggesting exchange with hot fluids, which is consistent with pervasive recrystallization of carbonates and remobilization of elements. However, the stratigraphic coherence of carbon isotopic compositions and the general lack of correlation between ␦ 13 C and either ␦ 18 O values or trace element concentrations supports a primary origin for 13 C-depleted carbonates, which are interpreted here to reflect anomalous oceanic compositions. The intimate association of thick carbonate units containing abundant carbonate debris flows with immediately underlying glacial strata indicates that chemical precipitation resulted from a rapid flux of carbonate alkalinity onto ocean margins during post-glacial transgression. Although these early Paleoproterozoic carbonates are similar to Neoproterozoic 'cap dolomites' in stratigraphic position and carbon isotopic compositions, the older post-glacial accumulations begin with limestone and lack many of the sedimentary structures typical of Neoproterozoic deposits. Furthermore, it is not understood why carbonates only occur above the middle of the three glacial horizons whereas these deposits are ubiquitous above Neoproterozoic diamictites. The differences might reflect lower overall carbonate saturation in early Paleoproterozoic oceans which contrasts sharply with Archean and later Paleoproterozoic intervals and higher siliciclastic inputs in rift environments, which shut down carbonate deposition.
2. Examples include velocity variations occurring over months to weeks [R. LeB Hooke, P. Calla, e... more 2. Examples include velocity variations occurring over months to weeks [R. LeB Hooke, P. Calla, et al., J. Glaciol. 35, 235 (1989)] and days to hours [A. Iken and R. A. Bindschadler, ibid. 32, 101 (1986)]. 3. S. M. Hodge, ibid. 13, 349 (1974); B. Kamb et al., J. Geophys. Res. 99, 15231 (1994); J. Harbor et al., Geology 25, 739 (1997). 4. A dense array of radio-echo sounding measurements
Isotopic chemostratigraphy has proven successful in the correlation of carbonate-rich Neoproteroz... more Isotopic chemostratigraphy has proven successful in the correlation of carbonate-rich Neoproterozoic successions. In successions dominated by siliciclastic rocks, chemostratigraphy can be problematic, but if thin carbonates punctuate siliciclastic strata, useful isotopic data may be obtained. The upper Pocatello Formation and lower Brigham Group of southeastern Idaho provide an opportunity to assess the potential and limitations of isotopic chemostratigraphy in overwhelmingly siliciclastic successions. The 5000 m thick succession consists predominantly of siliciclastic lithologies, with only three intervals that contain thin intercalated carbonates. Its depositional age is only broadly constrained by existing biostratigraphic, sequence stratigraphic and geochronometric data. The lowermost carbonates include a cap dolomite atop diamictites and volcanic rocks of the Pocatello Formation. The delta 13C values of these carbonates are distintly negative (-5 to -3), similar to carbonates that overlie Neoproterozoic glaciogenic rocks worldwide. Stratigraphically higher carbonates record a major positive delta 13C excursion to values as high as +8.8 within the carbonate member of the Caddy Canyon Quartzite. The magnitude of this excusion is consistent with post-Sturtian secular variation recorded elsewhere in the North American Cordillera, Australia, Svalbard, Brazil and Nambia, and exceeds the magnitude of any post-Varanger delta 13C excursion documented to date. In most samples, Sr-isotopic abundances have been altered by diagenesis and greenschist facies metamorphism, but a least-altered value of approximately 0.7076 supports a post-Sturtian and pre-Marinoan/Varanger age for upper Pocatello and lower Brigham rocks that lie above the Pocatello diamictite. Thus, even though available chemostratigraphic data are limited, they corroborate correlations of Pocatello Formation diamictites and overlying units with Sturtian glaciogenic rocks and immediately post-Sturtian successions in western North America and elsewhere.
High-resolution chemostratigraphy reveals an episode of enrichment of the redox-sensitive transit... more 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.
High-resolution geochemical analyses of organic-rich shale and carbonate through the 2500 million... more 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 Earth's 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.
Proceedings of the National Academy of Sciences, 2008
Recent geochemical data from Oman, Newfoundland, and the western United States suggest that long-... more 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 ␦ 13 Corg (carbon isotope composition of organic carbon) values but variable ␦ 34 SCAS (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 ␦ 13 Ccarb (carbon isotope composition of carbonate) excursions in the Ediacaran Period. The negative ␦ 13 Ccarb 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 Ϸ551 Ma (when Ediacaran oceans were pervasively oxidized) did evolution of oxygen-requiring taxa reach global distribution.
The recent proliferation of stratigraphic studies of ~13C variation in carbonates and organic C i... more The recent proliferation of stratigraphic studies of ~13C variation in carbonates and organic C in later Neoproterozoic and basal Cambrian successions ( ~ 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 02. 0301-9268/95/$09.50
In the Himalaya of central Nepal, uncertainty in the absolute depositional ages and the relative ... more In the Himalaya of central Nepal, uncertainty in the absolute depositional ages and the relative stratigraphic positions of several formations in the Proterozoic part of the Lesser Himalayan series has hindered structural mapping, east-west correlation of units and structures, and interpretations of basin architecture during both the Proterozoic and Paleozoic. At the stratigraphic base of the Lesser Himalayan series in
1] A large (11-15%) negative shift in d 13 C is observed in shallow water carbonates directly ben... more 1] A large (11-15%) negative shift in d 13 C is observed in shallow water carbonates directly beneath Neoproterozoic glacial deposits (or correlative disconformity) in northwest Namibia ascribed to a snowball Earth. Reproducibility and stratigraphic concordance of this anomaly in 16 sections across the ancient continental shelf support a primary origin, and field relations show it predates the fall in sea level associated with the Ghaub glaciation. We crudely estimate the duration of the isotopic shift as $0.6 Â 10 6 years from a simple thermal subsidence model. Similar or larger d 13 C anomalies are found directly beneath Neoproterozoic glacial units in Australia, Canada, China, Scotland, and Svalbard. After considering conventional interpretations for negative d 13 C anomalies, we conclude that a prolonged methane release to the atmosphere is most consistent with the timescale, magnitude, and geological context of the anomaly in Namibia. Counterintuitively, an anomalous methane flux that is sustained for 100s kyr may be consistent with a snowball glaciation.
Carbonates within the Rainstorm Member in the terminal Neoproterozoic Johnnie Formation of Death ... more Carbonates within the Rainstorm Member in the terminal Neoproterozoic Johnnie Formation of Death Valley, California record a remarkable negative carbon isotope anomaly -to a nadir of near -11‰ -that accompanies a dramatic rise in trace sulfate abundance and fall of carbonate associated sulfate δ 34 S values. The carbonates, including the laterally extensive Johnnie Oolite, were deposited during marine flooding atop a sequence boundary best observed in cratonward sections. The 12 C-rich alkalinity delivered to the seawater pool was likely associated with a dramatic rise in atmospheric O 2 , which resulted in 1) the oxidation of exposed continental shelf sediments rich in fossil organic matter and sulfides, and 2) the ventilation of the oceans. The carbon isotope anomaly is recorded in broadly equivalent successions that post-date known Marinoan glacial deposits and pre-date the Precambrian-Cambrian boundary in Oman, India, China, Australia, and Namibia, supporting the view that the oxygenation event was global. Large metazoan fossils (Ediacaran animals) first appear directly above this anomaly, suggesting that a critical threshold with respect to atmospheric O 2 had been crossed at this time. A negative excursion of similar magnitude occurs in overlying strata at the Precambrian-Cambrian boundary, which likely reflects similar processes.
The minor extent of sulfur isotope fractionation preserved in many Neoarchean sedimentary success... more The minor extent of sulfur isotope fractionation preserved in many Neoarchean sedimentary successions suggests that sulfate-reducing microorganisms played an insignificant role in ancient marine environments, despite evidence that these organisms evolved much earlier. We present bulk, microdrilled, and ion probe sulfur isotope data from carbonate-associated pyrite in the ~2.5-billion-year-old Batatal Formation of Brazil, revealing large mass-dependent fractionations (approaching 50 per mil) associated with microbial sulfate reduction, as well as consistently negative Δ(33)S values (~ -2 per mil) indicative of atmospheric photochemical reactions. Persistent (33)S depletion through ~60 meters of shallow marine carbonate implies long-term stability of seawater sulfate abundance and isotope composition. In contrast, a negative Δ(33)S excursion in lower Batatal strata indicates a response time of ~40,000 to 150,000 years, suggesting Neoarchean sulfate concentrations between ~1 and 10 μM.
Siliciclastic rocks of the Ruyang Group, southern Shanxi, and the broadly equivalent Gaoshanhe Gr... more Siliciclastic rocks of the Ruyang Group, southern Shanxi, and the broadly equivalent Gaoshanhe Group, Shaanxi, contain exceptionally well-preserved, large (Ø ≈ 150 μm) acanthomorphic acritarchs recently interpreted as late Neoproterozoic (Sinian, c. 800-544 Ma) in age. This biostratigraphic interpretation is based on the presence of large acanthomorphs in Sinian successions of South China and elsewhere and the perceived absence of
Sedimentary rocks in the western Anabar region, northwestern Siberia, preserve an exceptional rec... more Sedimentary rocks in the western Anabar region, northwestern Siberia, preserve an exceptional record of evolution and biogeochemical events near the Proterozoic-Cambrian boundary. Carbon isotopic data on petrographically and geochemically screened samples collected at 1 to 2 m intervals support correlation of the lower Anabar succession (Staraya Reckha and lower Manykai Formations) with sub-Tommotian carbonates of the Ust'-Yu-doma Formation in southeastern Siberia. In contrast, the upper Manykai and most of the overlying Medvezhya Formation appear to preserve a sedimentary and paleontological record of an evolutionary important time interval represented in southeastern Siberia only by the sub-Tommotian unconformity. Correlation of the Anabar section with other northern Siberian successions that contain well-dated volcanic rocks permits the estimate that the sub-Tommotian unconformity in southeastern Siberia spans approximately 3 to 6 m.y. Diverse small shelly fossils (but not archaeocyathans) previously thought to mark the base of the Tommotian Stage evolved sequentially throughout this earlier interval.
An enduring enigma of Neoprotero- zoic Earth history is the intimate associ- ation of glacial dia... more An enduring enigma of Neoprotero- zoic Earth history is the intimate associ- ation of glacial diamictites with typical warm-water carbonates. Among the many hypothesized explanations for this paleoclimatic dichotomy are high orbital obliquity, true polar wander, reduced solar luminosity, snowball albedo, CO2 drawdown, stagnant ocean overturn, and reinterpretation of diamictites as mega-impact ejecta. The Otavi carbonate platform on the Congo
Thick, unmetamorphosed successions of siliciclastic and carbonate rocks in eastern and western Si... more Thick, unmetamorphosed successions of siliciclastic and carbonate rocks in eastern and western Siberia preserve a record of Middle Riphean to Early Upper Riphean sedimentary environments and geochemistry. Consistent with data from other continents, our studies in the Uchur-Maya region in southeastern Siberia and the Turukhansk Uplift in northwestern Siberia suggest a first-order shift in l 13 C from values near 0‰ in the early Mesoproterozoic to values near +3.5‰ after about 1300 Ma. Over this same interval, primary 87 Sr/ 86 Sr values decrease from \ 0.7060 to B 0.7053. Combining lithologic, biostratigraphic, and geochemical data sets with available geochronologic constraints, we present a refined correlation between these two key Proterozoic successions in Siberia and add this dataset to a growing body of C and Sr isotopic data from this time interval. Carbon isotope chemostratigraphy from these regions supports the occurrence and timing of a first-order, 3.5‰ positive shift ca. 1250-1300 Ma, approximately coeval with the onset of active margin activity that predates the main phase of Rodinia assembly. Sr isotopic data may also be interpreted within the context of the evolving Mesoproterozoic tectonic regime. Available data suggest that no dramatic rise in 87 Sr/ 86 Sr heralds the main phase of Rodinia assembly in the terminal Mesoproterozoic, suggesting that significant juvenile crust was involved in mountain building, that relative hydrothermal flux from mid-ocean ridges remained high throughout the assembly of Rodinia and/or that increased continental runoff related to intense erosion of Grenvillian mountain belts terminated shortly after orogeny.
The Lapa Formation is a thick carbonate sequence (∼900 m) that constitutes the upper part of the ... more The Lapa Formation is a thick carbonate sequence (∼900 m) that constitutes the upper part of the Vazante Group on the São Francisco craton, Brazil. It conformably overlies a previously unrecognized glacial diamictite unit of poorly constrained age. The sequence, above the glacial unconformity, consists predominantly of organic-rich shale, subtidal rhythmic dolomicrites and microbialaminites, and intertidal stromatolites. Four boreholes, spanning different depositional settings, were sampled at highresolution and investigated for their petrographic and chemical criteria to evaluate their degree of preservation.
The early Paleoproterozoic Snowy Pass Supergroup of the Medicine Bow Mountains and Sierra Madre, ... more The early Paleoproterozoic Snowy Pass Supergroup of the Medicine Bow Mountains and Sierra Madre, Wyoming, USA and Huronian Supergroup, Ontario, Canada were deposited along the present-day southern flank of the Wyoming and Superior cratons. Whereas three discrete levels of glacial diamictite are developed in both successions, carbonate strata are known only directly above the middle diamictite (Vagner and Bottle Creek formations in Medicine Bow Mountains and Sierra Madre, respectively, and Espanola Formation in southern Ontario) in these thick correlative siliciclastics-dominated strata. The carbonates from each succession record negative ␦ 13 C values (−4.0 to −0.8‰, V-PDB) and attenuated carbon isotopic difference between organic and inorganic phases. Oxygen in carbonates is strongly depleted in 18 O suggesting exchange with hot fluids, which is consistent with pervasive recrystallization of carbonates and remobilization of elements. However, the stratigraphic coherence of carbon isotopic compositions and the general lack of correlation between ␦ 13 C and either ␦ 18 O values or trace element concentrations supports a primary origin for 13 C-depleted carbonates, which are interpreted here to reflect anomalous oceanic compositions. The intimate association of thick carbonate units containing abundant carbonate debris flows with immediately underlying glacial strata indicates that chemical precipitation resulted from a rapid flux of carbonate alkalinity onto ocean margins during post-glacial transgression. Although these early Paleoproterozoic carbonates are similar to Neoproterozoic 'cap dolomites' in stratigraphic position and carbon isotopic compositions, the older post-glacial accumulations begin with limestone and lack many of the sedimentary structures typical of Neoproterozoic deposits. Furthermore, it is not understood why carbonates only occur above the middle of the three glacial horizons whereas these deposits are ubiquitous above Neoproterozoic diamictites. The differences might reflect lower overall carbonate saturation in early Paleoproterozoic oceans which contrasts sharply with Archean and later Paleoproterozoic intervals and higher siliciclastic inputs in rift environments, which shut down carbonate deposition.
2. Examples include velocity variations occurring over months to weeks [R. LeB Hooke, P. Calla, e... more 2. Examples include velocity variations occurring over months to weeks [R. LeB Hooke, P. Calla, et al., J. Glaciol. 35, 235 (1989)] and days to hours [A. Iken and R. A. Bindschadler, ibid. 32, 101 (1986)]. 3. S. M. Hodge, ibid. 13, 349 (1974); B. Kamb et al., J. Geophys. Res. 99, 15231 (1994); J. Harbor et al., Geology 25, 739 (1997). 4. A dense array of radio-echo sounding measurements
Isotopic chemostratigraphy has proven successful in the correlation of carbonate-rich Neoproteroz... more Isotopic chemostratigraphy has proven successful in the correlation of carbonate-rich Neoproterozoic successions. In successions dominated by siliciclastic rocks, chemostratigraphy can be problematic, but if thin carbonates punctuate siliciclastic strata, useful isotopic data may be obtained. The upper Pocatello Formation and lower Brigham Group of southeastern Idaho provide an opportunity to assess the potential and limitations of isotopic chemostratigraphy in overwhelmingly siliciclastic successions. The 5000 m thick succession consists predominantly of siliciclastic lithologies, with only three intervals that contain thin intercalated carbonates. Its depositional age is only broadly constrained by existing biostratigraphic, sequence stratigraphic and geochronometric data. The lowermost carbonates include a cap dolomite atop diamictites and volcanic rocks of the Pocatello Formation. The delta 13C values of these carbonates are distintly negative (-5 to -3), similar to carbonates that overlie Neoproterozoic glaciogenic rocks worldwide. Stratigraphically higher carbonates record a major positive delta 13C excursion to values as high as +8.8 within the carbonate member of the Caddy Canyon Quartzite. The magnitude of this excusion is consistent with post-Sturtian secular variation recorded elsewhere in the North American Cordillera, Australia, Svalbard, Brazil and Nambia, and exceeds the magnitude of any post-Varanger delta 13C excursion documented to date. In most samples, Sr-isotopic abundances have been altered by diagenesis and greenschist facies metamorphism, but a least-altered value of approximately 0.7076 supports a post-Sturtian and pre-Marinoan/Varanger age for upper Pocatello and lower Brigham rocks that lie above the Pocatello diamictite. Thus, even though available chemostratigraphic data are limited, they corroborate correlations of Pocatello Formation diamictites and overlying units with Sturtian glaciogenic rocks and immediately post-Sturtian successions in western North America and elsewhere.
High-resolution chemostratigraphy reveals an episode of enrichment of the redox-sensitive transit... more 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.
High-resolution geochemical analyses of organic-rich shale and carbonate through the 2500 million... more 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 Earth's 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.
Proceedings of the National Academy of Sciences, 2008
Recent geochemical data from Oman, Newfoundland, and the western United States suggest that long-... more 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 ␦ 13 Corg (carbon isotope composition of organic carbon) values but variable ␦ 34 SCAS (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 ␦ 13 Ccarb (carbon isotope composition of carbonate) excursions in the Ediacaran Period. The negative ␦ 13 Ccarb 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 Ϸ551 Ma (when Ediacaran oceans were pervasively oxidized) did evolution of oxygen-requiring taxa reach global distribution.
The recent proliferation of stratigraphic studies of ~13C variation in carbonates and organic C i... more The recent proliferation of stratigraphic studies of ~13C variation in carbonates and organic C in later Neoproterozoic and basal Cambrian successions ( ~ 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 02. 0301-9268/95/$09.50
In the Himalaya of central Nepal, uncertainty in the absolute depositional ages and the relative ... more In the Himalaya of central Nepal, uncertainty in the absolute depositional ages and the relative stratigraphic positions of several formations in the Proterozoic part of the Lesser Himalayan series has hindered structural mapping, east-west correlation of units and structures, and interpretations of basin architecture during both the Proterozoic and Paleozoic. At the stratigraphic base of the Lesser Himalayan series in
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Papers by Alan Kaufman