scholarly journals C, O, and Sr isotopic variations in Neoproterozoic-Cambrian carbonate rocks from Sete Lagoas Formation (Bambuí Group), in the Southern São Francisco Basin, Brazil

2017 ◽  
Vol 47 (3) ◽  
pp. 521-543 ◽  
Author(s):  
Cristian Guacaneme ◽  
Marly Babinski ◽  
Gustavo Macedo de Paula-Santos ◽  
Antonio Carlos Pedrosa-Soares
Keyword(s):  
Carbonate Rocks ◽  
Carbonate Platform ◽  
Sr Isotope ◽  
Δ13c Values ◽  
Depositional Processes ◽  
Water Influx ◽  
Isotope Stratigraphy ◽  
Depositional Controls ◽  
Isotopic Variations ◽  
Marine Basin

ABSTRACT: High-resolution chemostratigraphic data of carbonates from the Sete Lagoas Formation (Bambuí Group) show large variations on the C, O, and Sr isotope compositions. Impure limestones at the base show primal δ13C values between -1.0 and 0‰, and δ18O values between -12.0 and -8.0‰. However, some dolostones demonstrate δ13C values varying from +2.8 to -6.8‰, highly radiogenic 87Sr/86Sr ratios (>0.7111), and low Sr concentrations (<350 ppm) related to post-depositional processes. In contrast, pure limestones at the top show very positive δ13C values between +8.3 and +12.8‰, δ18O values between -10.0 to -6.0‰, and 87Sr/86Sr ratios from 0.7073 to 0.7086, with high Sr concentrations (>900 ppm). They are linked to depositional controls on the carbonate platform, such as fluvial and/or submarine water influx, in which carbonates deposited on the proximal sector exhibit significant Sr isotopic variations and those on the distal sector were not subject to such controls, resulting in very homogeneous Sr isotope profiles. However, 87Sr/86Sr ratios of the distal carbonates are less radiogenic than carbonates expected for late Ediacaran (~0.7085). This discrepancy suggests a restricted marine basin without Sr isotopic homogenization with contemporary oceans and, in this case, global correlations based on Sr isotope stratigraphy are not reliable.

scholarly journals C-, Sr-isotope stratigraphy of carbonate rocks from the Southern Espinhaço Ridge, Minas Gerais, southeastern Brazil

2014 ◽  
Vol 86 (2) ◽  
pp. 633-648 ◽  
Author(s):  
LUCIO M.S. FRAGA ◽  
SORAYA C. NEVES ◽  
ALEXANDRE UHLEIN ◽  
ALCIDES N. SIAL ◽  
MARCIO M. PIMENTEL ◽  
...  
Keyword(s):  
Carbonate Rocks ◽  
Minas Gerais ◽  
Southeastern Brazil ◽  
Sr Isotope ◽  
Δ13c Values ◽  
Isotope Stratigraphy ◽  
Isotope Analyses

Neoproterozoic carbonate rocks comprise different stratigraphic units in the southern part of the Espinhaço Ridge, Minas Gerais, Brazil. C, O- and Sr-isotope analyses were carried out along four selected stratigraphic sections across these formations. These are: (i) the Rio Pardo Grande Formation in the upper portion of the Espinhaço Supergroup, sampled in section 3; (ii) Macaúbas Group laminated limestones (Tijucuçu Farm) and dolostone layers (Domingas Formation) have been respectively sampled along the so-called sections 1 and 2, and (iii) the lower stratigraphic units of the Bambuí Group, sampled in section 4. Laminated limestone samples from the Macaúbas Group have δ13C values as high as 10.9‰ decreasing up section to -1.1‰ and 87Sr/86Sr values vary from 0.7072 to 0.7076, a range commonly observed in Cryogenian rocks. In section 2, dolomitic samples exhibit 87Sr/86Sr from 0.7076 to 0.7077 while in section 3, 87Sr/86Sr from 0.7074 to 0.7079. In section 4, 87Sr/86Sr values are around 0.7080. The values of 87Sr/86Sr observed in carbonate samples from the Macaúbas Group are similar to those observed in the Sr-isotope secular curve for the Neoproterozoic. Carbonate samples from the base of the Bambuí Group correlate with Ediacaran fingerprints, after the Marinoan (ca. 635 Ma) glaciation.

scholarly journals Bio-chemostratigraphy of the Barremian-Aptian shallow-water carbonates of the southern Apennines (Italy): pinpointing the OAE1a in a Tethyan carbonate platform

Solid Earth ◽  
2012 ◽  
Vol 3 (1) ◽  
pp. 1-28 ◽  
Author(s):  
M. Di Lucia ◽  
A. Trecalli ◽  
M. Mutti ◽  
M. Parente
Keyword(s):  
Carbon Isotope ◽  
Deep Water ◽  
Carbonate Platform ◽  
Carbonate Platforms ◽  
Δ13c Values ◽  
Oceanic Anoxic Event ◽  
Strontium Isotope Stratigraphy ◽  
Anoxic Event ◽  
Anoxic Events ◽  
Isotope Stratigraphy

Abstract. Low biostratigraphic resolution and lack of chronostratigraphic calibration hinder precise correlations between platform carbonates and coeval deep-water successions. These are the main obstacle when studying the record of Mesozoic oceanic anoxic events in carbonate platforms. In this paper carbon and strontium isotope stratigraphy are used to produce the first chronostratigraphic calibration of the Barremian-Aptian biostratigraphy of the Apenninic carbonate platform of southern Italy. According to this calibration, the segment of decreasing δ13C values, leading to the negative peak that is generally taken as the onset of the Selli event, starts a few metres above the last occurrence of Palorbitolina lenticularis and Voloshinoides murgensis. The following rise of δ13C values, corresponding to the interval of enhanced accumulation of organic matter in deep-water sections, ends just below the first acme of Salpingoporella dinarica, which roughly corresponds to the segment of peak δ13C values. The whole carbon isotope excursion associated with the oceanic anoxic event 1a is bracketed in the Apenninic carbonate platform between the last occurrence of Voloshinoides murgensis and the "Orbitolina level", characterized by the association of Mesorbitolina parva and Mesorbitolina texana. Since these bioevents have been widely recognized beyond the Apenninic platform, the calibration presented in this paper can be used to pinpoint the interval corresponding to the Early Aptian oceanic anoxic event in other carbonate platforms of central and southern Tethys. This calibration will be particularly useful to interpret the record of the Selli event in carbonate platform sections for which a reliable carbon isotope stratigraphy is not available.

Seasonal and water-depth variations in sediment luminescence and in sedimentation from sediment trap samples at Gerlache Strait, Antarctic Peninsula

2009 ◽  
Vol 21 (5) ◽  
pp. 483-499 ◽  
Author(s):  
Glenn W. Berger ◽  
Sara Ante ◽  
Eugene W. Domack
Keyword(s):  
Sediment Trap ◽  
Trap Depth ◽  
Dry Mass ◽  
Luminescence Dating ◽  
Fine Silt ◽  
Sediment Fluxes ◽  
Depositional Processes ◽  
Infrared Stimulated Luminescence ◽  
Depositional Controls ◽  
Susceptibility Profiles

AbstractSediment trap arrays were deployed in Brialmont Cove and Andvord Bay, eastern Gerlache Strait, from December 2001–March 2003. The recovered sediments (representing instantaneous deposition from the viewpoint of luminescence dating) encompass all the annual and local glaciomarine depositional processes. Magnetic susceptibility profiles were used to infer seasonality in the trap cores, and thus to select subsamples for luminescence measurements. Multi-aliquot infrared stimulated luminescence (IRSL) apparent ages were used to assess the effectiveness of ‘clock zeroing’ (by daylight) of light sensitive luminescence within fine silt polymineral samples from each trap depth. IRSL apparent ages for 24 samples indicate that the largest age-depth differences occur with the autumn season samples at both trap sites, suggesting a previously unrecognized and regional (within the Gerlache Strait) change in depositional controls in the autumn compared to other seasons. The apparent ages also indicate some differences between the fjords, and a more complex oceanographic regime at Andvord Bay than at Brialmont Cove. Dry-mass sediment fluxes varied from 0.4 to 0.7 g cm-2 yr-1, with the largest flux at Brialmont Cove (∼0.7 g cm-2 yr-1) occurring in the bottom trap, whereas at Andvord Bay, the largest flux (∼0.6 g cm-2 yr-1) occurred in the middle trap (∼45 m above seafloor).

scholarly journals The emergence of subaerial crust and onset of weathering 3.7 billion years ago

2021 ◽  
Author(s):  
Desiree Roerdink ◽  
Yuval Ronen ◽  
Harald Strauss ◽  
Paul Mason
Keyword(s):  
Carbonate Rocks ◽  
Hydrothermal Fluid ◽  
Plate Tectonics ◽  
Chemical Weathering ◽  
Crystal Morphology ◽  
Sr Isotope ◽  
Fluid Mixtures ◽  
Mixing Ratios ◽  
Biosphere Evolution ◽  
Evolution Trend

&lt;p&gt;Reconstructing the emergence and weathering of continental crust in the Archean is crucial for our understanding of early ocean chemistry, biosphere evolution and the onset of plate tectonics. However, considerable disagreement exists between the various elemental and isotopic proxies that have been used to trace crustal input into marine sediments, and data are scarce prior to 3 billion years ago. Here we show that chemical weathering modified the Sr isotopic composition of Archean seawater as recorded in 3.52 to 3.20 Ga stratiform marine-hydrothermal barite deposits from three different cratons. We use a combination of barite crystal morphology, oxygen, multiple sulfur and strontium isotope data to select barite samples with the most seawater-like isotopic compositions, and subsequently use these in a hydrothermal mixing model to calculate a plausible seawater Sr isotope evolution trend from measured &lt;sup&gt;87&lt;/sup&gt;Sr/&lt;sup&gt;86&lt;/sup&gt;Sr data. From modeled mixing ratios between seawater and hydrothermal fluids required for barite precipitation and comparison of &lt;sup&gt;87&lt;/sup&gt;Sr/&lt;sup&gt;86&lt;/sup&gt;Sr in theoretical seawater-hydrothermal fluid mixtures with those recorded in the barite, we obtain a novel seawater Sr isotope evolution trend for Paleoarchean seawater that is much more radiogenic than the curve previously determined from carbonate rocks. Our findings require the presence and weathering of subaerial and evolved (high Rb/Sr) crust from 3.7 &amp;#177; 0.1 Ga onwards, and demonstrate that crustal weathering affected the chemistry of the oceans 500 million years earlier than previously thought.&lt;/p&gt;

Cryptic Sr and Nd isotopic variation across the Leinster Granite, southeast Ireland

1991 ◽  
Vol 128 (3) ◽  
pp. 251-256 ◽  
Author(s):  
Peter Mohr
Keyword(s):  
Isotopic Composition ◽  
Old Age ◽  
Sampling Scheme ◽  
Sr Isotope ◽  
Isotopic Variation ◽  
Geographic Position ◽  
Isotopic Pattern ◽  
Wide Range ◽  
Isotopic Variations ◽  
Range Of Values

AbstractRb–Sr isotope whole-rock data from the end-Caledonian Leinster Batholith define an errorchron age of 464±26 Ma, appreciably older than the accepted emplacement age of c. 405 Ma. This anomalously old age is the consequence of a highly variable initial Sr isotopic composition. Initial Nd ratios and TDM model ages also show a wide range of values. However, these isotopic variations are neither randomly distributed nor related to petrographic changes but instead reflect, in a simple way, geographic position about the axis of the batholith. The isotopic pattern revealed in the Leinster Batholith means that isochron ages obtained from S-type granites may be largely dependent on the sampling scheme adopted. The variable isotopic composition of the granite most likely resulted from either a heterogeneous metasedimentary source or hybridization of coeval, dominantly crustal melts.

Oxygen and carbon isotope stratigraphy of early Cambrian carbonates in southeastern Newfoundland and England

1992 ◽  
Vol 129 (3) ◽  
pp. 265-279 ◽  
Author(s):  
M. D. Brasier ◽  
M. M. Anderson ◽  
R. M. Corfield
Keyword(s):  
Carbon Isotope ◽  
Carbonate Rocks ◽  
Thermal Alteration ◽  
Carbon Isotope Stratigraphy ◽  
Oxygen And Carbon Isotope ◽  
Global Correlation ◽  
Granitic Intrusion ◽  
Isotope Stratigraphy ◽  
Siliciclastic Sediments ◽  
Reference Section

AbstractCarbonate rocks have been sampled through predominantly siliciclastic sediments above the Precambrian-Cambrian global stratotype level in southeastern Newfoundland to assess their potential for oxygen and carbon isotope stratigraphy. Comparable successions were sampled at Nuneaton and Comley in England. Greatly depleted δ18O signals are attributed to widespread thermal alteration during deep burial and granitic intrusion, including within the stratotype region. Carbon isotope ratios appear to have been less affected and these are described from nine sections. A provisional, composite δ13C curve is based on non-ferroan, pink nodular and bedded micrites. Several δ13C excursions occur in the fossiliferous Bonavista Group and allow the position of the Tommotian-Atdabanian boundary to be identified. Chemostratigraphic correlation of the new Precambrian-Cambrian boundary stratotype may, however, prove difficult because of the lack of suitable, well-preserved carbonates. The search must begin for a comparable reference section allowing global correlation of the boundary level using chemostratigraphy as well as biostratigraphy.

Carbon isotope stratigraphy and the problem of a pre-Tommotian Stage in Siberia

2001 ◽  
Vol 138 (4) ◽  
pp. 387-396 ◽  
Author(s):  
ARTEM KOUCHINSKY ◽  
STEFAN BENGTSON ◽  
VLADIMIR V. MISSARZHEVSKY ◽  
SHANE PELECHATY ◽  
PETER TORSSANDER ◽  
...  
Keyword(s):  
Carbon Isotope ◽  
Siberian Platform ◽  
Lower Cambrian ◽  
Isotopic Data ◽  
Sequence Stratigraphic ◽  
Anabar Uplift ◽  
Carbon Isotopic ◽  
The North ◽  
Isotope Stratigraphy ◽  
Isotopic Variations

Carbon isotopic oscillations are useful to elucidate the stratigraphy and biogeochemical events around the Precambrian–Cambrian transition. New isotopic data from the Manykaj and Emyaksin formations of the eastern Anabar Uplift (Siberia) help to correlate the Lower Cambrian and Neoproterozoic–Cambrian transitional beds across the Siberian Platform. The similarity of trends and amplitudes of the carbon isotopic curves, together with biostratigraphic and sequence-stratigraphic markers from the Anabar Uplift, provide a precise correlation with the southern part of the Siberian Platform. Diagenesis of argillaceous limestones of the Emyaksin Formation has apparently not affected the primary isotopic variations. The resulting curve is nearly identical in sections about 100 km apart in the Tommotian–Atdabanian portion of the formation. Relatively frequent and pronounced isotopic oscillations in the lower beds of the Emyaksin Formation fit between features I and II of the southern Siberian isotopic reference scale but are undetected therein owing to the depositional hiatus at the base of the Tommotian Stage in its type section. This confirms the transgressive onlap from the north suggested by previous studies, and makes the appearance of the Cambrian skeletal fossils on the Siberian Platform less abrupt. The hiatus in the south appears to embrace at least two biostratigraphic zones as recognized in the north. The case is strengthened for a pre-Tommotian Cambrian Stage in Siberia, the biostratigraphic framework for which has been elaborated earlier.

Combined Sm-Nd and Rb-Sr isotope systematics in the Donegal granitoids and their petrogenetic implications

1990 ◽  
Vol 127 (1) ◽  
pp. 75-80 ◽  
Author(s):  
C. S. Dempsey ◽  
A. N. Halliday ◽  
I. G. Meighan
Keyword(s):  
Continental Crust ◽  
Isotope Ratios ◽  
Amphibolite Facies ◽  
Sr Isotope ◽  
Isotopic Data ◽  
Isotopic Compositions ◽  
Isotopic Variations ◽  
Isotope Systematics

AbstractThe metaluminous to peraluminous granitoids of the Donegal batholith, northwest Ireland, were emplaced at c. 400 Ma into greenschist-amphibolite facies metasediments of the Dalradian Supergroup. Sm-Nd and Rb-Sr isotopic data are provided for eleven granitoid samples from six of the plutons and one specimen from the northeast granodiorite pluton of the Newry complex, Co. Down; the Donegal results reveal essentially similar initial Sr isotope ratios (0.7051–0.7068) but highly variable initial eNd values, −1.2 to −8.3 (and −0.5 for Newry). Certain granitoids have distinctive Nd isotopic compositions characteristic of the involvement of old, LREE-enriched continental crust in some cases or young crust and/or mantle-derived magmas in others. The Nd and Sr isotopic variations can be explained by a variety of mixing hypotheses.

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