scholarly journals Episode of intense chemical weathering during the termination of the 635 Ma Marinoan glaciation

2016 ◽  
Vol 113 (52) ◽  
pp. 14904-14909 ◽  
Author(s):  
Kang-Jun Huang ◽  
Fang-Zhen Teng ◽  
Bing Shen ◽  
Shuhai Xiao ◽  
Xianguo Lang ◽  
...  
Keyword(s):  
South China ◽  
Chemical Weathering ◽  
Snowball Earth ◽  
Carbonate Precipitation ◽  
Isotopic Data ◽  
Low Intensity ◽  
Cap Carbonates ◽  
Cap Carbonate ◽  
Carbonate Deposition ◽  
Global Precipitation

Cryogenian (∼720–635 Ma) global glaciations (the snowball Earth) represent the most extreme ice ages in Earth’s history. The termination of these snowball Earth glaciations is marked by the global precipitation of cap carbonates, which are interpreted to have been driven by intense chemical weathering on continents. However, direct geochemical evidence for the intense chemical weathering in the aftermath of snowball glaciations is lacking. Here, we report Mg isotopic data from the terminal Cryogenian or Marinoan-age Nantuo Formation and the overlying cap carbonate of the basal Doushantuo Formation in South China. A positive excursion of extremely high δ26Mg values (+0.56 to +0.95)—indicative of an episode of intense chemical weathering—occurs in the top Nantuo Formation, whereas the siliciclastic component of the overlying Doushantuo cap carbonate has significantly lower δ26Mg values (<+0.40), suggesting moderate to low intensity of chemical weathering during cap carbonate deposition. These observations suggest that cap carbonate deposition postdates the climax of chemical weathering, probably because of the suppression of carbonate precipitation in an acidified ocean when atmospheric CO2 concentration was high. Cap carbonate deposition did not occur until chemical weathering had consumed substantial amounts of atmospheric CO2 and accumulated high levels of oceanic alkalinity. Our finding confirms intense chemical weathering at the onset of deglaciation but indicates that the maximum weathering predated cap carbonate deposition.

How tillite weathering during the snowball Earth aftermath induced cap carbonate deposition

Geology ◽  
2012 ◽  
Vol 40 (11) ◽  
pp. 1027-1030 ◽  
Author(s):  
Sébastien Fabre ◽  
Gilles Berger
Keyword(s):  
Snowball Earth ◽  
Cap Carbonate ◽  
Carbonate Deposition

Combined paleomagnetic and isotopic data from the Doushantuo carbonates, South China: implications for the “snowball Earth” hypothesis

2004 ◽  
Vol 224 (3-4) ◽  
pp. 387-398 ◽  
Author(s):  
M. Macouin ◽  
J. Besse ◽  
M. Ader ◽  
S. Gilder ◽  
Z. Yang ◽  
...  
Keyword(s):  
South China ◽  
Snowball Earth ◽  
Isotopic Data

scholarly journals Chemostratigraphy of the lower Bambuí Group, southwestern São Francisco Craton, Brazil: insights on Gondwana paleoenvironments

2016 ◽  
Vol 46 (suppl 1) ◽  
pp. 145-162 ◽  
Author(s):  
Matheus Kuchenbecker ◽  
Marly Babinski ◽  
Antônio Carlos Pedrosa-Soares ◽  
Leonardo Lopes-Silva ◽  
Felipe Pimenta
Keyword(s):  
Biological Activity ◽  
Chemical Weathering ◽  
Southeastern Brazil ◽  
Sudden Increase ◽  
Freshwater Input ◽  
Isotopic Signatures ◽  
Mountain Ranges ◽  
Cap Carbonate ◽  
Carbonate Deposition ◽  
Regional Tectonic

ABSTRACT: The Bambuí Group, the most extensive carbonate-siliciclastic cover on the São Francisco craton, has been a matter of debate because of its potential correlations to global glacial events. Unfortunately, most available chemostratigraphic data came from samples collected on surface rock exposures, ever susceptible to the aggressive chemical weathering that characterizes the southeastern Brazil. On the other hand, we present here high-resolution chemostratigraphic studies based on C, O and Sr isotopic data from 53 samples collected along a weathering-free, continuous, 175 m thick sedimentary succession. This succession was recovered by borehole drilling in the southwestern São Francisco craton, where occur the Carrancas and Sete Lagoas formations, the lowermost units of the Bambuí Group. The drill cores reveal extremely irregular contacts between the basal diamictite and its basement, an Archaean foliated granodiorite. Geochronological and sedimentological data strongly suggest that the diamictite represents a lodgement till. This glaciogenic deposit is covered by a limestone succession which starts with impure carbonates showing aragonite pseudomorph fans and thin bands of black shale. The limestone pile grades to a marl-mudstone interval, which turns to a carbonate with biological components, succeeded by stromatolitic dolomite at the top. C and O isotopic signatures (referred to V-PDB) allow to the subdivision of the lower carbonate-pelite section into three intervals. The first isotopic interval corresponds to a cap carbonate, and displays negative values of δ13C (c . -4‰), and a large oscillation of the δ18O (-6 to -15‰). The Interval II shows a striking homogeneity in δ13C and δ18O, around 1‰ and -7‰, respectively. At the top, Interval III shows a large positive excursion of the δ13C (up to 8‰) and δ18O (-8 to -3‰) values. Unaltered 86Sr/87Sr ratios range from 0.7075 to 0.7077, mainly at the top of the section. The geochemistry of the carbonates is controlled by their terrigenous content (mostly quartz and clay minerals) which is concentrated in the lower units. Samples free of terrigenous contamination show Y/Ho ratios ranging from 25 to 50, suggesting a freshwater input during carbonate deposition. It is concluded that the diamictite has a glaciogenic origin and is covered by a cap carbonate. This pair has been identified along the basin and is related to one of the main Neoproterozoic glaciations. Discrepancy between the 86Sr/87Sr values and the global variation curves can be related to freshwater input during the carbonate deposition. Based on the regional tectonic context, the Bambuí Basin may have been a restricted marine basin, totally or partially surrounded by mountain ranges within Gondwana, in the Neoproterozoic/Paleozoic boundary. In its early stages, the sedimentation was influenced by a global glacial event, whose melting phase was responsible by freshwater input in the basin. The gradual rise of the temperature was followed by an increase of the biological activity. Finally, a sudden increase in the biological activity could have been driven by paleogeographic changes caused by the active tectonic.

U-Pb and Re-Os geochronology tracks stratigraphic condensation in the Sturtian snowball Earth aftermath

Geology ◽  
2020 ◽  
Vol 48 (6) ◽  
pp. 625-629 ◽  
Author(s):  
Alan D. Rooney ◽  
Chuan Yang ◽  
Daniel J. Condon ◽  
Maoyan Zhu ◽  
Francis A. Macdonald
Keyword(s):  
South China ◽  
Snowball Earth ◽  
Ionization Mass ◽  
Rift Basins ◽  
Authigenic Carbonates ◽  
Ice Volume ◽  
Accommodation Space ◽  
Cap Carbonates ◽  
Minimum Age ◽  
Os Isotope

Abstract The snowball Earth hypothesis predicts a strong hysteresis resulting in discrete multi-million-year glaciations followed by globally synchronous deglaciation. Here we present new U-Pb zircon and Re-Os sedimentary rock geochronology and Os isotope chemostratigraphy from post-Sturtian sequences in south China to test the synchroneity of deglaciation. High-precision chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon dates refine the minimum age of deglaciation to 660.98 ± 0.74 Ma, which is ∼2 m.y. older than previously reported. We also provide a new maximum age constraint on the onset of the Marinoan glaciation of 657.17 ± 0.78 Ma. A global compilation of new Os isotope chemostratigraphy reveals a large and systematic trend to unradiogenic values over &lt;1 m of stratigraphy. Together, these data indicate that the Mn-carbonates in south China are not cap carbonates that formed as a response to post-snowball alkalinity, but are authigenic carbonates that formed millions of years after deglaciation. Sturtian cap carbonates tend to be absent or more condensed than their younger Marinoan counterparts. We suggest that this reflects a combination of enhanced accommodation space in early Cryogenian underfilled rift basins, stronger hysteresis, larger ice volume, and/or higher CO2 levels needed for deglaciation of the longer Sturtian glaciation. Further, our findings indicate that the apparent diachroneity of deglaciation can be explained readily as a consequence of stratigraphic condensation, itself due to the large post-Sturtian glacioeustatic transgressive sequence that outpaced shallow-water carbonate deposition.

scholarly journals Ice-rafted dropstones in “postglacial” Cryogenian cap carbonates

Geology ◽  
2020 ◽  
Author(s):  
D.P. Le Heron ◽  
M.E. Busfield ◽  
C. Kettler
Keyword(s):  
Thin Section ◽  
Mass Flow ◽  
Cold Climate ◽  
Conventional Wisdom ◽  
Warm Water ◽  
Cap Carbonates ◽  
Cap Carbonate ◽  
Carbonate Deposition ◽  
Mass Flow Deposits ◽  
Paleoclimatic Significance

Dropstones of ice-rafted origin are typically cited as key cold-climate evidence in Cryogenian strata and, according to conventional wisdom, should not occur in postglacial, warm-water carbonates. In Namibia, the Chuos Formation (early Cryogenian) contains abundant dropstone-bearing intervals and striated clasts. It is capped by the Rasthof Formation, composed of laminites in its lower portion and microbial carbonates above. These laminites are locally found to contain pebble- and granule-sized lonestones in abundance. At the Omutirapo outcrop, meter-thick floatstone beds occur at the flanks of a Chuos paleovalley and are readily interpreted as mass-flow deposits. At Rasthof Farm, however, the clasts warp, deflect, and penetrate hundreds of carbonate laminations at both the outcrop and thin-section scale. We propose that these are dropstones, and we infer an ice-rafting mechanism. Evidence for vestigial glaciation concomitant with cap carbonate deposition thus merits a reappraisal of the depositional conditions of cap carbonates and their paleoclimatic significance.

scholarly journals Supplemental Material: Protracted northward drifting of South China during the assembly of Gondwana: Constraints from the spatial-temporal provenance comparison of Neoproterozoic–Cambrian strata

2020 ◽  
Author(s):  
Qiong Chen ◽  
Guochun Zhao ◽  
et al.
Keyword(s):  
South China ◽  
Detrital Zircons ◽  
Isotopic Data

Table S1: In-situ U-Pb ages and Hf isotopic data of detrital zircons from the Neoproterozoic–Cambrian samples from the western and eastern margins of South China.

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