查看更多>>摘要：Existing models for massively bedded barite (MBB) deposits (e.g., sedimentary exhalative and diagenetic/cold-seep) satisfy some geological and geochemical observations, but none explain the Paleozoic clustering of MBB deposits in Earth history. Here we bring seawater redox history into the picture and propose a sulfate-limited euxinic seawater (SLES) model where hydrothermally sourced Ba2+ accumulates as dissolved ion while other metal ions precipitate as insoluble metal sulfides. A subsequent encounter of this Ba2+-rich water mass with a sulfate-bearing one results in the deposition of MBB, thus physically separated from the accompanied metal sulfide deposits. We tested the SLES model using early Cambrian MBB deposits in South China through petrographic and isotope analyses. Syngenetic sphalerite and barite layers show a clear separation. A wide range of Sr-87/Sr-86 values (0.7082 to 0.7120) of the MBB supports a mixing of seawater and hydrothermal sources. A large range of delta S-34 values (32.2 to 61.1 parts per thousand) of the MBB and the occurrence of mineral hyalophane supports an overall sulfate-limited but sulfate-concentration temporally and spatially heterogeneous ocean. Our model established the genetic link between MBB deposits and the accompanied metal sulfide deposits in the Paleozoic. The dearth of MBB deposits before and after the Paleozoic is due to widespread ferruginous oceans with little sulfate and complete oxygenated oceans with too much sulfate, respectively. The Paleozoic clustering of the MBB deposits is a consequence of a critical redox transition in Earth history. (C) 2022 Elsevier B.V. All rights reserved.
Tsikos, HarilaosIreland, ThomasDauphas, NicolasHeard, Andy W....
15页
查看更多>>摘要：Earth's surface became permanently oxygenated during the Great Oxidation Episode, a geochemical transition between -2.43 and 2.22 billion years ago, but shallow-water cyanobacterial oases with molecular oxygen production likely existed for hundreds of millions of years before, in the otherwise anoxic and iron-rich oceans. Despite abundant geochemical evidence for elevated ambient oxygen in the Archean upper ocean, sites of active microbial oxygen production have not been geochemically characterized. We report geochemical and iron isotopic data for a horizon of iron-rich stromatolites in the 2.46-2.43 Ga Griquatown Iron Formation in South Africa deposited on the margin of an anoxic ferruginous basin. Bulk-rock and micro-sampled iron isotope data for the stromatolites indicate quantitative oxidation of iron delivered by deep upwelling currents, which is most readily explained by cyanobacterial communities inhabiting the stromatolites and producing local oxygen enrichments near the fair-weather wave-base. Modest enrichments in Mn and Ce indicate high oxidation potential in this stromatolitic setting. The iron-rich nature of the stromatolites indicates that upwelling iron sources in the early Paleoproterozoic oceans overwhelmed established iron-precipitation mechanisms in deeper basins that had generally maintained iron-poor conditions in shallow-marine peritidal zone during the Neoarchean. (C)& nbsp;2022 Elsevier B.V. All rights reserved.
查看更多>>摘要：The Indian Ocean accounts for over 20% of the global ocean volume, nearly at par with the Atlantic, and possesses a unique hydrography characterized by turn-over entirely through exchange with the Southern Ocean, Pacific and Atlantic. Despite its volumetric and hydrographic importance, the role of the Indian Ocean in glacial-interglacial carbon cycle dynamics remains poorly constrained. Radiocarbon dates on foraminifera from two marine sediment cores have been used to decipher past changes in the 'radiocarbon ventilation age' of deep waters from the Central Indian Ocean (CIO) basin. Time-series spanning the last 37 ka show coherent variations in both sediment cores, and indicate greatly enhanced ocean-atmosphere radiocarbon disequilibrium in the region during the last glaciation, with peak ocean-atmosphere radiocarbon age offsets occurring during Heinrich Stadial-1 (HS-1) and Heinrich Stadial-2 (HS-2). Uniquely, as compared to the bulk of existing radiocarbon data for the last deglaciation, CIO radiocarbon ventilation ages only approach modern values during the Holocene, with Benthic-Atmosphere (B-Atm) offsets remaining > 3000 C-14 yrs during the Bolling-Allerod period similar to 15 ka BP. The more gradual rejuvenation of the CIO is supported by parallel oxygenation indicators, as well as existing stable isotope data and Nd isotope trends. Together, the data suggest that the CIO was isolated from well-ventilated North Atlantic sourced deep waters during the last glacial, and particularly during Heinrich stadials 2 and 1. These findings underline the important role played by the Indian Ocean in deglacial carbon cycle change, particularly in the latter half of the last deglaciation. (C)& nbsp;2022 Elsevier B.V. All rights reserved.
查看更多>>摘要：The recent discovery of heavy Si isotopic compositions in both high-Na Tonalite-Trondhjemite-Granodiorite (TTG) and High-K Granite-Monzonite-Syenite (GMS) suites of early continental crust requires that a notable seawater-derived silica-rich component had been added to their respective protoliths prior to melting. Here we use the Ge/Si ratio as a complementary tracer to delta Si-30 in order to delineate the exact role of modal quartz and silicified basalts from the Archean seafloor among the primary controls of the early appearance of felsic melts on Earth. We have approached the question by (1) specifying the Ge/Si signatures of various Archean and post-Archean rock types by compiling the Ge-SiO2 data stored within the GEOROC database; (2) coupling Ge/Si investigation to silicon isotopes on a large selection of silicified and unsilicified altered mafic and ultramafic greenstones, felsic volcanics, TTG and GMS granitoids and mineral separates from TTGs of the Barberton Greenstone Belt (BGB) of South Africa. The GEOROC compilation demonstrates that Archean TTGs and granites display much lower Ge/Si (1.15 +/- 0.10 and 1.13 +/- 0.11 mu mol/mol, respectively) than post-Archean adakites, granites, tonalites and granodiorites (with average Ge/Si in the range of 1.64 to 1.85 mu mol/mol). This result is corroborated by the Ge/Si ratios we report from BGB rocks that formed prior to Kaapvaal craton stabilisation, including 3.5 Ga Theespruit Formation felsic volcanic rocks, 3.5-3.2 Ga TTGs and 3.2-3.1 Ga GMSs, all of which exhibit low ratios (0.68 +/- 0.23; 0.92 +/- 0.17; 1.05 +/- 0.19 mu mol/mol, respectively). Based on their low TTG-like Ge/Si, precratonic GMSs are dismissed as being derived from melting of both TTG and metasedimentary sources. Instead, TTGs and GMSs originated from similar Ge-depleted sources. Low Ge/Si ratios, coupled to heavy Si isotopic signatures (-0.14 parts per thousand & nbsp;& nbsp;< delta Si-30 < +0.27 parts per thousand & nbsp;) are also a characteristic feature of mafic and ultramafic BGB greenstones subjected to low-temperature hydrothermal seafloor alteration, especially at the interface between silicified (0.2 < Ge/Si < 1.2 mu mol/mol) and unsilicified (1.8 < Ge/Si < 3.1 mu mol/mol) portions of the altered Archean seafloor. We infer that both Na-rich and K-rich Archean felsic melts are derived from a unique class of protoliths: Ge-depleted metabasalts containing a significant modal proportion of supracrustal quartz generated by the silicification of the Eo-Paleoarchean basaltic seafloors. The transition from Na-rich to K-rich felsic melts in the BGB is assumed to be connected to a gradual increase of potassium as a key element associated with seafloor silicification. In contrast, younger (3.07-2.69 Ga), post-cratonic BGB granites have higher Ge/Si (1.93 & PLUSMN;0.23 mu mol/mol) and were generated through the reworking of a TTG-like basement, by incongruent melting of biotite and hornblende (1.79 < Ge/Si < 2.97 mu mol/mol) leaving an oligoclase-rich (0.64 < Ge/Si < 0.72) residue. The Earth changed from a prevalent Ge-depleted felsic crust in early-middle Archean times to a widespread Ge-enriched post-Archean crust, which emphasizes the importance of late Archean changes in the Earth dynamics. In particular, our data suggest the likelihood of generating primitive felsic continents by rather shallow melting processes, without the need for inducing high pressures by subduction, as long as Archean ocean floors were silicified. (c) 2022 Elsevier B.V. All rights reserved.
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