Boron and calcium isotope composition in Neoproterozoic carbonate rocks from Namibia: evidence for extreme environmental change

Kasemann, Simone A.; Hawkesworth, Chris J.; Prave, Anthony R.; Fallick, Anthony E.; Pearson, Paul Nicholas

doi:10.1016/j.epsl.2004.12.006

Cited by 156 publications

(94 citation statements)

References 35 publications

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“…Among these methane clathrate destabilisation by flooding of permafrost meadows (Kennedy et al, 2008) or a proposed elevation in atmospheric CO 2 concentrations, producing super-greenhouse warming to melt the ice sheets (Hoffman et al, 1998) are the most promising explanations. Rapid glacial meltdown would lead to a global transgression and super-greenhouse warming would result in intense continental weathering, producing an enormous alkalinity and nutrient flux into the oceans and a lowering in ocean pH (Hoffman et al, 1998;Kasemann et al, 2005). Recent work by Sansjofre et al (2011) postulated much lower pCO 2 levels for the Ediacaran, challenging the super greenhouse rapid meltdown hypothesis of previous authors.…”

Section: Introductionmentioning

confidence: 55%

Multiproxy constraints on alteration and primary compositions of Ediacaran deep-water carbonate rocks, Yangtze Platform, South China

Hohl

Becker

Herzlieb

et al. 2015

Geochimica et Cosmochimica Acta

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The occurrence of shallow and deep-water sedimentary facies has established the Yangtze Platform in South China as a key site for the study of Neoproterozoic ocean oxidation and Ediacaran animal evolution following the Marinoan glaciation. The Yanwutan section in Hunan Province is one of the few coherent sections on the Yangtze Platform where Ediacaran deep-water carbonate sediments (predominantly dolostones) are preserved together with organic carbon-rich shales. Here we present new major and trace element abundance data as well as Sr-, O-and C-isotope compositions of leachates from carbonates of the Doushantuo Formation. We evaluate the role of diagenetic modification of the carbonate rocks and constrain the redox evolution of Ediacaran seawater in space and time. 87 O of cap dolostones at many other sections were compromised by hydrothermal alteration. The overlying organic carbon poor micritic dolostone (unit II) shows negative Ce-anomalies that disappear towards the top of the unit. No Ce-anomalies occur in subsequent organic carbon-rich muddy dolostone units (units III to IV). These observations, enrichments in TOC that correlate with variations in redox-sensitive metals in the carbonates, negative d 13 C carb in units II to IV and the decoupling of d 13 C carb from d 13 C org argue for the existence of mostly anoxic deep-water at the Yangtze passive continental margin during the Ediacaran. The negative Ce-anomalies at the base of unit II (with negative d 13 C carb ) may reflect fluctuations towards suboxic or oxic conditions or an allochthonous origin of this unit. However, trace metal enrichments in carbonates of the same unit argue for reducing conditions in pore-water, whereas the carbonates may have preserved the REE+Y signatures inherited from suboxic-to oxic seawater. The trace element and negative d 13 C carb values in units II to IV are consistent with a stratified basin model with a large partially remineralised organic matter reservoir in anoxic bottom and pore-waters.

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Section: Introductionmentioning

confidence: 55%

Multiproxy constraints on alteration and primary compositions of Ediacaran deep-water carbonate rocks, Yangtze Platform, South China

Hohl

Becker

Herzlieb

et al. 2015

Geochimica et Cosmochimica Acta

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“…The strong covariation between ä 13 C and ä 18 O in almost all cap carbonates, their dolomite mineralogy, evidence of recrystallization, the unknown origin of most carbonate grains, abundance of diagenetic cements, and the wide scatter in isotopic values where numerous sections have been analysed, or that are not completely dolomitized, further suggest that a combination of primary and diagenetic processes may be important in determining the bulkrock value of cap carbonates . These caveats also apply to the interesting discovery of stratigraphic variations in B and Ca isotopes (Kasemann et al 2005). The secular (surface ocean) origin of the C isotopic excursion is a topic of active discussion with several possible mechanisms including Rayleigh fractionation of the atmosphere , temperature-or alkalinity-controlled fractionations (Higgins & Schrag 2003), upwelling of ä 13 C-depleted deep water (Grotzinger & Knoll 1995), or methane hydrate destabilization (Kennedy et al 2001a;Jiang et al 2003).…”

Section: Cap Carbonates and Deglaciationmentioning

confidence: 99%

Neoproterozoic glaciation in the Earth System

Fairchild

Kennedy

2007

JGS

213

116

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The Neoproterozoic contains severe glacial intervals (750–580 Ma) including two extending to low palaeomagnetic latitudes. Paucity of radiometric dates indicates the need for chronostratigraphic tools. Whereas the marine 87 Sr/ 86 Sr signatures show a steady rise, δ 13 C fluctuates, the most reproducible variations being negative signatures in carbonate caps to glacial units, but more diagenetic work is needed. Four conceptual models for the icehouse conditions are contrasted: Zipper-Rift Earth (diachronous glaciation related to continental rift margins), High-tilt Earth (high-obliquity and preferential low-latitude glaciation), Snowball Earth (extreme glaciation related to runaway ice–albedo feedback) and Slushball Earth (coexistence of unfrozen oceans and sea-level glaciers in the tropics). Climate models readily simulate runaway glaciation, but the Earth may not be able to recover from it. The Slushball state requires more extensive modelling. Biogeochemical models highlight the lack of CO 2 buffering in the Neoproterozoic and the likely transition from a methane- to a CO 2 -dominated climate system. Relevant processes include tropical weathering of volcanic provinces, and new land biotas stimulating both clay mineral formation and P delivery to the oceans, facilitating organic C burial. Hence a step change in the Earth System was probably both facilitated by organisms and responsible for moderating Phanerozoic climate.

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“…The total boron blank for this procedure was less than 1 ng (see Kasemann et al, 2005). In preparation for microanalysis, the carbonate samples and reference glasses were embedded in epoxy and polished.…”

Section: Sample Preparationmentioning

confidence: 99%

In situ boron isotope analysis in marine carbonates and its application for foraminifera and palaeo-pH

et al. 2009

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a b s t r a c t a r t i c l e i n f oThe boron isotope composition of marine carbonates such as foraminiferal tests and coral skeletons is increasingly being used to reconstruct seawater pH values and atmospheric pCO 2 concentrations spanning hundreds of thousands or even millions of years. However, inter specific and small scale (microns) intra specific isotopic variation, either due to life processes of an organism or patchy recrystallisation and dissolution in fossilised material, can limit the use of the boron isotope ratio as palaeo-pH recorder. One approach to address this problem is the combined use of in situ and high spatial resolution mass spectrometry and high precision bulk analytical techniques. While much effort has been invested to establish reference material for silicates little attention has been devoted towards characterizing and using carbonate material allowing direct isotopic comparisons between B isotope data produced on carbonates by different techniques and laboratories. Hence, here we present data on the boron isotope composition of carbonate material determined by multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS), thermal ionization mass spectrometry (TIMS), and secondary ionization mass spectrometry (SIMS). The new reference material permits the B isotope analysis of single foraminiferal chambers using SIMS and to empirically relate between seawater pH, the B isotope composition of O. universa and the predicted foraminiferal vital effect.

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Boron and calcium isotope composition in Neoproterozoic carbonate rocks from Namibia: evidence for extreme environmental change

Cited by 156 publications

References 35 publications

Multiproxy constraints on alteration and primary compositions of Ediacaran deep-water carbonate rocks, Yangtze Platform, South China

Multiproxy constraints on alteration and primary compositions of Ediacaran deep-water carbonate rocks, Yangtze Platform, South China

Neoproterozoic glaciation in the Earth System

In situ boron isotope analysis in marine carbonates and its application for foraminifera and palaeo-pH

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