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Silicon and Oxygen Stable Isotopes in Chert As Indicators of Diagenesis and Ocean Paleo-Environmental Conditions

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Silicon and Oxygen Stable Isotopes in Chert As Indicators of Diagenesis and Ocean Paleo-Environmental Conditions Silicon and oxygen stable isotopes in chert as indicators of diagenesis and ocean paleo-environmental conditions Dissertation zur Erlangung des Grades eines Doktors der Naturwissenschaften (doctor rerum naturalium) am Fachbereich Geowissenschaften der Freien Universität Berlin vorgelegt von Michael Tatzel Berlin, 2016 Erstgutachter: Prof. Friedhelm von Blanckenburg Zweitgutachter: Prof. Louis A. Derry Tag der Disputation: 14.04.2016 Eidesstattliche Erklärung Hiermit versichere ich, die vorliegende Dissertation selbstständig und ohne unerlaubte Hilfe angefertigt zu haben. Bei der Verfassung der Dissertation wurden keine anderen als die im Text aufgeführten Hilfsmittel verwendet. Beiträge von Koautoren zu publizierten und zur Publikation vorbereiteten Manuskripten sind im ‚Preface‘ dieser Arbeit dargelegt. Ein Promotionsverfahren zu einem früheren Zeitpunkt an einer anderen Hochschule oder bei einem anderen Fachbereich wurde nicht beantragt. Berlin, 18. Januar 2016 READ WRITE THINK DREAM (Geisel Library, UCSD) Who has no time to read the summary is referred to the twitter format version: When flint forms, silicon may keep its atomic mass and record sponge abundance. Oxygen loses heavy isotopes and records temperatures in the crust. Summary The conversion of amorphous opaline sediment to quartz chert is a very slow process that occurs on geological time scales of up to 20 million years. This diagenetic silicon dioxide (silica) phase transformation entails the conversion from opal-A (amorphous opal) via opal-CT (cristobalite-tridymite opal) to quartz and does not depend on whether silica was deposited as biogenic opal or precipitated inorganically from silicon- rich water. The involved early diagenetic dissolution and later diagenetic dissolution-precipitation reactions affect the silicon (Si) stable isotope composition of quartz, but the underlying isotope fractionation processes are largely unknown. This lack of knowledge hampers the interpretation of the silicon isotope composition of chert, the topic of this thesis. The first diagenetic reaction that occurs during opal maturation is the phase transformation of opal- A to opal-CT. This transformation is forming layered sedimentary rock referred to as porcellanite and commonly occurs during burial diagenesis at temperatures between 30 to 60°C. In pure, i.e., clay-free siliceous sediment (composed dominantly of opal-A), porcellanite layers can form already during early diagenesis. Such porcellanite layers are present in Pleistocene and Pliocene sediment of the Southern Ocean. These layers as well as their surrounding siliceous sediment were analyzed for their silicon isotope composition. I measured silicon stable isotope ratios of these sediments by multicollector ICP-MS after sample dissolution, and measured the silicon stable isotope ratios of porcellanite layers by femtosecond laser ablation multicollector ICP-MS (fs-LA-ICP-MS). Porcellanite layers are significantly enriched in heavy silicon isotopes (1.7 to 2.3‰ δ30Si, the normalized 30Si/28Si isotope ratio) relative to the surrounding siliceous sediment (-0.3 to 1.5‰ δ30Si). Scanning electron microprobe analyses following sequential NaOH extractions of three sediment samples reveal the preferential dissolution of diatoms relative to radiolarians. The preferential dissolution of diatoms that are enriched in isotopically heavy Si relative to Si from radiolarians and aluminum-silicate phases causes the depletion of 30Si in sediment and enrichment of 30Si in pore water relative to 28Si. Early diagenetically formed porcellanite layers precipitate from pore water as soon as the opal-CT solubility is exceeded. In situ Si isotope analyses of porcellanite layers using fs-MC-ICP-MS show anti- correlating Al/Si and δ30Si, documenting that early diagenetic aluminum silicate precipitation depletes pore water in light isotopes. These processes cause a shift of the initial bulk sediment Si isotope composition during early diagenesis. Therefore, the Si isotope composition of chert will also be shifted away from that of the initial bulk sediment. In the Precambrian, however, early diagenetic selective dissolution of opal was suppressed through high seawater Si concentrations. Precambrian cherts should therefore reflect the primary bulk Si isotope composition of the opaline precursor. Furthermore, isotopic shifts through diagenetic phase transformations will be minimal as exchange of Si between sedimentary layers is impeded in the presence of variable amounts of detrital minerals. The reason is that the solubility of siliceous sediment is reduced in the presence of detrital minerals. i Therefore, Si stable isotopes can be used to study paleo-environmental conditions in marine successions of Ediacaran through Cambrian times. Chert and siliceous shales from the ‘Lijiatuo’ section, Hunan, South China, were analyzed for stable Si isotopes, and major-and trace element concentrations. An overall decreasing trend of δ30Si values throughout the section is compatible with shifting proportions of the Si sources. We infer that high δ30Si values of up to 1.1‰ in the bottom part of the section (Late Ediacaran) represent dominant contribution of inorganic Si precipitation from seawater. Values decreasing down to -0.5‰ I assume to reflect an increasing contribution of light Si from siliceous sponge spicules. The relative proportion of sponges was quantified by mass balance calculations and geochemical inversion. An inferred late Ediacaran increase in sponge abundance correlates with Ce-anomalies, P- and Ba- enrichment in sediment. These geochemical proxy data suggests that seawater oxygenation occurred concurrently with the increased colonization of the continental slope by siliceous sponges. This multi-proxy evidence supports the hypothesis that the Late Neoproterozoic oxygen rise in seawater was significantly controlled by sponges. Sponges reduce dissolved and fine particulate organic matter from seawater through filter feeding. This way of feeding changes the redox structure of seawater and sediments and make sponges ‘ecosystem engineers’. Thus, the respiratory oxygen consumption is shifted towards greater water depths and shallow water habitats are becoming oxygenated. As another consequence the nutrient availability for other species is affected by changed fluxes of chemical elements between seawater and sediment. This mechanism called ‘ecosystem engineering’, possibly facilitated the Cambrian ‘explosion’, the rapid evolution of higher life on Earth. The oxygen (O) isotope composition (18O/16O) of chert is frequently used to reconstruct paleo- seawater temperatures. However, for most chert samples, oxygen isotope thermometry yields implausibly high temperatures. Two physico-chemical characteristics allow scrutinizing whether oxygen isotopes retain a diagenetic fingerprint. These are i) the strong effect of detrital minerals on the silica solubility, and by inference on the timing of silica phase transformations during sediment burial; and ii) the temperature- dependent equilibration of silica δ18O during dissolution-precipitation reactions. The set of Ediacaran to Cambrian chert and siliceous shale samples from ‘Lijiatuo’ section as introduced above was used to explore these dependencies. The normalized 18O/16O ratios of bulk chert ranges from 17.1 to 24.0‰, and the detrital mineral content is estimated to range from ca. 1 to 75 wt%. A mass balance calculation shows that the oxygen isotope composition of authigenic quartz ranges between 17.1 and 29.8‰. This result was verified by SIMS analyses of three detrital-rich samples. The δ18O value of quartz is correlated with the detrital mineral abundance. Raman spectroscopy of carbonaceous material reveals peak diagenetic temperatures around 260°C. The correlation between detrital mineral abundance and quartz δ18O provides evidence that chert δ18O records the temperature during opal-CT to quartz phase transformation. The preservation of this signature, despite burial to high diagenetic temperatures, confirms evidence that chert is resistant to isotopic exchange. A calculation of quartz formation rates at different temperature gradients suggests that the global ii trend of increasing chert δ18O through the Proterozoic results from quartz formation at increasing depths and decreasing temperatures as consequence of a decreasing geothermal gradient through cooling of the solid Earth. iii Zusammenfassung Die Umwandlung von amorphem Opal zu kristallinem Quarz-Chert (ein Silicium-reiches Sedimentgestein) ist ein sehr langsamer Prozess, der über geologische Zeiträume von mehr als 20 Millionen Jahren andauern kann. Die diagenetischen Phasenumwandlungen von Opal-A über Opal-CT zu Quarz finden dabei unabhängig davon statt, ob Silica (SiO2) als biogener Opal oder anorganisch aus Silicium (Si)-reichem Meerwasser ausgefällt wurde. Die Auswirkungen von frühdiagenetischer Opal-Auflösung und von diagenetischen Lösungs- Fällungsreaktionen auf die Silicium-Isotopensignatur von Quarz sind weitgehend unbekannt. Dieses fehlende Wissen erschwert die Interpretation der Si-Isotopenzusammensetzung von Chert als Paläo-Umwelt Indikator. Die erste diagenetische Reaktion, die bei der Opal Reifung stattfindet, ist die Opal-A Phasenumwandlung zum metastabilen Silica-Polymorph Opal-CT, welches das Sedimentgestein Porzellanit bildet. Diese Phasenumwandlung findet gewöhnlich zwischen 30 und 60°C statt. In sehr reinen, d.h. tonmineral-freien opalinen Sedimenten können Porzellanit-Lagen bereits bei der Frühdiagenese entstehen. Derartige
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