The cation-exchange purification technique used here does not remove anions (in our case, mostly Cl-, SO 4 2-and to a lesser extent NO 3-) from solutions. In this case, the addition of a known artificial matrix in excess in both the sample and standard solution can be used to dilute the natural concentration of the contaminant and to homogenize sample and standard matrices (doping method, Georg et al., 2006; Hughes et al., 2011). Indeed, dissimilar matrices will affect differently the plasma and ionization efficiency and will induce artificial bias in the delta measurements, invalidating the use of the standard-sample bracketing technique. In our samples, Cl-originating from seawater can be neglected compared to Cladded as HCl (Merck Suprapur) to dissolve the brucite; and as solutions were analyzed in a HCl matrix largely in excess (up to 0.5 mol L-1) compared to natural Cl-concentration. Similarly, the occurrence of NO 3-in seawater was resolved by the use of HNO 3 (Merck Suprapur, 0.5 mol L-1) as a solvent in both the samples and standards. For Depth Depth (m) (m)

Oceanic Anoxic Event 2 (Cenomanian-Turonian: ca 94 Ma) represents a major palaeoceanographic phenomenon that took place during an interval of extreme global warmth when large amounts of organic matter entered the marine burial record, probably triggered by increased availability of nutrients for planktonic biota. Three sections (Eastbourne, Sussex, UK; Raia del Pedale, Campania, Italy; and Tarfaya, Morocco) recording this event illustrate the influence on marine geochemistry of mafic volcanic rock-seawater interaction, anoxia to euxinia, and re-oxygenation and cooling during the socalled 'Plenus Cold Event'. The Eastbourne section represents the organiclean epicontinental pelagic deposits of the English Chalk; the Raia del Pedale section represents a shallow-water platform carbonate on the Tethyan continental margin, also largely devoid of organic matter; and the Tarfaya core represents an Atlantic margin site where cyclically bedded organic-rich sediments were well developed. Correlation between all three sections is readily achieved by biostratigraphy and carbon-isotope stratigraphy (d 13 C carb and d 13 C org ) over the Oceanic Anoxic Event 2 interval, represented by a characteristic broad positive carbon-isotope excursion. The stratigraphic range of the Plenus Cold Event, defined by the presence, in two discrete levels, of boreal fauna and an excursion to heavier oxygen-isotope values in the English Chalk, can be identified in Raia del Pedale and Tarfaya by using the carbon-isotope curve as a correlative tool. Similarly, a section in southern France allows its co-existing osmium-isotope excursion to relatively unradiogenic values to be placed in the context of the Oceanic Anoxic Event in all three analysed sections. A fall to lower osmium-isotope values clearly predated the onset of Oceanic Anoxic Event 2, as defined by the initial rise in carbon-isotope values, allowing the putative magmatic/mafic event as a trigger for the Oceanic Anoxic Event. An initial drop in sulphur-isotope ratios (d 34 S CAS ) at Eastbourne correlates with the osmium-isotope curve, suggesting that isotopically light sulphur could have been derived from a mafic igneous source. Re-oxygenation of sediments of all three investigated sections during the Plenus Cold Event is variably illustrated by change in cerium:calcium, iodine:calcium, molybedenum:calcium and uranium:calcium ratios, according to the redox behaviour of the elements in question and whether controls on seawater chemistry were local or global in nature. Changes in molybdenum-isotope ratios from Tarfaya and portions of the sulphur-isotope curve from Eastbourne and Raia del Pedale also indicate the probable presence of more oxygen-rich bottom waters during the Plenus Cold Event. Oxidation by such waters of previously deposited organic-rich shales, as well as loss of anoxic/euxinic sinks, is credited with temporarily enriching global seawater in a range of other redox-sensitive trace metals (for example, V, Cr, Co, Ni, Cu, Zn and Cd) during ongoing basalt-seawater int...

No abstract

Silicon (Si) isotope variability in Precambrian chert deposits is significant, but proposed explanations for the observed heterogeneity are incomplete in terms of silica provenance and fractionation mechanisms involved. To address these issues we investigated Si isotope systematics, in conjunction with geochemical and mineralogical data, in three well-characterised and approximately contemporaneous, $3.5 Ga chert units from the Pilbara greenstone terrane (Western Australia).We show that Si isotope variation in these cherts is large (À2.4& to +1.3&) and was induced by near-surface processes that were controlled by ambient conditions. Cherts that formed by chemical precipitation of silica show the largest spread in d 30 Si (À2.4& to +0.6&) and are characterised by positive Eu, La and Y anomalies and overall depletions in lithophile trace elements. Silicon isotope systematics in these orthochemical deposits are explained by (1) mixing between hydrothermal fluids and seawater, and/or (2) fractionation of hydrothermal fluids by subsurface losses of silica due to conductive cooling. Rayleigh-type fractionation of hydrothermal fluids was largely controlled by temperature differences between these fluids and seawater. Lamina-scale Si isotope heterogeneity within individual chemical chert samples up to 2.2& is considered to reflect the dynamic nature of hydrothermal activity. Silicified volcanogenic sediments lack diagnostic REE+Y anomalies, are enriched in lithophile elements, and exhibit a much more restricted range of positive d 30 Si (+0.1& to +1.1&), which points to seawater as the dominant source of silica.The proposed model for Si isotope variability in the Early Archaean implies that chemical cherts with the most negative d 30 Si formed from pristine hydrothermal fluids, whereas silicified or chemical sediments with positive d 30 Si are closest to pure seawater deposits. Taking the most positive value found in this study (+1.3&), and assuming that the Si isotope composition of seawater is governed by input of fractionated hydrothermal fluids, we infer that the temperature of $3.5 Ga seawater was below $55°C.

Silicon isotope ratios ( 28 Si, 29 Si and 30 Si) can be measured with high precision by multi-collector inductively coupled plasma mass spectrometers (MC-ICP-MS). However, the problematic extraction of silicon from geological materials has been a major disadvantage in previous silicon isotope studies with conventional gas source mass spectrometry, whereas available silicon isotope results obtained by MC-ICP-MS techniques have been mainly restricted to waters and high purity silica. We show here that high yields of silicon (>97%) can be achieved from samples ranging from pure silica to basaltic compositions (45-52 wt.% SiO 2 ) via a three-step digestion and purification procedure. Silicon isotope measurements, performed with a Finnigan Neptune MC-ICP-MS used in medium-resolution mode (resolving power: 2500), indicate that polyatomic interferences can be resolved and that both d 29 Si and d 30 Si can be determined with high accuracy and precision on interference-free peak plateaux in the mass spectrum. Instrumental blanks (20-65 mV) were reduced to acceptable values with a Cetac Aridus desolvating device fitted with a sapphire injector in the torch. Sensitivity in medium-resolution mode is in the range of B6 V per mg g À1 for 28 Si. d 29 Si and d 30 Si have been determined for silicon isotope standards IRMM-018 (d 30 Si = À1.75%), IRMM-018-76 (d 30 Si = À1.42%), Diatomite (d 30 Si = 1.34%) and Big Batch (d 30 Si = À10.52%), for USGS standards BHVO-2 (d 30 Si = À0.09%) and AGV-2 (d 30 Si = À0.01%), and for Aldrich pure silica powder (d 30 Si = À0.32%). Precision on d 30 Si is 0.18-0.41% (2 s.d.). Our combined procedure for sample preparation followed by high-resolution MC-ICP-MS analysis facilitates straightforward and safe measurement of silicon isotope ratios in silicate materials.