Martin Wille

Hydrogen sulphide release to surface waters at the Precambrian/Cambrian boundary.

Animal-like multicellular fossils appeared towards the end of the Precambrian, followed by a rapid increase in the abundance and diversity of fossils during the Early Cambrian period, an event also known as the ‘Cambrian explosion’. Changes in the environmental conditions at the Precambrian/Cambrian transition (about 542 Myr ago) have been suggested as a possible explanation for this event, but are still a matter of debate. Here we report molybdenum isotope signatures of black shales from two stratigraphically correlated sample sets with a depositional age of around 542 Myr. We find a transient molybdenum isotope signal immediately after the Precambrian/Cambrian transition. Using a box model of the oceanic molybdenum cycle, we find that intense upwelling of hydrogen sulphide-rich deep ocean water best explains the observed Early Cambrian molybdenum isotope signal. Our findings suggest that the Early Cambrian animal radiation may have been triggered by a major change in ocean circulation, terminating a long period during which the Proterozoic ocean was stratified, with sulphidic deep water.

Highly metalliferous carbonaceous shale and Early Cambrian seawater

We report evidence for the seawater origin of an extremely metal-enriched sulfide- and organic carbon–rich marker bed in a transgressive Early Cambrian black shale sequence along the passive margin of the Yangtze platform. The element concentration pattern in this marker bed suggests that it formed in a sediment-starved, stratified basin with a euxinic water column below an oxic surface layer. Biological activity was high in the surface layer, which was resupplied by communication with oxic oceans. The extremely low terrigenous input and the sulfate-reducing environment in the deeper part of the basin led to exceptionally high metal enrichments by factors of ∼107 with respect to modern seawater. The composition of the sulfidic rocks reflects the composition of the Early Cambrian oceans. The molybdenum isotope ratio suggests that during this time <35% of marine Mo was deposited in oxic sediments, and that suboxicanoxic marine environments were more widespread during the Early Cambrian than today.

Glacial Silicic Acid Concentrations in the Southern Ocean

Silicon Leakage Silicon is a major structural component of many marine organisms, whose chemistry is affected by oceanic nutrient distributions. To constrain nutrient changes since the last glacial period, Ellwood et al. (p. 1088, published online 21 October) measured the isotopic compositions of silicon obtained from the skeletons of deep-sea sponges found in deep cores from the Atlantic and Pacific sectors of the Southern Ocean and compared them to the silicon signatures in the skeletons of modern sponges. The results indicate that nutrient redistribution, related to iron fertilization from dust deposition, boosted the growth of organisms that transferred silicon to mid-latitudes during the last glacial period. Silicon isotope distributions in sponges contain the signature of ocean nutrient distributions during the last glacial period. Reconstruction of nutrient concentrations in the deep Southern Ocean has produced conflicting results. The cadmium/calcium (Cd/Ca) data set suggests little change in nutrient concentrations during the last glacial period, whereas the carbon isotope data set suggests that nutrient concentrations were higher. We determined the silicon isotope composition of sponge spicules from the Atlantic and Pacific sectors of the Southern Ocean and found higher silicic acid concentrations in the Pacific sector during the last glacial period. We propose that this increase results from changes in the stoichiometric uptake of silicic acid relative to nitrate and phosphate by diatoms, thus facilitating a redistribution of nutrients across the Pacific and Southern Oceans. Our results are consistent with the global Cd/Ca data set and support the silicic acid leakage hypothesis.

A solvent extraction technique for the isotopic measurement of dissolved copper in seawater.

Stable copper (Cu) isotope geochemistry provides a new perspective for investigating and understanding Cu speciation and biogeochemical Cu cycling in seawater. In this work, sample preparation for isotopic analysis employed solvent-extraction with amino pyrollidine dithiocarbamate/diethyl dithiocarbamate (APDC/DDC), coupled with a nitric acid back-extraction, to concentrate Cu from seawater. This was followed by Cu-purification using anion-exchange. This straightforward technique is high yielding and fractionation free for Cu and allows precise measurement of the seawater Cu isotopic composition using multi-collector inductively coupled plasma mass-spectrometry. A deep-sea profile measured in the oligotrophic north Tasman Sea shows fractionation in the Cu isotopic signature in the photic zone but is relatively homogenised at depth. A minima in the Cu isotopic profile correlates with the chlorophyll a maximum at the site. These results indicate that a range of processes are likely to fractionate stable Cu isotopes in seawater.

Isotopic evidence for oxygenated Mesoarchaean shallow oceans

Mass-independent fractionation of sulfur isotopes (MIF-S) in Archaean sediments results from photochemical processing of atmospheric sulfur species in an oxygen-depleted atmosphere. Geological preservation of MIF-S provides evidence for microbial sulfate reduction (MSR) in low-sulfate Paleoarchaean (3.8–3.2 billion years ago (Ga)) and Neoarchaean (2.8–2.5 Ga) oceans, but the significance of MSR in Mesoarchaean (3.2–2.8 Ga) oceans is less clear. Here we present multiple sulfur and iron isotope data of early diagenetic pyrites from 2.97-Gyr-old stromatolitic dolomites deposited in a tidal flat environment of the Nsuze Group, Pongola Supergroup, South Africa. We identified consistently negative Δ33S values in pyrite, which indicates photochemical reactions under anoxic atmospheric conditions, but large mass-dependent sulfur isotope fractionations of ~30‰ in δ34S, identifying active MSR. Negative pyrite δ56Fe values (−1.31 to −0.88‰) record Fe oxidation in oxygen-bearing shallow oceans coupled with biogenic Fe reduction during diagenesis, consistent with the onset of local Fe cycling in oxygen oases ~3.0 Ga. We therefore suggest the presence of oxygenated near-shore shallow-marine environments with ≥5 μM sulfate at this time, in spite of the clear presence of an overall reduced Mesoarchaean atmosphere.Oxidized sulfur, formed in photochemical reactions in an anoxic atmosphere, fuelled microbial sulfate reduction in Mesoarchaean oxygenated near-shore seas, according to sulfur and iron isotopes in pyrite.

Wille et al. reply

Replying to: S.-Y. Jiang et al. 459, 10.1038/nature08048 (2009)Jiang et al. present a new SHRIMP U–Pb zircon age of 532.3 ± 0.7 million years (Myr) ago for an ash bed in the lowermost black shale sequence of the Niutitang Formation, China, and claim that the data presented in our recent paper do not firmly support the idea that the biological and environmental changes at the Precambrian/Cambrian transition can be explained by a single global hydrogen sulphide (H2S) release event. Their new age seems to be supported by another recent SHRIMP investigation which indeed suggests that the Chinese metal-enriched sulphide layer does not represent the Precambrian/Cambrian boundary and shows that the redox history of both basins (Oman and South China) was much more complex.

A non-zircon Hf isotope record in Archean black shales from the Pilbara craton confirms changing crustal dynamics ca. 3 Ga ago

Plate tectonics and associated subduction are unique to the Earth. Studies of Archean rocks show significant changes in composition and structural style around 3.0 to 2.5 Ga that are related to changing tectonic regime, possibly associated with the onset of subduction. Whole rock Hf isotope systematics of black shales from the Australian Pilbara craton, selected to exclude detrital zircon components, are employed to evaluate the evolution of the Archean crust. This approach avoids limitations of Hf-in-zircon analyses, which only provide input from rocks of sufficient Zr-concentration, and therefore usually represent domains that already underwent a degree of differentiation. In this study, we demonstrate the applicability of this method through analysis of shales that range in age from 3.5 to 2.8 Ga, and serve as representatives of their crustal sources through time. Their Hf isotopic compositions show a trend from strongly positive εHfinitial values for the oldest samples, to strongly negative values for the younger samples, indicating a shift from juvenile to differentiated material. These results confirm a significant change in the character of the source region of the black shales by 3 Ga, consistent with models invoking a change in global dynamics from crustal growth towards crustal reworking around this time.