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The global biosphere is commonly assumed to have been less productive before the rise of complex eukaryotic ecosystems than it is today.sup.1. However, direct evidence for this assertion is lacking. Here we present triple oxygen isotope measurements ([increment].sup.17O) from sedimentary sulfates from the Sibley basin (Ontario, Canada) dated to about 1.4 billion years ago, which provide evidence for a less productive biosphere in the middle of the Proterozoic eon. We report what are, to our knowledge, the most-negative [increment].sup.17O values (down to -0.880/00) observed in sulfates, except for those from the terminal Cryogenian period.sup.2. This observation demonstrates that the mid-Proterozoic atmosphere was distinct from what persisted over approximately the past 0.5 billion years, directly reflecting a unique interplay among the atmospheric partial pressures of CO.sub.2 and O.sub.2 and the photosynthetic O.sub.2 flux at this time.sup.3. Oxygenic gross primary productivity is stoichiometrically related to the photosynthetic O.sub.2 flux to the atmosphere. Under current estimates of mid-Proterozoic atmospheric partial pressure of CO.sub.2 (2-30 times that of pre-anthropogenic levels), our modelling indicates that gross primary productivity was between about 6% and 41% of pre-anthropogenic levels if atmospheric O.sub.2 was between 0.1-1% or 1-10% of pre-anthropogenic levels, respectively. When compared to estimates of Archaean.sup.4-6 and Phanerozoic primary production.sup.7, these model solutions show that an increasingly more productive biosphere accompanied the broad secular pattern of increasing atmospheric O.sub.2 over geologic time.sup.8.Triple oxygen isotope measurements of 1.4-billion-year-old sedimentary sulfates reveal a unique mid-Proterozoic atmosphere and demonstrate that gross primary productivity in the mid-Proterozoic was between 6% and 41% of pre-anthropogenic levels., Author(s): Peter W. Crockford [sup.1] [sup.2] [sup.3] , Justin A. Hayles [sup.4] [sup.5] , Huiming Bao [sup.5] [sup.6] , Noah J. Planavsky [sup.7] , Andrey Bekker [sup.8] , Philip W. [...]

Volcanic eruptions release a large amount of sulphur dioxide (S[O.sub.2]) into the atmosphere (1,2). S[O.sub.2] is oxidized to sulphate and can subsequently form sulphate aerosol (3), which can affect the Earth's radiation balance, biologic productivity and high-altitude ozone concentrations, as is evident from recent volcanic eruptions (4). S[O.sub.2] oxidation can occur via several different pathways that depend on its flux and the atmospheric conditions (3). An investigation into how S[O.sub.2] is oxidized to sulphate--the oxidation product preserved in the rock record--can therefore shed light on past volcanic eruptions and atmospheric conditions. Here we use sulphur and triple oxygen isotope measurements of atmospheric sulphate extracted from tuffaceous deposits to investigate the specific oxidation pathways from which the sulphate was formed. We find that seven eruption-related sulphate aerosol deposition events have occurred during the mid-Cenozoic era (34 to 7 million years ago) in the northern High Plains, North America. Two extensively sampled ash beds display a similar sulphate mixing pattern that has two distinct atmospheric secondary sulphates. A three-dimensional atmospheric sulphur chemistry and transport model study reveals that the observed, isotopically discrete sulphates in sediments can be produced only in initially alkaline cloudwater that favours an ozone-dominated S[O.sub.2] oxidation pathway in the troposphere. Our finding suggests that, in contrast to the weakly acidic conditions today (5), cloudwater in the northern High Plains may frequently have been alkaline during the mid-Cenozoic era. We propose that atmospheric secondary sulphate preserved in continental deposits represents an unexploited geological archive for atmospheric S[O.sub.2] oxidation chemistry linked to volcanism and atmospheric conditions in the past., A close temporal correlation between large igneous provinces and major mass extinction events over the last 300 million years implies a potentially causal relationship between the two (6). There has [...]

The oxygen isotope composition of terrestrial sulfate is affected measurably by many Earth-surface processes. During the Neoproterozoic, severe 'snowball' glaciations would have had an extreme impact on the biosphere and the atmosphere. Here, we report that sulfate extracted from carbonate lenses within a Neoproterozoic glacial diamictite suite from Svalbard, with an age of ~635 million years ago, falls well outside the currently known natural range of triple oxygen isotope compositions and indicates that the atmosphere had either an exceptionally high atmospheric carbon dioxide concentration or an utterly unfamiliar oxygen cycle during deposition of the diamictites.

Author(s): Huiming Bao (corresponding author) [1]; Douglas A. Campbell [1]; James G. Bockheim [2]; Mark H. Thiemens [1] The dry valleys of Antarctica are some of the oldest terrestrial surfaces [...]

Understanding the composition of the atmosphere over geological time is critical to understanding the history of the Earth system, as the atmosphere is closely linked to the lithosphere, hydrosphere and [...]

Author(s): Huiming Bao (corresponding author) [1]; Mark H. Thiemens [1]; James Farquhar [1]; Douglas A. Campbell [1]; Charles Chi-Woo Lee [1]; Klaus Heine [2]; David B. Loope [3] The variation [...]

Mass-independent isotopic signatures for [Δ.sup.33]S, [Δ.sup.34]S, and [Δ.sup.36]S from sulfide and sulfate in Precambrian rocks indicate that a change occurred in the sulfur cycle between 2090 and 2450 million years ago (Ma). Before 2450 Ma, the cycle was influenced by gas-phase atmospheric reactions. These atmospheric reactions also played a role in determining the oxidation state of sulfur, implying that atmospheric oxygen partial pressures were low and that the rotes of oxidative weathering and of microbial oxidation and reduction of sulfur were minimal Atmospheric fractionation processes should be considered in the use of sulfur isotopes to study the onset and consequences of microbial fractionation processes in Earth's early history., The present-day sulfur cycle is strongly influenced by anthropogenic emissions, biological processes, and oxidative weathering of continental sulfides (1-3). It has been debated whether the sulfur cycle early in Earth's [...]