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The principles and applications of time‐of‐flight mass spectrometry involving instruments with independent (orthogonal) axes for ion generation and mass analysis are reviewed. This approach, generally referred to as orthogonal acceleration time‐of‐flight mass spectrometry, has proved particularly advantageous for the combination of continuous ionization sources with time‐of‐flight mass spectrometry. The history of the technique is briefly discussed along with the instrumental principles pertaining to all the stages of the instrumentation from ion source to detector. The applications of commercial and customized instruments are discussed for several ionization methods including electrospray, matrix assisted laser desorption/ionization, electron ionization, and plasma ionization. © 2000 John Wiley & Sons, Inc., Mass Spec Rev 19: 65–107, 2000

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After nearly a billion years with no evidence for glaciation, ice advanced to equatorial latitudes at least twice between 717 and 635 Mya. Although the initiation mechanism of these Neoproterozoic Snowball Earth events has remained a mystery, the broad synchronicity of rifting of the supercontinent Rodinia, the emplacement of large igneous provinces at low latitude, and the onset of the Sturtian glaciation has suggested a tectonic forcing. We present unique Re-Os geochronology and high-resolution Os and Sr isotope profiles bracketing Sturtian-age glacial deposits of the Rapitan Group in northwest Canada. Coupled with existing U-Pb dates, the postglacial Re-Os date of 662.4 ± 3.9 Mya represents direct geochronological constraints for both the onset and demise of a Cryogenian glaciation from the same continental margin and suggests a 55-My duration of the Sturtian glacial epoch. The Os and Sr isotope data allow us to assess the relative weathering input of old radiogenic crust and more juvenile, mantle-derived substrate. The preglacial isotopic signals are consistent with an enhanced contribution of juvenile material to the oceans and glacial initiation through enhanced global weatherability. In contrast, postglacial strata feature radiogenic Os and Sr isotope compositions indicative of extensive glacial scouring of the continents and intense silicate weathering in a post-Snowball Earth hothouse.rhenium-osmium geochronology | strontium isotopes | osmium isotopes | Windermere Supergroup | Neoproterozoic glaciation T he Snowball Earth hypothesis predicts that Neoproterozoic glaciations were global and synchronous at low latitudes and that deglaciation occurred as a result of the buildup of pCO 2 to extreme levels resulting in a "greenhouse" aftermath (1, 2). The temporal framework of Cryogenian glaciations is built on chemostratigraphy and correlation of lithologically distinct units, such as glaciogenic deposits, iron formation, and cap carbonates (3), tied to the few successions that contain volcanic rocks dated using U-Pb zircon geochronology (4). In strata lacking horizons suitable for U-Pb geochronology, Re-Os geochronology can provide depositional ages on organic-rich sedimentary rocks bracketing glaciogenic strata (5, 6). Moreover, Os isotope stratigraphy can be used as a proxy to test for supergreenhouse weathering during deglaciation (7). In a Snowball Earth scenario, we can make specific predictions for Cryogenian weathering: CO 2 consumption via silicate weathering should increase before glaciation, stagnate during the glaciation, and increase again during deglaciation. However, the use of a single weathering proxy to provide evidence for such a scenario, such as Sr isotopes from marine carbonates, is limited both by lithological constraints and an inability to distinguish between the amount of weathering and the composition of what is being weathered (8). The short residence time of Os in the present-day oceans (<10 ky) (9) provides a complementary higher resolution archive to Sr isotopes, and thus, insi...

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New Re-Os dates obtained from black shales overlying the Sturtian and Areyonga glacial deposits in southern and central Australia, respectively, challenge the prevailing consensus of three Neoproterozoic glaciations. The end of Sturtian glaciation in the Adelaide Rift Complex is constrained by a Re-Os date of 643.0 ؎ 2.4 Ma from the overlying Tindelpina Shale Member (basal Tapley Hill Formation). A Re-Os date of 657.2 ؎ 5.4 Ma for the basal Aralka Formation constrains the age of the underlying Areyonga glacial deposits in the Amadeus Basin. The Re-Os ages show that the Sturtian and Areyonga glacial deposits are younger than other radiometrically dated (ca. 685-750 Ma) Neoproterozoic glacial intervals previously regarded as possible correlatives. Thus, the ''Sturtian'' ice age was markedly diachronous, and/or there was more than one ''Sturtian''-type glaciation. Some Neoproterozoic glacial deposits may represent the products of regional and diachronous glaciation associated with protracted breakup of the supercontinent Rodinia rather than ''snowball'' or ''slushball'' Earth ice ages.

Chemical weathering consumes atmospheric carbon dioxide through the breakdown of silicate minerals and is thought to stabilize Earth’s long-term climate. However, the potential influence of silicate weathering on atmospheric pCO2 levels on geologically short timescales (103–105 years) remains poorly constrained. Here we focus on the record of a transient interval of severe climatic warming across the Toarcian Oceanic Anoxic Event or T-OAE from an open ocean sedimentary succession from western North America. Paired osmium isotope data and numerical modelling results suggest that weathering rates may have increased by 215% and potentially up to 530% compared to the pre-event baseline, which would have resulted in the sequestration of significant amounts of atmospheric CO2. This process would have also led to increased delivery of nutrients to the oceans and lakes stimulating bioproductivity and leading to the subsequent development of shallow-water anoxia, the hallmark of the T-OAE. This enhanced bioproductivity and anoxia would have resulted in elevated rates of organic matter burial that would have acted as an additional negative feedback on atmospheric pCO2 levels. Therefore, the enhanced weathering modulated by initially increased pCO2 levels would have operated as both a direct and indirect negative feedback to end the T-OAE.

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