Samples of river bank sediments and rocks were collected from across the entire study 3 site in order to constrain the elemental and isotopic composition of different lithologic end-4 members. Sub-samples of the river bank sediments were separated using a riffle splitter and 5 powdered in a ball mill. Rock samples were disaggregated using an agate mortar and pestle 6 before being ground in a ball mill. Only measurements of Na, Ca, Mg, and Sr concentrations are discussed in this paper. The vessel. During the reaction, the digestion vessels were heated from below with a hotplate. 25The liberated H 2 S gas was passed through a condenser and bubbled through a solution of 26

Dissecting ancient microbial sulfur cycling Before the rise of oxygen, life on Earth depended on the marine sulfur cycle. The fractionation of different sulfur isotopes provides clues to which biogeochemical cycles were active long ago (see the Perspective by Ueno). Zhelezinskaia et al. found negative isotope anomalies in Archean rocks from Brazil and posit that metabolic fluxes from sulfate-reducing microorganisms influenced the global sulfur cycle, including sulfur in the atmosphere. In contrast, Paris et al. found positive isotope anomalies in Archean sediments from South Africa, implying that the marine sulfate pool was more disconnected from atmospheric sulfur. As an analog for the Archean ocean, Crowe et al. measured sulfur isotope signatures in modern Lake Matano, Indonesia, and suggest that low seawater sulfate concentrations restricted early microbial activity. Science , this issue p. 703 , p. 742 , p. 739 ; see also p. 735

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