Sulfur isotopes in coal constrain the evolution of the Phanerozoic sulfur cycle

摘要

Sulfate is the second most abundant anion (behind chloride) inmodern seawater, and its cycling is intimately coupled to the cyclingof organic matter and oxygen at the Earth’s surface. For example,the reduction of sulfide by microbes oxidizes vast amounts of organiccarbon and the subsequent reaction of sulfide with iron producespyrite whose burial in sediments is an important oxygensource to the atmosphere. The concentrations of seawater sulfateand the operation of sulfur cycle have experienced dynamic changesthrough Earth’s history, and our understanding of this history isbased mainly on interpretations of the isotope record of seawatersulfates and sedimentary pyrites. The isotope record, however, doesnot give a complete picture of the ancient sulfur cycle. This is because,in standard isotope mass balance models, there are morevariables than constraints. Typically, in interpretations of the isotoperecord and in the absence of better information, one assumes thatthe isotopic composition of the input sulfate to the oceans hasremained constant through time. It is argued here that this assumptionhas a constraint over the last 390 Ma from the isotopic compositionof sulfur in coal. Indeed, these compositions do not deviatesubstantially from the modern surface-water input to the oceans.When applied to mass balance models, these results support previousinterpretations of sulfur cycle operation and counter recentsuggestions that sulfate has been a minor player in sulfur cyclingthrough the Phanerozoic Eon.