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Abstract

Sedimentary chert is the diagenetic end-product of amorphous silica that first transforms to opal-cristobalite/tridymite (CT) and subsequently to microcrystalline quartz via dissolution and reprecipitation reactions. The 18O/16O and 17O/16O ratios (i.e. triple oxygen isotope composition, δ18O, Δ’17O) of cherts record equilibration with the pore fluids at the diagenetic temperatures prevalent during the opal-CT to quartz transition. Pore fluid oxygen isotope gradients emerge when secondary minerals, e.g. clay minerals form during burial. Here, we demonstrate that chert triple oxygen isotope compositions not in equilibrium with modern like seawater can be explained by changes in pore water δ18O and Δ’17O without invoking any alteration or different paleo-seawater. We demonstrate this using a suite of Carboniferous cherts that we collected from the same stratigraphic level at five locations in the Rhenish Massif, Germany, spanning a range of inferred burial rates. The precursor sediments were deposited in the marine Rhenohercynian Basin and were subject to peak diagenetic temperatures of up to 279 ± 37 °C upon folding and thrusting. Their triple oxygen trajectory cannot be explained by any realistic paleo-seawater composition. Using our numerical silica diagenesis model, we find that the chert isotope signatures are explained by a formation temperature between 20–35 °C and a pore fluid composition of -7 to -14‰ δ18O consistent with a pore fluid gradient of -13.5 ‰ km-1 across the Rhenohercynian Basin. This suggests that triple oxygen isotope compositions of chert systematically record near-surface conditions and therefore represent a valuable resource for paleoenvironmental research.