Oxygen isotopes in cherts record decreasing paleo-heat flow on Shatsky Rise (W’ Pacific)

Oskar Schramm¹, Tommaso DiRocco¹, Andreas Pack¹, Patrick, J. Frings², Michael Tatzel¹

1 Geoscience Center, University of Göttingen

2 GFZ Potsdam

Sedimentary cherts form by the crystallization of amorphous silica during burial diagenesis. Their ¹⁸O/¹⁶O isotope ratio (δ¹⁸O_chert) is conventionally thought to record the combined influence of paleo-seawater temperatures and the δ¹⁸O of paleo-seawater. However, a recent model study argued that the kinetics of silica diagenesis control the temperatures at which quartz precipitates, such that δ¹⁸O_chert depends on paleo-heat flow during silica diagenesis. To provide an empirical underpinning to this interpretation, we investigated O-isotopes in the chert layers on top of the basalts on Shatsky Rise, a large igneous plateau in the Pacific Ocean. Here, we demonstrate that a ~ 5‰ δ¹⁸O_chert increase in these cherts going stratigraphically upwards was controlled by a decreasing heat flow over tens of millions of years, as predicted by a numerical model for silica diagenesis. Smaller variations within the overall trend can be explained by minor changes in sedimentation rate. This work provides evidence that δ¹⁸O_chert is sensitive to paleo-heat flow and that δ¹⁸O_chert discloses the prograde thermal histories of sedimentary basins and Earth’s thermal evolution.