Sedimentary Marcasite: a Proxy for Ocean Acidification?

Marcasite, the rarer FeS₂ mineral, is present in marine black shales spanning the Phanerozoic. This is puzzling because laboratory experiments and theory suggest that marcasite should only form in very acidic conditions (pH < 5.5) that are absent in modern marine sediment pore waters. Interestingly, at the times marcasite formed, proxies indicate that surface ocean pH was on average ~0.5 units lower than it is today. To investigate the environmental conditions leading to marcasite formation, we selected sediments of various lithologies deposited over a range of water depths across the mid-Cretaceous `OAE-2'. We used laser Raman microprobe analysis and scanning electron microscopy / energy dispersive X-ray spectroscopy to characterize and determine the relative abundances of acid-insoluble sulfur-bearing minerals across OAE-2 at the different sites. Marcasite is present at all sites and is neither exclusive to the traditional OAE-2 interval nor the black shale units. At a site with lower organic C/carbonate (Pont d'Issole), marcasite is often present as overgrowths on framboidal pyrite and as coarse cements in polyframboidal pyrite aggregates, suggesting that the marcasite postdates pyrite. At sites with higher organic C/carbonate (Cismon, Demerara Rise), marcasite and pyrite are either isolated or cemented by barite, with no indication that marcasite postdates pyrite. In summary, 1) low water column pH might be a prerequisite for diagenetic marcasite formation, and 2) higher organic C/carbonate (itself a function of low water column pH) likely results in faster pore water pH drawdown due to increased organic acid generation and lesser carbonate dissolution.