Rethinking Seawater Mo Isotope Mass‐Balance and the Sedimentary Mo Record

Abstract A lingering misconception is that seawater 98 Mo/ 95 Mo ratios should have increased more or less linearly with the oxygenation of Earth's oceans. At the root of this hypothesis is the generalization that oxidizing marine sediments have a stronger affinity for lighter‐mass Mo isotopes than their reducing counterparts. This is a gross oversimplification. Sediments from deep anoxic basins in the Baltic Sea that are affected by a metal oxide “shuttle” possess low 98 Mo/ 95 Mo ratios approaching those found on the well‐oxygenated abyssal seafloor. Rates of Mo accumulation in these Baltic Sea sediments are also three to four orders‐of‐magnitude faster. Conditions found in the redox‐stratified Baltic Sea were probably much more common at times in Earth's past, and if so would have helped drive high seawater 98 Mo/ 95 Mo ratios comparable to today even if they covered a small areal fraction of the global seafloor. This alternative model could help explain the development of high seawater 98 Mo/ 95 Mo ratios in Earth's past and has important implications for timeframes such as the Ediacaran‐Cambrian rise of animals.