Figure 7.

Steady‐state U isotope mass balance model results. (a) The fraction of oceanic U inputs removed into anoxic/euxinic sediments (horizontal axis) varies as a function of the fractionation factor (Δ_anoxic; vertical axis) between seawater and anoxic/euxinic sediments. (b) Steady‐state U isotope mass balance calculations show variations of seawater δ²³⁸U values as a function of anoxic seafloor area (%), keeping suboxic seafloor area fixed at 0% of total seafloor area and testing the sensitivity to possible Δ_anoxic values. In reality, suboxic seafloor area would co‐vary with anoxic/euxinic seafloor area; thus, this modeling exercise gives us the lowest estimation of anoxic/euxinic seafloor area [Colour figure can be viewed at http://wileyonlinelibrary.com]