Ferromanganese crusts as recorders of marine dissolved oxygen

The distinct triple oxygen isotope composition of tropospheric O₂ relative to seawater is the result of biogeochemical reactions (e.g. primary productivity, respiration), exchange with the stratosphere, and the relative size of different oxygen-containing reservoirs, namely O₂, O₃, and CO₂. This difference in isotopic composition gives tropospheric O₂ utility as a record of biogeochemical and atmospheric processes and may also be used for determining where in the rock record isotopic fingerprints of tropospheric oxygen may be preserved. The isotopic record of tropospheric oxygen in previous studies is largely limited to analyses of gas trapped in continental glaciers and a patchwork of other proxies, most notably the triple oxygen signature of sulfate. Here we show the uppermost layers of hydrogenetic, deep-ocean ferromanganese crusts from each of the major ocean basins have a triple oxygen isotope composition consistent with the direct incorporation of dissolved oxygen. The range of δ¹⁸O and Δ′¹⁷O in ferromanganese crusts suggests the Mn oxide endmember contains a near 50:50 mixture of oxygen from water and dissolved O₂. Our data indicate this signal also persists into older layers of the crusts, potentially preserving near 75 million years of the oxygen isotopic composition of the lower troposphere and subsequent deep-ocean respiration. Our analysis of oxygen isotope values, bulk chemistry, and estimated local dissolved oxygen for crust top samples reveals that variations in bulk chemistry ultimately exhibit more influence on the oxygen mass balance than changes in dissolved oxygen, presenting a challenge for unambiguous determination of local dissolved oxygen. Although analytical challenges remain, these widespread, layered deposits of ferromanganese crust may offer a viable path for future interrogation of the history or relative history of the oxygen cycle of the troposphere and deep ocean millions of years into the past.