Constraints on Early Paleozoic Deep-Ocean Oxygen Concentrations From the Iron Geochemistry of the Bay of Islands Ophiolite

• Main Authors: Stolper, Daniel A., Pu, Xiaofei, Lloyd, Max K., Christensen, Nikolas I., Bucholz, Claire E., Lange, Rebecca A.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: American Geophysical Union 2022
• Subjects: oxygen; ophiolites; iron redox; Earth history; hydrothermal; Geochemistry and Petrology; Geophysics; Macquarie Island
• Online Access: https://doi.org/10.1029/2021gc010196

The deep ocean is generally considered to have changed from anoxic in the Precambrian to oxygenated by the Late Paleozoic (∼420–400 Ma) due to changes in atmospheric oxygen concentrations. When the transition occurred, that is, in the Early Paleozoic or not until the Late Paleozoic, is less well constrained. To address this, we measured Fe³âº/ΣFe of volcanic rocks, sheeted dykes, gabbros, and ultramafic rocks from the Early Paleozoic (∼485 Ma) Bay of Islands (BOI) ophiolite as a proxy for hydrothermal alteration in the presence or absence of Oâ‚‚ derived from deep marine fluids. Combining this data with previously published data from the BOI indicates that volcanic rocks are oxidized relative to intrusive crustal rocks (0.35 ± 0.02 vs. 0.19 ± 0.01, 1 standard error), which we interpret to indicate that the volcanic section was altered by marine-derived fluids that contained some dissolved Oâ‚‚. We compare our results directly to the Macquarie Island and Troodos ophiolites, drilled oceanic crust, previously compiled data for ophiolitic volcanic rocks, and newly compiled data for ophiolitic intrusive rocks. These comparisons show that the BOI volcanic (but not intrusive) rocks are oxidized relative to Precambrian equivalents, but are less oxidized relative to Late Paleozoic to modern equivalents. We interpret these results to indicate that the Early Paleozoic ocean contained dissolved Oâ‚‚, but at concentrations ∼2.4× lower than for the Late Paleozoic to today. © 2022 The Authors. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. Issue Online: 05 July 2022; Version of Record online: 05 July 2022; Accepted manuscript online: 02 June 2022; Manuscript accepted: 17 May 2022; Manuscript revised: 24 February 2022; Manuscript received: 04 October 2021. DAS acknowledges support from an Esper Larsen Jr. ...