Abstract
Sedimentary redox proxies are usually employed to reconstruct the paleo-redox conditions of bottom water environments, assuming that porewater and bottom water dissolved oxygen concentrations are similar. Using a combination of geochemical and magnetic techniques, we investigate the relationship between iron speciation and mineralogy in recent (~1760–2009 CE) sediments retrieved from the Santa Barbara Basin (SBB) – a modern silled basin with low-oxygen (dissolved O2 < 10 μmol/kg) and sporadically anoxic (no O2 detected) bottom waters. Magnetic analyses reveal that biogenic magnetite is preserved in SBB sulfidic porewaters on at least decadal to centennial time scales. Highly-reactive Fe (oxyhydr)oxides remain preserved despite observed sulfidic porewaters, indicating incomplete pyrite conversion, and producing low Fepy [pyrite Fe]/FeHR ratios. We attribute this observation to restricted porewater reaction kinetics under high sedimentation rates. Our results also reveal non-steady state diagenesis caused by instantaneous depositional events (e.g., turbidites and flood layers). The most reducing water column suggested by Fe speciation coincided with the Macoma layer, where in situ colonization of hypoxia-intolerant bivalve shells argues for the most oxygenated bottom water in the 250 year record. A turbidite potentially introduced fresh unsulfidized FeHR that buffered upward-diffusing sulfide from underlying sediments. Subsequent pyrite precipitation following re-establishment of sulfidic porewaters could have facilitated a “false positive” interpretation. In comparison, redox-sensitive metal enrichments (MoEF, UEF, and ReEF) were not obscured by post-depositional diagenesis and appear to accurately record redox geochemistry at the sediment-water interface.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 95-109 |
| Number of pages | 15 |
| Journal | Chemical Geology |
| Volume | 519 |
| DOIs | |
| State | Published - Aug 5 2019 |
Bibliographical note
Funding Information:This work is supported by the National Science Foundation (grant number NSF OCE-1304327) awarded to I.H. the Scott Turner Award to Y.W. and by the Rackham Graduate School of the University of Michigan. Part of this work was performed as a US Visiting Student Fellow at the Institute for Rock Magnetism (IRM) at the University of Minnesota. The IRM is a US National Multi-user Facility supported through the Instrumentation and Facilities program of the National Science Foundation, Earth Sciences Division, and by funding from the University of Minnesota. We thank Mike Jackson and Peter Solheid for thoughtful discussions and technical support at the IRM. This is IRM publication 1708. We acknowledge Arndt Schimmelmann for helpful discussions. We appreciate comments from Andrew P. Roberts and other two anonymous reviewers, which greatly improved this manuscript. All geochemical data can be accessed from the Supplementary Information.
Funding Information:
This work is supported by the National Science Foundation (grant number NSF OCE-1304327 ) awarded to I.H., the Scott Turner Award to Y.W., and by the Rackham Graduate School of the University of Michigan . Part of this work was performed as a US Visiting Student Fellow at the Institute for Rock Magnetism (IRM) at the University of Minnesota. The IRM is a US National Multi-user Facility supported through the Instrumentation and Facilities program of the National Science Foundation, Earth Sciences Division, and by funding from the University of Minnesota. We thank Mike Jackson and Peter Solheid for thoughtful discussions and technical support at the IRM. This is IRM publication 1708. We acknowledge Arndt Schimmelmann for helpful discussions. We appreciate comments from Andrew P. Roberts and other two anonymous reviewers, which greatly improved this manuscript. All geochemical data can be accessed from the Supplementary Information.
Publisher Copyright:
© 2019 Elsevier B.V.
Keywords
- Diagenesis
- Iron speciation
- Magnetism
- Santa Barbara Basin
- Trace metals