Publication Date:
2022-05-25
Description:
Author Posting. © The Author(s), 2018. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in American Journal of Science 318 (2018): 527-556, doi:10.2475/05.2018.04.
Description:
Iron speciation and trace metal proxies are commonly applied together in efforts to identify
anoxic settings marked by the presence of free sulfide (euxinia) or dissolved iron (ferruginous) in the
water column. Here, we use a literature compilation from modern localities to provide a new empirical
evaluation of coupled Fe speciation and Mo concentrations as a proxy for pore water sulfide accumulation
at non-euxinic localities. We also present new Fe speciation, Mo concentration, and S isotope data from
the Friends of Anoxic Mud (FOAM) site in Long Island Sound, which is marked by pore water sulfide
accumulation of up to 3 mM beneath oxygen-containing bottom waters. For the operationally defined Fe
speciation scheme, ‘highly reactive’ Fe (FeHR) is the sum of pyritized Fe (Fepy) and Fe dominantly present
in oxide phases that is available to react with pore water sulfide to form pyrite. Observations from FOAM
and elsewhere confirm that Fepy/FeHR from non-euxinic sites is a generally reliable indicator of pore fluid
redox, particularly the presence of pore water sulfide. Molybdenum (Mo) concentration data for anoxic
continental margin sediments underlying oxic waters but with sulfidic pore fluids typically show
authigenic Mo enrichments (2-25 ppm) that are elevated relative to the upper crust (1-2 ppm). However,
compilations of Mo concentrations comparing sediments with and without sulfidic pore fluids underlying
oxic and low oxygen (non-euxinic) water columns expose non-unique ranges for each, exposing false
positives and false negatives. False positives are most frequently found in sediments from low oxygen
water columns (for example, Peru Margin), where Mo concentration ranges can also overlap with values
commonly found in modern euxinic settings. FOAM represents an example of a false negative, where,
despite elevated pore water sulfide concentrations and evidence for active Fe and Mn redox cycling in
FOAM sediments, sedimentary Mo concentrations show a homogenous vertical profile across 50 cm
depth at 1-2 ppm. A diagenetic model for Mo provides evidence that muted authigenic enrichments are
derived from elevated sedimentation rates. Consideration of a range of additional parameters, most
prominently pore water Mo concentration, can replicate the ranges of most sedimentary Mo
concentrations observed in modern non-euxinic settings. Together, the modern Mo and Fe data
compilations and diagenetic model provide a framework for identifying paleo-pore water sulfide
accumulation in ancient settings and linked processes regulating seawater Mo and sulfate concentrations
and delivery to sediments. Among other utilities, identifying ancient accumulation of sulfide in pore
waters, particularly beneath oxic bottom waters, constrains the likelihood that those settings could have
hosted organisms and ecosystems with thiotrophy at their foundations.
Description:
DSH, TWL, NJP, and CRT acknowledge support from the NASA Astrobiology Institute under
Cooperative Agreement No. NNA15BB03A issued through the Science Mission Directorate. Financial
support was provided to NR and TWL by NSF-OCE and an appointment to the NASA Postdoctoral
Program, as well as to BCG via a postdoctoral fellowship from the Agouron Institute. DSH was supported
by a WHOI postdoctoral fellowship.
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
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