271
release of exopolymeric substances (EPS) by phytoplankton (Passow 2000; Verdugo
and Santschi 2010). Marine snow may have formed within subsurface oil intrusions
as well as at the surface (Ziervogel et al. 2012; Passow et al. 2012; Passow 2014).
Consistent with this pulse, a visually distinctive, brown, fine-grained surface layer
in the top 1–2 cm with dark brown or black bands is seen in sediment cores affected
by the blowout.
What is the consequence of this pulse of organic-rich material once it reached the
seafloor? Our hypothesis is that it resulted in increased respiration of organic carbon
in the sediments, which in turn decreased oxygen concentration in sediment pore
waters. Since pore water oxygen was not measured, we use changes in the relative
concentration of two redox-sensitive elements, manganese (Mn) and rhenium (Re),
to constrain changes in the redox state of marine sediments following the blowout.
A brief review of the chemistry of the select redox-sensitive metals (Mn, Re) is
useful to allow interpretation of our data (e.g., Algeo and Rowe 2012; Morford and
Emerson 1999; Tribovillard et al. 2006; Crusius and Thomson 2000). The redox
chemistry of Mn in marine sediments is central to understanding redox conditions,
due to the importance of Mn cycling across the redoxcline in reducing environments. Mn exists primarily in two oxidation states: Mn(IV) and Mn(II). Mn(IV)
oxides are readily reduced to aqueous Mn(II) in reducing environments. Soluble
Mn(II) diffuses upward, and where pore water oxygen is present, it is then oxidized
to Mn(IV) oxide completing the Mn redox cycle (e.g., Froelich et al. 1979; Burdige
and Gieskes 1983; Gobeil et al. 1997). A distinct peak in bulk Mn typically marks
the top of the redoxcline. Recent findings of abundant pore water Mn(III) in hemiplegic sediments require a revision of this classic redox model to include one electron transfer reactions for the Mn cycle (Madison et al. 2013).
Rhenium is mobile under oxic conditions and precipitates under mildly reducing
and anoxic conditions (Crusius et al. 1996). It is ideally suited as a redox tracer
since its detrital concentration is very low relative to authigenic deposition; oxic
marine sediments have low concentrations of 0.4–0.6 ppb (Boyko et al. 1986; Koide
et al. 1986). Re is enriched in reducing sediments because dissolved Re(VII)O 4
−
is
reduced and precipitates in the solid phase, most likely as Re(IV)O 2 (Crusius et al.
1996). Re enrichment occurs under both anoxic (Colodner et al. 1993) and suboxic
conditions, below the zone of Fe reduction and prior to sulfate reduction (Crusius
et al. 1996; Morford et al. 2005).
In summary, oxic sediments are typically characterized by elevated Mn values,
while more reducing and anoxic sediments have relatively low Mn content and
elevated Re.
16.2 Analytical Approach
As part of the C-IMAGE I and II and Deep-C consortia, we collected sediment
cores at over 250 stations in the Gulf of Mexico (GoM) from August 2010 to
December 2017 (Larson et al. 2020). In this chapter, we present data from select
cores with an emphasis on the temporal evolution (Fig. 16.1). Cores were collected
16 Changes in Redox Conditions of Surface Sediments Following the Deepwater…
release of exopolymeric substances (EPS) by phytoplankton (Passow 2000; Verdugo
and Santschi 2010). Marine snow may have formed within subsurface oil intrusions
as well as at the surface (Ziervogel et al. 2012; Passow et al. 2012; Passow 2014).
Consistent with this pulse, a visually distinctive, brown, fine-grained surface layer
in the top 1–2 cm with dark brown or black bands is seen in sediment cores affected
by the blowout.
What is the consequence of this pulse of organic-rich material once it reached the
seafloor? Our hypothesis is that it resulted in increased respiration of organic carbon
in the sediments, which in turn decreased oxygen concentration in sediment pore
waters. Since pore water oxygen was not measured, we use changes in the relative
concentration of two redox-sensitive elements, manganese (Mn) and rhenium (Re),
to constrain changes in the redox state of marine sediments following the blowout.
A brief review of the chemistry of the select redox-sensitive metals (Mn, Re) is
useful to allow interpretation of our data (e.g., Algeo and Rowe 2012; Morford and
Emerson 1999; Tribovillard et al. 2006; Crusius and Thomson 2000). The redox
chemistry of Mn in marine sediments is central to understanding redox conditions,
due to the importance of Mn cycling across the redoxcline in reducing environments. Mn exists primarily in two oxidation states: Mn(IV) and Mn(II). Mn(IV)
oxides are readily reduced to aqueous Mn(II) in reducing environments. Soluble
Mn(II) diffuses upward, and where pore water oxygen is present, it is then oxidized
to Mn(IV) oxide completing the Mn redox cycle (e.g., Froelich et al. 1979; Burdige
and Gieskes 1983; Gobeil et al. 1997). A distinct peak in bulk Mn typically marks
the top of the redoxcline. Recent findings of abundant pore water Mn(III) in hemiplegic sediments require a revision of this classic redox model to include one electron transfer reactions for the Mn cycle (Madison et al. 2013).
Rhenium is mobile under oxic conditions and precipitates under mildly reducing
and anoxic conditions (Crusius et al. 1996). It is ideally suited as a redox tracer
since its detrital concentration is very low relative to authigenic deposition; oxic
marine sediments have low concentrations of 0.4–0.6 ppb (Boyko et al. 1986; Koide
et al. 1986). Re is enriched in reducing sediments because dissolved Re(VII)O 4
−
is
reduced and precipitates in the solid phase, most likely as Re(IV)O 2 (Crusius et al.
1996). Re enrichment occurs under both anoxic (Colodner et al. 1993) and suboxic
conditions, below the zone of Fe reduction and prior to sulfate reduction (Crusius
et al. 1996; Morford et al. 2005).
In summary, oxic sediments are typically characterized by elevated Mn values,
while more reducing and anoxic sediments have relatively low Mn content and
elevated Re.
16.2 Analytical Approach
As part of the C-IMAGE I and II and Deep-C consortia, we collected sediment
cores at over 250 stations in the Gulf of Mexico (GoM) from August 2010 to
December 2017 (Larson et al. 2020). In this chapter, we present data from select
cores with an emphasis on the temporal evolution (Fig. 16.1). Cores were collected
16 Changes in Redox Conditions of Surface Sediments Following the Deepwater…