276
noting that the redox boundary can migrate upward quickly but downward more
slowly due to relatively fast oxidation of organic matter in contrast to the slower
diffusion of oxygen (Gobeil et al. 2001).
No double peak for Mn is observed at select sites including DSH08 and PCB06 in
September 2011 and PCB06 in August 2012.The deeper relic Mn peak is subtle at
DSH08 in August 2012. This is likely due to the reduction of the Mn oxide associated with the relic peak under more reducing conditions. There is a wide range in
reported kinetics for the reduction of Mn oxide with first-order rate constants ranging from 0.002 d
−1
for pelagic sediments (Burdige and Gieskes 1983) to 8–60 d
−1
for coastal environments (Aller 1980). A pulse of organic carbon to the sea floor can
trigger reduction of Mn and Fe oxides within several days with a first-order rate
constant of 0.159 d
−1
(Magen et al. 2011), demonstrating that Mn oxides in surficial
sediments can be rapidly reduced. By August 2013 the relic Mn oxide peak is no
longer present in the majority of the sites; by August 2016 only one Mn oxide peak
is present at all sites.
16.3.4 Post-impact: Re Geochemistry
Re is the clearest indicator of reducing conditions, since detrital Re is very low, has
no known role in biogeochemical processes, and is not associated with cycling of
Mn oxides (Schaller et al. 2000; Morford et al. 2005). Before the event, downcore
Re is effectively constant at 0.5 ppb in surface sediments at PE09-04 (Fig. 16.2).
Below the Mn oxide peak, Re increases as sediments become more reducing with
depth.
Several features of the Re profiles provide insight into the evolution of reducing
conditions following the impact. A modest Re enrichment of 0.1–0.3 ppb typically
occurs between the two Mn peaks, and coincident with the Mn minimum, characteristically at 15–20 mm between August 2010 and February 2011 (Fig. 16.3a). This
infers reducing conditions relatively shallow in the sediment. This is consistent with
the massive pulse of organic-rich material to surface sediments during and following the MOSSFA event, which is rapidly respired, leading to rapid consumption of
pore water oxygen, reducing conditions, and a shoaled redoxcline. A distinct subsurface Re peak at 30–40 mm at DSH10 (Figs. 16.3a and 16.3b) and at 50–70 mm
at PCB06 is consistent with substantially more reducing conditions following the
sediment pulse as organic carbon degrades relatively rapidly in surface sediments.
16.3.4.1 Post-impact: Evolution of Re Enrichment
Three sites were sampled 9–10 times between August 2010 and August 2017, allowing us to determine how the event impacted surficial sediments and to constrain the
temporal evolution of reducing conditions. We are able to best describe the temporal
evolution of reducing conditions by measuring the change of Re over time.
D. W. Hastings et al.
noting that the redox boundary can migrate upward quickly but downward more
slowly due to relatively fast oxidation of organic matter in contrast to the slower
diffusion of oxygen (Gobeil et al. 2001).
No double peak for Mn is observed at select sites including DSH08 and PCB06 in
September 2011 and PCB06 in August 2012.The deeper relic Mn peak is subtle at
DSH08 in August 2012. This is likely due to the reduction of the Mn oxide associated with the relic peak under more reducing conditions. There is a wide range in
reported kinetics for the reduction of Mn oxide with first-order rate constants ranging from 0.002 d
−1
for pelagic sediments (Burdige and Gieskes 1983) to 8–60 d
−1
for coastal environments (Aller 1980). A pulse of organic carbon to the sea floor can
trigger reduction of Mn and Fe oxides within several days with a first-order rate
constant of 0.159 d
−1
(Magen et al. 2011), demonstrating that Mn oxides in surficial
sediments can be rapidly reduced. By August 2013 the relic Mn oxide peak is no
longer present in the majority of the sites; by August 2016 only one Mn oxide peak
is present at all sites.
16.3.4 Post-impact: Re Geochemistry
Re is the clearest indicator of reducing conditions, since detrital Re is very low, has
no known role in biogeochemical processes, and is not associated with cycling of
Mn oxides (Schaller et al. 2000; Morford et al. 2005). Before the event, downcore
Re is effectively constant at 0.5 ppb in surface sediments at PE09-04 (Fig. 16.2).
Below the Mn oxide peak, Re increases as sediments become more reducing with
depth.
Several features of the Re profiles provide insight into the evolution of reducing
conditions following the impact. A modest Re enrichment of 0.1–0.3 ppb typically
occurs between the two Mn peaks, and coincident with the Mn minimum, characteristically at 15–20 mm between August 2010 and February 2011 (Fig. 16.3a). This
infers reducing conditions relatively shallow in the sediment. This is consistent with
the massive pulse of organic-rich material to surface sediments during and following the MOSSFA event, which is rapidly respired, leading to rapid consumption of
pore water oxygen, reducing conditions, and a shoaled redoxcline. A distinct subsurface Re peak at 30–40 mm at DSH10 (Figs. 16.3a and 16.3b) and at 50–70 mm
at PCB06 is consistent with substantially more reducing conditions following the
sediment pulse as organic carbon degrades relatively rapidly in surface sediments.
16.3.4.1 Post-impact: Evolution of Re Enrichment
Three sites were sampled 9–10 times between August 2010 and August 2017, allowing us to determine how the event impacted surficial sediments and to constrain the
temporal evolution of reducing conditions. We are able to best describe the temporal
evolution of reducing conditions by measuring the change of Re over time.
D. W. Hastings et al.