Figure 6. Results of geochemical box modeling showing changes
in pyrite burial (Fpry), weathering fluxes
(Fw) needed to reproduce seen CAS trends in Monitor
Range for a 3mM SO4-2 scenario (panels
A and B) and a 5mM SO4-2 scenario
(panels C and D). Also shown are changes in sulfate concentrations
resulting from the associated weathering and pyrite burial fluxes
(panels B and D). Grey interval in all panels represents the HICE
interval. For additional sensitivity tests, see supplemental materials.
Changes to any single model parameter were unable to reproduce the fall
in δ34SCAS in the required timeframe
(supplemental material). However, model runs assuming moderate decreases
in pyrite burial in concert with an increase in weathering can generate
the observed δ34SCAS drop (Fig. 6). A
50–75% reduction in pyrite burial (i.e. 50% Fpyr and
25% Fpyr of initial burial rate, respectively) combined
with a 25–60% increase in the weathering flux (i.e., 125%
Fw and 160% Fw, respectively) produces
a ~10‰ drop using an initial marine sulfate
concentration of 5 mM. This scenario is the most parsimonious with the
initiation of sea-level fall, which would reduce shelf area and thus the
total aerial extent of pyrite burial, as well as increase the weathering
inputs. This change in sea level is a result of changes in global
climate and associated changes in thermohaline circulation at this time
(see section 5.3 below) may have increased marine oxygenation of the
Late Ordovician oceans, and further reduced global pyrite burial. These
combined effects would both contribute to the observed
δ34SCAS drop.
A starting marine sulfate concentration of 5 mM is consistent with
previous estimates for Late Ordovician seawater (Horita et al., 2002;
Jones and Fike, 2013). Model simulations starting with 3 mM marine
sulfate concentrations require smaller changes in the weathering and
pyrite burial fluxes (i.e., 25% reduction yields a value of 75%
Fpyr) to simulate the negative excursion (Fig. 6A).
However, we do not favor an initial marine sulfate concentration of 3 mM
or less due since it is the low end of estimates based on fluid
inclusions and previous Late Ordovician sulfur isotope modeling (Horita
et al., 2002; Hammarlund et al., 2012; Jones and Fike, 2013).
Furthermore, sensitivity tests with an initial 3 mM oceanic reservoir
show changes in seawater sulfate sulfur isotopes that are faster than
those documented from Upper Ordovician records in terms of reaching the
minimum value and the later return to baseline. Additionally, our model
places an upper constraint on Late Ordovician marine sulfate
concentrations, as our simulations with initial values of 10 mM or
greater cannot reproduce the observed sulfur isotope records unless
unreasonable changes in weathering and pyrite burial fluxes are
prescribed (supplemental material).
The observed negative δ34SCASperturbation requires a major decrease in pyrite burial, thus
potentially requiring a reduction in the global extent of euxinic
conditions (since pyrite burial is highly efficient under such
conditions). As a thought experiment, if we assume that most pyrite is
buried in euxinic settings, we can calculate a maximum estimate of
euxinic seafloor area. This approach is an oversimplification since
pyrite is also formed in reducing sediments overlain by oxic and anoxic
non-sulfidic waters. However, since pyrite burial in these settings is
less efficient, an even greater reduction in the area of the seafloor
subject to reducing conditions is required. Initial pyrite burial flux
for the late Katian required an Fpyr of 1.1 ×
1018 mol of S/Myr, compared to the modern global rate
of 0.67 × 1018 mol of S/Myr (Kurtz et al., 2003). The
extent of euxinic conditions in the modern oceans is estimated at
~0.15% of the global seafloor (Reinhard et al., 2013),
with reduced sulfur burial flux that would equate to
~3.1×1016 mol of S/Myr, most of which
occurs in the Black Sea (Neretin et al., 2001). This reduced sulfur
burial flux includes pyrite burial and any intermediate valence reduced
S species, as well as organically bound S, and is thus a maximum
estimate for reduced sulfur burial. Given this data from modern oceans
and the estimated late Katian pyrite burial rates, we can estimate the
extent of euxinic conditions in the late Katian at most was
~35× more than the modern, equating to an aerial extent
of approximately 5.3% (35 × 0.15% of the modern). We estimate that a
subsequent 75 to 50% reduction in pyrite burial—corresponding to the
minimum δ34SCAS values—would reduce
the maximum global estimate for the extent of euxinia to
~1.3 - 2.7% within the Hirnantian. We observe a shift
back to heavier δ34SCAS values in the
late Hirnantian-early Silurian, likely signaling a return to more
reducing conditions with the oceans.
Due to the susceptibility of carbonate-associated sulfate data to be
possibly compromised from various diagenetic processes (see above
section 5.1) that can lead to some variability within the recorded
δ34SCAS values, we performed a series
of sensitivity tests allowing for some degree of overprinting of primary
δ34SCAS values (Fig. S4–S6). These
tests reveal that our fundamental conclusions of decreased seafloor
euxinia do not change within a range of reasonable
δ34SCAS variations and possible
diagenetic overprinting, but simply affects estimated ranges of total
seafloor euxinia. Through careful sample selection and laboratory
treatment (i.e. preference of less permeable, low porosity micrite over
pack/grainstone, careful extraction procedures to avoid pyrite
oxidation) we have generated robust
δ34SCAS datasets that are in good
agreement with correlative and previously published
δ34SCAS datasets (Jones and Fike,
2013; Present et al., 2015). However, it is important to acknowledge
that diagenetic overprints are still possible but given the results of
our sensitivity tests, agreement of trends among sections, and
large-scale ( >5‰) trends within a section these secondary
processes cannot be the primary mechanisms responsible for the major
trends in data recorded from these study sites. Ultimately these model
results have produced conservative estimates for the extent of seafloor
euxinia in the Late Ordovician oceans.
5.3 Late Ordovician cascade
of redox, environmental, and biotic change
The negative excursion in δ34SCASrecorded from the late Katian is interpreted to indicate a reduction in
global euxinia, which is counter to the occurrence of undifferentiated
“anoxic” black shales (i.e. anoxic, ferruginous, or euxinic) found at
many locations across the globe during this interval (Melchin et al.,
2013). Our I/(Ca+Mg) trends indicate that sub-oxic to anoxic conditions
were locally pervasive at least in the sections we studied and by
inference may have been widespread in the late Katian oceans and
persisted into the Hirnantian. The explanation for this apparent
contradiction may lie with the fact that iodine and sulfur respond to
different types of reducing conditions, with iodine responding to
changes in redox near O2 reduction (i.e., non-sulfidic
anoxia), while sulfate reduction occurs in more reduced settings further
down the redox ladder (Froelich et al., 1978; Rue et al., 1997; Lu et
al., 2010). Additionally, these two proxies reflect different
spatiotemporal relationships, with iodine reflecting local water-column
conditions, while δ34SCAS values
record changes in sulfur cycling in the global oceans. In the discussion
that follows we focus on this new level of paleoredox specificity for
the Late Ordovician oceans in the context of coincident changes in the
environment, eustatic sea level, and the marine biosphere.
Significant changes in local and global marine redox conditions began in
the late Katian and were coincident with high sea level, elevated sea
surface temperatures (SSTs), and generally high levels of marine
biodiversity (Finney et al., 1999; Haq and Schutter, 2008; Trotter et
al., 2008; Rasmussen and Harper, 2011; Finnegan et al., 2011). There is
growing evidence from clumped oxygen isotopes and conodont
palaeothermometry that global average SSTs began declining in the latest
Katian with the initiation of Gondwanan ice sheet expansion (Trotter et
al., 2008; Finnegan et al., 2011). This relationship suggests that the
negative δ34SCAS excursion and the
implied changes in global average temperature were initiated by the
intensification of thermohaline circulation as a result of increased
deep-water formation around Gondwanan margins, consistent with
sedimentary indicators of upwelling in the Monitor Range section (i.e.,
bedded cherts and phosphates; Fig. 2) (Pope and Steffen, 2003).
Increased thermohaline circulation would have led to cooler globally
averaged SSTs and increased renewal of dissolved deep marine
O2, thus ventilating portions of previously euxinic
environments along continental margins, shifting the sulfidic chemocline
deeper and likely into the sediments in many regions. The ultimate
result was a reduced global pyrite burial flux. Thus, euxinic water
column conditions may have decreased globally in the latest Katian.
However, widespread sulfidic sediment pore waters may help explain the
mild enrichments in molybdenum concentrations and iron speciation
records of anoxic, sulfide limited, water-column conditions leading into
the Hirnantian as recorded in black shales/deep basinal settings on
Laurentia, Baltica, Avalonia, and peri-Gondwana (Hammarlund et al.,
2012; Hardisty et al., 2018).
The collective data suggest that a combination of cooling temperatures,
reduction of habitable space on shelves and in epeiric seaways due to
eustatic sea-level fall (Fig. S3), and our new evidence for possibly
widespread non-sulfidic anoxic marine conditions in many local basins
culminated in the first LOME pulse near the Katian-Hirnantian boundary.
Consistent with this hypothesis, the first appearance of bedded chert in
the Monitor Range section, suggesting an increase in local upwelling,
coincides with indicators of local sea-level fall (Finney et al., 1997)
and expansion of a local OMZ recorded in a drop in I/(Ca+Mg). There is
also evidence within previously published bulk nitrogen data (LaPorte et
al., 2009) for a shift toward more reducing conditions. This data shows
a trend to lighter δ15N values after our I/(Ca+Mg)
ratios drop to near 0 μmol/mol (Fig. 4D) where it is attributed to a
local increase in denitrification. Denitrification occurs after iodine
reduction on the redox ladder (Lu et al., 2010), consistent with the
observed relationship to our iodine data. Together these local redox
proxies suggest a progressive loss of oxygen in this local environment
prior to the Katian-Hirnantian boundary. Although many Hirnantian
localities show evidence of locally reducing conditions, the anomalously
high I/(Ca+Mg) ratios recorded from western Anticosti Island are likely
due to lowered local sea level allowing for changes in surface currents
and nutrient dynamics. The net result was well-oxygenated conditions in
very shallow waters that supported patch reef environments in this
region. Given the similarities in the iodine data across multiple basins
and our inferred global signatures in
δ34SCAS, we suggest that euxinia
likely decreased globally while, paradoxically, less severe anoxia
expanded globally in shallow settings thus non-sulfidic anoxia impacted
marine life leading into the first LOME pulse. This reduction in euxinic
conditions may be attributed to the observed global cooling of surface
waters and subsequent increased solubility of O2,
combined with enhanced thermohaline circulation, thus ventilating
previously euxinic portions of Late Ordovician oceans. This enhanced
ocean circulation may have in turn intensified local upwelling around
continental margins throughout the globe, thus leading to more local
primary productivity, enhancing global carbon burial and local anoxia,
as evidenced by I/(Ca+Mg) trends. These climatic and oceanographic
conditions during the late Katian–Hirnantian may have provided a unique
balance that resulted in expansion of anoxic non-sulfidic water masses,
but the increased oxygen solubility and circulation may have prevented
these water masses from being pervasively euxinic. Ultimately, these
marine redox conditions would have had a major impact on marine life in
productive continental margins and remaining shallow seaways.
Changes in global marine redox conditions associated with eustatic
sea-level rise have been invoked as a causal mechanism for the second
LOME pulse in the late Hirnantian (within the M. persculptusgraptolite biozone) (Harper et al., 2014). As Gondwanan ice sheets
melted and the late Hirnantian climate warmed (Finnegan et al., 2011),
marine stratification and chemocline migration during eustatic sea-level
rise likely played an important role in the second LOME pulse. The
δ238U records from carbonates on western Anticosti
Island (Bartlett et al., 2018)—along with δ98Mo and
δ238U data, Mo concentrations, and iron speciation
from organic-rich shale successions (Hammarlund et al., 2012; Zhou et
al., 2015; Zou et al., 2018; Stockey et al., 2020)—indicate a return
to widespread reducing conditions in global oceans during this time. Our
iodine and sulfur isotope datasets are consistent with a shift to more
reducing conditions. Specifically, late Hirnantian–early Silurian
I/(Ca+Mg) values indicate local anoxia at all three sections, and
δ34SCAS profiles from all sites trend
positively by ~ 7‰ (Fig. 5), indicating a return to
increased global pyrite burial. Increased reducing conditions along
continental margins during this time would have largely tracked eustatic
sea-level rise, warming sea surface temperatures would have led to
decreased O2 solubility and circulation. As OMZs
expanded from deep shelf/slope to shallower areas on the continental
shelf during the late Hirnantian–early Silurian this increased the
overall areal extent of seafloor overlain by anoxic and euxinic bottom
waters.
Conclusions
Paired iodine and sulfur isotope geochemistry reveal new spatiotemporal
relationships between marine non-sulfidic anoxia and euxinia associated
with the Late Ordovician Mass Extinction. Our I/(Ca+Mg) ratios are low
throughout this time interval in all sections, except for a set of high
values (average of 6 μmol/mol) recorded from the shallow patch reef
facies on western Anticosti Island. At the same time, our new
δ34SCAS records show a large negative
excursion of ~ 10‰ magnitude over the late
Katian–Hirnantian. Modeling of these
δ34SCAS records suggests that the
negative excursion was driven by moderate decreases in the pyrite burial
rates combined with modest increases in weathering. Our model also
suggests reductions of global seafloor euxinic conditions by
~ 3% from the late Katan into the Hirnantian. This is a
maximum estimate, but it is consistent with recent models of Hirnantian
to early Silurian global redox-based on other proxy data (Bartlett et
al., 2018; Stockey et al., 2020). Importantly, this transition does not
preclude the possibility of increasing oxygen deficiency as recorded in
iodine data in marginal settings due to enhanced upwelling as seen in
the Monitor Range. Additional data from redox-sensitive elements are
needed from multiple paleobasins to constrain the extent of these
non-sulfidic reducing conditions.
In sum, our multiproxy data and modeling indicate widespread ventilation
of marine environments followed by enhanced weathering during the late
Katian–early Hirnantian. This sequence of events likely resulted from
enhanced thermohaline circulation and growth of Gondwanan ice sheets
that cooled sea surface temperatures and potentially increased deeper
ocean oxygenation, therefore reducing euxinic conditions in the global
oceans. However, non-sulfidic anoxic conditions remained pervasive
throughout shallow shelf settings due to attendant increases in
productivity resulting from increased upwelling and ocean circulation.
These relationships indicate that a unique combination of reducing
marine conditions, climatic cooling, and glacioeustacy led to the first
LOME pulse. Subsequently, deglacial eustatic sea-level rise during the
late Hirnantian that coincided with warming temperatures, deoxygenation,
and decreased ocean circulation led to an expansion of global euxinic
conditions, broadly coincident with the second LOME pulse. Our study
sheds new light on the possibility of a complex and evolving redox
landscape reflecting the interplay of multiple interrelated
controls—with severe biotic turnover as a consequence. More generally,
these results hint at the improved perspective that can come by
integrating multiple local and global proxies from a wide distribution
of locations.
Acknowledgments
N.P.K would like to thank Chelsie Bowman, Randall Funderburk, Anders
Lindskog, and Westly Owings for their support and/or assistance in
sample collection and preparation. B.C.G. would like to thank Charles
Gill for assistance in sample collection in Nevada. We thank Dimitri
Kaljo for access and assistance in sample collection of the Kardla drill
core. This research was funded by the American Chemical Society
Petroleum Research Fund (grant ACS-PRF# 57487-DNI2 to S.A.Y.) and the
National Science Foundation (EAR-1748635 to S.A.Y. and J.D.O. and
EAR-0418270 to T.W.L.). Additional funds were provided by the NASA
Astrobiology Institute under Cooperative Agreement No. NNA15BB03A issued
through the Science Mission Directorate (to T.W.L.). This work was in
part performed at the National High Magnetic Field Laboratory, which is
supported by the National Science Foundation Cooperative Agreement No.
DMR-1157490 and the State of Florida. The authors declare no conflict of
interest.