Introduction
The Early Paleozoic, notably the Ordovician Period, hosts one of the
largest marine biodiversification events in Earth history, the Great
Ordovician Biodiversification Event (GOBE). Not long after this
proliferation of marine fauna, however, the second-largest mass
extinction in Earth history occurred, the Late Ordovician Mass
Extinction Event (LOME; (Harper et al., 2014)). The LOME resulted in the
loss of ~85% of marine species during two distinct
extinction pulses, with the first occurring at the Katian-Hirnantian
boundary, and the second in the late Hirnantian (Brenchley et al., 2001;
Harper et al., 2014). Traditionally, the first LOME pulse has been
associated with rapid global cooling and widespread glaciation that
resulted in major eustatic sea-level fall, creating widespread marine
habitat loss and ecologic shifts (Harper et al., 2014). The second LOME
pulse has been associated with sea-level rise and an expansion of anoxic
(potentially euxinic, anoxic, and sulfidic water column) conditions
(Hammarlund et al., 2012). However, recent studies have invoked
widespread anoxia/euxinia for both LOME pulses, indicating that the
redox conditions surrounding this event might be more complex than
initially understood (Zou et al., 2018). Associated with the LOME was a
major perturbation in the global carbon cycle, recorded as a positive
excursion in the marine carbon isotope record known as the Hirnantian
carbon isotope excursion (HICE) (Brenchley et al., 2003).
The primary causal mechanisms for the HICE has previously been
attributed to changes in carbonate weathering regimes during eustatic
sea-level fall (Kump et al., 1999) and/or enhanced burial of organic
matter associated with increased preservation resulting from decreased
marine oxygenation (Brenchley et al., 2003; Hammarlund et al., 2012;
Jones and Fike, 2013). Traditional sulfur (S) isotope approaches have
investigated the dynamics and extent of euxinic marine conditions during
the LOME. Pyrite sulfur isotope
(δ34Spyr) profiles across multiple
globally distributed paleobasins show nearly synchronous positive
excursions, suggesting a global perturbation, albeit with local
overprints to explain the variable magnitudes ~15-40‰.
Overall, the positive δ34Spyr shift
may reflect increased pyrite burial under widespread sulfidic conditions
(Hammarlund et al., 2012; Jones and Fike, 2013). However, the single
reported seawater sulfate (δ34SCAS)
record shows little variation in this global redox proxy throughout the
HICE, suggesting a minimal change in global pyrite burial rates over
this interval and thus a limited global expansion of euxinic conditions
(Jones and Fike, 2013).
Additionally, non-traditional paleoredox proxies have been applied to
assess the extent of widespread reducing conditions as a potential kill
mechanism for both LOME pulses. Specifically, uranium and molybdenum
(δ238U and δ98Mo) stable isotope
records have been interpreted to reflect global to regional changes in
marine redox, and have identified possible shifts towards more reducing
conditions—but at differing times during the Hirnantian (Zhou et al.,
2015; Bartlett et al., 2018). Models based on δ238U
data from eastern Laurentia suggest up to 15% of the total seafloor
area experienced anoxic conditions just before the end of the Ordovician
(Bartlett et al., 2018). Meanwhile, δ98Mo data from
South China have been interpreted to record local shifts from suboxic to
euxinic conditions prior to and during the early Hirnantian (Zhou et
al., 2015). However, each of these marine redox interpretations are
based on singular datasets within widely different depositional
environments. Moreover, delineating the timing of these inferred changes
in redox relative to one another remains problematic, as the lack of a
universally accepted, fully integrated Upper Ordovician biostratigraphic
scheme limits the resolution of correlations (see supplemental
information). Additionally, these two paleoredox proxies have different
specific responses due to their position on the redox ladder, i.e.
changes in increasingly reducing marine conditions, which creates
additional complications for understanding the onset of non-sulfidic
anoxia (δ238U) versus euxinia
(δ98Mo).
To better elucidate critical gaps in our understanding of the
mechanistic underpinnings for the LOME, we present new I/(Ca+Mg) ratios
and δ34SCAS datasets from three widely
distributed Upper Ordovician carbonate successions. Our study localities
were deposited in different paleocean basins and record the HICE along
with major fluctuations in eustatic sea level (Finney et al., 1997;
Young et al., 2010; Ghienne et al., 2014; Kiipli and Kiipli, 2020).
These new-paired geochemical datasets provide a more comprehensive
understanding and specificity of marine redox conditions (local and
global) and climate that led to the second-largest extinction event in
Earth history.
Background