5.1 Evaluation of diagenetic influences
Assessing I/(Ca+Mg) and δ34SCAS values
for potential diagenetic overprints is paramount to ensure the recorded
signals represent changes in seawater. Importantly, a recently published
study using δ44Ca and Sr/Ca ratios addressed the
extent of diagenetic influence on both the Monitor Range and western
Anticosti Island sections (Jones et al., 2020). This study suggests that
deeper water settings like that of Monitor Range and western Anticosti
Island, generally retain primary seawater geochemical signatures (i.e.
sediment buffered), while strata deposited in shallower water settings
are more likely to reflect geochemical signatures akin to sediment
porewaters (Jones et al., 2020). Unfortunately, extensive diagenetic
studies have not been performed on the Kardla drill core, Estonia,
however this section is interpreted to be deposited in a deeper shelf
setting (albeit shallower than the upper slope setting of the Monitor
Range section), suggesting that this section likely preserves mostly
primary geochemical signatures (Kaljo et al., 2011).
Meteoric diagenesis has been shown to decrease the concentrations of
both iodine (Lu et al., 2010; Hardisty et al., 2017) and sulfate in
carbonates, as freshwater typically contains lower concentrations of
these ions. However, there are no known processes that can increase
iodine in carbonates, and in the case of CAS, meteoric diagenesis itself
imparts a negligible isotopic effect (Gill et al., 2008), however early
diagenetic processes can still impart isotopic signatures. While there
is an abundance of low I/(Ca+Mg) values recorded in our datasets, we
interpret these as predominantly primary seawater signatures as
significant diagenetic alteration cannot explain the very high I/(Ca+Mg)
ratios, some of the highest in the early Paleozoic (Lu et al., 2018),
recorded within the shallow marine patch reef facies of the western
Anticosti Island section. These high I/(Ca+Mg) values were recorded
during a lower stand of sea level in the Hirnantian, a stratigraphic
interval, and carbonate facies that would have been most susceptible to
extensive diagenesis (Fig. SI 3B). Studies of Cenozoic carbonates from
the Great Bahamas Bank have shown that iodine concentrations may also be
reduced during early diagenesis in carbonate settings (Hardisty et al.,
2017). Intervals that were affected by meteoric diagenesis contained
I/(Ca+Mg) values close to 0 μmol/mol (Hardisty et al., 2017), likely
reflecting alteration by reducing fluids rather than primary seawater
values. While it is possible that processes similar to these may have
contributed to lowering general iodine concentrations in carbonates from
this study, if fluid migration were to greatly affect primary
geochemical signals it would be in the units that would have originally
contained the highest porosity and lowest permeability (i.e. the
shallow-water facies). In other words, the Lamframboise Member–Ellis
Bay Formation, Anticosti Island and the Saldus Formation, Estonia, by
this prediction would have low I/(Ca+Mg) values. However, these
respective intervals within our carbonate successions contain the
highest I/(Ca+Mg) values, while the lowest values are found in
fine-grained carbonate and clay-rich facies where porosity and
permeability would have likely inhibited fluid migration. Additionally,
these Bahamian drill cores have shown other evidence for meteoric
diagenesis in these intervals with near zero I/(Ca+Mg) values, including
carbon isotopic signatures that are significantly depleted compared to
the original aragonitic sediments that passively record primary seawater
(Swart and Oehlert, 2018), whereas the Late Ordovician carbon isotopic
data from our study sections do not show these types of signatures even
surrounding intervals of glacioeustatic exposure (Brenchley et al.,
2003; Desrochers et al., 2010; Young et al., 2010; Jones et al., 2016).
Geochemical crossplots are also a widespread tool used to assess the
fidelity of geochemical signatures, where correlating trends with high
R2 values can point to mixing of primary marine signal
with those from diagenetic alteration. Recrystallization of carbonates
during diagenesis can yield δ13C,
δ18O, δ34S and I/(Ca+Mg) signatures
that deviate from primary seawater values, reflecting a mixture of
primary and secondary sources and producing linear or asymptotic
relationships among the geochemical parameters (Ahm et al., 2018; Swart
and Oehlert, 2018). Here we have cross-plotted
δ18Ocarb,
δ13Ccarb, [CAS],
δ34SCAS, and I/(Ca+Mg) datasets, and
show weak to no correlations, indicating that complete diagenetic
overprint is absent from our datasets (supplemental material, Fig. S3).
The only crossplots that show significant correlation are I/(Ca/Mg) vs
δ18Ocarb and
δ18Ocarb vs
δ13Ccarb from Anticosti Island,
however, these trends simply reflect two distinct data populations that
internally do not correlate. Further, clear trends that continue across
formational boundaries and major facies changes suggest that geochemical
signatures found in these successions are largely primary and contain
limited diagenetic alteration.
The likelihood of bulk CAS to faithfully record primary seawater sulfur
isotope values has previously been called into question. Bulk
δ34SCAS may incorporate sulfate from
both primary and secondary carbonate phases potentially leading to more
“noise” in isotopic datasets, while
δ34SCAS from well-preserved brachiopod
carbonate components in the same section show more invariant values
(Present et al., 2015). Unfortunately, brachiopods within the sections
presented here are relatively rare and thus not a viable option for
performing high-resolution component-specific CAS measurements. Studies
comparing early Cenozoic bulk CAS, planktonic foraminiferal CAS, and
authigenic barite found that while species-specific foraminiferal data
yielded vital effects up to ±1‰ versus barite, bulk CAS faithfully
follow the recorded changes in secular
δ34Ssulfate in both duration and
magnitude (Toyama et al., 2020; Yao et al., 2020). Additionally,
δ34SCAS records presented here across
multiple paleobasins on separate paleocontinents from variable
bathymetric depths show biostratigraphically, well correlated
first-order trends (i.e. the fall δ34S in the late
Katian and a return to heavier values within the late
Hirnantian–Rhuddanian) also suggest preservation of primary seawater
signatures (Fig. 5). While local diagenetic processes have may
influenced the δ34SCAS data, it is
very unlikely that each of the study sections would experience similar
early and late diagenetic histories that resulted in similar first-order
trends. These local diagenetic histories can more likely explain the
smaller magnitude variations (~2-4‰) within and between
δ34SCAS records from the study sites.