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2017; Gernon et al., 2021). Increasing leading edge of continents by driving them by global changes in plate margin networks.
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87 Sr/ Sr ratios in Cenozoic marine lime- into advancing compressional states of sub- Increasing Sr/ Sr compositions in Cenozoic
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stones have been associated with decreases duction and collisions involving arcs and marine sediments have also been associated
in unradiogenic Sr flux related to lower continental blocks that favor crustal thicken- with glaciation (Palmer and Elderfield, 1985).
seafloor spreading rates (Van Der Meer et ing (Lee et al., 2013). Increases in crustal Snowball Earth deglaciation likely contrib-
al., 2014) and cooler ocean temperatures recycling and thickness identified in the zir- uted to the Paleoproterozoic and Neo-
(Coogan and Dosso, 2015), suggesting that con proxy data correlate with an increase in proterozoic Sr excursions (Sobolev and
the cause of increased Sr/ Sr ratios in the passive margin abundance (Bradley, 2008) Brown, 2019), but the data reviewed here
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oceans may be multifactorial. The ancient (Fig. 3C). These increases around the suggest that tectonism played a major role.
ocean crust record is in large part lost due to Proterozoic-Phanerozoic time interval also Collectively, the balance of these processes
subduction (Scholl and von Huene, 2009), correlate with a decrease in thermal gradi- is likely recorded in today’s continental rock
but our results suggest that increases in ents of high dT/dP metamorphism (Brown record by the great unconformities at the
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87 Sr/ Sr ratios in oceans have been strongly and Johnson, 2018). Collectively, these Precambrian-Phanerozoic and Archean-
influenced by a continental component. If patterns are consistent with superconti- Proterozoic boundaries (Windley, 1984;
the increases in the proportion of radio- nent break-up driving a greater proportion Peters and Gaines, 2012).
genic sources and sedimentary flux repre- of convergent margins into compressional
sent a coupled suite of processes as we advancing states and collisions with magma- BROADER IMPLICATIONS
contend, then they raise the question as to tism in thicker crust. High Sr/ Sr ratios in Our results suggest that increases in
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why convergent margin tectonism changed oceans during these periods are presumably 87 Sr/ Sr ratios in oceans occur when a
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during these time periods. Insight into this due to increases in the proportion of exposed greater proportion of continental crust is
issue comes from an examination of model- radiogenic sources and sedimentary flux thick and high, leading to increases in
ing and other proxy data sets to which we from the continents associated with a reorga- evolved felsic magmatism, radiogenic base-
now turn. nization of riverine drainage networks. ment exposure, and riverine sedimentary
The correlations between the zircon proxy flux. From a broader perspective, the results
SUPERCONTINENT PATTERNS AND data and processes involving subduction out- raise the important question of whether solid
SR FLUX lined above warrants an examination of the Earth processes play a fundamental role in
Modeling studies suggest that supercon- U/Yb ratio in the zircon data set. The U/Yb modulating global climate and atmospheric
tinent tenures should be marked by elevated ratio in zircon has been used as a proxy for oxygenation over geologic time scales. If the
temperatures in the underlying subconti- crustal reworking (Verdel et al., 2021) but correlations outlined above are representa-
nental mantle with convergent margins in may also reflect the amount of subducting tive of a global tectonic pattern as we con-
retreating states with arcs on thinner crust slab fluid addition in magmas, because U is tend, then the generation of continental relief
in outboard locations with respect to the a fluid-mobile large-ion lithophile element highlights the introduction of a significant
continents (Lenardic et al., 2011; Lee et al., (LILE; K, Sr, Rb, Cs, Ba, Pb, U) extracted silicate weathering sink associated with the
2013; Lenardic, 2016). Temporal consider- from slabs, and is, therefore, enriched rela- drawdown of CO and associated transi-
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ations based on multiple proxy data sets tive to HREE such as Yb (Barth et al., 2013). tion into periods of global glaciation
suggest that the lows in crustal recycling U/Yb increases correlate with the increases (Hoffman and Schrag, 2002) (Fig. 3). Uplift
and thickness identified in the zircon proxy in crustal assimilation and thickness we have associated with convergent margin tecto-
data overlap with the tenure of superconti- identified (Fig. 3C), consistent with a higher nism may therefore have further enhanced
nents over Earth’s history (Figs. 3A–3B) amount of crustal recycling and flux of sub- CO drawdown associated with the exhuma-
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(Bradley, 2011). For example, the low in duction fluid along a greater proportion of tion and weathering of rocks associated with
crustal recycling and thickness during the convergent margins during these periods. continental rifting and dispersal (Donnadieu
boring billion correlates with the tenure of We, therefore, conclude that the increases in et al., 2004; DeLucia et al., 2018). Continental
Nuna during a period dominated by high riverine Sr input into Earth’s oceans were uplift has also been previously postulated to
dT/dP metamorphism (Fig. 3B) and higher related to geodynamic changes in convergent be linked to steps in oxygenation of Earth’s
thermal gradients (Brown and Johnson, margin networks, which were required to atmosphere during the Paleoproterozoic and
2018). This pattern may reflect superconti- accommodate the birth and maturation of Neoproterozoic through enhanced erosion
nent insulation of the mantle (Brown and new ocean basins created by supercontinent and nutrient supply to the oceans, as well as
Johnson, 2018) associated with the devel- break-up and dispersal. These episodes were changes in the proportion of subaerial volca-
opment of hot back-arc environments associated with increased weathering and nism (Campbell and Allen, 2008; Gaillard et
(Hyndman et al., 2005) and a greater pro- erosion of radiogenic rocks along convergent al., 2011) (Fig. 3B). Oxygenation may have
portion of convergent margins in retreating margins and greater expanses of uplifted, fostered the decrease of CH , a potent green-
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states with arcs on thinner crust in outboard higher-elevation radiogenic crust found house gas (Fakhraee et al., 2019), while
localities (Roberts, 2013; Paulsen et al., along the leading edges of continents. uplifts along convergent margins promoted
2020; Tang et al., 2021). Increases in riverine Sr were likely amplified nascent glaciation in cooler, high-elevation
Supercontinent break-up, by contrast, by the exhumation of continental crust asso- habitats, providing further feedback (albedo)
should lead to a release of potential energy ciated with rifting (DeLucia et al., 2018), for a runaway global glaciation. The ultimate
stored in the underlying mantle (Lenardic, which is consistent with the general thesis drivers for these important steps in Earth’s
2016). Thermal release of the mantle induces supported here, that increases in riverine Sr evolution are controversial and likely
changes in the geodynamic state of the are primarily driven by tectonism induced involved a complex set of variables and

8 GSA TODAY | February 2022