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Continental Magmatism and Uplift as the

Primary Driver for First-Order Oceanic

87 Sr/ Sr Variability with Implications for
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Global Climate and Atmospheric Oxygenation

Timothy Paulsen, Dept. of Geology, University of Wisconsin Oshkosh, Oshkosh, Wisconsin 54901, USA, paulsen@uwosh.edu;
Chad Deering, Dept. of Geological and Mining Engineering and Sciences, Michigan Technological University, Houghton, Michigan
49931, USA; Jakub Sliwinski, Institute of Geochemistry and Petrology, Dept. of Earth Sciences, ETH Zurich, Zurich, 8092,
Switzerland; Snehamoy Chatterjee, Dept. of Geological and Mining Engineering and Sciences, Michigan Technological University,
Houghton, Michigan 49931, USA; and Olivier Bachman, Institute of Geochemistry and Petrology, Dept. of Earth Sciences, ETH
Zurich, Zurich, 8092, Switzerland

ABSTRACT 1983). According to standard models, conti- subsequently risen in association with the
Oceans cover 70% of Earth’s surface, set- nental crust is primarily formed by fluid differentiation of the crust, with rapid
ting it apart from the other terrestrial planets flux melting in the mantle wedge above increases during two principal intervals in
in the solar system, but the mechanisms driv- subducting hydrated oceanic plates as they the Precambrian, namely in the Paleo-
ing oceanic chemical evolution through time are recycled into the mantle. This is then proterozoic and Neoproterozoic (Shields
remain an important unresolved problem. followed by fractional crystallization of and Veizer, 2002) (Fig. 1A). Identifying
Imbalance in the strontium cycle, intro- mantle-derived magmas and/or partial potential drivers for these shifts in marine
duced, for example, by increases in conti- melting of preexisting crustal lithologies 87 Sr/ Sr ratios during the Precambrian is of
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nental weathering associated with mountain (Hawkesworth and Kemp, 2006; Moyen et widespread interest because of possible
building, has been inferred from shifts in al., 2021). Collectively, these “distillation” links to major perturbations in the global
marine carbonate Sr/ Sr ratios. There are, processes have led to the development of carbon cycle and hypothetical connections
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however, uncertainties about the spatial and a more felsic crust with a significant to changes in tectonism, climate, and
temporal patterns of crustal evolution in enrichment of incompatible elements, such atmospheric-oceanic oxygenation (Shields,
Earth’s past, particularly for the period lead- as rubidium and strontium, with respect 2007; Campbell and Allen, 2008; Sobolev
ing up to the Cambrian explosion of life. to the mantle as Earth has aged (Veizer, and Brown, 2019).
Here we show that U-Pb age and trace ele- 1989; McDermott and Hawkesworth, 1990). The average Sr isotopic composition of
ment data from a global compilation of detri- However, the questions of how the conti- the present oceanic crust is relatively uni-
tal zircons are consistent with marine car- nental crust has evolved chemically over form (~0.703), but the Sr isotopic composi-
bonate Sr/ Sr ratios, suggesting changes time and how it has influenced Earth’s tion of today’s continental crust (~0.73 on
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in radiogenic continental input into Earth’s oceans and atmosphere remain as funda- average) is spatially highly variable (~0.703
oceans over time. Increases in riverine Sr mental unresolved problems. to >0.73) due to a heterogeneous rock record
input were related to the break-up and dis- Earth’s present oceans have a uniform that includes juvenile and ancient, evolved
persal of continents, with increased weather- Sr isotopic composition ( Sr/ Sr = 0.7092) crustal components (Veizer and Mackenzie,
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ing and erosion of a higher proportion of that primarily reflects the balance between 2014). The average Sr isotopic composition
radiogenic rocks and high-elevation conti- radiogenic Sr input from weathering of the of today’s rivers (~0.711) reflects a balance
nental crust. Tectonic processes exert a continents and unradiogenic Sr input from of the weathering of such sources on a
strong influence on the chemical evolution of hydrothermal alteration of oceanic crust global scale (Veizer and Mackenzie, 2014),
the planet’s oceans over geologic time scales (Veizer and Mackenzie, 2014). Although but the dynamic nature of the solid Earth
and may have been a key driver for concomi- 87 Sr/ Sr ratios in marine carbonates are has likely led to changes in the proportion
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tant increases in atmosphere-ocean oxygen- better documented for Phanerozoic versus of radiogenic rocks being weathered on
ation and global climate cooling. Precambrian marine limestones, oceanic Earth’s surface over time. This notion is
87 Sr/ Sr ratios appear to have departed from supported by recent analyses of a global
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INTRODUCTION mantle values as early as ca. 2.8 Ga (Shields detrital zircon database, which have led to
Planetary differentiation has led to two and Veizer, 2002) (Fig. 1A). This transition the conclusion that, at least for the past 1.0
fundamental types of crust on Earth: (1) has been interpreted to mark a change Ga, increases in the Sr/ Sr ratios recorded
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continental, which tend to survive over long from mantle- to river-buffered oceans as in marine limestones generally coincide
periods acquiring ancient rock records and the continents rose and hydrothermal with decreases in the εHf composition of
evolved compositions; and (2) oceanic, circulation of oceanic crust decreased as zircons produced by increased magmatic
which tend to be juvenile and rapidly recy- heat dissipated from Earth with time (Veizer reworking of preexisting radiogenic crust
cled by subduction (Campbell and Taylor, and Mackenzie, 2014). Sr/ Sr ratios have (Bataille et al., 2017). Decreases in zircon
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GSA Today, v. 32, https://doi.org/10.1130/GSATG526A.1. CC-BY-NC.

4 GSA TODAY | February 2022