DO ZIRCON OXYGEN ISOTOPE COMPILATIONS DOCUMENT GLOBAL CLIMATIC AND TECTONIC EVENTS?

Zircon oxygen isotope ratios (δ¹⁸O) are commonly used as a proxy for the amount of supracrustal material incorporated into igneous melts and have been used to support models of global change driven by both climate and tectonics. Studies have taken temporal isotopic trends in global compilations as evidence for the inception of plate tectonics, the evolution of land plants, and outsized erosion. However, many studies do not incorporate geologic and geographic context, and instead depend on running averages or resampling methods. We interpret a newly georeferenced zircon δ¹⁸O data compilation including 381 published contributions of paired zircon U-Pb and δ¹⁸O analyses; we geospatially analyze 9,388 values from 1000 Ma to present and reconstruct their positions at time of formation in a continuous GPlates model. Results reveal stark differences in zircon δ¹⁸O signals preserved on different continents due to spatial, temporal, and sampling bias. The compilation shows positive and negative isotopic excursions, both of which have been interpreted as global phenomena. Sensitivity testing demonstrates that these apparent isotopic excursions are the product of disproportionate sampling of regional tectonic settings with distinctive zircon δ¹⁸O compositions. Low Tonian-Cryogenian δ¹⁸O previously interpreted as a signal of glacial meltwater, is related to ultra-high-pressure rocks of the Dabie Sulu orogen in central Asia, Egyptian ophiolites, and juvenile arc magmatism in Madagascar. Positive isotopic excursions in the Ediacaran-Cambrian and Devonian, previously interpreted to record Snowball Earth erosion and the rise of land plants, are the result of supracrustal sources of Gondwana. Negative isotopic values in the Neogene are the result of hypabyssal volcanic rocks associated with Yellowstone and Iceland and serve as a cautionary tale of bias that we can observe in the modern. Sampling bias in is analogous to Lagerstätten (sites of exquisite fossil preservation) and the “monograph effect” (outsized contributions from individual researchers), both of which result in artifacts of over-representation. Long-term zircon δ¹⁸O trends may broadly track the generation of granites in collisional settings and continental arcs through time, recording the melting of sediments and sedimentary rocks in individual orogens, and not flux of sediment subduction globally.