GLOBAL COMPILATIONS OF ZIRCON GEOCHEMISTRY: A BIASED VIEW INTO THE PROTEROZOIC–PHANEROZOIC TRANSITION

Global compilations of zircon initial Hf (εHft) and δ¹⁸O have been used to propose that extreme erosion associated with Neoproterozoic Snowball Earth lubricated subduction zones resulting in a change in the style plate tectonics. These studies relate negative εHft values and positive δ¹⁸O values from averaged global compilations to an increase in subduction of sediment and sedimentary rocks. However, this interpretation is inconsistent with modern low rates of erosion in Antarctica, the preservation of thick packages of Tonian and Cryogenian preglacial sedimentary sequences, and slow sedimentation rates inferred from Cryogenian glacial deposits. Here we reanalyze newly georeferenced global zircon geochemistry data sets to interrogate the data density in a spatial and temporal framework. We tie the data to GPlates to better understand geologic context of the extreme negative εHft and positive δ¹⁸O values that we hypothesize bias the global geochemical record. Results show that zircon geochemical records are significantly biased throughout Earth history. In particular, geochemical trends at the end of the Proterozoic are starkly different depending on geography, dominated by regions with higher data density due to both zircon production, preservation, and sampling. The latest Neoproterozoic negative excursion in zircon eHft is restricted to Gondwana, which we attribute to the introversion of Rodinia and collisions associated with supercontinent assembly involving thickening and melt extraction from Archean and Paleoproterozoic crust. The slight increase in δ¹⁸O is limited to data from Australia and South America, which is the result of extreme values from a select few local studies. The Late Neoproterozoic zircon δ¹⁸O values increase to early Tonian values after a large negative anomaly that is an artifact from anomalous altered late Tonian rocks in the Dabie Sulu orogen. A similar artifact is present in the near modern record where abundant data from the North American West bias the global composite to zircon δ¹⁸O values lower than mantle values. We find no evidence for increased sediment subduction associated with Snowball Earth. Instead, we associate secular changes in global averages to the topology of supercontinent assembly and bias from oversampled regional anomalies.