WE KNOW WHERE THE SEDIMENT CAME FROM – SOMEONE SHOULD TELL THE ZIRCONS

In the last decade there has been an exponential rise in the use of detrital zircon geochronology as a provenance tool. While detrital zircon ages (DZA) can, under favorable circumstances, provide significant constraints on provenance there are now several examples based on Modern sediment that suggests DZA may not be as diagnostic of source regions as previously thought. The most common example of bias in the zircon age record from Recent North American sediment is the near ubiquity of 1.2 – 1.0 Ga zircon, usually far in excess of the areal extent of exposed Grenville crust. This is the case for detrital zircon in alluvium from the French Broad River (N. Carolina and Tennessee) where exposure of Grenville crust makes up about 12% of the watershed area yet in most sections of the river the DZA are dominated by Grenville age peaks. Even more significantly, however, is that even in areas with virtually no Grenville crust exposed (e.g. Mississippi watershed) there are tributaries with close to 100% Grenville DZA. This significant bias is largely controlled by the extreme zircon fertility of Grenville granitoids – the exceptionally high Zr content of Grenville intrusions manifests itself in both extremely abundant and large zircon crystals that can be multiply recycled.

However, even in areas where sediment is derived virtually in situ there is a disconnect between the ages of zircon from bedrock and the local sediment. For example, in the Stepladder Mountains of SE California the area is virtually a point source of ~ 147 Ma granite, yet the sediment derived from the area, essentially within feet of the bedrock, has at most an overlap of 40% of DZA with pluton crystallization ages. Thus even though the sediment is forming on the source the sediment provenance would be incorrectly identified. The cause of this discrepancy, which we refer to as the “Stepladder Effect”, may be the result of aeolian contamination, of excessive xenocryst contribution, or some unidentified cause. Regardless, these results are a significant warning that DZA are not the ‘silver bullet’ of sediment provenance and multiple proxies should be employed for more robust identification of sediment source regions.