A New Bracketing Approach to Determine Depositional Ages and Paleoclimate Markers by Combined U–Pb Geochronology from Cryptotephra and Diagenetic Carbonate Concretions

Important geological research questions rely on accurate and precise absolute radiometric ages for the deposition of sedimentary strata: sediment accumulation and evolution rates, paleoclimate and paleogeographic reconstruction, etc.

For areas lacking frequent volcanic ash layers many studies use detrital zircon (DZ) maximum deposition ages (MDAs) from high-n laser ablation datasets to approximate true deposition ages (TDAs). However, there is a continuing discussion about the correct approach, spanning from rock types to sample to the statistical data treatment.

We propose a bracketing approach to resolve some of these issues. It can be applied in areas that fulfill three criteria: an active magmatic arc upwind, well-developed paleosols, diagenetic carbonate nodules. If a magmatic arc provides volcanic zircon and floodplain deposits develop into paleosols over a long enough time, the soils can capture and retain the ashfall zircons (develop into cryptotephra). This has been shown to provide higher numbers of deposition age-appropriate zircons than fluvial sandstones (e.g. Joeckel et al. 2019). Zircons found can only be as old or older than deposition.

Carbonate nodules forming in the soils can only be as young or younger than sediment deposition. It has been shown that carbonate layers and nodules can be dated to the same precision as laser ablation zircon data (e.g. Gulbranson et al. 2022). Especially specific early diagenetic carbonate phases hold high promise to provide precise dates because they can capture Uranium (see Ludvigson et al., this meeting).

This bracketing concept provides simple logical tests for DZ results and improves chronostratigraphic interpretations.