Likeness among detrital zircon populations—An approach to the comparison of age frequency data in time and space
The advent of new procedures for the rapid collection of large amounts of detrital zircon age data has created significant challenges to the representation, quantitative interpretation, and analysis of these results. Principal among these is the development of the most efficient and informative methods to accurately present and quantitatively compare numerous age data from large numbers of samples in a statistically valid, geologically meaningful, and easily assessable manner. Many detrital zircon studies have adopted a subjective-graphic approach to these tasks, primarily through the examination of vertically stacked probability density plots (PDPs). Although this methodology allows for quick visual assessment of changes in zircon age frequencies in space and/or time, it is fundamentally qualitative, and becomes rather impractical as numbers of analyses become large. More quantitative approaches to the comparison of PDPs (e.g., Kolmogorov-Smirnov test and multi-dimensional scaling) have also become more popular, but are rather computationally intense. In order to examine degrees of sameness among detrital zircon age populations in geographic and/or stratigraphic contexts, we develop a “likeness” metric that quantifies the degree of overlap between pairs of PDPs. We compare this metric to several others that have been proposed previously, and then evaluate its usefulness in describing source-to-sink changes in sample age populations by analyzing data from four published studies of detrital zircon age frequencies. Likeness among 26 Paleozoic samples from the Grand Canyon (southwestern United States) decreases with increasing stratigraphic separation, but stratigraphically contiguous sample likeness exhibits statistically insignificant upsection change. The degree to which these patterns are representative of other stratigraphic successions awaits further evaluation. Likenesses among 15 samples of modern fluvial sand from the Amazon River (South America), 11 stream samples from the French Broad River in the southern Appalachians (eastern United States), and 10 Quaternary coastal sand samples from eastern Australia decrease with increasing distance between samples; as might be expected, greater geographic disjunction results in greater dissimilarity. In contrast, likenesses among spatially contiguous fluvial and littoral sample ages all exhibit trends reflecting a greater down-system increase in likeness. These results suggest that sediment grain ages may exhibit increasing homogenization with transport in both fluvial and littoral systems. The extent to which these source-to-sink changes may characterize lateral variations in other sedimentary systems also awaits additional assessment. Nonetheless, each of these four examples demonstrates that the determination of sample-pair likenesses, and evaluation of their geographic and/or stratigraphic variation, serve to effectively highlight those aspects of differences in detrital zircon age frequencies that ultimately record the histories of geologic and geomorphic processes in both space and time.