Drowning in Geochemical Data: The Good, the bad, and the Ugly

Geochemical databases are placing unprecedented amounts of geochemical data at the fingertips of professionals and students. How these data are being used is taking an increasingly important role in shaping our thinking about the Earth. Databases have helped to expose (and eventually kill?) some long- cherished myths, such as the idea of the well-homogenized upper-mantle reservoir, and and they have made geochemical data accessible to geophysicists and enabled them to look at geochemistry with fresh eyes, leading to genuinely new insights. Yet, their very accessibility also makes them "dangerous tools" in the hands of the inexperienced. Statistical treatment of masses of geochemical data without, or with excessive, filtering can yield all sorts of "answers" we would probably be better off without. Data that are severely flawed (due to alteration or poor analytical quality, errors in published data, or errors during data entry) might not be easily identified by, say, a geodynamicist. Other dangers stem from overrepresentation of over-sampled locations and the general, but faulty, assumption of random sampling of the Earth. We will show examples where raw downloads of data from databases without extensive screening can yield data collections where the garbage swamps the useful information. We will also show impressive but meaningless correlations, e.g. upper-mantle temperature versus atmospheric temperature. The lesson is that screening is necessary. On the other hand, sound database compilations now demonstrate that average incompatible-element concentrations in global MORB are two to five times higher than published estimates. This fundamentally changes 30-year-old geochemical mass balance estimates of the mantle. OIBs are fundamentally similar to MORBs but are isotopically shifted, on average, to more "enriched" values. Mantle geochemistry is now fully consistent with dynamic models of "whole-mantle" circulation, with the likely exception of a relatively small, reservoir probably hidden near the base of the mantle. Among the most important changes triggered by the new databases are the following: (1) Driven by the database community, the standards required for reporting geochemical data in publications are changing in fundamental ways. Whereas it was common practice, especially in high-profile journals, to treat the data supporting the claims made in the published articles as relatively unimportant technical details, journal editors now require much more careful geochemical data reporting that includes complete data in numerical rather than graphical representation, information on sample locations, analytical methods, and uncertainty estimates. (2) Given the ever-increasing production rate of geochemical data, comprehensive use of geochemical data without relational databases is, or will soon be, impossible.