PHANEROZOIC SEDIMENT CYCLING, SEAWATER CHEMISTRY, CARBONATE MINERALOGY, AND CONTINENTAL DENUDATION - FRED WAS FIRST

Fred Mackenzie deserves full credit for several notions that are now central to current collective understanding of the evolution of continents and oceans. We briefly explore three of these. Mackenzie pioneered the numerical evaluation of rates of sedimentary rock cycling from mass-age data and formulated those relations to be anticipated for a constant or growing sedimentary reservoir. An example resides in recent attempts to interpret deep-sea sediment fluxes as records of both climate change and the evolution of planktic calcifiers. Although there is substantial question concerning past rates of seafloor generation and destruction which serves to exacerbate incertitude in deep sea sediment fluxes, the two plausible end-member scenarios for deep-sea accumulation are little different from “constant mass” and “linear accumulation” models proposed by Mackenzie in the early 1970s. Mackenzie was among the first to recognize that ocean chemistry evolves in concert with those major tectonic cycles that also shape the surface of the Earth. More importantly, he recognized that space and time are interchangeable; secular differences among carbonate phases in the rock record are also apparent as spatial differences in modern oceans; “calcite” and “aragonite” seas are perhaps best perceived merely as attributes of carbonate/chemical “facies” that migrate through space with time. Mackenzie was among the first quantitative hydro-geomorphologists. Although much current research is carried out in major orogens, rates of erosion across high mountain ranges may not even run a close second to rates of denudation imposed through human activity. The validity of this statement resides in quantitative representation of natural and anthropogenic rates of landscape change; Mackenzie developed the most important of these early in his academic career.