REVERSE WEATHERING IN SUBAQUEOUS, TROPICAL DELTAS

Reverse weathering was hypothesized by Mackenzie and Garrels in the mid-1960's as a likely component of elemental cycling in the oceans. Numerous examples of the neoformation and reconstitution of clay minerals at low temperatures in a range of marine sedimentary deposits have since been documented; however, it is still often assumed that marine clay authigenesis requires thousands of years and that the quantities formed are of minor significance. Over the last ~20 years, a broad spectrum of field measurements, laboratory experiments, and diagenetic models has demonstrated that rapid formation of authigenic clays takes place in the major tropical deltaic systems of S. America (Amazon - Guianas mudbelt) and Oceania (Gulf of Papua deltaic complex). Authigenic aluminosilicates in these deposits are typically Fe-rich and form in suboxic, nonsulfidic deposits underlying oxygenated waters over periods of months to years. The mobile deltaic topset and associated coastal deposits act as diagenetic batch reactors in which reactive Fe, Al -oxides supplied from tropical drainage basins are mixed with labile organic matter, biogenic silica, and seawater solutes. Sedimentary transport dynamics determine the magnitude and frequency of reworking, exposure, and reoxidation of debris, and are critical to maintaining suboxic diagenetic conditions, episodically entraining reactants, and promoting clay formation. Pore water distributions and solid phase compositions demonstrate that early diagenetic clay formation in tropical deltas can be major sinks for K⁺, F^-, biogenic Si, and probably Li⁺ and Mg²⁺, although regional differences in reactants, products, and sedimentary regime complicate global extrapolations. Depending on the exact assumptions, conservatively at least 10 - 20 % (range to > 60%) of the dissolved riverine delivery of these elements is stored in subaqueous, tropical deltas. Reports of rapid authigenic Fe-rich clay formation in many other high sedimentation rate nearshore regions, including the Mississippi delta and coastal deposits of Hawaii and Panama, confirm that reverse weathering is a very general and important early diagenetic process.