Paleoenvironmental Archives in Rock Rinds and Sand/Silt Coatings

Abstract

Organo-mineral chemical coatings in rock rinds and sands and silts, underappreciated research until recently, offer a wealth of paleoenvironmental records derived from chemical and biomineral weathering over varying time frames. Rock rinds as well as sands and silts in paleosols carry various detailed records ranging from thin weathered films in young (Holocene) samples to thicker, often overprinted records in recycled sands/rinds of Pleistocene and older age, some as old as Miocene and Late Oligocene. Whereas rinds on Earth are confined to the Late Paleogene/Neogene, similar examples on Mars extend into the Noachian (∼4 Ga). New research outlined here shows that coatings tend to produce altered microsurfaces conducive to microbe growth with fossil forms found in a variety of different microenvironments, including extreme polar and tropical locales. Organo-chemical coatings on sands in paleosols reveal a wealth of biomarkers important in paleoenvironmental reconstruction from pole to equator. Rock rinds, revealed here, are shown to carry weathering zones in microcosm that often bear a close similarity to juxtaposed paleosols, the latter carrying mature A/B/C/Cu horizonation, indicating that weathering forces affecting a substrate are fine-tuned to similar kinetic forces acting on clasts in surface deposits. Aeolian and cosmic airburst inputs to rock rinds and chemical reconstruction of Si/Al wreckages leading to clay mineral reconstruction are also found in some rind and paleosol records. Analyses of such coatings using the light microscope, scanning electron microscope/energy dispersive spectrometer, focused ion beam, and secondary ion mass spectrometer offer the prospect of revealing still more secondary organo-mineral compounds, whose relation to primary mineral weathering is of importance to understanding pedogenic and diagenetic processes. The sum total of environmental change over time, written or embedded in rinds and sand/silt coatings, when subjected to high-resolution microscopy offers a novel mineral archive at the nanometer level and above.