Reading the Walls of the Unfinished Cathedral: Insights to the Dynamic Denudation of Bermudan Eolianites and Paleosols.
Donald L. Johnson, Univ of Illinois, 713 S Lynn St., Champaign, IL 61820 and Diana N. Johnson, Geosciences Consultants, 713 So. Lynn St., Champaign, IL 61820.
Bermuda is a submerged mid-Tertiary volcanic seamount topped with cemented Quaternary calcareous eolian sand – eolianites, and shelly marine deposits. Five eolianites are recognized that are separated by Terra rossa-like, reddish-brown paleosols, enriched in eolian dust, that represent long time-periods. Weakly expressed, light-colored paleosols – called protosols, also occur and represent short time-spans. (Our definitions of soil and paleosol are: soil is lithic and/or organic material at or near the surface of planets altered by biological, chemical, and/or physical processes; paleosols are buried soils.) This study is based principally on morphological criteria displayed in numerous roadcuts, quarries, construction materials (brick-like stones cut from eolianite), and natural exposures (sea cliffs). Our interpretations are guided by the explanatory power of process vector analysis and biomantle theory, both central to dynamic denudation theory -- an alternative to conventional approaches for explaining landscapes (see Reference). Process vector analysis is a useful graphic, conceptual, semiquantitative-quantitative device that weights the relative effects and/or rates of two or more coacting processes. It is applicable to any science, but is particularly useful in providing explanatory insights to complex processes in soils. Its usefulness is maximized when joined with biomantle principles. A biomantle is the differentiated zone in the upper part of soils produced largely by bioturbation, aided by subsidiary processes (i.e., those usually emphasized in conventional genetic models -- shrink-swell, leaching-precipitation, eluviation-illuviation, biochemical transformations-translocations, etc.). A great many organisms bioturbate, at all scales, and they do so via four process styles: upward biotransfers, biomixing, cratering, and volume increases. All four styles operate in Bermuda, though comparatively few organisms are involved. Those bioturbationally dominant are multiple species of land crabs, land snails, ants and worms (endemics uncertain), a former tortoise, and various plants. Their collective impacts are documented in Bermudan eolianites and paleosols. The process promotes either one-layered biomantles -- those morphologically isotropic with regard to particle size, where bioturbated particles form a single layer, or two-layered biomantles – those morphologically anisotropic with regard to particle size, where bioturbated particles form two layers, the lower being a stonelayer of gravels (i.e., stone-line, nappes de gravats, etc.). For most soils the biomantle comprises their bioturbated upper part, the topsoils of grassland soils, the A and E horizons of forest soils, and if present their stonelayers. For certain soils the biomantle comprises the whole soil (entire soil is bioturbated). Bermuda paleosols and its surface soil qualify. The main genetic vectors are bioturbation and biochemical dissolutions and re-precipitations of calcium carbonate via leaching. Leaching is powered by organic acids that move downward, sometimes as “wetting fingers” that produce soil-filled vertical pendants, some with bioturbation signatures preserved within them. Visitors to Bermuda are confronted everywhere by stratified eolianites, and some contain paleosols, some with pendants. Where the stratified deposits lie in proximity to paleosols they often display bioturbationally-destratified zones, above and/or below the soils. The destratified zones may range in thickness from decimeters to a meter or more. And insofar as most buildings are made of bricks cut from eolianite, the bricks themselves show progressions from perfect geologic stratification to those totally bioturbated, with all intermediate stages (percentages are easily quantifiable). Excellent examples are displayed in the walls of the historic Unfinished Cathedral in St. George Parish, whose eolianite bricks exemplify the interpretive utility of the process vector method. As a philosophical footnote to our assessment, the way one interprets soils and sediments is very largely influenced by the theories and concepts employed. Theories guide and filter our observations, and they influence and prioritize the questions we ask. They also influence how we set up and strategize our research programs, and invariably they impart, consciously and unconsciously, a great influence on our conclusions – as our readings of the walls at St. George's confirm.Reference: Johnson, D.L., J. Domier, and D.N. Johnson. 2005. Reflections on the nature of soil and its biomantle. Annals, Association of American Geographers 95 (1):11-31.