THE GEOMORPHOLOGICAL VOICE OF THE BIOSPHERE
Vernadsky (1926) developed the concept of the biosphere as a planetary membrane that captures, stores, and transforms solar energy. The proportion of solar energy captured by the biosphere is small compared to that represented by climate processes, but large compared to other energy sources for landscape processes A tiny fraction of net primary productivity doing pedologic and geomorphic work (e.g., bioturbation, bioweathering, bioerosion, organic matter formation) is (as a global average) a greater energy input for landscape evolution than geophysical processes (Phillips, 2009a).
The soil and the biosphere have been characterized as an “excited membrane” or skin at the planetary surface stimulated by solar energy (Vernadsky, 1926; Nikiforoff, 1959). Can other aspects of landscapes—particularly landforms and topography—be characterized as an “excited membrane?”
Since Vernadsky (1926), many others have noted that Earth’s atmosphere is in chemical disequilibrium. Its composition is maintained by photosynthesis, respiration, and other biospheric processes. Organisms are not a necessary condition for an atmosphere to exist, or for most of the atmospheric constituents of Earth’s atmosphere to exist. However, the current general nonequilibrium chemical composition, which has remained in approximate steady-state for >2 Ga, is inseparably connected to the evolution of, and maintenance by, the biosphere. But while Earth’s atmosphere contains the signature of a biosphere, it cannot be said to represent any particular taxa.
Present composition of Earth’s atmosphere, which has been pretty consistent for the past 2 billion years.
Biotic influences on surface processes, landforms, and soils are pervasive, and tightly-coupled reciprocal interactions are reflected in plant-soil interactions, biogeomorphology, ecosystem engineering, and niche construction. At least some soils and landforms can be considered extended (composite) phenotypes (Phillips, 2009b; 2016). Similar to the atmosphere, Earth would have landforms and regolith in the absence of a biosphere (and did before the appearance of life). Some soil and landform elements that would exist on an abiotic surface are present. However, contemporary soils, landforms and landscapes in their current state would not exist without biota.
Section of the meandering Sabine River on the Louisiana/Texas border. Landforms such as this, both in particular and in general, have most emphatically not been consistent for the past 2 billion years!
However, even as species, communities, and ecosystems have been in long-term evolutionary flux, the atmosphere has been maintained in an approximate steady-state. This is clearly not the case for soils, landforms, and topography on geologic time scales. Global revolutions since the Archean have profoundly and irreversibly changed the biosphere, pedosphere,and toposphere, while modifications of atmospheric composition, and of the general global hydrosphere, have been moderated and have not been irreversible (Lenton and Watson, 2011).
In the atmosphere and hydrosphere, biotic changes are rapidly propagated throughout due to their global interconnectivity, at velocities of fluid flows measured in m sec-1. Soils and landforms, by comparison, are interconnected at more restricted spatial scales—subcontinental at the largest, and not infrequently over areas on the order of 100 m2 or less. Propagation of changes sometimes occurs at fluid velocities, but other processes (e.g., weathering and denudation) have rates often measured in units of m yr-1 to m ma-1. Soils and landforms therefore have much longer response and relaxation times to biologically driven change than the atmosphere-ocean system, and impacts are local and regional rather than global. One implication is that landforms and soils have a much richer “memory” of biosphere change (independently of whatever fossils they may contain) than the other spheres.
Phillips (2016) speculated that the toposphere and pedospere locally absorb most of the environmental effects of biota, thereby buffering the atmosphere, hydrosphere, and lithosphere from major changes. There are examples of pedological and geomorphological change producing negative feedbacks to mitigate changes in composition of the atmosphere/hydrosphere. Biologically driven shifts in atmospheric carbon dioxide concentrations result in climate change, which in turn accelerates or decelerates silicate weathering rates, either absorbing or releasing atmospheric CO2 to offset the biotically induced changes (e.g., Berner, 1992; Kump et al., 2000; Malkowski and Racki, 2009).
This raises some interesting questions with respect to Earth system evolution. The bio, litho-, atmo-, hydro-, topo-, and pedospheres coevolve at the global scale. Major biotic events have driven revolutions in the other spheres (Lenton and Watson, 2011), but the atmosphere and the global hydrological system seem to have been relatively steady-state at the global scale. The toposphere and pedosphere have not, and display substantially more spatial variability in responses than oceanic or atmospheric composition. This suggests that perhaps landforms and soils provide the major mechanisms or degrees of freedom by which Earth responds to biological evolution, at least within the context of the permanently oxygenated atmosphere and ocean that have existed for the past 2.4 Ga. There is some evidence to support this with respect to the carbon cycle and feedbacks among ecological processes, atmospheric and ocean chemistry, biotically-enhanced weathering, and soil and sedimentary carbon storage or release (Huggett, 1991; 1995; Lenton and Watson, 2011). Landforms and soils may thus be the “voice” of the biosphere as it authors planetary change, even if or when clear biotic signatures are lacking.
Berner RA. 1992. Weathering, plants, and the long term carbon cycle. Geochimica et Cosmochimica Acta 56: 3225-3231.
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Lenton T, Watson A. 2011. Revolutions That Made the Earth. Oxford University Press.
Malkowski K, Racki G. 2009. A global biogeochemical perturbation across the Silurian-Devonian boundary: Ocean-continent-biosphere feedbacks. Palaeogeography Palaeoclimatology Palaeoecology 276: 244-254.
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Phillips, J.D., 2016. Landforms as extended composite phenotypes. Earth Surface Processes & Landforms 41: 16-26.
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Vernadsky VI. 1926. The Biosphere. 1998 edition translated from Russian by D.B. Langmuir, edited by M.A.S. McMenamin and L. Margulis. New York: Nevramont.
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