Part I

My first, and abiding, interest in complex nonlinear dynamics arose in an effort to explain the extensive spatial variability in geomorphic and pedologic phenomena often found within short distances and small areas, in the absence of measurable variations in explanatory factors. Dynamical instability and chaos, whereby minor variations in initial conditions or effects of small, local disturbances become exaggerated over time, can explain this phenomenon. We have had considerable success over the past 25 or 30 years in this regard.

Soil profiles exposed on the Neuse River estuary shoreline, Croatan, N.C. Complex local spatial variability--despite uniform parent material--is evident. Dynamical instability and chaos in pedogenesis of the these soils was demonstrated nearly 25 years ago.

Evolution (I use the word here in its most general sense of long term historical development) of Earth surface systems is historically contingent and path dependent. This seems to be true of evolution of anything, but I will stick here to my supposed areas of expertise. The state of an Earth surface system (ESS; a landscape, ecosystem, etc.) is a function of generally applicable laws that ultimately determine the range of possibilities, geographically specific place factors (environmental constraints and opportunities), and history. While laws are general, if not universal, and apply to every ESS of a given type (e.g., stream channel, cave, mangrove swamp, soil profile, etc.), the place factors define the template in which those laws operate.

And then there is history.

The editor of Soil Science, Daniel Gimenez, known for his work on complex nonlinear dynamics and fractals in soils, recently suggested that I write a review paper for the journal updating my ideas on complexity in pedology and pedogenesis. It was an interesting challenge that had not otherwise occurred to me, and I'm glad I did it. The result was recently published as:

Phillips, J.D., 2017. Soil complexity and pedogenesis. Soil Science 182: 117-127 (or full text version here).

The abstract is below:

I just finished reading Paul Bogard's The Ground Beneath Us, (I recommend it), which among other things warns us yet again about the serious issues--environmental, economic, public health, food security--associated with over-reliance on chemical and fossil-fuel intensive industrial agriculture. It's a good 40-years-later follow-up to Wendell Berry's classic Unsettling of America: Culture and Agriculture (Sierra Club Books, 1977).

It also reminded me of a much more technical and difficult book I read a few years back, Jozef Visser's Down to Earth, subtitled "A Historical and Sociological Analysis of the Rise of 'Industrial' Agriculture and the Prospects for the Re-rooting of Agriculture in the Local Farmer and Ecology. Visser, who has graduate degrees in chemistry and a long career in agricultural chemistry, returned to graduate school later in life to produce this book, which is his dissertation from the University of Waginengen (Netherlands). A pdf is available free at the link above, and I recommend it.

Back in 1814, Pierre-Simon Laplace published a classic statement on causal determinism in science. If someone (a hypothetical or metaphorical demon, though Laplace's Demon is apparently a later embellishment; Laplace himself did not use the term) has perfect knowledge of the exact location and momentum of every atom in the universe, their future (and past) values at any time can be perfectly determined from classical mechanics.

The biogeomorphic impacts of organisms may differ at different stages in the development of landforms, ecosystems, or the individual organisms. I was thinking about this recently here along the shoreline bluffs of the Neuse River estuary, North Carolina, where I have been both looking at some soil profiles and enjoying the coastline.

There are at least five distinctly different biogeomorphic roles trees play along this shoreline--many more if you wanted to get more specific within these categories. The specifics are probably of only limited applicability elsewhere, but the general principle--multiple effects, which vary at different stages of both landform and vegetation development--is widely valid.

Trees and other vegetation grow thick and fast in this moist subtropical climate.

Stage 1A Surface Bioprotection

Trees (including canopy, roots, and litter) protect the ground surface from erosion and add organic matter to soil.

In studies of soil formation and landscape evolution, we often think in terms of a (over-) simplified "conveyor belt" model, where bedrock is weathered to create the raw material for soil formation at the base. Further up toward the ground surface, this weathered rock is progressively modified into soil. Thus, as you go from the base of the soil or weathering profile, material gets progressively more modified, and (in terms of soil rather than rock), older.

Anyone who's spent time in more than a few soil pits or road cuts knows that the conveyor belt is, at best, a loose approximation and often hardly applicable at all. Variations in properties of the rock or parent material, dynamical instabilities and positive feedbacks in weathering and other pedogenetic processes work in many cases to create increasingly variable and heterogeneous (both vertically and horizontally) regoliths over time. Critical processes operate in all directions (not just vertically), and moisture fluxes and biological activity follow preferential, self-reinforcing paths. Further, mass is added not just from weathering, but from deposition and organic matter, and removed by erosion, leaching, fire, and decomposition.

The attached paper (In Defense of Metanarratives:Extremal Principles, Optimality and Selection in Earth Surface Systems) was originally written in early 2015 and revised in April 2015, as an invited paper for a special issue of a geography journal. By mutual agreement with the guest editors, I withdrew the paper after deciding that I was unwilling/unable to satisfy some of the major recommendations of reviewers. The major, but by no means only, issues were that referees and guest editors felt I should more fully address history and philosophy of science issues and parse the definitions of principles, theories, narratives, etc. I felt that I could say what I was trying to say without getting into that stuff, which would have taken a lot of work on my part that would have seriously inhibited my studies on the (to me) far more interesting and important topics of how Earth surface systems actually work. After sitting on it for two years, and publishing bits and pieces of the ideas on optimality and selection in other contexts (but not the metanarratives part) I concluded that I am unlikely to ever resubmit it anywhere. But I did put a lot of work into writing the damn thing, so I am posting it online, for what it is worth.

Biogeomorphology considers the impacts of organisms on surface processes and landforms (e.g., biological weathering, effects of burrowing animals), and vice-versa (e.g., the role of landforms as habitat, effects of erosion on biota). We are particularly concerned these days, however, with reciprocal interactions, such as sediment trapping by vegetation, and the feedback effects of this deposition on plants and their habitat. We are also learning a lot about biogeomorphic ecosystem engineering (BEE), whereby biota modify the abiotic environment in ways that influence habitat or resources (positively or negatively) for themselves or other species, and biogeomorphic niche construction, where BEE effects influence selection pressures and biological evolution.

Some recent theoretical and modeling work on regolith and the so-called critical zone draws a distinction between the entire thickness of weathered material H and the mobile thickness h that is potentially (re)movable by erosion and mass wasting. As H > h this implies that in many cases there exists a subsurface immobile layer.

This distinction between a potentially laterally mobile and a fixed layer of weathered material is no doubt useful as a model assumption. It is also probably true, or close enough, for some very thick weathered mantles. And of course, the mobile:immobile distinction is self-evidently true during periods when a portion, but not all, of the regolith thickness is being transported.

Regolith in the flysch Carpathians, Czech Republic. Lighter upper layer is a landslide deposit. Lower material (the black layer is charcoal, apparently from a fire at some point) is weathered in place. It is all potentially mobile.