In honor of World Soil Day (December 5), a post about swamp soils, even though the theme this year is "Healthy Soils for Healthy Cities."
In 1984, the great soil geomorphologist Raymond Daniels and colleagues at N.C. State University published a bulletin for the N.C. Agricultural Research Service called Soil Systems of North Carolina. In it, they noted that the Dorovan soil series, an organic muck or mucky peat found along the lowermost reaches of coastal plain rivers, likely marked the soil effects of rising sea-level gradually encroaching upstream. I took a closer look in the field, and at soil surveys from the region, and decided they were on to something. I first looked into this from the other direction, so to speak, investigating the extent to which floodplain sediments in the lower rivers is derived from the upstream, Piedmont portion of rivers draining to the Carolinas coast (spoiler for the roughly 8.3 billion people who have not been following my work for the past 35 years: very little Piedmont sediment makes it to the lower rivers). Below is a map from a 1992 paper on the subject. The soils marked as "coastal" are those organic soils.
I revisited this in the 2020s in investigating various impacts and indicators of the upstream effects of sea-level rise on coastal plain rivers in South Carolina, North Carolina, and southeastern-most Virginia. I wanted to know how the upstream limit of the organic soils compared to that of other indicators. And more specific to the soil issue, a key question was how a river, swamp, or floodplain transitions--apparently relatively suddenly in some cases, judging from stratigraphy--from a muddy or sandy, mineral-dominated state to one that is overwhelmingly composed of organic matter?
Soil map showing transition from mineral Chastain soil series (symbol Ch) to the Dorovan mucky peat (symbols Dk, Do) along the Roanoke River near Williamston, N.C. From SoilWeb (https://casoilresource.lawr.ucdavis.edu/gmap/)
The three main soil series involved are the Dorovan, Hobonny, and Chowan series. The Dorovan and Hobonny differ only in their acidity; the Chowan series occurs where recent mineral deposition has buried the organic soil.
Example profiles (from SoilWeb). The layers labelled with an "O" are dominantly organic material, with the "a" indicating highly decomposed material.
An area of Hobonny soils, Thoroughfare Island, Waccamaw River near Conway, S.C.
In this study I found that the floodplain organic soils occur well within the areas influenced by coastal backwater effects.
I was able to work out a sequence of changes, as shown below, with the transition to organic soils highlighted. The short version is that the hydraulic effects of sea-level encroachment both inhibits downstream transport of mineral sediment, and increases the frequency and inundation of floodplains. The latter promotes the deposition of water-transported organic matter, and due to the fact that the vegetation is adapted to wetness, does not inhibit biomass production and litterfall from floodplain plants. The wet, anaerobic environment inhibits organic decomposition, and the organic material accumulates.
Organic matter and mud deposition along the Trent River, North Carolina.
Dorovan soil along the Black River, North Carolina.
The diagram below is a more detailed look at how the formation of the floodplain Histosols fits into the various changes now occurring as sea-level inches its way upstream.
Studying soil geography and transition problems like this could shed some light on wetland evolution and development, the little-understood dynamics of fluvial-to-estuarine transition zones, and sea-level impacts on rivers. The areas characterized by Dorovan and similar soils are part of landforms and ecosystems that have very high values for wildlife habitat, flood and storm protection, water quality, and recreation. They are also highly vulnerable not only to sea-level rise, but also rampant land development, in some cases at a nearly crazed pace (see Horry County, South Carolina for instance). While the wetlands themselves may be protected from the excavators and bulldozers, the adjacent wetlands are subject to their adverse impacts on water quality, habitat and hydrological connectivity, and space for migrating in response to sea-level change. The mineral-to-muck (or peat) shift is also an example of a system transition that may represent, and shed light on, the broader study of regime shifts and tipping points.
References:
Daniels RB, Kleiss HJ, Buol SW, Byrd HJ, Phillips JA. 1984. Soil systems in North Carolina. Raleigh (NC): North Carolina Agricultural Research Service. Bulletin 467. 77 p.
Phillips, J.D. 1992. The source of alluvium in large rivers of the lower Coastal Plain of North Carolina. Catena 19: 59-75.
I really appreciate the chronology of morphologic/hydrologic changes induced by SLR that you mapped out in your 2024 Hydrology paper. I had observed or conceptualized most of these pieces in groups, but I hadn't been able to put it together as thoroughly as you did. One of the readily-observable steps in this sequence is the lack of bank levees within the FETZ. This is a common feature across the NC and MD rivers I've explored. It is also evident in LiDAR data: see "Hydrologic versus biogeochemical controls on denitrification in tidal freshwetlands" (Estuaries and Coasts 2013).
ReplyDeleteThanks, Scott.
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