Every geomorphologist, soil scientist, farmer, backhoe operator or other person who digs holes in certain areas is familiar with them—floaters. I am not sure how widespread the terminology is, but the phenomenon is ubiquitous in areas where the regolith is derived mainly from underlying bedrock. Floaters are large rock fragments, unattached to underlying bedrock, within a soil or weathering profile.

Floaters in a limestone weathering profile, central Kentucky.

For farmers and excavators, floaters are mainly an annoyance. For pedologists and geomorphologists they can also be an annoyance. This is not only due to the difficulties they pose for digging and sampling, but because—particularly with augers, probes, and core samplers—they can easily be mistaken for underlying bedrock, resulting in underestimates of the depth and thickness of soils, regoliths, and weathering profiles.

But can these floaters tell us anything about weathering profile, critical zone, and regolith evolution?

This post continues the story of a collaboration with artist Petr Mores to combine scientific and artistic perspectives to tell stories of landscape evolution.

One of the first issues we encountered was that Petr, while wonderfully experienced in depicting nature from the ground up, was not accustomed to representing underlying soils and geology. Pavel and I, on the other hand (like many other pedologists and geomorphologists) are quite familiar with showing soils, regoliths, weathering profiles, and parent rock in the form of two-dimensional profiles and cross-sections, either in highly simplified forms or in annotated photographs. 

Mountain stream (Petr Mores)

Typical two-dimensional representations of soil profiles (by Carsten Lorz, from Phillips & Lorz, 2008).

A Czech mountain forest (Petr Mores)

Scientific communication is, in essence, storytelling. When our intended audience is restricted to other scientists of similar interests and expertise, we have both more and less freedom. More in the sense that a certain baseline knowledge base can be assumed—thus basic principles do not have to be reviewed, terms defined, and justifications made. While the language standards for rigor and precision are pretty strict, those for beauty and entertainment value are very low (though some voluntarily exceed these!). Less freedom in that there exist some pretty strict norms with respect to professionally and sociologically acceptable ways to communicate—if you want to get published, you’d better either adhere to these or give a damn compelling reason in those (rare) cases when you don’t. A couple of my recent posts linking storytelling to scientific norms in the geosciences, or attempting to: Earth science historical narrative plotlines, and an analysis of landscape evolutionary pathway stories. 

View from directly above a recently exposed area of the swamp paleosol, showing infilled fossil tree trunks (probably bald cypress).

The Flanner Beach formation is a Pleistocene formation that outcrops in eastern North Carolina, mainly along the Neuse River estuary. The formation is known to geoscientists mainly for its depiction of a transgressive sedimentary sequence, and for its rich store of fossils. 

At the base of the FB is a swamp paleosol; a dark, organic and clay-rich soil representing a freshwater swamp environment (more on this below). Above the swamp paleosol is a layer representing transition to a brackish environment, rich with fossil shells. This can be subdivided into zones containing fossils representing oligohaline, mesohaline, and polyhaline environments. Further up are stratified open-estuary sedimentary deposits, topped off by beach sands. The formation is about 200,000 years old. 

FURTHER THOUGHTS ON TROPICAL CYCLONE DELUGES

This post is the third in a trilogy pondering the very real possibility that we have entered a “new normal” with respect to tropical storms and hurricanes, focusing in particular on relatively small storms on the Saffir-Simpson scale producing prodigious amounts of rain in the eastern Carolinas (part 1; part 2). Here I offer some additional thoughts.

Flooding from Hurricane Matthew along U.S. 421 in Wilmington, NC (near the USS North Carolina Battleship Memorial)(Associated Press)

Compound flooding

In my previous post I suggested, as have others, that we may be entering a “new normal” with respect to increasing precipitation associated with tropical cyclones. In eastern North Carolina and in south Texas we have empirical evidence in the form of Hurricanes Matthew, Harvey, and Florence in 2016-2018. Basic climatological principles suggest that the ongoing global warming will produce this result, and climate models bear this out.

The 100 largest area-averaged, multiple day precipitation events in the U.S. record from 1949-2018 were examined by Kunkel and Champion (2019).  Hurricane Harvey was the single largest event for an area sized 50,000 km² and a duration of 4 days. Rainfall associated with Hurricane Florence ranked seventh. Almost all of the top 100 events occurred in the southeastern United States or along the Pacific coast. Hurricane Matthew (2016) resulted in 1-day rainfall records at Tarboro, Fayetteville, Lumberton, and Raleigh, NC and at Florence and Dillon, SC (Weaver et al., 2016). Hurricane Florence resulted in new peak streamflow records at 28 gaging stations in the Carolinas (Feaster et al., 2018).

I am looking out on my rain-soaked yard in Craven County, NC, where it sure seems wetter than normal. Indeed, data from the nearby weather station in New Bern shows 90 mm of rain so far this month, and 1648 so far this year—the averages for Dec. 20 are 55 mm since Dec. 1, and 1309 for the year.

But this ain’t nothin’, really. The real story in these parts is the increased precipitation from tropical cyclones. The largest floods in memory in many locations in eastern NC occurred in conjunction with Hurricanes Florence in 2018, Matthew in 2016, and Floyd in 1999. At many locations these three represent, in on order or another, the 3 largest floods ever recorded. The key question being asked is whether this is the “new normal;” whether more frequent and/or more powerful storms and rainfall events (relative to say, the 20^th century, are what we are going to get from now on. As one who suffered >$35K worth of uninsured water damage from Florence, I hope to hell not. But the evidence is not on my side.

U.S. Geological Survey Flood inundation map for Kinston, NC (Neuse River) for hurricane Matthew in 2016.

Large dams trap a great deal of river sediment. But in many cases this does not result in a significant reduction in sediment delivery by rivers to the coast. This is due largely to the fact that the lower reaches of many coastal plain rivers were sediment bottlenecks long before the dams were built, and did not deliver much sediment to the coast to start with, and to the long under-appreciated importance of sediment sources in the lower coastal plain and within the coastal zone.

This has been known, at least in some case studies, for 30 years. However, these case studies have done little to offset the conventional wisdom that because (A) dams trap sediment (100 percent of bedload and often >90 percent of suspended load), and (B) rivers are an important source of coastal sediments, then (C) sediment delivery to the coast has been reduced to the coastal zone since a proliferation of dam-building in the 1950s and 1960s, leading to problems such as beach erosion and wetland loss.