Soil morphology of an alluvial chronosequence from the Little River, North Carolina Coastal Plain, USA

Geomorphology 351 (2020) 106921

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Soil morphology of an alluvial chronosequence from the Little River, North Carolina Coastal Plain, USA Bradley E. Suther a,∗ , David S. Leigh b a b

Department of Geography and Anthropology, Kennesaw State University, Kennesaw, GA 30144, United States Department of Geography, University of Georgia, Athens, GA 30602-2502, United States

a r t i c l e

i n f o

Article history: Received 3 May 2019 Received in revised form 23 October 2019 Accepted 24 October 2019 Keywords: Argillic horizon Entisol Terrace Ultisol

a b s t r a c t A soil chronosequence on terraces of the Little River in the North Carolina Coastal Plain was characterized to evaluate age-related trends in pedogenesis. Five representative pedons per landform were studied from six late Quaternary fluvial surfaces, and temporal trends were assessed by regression of morphological parameters against previously reported terrace optical age estimates. Solum, B horizon, and Bt horizon thickness, subsoil clay content, and Buntley-Westin Index rubification have strong, positive correlations with age and together distinguish between soils of the floodplain (≤200 yr BP), the first terrace (9.9 ± 2.0 ka), intermediate surfaces (T2, T3b, T4; 17.4 ± 4.2–74.6 ± 10.4 ka), and the oldest terrace (T5b, 94.0 ± 15.9 ka). Over time, soils develop from Entisols with respective A-C and A-E-Bw-C profiles on the floodplain and first terrace to Ultisols with increasing argillic horizon thickness, rubification, and clay content on T2-T5b. Solum thickness, Bt horizon thickness, and clay content display linear increases through time, with soil thickening reflective of downward pedogenesis through permeable, coarse-grained alluvium. Increasing argillic horizon clay content is attributable to illuviation, and possibly also to a combined bioturbation-translocation process, whereby clays scattered throughout sandy parent sediments are delivered to the surface by bioturbation, then concentrated in B horizons by eluviation-illuviation. Rubification increases with age and reflects transformation and subsoil concentration of inherited free iron, rather than iron oxide formation from primary mineral weathering. T5b pedons display morphologies comparable to those of 200 ka to >1 Ma soils along Coastal Plain rivers that also drain the Appalachian Piedmont, probably because sandy, Coastal Plain-derived Little River alluvium has a higher hydraulic conductivity that promotes faster rates of downward weathering, clay translocation, and rubification than the more finely-grained sediments of Piedmont-draining rivers. Thus, textural differences related to sedimentary provenance should be recognized when interpreting chronosequences from the region. Although rates of pedogenesis differ among Coastal Plain alluvial soils, linear trends in the ≤100 ka Little River chronosequence may nonetheless correspond to the early, more linear phases of development in studies from the region that span much longer intervals of time. © 2019 Elsevier B.V. All rights reserved.

1. Introduction A soil chronosequence is a genetically-related group of soils developed on geomorphic surfaces of different age for which the soil-forming factors of climate, biota, relief, and parent material have been approximately equivalent over time (Jenny, 1941; Birkeland, 1999). Although these factors are never truly constant over the time span of most studies, in a valid chronosequence, time has such disproportional influence relative to the other factors that

∗ Corresponding author. E-mail address: [email protected] (B.E. Suther). https://doi.org/10.1016/j.geomorph.2019.106921 0169-555X/© 2019 Elsevier B.V. All rights reserved.

its effect on pedogenesis can be evaluated (Schaetzl and Thompson, 2015). In pedology, soil chronosequences remain the primary source for empirical data used to assess rates of soil development (Schaetzl et al., 1994) and test competing models of pedogenesis (Huggett, 1998). The time needed for soils to develop certain properties also has implications for restoration of degraded soils (Bockheim, 1980) and for future agricultural, soil conservation, and land management planning (Markewich and Pavich, 1991). In addition, in regions where relationships between soil properties and the age of geomorphic surfaces are established, soils are useful for dating and correlating landforms and surficial deposits (Pavich et al., 1984; Busacca, 1987; Dorronsoro and Alonso, 1994; Bockheim et al., 1996). In this context, chronosequence data may be employed in

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B.E. Suther and D.S. Leigh / Geomorphology 351 (2020) 106921

geologic mapping (Markewich et al., 1989) or used in tandem with other dating techniques to provide chronologic frameworks for archaeological investigations (Holliday, 2004). Given these benefits, soil chronosequences serve as valuable research tools in pedology, geomorphology, and applied soil science (Huggett, 1998). Compared to other regions of North America, a limited number of soil chronosequences have been characterized for fluvial terraces in the southeastern Coastal Plain (Markewich et al., 1987, 1988, 1989; Howard et al., 1993; Shaw et al., 2003). Even fewer chronosequences have been documented on alluvial terraces in the adjacent southeastern Appalachian Piedmont (Foss et al., 1981; Segovia, 1981; Layzell et al., 2012) and nearby Southern Blue Ridge mountains (Leigh, 1996). Thus, one objective of this paper is to improve understanding of soil development with time in alluvial valleys of the humid, subtropical southeastern United States. Another objective is to identify soil morphological parameters that distinguish alluvial deposits of different late Quaternary ages in the region. This investigation differs from previous work in the southeastern US in that the Little River basin is located entirely within the Sandhills province of the upper Coastal Plain (Fig. 1). Thus, the primary source material for soils in this study, with the exception of minor outcroppings of Piedmont rocks, consists of the highly weathered, sandy, quartz-rich sediments of the upper Coastal Plain (Conley, 1962a). Previous alluvial chronosequences in the southeastern Atlantic and Gulf Coastal Plains have been conducted on terrace soils along rivers that also drain the Piedmont and the Blue Ridge and/or Valley and Ridge provinces (Markewich et al., 1987, 1988, 1989; Howard et al., 1993; Shaw et al., 2003). Such soils have formed from finer-textured parent materials with relatively higher proportions of weatherable minerals than soils on terraces solely derived from the Coastal Plain (Daniels et al., 1999, p. 38-41). This parent material difference appears to influence age-related trends in pedogenesis. The present research also focuses on soils formed over a shorter duration of time (100 kyr) than the aforementioned studies. Chronosequences of Coastal Plain alluvial soils ranging in age from Holocene to Middle to Late Pleistocene (Shaw et al., 2003), Holocene to 1 Ma (Markewich et al., 1987, 1988, 1989), and 90 ka to 13 Ma (Howard et al., 1993) have been documented but provide relatively little information for distinguishing between soils dating from <1 to 100 ka. Furthermore, most age estimates for alluvial soils in prior research have been established by correlation with dated marine units, rather than by direct dating of fluvial sediments. In contrast, age control for this study is provided by an opticallystimulated luminescence (OSL) terrace chronology (Suther et al., 2011) with dates at or near pedon sampling locations, which provides greater confidence in soil age estimates. Given this context, data from the Little River facilitate better understanding of relationships between pedologic processes and time in the upper Coastal Plain. Also, our data will provide guidance for age estimates on fluvial deposits of similar provenance that fall within the <1 to 100 ka range elsewhere in the region where numerical ages for alluvial terraces are unavailable.

late Cretaceous to Tertiary age and from eolian sands that overlie the aforementioned sediments on some uplands (Daniels et al., 1999; Swezey et al., 2016). Uplands in the study area are comprised of late Cretaceous interbedded, nonmarine kaolinitic clays and clayey sands (Conley, 1962a). At the highest elevations, these sediments are capped by unconsolidated sands and gravels of fluvial and eolian origin that Conley (1962b) assigned to the Miocene and defined as the Pinehurst Formation. Recent studies indicate late Quaternary eolian sedimentation occurred in the upland sands of the Pinehurst Formation at both Fort Bragg (Leigh, 2008) and elsewhere in the Sandhills (Swezey et al., 2016). At lower elevations, late Cretaceous fine sands and clays of marine origin, along with limited exposures of Triassic sandstone and siltstone and Carolina Slate Belt rocks (slates and felsic tuffs), outcrop along the valley margins and on the hillslopes of major tributaries (Conley, 1962a). The valley segment that provides the geomorphic framework for this study is 17 km long, 2–3 km wide, and depicted in Fig. 2. In this location, Suther et al. (2011) delineated and dated by OSL a modern floodplain and five late Quaternary fluvial terraces of mappable extent. The mapping methodology and procedures for OSL dating of terrace alluvium are provided elsewhere (Suther et al., 2011) and therefore not repeated here, but Table 1 summarizes the geomorphic attributes and age estimates of fluvial surfaces. OSL ages for terrace alluvium range from 9.9 ± 2.0 (T1) to 94.0 ± 15.9 (T5b) ka (Suther et al., 2011; Table 1). At the floodplain locality, prehistoric floodplain vertical accretion deposits yielded an OSL age of 1.3 ± 0.3 ka, but Suther et al. (2011) cautioned that overestimation of dose rate may have resulted in underestimation of the true age of these sediments. They therefore interpreted a mid- to late-Holocene (≥1.3 ± 0.3 ka) to protohistoric age for presettlement floodplain alluvium. The top stratum of floodplain deposits, which occurs in the upper meter of profiles, consists of historical sediment deposited since widespread Euro-American occupation of the valley, approximately 200 yr BP (Suther et al., 2011). 2.2. Soils, land use, and climate Soils on the floodplain are mapped by the USDA Natural Resources Conservation Service (NRCS) Soil Survey as Entisols (Typic Fluvaquents) and Inceptisols (Cumulic Humaquepts, Fluvaquentic Dystrudepts), whereas soils mapped on terraces range from Entisols (Aquic Quartzipsamments) to Ultisols (Arenic and Typic Hapludults, Plinthic and Grossarenic Kandiudults) (Hudson, 1984; Wyatt, 1995). Longleaf pine (Pinus palustris) savanna occupies most of the study area, although agricultural and residential land uses are also present. The climate is humid subtropical (Peel et al., 2007), and the soil temperature regime is thermic (Daniels et al., 1999). Average annual precipitation is 115.8 cm, and the average annual temperature is 16.9 ◦ C, with respective average January and July temperatures of 6.0 ◦ C and 27.5 ◦ C (Arguez et al., 2010).

2. Study area

3. Methods

2.1. Regional setting and chronology

3.1. Site selection, sampling, and soil morphological analyses

The study area is located on the Fort Bragg Military Reservation in the valley of the Little River, which is a tributary to the Cape Fear River that drains 880 km2 of the Sandhills province of the upper Coastal Plain in North Carolina (Fig. 1). The Sandhills border the southern edge of the Piedmont and extend from North Carolina southwest into Georgia (Horton and Zullo, 1991). The region constitutes a highly dissected landscape of deeply-weathered, sandy, quartz-rich soils that formed from fluvial and marine deposits of

Soils and geomorphology of the study area were initially investigated in 2003–2004 in association with geomorphic mapping of the Little River valley (Suther et al., 2011) and a separate study of surficial processes influencing archaeological site burial at Fort Bragg (Ruiz and Leigh, 2005). These projects included inspection of numerous soil-stratigraphic profiles from auger borings, handdug holes, road cuts, and cut bank exposures at floodplain, fluvial terrace, hillslope, and upland landscape positions. This fieldwork

Table 1 Soils, ages, extent, and surface morphology of Little River fluvial terraces. Landforma

Mapped series and subgroupb

Field-verified series and subgroupc

OSL lab no.d

Mean OSL age (ka)d,e ± 2-sigma

OSL sample depth (cm)d

Average HARL (m)d,f

Proportion of study area (%)d,g

Terrace treadsd,h

Tread morphology Floodplain

Chewacla (Fluvaquentic Dystrudepts)

Terrace 1

Kenansville

Pactolus taxadjunct (Typic Quartzipsamments)

1.3 ± 0.3

200

1.2

1.4

flat to indeterminant ridge & swale

UGA-31CD475-T2

9.9 ± 2.0

90-120

3.0

9.1

indeterminant ridge & swale (some surfaces scrolled, others braided)

UGA-31CD475-110

17.4 ± 4.2

110

4.7

19.1

indeterminant ridge & swale (some surfaces scrolled, others braided)

—

NR

NR

NR

7.3

6.4

indeterminant ridge & swale (some surfaces possibly braided)

Wagram - site 1 (Arenic Kandiudults) Wagram taxadjunct - site 2 (Grossarenic Kandiudults)

UGA-TU5@90 UGA-TU5@190

40.0 ± 9.9 (site 1) 55.2 ± 15.2 (site 1)

90 190

9.6

16.0

UGA-T32A160

72.7 ± 13.1 (site 2)

160

UGA-T32A180

51.3 ± 12.2 (site 2)

180

UGA-T4-161

74.6 ± 10.4

161

15.6

23.2

NR

NR

NR

20.3

9.0

UGA-T5-135

94.0 ± 15.9

135

29.0

15.9

(Arenic Hapludults)

Tarboro taxadjunct (Typic Udipsamments)

Terrace 2

Lakeland (Typic Quartzipsamments)

Kenansville (Arenic Hapludults)

Terrace 3a

Kalmia (Typic Hapludults)

Terrace 3b

Blaney (Arenic Hapludults)

Terrace 4

Dothan (Plinthic Kandiudults)

Terrace 5a

Candor (Grossarenic Kandiudults)

Terrace 5b

Candor (Grossarenic Kandiudults)

Candor (Grossarenic Kandiudults)

—

Candor (Grossarenic Kandiudults)

flat with no depositional topography (one surface contains indeterminant ridge & swale)

where preserved, flat with no depositional topography (where eroded, only remnant ridges remain) where preserved, flat with no depositional topography where preserved, flat with no depositional topography (where eroded, only remnant knobs remain)

none

none

none to slight

slight

slight to moderate

moderate to high

moderate

B.E. Suther and D.S. Leigh / Geomorphology 351 (2020) 106921

UGA-LR@200

Degree of dissection

moderate to high

a Terraces 3 and 5 each have two components (“a” and “b”) for which the “a” component constitutes a lower and in some settings more eroded surface and the “b” component comprises a surface that is higher and in some areas more well preserved (Suther et al., 2011). b Soil series and the associated subgroup in USDA Soil Taxonomy at pit sampling locations, as mapped by the NRCS Soil Survey. c USDA Soil Taxonomy subgroup classification for soils on each terrace at pit sampling locations, and the soil series that best fits their description. Where one or more properties of the observed soil fall outside the range of characteristics of the most similar series, that soil is noted as a series taxadjunct, and the subgroup classification of the observed soil is given. For T3a and T5a, — = no data. d Previously reported by Suther et al. (2011). e Ages were obtained from 150 to 170 ␮m quartz. Equivalent dose was determined using the single-aliquot regenerative dose (SAR) protocol (Murray and Wintle, 2000). NR = not reported. f Average height above river level of landform surface determined from 2 m pixel-edge LIDAR DEM data. g Proportion of areal extent of the mapped segment of Little River valley (see Fig. 2) that is occupied by each landform. h Qualitative description of terrace tread morphology is based on visual inspection of LIDAR DEM data. Dissection was qualitatively assessed based on the number of intermittent and perennial streams, as mapped on USGS 7.5 topographic quadrangles, that originate on each landform.

3

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B.E. Suther and D.S. Leigh / Geomorphology 351 (2020) 106921

Fig. 1. Study area location in the Sandhills province of the upper Coastal Plain of North Carolina (main map) and within the southeastern United States (inset). Locations of the Little River and its major tributaries, as well as the map area for Fig. 2, are also shown.

provided a comprehensive view of soils in the area and informed the sampling design of the present study. For the chronosequence investigation, sampling sites were selected to isolate the influence of time on soil genesis and to minimize, to the maximum extent possible, variation in parent material and local relief among soils on different fluvial surfaces. Among terraces of different age, sample sites were located on a similar geomorphic component of each terrace surface (e.g., sandy flats, sand ridges, or sandy scroll bars), that prior reconnaissance suggested would be underlain by alluvial parent material of generally similar sedimentology and stratigraphy. In these areas, locations selected for sampling were typically situated on the upper part of the landform, in the most well-drained and least eroded positions, to reduce toposequence effects (Leigh, 1996). Drainageways and depressions were avoided (Markewich et al., 1989). Soil profiles examined during the aforementioned geomorphic mapping and geoarchaeological investigations provided context for the extent of soil spatial variation present on the geomorphic components of terraces that were of interest to this study. Five pedons representative of the soils typically found at these landscape positions on each terrace level were selected for sampling. One pedon, of the five from each surface, was sampled by either backhoe or hand-dug pits, and four profiles were evaluated by manual bucket auger boring. A complete morphological description was obtained for each pedon sampled by pit using standard NRCS terminology (Soil Science Division Staff, 2017), and an abbreviated description, comprising horizonation, texture, and color (moist Munsell), was completed for each auger boring. Pedons at pits were sampled in arbitrary ±10 cm increments within horizons, and bulk density samples were collected for each horizon. Where possible, the entire profile, including C horizon material, was sampled. Only samples

of the most pedogenically well-expressed B horizon (or the uppermost C horizon where B horizons were absent) were collected from auger borings. For use as a proxy for unweathered parent material, samples of “fresh” alluvium were obtained from the Little River channel bed at five separate locations. Particle size analysis was accomplished using the hydrometer and sieve methods (Gee and Bauder, 1986). Rubification values are based on the moist Munsell color and were calculated using the Redness Rating (Torrent et al., 1983) and the Buntley and Westin (1965) and Hurst (1977) indices.

3.2. Chronosequence parameters, age estimates, and statistical analyses Soils were assessed for chronosequence trends using solum, B horizon, and Bt horizon thickness; maximum percent clay by weight; and rubification. Clay content and rubification values were determined for each pedon from the most pedogenically wellexpressed interval of the most well-developed subsoil (typically B) horizon. Where B horizons were absent, the uppermost C horizon was used. OSL age estimates reported by Suther et al. (2011) for alluvium at the T1 (9.9 ± 2.0 ka), T2 (17.4 ± 4.2 ka), T4 (74.6 ± 10.4 ka), and T5b (94.0 ± 15.9 ka) sampling localities were obtained from either upper lateral accretion (T1, T2, T4) or lower vertical accretion (T5b) sands collected from between 90 and 161 cm depth within their respective stratigraphic profiles (Suther et al., 2011; Table 1). Because at these sites the time elapsed between the emplacement of OSL-dated alluvium, the cessation of terrace deposition, and the onset of soil formation was likely short in comparison to the total

B.E. Suther and D.S. Leigh / Geomorphology 351 (2020) 106921

5

Fig. 2. Geomorphic map of the Little River valley within the study area, modified from Fig. 2 of Suther et al. (2011). The inset depicts topographic cross-section A-A’ in the vicinity of the Terrace 1 and 2 (T1 and T2) sampling sites. Map units in the inset map are displayed at 40 % transparency so that terrace surface morphology is visible. Vertical exaggeration of shaded relief in both the main and inset maps is 3×.

duration of pedogenesis on each landform, OSL ages are assumed to provide reasonable estimates of soil age on T1, T2, T4, and T5b. A pair of OSL ages were respectively obtained for the T3b-1 and T3b-2 sites, which occur on independent T3b map units (Table 1; Suther et al., 2011). At T3b-1, terrace alluvium is composed of two allostratigraphic units: a lower unit (210-103 cm), composed of a truncated paleosol formed in lateral accretion sands; and an upper unit (103-0 cm) containing a sand splay deposit overlain by sandy lateral and vertical accretion sediments. Because the lower allounit contains a paleosol that formed prior to the unit’s erosional truncation (Suther et al., 2011), and because both the lower and upper allostratigraphic units appear to have experienced pedogenesis as a single soil since deposition of the upper unit, the age of basal T3b1 sediments (55.2 ± 15.2 ka) was used to estimate the cumulative duration of soil formation for T3b-1 pedons. At T3b-2, where alluvium occurs within a single allostratigraphic unit, age estimates of 72.7 ± 13.1 and 51.3 ± 12.2 ka were obtained from lateral accretion sands at the respective depths of 160 cm (in a C and Bt horizon) and 180 cm (within C horizon sediments). Because sediments at 180 cm were more homogeneous than those at 160 cm and lacked pedogenic alteration, the 51.3 ± 12.2 ka age is viewed as more reliable, and it is therefore used in chronosequence analyses. Relationships between morphological properties and age were evaluated using least squares linear regression. In this capacity, regression was not applied with the aim of developing predictive chronofunction models; instead, it was employed simply as an analytical tool to characterize potential age-related trends in soils. Raw (untransformed) data for individual properties were regressed against soil age, with the mean OSL age in years BP for each ter-

race serving as the independent variable. For floodplain soils, which are composed of multiple A-C horizon sequences, only data from the uppermost sequence in profiles were used. This uppermost A-C horizon sequence is typically contained within the upper 30–40 cm of historical vertical accretion sediment, which averages about 1 m in total thickness across the sampled floodplain map unit. As the entire thickness of historical sediment accumulated over the last approximately 200 yr (see Section 2.1), an age of 100 yr BP, representative of the time elapsed since the midpoint of this interval, was arbitrarily assigned to the uppermost horizon sequence. Soil parameters were also evaluated with respect to their occurrence in B or Bt horizons only, to determine if exclusive use of B or Bt horizon properties improves correlations with age (Mills, 2005). Additionally, for all parameters except soil thickness, regressions were performed with and without properties of modern channel bed sediment, to determine how inclusion of fresh alluvium (assigned an age of 1 yr BP) influences age correlations. 4. Results 4.1. Sedimentology and stratigraphy Soil-stratigraphic profiles, geomorphic context, particle size data, and previously reported OSL ages together indicate that the parent material of soils in this study consists of normally graded, Little River-derived late Quaternary alluvium (Tables 1 and 2, Fig. 2, this paper; Suther et al., 2011). Down-profile particle size distributions of the gravel and clay- and silt-free sand fractions (Fig. 3, Supp. Fig. 1) confirm that soils at the T1-T5b sampling pits formed

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B.E. Suther and D.S. Leigh / Geomorphology 351 (2020) 106921

Fig. 3. From left to right, horizons and plots of percent dry weight of gravel (of sample whole); sand, silt, and clay (of <2 mm); and whole phi interval sand fractions (of 0.063–2.0 mm) versus sample depth for soils at pit sampling locations on Terrace 1, Terrace 3b (site 1), and Terrace 5b of the Little River. All samples were collected from a vertical pit face except for the 200–360 cm depth interval of the T5b profile, which was sampled by hand auger beneath the depth of excavation at the pit location. The interpreted base of late Quaternary Little River alluvium is labeled on the gravel content plots for the T3b-1 and T5b sites. Beneath this contact, either Coastal Plain sediments or older alluvial fills occur that predate Late Pleistocene Little River deposits. The base of late Quaternary alluvium was not reached at the Terrace 1 sampling location.

Table 2 Abbreviated profile descriptions for soils at sampling pits and particle size distributions of modern channel bed sediment. Depth (cm)

Moist matrix colora

(>2 mm)b (%)

Sandc (%)

Siltc (%)

Clayc (%)

Texture classd

Structuree

Bulk density (g/cm3 )

CB 1 CB 2 CB 3 CB 4 CB 5

n/a n/a n/a n/a n/a

n/a n/a n/a n/a n/a

2.5Y 5/4 10YR 6/4 10YR 6/4 10YR 4/3 2.5Y 5/4

78.1 1.5 0.6 0.0 0.1

99.7 99.8 99.9 95.9 99.7

0.3 0.2 0.1 2.9 0.3

0.0 0.0 0.0 1.3 0.0

grxcos cos s cos s

n/a n/a n/a n/a n/a

n/a n/a n/a n/a n/a

FP

A C1 C2 Ab C’1 C’2 C’3 A’b C”1 C”2 A”b C”’

0-7 7-24 24-37 37-47 47-58 58-78 78-100 100-114 114-130 130-170 170-187 187-220+

10YR 4/2 10YR 6/4 10YR 5/4 10YR 4.5/4 10YR 7/3 10YR 6/6 10YR 5.5/6 10YR 5/4 2.5Y 6/4 10YR 5/4 10YR 4/3 10YR 4/4

0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.0 0.0 0.0 0.0

85.9 92.1 84.6 92.0 97.5 95.0 91.9 94.0 96.7 95.3 84.7 78.5

9.6 5.1 12.1 5.2 1.8 3.3 5.5 4.2 2.1 3.2 12.0 15.1

4.5 2.7 3.2 2.8 0.7 1.7 2.6 1.8 1.3 1.5 3.3 6.4

ls s ls fs s s s s s s ls ls

wk med gr wk med gr wk med gr wk med gr sg wk fn gr wk med gr wk fn gr wk fn gr wk fn gr mod fn gr mod fn gr

1.00 1.31 1.35 1.38 1.38 1.42 1.44 — — — — —

T1

Ap E1 E2 Bw BC C1 C2 C3 C4

0-8 8-19 19-62 62-81 81-88 88-109 109-128 128-166 166-211+

10YR 3/2 2.5Y 5/4 10YR 6/4 10YR 6/6 10YR 5/8 10YR 6/6 10YR 6/4 10YR 7/6 10YR 7/6

0.1 0.1 0.1 1.3 5.4 30.2 15.4 21.5 45.3

94.1 93.1 92.3 91.2 94.4 96.7 97.9 97.9 97.7

3.9 4.9 6.2 6.9 4.1 2.6 1.3 0.9 1.3

2.0 2.0 1.5 2.0 1.5 0.7 0.8 1.2 1.0

s s s s cos grcos grs grcos vgrcos

wk fn gr wk fn gr wk fn gr wk med sbk wk med gr sg sg sg sg

— 1.41 1.50 1.54 1.45 1.73 1.42 1.44 1.71

T2

A EA E Bt C Csm C’

0-10 10-20 20-60 60-100 100-125 125-140 140-160+

10YR 4/3 2.5Y 5/4 2.5Y 6/4 7.5YR 5/7 10YR 6/6 5YR 5/8 2.5Y 6/3

0.2 0.1 1.1 1.6 8.3 33.0 49.2

85.5 87.7 83.7 77.9 96.1 93.8 94.1

11.4 9.8 12.2 10.7 1.1 1.2 2.0

3.0 2.5 4.1 11.4 2.8 5.0 3.9

ls s ls sl cos grcos vgrcos

wk fn gr wk fn gr wk fn gr mod med sbk sg sg - cemented sg

1.28 1.46 1.56 1.53 1.71 — —

T3bg (site 1)

Ap E1 E2 Bt C Btb1 Btb2 Btb3 C’ 2C

0-18 18-39 39-67 67-98 98-103 103-116 116-140 140-166 166-210 210-230+

2.5Y 3/1 2.5Y 6/4 2.5Y 7/4 10YR 5/6 2.5Y 7/3 & 2.5Y 7/4 10YR 5/6 10YR 7/4 (dep) 10YR 7/4 (con, dep) 2.5Y 7/4 (con, dep) 2.5Y 7/1 & 5YR 6/4

0.4 0.6 3.8 6.6 3.3 1.7 2.3 4.0 8.0 0.0

89.3 90.8 91.9 79.5 96.0 80.9 68.6 67.4 98.3 11.1

8.3 7.3 6.4 4.7 2.2 6.5 13.6 19.2 0.2 46.6

2.5 1.9 1.7 15.8 1.7 12.6 17.8 13.4 1.4 42.2

s s s sl s sl sl sl cos sic

wk med gr wk fn sbk wk fn sbk mod fn sbk sg mod fn sbk mod fn sbk mod fn sbk sg ma

1.40 1.64 1.58 1.67 — 1.60 1.75 1.71 1.55 —

T3b (site 2)

Ap E1 E2 Bt1 Bt2 C & Bt

0-6 6-42 42-73 73-100 100-147 147-170

C1 C2 C3

170-190 190-227 227-235+

10YR 3/2 2.5Y 5/4 2.5Y 6/4 10YR 5/8 (con) 10YR 5/8 2.5Y 7/3 (C) 10YR 5/8 (Bt) 10YR 7/8 (con, dep) 10YR 7/6 (con,dep) 10YR 7/1

0.1 0.2 0.3 0.5 2.4 2.9 (C) 1.4 (Bt) 5.5 7.3 14.4

89.5 89.6 88.3 88.5 95.8 99.1 (C) 96.8 (Bt) 98.6 98.5 99.3

9.0 8.6 9.4 5.4 1.5 0.5 (C) 0.9 (Bt) 0.3 0.5 0.2

1.5 1.8 2.3 6.1 2.7 0.4 (C) 2.3 (Bt) 1.1 1.0 0.5

s s s s s s to cos s to cos cos cos cos

wk fn gr sg sg wk med sbk wk fn sbk sg (C) wk med sbk (Bt) sg sg sg

1.43 1.46 1.51 1.62 1.54 1.38 — 1.37 1.33 —

f

B.E. Suther and D.S. Leigh / Geomorphology 351 (2020) 106921

Horizon

Landform

7

8

Table 2 (Continued) Depth (cm)

Moist matrix colora

(>2 mm)b (%)

Sandc (%)

Siltc (%)

Clayc (%)

Texture classd

Structuree

Bulk density (g/cm3 )

T4

Ap E Bt BE E’ B’t1 B’t2 B’t3 B’t4 B’t5

0-11 11-54 54-90 90-130 130-166 166-204 204-252 252-287 287-307 307-347

2.5Y 3/1 2.5Y 6/4 10YR 5/8 10YR 6/6 & 10YR 6/8 10YR 7/6 & 2.5Y 7/4 7.5YR 5/8 (dep) 7.5YR 5/8 (con) 10YR 7/6 (con, dep) 10YR 6/8 10YR 7/1 & 10YR 7/2

0.1 0.5 1.9 4.2 9.8 27.1 6.2 2.4 2.1 0.2

87.7 87.0 85.6 90.8 90.4 84.6 82.8 74.2 80.3 51.4

10.8 11.4 9.1 7.4 8.0 5.7 4.7 6.5 4.3 20.5

1.5 1.6 5.3 1.8 1.6 9.7 12.5 19.3 15.4 28.1

s s ls s s grls ls sl sl scl

wk fn gr wk fn gr wk med sbk wk med gr sg wk med sbk wk to mod med sbk mod med sbk mod med sbk —

1.42 1.62 1.66 1.59 1.55 1.66 1.66 — — —

T5bi

A Ap E Bt E’1 E’2 B’t Btx1

0-15 15-40 40-63 63-87 87-113 113-134 134-145 145-180

0.0 0.1 0.0 0.2 0.2 0.3 0.4 0.7

90.0 89.0 84.0 83.1 87.8 88.8 82.4 77.4

7.5 9.3 13.3 10.7 9.7 8.7 11.6 14.6

2.5 1.8 2.7 6.3 2.5 2.5 6.0 8.0

s s ls ls s s ls sl

wk fn gr wk med gr wk fn gr wk med sbk wk med sbk wk fn gr to sg wk med sbk wk med sbk

1.44 1.58 1.63 1.63 1.56 1.52 1.75 1.85

Btx2

180-256

2.5Y 3/2 2.5Y 4/2 2.5Y 5/4 10YR 5/8 10YR 6/6 (con, dep) 10YR 6/6 (con, dep) 10YR 5/6 (con) 10YR 5/8, 2.5Y 6/3, 2.5Y 6/2 (con, dep) 2.5YR 4/8, 10YR 5/8, 10YR 6.5/1 (con, dep)

1.3

70.3

8.1

21.6

scl

wk med sbk to ma

—

Btx3

256-274

6.9

72.0

10.5

17.5

sl

wk med sbk to ma

—

Btx4 Btx5

274-288 288-300

7.6 11.5

75.2 68.6

11.8 11.5

13.0 19.9

sl sl

wk med sbk to ma wk med sbk to ma

— —

Btx6

300-318 318-360

2.5YR 4/8, 5YR 5/8, 10YR 6/2, 10YR 6/1 (con, dep) 7.5YR 6/6 (con) 2.5Y 4/8, 7.5YR 6/8, 10YR 7/1, 10YR 7/2 (con, dep) 10YR 7/2 (con) 10YR 7/2 (con)

20.5 5.4

61.1 51.6

12.5 19.4

26.4 29.0

grscl scl

wk cs abk to ma —

—

h

T5b (cont’d)

a

Abbreviations: (con) = redox concentrations present, (dep) = redox depletions present. Percentages based on dry wt. of sample whole. c Percentages based on dry wt. of <2 mm fraction. d Abbreviations: gr = gravelly, vgr = very gravelly, grx = extremely gravelly, cos = coarse sand, fs = fine sand, s = sand, ls = loamy sand, sl = sandy loam, scl = sandy clay loam, sic = silty clay. e Abbreviations: wk = weak, mod = moderate, fn = fine, med = medium, cs = coarse, gr = granular, sbk = subangular blocky, abk = angular blocky, sg = single grained, ma = massive. Here and elsewhere, n/a = not applicable; — = no data. f Channel bed (CB) samples: gravel point bar (1), sand point (2) or mid-channel (3,5) bars, sand on clay outcrop (4). g OSL sample UGA-TU5@90 reported by Suther et al. (2011) was obtained from a depth of 90 cm in this C horizon, about 1 m away from the described profile, where the C horizon occurs at a slightly shallower depth. The lower boundary of Little River alluvium in this profile is at 210 cm. h Description is composited from two pedons for illustrative purposes. Ap-B’t2 horizons are taken from T4, pedon 1, and B’t3-B’t5 horizons are taken from T4, pedon 2 (Suther, 2006). Auger refusal on gravel at 220 cm in pedon 1 prevented description of the entire profile. Lower boundary of Little River alluvium at the pedon 2 location is 307 cm. i Lower boundary of alluvium in this profile is at 318 cm, within the Btx6 horizon. b

B.E. Suther and D.S. Leigh / Geomorphology 351 (2020) 106921

Horizon

Landform

B.E. Suther and D.S. Leigh / Geomorphology 351 (2020) 106921

in sandy alluvial materials of comparable sedimentology. Fluvial deposits at these sites consist of gravelly to very gravelly bedload and lower channel bar sands that fine upward into sandy upper lateral accretion sediments overlain by vertical accretion sands and silts. Through this sequence, very coarse (1.0–2.0 mm) and coarse (0.5–1.0 mm) sand contents typically parallel trends in the gravel fraction and decrease in the upwards direction in the top half to top third of most columns. In the same depth interval and direction, fine (0.125-0.250 mm) and very fine (0.063-0.125 mm) sand contents either increase or remain constant (Fig. 3, Supp. Fig. 1). Similar grain size trends were observed in terrace profiles examined by auger boring. Among T1 deposits, whose vertical particle size distributions have not been altered by illuvial clay accumulation, the <2 mm fraction of vertical accretion sediments at the sampling pit location contains ∼1–2 % clay; 3–7 % silt; and 91–94 % sand, with medium, fine, and very fine sand together comprising more than half of total sand (Table 2, Fig. 3). Floodplain deposits at sampling locations differ from those at the T1-T5b sites, in that floodplain profiles contain thicker, more finely-grained vertical accretion facies (Table 2, Supp. Fig. 1). At the floodplain pit, the examined stratigraphic section is composed entirely of historical (0–100 cm) and prehistoric (at >100 cm depths) vertical accretion facies that together exceed 220 cm in total thickness. Cutbank exposures revealed floodplain overbank alluvium is underlain by gravelly to sandy lateral accretion deposits similar to those observed in T1-T5b profiles, but pit excavation and auger boring depths did not penetrate these sediments at floodplain sampling sites. Sediments at the floodplain pit are predominantly medium, fine, and very fine sand, with negligible gravel content and a <2 mm fraction composed of 2–15 % silt and 1–6 % clay (Table 2, Supp. Fig. 1). Profiles in floodplain auger borings displayed similar particle size distributions (Fig. 4D, this paper; Suther, 2006). Despite these sedimentary differences with terrace alluvium, in our view, the overall sandy character of floodplain sediments, typified by 85–98 % sand by weight in most samples, is sufficiently similar to that of terrace soil parent material to justify inclusion of floodplain pedons in the chronosequence. 4.2. Soil morphology Entisols are present on the floodplain and T1, whereas various Ultisols occur on T2-T5b. Floodplain soils are Typic Quartizpsamments with A-C-Ab-C’ profile sequences (Tables 1–3). Evidence of pedogenic development in floodplain pedons is mainly limited to A horizon formation and the partial “blurring” by biological activity of bedding planes and sedimentary contacts within upper portions of the stratified alluvial parent material. Uppermost C horizons in floodplain profiles have textures of fine sand to sand and display moist Munsell matrix colors ranging from 10YR 4/3-10YR 6/4. Soils on T1 are Typic Udipsamments and are slightly more developed than floodplain soils, containing eluviated (E) horizons above incipient B (Bw) horizons that lack clay bridges and coatings but show signs of rubification. Bw horizons have weak, medium subangular blocky structure and sand to loamy sand textures that are too coarse to satisfy the cambic diagnostic horizon textural requirements in US Soil Taxonomy (Soil Survey Staff, 2014). T2 soils are Arenic Hapludults and are the youngest soils in the chronosequence (17.4 ± 4.2 ka) with argillic horizons. T2 soils typically have A-E-Bt-C profile sequences, although one of the five pedons examined has an A-E-Bw-C profile similar to T1 soils. Bt horizons are typically 30–60 cm thick, have 7.5YR hues and 14–19 % clay by weight, and contain sand grains that are coated and bridged with clay. The structure of T2 Bt horizons is only slightly more developed than that of T1 Bw horizons. Terrace 3b soils are Arenic Kandiudults (T3b-1) and Grossarenic Kandiudults (T3b-2) and typically have A-E-Bt1-Bt2-C horizona-

9

tions, with additional Bt subhorizons (T3b-1 pedons) or a C and Bt horizon (T3b-2 profiles) sometimes present (Tables 1–3). Argillic horizons range from 35 to 95 cm in cumulative thickness; have 10YR hues, 7–20 % clay, and weak to moderate subangular blocky structure; and show evidence of clay illuviation in the forms of coatings and bridgings of sand grains and films on ped faces. T3b-2 pedons are the only soils in this study that display prominent pedogenic lamellae, which compose about two-thirds of the Bt2 horizon and one-fourth of the immediately underlying C and Bt horizon at the pit sampling location (Suther, 2006). Lamellae in this pedon are 1–3 cm thick, composed of 10YR 5/8 loamy sand with 3–9 % clay, and contain sand grains that are coated and bridged with clay. Similar lamellae were observed in three of the other four T3b-2 pedons. Soils on T4 and T5b are Grossarenic Kandiudults and are the most well-developed soils in the chronosequence. They typically feature bisequal profiles with Ap-E-Bt-E’-B’t1-B’t2 (T4) or A-E-BtE’-B’t-Btx1-Btx2 (T5b) horizon sequences. Argillic horizons have 7.5YR-2.5YR hues, 14–37 % clay, and a cumulative thickness that ranges from 100 to >200 cm. In general, T5b soils are more welldeveloped than T4 profiles and contain multiple variegated, dense, compact Btx horizons, with 5YR-2.5YR hues and weak medium subangular blocky to nearly massive structure. Btx horizons contain substantial amounts of brittle material of firm rupture resistance but do not qualify as diagnostic fragipans in the US Soil Taxonomy because they are composed of <60 % brittle matrix and contain very little material that slakes in water (Soil Survey Staff, 2014). In T4 and T5b profiles where the complete thickness of terrace alluvium was observed, argillic horizons are present to the base of Little Riverderived sediment and extend into underlying material, which at sampling locations consists either of older alluvial fill or Coastal Plain deposits of fluvial or marine origin. 4.3. Chronosequence trends An increase in soil development is apparent across the chronosequence. Solum thickness; B and Bt horizon thickness; maximum percent clay; and rubification as measured by the Buntley-Westin Index have strong, positive relationships with mean age estimates and show the highest correlations with age among the evaluated soil properties (R2 ≥ 0.70; Table 4, Fig. 4). Hurst Index, Redness Rating, and Munsell hue values display more moderate correlations with age, at least in the context of linear models. Including properties from the most well-expressed subsoil horizons of all pedons in the chronosequence (Table 4, Fig. 4), rather than focusing exclusively on B or Bt horizons and assigning zero values to younger soils that lack these horizons (Supp. Table 1), provides stronger correlations with age for every variable except maximum percent clay. Inclusion of modern alluvium in regressions also improves correlations with age for clay content, as well as for rubification parameters. The forms (shapes) of regression plots suggest that some properties display linear relationships with mean age, while others do not (Fig. 4). Visual interpretation of Fig. 4A and 4D indicate solum thickness and maximum percent clay exhibit approximately linear increases with mean age. Regressions for both cases pass the Spearman rank correlation test for constancy of variance in residuals (Table 4), which provides additional evidence that a linear model appropriately characterizes these relationships. Regressions of B and Bt horizon thickness versus mean age fail the test for constancy of variance, perhaps owing to the lack of complete thickness measurements for these horizons in most T5b soils (see Fig. 4B-C), but both parameters nonetheless display roughly linear increases with age, within the scatter of observations. Given the full breadth of 2␴-error in OSL age estimates, it is possible that rubification, in addition to clay content and thickness

10

Table 3 Major pedogenic features of Little River soils by geomorphic surface. Surface and estimated age (ka)

Characteristic T1 (9.9 ± 2.0)

T2 (17.4 ± 4.2)

T3b - site 1 (55.2 ± 15.2)

T3b - site 2 (51.3 ± 12.2)

T4 (74.6 ± 10.4)

T5b (94.0 ± 15.9)

A-C-Ab-C’

Ap-E-BwBC-C

A-E-Bt-C

Ap-E-Bt-C Btb1-Btb2-C’

Ap-E-Bt1-Bt2C&Bt-C

Ap-E-Bt-E’B’t1-B’t2a

A-E-Bt-E’B’t-Btx1-Btx2a

E horizon thickness (cm)b,c

0

50 (28-60)

46 (24-65)

59 (45-89)

62 (41-72)

67 (21-96)

60 (20-95)

B horizon thickness (cm)b,c

0

24 (18-30)

36 (20-62)

78 (65-95)

48 (35-74)

141 (95-205)

208d (>55->115)

Bt horizon thickness (cm)b,c

0

0

32 (0-62)

78 (65-95)

48 (35-74)

141 (95-205)

208d (>55->115)

Solum thickness (cm)b,c

6 (3-8)

81 (75->105)

95 (90->110)

164 (145-175)

134 (107-158)

213 (128-307)

318d (>150->240)

Maximum subsoil clay content (% <2 mm)

3.0 (2.2-3.7)

4.5 (3.5-7.9)

14.1 (4.5-18.8)

18.1 (15.9-20.3)

11.4 (6.5-21.5)

23.9 (14.3-36.5)

26.5 (24.8-29.3)

Structure of subsoil horizon with maximum claye

wk med gr

wk med sbk

wk to mod med sbk

mod fn sbk

wk med sbk

mod med sbk

wk med sbk to ma

Munsell hue of reddest subsoil horizonf

10YR

10YR

7.5YR (3)

10YR

10YR

7.5YR (4)

5YR (4)

10YR (1)

2.5YR (1)

10YR (2) a

Additional B’t (T4) or Btx (T5b) horizons occur in some profiles. b For all surfaces except T5b, average values are given for E, B, and Bt horizon thickness; solum thickness; and clay content. Range values are reported in parentheses. The base of the solum was not reached in several profiles on T1 and T2. For these soils, minimum solum thickness measurements that exceed the thicknesses of other sola on the same surface are reported as “greater than” values at the upper end of the range. c For an individual pedon, the properties of E, B, and Bt horizon thickness respectively refer to the cumulative thickness of all subhorizons of each horizon type within the profile. d Because the entire solum was observed at only one location on T5b (pedon 1), B horizon, Bt horizon, and solum thickness values are reported for that pedon instead of an average value for the surface. The range of minimum measurements obtained for these properties from the other four pedons are given in parentheses. e See Table 2 for definitions of abbreviations. f If the Munsell hue of the reddest subsoil horizon varied among the five pedons examined per surface, each observed hue is given, followed by the number of pedons with that hue in parentheses.

B.E. Suther and D.S. Leigh / Geomorphology 351 (2020) 106921

FP (0.0 to 0.2) Typical horizon sequence

Table 4 Selected statistics for linear regressions of soil morphological properties versus mean age for the most well-expressed subsoil horizon per pedon for a given parameter. R2

P (Significance of F-ratio)

b1 a

b0 a

nb

Normality of soil propertyc

Normality of errorsd

Constancy of variancee

Form of relationshipf

solum thicknessg B thicknessg Bt thicknessg % clayh Buntley-Westin Indexh

0.83 0.75 0.78 0.74 0.70

<0.001 <0.001 <0.001 <0.001 <0.001

0.00260**** 0.00166**** 0.00180**** 0.00025**** 0.00025****

21.94* −1.69 −11.42 3.11** 13.59****

25 30 30 39 40

p f f f f

p f p f p

p f f p p

Hurst Indexh

0.56

<0.001

0.00015****

22.01****

40

f

p

f

Redness Ratingh

0.44

<0.001

0.00007****

−0.77

38

f

f

f

Munsell Hueh

0.45

<0.001

0.00004****

10.39****

38

f

f

f

linear linear linear linear nonlinear (?) (single, three parameter exponential growth: y = y0 + aebx ) nonlinear (single, three parameter exponential decay: y = y0 + ae−bx ) nonlinear (single, two parameter exponential growth: y = aebx ) nonlinear (quadratic polynomial: y = y0 + ax + bx2 )

a Statistical significance for t-statistics associated with b1 (slope of regression line) and b0 (y-intercept) is indicated for a given significance level as follows: **** = 0.001, *** = 0.01, ** = 0.05, * = 0.1, no asterisk = significance at greater than 0.1. b n varies with respect to the inclusion or exclusion of channel alluvium and the exclusion of pedons where only a minimum value was measured. c Result of Shapiro-Wilk test of normality of soil property. Properties with p-values of >0.05 for incorrectly rejecting the null hypothesis of a normal distribution pass the test. For this and other tests: p = passed, f = failed. d Result of Shapiro-Wilk test of assumption of normal distribution of errors about regression line. Regressions with p-values of >0.05 for incorrectly rejecting the null hypothesis of normally distributed errors pass the test. e Result of Spearman rank correlation test of assumption of constant variance of errors. Regressions with p-values of >0.05 for incorrectly rejecting the null hypothesis of constant error variance pass the test. f Where nonlinear relationships exist between soil properties and mean age estimates, the function type and generalized equation of the applicable model are provided in parentheses. All linear relationships follow the equation y = b0 + b1 x. g Sample set excludes samples of modern channel alluvium. h Sample set includes samples of modern channel alluvium. Two samples of modern channel alluvium with hues of 2.5Y were excluded from regressions of Redness Rating and Munsell hue. Redness Rating applies to soils and sediments with a minimum redness of 10YR. Analysis of Munsell hue was restricted to materials having the same hue unit (YR).

B.E. Suther and D.S. Leigh / Geomorphology 351 (2020) 106921

Soil property

11

12

B.E. Suther and D.S. Leigh / Geomorphology 351 (2020) 106921

Fig. 4. Soil morphological properties plotted as a function of mean age. (A) solum thickness; (B) B horizon thickness; (C) Bt horizon thickness; (D) maximum percent clay; (E) Buntley-Westin rubification index; (F) Hurst rubification index; (G) Redness Rating; (H) Munsell Hue. Values in plots D–H represent the most pedogenically well-expressed part of the subsoil with respect to that parameter and include measurements from modern channel alluvium. Results of simple linear regression are shown. Gray dashed lines depict 95 % confidence intervals. P-values apply to the F-statistic of each regression. Error bars represent two standard deviations of the OSL age estimates and are only shown for data points included in regressions.

parameters, also increases in a linear fashion over time (Fig. 4E-H). However, when plotted against mean ages, Hurst Index, Redness Rating, and Munsell hue values exhibit clear nonlinear trends with respect to the duration of pedogenesis. This interpretation

is corroborated by the fact that regressions involving these three variables fail the assumption of constant error variance (Table 4). Because linear relationships with time cannot be ruled out for the aforementioned properties due to age estimate uncertainties, linear regression is used in this study as the primary tool for eval-

B.E. Suther and D.S. Leigh / Geomorphology 351 (2020) 106921

13

tionship for this variable (Table 4), but the model provided little improvement over simple linear regression (R2 = 0.73 vs. R2 = 0.70). Error in age estimates and resulting uncertainty surrounding the precise form of soil-age relationships, as well as assumption violations of least-squares regression (e.g., non-normal distributions of dependent variables and regression residuals; Table 4), prevent use of the models described in this study as definitive, predictive chronofunctions. Nonetheless, Little River data provide valuable insights into the degree, direction, and rates of pedogenesis in upper Coastal Plain soils during late Quaternary time. 5. Discussion 5.1. Reliability of soils as age indicators

Fig. 5. Results of nonlinear regression of (A) Hurst rubification index, (B) Redness Rating, and (C) Munsell hue values against mean age. Gray dashed lines represent the 95 % confidence intervals, and p-values apply to the F-statistic for each regression. Plotted values were obtained from the most rubified subsoil horizon and include measurements from modern channel alluvium. Data points are displayed at 60 % transparency and appear gradationally darker as the number of pedons per surface with the same numerical value for each index increases.

uating chronosequence data. Nevertheless, it is our view that (1) mean OSL ages (Table 1) represent the best estimates for the ages of soils developed in Little River terrace alluvium, and (2) assessment of chronosequence data should include explicit evaluation of soil properties with respect to mean ages, using additional exploratory functional forms as appropriate. Therefore, nonlinear functions that provide an improved fit, relative to linear models, between rubification values and mean age are suggested (Table 4). Relationships between subsoil rubification and mean age appear to be best expressed by a single, three parameter exponential decay function, in the case of Hurst Index values (R2 = 0.80); a single, two parameter exponential growth model, with respect to the Redness Rating (R2 = 0.79); or a quadratic polynomial equation, for Munsell Hue values (R2 = 0.73) (Fig. 5A-C). The plot of Buntley-Westin Index values versus mean age (Fig. 4E) suggests that a single, three parameter exponential growth function might better characterize the age rela-

Solum, B horizon, and Bt horizon thickness; maximum percent clay; and Buntley-Westin Index rubification are the best age indicators for Little River soils. Solum thickness has the highest correlation with age (R2 = 0.83, Fig. 4A) and distinguishes historical floodplain (0–200 yr BP) from T1 and older (≥9.9 ± 2.0 ka) deposits. This parameter may also be useful for differentiating terminal Pleistocene terraces from older ones, with over half of pedons on T2 and younger surfaces (≤17.4 ± 4.2 ka) having sola ≤1 m thick and all pedons on T3b-T5b (51.3 ± 12.2–94.0 ± 15.9 ka) displaying sola >1 m in thickness. However, minimum measurements of 100+ cm in three of the ten T1 and T2 pedons indicate that these soils may have sola thicknesses similar to T3b profiles. Bt horizon thickness is also strongly correlated with age (R2 = 0.78), and the presence of a Bt horizon distinguishes most T2 and all T3b, T4, and T5b soils from those on T1 (Fig. 4C, Table 3). B horizon thickness has a similar correlation with age (R2 = 0.75, Fig. 4B) and provides the simplest method for distinguishing floodplain soils, which lack B horizons, from T1 pedons, which all contain Bw horizons (Table 3). However, neither B nor Bt horizon thickness are useful for separating T2 soils from those on T3b (51.3 ± 12.2–55.2 ± 15.2 ka). Minimum measurements and intraterrace variability prevent clear discrimination between the T3b, T4, and T5b surfaces using soil thickness parameters, but pedons with extremely thick sola (>3 m) and argillic horizons (>2 m) are restricted to T4 and T5b (≥74.6 ± 10.4 ka). Maximum percent clay also exhibits a strong, positive relationship with age (R2 = 0.74, Fig. 4D), but intra-surface variation limits its usefulness for separating individual terrace levels. Clay content does, however, distinguish the youngest soils (floodplain, T1) from the oldest ones (T4, T5b). Respective Buntley-Westin Index values permit separation of T1-T4 soils (18–32) from younger floodplain (9–12) and older T5b (40–48) pedons (Fig. 4E). Increasing redness with age (R2 = 0.70) reflects a strong correlation between subsoil color and pedogenic iron oxide accumulation over time. Percent dithionite-extractable iron (Suther, 2006, p. 49) shows a similar temporal trend and registers respective correlation coefficients of 0.82 and 0.79 with Buntley-Westin Index and Munsell hue values. Redness Rating and Munsell hue clearly differentiate T5b pedons from younger soils but do not distinguish soils by terrace level for the floodplain through T4 surfaces. 5.2. Intra-terrace soil variability This study does not characterize the full range of soil variability across each terrace but instead evaluates the degree of soil development typical of comparable geomorphic components of terrace surfaces that are underlain by similar parent materials. Among representative pedons from these locations, a range of soil morphological expression exists on each terrace level. Coastal Plain soils are

14

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known for their high spatial variability (Daniels and Gamble, 1967), and a number of workers have investigated its causes. Daniels and Gamble (1967) and Daniels et al. (1967, 1968, 1971) demonstrated that much of the soil variability on the North Carolina Coastal Plain results from the influence of water table regime, topography, and landscape dissection on pedogenesis, with the greatest variability found near the dissected edges of flat uplands (“dry edges”) and at the transition from moderately-well to poorly drained soils in the vicinity of shallow depressions on upland surfaces (“wet edges”). More recently, Phillips (1993a, b) and Phillips et al. (1994) utilized nonlinear dynamical systems and deterministic uncertainty concepts to explain more extreme short-range variability in surface (A + E) horizon thickness and soil series classification that they observed superimposed on the broader scale soil-landscape relationships documented by Daniels and colleagues. Phillips (1993b) and Phillips et al. (1994) attributed this variation to perturbations from tree throw and faunalturbations that they believed were able to persist and grow over time to produce the soil spatial variability of the modern landscape. For Little River soils, some intra-terrace variability appears to result from measurable variation in parent material and local relief. In other instances, spatial variation occurs in the absence of clear variation in soil forming factors and may reflect morphological variability caused by past bioturbations or site-to-site differences in initial conditions that are no longer apparent (Phillips et al., 1994). On T1, T2, and T3b-2, some variability is clearly attributable to differences in parent material sedimentology. For these sites, B horizon sand:silt ratios and/or clay- and silt-free sand fraction distributions indicate that several pedons developed in alluvium that was either more finely- or coarsely-grained than that of other soils on the same surface. Such textural differences likely contribute to outlier subsoil clay contents that are either substantially higher (T1, pedon 3, 7.9 %; T3b-2, pedon 5, 21.5 %) or lower (T2, pedon 5, 4.5 %) than those of other profiles on the same terrace (Fig. 4D; Suther, 2006, p. 162–181). Lack of clay in the parent alluvium of pedon 5 on T2 probably also inhibited argillic horizon formation in this profile, and, in turn, resulted in a zero measurement that increases intra-surface variation in Bt horizon thickness among T2 soils (Fig. 4C). Drainage effects resulting from small site-to-site topographic differences also may contribute to intra-surface variation, especially in rubification. On T3b-1, the reddest Buntley-Westin (24 vs. 18) and Hurst (12.5 vs. 16.7–20) Index values were associated with soils at the highest positions across a well-drained, sandy flat with ∼40 cm of local relief. On T1 and T2, within-terrace variability in rubification probably reflects the influence of “edge effect,” or the tendency of soils near the dissected edge of terraces to have redder B horizons with higher clay content than soils farther inland on the same surface, owing to a locally deeper water table (Daniels and Gamble, 1967). Among T1 and T2 soils, which were situated on scroll bars, most pedons within 30 m of terrace scarps or streamdissected edges of scroll bar ridges displayed greater rubification than profiles farther inland. The extent to which soil variability reflects the persistence (or growth) of disturbances from past bioturbation, the amplification of minor site-to-site differences in initial conditions, or simply random noise is often difficult to ascertain (Schaetzl and Anderson, 2005, p. 341), and this holds true along the Little River. But given that such phenomena have been identified as possible explanations for local-scale variation elsewhere in the region (Phillips, 1993b; Phillips et al., 1994; Phillips, 2001), the capacity of these mechanisms to contribute to intra-terrace variability here deserves recognition. Growth over time of perturbations from biota offers a plausible explanation for intra-surface variability in the chronosequence that occurs in the absence of local variation in topography, drainage, or parent material. This mechanism is particularly perti-

nent to variation in clay content and solum and subsoil thickness, as variability in related properties (profile horizonation, A + E horizon thickness, vertical clay distribution) have been linked to faunal- and floralturbation in other Coastal Plain landscapes (Phillips, 1993b; Phillips et al., 1994). The influence of bioturbation on study area soils, particularly with respect to argillic horizon formation, is evaluated in Section 5.3.2. Although morphological parameters differentiate between pedons of the floodplain, first terrace, intermediate surfaces (T2T4), and T5b (see Section 5.1), intra-terrace soil variability prevents reliable discrimination of individual terrace levels among the T2, T3b, and T4 surfaces. Despite these limitations, data indicate clear age-related trends in the development of Little River soils. 5.3. Soil genesis trends 5.3.1. Soil thickness Solum, B horizon, and Bt horizon thickness have progressively increased over time (Fig. 4A-C). Soil thickening in this environment probably reflects the progressive downward movement of weathering through thick, coarse-grained alluvium, whose high permeability promotes deep percolation. Profile and OSL age data support an approximately linear increase in solum thickness for soils ≤55.2 ± 15.2 ka. Within this time frame, there appears to be an interval of early soil formation related to initial E and B horizon development when profiles thicken more rapidly (Fig. 4A). Given the mean OSL ages for prehistoric floodplain, T1, and T2 soils, E and Bw horizons form within 1–10 kyr, and argillic horizons develop after 10–17 kyr (Tables 1–3; Fig. 4B). The time required for argillic horizon development along the Little River is similar to that observed by Leigh (1996) on alluvial terraces in the southern Blue Ridge but is considerably longer than argillic horizon formation times of 4–6 kyr documented by Foss et al. (1981); Segovia (1981), and Layzell et al. (2012) in the southeastern Piedmont. Making definitive interpretations about trends in solum and argillic horizon thickness for ≥74.6 ± 10.4 ka soils is more difficult. Solum and Bt horizon thicknesses were measured for four pedons on T4 and one pedon on T5b where the entire thickness of Little River-derived alluvium was observed (Fig. 4A-C). These data, which document respective solum and argillic horizon thicknesses of 128–318 cm and 95–208 cm among T4 and T5b soils, indicate that profiles continue to deepen over 70–100 kyr timescales (Table 3). At these sites, pedogenesis extends below the contact between Little River alluvium and underlying deposits (Table 2, Fig. 3, Supp. Fig. 1). Whether soil horizons beneath this contact reflect pedogenesis that occurred before, after, or both before and after deposition of T4 and T5b sediment cannot be determined from available data, so these horizons were excluded from chronosequence analyses. However, if the pedogenesis postdates T4 and T5b deposition, then even solum and argillic horizon measurements from locations where the full thickness of Little River sediment was examined may only represent minima with respect to soil thickness indices. Whereas ∼100 ka soils with sola in excess of three meters thick may seem unreasonable, these values are consistent with soil thicknesses of five to nine meters on Pliocene to Miocene age surfaces in the upper Coastal Plain of North Carolina (Daniels et al., 1978) and Georgia (Pavich et al., 1981). 5.3.2. Subsoil clay content and argillic horizon formation Subsoil clay contents show a gradual increase over time, culminating in respective argillic horizon clay maxima of 14–37 % and 25–29 % on T4 and T5b (Fig. 4D). Because parent material clay content is low, most Bt horizon clay must have accumulated by pedogenic processes or by eolian influx. In T2-T5b pedons, clay films coat and bridge sand grains in most Bt horizons, and thin, discontinuous clay films occur on ped faces in

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the argillic horizons of T3b-1, T4, and T5b soils (Suther, 2006). These observations are consistent with findings of prior studies (Gamble et al., 1970; Cabrera-Martinez et al., 1989) that demonstrate illuviation is a key factor in subsoil clay enrichment and the formation of sandy/loamy texture contrasts at E-B horizon boundaries within many Coastal Plain Ultisols. A meaningful proportion of the clay content in argillic horizons was likely sourced from inherited clays that were originally contained in the upper parts of terrace deposits, subsequently eluviated from A and E horizons, and added to inherited fines already present in the subsoil. However, parent material proxies for T2-T5b soils indicate that the <2 mm fractions of vertical accretion alluvium probably contained only 1–6 % clay (see Section 4.1), while the clay content of lateral accretion sediments was even lower (1–4 %; see channel alluvium and T1-T3b C horizons, Table 2). Such small amounts of clay, combined with the fact that, on average, “eluvial” (A + E) horizons comprise less than half of the solum thickness in T2-T5b profiles, suggest that inherited fines do not fully account for argillic horizon clay contents in some soils. Specifically, this appears to be the case for T3b-1, T4, and T5b pedons, which have maximum clay contents from 14 to 37 % and cumulative Bt horizon thicknesses of 65–208 cm (Figs. 3 and 4C-D, Supp. Fig. 1). If clays already present at A, E, and B horizon positions in terrace sediments prior to the onset of soil development are together insufficient to account for argillic horizon clay contents, then the remaining clays must have originated from another source; either from the weathering of silt- and sand-sized minerals or of sand grain clay coats in the parent alluvium, eolian inputs, bioturbative additions of fine-grained material (e.g., Phillips, 2004), or from some combination of these processes. Secondary minerals produced by weathering of silt- and sandsized silicates likely account for some argillic horizon clays, but they are probably not the main source. With age, Suther (2006, p. 125–126) found that Little River B horizons display a progressive decrease in the ratio of bases (CaO, K2 O, MgO, Na2 O) to alumina (Al2 O3 ) and other resistant oxides in the whole soil (<2 mm) and also exhibit declines in base concentrations in the fine sand fraction (0.125-0.250 mm). Whereas these geochemical changes suggest that primary mineral weathering is occurring in the chronosequence, the base concentrations of unaltered parent material indicate that the abundance of weatherable minerals is too low to produce large amounts of secondary clay. Parent material proxies for terrace Ultisols, including both the <2 mm and fine sand fractions of historical floodplain alluvium and “fresh” channel bed sediment, contain ≤1 % bases by weight on an oxide basis (Suther, 2006, p. 131-132). Such geochemistry suggests that even prior to subsoil development, profiles contained relatively small quantities of weatherable minerals in the silt and sand fractions. This interpretation is consistent with the siliceous mineralogies typical of floodplain soils along Coastal Plain-draining streams like the Little River that are derived mainly from Coastal Plain sediments (Daniels et al., 1999, p. 39-41). Thus, it is unlikely that large quantities of clay were supplied to B horizons via neoformation from weathering of sand- and silt-sized primary minerals. It is also unlikely that neoformation from weathering of sand grain clay coatings in surface horizons was a major source of fines, because only minor amounts of clay occur as sand grain coatings in parent alluvium. Inspection under 10X magnification of sands from typical C horizons revealed that 77–95 % of grains are either uncoated or contain clay coats on <5 % of grain surfaces, and no grain surfaces have >30 % coated with clay (Supp. Table 2). Given that eolian sedimentation occurred in the inland Coastal Plain during the Late Pleistocene, producing riverine eolian dune fields sourced from paleo-floodplains (Daniels et al., 1969; Markewich and Markewich, 1994; Ivester and Leigh, 2003; Leigh et al., 2004; Swezey et al., 2013) and upland dunes and sand sheets

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within the Carolina Sandhills (Leigh, 2008; Swezey et al., 2016), one might wonder if Little River soils have been enriched by eolian fines. However, if pedons received dust inputs, eolian fines do not appear to have included much silt. Silt depth distributions in profiles generally follow relative abundance trends of the fine and very fine sand fractions, and no pedons have silt caps (Fig. 3 and Supp. Fig. 1), suggesting that the silt content mainly reflects the sedimentology of the parent alluvium rather than eolian sources. Given the greater mobility of clay-sized particles in soils, the possibility of eolian clay additions is more challenging to evaluate. Phillips (2004) rejected eolian dust as a major clay source in Ultisols on the North Carolina lower Coastal Plain, based on the absence of appreciable fines in nearby relict coastal dunes of comparable age (77 ka; Phillips et al., 1996). This interpretation is probably also valid along the Little River, and it is further supported by opticallydated Coastal Plain riverine dunes of similar or greater age than the present chronosequence that also contain little clay. Along the Canoochee and Ohoopee rivers in southeastern Georgia, Ivester et al. (2001) documented dunes dating to OIS 6 or earlier intervals that could have potentially received intraregional Late Pleistocene dusts blown downwind from dune fields along rivers farther to the west (see Fig. 1 of Ivester and Leigh, 2003), given that W to WSW paleowinds prevailed in Georgia during periods when dune sedimentation occurred (Markewich et al., 2015, p. 172). Presumably, the dunes would have also been susceptible to the influx of extraregional dust during at least some portions of the late Quaternary, as African dust is known to reach the southeastern US under the modern climate (Prospero et al., 2001; Muhs et al., 2009). Additionally, the maximum clay content in dune sediments, which occurs in the >120 ka Canoochee dunes, is only 3.2 %. Of this clay, at least some appears to be supplied by weathering of silt- and sand-sized potassium feldspars, not eolian dust (Ivester and Leigh, 2003). Similarly low clay contents occur in riverine dunes in eastern North Carolina (Daniels et al., 1969) and in upland dunes on Fort Bragg, including in a 22.7 ± 5.9–24.1 ± 6.1 ka dune on an interfluve 8 km southwest of our sites (Leigh, 2008; Supp. Fig. 2). Unless clay dust additions to dunes across the Southeast were similarly destroyed by weathering or removed by illuvial translocation to groundwater or underlying strata, the lack of clay in Pleistocene dunes at the regional scale argues against dust as a major clay contributor to argillic horizons. Because the combined contributions of inherited clays, neoformation, and eolian additions do not fully account for the argillic horizon clay contents of some Little River soils, another mechanism of enrichment should be considered. A possible explanation is offered by Phillips (2007), who argues that no single process (e.g., eluviation-illuviation, mineral weathering, or bioturbation) alone is sufficient to explain textural contrasts between sandy surface (A + E) and loamy Bt horizons of Ultisols on the lower Coastal Plain, 190 km east of our sites. Phillips hypothesized that a combination of clay translocation from A and E to subsoil horizons, along with bioturbative delivery of fines to the surface from deeper within the parent material, could explain differences in surface versus subsoil textures. In this model, “conveyor belt” species (e.g., ants, termites; Johnson et al., 2005) deliver clay and mixed grain size material from lenses of fines scattered throughout a sandy parent material to surface horizons; translocation by percolating water concentrates clays in argillic horizons; and bioturbation maintains soil structure and macroporosity, which facilitates vertical water movement that allows illuviation to continue through time despite an increasing subsoil clay content. A coupled bioturbation-translocation process provides a potential source for subsoil clays not explained by other causes, as this mechanism’s requisite components are present. Clay films and/or bridges are found in every pedon with an argillic horizon and clearly indicate that clay translocation is occurring. Fines sources are also available. Although very sandy, C horizon alluvium contains clay

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beds and drapes that could supply fines, and greater quantities of clay occur in subjacent Coastal Plain sediments. These underlying deposits have sand to sandy loam textures but also contain large clay lenses (Fig. Mu 3.2 of Daniels et al., 1999) that at some sites are situated immediately beneath alluvium at depths as shallow as 210–230 cm (see 2C horizon of T3b-1 profile, Fig. 3). Finally, bioturbation is ubiquitous in the sandy soils of the southern Coastal Plain, including in the Sandhills (Leigh, 1998, 2001), based on a growing body of geoarchaeological (Peacock and Fant, 2002; Bush and Feathers, 2003), soil geomorphic (Phillips et al., 1994), and entomological (Tschinkel, 2015, 2016) evidence. If bioturbation-translocation processes are operating, bioturbators would have to be capable of moving fine-grained sediment upward without disrupting vertical trends in the coarse-grained component of alluvium. Fining upward trends in the gravel (>2 mm) and very coarse sand (1–2 mm) fractions appear intact in soil pits (Fig. 3, Supp. Fig. 1), and similar trends were evident in borings. Normal grading is particularly pronounced in the >2 mm fraction of pedons at sampling pits, where gravel is virtually absent to depths of at least 30–90 cm (Table 2, Fig. 3, Supp. Fig. 1). Given that bioturbation can allow coarse fragments to settle downward in sandy soils (Leigh, 2001), one might question if this depth distribution is itself caused by bioturbative processes, but this does not appear to be the case. Gravels in T2-T5b deposits exhibit fining upward trends crudely similar to those of younger sediments, including those of the T1 pit (Fig. 3) and floodplain and T1 deposits encountered in cutbanks. These younger sediments feature fining upward gravels nearly completely contained within C horizons that have been relatively undisturbed, based on the presence of intact sedimentary structures (Suther, 2006, p. 162-163). This suggests that the fining upward trends of gravels in older soils have a sedimentary origin and have not been substantially modified by post-depositional processes. Thus, any organisms supplying fines to surface horizons from deeper sediments would have to do so without displacing much gravel. Among faunalturbators, both ants and termites would satisfy this requirement (Halfen and Hasiotis, 2010). Phillips (2007) notes both faunalturbation and floralturbation deliver fine-grained materials to the surface that promote argillic horizon development in lower Coastal Plain soils. He indicates ants and termites are the most important fauna involved. At the regional scale, the importance of bioturbation by ants is also emphasized by Tschinkel (2015), who states that together, ground nesting ant species may be the most important agents of faunalturbation in sandy Coastal Plain settings. The fungus gardener ant (Trachymyrmex septentrionalis) makes nests ∼0.5-1.5 m deep and can annually deliver 0.5–1.5 t of sandy material to the ground surface in Florida pinewoods landscapes (Seal and Tschinkel, 2006). Other species, such as the Florida harvester ant (Pogonomyrmex badius) and winter ant (Prenolepsis imparis) construct deeper nests that may reach maximum respective depths of ∼3 and ∼4 m in sandy Florida Panhandle soils (Tschinkel, 2003). In such settings, Tschinkel (2015) estimated harvester ants at a density of 16 colonies per ha could produce 64 t of biomantled material per 1000 yr, with 10–15% of material delivered from depths >100 cm and about 2 % (1.2 t) sourced from >200 cm depth, suggesting that some species might be capable of delivering fines to surface horizons from great depth. Phillips (2007) did not identify the bioturbating ant and termite species at his sites. But among 20 Ultisol pedons on the Pamlico terrace, insect burrows were found to extend into C horizons in 14 of 20 cases and encountered in underlying stratified marine sediments at an additional five sites (Phillips, 2007). Phillips (2007) states that tree tip mounds also supply fines at sites with hardwoods, but he indicates tree throw is less important under pines, which are more likely to suffer trunk break than uprooting in high winds. A moderate amount of uprooting was evident near our sites, and, in the T3b-1

sampling pit, an apparent rootball remnant uprooted from the paleosol at this location was observed in the pit face one meter away from the profile we sampled that locally increased clay content at 65–100 cm depth. However, in the upper parts of most profiles, the absence of gravels, which are typically brought upwards by uprooting (Small et al., 1990), suggests that tree throw has not been a major fines contributor for the majority of our pedons. Characterizing pedologic effects of bioturbation was beyond the scope of the present research, and we caution that our data do not allow direct assessment of bioturbative fines contributions to Little River profiles. With that caveat in mind, we highlight the above examples to underscore (1) the amount and rate of material delivered by bioturbation to the surfaces and upper horizons of some Coastal Plain soils; (2) the depth of material sourcing achieved by some organisms in the region; and (3) the possibility that bioturbation-translocation processes may account for a significant proportion of argillic horizon clay content in settings that are inconsistent with other sources of fines. Along the Little River, where B horizon clay content is not fully explained by “within solum” sedimentary inheritance, clay neoformation is limited because of the “preweathered” nature of the parent material, and available evidence argues against dust as a major clay source; we view the translocation of clay from surface to subsurface horizons, operating in tandem with bioturbative delivery of fine-grained material to the soil surface, as a viable source of fines that may contribute to argillic horizon development.

5.3.3. Rubification Rubification increases and may have changed episodically through time. Although temporal trends vary by index, both Buntley-Westin and Hurst Index values (Fig. 4E-F) show substantial differences in redness between floodplain and T1 profiles, comparable rubification among T1-T4 soils, and greatest redness in T5b pedons. Subsoil rubification is strongly correlated to free iron content (see Section 5.1). Increasing redness and extractable iron are thought to mainly reflect the translocation (and transformation) of inherited free iron and its concentration in B horizons over time, rather than the production of iron oxides by primary mineral weathering. This interpretation is consistent with that of Daniels et al. (1975), who attributed landscape-scale variability in extractable iron and rubification in Coastal Plain Paleudults to variation in water table depth and oxidation-reduction site history, rather than to differential weathering or spatial variation in parent material iron content. In pedons near upland terrace edges, Daniels et al. (1975) argued that deep water tables and long-term oxidation prevented the loss of inherited iron and promoted E horizon development and illuvial clay and iron oxide accumulation. As a result, soils at these positions displayed greater B horizon extractable iron, rubification, and clay content than pedons at poorly drained terrace interiors, where high water tables and long-term reduction inhibited illuviation and caused free iron losses. Similar to the soils of Daniels et al. (1975), most free iron in B horizons does not appear to be sourced from primary mineral weathering. Total chemistry of both the whole soil and fine sand fractions of modern channel bed and historical overbank alluvium indicate that the abundance of weatherable minerals is low in soil parent material (see Section 5.3.2). Furthermore, Suther (2006, p. 132-133) found that, whereas dithionite iron in the <2 mm fraction of subsoils increases from lower to higher terraces, iron bound in primary detrital minerals (Fe2 O3 -FeD ) remains essentially stable across the chronosequence. Given the comparable parent material from terrace to terrace, this suggests there is little change in primary iron content, and thus little primary mineral iron released by weathering, among older versus younger soils.

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Daniels et al. (1975) implicated free iron aggregates and coatings in parent sediments as the primary iron source in their study, and a similar origin is likely for Little River pedons. Although sands in C horizons are mostly free of clay and iron coatings, floodplain C horizon sediments, which consist of vertical accretion alluvium, are yellowish-brown with predominantly 10YR hues, values of 4–7, and chromas of 3–6. These colors indicate the presence of goethite (Schwertmann and Taylor, 1989) and suggest that the silt and clay fractions of vertical accretion sediments were an important free iron source. As suggested by Daniels et al. (1975) for their “dry edge” soils, inherited free iron in the upper parts of well-drained Little River deposits were probably concentrated in the subsoil via clay translocation during E and B horizon formation. If a coupled bioturbation-translocation process is supplying additional clay derived from deeper strata to subsoils (see Section 5.3.2), this mechanism would also deliver additional free iron, as most iron oxides occur in the clay fraction of soils and sediments (Schwertmann and Taylor, 1989). Some free iron may have also accumulated via plant uptake of soluble iron (Daniels et al., 1975) before terracing of surfaces, when local water tables were still shallow. Increasing rubification also reflects transformations in pedogenic iron oxides. Suther (2006, p. 132-133) found that the ratio of subsoil oxalate- to dithionite-extractable iron decreases from lower to higher terraces, indicating that poorly crystalline iron constitutes a declining proportion of total free iron over time. This trend probably reflects the increasing abundance of crystalline iron oxides, especially goethite, in T1-T5b soils, as well as hematite, the presence of which is indicated by 5YR-2.5YR hues (Schwertmann and Taylor, 1989) in T5b Btx horizons. Markewich et al. (1989) showed that both rubification and iron contents are highly correlated with age in Entisol-InceptisolUltisol pathway soils of the eastern US, suggesting that increasing rubification across the present chronosequence is at least partly age-related. Although the precision of age estimates is too low to support definitive conclusions about variation in rates of rubification, comparison of redness values with mean ages suggests that reddening may have developed episodically, with phases of rapid change (Fig. 5A-C). In addition to age, these trends may indicate the influence of changes in soil drainage and/or paleoclimate. The initially rapid increase in redness indicated by floodplain versus T1 Buntley-Westin and Hurst Index values (Fig. 4E-F) results from E horizon formation and the accumulation of pedogenic iron oxides in T1 Bw horizons. These processes are time-dependent but may have also been promoted by internal drainage improvements that likely occurred after terracing of the T1 surface. Rubification similarities among T1-T4 soils may indicate that initial terracing of surfaces promotes formation of iron oxides that are stable at well-drained sites over 10–75 kyr timescales. Topography-related drainage effects may have also influenced rubification of T5b Btx horizons. Since T5b is 29.0 m above river level and >14 m above the next highest terrace (Table 1), its soils are probably in the most well-drained location in the valley. Relative to lower elevations, this setting would favor deep oxidation, the transformation of poorly crystalline free iron phases to crystalline iron oxides, and hematite formation. Such effects may have been amplified if T5b was positioned higher above the river for a longer period of time than the other terraces, as implied by interterrace mean rates of incision reported by Suther et al. (2011, p. 85) that suggest downcutting between deposition of T5b and T4 was possibly more rapid than incision following the deposition of subsequent terraces. Alternatively (or additionally), T5b pedons may have been exposed to more intense weathering conditions than younger soils. Given that T5b pedons date to 94.0 ± 15.9 ka (78.1–109.9 ka), it is possible they experienced the latter part of the last interglacial (OIS

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5d-5a, 115-75 ka), a period that was known to have been warmer and wetter in the southeastern US than the last glacial, based on pollen records from Lake Louise, Florida (Watts, 1971) and the Hybla Valley, Virginia (Litwin et al., 2013) that latitudinally bracket the study area within the eastern US. If so, in addition to age and topographic effects, T5b rubification may reflect a more intensely leached weathering environment associated with conditions of the last interglacial.

5.4. Comparisons with previous studies Increases with age in solum and Bt horizon thickness, clay content, and rubification are generally consistent with the findings of previous Coastal Plain chronosequences. Markewich et al. (1989) reported that solum and Bt horizon thickness, clay mass, and rubification of soils on fluvial and marine deposits dating from the Holocene to 1 Ma show positive, logarithmic trends with age. Howard et al. (1993) similarly observed logarithmic increases with age in solum thickness, rubification, and clay content for 90 ka to 13 Ma soils along the James River (Virginia). The most pronounced difference between Little River soils and those of previous studies is their rate of morphological development. T5b soils (94.0 ± 15.9 ka) have 2.5YR-5YR hues and 25–29 % clay, and one deep boring on T5b revealed a solum thickness of 318 cm and cumulative argillic horizon thickness of 208 cm (Table 2, Fig. 4A-C). Solum and argillic horizon thickness minima respectively range from 150-240+ cm and 55-115+ cm in the four other T5b pedons. Soils of similar age (60–120 ka) of Markewich et al. (1989) and Howard et al. (1993) have 10YR-7.5YR hues, 17–30 % clay, and sola and Bt horizon thicknesses that respectively vary from 114 to 147 cm and 51 to 120 cm. Ages of 200 ka-1 Ma are required for the soils of Markewich et al. (1989) to exhibit the solum and argillic horizon thicknesses and rubification of the oldest Little River soils. Along the James River, ages of 1.2–4.4 Ma appear necessary to attain similar rubification values; ≥1.2 Myr of pedogenesis are needed to achieve sola thicknesses comparable to T5b solum thickness minima; and no soils exhibit argillic horizon thicknesses similar to the 208 cm measurement from the T5b pit location. Faster rates of pedogenesis along the Little River likely reflect the coarse (∼85-99 % sand), quartz-rich alluvium from which soils developed. Comparisons of C horizon material from Little River pedons with that of soils of prior studies indicate that the latter developed from considerably finer-textured material, ranging from sands to clay loams and clays (Markewich et al., 1989; D29-D34) and sandy loam to clay (Howard et al., 1993;). This textural difference is related to differences in source area. Little River terraces are derived from the sandy sediments of the upper Coastal Plain Sandhills, whereas deposits of other studies are derived from both Coastal Plain deposits and the finer-grained sediments of the Piedmont, Blue Ridge, and Ridge and Valley. Because sand has a higher hydraulic conductivity than loamy materials, Little River soils likely developed with better internal drainage than those along rivers that also drain Appalachian provinces, which would promote faster rates of downward weathering, clay translocation, and rubification. A more freely-drained pedoenvironment that favors vertical water movement and lessivage, combined with possible bioturbative delivery of additional, translocatable fines to surface horizons, might also explain why Little River subsoils have about the same clay content as soils of similar age along rivers that drain the Piedmont, even though the latter are derived from more clay-rich materials with greater abundances of weatherable minerals. The difference in rates of soil development among the above chronosequences demonstrates that considerable regional differences exist in Coastal Plain alluvial soils that are likely related to parent material texture.

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Little River chronofunctions also differ from those of previous Coastal Plain chronosequences. Little River pedons show linear increases with age in clay content and thickness parameters, while displaying temporal rubification trends that are best modeled by either exponential growth or decay functions. No property appears to have reached a “steady state” of maximum development. In contrast, similar parameters typically show positive, logarithmic trends with age of the form Y = a + b log X in the studies of Markewich et al. (1989) and Howard et al. (1993). Such logarithmic functions effectively represent initially rapid development that later slows to approach an asymptote or steady state (Bockheim, 1980). Given the short duration of the present chronosequence (∼100 kyr) relative to that of the aforementioned studies (1 to >10 Myr), it may be that the linear increases in soil thickness parameters and clay content along the Little River correspond to the more rapid and linear earlier phases of development documented in other Coastal Plain chronosequences that span much longer time intervals. 5.5. Geomorphic and geoarchaeological survey implications Terrace height is the most reliable relative age indicator for Little River alluvial deposits (Suther, 2006), but soil properties are also strongly correlated with age. Thus, although overlap in soil morphology among vertically adjacent surfaces precludes identification of a discrete terrace level in some cases, in regional settings where terraces are unpaired or discontinuous, or where neotectonics or eustasy have obscured relationships between elevation and terrace age, soil properties used in combination with crosscutting relationships and numerical dating should facilitate the best possible geomorphic mapping. Whereas soil properties do not differentiate terminal Pleistocene and younger deposits, which may contain buried artifacts, from older terraces, soils still may be useful to geoarcheological surveys. In this setting, the presence of an argillic horizon does not rule out the possibility that alluvium contains buried artifacts. But the lack of an argillic horizon suggests a maximum age of 17.4 ± 4.2 ka (Fig. 4C), and the lower part of this interval is within the age range where potential for buried artifacts should be considered. 6. Conclusions Solum, B horizon, and Bt horizon thickness, maximum percent clay, and Buntley-Westin rubification exhibit strong, positive correlations with age and are the most meaningful relative age indicators for Little River terraces. Buntley-Westin rubification distinguishes between soils of the historical floodplain (≤200 yr BP), terraces of intermediate age (T1-T4, ca. 10–75 ka), and the oldest surface (T5b, ca. 94 ka); presence of a Bw horizon discriminates T1 versus floodplain profiles; and argillic horizons differentiate most T2 and all T3b-T5b soils from those on the first terrace. However, intra-terrace soil variability produces inter-terrace overlap in most properties that prevents the discrimination of individual terrace levels among the T2-T4 surfaces. Over time, soils develop from Entisols with A-C or A-E-BwC horizon sequences to Ultisols with increasing argillic horizon thickness, clay content, and rubification. Solum, B horizon, and Bt horizon thickness exhibit linear increases through time that reflect the gradual downward progression of weathering through permeable, coarse-grained alluvium. Subsoil clay content also increases gradually with age. Although some fines have been illuviated from the upper parts of terrace deposits, clay contents of argillic horizons that are disproportionally high relative to that of terrace alluvium suggest that T3b-1, T4, and T5b soils received additional clays from another source. Quantities of fines supplied by neoformation and atmospheric dust are too low to account for the clay contents of

these pedons. Alternatively, a combined bioturbation-translocation mechanism, whereby fines scattered throughout sandy parent sediments are delivered to the surface by ants and termites, then concentrated in B horizons by eluviation-illuviation, provides a plausible clay source. Increasing rubification through time reflects the transformation and subsoil concentration of inherited free iron, rather than production of iron oxides from mineral weathering. In addition to greater age, the advanced rubification of T5b pedons relative to younger soils may reflect both episodic and long-term improvements to drainage resulting from river incision and/or exposure of T5b profiles to the more intensively leached conditions of the last interglacial. T5b pedons (94.0 ± 15.9 ka) have Bt horizon thickness, clay content, and redness values comparable to those of 200 ka to >1 Ma soils along Coastal Plain rivers that also drain the Appalachian Piedmont, probably because sandy, Coastal Plain-derived Little River alluvium has a higher hydraulic conductivity that promotes faster rates of downward weathering, clay translocation, and rubification than the more finely-grained sediments of Piedmont-draining rivers. Thus, considerable differences exist in rates of pedogenesis among Coastal Plain alluvial soils likely related to texture that should be recognized when interpreting chronosequences from the region. Clay content and soil thickness parameters display linear increases with age along the Little River, whereas similar properties exhibit logarithmic increases with time in 1 to >10 Ma chronosequences from Piedmont-draining Coastal Plain streams. Although rates of development have clearly differed among these soils, linear trends in the ≤100 ka Little River pedons may nonetheless correspond to the earlier, more linear stages of pedogenesis in other Coastal Plain chronosequences that span much longer periods of time. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements University of Georgia grant “Geomorphic Processes Influencing Archeological Site Burial at Ft. Bragg” (DACA88-99-D-0002-0025) from the US Army Corps of Engineers funded excavations. The Kennesaw State University College of Humanities and Social Sciences, Graduate College, and Office of the Provost and Vice President for Academic Affairs provided funding for equipment used in the sand grain clay coat analysis. We thank the Department of the Army and Mr. H. Davis for access. James Rogers, Nicole Brannan, Joe Herbert, and Amy Woodell provided field assistance. J. Phillips and two anonymous reviewers provided valuable input to earlier versions of this manuscript. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.geomorph. 2019.106921. References Arguez, A., Durre, I., Applequist, S., Squires, M., Vose, R., Yin, X., Bilotta, R., 2010. NOAA’s U.S. Climate Normals (1981-2010). Annual/seasonal and monthly normals (Fayetteville Pope AFB, NC, US). NOAA National Centers for Environmental Information, http://dx.doi.org/10.7289/V5PN93JP, accessed 4/8/19.

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