Submerged Archaeoanthrosols of the estearias (stilt villages) in Maranhão: A new frontier in Pedoarchaeology in Brazil

ABSTRACT

The submersed anthrosols of the estearias in Maranhão exhibit unique characteristics that expand our understanding of anthropogenic soils in complex hydromorphic and fluviolacustrine environments. This study aimed to characterize these soils morphologically, physically, and chemically, and to compare them with other Amazonian Archaeological Dark Earths (ADEs) from upland and floodplain environments. A database of 160 anthropic horizons from 71 profiles in eight studies was compiled from theses and dissertations, allowing detailed comparative analyses. Principal Component Analysis and Pearson correlation were applied to identify the main soil properties associated with each environment. The results indicate that, beyond anthropogenic additions, prolonged flooding and redox-affected conditions play a crucial role in soil formation. When combined with horizons containing ceramic artifacts, charcoal, and organic sediments, these features suggest that the estearias soils formed under distinct dynamics compared to typical ADEs. The novel association of anthropogenic and organic horizons in submerged estuarine soils highlights the complexity of environmental and human interactions in the region. These findings support the need for revising ADE classification criteria, particularly regarding the exclusive use of available phosphorus as a diagnostic parameter. Complementary studies, including micromorphology, mineralogy, and spatial and temporal analyses, are essential to understand the genesis and dynamics of these soils. Despite their limited agricultural potential, these soils represent a valuable scientific and cultural heritage.

Keywords
anthropization; hydromorphism; Anthropogenic Soils; paleoenvironments; Archaeological Dark Earth

INTRODUCTION

For at least 11,000 years BP, the Amazonian lowlands have been inhabited by hunter-gatherer groups whose progressive adaptation to the hot, humid tropical environment was essential to their survival and development. Their earliest records consist of cave shelters, such as those found in Carajás and Monte Alegre (PA), and later shell mounds (sambaquis) devoid of ceramics or agricultural traces. Over time, these sites evolved into more recent adaptations, often overlapping settlements, characterized by more complex archaeological soils formed through diversified cultures, with much broader geographic distribution patterns (Meggers, 1979; Roosevelt, 1992; Neves, 2000). Research across multiple disciplines seeks to understand not only the relationship between current landscape features and anthropogenic modifications but also how landscapes influenced the process of human occupation in the Amazonian environment (Miller and Nair, 2006; Arroyo-Kalin, 2010; Roosevelt, 2013; Maezumi et al., 2018; Oliveira et al., 2020).

Soils altered by pre-Columbian human activity are among the primary evidence of anthropogenic influence in the humid tropical environment. Studies indicate that the formation of these soils resulted from the accumulation of organic and mineral residues generated during the process of sedentism, which, over time, acted as an agent of change in their natural properties. The most internationally recognized anthropogenic soils to date are the Archaeological Dark Earths (ADEs), known for their dark coloration and traces of prolonged human occupation, including ceramic and lithic artifacts as well as partially carbonized residues. From a pedological perspective, ADEs are distinguished by their high concentrations of phosphorus, calcium, magnesium, zinc, and manganese, in addition to lower acidity, strong biological structuring, and remarkable resilience, and these sharply contrast with adjacent soils, which are generally nutrient-deficient and acidic (Costa and Kern, 1999; Lehmann et al., 2003; Schaefer et al., 2004; Kämpf and Kern, 2005; Kern et al., 2009; Garcia et al., 2015; Lombardo et al., 2022).

Most of the knowledge generated about ADEs is based on the physiographic context of the Amazon and Madeira River valleys, where early studies suggested that these soils were restricted to well-drained areas, known as "terra firme" (upland). However, subsequent research has documented their presence in floodplains, known as “várzea” (floodplain) environments (Stenberg, 1998; Myers et al., 2003; Souza et al., 2009; Fraser et al., 2011b; Macedo et al., 2019). This discovery revealed that ADEs sites were distributed throughout the complex Amazonian biosystem, embedded in a wide variety of soils and landscapes (Woods and McCann, 1999; Kern et al., 2003). Given their interconnected and complementary nature, it is estimated that ADEs cover approximately 3 % of the Amazon region, appearing in patches ranging from 0.5 to 350 hectares. However, these estimates may be questioned due to the lack of detailed soil surveys in the region (Erickson, 2003; Sombroek et al., 2003; Campos et al., 2011; McMichael et al., 2014).

In the transitional areas between the Amazon region, the coastal environment, and adjacent biomes, more recent research has sought to identify new occurrences of Anthrosols, not only to understand their genesis but also to integrate the findings with ethnographic data from these sites, shedding light on the dynamics of the communities that formed them (Schaan et al., 2006; Schmidt et al., 2014, 2023; Arregui, 2022). A notable example is the estearias Archaeological Sites, located in the Maranhão Lowland microregion (Maranhão State), which represents the western coastal sector of the Amazonian environment. These archaeological sites contain ceramic and lithic artifacts, as well as accumulations of charcoal and organic material in a soil layer that can exceed 0.40 m in thickness, exhibiting several morphological similarities to ADEs (Navarro, 2016, 2018a,b).

These sites, partially or entirely submerged in lakes and rivers, are characterized by wooden stakes driven into hydromorphic soils, known as esteios, from which the term estearias is derived. These structures represent the remains stilt villages (palafitas) that, according to radiocarbon dating, existed nearly 7,000 years BP, with their main occupation period dating back to 800-1100 BP, corresponding to the late pre-Columbian Amazonian occupation (Navarro, 2017, 2018b, 2022; Navarro et al., 2020). Most of these sites remain submerged year-round, but during a brief dry season (October to December), water levels recede enough to expose the soil surface and the esteios, revealing ceramic and lithic fragments, as well as organic residues on the surface and in the subsurface.

Despite the first mentions of the estearias dating back to the mid-18th century, these archaeological sites remain among the least studied in Brazil (Navarro, 2017, 2018a; Navarro and Roosevelt, 2021). The associated submerged anthropogenic soils developed under conditions of water saturation and reduction, yet no pedological characterization or comparison with other Anthrosols found in upland or other Amazonian environments has been conducted to date. This study aimed to characterize the estearias Anthrosols, analyzing their main morphological, physical, and chemical properties and comparing them with other anthropic soils from different Amazonian environments to assess the similarities and differences in ADEs under varying formation conditions.

MATERIALS AND METHODS

Study area characterization

Maranhão Lowland is located within the eastern limits of the Amazon biome, in the state of Maranhão, and comprises an extensive Environmental Protection Area (APA, the Portuguese acronym) of approximately 1,775,035.6 ha (Figure 1). It is surrounded by the flattened terrain of the Bacabal Submarine Surface, characterized by coalescing estuaries of the Mearim, Turiaçu, Itapecuru, and Pindaré rivers, lakes, swamps, floodable fields, and a mix of land uses (agroecosystems) (Conceição et al., 2013; Dantas et al., 2013; Farias Filho et al., 2013). According to the Köppen classification system, the climate of the Maranhão Lowland is of type Aw – tropical with concentrated rainfall from January to July and a dry season from August to December (Menezes, 2020). Precipitation ranges from 1,600 to 2,500 mm per year, and the average annual temperature is consistently above 18 °C (Baristella et al., 2013).

Figure 1
Location map of the Lontra, Formoso, and Encantado archaeological sites in the Maranhão Lowland region, Maranhão State, Brazil. Main hydrographic features of the area are highlighted.

Regarding geology, the Maranhão Lowland is located between the Barreirinhas and São Luís sedimentary basins, consisting of Tertiary rock substrate and extensive Quaternary sediment coverage, especially from the Holocene. Neotectonics is significant in the area, affecting clayey and sandy formations that form estuaries and plains with straight elevation changes. The relief is flat, with vast areas prone to flooding due to river dynamics. The Maranhão Lowland hosts various ecosystems, including forests, savannas, and floodplain fields, with a complex hydrographic network that shapes local vegetation and biodiversity (Ab'Saber, 2006; Suguio, 2010; Machado and Torres, 2012; Rocha et al., 2015).

Within this area, three estearia sites were selected, where the stilts do not remain submerged year-round: Lontra, Formoso, and Encantado. At these sites, six soil profiles were described and collected: two from each site, one in the anthropized area and the other in a position without evident anthropic influence (Figure 2). The profiles were described and collected according to the method described in Santos et al. (2015).

Figure 2
Anthropogenic soils of the estearias, non-anthropogenic soils, occurrence environment, and artifacts found. (a) and (b): Lontra site; (c) and (d): Formoso site; (e) and (f): Encantado site; (g): Ceramic artifact.

Laboratory analyses

For the chemical and textural analyses, the collected samples were air-dried, crushed, and sieved through a 2 mm mesh to obtain the air-dried fine earth fraction (ADF). Chemical characterization included the following parameters: pH(H₂O) at a soil solution ratio of 1:2.5; available and residual phosphorus; exchangeable bases (Ca²⁺, Mg²⁺, K⁺, and Na⁺), exchangeable aluminum (Al³⁺), and potential acidity (H+Al). From these data, sum of bases (SB = Ca²⁺ + Mg²⁺ + K⁺ + Na⁺), cation exchange capacity (CECp = BS + (H+Al)), base saturation (BS= (SB/CECp) × 100), and exchangeable aluminum saturation (Al S. = Al³⁺ / (SB + Al³⁺) × 100) were calculated. Total organic carbon (TOC) was determined by the wet oxidation method, using the Walkley-Black adapted method, which involves the oxidation of organic carbon with potassium dichromate (Teixeira et al., 2017).

Granulometric analysis was performed using 10 g of ADF with 50 mL of 0.1 mol L^-1 NaOH and 150 mL of deionized water, dispersed for 16 h at 50 rpm. Sand fraction was separated using a 0.053 mm mesh sieve and subdivided into fine sand (0.05 to 0.02 mm) and coarse sand (0.2 to 2 mm). Clay and silt suspension was transferred to a 500 mL graduated cylinder, completed with water for sedimentation according to Stokes Law. The clay fraction was obtained by the pipette method, and silt was determined by difference (Teixeira et al., 2017).

Comparative analyses with other anthropogenic soils in Brazil

For comparative analysis with other Archaeological Dark Earths (ADEs), graduate research studies available in national academic repositories, such as the Digital Library of Theses and Dissertations (BDTD) and institutional repositories of Brazilian universities, were consulted. The selection of studies was conducted through keyword searches (such as Amazonian Dark Earths, “Terra Preta de Índio”, and anthropic soils), followed by a full review of the documents to verify their suitability for the proposed objective. Only studies that provided detailed information on morphological, physical, and chemical properties, allowing comparison with the data obtained in this study, were included. Studies that lacked such information or dealt with anthropic soils outside the Amazonian context were excluded. In total, 160 anthropic horizons were considered, corresponding to 71 profiles from eight studies, whose titles and authors are listed in table 1.

Table 1
Number of profiles and studies from which the data were extracted

No. of profiles	Title of the dissertation or thesis	Author and year
3	Gênese, química, mineralogia e micromorfologia de solos da Amazônia Ocidental	Lima (2001)
8	Caracterização pedológica de arqueo-antropossolos no Brasil: sambaquis da Região dos Lagos (RJ) e Terras Pretas de Índio na região do baixo rio Negro/Solimões (AM)	Correa (2007)
8	Atributos fisícos, quimícos e mineralógicos de solos com horizonte antrópico (Terra Preta de Índio) em áreas de várzea do Rio Solimões, AM	Macedo (2014)
17	Gênese, mineralogia, micromorfologia e formas de fósforo em arqueo-antropossolos da várzea do Rio Amazonas	Souza (2011)
6	Características de atributos do solo em ambientes da região sul do estado do Amazonas	Aquino (2014)
2	Pedogênese e indicadores pedoarqueológicos em Terra Preta de Índio no município de Iranduba - AM	Macedo (2014)
4	Caracterização de solos antrópicos e não antrópicos ao longo de uma topossequência no município de Apuí-AM	Santos (2014)
6	Caracterização e Classificação de Solos com horizonte antrópico no Sul do Estado de Rondônia	Cavassani (2018)
17	Terras Pretas Arqueológicas no Médio Amazonas e Alto Solimões: química, mineralogia, micromorfologia e idade	Miranda (2018)

Data from the literature were then divided into two main Amazonian ecosystem environments, uplands and floodplains, and compared with the soils from the Maranhão Lowland. Principal Component Analysis and Pearson Correlation were applied, along with visualizations using scatter plots, boxplots, and histograms, to identify the main soil properties associated with each analyzed environment. This approach ensures that the comparative dataset is robust, comprehensive, and directly comparable with the Maranhão Lowland soils studied here. The analyses aimed to assess the primary differences between these environments and the variability of Amazonian Archaeological Dark Earths. The data were analyzed using the R statistical software (R Development Core Team, 2024).

RESULTS

General characteristics of Archaeological Anthrosols and associated soils

Profiles exhibiting signs of anthropization at the three sites presented anthropic A horizons (Au) characterized by dark colors (low value and chroma), cumulative thickness exceeding 0.20 m, and the presence of microfragmented charcoal as well as ceramic and lithic artifacts (Table 2). Two of these anthropic horizons (P1 and P5) were predominantly organic, classifying them morphologically as histic-sapric (H). Due to their mixed characteristics, these horizons were classified as Hu, combining both histic and anthropic properties.

Table 2
Morphological characteristics of the soils described at the Lontra, Formoso, and Encantado sites

Sites	Horizon	Layer	Structure1	Munsel Color
				Dry	Moist
		m
Lontra	P1 - Gleissolo Tiomórfico Órtico típico, A antrópico/ Dystric Stagnic Umbric Thionic Gleysols
	Hu	0.00 – 0.17	mo g/m ba; frc peq bs; gs	5YR 3/2	10YR 2/1
	Au	0.17 – 0.32	mo g/m ba; frc peq bs; gs	5YR 2.5/1	10YR 2/1
	Cg	0.32 – 0.54	mo peq ba	5YR 5/1	5YR 3/1
	Cgj1	0.54 – 0.73	mc	10YR 5/1 (2.5Y 8/8)2	10YR 2/2 (2.5Y 6/8)2
	Cgj2	0.73 – 0.90+	mc	10YR 5/1 (2.5Y 8/8)2	10YR 2/2 (2.5Y 6/8)2
	P2 - Gleissolo Melânico Tb Distrófico típico/ Dystric Stagnic Umbric
	A1	0.00 – 0.13	fr gr/med/pq bs	5YR 5/1	10YR 2/1
	A22	0.13 – 0.25	fr gr/md/pq bs	5YR 4/1	10YR 2/1
	Cg1	0.25 – 0.68+	mc	10YR 7/1	10YR 5/1
Formoso	P3 - Gleissolo Háplico Ta Eutrófico típico, A antrópico/ Eutric Stagnic Umbric Gleysols
	Au1	0.00 – 0.20	mo pq/md bs	10YR 3/2	10YR 2/1
	Au2	0.20 – 0.40	mo pq/md bs	10YR 4/2	10YR 2/2
	Cg1	0.40 – 0.60	mc	10YR 5/1	10 YR 4/1
	Cg2	0.60 – 0.80+	mc	10YR 6/1	11YR 4/1
	P4 - Organossolo Háplico Sáprico térrico/ Dystric Stagnic Umbric Gleysols
	H1	0.00 – 0.18	mo med/pq/mp bs	7.5YR 4.5/1	7.5YR 2.5/1
	H2	0.18 – 0.66	mc	5YR 3/1	5YR 3/1
	HC	0.66 – 0.84	mc	2.5YR 3/1	2.5YR 2.5/1
	Cg	0.84 – 1.00+	gs	7.5YR 6/4	7.5YR 4/3
Encantado	P5 - Organossolo Háplico Hêmico térrico, A antrópico/ Hemic Histosols
	Au1	0.00 – 0.15	fo med/pq bs	10YR 4/3	10YR 2/1
	Au2	0.15 – 0.22	fo med/pq bs	10YR 4/4	10YR 2/1
	2Hu	0.22 – 0.31	fc peq/mp bs	5YR 3/2	2.5YR 2.5/1
	3H	0.31 – 0.37+	mc	2.5YR 2.5/1	10YR 2/2
	P6 - Organossolo Háplico Hêmico térrico/ Hemic Histosols
	H1	0.00 – 0.14	mo me/pq/mp bs	7.5YR 2.5/2	10YR 2/1
	H2	0.14 – 0.37	mo me/pq bs	7.5YR 3/1	7.5YR 2.5/1
	2C	0.37 – 0.53	mo pq/mp; gs	10YR 6/4	10YR 4/4
	3H	0.53 – 0.73+	mc	2.5YR 4/3	2.5YR 3/2

• ⁽¹⁾
  Structure: Degree of development: fr (weak), mo (moderate), ft (strong). Size: mp (very small), p (small), m (medium), g (large), mg (very large). Type: gs (simple grains), gr (granular), bs (subangular blocks), lm (laminar), mc (massive).
• ⁽²⁾
  Color of the mottles.

At the Lontra site, the anthropic profile also contained gleyed horizons (Cg) with abundant medium-to-large yellowish mottles and a hydrogen sulfide odor, indicating the presence of sulfides. In contrast, non-anthropic profiles displayed subsurface gleyed horizons and dark surface horizons resulting from organic matter accumulation in periodically flooded environments, where histic horizons were common. Profile depths were generally limited to approximately 1.00 m, with variable structure and consistency among horizons.

Particle size analysis revealed a wide range of textures. In anthropic soils (P1, P3, and P5), sand, silt, and clay fractions ranged from 25 to 540 g kg^-¹, 123 to 492 g kg^-¹, and 210 to 803 g kg^-¹, respectively. In non-anthropic profiles (P2, P4, and P6), the corresponding ranges were 29–847 g kg^-¹ for sand, 49–396 g kg^-¹ for silt, and 103–853 g kg^-¹ for clay (Table 3). Fine fractions (silt and clay) predominated at Lontra and Encantado, whereas sand was more abundant at Formoso. Overall, anthropic horizons contained a higher proportion of sand, particularly fine sand.

Table 3
Granulometric composition and textural classification of the soils from the estearias

Sites	Horizon	Granulometry					Soil texture
		CS(1)	F. S(2)	T. S.(3)	Silt	Clay
		––––––––––––––– g kg-1 –––––––––––––––
Lontra	P1 - Gleissolo Tiomórfico Órtico típico, A antrópico/ Dystric Stagnic Umbric Thionic Gleysols
	Hu	50	64	114	276	610	Very clayey
	Au	18	114	132	382	486	Clay
	Cg	15	254	269	337	394	Clay loam
	Cgj1	39	16	55	454	491	Silty clay
	Cgj2	6	19	25	492	483	Silty clay
	P2 - Gleissolo Melânico Tb Distrófico típico/ Dystric Stagnic Umbric
	A1	18	47	65	396	539	Clay
	A2	7	49	56	276	668	Very clayey
	Cg1	140	62	202	267	532	Clay
Formoso	P3 - Gleissolo Háplico Ta Eutrófico típico, A antrópico/ Eutric Stagnic Umbric Gleysols
	Au1	10	504	514	245	241	Sandy clay loam
	Au2	11	422	433	296	271	Clay loam
	Cg1	18	522	540	250	210	Sandy clay loam
	Cg2	39	497	536	228	236	Sandy clay loam
	P4 - Organossolo Háplico Sáprico térrico/ Dystric Stagnic Umbric Gleysols
	H1	16	344	360	258	382	Clay loam
	H2	165	100	265	187	548	Clay
	HC	97	28	125	168	707	Very clayey
	Cg	124	723	847	49	103	Loamy sandy
Encantado	P5 - Organossolo Háplico Hêmico térrico, A antrópico/ Hemic Histosols
	Au1	27	155	182	285	533	Clay
	Au2	27	271	298	371	332	Clay loam
	2Hu	20	9	29	311	659	Very clayey
	3H	23	52	75	123	803	Very clayey
	P6 - Organossolo Háplico Hêmico térrico/ Hemic Histosols
	H1	12	25	37	205	758	Very clayey
	H2	23	16	39	107	853	Very clayey
	2C	70	225	295	366	339	Clay loam
	3H	119	13	132	171	696	Very clayey

• ⁽¹⁾
  C. S.: Coarse Sand;
• ⁽²⁾
  F. S.: Fine Sand;
• ⁽³⁾
  T. S.: Total Sand.

Soils in the estearias were generally acidic (pH <5.5). At Formoso and Encantado, anthropic horizons were less acidic than non-anthropic ones. At Lontra, anthropic horizons were extremely acidic (pH 2.4–3.7), while non-anthropic soils remained around pH 4.0. Total organic carbon (TOC) varied considerably, exceeding 80 g kg^-¹ in the non-anthropic profiles of Formoso and Encantado (P4 and P6). Both anthropic and non-anthropic profiles at Encantado exhibited histic horizons (3H) at their bases, with TOC contents of 232 and 322 g kg^-¹, respectively, indicating burial of organic matter (Table 4).

Table 4
Chemical properties of the soils described at the Lontra, Formoso, and Encantado sites

Sites	Horizon	Layer	pH	TOC	P	K	Ca2+	Mg2+	Na+	Al3+	H + Al
		m	H2O	g kg-1	––– mg kg-1–––		–––––– cmolc kg-1 ––––––
Lontra	P1 - Gleissolo Tiomórfico Órtico típico, A antrópico/ Dystric Stagnic Umbric Thionic Gleysols
	Hu	0.00 – 0.17	3.4	116.01	9.4	59	2.09	3.47	0.00	8.78	34.7
	Au	0.17 – 0.32	3.68	65.08	105.1	81	1.94	3.35	0.00	6.97	25.2
	Cg	0.32 – 0.54	2.42	42.34	4.9	60	1.56	2.81	0.00	5.56	16.4
	Cgj1	0.54 – 0.73	2.53	60.73	0.5	31	8.21	7.74	0.00	24.72	50.4
	Cgj2	0.73 – 0.90+	3.4	64.39	1.6	14	5.59	10	0.00	20.22	37.4
	P2 - Gleissolo Melânico Tb Distrófico típico/ Dystric Stagnic Umbric
	A1	0.00 – 0.13	3.91	78.42	70.3	102	2.07	3.44	0.00	5.6	23.7
	A2	0.13 – 0.25	4.08	82.77	45.9	126	1.77	2.79	0.00	6.17	22.6
	Cg1	0.25 – 0.68+	4.31	9.69	2.4	56	1.42	3.01	0.00	4.28	8.1
Formoso	P3 - Gleissolo Háplico Ta Eutrófico típico, A antrópico/ Eutric Stagnic Umbric Gleysols
	Au1	0.00 – 0.20	4.95	49.88	46.4	119	4.47	4.23	0.01	0.69	7
	Au2	0.20 – 0.40	4.78	34.40	77.1	159	4.64	4.99	0.00	0.58	6.8
	Cg1	0.40 – 0.60	4.8	31.67	102.4	108	4.44	3.32	0.00	0.31	5.6
	Cg2	0.60 – 0.80+	5.09	16.07	45.4	124	4.39	3.21	0.00	0.37	6
	P4 - Organossolo Háplico Sáprico térrico/ Dystric Stagnic Umbric Gleysols
	H1	0.00 – 0.18	3.91	84.86	5.3	123	5.59	5.29	0.01	1.25	16.9
	H2	0.18 – 0.66	3.96	184.69	0.8	151	9.46	8.66	0.00	1.22	42.2
	HC	0.66 – 0.84	4.06	147.91	1.2	270	7.41	9.35	0.00	1.66	34.4
	Cg	0.84 – 1.00+	3.97	4.87	1.6	35	0.57	0.62	0.00	0.44	2.1
Encantado	P5 - Organossolo Háplico Hêmico térrico, A antrópico/ Hemic Histosols
	Au1	0.00 – 0.15	4.11	67.75	10.1	134	2.95	4.37	0.00	2.14	15.8
	Au2	0.15 – 0.22	4.77	19.90	11.5	137	2.01	3.55	0.00	0.83	8.8
	2Hu	0.22 – 0.31	4.64	229.87	5.1	121	9.53	11.59	0.00	0.25	26.9
	3H	0.31 – 0.37+	4.52	231.90	3.4	112	6.8	8.91	0.00	0.23	27.3
	P6 - Organossolo Háplico Hêmico térrico/ Hemic Histosols
	H1	0.00 – 0.14	3.75	140.66	5.9	75	3.85	5.03	0.00	4.38	28.4
	H2	0.14 – 0.37	4.4	228.89	3.4	58	8.48	8.88	0.00	1.29	30.7
	2C	0.37 – 0.53	5.32	17.87	12	117	2.81	3.82	0.00	0.19	9.5
	3H	0.53 – 0.73+	4.14	322.10	1.4	93	10.23	10.44	0.00	0.83	40.4

Available phosphorus (P) in anthropic horizons ranged from 5.1 to 105.1 mg kg^-¹, with lower values at Encantado, while non-anthropic profiles were naturally P-poor. Potassium (K) ranged from 14 to 270 mg kg^-¹, without a clear pattern related to anthropization. Ca²⁺ and Mg²⁺ varied from 0.57–10.23 and 0.62–11.59 cmol_c kg^-¹, respectively, showing no significant differences between anthropic and non-anthropic profiles. Sodium (Na) was negligible in all profiles, likely due to limitations of the Mehlich-1 extractant. Exchangeable acidity (Al³⁺) was highest at Lontra, whereas potential acidity (H+Al) followed the same trend as TOC.

Effective cation exchange capacity (CTCe) ranged from 6.74 to 40.75 cmol_c kg^-¹ in anthropic horizons and from 1.72 to 21.74 cmol_c kg^-¹ in non-anthropic profiles, while potential CEC (CTCp) ranged from 13.64 to 66.43 and 6.38 to 61.31 cmol_c kg^-¹, respectively (Table 5). Only the anthropic profile at Formoso showed base saturation (V%) above 50 %, whereas aluminum saturation (m%) was highest at Lontra (>49 %).

Table 5
Chemical characteristics and micronutrient concentrations of soils described at the Lontra, Formoso, and Encantado sites

Site	Horizon	Layer	SB(1)	CECe(2)	CECp(3)	B.S. (4)	Al S. (5)	Micronutrients
								S	Zn	Cu	Fe	Mn
		m	––––––– cmolc kg-1 –––––––			––––– % –––––		––– mg kg-1–––
Lontra	P1 - Gleissolo Tiomórfico Órtico típico, A antrópico/ Dystric Stagnic Umbric Thionic Gleysols
	Hu	0.00 – 0.17	5.71	14.49	40.41	14.1	60.6	1104	18.54	0.32	95.4	28.4
	Au	0.17 – 0.32	5.50	12.47	30.70	17.9	55.9	819.7	20.72	0.23	23.9	23.7
	Cg	0.32 – 0.54	4.52	10.08	20.92	21.6	55.2	442.2	3.15	1.00	79.8	13.8
	Cgj1	0.54 – 0.73	16.03	40.75	66.43	24.1	60.7	4131.8	13.69	2.79	1253.6	50.4
	Cgj2	0.73 – 0.90+	15.63	35.85	53.03	29.5	56.4	3795.8	39.7	2.01	1587.4	83.2
	P2 - Gleissolo Melânico Tb Distrófico típico/ Dystric Stagnic Umbric
	A1	0.00 – 0.13	5.77	11.37	29.47	19.6	49.3	490.6	29.6	0.36	74	30.6
	A2	0.13 – 0.25	4.88	11.05	27.48	17.8	55.8	129.7	22.6	0.36	70.7	28.2
	Cg1	0.25 – 0.68+	4.57	8.85	12.67	36.1	48.4	43.9	2.55	0.96	100.7	15.6
Formoso	P3 - Gleissolo Háplico Ta Eutrófico típico, A antrópico/ Eutric Stagnic Umbric Gleysols
	Au1	0.00 – 0.20	9.01	9.70	16.01	56.3	7.1	33.7	11.57	2.07	742.1	39
	Au2	0.20 – 0.40	10.04	10.62	16.84	59.6	5.5	41.1	13.38	2.70	918.1	43
	Cg1	0.40 – 0.60	8.04	8.35	13.64	58.9	3.7	52.4	16.19	2.50	848	49.3
	Cg2	0.60 – 0.80+	7.92	8.29	13.92	56.9	4.5	33.5	12.71	2.29	1730.3	73.4
	P4 - Organossolo Háplico Sáprico térrico/ Dystric Stagnic Umbric Gleysols
	H1	0.00 – 0.18	11.20	12.45	28.10	39.9	10.0	457.9	3.97	0.82	516.2	56.6
	H2	0.18 – 0.66	18.51	19.73	60.71	30.5	6.2	755.1	8.50	0.85	1018	127.3
	HC	0.66 – 0.84	17.45	19.11	51.85	33.7	8.7	267.7	8.19	1.46	744.1	72.2
	Cg	0.84 – 1.00+	1.28	1.72	3.38	37.9	25.6	70.2	6.58	0.37	114	6.9
Encantado	P5 - Organossolo Háplico Hêmico térrico, A antrópico/ Hemic Histosols
	Au1	0.00 – 0.15	7.66	9.80	23.46	32.7	21.8	632.3	34.1	0.58	269.5	58.7
	Au2	0.15 – 0.22	5.91	6.74	14.71	40.2	12.3	154.6	39.1	0.60	193.7	27.5
	2Hu	0.22 – 0.31	21.43	21.68	48.33	44.3	1.2	897.2	57.3	0.25	140.2	111.3
	3H	0.31 – 0.37+	16.00	16.23	43.30	37.0	1.4	664.6	27.1	0.22	206.8	149.0
	P6 - Organossolo Háplico Hêmico térrico/ Hemic Histosols
	H1	0.00 – 0.14	9.07	13.45	37.47	24.2	32.6	793.9	24.8	0.73	425.8	72.6
	H2	0.14 – 0.37	17.51	18.80	48.21	36.3	6.9	332.3	27.1	0.25	109.6	82.4
	2C	0.37 – 0.53	6.93	7.12	16.43	42.2	2.7	92.9	18.42	1.23	169.6	15.2
	3H	0.53 – 0.73+	20.91	21.74	61.31	34.1	3.8	1142.8	26.3	0.28	111.9	112.3

• ⁽¹⁾
  SB: Sum of bases;
• ⁽²⁾
  CECeff: Effective cation exchange capacity;
• ⁽³⁾
  CECpot: Potential cation exchange capacity;
• ⁽⁴⁾
  B.S.: Base saturation;
• ⁽⁵⁾
  Al S.: Aluminium saturation.

Sulfur (S) contents were particularly high in P1 (Cgj1 and Cgj2: 4131.8 and 3795.8 mg kg^-¹), while other profiles ranged from 130 to 11,423 mg kg^-¹, except P3 (<35 mg kg^-¹), apparently correlating with TOC. Anthropic soils were enriched in Zn, while Cu distribution was variable with no consistent pattern. Available Fe ranged from 23.9 to 1730.3 mg kg^-¹, and Mn levels were low, neither showing a clear relationship with anthropization (Table 5).

According to the Brazilian Soil Classification System (SiBCS), only profiles P1 and P3 met the criteria for anthropic horizons. Profile P5, although displaying an anthropic A horizon and histic horizons containing artifacts, did not reach the minimum levels of available phosphorus and was classified as an Organossolo with anthropic characteristics. Non-anthropic profiles were classified according to their properties, resulting in Gleissolos and Organossolos of different histic or anthropic subtypes (Table 6). Under WRB criteria, none of the anthropic horizons qualified as pretic horizons, whereas non-anthropic profiles were classified as Gleysols and Histosols, reflecting their hydromorphic and organosolic properties.

Table 6
Classification of anthropic and non-anthropic soils described and collected at the Lontra, Formoso, and Encantado sites

Profile	SiBCS Classification	WRB Calssification
Lontra Site
P1	Gleissolo Tiomórfico Órtico típico, A antrópico	Dystric Stagnic Umbric Thionic Gleysols (Clayic, Humic, Uterquic).
P2	Gleissolo Melânico Tb Distrófico típico	Dystric Stagnic Umbric Gleysols (Clayic, Humic, Uterquic).
Formoso Site
P3	Gleissolo Háplico Ta Eutrófico típico, A antrópico	Eutric Stagnic Umbric Gleysols (Loamic, Humic, Uterquic)
P4	Organossolo Háplico Sáprico térrico	Dystric Stagnic Umbric Gleysols (Clayic, Humic, Uterquic)
Encantado Site
P5	Organossolo Háplico Hêmico térrico, A antrópico	Hemic Histosols (Dystric, Ortomineralic)
P6	Organossolo Háplico Hêmico térrico	Hemic Histosols (Dystric, Endomineralic, Mulmic)

Principal Component Analysis and the relationships of soil properties in the Estearias and Other Archaeological Dark Earths

Principal Component Analysis (PCA) (Figure 3) indicated that available P, thickness of the anthropic horizon, pH, TOC, Al³⁺, and clay content were the most relevant properties. The first two components explained 55.2 % of the variance, distributing the samples according to the characteristics of the different environments (Figure 4). The fluviolacustrine environment of the estearias showed high levels of Al³⁺ and clay, while pH displayed an inverse pattern.

Figure 3
Principal Component Analysis (PCA) for anthropogenic horizons in ADEs located in the Maranhão Lowland, Amazonian Upland and Floodplain.

Correlations among these properties were mostly weak to moderate, although specific patterns emerged for each environment (Figure 4). Across the entire dataset, the strongest correlations were between pH and Al³⁺ (r = -0.578) and between clay and Al³⁺ (r = 0.344), highlighting the strong association between acidity-related properties. In the estearia soils, correlations were particularly pronounced for clay (r = -0.638 and 0.455 with pH and Al³⁺, respectively).

Figure 4
Correlations between the principal components identified in the anthropogenic horizons of ADEs from the Maranhão Lowland, Amazonian Upland and Floodplain.

In the Maranhão Lowland, clay was also strongly correlated with TOC (r = 0.798), emphasizing its key role in protecting organic compounds. In floodplain soils, the negative correlation between TOC and pH (r = -0.377) was notable. The correlation between clay and anthropic horizon thickness was positive only in floodplain environments (r = 0.427). Available P showed positive correlations with pH (r = 0.289) and negative correlations with TOC (r = -0.212). In the estearia soils, P was negatively correlated with both TOC (r = -0.414) and clay (r = -0.427). Conversely, in floodplain soils, the correlation between P and clay was positive (r = 0.379).

The soil texture triangle (Figure 5) highlighted distinct patterns among the three environments. In the uplands, horizons displayed high variability, ranging from sandy and sandy loam to clay. Floodplain horizons were less variable, concentrating in intermediate classes such as loam and clay loam. In the fluviolacustrine estearia environment, more clayey classes predominated, ranging from clay to silty clay.

Figure 5
Granulometry distributed across the different textural classes of anthropic horizons studied by various authors in different environments.

Figure 6 shows the percentage distribution of soil classes across the three environments. In floodplain (V), Neossolos Flúvicos and Gleissolos Háplicos predominated, with a minor contribution from Cambissolos Flúvicos. In uplands (TF), the greatest diversity of soil classes was observed. In the fluviolacustrine estearia environment (FL), Gleissolos and Organossolos were dominant.

Figure 6
Soil classes with anthropic horizons studied by different authors. Amazonian Floodplain (AF), Amazonian Upland (AU), and Maranhão Lowland (ML) environments.

DISCUSSION

Estearias soils: Morphological, textural, and chemical properties

Estearia soils of Maranhão remain submerged for nearly ten months each year, and even during the dry season, the water table rarely drops below 0.80 m, limiting profile depth to around 1.00 m. These conditions promote intense hydromorphism, resulting in darkened horizons with high TOC contents due to organic matter accumulation and reduced decomposition rates (Van Den Broek and Van Der Marel, 1969; Mafra et al., 2007; Amendola et al., 2018; Marschner, 2021). In the mottles of P1, yellow to orange coloration suggests the possible presence of jarosite, an FeSO₄ mineral formed through sulfide oxidation under restricted drainage, typical of swamps or mangrove environments (Prada-Gamero et al., 2004; Bomfim et al., 2015; Gomes et al., 2016).

Seasonal sediment deposition strongly influences textural variability, a feature common in alluvial soils such as those in the Maranhão Lowland (Lima et al., 2006; Valladares, 2009; Campos et al., 2012; Cipriano-Silva et al., 2020). Mudflats associated with mangroves, similar to the Cgj horizons of P1, exhibit stratified layers ranging from fine sand to clay, reflecting depositional energy dynamics (Mendes, 2005). This sedimentary heterogeneity is reflected in structural and consistency differences between surface and subsurface horizons, evidencing genesis from diverse sedimentary sources (Lima et al., 2006; Montenegro and Montenegro, 2006; Cipriano-Silva et al., 2020).

Anthropic profiles showed higher sand contents, likely linked to fire management and the presence of ceramic debris discarded by pre-Columbian communities (Smith, 1980; Teixeira and Martins, 2003; Teixeira et al., 2009; Barros et al., 2016; Schaefer et al., 2025). In well-drained contexts, charcoal fragments and high temperatures can stabilize fine particles and organic residues into small aggregates, increasing the sand fraction in anthropic soils (Ketterings et al., 2000; Mataix-Solera et al., 2011; Bento-Gonçalves et al., 2012; Agbeshie et al., 2022). In the estearias, however, hydromorphism limits this effect, and the anthropic influence on sand content is mainly evident in the abundant charcoal within the Au and Hu horizons. This indicates that biomass burning primarily occurred at the surface of stilt-house platforms for food and ceramic preparation, as supported by the reddened and carbonized clay blocks observed in P6 (Figure 7), since the waterlogged soils for most of the year would have prevented significant thermal impacts.

Figure 7
Soil block with clear signs of carbonization and oxidation removed from the anthropic profile of the Encantado site (P6). (a) Position of the clay layer in the profile; (b) Enlargement of the oxidized face; (c) Soil block and charred material removed from the profile.

The estearia soils exhibit a predominance of fine sand over coarse sand, indicative of low-energy floodplains (Lima et al., 2006, 2007), typical of fluvio-lacustrine systems, which likely supported fishing as a primary food source for the communities inhabiting the stilt villages (Navarro, 2017, 2018ab; Navarro and Roosevelt, 2021). Extended flooding periods allow fine particles to remain suspended for longer and settle gradually, producing more clayey surface horizons. This surface clay accumulation reflects both environmental conditions and the prolonged lacustrine phase, whereas the high silt content points to low pedogenetic intensity, as sediment deposition exceeds the rate of genetic horizon development (Valladares, 2009; Guimarães et al., 2013; Farias Filho et al., 2020).

At the Formoso (P3) and Encantado (P5) sites, anthropic soils were less acidic than non-anthropic soils, a pattern observed in ADEs on upland but not in floodplain settings (Lima et al., 2002; Schaefer et al., 2004; Falcão et al., 2009; Macedo et al., 2017). This reduced acidity is attributed to the incorporation of ashes rich in Ca, K, and Na from the combustion of organic matter (Woods, 2003). Conversely, pH values in the Cgj1 and Cgj2 horizons suggest a thiomorphic character (mangrove influence), resulting from H₂SO₄ formation due to sulfide oxidation during flooding and drying cycles, redistributing compounds along the profile (Ferreira et al., 2007a,b; Combatt et al., 2013; Lee et al., 2021). Thus, in floodplains, the anthropization effect on pH is controlled by hydromorphism and the presence of native sulfides (Woods, 2003; Schaefer et al., 2025).

Total organic content was highly variable and unevenly distributed between surface and subsurface horizons, reflecting both anthropogenic additions and natural accumulation in reducing environments, consistent with the criteria for histic classification (Santos et al., 2018). Evidence suggests that original organic horizons were buried, while anthropic horizons formed over sediments richer in organic matter, representing an association between anthropic and organic soils.

Available phosphorus (P), a required criterion for anthropic horizons (>30 mg kg^-¹) (Santos et al., 2018; IUSS Working Group WRB, 2022), was not consistently reached, but the presence of lithic and ceramic artifacts near stilt villages supports and integrated with morphological features, demonstrating a clear anthropization processes. The articulated position of some artifacts and their direct association with structural elements indicate that part of the material remains in situ, although partial reworking or secondary transport by hydrological events cannot be completely ruled out, a characteristic of dynamic fluvio-lacustrine systems.

Low P levels may reflect community dietary habits, losses during flood cycles, or natural acidity favoring P fixation to Al (Glaser et al., 2001; Collins et al., 2002; Kämpf and Kern, 2005; Silva et al., 2011; Kendall et al., 2018; Mehmood et al., 2018; Cavassani et al., 2021). Phosphorus variability may also be linked to sediment dispersion and inheritance from previously weathered material (Lima et al., 2002; Macedo et al., 2019; Schaefer et al., 2025).

Exchangeable K, Ca, and Mg values reflect interactions between parent material, natural acidity, and deposition of organic or ceramic residues. Elevated K⁺ levels may derive from ceramic fragments rich in potassium (Lima et al., 2002; Correa, 2007; Falcão et al., 2009; Barros et al., 2012), while low Ca²⁺ and Mg²⁺ are associated with acidity that rapidly dissolves fish bones and spines (Smith, 1980; Schaefer et al., 2004; Kämpf and Kern, 2005; Novotny et al., 2009; Souza et al., 2009, 2016; Macedo et al., 2017). The H+Al follows the same pattern as TOC, indicating that most potential acidity results from organic matter decomposition (Falcão and Borges, 2006; Ebeling et al., 2008; Bento et al., 2020).

Sulfur concentrations in the Cgj1 and Cgj2 horizons of P1 indicate sulfide influence and suggest ancient mangrove ecosystems. The combination of high organic matter, anaerobic conditions, acidic pH, and elevated Al³⁺ favors sulfate reduction to sulfides, supporting the hypothesis that these areas were mid-Holocene paleomangroves, evolving into productive lacustrine systems before probable partial drainage at the end of the Holocene (Prada-Gamero et al., 2004; Vasconcelos et al., 2014; Bomfim et al., 2015; Kida and Fujitake, 2020; Moraes et al., 2021, 2022).

Zinc enrichment follows patterns observed elsewhere and is linked to decomposition of plant materials used in stilt villages, incorporated after site abandonment (Kern and Kämpf, 1989; Costa and Kern, 1999; Lima et al., 2002; Lehmann et al., 2003; Woods, 2003; Souza et al., 2009; Fraser et al., 2011a; Barbosa et al., 2020; Bento et al., 2020). Available Fe tends to be lower due to complexation with organic matter and redox dynamics in hydromorphic soils (Correa, 2007; Cunha et al., 2009; Frohne et al., 2011; Shaheen et al., 2019).

According to the SiBCS, only profiles P1 and P3 met the criteria for anthropic horizons. Profile P5, despite having an anthropic A horizon and H horizons with artifacts, did not reach the minimum available P and was thus classified as an Organossolo with anthropic traits. Other profiles were classified based on their properties, resulting in Gleissolos and Organossolos of various histic or anthropic subtypes (Table 6). Following WRB criteria, none of the anthropic horizons qualified as pretic horizons, whereas non-anthropic profiles were classified as Gleysols and Histosols, reflecting their hydromorphic and organosolic properties.

Archeoanthrosols across distinct Amazonian environmental contexts

In the fluvio-lacustrine estearia environment, high Al³⁺ and clay contents, grouped in the analysis, are associated with low pH, highlighting the characteristic acidity of these soils. This acidity reflects Al³⁺ saturation and the hydromorphic conditions of frequently flooded areas, within an acidic, nutrient-poor, and pre-weathered setting of the Barreiras Group domain (Mafra et al., 2007; Vasconcelos et al., 2014; Amendola et al., 2018; Corrêa et al., 2023). Although clay production through weathering in anthropic soils is possible, the correlation between clay content and acidity suggests that proton retention, enhanced by TOC and sulfurization processes, is the primary mechanism driving acidification in this environment.

In contrast, in the Amazonian floodplain, the thickness of anthropic horizons and P contents emerge as key factors, with thicker horizons showing higher P concentrations, reflecting the disposal of nutrient-rich materials and the practices and population densities of the communities that once inhabited these areas (Smith, 1980; Glaser et al., 2001; Kern et al., 2003; Kämpf and Kern, 2005). In these soils, the negative relationship between TOC and pH is more pronounced, indicating that naturally more alkaline soils become increasingly acidified as organic matter accumulates, intensifying organic acidification processes.

Upland soils exhibit greater variability, particularly regarding TOC, highlighting the combined influence of pedogenetic and biological factors, which is accentuated by efficient drainage and continuous cultivation after site abandonment. The depositional influence on clay accumulation is also evident in the correlations between clay content and the thickness of anthropic horizons. In the estearias, these correlations reflect natural clay retention in saturated environments, whereas in the floodplain, the positive correlation indicates depositional conditions with longer duration and higher energy, emphasizing distinct formation dynamics across environments (Lima et al., 2007; Campos et al., 2012; Cipriano-Silva et al., 2020).

The integrated interpretation of correlation analyses, the soil texture triangle, and environmental context suggests that in estearia anthropic horizons, the combination of high-water saturation, accumulation of clay and TOC, and sulfurization processes governs acidification. In floodplain and upland soils, other factors, such as sediment deposition, population density, and post-abandonment management, predominate in controlling chemical and textural dynamics, evidencing distinct pedogenetic trajectories across environments.

Implications for the classification of anthropic soils in the estearias

Differences observed in the anthropic soils of the estearias raise questions regarding the adequacy of the criteria currently employed in classification systems. Although these soils share elements with ADEs, their unique characteristics, such as persistent hydromorphism, the occurrence of histic horizons, and, in some cases, pronounced acidity, indicate marked distinctions in soils that have been generically referred to as “Terra Preta de Índio”. These distinctions should be considered when recognizing Antrossolos at the order level in the SiBCS.

The presence of an Organossolo with anthropic characteristics warrants special attention, as such an occurrence has not previously been documented in studies examining ADEs to date. Despite not fitting the conventional definitions of these dark earths, these soils retain unequivocal traces of past human activity, particularly in horizon organization and artifact accumulation. This finding broadens the spectrum of recognized archaeopedological situations, demonstrating that anthropic influence is not restricted to upland environments.

Identified soil classes directly reflect the hydromorphic conditions of the Amazonian floodplain. Gleissolos, and less frequently Organossolos, predominate under water-saturated conditions, favoring both organic matter accumulation and sulfide oxidation processes. In upland areas, a greater diversity of classes is observed, associated with the heterogeneous distribution of ADEs, ranging from well-drained zones to areas where partial drainage restriction does not result in full hydromorphism. Consequently, the coexistence of anthropic horizons across different pedoenvironmental contexts demonstrates that the variability of these formations extends beyond the concepts traditionally associated with ADEs, with direct implications for refining soil classification.

CONCLUSIONS

Estearia soils (under stilt villages) of Maranhão exhibit unique characteristics that expand the understanding of Anthrosols in complex hydromorphic environments, particularly in fluviolacustrine settings. In addition to anthropic inputs, hydromorphism and flooding cycles play a fundamental role in the formation of these soils, directly influencing their properties. When associated with anthropic horizons containing ceramic artifacts, charcoal, and organic sediments, these features demonstrate that estearias exhibit a distinct formation dynamic from typical Amazonian Archaeological Dark Earths (ADEs), both in upland and floodplain areas.

The superposition of anthropic and organic soils in former mangrove ecosystems underscores the complexity of environmental and human interactions in the region. The data highlight the need to revise the criteria for classifying ADEs, particularly regarding phosphorus content as a key determining parameter. These findings emphasize the archaeological and environmental relevance of the estearia sites, contributing to the understanding of the practices and adaptations of pre-Columbian communities that inhabited Amazonia.

Further attention is drawn to the importance of complementary investigations, both spatial (well-drained surrounding soils) and temporal (geochronology of occupation), as well as micromorphological and mineralogical analyses, which may deepen knowledge of the genesis and dynamics of these soils. Although little studied and unsuitable for agriculture, these soils represent a valuable scientific and cultural heritage.

ACKNOWLEDGMENTS

We thank Museu de Solos Dokuchaev (UEMA), Laboratório de Arqueologia (LARq/UFMA), and Núcleo Terrantar (UFV) for logistical support, as well as the Graduate Program in Soils and Plant Nutrition (PPGSNP/UFV) for research support.

• How to cite:
  Lalas A, Schaefer CEGR, Ker JC, Navarro AG, Siqueira RG, Firmino FHT, Silva MB. Submerged Archaeoanthrosols of the estearias (stilt villages) in Maranhão: A new frontier in Pedoarchaeology in Brazil. Rev Bras Cienc Solo. 2026;50nspe1:e0250065. https://doi.org/10.36783/18069657rbcs20250065
• FUNDING
  This study was conducted with the support of Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001.

DATA AVAILABILITY

The data will be provided upon request.

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Edited by

• Editor:
  Marcos Gervasio Pereira https://orcid.org/0000-0002-1402-3612 and Fabio Soares de Oliveira https://orcid.org/0000-0002-1450-7609

Publication Dates

• Publication in this collection
  27 Apr 2026
• Date of issue
  2026

History

• Received
  28 Feb 2025
• Accepted
  14 Oct 2025