Human influences on the present denudation rates of the Paulista Peripheral Depression, Brazil

Influências humanas nas taxas atuais de denudação da Depressão Periférica Paulista, Brasil

ABSTRACT

Chemical weathering and soil erosion in large or small watersheds are used to understand the effect of evolution processes on the Earth’s surface and climate. Currently, human-driven land use changes have substantial effects on landscape changes over a range of temporal and spatial scales. In this context, the Sorocaba River basin, São Paulo State, Brazil, is an ideal study area to assess human influence on the present denudation rates of the Paulista Peripheral Depression (PPD). Twelve fluvial water sample collections were carried out at the Sorocaba River mouth, covering one complete hydrological cycle (Jun/2009 to Jun/2010). In the same period, 46 rainfall events were sampled.

All sampleswere analyzed for dissolved cations, anions and silica, and total dissolved solids (TDS). Total suspended solids (TSS) were measured only in fluvial samples. The export of TDS and TSS occurs mostly during the wet season, accounting for ca. 60% and 87% of the total dissolved solids and total suspended sediment transported in the hydrological cycle studied (2009/2010), respectively. The total annual specific flux of TSS was 74.27 t/km2/year (including soil erosion by agricultural activities), with a small portion being derived from the anthropogenic contributions (ca. 2%).

The total annual specific flux of TDS (74.43 t/km2/year) was similar to the TSS, but after the correction of the atmospheric inputs and anthropogenic contributions (ca. 18 and 29%, respectively), this value decreased to 39.35 t/km2/year. The chemical weathering rate was 7.1 m/Myear and this process tended towardsmonosialitization (RE=2.6),with an atmospheric/soil CO2 consumption rate of 3.3 Å~10-5 mol/km2/year. The difference between the chemicalweathering and soil removal rates (7.1 and 49.6 m/Myear, respectively) indicated that the soil thickness reduction occured in the Sorocaba River basin. The climatic control on chemical weathering and soil removal rates was clearly evidenced, with the highest values of denudation occurring in the wet and hot climatic conditions.

However, the chemical weathering processes (RE index) was not sensitive to climatic controls. In order to assess human influence on chemical weathering and soil removal rates in the PPD, the resultswere comparedwith otherworks undertaken in this vast geomorphological province. Even considering the uncertainties associated with a number of data points, the chemicalweathering and soil removal rates in the PPD observed in this study were approximately 4 and 7.5-fold higher than these natural denudation rates, respectively, evidencing the effect of recent land use changes on the present denudation rates in the PPD. Thus, this study reinforces the complexity of the human-landscape systems in São Paulo State and increases the values of long-term landscape evolution rates.

Keywords: Landscape evolution, Water-rock interactions, Human-landscape system, Brazil

RESUMO

Intemperismo químico e erosão do solo em grandes ou pequenas bacias hidrográficas são usados ​​para entender o efeito dos processos de evolução na superfície e no clima da Terra. Atualmente, as mudanças no uso da terra causadas pelo homem têm efeitos substanciais nas mudanças da paisagem em uma gama de escalas temporais e espaciais. Neste contexto, a bacia do rio Sorocaba, estado de São Paulo, Brasil, é uma área de estudo ideal para avaliar a influência humana nas taxas atuais de denudação da Depressão Periférica Paulista (PPD).

Doze coletas de amostras de água fluvial foram realizadas na foz do rio Sorocaba, cobrindo um ciclo hidrológico completo (jun/2009 a jun/2010). No mesmo período, 46 ​​eventos de chuva foram amostrados. Todas as amostras foram analisadas para cátions dissolvidos, ânions e sílica, e sólidos dissolvidos totais (SDT). Sólidos suspensos totais (SST) foram medidos apenas em amostras fluviais. A exportação de SDT e SDT ocorre principalmente durante a estação chuvosa, representando ca. 60% e 87% do total de sólidos dissolvidos e sedimentos suspensos transportados no ciclo hidrológico estudado (2009/2010), respectivamente.

O fluxo específico anual total de SST foi de 74,27 t/km /ano (incluindo erosão do solo por atividades agrícolas), com uma pequena parcela derivada de contribuições antropogênicas (aproximadamente 2%). O fluxo específico anual total de SST (74,43 t/km/ano)  ^/ / ano) foi semelhante ao TSS, mas após a correção das entradas atmosféricas e contribuições antrópicas (aproximadamente 18 e 29%, respectivamente), esse valor diminuiu para 39,35 t/km² ^/ ano.

A taxa de intemperismo químico foi de 7,1 m/M²/ano e esse processo tendeu à monossialitização (RE _{= 2,6), com uma taxa de consumo de CO²}  atmosférico/solo de 3,3 × 10³ ^ol  /km²^/ R _E/ano. A diferença entre as taxas de intemperismo químico e remoção de solo (7,1 e 49,6 m/Mano, respectivamente) indicou que a redução da espessura do solo ocorreu na bacia do Rio Sorocaba.

O controle climático sobre as taxas de intemperismo químico e remoção de solo foi claramente evidenciado, com os maiores valores de denudação ocorrendo nas condições climáticas úmidas e quentes. No entanto, os processos de intemperismo químico ( índice) não foram sensíveis aos controles climáticos. A fim de avaliar a influência humana nas taxas de intemperismo químico e remoção de solo na PPD, os resultados foram comparados com outros trabalhos realizados nesta vasta província geomorfológica.

Mesmo considerando as incertezas associadas a vários pontos de dados, as taxas de intemperismo químico e remoção de solo na PPD observadas neste estudo foram aproximadamente 4 e 7,5 vezes maiores do que essas taxas de denudação natural, respectivamente, evidenciando o efeito das mudanças recentes no uso da terra nas taxas de denudação atuais na PPD. Assim, este estudo reforça a complexidade dos sistemas humano-paisagem no Estado de São Paulo e aumenta os valores de longo prazo  taxas de evolução da paisagem a longo prazo .

Palavra-chave: Evolução da paisagem, Interações água-rocha, Sistema humano-paisagem, Brasil

Introduction

The dissolved and suspended loads carried by rivers to the oceans are generally related to several processes and interactions involving climatic, hydrological, physical, chemical and biological conditions. Chemical weathering is a typically a destructive process, where the primary minerals of the bedrock are chemically weathered in secondary minerals due to chemical reactions from the joint action of water and the atmospheric/soil CO₂. This process releases the soluble ions that supply the dissolved load to the rivers and the residual products of the chemical weathering provide the materials that constitute the soil profile (Millot et al., 2002).

On the other hand, soil erosion promotes a reduction in the soil thickness and the material removed is subject to the transportation and sedimentation processes. The sediment load (suspended load and bed load) carried by the rivers is related to the soil erosion. In this work, the sediments load will be considered only as the suspended load. In synthetic terms, chemical weathering supplies the dissolved load to rivers, whereas soil erosion supplies the solid loads, and these processes play an important role in the evolution of the Earth’s surface (Négrel et al., 2007). Thus, the present denudation rates can be measured using the dissolved and suspended load carried by rivers.

The watershed is characterized as an important unit for conducting hydrochemical studies related to chemical weathering and soil removal rates. The changes that occur in the drainage area are reflected in the quality of surface water and can be evidenced by the monitoring of river systems (Mortatti and Probst, 2003) from the assessment of loads of cations, anions and dissolved silica and suspended particulate matter. The first studies to investigate the nature and composition of the suspended and dissolved load transported by rivers were carried out in the 1960s and 1970s (e.g., Barth, 1961; Gibbs, 1967, Gibbs, 1970; Johnson et al., 1968; Tardy, 1968, Tardy, 1969, Tardy, 1971; Martin and Meybeck, 1979).

Since then, many studies have been developed, including those that used mass balance models and adjusted atmospheric and anthropogenic contributions where necessary, to identify the fractions from the soil denudation and/or rock weathering processes and determine their respective rates (e.g., Meybeck, 1987; Stallard and Edmond, 1987; Lasaga et al., 1994; White and Blum, 1995; Gaillardet et al., 1997, Gaillardet et al., 1999; Boeglin and Probst, 1998; Dessert et al., 2001; Millot et al., 2002; Dessert et al., 2003; Mortatti and Probst, 2003; Oliva et al., 2003; Walling and Fang, 2003; West et al., 2005; Louvat et al., 2008; Moon et al., 2014).

However, the natural landscape evolution has been modified by different human activities (Murray et al., 2009). These human-landscape systems generate several impacts on the temporal and spatial scales (Harden et al., 2014). Land use/land cover changes (LULCC) are responsible for the increase of soil erosion, a process naturally influenced by the climate, soil, vegetation, and relief (Summerfield, 1991). Montgomery (2007) proposed that the soil removal rates in plowed agricultural fields are 1–2 orders of magnitude greater than rates of soil production, erosion under native vegetation and long-term geological erosion.

Studies on land use changes have been developed in America, Asia, Africa and Europe (Van Rompaey et al., 2002; Latocha and Migon, 2006; Latocha, 2009; Mhangara et al., 2012; Mugagga et al., 2012; Bruschi et al., 2013; García-Ruiz et al., 2013; Kim et al., 2014; Sanyal et al., 2014; Sun et al., 2014; Gessesse et al., 2015; Karamesouti et al., 2015; Serpa et al., 2015; Hernández et al., 2016; Kosmas et al., 2016; Latocha et al., 2016; Tarolli and Sofia, 2016; Zhang and Shanggaun, 2016; Zope et al., 2016).

In Brazil, only a few studies have documented the impacts of land use changes in rural or urban areas (Rodrigues, 2005; Costa and Bacellar, 2007; Simon and Cunha, 2008; Beskow et al., 2009; Latrubesse et al., 2009; Minella et al., 2009; Perez Filho and Quaresma, 2011; Moraes et al., 2012; Oliveira et al., 2015; Rosolen et al., 2015; Borrelli et al., 2017; Lupinacci et al., 2017; Couto Júnior et al., 2019).

Precambrian Cratons, Orogenic Belts and Phanerozoic Sedimentary Basins are the main geodynamic structures in Brazil territory, with different Geomorphological Provinces (Fig. 1a). Three geomorphological provinces have been recognized locally in the São Paulo State: the Atlantic Plateau (AP), the Paulista Peripheral Depression (PPD) and the Occidental Plateau (OC) (Fig. 1b).

According to Almeida (1964), the PPD was formed during the Neogene and Pleistocene periods, between the Paraná CFB (continental flood basalt) province of the Paraná Basin and the igneous/metamorphic rocks of Mantiqueira Orogenic Belt (Fig. 1b) and mainly consists of clastic sedimentary rocks from the Paraná Basin, and represents a sculpted. The Sorocaba River basin (Fig. 1c) is located in the southeastern part of the São Paulo State.

Since its occupation in the seventeenth century, the Sorocaba River basin has gone through successive cycles of development and has seen the diversification of human activities, and is currently under strong anthropic pressure, with a population of ca. 1 million inhabitants, 1850 enterprises and large agricultural areas (IPT, 2006; IBGE, 2010). The LULCC in the Sorocaba River basin may affect the present denudation rates in this watershed, the mouth of which is located in the PPD. Thus, this work aim to assess human influences on the present chemical weathering and soil removal rates in the PPD, obtained through dissolved and suspended loads, respectively, and carried by Sorocaba River.

Section snippets

Study area

The Sorocaba River basin covers an area of 5269 km² and is located between latitudes 23 and 24°S and longitudes 47 and 48°W (Fig. 1c). The average annual temperature ranged from 18 to 22 °C (IPT, 2006). According to the Köppen (1948) classification, the climate is a Cwa type, characterized by the predominance of higher values of rainfall and discharge in the summer period (October to March) in relation to dry periods (April to September). The pluviometric and fluviometric historical data (DAEE, 

Methods

According to Moon et al. (2014), the assessment of chemical weathering and soil removal rates is necessary to consider the sampling frequency and variability in the river discharge, concentration and contribution of different sources. In the same study, the author suggests at least 10 and preferably 40 temporal chemical data points with synchronous discharge from each river, with uncertainties within −28 to 22%, −22 to 16% and −14 to 15% with 10, 24 and 40 samples (50% confidence). Twelve

Contribution of different sources to fluvial fluxes

The contribution of different sources to fluvial fluxes of dissolved cations, anions and silica, as well as the TSS and TDS, were calculated using a mass balance model expressed in Eq. (2), modified from White and Blum (1995). The model used did not consider the fluxes resulting from the biomass change and derived from the ionic exchange sites in clay minerals because they have negligible contributions, close to “zero”, according to White and Blum (1995).where: 

Chemistry of surface waters

The results of Q, pH, EC, T and concentrations of dissolved ions, SiO₂, TDS and TSS, with their respective averages for the study period, are in Table 1. The Q values showed seasonal variation in consonance with the historical data of the monthly average (Fig. 2), with an average discharge for the study period of 146.62 m³/s. The Sorocaba River waters presented a pH ranging from 6.9 to 7.3 during the study period (average of 7.0). The EC showed significant seasonal variation (average of

Seasonality and contribution of different sources to fluvial fluxes

The fluvial fluxes integrate the contributions of the chemical weathering and soil removal that occur in the watershed, and also the atmospheric inputs and, in some regions, the anthropogenic inputs associated with pollution from industrial, agricultural and/or urban activities, which can reach the river in punctual or diffuse contribution forms (Stallard and Edmond, 1981; Probst et al., 1994; Mortatti et al., 1997; Conceição and Bonotto, 2004; Hissler and Probst, 2006; Conceição et al., 2015a; 

Conclusion

This study allowed a better understanding of the chemical weathering of rocks and the soil erosion processes that occur in a tropical region of South America, through the evaluation of the fluvial transport dynamics in the Sorocaba River basin. The concentration of dissolved cations, anions and silica increased during the period with lower discharge. The most abundant cations and anions in surface waters were Ca²⁺ and HCO_3⁻. The TSS concentration was directly related to the discharge and

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.

Acknowledgments

The authors are grateful to the Fundação de Amparo à Pesquisa do Estado de São Paulo (Process No. 08/57104-4 and 08/09369-9), and to the Conselho Nacional de Desenvolvimento Científico e Tecnológico, (Process No. 134169/2009-3) for their financial support, and to the Stable Isotope Laboratory of the Center for Nuclear Energy in Agriculture, São Paulo, Brazil for the research infrastructure. In particular, Dr. Achim A. Beylich and two anonymous referees are thanked for their detailed and

References (117)

T.F.W. Barth Abundance of the elements, areal averages and geochemical cycles Geochim. Cosmochim. Acta (1961)

S. Beskow et al. Soil erosion prediction in the Grande River Basin, Brazil using distributed modeling Catena (2009)

J.I. Boeglin et al. Physical and chemical weathering rates and CO₂ consumption in a tropical lateritic environment: the upper Niger basin Chem. Geol. (1998)

V.M. Bruschi et al. Analysis of geomorphic systems’ response to natural and human drivers in northern Spain: implications for global geomorphic change Geomorphology (2013)

C. Dessert et al. Erosion of Deccan Traps determined by river geochemistry: impact on the global climate and he ⁸⁷Sr/⁸⁶Sr ratio of seawater Earth Planet. Sci. Lett. (2001)

C. Dessert et al. Basalt weathering laws and the impact of basalt weathering on the global carbon cycle Chem. Geol. (2003)

J.M. Edmond High precision determination of titration alkalinity and total carbon dioxide content of seawater by potentiometric titration Deep-Sea Research Part I: Oceanogr. Res. Pap. (1970)

A.M. Fernandes et al. Chemical weathering rates and atmospheric/soil CO₂ consumption of igneous and metamorphic rocks under tropical climate in southeastern Brazil Chem. Geol. (2016)

J. Gaillardet et al. Chemical and physical denudation in the Amazon river basin Chem. Geol. (1997)

J. Gaillardet et al. Global silicate weathering and CO₂ consumption rates deduced from the chemistry of large rivers Chem. Geol. (1999)