Chemical weathering rates and atmospheric/soil CO2 consumption of igneous and metamorphic rocks under tropical climate in southeastern Brazil

Taxas de intemperismo químico e consumo de CO2 atmosférico/do solo em rochas ígneas e metamórficas sob clima tropical no sudeste do Brasil.

ABSTRACT

Chemical weathering rates and atmosphere/soil CO₂ consumption of igneous and metamorphic rocks under tropical climate in southeastern Brazil were evaluated using the chemical composition of surface waters and fresh rocks and soil (horizon C) in the Upper Sorocaba River basin. Surfacewater sampleswere collected between June/2009 and June/2010, and analyses were performed to assess pH, electrical conductivity (EC), temperature and total dissolved solids (TDS), including Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, SO₄ ²⁻, PO₄ ³⁻, NO₃ − and SiO₂. Fresh rocks and C horizon samples were also collected, taking into account their geological context, abundance and spatial density, to analyze major elements and mineralogy. The concentration of TDS and dissolved cations, anions and silica increased during the dry period in relation to the wet period, and the same behavior was observed for pH, EC and temperature. After corrections of anthropogenic contributions (ca. 21 t/km2/yr) and atmospheric inputs (ca. 19 t/km²/yr), the annual flux due to chemical weathering involving the igneous and metamorphic rocks was ca. 29 t/km2/yr. The CO₂ atmospheric/soil consumption in the Upper Sorocaba River basin was ca. 0.2 Å~ 106 mol/km2/yr, and when extrapolated to the entire Mantiqueira Orogenic Belt, accounted an estimated consumption of 0.07 Å~ 1012 mol/yr, representing 0.6% of the total CO₂ consumption flux derived from global average silicate weathering. The chemical weathering rates of igneous and metamorphic rocks in the Upper Sorocaba River basin were estimated at 15 m/My, respectively. The main weathering process in this watershed was the monossialitization, with partial hydrolyses of bedrock minerals, except quartz, which was not weathered and remained in the soil profile. The annual specific flux derived from igneous and metamorphic rocks at Upper Sorocaba River basin could be compared with watersheds in tropical climates. However, this value is higher than in other North American, European, Asian and African granitoid watersheds, and lower than in montane watersheds.

Introduction

The determination of the chemical weathering and physical erosion is of interest to geoscientists, as this phenomenon provides the parameters needed for better soil exploration, in order to assist establishing agricultural fields and human settlements. Besides, the balance between these phenomena contributes to the geomorphological modeling of the Earth’s surface. Chemical weathering of fresh rocks is the main mechanism of CO₂ consumption from the atmosphere and, consequently, has the basic function of moderating the Earth’s climate. The physical erosion is related to soil loss, acts on the weathered surface by removing the cover and carrying the particulate matter. In both processes, the dissolved and particulate materials are transported through rivers to the oceans, resulting in the deposition of Ca and Mg carbonates (and smaller amounts of Fe and Mn) and sediments, respectively.

The chemical weathering can be evaluated from the silica mass balance or from models using the dissolved sodium, calcium, potassium, magnesium and silica and total dissolved load concentrations, where inputs from rainfall require corrections (Amiotte-Suchet and Probst, 1993, Bain et al., 2001, Boeglin and Probst, 1998, Boeglin et al., 1997, Clow and Drever, 1996, Dessert et al., 2001, Dessert et al., 2003, Dupré et al., 2003, Edet et al., 2013, Gaillardet et al., 1997, Gaillardet et al., 1999, Gao et al., 2009, Gibbs, 1967, Grasby and Hutcheon, 2000, Gurumurthy et al., 2012, Johnson et al., 1968, Kattan et al., 1987, Land et al., 1999, Laraque et al., 2013, Li and Zhang, 2008, Li et al., 2014, Liu et al., 2016, Louvat, 1997, Louvat and Allègre, 1997, Louvat and Allègre, 1998, Louvat et al., 2008, Millot et al., 2002, Nkounkou and Probst, 1987, Oliva et al., 2003, Pacheco and Van der Weijden, 2002, Peray, 1998, Probst, 1986, Probst et al., 1994, Semhi et al., 2000, Stallard and Edmond, 1981, Stallard and Edmond, 1983, Stallard and Edmond, 1987, Tardy, 1968, Tardy, 1969, Tardy, 1971, Van der Weijden and Pacheco, 2006, Wang et al., 2016, White and Blum, 1995, Wu et al., 2013, Xu and Liu, 2010, Zakharova et al., 2007, Zhang et al., 2016).

The Earth’s upper continental crust is constituted of ca. 25% for granitoid rocks, with their chemical weathering controlled mainly for temperature and runoff (Oliva et al., 2003). The Brazilian Southeast region is marked by the presence of Mantiqueira and Tocantins orogenic systems, where outcrops igneous and metamorphic rocks, and part of the São Francisco Craton (Fig. 1a). Due to their areal extent and the strong susceptibility to chemical weathering of these rocks under tropical climate, these areas play an important role in the global consumption of CO₂. Surprisingly, no chemical weathering rates have been proposed for igneous and metamorphic rocks under tropical climate in these orogenic systems located in South America. Only few studies have focused on the chemical weathering rates of silicate rocks under different climates in Brazil, i.e. (a) equatorial climate: silicate rocks (igneous, metamorphic and sedimentary) in the giant Amazon River basin (Mortatti and Probst, 2003); (b) tropical climate: high grade metamorphic rocks of granulite facies in Bahia State (Moreira-Nordemann, 1980), Poços de Caldas alkaline rocks in Minas Gerais State (Bonotto et al., 2007) and sedimentary (mainly sandstones and mudstones – Conceição and Bonotto, 2003, Conceição and Bonotto, 2004) and basalts (Conceição et al., 2015) rocks from Paraná Sedimentary basin in São Paulo State; (c) semi-arid climate: metamorphic (gneiss, amphibolite and quartzite) in Bahia State (Moreira-Nordemann, 1984).

A study involving a mass-balance in a large basin would allow to estimate the dissolved material flux of the Mantiqueira and Tocantins Orogenic Belts, characterizing and quantifying the respective chemical weathering rates and atmospheric/soil CO₂ consumed during the water-rock/soil interaction process. However, this research would be very complex because of the different types of rocks, climate, soil thickness, reliefs and human activities in every region of this great basin. Studies of chemical weathering and atmospheric/soil CO₂ consumption in small watersheds, with one single type of rock, allow understanding the control parameters these processes and show that chemical dynamics in these areas can be applied globally (Millot et al., 2002). Thus, a small watershed could be chosen to assess the current chemical weathering rates and atmospheric/soil CO₂ consumption of igneous and metamorphic rocks under tropical climate in Brazil.

In this context, the Upper Sorocaba River basin, located in the eastern part of São Paulo State, Brazil, is an ideal study area to assess the chemical weathering rates and atmospheric/soil CO₂ consumption of igneous and metamorphic rocks under tropical climate from the Ribeira Belt, which is composed for metamorphic rocks of the Embu Complex and the São Roque Group, with their associated granites (Fig. 1b). Also, the influences of rainwater and human activities on the annual flux of elements or compounds transported by the Upper Sorocaba River were explained. In addition, after these necessary corrections, the main chemical weathering processes and associated CO₂ flux were modeled using the weathering index defined by Tardy (1971) and the mass balance between fluvial and pluvial transport of Ca, Mg, K and Na, respectively. Thus, our results provide new insight into the chemical weathering and atmospheric/soil CO₂ consumption of granitoid rocks in South America, which can be used to complement global knowledge about the water-rock interaction process in granitoid watersheds under different climates conditions in future studies.