Changes in insolation driven by precession and obliquity are considered the major driver of tropical precipitation on orbital time scales, and responsible for vegetation and physical landscape changes during the Late Holocene over tropical South America. Here we investigate the environmental changes in the karst region of Chapada do Apodi - Northeastern Brazil (NEB), using a multi-proxy approach including carbon (δ13C), oxygen (δ18O) and strontium (87Sr/86Sr) isotopic analyses on speleothems from different caves, carbonate bedrock, and clastic cave deposits. This approach reveals that the balance between soil formation and erosion and their alternating impact on vegetation and precipitation changes occurred in response to variations in the position and intensity of the Intertropical Convergence Zone (ITCZ) over the region. The high δ13C and δ18O and low 87Sr/86Sr values at 4,200 yrs BP indicate a massive episode of soil erosion, resulting in the exposure of carbonate bedrocks over a large area of the karst terrain. This event marks the beginning of the Meghalayan chronozone, characterized as the aridification of this region, decline in soil production, drying out of underground drainages, and increased dominance of dry-adapted flora species, characteristic of a more open vegetation (caatinga). We investigated if the Holocene climatic changes affected human occupation in the NEB and found that the overall demographic course is virtually identical to the well-established curve characterized by population deflation during Middle Holocene.
Bibliographical noteFunding Information:
We thank Christian Millo, Alyne Barros M. Lopes and Osmar Antunes (LES-IGc- USP, Brazil ), and Luiz Mancini (LAIS-IGc- UNB, Brazil ) for their support during the analyses, André Zular (IGc-USP, Brazil) for support during the field trip and grain-size analysis, Johan Etourneau (Université de Bordeaux, France) for comments about this paper. We are grateful to Leda Zogbi for the Trapiá cave map and to Professor Dr. Francisco Hilário Bezerra (UFRN, Brazil) and A.N.P., Brazil for aerial images, and Carlos Mazocca (IGc-USP, Brazil) for images editing. We thank Jocy Brandão Cruz, Diego de Medeiros Bento, José Iatagan Mendes de Freitas, Darcy José dos Santos, Uilson Paulo Campos (CECAV/RN), Antônio Idaelson do Nascimento and Geilson Góes Fernandes for all support in the field trip, information and data about the caves. This work was supported by the FAPESP , Brazil through PIRE NSF- FAPESP [ 2017/50085–3 to F.W.C], as well as the fellowships to G.U. [ 2020/02737–4 ]. V.F.N [ 2016/15807–5 ], A.S. [ 2017/16451–2 ] and A.C.B. [2019/12981–2], and the regular research grant [2019/15914–4 to A.S.]. The NSF, United States support through grants [AGS-1303828 and OISE-1743738] to MV and 1103403 to R.L.E and H.C. is acknowledged. The NSFC, China support through grant [ NSFC 41888101 ] to H.C. is acknowledged. N.M.S. acknowledges the support of CNPq, Brazil [Grants 423573/2018–7 ; 308769/2018–0 ] and CAPES, Brazil [Grant 88887.310301/2018–00 ]. G.U. is grateful to CAPES for the PhD and PosDoc fellowships through the Programa de Pós-Graduação em Geoquímica e Geotectônica at Universidade de São Paulo, Brazil.
The other possibility for high 87Sr/86Sr values from Early to Middle Holocene in NEB is the enhanced weathering in response to a wetter climate combined with reduced erosion rates, which would promote soil layer thickening above the cave. This scenario would result in a higher availability of radiogenic Sr in thicker soils above the cave (Fig. 4). High rates of soil accumulation could also explain the more negative ?13C values of speleothems observed in the same period, which would have increased the contribution of more depleted carbon derived from organic material in the soil (Burns et al., 2002; Genty et al., 2003; Cruz et al., 2006; Denniston et al., 2017; Jones et al., 2018). This assumption is supported by the presence of very negative ?13C in bulk organic matter (OM) of soils, which varies from ?24 to ?32? in the areas covered by caatinga dry forest in NEB (Pessenda et al., 2010). However, the ?13C of speleothems could not be used to investigate changes of the vegetation type, as the isotopic values in the OM of these soils show a similar range to the ones covered by rainforest in coastal humid areas in Brazil (e.g. Calegari et al., 2017; Buso Junior et al., 2019).In addition, the marine record offshore NEB also suggests opening of forest during the Late Holocene, as inferred from the increase in C4 grass species at the time when the caatinga forest retracted and a semi-arid climate was established in the region (Mulitza et al., 2017, Fig. 4i). This notion is supported by vegetation model simulations that show dominance of savanna in the Middle Holocene, while open shrubland, typical of caatinga dry forest, expanded in NEB during the last 4,000 yrs BP (Maksic et al., 2018).We thank Christian Millo, Alyne Barros M. Lopes and Osmar Antunes (LES-IGc-USP, Brazil), and Luiz Mancini (LAIS-IGc-UNB, Brazil) for their support during the analyses, Andr? Zular (IGc-USP, Brazil) for support during the field trip and grain-size analysis, Johan Etourneau (Universit? de Bordeaux, France) for comments about this paper. We are grateful to Professor Dr. Francisco Hil?rio Bezerra (UFRN, Brazil) and A.N.P. Brazil for aerial images and Carlos Mazocca (IGc-USP, Brazil) for images editing. This work was supported by the FAPESP, Brazil through PIRE NSF-FAPESP [2017/50085?3 to F.W.C], as well as the fellowships to G.U. [2020/02737?4]. V.F.N [2016/15807?5], A.S. [2017/16451?2] and A.C.B. [2019/12981?2], and the regular research grant [2019/15914?4 to A.S.]. The NSF, United States support through grants [AGS-1303828 and OISE-1743738] to MV and 1103403 to R.L.E and H.C. is acknowledged. The NSFC, China support through grant [NSFC 41888101] to H.C. is acknowledged. N.M.S. acknowledges the support of CNPq, Brazil [Grants 423573/2018?7; 308769/2018?0] and CAPES, Brazil [Grant 88887.310301/2018?00]. G.U. is grateful to CAPES for the PhD and PosDoc fellowships through the Programa de P?s-Gradua??o em Geoqu?mica e Geotect?nica at Universidade de S?o Paulo, Brazil.
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- Demographic impacts
- Northeastern Brazil
- Stable isotopes