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.
- Demographic impacts
- Northeastern Brazil
- Stable isotopes