Two Millennia of South Atlantic Convergence Zone Variability Reconstructed From Isotopic Proxies

Abstract

Most reconstructions of the South American Monsoon System (SAMS) over the last two millennia are based on δ18O records from locations at high‐elevation sites in the Andes, which are not influenced by the South Atlantic Convergence Zone (SACZ). Yet the SACZ is a key driver of SAMS variability over much of Brazil. Here we use two new δ18O records from speleothems sampled in the central and southwestern portions of the SACZ core to show that the SAMS was not varying in phase over the entire tropical continent during the last two millennia. In fact, speleothem records located to the northeast of the SACZ record precipitation variations that are antiphased with similar records on the opposite side of the SACZ, in particular during the Little Ice Age period, while records close to the core of the SACZ axis show no significant departure from the mean state during this period.

Late Quaternary Variations in the South American Monsoon System as Inferred by Speleothems—New Perspectives using the SISAL Database

“…One possible driver of the variability in the Lapa Grande δ 18 O time series are changes in regional rainfall amounts associated with latitudinal fluctuations of the SACZ (Figure 2). Latitudinal fluctuations and shifts of the SACZ affect rainfall amounts in boundary regions, while precipitation amounts in the centre of the SACZ are inferred to be constant [22]. Thus, Lapa Grande's position in the centre of the SACZ, and steady mean δ 18 O values of Lapa Grande speleothems during the Holocene, imply little changes in precipitation amount at this site and suggest it was under the influence of the SACZ during the entire Holocene.…”

“…Warm conditions in the NH force a northward positioning of the ITCZ that weakened the convective activity of the SAMS. This weakening was exhibited as an increase in speleothem δ 18 O values during the MCA in the records: Diva Cave from Northeast Brazil [19], Pau d'Alho and Jaraguá caves from mid-western Brazil [18,22], Palestina [28] and Cascayunga caves [13] from the Peruvian Andes. However, speleothem records from Tamboril [32] and São Bernardo [22] caves located in central Brazil, and Cristal Cave [7] from southeast Brazil do not show a significant anomaly during the MCA.…”

“…This weakening was exhibited as an increase in speleothem δ 18 O values during the MCA in the records: Diva Cave from Northeast Brazil [19], Pau d'Alho and Jaraguá caves from mid-western Brazil [18,22], Palestina [28] and Cascayunga caves [13] from the Peruvian Andes. However, speleothem records from Tamboril [32] and São Bernardo [22] caves located in central Brazil, and Cristal Cave [7] from southeast Brazil do not show a significant anomaly during the MCA. Conversely, the speleothem records from the Umajalanta-Chiflonkhakha Cave system in the eastern Bolivian Andes [14] suggest increased convective activity during the MCA period, with decreasing speleothem δ 18 O values, which was interpreted as a result of a different mechanism (not ITCZ/SAMS system), implying that moisture was sourced mainly from the southern tropical Atlantic [14].…”

“…Most of the records only have δ 18 O data, their resolution is approximately annual, and, with exception of the ALHO6 stalagmite that is annually laminated, all speleothems' age models are based on U/Th ages. The records included are DV2 (Diva Cave), TR5 (Torrinha Cave), LD12 (Lapa Doce Cave) from northeast Brazil [19]; CR1* (Cristal Cave) from southeast Brazil [7]; SBE3 (São Bernardo Cave), SMT5 (São Matheus Cave), TM0 (Tamboril Cave) from central Brazil [22,32]; ALHO6 (Pau d'Alho Cave), CUR4 (Curupira Cave) from middle-western Brazil [18,22]; PAR1 (Paraiso Cave) from eastern Amazonia [29]; CAS-A (Cascayunga), CAS-D (Cascayunga), PAL3 (Palestina Cave), PAL4 (Palestina Cave) from the Peruvian Andes [13,28,31] and Boto1 (Chiflonkhakh Cave), Boto3 (Chiflonkhakh Cave), Boto7, (Chiflonkhakh Cave), Boto 10 (Umajalanta Cave) from the Bolivian Andes [14].…”

Vegetation, rainfall, and pulsing hydrology in the Pantanal, the world’s largest tropical wetland

“…During the wet season, from October to April, low level convergence over southern South America associated with the ITCZ results in the advection of moist air masses via northwesterly winds derived from the Amazon Basin [10,26,27]. Rainfall variability is thought to have strong linkages to ENSO; however, recent studies have also pointed to the occurrence of quasi-periodic heavy rainfall events associated with the South American Convergence Zone (SACZ), which are driven by the southern Atlantic and the Madden-Julien Oscillation [26,28,29].…”