Spatiotemporal Intertropical Convergence Zone dynamics during the last 3 millennia in northeastern Brazil and related impacts in modern human history

Abstract

Changes in tropical precipitation over the past millennia have usually been associated with latitudinal displacements of the Intertropical Convergence Zone (ITCZ). Recent studies provide new evidence that contraction and expansion of the tropical rain belt may also have contributed to ITCZ variability on centennial timescales. Over tropical South America few records point to a similar interpretation, which prevents a clear diagnosis of ITCZ changes in the region. In order to improve our understanding of equatorial rain belt variability, our study presents a reconstruction of precipitation for the last 3200 years from the northeastern Brazil (NEB) region, an area solely influenced by ITCZ precipitation. We analyze oxygen isotopes in speleothems that serve as a faithful proxy for the past location of the southern margin of the ITCZ. Our results, in comparison with other ITCZ proxies, indicate that the range of seasonal migration, contraction, and expansion of the ITCZ was not symmetrical around the Equator on secular and multidecadal timescales. A new NEB ITCZ pattern emerges based on the comparison between two distinct proxies that characterize the ITCZ behavior during the last 2500 years, with an ITCZ zonal pattern between NEB and the eastern Amazon. In NEB, the period related to the Medieval Climate Anomaly (MCA – 950 to 1250 CE) was characterized by an abrupt transition from wet to dry conditions. These drier conditions persisted until the onset of the period corresponding to the Little Ice Age (LIA) in 1560 CE, representing the longest dry period over the last 3200 years in NEB. The ITCZ was apparently forced by teleconnections between Atlantic and Pacific that controlled the position, intensity, and extent of the Walker cell over South America, changing the zonal ITCZ characteristics, while sea surface temperature changes in both the Pacific and Atlantic stretched or weakened the ITCZ-related rainfall meridionally over NEB. Wetter conditions started around 1500 CE in NEB. During the last 500 years, our speleothems document the occurrence of some of the strongest drought events over the last centuries, which drastically affected population and environment of NEB during the Portuguese colonial period. The historical droughts were able to affect the karst system and led to significant impacts over the entire NEB region.