Heinrich Stadials recorded in Northeast Brazilian Stalagmites

Abstract

During the last glaciation, periods of extreme cooling triggered massive freshwater and iceberg discharges into the North Atlantic. These periods of cooling, referred to as Heinrich Stadials, are believed to have caused an abrupt reorganization of atmospheric and oceanic circulation. Studying the behavior of tropical precipitation systems during Heinrich Stadials is important to understanding the response of tropical hydroclimate change to North Atlantic climate anomalies. In addition, tropical precipitation records may provide insight into the detailed timing and structure of Heinrich Stadials. Doing so is critical to understanding the underlying causes of Heinrich Stadials and their associated discharge events. The position of the Inter-Tropical Convergence Zone (ITCZ) plays a key role in the locality and amount of tropical precipitation worldwide. Paleo-records suggest that Heinrich Stadials triggered a southward migration of the ITCZ. The semiarid region of Northeast (NE) Brazil is located immediately south of the modern-day Atlantic ITCZ position. Short-lived speleothems that decorate the dry caves of NE Brazil suggest past periods of intense rainfall due to southerly migrations of the ITCZ. Previous studies have linked NE Brazil speleothem growth phases to Heinrich Stadials (Wang et al. 2004). Thus, reconstructing NE Brazilian pluvial periods will provide important insight into the chronology and structure of Atlantic ITCZ migrations associated with Heinrich Stadials. Here, we present a high-resolution, absolute dated, multi-stalagmite record of Heinrich Stadial (HS) 1, 4, 5 and 6 as recorded in NE Brazilian stalagmites. This thesis will focus on HS4 and HS1 time periods. Results show a detailed anti-correlation between NE Brazil and Northern Hemisphere subtropical records during HS4 and HS1, such as the Hulu Cave record from China (Wang et al. 2001). The synchronicity of these two distant records suggests a rapid transmission of atmospheric signals, likely through the global migration of the ITCZ. In addition, the 2-phased precipitation structure of HS1 and HS4 recorded in NE Brazil may reflect 2-stepped cooling observed in North and mid-Atlantic sea surface temperatures (Bond et al. 1992; Martrat et al. 2007). This supports the hypothesis that climate-ocean forcings were the underlying cause of Heinrich Events and highlights the relationship between mid-Atlantic sea surface temperatures and the position of the Atlantic ITCZ during the last glaciation. Arid NE Brazil is situated between the Amazon and the Atlantic rainforests. Evidence suggests that the pluvial periods associated with Heinrich Stadials promoted an ecological "bridge"� between both rainforests. This bridge may have permitted the transfer of species between rainforests. In this thesis, we compare NE Brazil speleothem precipitation records to NE Brazil palynological marine records to suggest the precise timing of rainforest expansion during HS1. Characterizing these NE Brazil pluvial anomalies are critical in understanding the link between climate change and the response of environmental systems.