Browsing by Subject "ITCZ"

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    Heinrich Stadials recorded in Northeast Brazilian Stalagmites
    (2015-05) Wendt, Kathleen
    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.
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    Multi-Proxy Evidence Of Southward Migration Of The Intertropical Convergence Zone During Heinrich Stadial 1
    (2023-08) Hansen, Wyatt
    Heinrich event 1 (H1) and its associated stadial period (HS1) resulted in an aridification in the northern hemisphere subtropics and wetter conditions in the Southern Hemisphere subtropics (Deplazes et al.,2013; Wang et al., 2001). Pluvial periods in Northeast (NE) Brazil (Auler et al., 2004) associated with Heinrich Stadials are characterized by a two-phased increase in rainfall amount and a transition to a wetter and more diverse vegetation regime (Dupont et al., 2010) attributed to a southward shift of the Intertropical Convergence Zone (ITCZ). This two-phased structure is well documented from palynological and sediment core records but lacks the high-precision dating required to fully understand the exact timing and duration of the pluvial period. This issue is shared with IPCC models which have a difficult time characterizing the precipitation response in the tropics to modern climate change. By fully understanding past pluvial responses to climate forcings, climate models and mitigation strategies can be improved.Heinrich tuned growth intervals of speleothems in Toca da Barriguda cave in NE Brazil capture past pluvial periods. This study presents the U-Th dated speleothem record, TBR-14, from Toca Da Barriguda in NE Brail during HS1. The main growth interval of TBR-14 extended from 17,029 ±76 years B.P. (before 1950 A.D.) to 15,850 ±90 years B.P. The record consists of oxygen isotopes (δ18O), trace elements, and confocal fluorescence during the pluvial period. The resulting record reveals the timing and structure of the NE Brazil pluvial anomaly associated with HS1 at an unprecedented resolution and precision. δ18O displays a two-phased shift in values related to the latitudinal range of the ITCZ. The oxygen isotope structure is characterized by an initial wet period lasting until 16,166 ± 61 years B.P (-4 to -6 ‰ ?18O), followed by a wetter period ending at 15,850 ± 90 years B.P (-6 to -8 ‰ ?18O). The onset of the wetter period occurred abruptly with a dramatic drop in ?18O (~1.4‰) in two years. The fluorescent banding structure, prior calcite precipitation (PCP) indices, and phosphorus/calcium (P/Ca) ratios indicate that the transition resulted in the wettest period of NE Brazil during HS1. The isotopic drop occurs, within error, at the same time and rate that China shifts abruptly to a drier climate (Treble et al., 2007). On a broader timescale, PCP indices, P/Ca, and confocal banding depict decreased aridity and precipitation changes. These changes are consistent with the ecological bridge hypothesis, which attributes vegetational changes recorded in regional records for the cause of periodic species exchange between the Atlantic and Amazon Rainforests.