Modern terrigenous sediment sources and past changes in precipitation over tropical South America recorded in terrigenous sediments deposited in the western equatorial Atlantic

Author(s)

Júlia Grigolato Iani

• Document type: Master's Dissertation
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto de Geociências (IG/BT)
• Defense date: 2023-09-22

Examining board members

Cristiano Mazur Chiessi; Thiago Pereira dos Santos; Marília Harumi Shimizu

• Advisor: Cristiano Mazur Chiessi

Abstract

The effect of the slowdown of the Atlantic Meridional Overturning Circulation (AMOC) on precipitation in the Amazon Basin is a crucial issue for the future of the Amazon rainforest. At the end of the Quaternary, the AMOC strength was significantly reduced during the Heinrich Stadials (HS). These millennial-scale periods of AMOC decline offer a valuable opportunity to examine and assess the impact of AMOC intensity on Amazonian precipitation. Despite the efforts made during the last decade to understand the effects of HS on the Amazon Basin, the response of this transcontinental river system to changes in AMOC strength during the most recent HS (i.e., HS0 or Younger Dryas, 12.9 11.7 cal ka BP) remains uncertain. This masters dissertation aimed at characterizing how changes in AMOC strength affected Amazonian precipitation during the last 28,000 years. To achieve this objective, in a first step, major elements in suspended sediments from the Negro, Solimões and Amazonas Rivers were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES) covering a complete annual hydrological cycle, in order to evaluate the current fluvial dynamics of the Amazon Basin. These data provide a solid basis for the second stage of the dissertation, namely the paleoclimatic reconstruction of the Amazon Basin, carried out with emphasis on the Heinrich Stadial 1 (HS1) (18.6 14.7 cal ka BP) and the Younger Dryas. The paleoclimate reconstruction was based on two marine sediment cores collected from the western equatorial Atlantic, radiocarbon dated, analyzed by energy dispersive X-ray fluorescence (ED-XRF) and interpreted with the aid of principal component analysis. Interpretation of the data produced for the Younger Dryas was also aided by a compilation of hydroclimatic records from the Amazon Basin and its surroundings, as well as outputs from the transient TraCE-21ka run of the numerical climate model CCSM3. The results show that HS1 had two distinct phases: HS1a between 18.6 and 16.7 cal ka BP and HS1b between 16.7 and 14.7 cal ka BP. HS1a is characterized by high values of ln(Ti/Ca) and ln(Ti/Al), and low values of ln(Al/K) and ln(Fe/K), whereas HS1b is characterized by relatively lower values of ln(Ti/Ca) and ln(Ti/Al), and relatively higher of ln(Al/K) and ln(Fe/K). The differences between HS1a and HS1b are related to a shift in the main locus of precipitation: the central And es during HS1a and the southeastern lowlands of the Amazon Basin during HS1b. The change in the main locus of precipitation over the Amazon Basin was likely driven by a marked decrease in AMOC strength during HS1b, that reached its lowest level during the last deglaciation. Results for the Younger Dryas suggest distinct precipitation responses over the Amazon Basin. While its beginning is marked by an increase in ln(Ti/Ca) and ln(Al/K) and a decrease in ln(Fe/K) and ln(Ti/Al), the remainder of the event presents lower values of ln(Ti/Ca) and ln(Ti/Al) and higher values of ln(Al/K) and ln(Fe/K). We suggest that the decrease in austral summer insolation that characterized the period between HS1 and the Younger Dryas weakened the South American Monsoon System, and the Intertropical Convergence Zone gradually became the main source of moisture for the Amazon Basin. These atmospheric features influence precipitation over the Amazon Basin in distinct ways. The different responses of Amazonian precipitation during HS1 and the Younger Dryas are, therefore, related to the slowdown of the AMOC and changes in austral summer insolation. The variation in precipitation response in each event and sub-event reinforces the importance of paleoclimate studies in the Amazon Basin and the need to compartmentalize it in terms of its high complexity and continental dimension. (AU)

• FAPESP's process: 21/00853-0 - Modern terrigenous sediment sources and past changes in precipitation over tropical South America recorded in terrigenous sediments deposited in the western tropical Atlantic
• Grantee: Júlia Grigolato Iani
• Support Opportunities: Scholarships in Brazil - Master