A comprehensive study of precipitation variability in convergence zones through trace elements in speleothems

• Grant number: 25/03747-7
• Support Opportunities: Scholarships abroad - Research Internship - Doctorate
• Start date: May 26, 2025
• End date: August 29, 2025
• Field of knowledge: Physical Sciences and Mathematics - Geosciences
• Principal Investigator: Francisco William da Cruz Junior
• Grantee: Nathalie Melissa Martinez Medina
• Supervisor: Juan Pablo Bernal Uruchurtu
• Host Institution: Instituto de Geociências (IGC). Universidade de São Paulo (USP). São Paulo , SP, Brazil
• Institution abroad: Universidad Nacional Autónoma de México, Juriquilla (UNAM), Mexico
• Associated to the scholarship: 23/14903-4 - The influence of the intertropical convergence zone on the South American Summer Monsoon during the last glacial period, BP.DR

Abstract

Paleoclimate reconstructions in South America strongly rely on the interpretation of speleothem ¿18O values. However, these records are often modulated by more than one process, complicating their interpretation, particularly in convergence zones where remotely sourced air masses interact. Emerging records from central Brazil and northern Venezuela underlying the South Atlantic Convergence Zone (SACZ) and the Intertropical Convergence Zone (ITCZ) show an intriguing nearly neutral trend in oxygen isotope values during the Holocene period. Which contrast the expected ¿18O trend towards depleted (enriched) values following the austral (boreal) summer insolation pacing. These findings underscore the need for complementary local effective moisture proxies such as trace elements whose supply vary according to the prevalent hydroclimate and environmental conditions. Indeed, this apparent mid- to late- Holocene divergent rainfall history between the SACZ region and the SAMS domain has been recently challenged by 86Sr/87Sr records interpreted as residence time within the vadose zone above the cave. Whereas, local proxies in northern Venezuela have yet to be developed for comparison with the iconic titanium concentration record from the Cariaco Basin. Therefore, the main goal of this research is to contribute to the understanding of the local hydroclimate variability in convergence zones, by analyzing trace elements in speleothem samples collected in caves underlying the SACZ and the ITCZ. The analysis will be conducted using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) at the Centro de Geociências da Universidad Nacional Autónoma de México (UNAM) under the guidance of Juan Pablo Bernal PhD, a leading specialists in the analysis of speleothem trace elements. It is expected to compare the Sr/Ca results obtained by LA-ICP-MS with those previously measured by X-ray Fluorescence (XRF). And determine, based on the correlations between Sr/Ca, Mg/Ca and Ba/Ca whether prior calcite precipitation has modulated their variability during speleothem growth. Additionally, we will investigate climate events in the central Brazil lowlands by identifying elements such as Fe, Si, Mn, and Al, which are typically associated with detrital minerals associated with cave drainage paleofloods. This approach will provide independent evidence of local hydroclimate conditions and, in particular, extreme rainfall events.