Analyzing Environmental and Climate Records in Portuguese Speleothems using Ultra-high Sensitivity Magnetic Microscopy

MIT PIs:
Eduardo Andrade Lima – Principal Research Scientist Dept. of Earth, Atmospheric and Planetary Sciences

PT PIs:
Eric Claude Font – Faculdade de Ciências e Tecnologia da Universidade de Coimbra (FCTUC)

Abstract: Speleothems, defined as secondary mineral deposits formed in caves, are excellent recorders of climate during the Quaternary and may have continuously recorded variations in the Earth’s magnetic field during their growth period. Their age can be determined precisely using U-Th series disequilibrium dating, and the magnetic, geochemical and mineralogical signatures preserved in their thin laminations can provide high-resolution climate- and environmental proxy time series at subannual to millennial time-scales [1-3]. Speleothems host magnetic minerals that originate from the soils and rocks above a cavern system. As these magnetic minerals are incorporated into actively growing stalagmites, the grains acquire a detrital remanent magnetization (DRM), which has the ability to accurately record the direction and relative intensity of the Earth’s magnetic field and can be readily measured using standard and superconducting quantum interference device (SQUID)-based rock magnetometers [4-8]. Magnetic studies of speleothems also provide high-resolution records of climate variability by linking rock-magnetic properties to climate and environmental forcing parameters acting on soils [9-11].

However, applications on the use of speleothem magnetic properties as archives of climate change are at an embryonic stage and require a great deal of maturation before they can be comparable to traditional studies. A major limitation in correlating the magnetic signal of speleothem with standard environmental and climate proxies relies is the limited temporal scale (i.e., sample size) of paleomagnetic speleothem samples. Owing to their very weak magnetization, larger sample volumes are required to ensure detection by standard SQUID rock magnetometers, thereby averaging magnetic information over large time periods. For example, paleotemperature and paleoprecipitation measured by oxygen isotope composition are measurable at the sub-millimeter scale (i.e., annual to sub-annual scale) in the calcite laminae, whereas standard rock- and paleo- magnetic analyses usually require millimeter to centimeter sized (i.e., decadal to millennial scale) samples. Here we address this challenge by using the MIT scanning SQUID microscope and the Quantum Diamond Microscopes (QDM) available at MIT and Harvard University to investigate the magnetic signature of Portuguese speleothems recently collected by the Portuguese team. These recently developed techniques provide outstanding mapping of speleothem magnetization at a micrometer to sub-millimeter scale (Appendix I).

Our main objectives are to explore: i) the link with environmental forcing parameters acting on soils (paleoprecipitation, temperature); and ii) to test the use of speleothem as potential paleo-fire archives. We will base our methodology on a multidisciplinary approach combining magnetic, geochemical (including mercury), petrographic and mineralogical analyses, which leverages a newly formed collaboration between the project leader Eric Font (UC, IDL), Eduardo Andrade Lima (MIT) and an international consortium of research groups specializing in speleothem studies, including young Portuguese researchers. Results will be integrated into global isotope database (e.g. SISAL, [12]). We will be able to calibrate our database with temperature, precipitation and other climate constrains from regional historical and instrumental records over the last century (available at IPMA). For pre-instrumental periods, we intend to calibrate our data with sedimentary and ice records and with speleothems from North and South America currently studied by our international collaborators.

This proposal also connects with a larger ongoing collaborative project conducted by our international partners to investigate speleothem paleomagnetism (National Science Foundation grants # 2044806, 2044535, and 2044506). The implications of the expected results are as follows: i) consolidate our national and international competitiveness in this area of the geosciences, including the innovation and knowledge transfer between Portuguese institutions, MIT and our international partners; ii) provide innovative scientific tools to unravel recent climate and environmental changes in western Iberia; iii) explore the occurrence and frequency of forest fires in Portugal, thus contributing to fire risk assessment and management, and its associated impacts on human health; iv) promote the touristic value and natural heritage of the caves in Portugal and create outreach materials for schools and society; and v) provide scientific and societal contributions towards United Nation’s Sustainable Development Goals 6 (Water), 13 (Climate changes), and 15 (Terrestrial ecosystems).