Magnetic fingerprinting of modern continental sediments in Northeastern Brazil

Northeastern Brazil (NEB) is one of the most hydroclimatically sensitive regions in South America. Its globally semi-arid hydroclimate is shaped by the seasonal migration of the Intertropical Convergence Zone (ITCZ). Paleoclimate records documented a southward shift of the mean ITCZ position and intensified precipitation over NEB during millennial-scale events, which mobilized large quantities of detrital material transported to the adjacent Atlantic margin.

Environmental magnetism offers a non-destructive, high-resolution approach to assess the sediment provenance, weathering intensity, and mineralogical transformations. Magnetic minerals such as magnetite, hematite, and goethite carry unique coercivity and thermal signatures that reflect their formation and transport history. Few paleoclimate studies showed an increase in high-coercivity minerals in NEB marine sediments during past millennial-scale events, which may reflect enhanced riverine input from intensely weathered continental regions. However, the interpretation of magnetic records is limited by the absence of modern reference datasets from upstream continental sources.

Here we provide the first comprehensive rock-magnetic characterization of modern NEB continental sediments to better trace their provenance and improve the paleoclimatic interpretation of magnetic records in marine sediment cores. We investigated the magnetic mineralogy of about 80 modern sediment samples collected within the Parnaíba and the Maranhão hydrological systems using a suite of environmental magnetic techniques, which includes the acquisition and demagnetization of the Natural, Anhysteretic and Isothermal remanent magnetizations (NRM, ARM, IRM), stepwise thermal demagnetization of 3-axes IRM, hysteresis loops, backfield IRM curves with unmixing of coercivity spectra and thermomagnetic curves.

First results highlight the diversity of modern magnetic signatures within the Parnaíba and the Maranhão basins. Different mixing proportions of low-coercivity minerals such as magnetite versus high-coercivity minerals such as hematite and goethite seem to reflect contrasting source conditions within NEB in terms of rainfall amount, weathering intensity and lithology. In addition, while samples dominated by magnetite are abundant in regions with a crystalline bedrock and in downstream areas close to river mouths, samples with a high proportion of high-coercivity minerals (hematite, goethite) dominate in upstream NEB regions. Therefore, a grain-size sorting process may also be at play along the Parnaíba and the Maranhão hydrological systems and contribute to explain the spatial differences in modern magnetic mineralogy observed within NEB.