Rock Magnetic Record of Environmental Changes over the Last 40.000 Years in the Westermost Mediterranean: the Alboran Sea Sedimentary Record

We present here a high-resolution rock magnetic study of deep-sea marine sediments from the Alboran Sea (Westernmost Mediterranean). The analyzed sediment record, core GP03, 889 cm in length, covers the last 40.000 years from upper Pleistocene glacial times through the Last Glacial Maximum, subsequent deglaciation, Younger Dryas cooling reversal and the whole Holocene up to the current Industrial Period. The sediment core was sampled at high resolution, and 379 samples were measured for their bulk magnetic susceptibility, frequency-dependent susceptibility, hysteresis cycles and Isothermal Remanent Magnetization (IRM) direct and reverse acquisition curves. A subset of 125 samples were subjected to thermomagnetic experiments up to 700º C in order to reveal Curie and thermal transformation temperatures and to support the identification of the precise magnetic mineralogy. The variations in the magnetic mineralogy along the analyzed record, together with previously published geochemical data, have allowed to track the main paleoenvironmental and paleoclimatic changes in the studied region over the last 38 ka, as well as documenting a very strong reductive dissolution horizon affecting magnetic phases in the Younger Dryas sediments, which points to a significant deoxygenation event. A strong correlation of magnetic parameters and stadial-interstadial fluctuations during the last glaciation, especially between 25 and 38 ka, is observed, pointing to variations in riverine vs. aeolian terrigenous input. In modern sediments (approximately the last 200 years), a sharp increase in magnetite abundance in the sediments has been recognized, coinciding with the timespan of the Industrial Period. We conducted magnetic extraction to concentrate the ferromagnetic fraction of these sediments, followed by Scanning Electron Microscopy and EDS analysis, and identified strongly ferromagnetic microspherules with textures typical of rapid crystallization from high temperature melts. These microspherules, rich in magnetite, are indistinguishable from typical fly ash magnetic microspherules of industrial origin, which support they are the magnetic fingerprint of anthropogenic industrial ferromagnetic phases in this marine setting that superimposed on the natural paleoenvironmental changes during the uppermost Pleistocene and Holocene.