HYDROTHERMAL IRON IN THE SANTORINI CALDERA – INSIGHTS FROM IRON SPECIATION AND IMPLICATIONS FOR ANCIENT BANDED IRON FORMATIONS

Banded iron formations are important mineral resources and often used in ancient Earth environmental reconstructions, though their depositional mechanisms remain poorly known. Santorini, an active volcanic system in the South Aegean Volcanic Arc (SAVA), hosts modern intra-caldera iron- and silica-rich hydrothermal vents. Metagenomic studies indicate iron oxide biomineralization in the sediments and iron-rich bacterial mats. Milos, another volcanic system in the SAVA, hosts previously studied Algoma-type ~2-million-year-old iron formations as well.

International Ocean Discovery Program (IODP) Expedition 398 drilled sites within the Santorini caldera and outside in nearby extensional basins with the goal of recovering sediments and volcanic material to characterize the formation and eruptive history of the Christiana-Santorini-Kolumbo system. As part of this expedition, we recovered sediments from two sites: U1595 within the Santorini caldera and U1599 in the nearby Anafi Basin. We analyzed their pore water chemistry and solid phase iron speciation to reconstruct the past variability and spatial extent of hydrothermal activity, geochemical signals preserved in the sediments, and diagenetic effects on discrete iron-rich layers. In intra-caldera sediments, which were deposited after the ~1650 BCE caldera forming Minoan eruption, we identified two intervals with peaks in iron content: the uppermost sediments and between 55-60 meters below seafloor. This deeper interval directly underlies an eruption sequence in 726 CE, suggesting enhanced iron-rich hydrothermal activity prior to volcanic events. Sediments recovered outside the caldera are significantly older (up to 5.33 Ma) but still have a multitude of discrete iron-rich intervals evident in both the sediment chemistry and as visually identifiable cm-scale horizontal layers in the cores.

By comparing the two sites, we are better constraining the timing of hydrothermal activity, the spatial extent of hydrothermal deposits, and the preservation of iron-rich layers through diagenesis. These findings, combined with additional analyses performed during IODP Expedition 398, are yielding new insights into the mechanisms and environmental controls on volcanically derived iron formations, which in turn are giving us greater clarity in our interpretations of analogous deposits in the rock record.