BANDED IRON-DEPOSITING SHALLOW SUBMARINE HYDROTHERMAL VENT FIELDS ON THE HELLENIC VOLCANIC ARC (Invited Presentation)

For much of Earth's history, the oceans were rich in dissolved ferrous iron, leading to seafloor deposition of banded iron formations (BIFs) as far back as ~3800 million years (Myr) ago) and as recently as ~700 Myr ago, during the Neoproterozoic Snowball Earth. BIFs are thought to be a major feature of the Precambrian oceans, and that they preserve the distinct chemistry of the past seawater from which they precipitated and the changing redox state of the oceans and atmosphere in their spectacular Fe-Si-rich layers. Milos, one of a series of emergent volcanic Islands on the Hellenic Volcanic Arc in the Aegean Sea, hosts modern BIF analogues that are at most, ~2.0 Myr old. These modern BIFs are associated with biological activity in the form of microfossils and a low organic carbon content presenting a photosynthetic isotopic signature. Episodic hydrothermal activity is implicated in the formation of the distinct alternating Fe-Si-rich layers, with their mineralogical, physical and chemical properties being comparable to Precambrian BIFs. The similarities between these unmetamorphosed rocks and the Precambrian BIF deposits, open a unique window for deciphering the processes that led to the formation and termination of BIF deposition in deep time. They provide an invaluable resource for constraining past environmental conditions because they have not been falsified by the havocs of time; capturing biogeochemical processes that likely prevailed in similar settings during the Precambrian. Active hydrothermal activity on the coast of Milos differentiates into habitats similar to the sulfidic, ferruginous and oxic settings that became widespread following the first rise of atmospheric oxygen, 2450 Myr ago, during the Great Oxidation Event (GOE). These chemical gradients demonstrate that nutrient and trace element cycling in the Post-GOE sulfide-rich continental margins, relative to the ferruginous/oxic settings where the iron oxides that formed BIFs precipitated, would have assembled distinct microbial communities as a function of their unique chemical composition. The data demonstrate that extinct and active hydrothermal activity on the Coast of Milos Island may serve as Fe and S analogue systems for deciphering the biogeochemical cycling of trace elements in Earth’s early oceans.