Question

### Banded Iron Formations

Banded Iron Formations (BIFs) are ancient sedimentary rocks consisting of alternating layers of iron-rich minerals (such as magnetite) and silica-rich chert. Two geologists propose different mechanisms for how dissolved ferrous iron ($Fe^{2+}$) in the Precambrian oceans was oxidized to insoluble ferric iron ($Fe^{3+}$) to form these deposits approximately 2.5 billion years ago.

Geologist 1
BIFs were formed through biological activity. Early photosynthetic cyanobacteria in shallow marine waters produced molecular oxygen ($O_2$) as a byproduct of photosynthesis. This free oxygen reacted with dissolved $Fe^{2+}$ in the water, oxidizing it to $Fe^{3+}$, which precipitated out of solution as iron oxides. This process occurred primarily in shallow coastal regions where sunlight was abundant.

Geologist 2
BIFs were formed through abiotic (non-biological) photochemical processes. The oxidation of $Fe^{2+}$ to $Fe^{3+}$ occurred without the involvement of living organisms or free oxygen. Instead, ultraviolet (UV) radiation from the Sun penetrated the Earth's early atmosphere, which lacked a protective ozone layer. This UV light directly catalyzed the photo-oxidation of dissolved $Fe^{2+}$ in the upper ocean layers, leading to the precipitation of iron oxides.

Based on the viewpoints of Geologist 1 and Geologist 2, which of the following is a shared assumption of both geologists regarding the ancient oceans during the period when Banded Iron Formations were deposited?