Sweden emerges

The Proterozoic Eon began 2.5 billion years ago. Its name means “early life” but we now know that life existed long before the start of this eon. This eon starts and ends dramatically. It begins with oxygenation of Earth’s atmosphere and it ends with Snowball Earth, a time of widespread, perhaps global glaciation. Bracketed by these pivotal events is a time geologists refer to as either “the boring billion” or Earth’s “middle age”. Arguably, nothing happened for one billion years. Or perhaps, we simply don’t know enough yet to know what happened. 

Our story begins in the early Proterozoic Eon, before the boring billion but after the first rise of oxygen in Earth’s atmosphere. The rocks on Utö are 1.9 billion years old. They record Sweden’s emergence from the sea. Much of Sweden is made of layers of volcanic ash and other products of explosive volcanism which alternate with a suite of chemically precipitated rocks, ranging from carbonates, which are rocks made of calcite [chemical formula: CaCO₃] or dolomite [chemical formula: CaMg(CO₃)₂], to iron formations, which are rocks made mostly of iron oxides such as hematite and magnetite. 

Artist’s impression of the early shores of Sweden by Susanne Hjerp

The early shores of Sweden were flourishing with microbial life, remnants of which remain as finely laminated microbial mats and dome-shaped mounds called stromatolites which are formed by colonies of cyanobacteria. Aided by sunlight, these organisms combined water and carbon dioxide into carbohydrates and oxygen in a reaction called photosynthesis. The steady production of oxygen may have contributed to the rise of oxygen in Earth’s atmosphere which started a few hundred million years earlier. 

Oxygenation of the atmosphere transformed soluble (ferrous) iron (Fe²⁺) to insoluble (ferric) iron (Fe³⁺). This led to widespread precipitation of iron formations.