Ancient Greenland Rock Reveals Early Ocean Sedimentation Through Potassium Isotopes

What is it about?

In this study we investigate a 3.8‑billion‑year‑old quartz–pyroxene rock from Akilia Island, Greenland — one of the oldest rock formations on Earth. For decades scientists have debated whether this rare band of rock originated as volcanic material or as sediments laid down in an ancient ocean. To resolve this question, we measured both the amount of potassium in the rock and the proportion of its different forms (isotopes) and compared these results with those from other ancient iron‑rich rocks. We found that layers with very little potassium have isotopic signatures similar to seawater, while clay‑rich layers contain more potassium and lighter isotopes. The Akilia rock itself was originally potassium‑poor and chemically similar to material that precipitated from seawater, but it was later modified by fluids from nearby rocks. Our findings show that this banded rock is a metamorphosed marine sediment rather than an igneous intrusion and that sedimentary processes were already active on the early Earth.

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Why is it important?

Understanding how and when sediments accumulated on Earth helps us reconstruct early ocean chemistry, nutrient cycles and the conditions that may have supported the emergence of life. The Akilia quartz–pyroxene rock has been cited as one of the oldest possible repositories of early biological signatures, so clarifying its origin is crucial. By using potassium isotopes — a new tool for paleogeochemical studies — we demonstrate that the heavy potassium in ancient seawater was balanced by removal of the lighter form into clays. This insight not only resolves a long‑standing debate about this unique Greenland rock but also opens up a way to trace ocean evolution and continental weathering over billions of years.

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