Passage V

Two scientists discuss whether a snowball Earth (Earth completely covered with glacial ice) existed from 710 million years ago (mya) to 700 mya, a period during which ail the continents were located on or near the equator.

Scientist 1

A million years before 710 mya, glacial ice covered all of Earth's surface except the low latitudes (30°N to 30°S). Over the next million years, continuing rapid rock weathering in the low latitudes removed CO₂ from the atmosphere, and the ice at high latitudes reflected incoming solar radiation. Both processes cooled Earth, which allowed the ice to spread and cover the remainder of Earth 's surface by 710 mya. Soon thereafter, the 1-kilometer-thick ice cover caused the extinction of al photosynthetic organisms. Sediments deposited at the low latitudes by glaciers during the period from 710 mya to 700 mya have been found.

Banded iron formations (BIFs) that formed in the million years after 700 mya at the low latitudes have been found. The BIFs formed only in ocean water that was saturated with Fe²⁺, an ion of iron that reacts readily with atmospheric O₂. Saturation with Fe²⁺ was possible only if an ocean had been cut off from atmospheric O₂ for millions of years by thick ice. Once the ice melted, the Fe²⁺ reacted with O₂ to form BIFs.

Scientist 2

From 710 mya to 700 mya glacial ice covered all of Earth's surface except the low latitudes. The low latitudes remained mostly ice-free over this period because the continents there absorbed enough solar radiation to keep the average temperature above 0°C. Although rapid rock weathering was removing CO₂ from the atmosphere, wide spread, continual volcanic eruptions at low latitudes simultaneously added an equal amount of CO₂ to the atmosphere.

Fossils of photosynthetic organisms from this period have been found. Some of the sediments from this period were deposited at low latitudes when floating icebergs that broke off of glaciers at high latitudes melted and dropped sediment that had been trapped in the ice. BIFs did form as described by Scientist 1, but only in the few isolated ocean basins at low latitudes that had been completely covered with thick ice over this period.