Carbon dioxide may have acted as Earth’s ‘thermostat’ since earliest timesBy Steve Koppes
A greenhouse gas that has become the bane of modern society may have saved Earth from completely freezing over early in the planet’s history, according to the first detailed laboratory analysis of the world’s oldest sedimentary rocks.
Scientists have for years theorized that high concentrations of greenhouse gases could have helped Earth avoid global freezing in its youth by allowing the atmosphere to retain more heat than it lost. A team of scientists at Chicago and the University of Colorado has now analyzed ancient rocks from the eastern shore of Hudson Bay in northern Quebec, Canada, and found the first preliminary field evidence supporting this theory.
“Our study shows the greenhouse gas that could have sustained surface temperatures above freezing 3.75 billion years ago may have been carbon dioxide,” said Nicolas Dauphas, Assistant Professor in Geophysical Sciences and the College. Dauphas and his co-authors Nicole Cates and Stephen Mojzsis of the University of Colorado, and Vincent Busigny, now of the Institut de Physique du Globe in Paris, will present their data in the Thursday, Feb. 15 issue of the journal Earth and Planetary Science Letters.
The study, led by Dauphas, helps explain how the Earth may have avoided becoming frozen solid early in its history, when astrophysicists believe the sun was 25 percent fainter than today. Previous studies have shown that liquid water existed at the Earth’s surface, even though the weak sun should have been unable to warm the Earth above freezing conditions. But high concentrations of carbon dioxide or methane could have warmed the planet.
Discovered in 2001 by a team of Canadian scientists, the Quebec rocks are among the oldest-known in Earth’s 4.5-billion-year history. Slow-acting geologic processes destroy and recycle the Earth’s crust on vast time scales, leaving only scraps of land containing remains of the planet’s oldest rocks.
The only other outcropping of rocks that are about as old occur in western Greenland. Scientists have studied those rocks exhaustively for more than three decades. But the limited extent of the rocks of this antiquity may have provided only a biased view of the early Earth, Dauphas said.
Mojzsis and Cates revisited the Canadian site to pursue new, but as yet unrealized, opportunities for analysis and comparison. “It is a grand landscape of water, wind and rock carved by glaciers and only lightly touched by the people who live there,” Mojzsis said. But the region would have looked much different 3.8 billion years ago.
“At that time, it would have appeared to be a totally alien world to us, with a dense atmosphere of carbon dioxide and methane that would have imparted a reddish cast to the sky, and deep, dark greenish-blue oceans of iron-rich water washing onto beaches of small continents scattered across the globe,” Mojzsis said.
The Chicago-Colorado scientists focused their analysis on rocks they suspected contained chemical sediments that precipitate like salt from seawater. First they dissolved the rock, separating iron oxides and iron carbonates from other constituents. Then they used a mass spectrometer to measure the isotopic composition of the iron. All iron atoms have 26 protons at their core, but they can be accompanied by a varying number of more numerous neutrons.
“Iron has several isotopes, and the ratio of these isotopes changes from one rock to another,” Dauphas explained. “Sediments that formed by precipitation from seawater have a very distinct signature of iron isotopes.” After analyzing the iron composition of the rocks, Dauphas noted, “We found that indeed they had the typical signature of something that formed by precipitation in a marine setting.”
The iron probably was released with other metals in hydrothermal vents, called black smokers, which are found along mid-ocean ridges where molten lava emerges on the sea floor to create new oceanic crust. In today’s oxygen-rich oceans, the iron rapidly precipitates and concentrates near these vents. But in the oxygen-starved oceans of 3.8 billion years ago, oceanic currents could transport the iron long distances, before becoming partially oxidized and deposited in sea-floor sediments. Some of these sediments survive today as banded iron formations.
Previous research on the rocks from Greenland already had revealed the existence of ocean water at that early stage in Earth’s history, known as the Eoarchean Period. But the Canadian rocks showed something else: the first hints that Eoarchean oceans also contained iron carbonates. Iron carbonates can only form in an atmosphere that contains far higher levels of carbon dioxide than are found in Earth’s atmosphere today, Dauphas said. This carbon dioxide would have played an important role as a planetary thermostat in the support of life on Earth.
“If it gets cold, ice caps form, chemical weathering decreases, carbon dioxide accumulates in the atmosphere, which increases the greenhouse effect and surface temperatures. If it gets hot, the rate of chemical weathering increases, the rate of burial of sedimentary carbonates increases, and the amount of carbon dioxide in the atmosphere and surface temperatures decrease,” Dauphas said.
Other factors are involved in this simplified scheme. “Still, it is possible that such a thermostat was at work as early as 3.75 billion years ago,” he said.