Carbon Dioxide Levels Controlled By Degassing and Chemical Weathering Over Time
26 Dec, 2006 05:08 pm
Carbon dioxide is a potent greenhouse gas and its concentration in our atmosphere is naturally controlled over geologically long (millions of years) timescales by two key Earth processes: degassing of the interior of the Earth via volcanic eruptions (which adds CO2 to the atmosphere) and chemical weathering of the rocks that make up the Earth's crust (which removes CO2 from the atmosphere).
Observations of modern environments show that, in addition to temperature and overall wetness, rates of chemical weathering are also strongly correlated with the amount of physical erosion - the break up of rock by landslides and the grinding action of glaciers. This is due to the fact that the physical breakdown of rock increases the surface area available to chemical weathering, and this effect may over-ride the temperature control on the rate of chemical weathering. This latter process could, in theory, lead to a switch in the sign of the chemical weathering-CO2 feedback. In that case, when cold conditions produce ice ages, chemical weathering rates and CO2 removal from the atmosphere could speed up, actually helping to maintain cold conditions in the long term.
Scientists speak of the background climate of the Earth over the past 3 million years or so – that of the most recent epoch of Earth history, the Pleistocene - as being in an “icehouse” state. Before about 3 million years or so (perhaps a little longer, as there is still some debate about the exact timing) the only ice-sheet on the planet covered Antarctica. In contrast, the past 3 million years has been characterised by ice-sheets at both Poles. What is more, the exact size of the Northern Hemisphere ice sheets has changed dramatically, from situations like the present-day with just a small ice-cap over Greenland (so-called interglacials) to a radically different scenario (so-called glacials) with much larger ice-caps over North America and Eurasia (extending as far south as southern Britain). During the Pleistocene each full glacial-interglacial cycle seems to have lasted about 100 thousand years.
While most scientists agree that the radical swings in climate in and out of glacial periods during the Pleistocene are due to small variations in the way the Earth orbits the Sun, one of the big questions in Earth science right now is the precise role the carbon cycle has played in both amplifying the glacial-interglacial changes and in causing the long-term descent into a state with Northern Hemisphere ice at all. The processes involving chemical weathering outlined above are part of this question. The problem is that all these issues are poorly quantified. In particular, it is not known whether recent glacial periods of Earth history are accompanied by lower chemical weathering rates due to the cold, dry conditions or faster rates due to intense glacial grinding of rocks. What is well known is that, almost everywhere on the planet, physical erosion (measured by the rates of sediment accumulation) has increased up to 5 fold in the last 5 million years or so as a consequence of the waxing and waning of ice sheets in the Northern Hemisphere .The goal of our paper was to ascertain whether chemical weathering rates increased as well.
Unfortunately there are no direct records of the amount of chemical weathering that occurred in the past. Instead we have to use indirect methods, in our case the isotopic composition of the element Pb in oceanic ferromanganese crusts from the North Atlantic. These are metallic marine deposits that precipitate directly from seawater pretty much everywhere in the oceans where sediments don’t accumulate. They grow extremely slowly (around 1 mm per million years) so that, in order to examine how the Pb isotopic composition has changed over the last few glacial-interglacial cycles – in our record a period of 550 thousand years, we had to microsample them using a laser ablation system. By combining our new data with a simple model of the changes in the Pb isotope composition of the deep North Atlantic Ocean in response to chemical weathering variation, we found that chemical weathering rates were two or three times lower in the glaciated continental interior of the North Atlantic region during the colder glacial periods than during the intervening warmer interglacial periods. This is principally due to the rapid weathering of fresh material exposed during interglacials when the ice sheets retreated. This observation largely confirms the idea that wamer and wetter conditions during interglacials lead to increased weathering rates. However, at the same time as ice sheets covered much of North America and Northern Europe sea level was around 130 m lower than it is today, a consequence of the fact that much more water was trapped in continental ice-sheets. An indirect result is that, during glacials, continental shelves (such as the North Sea), and their covering of easily weatherable marine sediments, are exposed to the agents of chemical weathering. We estimate that the decrease caused by the growth of ice sheets is roughly balanced by the increase in chemical weathering caused by the exposure of the continental shelves, indicating that chemical weathering rates remained relatively constant on glacial-interglacial timescales.
A more interesting corollary of our analysis – but one that will require further work to confirm – is that on timescales longer than a single glacial-interglacial cycle, rapid chemical weathering during warm interglacial periods of rock ground up during each cold glacial period, results in a long-term increase in chemical weathering rates and a long-term net draw down of atmospheric CO2 into ocean sediments. We suggest that this process may create a positive feedback on global climate that, once initiated, promotes additional cooling and further glaciation, possibly serving to prolong icehouse periods.
Foster, G.L. and Vance, D. Nature 444, 918-921. doi:10.1038/nature05365. 14 December 2006.