Whether or not you believe that the Earth was totally encased in ice up to four times during the Neoproterozoic Era, there is convincing evidence that ice sheets did extend to the tropics during such “Snowball” episodes. How such extremely cold episodes came to prevail for several million years has been the subject of debate for 5 years, since Harland’s notion of global glaciations was resurrected by palaeomagnetic evidence for the low latitudes of Neoproterozoic glaciogenic rocks. Ice extending almost to the Equator, even if just on the continents, would have driven down global temperatures simply because it would have reflected away solar radiation. Increased albedo helps explain why frigid conditions lingered, but some other cooling mechanism must first have encouraged the widespread formation of ice sheets. Essentially, the supply of the “greenhouse” gas CO2 by volcanic activity must have been outstripped by burial or solution of carbon in some form. The two usually identified candidates are increased deposition of carbonate sediments and the accumulation of unoxidised organic carbon in sea-floor muds. It is the first of these that dominates climate control today, by the accumulation of carbonate shells of marine plankton, and that has probably prevailed since foraminifera and coccolithophores began to proliferate in the Mesozoic. No shelled organisms existed during the Precambrian, so a major factor in damping down climate fluctuations was missing before the start of the Phanerozoic. This crucial difference between the modern and Precambrian world focussed the attention of Andy Ridgwell, Martin Kennedy (University of California) and Ken Caldeira (Lawrence Livermore National Laboratory) in seeking an explanation for “Snowball” events (Ridgwell, A.J. et al. 2003. Carbonate deposition, climate stability and Neoproterozoic Ice Ages. Science, v. 302, p. 859-862).
Carbonate sediments are plentiful in the Precambrian record. Some formed as a result of organic action (stromatolitic limestones) and others show evidence for direct, inorganic precipitation of carbonates from sea water. The latter indicate sea water in which calcium and carbonate/bicarbonate ions exceeded the solubility of calcite and the ability of organic activity to remove calcite from solution. Evidence for such extreme oversaturation is rare, but the cap carbonates that overlie Neoproterozoic glaciogenic rocks are important examples. The key area of carbonate deposition has always been on shallow continental shelves, the main secreters of carbonates during the Precambrian having been blue-green bacteria that can photosynthesise only in shallow water. Falls in sea-level or a reduction in the area of shelves during the Phanerozoic reduced this sink for CO2 in the build-ups of coral and shelly limestones, but plankton of the open oceans continued to accumulate on the deep sea floor. Because calcite can be dissolved at depth, the deepest sea floor does not contain much carbonate. However, a fall in sea level, increases the area suitable for deep-water burial of shelly material, because the carbonate compensation depth or lysocline also falls. In the absence of shelly plankton, this modern balancing mechanism for ocean chemistry did not exist during the Precambrian. Superficially, it might seem that a reduction in the area of shelf deposition of carbonates, brought on by a sea-level fall, would allow CO2 to build up in the atmosphere, driving towards warmer conditions. However the way in which atmospheric carbon dioxide is related to dissolved carbonate (CO32-) and bicarbonate (HCO3–) ions tells a very different story. This is the equilibrium: CO2 + CO32- +H2O = 2HCO3–. Less carbonate accumulation on reduced continental shelves would drive up the carbonate-ion concentration of sea water, and also its pH. So, according to Le Chatelier’s Principle, the equilibrium proceeds to the right and adds to the more soluble bicarbonate ions in sea water. This consumes CO2, and drives down the “greenhouse” effect. Ridgwell and colleagues developed a model around this equilibrium, and applied it to conditions of falling sea level when carbonates were only deposited on continental shelves. Their results show that decreased shelf-carbonate burial during a period of sea-level fall would rapidly drive down the warming effect of atmospheric carbon dioxide. Combined with the lower solar energy output during the Neoproterozoic, that would be sufficient to create protracted periods of frigidity. Alkalinity of the oceans would increase through periods of glaciation, so that once sea-level rose, massive carbonate precipitation would form cap carbonates on the newly inundated shelves, thereby reducing the oceanic drawdown of CO2.
Ridgwell et al’s model is not easy to grasp, and relies on its initiation by falling sea-level. Either that resulted from build up of continental glaciers because of some other climatic mechanism, or internal processes increased the volume of the ocean basins. An example of the last is a decrease in sea-floor spreading, when cooling of the lithosphere increases it density so that it sags down. Periods of accelerated creation of oceanic lithosphere displace sea water upwards, and perhaps that might explain an increase in shelf areas, which would allow warming according to the new model. The model also needs special pleading to account for the 1 billion-year absence of glaciation before the period of Snowball events. The authors suggest that it could have been prevented by much wider shelves during earlier times, but without quoting evidence.
Continental erosion and climate
Maureen Raymo suggested in 1988 that long term climate change was modulated by the rise of mountain chains and their erosion and weathering. This is because chemical weathering of silicate minerals is a net consumer of atmospheric carbon dioxide. Raymo’s hypothesis, based on T.C. Chamberlin’s theory of glaciation, has set climatically concerned geochemists to analysing the trace element content of river water in many mountainous regions, because those such as strontium are proxies for the amount of weathering going on today. Others have looked at the flux of elements into seawater through the Phanerozoic in particular, by analysing marine carbonates, to see if the ups and down’s of water composition through time match the record of climate change. These time series do suggest some matching, but not precise enough for all to agree with the hypothesis. Measurements of river-water composition have also met set-backs. Much of the weathering flux from mountains seems to stem from dissolution of carbonate rocks, and that does not lead to long-term loss of CO2 from the atmosphere. In a bid to resolve the contributions of carbonates and silicates, Andrew Jacobson and Joel Blum of the University of Michigan have studied the flux from part of the Alps of New Zealand’s South Island (Jacobson, A.D. & Blum, J.D. 2003. relationship between mechanical erosion and atmospheric consumption in the New Zealand Southern Alps. Geology, v. 31, p. 865-868). Their area is a good choice because the New Zealand Alps are actively rising, precipitous and drenched with continual heavy rain and snowfall. Moreover, they offer something that the Andes and Himalaya do not; the rocks are pretty uniform. What they find will not please Raymo’s followers. As in many mountain ranges, mechanical erosion favours carbonate weathering over that of the CO2 sequestering alteration of silicates. With a low ratio of silicate:carbonate chemical weathering, mountain building in New Zealand does draw down carbon dioxide, but only by a factor of about 2. They conclude that more stable areas with lower relief are more likely to affect climate. Although chemical weathering in them is lower than in mountains, that of silicates is far higher than for carbonates. Moreover, active mountain ranges are minuscule compared with the extent of more subdued land. It seems likely from Jacobson and Blum’s findings that the major control of weathering over climate depends to a large degree on where continents are located relative to warm, humid climatic zones. For much of the early Cenozoic, the dominantly crystalline Precambrian shields of India, Africa, Australia and South America straddled the Equator, and witnessed intense weathering. Maybe that relationship helped draw down carbon dioxide, and gradually cooled the planet from the hot and humid climate of the late Mesozoic.