One process connected to long-term climate change is the way that weakly acid rainwater (containing dissolved CO2) weathers silicates in continental rocks, one product being carbonate in soils. The process should draw CO2 from the atmosphere, thereby reducing its “greenhouse” effect. The idea is by no means new, but received a boost in the mid 1990’s from Maureen Raymo’s suggestion that fluctuations in the strontium-isotope composition of the oceans through geological time should be a proxy for changes in the rate of continental weathering. The 87Sr/86Sr of marine carbonates does show clear correlation with long-term climate shifts during the Phanerozoic..
Continental weathering should increase as topographic relief becomes greater through mountain building episodes. The Himalaya’s rise through the late-Tertiary has been suggested as a major influence over climatic deterioration, partly by its effect on the Asian monsoon and partly as a huge site for the sequestration of atmospheric CO2 by chemical weathering. Himalayan rivers have enormous flows and equally large sediment and dissolved element loads. In particular they carry far more strontium than other rivers, and it has a highly radiogenic content of 87Sr. There are three means of attaining these levels: from average continental crust which has a higher 87Sr/86Sr ratio than oceanic crust (the other main source of seawater strontium); from strontium rich limestones that acquired their isotopic signatures from the ocean when they were deposited; or from sources with unusually high 87Sr/86Sr ratios. The Himalaya are well known for carbonate sediments, and for granites formed by melting of deeper, older continental material that gives them very high proportions of radiogenic strontium. Recent work now shows that a significant contribution of highly radiogenic strontium to Himalayan rivers is hydrothermal activity (Evans, M.J. et al. 2001. Hydrothermal source of radiogenic strontium to Himalayan rivers. Geology, v. 29, p. 803-806). Hot springs feeding a major tributary of the Ganges contribute up to 30% of its strontium load, and incidentally a great deal of CO2. Both result from hydrothermal alteration of deeper rocks, and are unrelated to weathering if the water involved emanates from the deep crust. It seems that these waters are recycled rainwater, so this is a case of a high-temperature chemical weathering. Whatever, it further complicates the original notion of linkage between mountain building and climate.
Methane and Snowball Earth
The well-publicized “Snowball Earth “ model for Neoproterozoic glaciogenic rocks that occur at tropical palaeolatitudes has to involve an escape mechanism from global frigidity. Without some means of warming, the high albedo of widespread ice would have locked the Earth into perpetual glaciation, which of course did not happen.
The main proponents of the model, Paul Hoffman and Dan Schragg of Harvard University suggested a gradual build up of volcanogenic CO2 during “Snowball” conditions, when a dry atmosphere would have retained the “greenhouse” gas instead of its being sequestered to the oceans and carbonate rocks by acid rain and continental weathering. Gradually, atmospheric temperatures would have risen due to trapping of outgoing, long-wave radiation by CO2. This simple aspect of the model leads to scenarios where warming overruns once ice sheets disappeared, to give extremely high-temperature conditions. Using carbon-isotope data from marine carbonates is a means of supporting or refuting this escape mechanism, and also of detecting the influences of other components of the carbon cycle. Carbonates take up carbon dissolved in seawater without fractionating its different isotopes, and provide measures of the degree to which organic processes did contributed to fractionation. Cell processes preferentially take up 12C, and if large masses of undecayed organic matter ends up in seafloor sediments, the proportion of “heavier” 13C (indicated by the standardized ratio of the two main isotopes d13C) increases in seawater and the atmosphere. Carbon of mantle origin, that emerges as volcanic CO2, has a constant d13C of about -5‰. So these two processes contribute to an isotopic balance, which for most of the Mesozoic and Cenozoic Eras established a d13C of between 0 and +4 ‰ in sea water and limestones. This is interpreted as a sign that the recent carbon cycle achieved a balance between volcanic additions and organic carbon burial weighted towards trapping of undecayed carbohydrate in sea-floor sediments. Explanations for broad climate changes since 250 Ma therefore rely more on other mechanisms than on the carbon cycle
The most comprehensive study of Neoproterozoic carbon (Walter, M.R. et al. 2000. Dating the 840-544 Ma Neoproterozoic interval by isotopes of strontium, carbon and sulfur in seawater, and some interpretative models. Precambrian Research, v. 100, p. 371-433) does indeed show dramatic see-sawing of d13C through supposed “Snowball” events, from highly positive values (<+10‰) before glaciogenic sedimentation to highly negative (>-10‰) in the immediate aftermath. However, few data were available from within glaciogenic sediments, and resolution is insufficient to detect tell-tale trends. The key approach needs detailed carbon isotopes through a single event, and such data appeared recently for the famous Neoproterozoic glaciogenic-cap carbonate sequence of Namibia (Kennedy, M.J. et al. 2001. Are Proterozoic cap carbonates and isotopic excursions a record of gas hydrate destabilization following Earth’s coldest intervals. Geology, v. 29, p. 443-446)
Kennedy et al. measure d13C in carbonate cements in the glaciogenic diamictites, in overlying cap carbonates and in cement to later clastic rocks. Interestingly, there is little sign of a gradual decrease in 13C through the glaciogenic rocks. Constant oceanic carbon composition would be expected if no volcanic CO2 entered seawater during frigid, dry conditions, and living processes were minimal. In the cap carbonates d13C plummets from +3‰ to -4‰. One simple explanation would be massive “rain-out” of volcanic CO2 (d13C of -5‰) that had built up in the air during the “Snowball” episode.