Web Supplement 25.4 25.4 PAST VARIATIONS IN CLIMATE AND ATMOSPHERIC CARBON DIOXIDE LEVELS The geologic carbon cycle was described in Chapter 25.1 as providing the negative feedback loop responsible for stabilizing P over time scales of 105 to 107 y. In CO2 this cycle, the burial of CaCO3 and SiO2 in ocean sediments fuels decarbonation reac- tions that produce CO2 and CaSiO3. The CO2 is degassed through volcanoes and the CaSiO3 is uplifted onto land. Weathering of uplifted CaSiO3 by atmospheric CO2 supplies bicarbonate, silicate, and calcium ions to river runoff. In the modern ocean, plankton transform these ions back into CaCO3 and SiO2. If the rate of weathering is limited by P , increasing P should lead to enhanced uptake rates as a result of enhanced CO2 CO2 weathering, thereby providing a negative feedback that acts to stabilize P and the CO2 sizes of the other crustal carbon reservoirs. Nevertheless, large transient swings in P CO2 have occurred during various periods of Earth’s history. A few have led to long-term reorganizations of the global carbon cycle. Paleoceanographers are particularly inter- ested in studying these events as they provide some clues as to what changes we can expect as a result of our anthropogenic perturbations. A key to understanding these past swings is recognizing the important role of tectonism in the geologic carbon cycle. First, it determines the rates and loca- tions of the decarbonation reactions and of uplift. Second, it determines the spatial extent of shallow shelves. The latter was critically important prior during the Pre- cambrian and first half of the Phanerozoic because carbonate burial was restricted to the shallow shelves. In the mid-Phanerozoic, the evolution of pelagic calcifiers enabled burial of carbonate in deep-sea sediments. As discussed in Chapter 15.6, deep-sea sedimentary carbonate participates in a stabilizing feedback loop called cal- cite compensation that operates over time-scales of 104 y. In the present-day ocean, sedimentary carbonate deposition is concentrated in the Atlantic and Indian Oceans (Figure 15.5), whereas subduction, and, hence, decarbonation, is occurring primar- ily in the Pacific Ocean. Thus, the stability afforded by the geologic carbon cycle will occur only over time scales long enough to capture a supercontinent cycle. The stabilizing feedback of the geologic carbon cycle is also predicted to be of minimal 1 2 Web Supplement 25.4 effect during periods of tectonic stability due to the lack of mountain building and subduction. Tectonism is also important in the geological carbon cycle because it determines land-mass distributions. In the present day, most of the land mass is located in north- ern hemisphere mid-latitudes. A land-mass distribution in which most of the continents are located at tropical latitudes would be expected to result in a cold climate due to enhanced continental weathering rates. Conversely, our present geography should sup- port a relatively warm climate. This was not the case, at least prior to human-induced warming. The probable explanation is that the geological carbon cycle’s stabilizing feed- back is not presently working well. As noted earlier, most subduction is occurring in the Pacific where the sediments have a low carbonate content, thus disabling the geologic carbon cycle’s metamorphic decarbonation linkage. On the other hand, the presence of large land masses in the mid-latitudes does provide some climate stability in that an extreme cooling event leads to the formation of continental ice sheets. This slows continental weathering, permitting volcanic CO2 to reaccumulate and warm the atmosphere. In the following subsections, the long-term evolution of the global carbon cycle is discussed along with the most likely explanations for the large transient swings in P CO2 that have occurred sporadically throughout Earth’s history. The marine carbon cycle has played an important role in the past and is likely to determine the degree to which our perturbations of the global carbon cycle will result in global climate change. 25.4.1 The Carbon Cycle in the Precambrian When viewed over the long term, atmospheric CO2 levels have been in decline since the early Precambrian. As shown in Figure W25.1, P is estimated to have dropped CO2 by a factor of 100 to 10,000 over Earth’s history, indicating a large-scale relocation of carbon in the crustal-ocean-atmosphere factory. During the Hadean (4.6 to 3.8 bybp), the global carbon cycle was controlled solely by geology in which the reactions shown in Eqs. 25.1 through 25.8 stabilized P through the formation of carbonates via abio- CO2 genic precipitation in warm, shallow-water environments. This stabilizing feedback was important as the Sun’s solar luminosity has been increasing (slowly) over time. On the early Earth, solar luminosity was only 70% of the present day, so a long-term increase in insolation should lead to an overall decline in P assuming that warming temperatures CO2 enhance weathering rates. The advent of life around 3.8 bybp added a new crustal reservoir to the global carbon cycle, that of sedimentary organic matter. (Land plants did not evolve until the Phanero- zoic.) The accumulation of organic matter in a sedimentary reservoir contributed to the overall trend of declining P . One of the earliest life forms were the methanogenic CO2 archaeans. These microbes are thought to have converted virtually all of the primordial volcanic H2 in the atmospheric to methane via reaction with atmospheric CO2. Methane is a far more efficient greenhouse gas than CO2 (Table 25.2). The replacement of CO2 with CH4 is thought to have kept the early Earth’s atmosphere warm and its surface ice free. Prior to the evolution of methanogens, P levels were not high enough to keep CO2 Web Supplement 25.4 3 107 104 Ocean-covered earth Huronian glaciation 106 (5 to 208C) 103 bar) Neoproterozoic m glaciation 105 (5 to 208C) 102 30% Solar flux 8 reduction (0 C) 104 concentration (PAL) 2 10 partial pressure ( Mt Roe palaeosol 2 CO Constraints provided by the 103 CO Ruyang microfossil analyses 1 Terrestrial C3 photosynthesis 102 0.5 1.5 2.5 3.5 4.5 Time before present (Gyr) FIGURE W25.1 Atmospheric PCO2 relative to present atmosphere level (PAL) in the Archaean and Proterozoic based on analysis of microfossils (symbols). Shaded area was obtained from modeling. Source: After Kaufman A. J., and S. Xiao (2003). High CO2 levels in the Proterozoic atmosphere estimated from analyses of individual microfossils. Nature 425, 279–283. Earth ice free as the Sun’s luminosity was still quite low. Thus, the methanogens added an important element to the carbon cycle that acted on global climate. Methane production by the methanogens did not lead to a runaway greenhouse effect, as a negative feedback was established through the interaction of UV radiation with CH . At high P , UV radiation induces the formation of a photochemical haze 4 CH4 that reflects insolation. This methane thermostating ended around 3.0 bybp with the advent of oxygenic photosynthesizers. Their production of O2 led to the oxidation of atmospheric CH4. With the loss of this potent greenhouse gas, the planet entered a period (mid-to-late Proterozoic) in which several global glaciation events occurred. These are referred to as “ice-houses” or “Snowball Earths.” As discussed later, several other causative factors contributed to the occurrence of these ice-house conditions. In contrast to the climate variability of the Proterozoic, no Snowball Earths have occurred since the beginning of the Phanerozoic 550 mybp. The switch back to climate stability is attributed to the advent of multicellular lifeforms and, in particular, to the calcite compensation feedback made possible by evolution of biocalcifying marine organisms. 25.4.2 Snowball Earths Many Snowball Earth episodes are known to have occurred during the Proterozoic. They are recorded as lithified glacial sedimentary deposits, called tillites, accompanied by banded iron formations (oxidized iron) overlain by very thick carbonate layers, 4 Web Supplement 25.4 called cap carbonates.1 These carbonate deposits are typically 3 to 30 m thick and occur on platforms, shelves, and slopes worldwide. The global distribution of these unique sediments suggest that during a Snowball Earth episode, all the land masses and most, if not all, of the oceans were covered by ice. The first series of Snowball Earths occurred between 2.2 and 2.45 bybp at the beginning of the Proterozoic (Huronian and Makganene glaciations). The second series took place during the late Proterozoic (the Sturtian glaciation 710 mybp, the Varanger-Marinoan 635 mybp, and the Gaskiers 582 mybp). This latter period is referred to as the Cryogenian. Evidence suggests that the entire oceans froze during the Cryogenian. Life probably survived under thin ice at equatorial latitudes and at hydrothermal vents. Entering Snowball Conditions Various combinations of four factors are thought to have led to the runaway cooling that produced Snowball Earth conditions in the Proterozoic. Briefly, these factors are (1) high weathering rates, (2) increased volcanic activity leading to the formation of continental flood basalts, (3) the passage of Earth through a giant molecular cloud every 140 my, and (4) the loss of atmospheric methane following the rise of oxygenic photosynthesis. (The last would have served to cause only the first series of Snowball Earths.) Once cooling had started, a positive feedback is postulated to have occurred when the ice sheets reached some critical latitude due to their cumulative high albedo, i.e., the ice sheets reflect insolation back into space, thereby preventing its absorption by the greenhouse gases. High weathering rates are thought to have been caused by three factors: (1) a pre- ponderance of continents in the tropics, where it is hot and wet; (2) the breakup of supercontinents; and (3) the formation of supermountain chains.