Our planet is habitable because the carbon cycle maintains a balance between the supply and the consumption of atmospheric pCO2. Plate tectonics influence the long-term (>105 yr.) cycling of carbon, controlling both the rate of volcanic CO2 outgassing and the removal of CO2 through the exhumation of weatherable rocks. Earth has undergone several periods of large-scale volcanic eruptions termed Large Igneous Provinces (LIPs). Three of the largest and most recent LIPs are the Deccan Traps at 65 Myr, Central Atlantic Magmatic Province (CAMP) at ~200 Myr, and Siberian Traps at ~250 Myr. Various researchers have postulated that volcanic outgassing associated with the emplacement of each LIP led to large-scale environmental/climatic changes, which are closely linked to mass extinctions and subsequent evolutionary recovery. However, a major tenet of these hypotheses, that large-scale outgassing is associated with LIPs, has yet to be demonstrated.
Here, PIs will reconstruct atmospheric CO2 levels using stable isotope measurements from sediments deposited before, during, and after the emplacement of the CAMP. They make use of a ?paleo-barometry? method that is based on stable carbon isotope values in both the inorganic and organic phases of paleosols (e.g., Cerling et al. 1999). Variations in the carbon isotope values respond directly to the levels of CO2 in the atmosphere. In the Newark and Hartford Basins, numerous paleosol horizons are found interbedded with igneous rocks of the CAMP; such superposition combined with cycle stratigraphy allows very little age uncertainty. These paleosol isotope reconstructions will test the hypothesis that atmospheric pCO2 increases with LIP emplacements. An under appreciated consequence of LIP emplacement is that well after the initial injection of CO2, long-term weathering of the CAMP rocks may lower atmospheric CO2 below pre-event levels. PIs sampling strategy will collect data to test whether chemical weathering of CAMP volcanic rocks resulted in a long-term decrease in pCO2.
The work supported and produced by this grant shows a clear link between the extrusion of large volumes of lava and atmospheric pCO2. Volcanologists have known for a long time that CO2 is one of the primary gases released during volcanic eruptions. In general, the amount of CO2 released is balanced by weathering reactions that keep the Earth pCO2 and hence climate in check. One of the unique things about our planet is that it has a feedback mechanism to keep climate in check. For example, if the Earth's atmosphere gets jolted with extra CO2, the rate of weathering increases and returns the atmospheric pCO2 back to its preferred state. Conversely if pCO2 decreases too quickly the Earth cools and the weathering reactions slow. This is referred to as Earth's thermostat. The eruption of large volumes of lava in a short interval (<10,000 years) at one time was thought to account for warmer planetary climates. These intervals are referred to as Large Igneous Provinces. Our work set out to determine the short-term (1,000-300,000 year) and long-term (>500,000 year responses to the volcanism associated with the Central Atlantic Magmatic Province (CAMP). The volume of lava erupted during CAMP was ~2–3 × 106 km3, making it one of the most voluminous of the large igneous events. In eastern North America, 3 lava flows are identified. In Morocco, 4 distinct lava flows are found. We reconstructed atmospheric pCO2 levels before during and after the emplacement of the CAMP lavas. We used stable isotope measurements from soil carbonates to do this. Our results show: 1) that inmediately following each of the 3 lava flows, atmospheric pCO2 values doubled. 2) that over the next 200,000 to 300,000 years, pCO2 values decreased to pre-eruptive values. 3) we also found a 4th doubling of pCO2 above the last lava found in eastern North America, confirming the Moroccan geology that there were four large flows during the CAMP. For geographical or other reasons, the 4th flow did not reach eastern North America. 4) that over the long-term (>500,000 years) atmospheric pCO2 decreased significantly below pre-eruptive values. These results are significant for understanding the important driving forces for our planet's climate. Geochemical models of the weathering reactions had predicted that a pCO2 increase would require 150,000 to 300,000 years to decay back to background levels. The excellent time scale for the CAMP eruptions and intervening sediments in eastern North America constrain the models. Our results confirm most model estimates and provide real data against which the models can be tested. Perhaps a more subtle but significant finding is that the weathering of lavas greatly increases weathering reactions such that large-scale eruptions ultimately cause pCO2 to decrease and the planet to cool. This line of evidence has been under appreciated and may help us to unravel why the planet warmed and cooled at times in the past. In the end, our planet has a climatic thermostat built into it. We showed that increased volcanism will lead to a short-term increase in atmospheric CO2. To cause long-term deviations in our climate, such as the ice ages over the past 35 million years, our work emphasizes that we need to consider more closely the effects of weathering of large scale lavas. As our society debates issues associated with increasing atmospheric CO2, it is imperative to fully the geologic reaction to higher CO2. While the scale of our study may seem too long for the current debate, history shows that better informed decisions require input from all arenas, not just the past 1000 years.
