Chicxulub and Climate: Investigating the Climate Sensitivity to Stratospheric Injections of Impact-generated S-bearing Gases

This work uses an atmospheric singlecolumn-model, SCCM, coupled to a sulfate aerosol model to provide an initial assessment of the climate sensitivity to stratospheric sulfate aerosols produced by the reaction of S-bearing gases and water vapor released in the Chicxulub impact event. As expected, the formation of large amounts of sulfate aerosols in the stratosphere results in a strong heating (tens to hundreds of degrees) of the stratosphere accompanied by cooling (few degrees) at the Earth’s surface. Temperature excursions appear to be much stronger on continental interiors than on the oceans, while at coastal locations temperatures are mitigated by the near proximity of large masses of water. Chicxulub Impact. The Chicxulub structure, on the Yucatán Peninsula, Mexico, was produced 65 Myr ago, in coincidence with the large Cretaceous-Tertiary mass extinction [1]. The impact occurred on a partially submerged platform consisting of a thick (3 km) sequence of carbonates and evaporites overlying a continental crust, and produced a multiring structure, whose transient cavity diameter is estimated to be around 100 km [2]. The extension of the evaporitic deposits in the sediments is not well constrained, ranging from about 50% [3] to 30% [4], although lower limits of 10% have also been suggested [5]. A shallow sea (few tens of meters) was covering the region, suggesting that the sedimentary layer was probably saturated with water. In addition to the direct short-term effects of the impact (e.g., see [6]), the presence of evaporites in a watersaturated sedimentary layer may have caused, through the release in the atmosphere of large loads of Sbearing gases and water vapor, a long lasting, strong, and abrupt climate shift, possible key to the K/T boundary mass extinction. Hydrocode simulations [7,8] indicate that the amount of S injected in the stratosphere ranged between about 75 and 270 Gt, depending on projectile type and impact speed, with a lower limit of 25 Gt under the assumption that evaporites constituted only ∼10% of the sedimentary layer. Previous estimates relative to these loads suggest a climate forcing about two orders of magnitude stronger than the well-known Pinatubo volcanic eruption, persisting globally for about two years after the impact [9]. For comparison, the climate forcing associated with the largest estimated CO2 release from the sedimentary layer is about half, in magnitude, that of Pinatubo (but of opposite sign, implying heating). Model. The atmospheric model used for this work is a Single Column Model version of the National Center for Atmospheric Research (NCAR, Boulder, CO) Community Climate Model Version 3 (CCM3), SCCM, coupled to the Sulfate Aerosol Model developed for this investigation [9]. Sulfate Aerosol Model. The Sulfate Aerosol Model is used to simulate the evolution of the sulfur-bearing gases injected in the stratosphere by the impact event. Concentrations of SO2, H2O, and sulfate aerosols are distributed uniformly over the globe (partially justified by the fast worldwide expansion, well beyond the stratosphere, of the impact plume). The microphysical model for the formation and evolution of the stratospheric aerosols [9] follows that described in [10]. The aerosols are continuously formed by combining impact-produced SO2 and SO3 with H2O, and their evolution is described by processes like coagulation, growth, gravitational settling, and diffusion for various particle sizes. Single Column Model. To investigate short-term climate sensitivity to stratospheric loads of sulfate aerosols we coupled the Sulfate Aerosol Model to SCCM. SCCM is a one-dimensional model equivalent to a single vertical column of the more complete threedimensional global climate model CCM3 [11]. In this model, the physical processes (such as radiative transfer and moist convection) are parameterized in isolation from the rest of the large-scale model (therefore lacking the horizontal feedbacks that occur in complete three-dimensional models of the atmosphere). Horizontal advection is supplied by pre-determined “usersupplied” boundary conditions. While lacking the more complete feedback mechanisms available to an atmospheric column imbedded in a global model, the single column model is computationally inexpensive, providing a quick first look at the response of the system to the introduced forcing. Climate Sensitivity. Climate sensitivity is defined as the mean change in global surface temperature that occurs in response to a specified radiative forcing. In this study, we take a more general approach and examine the temperature changes induced by S-bearing Lunar and Planetary Science XXXIII (2002) 1269.pdf