ALBERT

Description: ABSTRACT [1]  The global debris layer created by the end-Cretaceous impact at Chicxulub contained enough soot to indicate that the entire terrestrial biosphere had burned. Preliminary modeling showed that the reentry of ejecta would have caused a global infrared pulse sufficient to ignite global fires within a few hours of the Chicxulub impact. This heat pulse and subsequent fires explain the terrestrial survival patterns in the earliest Paleocene, since all the surviving species were plausibly able to shelter from heat and fire underground or in water. But new models of the global infrared heat pulse, as well as the absence of charcoal, and the presence of noncharred organic matter, have been said to be inconsistent with the idea of global fires that could have caused the extinctions. It was suggested that the soot in the debris layer originated from the impact site itself because the morphology of the soot, the chain length of PAH's, and the presence of carbon cenospheres were said to be inconsistent with burning the terrestrial biosphere. These assertions are either incorrect or have alternate explanations that are consistent with global firestorms. We show that the apparent charcoal depletion in the K-Pg layer has been misinterpreted due to a failure to correct properly for sediment deposition rates. We will also show that the mass of soot potentially released from the impact site is far too low to supply the observed soot. But global firestorms are consistent with both data and modeling. © 2013 American Geophysical Union. All rights reserved.

Description: [1]  The Chicxulub asteroid impact produced massive extinction in terrestrial environments most likely through an intense heat pulse and subsequent widespread fires. Aquatic environments were shielded from this heat and fire but nevertheless showed massive extinction in marine environments and, for reasons unexplained, far less extinction in freshwater environments. Extinction in marine environments resulted from the effects of an “impact winter” caused by dust and smoke in the atmosphere that extinguished sunlight at the Earth's surface for a period of months to years. The resulting cessation of photosynthesis caused a globally extensive extinction of phytoplankton taxa. Because aquatic ecosystems, unlike terrestrial environments, are strongly dependent on daily photosynthetic output by autotrophs, loss of phytoplankton likely caused catastrophic mortality and extinction in aquatic ecosystems. Other potential causes of mortality in aquatic ecosystems include lower ambient temperatures and anoxia due to the lack of photosynthetic oxygen. Inland waters, although probably subject to high mortality, showed lower proportionate extinction than marine environments probably because of the greater potential among the freshwater taxa for dormancy, the greater efficiency of reaeration by rapid flow to offset oxygen demand, abundant thermal refugia fed by groundwater at moderate temperatures, and pre-adaptation of freshwater taxa to a great degree of environmental variability. In addition, detrital feeders appear to have had low extinction rates in either marine or freshwater environments, but again freshwater taxa would have been favored by higher renewal rates of detrital organic matter as a result of their direct hydrologic contact with soil.

Description: Playa wetlands are important areas of soil organic carbon and nutrient storage in drylands. We conducted this study to assess how catchment biophysical variables control soil organic carbon and nitrogen in playas and how playas function differently than upland ecosystems. We found that playa organic carbon and nitrogen corresponded primarily with catchment vegetation cover and secondarily with catchment area, slope, and soil texture. The effect of increased organic matter production associated with high catchment vegetation cover overshadowed the potential effect of reduced runon. We also found carbon and nitrogen profiles to be significantly shallower in playas than uplands. This trend correlated with evidence of sedimentation and shallow-rooted plants in playas. Uplands had a deeper C and N profile, which correlated with organic matter being generated by deeply-rooted vegetation. In playas, C:N ratios remained constant through depth but in uplands, C:N ratios increased through depth. We found evidence that differences in rooting depth distributions and soil texture may explain these C:N variations between uplands and playas. In uplands, clay concentration increased with depth; whereas in playas, clay concentration did not change with depth, which highlighted the important role of sedimentation in these ecosystems. Our results suggest that small changes in playa catchment vegetation cover in response to climate change or grazing intensity would greatly impact playa soil organic carbon and nitrogen stocks. This effect would be due to the playa soils dependence on allochthonous organic matter and the large upland area that drains into playas.