THE ROLE OF BACTERIAL RESPIRATION RATES IN THE CARBON ISOTOPIC SHIFTS ASSOCIATED WITH GLOBAL CLIMATIC CHANGE AND MASS EXTINCTION

Many ad hoc explanations have been offered for global carbon isotopic excursions discovered in the marine sedimentary record. Largely unnoticed has been the coincidence of large positive excursions with global cooling and large negative excursions with global warming (thus, the excursions roughly parallel those for oxygen isotopes). Most of the carbon isotopic excursions began at the time of a mass extinction and peaked slightly later. Thus, it appears that such excursions were triggered by environmental changes associated with mass extinction. Particulate organic matter in the ocean is subject to any of three fates: consumption by herbivores, consumption by bacteria, or burial. Carbon isotope excursions have most often been attributed to changes of productivity or, more often, of burial rates for isotopically light carbon. The rate of carbon burial on land, except in late Paleozoic coal swamps, has been too low to be of global significance. The same is true of carbon burial in the deep sea. The dominant locus of carbon burial is the ocean margin: continental shelves and slopes, where today bacteria on average respire about 75% of the organic carbon, while the remainder is buried. The role of these bacteria in remineralization, though always enormous, has inevitably varied greatly in magnitude due to global climatic changes. Bacterial respiration rates are positively correlated with temperature, adhering roughly to the Arrhenius Q10 exponential pattern for chemical reactions. During “icehouse” times, such as the present, subdued bacterial respiration permits a larger fraction of particulate organic carbon to be buried than during “greenhouse” times. Therefore, rapid global cooling yields a shift toward isotopically heavier carbon in the carbon dioxide of seawater. Rapid global cooling has the opposite effect. Subsequent climatic change or negative feedbacks cause these excursions to be expressed as spikes on the isotopic curve. Nonetheless, global changes in respiration rates by marine bacteria can account for both the positive and negative carbon isotopic excursions that are often associated with climatic shifts and mass extinctions throughout the Phanerozoic marine stratigraphic record. This mechanism can also account for the carbon isotopic shifts associated with Pleistocene climatic oscillations.