Astrobiology Science and Technology for Exploring Planets (ASTEP)

PI: Bralower, Timothy

Periods of recovery from mass extinctions provide a means to discover how complex ecosystems and biogeochemical systems are assembled. Many mass extinctions are followed by an “initial recovery phase” dominated by a few species, and characterized by drastically reduced productivity, reduced grazing and unusual biogeochemical conditions. This “boom-bust” community is succeeded by an “intermediate recovery phase” in which species dominance is reduced and productivity proxies such as surface-to-deep carbon isotope gradients are restored. A “final full recovery phase” may follow much later when species diversity is restored to pre-extinction levels, often long after basic ecosystem services and chemical cycling seem to have fully recovered. Major mass extinctions seem to ‘reset’ the global ecosystem back to a “Precambrian-like” microbially-dominated state which subsequently recovers by a combination of physical environmental change and revolution of extinguished ecologies. Consequentially, extinctions give us a window into the structural development of complex ecosystems.

The dynamics of recovery events has received only cursory, and mostly descriptive, study except from a theoretical stand point. How does the interplay of extinction selectivity, species richness, the re-evolution of key ecological strategies, and restoration of coupling between plankton and benthos contribute to these stages of recovery? Here, we propose an integrated study to test the role of biological and environmental variables in the structure of the recovery from the Cretaceous-Paleogene mass extinction.

Our project will be a detailed investigation of community structure during the Cretaceous/Paleogene boundary mass extinction and the subsequent recovery focusing on fossil groups that represent three different trophic levels, planktonic primary producers (calcareous nannoplankton), planktonic consumers (planktonic foraminifera), and benthic consumers (bivalves). Our investigations will reconstruct population structure for all three groups using detailed assemblage counts and analyses of evenness and diversity, as well as geochemical and biological proxies for biogeochemical evolution of oceanic ecosystems. We will compare the timing and rates of recovery of the different groups and major changes in population structure in an attempt to determine the environmental and ecological controls on speciation. Previous studies have focused largely on the 200-300 kyr interval on either side of the boundary, whereas our study will encompass the interval from just before the extinction to about 4 million years after the extinction, the time period in which most of the major structural features of marine ecosystems appear to stabilize. We will investigate sections across a spectrum of environments, ranging from the open ocean to the shelf, and we will use geochemical proxies for biological production to reconstruct the re-assembly of oceanic biogeochemical pathways and environmental structure. Our goal is to provide a unique, ecosystem-wide assessment of the Cretaceous/Paleocene boundary mass extinction and the subsequent recovery that should have broad relevance to the study of other major mass extinctions. We expect that the dynamics of the K/P recovery may also bear on the late Precambrian development of complex communities of multicellular organisms and the consequent stabilization of global climate and carbon cycle fluctuations.