Phanerozoic ecosystems paleobiology through the lens of carbonate sediments

Carbonate sedimentation has occurred continuously on Earth since the Archaean. These sediments are important archives of Earth's geochemical and biological histories. The three chapters in this work explore the geobiology and geochemistry of Earth's sedimentary carbonate cover at different temporal and spatial scales. Chapter 2: Reduction in animal abundance and oxygen availability during and after the end-Triassic mass extinction. I reconstructed the redox history of shallow marine ecosystems across the end-Triassic mass extinction and coeval changes in the abundance of biomineralizing fauna in limestones across the Triassic/Jurassic strata. The results indicate a close relationship between the oxygen content of sea water and the ability of organisms to build their shells. With this work, we demonstrated that the water column redox became reducing at the end of the Triassic period. The sustained anoxia reduced the overall carrying capacity of marine ecosystems making the ecosystems not only less diverse but also rare. Chapter 3: Seashells through time: A Phanerozoic record of the abundance of skeletal organisms Secular changes in the abundance of skeleton-forming organisms in the marine realm are biologically important, with implications for understanding patterns in Phanerozoic marine diversity, primary productivity, resource utilization, faunal abundance, and overall energetics of marine ecosystems, but are challenging to quantify. In this study, we use compositional data from marine limestones to develop a more direct record of Phanerozoic variation in the abundance of skeletal forming organisms. We found that the abundance of skeletal grains in marine limestones has increased across the Phanerozoic. The increase in abundance closely follows the marine biodiversity curve indicating a close coupling of biodiversity and abundance in marine ecosystems across time. Chapter 4: Large variations in redox and diagenesis sensitive trace metals in sedimentary carbonates: Insights from micron scale geochemistry Stable isotope ratios of carbon, oxygen, and major (Sr, Mn, Mg, Fe) and trace elements (Mo, U) concentrations preserved in carbonates have been used to interpret chemistry of Earth's climate and major transitions in biological history, like evolution of multicellular life, biomineralization, and mass extinctions. These carbonates deposited in shallow marine environments get influenced by early marine and meteoric diagenetic processes which stabilize carbonate sediments through precipitation of interstitial authigenic cements. This chapter explores the micron scale variability in geochemistry of authigenic cements to track the pore water chemistry of ancient sea floor environments. We found large variations within and across different carbonate components (micrite, microbialite, oncoid, skeletal grain, cement) in our samples. The results indicate that microbial metabolisms like sulphate and iron reduction could have significant influence on the bulk geochemistry of ancient carbonates through authigenic carbonate production at the sediment-water interface.

Keywords: Sustainability.
Geological and Environmental Sciences.
Advisor: Payne Jonathan L.
Lehrmann Dan
Mukerji Tapan
Sperling Erik