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The Eocene Oligocene Transition: productivity bloom or short-circuit in fishes?

  • Author(s): Zill, Michelle Elizabeth
  • Advisor(s): Norris, Richard D.
  • et al.
Abstract

The Eocene-Oligocene Transition (EOT) from 35-33 Ma, centered at 33.9 million years ago (Ma), marks the transition from a Greenhouse to an Icehouse earth. It is associated with the appearance of bulk-feeding whales, and the widespread increase in opal sedimentation in the earliest Oligocene, and has been interpreted to record the initiation of a highly productive Southern Ocean ecosystem. We measured accumulation rates of pelagic fish teeth and shark denticles (ichthyoliths) in a global array of ocean cores that span the EOT to test the impact of this transition on mid-level pelagic consumers, which could serve as trophic links between the opal-producing diatoms and larger, predatory whales. We find that there is no increase in fish productivity across the Eocene Oligocene Transition in any of our records; indeed, few records show any changes in fish production associated directly with the E/O boundary or the Oi-1 glaciation event at 34 Ma. Moreover, we find that export productivity was lower in our Southern Ocean sites compared to lower latitude sites in both the Atlantic and tropical Pacific for the duration of the records. Ecosystem models predict that diatom-based food webs should support abundant top predators. However, with reduced fish productivity, we speculate that diatoms instead formed the base of a food web which short-circuited fishes, perhaps feeding krill and other seasonally blooming zooplankton that in turn directly supported seasonally present top predators such as large whales.

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