Annette Olivarez Lyle

Regional climate shifts caused by gradual global cooling in the pliocene epoch

The Earths climate has undergone a global transition over the past four million years, from warm conditions with global surface temperatures about 3 °C warmer than today, smaller ice sheets and higher sea levels to the current cooler conditions. Tectonic changes and their influence on ocean heat transport have been suggested as forcing factors for that transition, including the onset of significant Northern Hemisphere glaciation ∼2.75 million years ago, but the ultimate causes for the climatic changes are still under debate. Here we compare climate records from high latitudes, subtropical regions and the tropics, indicating that the onset of large glacial/interglacial cycles did not coincide with a specific climate reorganization event at lower latitudes. The regional differences in the timing of cooling imply that global cooling was a gradual process, rather than the response to a single threshold or episodic event as previously suggested. We also find that high-latitude climate sensitivity to variations in solar heating increased gradually, culminating after cool tropical and subtropical upwelling conditions were established two million years ago. Our results suggest that mean low-latitude climate conditions can significantly influence global climate feedbacks.

Eocene biogenic silica accumulation rates at the Pacific equatorial divergence zone

[2] The Eocene biosiliceous oozes of the tropical Pacific are an easily recognizable sediment type having a reddish brown color and sugary texture. These sediments are dominated by radiolarian tests, with traces of terrigenous debris, hydrothermal oxides, a few diatoms and little if any carbonate. The paucity of carbonate is a result of the very shallow Pacific calcite compensation depth (CCD) of the Eocene (3300 m) [Rea and Lyle, 2005, and references therein]. The dominance of radiolarian tests over those of diatoms is an enigma in that diatoms usually dominate siliceous-rich sediments of the Oligocene and Neogene. The opal-rich character of these Eocene sediments is commonly associated with the development of cherts within the sections. [3] These unusual characteristics of Eocene radiolarian oozes have prompted some authors to puzzle over how so much silica could have been delivered to the sea floor during