Mid Eocene Climantic Optimum Triggered 500k Year Hypoxic Bottom Waters in Wstern Tethys

Paleoenvironmental changes during the Middle Eocene Climatic Optimum (MECO) and its aftermath: The benthic foraminiferal record from the Alano section (NE Italy)

Authors:

1. F. Boscolo Galazzo (a)
2. L. Giusberti (a)
3. V. Luciani (b)
4. E. Thomas (c, d, e)

Affiliations:

a. Department of Geosciences, Via G. Gradenigo 6, University of Padova, Italy

b. Department of Physics and Earth Sciences, Via G. Saragat 1, University of Ferrara, Italy

c. Department of Geology and Geophysics, Yale University, New Haven, CT, USA

d. Department of Earth and Environmental Sciences, Wesleyan University, Middletown, CT, USA

e. School of Earth Sciences, University of Bristol, UK

Abstract:

The Middle Eocene Climatic Optimum (MECO) was one of the most severe, short-term global climate perturbations of the Cenozoic that occurred at ca. 40 Ma and was characterized by a gradual 4-6°C temperature increase of intermediate and deep-waters. We investigated the response to the MECO of the deep-sea ecosystem in the central-western Tethys, through a quantitative study of bathyal benthic foraminiferal assemblages in the expanded and continuous Alano section (northeastern Italy), for which data on stratigraphy, lithology, isotope and trace element geochemistry, and calcareous microplankton were available. During the gradual warming of MECO (lasting between 350–650 kyr) marine export productivity increased, causing a significant but transient restructuring of benthic foraminiferal faunas, which changed gradually from assemblages typical for oligo-mesotrophic sea floor conditions to assemblages indicative of more eutrophic conditions. Just after the peak MECO conditions, which lasted less than 100 kyr, a prolonged phase of environmental instability (~ 500 kyr) occurred, marked by even more highly increased export productivity leading to bottom-water oxygen depletion, as reflected in deposition of organic-rich sediments and multiple peaks of bi-triserial opportunistic benthic foraminiferal taxa, including buliminids, bolivinids and uvigerinids. The high productivity may have been caused by a strong influx of nutrient-bearing fresh water into the basin, due to the increased vigour of the hydrological cycle during the warm period, and this increased fresh-water influx might have been a factor in enhancing water column stratification, thus exacerbating the hypoxic conditions, which persisted about 400–500 kyr. After deposition of the organic-rich layers the environmental perturbation ended, and benthic foraminiferal assemblages recovered while conditions became very similar to what they were before the MECO. The environmental disturbance during and directly after the MECO thus strongly but transiently affected benthic foraminiferal assemblages in the central western Tethys.

Highlights:

1. We studied benthic foraminifera from a Tethyan section spanning the MECO.

2. Benthic foraminifera reflect paleoenvironmental changes across MECO.

3. Foraminiferal changes suggest variations in productivity and sea-floor oxygenation.

4. Sea-floor eutrophication and oxygen depletion follow the MECO peak warming.

5. No permanent effects on benthic foraminifera have been detected.

 

Evolution of High Crowned Teeth Not Tied to Grass Consumption

Dining on field grasses would be ruinous to human teeth, but mammals such as horses, rhinos and gazelles evolved long, strong teeth that are up to the task.

New research led by the University of Washington challenges the 140-year-old assumption that finding fossilized remains of prehistoric animals with such teeth meant the animals were living in grasslands and savannas. Instead it appears certain South American mammals evolved the teeth in response to the gritty dust and volcanic ash they encountered while feeding in an ancient tropical forest.

The new work was conducted in Argentina where scientists had thought Earth's first grasslands emerged 38 million years ago, an assumption based on fossils of these specialized teeth. But the grasslands didn't exist. Instead there were tropical forests rich with palms, bamboos and gingers, according to Caroline Strömberg, UW assistant professor of biology and lead author of an article in Nature Communications.

"The assumption about grasslands and the evolution of these teeth was based on animal fossils," Strömberg said. "No one had looked in detail at evidence from the plant record before. Our findings show that you shouldn't assume adaptations always came about in the same way, that the trigger is the same environment every time."

To handle a lifetime of rough abrasion, the specialized teeth – called high-crowned cheek teeth – are especially long and mostly up in the animals' gums when they are young. As chewing surfaces of the teeth wear away, more of the tooth emerges from the gums until the crowns are used up. In each tooth, bone-like dentin and tough enamel are complexly folded and layered to create strong ridged surfaces for chewing. Human teeth have short crowns and enamel only on the outside of each tooth.

In Argentina, mammals apparently developed specialized teeth 20 million years or more before grasslands appeared, Strömberg said. This was different from her previous work in North America and western Eurasia where she found the emergence of grasslands coincided with the early ancestors of horses and other animals evolving specialized teeth. The cause and effect, however, took 4 million years, considerably more lag time than previously thought.

The idea that specialized teeth could have evolved in response to eating dust and grit on plants and the ground is not new. In the case of Argentine mammals, Strömberg and her co-authors hypothesize that the teeth adapted to handle volcanic ash because so much is present at the study site. For example, some layers of volcanic ash are as thick as 20 feet (six meters). In other layers, soils and roots were just starting to develop when they were smothered with more ash.

Chewing grasses is abrasive because grasses take up more silica from soils than most other plants. Silica forms minute particles inside many plants called phytoliths that, among other things, help some plants stand upright and form part of the protective coating on seeds.

Phytoliths vary in appearance under a microscope depending on the kind of plant. When plants die and decay, the phytoliths remain as part of the soil layer. In work funded by the National Science Foundation, Strömberg and her colleagues collected samples from Argentina's Gran Barranca, literally "Great Cliff," that offers access to layers of soil, ash and sand going back millions of years.

The phytoliths they found in 38-million-year-old layers – when ancient mammals in that part of the world developed specialized teeth – were overwhelmingly from tropical forests, Strömberg said.

"In modern grasslands and savannas you'd expect at least 35 to 40 percent – more likely well over 50 percent – of grass phytoliths. The fact we have so little evidence of grasses is very diagnostic of a forested habitat," she said.

The emergence of grasslands and the evolution of specialized teeth in mammals are regarded as a classic example of co-evolution, one that has occurred in various places around the world. However, as the new work shows, "caution is required when using this functional trait for habitat reconstruction," the co-authors write.