Study finds ocean warmth and atmospheric CO2 concentrations were decoupled during Miocene period; suggests role of ocean circulation, clouds and water vapor feedback

7 June 2012

A new study based on deep-sea sediment cores, published in the journal Nature, has found that during the Miocene period ~12–5 million years ago, climate was decoupled from atmospheric carbon dioxide concentrations. During that time, temperatures across a broad swath of the North Pacific were 9–14 °F warmer than today, while atmospheric carbon dioxide concentrations remained low—200–350 ppm, i.e., near values prior to the Industrial Revolution.

Most studies of Earth’s climate have consistently found a strong correlation between global climate and atmospheric carbon dioxide—i.e., during warm periods, high concentrations of CO₂ persist, while colder times correspond to relatively low levels.

Here we present quantitative geochemical sea surface temperature estimates from the Miocene mid-latitude North Pacific Ocean, and show that oceanic warmth persisted throughout the interval of low pco₂ ~12–5 Myr ago. We also present new stable isotope measurements from the western equatorial Pacific that, in conjunction with previously published data, reveal a long-term trend of thermocline shoaling in the equatorial Pacific since ~13 Myr ago.

We propose that a relatively deep global thermocline, reductions in low-latitude gradients in sea surface temperature, and cloud and water vapor feedbacks may help to explain the warmth of the late Miocene. Additional shoaling of the thermocline after 5 Myr ago probably explains the stronger coupling between pco₂, sea surface temperatures and climate that is characteristic of the more recent Pliocene and Pleistocene epochs.

—LaRiviere et al.

The research team, led by Jonathan LaRiviere and Christina Ravelo of the University of California at Santa Cruz (UCSC), generated the first continuous reconstructions of open-ocean Pacific temperatures during the late Miocene epoch—a time of nearly ice-free conditions in the Northern Hemisphere and warmer-than-modern conditions across the continents.

The team used an organic compound—unsaturated alkenone—as their “fossil thermometers.” The compound is produced by tiny phytoplankton and preserved in cores of ocean sediment drawn from the mid-latitude Pacific Ocean basin. The cores were retrieved by marine scientists working aboard the drillship JOIDES Resolution. Ratios of the compound preserve a record of the water temperature in which the plankton lived.

These data provide the first evidence that late Miocene sea surface temperatures were significantly warmer than today across a large swath of the North Pacific. The research team found that sea surface temperatures appeared to be highest in the early part of the late Miocene (around 12 to 13 million years ago), and gradually cooled throughout the late Miocene.

It’s a surprising finding, given our understanding that climate and carbon dioxide are strongly coupled to each other. In the late Miocene, there must have been some other way for the world to be warm. One possibility is that large-scale patterns in ocean circulation, determined by the very different shape of the ocean basins at the time, allowed warm temperatures to persist despite low levels of carbon dioxide.

—Jonathan LaRiviere

The researchers also looked at changes in the late Miocene thermocline, or the ocean layer where warmer, shallow waters meet colder, deeper waters. By comparing oxygen isotope data retrieved from a variety of fossil plankton species that thrive at different ocean depths, they found that the depth of the thermocline has been growing shallow over the past 13 million years.

The scientists suggest that the deep thermocline—much deeper than at present—resulted in a distribution of atmospheric water vapor and clouds that could have maintained the warm global climate.

The results explain the seeming paradox of the warm—but low greenhouse gas—world of the Miocene.

—Candace Major, program director in NSF’s Division of Ocean Sciences

Several major differences in the world’s waterways could have contributed to the deep thermocline and the warm temperatures of the late Miocene. For example, the Central American Seaway remained open, the Indonesian Seaway was much wider than it is now, and the Bering Strait was closed. These differences in the boundaries of the world’s largest ocean, the Pacific, would have resulted in very different circulation patterns than those observed today.

By the onset of the Pliocene epoch, about five million years ago, the waterways and continents of the world had shifted into roughly the positions they occupy now.

That also coincides with a drop in average global temperatures, a shoaling of the thermocline, and the appearance of large ice sheets in the Northern Hemisphere—in short, the climate humans have known throughout recorded history.

Other co-authors of the paper are Allison Crimmins of UCSC and the US Environmental Protection Agency; Petra Dekens of UCSC and San Francisco State University; Heather Ford of UCSC; Mitch Lyle of Texas A&M University; and Michael Wara of UCSC and Stanford University.

Resources

• Jonathan P. LaRiviere, A. Christina Ravelo, Allison Crimmins, Petra S. Dekens, Heather L. Ford, Mitch Lyle & Michael W. Wara (2012) Late Miocene decoupling of oceanic warmth and atmospheric carbon dioxide forcing. Nature. doi: 10.1038/nature11200