What Caused the Ice Ages? Tiny Ocean Fossils Offer Key Evidence
The last million years of Earth history have been characterized by frequent glacial-interglacial cycles; these are large swings in climate that are linked to the growing and shrinking of massive, continent-spanning ice sheets.
These cycles are triggered by subtle oscillations in Earth's orbit and rotation. Yet, the orbital oscillations are too subtle to explain the large changes in climate.
According to Daniel Sigman, the Dusenbury Professor of Geological and Geophysical Sciences, at Princeton University, the cause of the Ice Ages is one of the great unsolved problems in geosciences. "Explaining this dominant climate phenomenon will improve our ability to predict future climate change."
Almost half a century ago, in the 1970s, scientists discovered that the concentration of the atmospheric greenhouse gas carbon dioxide (CO2) was about 30 percent lower during the Ice Ages. That prompted theories that the decrease in atmospheric CO2 levels is a key ingredient in the glacial cycles.
However, the causes of the CO2 change remained unknown. Some data suggested that, during the Ice Ages, CO2 was trapped in the deep ocean, but the reason for this was debated.
During Ice Ages, changes in the surface waters of the Antarctic Ocean worked to store more CO2 in the deep ocean
Recently, an international research collaboration led by scientists from Princeton University and the Max Planck Institute for Chemistry (MPIC) have found evidence indicating that during Ice Ages, changes in the surface waters of the Antarctic Ocean worked to store more CO2 in the deep ocean. The research was published in the journal Science.
The researchers used sediment cores from the Antarctic Ocean in order to generate detailed records of the chemical composition of organic matter trapped in the fossils of diatoms -- floating algae that grew in the surface waters, then died and sank onto the sea floor. According to the research paper, their measurements provide evidence for systematic reductions in wind-driven upwelling in the Antarctic Ocean during the Ice Ages.
In the paper, researchers say that for decades they have known that the growth and sinking of marine algae pumps CO2 deep into the ocean, a process often referred to as the biological pump.
The biological pump is driven mostly by the tropical, subtropical, and temperate oceans; and is inefficient closer to the poles, where CO2 is vented back to the atmosphere by the rapid exposure of deep waters to the surface.
The worst offender is the Antarctic Ocean: The strong eastward winds encircling the Antarctic continent pull CO2-rich deep water up to the surface, leaking CO2 to the atmosphere.
The potential for a reduction in wind-driven upwelling to keep more CO2 in the ocean, and thus to explain the Ice Age atmospheric CO2 drawdown, has also been recognized for decades. Until now, however, scientists have lacked a way to unambiguously test for such a change.
The Princeton-MPIC research collaboration has developed such an approach, using tiny diatoms. Diatoms are floating algae that grow abundantly in Antarctic surface waters, and their silica shells accumulate in deep sea sediment. The nitrogen isotopes in diatoms' shells vary with the amount of unused nitrogen in the surface water.
Research revealed the evolution of nitrogen concentrations in Antarctic surface waters over the past 150,000 years
The Princeton-MPIC team measured the nitrogen isotope ratios of the trace organic matter trapped in the mineral walls of these fossils, which revealed the evolution of nitrogen concentrations in Antarctic surface waters over the past 150,000 years, covering two Ice Ages and two warm interglacial periods.
"Analysis of the nitrogen isotopes trapped in fossils like diatoms reveals the surface nitrogen concentration in the past," said Ellen Ai, first author of the study and a Princeton graduate student working with Sigman, and with the groups of Alfredo Martínez-García and Gerald Haug at MPIC.
"Deep water has high concentrations of the nitrogen that algae rely on. The more upwelling that occurs in the Antarctic, the higher the nitrogen concentration in the surface water. So, our results also allowed us to reconstruct Antarctic upwelling changes."
The data were made more powerful by a new approach for dating the Antarctic sediments. Surface water temperature change was reconstructed in the sediment cores and compared with Antarctic ice core records of air temperature.
"This allowed us to connect many features in the diatom nitrogen record to coincident climate and ocean changes from across the globe," said Alfredo Martínez-García. "In particular, we are now able to pin down the timing of upwelling decline, when climate starts to cool, as well as to connect upwelling changes in the Antarctic with the fast climate oscillations during Ice Ages."
This more precise timing allowed the researchers to home in on the winds as the key driver of the upwelling changes.
New theory for the origin of the Ice Ages
The new findings also allowed the researchers to disentangle how the changes in Antarctic upwelling and atmospheric CO2 are linked to the orbital triggers of the glacial cycles, bringing scientists a step closer to a complete theory for the origin of the Ice Ages.
"Our findings show that upwelling-driven atmospheric CO2 change was central to the cycles, but not always in the way that many of us had assumed," said Daniel Sigman. "For example, rather than accelerating the descent into the Ice Ages, Antarctic upwelling caused CO2 changes that prolonged the warmest climates."
The researchers' findings also have implications for predicting how the ocean will respond to global warming. Computer models have yielded ambiguous results on the sensitivity of polar winds to climate change.
The researchers' observation of a major intensification in wind-driven upwelling in the Antarctic Ocean during warm periods of the past suggests that upwelling will also strengthen under global warming. Stronger Antarctic upwelling is likely to accelerate the ocean's absorption of heat from ongoing global warming, while also impacting the biological conditions of the Antarctic Ocean and the ice on Antarctica.
According to Ellen Ai, the new findings suggest that the atmosphere and ocean around Antarctica will change greatly in the coming century." However, she said that "because the CO2 from fossil fuel burning is unique to the current times, more work is needed to understand how Antarctic Ocean changes will affect the rate at which the ocean absorbs this CO2."
A brief description of the Ice Ages
Science has recorded five significant Ice Ages throughout the history of planet Earth up to the present:
Huronian glaciation: 2.4 to 2.1 billion years ago
The Huronian glaciation was a glaciation during the Siderian and Rhyacian periods of the Paleoproterozoic Era. The Huronian glaciation followed the Great Oxygenation Event, a time when increased atmospheric oxygen decreased atmospheric methane. The oxygen combined with the methane and formed carbon dioxide and water, which do not retain heat as well as methane does. As a consequence, this glaciation led to a mass extinction on Earth.
Cryogenian glaciation: 850 to 635 million years ago
The Cryogenian forms the second geologic period of the Neoproterozoic Era, preceded by the Tonian Period and followed by the Ediacaran. The Sturtian and Marinoan glaciations occurred during this period, which are the greatest Ice Ages known to have occurred on planet Earth.
Andean-Saharan glaciation: 460 to 430 million years ago
The Andean-Saharan glaciation occurred during the Paleozoic Era, during the late Ordovician and the Silurian period. According to Eyles and Young, "A major glacial episode at c. 440 Ma, is recorded in Late Ordovician strata (predominantly Ashgillian) in West Africa (Tamadjert Formation of the Sahara), in Morocco (Tindouf Basin) and in west-central Saudi Arabia, all areas at polar latitudes at the time. From the Late Ordovician to the Early Silurian the centre of glaciation moved from northern Africa to southwestern South America."
Karoo Ice Age: 360 to 260 million years ago
The late Paleozoic ice-house, formerly known as the Karoo Ice Age, was between 360–260 million years ago (Mya) during which large land-based ice-sheets were present on Earth's surface. It was the second major glacial period of the Phanerozoic.
Quaternary glaciation: 2.6 million years ago to present
The Quaternary glaciation, also known as the Pleistocene glaciation, is an alternating series of glacial and interglacial periods during the Quaternary period that began 2.58 million years ago, and is still ongoing.