December 5, 2005

Ancient sediments show influence of southern ocean circulation on climate

By Emily Saarman

About 34 million years ago, the Earth's climate transitioned from a "greenhouse climate" to the "icehouse climate" of today, forming a massive ice sheet on the Antarctic continent. 

Linda Anderson analyzed the chemical properties of seafloor sediments laid down millions of years ago to piece together a picture of how Southern Ocean circulation may have looked deep in the past.

A new study by Linda Anderson, an ocean sciences researcher at UCSC, suggests that oceanographic features in the Southern Ocean--the intensity of current flow and the amount of stratification (the formation of distinct layers at different depths)--may have played a key role in the transition. 

Anderson analyzed the chemical properties of seafloor sediments laid down millions of years ago to piece together a picture of how Southern Ocean circulation may have looked deep in the past. The periods covered in her study include the transition from the Eocene to the Oligocene epochs about 33 million years ago and a similar transition between the Oligocene and Miocene epochs about 23 million years ago. These transitions coincided with a configuration of Earth's orbit around the Sun that facilitated ice growth. Some additional factor on Earth, however, amplified the climate response during these transitions.

The geological record suggests that this additional factor was a reduction of greenhouse warming due to a decrease in atmospheric carbon dioxide. The Southern Ocean may have played a role in the drawdown of atmospheric carbon dioxide through its influence on the global carbon cycle, Anderson said.

Her study suggests that, unlike today, the mixing of the Southern Ocean around the time of the climate shift was neither as intense nor as deep as it is now. As a result, the ocean was more stratified and regional characteristics of deep and intermediate waters were maintained. This layered structure may have had important consequences for global carbon cycling, setting the stage for the transition from greenhouse to icehouse.

Phytoplankton--tiny plants growing in the surface waters--use carbon dioxide from the atmosphere and transform it into organic carbon. When the plants (or animals that feed on them) die and decompose, most of the organic carbon is recycled, but a small amount is buried within the deep ocean. In a layered ocean, dead organisms can drift into the deeper layers before they decompose, effectively burying the organic carbon in the depths.

Shifting continents have gradually changed the Southern Ocean over the past 30 million years so that water now whips around Antarctica in a strong current known as the Antarctic Circumpolar Current. The strength of the current blocks the influx of nutrient-poor surface water and, as this current squeezes through the narrow passage between South America and Antarctica, it mixes the water from top to bottom. As a result, most of the organic carbon formed within the Southern Ocean today is oxidized before it can be buried.

The impact of organic carbon burial on the atmospheric carbon dioxide depends on the relative burial of organic carbon to total carbon. The types of organisms that fix organic carbon are important, because some form shells of inorganic carbon in a process that releases carbon dioxide. Regional characteristics of the water affect which organisms are favored ecologically by controlling the nutrient content of the surface ocean. The integral relationship between biology (the organisms that fix the organic carbon) and the physical structure of the ocean (both the delivery of nutrients and removal of organic carbon for burial) ultimately control the atmospheric carbon dioxide, Anderson said.

As scientists search for ways to remove carbon from the atmosphere and reduce global warming, understanding organic carbon burial mechanisms of the past and the role of the structure of the ocean will give us important insights into how the global system may behave in the future, Anderson said.

"We have a long way to go to understand the modern Earth system, but because we only have one Earth, we must turn to the geological record to provide us with test cases," she said.

 

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