Ancient oceans experienced a surge in biological productivity

BOSTON—In work that could improve understanding of future climate change, University of Michigan researchers have documented a global-scale increase in oceanic biological productivity that occurred between about 6 million and 4 million years ago, during the late Miocene and early Pliocene epochs of geological history.

Graduate student Casey Hermoyian and Prof. Robert M. Owen of the U-M Department of Geological Sciences discovered the “biogenic bloom” by measuring levels of phosphorus in marine sediment cores collected from the Atlantic and Indian Oceans. Hermoyian reported their findings at the spring meeting of the American Geophysical Union.

The waxing and waning of biological productivity in ancient oceans offers insights into climate change, Owen explains. Biological productivity is a measure of the amount of biomass (total living matter) produced in a given time. In oceans, biomass is produced mainly by photosynthesis: microscopic organisms (plankton) capture energy from the sun and use it to convert carbon dioxide and dissolved nutrients, such as phosphorus, into biomass. In the process, oxygen is released into the atmosphere as a byproduct.

By studying patterns of biological productivity, paleoceanographers can make inferences about climate, which is affected by changing levels of atmospheric gases. For example, a long period of high biological productivity could lead to a net loss in carbon dioxide from the atmosphere, which in turn could cause the Earth to cool—an anti-greenhouse effect.

“A basic paradigm of earth science is that the present is the key to the past. But in fact, in many cases the past also is the key to the future,” says Owen. “One of the ways in which we try to understand the present-day climate and especially the future climate is by going back in the geologic record to see if we can discern the causes and effects of what occurred. And the more insight we can get from the past, the better we know how processes are working today—and more important, how they’re going to work in the future.”

Earlier investigations by several researchers had suggested that biological productivity rose dramatically in the Indo-Pacific region of the world ocean during the late Miocene and early Pliocene. Hermoyian and Owen wanted to find out whether this biogenic bloom occurred only locally or was a worldwide phenomenon. The key to the answer lay in the pattern of phosphorus distribution during the time of the bloom, especially in areas of relatively low biological productivity.

“You can think of the distribution of phosphorus and other nutrients like the distribution of wealth,” says Owen. “One possibility for what occurred during the biogenic bloom is that there was not an overall global increase in productivity, but instead, changing ocean circulation patterns shifted nutrients away from the nutrient-rich Atlantic Ocean to the usually nutrient-poor Indian Ocean. Around my lab we began referring to this possibility as the ‘Robin Hood’ hypothesis.”

A second possibility is a “rich-get-richer” or “Sheriff of Nottingham” scenario, in which ocean circulation changes caused areas that already were nutrient-rich to become even richer by transferring nutrients from nutrient-poor areas.

A true global-scale biogenic bloom would have occurred only if nutrients increased throughout the world ocean, a situation that Owen’s group dubbed the “Camelot” hypothesis.

To distinguish among the three hypotheses, the researchers measured phosphorus accumulation rates in core samples from areas of the Indian and Atlantic Oceans where productivity is usually low. Their results show that biological productivity increased by two to 30 times above background levels throughout the world ocean during the late Miocene and early Pliocene. This is significant because it means that the biogenic bloom occurred on a global, rather than a regional scale, and that it was not linked to a shift of nutrients but instead to a worldwide, overall increase in nutrient supply. In other words, the biogenic bloom was a lot like Camelot.

Now the researchers are trying to understand what caused the increase in nutrients. They suspect that rapidly rising mountains played an important role.

“We know that both the Himalayas and the Andes were rising dramatically during this time,” says Owen. “When mountains get higher, they act as a barrier. As the wind tries to move along, it hits the barrier and flows up. Wind that crosses oceans carries a lot of moisture with it, and when it goes up it cools off, it dries out, and it dumps its moisture.” The resulting rainfall flushes soils and nutrients into rivers, which empty into oceans.

But rising mountains may not tell the whole story. Many of the productivity increases that Hermoyian and Owen observed were characterized by two closely-linked peaks, suggesting that a second factor independently influenced the increase, or was triggered by the first.

“We don’t have a definitive answer yet,” Owen admits. “In my gut I feel the nutrient increase is certainly linked to the uplifting of the Himalayas and the Andes, but we have to keep looking at other possibilities until we know for sure.”

Robert M. Owen