Multi-disciplinary project seeks clues to absence of benthic life in paleological record
Posted October 24, 2008
The bottom of McMurdo Sound is teeming with life — from brittle stars to scallops to wildly diverse single-celled critters called foraminifera, many of which build hard body parts or shells out of calcium carbonate.
That’s the story today. But what happened in the ocean millions of years ago? That’s a hard question to answer. For some reason, there are few signs of these critters in the fossil records that geologists and other scientists study in sediment cores taken from below the seafloor.
“It’s almost like a disconnect from the life of today with the life of the past,” observed Molly Miller, a geology professor in the Department of Earth and Environmental Sciences at Vanderbilt University and one of the project’s principal investigators for the National Science Foundation-funded study.
Unraveling the mystery is important for a number of reasons, including understanding the evolution of the Antarctic benthic ecosystem through the millennia. Yet the absence of the seafloor-hugging organisms is also of interest because many of them are calcifying animals that form shells and skeletons made of the calcium carbonate materials calcite or aragonite.
Scientists can use their fossilized remains to reconstruct things like past climate and ocean circulation. However, perhaps more importantly, understanding the conditions under which the remains of calcifying organisms dissolve will also help climate change scientists predict what might happen to the modern-day calcifiers, as the oceans become more acidic from absorbing atmospheric carbon dioxide.
In addition, calcium carbonate material naturally dissolves faster in cold, polar waters — one of the key factors the scientists believe may have caused disruptions in the fossil record.
“In tropical zones, calcium carbonate should stick around for a while,” explained Sally Walker, a paleontologist from the University of Georgia in Athens who studies the process of fossilization. “In cold, polar conditions, you would think that calcium carbonate would rapidly dissolve. We do have a record of it in some of the cores, so we know it can fossilize in cooler conditions.”
In effect, while interested in the process of fossilization, an area of study called taphonomy, Walker, working with Miller and Sam Bowser from the New York State Department of Health’s Wadsworth Center in Albany, are also studying the conditions that destroy the remains of the marine animals and foraminifera, or forams, a group of creatures often used by micropaleontologists to study past climate conditions.
“To understand the fossilization process makes you realize this connection between the decay of calcium carbonate and how rapidly it can decay, and then how rapidly it can get into the chemical carbon cycle,” said Walker, a principal investigator for the project making her first trip to the Antarctic.
The scientists believe that in addition to the cold polar water, the ice-covered marine habitat may play a role in destroying the animal remains before they can fossilize. For example, perhaps advancing glaciers ground away shells and other material. Or as Walker put it, “We’re looking at the effects of ice on the fossil record of Antarctica.”
“This project is geobiology, an emerging field of science looking at the connections between biology and geology,” said Tom Wagner, program manager of Antarctic Earth Sciences in the National Science Foundation’s Office of Polar Programs, which is funding the research.
To put their theories to the test, the scientists will conduct a variety of experiments from a field camp at Explorers Cove in New Harbor. The camp, located across McMurdo Sound from the U.S. Antarctic Program’s McMurdo Station, has been used for a number of seasons by Bowser and his team of science divers.
A cell biologist, Bowser studies forams and their place in the benthic food web. Another principal investigator on the project, Bowser is also interested in forams’ possible applications in area such as nanotechnology and biomedicine.
The collaboration is an interesting one. Bowser brings his general expertise in the local benthic ecology to the table, while Miller is an expert in bioturbation (the mixing and displacement of sediments by fauna) and Walker measures the rate of dissolution for the shell-secreting animals.
“Drill cores taken of sediments found on Antarctica’s continental shelves provide our most important records of deep-time climate change,” Wagner explained. “And they should be teeming with remnants of life from the sea bed but they aren’t. Why not?
“It’s important information because it would tell us about past ecosystems while providing another perspective on climate change,” he added. “ It could be that they aren’t preserved, but it could also be that we just don't know how to interpret the records that we have. And that’s what makes this project so exciting — it could lead to a total reappraisal of Antarctica’s past.”
Miller’s role will be to understand the relationship between the benthic fauna and the sediments on which and in which they live. She wants to know how they churn and affect the sediments, which is a function of how many animals are present and how fast the sediment accumulates. Most of the sediment cores drilled in the region — such as from the recent ANDRILL program and the older Cape Roberts project — show little bioturbation, according to Miller.
“We’re actually doing a direct approach,” she explained. “We’re collecting animals that live there, putting them in constrained little aquaria with sediment, and we’re looking at how they mess it up.” Divers will also put out sediment traps on the seafloor to see how much sediment accumulates during the course of the fieldwork. Some traps will remain out for two years until the project resumes in 2010.
The aquariums, which are only about 2.5 centimeters wide, include screens that allow ocean water to move through the chamber. “It’s basically a little microcosm of the natural world,” Miller said.
At the end of the season, divers will collect the aquariums, which the scientists will then freeze and return to the laboratory, where they’ll X-ray the tanks to detect any churning of the sediments by the animals. Then they can compare the results with X-rays taken of sediment cores.
“We’re just unclear as to the extent by which that [bioturbation] process is going on and has been going on,” Miller said. “It looks like it’s not too extensive in the cores in the ANDRILL and Cape Roberts projects.”
Walker will use techniques developed for a long-term experiment in the tropics that she’s involved in called the Shelf and Slope Experimental Taphonomy Initiative (SSETI). For her purposes, it will be important to use a range of calcifying organisms to determine different rates of dissolution because the animals create different skeletal types in various ways. Some, for instance, use organic materials in their skeletal makeup, which may slow dissolution by the cold water.
“Just like a skyscraper is built differently than a ranch-style home, animals do that, too,” Walker said. “I would expect a spectrum of dissolution [rates] in the short term and the long term.”
For example, forams build complex shells of calcite that seem to outlast a snail shell of aragonite, according to Walker. She will also use instrumentation employed by coral reef researchers, a water-quality-monitoring sonde, to measure water temperature, acidity, nutrient levels and other parameters to constrain the conditions under which the decay occurs.
More generally, the scientists will track the relative abundance of species in the underwater region. “Even in a place like Explorers Cover [there are] components of the fauna that are not very well known at all,” Miller noted.
“The implications of this project are greater because of the diversity of people involved,” she added.
NSF-funded research in this story: Sam Bowser, New York State Department of Health, Award No. 0739583; Molly Miller, Vanderbilt University, Award No. 0739496; Sally Walker, University of Georgia, Award No. 0739512.
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