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Friday, June 20, 2008

Can Changes in Sea Level Cause Periods of Mass Extinction?—A Galaxy Exclusive

For years scientists have been trying to find explanations for some of the world’s most mysterious extinction events. Since life began on Earth some 3.5 billion years ago, there have been as many as 23 mass extinction events. Over the past 540 million years, there have been five well-documented mass extinctions, primarily of marine plants and animals, with a stunning 75-95 percent loss of species. Sometimes scientists have been able to pin down the likely culprits for these dramatic events, but often, no clear answer has ever been determined…at least not until now.

Recent research suggests that the size of the ocean itself has been one of the most constant and relentless decider of fates over the last 500 million years. Shanan Peters, lead author of the study, says that sea level has been, and will continue to be, one of the most influential factors in determining Earth’s biology.

Peters' research, which recently appeared in the journal Nature, provides an intriguing perspective on one of nature’s most pervasive mysteries. Most of us correlate Earth's periodic mass extinctions with dramatic and sudden events like a blazing asteroid or a sky-blackening super volcano—the kind of things that have been linked to the demise of the dinosaurs. While dramatic events like these do appear to have played major roles in some extinction periods, they certainly haven’t accounted for all of them. Peters says that is where sea level comes into play. His research provides evidence that convincingly fills in the gaps.

“The oceans might seem to be rather static, but sea levels, in fact, change quite dramatically over geological time. In fact, the rocks right here in Madison, Wisconsin—pretty much in the center of our continent—were in fact deposited in one of these shallow seas that covered much of the present day land area almost 500 million years ago…My results show that the expansion and contraction of these sea waves had a big impact on controlling which animals lived and which animals died…When these seas expand and contract, many of the animals that lived in them were forced to deal with all sorts of environmental changes, and many of those changes caused some animals to go extinct.”

As part of his research, Peters measured two principal types of marine shelf environments found in the rock record. Specifically he looked at the sediments derived from erosion of land and sediments composed primarily of calcium carbonate, which is produced in-place by shelled organisms and by chemical processes. Peters noted the differences in sediment stability, temperature and the availability of nutrients and sunlight and found that dramatic sea level changes appear to correlate with extinction patterns.

But does any of this apply to us today, and if so, how? National Science Foundation (NSF) Program Manager Rich Lane believes it does. "This breakthrough speaks loudly to the future impending modern shelf extinction due to climate change on Earth," says Lane.

Lane is referring to the fact that many climate experts fear a warming climate will cause sea levels to rise with devastating consequences. The Daily Galaxy asked Peters how climate affects sea levels.

“Climate change is linked to sea level for two reasons. Most important, changes in the amount of water stored as ice on land affects the amount of water in the oceans. If all of the ice currently on land melted, average sea level would rise by about 70 m,” Peters explained to Rebecca Sato of The Daily Galaxy. “The second link between sea level and climate is global temperature. As the earth warms, water expands, and this expansion makes sea level rise. The magnitude of this effect is small, on the order of a few meters.”

Peters agrees that rising sea levels will have a biological impact in the relatively near future, but how this change will affect us depends on several different factors.

“Climate change WILL result in a change in sea level. My study, and many others that have focused in great detail on individual events, really do help to understand the biological effects of the environmental changes that go along with sea level change. The overall lesson from the past is that the effect of a given change in sea level depends on the rate of that change and on the state of environments at the time the change occurs,” Peters tells The Daily Galaxy.

However, there’s more too it than that. He also explains that the relatively modest change in sea level expected to accompany global warming isn’t necessarily going to harm marine life, at least not to the extent that some of the more dramatic sea level changes may have caused in the past.

“If sea level rise is fast enough, shallow carbonate environments that need to be in the photic zone (things like coral reefs), can be drowned, and this can result in a temporary or long-term decline in the abundance and diversity of animals like corals. However, sea level rises also expand the areas of shallow seas and can have an overall positive impact on marine biodiversity.” Peters told The Daily Galaxy. “In fact, most of the major mass extinctions in the past have been associated with the draining of large shallow seas that once covered much of the present-day land surface. Only a few cases of abrupt sea level rise, often in conjunction with the spread of anoxic marine bottom waters, appear to have been important drivers of extinction.”

So, it’s really not marine life that Peters is predominantly worried about in regards to any future rise in sea level. He explained to The Daily Galaxy that overall the biosphere could handle the sea level rise that is predicted to occur with global warning. He isn’t so sure, however, that humans will be so lucky.

“In my opinion, a much bigger concern with respect to global warming-induced sea level rise is the human impact. A very large fraction of the world's population and infrastructure is within a few meters of sea level. Thus, even a small rise is going to wreak almost unimaginable havoc,” Peters told The Daily Galaxy. “In fact, I think the impending sea level rise is the most serious threat posed by global warming. The biosphere is well-conditioned to deal with the magnitude of sea level rise that we are likely to induce. We, as a society, are not.”

Posted by Rebecca Sato.

Original here

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