Video – Sea Change: How High Will Waters Rise?
Where on Earth would you look for a Pliocene beach? Since sea level is thought to have been higher in that era, it’s a good guess that a Pliocene beach would be above the elevation of today’s shoreline. Of course, in places where the land itself has dropped down, a Pliocene-era beach could be underwater today. Such sites would be hard to study. So the Pliomax team looks just inland of current shorelines. How far inland depends on many factors, such as the local gradient of the land and elevation changes of the land — either up or down — since the Pliocene.
The Pliomax team finds old beaches by looking for fossilized remains of flora and fauna that live close to shorelines. The Pliocene was recent enough such that organisms back then were nearly identical to those of today. The team searches for shells and corals, coastal species that frequently become fossils. The scientists also seek out physical indications of shorelines turned to stone, such as rippled sandstone and salt-marsh clays.
In most places, tell-tale signs of Pliocene beaches no longer remain. Perhaps the conditions for making fossils weren’t right. Or, perhaps, geologic forces acting over millions of years, such as scouring by wind, erosion by water and the advance and retreat of ice-age glaciers, obliterated all beach traces. In some places, fossil shorelines were later covered and hidden by meters-thick drifts of windblown or waterborne sand and soil. Since so many factors conspire to obscure the location of past beaches, the Pliomax team chooses its field sites with exquisite care and lots of background research.
Paul Hearty, an expert on identifying ancient shorelines, leads many of the team’s field expeditions. Hearty has spent most of his career seeking out beaches and coral left behind during the brief warm periods of the last million years, periods like today. The process used to find Pliocene beaches is about the same. Hearty and his teammates rarely dig for fossils. The don’t have the time or the heavy equipment for that. Instead, they look on the surface for fossils that have been uncovered by rainfall or streams. And they let others do the digging, by exploring road cuts and gravel pits.
In the field, the researchers guess the age of corals, shells and other fossil finds. Clues they consider include location, the particular species of organism and the state of preservation. Paul Hearty whacks stones with his rock hammer and gauges age from the tone of the ping. Sometimes he wets a fleck chipped off a stone to make the grains of the mineral easier to see under his hand lens. Such field techniques help the scientists avoid returning home bearing a crate of useless rubble. But it takes painstaking chemical and physical testing back in the laboratory to be certain of the age of field specimens.
The Pliomax team measures the elevation above current sea level of the Pliocene deposits they identify using a precision GPS with a dish-shaped antenna strapped on like a backpack. With this device they can pinpoint their location to within a few centimeters in three dimensions, about 100 times more precisely than the readings of a typical car GPS . Later, when they know the age of each beach explored, the team will correct the elevation measurement for changes that have occurred in the height of the land since the Pliocene. Calculating correction factors for elevation is a complex task, requiring knowledge of the many forces that warp the continental and oceanic surface of the Earth.