A Lamont-Doherty scientist and his colleagues are laying the groundwork for worldwide study of why oceans rise and fall

Ocean drilling tells tales of shorelines past

Larry Krumenaker

You've heard that global warming is going to raise the ocean levels, turn New York City into a new Amsterdam, make the Tappan Zee another Zuider Zee, and change New Jersey into one big shallow wading pool.

Big deal. Been there; done that; got the dirt to prove it.

Dr. Gregory Mountain of Lamont-Doherty and Professor Ken Miller of Rutgers head a team of scientists in a pioneering study to chart the level of the sea through time. We know that the sea is rising a foot a century; should we be concerned? Is the change anthropogenic, or has it happened before? To answer these questions, Mountain reasons, we must "understand nature prior to man's appearances," determining where the shorelines were at any past date.

An observer standing today on the waterline at Atlantic City would see the ground slope 1 meter per kilometer, both landward and seaward. At other times, that slope would be the same but the sea level would differ. But when did these changes happen, and where were the shorelines then? More important: Were these local changes, or did global conditions affect the sea level everywhere at the same time?

To answer those questions, Mountain guided the scientific staff on board the drilling ship Resolution, operated by the Ocean Drilling Program (ODP), in 1993 at a point 110 miles east on the continental shelf's edge. Miller alternated between the ship and a land-based team, which drilled several holes along the shoreline. Underneath the two sites, situated on a line or transect, lie rock layers recording the shoreline's history and the erosional deposits that made Jersey and its continental shelf.

Because the rock layers are tilted, sediments from different geological eras reach the surface at different places. Drilling out cores of rocks from holes all along the transect allows the reconstruction of a nearly continuous history of shoreline action. It's not the first attempt to extract sea-level history from rock cores, but nobody has mounted such a massive effort to garner a long and complete record of global sea levels. Past efforts have been small and localized.

New Jersey's continental shelf is ideal for this study: Whether high and dry or drowned, it's been geologically stable for millions of years. Once this chronology is made, scientists will set out for New Zealand, Brazil, South China, and other areas of similar geologic age but with different tectonic histories to see if the results hold worldwide. New Jersey will be the baseline for their comparisons.

Getting the dirt, 10 meters at a time

Mountain's group obtains cores with a drill bit attached to a train of pipes. Within each outer pipe, which holds the hole firm, an inner pipe plunges downward until another length of compressed ancient sands, muds, or rocks fills the pipe and is pulled to the surface.

At sea, the Resolution looks like an oil tanker with an Eiffel Tower (the drilling rig) standing amidships. Since the outer pipes are flexible steel, the ship can ride the waves without snapping the links. Researchers bring new 10-meter-long cores to the surface, encase them in plastic tubing, and lug them to a deck for marking and identification. Each core then goes inside the ship to various labs where it is X-rayed, gamma-ray-scanned, cut in half, photographed, sampled, sliced, and stored under refrigeration. When everything is working correctly, researchers can bring a new core into the ship every 10-30 minutes, 24 hours a day.

On shore, the process is similar but smaller. A drill rig brings up cores a few meters long; engineers and graduate students hand off the cores for initial examination, photography, and wrapping for shipment to storage at Rutgers or Lamont. Unlike their offshore colleagues, Miller and crew do not go as deep into the Earth and can go home for the night.

The researchers safeguard the drilling environment in two important ways. During the planning and drilling stages, they check for hydrocarbons (which can explode upward to damage both the ship and the site) and avoid drilling near them. After the procedure, the team fills the hole with mud and caps it with hydraulic cement, leaving the seabed essentially unchanged.

As a first estimate, proprietary data from holes drilled by Exxon in 1967 established a curve of sea level rises and falls but with nowhere near the accuracy of scientific dating.1 (Oil exploration is not scientific surveying; scientists take whole cores, whereas oilmen take thumbnail-sized cuttings. Geologists know the depth of samples to within centimeters, but oil companies only need to know within tens of feet.) Despite methodologic differences, preliminary results from ODP drilling seem to confirm Exxon's finding that the sea level during the past 35 million years rose and fell with roughly 1.5 million-year cycles. The science party came to this conclusion when they determined that gaps in the rock record drilled on land matched the age of displaced shallow-water sediment they drilled on the continental slope. The only sensible explanation, they argued, was that these represented times of lowered sea level, and sediment eroded from the shoreline and the once-flooded continental shelf reached the slope during submarine avalanches and other abrupt processes.

Mountain and Miller consider that neither their data nor Exxon's can yet determine the range of the highs and lows of these sea level cycles. Could the difference be as much as 150 meters, as Exxon believes, or just a few tens of meters as Mountain suggests? "If these sea level changes were 30 meters instead of 150," he responds, "we know of several mechanisms to explain why: thermal expansion and contraction of sea water, decanting groundwater from the continents, the rise and fall of small bits of seafloor, and the erosion of mountains and sediment deposition into ocean basins. But for changes approaching 150 meters every 1.5 million years, the only known mechanism is the melting and freezing of enormous volumes of glacial ice. Until next July's drilling, we're only halfway towards solving this puzzle; we've confirmed that sea level changes, but we don't know 'why' because we haven't nailed down the 'how much.'"

The ice sheet cometh, then melteth

Perhaps 50 million years ago Earth had no ice; the East Coast beaches ranged as far inland as Harrisburg, Pa. Seafloor spreading, the creation deep within the Earth of new seabed material and its subsequent dispersal, was the main mechanism raising the sea levels that high. Produced more prodigiously then than today, the new material would take up more volume in deep ocean basins, flooding seawater across more landscapes. If you raise the seafloor by filling it with dirt, the surface must rise as well. Most of New Jersey would have been hundreds of meters below the waves.

Miller says: "After the peak of seafloor spreading 80 million years ago, the rate has decreased and sea level has very slowly lowered. But superimposed on that first-order trend are some high-frequency [changes] going on." Seafloor spreading is still the primary influence on sea levels, but secondary influences (particularly climate) can temporarily enhance or inhibit its effect.

The Lamont cores cover a time when ice ages alternated with subtropical interglacial periods, 10-34 million years ago. Around 35 million years ago, something changed the "hothouse world" to an "icehouse world," including the formation of the Antarctic ice sheets. About 20 million years ago, Jersey's climate resembled modern South Carolina's, but with (or because of) a higher carbon dioxide content. Temperatures cooled off until, 10 million years ago, it was just slightly warmer than today; river deltas carved canyons, many of which still exist today offshore, and left "garbage dumps" of debris on the continental shelf.

The ODP cores indicate that the Exxon low-sea-level periods correspond to ice ages. At each drop in sea level, oxygen isotope indicators for rapid ice buildup appear at the same time. Ice ages solidify much of the Earth's water, change the ratio of oxygen isotopes to be found, and lower the sea level. A surprise was that each ice buildup produced a gap in the sedimentary record. There were no dumpings of land material. While the ocean drilling sites probably never saw the light of day, they also didn't receive any washouts from the shore.

"The Exxon record suggests that the rise and fall was on the order of 140 meters in the mid-Oligocene 30 million years ago, and we say it is about 30-50 meters, so they are wrong by almost a factor of three." Miller summarizes the record of the past 1 million years as evidencing eight to 10 changes in sea level, each around 100 meters in range, and as many as 50 smaller, 40-meter-range changes.

A mere 125,000 years ago, most of New Jersey was above sea level. "There is a slight ridge, an elevation difference, about where the Garden State Parkway runs, and that may in truth mark the old paleo-shoreline," Miller points out. Then the last ice age struck.

Glacial ice also depresses the land it sits on. The most recent ice, Miller says, "covered most of Canada and came down as far south as Woodbridge Center Mall and New York City, including [burying] the Empire State Building," though it would not have covered the World Trade Center's upper floors. The land south of the ice compensated for this continental depression by rising in altitude, pushed up like dough by the nearby ice mass, and remained that way for thousands of years, a rare change in slope.

It would have been a long, cold trip to the beach some 18,000 to 21,000 years ago. The sea was 120 meters below its present level, partially due to the artificially high shore. This shoreline would have been near the edge of the continental shelf, around 120 miles farther out than it is today. Since then, the sea has been returning to pre-ice-age levels, all due to the melting of this most recent sheet. "That's what happens when you melt all that ice," says the Rutgers geologist. "It wasn't a linear melting. Rick Fairbanks [of Lamont] has shown it occurred in two major meltwater pulses, which resulted in large and rapid changes in sea level on the order of 30-40 meters in a thousand years [around 10 and 14,000 years ago]. There's melting of ice sheets still going on today."

The non-ice-covered terrain not only was scoured of material, creating the large record of deposits offshore, but also sank back down, allowing the ocean to c reep back in. Today, shoreland continues to sink, and the sea level to rise, apparently enhanced by melting southern polar ice. Should greenhouse effects cause the rises predicted, the Garden State Parkway could once again--and soon--be a shore road.

The answer to questions about how much the sea level changes, and where it was between a few thousand and 10 million years ago, may lie on the transect between the two sites. Other team members, Lamont geologist Nicholas Christie-Blick and his colleague James Austin Jr. of the University of Texas, take their turn at sea to bring back data in the summer of 1997. With luck, one of those holes may show the evidence of sediment that actually was above sea level, helping the ODP team further clarify the ancient background underlying the changes we observe today in our planet's oceans.

Related link...

  • Joint Oceanographic Institutions for Deep Earth Sampling homepage

    1 Haq BU, Hardenbol J, Vail PR. Chronology of fluctuating sea levels since the Triassic (250 million years ago to present), Science 235 (1987):1156-1167.

    LARRY KRUMENAKER is a science and technology writer whose writing has appeared in Science, The Sciences, The Scientist, 21stC, and other publications. He is the author of the Oryx Guide to Astronomy Resources (Phoenix: Oryx Press, forthcoming) and Net.Journal Directory: The Catalog of Full Text Periodicals Archived on the World Wide Web (Hillsdale, NJ: Hermograph, 1997).

    PHOTO CREDITS: Larry Krumenaker.