26 February 2004
Hawaii's drowned coral reefs--
victims of sudden climate change?
Over the next 50 to 100 years, global sea-level rise could become a major environmental and social problem. Although sea levels are rising slowly at present, recent research suggests that periods of very rapid sea-level rise have occurred in the past, probably due to the breakup of large ice sheets. In the March 2004 issue of Geology, MBARI Postdoctoral Fellow Jody Webster presents new evidence for rapid sea-level rise based on studies of drowned Hawaiian coral reefs.
To predict future changes in sea level, scientists must first understand changes that have occurred in the past. Global sea levels have been rising for about the last 19,000 years, ever since the peak of the last ice age. However, determining exactly how fast this rise occurred has been difficult, because some coastal areas have been rising while others have been sinking during this time.
Webster chose the northeast coast of the island of Hawaii for his study because he hoped the drowned reefs in this area would provide detailed information on sea-level changes since the last ice age. He has spent the last two years examining video and samples from MBARI's 2001 Hawaii expedition, trying to determine exactly when, how, and why these reefs drowned.
Coral reefs "drown" when the water above them becomes so deep that they do not receive enough sunlight to support the colonies of photosynthetic algae that the corals rely on for growth. Coral reefs can keep up with a gradual rise in sea level by growing upward toward the surface. However, if sea level rises more rapidly than a reef can grow, the reef will eventually drown and be overgrown by deep-water coralline algae.
Webster documented this transition from shallow corals to deep-water algae in the Hawaiian reefs by analyzing samples of drowned reef collected by MBARI's remotely operated vehicle Tiburon. He also determined when the reefs drowned by measuring the ages of coral and deep-water algae using concentrations of Uranium, Thorium, and Carbon-14 isotopes. He then constructed a chart showing the changes in relative sea level at this site over time. After compensating for the upward growth of the reef and the sinking of the island under the weight of erupted lava, Webster was able to estimate the effects of global sea-level rise.
Webster is particularly interested in one particular drowned reef whose crest is about 150 meters below sea level. Samples from this reef indicate it was actively growing until about 14,700 years ago. At that point, global sea level apparently increased too rapidly for reef growth to keep up. These findings echo previous studies, which showed rapid rises in sea level between 14,200 and 14,700 year ago in locations as far apart as Barbados and Southeast Asia.
These data support the hypothesis that a sudden rise in global sea level occurred between 14,200 and 14,700 years ago (the exact timing and extent of this rise are still a matter of some controversy). This sudden rise has been labeled "meltwater pulse 1A" because it is thought to reflect the rapid melting of one or more ice sheets, such as those presently covering parts of Greenland and Antarctica.
If a similar "meltwater pulse" occurred today, it would threaten coastal cities around the world. Webster notes that such an event is not inconceivable. In 2002, the Larsen B ice shelf in Antarctica broke apart, releasing a chunk of ice larger than the entire state of Rhode Island. Since the Larsen B shelf was already floating in seawater, its collapse did not add significantly to global sea-level rise. However, according to Webster, "This event really woke people up. They began to realize that such things could happen very, very quickly, in a matter of months, not just over geologic time."
He adds that, "Given continued concerns about the impact of global warming on the ice sheets, gathering more data from the geological record about past events is critical." To this end, he plans to continue his work on submerged coral reefs in Hawaii and around the world.
Webster, J.R., D.A. Clague, K. Riker-Coleman, C. Gallup, J.C. Braga, D. Potts, J.G. Moore, E.L. Winterer, and C.K. Paull (2004), Drowning of the -150 m reef off Hawaii: A casualty of global meltwater pulse 1A?. Geology, 32:3, 249-252; doi: 10.1130/G20170.1
For more information, contact: Kim Fulton-Bennett , firstname.lastname@example.org, 831-775-1835