High Water Mark
Part of the Climate Change exhibition.
Part of the Climate Change exhibition.
The seas have risen and fallen many times in Earth's history, and fossilized coral can be silent evidence of higher sea level in the past. Although living coral grows underwater, researchers may collect fossil specimens several feet above current sea level in some places.
The reef that yielded this sample flourished about 125,000 years ago, during the last interglaciation--a warm period between major ice-sheet advances. During this 20,000-year interval, the ice sheets of Greenland and perhaps of the Antarctic were smaller than today. Meltwater from those ice sheets accounts for the higher sea level that allowed these corals to grow. Colonies of two types of coral are displayed together here, as they grew: brain coral (Diploria sp.) and star coral (Montastrea sp.). The coral was collected several meters above sea level in Grassy Key, Florida. It is from a geological formation known as the Key Largo Limestone.
The "steps" leading down to the current shoreline of San Clemente Island, California, were created during several interglacial periods when sea level was much higher. The island itself was slowly rising—a phenomenon called isostatic rebound. So, as the ocean rose during interglacials, fell during glacials and rose again as glaciers retreated, it cut beaches at successively lower levels. The lowest major terrace was cut at the same time the coral on display was growing in the Florida Keys.