Dead zones may record river floods

From Denver, at a meeting of the Geological Society of America

The remains of sediment-dwelling microorganisms beneath periodically oxygen-depleted ocean waters could enable scientists to determine when nearby rivers flooded for extended periods.

Huge areas of anoxic water occur offshore near the mouths of rivers, says Lisa E. Osterman of the U.S. Geological Survey in Reston, Va. These areas often form because the rivers carry large amounts of agricultural fertilizers and other nutrients into the sea, where they fuel immense blooms of algae and other plankton at the ocean’s surface (SN: 6/5/04, p. 360: Dead Waters). When those organisms die and fall to the seafloor, their natural decomposition depletes dissolved oxygen in the water.

Fish can swim away from these so-called dead zones, but organisms that live in the seafloor can’t. The microorganisms that live in some sediments off the coast of Louisiana provide a good record of the anoxic zone created by the Mississippi River, says Osterman. In recent years, when this dead zone was extensive and long lasting, the populations of three particular anoxia-tolerant species exploded in sediments of these areas, compared with populations of anoxia-sensitive species.

However, analyses of those sediments indicate that anoxic waters blanketed the area several times before the 1960s, when farmers began to use increasing amounts of artificial fertilizers. Those earlier dead zones probably arose in years when the Mississippi River carried abnormally large volumes of water into the Gulf of Mexico, says Osterman.

When these massive flows of fresh water spilled out of the river and formed a thick layer atop the denser salt water of the Gulf, oxygen couldn’t reach the underlying ocean water efficiently. Indeed, the four periods in which anoxia-tolerant sediment dwellers were most prevalent—1823, 1848 through 1850, 1882, and 1890—match intervals of high river flow, as measured by instruments in place at Vicksburg, Miss.

Population trends among the microorganisms in older sediments may enable scientists to infer periods of high Mississippi River flows for times before 1817, when the Vicksburg instruments were installed. Similar analyses for other anoxia-prone regions could detect ancient periods of river flooding, even in eras when no observers or instruments were present to record the flows, Osterman says.

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