RECORDS OF NUTRIENT-ENHANCED COASTAL OCEAN PRODUCTIVITY IN SEDIMENTS FROM THE LOUISIANA CONTINENTAL-SHELF

Shelf sediments from near the mouth of the Mississippi River were coll ected and analyzed to examine whether records of the consequences of a nthropogenic nutrient loading are preserved. Cores representing approx imately 100 yr of accumulation have increasing concentrations of organ ic matter over this period, indicating increased accumulation of organ ic carbon, rapid early diagenesis, or a combination of these processes . Stable carbon isotopes and organic tracers show that virtually all o f this increase is of marine origin. Evidence from two cores near the river mouth, one within the region of chronic seasonal hypoxia and one nearby but outside the hypoxic region, indicate that changes consiste nt with increased productivity began by approximately the mid-1950s wh en the inorganic carbon in benthic forams rapidly became isotopically lighter at both stations. Beginning in the mid-1960s, the accumulation of organic matter, organic delta(13)C and delta(15)N all show large c hanges in a direction consistent with increased productivity. This las t period coincides with a doubling of the load of nutrients from the M ississippi River, which levelled off in the mid-1980s. These data supp ort the hypothesis that anthropogenic nutrient loading has had a signi ficant impact on the Louisiana shelf.