ABSTRACT

Total carbon, organic carbon, and total sulfur were measured in 696 surficial sediment samples collected from the Maryland portion of Chesapeake Bay between 1976 and 1981. The total carbon content of Bay sediments is comprised almost entirely of organic carbon. Preliminary findings indicate that the total sulfur content is preponderantly inorganic. Contour maps of the data reveal two distinct carbon/sulfur regions: the northern bay and the middle bay, separated at 39°03N latitude.

The organic carbon content of sediments in the northern hay spans nearly the full range of measured values, from 0.1 to 10.5 percent, averaging 3.3 percent. Of all the grain-size parameters, carbon is most strongly associated with silt (r = 0.65) in these sediments. The strength of the correlation is related to the terrigenous nature of the organic carbon, composed of particulate coal and plant detritus. In the middle bay, carbon concentrations range from 0 to 7.1 percent around a mean of 1.7 percent. Carbon, derived mainly from primary productivity, is more strongly associated with clay (r = 0.91) than with silt (r = 0.72). Within both areas, the correlation between carbon content and water depth is poor.

The areal distribution of sulfur in bay sediments is strongly affected by the availability of sulfate in the water column. In the northern bay, sulfate concentrations in the overlying water are sufficiently low (< 10 mM) to limit the rate at which carbon is oxidized by sulfate-reducing bacteria. Consequently, the sulfur content of the sediments is also low (x = 0.32%). The ratio of carbon to sulfur (C/S = 10.3) in northern bay sediments is similar to that of freshwater lake sediments. Sulfate availability is not a limiting factor in sulfate reduction in the middle bay. The ample supply of sulfate produces a higher mean concentration of sedimentary sulfur (x = 0.64%) and a C/S ratio of 2.7, within the range of the normal marine ratio of 2.8 ± 1.5.

Carbon and sulfur distributions, in conjunction with interstitial water data, are used to categorize the two regions according to Berner's (1981b) geochemical classification of sedimentary environments. The geochemical environment of the northern bay is anoxic-methanic, and that of the middle bay, anoxic-sulfidic.