ABSTRACT: Syndepositional Dissolution of Shallow-Marine Carbonates

Lynn M. Walter, Linda Bonnell, William P. Patterson

Although early marine cementation is well documented in reefs and carbonate sands, pore-fluid geochemistry of finer grained, more organic-rich carbonate sediments from lagoonal and back-reef environments in the Florida Keys demonstrates that dissolution, rather than cementation, is common. Fine-scale (2 cm) sampling and analyses of pore fluids and sediments as well as determination of sulfate reduction rates using radiotracer ³⁵S techniques was conducted for the upper 30 to 100 cm of carbonate sediments from Florida Bay and across the inner shelf of the Atlantic reef tract to better document this reaction.

Pore fluids have elevated Ca/Cl ratios relative to overlying seawater (up to 30% increase) indicating carbonate dissolution occurs within sediment. Covariant increases in Ca/Cl ratios and total dissolved CO₂ links carbonate dissolution to organic matter decomposition. Most pore fluids are functionally anoxic based on the presence of H₂S and ammonia and significant depletions in SO₄. Sulfate reduction rates decrease with depth, but are relatively rapid compared to rates determined in nearshore terrigenous clastics by other workers. Low degrees of SO₄ reduction in the face of rapid in-situ rates of SO₄ reduction demand rapid chemical exchange rates. Moreover, at these organic matter decomposition rates, sustaining organic carbon lev ls of between 1-2 wt.% requires rapid particle mixing to introduce fresh organic matter to sediment. Because sediment bioirrigation by callianassid shrimp and other organisms can be intense, the sediment-pore fluid system, in fact, is both chemically and mechanically open. This situation provides fresh influx of organic matter to the sediment column, sustaining rapid rates of microbial metabolism, and leads to significant dissolution losses within the upper 20 cm of sediment.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990