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PSTransgressive Recycling Produces Organic-Rich Carbonate Muds*

By

Harold R. Wanless 1, Brigitte Vlaswinkel 2, and Kelly L. Jackson 2

Search and Discovery Article #40158 (2005)

Posted July 20, 2005

 

*Poster presentation at AAPG Annual Convention, with SEPM, Calgary, Alberta, June 19-22, 2005.

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1Department of Geology, University of Miami, P.O. Box 249176, Miami, FL 33124, telephone: 305-284-2697 ( [email protected])

2Rosenstiel School Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149

 

Abstract

Typical carbonate muds in the coastal bays of south Florida contain 2-10% particulate organics. Carbonate muds formed in response to historical and Holocene transgressive recycling contain 20-40% organics.

Historical recycling is in response to a rapid sea level rise (25cm) that began about 1930. Organic-rich carbonates are rapidly filling lake and lagoon depressions in the coastal complex of Northwestern Florida Bay, Cape Sable, Mangrove Coast and 10,000 Islands as they are opening to increased sediment input. The organic component is composed of varying mixtures of algal/cyanobacterial, mangrove root peat, and freshwater marsh sedge detritus depending on the particulate organic matter sources at a site. Algal/cyanobacterial organics are provided by mats growing on recycled deposits, from recycling older carbonate and organic muds, and from diatom blooms in response to higher nutrients in the transgressive water column. Mangrove organics are provided by both shore erosion of mangrove peats and post-hurricane decay of peats beneath collapsed mangrove forests. Freshwater sedge organics are provided by collapse and decay of freshwater marsh peats as a result of saline water intrusion.

Organic composition in carbonate muds varies both along the Florida Bay, Mangrove Coast, and 10,000 Islands coastal complex of southern Florida and within the coastal complex of local areas.

Rapid transgressive recycling of organic-rich carbonate sediment also occurred following a small rapid rise in sea level about 2500-2400 years before present. This sea-level-induced transgression triggered both a 100km-long rapidly shallowing carbonate mud tidal flat coastline and produced extensive channel- and lake-filling organic-rich carbonate mud sequences.

 

 

Take Home Message

Tropical carbonate sedimentation is commonly associated with communities which produce large amounts of organic matter in the form of:

  • Seagrass roots and detritus in marine lagoons,

  • Mangrove or Spartina peats in saline coastal swamps & marshes,

  • Sawgrass peats in freshwater marshes.

During times of rising sea level, these vascular plant organic root and litter peats may be recycled in response to erosion and re-deposition by prevailing processes and hurricane events. As this happens, biological processes (decay and nutrient blooms) will convert much of the organic matter into algal, cyanobacterial, bacterial and fungal remains.

Currently, this rapid recycling is occurring along the southwest Florida coast. Re-deposition at rates of 1-15 cm per year is rapidly filling coastal lagoons. Meter-scale sediment sequences are rapidly forming which contain 3-33 percent particulate organic matter. A significant portion of this has been converted from tree, sedge, grass and associated root organics to algal and microbial organics.

Small pulses of rapid sea level rise are common in the late Holocene and probably throughout geologic history. This may be a significant process to deposit and preserve algal and microbial matter in coastal sequences.

Even in times before land plants and sea grasses, small, quick rises in sea level would have triggered rapid erosion and deposition of algal and cyanobacterial organics in muddy coastal systems.