A comparative study of modern carbonate mud in reefs and carbonate platforms: Mostly biogenic, some precipitated

Carbonate mud from reefs and carbonate platforms in six locations of the Atlantic, Indian, and Pacific Oceans (Belize, Bahamas, Florida, the Maldives, French Polynesia, Great Barrier Reef) was systematically and quantitatively analyzed with regard to texture, composition, mineralogy, and geochemistry. Mud composition shows considerable variability, however, the data supports the contention that these muds are largely derived from the breakdown of skeletal grains and codiacean algae. Only mud from the Bahamas and northern Belize, areas which are characterized by common whitings, is interpreted to be mainly inorganically precipitated. Three grain-size fractions (63–20 μm, 20–4 μm, < 4 μm) from twelve samples of mud were investigated by scanning electron microscopy (SEM) to identify grains, X-ray diffraction to measure relative abundances of carbonate minerals, atomic absorption spectroscopy (AAS) to determine strontium concentration, and mass spectrometry in order to measure stable-isotope ratios of carbon and oxygen. The coarser grain-size fractions 63–20 and 20–4 μm are dominated by skeletal fragments with the exception of the Bahaman samples that are composed of peloids. The grain-size fraction < 4 μm is characterized by the occurrence of small aragonite needles, nanograins, and coccoliths. Coccoliths are common in deeper lagoonal settings of the open ocean settings (Maldives, French Polynesia). The geochemistry of the < 4 μm fraction indicates algal and skeletal origins for most of the samples because strontium concentrations range between 2000–8000 ppm and the δ13C values are not as high as in non-skeletal grains. The Bahaman samples, however, exhibit the highest aragonite contents and strontium concentrations, suggesting physico-chemical precipitation. The northern Belize and Great Barrier Reef samples show the highest magnesium calcite values and, accordingly, produced the lowest aragonite and strontium measurements. The high-magnesium calcite portion of the northern Belize mud is either precipitated or due to abundant micritized skeletal grains (e.g., foraminifera): more studies are needed to verify the origin. In the case of the Great Barrier Reef sample, coralline algae appear to be the source of abundant high-magnesium calcite. This study emphasizes that from a global perspective, modern muds in reefs and carbonate platforms exhibit different compositions but are in many cases biologically derived. Even though the composition of modern carbonate muds varies among the six locations investigated, they may serve as analogs for the formation of muds in Cenozoic and Mesozoic reefs and carbonate platforms. Limitations of the interpretation of carbonate-mud origin include the difficulty of identifying, quantifying, and analyzing small grains, the ease with which small grains and crystals are diagenetically altered to microsparitic limestone, and the fact that several modern producers of carbonate mud did not exist in the geological past.