A Comparative Study of the Origin of Carbonate Mud in Reefs and Carbonate Platforms Using Modern Samples From the Atlantic, Pacific and Indian Oceans
Gischler, Eberhard; Dietrich, Sarah
Fine-grained carbonates are common in the rock record. The origin of modern carbonate mud, which serves as a model for mudstone and wackestone formation, is a fundamental and controversially discussed issue. Detrital, algal, and inorganic origins and combinations of these are being discussed, to mention the most common. Existing studies have usually concentrated on one location. Among these, Bahamian muds have probably received most attention. In this study, a comparative approach was chosen. Twelve modern carbonate mud samples from the Bahamas, Florida, Belize, French Polynesia, the Great Barrier Reef, and the Maledives were compared based on their composition, mineralogy, and geochemistry. The aim of this study is to analyze systematically whether carbonate mud has biogenic or abiogenic origins. The composition of the 12 samples was evaluated qualitatively under SEM. Mollusk fragments, foraminiferal tests, peloids, and tunicate spicules dominate the grain-size fractions 63-20 µm and 20-4 µm. Aragonite needles, nanograins, and coccoliths are common in the fraction <4 µm. Coccoliths are most common in the open ocean settings (Maldives, Polynesia). The amount of aragonite in the 20-4 µm and <4 µm fractions is 86% on average. In the 63-20 µm fraction it is 92%. Highest aragonite percentages were observed in the Bahamas samples. Trace element concentration of strontium in aragonite averages 7,000 ppm in the 63-20 µm grain-size fraction, 5,000 ppm in the 20-4 µm fraction, and 5,600 ppm in the <4 µm fraction. These strontium values suggest a biogenic (skeletal; algal) source of the grains. If values were higher, like 9.000-10.000 ppm, an abiogenic source would be likely. Bahamas samples reach the highest strontium concentrations (8,200 ppm). Stable isotope ratios of oxygen and carbon range from +0,6‰ to +4,4‰ (δ13C) and from -2,65‰ to +0,3‰ (δ18O). Moderate δ13C values indicate that the majority of grains have skeletal and algal origins. Only Bahamas samples display heavy δ13C values. In conclusion, most of the carbonate mud analyzed is presumably of biogenic origin. However, the <4 µm fractions from the Bahamas and northern Belize differ from the rest. Interestingly, these samples are from areas with common whitings, which are probably indicative of carbonate precipitation. Thus, in a global context, the Bahamas which are so often used as model example of modern carbonate mud formation, appear to represent the exception rather than the rule.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013