The Strontium Anomaly in Modern vs. Ancient Dolomites: Geochemical Study of Carbonate Sediments and Associated Fluids in Hypersaline Coastal Lagoons, Região dos Lagos, Brazil and Coastal Sabkhas, Dohat Faishakh, Qatar
Abstract
In his classic paper on the trace-element geochemistry of dolomite, Lynton S. Land (1980) noted that "most ancient dolomite is trace-element-depleted relative to Holocene analogs". This observation remains particularly relevant with respect to the strontium composition of ancient vs. modern dolomite. The typical Sr content of ancient dolomites is on the order of a few hundred ppm, whereas Holocene dolomites precipitating under hypersaline conditions are often more enriched in Sr with values exceeding 1000 ppm. If modern hypersaline analogs are to be useful for interpreting ancient carbonate sequences, this quantitative difference in Sr composition of ancient vs. modern dolomite needs to be studied using a process-based approach in specific modern environments to characterize the factors leading to Sr incorporation in the carbonates and subsequent early diagenesis resulting in the removal of Sr ions.
We present an investigation of the Sr content of Mg-carbonates and dolomite sediments, associated pore fluids and regional waters from three hypersaline coastal lagoons (Lagoa Vermelha, Brejo do Espinho and Lagoa Salgada), located in the Região dos Lagos east of Rio de Janeiro, Brazil. Because these lagoons are rare examples of modern dolomite precipitation in a hypersaline environment, they have been studied intensively during the past 30 years, revealing the importance of microbial mediation in the precipitation of the carbonate sediments. Our geochemical analyses indicate that sediments containing predominately high Mg-calcite contain some of the highest concentrations of Sr with values averaging 2000 ppm, whereas the 100% dolomite intervals are slightly depleted with Sr values approaching 1000 ppm. The pore fluids and regional waters are not enriched in Sr with maximum values reaching 20 ppm. We attribute the elevated Sr concentrations to the biotic influence on carbonate precipitation in the lagoons. However, large Sr-anomalies in ancient dolomites have been interpreted as the product of secondary replacement of aragonite during dolomitization processes. We are testing this hypothesis in the coastal sabkhas of Dohat Faishakh, Qatar, where aragonite is the dominate marine carbonate being replaced by dolomite precipitation under extreme hypersaline conditions. Land, Lynton S. (1980). SEPM Spec. Pulb. No. 28, 87-110.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMPP54A..08M
- Keywords:
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- 0473 Paleoclimatology and paleoceanography;
- BIOGEOSCIENCESDE: 1051 Sedimentary geochemistry;
- GEOCHEMISTRYDE: 3675 Sedimentary petrology;
- MINERALOGY AND PETROLOGYDE: 4863 Sedimentation;
- OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL