|
The composition of atypical tropical carbonate sediments that form in high-nutrient settings is reminiscent of cool-water or non-tropical carbonates. Misinterpreting such deposits in the rock record results in misleading climatic and paleoceanographic interpretations. Here we present a modern example of a high-nutrient tropical setting with carbonate production and sedimentation. The shelf off northern Mauritania is influenced by oceanic upwelling. The upwelling, nutrient-enriched waters warm up on the broad shelf to temperatures typical for many tropical environments. Another peculiarity of this shelf area is that Trade Winds introduce high amounts of eolian dust from the Sahara, further fertilizing the environment. The carbonate grain associations of this depositional setting reflect these conditions. The sediments collected in water depths between 10 and 150 m waterdepth are characterized by heterozoan carbonate grain associations. They vary from clean coarse-grained, almost pure carbonate sediments to fine-grained sediments with siliciclastic (dust) contents of about 50%.
Carbonate components include abundant molluscs, worm tubes and bryozoans, as well as foraminifers and arthropods, elements that are also abundant in extratropical sediments. While planktonic foraminifers represent the upwelling element, other components (e.g. molluscs) demonstrate the tropical origin of the sediment. The high-nutrient (and thus also low light-penetration) conditions are reflected in the fact that hermatypic shallow-water corals are absent as are calcareous green algae.
The Mauretanian sediments represent an environment that is rare in the modern world but might have been more common in the geologic past when global temperatures were higher. On the basis of facies analyses, analogous sediments from the rock record could be misinterpreted as extratropical carbonates when using a high taxonomic level. Closer study allows to distinguish high-nutrient carbonate sediments from extra-tropical sediments, thus improving paleoclimatic reconstruction.
|
