Origin of facies zonation in microbial carbonate platform slopes: Clues from trace element and stable isotope geochemistry (Middle Triassic, Dolomites, Italy)

Limestones containing radiaxial fibrous cements were sampled along the southern slope of the late Anisian (Middle Triassic) Latemar carbonate platform in the Dolomites, northern Italy. The Latemar upper slopes comprise massive microbial boundstone, whereas lower slopes are made of clinostratified grainstone, rudstone and breccia. Samples are representative of a seawater column from near sea-level to an aphotic zone at about 500 m water depth. Radiaxial fibrous cements were analyzed for carbon (δ13C) and oxygen (δ18O) stable isotopic composition, as well as major and trace element content, to shed light on the origin of the slope facies zonation. The δ13C vary between 1·7‰ and 2·3‰ (Vienna Pee-Dee Belemnite), with lowest values at palaeo-water depths between 70 m and 300 m. Radiaxial fibrous cements yielded seawater-like rare earth element patterns with light rare earth element depletion (NdSN/YbSN ≈ 0·4), superchondritic yttrium/holmium ratios (≈55) and negative cerium anomalies. Cadmium reaches maximum values of ca 0·5 to 0·7 μg/g at palaeo-water depths between 70 m and 300 m; barium contents (0·8 to 1·8 μg/g) increase linearly with depth. The downslope patterns of δ13C and cadmium suggest increased nutrient and organic matter contents at depths between ca 70 m and 300 m and point to an active biological pump. The peak in cadmium and the minimum of δ13C mark a zone of maximum organic matter respiration and high nutrient and organic matter availability. The base of this zone at ca 300 m depth corresponds with the transition from massive microbial boundstone to clinostratified grainstone, rudstone and breccia. The microbial boundstone facies apparently formed only in seawater enriched in organic matter, possibly because this organic matter sustained benthic microbial communities at Latemar. The base of slope microbialites on high-relief microbial carbonate platforms may be a proxy for the depth to maximum respiration zones of Palaeozoic and Mesozoic periplatform basins.