Origin of facies zonation along the slope of the Middle Triassic Latemar carbonate platform (Dolomites, Italy): clues from trace elements and stable isotope geochemistry

Many Paleozoic and Mesozoic high-relief carbonate platforms show a subdivision of the slope facies. The upper slopes are comprised of massive to poorly clinostratified microbial boundstone, whereas clinostratified deposits of grainstone, rudstone, and breccia are common lower slope facies. At present, the reasons why such slope facies subdivision exist are not fully understood, and the environmental conditions along these slopes are only poorly constrained. In order to shed light on the origin of the facies zonation of microbial platforms, marine fibrous cements lining cm-scale primary cavities were sampled along a well exposed slope of the Latemar platform at different paleo-water depths, and were analyzed for their C and O stable isotope composition. Rare earth element (REE), Y, Ca, Mg, Fe, Mn, Cr, Co, Cd, Cu, Ba and Sr contents were also analyzed, using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The fibrous cements are characterized by superchondritic Y/Ho ratios, negative Ce anomalies and PAAS-normalized patterns that are depleted in light REE, indicating precipitation in oxygenated seawater. The highest 13C values were recorded in samples from shallow depths, and then reach a minimum at paleo-water depths of around 50-200 m. Cadmium (Cd), a trace metal with a nutrient-like behavior, is most abundant within this same depth range. A correlation between 13C and Cd points to an active biological pump in the water column flanking the slope. Photosynthesizing primary producers were active in the first few tens of m of the water column, as reflected by relatively higher 13C carbonate values. A respiration maximum is indicated between 50-200 m water depth by relatively high Cd, and a 13C which is ca. 0.5 ‰ lower with respect to the surface water. Below 200 m water depth, Cd and 13C values rebound, and slope facies switch from microbialitic to detrital (grainstone and rudstone). The correlation of nutrient tracers with the downslope facies transition suggests that the microbial communities that constitute the boundstone facies were likely nutrient-limited at Latemar, and could only proliferate at shallow water depths, where nutrients were not yet depleted by respiration. This could explain the subdivision of the slope into a upper (microbial) and a lower (“detrital”) facies belt, and suggests that this facies transition, commonly observed in Paleozoic and Mesozoic microbial platforms, may mark the depth to the oxygen minimum zone in coastal water columns.