A snapshot of Middle Triassic seawater from the geochemistry of marine cements along a steep carbonate platform slope – Latemar platform, Dolomites, Italy

Present day ocean water is stratified, with a well ventilated mixed layer at the surface and a zone depleted of oxygen underneath, locally affected by permanent hypoxia (oxygen minimum zone, OMZ). In the mixed layer, primary production occurs through photosynthesis; organic remains from primary producers then sink in the water column, and become the nutrition of heterotrophic plankton, which concentrates immediately below. Respiration consumes oxygen and constitutes a dominant reason why OMZs develop. The transfer of organic matter from the photic mixed layer to intermediate waters, known as “biological pump”, is reflected by a distinct pattern of the δ13C values of dissolved inorganic carbon (DIC). The 12C isotope is preferentially fixed as CO2 in the tissues of primary producers via photosynthesis, and in surface waters the DIC remains enriched in 13C. Below the mixed layer, prevailing respiration processes release the carbon fixed by primary producers, adding 12C to the intermediate seawater DIC pool. This pattern, distinctive in modern oceans, could be hardly documented in the fossil record. We use the trace element and stable isotopic composition of radiaxial fibrous calcite cements from the Latemar carbonate platform in the Dolomites as proxies for the oxygenation and nutrient zonation of the Middle Triassic seawater column. Fibrous cements lining cm-scale primary cavities were sampled along a well exposed slope of the Latemar platform at different paleo-water depths, and analyzed for their rare earth element (REE), Y, Ca, Mg, Fe, Mn, Cr, Co, Cd, Cu, Ba and Sr contents using LA-ICP-MS. The cement carbonates were also analyzed for their C and O stable isotope composition. Superchondritic Y/Ho ratios, negative Ce anomalies and PAAS-normalized patterns that are depleted in light REE indicate precipitation in well-oxygenated seawater through the range of slope paleo-depths. Instead, 13C values are highest in shallow water, 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. The correlation between 13C and Cd points to an active biological pump in the water column flanking the slope, with the respiration maximum between 50-200 m water depth marked 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 the slope facies switch from microbialitic to detrital grainstone and rudstone. The correlation of the nutrient tracers with the downslope facies switch suggests that the microbial communities were likely nutrient-limited at Latemar, and did not colonize the lower portion of the slope because it was in contact with a seawater depleted in nutrients.