Radiocarbon measurements and loose-sediment quantification of seafloor sediment samples off northern Mauritania

Heterozoan carbonates are typical for extratropical sedimentary systems. However, under mesotrophic to eutrophic conditions, heterozoan carbonates also form in tropical settings. Nevertheless, such heterozoan tropical sedimentary systems are rare in the modern world and therefore are only poorly understood to date. Here a carbonate depositional system is presented where nutrient-rich upwelling waters push onto a wide shelf. These waters warm up in the shelf, giving rise to the production and deposition of tropical heterozoan facies. The carbonate facies on this shelf are characterized by a mixture of tropical and cosmopolitan biogenic sedimentary grains. Study of facies and taxonomy are the key for identifying and characterizing tropical heterozoan carbonates and for distinguishing them from their coolwater counterparts, in particular in the past where the oceanography cannot be determined directly.

Experiment: Competition between calcifying and noncalcifying temperate marine macroalgae under elevated CO2 levels

Since pre-industrial times, uptake of anthropogenic CO2 by surface ocean waters has caused a documented change of 0.1 pH units. Calcifying organisms are sensitive to elevated CO2 concentrations due to their calcium carbonate skeletons. In temperate rocky intertidal environments, calcifying and noncalcifying macroalgae make up diverse benthic photoautotrophic communities. These communities may change as calcifiers and noncalcifiers respond differently to rising CO2 concentrations. In order to test this hypothesis, we conducted an 86?d mesocosm experiment to investigate the physiological and competitive responses of calcifying and noncalcifying temperate marine macroalgae to 385, 665, and 1486 µatm CO2. We focused on comparing 2 abundant red algae in the Northeast Atlantic: Corallina officinalis (calcifying) and Chondrus crispus (noncalcifying). We found an interactive effect of CO2 concentration and exposure time on growth rates of C. officinalis, and total protein and carbohydrate concentrations in both species. Photosynthetic rates did not show a strong response. Calcification in C. officinalis showed a parabolic response, while skeletal inorganic carbon decreased with increasing CO2. Community structure changed, as Chondrus crispus cover increased in all treatments, while C. officinalis cover decreased in both elevated-CO2 treatments. Photochemical parameters of other species are also presented. Our results suggest that CO2 will alter the competitive strengths of calcifying and noncalcifying temperate benthic macroalgae, resulting in different community structures, unless these species are able to adapt at a rate similar to or faster than the current rate of increasing sea-surface CO2 concentrations.