Distribution of metal(loid)s in size fractions of carbonate-rich marine sediments: Detrital vs. authigenic origin of carbonates

Two marine carbonate-rich sediments were studied to gain insight into the role of carbonate minerals of different origin in the distribution and occurrence of metal(loid)s. Successive fractions of surficial sediments were separated according to size (<8 μm, <4 μm, <2 μm, <1 μm and <0.45 μm) and the changes in their properties and multi-element composition were investigated. Both sediments showed low levels of metal(oid)s, as a result of their high carbonate content. The size-related concentrations of elements in the sediment fractions varied according to the ratio of carbonates to aluminosilicates, which was governed by the carbonate origin. In sediment with detrital calcite, the coarse-grained calcite particles were efficiently removed already from the <8 μm fraction, leading to a prevalence of aluminosilicates and a significant size-dependent increase in the concentrations of most elements in finer fractions. In sediment with authigenic aragonite, fine aragonite particles were present in all fractions, diluting the aluminosilicate component and lowering the content of metals. A significant increase in the concentrations of most metals occurred only in the finest fraction (<0.45 μm), where the content of carbonates eventually decreased. Similar size-dependent trends of Sr and U in sediment with authigenic carbonates indicated the important role of aragonite precipitation in their sequestration. The redox sensitive elements As, Mo, Sb and U were significantly associated with sedimentary organic matter and affected by Fe and Mn cycling under reducing conditions, prevailing in the investigated sediments. Fractionation by particle size revealed significant differences in the sequestration and behavior of the elements in these seemingly similar carbonate sediments, which remain undetected when considering the bulk sediment.