Evolution of the trace element contents (Sr and Mn) of hemipelagic carbonates from the Zyumaia Paleocene section (Gipuzkoa, Spain): implications for the knoledge of seawater chemistry during the Selandian
Résumé
Well exposed and stratigraphically well constrained by numerous studies, the Zumaia section is one of the best places to conduct studies on the Palaeocene in basin facies. Thus, this section has been chosen [Schmitz et al., 2011] as a stratotype of Selandian basal and terminal limits (GSSP: Global boundary Stratotype Section and Point). The sediments consist of carbonate hemipelagites interbedded with fine carbonate (Maastrichtian to Selandian) and siliciclastic (Thanetian to Eocene) turbidites.
The purpose of this work is to geochemically characterize the Selandian by trace element contents (strontium and manganese) and to try to assess the chemical composition of seawater during the Paleocene. Analysis of various separated granulometric fine fractions show that hemipelagic sediments from the Zumaia section present a high preservation quality of the original records of trace-element contents. Late burial diagenesis plays only a minor role and geochemical breaks are not reducible to a change in the nature of carbonate producers.
The strontium contents of Paleocene sediments require that the Sr/Ca ratio of seawater was lower than that in the present ocean. The Selandian is characterized by a positive excursion of the strontium curve. This accident is also recognized in several worldwide sections and is related to the platform/basin carbonate sedimentation budget and the intensity of oceanic hydrothermalism.
The Mn content of hemipelagites is very high and can reach 2500–3000 ppm in the Paleocene. A comparison of analyses by atomic absorption spectrometry (AAS) and electron paramagnetic resonance (EPR) shows that both Mn2+ (in the calcite lattice) and Mn4+ (as oxide micro nodules) coexist. The Mn content fluctuations are related to the opening phases of the North Atlantic during the Paleocene by submarine volcanism and hydrothermalism in the North Atlantic Igneous Province (NAIP).