Analyses of Be and Nd isotopes confirm that the hardgrounds drilled and dredged at the seafloor at Site 1196 as well as the subsurface hardground at Site 1194 are ferromanganese crusts that have incorporated the trace metal isotope composition of ambient seawater. The strong currents that have swept the Marion Plateau and that have prevented sedimentation and caused large hiatuses in the sedimentary sequence were the prerequisite that these crusts formed by precipitation from seawater. Major and minor element compositions suggest diagentic alteration on small spatial scales while preserving primary growth structures. Dating attempts with Be isotopes suggest that surface hardgrounds have grown rapidly within the most recent few 100 k.y. Given that sedimentation probably stopped in early Pliocene time, this implies that hardground crust formation only started after the SMP top had already been exposed to the seafloor for a few million years. The age of 8.7 ± 0.5 Ma obtained by Be isotope dating for the deep hardground crust at 117 m subsurface depth at Site 1194 is in excellent accordance with biostratigraphic and seismic stratigraphic age assignment of the hiatus (7.7–11.8 Ma) during which the crust formed.
Our results suggest that hardground crusts have been diagenetically altered but may have nevertheless preserved the initial seawater-derived Be and Nd isotope composition. The hardgrounds represent relatively fast growing Fe-Mn precipitates that only cover short periods of time during long sedimentary hiatuses and are therefore unsuitable to determine the durations of the hiatuses.