Improving confidence in ferromanganese crust age models: A composite geochemical approach

Accurate age models for marine ferromanganese (Fe-Mn) crusts are essential to understand paleoceanographic changes and variations in local environmental factors affecting crust growth rate and their lateral continuity. However, no absolute method exists for dating these deposits beyond the age of 10 Myr, which requires the combination of a number of approaches. Here, we present a composite age model for a 15 cm thick Fe-Mn crust sample obtained by unique core drilling using a remotely operated vehicle at a water depth of 1130 m, on the summit of Tropic Seamount, in the north-east Atlantic. The age model is based on cross-validation of laser-ablation U-Pb dating, Co-chronometry and Os isotopes. These enable robust calibration of the age-depth model using the Bayesian statistical modelling of Markov Chain Monte Carlo (MCMC) simulations. The results show that this Fe-Mn crust commenced growth in the Late Cretaceous between 73 and 77 Ma, and grew at a rate between 1 and 24 mm/Myr, averaging 4 mm/Myr. The phosphatised carbonate substrate, capping Tropic Seamount and underlying most of the Fe-Mn crusts, yields a U-Pb age of 84 +/- 4 Myr, and provides the upper age limit for the model. Less radiogenic excursions of Os-188/Os-187 in the vertical profile through the crust permit the identification of key inflection points in the Os isotope seawater curve at the Eocene-Oligocene and Cretaceous-Paleogene transitions. Growth rates estimated from the empirical Co-chronometer are combined with the age envelope defined by the Os data and used to validate the MCMC simulations. The model identifies five hiatuses that occurred during the Pliocene (2.5 +/- 1.9-5.3 +/- 1.7 Ma), Early Miocene (16 +/- 1-27 +/- 2 Ma), Oligocene (29 +/- 2-32 +/- 1 Ma), Eocene (41 +/- 2-52 +/- 0.6 Ma), and the Late Paleocene (55 +/- 1-59 +/- 1.4 Ma). A major phosphatisation event affecting the Fe-Mn core can be dated to the Late Eocene (38 +/- 1.2 Ma), which coincides with a recorded change in the global oceanic system, from warm and sluggish circulation to cold and vigorous thermohaline-driven meridional overturn at the onset of Antarctic glaciation.