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

Bramley Murton,Ian Parkinson,Simon Chenery,Matthew Horstwood,Pierre Josso,Paul Lusty

doi:10.1016/j.chemgeo.2019.03.003

Bramley Murton, Ian Parkinson + Show 4 more

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https://doi.org/10.1016/j.chemgeo.2019.03.003

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Abstract

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 188Os/187Os 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.

Highlights

Ferromanganese (Fe-Mn) crusts are found in all oceans at depths between 400 and 7000 m beneath sea level, where they accumulate on exposed hard rock substrates, such as the flanks and summit of seamounts, ridges and sediment-free plateaus (Josso et al, 2017; Koschinsky and Hein, 2017; Lusty et al, 2018)
We examine current best-practice and the importance of using multiple techniques (LA-ICP-MS U-Pb dating, Os isotopes, and Co-chronometry) to cross-validate results and produce more robust age models for Fe-Mn crusts
All measurements for the validation material and sample are presented in Supplementary data 4

Summary

Introduction

Ferromanganese (Fe-Mn) crusts are found in all oceans at depths between 400 and 7000 m beneath sea level (mbsl), where they accumulate on exposed hard rock substrates, such as the flanks and summit of seamounts, ridges and sediment-free plateaus (Josso et al, 2017; Koschinsky and Hein, 2017; Lusty et al, 2018). The potential relative contribution of biogenic, biologically-induced or abiogenic processes to crust formation remains uncertain (Templeton et al, 2009; Wang and Müller, 2009) Their high specific surface area (mean 325 m2/g), dipolar charge and long exposure-time to seawater, mean Fe-Mn oxyhydroxide particles are effective at scavenging dissolved trace elements (Hein et al, 2000). These properties, combined with surface-enhanced oxidation reactions that favour retention of redox-sensitive elements, make Fe-Mn crusts a net sink for many elemental species dissolved in seawater. Their processes of formation mean they represent condensed archives of the compositional evolution of seawater through time, recording climatic and geomorphological changes, such as periods of tropical weathering, glacial erosion and the opening and closing of pathways between

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