Osmium Isotope Straigraphy of Ferromanganese Crusts

To interpret the changes in isotopic compositions recorded in hydrogenetic ferromanganese (Fe-Mn) crusts over time it is essential to calibrate them in terms of time. The ¹⁰Be method is only reliable for the first 10 Myr. For older parts of the crusts the Co-constant flux method is used. Both approaches however, will fail to account for any growth hiatus or erosion in the sections older than 10 Ma. Attempts at using Sr isotope stratigraphy failed because of post-depositional exchange. For osmium (Os) isotopes on the other hand, calculations of the rate of post-depositional exchange suggest that long-term records in Fe-Mn crusts are reliable. This would allow the ¹⁸⁷Os/¹⁸⁸Os profile of any hydrogeneous Fe-Mn crust to be fitted against the ¹⁸⁷Os/¹⁸⁸Os seawater record established for the last 80 Myr. This stratigraphic method would determine the age of crusts at any depth and identify changes in growth rate, cessation of growth and/or intervals of crust erosion. We tested this hypothesis on the hydrogeneous crust CD29-2 from the Central Pacific Ocean which had been subject to many previous radiogenic isotope studies. CD29-2 is a 105mm thick crust with a growth rate of 2.1mm/Myr, as determined from ¹⁰Be/⁹Be ratios and the Co-constant flux method. This gives a minimum age of 50 Ma for the lowermost portions of the crust. Samples were taken every 2mm through the crust which results in a time-spacing of 1Myr assuming a constant growth. For each sample the ¹⁸⁷Os/¹⁸⁸Os ratio and the ¹⁸⁷Os concentration ([¹⁸⁷Os]) were determined by ID-NTIMS. The [¹⁸⁷Re] was measured by MC-ICPMS, allowing correction for ¹⁸⁷ Re-decay. The corrected ¹⁸⁷Os/¹⁸⁸Os ratios were compared to the seawater record. Using the Be and Co time scales, the ¹⁸⁷Os/¹⁸⁸Os curve obtained from the crust shows a distorted version of the established seawater record. A good match is found if three hiatuses are allowed. The first hiatus of 15 Myr is assigned to the period between 13 and 28 Ma, a second one of 3 Myr to the period between 32 and 35 Ma and a last one of 4 Myr to the period between 35 and 39 Ma. The total elapsed time in these hiatuses is 22 Myr, which increases the age of the base of the crust from 50 to 72 Ma. The crust record thus covers the K-T boundary at 65 Ma where it shows the now well known Os isotopic drop. This new dating technique is the first one to date reliably hydrogenetic Fe-Mn crusts older than 10 Ma and is only limited in time by the oldest data of the ¹⁸⁷Os/¹⁸⁸Os seawater record, 80 Ma at present. It confirms the dating obtained in CD29-2 by the Be method for the first 10 Myr.