Program > By author > Ishikawa Akira

The impact event at the Cretaceous-Paleogene (K-Pg) boundary, ~66 million years ago, formed ~200-km diameter Chicxulub impact structure on the Yucatán Peninsula of the Gulf of Mexico [1]. The asteroid impact delivered osmium (Os) into the ocean, which records globally as a negative Os isotope (187Os/188Os) shift in the pelagic carbonate sequences for ~200 thousand years (kyr) after the impact [2]. This impact-induced changes in ocean chemistry provides a unique means of constraining the time scale of recovery after the environmental perturbation [3]. However, this approach has been established with the data from only three pelagic sites among the many reported the K-Pg boundary sections. Here we report new record of 187Os/188Os ratios and concentrations of highly siderophile elements (HSEs: Os, Ir, Ru, Pt, Pd, Re) in the earliest to middle Paleocene limestone deposited on top of the Chicxulub impact structure recovered during the IODP-ICDP Expedition 364 [4].

The HSE concentrations throughout the earliest to middle Paleocene limestone show remarkably lower than those in the gray-green marlstone, which contains evidence for asteroid/chondritic material [5]. The CI-chondrite normalized HSE patterns in the Paleocene limestone exhibit concentrations and signatures typical for the crustal materials, following a tilted pattern with low Os-Ir-Ru and higher Pt-Pd-Re concentrations.

Age-corrected 187Os/188Os ratios are low in the earliest Paleocene limestone (~0.19) and then increase gradually. Subsequently, 187Os/188Os ratios recovered to steady state (~0.45) around ~2.5 million years (myr) after the impact. The 187Os/188Os ratios of the limestone within the Chicxulub impact basin are consistent with the values previously reported from the pelagic sites in that the ratios recover from low (~0.17 to 0.2) to steady state (~0.4) after the K-Pg impact [2]. However, the recovery time for Os isotope ratios is quite different, 187Os/188Os profile recorded in the impact basin remain lower than the expected values compared to those of the pelagic sites for at least the first ~1 myr of the Paleocene [2].

Our results suggest that the Paleocene limestone accumulated in the Chicxulub impact structure recorded continuous but unique temporal evolution of seawater Os isotopic composition after the K-Pg impact. A possible mechanism to achieve the delayed recovery of 187Os/188Os values within the impact basin demands (1) only small influx of radiogenic Os in global seawater, and (2) a large input of unradiogenic Os from impact ejecta deposited in the Gulf of Mexico and on Yucatán Peninsula, or (3) input from the impact melt sheet underneath the central basin via venting of hydrothermal fluids [6].

References

[1] Gulick, S.P.S., Barton, P.J., Christeson, G.L., Morgan, J.V., McDonald, M., Mendoza-Cervantes, K., et al. (2008) Importance of pre-impact crustal structure for the asymmetry of the Chicxulub impact crater. Nat. Geosci. 1, 131-135.

[2] Ravizza, G. and VonderHaar, D. (2012) A geochemical clock in earliest Paleogene pelagic carbonates based on the impact-induced Os isotope excursion at the Cretaceous-Paleogene boundary. Paleoceanography 27, PA3219.

[3] Lowery, C.M., Bralower, T.J., Owens, J.D., Rodríguez-Tovar, F.J., Jones, H., Smit, J., et al. (2018) Rapid recovery of life at ground zero of the end-Cretaceous mass extinction. Nature 558, 288-291.

[4] Morgan. J.V., Gulick, S.P.S., Bralower, T., Chenot, E., Christeson, G., Claeys, P., et al. (2016) The formation of peak rings in large impact craters. Science 354, 878-882.

[5] Goderis, S., Sato, H., Ferrière, L., Schmitz, B., Burney, D., Kaskes, P., et al. (2021) Globally distributed iridium layer preserved within the Chicxulub impact structure. Sci. Adv. 7, eabe3647.

[6] Kring, D.A., Tikoo, S.M., Schmieder, M., Riller, U., Rebolledo-Vieyra, M., Simpson, S.L., et al. (2020) Probing the hydrothermal system of the Chicxulub impact crater. Sci. Adv. 6, eaaz3053.