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. 2021 Feb 24;7(9):eabe3647.
doi: 10.1126/sciadv.abe3647. Print 2021 Feb.

Globally distributed iridium layer preserved within the Chicxulub impact structure

Steven Goderis  1 , Honami Sato  2   3 , Ludovic Ferrière  4 , Birger Schmitz  5 , David Burney  6 , Pim Kaskes  7   8 , Johan Vellekoop  7   9 , Axel Wittmann  10 , Toni Schulz  11   12 , Stepan M Chernonozhkin  13 , Philippe Claeys  7 , Sietze J de Graaff  7   8 , Thomas Déhais  7   8 , Niels J de Winter  7   14 , Mikael Elfman  5 , Jean-Guillaume Feignon  11 , Akira Ishikawa  15   3 , Christian Koeberl  11 , Per Kristiansson  5 , Clive R Neal  6 , Jeremy D Owens  16 , Martin Schmieder  17   18 , Matthias Sinnesael  7   19 , Frank Vanhaecke  13 , Stijn J M Van Malderen  13 , Timothy J Bralower  20 , Sean P S Gulick  21   22   23 , David A Kring  18 , Christopher M Lowery  21 , Joanna V Morgan  24 , Jan Smit  25 , Michael T Whalen  26 ; IODP-ICDP Expedition 364 Scientists
Affiliations

Globally distributed iridium layer preserved within the Chicxulub impact structure

Steven Goderis et al. Sci Adv. .

Abstract

The Cretaceous-Paleogene (K-Pg) mass extinction is marked globally by elevated concentrations of iridium, emplaced by a hypervelocity impact event 66 million years ago. Here, we report new data from four independent laboratories that reveal a positive iridium anomaly within the peak-ring sequence of the Chicxulub impact structure, in drill core recovered by IODP-ICDP Expedition 364. The highest concentration of ultrafine meteoritic matter occurs in the post-impact sediments that cover the crater peak ring, just below the lowermost Danian pelagic limestone. Within years to decades after the impact event, this part of the Chicxulub impact basin returned to a relatively low-energy depositional environment, recording in unprecedented detail the recovery of life during the succeeding millennia. The iridium layer provides a key temporal horizon precisely linking Chicxulub to K-Pg boundary sections worldwide.

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Figures

Fig. 1
Fig. 1. Location map.
(A) Paleogeographic reconstruction for the Late Cretaceous with K-Pg ejecta sites distal (>5000 km) to very proximal (<500 km) to the Chicxulub impact structure, displaying sites with a detected Ir anomaly [values based on (3, 7)]. This map is redrawn after (55) and updated according to (56). All distances as paleogeographic distances between the K-Pg boundary sites and the Chicxulub impact structure. (B) Iridium anomaly as measured by (1) at the distal Gubbio K-Pg boundary site, Italy. (C) Stars designate positions of drill cores on the Yucatán peninsula mentioned in the text (Y6, Yucatán 6; C1, Chicxulub 1; Yax-1, Yaxcopoil-1), including International Ocean Discovery Program (IODP)–International Continental Scientific Drilling Program (ICDP) Expedition 364 (green star). (D) Schematic cross section of the Chicxulub crater, with the location of Site M0077 based on (57). Photo credit for the image in (B): Heiko Pälike, MARUM-Center for Marine Environmental Sciences, University of Bremen.
Fig. 2
Fig. 2. IODP-ICDP Expedition 364 core overview and geochemical profiles.
(A) Lithology recovered from drill core at Site M0077 from 505 mbsf to total depth, with Paleogene sediments (gray), suevite (purple), impact melt rock (green), felsic (granitic) basement (pink), and pre-impact dikes (yellow), adapted after (22). (B) Subdivision of the upper peak-ring interval into various lithological units following (23). (C) Concentration profiles for chromium and nickel, data from (23). Nickel concentration peak detailed in Fig. 3. (D) Inset highlighting Core 40R-1, with core intervals displaying siderophile element enrichments, as indicated by μXRF mapping. The high-resolution line scan photo is from the onshore science party (23).
Fig. 3
Fig. 3. Chemostratigraphy of Core 40R-1.
Profile of iridium concentrations (A), nickel concentrations (B), and initial 187Os/188Os (C) relative to lithological units, proposed depositional mechanism, foraminiferal zone (24), relative timing after impact, and sulfide mineral intervals. Symbol legends designate the laboratories where the respective siderophile element concentrations were determined. Uncertainties on Ir concentrations and 187Os/188Os are expressed as 2 SE or 2σ for data determined by mass spectrometry and instrumental neutron activation analysis iridium coincidence spectrometry, respectively (mostly contained within the symbols). The interval of highest Ir enrichment is highlighted in blue, which corresponds to the white dashed lines in the enlarged core photograph. The high-resolution line scan photo is from the onshore science party (23). While the siderophile element enrichment in the gray-green marlstone and at the top of the transitional unit reflects the deposition of meteoritic matter, the elevated concentrations at the bottom of the transitional unit result from sulfide mineralization following hydrothermal activity in the impact basin. The highest HSE enrichment at the base of the gray-green marlstone (~616.58 mbsf) is interpreted to record the settling of Ir-rich dust, estimated to have been deposited within a few decades after the impact event, and subsequently reworked into a broader interval.
Fig. 4
Fig. 4. Cl-Chondrite normalized HSE patterns.
Green and blue lines denote samples from the base of the gray-green marlstone and at the top of the transitional unit (616.60 mbsf), respectively. Gray and black lines designate other intervals in Core 40R-1. For comparison, the admixture of 1% of CM carbonaceous chondrite to upper continental crust (UCC) was calculated and is shown in purple. HSE concentrations for CI and CM chondrites, UCC, and distal K-Pg boundary sections are from (8, 38, 58). The paleogeographic locations of the different K-Pg boundary sections shown here are highlighted in Fig. 1.
Fig. 5
Fig. 5. Double logarithmic plot of Cr versus Ir concentrations.
Data for global K-Pg sections (7), Chicxulub fossil meteorite fragments (13), Yaxcopoil-1 and Yucatán 6 suevite and impact melt rocks (31), and average data compiled for Core 40R-1 samples, compared to the Cr/Ir ratios in various terrestrial lithologies and chondrites. The gray field indicates mixing trajectories between chondritic projectiles and common terrestrial targets. CC, continental crust; PUM, primitive upper mantle [(59) and references therein].
Fig. 6
Fig. 6. Os isotope ratios versus Os concentrations.
187Os/188Os, not corrected for radiogenic ingrowth, versus Os concentrations determined for the IODP-ICDP Expedition 364 Core 40R-1 samples, impact melt rocks, melt clasts, and lithic clasts from Yaxcopoil-1 (32), Chicxulub 1 impact melt rock (30), and Beloc impact glass (30). The curve represents the calculated mixing line between CI carbonaceous chondrite and the UCC, with admixture of up to 5% CI chondrite. The mixing trajectory between UCC and PUM is indicated by gray solid squares (1 to 50%). Because of low Re/Os ratios, chondritic materials typically yield unradiogenic 187Os/188Os ratios of ~0.13, with limited variation between different types of chondrites (58). Most samples of Core 40R-1 plot close to the mixing line between chondrite and UCC, similar to Yaxcopoil-1 impactites and Beloc impact glass. The highest meteoritic contribution of ~0.1% is measured at the base of the gray-green marlstone. The samples from the lower part of the transitional unit (downward from ~617.32 mbsf) plot to the right of this mixing line, as Os was likely concentrated through hydrothermal activity and sulfide mineralization. This plot is based on (59) and references therein.
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