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The end- mass extinction event at the impact area: A rapid macrobenthic diversification and stabilization

Francisco J. Rodríguez-Tovar1, Christopher M. Lowery2, Timothy J. Bralower3, Sean P.S. Gulick2,4,5, and Heather L. Jones3 1Departamento de Estratigrafía y Paleontología, Univ. Granada, Spain 2Institute for Geophysics, Jackson School of Geosciences, Univ. Texas at Austin, USA 3Department of Geosciences, Pennsylvania State Univ. USA 4Center for Planetary Systems Habitability, Univ. USA 5Department of Geological Sciences, Jackson School of Geosciences, Univ. Texas at Austin, USA

Ichnological analysis at the Chicxulub impact crater, México (IODP/ICDP; Site M0077), showed a surprisingly rapid initial tracemaker community recovery after the K-Pg mass extinction event. Full recovery was also rapid, with a well-developed tiered community within ∼700 k.y. Several stages of recovery were observed, with distinct phases of stabilization and diversification, ending with a trace fossil assemblage of abundant Zoophycos, Chondrites, and Planolites. Rapid recovery suggests that favorable paleoenvironmental conditions were quickly reestablished within the impact basin. Comparison with end- extinction reveals that postextinction recovery was significantly faster 1 after the K-Pg event. The end-Cretaceous mass extinction event at the impact area: A rapid macrobenthic diversification and stabilization Rodríguez-Tovar et al.

The K-Pg mass extinction, 66.0 Ma (Renne et al., 2013), was one of the most important events in End-Cre. ~66 Myr the Phanerozoic, severely altering the evolutionary and ecological history of biotas (Schulte et al., 2010). Caused by paleoenvironmental changes End-Dev. End-Ord. End-Per. associated with the impact of an asteroid (Alvarez End-Tri. et al., 1980) on the Yucatán platform in the southern , which formed the Chicxulub impact crater (Hildebrand et al., 1991). Prolonged impact winter resulting in global darkness and cessation of photosynthesis, and acid rain are considered as major killing mechanisms. Major groups disappeared across the boundary (e.g., the nonavian , marine and flying reptiles, ammonites, and ), and other groups suffered severe species level extinction, including planktic foraminifera, and calcareous nannofossils. Thus, the end- Cretaceous impact event had a major importance in the evolution of life in the Earth from the Paleogene.

2 The end-Cretaceous mass extinction event at the impact area: A rapid macrobenthic diversification and stabilization Rodríguez-Tovar et al.

To evaluate the significance of the asteroid impact in the K- Pg mass extinction in April and May 2016, Expedition 364 of the IODP/ICDP drilled into the Chicxulub peak ring and recovered ~130 m of impact deposits which provide a record of the recovery of life in a sterile zone.

Analysis of trace fossils reveals the effect of impact-driven environmental changes on the macrobenthic tracemaker community. Ichnological research is being increasingly used as a tool to study the “Big Five” mass extinctions, with special attention to the K-Pg impact mass extinction event 3 (Lavandeira et al., 2016). The end-Cretaceous mass extinction event at the impact area: A rapid macrobenthic diversification and stabilization Rodríguez-Tovar et al.

Initial recovery years after the impact event

K-Pg boundary (top of peak ring at 618m)

Ichnological data, integrated with planktic foraminifera and calcareous nannoplankton datasets, revealed that life reappeared in the basin just years after the impact. Clear, discrete trace fossils, including Planolites and Chondrites, are registered in the sediments deposited just immediately after the event (Lowery et al., 2018).

Establishment of life within marine impact craters is controlled more by circulation (and thus crater geometry) than by the magnitude of the impact or global environmental effects.

To follow up on this study, ichnological research has been conducted to investigate the INITIAL DIVERSIFICATION, EVOLUTION, RESTRUCTURING, AND STABILIZATION OF THE MACROBENTHIC COMMUNITY FOLLOWING THE IMPACT EVENT (Rodríguez-Tovar et al., 2020). 4 The end-Cretaceous mass extinction event at the impact area: A rapid macrobenthic diversification and stabilization Rodríguez-Tovar et al.

- First phase of diversification to ~45 k.y. after the K-Pg impact event: increase in abundance and size of traces and development of an initial community with Planolites, Chondrites, and Palaeophycus. - Prolonged phase of stabilization: changes only in their relative abundance, until ~640–700 k.y. - Second phase of diversification: appearance of Zoophycos. Highest diversity, abundance, and size of traces, with a community of Zoophycos, Chondrites, Planolites, and Palaeophycus within ~700 k.y. - Phase of consolidation/dominance: community maintenance through at least ~1.25 m.y. after the K-Pg.

Sedimentological and ichnological features through the cores,. Ch—Chondrites; Pa—Palaeophycus; Pl— Distribution of ichnotaxa Planolites; Zo—Zoophycos; BI—bioturbation index (0–4). and proposed recovery model of macrobenthic tracemaker community after the K-Pg impact event with differentiated phases.

5 The end-Cretaceous mass extinction event at the impact area: A rapid macrobenthic diversification and stabilization Rodríguez-Tovar et al.

Alvarez, L.W. et al., 1980. Science, 208, 1095– 1108, Hildebrand, A.R. et al., 1991. Geology, 19, 867– 871. Labandeira, C. et al., 2016. The Trace Fossil Record of Major Evolutionary Events: Topics in Geobiology: Berlin, Springer, 265–300. Lowery, C.M. et al., 2018. Nature, 558, 288–291. Renne, P.R. et al., 2013. Science, 339, 684–687. Rodríguez-Tovar, F.J. et al., 2020. Geology, https://doi.org/10.1130/G47589.1 Schulte, P. et al., 2010. Science, 327, 1214–1218.

 Ichnological analysis supports the rapid complete recovery of the macrobenthic tracemaker community after the K-Pg mass extinction event within the Chicxulub impact crater.  Tracemakers first appeared in the crater within a few years of the impact, and a mature, multitiered macrobenthic community was established ∼700 k.y. after the impact.  Comparison with the end-Permian extinction reveals similarities in the structure and progression of recovery, although the K-Pg recovery was significantly faster.  This rapid recovery may have been related to the shorter duration of paleoenvironmental changes related to the K-Pg event and the different effects of the two events on benthic communities.  One of the primary lingering effects of the Chicxulub impact was a global shift in export productivity, and thus changes in ichnodiversity and abundance of trace fossils responded primarily to variations in the flux of organic matter to the seafloor during the early .

This research used samples and data from the IODP/ICDP. The research was funded by Projects CGL2015–66835-P (Secretaría de Estado de I + D+I) and B-RNM- 072-UGR18 (Fondo Europeo de Desarrollo Regional de Andalucía) (Rodríguez-Tovar), U.S. National Science Foundation grants OCE-1737351 (Gulick, Lowery) 6 and OCE-1736951 (Bralower), and postexpedition awards from the IODP (Gulick, Lowery, Bralower).