Rapid recovery of life at ground zero of the end- Cretaceous mass extinction (original) (raw)

The Cretaceous/Palaeogene mass extinction eradicated 76% of species on Earth 1,2. It was caused by the impact of an asteroid 3,4 on the Yucatán carbonate platform in the southern Gulf of Mexico 66 million years ago 5 , forming the Chicxulub impact crater 6,7. After the mass extinction, the recovery of the global marine ecosystem-measured as primary productivity-was geographically heterogeneous 8 ; export production in the Gulf of Mexico and North Atlantic-western Tethys was slower than in most other regions 8-11 , taking 300 thousand years (kyr) to return to levels similar to those of the Late Cretaceous period. Delayed recovery of marine productivity closer to the crater implies an impact-related environmental control, such as toxic metal poisoning 12 , on recovery times. If no such geographic pattern exists, the best explanation for the observed heterogeneity is a combination of ecological factors-trophic interactions 13 , species incumbency and competitive exclusion by opportunists 14-and 'chance' 8,15,16. The question of whether the post-impact recovery of marine productivity was delayed closer to the crater has a bearing on the predictability of future patterns of recovery in anthropogenically perturbed ecosystems. If there is a relationship between the distance from the impact and the recovery of marine productivity, we would expect recovery rates to be slowest in the crater itself. Here we present a record of foraminifera, calcareous nannoplankton, trace fossils and elemental abundance data from within the Chicxulub crater, dated to approximately the first 200 kyr of the Palaeocene. We show that life reappeared in the basin just years after the impact and a high-productivity ecosystem was established within 30 kyr, which indicates that proximity to the impact did not delay recovery and that there was therefore no impact-related environmental control on recovery. Ecological processes probably controlled the recovery of productivity after the Cretaceous/Palaeogene mass extinction and are therefore likely to be important for the response of the ocean ecosystem to other rapid extinction events. The recent joint expedition of the International Ocean Discovery Program and International Continental Drilling Program (hereafter, Expedition 364) recovered what is, to our knowledge, the first record of the few hundred thousand years immediately after the impact within the Chicxulub crater. Site M0077, which was drilled into the peak ring of the crater 7 (Extended Data Fig. 1), sampled an approximately 130-m-thick, generally upward-fining suevite (that is, melt-bearing impact breccia) overlying impact melt rocks and fractured granite 17. The boundary between the suevite and overlying earliest-Palaeocene pelagic limestone is in core 40-1 (Fig. 1), and comprises a 76-cm-thick upward-fining, brown, fine-grained micritic limestone that we term the 'transitional unit'. The lower portion of the transitional unit is laminated below 54-cm core depth and contains no trace fossils (Fig. 1 and Extended Data Fig. 2). The laminations are thin, graded beds with sub-millimetre-scale cross-bedding that indicates bottom currents, and are likely due to the movement of wave energy-including tsu-nami and/or seiches-in the days after the impact. The fine grain size (primarily clay to silt, with some sand-sized grains concentrated in the graded beds) suggests that much of the material in the transitional unit was deposited from resuspension and settling. The transitional unit is overlain by a white pelagic limestone. The lowermost sample taken in this limestone (34 cm core depth) contains the planktic foraminifer Parvularugoglobigerina eugubina (which marks the base of Zone Pα), other foraminifer of the same genus (P. extensa, P. alabamensis) and Guembelitria cretacea. Because many other species that originate within Zone Pα first appear a few centimetres higher in the section (31-32 cm), we conclude that the base of the limestone lies very near the base of this zone, 30 kyr after the impact 18. Biostratigraphy and basic assumptions about depositional and crater processes indicate that the transitional unit was deposited between several years and 30 kyr after impact (Fig. 2). To better constrain this, we use the abundance of extraterrestrial 3 He to determine sediment accumulation rates (see Methods). This proxy provides a firm upper limit of 8 kyr for deposition, assuming none of the 3 He is reworked. If even a small amount of 3 He is reworked (which is very likely given the prevalence of reworked microfossils and impact debris), then the transitional unit was deposited in a period of time of less than about 2 8 8 | N A t U r e | V O L 5 5 8 | 1 4 J U N e 2 0 1 8