SYN- OR POSTDEPOSITIONAL DEGRADATION OF PALYNOMORPHS IN THE CHICXULUB POST-IMPACT CRATER FILL, A CONSEQUENCE OF HYDROTHERMAL VENTING?

The Chicxulub K-Pg boundary crater is the best-preserved large impact structure on Earth. This crater provides rare insight in biological recolonization on a regional to global scale. A central theme of IODP Expedition 364, recently drilled into the Chicxulub peak ring, is the recovery of life after this impact. One of the few phytoplankton groups that suffered no extinctions across the K-Pg boundary is the organic-walled cyst-producing dinoflagellates. As a major surviving group, dinoflagellates likely played a major role in the short-term biological recovery, directly following the impact. Therefore, the post-impact infill of the Chicxulub impact crater was analyzed for palynological content, in an attempt to study the dinoflagellate recovery within the Chicxulub impact structure.

Unfortunately, the preservation of palynomorphs proved to be very poor in the post-impact deposits. In fact, while pyrite was frequently abundant, presumably indicative of the deposition of organic matter, most investigated samples were completely barren for palynomorphs. The absence of palynomorphs is therefore likely the result of syn- or post-depositional degradation of organic matter. Our analyses show that these degradational conditions persisted at least throughout the first 4 million years of the Paleocene (617.13-608.02 mbsf). It remains uncertain what caused these degradational conditions. Possibly, the emplacement of an impact-induced local hydrothermal system played a role. Modeling the thermal evolution of the Chicxulub system suggests that the lifetime of such a hydrothermal system could range up to 3 million years (Abramov and Kring, 2007). The greatest hydrothermal alteration is expected to have occurred in the peak ring, because it rises along the edge of the central melt sheet. Hence, potentially, the hydrothermal system was hotter and persisted longer in the vicinity of the peak ring.

From the interval representing the PETM, as well as from the overlying early Eocene sequence, black shales were recovered. These black shales provide a better preservation of palynomorphs, including characteristic early Eocene dinoflagellate cyst taxa like Apectodinium. The black shale deposition during Early Eocene hypothermals suggests continental margin anoxia, potentially driven by nutrient feedbacks (Sluijs et al., 2014).