The recovery of the biological pump across the K/Pg boundary in the GSSP of El Kef, Tunisia

<p>The study of Earth’s Big Five mass extinctions provides insight into the resilience of ecosystems to environmental perturbations. Earth’s most recent mass extinction at the Cretaceous/Paleogene boundary (K/Pg) was caused by the impact of an asteroid in the Yucatan peninsula rather than by intense volcanism. Mass extinctions among marine calcareous nannoplankton heavily disrupted the marine food web resulting in a severe weakening of the ocean’s biological pump. The timing and heterogeneous nature of the recovery of the biological pump remain poorly resolved in the neritic zone in the aftermath of the impact. Here, we address the evolution of the biological pump across the K/Pg at the Global Boundary Stratotype Section (GSSP) at El Kef, Tunisia using high-resolution compound-specific carbon isotope records (<em>δ</em><sup>13</sup>C<sub>biomarker</sub>) of non-calcareous marine phototrophs from an outer shelf to upper bathyal setting of the southwestern Tethys Ocean. We use <em>δ</em><sup>13</sup>C<sub>biomarker</sub> to reconstruct ε<sub>p</sub>, which is a function of the community structure of marine phototrophs, their rate of carbon fixation, and the concentration and isotopic composition of aqueous CO<sub>2</sub>. We then use our <em>ε</em><sub>p</sub> record to constrain the recovery of the biological pump in this region while considering the composition of marine phytoplankton, the assemblage and isotopic composition of benthic foraminifera, state-of-the-art physiological models for <em>ε</em><sub>p</sub>, and carbon cycle simulations using cGENIE. Our results indicate that the recovery of the biological pump in the outer shelf-upper bathyal zone likely outpaced the recovery in the open ocean. This is in agreement with the selective extinctions among phytoplankton at the K/Pg, with most survivors that would later repopulate open-ocean sites being adapted to neritic environments.</p>

Evidence for contemporaneous Deccan volcanic aerosol deposition preceding the K-Pg boundary at El Kef, Tunisia

<p>The cause of the Cretaceous-Paleogene extinction remains debated between an asteroid impact and volcanism. Precise geochronology showed that the extinction coincided with a voluminous phase (Poladpur eruption) of Deccan volcanism (Schoene et al., 2019). Paleontological evidence indicates that microfossil diversity declined about 300,000 years before the K-Pg boundary, synchronous with the onset of Deccan volcanism (Keller et al. 2009). High concentrations of Ir in the K-Pg boundary supported the asteroid hypothesis but recent work indicates that siderophile accumulation at the K-Pg in El Kef is secondary (Humayun et al., this conf.). Here, we critically examine existing element data for the K-Pg boundary and examine new results at the El Kef site, Tunisia, for volcanogenic volatile element accumulation associated with the contemporaneous Deccan eruptions. In this study, we analyzed 60 elements by laser ablation ICP-MS in search of these volcanic aerosol enrichments in the K-Pg sediments at El Kef, Tunisia. A study of siderophile element distribution at global K-Pg sites found that the Ru/Ir ratio is sub-chondritic. Mixing of upper continental crust (Ru/Ir> CI) with a chondritic impactor fails to explain this trend. Volcanic aerosol emissions for Ir are well known but there is less data available for Ru. Relative emission rates of Ru were found to be lower than those of Ir for the Kudryavy volcano (Yudovskaya et al., 2008), so a possible explanation of the sub-chondritic Ru/Ir ratio observed in global K-Pg sites involves deposition of volcanic aerosols in sediments. We also modeled the effect of adding volcanic aerosols to sediments approximated compositionally as upper continental crust (UCC) to find that Re, Cd, Os and Ir are the first elements to become enriched in sediments by volcanogenic aerosol deposition. Sediments from El Kef below the K-Pg boundary are enriched in both Re and Cd. On a plot of Cd vs. Re, the K-Pg sediment from El Kef falls on a mixing line between volcanic aerosol (Erta Ale volcano) and UCC. Sediment at 3 cm above the K-Pg boundary has little enrichment of either Cd or Re, interpreted here to indicate that this sediment was deposited in the interlude between the Poladpur and the Ambenali eruption phases of the Deccan. The availability of chemical proxies of volcanogenic aerosol deposition in sediments enables direct correlation between fossil evidence and the contemporaneous intensity of volcanic outgassing, the likely destroyer of life by the Deccan eruptions (Keller et al., 2020).</p>

Pyrite Oxidation Controlled Siderophile Element Accumulation on the K-Pg Boundary at El Kef, Tunisia

<p>In a transformative contribution, Alvarez et al. (1980) discovered the iridium anomaly at several K-Pg boundary locations that they attributed to an extraterrestrial impact that triggered the end-Cretaceous extinction. The absence of a suitable mechanism by which to concentrate siderophile elements in the boundary clay drove the argument for an extraterrestrial origin of the iridium. They made the observation that fallout from a fireball would be expected to create a uniform distribution of Ir in the clay layer and puzzled over the scale of lateral variation in Ir observed even then. A detailed global study of the siderophile element (Ru, Rh, Pd, Ir, Pt Au) distribution at the K-Pg boundary found non-chondritic patterns concluding that some post-depositional process(es) must have affected the elemental distribution (Goderis et al., 2015). Such processes would mobilize siderophile elements into the surrounding strata. Here, we applied laser ablation ICP-MS, a microanalytical technique, to investigate the distribution of 60 elements, with an emphasis on the siderophile elements, in a vertical transect at the K-Pg boundary at El Kef, Tunisia, to search for elemental transport in or out of the K-Pg clay layer. The K-Pg boundary at El Kef consists of irregular mixed layers of clay, goethite and gypsum with marls above and below. The siderophile elements are concentrated in the goethite-rich component with a distinctly terrestrial crust pattern, albeit super-enriched, with prominent negative Pt anomalies indicative of deposition from an oxidized solution. The Fe/Se ratio indicates an origin of the goethite by oxidation of sedimentary pyrite. Iron oxyhydroxides are effective substrates for the binding of trace metal oxyanions from solution. The extreme enrichment of siderophile elements reflects long-term concentration of siderophile elements from percolating oxidized groundwaters at El Kef. The sulfuric acid produced by pyrite oxidation was neutralized by calcium carbonate in the marls to form gypsum. Selenium (normally a sulfur analog) is undetectable in the El Kef gypsum endmember (Se/S~0), unlike marine gypsum, supporting a formation by pyrite oxidation. This observation potentially explains the ubiquitous non-chondritic siderophile patterns observed globally and the variable Ir enrichments that puzzled geochemists since Alvarez et al. (1980). In view of this observation, siderophile element enrichment in the K-Pg layer can no longer be taken as unambiguous evidence of an extraterrestrial impact.</p>

Blooms of aberrant planktic foraminifera across the K/Pg boundary in the Western Tethys: causes and evolutionary implications

We report a detailed study of the different categories and types of abnormal morphologies in planktic foraminifera recognizable in the lowermost Danian, mainly from the El Kef and Aïn Settara sections, Tunisia. Various types of abnormalities in the test morphology were identified, including protuberances near the proloculus, abnormal chambers, double or twinned ultimate chambers, multiple ultimate chambers, abnormal apertures, distortion in test coiling, morphologically abnormal tests, attached twins or double tests, and general monstrosities. Detailed biostratigraphic and quantitative studies of the Tunisian sections documented a major proliferation of aberrant planktic foraminifera (between approximately 5% and 18% in relative abundance) during the first 200 Kyr of the Danian, starting immediately after the Cretaceous/Paleogene (K/Pg) boundary mass extinction (spanning from theGuembelitria cretaceaZone to the lower part of theP. pseudobulloidesZone). This contrasts with the proportionately low frequency of aberrant tests (generally <2%) identified within the uppermost Maastrichtian, suggesting more stable environmental conditions during the last ~50–100 Kyr of the Cretaceous. Two main pulses with abundant aberrant tests were recognized in the earliest Danian, the one recorded in the well-known K/Pg boundary clay being the more intense of those (maxima of >18%). These main pulses of aberrants coincide approximately with relevant quantitative and evolutionary turnovers in the planktic foraminiferal assemblages. In this paper, we explore the relation of these high values of the foraminiferal abnormality index with the environmental changes induced by the meteorite impact of Chicxulub in Yucatan, Mexico, and the massive eruptions of the Deccan Traps, India.