Geochemical characteristics of carbonaceous chalk near the Cretaceous/Paleogene transition, central Jordan: Strong metal enrichment of redox-sensitive and biophile elements from remineralized calcitic plankton

Carbonaceous calcitic chalk from a 50-m stratigraphic section of the Maastrichtian-Danian Muwaqqar Chalk Marl (MCM) Formation, Jordan, has elevated concentrations of biophile and/or redox-sensitive trace metals (Mo, Ni, V, U, Cr, Cd, Ag, Se, and P) which indicate enhanced paleo-bioproductivity, biodegradation and remineralization under reducing conditions. Very low concentrations in Mn and Ba suggest a relatively open system for components mobile under reducing conditions. The last 800,000 years to the K/Pg boundary are not exposed in Jordan, but the stratigraphic 20-m-interval below and above the hiatus of the K/Pg transition (n = 8) with 2.6 ± 1.8 wt% organic C is particularly metal-enriched with average bulk-rock contents of 132 ppm Mo, 177 ppm Ni, 363 ppm V, 576 ppm Zn, 39 ppm Se, 1.4 ppm Ag, 106 ppm Cd, 474 ppm Cr. These concentrations are dominantly of authigenous origin, when normalized to PAAS, i.e. 100 % Mo, 100% Ag, 100% Cd, 96% Cr, 95% Ni, 95% Zn, 80% V authigenous, and testify to ≥90% of biodegradation of planktonic debris by anaerobic bacterial activity at or near the seafloor. The depositional environment is euxinic, and Mo/TOC ratios from <10 to around 100 (as well as analogous trends for Ni/TOC and Cr/TOC) indicate an evolution from initially restricted hydrographic conditions to unrestricted open-sea conditions towards the K/Pg transition. The common paleoenvironmental proxies for black shale such as V, V/(V + Ni) and V/Cr ratios, indicate oxic depositional conditions for the studied chalk, which are misleading because the low illite content in the calcareous ooze does not allow sufficient retention of V in this chalk. REE patterns identify a seawater and a clastic component to about equal amounts on average, with authigenous ΣREE of 23 ± 19 ppm (n = 20). The negative Ce anomaly of the authigenous component is −0.6 ± 0.2 (log PAAS normalized), compared to recent seawater with −1.1. The geochemical composition of the carbonaceous calcitic chalk sequence is an expression of the general Cretaceous-Paleogene upwelling regime on the southern passive margin of the Neo-Tethys Ocean in the Cretaceous-Palaeogene (K-Pg) greenhouse environment.