Positive Eu anomaly development during diagenesis of the K/T boundary ejecta layer in the Agost section (SE Spain): implications for trace‐element remobilization

Abstract

K/T boundary geochemical anomalies have been used previously to support the impact event at the end of the Cretaceous. However, impact models and assessment of the extraterrestrial contribution to the boundary sediments should also consider the diagenetic alteration of the impact signatures. Mineralogical and geochemical studies centring on redox proxies reveal differences in trace‐element concentrations at Agost and Caravaca (SE Spain), two of the most complete K/T boundary sections. These differences probably derive from variations in the diagenetic evolution of the ejecta layer. Several redox proxies, such as extensive pyrite formation, high authigenic uranium concentration and positive Eu anomalies, indicate very strong reducing conditions at Agost. Positive Eu anomalies are extremely unusual in sediments, and in this case are interpreted to indicate a highly reducing environment. In such conditions, certain trace elements such as Ir, may have been remobilized, thereby masking the original signature of the impact.

Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling

“…In addition, high concentrations of U and Th were found in the YDB sediment at six of six Clovis-age sites analyzed and in four of four Bays with a paleosol, just as were found in the impact layers at Chesapeake Bay (38) and the K/T (see SI Fig. 15) (45). Because the heavy minerals, zircon, monazite, and garnet, along with Ti-rich minerals, such as titanite, ilmenite, and rutile, sometimes contain high concentrations of U and Th, we investigated whether lag deposits of those minerals might be the source of high radioactivity.…”

Trace element concentrations in the Mexico‐Belize ejecta layer: A link between the Chicxulub impact and the global Cretaceous‐Paleogene boundary

“…Analogous positive anomalies of Eu and Ce have been recorded in K/T boundary clay samples and potassium feldspar spherules from Agost, Spain. The Eu anomaly was interpreted to indicate a reducing environment (Martinez-Ruiz et al 1999). However, the Albion Formation ejecta blanket is deposited in a terrestrial environment and the samples in question show no sign of reducing conditions.…”

“…6) in the lower part of the spheroid bed in Albion Island is, however, consistent with characteristics of K/T boundary layer elsewhere (e.g., Spain, Denmark, and New Zealand). Various explanations have been provided for the enrichment of the non-meteoritic elements As, Sb, and Zn from the distal K/T boundary layer (Smit and ten Kate 1982;Strong et al 1987;Schmitz 1992;Martinez-Ruiz et al 1999). Although elevated values of As and Sb are not ubiquitous at the K/T boundary (Ebihara and Miura 1996), the anomalies are distributed to such an extent that they are most likely linked to a global dispersal event.…”

“…Investigations of ejecta deposits in northeast Mexico have revealed barium anomalies just below the iridium anomaly (Stüben et al 2005). In the K/T boundary layer in Agost, Spain, the Ba anomaly occurs within the boundary layer (Martinez-Ruiz et al 1999). Additionally, at Brazos River, Texas, USA, peaks of sulfur were discovered in a spherulebearing unit (Heymann et al 1998).…”

“…Enrichment in Ni, Co, Cr, and in the non-meteoritic elements As, Sb, Se, and Zn also characterizes the global boundary layer (e.g., Smit and ten Kate 1982;Schmitz 1985Schmitz , 1992Schmitz et al 1988;Gilmour and Anders 1989;Ebihara and Miura 1996;Strong et al 1987;Martinez-Ruiz et al 1999. However, the exact relation of the Chicxulub impact to the 65 Myr old K/T boundary sediments is still debated.…”