Rock magnetic signature of the Cretaceous‐Tertiary boundary

Abstract

Black magnetic microspherules, commonly found in Cretaceous‐Tertiary (K‐T) boundary sediments and believed to be of an impact melt origin, were found to be responsible for a strong magnetic anomaly at the K‐T boundary in Petriccio, Italy, and in almost all Deep Sea Drilling Project (DSDP) sediment cores investigated in which the K‐T boundary was present. The enriched concentrations of magnetic minerals have been detected by measurements of the magnetic susceptibility which is in a good approximation proportional to the magnetic microspherule content. A rock magnetic characterization of extracted microspherules by hysteresis, Isothermal Remanent Magnetization acquisition, demagnetization and thermomagnetic measurements describes the magnetic minerals in the microspherules as ferrites with a near magnetite composition and ‘soft’, i.e. multidomain, magnetic behavior, in agreement with earlier nonmagnetic studies. On the contrary, volcanic ash from Hawaii has distinctively ‘harder’ magnetic properties, an important finding with regard to the proposed alternative, a volcanic origin of the well‐known Ir‐anomaly at the K‐T boundary. It is suggested here that from magnetic susceptibility scanning of numerous cross‐sections of the K‐T boundary a distinct worldwide pattern might evolve that can potentially reveal the impact/volcano site because of a presumed higher microspherule concentration near the source. The advantage of mapping the microspherule distribution over the previous unsuccessful attempt using Ir‐concentrations lies in the large size (hundreds of microns) of the microspherules with relatively short airborn life times while the iridium might have adhered to small dust particles that circled the globe several times before settling.