Characterization of n-alkanes, pristane and phytane in the Cretaceous/Tertiary boundary sediments at Kawaruppu, Hokkaido, Japan.

Abstract

Normal alkanes from C12 to C36, pristane and phytane were identified in the Cretaceous/Tertiary (K/T) boundary sediments at Kawaruppu, Hokkaido, Japan. These compounds were found at the levels of sub to a few nmol g-1. Total concentrations of the n-alkanes within the boundary claystone were generally one third to one half of those in the sediments above and below the claystone. The smaller concentrations within the claystone were mainly due to smaller concentrations of longer chain n-alkanes (C25 to C31) than in the sediments above and below it. The longer chain n-alkanes were likely from terrestrial plants and reflected their small population at the end of Cretaceous. The concentration of longer chain n-alkanes decreased rapidly at the base of the boundary claystone, indicating the sudden cease of terrestrial n-alkane input, continued low to the horizon at the upper two thirds of the claystone and then, started increasing toward the top of the claystone. The n-alkane concentration change indicates the period of the small input of terrestrial n-alkanes to be ca. 7,000 years at most and the recovery period to be ca. 2,000 years. The depth distribution pattern of pristane plus phytane concentrations in the K/T sediments roughly resembles that of the n-alkanes.

Thiophenes in the Cretaceous/Tertiary boundary sediments at Kawaruppu, Hokkaido, Japan.

“…The abundance continues to be low over the lower two third portion (0.8 to 10.6 cm) of the claystone and then gradually increases from the horizon at the upper one third (10.6 cm) of the claystone to the sediments above the boundary claystone. The concentration of the n-alkanes also decreased at the lowest horizon in the boundary claystone from those in the sediments below the boundary claystone (Mita and Shimoyama, 1999a). This suggests that the lower concentrations of the nalkanes within the boundary claystone are related to decreased input of terrestrial organic matter compared to the sediments above and below it.…”

“…However, its concentration was low below and over the lower two thirds of the claystone, but increased in the upper one third. This distribution pattern resembles that of the longer chain n-alkanes (Mita and Shimoyama, 1999a). The depth profiles of the thiophene and alkane concentrations possibly reflect the massive extinction of organisms.…”

“…3, together with the concentrations of the longer chain n-alkanes from C 27 to C 29 (Mita and Shimoyama, 1999a). The depth profile of the C 25 HBI thiophene is different from those of the C 25 HBI alkane and the n-alkanes.…”

“…In the present study, we detected a C 25 highly branched isoprenoid (HBI) thiophene in the sediments below the boundary claystone, and polynuclear aromatic thiophenes below, within and above the claystone. The depth distribution of these thiophenes is compared with that of the C 25 HBI alkane detected, the longer chain nalkanes (Mita and Shimoyama, 1999a), and sulfide concentration (Kajiwara and Kaiho, 1992). Furthermore, the presence of the C 25 HBI thiophene is discussed in relation to the massive extinction of diatoms, and the polynuclear aromatic thiophenes to the redox conditions during the deposition and the diagenesis of the K/T boundary sediments at Kawaruppu.…”

“…In this connection, organic geochemical studies on the Cretaceous/Tertiary (K/T) boundary sediments have found evidence for both extraterrestrial organic compounds (Zhao and Bada, 1989) and wildfires triggered by the bolide impact (Wolbach et al, ously, organic geochemical studies of amino acids (Mita et al, 1996) and dicarboxylic acids (Mita et al, 1998) in the sediments suggested the possibility of both extraterrestrial as well as terrestrial sources of these compounds. Characteristic distributions of aliphatic hydrocarbons (Mita and Shimoyama, 1999a), and polycyclic aromatic hydrocarbons (PAHs) (Mita and Shimoyama, 1999b) in the sediments were reported in relation to the massive extinction and wildfires, respectively. Other published studies of the sediments dealt with planktonic foraminifera Kaiho, 1992), palynomorphs , sulfur isotopes (Kajiwara and Kaiho, 1992) and Ir accumulation and clay minerals (Tasaki et al, 1992).…”