Identification and Characterization of the Cretaceous/Tertiary Boundary at ODP Sites 886 and 803 and DSDP Site 576

Abstract

The Cretaceous/Tertiary (K/T) boundary Ir anomaly has been identified in deep-sea clays at three new localities in the North Pacific: Ocean Drilling Program Site 886 near the Chinook Trough (44°41.4'N, 168°14.4'W), ODP Site 803 on the Ontong-Java Plateau (2°26.0'N, 160°32.5'E), and Deep Sea Drilling Project Site 576 in the western North Pacific (32°21.4'N, 164°16.5'E). At each of these sites, the K/T boundary is characterized by a large Ir anomaly, coincident with minor enrichments of Cr and abundant shocked quartz and magnesioferrite spinel. Spheroidal material has also been observed in K/T boundary sediments at ODP Site 886 and DSDP Site 576. The magnitude of the Ir anomaly is comparable to that observed at other marine sites around the world, with estimated net fluences of 52, 140, 130, and 90 ng Ir cm" 2 in Holes 886C, 803D, 576, and 576B, respectively. At Sites 803 and 576, some biostratigraphic control was available from poorly preserved nannofossils and ichthyoliths, respectively. At Site 886, it was necessary to survey Ir concentrations across a 18.1-meter-thick unit of clay (54.3 to 72.4 mbsf) known to record sedimentation from 80 to 10 Ma.

Reconstructing the marine 187Os/188Os record and the particulate flux of meteoritic osmium during the late Cretaceous

“…It is noteworthy that the Ir maximum (Table 1: Sample 8H-1 143-144) is offset below the 187 Os/ 188 Os minimum (Table 1: Sample 8H-1 131-132) by 12 cm. These results are consistent with previous work showing that the boundary interval is disturbed with a broad Ir anomaly and extraterrestrial particles distributed over approximately 60 cm (Kyte et al, 1995). Below the KTB, Ir concentrations decline and Pt/Ir ratios increase to values that are similar to those measured in recent (Cave et al, 2003) and late Cretaceous (Ravizza et al, 2001) metalliferous sediments.…”

“…It is likely that these differences result from heterogeneities among splits from the same sample powder. While it is possible that systematic problems with spike sample equilibration during the fusion could be responsible for the poor reproducibility, this is unlikely because measured Ir concentrations are similar to those reported previously by Kyte et al (1995) (Fig. 2).…”

“…Complementary Pt and Ir data are reported in order to better constrain the platinum group element (PGE) source in these sediment samples. Previously published data constrain the age of the sediments to fall between 65.5 Ma (KTB: Kyte et al, 1995) and 80 Ma (Keller et al, 1995). Note that here the informal but widely recognized term KTB is used to refer to the Cretaceous-Paleogene Boundary.…”

Projectile Identification in Terrestrial Impact Structures and Ejecta Material

“…The mixing of a chondritic projectile with an original crustal composition results in a decrease of the Cr/Ir ratio, depending on the degree of projectile contamination (as illustrated in Fig. 15.1 for the Popigai and Chicxulub impact melts and selected K-Pg boundary samples; Alvarez et al, 1980;Kyte et al, 1980Kyte et al, ,1995Smit and Hertogen, 1980;Kyte, 1998;Tagle and Claeys, 2005;Tagle and Hecht 2006;.…”

“…PUM: primitive upper mantle; MORB: mid-ocean ridge basalt; UCC: upper continental crust; CC: continental crust. Popigai impact melt rock data from (1) Tagle and Claeys (2005); Chicxulub (from the Yaxcopoil-1 drill core) melt data from (2) ; and K-Pg boundary data from (3) Kyte (1998), (4) Kyte et al (1995), (5) Smit and Hertogen (1980), (6) Alvarez et al (1980) and (7) Kyte et al (1980). than 1 wt% of bulk meteoritic component (Table 15.2).…”