11:00 AM - 1:00 PM
[MIS02-P07] Color type, size distribution, and flux of Australasian microtektite at IODP Site U1426 in the central Japan Sea, and their potential significance as a stratigraphic marker in the Northeastern Pacific
Keywords:Australasian tektite event, microtektite, MIS 20
Australasian Tektite Event [AATE] is a probable large impact event that occurred somewhere in Indochina Peninsula approximately at 0.8 Ma, and characterized by the widest tektite strewn field extending approximately 12,000 km south-southward to the Antarctic Peninsula. On the other hand, currently recognized north-northwestward distribution limit is only 500 km from the hypothetical impact site reaching to the southernmost part of China, although it is difficult to identify a microtektite layer in on-land sections. In the western Pacific margin, the northern limit of microtektites distribution is in the northernmost part of the Philippine Sea at 30 N, approximately 3,400 km northeastward from the hypothetical impact site. Recently, we found Australasian microtektites at sites U1422 in the northern part and site U1426 in the central part of the Japan Sea exactly at the same horizon according to the stratigraphic correlation of dark and light layers that reflected millennial-scale palaeoceanographic changes in the sea in association with East Asian monsoon variability (Tada et al., 2018). According to the high-resolution age model, the microtektite layer is in the middle part of MIS 20 slightly before the glacial maximum. The estimated age is 0.808 Ma which is slightly older than previously estimated age of 0.788+0.03~0.800 +0.06 Ma.
At site U1426, approximately 95% of microtektites are spheres and spheroids with minor amounts of dumbbell, tear drop, bullet shapes, and aggregates of two spheres. We recognized 4 types of color for spherical microtektites; 1) gray to black gray translucent to opaque (44.6%), 2) colorless transparent (40.9%), 3) pale-yellow to olive yellow transparent to translucent (13.8%), and 4) creamy white opaque (0.7%). Size distribution of pale-yellow type is distinctly different from other color types. Namely, grain size of pale-yellow type tends to be larger than that of other types, and characterized by trimodal size distribution with the modes at ~55, ~78, and ~115um, whereas grain size distributions of other types are unimodal with modal grain sizes of ~53um for gray type, ~58um for colorless type, and ~83um for creamy white type. Largest diameter is ~200um for pale-yellow type while that for other types are less than ~160um. Microcraters are commonly observed on their surfaces of pale-yellow type spheres larger than 160um while surfaces of other color types of spheres are smooth.
These results suggest at least the largest(~115um) population of pale-yellow type represents microtektites travelled all the way from the impact site. On the other hand, gray type microtektites seem to contain tiny black opaque particles (crystallites?) of submicron size. It is possible that gray type microtektites are actually microkrystites, and were condensed from the vapor plume. It is also possible that colorless type and the smallest grainsize population of pale-yellow type were also condensed from the vapor plume judging from their similar grain size. Further detailed observation is necessary to clarify their origin.
The result of this study suggests that Australasian microtektites are widely distributed in the Northwestern Pacific than previously thought, but were not found previously because of their small sizes of less than 100um. It could be used as a good marker horizon approximately 30 ky before the Brunhes/Matsuyama bounday.
Flux estimate of each color type of the microtektites will be also presented.
At site U1426, approximately 95% of microtektites are spheres and spheroids with minor amounts of dumbbell, tear drop, bullet shapes, and aggregates of two spheres. We recognized 4 types of color for spherical microtektites; 1) gray to black gray translucent to opaque (44.6%), 2) colorless transparent (40.9%), 3) pale-yellow to olive yellow transparent to translucent (13.8%), and 4) creamy white opaque (0.7%). Size distribution of pale-yellow type is distinctly different from other color types. Namely, grain size of pale-yellow type tends to be larger than that of other types, and characterized by trimodal size distribution with the modes at ~55, ~78, and ~115um, whereas grain size distributions of other types are unimodal with modal grain sizes of ~53um for gray type, ~58um for colorless type, and ~83um for creamy white type. Largest diameter is ~200um for pale-yellow type while that for other types are less than ~160um. Microcraters are commonly observed on their surfaces of pale-yellow type spheres larger than 160um while surfaces of other color types of spheres are smooth.
These results suggest at least the largest(~115um) population of pale-yellow type represents microtektites travelled all the way from the impact site. On the other hand, gray type microtektites seem to contain tiny black opaque particles (crystallites?) of submicron size. It is possible that gray type microtektites are actually microkrystites, and were condensed from the vapor plume. It is also possible that colorless type and the smallest grainsize population of pale-yellow type were also condensed from the vapor plume judging from their similar grain size. Further detailed observation is necessary to clarify their origin.
The result of this study suggests that Australasian microtektites are widely distributed in the Northwestern Pacific than previously thought, but were not found previously because of their small sizes of less than 100um. It could be used as a good marker horizon approximately 30 ky before the Brunhes/Matsuyama bounday.
Flux estimate of each color type of the microtektites will be also presented.