Implication of spatiotemporal distribution of black shales deposited during the Cretaceous Oceanic Anoxic Event-2

Abstract

Oceanic Anoxic Events (OAEs) mark the contemporaneous deposition of organic-rich marine sediments termed "black shales" in the wide areas of the oceans. An anoxic event that occurred at the Cenomanian- Turonian boundary, OAE-2, has been recognized as one of the largest events in the Cretaceous. Carbon isotopic compositions (δ¹³C) of sedimentary carbonate and organic matter exhibit a positive excursion across the OAE-2, reflecting an enhanced burial rate of ¹³C-depleted organic carbon during the event. Here we compile a spatiotemporal distribution of black shales on the basis of their onset timings relative to the δ¹³C excursion as a time-control reference, and discuss the "spreading patterns" of black shale deposition. The patterns suggest that the deposition of black shales started from marginal regions of the southern North Atlantic and the Western Interior Seaway in North America, and spread to the northern North Atlantic and Tethys Sea. Strangely, the black shales whose onset corresponds to that of the δ¹³C excursion have not been found in many locations. Furthermore, extensive deposition of black shales in the Tethys and some sites in the North Atlantic occurred significantly after the major shift of the δ¹³C excursion. Sediments in the largely unexplored Pacific basin may be the missing link in the temporal relationship between the black shale deposition and the isotopic excursion.