[1] During the Arctic Coring Expedition (ACEX), a 428-m-thick sequence of Upper Cretaceous to Quaternary sediments was penetrated. The mineralogical composition of the upper 300 m of this sequence is presented here for the first time. Heavy and clay mineral associations indicate a major and consistent shift in provenance, from the Barents-Kara-western Laptev Sea region, characterized by presence of common clinopyroxene, to the eastern Laptev-East Siberian seas in the upper part of the section, characterized by common hornblende (amphibole). Sea ice originating from the latter source region must have survived at least one summer melt cycle in order to reach the ACEX drill site, if considering modern sea ice trajectories and velocities. This shift in mineral assemblages probably represents the onset of a perennial sea ice cover in the Arctic Ocean, which occurred at about 13 Ma, thus suggesting a coeval freeze in the Arctic and Antarctic regions.

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This paper is based on the study of the new geological, isotopic, and geochemical data that were obtained in the last years on the igneous and metamorphic rocks of East Antarctica, dated from Late Mesoproterozoic to Early Paleozoic. This period of its geological history is believed, based on plate-tectonic paleoreconstructions, to include, first, the origin and breakup of the hypothetical Rodinia Supercontinent and, later, the formation of a new supercontinent, Gondwana. Our review and critical study of extensive foreign literature, as well as the results of our own field and laboratory studies of crystalline rocks from the East Antarctica crystalline basement, suggested a different idea: the supercontinental lithospheric block, including the continents of the Gondwana group, had remained intact throughout the Proterozoic, and perhaps throughout the whole of the Precambrian. Intensity-variable extension pulses, which acted on that primary continental lithosphere, might have caused the origin of some local ocean openings, yet did not result, up to the end of the Mesozoic, in the complete breakup of this sialic megablock with any large-scale spreading development.

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