Zircon geochronology of bottom rocks in the central Arctic Ocean: analytical results and some geological implications

In the past few years sampling of deepwater seabed gained an increasingly important role in studying geological structure of the Arctic Ocean. A common concept of virtually uninterrupted pelagic drape in the Amerasia Basin and exclusively ice-rafted nature of all clastic components that occur in bottom sediments was challenged by recent discoveries of bedrock exposures in the sea floor, while correlation of results of analytical study of bottom samples collected by the Russian expeditions in 2000, 2005 and 2007 with bathymetric environments at respective sites suggested that certain dredged and cored coarse rock fragments appeared meaningful for bedrock characterization if even the source sub-pelagic outcrop was not positively documented. The first results of age determinations of detrital zircons that were extracted from coarse fragments of lithic sedimentary rocks resting on the seabed and in the immediate sub-bottom, as well as of zircons from fragments of magmatic/metamorphic rocks and of zircon grains separated directly from sub-pelagic unlithified sediments are in agreement with published interpretations of the Lomonosov Ridge bedrock as composed of Mesozoic terrigenous sequences; the presence of an older Neoproterozoic(?) – Early-Middle Paleozoic basement is also possible. The Mendeleev Rise bedrock, too, is believed to mainly consist of Paleozoic-Early(?) Mesozoic sedimentary superstructure that may locally rest on the Earliest Paleozoic or even older units. Basaltic rocks likely to originate from the High Arctic Large Igneous Province (HALIP) has not so far been found among the collected fragments but limited loose zircon grains probably derived from broadly contemporaneous magmatic products were recorded in sub-pelagic sediment along with dropstones of variably metamorphosed Precambrian mafic and granitoid rocks. INTRODUCTION Great progress in acquisition of new bathymet-ric and geophysical data relevant to understanding the geological structure and history of the Arctic Ocean, including the tectonic nature of enigmatic Central-Arctic bathymetric highs, was achieved in recent years by the Arctic countries through their programs for delineation of respective extended continental shelves. However, only limited direct geological information was obtained on the compo-sition of sub-bottom bedrock concealed by almost continuous drape of young sediments. Only at a few sites can the lithic fragments recovered by bottom sampling be interpreted with sufficient confidence as representing in situ submarine bedrock, while in most cases they are regarded ice rafted debris (IRD) of questionable derivation. In search of provenance of lithic and mineral clastic components in bottom sediments we conducted age determinations on zircon crystals of two categories: (1) extracted from the rock fragments and (2) separated directly from hemipelagic sediments. In this paper we present the results of more than 700 zircon U-Pb age measurements completed before 2012. The samples labeled AF00, AF05, AF07 were collected during MS " Akademik Fedorov " cruises Arctic-2000, 2005, 2007, those marked ALR07 were acquired in 2007 on board NIB (nuclear icebreaker) " Rossiya " , and two specimens designated BC were selected for the analysis from clastic material sampled by RV " Polarstern " in the course of ARK-XXIII/3-2008 cruise

Mineralogical composition of the upper 300m of Hole 302-M0002A

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