The Quaternary history of Beringia and of the Arctic-Pacific marine connection via the Bering Strait is poorly understood because of the fragmentary stratigraphic record from this region. We report new borehole and seismic-reflection data collected in 2006 in the southwestern Chukchi Sea. Sediment samples were analyzed for magnetic properties, grain size, heavy minerals, and biostratigraphic proxies (spores and pollen, foraminifers, ostracodes, diatoms, and aquatic palynomorphs). Two shallow boreholes drilled between the Chukotka Peninsula and the Wrangel Island recovered sediments of two principal stratigraphic units with a distinct unconformity between them. Based on predominantly reverse paleomagnetic polarity of the lower unit and pollen spectra indicative of forested coasts and climate warmer than present, the age of this unit is estimated as Pliocene to early Pleistocene (broadly between ca. 5 and 2 Ma). Attendant sedimentary environments were likely alluvial to nearshore marine. These deposits can be correlated to the seismic unit infilling valleys incised into sedimentary bedrock across much of the study area, and possibly deposited during a transgression following the opening of the Bering Strait. The upper unit from both boreholes contains Holocene 14 C ages and is clearly related to the last, postglacial transgression. Holocene sediments in Borehole 2 indicate fast deposition at the early stages of flooding (between ca. 11 and 9 ka) to very low deposition, possibly related to expansive sea ice. Closer to shore, deposition at Borehole 1 resumed much later (ca. 2 ka), likely due to a change in the pattern of coastal erosional processes and/or the demise of a landbridge between the Chukotka Peninsula and the Wrangel Island inferred from studies on mammoth distribution.

Dating of marine sediments and faunal remains they contain in stratotype and reference sections by the methods of infrared optically stimulated luminescence (IR-OSL) of K-feldspar, optically stimulated afterglow (OSA) of quartz, electron spin resonance (ESR), and 230Th/U provides new constraints on deposition in the Yenisei mouth during the Kazantsevo interglacial. The luminescence and U–Th ages in the 120–68 ka range and 93–70 ka ESR ages show that the deposition spanned the whole marine isotope stage (MIS) 5. The sediment structures and textures, grain sizes and mineralogy, and faunal records indicate tidal and shelf deposition environments. The sampled assemblages of marine mollusks comprise taxa that typically live in relatively shallow and warm water, as well as abundant subarctic and boreal species, including the Arctica islandica index species. The variations of faunal patterns, more likely, had facies rather than climatic controls, while the sediments were deposited during transgression, in a warm climate, when the area was ice-free.

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