FINE-SCALE ANALYSIS OF LAMINATED SEDIMENTS FROM MARINE ISOTOPE STAGE 11 (425 KA) IN THE BERING SEA, IODP EXPEDITION 323, SITE U1345

Lamination deposition in marine sediments is favored by anoxic bottom water conditions that limit secondary productivity and bioturbation and instead preserve a high-resolution climatic signal. Anoxia is caused by a variety of factors including excessively high primary productivity or expansion of the oxygen minimum zone. Laminations are preserved across the North Pacific and its marginal seas at several glacial terminations, but a detailed analysis of laminae composition does not yet exist. Here we examine a 3.5 m long deposit from the shelf-slope break of the Bering Sea, Site U1345, dated to the beginning of Marine Isotope Stage (MIS) 11 (425-413 ka). These laminations are composed of 3 alternating distinct laminae types, distinguished by their grey, dark brown, and light brown colors. Microfossils are abundant in all laminae and include diatoms, calcareous nannofossils, and foraminifera. We aim to determine if the laminations are sedimentary varves showing annual to seasonal variability or longer-scale alternations of high productivity events. If these are varves, they will supply a high-resolution record of climate variability during a transitional warming period. Laminations have been counted using scanned images and compared with the existing age model. Seven facies describing laminae patterns and bioturbation levels have been identified. Smear slides from every lamina contained in three 10 cm half-rounds from core U1345D-13H have been made, and encompass all facies. U-channels were also taken from each half-round, embedded in resin, and made into thin sections for subsequent SEM analysis. Preliminary results indicate that individual laminae are not seasonally nor annually deposited, and instead represent 7.2, 2.9, and 2.0 years per lamination at 409, 414, and 420 ka, respectively. However, analysis of lamination counts may be hindered by the poorly constrained age model that is defined by limited tie-points. Sedimentation rates are also almost certainly not constant throughout the core, providing another source of error when analyzing laminae counts. Diatom counts and SEM analysis will provide more information on the differences in laminae composition of each facies. Seasonal or sub-annual deposition will still be considered if regularly recurring changes are seen in diatom assemblages.