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doi:10.2204/iodp.proc.341.205.2019

Introduction

In 2013, Integrated Ocean Drilling Program (IODP) Expedition 341 operated in the Gulf of Alaska with a program designed to study the interplay between climate, surficial process dynamics, and tectonics of the St. Elias mountain range through coring and wireline logging at five sites (Jaeger et al., 2014a). Site U1418 is located on an elevated region of the Syurveyor Fan at 3677 m water depth and lies below the Alaska Current, which forms the eastern boundary of the Alaska Gyre. This area has seasonal high chlorophyll concentrations that are associated with eddies in the Alaska Current (Brickley and Thomas, 2004). The positioning of Site U1418 made it possible to study both the impact of glacial–interglacial cycles on the development of the Surveyor Fan and local paleoceanography.

A total of six holes were cored at Site U1418, ranging in depth from 17.0 to 948.7 meters below seafloor (mbsf). The cores recovered a sequence of Late Pleistocene–Holocene (0–1.2 Ma) mud with interbedded silt, mud with interbedded clast-poor diamict, mud, and muddy diamict (Jaeger et al., 2014b). Several microfossil groups were present at this site, including planktonic and benthic foraminifers, diatoms, radiolarians, and calcareous nannofossils (Jaeger et al., 2014b). Calcareous nannofossils were identified intermittently in the uppermost ~266 m core composite depth below seafloor (CCSF-A) and less frequently from 266 to 600 m CCSF-A (Jaeger et al., 2014b).

The presence and species abundance of calcareous nannofossils reflect their affinity for particular surface water properties, such as temperature, nutrient concentrations, and salinity. Through studying the nannofossil assemblage, changes in ocean conditions and primary productivity can potentially be reconstructed. For example, the species Gephyrocapsa muellerae and Coccolithus pelagicus are both associated with cool surface waters during the Holocene–recent (Ziveri et al., 2004) and can be used as temperature proxies. The carbonate content of sediments can also provide information about calcareous plankton productivity and bottom water carbonate saturation. During Pleistocene glacial–interglacial cycles, carbonate content of deep-sea sediments has been shown to vary with climate state, impacting bottom water chemistry. One example of this comes from the southern Atlantic (Ocean Drilling Program Site 1089) where higher carbonate content is associated with glacial periods and is driven by shelf-deep sea carbonate fractionation (Hodell et al., 2001). Alternatively, the carbonate content maybe the result of surface water productivity rather than preservation.

Here we present calcareous nannofossil assemblage data, including group abundance and preservation, and calcium carbonate content for 0–266 m CCSF-A at Site U1418. This interval covers the past ~200 ky based on the shipboard age model and encompasses the last two glacial cycles. The goal of this project was to link the nannofossil record to changes in calcium carbonate deposition over glacial and interglacial intervals and to resolve interplay between primary production and climate.

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