The recent marine sedimentary record of Baranof Island, Southeast Alaska - implications for paleoclimate reconstructions

Modern and paleoclimate studies suggest a correlation between Pacific decadal climate variability and marine ecosystem productivity, but are generally limited by either short periods of observation or low temporal resolution. Long, annually resolved paleoclimate time-series data are thus critical for assessing this correlation and understanding the full range of Pacific climate variability and its impacts. Baranof Island is located in the Alexander Archipelago of Southeast Alaska, and contains many temperate ice-free fjords with shallow sills that enhance organic matter preservation by restricting oxygenation of bottom waters. Multicore samples EW0408-32MC and 43MC were recovered from two fjords on Baranof Island, and analyzed to determine how recent sedimentation patterns relate to the instrument record as a first step towards reconstructing high-latitude Pacific climate at annual timescales. A combination of radiometric ¹³⁷Cs and excess ²¹⁰Pb geochronometry, 3D computerized tomography (CT), and high-resolution Avaatech scanning XRF geochemical analyses were used to investigate this relationship. Scanning XRF data were collected every 2 mm on core 32MC, while 43MC was measured at 0.2 mm intervals. Core 32MC is composed of a diffusely laminated to bioturbated clay with a maximum apparent steady-state sedimentation rate of ~5±1 mm/yr, while core 43MC is a strongly laminated diatom ooze with a maximum apparent sedimentation rate of ~6±0.5 mm/yr. Using conservative estimates of accumulation to assess basal ages of the cores yields approximately ~AD 1930 for 32MC, and ~AD 1900 for 43MC. Scanning XRF centered natural log-ratio transformed [clr] element intensities indicate 32MC is controlled by a balance between detrital (Al, Si, K, Ti, Fe, and Ca) and biogenic components (S and Br), and isolated peaks in clr Ca data correspond with CT-visible shell debris. Core 43MC is more complex, with both XRF and CT scans indicating four distinct lithologies: (i) millimeter-thick laminae; (ii) a potentially seismogenic turbidite; (iii) a fluvial hypopycnal deposit; and (iv) a subsurface bioturbated horizon. The contrasting natures of these two multicores collected from adjacent fjords argue for strong local controls on sedimentation (e.g. sill depth, fluvial discharge, and productivity). Preliminary analysis of the clr Br data (a proxy for organic carbon) from EW0408-32MC shows striking similarities to both the winter anomaly pattern of the North Pacific Index (NPI), as well as a modeled net primary productivity summer anomaly dataset. Both the NPI and Pacific marine productivity are known to vary due to dynamics associated with the Pacific Decadal Oscillation, suggesting that the recent sedimentary record of Baranof Island is a faithful recorder of regional climate signals.