Multi-proxy record of ocean-climate variability during the last two millennia on the Mackenzie Shelf, Beaufort Sea

A 2,000 year-long oceanographic history, in sub-centennial resolution, from a Canadian Beaufort Sea continental shelf site (60 meters water depth) near the Mackenzie River outlet is reconstructed from ostracode and foraminifera faunal assemblages, shell stable isotopes (delta O-18, delta C-13) and sediment biogenic silica. The chronology of three sediment cores making up the composite section was established using Cs-137 and Pb-210 dating for the most recent 150 years and combined with linear interpolation of radiocarbon dates from bivalve shells and foraminifera tests. Continuous centimeter-sampling of the multicore and high-resolution sampling of a gravity and pis-ton core yielded a time-averaged faunal record of every similar to 40 years from 0 to 1850 CE and every similar to 24 years from 1850 to 2013 CE. Proxy records were consistent with temperature oscillations and related changes in organic carbon cycling associated with the Medieval Cli-mate Anomaly (MCA) and the Little Ice Age (LIA). Abundance changes in dominant microfossil species, such as the ostracode Paracyprideis pseudopunctillata and agglutinated foraminifers Spiroplectammina biformis and S. earlandi, are used as indicators of less saline, and possibly corrosive/turbid bottom conditions associated with the MCA (similar to 800-1200 CE) and the most recent similar to 60 years (1950-2013). During these periods, pronounced fluctuations in these species suggest that prolonged seasonal sea-ice melting, changes in riverine inputs and sediment dynamics affected the benthic environment. Taxa analyzed for stable oxygen isotope composition of car-bonates show the lowest delta O-18 values during intervals within the MCA and the highest during the late LIA, which is consistent with a 1 degrees to 2 degrees C cooling of bottom waters. Faunal and isotopic changes during the cooler LIA (1300-1850 CE) are most apparent at similar to 1500-1850 CE and are particularly pronounced during 1850 to similar to 1900 CE, with a similar to 0.5 per mil increase in 818O values of carbonates from median values in the analyzed taxa. This very cold 50-year period suggests that enhanced summer sea ice suppressed productivity, which is indicated by low sediment biogenic silica values and lower delta C-13 values in analyzed species. From 1900 CE to present, declines in calcareous faunal as-semblages and changes in dominant species (Cassidulina reniforme and P. pseudopunctillata) are associated with less hospitable bottom waters, indicated by a peak in agglutinated foraminifera from 1950-1990 CE.

Automated summary

A 2,000 year-long oceanographic history was reconstructed from a Canadian Beaufort Sea continental shelf site using various proxies. The records revealed temperature oscillations and changes in organic carbon cycling associated with the Medieval Climate Anomaly and the Little Ice Age. Changes in faunal and isotopic composition indicated variations in bottom water conditions, with the most significant changes occurring during the cooler period of the Little Ice Age.