NOAA/WDS Paleoclimatology - Arctic Northwest Atlantic 646 Year Coralline Algae Sea Ice Record

STUDY NOTES: Samples of Clathromorphum compactum were live collected at Arctic Bay, Nunavut, Canada (73.0174°N; 85.1536°W, Sample AB1, 17m depth, lifespan 1779-2009) and off Kingitok Island, Labrador, Canada (55.3983°N; 59.8467°W, Sample Ki1, 17m depth, lifespan 1851-2011; Sample Ki2, 15m depth, lifespan 1365-2011). Mg/Ca ratios were measured along transects extending over the entire lifespan of each specimen using a JEOL JXA 8900 RL electron microprobe at the University of Göttingen, Germany. Samples were obtained along transect lines oriented perpendicular to the plane of calcite accretion. At each interval the specific subsample location was selected manually by moving the stage no more than 20 µm laterally from the transect line to avoid unsuitable sample locations (i.e., conceptacles= reproductive structures and uncalcified cell interiors). The relative mean standard deviations of multiple standard measurements were found to be no larger than 1.0 % for MgO and 1.2 % for CaO (2-sigma). Counting statistics errors vary between 1.0 relative % and 2.9 relative % for MgO and between 0.40 relative % and 0.62 relative % for CaO (2-sigma). Algal growth-increment widths were calculated from widths of annual Mg/Ca cycles. Reconstructions were standardized to have a mean of zero and unit variance. A combined algal record was calculated by averaging the normalized annual growth rates and annually-averaged Mg/Ca ratios from all three samples.
ABSTRACT SUPPLIED BY ORIGINATOR: Northern Hemisphere sea ice has been declining sharply over the past decades and 2012 exhibited the lowest Arctic summer sea-ice cover in historic times. Whereas ongoing changes are closely monitored through satellite observations, we have only limited data of past Arctic sea-ice cover derived from short historical records, indirect terrestrial proxies, and low-resolution marine sediment cores. A multicentury time series from extremely long-lived annual increment-forming crustose coralline algal buildups now provides the first high-resolution in situ marine proxy for sea-ice cover. Growth and Mg/Ca ratios of these Arctic-wide occurring calcified algae are sensitive to changes in both temperature and solar radiation. Growth sharply declines with increasing sea-ice blockage of light from the benthic algal habitat. The 646-y multisite record from the Canadian Arctic indicates that during the Little Ice Age, sea ice was extensive but highly variable on subdecadal time scales and coincided with an expansion of ice-dependent Thule/Labrador Inuit sea mammal hunters in the region. The past 150 y instead have been characterized by sea ice exhibiting multidecadal variability with a long-term decline distinctly steeper than at any time since the 14th century.