Thursday, February 17, 2011, 4:30PM - 5:30PM
Full title: The 8.2 ka event on the NW Iceland Shelf as recorded in paired d18O and Mg/Ca measurements of the benthic foraminifer Cibicides lobatulus.
Abstract: The 8.2 ka event was a short-lived climate excursion that holds important information about causes of abrupt climate changes during the otherwise climatically stable Holocene epoch. The 8.2 ka event was first documented in Greenland’s ice core records as a sudden drop surface air temperatures, ranging from 3 to 8°C degrees, depending on various reconstructions. The cooling has been attributed to the catastrophic drainage the proglacial lakes Agassiz and Ojibway during the final deglaciation of the Laurentide ice sheet into the North Atlantic. Unequivocal proof that freshwater from theses proglacial lakes triggered the 8.2 ka event climate excursion is lacking. And there is limited knowledge of how the 8.2 ka event is expressed in the ocean--in timing, in duration, and in rates of temperature and salinity changes--because open ocean sedimentation rates are generally low and thus provide poor temporal resolution compared to the highly resolved Greenland ice core records.
We aimed to determine the timing of the onset and termination of the 8.2 ka event, the evolution and magnitude of temperature change, and whether the cooling was accompanied by a freshening in the marine realm by sediment core MD99-2266 from the mouth of Ìsafjarðardjúp, the largest fjord in NW Iceland. MD99-2266 contains 38 m of Holocene sediment. We concentrated the analysis on the interval from 8400 to 7600 cal yr BP that likely captured the 8.2 ka event signal. The chronology for MD99-2266 is based on 20 radiocarbon dates and the depth of the Saksunarvatn tephra. The interval from 7600 to 8400 cal years BP is bracketed by dates at 1398-1399 cm (7315±75 cal yr BP, 6390±20 14C years) and at 2237-2238 cm (8816±85 cal yr BP, 7885±15 14C yr); three additional dates lie within this interval. The high sedimentation rates in our record allowed us to subsample every 10 cm at an ~18-year resolution. We reconstructed temperature by measuring Mg/Ca of Cibicides lobatulus, a benthic foraminifer. To convert the Mg/Ca ratios to temperatures, we developed a new Mg/Ca temperature calibration for C. lobatulus based on 26 surface sediment samples and core tops from 16 locations in the high latitude North Atlantic.
We reconstructed the oxygen isotopic composition of seawater (d18Osw) to detect any freshening signal by taking the Mg/Ca derived temperatures to separate the d18Osw contribution from the oxygen isotopic composition of C. lobatulus (d18Ocalcite) in the same samples. All of our proxies indicate a prominent climate excursion, coinciding with the 8.2 ka event as registered in the GISP2 ice core record. Mg/Ca derived temperatures showed a cooling of ~2.7-4°C between ~8300 and 8170 cal yr BP accompanied by lighter d18Osw values of ~0.6-1.1‰ between ~8260 and 8220 cal yr BP. The percentages of the arctic species, Cassidulina reniforme, spiked from a mean of 5% to a high of 11% at ~8235 and 8240 cal yr BP; the percentages of the arctic species, Elphidium excavatum f. clavata, spiked from a mean of 4% to a high of 15% at ~8240 cal yr BP. Carbonate weight percent, a proxy for bioproductivity on the Icelandic shelf dropped during the cold event. The interval of low CaCO3 started at ~8260 cal yr BP with a drop from ~ 16% to ~10%; the lowest CaCO3 of 9.6 % occurred at 8240 cal yr BP. The interval of low CaCO3 ended at ~8220 cal yr BP. The 8.2 ka event lasted ~170 years with a 40-year peak cooling interval in our record, thus matching the 8.2 ka event as recorded in Greenland ice core records.