215 Paleoclimatology/Paleoceanography II

Wednesday, 31 October 2007
1:30 PM - 5:30 PM

USING MG/CA AND STABLE LIGHT ISOTOPE RATIOS OF A BENTHIC FORAMINIFERAL SPECIES TO DETECT THE 8.2 KA COOLING EVENT ON THE ICELANDIC SHELF

Paper number: 215-15
Program: Technical Sessions
Time: 5:00 PM – 5:15 PM

Authors

Abstract

Greenland ice cores record a 6°C cooling, lasting ~100 years, around 8.2 ka BP. The 8.2-ka cooling event has been associated with a final, catastrophic draining of proglacial lakes Agassiz and Ojibway, which released 163,000 km3 of fresh water into the North Atlantic. One of the unresolved problems is whether the 8.2 event affected the Icelandic shelf and left an imprint in the marine record. The 8.2 event was not recorded in the d18O of foraminiferal calcite in a Calypso piston core MD99-2266 (66°13'77”N, 23°15'93”W, water depth 100 m) retrieved in NW Iceland when sampled at a resolution of ~100 years, a commonly used sampling resolution. We argued that the poor sampling resolution could easily have missed the 8.2 event. We took advantage of the high sedimentation rates of the core and sampled at ~15-24 year resolution between 7700 and 8400 cal yr BP. The chronology in this interval is well constrained by 6 AMS 14C dates. We also argued that the d18O (a function of temperature and salinity) might not have recorded the event, because a cooling would result in heavier isotopes and the freshening in lighter isotopes. The opposing signals of cooling and freshening might dampen the d18O record.

We proposed to analyze the Mg/Ca ratio (a function solely of temperature) of Cibicides lobatulus, a benthic, epifaunal foraminifera that occurs in high percentages in the core. We present a new provisional calibration curve for the Mg/Ca of C. lobatulus against temperature (T), Mg/Ca=1.1031+0.12905 T (R=0.88), which is based on the analysis of 12 shallow (100-350 m) surface sediment samples from the SW/N Iceland and Greenland shelves with a robust temperature data set ranging from 0.6-7.2° C. The d18O composition became heavier by 0.2‰, which has been suggested to indicate a ~1°C cooling. The quantitative temperature reconstruction based on the Mg/Ca analysis showed a ~3°C cooling at 8.2 ka, lasting ~100 years. As hypothesized the d18O signal was dampened because of the opposing effect of cooling and freshening. We calculated the d18O of seawater using the Lynch-Stieglitz (2003) equation: d18O(foraminifera)-d18O(seawater)+0.27=-0.21T+3.38. The results showed a lighter d18O composition of ~0.4‰, which indicates a response to fresh water inflow. We also present the mineral and foraminifera records. This project was funded by a GSA student grant.