Early Holocene marine incursion and a freshened Tyrrell Sea in Hudson Bay Lowlands, Canada

The Tyrrell Sea developed in the Hudson Bay Lowlands after the breakup of the ice saddle of the Laurentide Ice Sheet over Hudson Bay in the early Holocene. Coeval with its incursion was the final drainage of glacial Lake Agassiz-Ojibway which many hypothesize freshened the North Atlantic, causing significant cooling at 8.2 ka. However, little is known about the freshening of this inland sea itself. This paper presents our study of the sedimentology and stable isotope geochemistry on the extensively exposed Tyrrell Sea deposit in the open pit of the former Victor diamond mine near the Attawapiskat River. The sediment succession indicates that the ice saddle covered the study area prior to its breakup and the final drainage of Lake Agassiz-Ojibway, differing from previous interpretations that placed this site in the lake about 135 km south of the ice margin. Oxygen isotope data from benthic foraminifera (Haynesina orbicularis) indicate that the drainage of Lake Agassiz-Ojibway freshened the Tyrrell Sea with low δ¹⁸O (1.18‰ VPDB) for a short period from 8.1 ± 0.3 to 8.07 ± 0.3 cal ka BP. After regaining its original isotope composition with elevated δ¹⁸O (1.7–2.2‰) at 8.07 ± 0.3 to 7.8 ± 0.2 cal ka BP, the sea underwent a major freshening for more than a millennium at 7.8 ± 0.2 to 6.6 ± 0.2 cal ka BP, whereby δ¹⁸O was lowest (0.36–1.15‰). This millennial-scale freshening occurred during the accelerated recession of the ice sheet as previous ¹⁰Be dating work indicates and likely resulted from large meltwater influx from the ice. Subsequent reduction in meltwater influx slowed ocean freshening and raised δ¹⁸O slightly (0.73–0.83‰) until the Tyrrell Sea withdrew from this site around 6.1 ± 0.5 cal ka BP. Although the initial, short-lived freshening agrees in chronology with the 8.2 ka cooling, the later, millennial-scale freshening has no corresponding oxygen isotope and climate anomalies in the Labrador Sea sediment and Greenland ice cores, respectively. This inconsistency casts doubts on the concept that the brief freshening of the North Atlantic by the drainage of Lake Agassiz-Ojibway could have caused the cooling.