Recent (1930s) natural acidification and fish-kill in a lake that was an important food source for an Inuit community in northern Québec, Canada

Isiurqutuuq Lake was an important food source for the Inuit community of Akulivik (northern Québec, Canada) until the 1930s when the fish disappeared. Their disappearance was attributed to explosives used by prospectors. Modern lake water is acidic (pH 4.5), and most life forms are absent. A unit of sulphide-rich graphitic shale occurs throughout the watershed. The oxidation of sulphide minerals produces acidic water, rich in metals, including Zn, Cu and Ni. The lake is less than 15 m above sea level and close to the Hudson Bay coast. Ice sheets, which remained until about 8000 BP, depressed the region 150 m below the present levels, and when ice melted Isiurqutuuq Lake was part of the sea floor. Isostatic uplift exposed the sulphide zone on the north shore of the lake at ∼2500 BP, followed by exposure of the lake. Weathering and oxidation of the sulphides was slow at the start, but increased steadily as frost heaving and fracturing permitted access of water and air to the sulphide zone. Diatom analysis of lake sediment cores reveals striking changes up-section. A clay unit at the base contains a marine-littoral diatom flora. Following uplift and marine regression, freshwater diatoms became established, dominated by circumneutral and alkaliphilous taxa. The subsequent appearance and increasing abundance of acidophilous diatoms reveal increasing acidification which culminates in the top 2 cm. The modern lake-bottom sediments are dominated by acidobiontic species that develop at pH values <5.5. This relatively late acidification is attributed to the initial buffering of acid by marine shells deposited when the lakebed was beneath Hudson Bay. Only when the shells were consumed did the lake become acidic. The scientific evidence is consistent with the observation of the Inuit population that the fish died out in the 1930s. Remediation of the problem is possible by neutralising the acid with modest addition in the lake of the abundant shell sand found in the Akulivik area.