Abstract
Lake Myvatn, Iceland, is one of the most biologically productive lakes in the northern hemisphere, despite seasonal ice cover. Hydrothermal and groundwater springs make up the dominant source to this lake, and we investigate their Mg isotope ratio to assess the effect of mid-ocean ridge hydrothermal springs, which are the primary modern sink of seawater magnesium. We also examine a time series in the only outflow from this lake, the Laxa River, to assess the effects of seasonal primary productivity on Mg isotopes. In the hydrothermal waters, there is a clear distinction between cold waters (largely unfractionated from primary basalt) and relatively hot waters, which exhibit over 1‰ fractionation, with consequences for the oceanic mass balance if the hydrothermal removal of Mg is not fully quantitative. The outflow Mg isotopes are similar to basalts (δ26Mg = −0.2 to −0.3) during winter but reach a peak of ∼0‰ in August. This fractionation corresponds to calcite precipitation during summer in Lake Myvatn, preferentially taking up light Mg isotopes and driving the residual waters isotopically heavy as observed, meaning that overall the lake is a CO2 sink.
Highlights
Chemical weathering of continental silicate rocks is the primary long-term drawdown process of atmospheric CO2 (Walker et al, 1981; Berner et al, 1983; Chamberlin, 1899)
We present Mg isotope data from high- and low-temperature groundwaters that dominantly source Lake Myvatn, Iceland
We report Mg isotope data from a time series from the single outflow from Lake Myvatn, the Laxa River
Summary
Chemical weathering of continental silicate rocks is the primary long-term drawdown process of atmospheric CO2 (Walker et al, 1981; Berner et al, 1983; Chamberlin, 1899). Continental weathering provides many critical nutrients to the coastal oceans (e.g., Fe, P), which fertilize organic carbon growth (Gíslason et al, 2006). Continental weathering and erosion processes deliver particulate clays and reactive iron to the oceans, which enhance organic carbon burial (Kennedy and Wagner, 2011; Lalonde et al, 2012; Kennedy et al, 2014; Hawley et al, 2017). Weathering controls the climate both on short timescales (via organic carbon growth and burial) and long timescales. Chemical weathering affects the carbonate saturation state of the oceans on millennial timescales, affecting its pH and ability to store CO2 (Archer et al, 2000)
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