Using Mg Isotopes to Estimate Natural Calcite Compositions and Precipitation Rates During the 2010 Eyjafjallajökull Eruption

Abstract

Chemical weathering of silicate rocks is a key control on the long-term climate, via drawdown of atmospheric CO 2. Magnesium isotopes are increasingly being used to trace weathering, but are often complicated by several coincident fractionating processes. Here we examine Mg isotope ratios of waters stemming from beneath lava flows from the 2010 Eyjafjallajökull eruption. Travertine calcite was observed directly precipitating from these high-TDS (total dissolved solids) waters, and were also sampled. This system therefore provides the opportunity to study natural Mg isotope fractionation by calcite. Riverine δ 26 Mg increase from −2.37 to +0.43% with flow distance, as isotopically light travertine precipitates (δ 26 Mg = −3.38 to −3.94%). The solution Mg isotope ratios also co-vary with pH, calcite saturation indices and Sr/Ca ratios, strongly indicating that they are dominantly controlled by carbonate precipitation. Using experimental isotopic fractionation factors and the measured δ 26 Mg values, we can predict the compositions of the precipitated travertines that are within uncertainty of the directly measured travertines. Hence, in some systems, Mg isotopes can be used to quantify carbonate precipitation.

Hydrothermal and Cold Spring Water and Primary Productivity Effects on Magnesium Isotopes: Lake Myvatn, Iceland

“…Calcite precipitation has previously been observed driving riverine Mg isotope ratios heavy in Iceland during highly supersaturated outflow from the 2010 Eyjafjallajökull eruption (Pogge von Strandmann et al, 2019c). In this case, calcite SI reached values of > 1.5, travertine precipitation was observed, and δ 26 Mg changed by over 2.5 .…”

“…Overall, this calcite precipitation is an order of magnitude less than the Si sequestration by diatom growth of ∼7,500 t/year (Opfergelt et al, 2011), showing that the phytoplankton (including diatoms) in Myvatn can have a significantly larger effect on the concentration of many elements compared to calcite precipitation. In comparison, the stream draining the Eyjafjallajökull eruption exhibited a Mg isotope-based calcite precipitation rate of 3,200 t/year (Pogge von Strandmann et al, 2019c).…”

“…Given that the groundwater influx rate into Lake Myvatn is fairly constant at 33 m 3 /s, it follows that the outflow rate through the Laxa River is broadly the same rate (Olafsson, 1979a;Opfergelt et al, 2011). Assuming that Mg isotopes are solely being fractionated by calcite formation, with a known fractionation factor of α = 0.9965 (Mavromatis et al, 2013;Saenger and Wang, 2014;Pogge von Strandmann et al, 2019c), up to 8% of dissolved Mg is lost into calcite (Pogge von Strandmann et al, 2019c). Following the calculations detailed in Pogge von Strandmann et al (2019c), using the Laxa outflow rate and the volume of Lake Myvatn (∼9.25 × 10 7 m 3 ), the calcite precipitation rate reaches a maximum of ∼53 t/month (∼6 mmol/month/m 3 ) in August.…”

“…While a considerable body of literatures exists on Mg isotope behavior in rivers and occasionally groundwaters, much less is known about Mg isotope fractionation in continental hydrothermal fluids (Tipper et al, 2006b;Huang et al, 2018;Oelkers et al, 2019;Pogge von Strandmann et al, 2019c). This is potentially also important for the removal of Mg from seawater.…”

Triple Isotope Fractionation Exponents of Elements Measured by MC-ICP-MS—An Example of Mg

“…Table S1). (B) Two standard deviations on the slope of regression lines extracted from published data sets 1, 29 2, 18 3, 30 4, 31 5, 32 6, 15 7, 33 8, 34 9, 35 10, 36 11, 37 12, 38 13 (this study).…”