Abstract
Abstract. Magnesium is an element critically involved in the carbon cycle, because weathering of Ca-Mg silicates removes atmospheric CO2 into rivers, and formation of Ca-Mg carbonates in the oceans removes carbon from the ocean-atmosphere system. Hence the Mg cycle holds the potential to provide valuable insights into Cenozoic climate-system history, and the shift during this time from a greenhouse to icehouse state. We present Mg isotope ratios for the past 40 Myr using planktic foraminifers as an archive. Modern foraminifera, which discriminate against elemental and isotopically heavy Mg during calcification, show no correlation between the Mg isotope composition (δ26Mg, relative to DSM-3) and temperature, Mg / Ca or other parameters such as carbonate saturation (ΔCO3). However, inter-species isotopic differences imply that only well-calibrated single species should be used for reconstruction of past seawater. Seawater δ26Mg inferred from the foraminiferal record decreased from ~0‰ at 15 Ma, to −0.83‰ at the present day, which coincides with increases in seawater lithium and oxygen isotope ratios. It strongly suggests that neither Mg concentrations nor isotope ratios are at steady state in modern oceans, given its ~10 Myr residence time. From these data, we have developed a dynamic box model to understand and constrain changes in Mg sources to the oceans (rivers) and Mg sinks (dolomitisation and hydrothermal alteration). Our estimates of seawater Mg concentrations through time are similar to those independently determined by pore waters and fluid inclusions. Modelling suggests that dolomite formation and the riverine Mg flux are the primary controls on the δ26Mg of seawater, while hydrothermal Mg removal and the δ26Mg of rivers are more minor controls. Using Mg riverine flux and isotope ratios inferred from the 87Sr / 86Sr record, the modelled Mg removal by dolomite formation shows minima in the Oligocene and at the present day (with decreasing trends from 15 Ma), both coinciding with rapid decreases in global temperatures.
Highlights
The weathering of Ca-Mg silicates sequesters atmospheric CO2 and controls long-term climate (Walker et al, 1981; Berner et al, 1983; Kump et al, 2000)
Mg isotope ratios of G. truncatulinoides, G. sacculifer and G. ruber are within the range of previous studies (Chang et al, 2004; Pogge von Strandmann, 2008; Wombacher et al, 2011)
The new data for G. inflata are similar to G. truncatulinoides, its nearest relative in the study with the most similar habitat
Summary
The weathering of Ca-Mg silicates sequesters atmospheric CO2 and controls long-term climate (Walker et al, 1981; Berner et al, 1983; Kump et al, 2000). The history of weathering on land is partly reflected by the Mg evolution of the ocean, recorded by marine carbonates. Given that Mg is critically involved in the long-term carbon cycle, studying Mg isotopes offers advantages over trace elements that are not involved so directly, such as strontium or osmium isotopes (Palmer and Edmond, 1992; Levasseur et al, 1999; Georg et al, 2013). The dominant Mg source to the oceans is thought to be continental weathering, via rivers and groundwater, while the main sinks are removal by hydrothermal fluids (primarily by high-temperature interaction with basalt, which is thought to quantitatively strip Mg from fluids), dolomite formation and low-temperature clay formation during alteration of the oceanic crust (Holland, 2005).
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