Modern and Cenozoic records of seawater magnesium from foraminiferal Mg isotopes

Abstract

Abstract. Magnesium is an element critically involved in the carbon cycle, because weathering of Ca-Mg silicates removes atmospheric CO 2 into rivers, and formation of Ca-Mg carbonates in the oceans removes carbon from the oceanatmosphere 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 (δ 26 Mg, relative to DSM-3) and temperature, Mg / Ca or other parameters such as carbonate saturation ( CO 3 ). However, inter-species isotopic differences imply that only well-calibrated single species should be used for reconstruction of past seawater. Seawater δ 26 Mg 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 δ 26 Mg of seawater, while hydrothermal Mg removal and the δ 26 Mg of rivers are more minor controls. Using Mg riverine flux and isotope ratios inferred from the 87 Sr / 86 Sr 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.

Effects of cone combinations on accurate and precise Mg‐isotopic determination using multi‐collector inductively coupled plasma mass spectrometry

“…The δ 26 Mg values of a seawater reference material (NASS‐6) and two rock reference materials (AGV‐2 and BHVO‐2) were also determined. The results (Table ) are in good agreement with the previous published values …”

“…employed to provide ubiquitous traceability in the fields of geochemistry, speleology, hydrology, oceanography, paleoclimatology, cosmochemistry, and biology, [3][4][5][6][7][8][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] such as understanding biogeochemical processes, 6,14,17,28 tracing fluid/melt-mineral interaction, 29 characterizing chemical weathering, 5,11,12 exploring earth and planetary evolution, 3,4,13 and reconstructing climate history. 5,15,16 To date, a range of~6‰ in the 26 Mg/ 24 Mg ratios has been observed in various types of natural samples. 9,[30][31][32] However, although analytical methods for the measurement of Mg isotopes have been significantly improved, a recent inter-laboratory study 33 reported a~0.4‰ difference in δ…”

Constraining dolomitization by Mg isotopes: A case study from partially dolomitized limestones of the middle Cambrian Xuzhuang Formation, North China

“…There is no constraint on the Mg isotopic composition of Cambrian seawater. It is proposed that Cenozoic seawater has limited variation in δ 26 Mg based on the study of bulk carbonate samples [ Higgins and Schrag , ], while larger fluctuation of Cenozoic seawater δ 26 Mg (0‰∼−0.8‰) is obtained from the study of foraminifera tests [ Pogge von Strandmann et al ., ]. In either case, the estimated δ 26 Mg of Cenozoic seawater is lower than the simulated , suggesting that dolomitization fluids should be more 26 Mg‐enriched than seawater, such as compositionally modified seawater or subsurface brine.…”

The Li isotope composition of marine biogenic carbonates: Patterns and mechanisms

“…This process would lead to high δ 7 Li (high Δ 7 Li physiol ) and low Li/Ca (low β Li ) of carbonates. Removal of Mg from the calcification reservoir through Mg specific channels has previously been suggested for foraminifera (Bentov and Erez, 2006;Zeebe and Sanyal, 2002) to explain their low Mg/Ca value relative to seawater, even if the relevance of this process is debated (Pogge von Strandmann et al, 2014;Wombacher et al, 2011). Indeed, high Mg content in fluids inhibits calcite precipitation (e.g.…”