Calcium and magnesium isotope systematics in rivers draining the Himalaya-Tibetan-Plateau region: Lithological or fractionation control?

Tipper, Edward T.; Galy, Αlbert; Bickle, M. J.

doi:10.1016/j.gca.2007.11.029

Cited by 200 publications

(151 citation statements)

References 66 publications

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“…In line with the findings in rivers draining the Himalayan region (e.g. Galy et al 1999;Jacobson et al 2002;Tipper et al 2008;Bickle et al 2015;Chapman et al 2015), many more NCHT rivers are affected by secondary calcite precipitation. For instance, based of the chemistry of the dissolved species, around 2/3 of the Ca released in the catchment of the Yellow River by the weathering of rock has precipitated as calcite.…”

Section: Resultssupporting

confidence: 84%

Sequestration of carbon as carbonate in the critical zone: insights from the Himalayas and Tibetan Plateau

2017

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The carbon pool stored in soil carbonate is comparable to the soil organic carbon. Therefore, secondary calcite precipitation in supersaturated catchment could be an important, yet poorly constrained, carbon sink within the modern global carbon cycle. The chemical analysis of some dissolved species transported by rivers, such as elevated Sr/Ca and Mg/Ca ratios but also heavy stable Ca isotopic compositions, witness the formation of secondary calcite in rivers draining arid regions. However, in areas affected by active tectonics and rapid physical erosion, co-variations in the fluvial Sr/Ca and Mg/Ca ratios could also be related to incongruent carbonate weathering processes. Here, we present a model to assess the roles played by incongruent carbonate dissolution and secondary calcite precipitation in modern weathering processes. We tested and applied the model to rivers draining the Himalayan-Tibetan region. The results suggest that regional aridity in the drainage basin promotes carbon sequestration as secondary carbonate but that for a given runoff, incongruent dissolution of carbonate possibly related to rapid physical erosion amplifies such sequestration. The isotopic compositions ( 13 C/ 12 C and 18 O/ 16 O) of detrital carbonate transported by the main rivers in South and South-East Tibet imply that around 1% of the suspended material transported by those rivers corresponds to secondary carbonate and can represent between 5% and 15% of the alkalinity flux. Most of these alkalinity transported as particulate material is, nevertheless related to the weathering of carbonate lithologies and is also subjected to dissolution prior its final storage in sedimentary basins. However, on glacial-interglacial timescale this will amplify the significant role of mountain weathering on climatic variations.

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Section: Resultssupporting

confidence: 84%

Sequestration of carbon as carbonate in the critical zone: insights from the Himalayas and Tibetan Plateau

2017

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“…5). These changes are fairly large, varying between −3 and +0.5 ‰, which is approximately the modern range of rivers Tipper et al, 2006bTipper et al, , 2008Tipper et al, , 2012de Villiers et al, 2005;Huang et al, 2012;Wimpenny et al, 2011). However, given that the dominant control over the δ 26 Mg of rivers is the lithology undergoing weathering (isotopically light carbonates, and heavy silicates), this would require that during most of the Oligocene and late Miocene rivers, were dominated by carbonate weathering, Figure 5.…”

Section: A Cenozoic Seawater Reconstruction Based On Foraminiferal Camentioning

confidence: 97%

“…Rather than simply solving the flux mass balance, the isotope mass balance equation is solved for F sed , using the same assumptions for the hydrothermal flux and riverine flux as in the first model. This then requires a "known" riverine isotope ratio through time, which is constrained from the river 87 Sr / 86 Sr of Lear et al (2003) Tipper et al, 2008). Clearly, this approach also assumes that Sr isotopes are not anomalously influenced by radiogenic Himalayan carbonates (Palmer and Edmond, 1992), although it has been suggested that the radiogenic Himalayan flux is due to mixing between unradiogenic carbonate and radiogenic silicate weathering (Galy et al, 1999).…”

Section: Modelling Seawater Mg Using Mg Isotopesmentioning

confidence: 99%

Modern and Cenozoic records of seawater magnesium from foraminiferal Mg isotopes

2014

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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.

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“…The magnesium isotopic compositions of river waters are highly heterogeneous. [19,37,38] The flux-weighted d 26 Mg of global runoff has been estimated at À1.09%, based on sampling of 45 rivers across the globe, [37] which is lower than the average d seawater at time scales matching riverine input. [5] Furthermore, although Mg in calcite has a consistently lower d 26 Mg value than Mg in dolomite by approximately 2%, [5] considering that the Mg content of calcite is significantly lower than that of dolomite, dolomite precipitation may be an important control on seawater Mg isotopic composition.…”

Section: Controlling Factors On Mg Content and Isotopic Composition Omentioning

confidence: 99%

Homogeneous magnesium isotopic composition of seawater: an excellent geostandard for Mg isotope analysis

Ling

Sedaghatpour

Teng

et al. 2011

Rapid Comm Mass Spectrometry

145

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The magnesium (Mg) isotopic compositions of 40 seawater samples from the Gulf of Mexico and of one seawater sample from the southwest Hawaii area were determined by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) to investigate the homogeneity of Mg isotopes in seawater. The results indicate that the Mg isotopic composition of seawater from the Gulf of Mexico is homogeneous, both vertically and horizontally, with average values for d 25 Mg = À0.43 AE 0.06 (2SD, n = 90). The magnesium isotopic composition of seawater is principally controlled by river water input, carbonate precipitation and oceanic hydrothermal interactions. The homogeneous Mg isotopic composition of seawater indicates a steady-state budget in terms of Mg isotopes in oceans, consistent with a long Mg residence time (~13 Ma). Considering that seawater is homogeneous, readily available in large amounts, can be easily accessed and processed for isotopic analysis, and has an isotopic composition near the middle of the natural range of variation, it is an excellent geostandard for accuracy assessment to rule out analytical artifacts during high-precision Mg isotopic analysis.

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Calcium and magnesium isotope systematics in rivers draining the Himalaya-Tibetan-Plateau region: Lithological or fractionation control?

Cited by 200 publications

References 66 publications

Sequestration of carbon as carbonate in the critical zone: insights from the Himalayas and Tibetan Plateau

Sequestration of carbon as carbonate in the critical zone: insights from the Himalayas and Tibetan Plateau

Modern and Cenozoic records of seawater magnesium from foraminiferal Mg isotopes

Homogeneous magnesium isotopic composition of seawater: an excellent geostandard for Mg isotope analysis

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