Modern and Cenozoic records of sea water magnesium from foraminiferal Mg isotopes

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doi: 10.5194/bg-11-5155-2014
Author(s): Pogge von Strandmann, P. A. E.; Forshaw, J.; Schmidt, D. N.
Author Affiliation(s): Primary:
University of Bristol, School of Earth Sciences, Bristol Isotope Group, Bristol, United Kingdom
Volume Title: Biogeosciences
Source: Biogeosciences, 11(18), p.5155-5168. Publisher: Copernicus GmbH on behalf of the European Union, Katlenburg-Lindau, International. ISSN: 1726-4170
Note: In English. 92 refs.; illus., incl. 1 table
Summary: 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 per mil at 15 Ma, to -0.83 per mil 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.
Year of Publication: 2014
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 12 Stratigraphy, Historical Geology and Paleoecology; Alkaline earth metals; Atlantic Ocean; Biomineralization; Calcification; Calcite; Calcium; Carbonate ion; Carbonates; Cenozoic; Chemical composition; Chemical ratios; Climate change; Concentration; Controls; Cores; Foraminifera; Geochemical cycle; Geochemistry; Globigerina; Globigerinacea; Globigerinidae; Holocene; Hydrochemistry; Invertebrata; Isotope fractionation; Isotope ratios; Isotopes; Leg 208; Magnesium; Marine environment; Metals; Mg-26/Mg-24; Mg/Ca; Microfossils; Models; Modern; ODP Site 1262; ODP Site 1263; ODP Site 1264; Ocean Drilling Program; Oligocene; Paleo-oceanography; Paleoclimatology; Paleogene; Protista; Provenance; Quaternary; Reconstruction; Rotaliina; Sea water; Sinks; South Atlantic; Sr-87/Sr-86; Stable isotopes; Strontium; Temperature; Tertiary; Upper Holocene; Walvis Ridge
Coordinates: S290000 S270000 E0030000 E0013000
Record ID: 2016104797
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Copernicus Gesellschaft, Katlenburg-Lindau, Germany