Santiago Ramos
Danielle
Santiago Ramos
Danielle
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ArticleA Mg isotopic perspective on the mobility of magnesium during serpentinization and carbonation of the Oman Ophiolite(American Geophysical Union, 2020-12-04) de Obeso, Juan Carlos ; Santiago Ramos, Danielle ; Higgins, John A. ; Kelemen, Peter B.Alteration of mantle peridotite in the Samail ophiolite forms secondary minerals, mainly serpentine and Mg‐rich carbonates. Magnesium accounts for ∼25 – 30% of peridotite mass and its mobility can be used to trace this alteration. We report the first set of Mg isotope measurements from peridotites and their alteration products in Oman. Partially serpentinized peridotites have Mg isotope ratios that are indistinguishable from estimates for the average mantle and bulk silicate earth (δ26Mg = −0.25 ± 0.04‰). However, more extensively altered peridotite samples show large shifts in Mg isotopic composition. The range of δ26Mg values for our suite of alteration products from the mantle section is ∼4.5‰ (from −3.39‰ to 1.19‰), or >60% of the total range of terrestrial variability in δ26Mg values. Serpentine veins are typically enriched in 26Mg (max δ26Mg value = 0.96‰) whereas Mg‐carbonate veins are associated with low 26Mg/24Mg ratios (magnesite δ26Mg = −3.3‰, dolomite δ26Mg = −1.91‰). Our preferred explanation for the range in δ26Mg values involves coprecipitation of serpentine and carbonates at water‐to‐rock ratios >103. The coincidence of alteration products characterized by δ26Mg values that are both lower and higher than bulk silicate Earth and the finite 14C ages of the carbonates suggest that both serpentinization and carbonation are ongoing in Oman. Rates of calcite precipitation in travertines inferred from Δ26Mgcal‐fl suggest that travertine formation in Oman sequesters a total of 106–107 kg CO2/yr, consistent with previous estimates.
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ArticleGroundwater discharge impacts marine isotope budgets of Li, Mg, Ca, Sr, and Ba(Nature Research, 2021-01-08) Mayfield, Kimberley K. ; Eisenhauer, Anton ; Santiago Ramos, Danielle ; Higgins, John A. ; Horner, Tristan J. ; Auro, Maureen E. ; Magna, Tomas ; Moosdorf, Nils ; Charette, Matthew A. ; Gonneea, Meagan E. ; Brady, Carolyn E. ; Komar, Nemanja ; Peucker-Ehrenbrink, Bernhard ; Paytan, AdinaGroundwater-derived solute fluxes to the ocean have long been assumed static and subordinate to riverine fluxes, if not neglected entirely, in marine isotope budgets. Here we present concentration and isotope data for Li, Mg, Ca, Sr, and Ba in coastal groundwaters to constrain the importance of groundwater discharge in mediating the magnitude and isotopic composition of terrestrially derived solute fluxes to the ocean. Data were extrapolated globally using three independent volumetric estimates of groundwater discharge to coastal waters, from which we estimate that groundwater-derived solute fluxes represent, at a minimum, 5% of riverine fluxes for Li, Mg, Ca, Sr, and Ba. The isotopic compositions of the groundwater-derived Mg, Ca, and Sr fluxes are distinct from global riverine averages, while Li and Ba fluxes are isotopically indistinguishable from rivers. These differences reflect a strong dependence on coastal lithology that should be considered a priority for parameterization in Earth-system models.
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ArticleCenozoic global cooling and increased seawater Mg/Ca via reduced reverse weathering(Nature Publishing Group, 2017-10-10) Dunlea, Ann G. ; Murray, Richard W. ; Santiago Ramos, Danielle ; Higgins, John A.Authigenic clay minerals formed on or in the seafloor occur in every type of marine sediment. They are recognized to be a major sink of many elements in the ocean but are difficult to study directly due to dilution by detrital clay minerals. The extremely low dust fluxes and marine sedimentation rates in the South Pacific Gyre (SPG) provide a unique opportunity to examine relatively undiluted authigenic clay. Here, using Mg isotopes and element concentrations combined with multivariate statistical modeling, we fingerprint and quantify the abundance of authigenic clay within SPG sediment. Key reactants include volcanic ash (source of reactive aluminium) and reactive biogenic silica on or shallowly buried within the seafloor. Our results, together with previous studies, suggest that global reorganizations of biogenic silica burial over the Cenozoic reduced marine authigenic clay formation, contributing to the rise in seawater Mg/Ca and decline in atmospheric CO2 over the past 50 million years.