Mg/Ca thermometry in planktic foraminifera: Improving paleotemperature estimations for G. bulloides and N. pachyderma left

Abstract

Planktic foraminiferal Mg/Ca ratios have become a fundamental seawater temperature proxy in past climate reconstructions, due to the temperature dependence of Mg uptake into foraminiferal calcite. However, empirical calibrations for single species from methodologically consistent data are still lacking.Here we present species-specific calibrations of Mg/Ca versus calcification temperature for two commonly used species of planktic foraminifera: Globigerina bulloides and Neogloboquadrina pachyderma left, based on a series of Southern Ocean and North Atlantic core tops. Combining these new data with previously published data, we derive an integrated G. bulloides Mg/Ca-temperature calibration for mid and high latitudes of both hemispheres between 2 and 188C, where Mg/Ca 5 1.006 6 0.032 * e 0.065 6 0.003 * Tiso (R 2 5 0.82). G. bulloides is found to calcify deeper in the Southern Ocean ($ 200 m) than in the North Atlantic (top 50 m). We also propose a Mg/Ca temperature calibration to describe the temperature response in N. pachyderma left that calcified away from the influence of sea ice in the Southern Ocean, valid between $ 21 and 98C, of the form Mg/Ca 5 0.580 6 0.016 * e 0.084 6 0.006*Tiso (R 2 5 0.70). These calibrations account for uncertainties on Mg/Ca measurements and calcification temperature that were carefully estimated and propagated using Monte Carlo iterations. The 1r propagated error in Mg/Ca-derived temperatures is 1.18C for G. bulloides and 0.98C for N. pachyderma left for the presented data sets. Geographical extension of genotypes must be assessed when choosing to develop regional or global calibrations.

Relationships Between Temperature, pH, and Crusting on Mg/Ca Ratios in Laboratory‐Grown Neogloboquadrina Foraminifera

“…Our ontogenetic calcite‐specific Mg/Ca:temperature relationship yields temperatures indistinguishable from those at the presumed calcification depth for both Bering Sea intervals (6°C at 14.5 ka and 5°C at 17.8 ka). By contrast, while several previously published Mg/Ca:temperature relationships produce temperatures within 2°C of assumed 50 m temperature at 14.5 ka (6°C) (Jonkers et al, ; Nürnberg, ; von Langen et al, ), all result in lower temperatures at 17.8 ka (5°C) by 1–11°C (Figure ) (Jonkers et al, ; Kozdon, Eisenhauer, et al, ; Nürnberg, ; Vázquez Riveiros et al, ; von Langen et al, ). This could be partly the result of a 10% increase in crust calcite in this interval, as described above.…”

“…In the Mg/Ca:temperature relationship derived from N. pachyderma cultured at 6, 9, and 12 ° C, Mg/Ca ratios were higher than those estimated from previously published N. pachyderma Mg/Ca:temperature relationships from core tops and sediment traps (Hendry et al, ; Jonkers et al, ; Kozdon, Eisenhauer, et al, ; Nürnberg, ; Vázquez Riveiros et al, ), and different from those in N. incompta (Figure S1 in the supporting information). Our results, using individual shell LA‐ICP‐MS profiles, yield the following relationship (Figure ) where T is temperature: …”

“…The resulting Mg/Ca:temperature relationship cannot be constrained beyond linear, as has been published for other limited temperature ranges studied in N. pachyderma (Hendry et al, ; Kozdon, Eisenhauer, et al, ). However, an exponential fit, as reported elsewhere (Jonkers et al, ; Mashiotta et al, ; Nürnberg, ; Vázquez Riveiros et al, ), may be more appropriate over a larger range of temperature.…”

“…1, Nürnberg et al (); Ref. 2, Vázquez Riveiros et al (). Error on temperatures calculated using equation are ±1.6°C, based upon standard error around mean Mg/Ca at each treatment, averaging ±0.2 Mg/Ca mmol/mol.…”

Reconstruction of North American drainage basins and river discharge since the Last Glacial Maximum

“…Thus, it is a good predictor of changes in drainage basin area, but is somewhat less effective at predicting past river discharge, especially when that discharge is strongly influenced by ice-sheet melt. Discharge inferences continue to be aided by the growing database of paleoceanographic proxy records (e.g., Keigwin et al, 2006;Obbink et al, 2010;Williams et al, 2012;HillaireMarcel et al, 2013;Taylor et al, 2014;Gibb et al, 2014;Gil et al, 2015), improved geochemical techniques to produce richer records that also reduce or quantify uncertainties (e.g., Vetter, 2013;Spero et al, 2015;Khider et al, 2015;Vázquez Riveiros et al, 2016), and continuing work to provide quantitative freshwater discharge estimates based on these data (Carlson et al, 2007a;Carlson, 2009;Obbink et al, 2010;Wickert et al, 2013). Particularly important are records of past water isotopes, past water temperatures, and geochemical signatures of provenance.…”