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

Abstract

Mg/Ca ratio paleothermometry in foraminifera is an important tool for the reconstruction and interpretation of past environments. However, existing Mg/Ca:temperature relationships for planktic species inhabiting middle‐ and high‐latitude environments are limited by a lack of information about the development and impact of low‐Mg/Ca ratio “crusts” and the influence of the carbonate system on Mg/Ca ratios in these groups. To address this, we cultured individual specimens of Neogloboquadrina incompta and Neogloboquadrina pachyderma in seawater across a range of temperature (6°–12°C) and pH (7.4–8.2). We found by laser ablation inductively couple mass spectrometry analyses of shells that culture‐grown crust calcite in N. incompta had a lower Mg/Ca ratio than ontogenetic calcite formed at the same temperature, suggesting that temperature is not responsible for the low‐Mg/Ca ratio of neogloboquadrinid crusts. The Mg/Ca:temperature relationship for ontogenetic calcite in N. incompta was consistent with the previously published culture‐based relationship, and no significant relationship was found between Mg/Ca ratios and pH in this species. However, the Mg/Ca ratio in laboratory‐cultured N. pachyderma was much higher than that reported in previous core top and sediment trap samples, due to lack of crust formation in culture. Application of our ontogenetic calcite‐specific Mg/Ca:temperature relationships to fossil N. pachyderma and N. incompta from five intervals in cores from the Santa Barbara Basin and the Bering Sea shows that excluding crust calcite in fossil specimens may improve Mg/Ca‐based temperature estimates.

Calibration and Validation of Environmental Controls on Planktic Foraminifera Mg/Ca Using Global Core‐Top Data

“…N . pachyderma produced the most complex calibration, within which the inclusion of Ω shallow is only partially consistent with previous independent work (Davis et al, ; Hendry et al, ; Jonkers et al, ). Similarly, the exclusion of salinity is consistent with early studies that suggested salinity to be of secondary importance (Nurnberg, ), but its influence has not been specifically isolated in culture studies.…”

“…While many of the concerns discussed above still hold for this combined relationship, it is noteworthy that it has considerably greater skill than either of the optimal relationships for the individual species and would therefore be more appropriate to apply to independent Neogloboquadrina foraminifera regardless of species. Furthermore, equation is generally more consistent with independent results (Davis et al, ; Jonkers et al, ; von Langen et al, ), suggesting that it is more realistic in addition to being more skillful. We therefore see equation to have greater utility for paleoceanographic studies and suggest that it be used in place of either equation or .…”

“…However, the selection of Ω deep and exclusion of temperature are not consistent with suggestions that N . pachyderma is among the most dissolution‐resistant planktic foraminifera (Malmgren, ) or with culture‐based evidence for a strong T dependence (Davis et al, ). Along with its marginal skill, this casts considerable doubt on the usefulness of equation , and we cannot recommend that it be applied in paleoceanographic contexts.…”

“…Additional confounding variables that may inhibit robust N . pachyderma calibrations include volcanic ash (Meland et al, ; Morley et al, ), mixing of glacial and Holocene foraminifera at low sedimentation rate sites (Meland et al, ), variations in the proportion of low‐Mg outer crust, which can comprise up to 90% of calcite (Davis et al, ; Lohmann, ) and up to seven different N . pachyderma genotypes (Darling et al, ).…”

“…Somewhat higher validation statistics for N . incompta suggest greater predictive skill, but the selection of salinity rather than temperature as an optimal variable is surprising given two independent culture studies suggesting a temperature dependence similar to that of other species (Davis et al, ; von Langen et al, ). While these studies did not explicitly evaluate salinity and therefore do not provide robust evidence disproving a salinity effect, both present convincing evidence for a temperature influence.…”