Understanding the Effects of Dissolution on the Mg/Ca Paleothermometer in Planktic Foraminifera: Evidence From a Novel Individual Foraminifera Method

Abstract

It is well documented that partial dissolution of planktic foraminiferal tests results in a reduction of Mg/Ca ratios, and hence of inferred calcification temperatures; however, traditional analysis techniques have made it difficult to identify the exact mechanism through which Mg is lost. Three hypotheses have been proposed as models for Mg loss for a given extent of dissolution: (1) a percent loss of Mg in individuals, (2) a molar loss of Mg in individuals, and (3) a loss of the highest‐Mg (warmest) individuals from a population. It is vital to better constrain these models as they have very different implications for Mg/Ca paleotemperature dissolution corrections. Here we use a novel individual foraminifera Mg/Ca method to examine the effects of dissolution on the Mg/Ca paleothermometer in three species of planktic foraminifera, Globigerinoides ruber, Neogloboquadrina dutertrei, and Pulleniatina obliquiloculata, from a depth transect of core tops on the Ontong Java Plateau in the western equatorial Pacific. With the exception of the most heavily dissolved population of P. obliquiloculata, our data best support a percent Mg loss model as indicated by the preservation of inferred temperature distribution shapes among the sampled populations and the close fit of the simulated percent Mg loss model to the observed data. Coupled with estimates for foraminiferal dissolution, identification of the percent Mg loss model will allow for more accurate dissolution corrections in Mg/Ca paleothermometry work.

Micro‐CT Scanning of Tests of Three Planktic Foraminiferal Species to Clarify DissolutionProcess and Progress

“…Of the tests of the three species in this study, the parts of N. dutertrei tests characterized by high Mg/Ca ratios are considered to be the most sensitive to dissolution (Dekens et al, 2002). The selective dissolution of the ontogenetic calcite layer on the inner side of the chamber wall with higher Mg/Ca ratios and preservation of calcite crust on the outer side of the chamber wall with lower Mg/Ca ratios seems to effectively reduce the Mg/Ca ratio of N. dutertrei as dissolution progresses as shown in Rongstad et al (2017). If the high Mg/Ca layer in the test was prone to dissolution, the variation in Mg/Ca ratios due to calcification temperature could influence the process and sensitivity of test dissolution.…”

Midlatitude Southern Hemisphere Temperature Change at the End of the Eocene Greenhouse Shortly Before Dawn of the Oligocene Icehouse

“…It cannot be explained by sensitivity to error in our foraminiferal Mg/Ca results: a one standard deviation (0.55 mmol/mol) change from our average measured pre‐Step 1 foraminiferal Mg/Ca value results in a surface seawater δ 18 O change of less than ±0.27‰ using the B values of both Lear et al () and Martin et al (). The discrepancy cannot be explained by partial dissolution of our sample material; dissolution would decrease foraminiferal Mg/Ca (e.g., Rongstad et al, ), which would lead to lower seawater temperatures, and, thus, lower calculated values for latest Eocene surface seawater δ 18 O. Areas where recrystallization or clay contamination has occurred can be recognized and removed from LA‐ICPMS Mg/Ca data (see supporting information).…”

Investigating ENSO‐Related Temperature Variability in Equatorial Pacific Core‐Tops Using Mg/Ca in Individual Planktic Foraminifera

“…Planktic foraminiferal Mg/Ca is a well‐established tool for reconstructing past ocean temperatures (e.g., Nürnberg et al, ; Lea et al, ; Elderfield & Ganssen, ; Anand et al, ). We model the foraminiferal Mg/Ca data in an inverse sense, converting Mg/Ca to calcification temperatures, because that is the task facing paleoceanographers; and because Mg/Ca‐inferred temperature variability is insensitive to partial dissolution, in contrast to Mg/Ca variability (Rongstad et al, ). Inferred temperatures were calculated from Mg/Ca ratios using the Anand et al () sediment trap temperature calibrations, specifically their geometric mean method for G. ruber white (250–350 μm) giving a 10% increase in Mg/Ca per °C, and their assumed exponential constant method for N. dutertrei (350–500 μm) giving 9% per °C.…”

“…While partial dissolution results in a reduction of planktic foraminiferal Mg/Ca ratios (Brown & Elderfield, 1996;Dekens et al, 2002;Regenberg et al, 2014;Rosenthal & Lohmann, 2002), G. ruber and N. dutertrei lose Mg to partial dissolution as a percentage of the Mg initially present, resulting in no significant change in calculated temperature variance or temperature distribution shape (Rongstad et al, 2017). Therefore, a a Detailed information pertaining to calibrated radiocarbon ages, including ΔR values and accession numbers, are located in the supporting information text and in Table S1.…”