Incorporation and preservation of Mg in Globigerinoides sacculifer: implications for reconstructing the temperature and ¹⁸O/¹⁶O of seawater

Abstract

Abstract. Using bathymetric transects of surface sediments underlying similar sea surface temperatures but exposed to increasing dissolution, we examined the processes which affect the relationship between foraminiferal Mg/Ca and 5180. We found that Globigerinoides saccculifer calcifies over a relatively large range of water depth and that this is apparent in their Mg content. On the seafloor, foraminiferal Mg/Ca is substantially altered by dissolution with the degree of alteration increasing with water depth. Selective dissolution of the chamber calcite, formed in surface waters, shifts the shell's bulk Mg/Ca and 5180 toward the chemistries of the secondary crust acquired in colder thermocline waters. The magnitude of this shift depends on both the range of temperatures over which the shell calcified and the degree to which it is subsequently dissolved. In spite of this shift the initial relationship between Mg/Ca and 5180, determined by their temperature dependence, is maintained. We conclude that paired measurements of 5180 and Mg/Ca can be used for reconstructing 518Owater, though care must be taken to determine where in the water column the reconstruction applies.

Planktonic foraminiferal Mg/Ca as a proxy for past oceanic temperatures: a methodological overview and data compilation for the Last Glacial Maximum

“…The cause of this reduction is thought to be the preferential dissolution of high Mg/Ca regions of the test i.e. those formed in warmer waters (Brown and Elderfield, 1996;Rosenthal et al, 2000). The effect of partial dissolution is therefore a biasing of mean test Mg/Ca values towards colder temperatures.…”

“…During dissolution, decreasing Mg/Ca is thought to represent the preferential removal of calcite formed at warmer temperatures (Brown and Elderfield, 1996;Rosenthal et al, 2000). The region of calcite within a foraminiferal test with the highest Mg/Ca ratio probably reflects the warmest temperature experienced by that individual and therefore may give the best impression of near surface temperatures.…”

“…4) (Lorens et al, 1977;Rosenthal and Boyle, 1993;Russell et al, 1994;Brown and Elderfield, 1996;Rosenthal et al, 2000). The cause of this reduction is thought to be the preferential dissolution of high Mg/Ca regions of the test i.e.…”

“…More recently Mg/Ca thermometry has been greatly refined and applied almost routinely to paleoceanographic questions concerning temperature variability through time (e.g. Nu¨rnberg et al, 1996;Hastings et al, 1998;Lea et al, 1999Lea et al, , 2000Lea et al, , 2002Mashiotta et al, 1999;Elderfield and Ganssen, 2000;Rosenthal et al, 2000;Koutavas et al, 2002;Stott et al, 2002;Pahnke et al, 2003;Rosenthal et al, 2003;Visser et al, 2003). Mg/ Ca ratios of other calcifying organisms have also been shown to be sensitive to changes in temperature.…”

Pleistocene sea-surface temperature evolution: Early cooling, delayed glacial intensification, and implications for the mid-Pleistocene climate transition

“…Bias may be introduced due to preferential loss of Mg by dissolution of Mg-rich calcite (e.g. Brown and Elderfield, 1996;Rosenthal et al, 2000) but can be corrected for by using estimates of carbonate ion concentration (Dekens et al, 2002). Variations in seawater Mg/Ca also need to be considered when estimating secular changes in ocean temperature from foraminiferal Mg/Ca.…”