Core top calibration of Mg/Ca in tropical foraminifera: Refining paleotemperature estimation

Abstract

[1] Optimal use of Mg/Ca as a paleotemperature proxy requires establishing calibrations for different species of foraminifera and quantifying the influence of dissolution. To achieve this goal, we have measured Mg/Ca and d 18 O in a series of tropircal and subtropical core tops, including four depth transects: the Ceara Rise, the Sierra Leone Rise, and the Rio Grande Plateau in the Atlantic, and the Ontong Java Plateau in the Pacific, focusing on spinose mixed layer dwelling species Globigerinoides ruber and Globigerinoides sacculifer, and nonspinose thermocline dwelling Neogloboquadrina dutertrei. Shell Mg/ Ca in G. sacculifer is 5-15% lower than in G. ruber, while N. dutertrei Mg/Ca is 49-55% lower than in G. ruber. This statistically significant offset has allowed us to establish different calibrations for each species. Multilinear regression analysis was used to develop calibration equations that include a correction term for the dissolution effect on Mg/Ca in foraminiferal calcite. Presented in this paper are two sets of calibrations; one set using core depth as a dissolution correction and another using ÁCO 3 2À as a dissolution parameter. The calibrations suggest that G. ruber is the most accurate recorder of surface temperature, while G. sacculifer records temperatures below the surface at 20-30 m. The depth habitat of N. dutertrei is more uncertain, owing to the wide range in habitat depths depending on hydrographic conditions, but on average, Mg/Ca and d 18 O data suggest it is at $50 m. Of the three species, N. dutertrei is the most sensitive to dissolution (up to 23% decrease in shell Mg/Ca per km), while G. sacculifer is the most resistant.

Holocene and Eemian sea surface temperature trends as revealed by alkenone and Mg/Ca paleothermometry

“…The Mg/Ca ratio measured on surface-dwelling planktonic foraminifera is a well-established SST proxy that has been internationally calibrated by 25 laboratories (Rosenthal et al, 2004;Greaves et al, 2008). Systematic biases owing to, e.g., ecological behaviour of planktonic foraminifera (Regenberg et al, 2009), local hydrologic characteristics such as calcite saturation levels at depth (Dekens et al, 2002), and different cleaning protocols used to remove the coating of Mg-rich contaminant phases (Rosenthal et al, 2004) have led to a wide range of calibration equations to convert Mg/Ca into temperature. As for the alkenones results, we report the Mg/Ca-based SST data as they were estimated in the original publications.…”

Indian Ocean circulation and productivity during the last glacial cycle

“…G. sacculifer is a dissolution-resistant mixed-layer foraminifera that lives at a depth of ~20 m ( (Dekens et al, 2001)) and exhibits nearly uniform distribution of Mg between various chambers, suggesting minimal habitat migration ( (Eggins et al, 2003)). However, a terminal-sac-like chamber forms in deeper water during gametogenesis, and it is necessary to avoid specimens with this chamber for studies related to mixed-layer processes.…”

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

“…This uncertainty may be compared with their calculated glacial-interglacial temperature changes of up to 5 1C over the last 450 ka. Dekens et al (2002) susceptibility of a particular species to dissolution. For example, on the OJP N. dutertrei showed a decrease in Mg/Ca of 20% per km increase in water depth (Dekens et al, 2002).…”

“…Dekens et al (2002) susceptibility of a particular species to dissolution. For example, on the OJP N. dutertrei showed a decrease in Mg/Ca of 20% per km increase in water depth (Dekens et al, 2002). This may be compared to a response in G. sacculifer of 5% per km.…”

“…Since the solubility of carbonate tends to increase with depth at any location, core-top material from greater depths will be prone to alteration of Mg/Ca. Dekens et al (2002) produced a core-top calibration using samples from the tropical Pacific and Atlantic. Their calibration included a correction term for dissolution effects on Mg/Ca in foraminifera which we discuss in Section 5.…”