Application of the laser ablation Mg/Ca ratio from planktonic foraminifera to paleotemperature and stratification reconstructions: example from the Cabo Frio Upwelling System (Brazil) during the Holocene

Abstract:

This study aimed to reconstruct paleotemperatures through Mg/Ca ratio in Globigerinoides ruber and Globigerina bulloides using the laser ablation and ICP-MS technique (LA-ICP-MS) in a core sampled from a western boundary upwelling system. The mean Mg/Ca ratio from the last three chambers was measured in seven tests of G. ruber and G. bulloides .Paleotemperatures were reconstructed using the equations of Anand et al (2003) for G. ruber and Mashiotta et al. (1999) for G. bulloides. The last chamber from G. ruber recorded Mg/Ca values which temperatures varied between 19 and 22°C. The temperatures obtained from the last chamber were lower than from the others two chambers, which temperatures varied between 24 and 26°C. This difference suggests the specie migrate to cold deeper waters during the calcification of the last chamber. It indicates that reconstructions based on the last chamber confronted with the mean of others two chambers could be used to estimate past water stratification. The Mg/Ca values of G. bulloides were not so different among the chambers, generating paleotemperatures between 18 and 22°C. However, G. bulloides’s individual variation coefficient was higher (14 – 48%) than G. ruber (12 – 28%), indicating the specie tolerates higher thermic amplitude due to its behavior related to searching by food. Cold surface waters and warmer sub-surface waters occurred between 9.0 and 7.2 kyr BP and the opposite occurring after 7.2 ka BP. The G. bulloides’s estimated cold episodes were marked before 8,0 ka BP, between 6.0 and 5.5 kyr BP, between 4.5 and 4.0 kyr BP and after 2.0 ka BP, which could be associated to upwelling intensity. The divergence of G. bulloides and G. ruber’s last chamber paleotemperatures suggest the species may not inhabit during the same time, which demands caution in stratification reconstructions based on these two species.