INFLUENCE OF SALINITY ON PLANKTIC FORAMINIFERAL MG/CA AS DEDUCTED FROM A RECONSTRUCTION OF THE SHOALING OF THE ISTHMUS OF PANAMA

Samples from ODP Site 1000 from a waterdepth of 916 m were used to reconstruct a Mg/Ca-record for the Early Pliocene Caribbean Sea. Analyses were done on the planktonic foraminifer Globigerinoides sacculifer. By comparing Mg/Ca values and d¹⁸O stable isotopes of G.sacculifer from the same samples we conclude that Mg/Ca is not only recording the temperature history in the Caribbean but also displays a considerable salinity-effect due to the shoaling of the Isthmus of Panama. The interval prior to 4.6 Ma, when there still existed a free exchange of upper ocean watermasses between the Pacific and the Atlantic, shows Mg/Ca and d¹⁸O-records which are negatively correlated to each other, with SSTs varying between 26.5°C and 29°C. After 4.6 Ma, when the Panamanian sill shoaled to a waterdepth of less than 100 m, the Mg/Ca and d¹⁸O records show a positive correlation. Unrealistic cyclicities with Mg/Ca-temperature variations of up to 5°C and maximum temperatures over 34°C suggest an additional factor influencing the Mg/Ca-record. The positive correlation between Mg/Ca and d¹⁸O implies that maximum temperatures are accompanied by maximum salinities. Since increasing salinity is known to increase the Mg/Ca-ratio in foraminifers by 4-10%/1 psu (Nuernberg et al.’96, Lea et al.’99), we conclude that salinity changes of up to 4 psu have increased Mg/Ca-ratios by as much as 20-30% after 4.6 Ma. To determine the absolute salinity change during this interval, we extracted the temperature and ice-effect signals from the d¹⁸O-record. We used the 2-2.5°C cyclicity from prior to 5.6 Ma in the Mg/Ca-record to extract the temperature signal from the d¹⁸O-record. The benthic d¹⁸O-record from Site 846 was used as an initial approach to assess the ice-effect. The resulting Mg/Ca temperature reconstruction shows an increase in average SST after 4.6 Ma from 26.5°C to 28.5°C.