ESR DATING BENTHIC FORAMINIFERA FROM THE CEARá RISE, CENTRAL ATLANTIC

Foraminiferal analyses can provide paleonenvironmental data about changes in ocean chemistry, ocean circulation, and world climate, if they can be dated accurately. Reliable dating requires dates from at least two methods with different theoretical assumptions. Until now, foraminifera older than 50 ka could only be dated by ²³⁰Th/²³⁴U, providing that they contained enough U.

Electron spin resonance (ESR) dating can date calcite from planktonic forams, molluscs, and corals up to at least 200 ka in age, but ESR had not been tested on benthic forams. ESR dating uses a radiation-sensitive signal formed in the mineral that grows in response to sedimentary and internal radiation. A new protocol for ESR dating benthic forams was developed and tested using two samples of Nuttalides umbonifera and Cibicidoides wuellerstorfi specifically picked for ESR dating from Core EW9209-1JPC drilled in Pleistocene sediment on the Ceará Rise, central Atlantic. The core was drilled on northeastern slope of the Ceará Rise at 5ºN and 43ºW in 4056 m of water. Sediment from 36 slices from the core was tested for species abundance for Nuttalides umbonifera and Cibicidoides wullerstorfi. To obtain sufficient forams, slices 10 cm in thickness were picked for N. umbonifera and C. wullerstorfi and combined for analysis. Due to the small sample sizes, accumulated dose was calculated by ramping, in which successive irradiations from a ⁶⁰Co γ source for the aliquots are followed by ESR analysis at 5 mW power to obtain at least 10 points on the growth curve. Six associated bulk sediment samples (Table 1) were analyzed for U, Th, and K concentrations with NAA to assess the external dose rates. Both samples are still being ramped with more irradiation to improve the accuracy of accumulated dose calculation. With three points on the growth curve, BF1 (at 478-492 cm depth) yielded an accumulated dose of 117.8 ± 17.2 krad. With eight points on the growth curve, BF2 (at 560-570 cm) yielded an accumulated dose of 145.3 ± 9.0 krads, which corresponds to a date of 164 ± 20 ka. This age agrees well with the known ages for the sediment at this core depth. Although other tests will ascertain the reliability of this new application generally, this is the first successful test which uses ESR to date benthic foraminifera.