# Chamber‐by‐Chamber Measurements of Planktonic Foraminiferal Mg, Sr, and Na to Ca Ratios With Femtosecond LA‐ICP‐MS

## TL;DR

This paper uses advanced laser technology to measure trace elements in foraminifera shells to better understand ancient ocean conditions.

## Contribution

A new method using femtosecond LA-ICP-MS is developed for precise chamber-by-chamber trace element analysis in foraminifera.

## Key findings

- Mg/Ca and Sr/Ca ratios were measured with better than 5% precision, suitable for paleoceanographic reconstructions.
- Oxidative cleaning with perchloric acid and hydrogen peroxide improved reproducibility and impurity removal.
- G. menardii showed a ~7°C chamber-to-chamber temperature difference, suggesting vertical migration in the ocean.

## Abstract

Distribution patterns of foraminifera are controlled by environmental parameters such as temperature, salinity, and nutrient concentrations in each water mass. Since trace elements to Ca ratios of marine microfossil calcite test of foraminifera record environmental and ecological habitat information, we used femtosecond (fs) LA‐ICP‐MS to obtain accurate chamber‐by‐chamber Mg/Ca, Sr/Ca, and Na/Ca of four foraminifera species to clarify the impact of foraminiferal depth migration on paleoceanographic reconstruction. The Mg/Ca and Sr/Ca ratios were measured with precision better than 5%, fulfilling the accuracy typically required for paleoceanographic reconstructions. We also examined the differences in element ratios due to the pretreatment cleaning methods for extracting accurate paleoceanographic information.

The fsLA‐ICP‐MS has the advantage of less matrix and instrumental element fractionation effects on elements with high condensation temperatures. We also applied the use of multiple carbonate standard materials for concentration standardization.

The fsLA‐ICP‐MS analysis was optimized using a spot size of 30 μm or larger with a laser repetition frequency of 5 to 15 Hz in a circular analytical trajectory. A comparison between ultrasonic and oxidative cleaning protocols revealed that oxidative test cleaning with perchloric acid and hydrogen peroxide achieved higher reproducibility and more efficient impurity removal compared to ultrasonic cleaning with ultrapure water and methanol. Repeated analysis on the same chambers of two species,
O. universa
and P. obliquiloculata, yielded mean relative standard deviations for Mg/Ca and Sr/Ca of < 5%.

A quantitative method was rapidly developed for determination of Mg, Sr, and Na to ratios of biogenic carbonates of foraminifera. T. sacculifer showed no chamber‐by‐chamber Mg/Ca variation in calcifying temperature, but average test Mg/Ca temperature decreased by 1.4°C with the addition of the final sac‐like chamber and final calcite layer. G. menardii showed a ~7°C difference among chambers suggesting upward migration in the shallow part of the thermocline.