1. Direct Radiocarbon Analysis of Carbonates by Laser Ablation coupled with AMS
Christiane Yeman

2. Content
Introduction to Radiocarbon in carbonates and Radiocarbon Dating
Laser Ablation AMS
Setup and Improvements
Sampling Methods
Data Evaluation
Applications
Summary and Outlook

3. Radiocarbon (14C) in the environment
Hua et al, Radiocarbon, 2013
14C bomb pulse in the atmosphere from tree rings
Andrews et al., NRC RP, 2016
14C bomb pulse in the ocean from a coral
Welte, Phd thesis, 2015

4. Accelerator Mass Spectrometry (AMS) for 14C analysis
200kV Accelerator
Low Energy Magnet
High Energy Magnet
2 m
12C Faraday cup
14C/12C
Ion Source
Electrostatic
Deflector
14C detector
Synal et al., NIM, 2007
Introduction to Radiocarbon Dating

5. Conventional sample preparation for 14C measurements in carbonates
1. Microdrilling to extract samples
2. Add phosphoric acid to produce CO2
=> start graphitisation (3-4 hours)
Graphitisation
1 - 15mg of CaCO3
counts
(2 0/00 precision)
20 samples in 1 week
Conversion to CO2
mg of CaCO3
counts
(5 0/00 precision)
20 samples in 2.5 days
Ruff et al., Radiocarbon, 2007
Sample preparation is
Time consuming
Vulnerable to contamination
Introduction to Radiocarbon Dating

7. What is Laser Ablation? ArF Laser: Pulsed, 8ns 193nm 20 – 250Hz
CO2
CO2
CO2
ArF Laser:
Pulsed, 8ns
193nm
20 – 250Hz
12mJ max. energy
50% CO2 conversion

8. Laser Ablation AMS Setup
Cell window
He + CO2
Welte, PhD thesis, 2015
Problems:
Short lifetime of the cell window
Low fluences on the sample
Low CO2 production
Low ion currents

9. Improvements on the LA-setup
22.5°
He + CO2 to AMS
CO2
Original Setup
Improved Setup
Energy on sample
0.8 – 2 mJ
0.8 – 2.5 mJ
Spot size
110 x 680 µm2
75 x 140 µm2
Fluence on sample
J/cm2
7 – 19 J/cm2
Ion currents
3 – 6 µA
6 – 9 µA

10. LA-AMS setup at the Laboratory of Ion Beam Physics MICADAS Laser
LA setup
Laser
Sample Box
Sample Box
Ablation cell
Sample holder
Pressure sensor

11. Laser Sampling Methods
Layer scan - ablating within the growh layer
Survey scan - ablating along the growth axis
c) Precision scan -Zig Zag pattern within a growth layer with displacement dx along the growth axis
Welte, Phd thesis, 2015
Welte 2015

12. Data Evaluation Laser on Laser off v = 10 – 20 µm/s
Molecules substraction
Blank substraction
13C/12C mass fractionation
correction
Standard normalization
=> Assigning the 14C data to a location on the sample
v = 10 – 20 µm/s

13. Application 1: Arctica Islandica
Arctica Islandica RWL7 captured in 1988
First measurement with the improved setup
v=15µm/s , E=1,5mJ, spot size 75 x 140 µm2
analysis time 20min
- Single cycles
- 5 cycles weighted mean
(subsamples)
14 mm
14C bomb peak is revealed by LA-AMS measurement
Collaboration with Rob Witbaard, NIOZ

14. Application 2: Otolith of a Red Snapper How old is the fish?3 1 2
6.5 mm
1: one survey + two precision scans, precision 1.5-3%
2: two survey scans, precision 2-3%
3: one survey scan, precision 2-3%
spatial resolution of subsamples: 500 μm
Collaboration with Allen Andrews, NOAA Fisheries Hawaii

15. Application 2: Otolith of a Red Snapper How old is the fish?
The fish is older than 50 years.
Collaboration with Allen Andrews, NOAA Fisheries Hawaii

16. Summary: LA-AMS… Outlook
establish 14C profiles of carbonates in less than an hour
Spatial resolution down to 100µm
Ideal for small samples
Modified Setup:
smaller spotsizes and higher energies on the sample
=> higher fluences on the sample and higher CO2 production
=> higher ion currents and 14C counts lead to improved precision
Outlook
Otoliths, Corals, Stalagmites, Shells are waiting to be measured
Use LA-AMS for larger case studies
Explore applicability on organic materials

17. Thanks for your attention!
The LA-AMS TEAM
Marcus Christl, Lukas Wacker, Hans-Arno Synal, Caroline Welte
Bodo Hattendorf, Joachim Koch, Detlef Günther
Trace Element and Micro Analysis