The determination of stable O, C, N and Si isotopes is an important method for characterizing (paleo-) environmental changes and for the age classification of time series based on oxygen isotope stratigraphy, particularly in the Geosciences. On one hand, our analysis allows the determination of δ¹⁸O and δ¹³C on biogenic carbonates from marine sediment archives. On the other hand, we determine the δ¹⁸O values in ice and water samples from the Arctic and Antarctic, as well as the δ¹³C values of dissolved inorganic carbon in the water of the polar oceans. In addition, our laboratories offer the possibility of determining δ¹⁸O and δ³⁰Si in biogenic opal (diatoms; radiolarians, sponge needles). Currently we are setting up a method to analyze the δ¹³C and δ¹⁵N composition of marine sediment. 

The scientific topics we work on as a facility are interdisciplinary and include geological, geochemical, palaeoceanographic, biological and oceanographic questions on time scales from seasons to millions of years.

Our facility is equipped with the following systems:

The laboratories can be visited by prior arrangement. We are happy to answer any questions you may have. If you require analyses in our laboratories, please contact Prof. Dr. Ralf Tiedemann. Depending on capacity, the samples will be analyzed as quickly as possible.

Measuring principle

The O- and C-isotope ratios are not analyzed individually, but are derived from the isotopology of CO₂. Signal intensities of mass/charge ratios (m/z) 44, 45, 46 reflect the abundances in ¹²C¹⁶O¹⁶O, ¹³C¹⁶O¹⁶O and ¹²C¹⁸O¹⁶O. δ¹³C is calculated via the intensity ratios 45:44; δ¹⁸O via the ratio 46:44.

During the measurement, the sample is analyzed against a reference gas with a known isotopic composition. The results are calibrated to internationally recognized standards. Isotope ratios from carbonate samples are calibrated against the VPDB standard, a standard that is identical to the previously used PDB standard from the Cretaceous Pee Dee Belemnite Formation in South Carolina. Water samples are measured against V-SMOW (Vienna Standard Mean Ocean Water). We have our own well-reproducible in-house standards for both measurement methods.

Measurement of δ¹³C + δ¹⁸O in biogenic carbonates (Rooms: Building D-1330, D-1520)

Two isotope mass spectrometers (IRMS) (one MAT253 / one MAT253plus) are available for the measurement of δ¹³C and δ¹⁸O in biogenic carbonates. They are both coupled to a Thermo Fisher Scientific Kiel IV carbonate extraction unit (Fig. 1) in order to purify the resulting CO₂ gas and automatically feed it into the IRMS after the addition of phosphoric acid.

Fig 1: left: MAT253 with Kiel - IV; right: Kiel - IV oven with sample vials and phosphoric acid.

Stable oxygen and carbon isotopes of biogenic carbonates are analyzed with an accuracy of δ¹³C < 0.04‰ and δ¹⁸O < 0.08‰ (Fig 2). For an analysis, 20 - 100 µg of carbonate are required. The pure analysis time of a sample carousel with 46 samples (12 x house standard + 34 samples) is 27.5 hours.

Fig 2: δ¹³C and δ¹⁸O analysis data of the SHK-2020 house standard for the MAT253 and MAT253 plus analyzer systems.

The MAT253 plus is equipped with additional detectors that also allows the analysis of the isotope abundance of mass 47 (¹⁶O¹³C¹⁸O). This provides the prerequisite for analyzing "clumped isotopes - Δ47" in carbonate samples, a method that we would like to use in the future.

Measurement of δ¹³C on inorganic dissolved carbon (DIC) in seawater (Room: Building D-1340)

The measurement of stable carbon isotopes of dissolved inorganic carbon (DIC) in waters from the polar seas is routinely carried out in our laboratories. This parameter is being determined via a Gasbench plus coupled to a DeltaQ from Thermo Fisher Scientific. This system is used to measure δ¹³C from DIC in water samples and carbonates (as a calibration standard) (accuracy < 0.08 ‰; Fig 4).

Fig 3: GasBench plus - DeltaQ from ThermoFisher Scientific.

Fig 4: δ¹³C analysis data of the house standard SHK-2020 as well as different control standards for the GasBench plus - DeltaQ analyzer system.

The autosampler of the Delta comprises 96 positions. Routinely 48 positions are used (9 calibration standards, 3 control standards, 36 samples). A sample volume of 1 mL is required for the individual analysis. After flushing the headspace with helium the sample is spiked with phosphoric acid. After a reaction time of 24 hours, the 48 sample vials are analyzed over a period of 12 hours.

δ¹⁸O of seawater (Room: Building D-1390)

The δ¹⁸O analysis of seawater samples, mainly from polar regions, is carried out using the Isoprime precsION with Dual Inlet and AquaPrep unit from Elementar (Fig. 5).

Fig 5: Precision + Dual Inlet and AquaPrep Unit from Elementar.

A SD of < 0.05 ‰ is achieved through routine analyses. The IA-RO63 standard from the iso-Analytical laboratory is analyzed to check the data quality (Fig. 6).

Fig 6: δ¹⁸O analysis data of the house standards Ocean and DML as well as data of the control standard IA-RO63. It can be seen that the SD of the DML house standard is higher, but it is still within the specification of the analysis system. The transparent background covers the mean ± SD (0.05 ‰).

The samples (200 µL per individual measurement) are equilibrated in a sample rack at a constant temperature and transferred to the Dual Inlet using an autosampler. The samples are analyzed after 9 hours equilibration time. A measurement sequence comprises 60 standards and 60 samples and requires a period of 36 hours.

δ¹⁸O and δ³⁰Si from Opal (Room: Building D-0010)

In this laboratory, combined oxygen (δ¹⁸O) and silicon isotope measurements (δ³⁰Si) are carried out on biogenic opal (diatoms, radiolarians, sponge needles) (Fig. 7). With the new method implemented at the AWI, changes in the water mass structure (e.g. surface water stratification due to meltwater input) and changes in the supply of nutrient silicon and its use in polar and subpolar ocean regions (e.g. Southern Ocean, North Pacific) can be reconstructed on geological time scales.

For the measurements, pure diatom-, radiolarian- or sponge needle opal is separated and enriched with the aid of complex purification steps. To remove the hydroxyl groups in the opal structures, approx. 2 mg of purified biogenopal is dehydrated at a temperature of 1100°C (Fig. 7c). The dehydrated samples are then vaporized in a sealed reaction chamber using a CO₂ laser. O₂ and SiF₄ are formed by the addition of BrF₅ gas (laser fluorination) (Fig 8). Measurements of δ¹⁷O and δ¹⁸O are carried out on the released O2 using a Sercon 2020 mass spectrometer. The SiF₄ gas is transferred into borosilicate glass tubes. The isotope ratio of δ²⁹Si and δ³⁰Si is then measured from the gas using a Finnigan MAT 252 mass spectrometer (for detailed descriptions see Chapligin et al. 2010, Maier et al. 2013, Abelmann et al. 2015).

This method, with which δ¹⁸O and δ³⁰Si can be measured on the same sample, is used in close collaboration with the isotope laboratory in Potsdam. Sample preparation and δ³⁰Si measurements are carried out in the opal isotope laboratory at AWI Bremerhaven (external precision δ³⁰Si: 0.12‰). A system for laser fluorination and measurement of δ¹⁸O is currently being put into operation (internal precision: δ¹⁷O < 0.1‰, δ¹⁸O < 0.03‰).

Fig 7: Scanning electron microscope images of (A) two diatom species and (B) two radiolarian species. (C) Schematic representation of the opal structure (SiO₂*nH2O) with an inner layer of tetrahedra (Si-O-Si compounds) and an outer aqueous layer (Si-OH compounds) (opal structure modified after Perry, 1983). Since the aqueous "outer layer" may contain exchangeable oxygen, dehydration must take place before isotope analysis. Compiled by Edith Maier/AWI.

Fig 8: Flowchart of the instrumentation setup for combined oxygen and silicon isotope analyses.

δ¹³C and δ¹⁵N of organic material from marine sediments

The EA IsoLink CN - MAT253 mass spectrometer enables the determination of δ¹³C and δ¹⁵N in the organic matter of marine sediments. The δ¹³C and δ¹⁵N values can be used to estimate the proportion of terrigenous deposited versus marine-formed organic matter. In addition, the δ¹⁵N values of the organic matter also provide information on the efficiency of the biological pump if terrigenous input can be excluded. The δ¹³C and δ¹⁵N values of organisms, on the other hand, characterize the sources of nitrogen and carbon uptake and thus allow conclusions to be drawn about their nutrition. The external precision is better than 0.1 ‰ for δ¹³C and better than < 0.15 ‰ for δ¹⁵N. The autosampler can be equipped with 96 positions.