Climate change talk: Application of boron isotopes using LA-MC-ICP-MS for pH
Jan Fietzke (GEOMAR)
Both ocean warming and acidification caused by the antropogenic emission of carbon dioxide (CO2)are the major challenges to marine ecosystems. In particular marine calcifiers are expecteded to be affected in their ability to produce skeletal hard parts when facing an environment with continuously decreasing calcium carbonate (CaCO3) saturation state.
To evaluate the performance and possible strategies of acclimatisation/adaptation of marine calcifying organisms the reconstruction of ambient seawater pH is essential. This reconstruction is in particular important for the last decades and centuries as it allows for a comparsion of natural and antropogenic pH variability.
Boron stable isotopes ( δ 11B) revealed from carbonates have been used as a proxy for seawater pH for many years. Using a recently published LA-MC-ICP-MS method  we analysed the spatial distribution of boron isotopes in different marine calcifying organisms:
- corals cultured in the lab at different pH
- bivalves from another pH culturing experiment
- a specimen of Clathomorphum nereostratum a long-lived crustose coralline alga that exhibits annual growth increments.
Culturing experiments are a common practice to provide the base for proxy calibration. In this particular case the pH treatment was chosen to calibrate the boron isotope proxy for pH in corals and bivalves. The corals showed a behaviour which compares well with published data from lab and field studies. The bivalves lower their internal pH releative to the ambient sea water.
Our data suggest, the δ 11B composition of shells is in accord with the internal pH. Nevertheless, bivalves are no suitable archives for sea water pH reconstruction. Crustose coralline algae have recently been introduced as climate (SST) recorders in high-latitude shallow-water habitats. The particular specimen used in this study was collected alieve off the coast of Attu island (Aleutian Islands) in summer 2004. With an annual growth of 400-500µm the 6cm long profile covers more than the entire 20th century.
Here we present the first high-resolution 2D-images of boron isotopes providing a precision and accuracy close to analytical bulk techniques for a spatial resolution of 100µm. The combination of electron microprobe elemental mappings and LA-MC-ICP-MS isotopic images now allows for a detailed reconstruction of two seawater key parameters: SST from Mg/Ca and pH from δ 11B, respectively.
Our data show that long-term pH-decrease is recorded in the skeleton of our sample specimen. Between 1900 and the δ boron isotopes indicate a drop of ~0.08(1) pH units which is in accord with the value expected from atmospheric CO2 time-series data. A seasonal cycle of pH-variability (up to 0.1 pH units) is recorded too with highest values during late spring/early summer. His is most likely a result of the CO2 consumption during the spring microalgal bloom. The latter is in agreement with instrumental data from a nearby location.
During the spring bloom in May a rapid pH increase of up to 0.15 units was measured.