ZINCCARB · Zinc isotopes as tracers of nutrient cycling and carbon uptake by the past oceans

Final Activity Report Summary - ZINCCARB (Zinc isotopes as tracers of nutrient cycling and carbon uptake by the past oceans)

The growth of algae, i.e. phytoplankton, in marine surface waters is of major importance in the regulation of atmospheric CO2 because they incorporate carbon and the subsequent transport of dead biomass to deeper waters extracts CO2 out of the atmosphere. This process is known as the biological pump, and the availability and usage of the major nutrients, such as phosphorus (P) and nitrogen (N), is the dominant control on the strength of this pump in most oceanic areas. Iron (Fe) and zinc (Zn) are among the essential trace metal micronutrients for phytoplankton; these trace metals though are highly depleted in specific parts of the surface ocean that are replete in major nutrients, such as the Southern Ocean. These areas are the so-called high nutrient low chlorophyll (HNLC) zones. Thus, in these HNLC zones it is not the major nutrients that limit the phytoplankton growth, but the availability of the minor nutrients Fe and Zn. The proposed release of these HNLC zones from trace metal limitation, thereby increasing the strength of the biological pump and the drawdown of CO2 from the atmosphere, is one of the key hypotheses to explain lower atmospheric CO2 during glacial periods, as recorded in ice-cores. Nevertheless, this hypothesis has been difficult to prove due to a lack of robust methods to reconstruct the biological pump of the past. New techniques in isotope geochemistry have shown that phytoplankton organic material preferentially incorporates light Zn isotopes, which is expected to leave residual seawater Zn-isotopically heavy. The isotopic heaviness of the residual seawater Zn could potentially track the degree of trace metal depletion in the surface ocean. Replete Zn conditions would leave the seawater relatively unfractionated, whereas Zn isotopes would be progressively more fractionated, with heavier Zn isotopes corresponding to increasing Zn depletion. Thus, if a suitable archive of surface seawater Zn isotopes could be identified, it would be possible to track the degree of trace metal availability in HNLC zones in the past. We investigated the use of diatom opal, i.e. the skeleton of a particular type of marine algae which was dominantly made of silicon, as a record of the Zn isotopic composition of surface seawater. While the organic part of diatoms fractionated Zn isotopes the silicate skeleton, i.e. frustules, potentially incorporated Zn with the seawater Zn isotope composition. A fraction of these diatom frustules sank through the ocean, was deposited on the seafloor and could be sampled through ocean drilling expeditions. We also developed cleaning and measurement protocols for the reliable extraction of the Zn isotope compositions of diatom material and measured Zn isotopic compositions in cleaned diatom frustules from a sequence of core-top samples across the Southern Ocean. The core-top sample transect that we measured represented diatom material deposited during very recent ocean conditions and spanned samples with high diatom burial rates, from areas with relatively high Zn availability, to samples with progressively lower diatom burial rates and lower Zn availability. All the diatom frustules from the core-top samples exhibited Zn isotopic compositions heavier than the continental input of unfractionated Zn, as expected of surface waters in highly trace metal-depleted HNLC zones. Furthermore, the Zn isotope composition tracked decreasing diatom opal burial rates with progressively heavier Zn isotope compositions. These results convincingly suggested that Zn isotopes in diatom frustules could potentially serve as a record of past trace metal availability in HNLC zones and could offer a method to examine whether release from trace metal limitation in HNLC zones could explain the observed lower atmospheric CO2 during glacial periods.

Data: CORDIS, © European Union