3.2 OCEANOGRAPHY AND TRACER MEASUREMENTS
3.2.2 Observation of stable noble gas isotopes and transient tracers (CFCs) glacial melt water fractions and volumes and quantify the composition of newly
formed WSBW at their sources. We will use the transient tracer (chlorofluorocarbon, CFC) measurements to determine the time scales of residence and circulation to quantify the formation rates of glacial melt water (basal melting rates) and of WSBW formation rates. We will use CFC data to estimate the anthropogenic carbon content and uptake in the recently formed WSBW. We will also incorporate available historic tracer data to assess possible temporal variability of glacial melting, WSBW composition, formation, and anthropogenic carbon uptake.
Approach and methods
The oceanic measurement of the low-solubility and stable noble gases helium (3He, 4He) and neon (Ne) provide a useful tool to identify and to quantify basal glacial melt water. Atmospheric air with a constant composition of these noble gases is trapped in the ice matrix during formation of the meteoric ice. Due to the enhanced hydrostatic pressure at the base of the shelf ice, these gases are completely dissolved in the water, when the ice is melting from below. This leads to an excess of D4He = 1060% and DNe = 770% in pure glacial melt water (the D stands for the noble gas excess over the air-water solubility equilibrium). Frontal or surface melt water would equilibrate quickly and not lead to any noble gas excess in the ocean water. With an accuracy of <0.5% for He measurements performed at the IUP Bremen, basal glacial melt water fractions of <0.05% are detectable. The
3He/4He isotope ratio provides additional information. In Antarctic shelf water the ratio is low in comparison to ratios in WDW (the WDW has a maximum in 3He/4He) and provide complementary information of the composition of WSBW. Finally, primordial helium (mantle helium with a far higher 3He/4He ratio, d3He ≈ 800%) enters the ocean from spreading regions of submarine ridge systems or other hydrothermal active sites like hydrothermal vents or submarine volcanoes.
The anthropogenic transient trace gases chlorofluorocarbons (11 and CFC-12) allow estimating the time scales of the renewal and ventilation of inner oceanic water mass transport. They enter the ocean by gas exchange with the atmosphere.
Since the evolution of these transient tracers in the ocean interior is determined on first order by their temporal increase in the atmosphere and subsequently by advection in the ocean interior, they allow quantifying the time scales of deep and bottom water transport and formation. In a higher order approach, using the so called Transit Time Distribution (TTD) method (or water mass age spectra), they allow determining the integrated advection and mixing time scale of a water mass.
These CFC and TTD method based time scales of ventilated water masses integrate residence, circulation, and transport and on the shelf, slope, and deep basin and allow determining water mass ventilation and formation rates. Combined CFC based time scales with noble gas and OMP based melt water inventories allow calculating basal glacial melting rates and the basal glacial melting induced WSBW formation rates.
Additionally, the CFCs and TTD method can be used to estimate the anthropogenic carbon content in WSBW by applying the CFC based TTDs to the well known atmospheric anthropogenic carbon history. That method is very reliable particularly in deep and bottom water and it is fully independent of carbon measurements and back calculating methods, which require additional geochemical observations or linear regression methods (which need carbon measurements from at least two different times of observations at the same location and which are not available in the area of our investigation).
Work at sea
We took 540 water samples for stable noble gas isotopes (3He, 4He, Ne) in copper tubes from 68 stations (profiles usually from bottom to surface; see black circles in Fig. 3.2.1.1b). For the transient tracers (chlorofluorocarbons, CFC-11, CFC-12), we took 1,080 samples on 94 stations (all full bottom-surface-profiles; see red dots in Fig. 3.2.2.1).
Fig. 3.2.2.1: Map of the Filchner Trough with noble gas and CFC stations during Polarstern PS82 (ANT-XXIX/9). Red dots are positions of combined noble gas and CFC
stations, green dots are positions of only CFC stations.
Oceanic water samples for helium isotopes and neon were stored from the CTD/
water bottle system into gas tight copper tubes, which are clamped of at both sides. The noble gas samples are analyzed later in the IUP Bremen noble gas mass spectrometry lab. The copper tube water samples are processed in a first step with an ultra high vacuum gas extraction system. Sample gases are transferred via water vapour into a glass ampoule kept at liquid nitrogen temperature. For analysis of the noble gas isotopes the glass ampoules are connected to a fully automated ultra high vacuum mass spectrometric system equipped with a two-stage cryogenic trap system. The system is regularly calibrated with atmospheric
3.2.2 Observation of stable noble gas isotopes and transient tracers (CFCs) air standards (reproducibility better ±0.2%). Also measurement of blanks and linearity are done.
For the transient tracers (CFC) water samples from the CTD/water bottle system were collected into 100 ml glass ampoules and are flame sealed after a CFC free headspace of pure nitrogen had been applied. The CFC samples are later analysed in the CFC-laboratory at the IUP Bremen. The determination of CFC concentration will be accomplished by purge and trap sample pre-treatment followed by gas chromatographic (GC) separation on a capillary column and electron capture detection (ECD). The amount of CFC degassing into the headspace will be accounted for during the measurement procedure in the lab. The system will be calibrated by analyzing several different volumes of a known standard gas. Additionally the blank of the system will be analyzed regularly.
All samples will be shipped home after the expedition and will be analysed in the UHB-IUP noble gas and CFC laboratories. The measurements are expected to be completed one year after arrival in our home lab in Bremen. A careful data quality check will be carried out then.
Expected results
The 3-dimensional spatial coverage with CFC and noble gas samples in the Filchner Trench region is higher than from previous expedition. If this spatially dense and almost synoptic station distribution is sufficient to estimate realistic basal melt rates from the Filchner Ice Shelf, i.e. if we have covered the total dense water that was formed at Filchner to integrate the total glacial melt water volume, can be judged only, when the final noble gas and CFC data, together with the final hydrographic data, are available. However, we expect that we have covered sufficiently the area, where recently formed WSBW is transferred on the shelf northward and down the slope into the deeper Weddell Basin and also possibly westward. This will allow us to estimate the glacial melt water fraction and its inventory in WSBW and to assess the formation and export rates of WSBW.
Data management
All our data will be made public on the PANGAEA Data Publisher for Earth &
Environmental Science as soon as we have them available (approx. one year after the cruise), carefully quality controlled, and published in a peer reviewed journal.
Our cooperation partners will receive the data as soon as the final data set is available.