Stable Isotopic Study of Southern Ocean Surface Waters: An Update

By: R. Srivastava*, R. Ramesh** and S. M. Pednekar***
Surface water salinity (S) and δ18O measurements were repeated again in another cruise in 2009 CE after our first work in 2006 CE, to understand the temporal variability, if any, in the salinity- δ18O relationship and hence the surface water properties of the Southern Ocean. Data show more scatter in the δ18O values during 2009 CE relative to 2006 CE. Also a significant difference in the intercepts of the S- δ18O relations between 2009 CE and 2006 CE was detected.
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Introduction

The Southern Ocean, defined as the region between the south of 60°S and Antarctic, is an important region that affects the climate of the Earth. The main bottom and intermediate water masses of the world ocean originate here. Despite its close relationship with changing atmospheric conditions, the vertical and horizontal structures of the Southern Ocean, as a whole, have been studied only to a limited extent, although some detailed studies exist for some specific locations (Archambeau, et al., 1998, Aoki et al., 2003 and 2005). Stable isotopes of oxygen and hydrogen have been used as very reliable tracers for hydrological processes for a long (Dansgaard, 1964, Gat, 1996, Meredith et al., 1999, Paul et al., 1999). In the modern ocean they have been used as tracers for evaporation/precipitation, melting of sea ice, glacial and river runoff, deep ocean water masses, and deep-water formation processes. The isotopic compositions at various stages of the hydrological cycle help constrain different water masses as well as their movement. Several expeditions have been made earlier to explore the Indian Ocean as well as the Southern Ocean. But due to high spatial and temporal variability this region needs more studies to characterise its physical (such as temperature and salinity), chemical, and isotopic properties adequately. A combined study of stable hydrogen (δD) and oxygen (δ18O) isotopes and salinity can be ideal to monitor various processes happening in the oceans.

The data presented here were collected during the expedition to the Southern Ocean, Antarctic on board R/V ‘Akademik Boris Petrov’ (ABP 35) during February, March and April 2009. A vast area (from 8°N to 66°S and 45°E to 68°E) was sampled. Several ocean surface water samples have been collected at each degree interval of latitude. The longitudinal coverage is less in comparison to the latitudinal. The cruise track is shown in Figure 1.

 

 

Materials and Methods

Ocean surface water samples were collected with the help of a small, clean plastic bucket. This was stored in 100 ml plastic bottles with tight-fitting double caps to prevent evaporation. Salinity measurements were done onboard using an Autosal.

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An isotope ratio mass spectrometer (Geo 20-20) is used for isotopic analysis. We equilibrated the water with CO2 gas for δ18O analysis whereas for δD analysis we equilibrated water samples with H2 gas. Since H2 takes a lot of time for equilibration, platinum coated beads (known as Hoko Beads) are used as a catalyst. This equilibrated gas goes into the mass spectrometer for further isotopic analysis. The isotopic compositions are expressed in the conventional units defined by:

δ = [( Rsample/Rstandard) – 1] * 1000

 

Where R = 18O/16O or D/H. We used VSMOW as a reference. Standard deviation of these measurements is about ±0.15‰ (n=61) for δ18O whereas ±2‰(n=58) for δD.

 

 

Results and Discussion

A comparative study has been made for δ18O using previous data (Srivastava et al., 2007) to see any change in the past 3 years (2006-2009). Latitudinal variation of salinity and oxygen isotopic composition (δ18O) of ocean surface water for the year 2009 is shown in Figure 2.

Data pertaining to forward and return journeys are plotted using different symbols. Mostly, similar trends were observed during the forward and return journeys in salinity and δ18O. More fluctuations were observed in δ18O values than in salinity. Also fluctuations are more in δD than δ18O as is expected. There is a sharp decrease in salinity and δ18O values around 45°S which confirms the finding of Srivastava et al. (2007). More fluctuations in the δ18O surface water may be due to the strong turbulence which was observed during the expedition.

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A comparison of δ18O values of ocean surface water for 2006 and 2009 CE is shown in Figure 3. δ18O values of ocean surface water for 2006 and 2009 show similar trends with larger fluctuations during 2009. These data also show that δ18O values, on the average,  have shifted more towards the negative values.  The effect of global warming and ice melting shows up in these measurements clearly.

A scatter plot between δ18O and δD for 2006 and 2009 is presented in Figure 4. Again high scatter during 2009 makes regression coefficient poor (r2=0.48). Data demonstrates significant difference in the intercept between 2009 and 2006.  Slope of the regression is close to the slope of meteoric water line whereas intercept is quite low.  This is because more ice seems to have melted and mixed with sea water (Singh et al., 2011).

 

 

Conclusion

The present data confirm the major findings of the previous studies but the scatter seems to have increased, due to more melting in Antarctic. Further careful monitoring would be required for confirming this result.

 

 

Acknowledgements

We thank M. Tiwari for his critical comments.

 

 

References

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  8. Srivastava, R., Ramesh, R., Prakash, S., Anilkumar, N. and Sudhakar, M. (2007), ‘Oxygen isotope and salinity variations in the Indian sector of the Southern Ocean’, Geophysical Research Letters, 34: L24603, doi: 10.1029/2007GL031790.
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