Hg speciation along 40°S

Arne Bratkic, Mitja Vahcic, Milena HorvatJožef Stefan Institute

LjubljanaSlovenia

Overview

• Introduction:– goals and hypothesis

• What we measured– sample list

• How we measured– determination of Hg species

• Results• Some conclusions

Hg cycle in water and sources to water bodies

Sources of Hg to open ocean include river discharges, wet and dry atmospheric deposition, sediments and hydrothermal vents in geologically active areas. Except last, the rest can be heavily impacted by human activities.

Goals

• JC068 is first sampling (for Hg) in such detail in South Atlantic

• to determine Hg speciation in water column with high vertical resolution for use in Geotreaces and Global Mercury Observation System projects

• collect additional samples to further elucidate open ocean Hg cycle

Hypothesis

• are double MeHg peaks, as a result of primary production and mineralization of organic matter, observable?

• to detect Hg contamination of antarctic waters, due to Hg depletion events over Antarctica, ice melting and river transport

Sample listWater Onboard Laboratory

THg 380 126 (NF,F)

MeHg - 126 (NF,F)

DGM 357 -

DMeHg - 20

Sample THg MeHg

Sediments (1cm thick slices)

80 80

Biomass - -

Determination of Hg speciesSampling

Hg0, DMeHg

Isolation(bubbling / preconcentration)

Hg2+, MeHg

Sample preparation(filtration, stabilization, storage)

Isolation(distillation, extraction, oxydation/reduction)

Hg species separation(GC, HPLC)

Detection(AFS HP, AAS HP, ICP MS)

MeHg: LOD 0,75pg/L (3.5fM)

Derivatization(ethylation)DMeHg

Hg0

Results - THg

0.00 0.50 1.00 1.50 2.00 2.50 3.000

500

1000

1500

2000

2500

3000

3500

4000

Ship

Lab NF

THg [ng/L]

dept

h [m

]

10% error bars

• onboard measurements give better reproducibility and are more reliable

• contamination issues with inappropriately clean reagents, work area or during transport

• (vacuum packing is a good option)

portable Hg lab oratory on canadian icebreaker Amundsen

Results - THgaverage: 0.32 ng/Lstdev: 0.2 ng/L

red circles denote possible increase due to atmospheric deposition and DGM oxidation

there is no apparent trend throughout the water column and no correlations with measured nutrients

yellow circle may show increase due to Mid-Atlantic Ridge vulcanism

Comparison with other studies

• ratio MeHg : THg = <1 – 18%• 2-48pg/L MeHg (9-223fM)• 0.08-2.54 ng/L THg (0.37-11pM)

Mason et al., 2012

Results-MeHgred circle: MeHg conc. are higher from 1200m downward. This is possibly anthropogenic signal or a consequence of intensive haterotrophic degradation

red circles: peak in MeHg is suggesting association with particulate matter in bioactive layer

yellow circles: anthropogenic signal?

upwelling zone, source of MeHg?

MeHg is expected to be higher in two layers of intensive bioactivity

Heimburger et al., 2012

Results - MeHg

J A S O N D J F M A M J J A S O N D J F M A M J

2007 2008 2009

Heimbürger et al., 2010

J A S O N D J F M A M J J A S O N D J F M A M J

2007 2008 2009

500m

Open-ocean water column MeHg production coupled to Corg remineralization (Cossa et al., 2009; Sunderland et al., 2009)

MeHg is tightly coupled to food web basis structure (Heimbürger et al., 2010)

seasonality

Results - DGM

generally higher DGM values in UCDW and AABW than in NADWred circle: influence of MAR or tectonic processes in Argentinian basin?

surface waters are depleted of DGM, signifying its loss to the atmospherehigher values below photic zone may be consequence of heterotrophic presence activity

Results - DGM

DGM is highly correlated with nitrate, and also phosphate and silicate (r values 0.88, 0.89 and 0.92, respectively)Below 1000m is uniformly distributed through water masses and slightly increasing towards bottom.

values above 1000m

values below 1000m

values for DGM range from 60 to 100 pg/L in ocean water column and represent from 0.9% to 56% of THg

Results - DMeHg

• ranges from 0.09 to 8.28pg/L

• DMeHg values are higher below 1000m and highest in UCDW and above bottom

• represents up to 1,8% of THg

10 20 30 40 50 60 70 80 900

1000

2000

3000

4000

5000

6000

THGMeHgDMeHgDGM

Hg species concentration

dept

h [m

]

0.2 0.3 0.4 0.5 0.60

1000

2000

3000

4000

5000

6000

THg [ng/L]

Mason et al., 2012

Conclusions

• these are first vertical high resolution quality data for this oceanic region of Atlantic

• we did not observe significant patterns in THg• MeHg is produced within ocean water column

and is likely associated with particulate matter of biogenic origin

Conclusions

• surface waters are depleted in DGM but enriched in deep waters

• significant positive correlation between DGM and nutrients (r≤83) are very interesting

Future work

• THg and MeHg in sediments• determination of microbial community and

functional genes • comparison of data on nitrogen isotopes, He…

Thank you

• special thanks to Gideon for inviting us to the cruise!

• with many thanks to Vesna, Kristina and Joze for help with the measurements!