1. Wet and dry deposition fluxes of major ions and trace elements to the South Atlantic
Rosie Chance, Alex Baker, Tim Jickells, Alina Marca
School of Environmental Sciences, University of East Anglia.

2. Overview Introduction Sampling and analytical methods Results
GEOTRACES objectives wrt. aerosol
Carcass Island
Sampling and analytical methods
Results
Trace metals
Macronutrients
Nitrate isotopes
Titanium in seawater
Summary and future work
Brief introduction to GEOTRACES – Sampling – methods – results – trace metal fluxes & nitrate and ammonium fluxes, wet vs dry – the bigger picture, how results fir into larger databases – summary – future work - acknowledgements

3. Introduction
GEOTRACES
GEOTRACES objectives
Constrain magnitude and spatial distribution of atmospheric flux of TEIs to the surface ocean.
Global database quantifying sources of micronutrients for use in models
Global-scale ground-truthing of aerosol deposition models
Establish range of fractional solubility of key TEIs
Already heard about in this session (??)
An International Study of the Marine Biogeochemical Cycles of trace Elements and their Isotopes
SCOR (Scientific Committee on Ocean Research) UK contribution NERC funded, one large consortium bid and several smaller standard grants.
Trace elements and isotopes play important roles in the ocean as nutrients, as tracers of processes now and in the
past, and as contaminants.
e.g. iron is a limiting micronutrient, may have heard of iron fertilisation?
e.g. Copper can be toxic to phytoplankton
e.g. lead and mercury anthropogenic pollutants
Have a role in regulating the carbon cycle; sensitivity of these biogeochemical cycles to global change very poorly understood.
Atmospheric deposition is an important, but poorly quantified, mode of transport of low-solubility TEIs from the continents to the surface waters of the ocean. For micronutrients such as Fe, Zn and Co it may be the critical pathway for maintaining biologically necessary concentrations of these elements in surface waters of the open ocean.
Additionally, atmospheric transport is an important vector for transferring anthropogenic materials from the continents to the open ocean.
Atmospheric deposition is highly variable spatially and temporally, needs to be understood on physical scales that cover major hydrographic provinces, and temporal scales from seasonal to thousands of years.
Aim to make quantitative comparison of atmospheric deposition of TEIs to the surface ocean with other inputs such as freshwater run-off, marginal sediments, hydrothermal vents, in order to elucidate the relative importance of atmospheric deposition.
GEOTRACES recommended undertaking coarse resolution global surveys of ocean surface waters and atmospheric aerosols in the major ocean basins passing through the major mineral aerosol gradients. Natural temporal variability of atmospheric deposition can be exploited at time-series stations.
UK Contribution – focus on south Atlantic, transect plume arising from south America around 40degS

4. Why the south Atlantic? Baker (UEA) – Atlantic database Introduction
Patagonian dust
Why the south Atlantic?
Baker (UEA) – Atlantic database
Potential dust source:
Patagonian dust arises in the arid parts of southern South America, and is thought to be a significant source of micronutrients to the south Atlantic. Ice core records indicate large fluctuations in dust supply over glacial-interglacial timescales, yet very little is known about modern day fluxes.
One of the areas of interest identified in the GEOTRACES Science plan was the south Atlantic and Southern Ocean downwind of Patagonia, which is a high nutrient/low chlorophyll region with extremely low but variable mineral aerosol inputs. Much of my recent work sampling aerosol has focused on this area.
Scant coverage in existing databases, note in Prosperos network although there is a station in the Falklands no samples were collected there.

5. Field sampling Methods Sampling Three research cruises:
Detailed survey of eastern south Atlantic during 357 in October – November 2010
Full crossing of the Atlantic at 40deg S in early 2012 (JC68)
Tropical Atlantic cruise D361, high dust loading from Sahara
Time series station on Carcass island in the Falklands
Today I will compare results from Carcass island, D357 and for contrast D361.

6. Carcass Island: A new time series station
Methods
Sampling
Carcass Island: A new time series station
In operation:
Sept 2010 – April 2011
Oct 2011 – April 2012
Weekly bulk aerosol samples
The problem with dust events is they are episodic! Cruises can only give you a snap shot, so time series monitoring at a station or stations is very valuable.
Carcass island – western island, obliging locals, infrastructure = generator shed, relatively easy access.
High volume aerosol samples were collected weekly, September 2010 to April 2011 and October 2011 to April Five visits to Carcass Island were made, between which sampling was conducted by the local landowner.
Sampling was automatically controlled such that only air from a ‘clean marine’ sector was sampled.
During the second year of operation an ozone monitor and total deposition collector were added to the station.

7. Challenges…. Methods Sampling Johnny rooks / striated caracara
Total passive deposition sampler

8. Deposition flux = Catm • Vd
Methods
Extraction & Analysis
Sample analysis
Analyte
Method
Nitrate, sulphate, chloride
Ammonium
Ion chromatography
Nitrate isotopes
Bacterial denitrification + IRMS
Sodium, potassium, calcium, magnesium
ICP-OES
Trace metals:
Fe, Al, Mn, Zn, V, Ti, Co, Ni, Cu, Ag, Cd, Pb, Th
ICP-OES & ICP-MS
Total metals
INAA
Major ions:- aqueous extractions
“Soluble metals” – ammonium acetate extraction, pH 4.7
Several rarely measured metals e.g. Silver <LoD
Results are blank corrected
Calculate: atmospheric concentrations = (total amount on filter – blank)/air vol
Deposition flux calculated using deposition velocity – for size segregated samples, select Vd as function of particle size and wind speed, for bulk samples, use constant value, here chose 0.3 cm s-1 which is quite commonly used so results comparable.
Choice of Vd a significant source of uncertainty in flux calculations....
Deposition flux = Catm • Vd

9. Dry deposition of soluble trace metals
Results
Trace metals
Dry deposition of soluble trace metals
nmol m-2 day-1
<LoD
First – “dusty” elements
Boxes show median and quartiles for deposition of Soluble metals; whiskers show range of data. Note log scale
D357 and carcass both quite similar – very low – D361 samples at least an order of magnitude higher
Carcass in general slightly higher than D357
Caution – at least some samples <LoD, therefore lower range represents LoD and actual fluxes lower than this, plus LoD not necessarily the same for all cruiises as dependent on calibration used. Will rerun some samples, but anticipate still having some <LoD values.
Red boxes indicate all values <LoD for at least one campaign – annoyingly the ones you are probably interested in
Th data in particular almost all <LoD

10. Dry deposition of soluble trace metals
Results
Trace metals
Dry deposition of soluble trace metals
<LoD
nmol m-2 day-1
Second – anthropogenic elements
Many more samples <LoD
Less pronounced difference between the campaigns

11. Spatial distribution e.g. Al
GEOTRACES
Spatial distribution e.g. Al

12. Qualitative differences in aerosol chemical composition
Results
Principal component analysis
Qualitative differences in aerosol chemical composition

13. Results: Air mass origin
Back trajectories
Results: Air mass origin
CARCASS
D357
5 day back trajectories
Very similar air masses throughout out the year – westerly winds crossing southern Patagonia
During D357, all air sampled was remote marine in type, again westerly winds as expected
More variable air mass origins during tropics cruise, results shown here are for Saharan air masses
Note, no indications of patagonian dust event during Carcass yr 1
D361
5 day back trajectories calculated using NOAA HYSPLIT model

14. JC68 Back trajectories

15. GEOTRACES
Aerosol optical depth
D361
D357
JC68
Microtops data confirms expected trends.

16. Contribution of wet deposition
Results
Trace metals
Contribution of wet deposition
nmol m-2 day-1
CARCASS
D357
D361 Fe
wet
dry
ratio
967
948†
1.0
2109
243*
8.7
4741
3127
1.5 Al
2068
326†
6.3
1845 76
24.3
14101
13064
1.1 Mn 64
18†
3.6 83 7
11.9
233 70
3.3 Zn
223
16†
13.9
1785 8*
3104
54*
57.5
SOLUBILITY
Relative contribution of wet deposition tends to increase with solubility of element
Wet deposition is always at least equal to dry deposition, usually much larger
Relative contribution also depends on air mass type and origin etc –differences between the campaigns. Wet deposition appears to be most important for the very remote marine air sampled during D357. However, this might be a consequence of only a few short 9high concentration) rain events being sampled during the cruise, despite a relatively high climatological rainfall rate? Very small sample set makes it difficult to account for such things.
† Estimated from soluble fluxes * Based on max total metal concentration

17. Macronutrient deposition
Results
Macronutrients
Macronutrient deposition
 Wet deposition
Climatology data
(Baker et al., 2010).
μmol m-2 day-1
NH4+
NO3-
West
East
0.8
2.5
Compares to fluxes of and nitrate and ammonium respectively, estimated for western atlantic (Baker et al., 2010 – climatology)
Note highest value is for sample 09, which also has anomalous nss Ca – suspect/different, noted as very clean... Excluding this, max is 3.2, median 1.5.
These values are low for nitrate, and high for ammonium – why?
Conversely - during D357, values of 1.6 and 0.5 nitrate and ammonium respectively. In agreement to Baker et al climatology results for south east Atlantic (2.5 and 0.8 umol m-2 d-1)

18. Nitrate stable isotopes: Carcass Island
Results
Nitrate isotopes
Nitrate stable isotopes: Carcass Island
Negative delta 15 N nitrate, range 0 to nearly -8 ppt.
Within range observed for natural NO3 sources and/or open ocean signal. Positive values generally indicative of continental run off and polluted air.
Suggestion of links with nitrate concentrations and wind speed – as season progresses nitrate concs fall around Nov-December, wind speeds increase and minimum delta 15 N observed.

19. Nitrate stable isotopes: D357
Results
Nitrate isotopes
Nitrate stable isotopes: D357
Negative delta 15 N nitrate, range 0 to nearly -8 ppt.
Within range observed for natural NO3 sources and/or open ocean signal. Positive values generally indicative of continental run off and polluted air.
Suggestion of links with nitrate concentrations and wind speed – as season progresses nitrate concs fall around Nov-December, wind speeds increase and minimum delta 15 N observed.

20. Climatological approach
Results
The bigger picture
Climatological approach
Shipboard sampling = “snapshot”
Decoupled from water column
 Flux estimates from larger data sets, where available!
Aerosol databases:
GEOTRACES
COST Action 735
Plans for D357 data – combine with others from this region, estimate fluxes following climatological approach.
Baker et al., GBC.

21. Titanium in the water column (JC68)
Details in cruise report

22. Summary
Preliminary TM and nutrient fluxes available (wet, dry soluble and dry total)
Dry deposition fluxes very low in the south Atlantic
Wet deposition fluxes equal to or larger than dry deposition
Future work
Sample analysis:
JC68 cruise
Carcass Island yr 2, including volcanic ash
Links to water column data?
Data compilation/synthesis papers
East Atlantic
West Atlantic?

23. Acknowledgements
Thanks to all who have helped make the project possible, especially….
Rob and Lorraine McGill
Michael Clarke, Clive Wilkinson, Nigel Bishop, Arlette Betts and many others in the Falklands.
British Antarctic Survey
Officers, crew and science party of D357, D361 and JC68