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.

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

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

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.

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.

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 2012. 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.

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

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

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

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

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

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

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

JC68 Back trajectories

GEOTRACES Aerosol optical depth D361 D357 JC68 Microtops data confirms expected trends. http://aeronet.gsfc.nasa.gov/new_web/maritime_aerosol_network.html

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

Macronutrient deposition Results Macronutrients Macronutrient deposition  Wet deposition Climatology data (Baker et al., 2010). μmol m-2 day-1 NH4+ NO3- West 0.5-0.9 1.9-2.5 East 0.8 2.5 Compares to fluxes of 1.9 -2.5 and 0.5-0.9 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)

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.

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.

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., 2010. GBC.

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

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?

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