Trace Metal Iron (Fe), an important element to measure at sea. Dr. Thato Nicholas Mtshali

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1 Trace Metal Iron (Fe), an important element to measure at sea Dr. Thato Nicholas Mtshali Southern Atlantic Ocean and Antarctic Seminar (Cape Town): 5 December 2017

2 A CSIR-led multidisciplinary and multi-institutional program aimed at understanding the carbon (CO 2 ) climate systems in the Southern Ocean.

3 We do: integrated Earth systems scale climate research in the Southern Hemisphere in order to answer climate-carbon-ecosystem questions at appropriate time & space scale. Platform integration: our novel research capabilities

4 SOCCO Why Autonomous Platforms are necessary to close temporal spatial scale gaps Q? Inter-annual

5 The domain we work in.. (south Atlantic sector of the Southern Ocean)

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8 Phytoplankton biomass (Chl-a as a proxy). Seasonal evolution of phytoplankton biomass is attributed to contrast in : Light limitation that vary seasonally Seasonal cycle of nutrients availability and changed in MLD. Winter PP Understanding Summer PP

9 Why trace metal Fe. Despite having ample amount of NO3, PO4 and SiO4, and varying PAR availablity PP is very low.

10 Why trace metal Fe. It has been shown that bioavailable Fe (dfe) is one of the limiting micronutrient for phytoplankton growth in the SO. Low supply, solubility in oxygenated ocean and its low concentration (< 1.0nM) Tested 13 Geo-engineering ocean Fe fertilization experiments. High Nutrient-Low Chlorophyll HNLC

11 Why trace metal Fe. The element plays an important role in different phytoplankton metabolic processes: Synthesis of Chlorophyll-a (blooms) Photosynthetic and electron transport chain Reduction of CO 2 and macronutrients NO 3 and PO 4 3- Light reaction Dark reaction

12 It is supplied (internally and externally) by multiple mechanisms that vary seasonally. Physical processes Boyd and Ellwood 2010.

13 Once Fe enters the Ocean from above.. Organic ligands ReDox Fe 3+ Fe 2+ Removal processes Biological uptake By phytoplankton Scavenging By sinking organic particles Detritus Scavenging

14 Fe and Climate. It plays a major role in facilitating Air/Sea CO 2 exchange between the ocean and the atmosphere through phytoplankton photosynthesis processes. Light reaction Dark reaction How climate change will alter the biogeochemical cycling of Fe remains an open question.

15 Winter measurements are essential in understanding Fe biogeochemistry over seasonal cycles in the Southern Ocean. Winter observation-improve BGCM parameter to cover full seasonal Fe-BGC. Winter Summer Tagliabue and Mtshali et al., (2012); Ellwood et al., (2008); Tagliabue et al., (2014)

16 SOCCO SOSCEx III (winter and summer) cruises PS1 and PS2 PS2 Bio2 Bio3 Bio4 PS1 Trace metal Fe stations (winter and summer) PS1 (42.7 S; 8.7 E) PS2 (45.0 S; 6.4 E) Bio2 (46.0 S; 5.2 E) Bio3 (50.0 S; 1.03 E) Bio4 (55.1 S; 0.00 ) Late winter July 2015 Early-summer December 2015 Mid-summer January 2016 Late-summer February 2016

17 Trace metal clean facilities

18 SOSCEx III Winter vs Summer dfe profiles * Mtshali et al., 2017 and 18 (two papers in progress)

19 Seasonal depletion of DFe in the summer subsurface reservoir in the SAZ: paper in progress T.N. Mtshali, N.R. van Horsten, S. Nicholson, A. Roychoudhury, S.J. Thomalla,, T.J. Ryan-Keogh, W. Joubert, E. Buciarelli, G. Sarthou, A. Tagliabue and P.M.S. Monteiro. SAZ PS1 (42.7S; 8.7E) a) b) c) Winter MLD ( 200m), Euphotic depth ( 75m) Nutrient-like profile A key feature of all the profiles was a distinct seasonal variability of dfe concentrations in the upper 200 m depth.

20 We divided the water column (0-200m) into two depth limits: 1. The euphotic zone (0-75m) - where biological activity is expected to maintain low surface DFe concentrations over the season. 2. The subsurface reservoir (75-200m) below the productive euphotic zone where no entrainment will add DFe. 3. This allows us to investigate seasonal changes in the DFe inventories.

21 Seasonal decline in dfe inventories within the depth limits (July February) Within summer season no significant differences between Dec - Jan, rapid decline from Jan Feb. Q: What control seasonal and temporal decline in inventories within the a) surface and b) subsurface reservoirs?

22 Seasonal PP and Fe requirements minimum in winter increases to maximum in January, followed by a decline in February (wc and MLD) 3.27 µmolfe:molc ratio: Strzepek et al., 2011/12

23 Surface decline: due to biological Fe uptake by phytoplankton Subsurface decline: scavenging as biomass builds up, adsorption of Fe onto detrital biological matter results in the removal/transport of dfe from the subsurface to the deep waters by settling of biogenic debris and consequently remineralized at depth. 1-D Biogeochemical model: Physical processes: summer storm driven mixing provides relief from phytoplankton Fe limitation through the entrainments beneath the productive layer (MLD) Nicholson et al., Conclusion: Subsurface Fe reservoir is important to drive summer PP

24 Thank you Any questions??

25 GoodHope SOSCEX Southern Ocean Seasonal Cycle Experiment 2 Ocean Sections (GO-SHIP / GEOTRACES) Winter 2018 Spring Summer Process study locations 3 Combined Glider deployments Sustained long term observations CO2 (SOCAT) Themes Air-Sea Fluxes Biological Carbon Pump Decadal changes in ocean interior Physics to Predators Sea-Ice heat fluxes and engineering