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Is the Benguela Upwelling System pulling its weight in terms of global carbon sequestration into the deep ocean? Howard

Is the Benguela Upwelling System pulling its weight in terms of global carbon sequestration into the deep ocean? Howard Waldron, Pedro Monteiro and Neil Swart. Three approaches: Regional and annual scale. Flows of NO3-N converted to carbon.

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Is the Benguela Upwelling System pulling its weight in terms of global carbon sequestration into the deep ocean? Howard

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  1. Is the Benguela Upwelling System pulling its weight in terms of global carbon sequestration into the deep ocean? Howard Waldron, Pedro Monteiro and Neil Swart

  2. Three approaches: • Regional and annual scale. Flows of NO3-N converted to carbon. • Compartmentalization of space scale – transport of DIC, POC and DOC • Bottom nepheloid layer (POC) and importance of DOC Upwelling Index Gate Hypothesis Modeling x-shelf advection of POC

  3. “A” - Imported Potential New Production“B” - Local Potential New Production“C” - Southern Benguela Potential New Production“D” - Off-Shelf Export of Potential New Production“E” - Carbon Sink over Continental Shelf“F” - Long-Term Sequestration in Shelf Sediments

  4. Providing estimates for three of the variables gives a solution, algebraically, for each of the remaining three. • Southern Benguela Potential New Production (C). 5.6 x 1013gC.y-1. Obtained from an upwelling index. SST v NO3 – Satellite imagery – CTW activity. Amount of NO3-N available to EZ over 12 month periods converted to carbon (Redfield). • Local Potential New Production (B). 1.7 x 1013 gC.y-1. Obtained from the difference in [NO3-N] between SACW and water which upwells: 14 mmol.m-3 is enriched (on average) by 6 mmol.m-3 as it passes over the shelf. • Off-Shelf Export (D). 0.2 x 1013 gC.y-1. Obtained from satellite images of SST: Calculation of EZ NO3-N occurring in upwelling-derived water seaward of the 350m isobath.

  5. Since estimates have been provided for “C”, “B” and “D” -- Imported Potential New Production (A) =C - B = 3.9 x 1013 Shelf Sink (E) = C - D = 5.4 x 1013 Shelf Sequestration (F) = E - B = 3.7 x 1013 0.2 C D 5.6 E 1.7 B 3.9 F A 5.4 3.7

  6. Are these algebraically derived estimates realistic??? C - Southern Benguela Potential New Production (5.6 x 1013gC.y-1) 15N estimates of new production from southern Benguela research cruises (Probyn and Waldron). Typical uptake rates in new, mature and aged upwelled water applied to satellite-derived areal extent of such waters for an annual period (excluding winter): 1.4 x 1013gC.y-1 Brown et al. (1991) estimated total primary Production for the southern Benguela: 7.6 x 1013gC.y-1 Waldron (5.6) - a system-wide average f-ratio of 0.7 (Too high) Probyn and Waldron (1.4) – a system-wide f-ratio of 0.2 (More relistic)

  7. A - Imported New Production (3.9 x 1013)Based on SACW [NO3], the volume flux of SACW requiredto sustain this rate of new productionis between 1.1 and 1.5 Sv.Stramma & Peterson (1989) - Coastal Upwelling in the Benguela between 24 and 32 deg South = 2 Sv.E - Sink of Potential New Production over the Continental Shelf (5.4 x 1013)Dimensions of continental shelf and rate of potential new productioncombine to invoke a mean daily rate of sinking of 1.5 gC.m-2d-1.Bailey (1987) working with sediment traps obtained a rangeof 3.7 - 4.8 gC.m-2d-1 at or close to upwelling centres.

  8. F - Sequestration in continental shelf sediments (3.7 x 1013)Not easily tested because of a lack of independent sources of evidence.Organic carbon content of sediment and rate of deposition may not prove supportive.Swart (MSc) revisited the Shelf Edge Exchange Hypothesis (SEEP) but in the context of an upwelling system. SEEP

  9. The estimate of potential new production (5.6 x 1013 gC.year-1)probably represents a maximum.WHY?1. Potential New production assumes the total assimilation of all available nitrate.2. The upwelling proxy (dynamically driven sea level fluctuation) treated Winter in the same way as the rest of the year which was too simplistic. Revised estimate and comparison. Original Revised From 15N (Probyn and Waldron) Excl. Winter 5.6 4.2 1.4 x 1013 gC.year-1.

  10. Schematic representation of DIC flux Monteiro: CMTT Book Chapter

  11. Zones of concentrated wind-stress curl – upwelling centres Monteiro et al., 2006, 2008

  12. Atmosphere A1 CO2 1.7 CH4 101 106 B1 C1 D1 6.33 B2 POC 117 1230 C2 6.2 DOC B3 1.04 1340 D2 1230 Inner Shelf DIC 0.52 D3 1340 D3 0.52 POC NB 1340T DIC flux 117T upwells LS1 LS2 Outer Shelf Slope 0.52 Monteiro: CMTT Book Chapter Southern Benguela Carbon Fluxes

  13. Atmosphere A1 CO2 1.1 0.9 CH4 367 371 B1 C1 D1 20.1 B2 POC 396 963 C2 6.0 DOC B3 7.24 1340 D2 963 Inner Shelf DIC 3.62 D3 1340 D3 3.62 1340T DIC flux 396T upwells POC LS1 LS2 Outer Shelf Slope 0.52 Monteiro: CMTT Book Chapter Northern Benguela Carbon Fluxes

  14. Substantial import of DIC onto the continental shelf Small export of POC and DOC 99% of imported DIC is re-exported 1% constitutes an export flux Net sink for southern Benguela: 1012 gC.year-1

  15. Monteiro et al., 2005: Continental Shelf Research Organic-rich bands of sediment inshore – material transported offshore

  16. Belts of POC exporting offshore continually interacting with the physics: Internal tide and the double shelf break in the northern system.

  17. Lateral Carbon Export from the Benguela: the context Sediment TOC (%)

  18. Key research questions: • Is there a flux of organic carbon from the continental shelf of the Benguela to the deep ocean, and what is its magnitude?

  19. Research strategy and methods • Biogeochemistry • POC/N • C & N stable isotopes • DOC/N

  20. Results: Evidence of POC enriched bottom nepheloid layers

  21. Turbidity as a proxy for POC: developing a time-series Individual cruises & depths All observations

  22. Turbidity time-series: evidence of cross shelf PM propagation

  23. Turbidity time-series: evidence of cross shelf PM propagation Shelf break

  24. Modelling cross-shelf POC advection POC(x) = POC(0) e - (λ / u).x

  25. Quantifying the annual flux POC flux = (∆POC/∆x) × u 5.9×1010 g C.yr-1

  26. DOC: Along isopycnal export from the Benguela system? DOC flux = (∆DOC/∆x) × u 8×1011 g C.yr-1

  27. Swart’s conclusions on lateral carbon flux from the Benguela • Organic carbon rich BNLs perennial on the S.B. shelf • DOC concentrations are >> POC concentrations and lead to a 10 x greater off-shelf flux in the BNL. • Lateral carbon export in the BNL achieves about 25% of that required to make the S.B. system carbon neutral w.r.t. the atmosphere.

  28. Carbon sequestration summary: Waldron: 70% of 4.2 x 1013 gC.year-1 = 2.94 x 1013 gC.year-1 Monteiro: 1 x 1012 gC.year-1 Swart: 8.59 x 1011 gC.year-1 Benguela pulling its weight? Hutchins et al. (2000) quoted therein – Coastal upwelling areas: 0.5 – 1% of ocean surface Support 11% of total global new production i.e 7.9 x 1014 gC.year-1 Southern Benguela new production as % age of total coastal uw new production: Waldron (4.2 x 1013 gC.year-1) = 5.3% Monteiro (2 x 1012 gC.year-1) = 0.25% Battle et al. (2000) - Total ocean’s ability to sequester carbon – 2.6 x 1015 gC.year-1 Southern Benguela contribution: Waldron estimate: 1.1% Monteiro estimate: 0.04 % Swart estimate: 0.03%

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