Is the Benguela Upwelling System pulling its weight
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 University of Cape Town 1
Three approaches: • Regional and annual scale. Flows of NO 3 -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 University of Cape Town 2
“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 University of Cape Town 3
Providing estimates for three of the variables gives a solution, algebraically, for each of the remaining three. n Southern Benguela Potential New Production (C). 5. 6 x 1013 g. C. y-1. Obtained from an upwelling index. SST v NO 3 – Satellite imagery – CTW activity. Amount of NO 3 -N available to EZ over 12 month periods converted to carbon (Redfield). n Local Potential New Production (B). 1. 7 x 1013 g. C. y-1. Obtained from the difference in [NO 3 -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. n Off-Shelf Export (D). 0. 2 x 1013 g. C. y-1. Obtained from satellite images of SST: Calculation of EZ NO 3 -N occurring in upwelling-derived water seaward of the 350 m isobath. University of Cape Town 4
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 D C E 3. 9 A 5. 4 F B 5. 6 1. 7 3. 7 University of Cape Town 5
Are these algebraically derived estimates realistic? ? ? C - Southern Benguela Potential New Production (5. 6 x 1013 g. C. y-1) 15 N 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 1013 g. C. y-1 Brown et al. (1991) estimated total primary Production for the southern Benguela: 7. 6 x 1013 g. C. 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) University of Cape Town 6
A - Imported New Production (3. 9 x 1013) Based on SACW [NO 3], the volume flux of SACW required to sustain this rate of new production is 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 production combine to invoke a mean daily rate of sinking of 1. 5 g. C. m-2 d-1. Bailey (1987) working with sediment traps obtained a range of 3. 7 - 4. 8 g. C. m-2 d-1 at or close to upwelling centres. University of Cape Town 7
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 University of Cape Town 8
The estimate of potential new production (5. 6 x 1013 g. C. 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 5. 6 Revised Excl. Winter 4. 2 From 15 N (Probyn and Waldron) 1. 4 x 1013 g. C. year-1. University of Cape Town 9
Schematic representation of DIC flux Monteiro: CMTT Book Chapter
Zones of concentrated wind-stress curl – upwelling centres Monteiro et al. , 2006, 2008
Atmosphere A 1 CO 2 CH 4 1. 7 101 106 B 1 B 2 C 1 117 B 3 D 1 6. 33 POC 1230 1. 04 Inner Shelf 1340 0. 52 6. 2 C 2 DOC D 3 1340 0. 52 NB 1340 T DIC flux 117 T upwells LS 1 Outer Shelf Monteiro: CMTT Book Chapter D 2 1230 LS 2 POC Slope DIC D 3 0. 52 Southern Benguela Carbon Fluxes
Atmosphere A 1 0. 9 CO 2 CH 4 1. 1 367 371 B 2 C 1 396 B 3 D 1 20. 1 POC 963 7. 24 Inner Shelf 1340 3. 62 1340 T DIC flux 396 T upwells DOC D 2 963 D 3 1340 3. 62 LS 1 Outer Shelf Monteiro: CMTT Book Chapter 6. 0 C 2 LS 2 POC Slope DIC D 3 0. 52 Northern Benguela Carbon Fluxes
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 g. C. year-1
Monteiro et al. , 2005: Continental Shelf Research Organic-rich bands of sediment inshore – material transported offshore
Belts of POC exporting offshore continually interacting with the physics: Internal tide and the double shelf break in the northern system.
Lateral Carbon Export from the Benguela: the context Sediment TOC (%)
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?
Research strategy and methods Biogeochemistry POC/N C & N stable isotopes DOC/N
Results: Evidence of POC enriched bottom nepheloid layers
Turbidity as a proxy for POC: developing a time-series All observations Individual cruises & depths
Turbidity time-series: evidence of cross shelf PM propagation
Turbidity time-series: evidence of cross shelf PM propagation Shelf break
Modelling cross-shelf POC advection POC(x) = POC(0) e - (λ / u). x
Quantifying the annual flux POC flux = (∆POC/∆x) × u 5. 9× 1010 g C. yr-1
DOC: Along isopycnal export from the Benguela system? DOC flux = (∆DOC/∆x) × u 8× 1011 g C. yr-1
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.
Carbon sequestration summary: Waldron: 70% of 4. 2 x 1013 g. C. year-1 = 2. 94 x 1013 g. C. year-1 Monteiro: 1 x 1012 g. C. year-1 Swart: 8. 59 x 1011 g. C. 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 g. C. year-1 Southern Benguela new production as % age of total coastal uw new production: Waldron (4. 2 x 1013 g. C. year-1) = 5. 3% Monteiro (2 x 1012 g. C. year-1) = 0. 25% Battle et al. (2000) - Total ocean’s ability to sequester carbon – 2. 6 x 10 15 g. C. year-1 Southern Benguela contribution: Waldron estimate: 1. 1% Monteiro estimate: 0. 04 % Swart estimate: 0. 03%
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