Carbon Exchanges between the Continental Margins and the Open Ocean

Transcription

1 Carbon Exchanges between the Continental Margins and the Open Ocean Outline: 1. Introduction to problem 2. Example of how circulation can export carbon to open ocean 3. Example of how particle transport can export carbon 4. Example of fresh water pump 5. Challenges for constraining exchanges between continental margins and open ocean Colorado Springs, CO Jan. 23, 2007

2 Model variability is consistent with patterns observed with satellites John Ryan, MBARI

3 Recent compilation of EU fluxes by Borges et al. (2006) CO 2 fluxes on continental shelves may be as large as terrestrial biosphere but tend to be offset by estuary fluxes

4 Coastal Ocean Metabolic State: CO 2 Sink or Source? Net Autotrophic Net Heterotrophic CO 2 CO 2 Carbon Carbon Carbon Carbon OM OM Carbon Carbon Coastal Ocean CO 2 Sink Coastal Ocean CO 2 Source 1. transport of organic matter (OM) from land to the coastal oceans. 2. net transport of OM from the coastal ocean to open ocean ( continental shelf pump ). 3. Carbon lost to sediments 4. exchange of atmospheric CO 2 across the air/sea interface.

5 Rivers deliver about 0.45 PgC/yr of terrestrial POC and DOC to coastal oceans but only 0.1 PgC/yr is preserved in coastal sediments. If the margins are close to neutral in terms of air-sea exchange, then this material must be exported to the open ocean. Courtesy S. Lohrenz

6 Example 1 of continental shelf pump: circulation in North Sea This region is a moderate sink for atmospheric CO 2 : Calais, France CO 2 Algae Hamburg, Germany air sea CO 2 Algae Bergen, Norway 8.4*10 12 g C yr -1 Zooplankton Zooplankton shallow southern North Sea bacterial respiration CO 2 POC Atlantic Exchange deep northern North Sea South - North section through the North Sea Slides courtesy of H. Thomas

7 DIC observation on South - North section 0 o o N /1 W, St. 1e 0 o o 57 N /2.25 E, St Depth [m] 40 Depth [m] DIC [ μmol kg ] DIC [ μmol kg ] summer winter = removal by photosynthesis = extra CO by mineralisation 2 Slides courtesy of H. Thomas

8 Increase of specific DIC during transport through the North Sea Western Inflow North Sea Budget This region is a moderate sink for atmospheric CO 2 : 8.4*10 12 g C yr -1 Bozec et al., 2005a

9 Increase of specific DIC during transport through the North Sea Eastern Outflow North Sea Budget This region is a moderate sink for atmospheric CO 2 : 8.4*10 12 g C yr -1 Bozec et al., 2005a

10 Increase of specific DIC during transport through the North Sea Inflow vs. Outflow North Sea Budget This region is a moderate sink for atmospheric CO 2 : 8.4*10 12 g C yr -1 27% total C input Rivers 32% C input Baltic Sea 41% C input Total C input 31.1*10 12 g C yr -1 3% exported to sed. 97% exported to Atlantic Bozec et al., 2005a

11 Do we have a similar analog with the Gulf of Mexico and the Florida Current?

12 Example 2 of continental shelf pump: POC Budget for Oregon Coast Data set collected by B. Hales group includes all the information needed for constraint of O 2 and POC mass balances at Cape Perpetua 44º 13 N Supersucker! Conducted 2 survey cruises 75 days apart to assess budget changes

13 Budget term Estimation method Value (mmol m -3 Cross-shelf transport Along-shore transport Temporal change Consumption + burial in sediments Net production Budget imbalance Mean Ekman transport multiplied by surface POC at shelfbreak; POC in upwelled waters assumed = 0 Difference between mean cross-shelf integrated POC fluxes at CH and CP divided by distance between CH and CP Difference between 5/28 and 8/12 cross-shelf integrated POC at CP, divided by 75 days day -1 ) Uncertainty (mmol m -3 day -1 ) Uncertainty estimation method Hartnett and Devol (2003) ± % uncertainty Stoichiometric conversion of O 2 production with PQ = 1.5 POC Balance Summary Extreme ranges in observed surface and upwelled POC, extreme uncertainties in calibration, and uncertainty in Ekman transport Extreme ranges in observed alongshore POC flux, extreme uncertainties in calibration Extreme ranges in observed average POC at CP, accounting for short-term variability, and calibration uncertainties Uncertainties in P O2 propagated by uncertainty in PQ (1.3 Š PQ Š 1.5) Propagation of above uncertainties, accounting for correlated errors From B. Hales

14 May POC Balance Summary There must be a net export of particles August Bottom line: O 2 balance requires 1.5 mmol m -3 day -1 of OC production. From B. Hales If not exported, section-average POC in August should be >100 mmol m -3 greater than in May. Identified POC exports can not account for O 2 based POC production Where is the POC going?

15 Example 3 of cont. shelf pump: freshwater pump/cascade flows A. Chen noted that in East China Sea coastal productivity is higher than can be explained from river nutrients alone. Also, river materials were more extensively dispersed during strong river outflow and confined to upper water column. CDOM in the Mississippi River Delta These phenomena were attributed to large fresh water discharge to coastal region creating surface transport to open ocean and pulling intermediate depth open ocean water onto shelf the fresh water pump. Ivanov (2004) has suggested that cascade flows can transport Sv/100km of shelf edge water into the open ocean suggesting that shelf water residence times of ~ 6mo. Figure from S. Lohrenz

16 Significant cross-shelf C exchange has been suggested for at least two decades (sensu Walsh, 1988); why is this still uncertain? Multiplicity of modes that frequently present what appear to be contradictory evidence: Reimers et al., 2004, CBED Workshop Report

17 One possible way forward: Conduct large scale control volume budget analysis for the Gulf of Mexico. This approach requires a multidisciplinary program like NACP that can provide an integrated picture from dry land to open ocean. I look forward to discussions this week as we work to develop a more detailed strategy.

18 Thank you!