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From Landscapes to Oceans Ocean margins and the global (and North American) carbon cycle

From Landscapes to Oceans Ocean margins and the global (and North American) carbon cycle Burke Hales

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From Landscapes to Oceans Ocean margins and the global (and North American) carbon cycle

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  1. From Landscapes to Oceans Ocean margins and the global (and North American) carbon cycle Burke Hales Wei-Jun Cai, Greg Mitchell, Chris Sabine, Oscar Schofield, Jim Bauer, Francisco Chavez, Jackie Grebmeier, Rick Jahnke, Val Klump, Steve Lohrenz, Nick Bates, C-T. Arthur Chen, Miguel Goni, Niki Gruber, Brent McKee, Clare Reimers, Taro Takahashi, and NACM workshop* participants. *Draft report available at ftp.oce.orst.edu in pub/hales/nacm via ftp with anonymous login

  2. The current atmospheric CO2 increase is driven by small net imbalances in large gross fluxes Sabine et al., 2004 Despite the small gross C fluxes in the small area of the ocean margins, the net fluxes may be significant.

  3. Ocean margins are the ‘bridge’ between terrestrial landscapes and the pelagic environment where: Terrestrial carbon is delivered, deposited, decomposed, Most organic carbon input to ocean interior occurs, Autotrophic/heterotrophic balance of the pelagic ocean is determined. Terrestrial Environment Pelagic Oceans Coastal Oceans Estuaries Surface Ekman transport Air-sea exchange Rivers Tidal exchange NCM Net Community Metabolism ‘Mixing’ Particle export Upwelling Sediment burial Deep water formation Groundwater Ocean Margins

  4. primary productivity, Where disproportionate shares of global ocean: Biomass, (Behrenfeld and Falkowski, 1997) or export productivity, not to mention fisheries, are found (Muller-Karger et al., 2005)

  5. But coastal systems are highly variable! Whole-ocean dynamic ranges are seen in short (<days, <km) time, space scales! Hales et al., 2005 Friederich et al., 2005 And coastal systems can be significantly affected by longer time-scale forcings (e.g., ENSO, PDO)

  6. months hours days years decades Time Climate change Tides Weather, Events Seasons ENSO NAO/PDO Cai et al. 2006 With the type of variability seen in ocean margins… …extrapolation using sparse datasets is risky! Surface mixed, Bottom boundary layers Estuaries, Plumes Fronts, jets Shelf widths, Eddies PDO Sediments Space meters 100 km 1 km 10 km 1000 km

  7. Margin air-sea CO2 flux study epitomizes shortcomings in our understanding. Tellus B 51 1999 Is there a continental shelf pump for the absorption of atmospheric CO2? By SHIZUO TSUNOGAI, SHUICHI WATANABE and TETSURO SATO In the last 10-15 years, numerous publications have argued for Pg-scale annual air-sea CO2 fluxes from the coastal oceans. They just can’t agree on the sign (e.g. Ducklow and McAllister, 2004; Smith and Hollibaugh, 1993).

  8. High variability and undersampling make even the best attempts to synthesize available data risky. Cai et al. 2006 Borges, 2005 “Province”-based extrapolations from regional studies to global flux estimates

  9. But isn’t there better coverage in North American margins? LDEO (MBARI, OSU, AOML, UGA, UMT,…) databases contain ~106 coastal surface pCO2 measurements Monthly-mean fluxes were calculated for each 1° x 1° pixels within ~3° from the coastline Integrating fluxes from ‘coastal’ pixels, the bottom line, in Tg C yr-1: Total: +2 ± 35 Mexico: +45 ± 14 US: -21 ± 18 Canada: -22 ± 27 Takahashi et al. SOCCR CH. 15

  10. Is the net flux really nearly zero? Maybe. But this result is sensitively dependent on near-cancellation of large sources and large sinks, which occur in EXTREMELY low sample-density regions.

  11. The margin air-sea CO2 flux may be the least of our concerns… What about off-shelf export to the deep ocean? Hales et al. 2006 suggested a margin-ocean interior POC export from the OR coast that was 10x larger than their 2005 estimate of (large) air-sea CO2 uptake. Model results for the Pacific show an air-sea CO2 flux of equivocal sign, but a persistent, large POC export flux: From Gruber et al. unpubl; ROMS 0.5° resolution with FGM

  12. Significant cross-shelf C exchange has been suggested for at least two decades (sensu Walsh, 1988); why is this still uncertain? Direct measurement impossible? Multiplicity of modes: Offshore surface water transport--Usually contains lower C than compensating onshore flow Lateral exchange-- C gradients wrong sign Deep water formation-- rare Particle export-- extremely difficult to measure Where exactly is the coastal/pelagic boundary?

  13. The margin air-sea CO2 flux may be the least of our concerns… What about delivery of terrestrial C? 7-8 North American Riverine TOC Flux Estimates by Region (Tg C y-1) 5-10 2-2.5 TOTAL: 21-30 1-1.5 2-2.5 Sources: Mulholland and Watts (1982), Meybeck (1982), Meybeck (1993), Ludwig et al (1996), Aitkenhead and McDowell (2000), and references therein 4-5

  14. Maybe there is some uncertainty about ‘details’ of terrestrial C flux, but why are there still big picture uncertainties? Discharge from rivers and delivery to the ocean are not necessarily equal. Rivers discharge high OC fluxes to their estuaries. What is actually delivered to the coastal ocean through the estuary mouth? How is the small net delivery flux distinguished from large gross tidal fluxes? Estuarine processing? Net diagenesis of marine and terrestrial C in tidal regions is not well understood. Where exactly is the estuary mouth? Mississippi (Tarbert Landing) 495 km from coast McKee, 2005

  15. The margin air-sea CO2 flux may be the least of our concerns… What about exchange through the sea floor? Groundwater Most preservation of C in marine sediments (burial) occurs in margin sediments, especially deltas Gross fluxes are relatively low, but potentially longer timescales for net fluxes Multiple significant modes of exchange across the sediment-water interface Pelagic benthic study could focus on 1-D processes, i.e. steady diffusion, deposition, & burial in fine-grained sediments. Margin benthic study must consider a variety of transport processes in hetero-geneous sediments in multiple dimensions From Reimers et al., 2004

  16. What about exchange through the sea floor? Where exactly is the sediment water interface? Greatly complicated by the dynamic nature of the coastal seafloor itself. Changes in input functions (sediment delivery, biological productivity) and deposition conditions (wave, current energy) can dramatically alter the seafloor. SeaWiFS Images of Hurricane Floyd, September 15 and 16, 1999 (James G. Acker, NASA)

  17. The margin air-sea CO2 flux may be the least of our concerns… What about net community metabolism? Globally, less than 10% of terrestrial OC discharged by rivers can be accounted for in coastal sediments. It’s probably not hiding in sediments someplace else. Is the remainder respired? Close juxtaposition of extreme redox states, mineral interfaces may make for super-efficient degradation of OC (Aller, 1998). Respiration of terrestrial OC was a large motivation for Smith and Hollibaugh’s claim of margin CO2 emission. Respiration of allochthonous OC, even by abiotic processes (sensu Benner), can lead to net heterotrophy even in illuminated, high-nutrient waters. But sediment respiration is difficult to measure (given flux complications shown before), and water column C oxidation rate measurements are rare.

  18. What about net community metabolism? Can autotrophy be relegated to planktonic photosynthesis in surface waters? Probably not. Epifaunal benthic photosynthesis Macro-algae 2000 Nelson, unpublished Salt marshes Mangroves

  19. Summary Net C fluxes across key boundaries in the continental margins are largely unconstrained. While the air-sea CO2 flux has garnered much attention, there are others that are potentially larger and less well-constrained. These include the net transport of carbon across the coastal/pelagic, estuary/ocean, water-column/seafloor boundaries. Net reaction/transformation of C in the coastal margins is unquantified. Primary productivity, e.g., is nearly meaningless given the wide range of f-ratios, diversity of C sources, close-coupling of sediments and water column in this environment. Many estimates of the above are based on imbalances in incomplete budgets.

  20. Summary These uncertainties are due in large part to high amplitude variability over a wide range of spatial and temporal scales, to diversity of processes driving this variability, and to issues regarding definitions of relevant space and time frames. Observational and synthetic capabilities based on traditional approaches to pelagic research probably need to be re-thought. The “integrationists” (the top-downers) and the “constructionists” (the bottom-uppers) have to coordinate at the earliest stages.

  21. NACP and OCCC implementation strategies recognized need for margin studies; recommended a synthesis and planning workshop: NORTH AMERICAN CONTINENTAL MARGINS (NACM) THE OCCC/NACP COASTAL CO2 WORKSHOP Boulder, CO; Sept. 21-23, 2005 Lead Organizer: Burke Hales (OSU) Organizing Committee: Wei-Jun Cai, Greg Mitchell, Chris Sabine, Oscar Schofield 50 participants 37 scientific inst. 3 gov. agencies 6 countries Draft report available now via ftp at: ftp.oce.orst.edu in pub/hales/nacm -- use anonymous login

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