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

The current atmospheric CO2 increase is driven by small net imbalances in large gross fluxes

Despite the small gross C fluxes in the small area of the ocean margins, the net fluxes may be significant.

Sabine et al., 2004

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.

Rivers

EstuariesCoastal Oceans

‘Mixing’

Upw

ellin

g

Dee

p w

ater

form

atio

n

Particleexport

Sediment burial

Pelagic Oceans Terrestrial Environment

Groundwater

Tidal exchange

Ocean Margins

Surface Ekman transport

Air-sea exchange

Net Community Metabolism

NCM

Biomass,

primary productivity,

or export productivity,

(Behrenfeld and Falkowski, 1997)

(Muller-Karger et al., 2005)

Where disproportionate shares of global ocean:

not to mention fisheries, are found

But coastal systems are highly variable! Whole-ocean dynamic ranges are seen in short (<days, <km) time, space scales!

And coastal systems can be significantly affected by longer time-scale forcings (e.g., ENSO, PDO)

Hales et al., 2005

Friederich et al., 2005

With the type of variability seen in ocean margins…

Cai et al. 2006

…extrapolation using sparse datasets is risky!

hours days months years decades

1 km 10 km 100 km 1000 km

Tides Weather,Events

Seasons ENSO NAO/PDO

meters

Climatechange

Surface mixed,Bottom boundarylayers

Estuaries,Plumes

Fronts,jets

Shelf widths,Eddies

PDOSediments

Space

Time

Tellus B 51 1999Is there a continental shelf pump for the absorption of

atmospheric CO2?

By SHIZUO TSUNOGAI, SHUICHI WATANABE and TETSURO SATO

Margin air-sea CO2 flux study epitomizes shortcomings in our understanding.

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).

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

Integrating fluxes from ‘coastal’ pixels, the bottom line, in Tg C yr-1:Total: +2 ± 35

Mexico: +45 ± 14US: -21 ± 18

Canada: -22 ± 27

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

But isn’t there better coverage in North American margins?

Takahashi et al. SOCCR CH. 15

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.

From Gruber et al. unpubl; ROMS 0.5° resolution with FGM

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:

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:

Deep water formation-- rare

Offshore surface water transport--Usually contains lower C than compensating onshore flow

Particle export-- extremely difficult to measure

Lateral exchange-- C gradients wrong sign

Where exactly is the coastal/pelagic boundary?

4-5

2-2.5

1-1.5

2-2.5

7-8

5-10

North American Riverine TOCFlux Estimates

by Region(Tg C y-1)

Sources:Mulholland and Watts (1982),Meybeck (1982),Meybeck (1993),Ludwig et al (1996),Aitkenhead and McDowell (2000),and references therein

TOTAL: 21-30

The margin air-sea CO2 flux may be the least of our concerns…What about delivery of terrestrial C?

Mississippi (Tarbert Landing) 495 km from coast

McKee, 2005

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?

From Reimers et al., 2004

The margin air-sea CO2 flux may be the least of our concerns…What about exchange through the sea floor?

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

Groundwater

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

SeaWiFS Images of Hurricane Floyd, September 15 and 16, 1999

(James G. Acker, NASA)

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.

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.

2000 Nelson, unpublished

What about net community metabolism?Can autotrophy be relegated to planktonic photosynthesis in surface waters?

Probably not.

Epifaunal benthic photosynthesis

Macro-algae

Mangroves

Salt marshes

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.

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.

Boulder, CO; Sept. 21-23, 2005

Lead Organizer: Burke Hales (OSU)

NORTH AMERICAN CONTINENTAL MARGINS (NACM) THE OCCC/NACP COASTAL CO2 WORKSHOP

6 countries

50 participants

37 scientific inst.

3 gov. agencies

Organizing Committee: Wei-Jun Cai, Greg

Mitchell, Chris Sabine, Oscar Schofield

NACP and OCCC implementation strategies recognized need for margin studies; recommended a synthesis and planning workshop:

Draft report available now via ftp at:

ftp.oce.orst.edu in pub/hales/nacm -- use anonymous login