The Flux of Particulate Material in the Ocean and New Insight into the Mechanisms that Drive it

S G Wakeham Skidaway Institute of Oceanography, 10 Ocean Science Circle, Savannah, GA 31411

C Lee, R A Armstrong and J K Cochran

Marine Sciences Research Center, Stony Brook University, Stony Brook, NY 11794

J-C Miquel International Atomic Energy Agency, Marine Environment Laboratory, Monaco

Chapman Conference on The Role of Marine Carbon and Calcite Fluxes in Driving Global Climate Change, Past and Present

Abstract

Particulate (organic and inorganic) matter (PM) produced in surface waters of the ocean is extensively degraded in the water column, largely in the Twilight Zone, with only a fraction reaching the sea floor to be preserved in the sediments. Yet the balance between the extent of organic matter degradation and sequestration of any surviving carbon in the deep ocean and sediments affects global carbon cycling and is the basis by which past oceanic conditions and global climate may be inferred from the sediment record. It is therefore critical to understand the fate of PM. Our analysis of JGOFS data shows that particulate organic carbon (POC) flux correlates with and may be predicted from the flux of mineral material (opal, carbonates, and dust). This implies that there must be strong physical relationships between organic matter and mineral ballast and between degradation of organic matter and dissolution of mineral material. The sinking dynamics of particulate matter through the water column depend on the relative ratios of lower density organic matter and higher density mineral ballast and on the mechanisms controlling the behavior of these two phases as they affect particle integrity. Our recent work has involved developing novel sampling and multitracer approaches to begin to characterize the in-situ sinking and degradation/dissolution behavior of PM.

Flux units: 1015 gC/y

Pool units: 1015 gC

Ocean Carbon Cycle

(from Doney, S.C. and D. Schimel 2002. Global change - The future and the greenhouse effect. Encyc. Life Sci., Macmillan Publ. Ltd., www.els.net)

Intermediate and deep ocean

38,100

Simplified Biological Pump

activevertical migration

fixation of C, Nby phytoplankton

grazing

egestion

passivesinking ofPOC, PIC

aggregate formation

respiration

excretion

excretion

respiration

physical mixingof DOC

consumption,repackaging

CO2N2

Seabed

Base of euphotic zone

decomposition

break up

(bacteria) (zooplankton)

Lateraladvection

OCTET

Biological removal of carbon from the atmosphere and storing it in the deep-sea and sediments

(from OCTET Report, 2000)

Martin Open Ocean Composite Curve

(Martin, J.H., G.A. Knauer, D.M. Karl, W.W. Broenkow. 1987. VERTEX: carbon cycling in the northeast Pacific. Deep-Sea Res. 34: 267-285)

Early (pre-JGOFS) organic geochemical flux studies (using very diverse sampling) show “exponentially”-decreasing

organic matter flux with increasing depth in the water column and suggested differing reactivities towards degradation

(Wakeham, S. G. and C. Lee. 1993. Production, transport, and alteration of particulate organic matter in the marine water column. In: Organic Geochemistry (M. Engel and S. Macko, eds.), Plenum Press, pp. 145‑169)

(Sheridan C.C., C. Lee, S.G. Wakeham, and J.K.B. Bishop. 2002. Suspended particle organic composition and cycling in surface and midwaters of the equatorial Pacific Ocean. Deep-Sea Res. I 49: 1983-2008)

Organic Biomarkers as Diagenetic Indices

Percent of Organic Carbon

105 m Trap

1000 m Trap

>3500 m Trap

Sediment

0 20 40 60 Plankton

Lipid

Carbohydrate

Uncharacterized

POC Flux, mg/m d

1.00.01 80 100

Amino Acid

100

2

POC flux and major biochemical abundances –Equatorial Pacific. How much OC could we account for?

(Wakeham, S. G., C. Lee, J. I. Hedges, P. J. Hernes and M. L. Peterson. 1997. Molecular indicators of diagenetic status in marine organic matter. Geochim. Cosmochim. Acta. 61: 5363-5369)

(Wakeham, S. G., M. L. Peterson, J. I. Hedges and C. Lee. 2002. Lipid biomarker fluxes in the Arabian Sea: with a comparison to the Equatorial Pacific Ocean. Deep-Sea Res. II. 49: 2265-2301)

This selectivity results from a wide range of apparent reactivities for individual compounds, where the most

reactive are poorly preserved, e.g., Arabian Sea.

Normalized Flux (% of flux at 500 m)

500 m Trap

Surface sediment

C25 HBI alkenes

C25 HBI alkenes

27

31

35

Selectivity is related to i) molecular structure and/or ii) matrix effects

highly unsaturated = highly reactive (diatom)

saturated = nonreactive (vascular plant)){(Wakeham, S. G., M. L. Peterson, J. I. Hedges and C. Lee. 2002. Lipid biomarker fluxes in the Arabian Sea: with a comparison to the Equatorial Pacific Ocean. Deep-Sea Res. II. 49: 2265-2301)

What’s next? We now know: i) fluxes decrease with depth; ii) compositions (may) change with depth; iii) degradation/preservation are selective. But…what happens (to organic

matter) in the “twilight zone”?

Upper Water Column

Twilight Zone

Deep Sea

OCTET

The Twilight Zone

“There is a fifth dimension beyond that which is known to man. It is a dimension as vast as space and as timeless as infinity. It is the middle ground between light and shadow, between science and superstition, and it lies between the pit of man's fears and the summit of his knowledge. This is the dimension of imagination. It is an area which we call the Twilight Zone.”

Rod Serling, The Twilight Zone, 1959

The mesopelagic zone, between ~100 - 800 meters, is sometimes referred to as the “Twilight Zone”, partly because it is the transition zone between depths that receive sunlight and those that do not, and partly because of the mystery behind many of the processes occurring there.

What shouldWhat should we know about sinking particles? we know about sinking particles?

Are ballast minerals a key to predicting carbon export?

What role does aggregation play in sinking?

Are ballast and aggregation equally important throughout the water column?

Do minerals physically protect a fraction of their associated total organic matter?

Organic carbon fluxes decrease with depth to varying degrees at different locations.

The percent of total mass made up by organic carbon reaches a constant value at depth, ~5%.

Carbon fluxes and concentrations behave differently

(Armstrong R. A., C. Lee, J.I. Hedges, S. Honjo and S.G.Wakeham. 2002. A new, mechanistic model for organic carbon fluxes in the ocean based on the quantitative association of POC with ballast minerals. Deep-Sea Res. II, 49: 219-236)

Model of Labile and Protected POC

unprotected/labile

protected

Is POC partitioned between a pool that is accessible to hydrolytic enzymes (unprotected [surface coatings?]) vs. a pool that is inaccessible (protected inside the particle matrix) to the same enzymes?

(Armstrong R. A., C. Lee, J.I. Hedges, S. Honjo and S.G.Wakeham. 2002. A new, mechanistic model for organic carbon fluxes in the ocean based on the quantitative association of POC with ballast minerals. Deep-Sea Res. II, 49: 219-236)

(Klaas C. and D.E Archer. 2002. Association of sinking organic matter with various types of mineral ballast in the deep sea: Implications for the rain ratio. Global Biogeochem. Cycles 16: Art. No. 1116)

Which mineral ballast correlates most with POC flux?

The next step---

The importance of mineral ballast on sinking and preservation

Organic matter will not sink without mineral ballast[ρOM ~0.9-1.05; ρcarbonate 2.3; ρsilica 2.5 g/cc]

OM-mineral aggregates can sink, but dissolution of the mineral ballast may slow the sinking, and/or degradation of an organic “glue”

may allow aggregates to disaggregate and slow sinking

MedFlux is testing the relationship between ballast and POM behavior

DYFAMED – French JGOFS site, 10 years data

Near-shore (52 km from Nice)

Deep water (2300 m)

Free of coastal influence

Seasonality in biological structure (seasonality in mineral ballast types)

Seasonality of POM fluxes

Close to Monaco’s IAEA lab

MedFlux

(Peterson, M.L., S.G. Wakeham, C. Lee, J.C. Miquel and M.A. Askea. 2005. Novel techniques for collection of sinking particles in the ocean and determining their settling rates. Limnol. Oceanogr. Methods, submitted.)

In 2003, mass flux peaked after the spring bloom and rapidly decreased with time at both 200 and 800 m.

We measured the percent organic carbon in the trap samples. The percent organic carbon is higher when

mass fluxes are lower.

MedFlux 2003

At 200 m, highest particle flux occurs at rates between 200-500 m/d.

Percent organic carbon is higher at lower settling velocities

MedFlux 2003

(Peterson, M.L., S.G. Wakeham, C. Lee, J.C. Miquel and M.A. Askea. 2005. Novel techniques for collection of sinking particles in the ocean and determining their settling rates. Limnol. Oceanogr. Methods, submitted.)

Principal components analysis (PCA) is a multivariate ordination technique that reduces the number of variables

in a data set by constructing “latent variables”, or axes, through which maximum variability in a data set is explained.

MedFlux 2005

Do particles sink faster at depth?

There is a suggestion that the settling velocity spectrum shifts with depth: particles collected at 1800 m have higher average settling velocities than particles at 200 and 400 m.

“Unified Ballast-Aggregation Theory of Export”

If mineralized plankton aggregate faster, then mineralized plankton would be preferentially exported from the euphotic zone, and aggregation could be considered as the first step in the association between carbon and minerals in sinking particles. Ballast may become more important at depth.

• This narrows the possible theories for preservation at depth.

• This makes the current acidification of the ocean even more worrisome as increasing acidification dissolves forams and coccolithophorids so that there might be less flux, less organic C export, and thus less CO2 permanently removed from the surface ocean.