Transcript (to organic matter) in the “twilight zone”?

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.
Ocean Carbon Cycle
Pool units: 1015 gC
Flux units: 1015 gC/y
Intermediate
and deep ocean
38,100
(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)
Biological removal of carbon from the atmosphere
and storing it in the deep-sea and sediments
Simplified Biological Pump
CO2
N2
fixation of C, N
by phytoplankton
physical mixing
of DOC
respiration
grazing
excretion
aggregate
formation
egestion
Lateral
advection
break up
Base of euphotic zone
passive
sinking of
POC, PIC
decomposition
(bacteria)
consumption,
repackaging
(zooplankton)
Seabed
active
vertical
migration
respiration
excretion
OCTET
(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)
Organic Biomarkers as Diagenetic Indices
(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)
POC flux and major biochemical abundances –
Equatorial Pacific. How much OC could we account for?
2
Percent of Organic Carbon
POC Flux, mg/m d
0.01 1.0
0
100
Plankton
20
40
60
80
100
Amino Acid
105 m Trap
1000 m Trap
Uncharacterized
Lipid
>3500 m Trap
Sediment
Carbohydrate
(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)
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)
0
-1000
OC
25:3 HBI
37:2 alkenone
24-methylene cholesterol
C30-diol
12-hydroxy-C28 acid
-2000
-3000
(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)
Selectivity is related to i) molecular structure
and/or ii) matrix effects
C25 HBI alkenes
500 m Trap
highly unsaturated = highly reactive (diatom)
31
saturated = nonreactive (vascular plant))
27
Surface sediment
35
C25 HBI alkenes
(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 should 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?
Carbon fluxes and concentrations
behave differently
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%.
(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
Is POC partitioned
between a pool
that is accessible to unprotected/
labile
hydrolytic enzymes
(unprotected [surface
coatings?]) vs. a pool
that is inaccessible
(protected inside the
particle matrix) to the protected
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)
Which mineral ballast correlates most with POC flux?
(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)
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
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
(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.)
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.
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.
MedFlux 2005
“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.