Fueling the ocean

biological pumpJorge Sarmiento

& Jennifer SimeonPrinceton University

Fueling the ocean biological pump

I. IntroductionA. The problem and a hypothesisB. Support for the hypothesis

1. From model simulations2. From observations (Si* and water mass analysis)

II. Gaining insights from model simulations

III. ImplicationsIV. Future research

The Problem• Sediment traps suggest that ~one-third of

the particulate organic matter flux at 200 m continues past the base of the main thermocline (defined as = 26.8)

• If nitrate lost by the above particle sinking were not replaced, the thermocline nitrate would be depleted within ~50 years!

• QUERY: How do nutrients return from the deep ocean to the thermocline?

(Sarmiento et al., Nature, 2004)

Hypo-thesis:

The main return pathway for nutrients from the deep ocean is Subantarctic Mode Water (SAMW)

Support from model

simulations

RESULT: ~Three-quarters of biological production N of 30°S is controlled by nutrients fed in from the south. Most of the effect occurs in the density interval corresponding to SAMW and upper AAIW ( < 27.3; LL model; Marinov et al., Nature, 2006)

“normal”

Nutrient depletion south of 30°S

Primary evidence:

Export production

(Pg C yr-1 deg-1)

Support from Si*

Observations

SAMW forms in deep wintertime mixed layers in the Southern

Ocean spanning the Subantarctic

Front

Density increases from 26.5 to 27.1 in an eastward

circuit from W. Atlantic Ocean

(McCartney, 1977)

Fronts = STF (N&S), SAF, & PF

Zones = SAZ and PFZ

Southern Ocean Nitrate and Silicic Acid Distributions

•An unusual characteristic of the waters spanning the Subantarctic Front is their high nitrate and low silicic acid concentrations.

•We find that Si* = Si(OH)4 - NO3- is an excellent tracer of

these low silicic acid high nitrate surface waters

Si* and wintertime mixed layer depth

Note that low Si* region (blue on left) matches deep wintertime mixed layer where SAMW forms (red on right).

Si* on 26.8 (~SAMW) isopycnal shows global extent of the SAMW influence

•This isopycnal surface is at the depth of the NPIW (North Pacific Intermediate Water), which forms in the Sea of Okhotsk and "mixed water region" between the Kuroshio and Oyashio Currents. Tidal mixing may play a central role there.

There is nowhere else at the surface of the ocean where Si* is negative

But why is Si* so negative in this band?

`

-When stressed (e.g., by iron or light limitation), diatoms tend to build more silicified shells, leading to a Si to NO3 uptake ratio of 2:1 and higher [Hutchins and Bruland, 1998; Takeda, 1998.]

-When diatoms have adequate light and nutrients, they tend to take up Si and nitrate in a ratio close to 1:1

-Hypothesis: iron or light stress in Southern Ocean leads to high Si to NO3 uptake ratio, which generates negative Si*

Schematic of nutrient cycle in Southern Ocean

Support from water type

analysis

Plancherel et al. (pers. comm.)

water type analysis

• Si* on WOCE Indian Ocean I8S+I9N section (Western Indian Ocean)

• SAMW water type fraction (STMW water type above, AAIW water type below)

Fueling the ocean biological pump

I. Introduction

II. Gaining insights from model simulations

A. Tagged water type simulations

B. Tagged phosphate simulations

III. Implications

IV. Future research

Tagged water type simulations

SAMW

AAIW

Northern

Tropical

Tagging water types

•Dye tracers are used to determine the relative contribution of four water types (black) to the main thermocline (blue)

•Tracer is set to 1 in black area, set to 0 in white area, conserved in blue area.

Fractional contribution of different water types to the main thermocline ( < 27.4)

(LL model)

SAMW

North

Tropical

AAIW

Fractional contribution of different water types to the main thermocline. Average above = 26.5

(LL model)

“Typical” model

Kv = 0.6 cm2 s

-1 AI = 2000 m2 s-1

HH

Low vertical mixing model

Kv = 0.15 cm2 s

-1 AI = 1000 m2 s-1 LL-low Kv

High wind model

LL with ECMWF winds (higher over Southern Ocean), narrowed Drake Passage, higher surface S in Weddell & Ross Seas, 50 cm2 s-1 between top two layers, and 1.3 cm2 s-1 in Southern Ocean.

P2A-high wind

Three models were used:

⎟⎟⎠

⎞⎜⎜⎝

⎛−+=

S

y

Ix

s

Sxv

L

DAL

f

L

D

AKpDg

ρ

τ

ρε

δ 2

Gnanadesikan (1999)0

1

2

LL-low Kv

HH

P2A-high wind

Meridional overturnin

g (Sv)

NADW

NADW

NADW

Transport in waters of < 27.4

P2A-high winds

HH

LL-low KvNorthward flow

Southward flow

MODEL SAMW TROPICAL NPAC AAIW NATL HH 0.450 0.291 0.189 0.033 0.028

LL-low Kv 0.592 0.070 0.285 0.021 0.029

P2A-high wind 0.700 0.047 0.189 0.016 0.038

Fractional contributions of water types to the upper thermocline ( <

26.5) by different models

Annual, global average at Year 400

Simulating a strong SAMW influence requires low vertical mixing and high Southern Ocean winds

Which model is more “realistic”?

Observations

Pacific radiocarbon at 150°W (P16) favors P2A. LL has too low deep concentrations. HH has too low surface concentrations.

However, observational analyses favor low latitude upwelling (HH)

We plan to explore localized vertical mixing in regions of strong interactions between tides, internal waves, and rough topography as an alternative mechanism for low latitude upwelling.

Tagged phosphatesimulations

Phosphate partitioning in nutrient model

Total

SAMW

Tropical

NPAC South

NAtl

AAIW

Remin

(LL model)

Phosphate partitioning in LL model - average

above 26.5

Total

SAMW

Tropical

NPAC

Remin

Phosphate end-members(fractional contribution above 26.5)

Model new production: Contribution from each end-member

LL

HH

P2A

Net Phosphate

flux through 26.5

Red is positive

(upwards)

(mmol m-2 y-

1)

Conclusions(1) Fueling the biological pump

SAMW accounts directly for about 20% of biological production in the world ocean.

Indirectly (including remineralized production) SAMW and AAIW together account for more than two-thirds of biological production north of 30°S - most of this is due to SAMW.

The NPIW accounts for North Pacific nutrient return(2) Processes controlling the rate of SAMW formation

• Low interior vertical mixing shifts NADW return flow from low latitudes to Southern Ocean (and North Pacific)

• High Southern Ocean winds increase upwelling in Southern Ocean, shifting it away from North Pacific and tropics.

(3) Mechanisms & pathways by which SAMW enters the upper thermocline

• Primarily by advection along isopycnals from southeast corner of subtropical gyres followed by upwelling along boundaries

• Small amount of surface (Ekman) transport to north

Fueling the ocean biological pump

I. IntroductionII. Gaining insights from model

simulationsIII. Implications

A. For global diatom productionB. For deep trapping of Si(OH)4

C. For global warming response

IV. Future research

For global diatom

production

Silicic acid to nitrate supply ratio across 100 m

€

J opal

J organic nitrogen

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟=

Si(OH)4 100 −200 m− Si(OH)4 0 −100 m( )∑

NO3-

100 −200 m− NO3

-

0 −100 m( )∑

For deep trapping of

Si(OH)4

WOCE “Conveyor Belt” Sections

Nitratemol/kg)

Silicic Acidmol/kg)

Regional analysis of results from Schlitzer adjoint model

Preservation fraction enhancement: fi2000 m/133 m=fAverage2000m/133m⋅εipreservationwhere

fAverage2000m/133m=ΦTotal(2000m)ΦTotal(133m)εipreservation=Φi(2000m)Φi(133m)fAverage2000m/133m

Export production enhancement: fi133 m=fiArea⋅εiproductionwhere

fiArea=AreaiAreaTotalεiproduction=Φi(133m)ΦTotal(133m)fiArea

Φi(2000m)=ΦTotal(133m)⋅fi133m⋅fi2000m/133m

Sarmiento et al. (2007)

Opal flux analysis

Sarmiento et al. (2007)

Organic nitrogen flux analysis

Sarmiento et al. (2007)

Summary of implications

• The low silicic acid to relative to nitrate of SAMW represents a key factor determining Si limitation of diatoms in low latitudes.

• The high silicic acid relative to nitrate of the deep ocean is due to high export in the Southern Ocean, not to a globally distributed deep dissolution of opal.

• GFDL climate model shows modest response of the SAMW return path to global warming. Paleo-implications examined by Brzezinski et al. (2002) & Matsumoto et al. (2002).

Fueling the ocean biological pump

I. IntroductionII. Further insights from model simulationsIII. ImplicationsIV. Future research

A. Further work with water mass analysis (Y. Plancherel)

B. Pathways from thermocline to surface (J. Palter)C. Return pathway of MOC: exploration of localized

mixing mechanisms using 14C and 3He (D. Bianchi)D. Global warming response (A. Gnanadesikan, J.

Simeon, E. Galbraith)

Watermass budgeting in GFDL CM2.1 coupled climate

model

Net transformation into density class= Flow out on east

– Flow in on west (fixed at 80°E)– Flow in on North

Budget for years 300-320 of 1860 control south of 30°S

Formation of Mode-Intermediate waters from mixing light and dense waters (light waters predominate)

Dense to light transformation

Net lightening of dense water

Comparison with 1%/yr to 2X CO2 run (Control fluxes are bold lines)

More intermediate water formation from dense water south of Australia.

Net lightening essentially unchanged

Less transformation of light to intermediate waters north of Kerguelen.

Summary of global warming simulations

• Upwelling transformation of dense water essentially unchanged.

• Eventual fate of deep water (mode/intermediate vs. lighter waters) changes somewhat.

• Transformation in Southern Indian vs. Australia shows major shifts.