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Fueling the ocean biological pump. Jorge Sarmiento & Jennifer Simeon Princeton University. Fueling the ocean biological pump. Introduction The problem and a hypothesis Support for the hypothesis From model simulations From observations (Si* and water mass analysis)
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Fueling the ocean biological pump Jorge Sarmiento & Jennifer Simeon Princeton University
Fueling the ocean biological pump • Introduction • The problem and a hypothesis • Support for the hypothesis • From model simulations • From observations (Si* and water mass analysis) • Gaining insights from model simulations • Implications • 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?
Hypo-thesis: The main return pathway for nutrients from the deep ocean is Subantarctic Mode Water (SAMW) (Sarmiento et al., Nature, 2004)
Primary evidence: Export production (Pg C yr-1 deg-1) Nutrient depletion south of 30°S “normal” 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)
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 sq = 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?`
Schematic of nutrient cycle in Southern Ocean -When diatoms have adequate light and nutrients, they tend to take up Si and nitrate in a ratio close to 1:1 -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.] -Hypothesis: iron or light stress in Southern Ocean leads to high Si to NO3 uptake ratio, which generates negative Si*
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 • Introduction • Gaining insights from model simulations • Tagged water type simulations • Tagged phosphate simulations • Implications • Future research
Tagging water types SAMW • 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. AAIW Northern Tropical
Fractional contribution of different water types to the main thermocline (sq < 27.4) SAMW AAIW Tropical North (LL model)
Fractional contribution of different water types to the main thermocline. Average above sq = 26.5 (LL model)
Three models were used: 2 1 0 Gnanadesikan (1999)
Meridional overturning (Sv) NADW HH NADW LL-low Kv NADW P2A-high wind
Transport in waters of sq < 27.4 P2A-high winds LL-low Kv Northward flow HH Southward flow
Fractional contributions of water types to the upper thermocline (sq< 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”? Pacific radiocarbon at 150°W (P16) favors P2A. LL has too low deep concentrations. HH has too low surface concentrations. Observations
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.
Phosphate partitioning in nutrient model Total Remin SAMW AAIW Tropical NAtl NPAC South (LL model)
Phosphate partitioning in LL model - average above 26.5 Total SAMW NPAC Remin Tropical
Net Phosphate flux through 26.5Red is positive (upwards)(mmol m-2 y-1) LL HH P2A
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 • Introduction • Gaining insights from model simulations • Implications • For global diatom production • For deep trapping of Si(OH)4 • For global warming response • Future research
Nitrate (mmol/kg) Silicic Acid (mmol/kg)
Regional analysis of results from Schlitzer adjoint model 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 • Introduction • Further insights from model simulations • Implications • Future research • Further work with water mass analysis (Y. Plancherel) • Pathways from thermocline to surface (J. Palter) • Return pathway of MOC: exploration of localized mixing mechanisms using 14C and 3He (D. Bianchi) • 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) Less transformation of light to intermediate waters north of Kerguelen. More intermediate water formation from dense water south of Australia. Net lightening essentially unchanged
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.
