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Complementary Constraints from Carbon and Nitrogen Isotopes on the LGM Ocean’s Biological Pump
Andreas Schmittner and Christopher Somes College of Earth, Ocean, and Atmospheric Sciences
Oregon State University
δ13C
I’ll tell you about
circulation δ15N
I’ll tell you about iron
fertilization Δ14C
I’ll tell you about
ventilation
Funded by NSF’s Marine Geology and Geophysics Program
IPODS Workshop Aug. 27, 2016, Utrecht
Goals• Explore effects of changes in physics (e.g.
circulation, sea ice) and biology (e.g. iron fertilization) on LGM distributions of δ13C, δ15N
• Compare model solutions to reconstructions
• Was the LGM biological pump stronger?
Schmittner and Somes (2016) Paleoceanography, doi:10.1002/2015PA002905.
Detritus
DIC
PO4
NO3
O2
Photosynthesis
Fast Recycling
13C
Water ColumnDenitrification
Sinking
15N15N13C
Grazing
Mortality
Remineralization
Excretion
CO2O2
13C
N-Fixation
N2δ15NN2=0
BenthicDenitrification
(Fe)
εBD ! −4‰
εWCD ! −25‰
εP ! −5‰
εP
εP
εE = −6‰
εAS
CaCO3
δ13C ! 2‰13C
δ13CDIC ! 2‰
Phytoplankton
15N13C δ
13C ! −20‰
Diazotrophs
15N13C δ 13C ! −12‰
δ13C ! −20‰
Zooplankton
15N13C δ
13C ! −20‰
14CDecay
δ 13CCO2 ! −6.5‰
δ 13CDIC ! 0.5‰
DOM
15N13C δ
13C ! −20‰
εP ! −20‰
εNF ! −1‰δ 15NNO3 ! 9‰
δ 15NNO3 ! 5.5‰
δ 15N ! −1‰
δ 15N ! 4‰
δ 15N ! 7‰
δ 15N ! 5‰
δ 15N ! 6‰
MOBI Model of Ocean Biogeochemistry and Isotopes Somes et al. (2013, 2015) Schmittner et al. (2015)
Numerical Experiments• Pre-industrial Control (piCtrl) μmax = 0.6 d-1
• 6 idealized LGM experiments
• same physics: PMIP3-like boundary conditions (ice sheets, CO2), δ13CCO2 = -6.5‰
• different maximum phytoplankton growth rates μmax = 0.6 - 1.2 d-1 to mimic iron fertilization (LGM0.6, LGM0.7, … LGM1.2)
Simplifications
• No spatial differences of iron fertilization effects on growth rates
• No changes in winds or tides
• No sea level effects on benthic denitrification
• No interactive sediments (global ALK = const, DIC will be too low)
Results (Physics)
• ΔSAT = -4.3 K
• Sea ice cover increased by 70%
• AMOC weaker (11 Sv) and shallower than piCtrl (17 Sv)
• Deep 14C-ages increased by 600 years
Annan and Hargreaves (2013)
Roche et al. (2012)
Sarnthein et al. (2013) Skinner et al. (previous IPODS mtg)
briefly: not inconsistent with reconstructions
Global Mean Quantities
Global Mean Vertical Profiles (horizontally averaged)
DIC = Cpre + Corg + CCaCO3
δ13CDIC = δ13Cpre + δ13Corg
︷Biological
Pump
Corg=RC:OAOU
“Physics” ΔCorg = 280 Pg
“Iron Fertilization” ΔCorg = 400 Pg
ΔCorg vs Δδ13C R=-0.89
δ13C is a proxy of ΔCorg
ΔCorg vs Δδ13C R=-0.97
“Physics” shallower AMOC increases vertical gradient in Atlantic
“Iron Fertilization” further
increases vertical gradient in Atlantic
and decreases δ13C in Indian &
Pacific
δ13C
OxygenSimilar to Corg
“Physics” increased O2 in thermocline, smaller suboxic zones (green lines), decreased O2 in deep
ocean
“Iron Fertilization” decreases oxygen in lower
thermocline and deep ocean, increases suboxic
zones
Huge suboxic zone in North Pacific inconsistent with reconstructions (Jaccard
and Galbraith, 2011)
Nitrogen Cycle
“Physics” decreases water-
column denitrification and N-fixation, increases N
inventory
“Iron Fertilization” decreases surface nutrients, increases
WCD and NF, decreases N
inventory
“Physics” decreases δ15N
“Iron Fertilization” increases δ15N
δ15N
Comparison to Reconstructions
• δ15N reconstructions from Nitrogen Cycle in the Ocean Past and Present (NICOPP) compilation (Galbraith et al., 2013)
• δ13C reconstructions from Peterson et al. (2014)
R RMSE
0.33 2.1
0.34 1.8
0.21 2.9
0.45 1.3
0.21 2.4
0.21 3.2
R RMSE
R RMSE
0.75 1.140.63 1.48
R RMSE
0.78 0.770.78 0.76
σobs=0.52‰
Conclusions
• N and C isotopes provide complementary constraints on the biological pump
• Models with weaker and shallower AMOC and moderately increased iron fertilization fit observations best
• ΔCorg ~ 500 - 700 Pg
δ13C
I’ll tell you about Corg
δ15N
I’ll tell you about iron
fertilization
Ongoing and Future Work
• Effects on CO2 (Samar Khatiwala)
• Iron and Circulation (Juan Muglia)
• LGM Nitrogen Cycle (Somes et al., in prep for Frontiers in Marine Sciences)
• Early Deglaciation
Dank je !
🌷
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