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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