Experiment 2
200 0
Experiment 3
100 30
Experiment 1
0.6 1.9
W1 W2 W3 W4 W5 W6 W7
0 2 4 6 8 10 12 14 16 18 20
Basins
NPSTANDARD
Surface N:P ratios
Nitrogen dynamics during the Cenomanian-Turonian oceanic Anoxic Event 2:
A model study for the proto-North Atlantic
I. Ruvalcaba Baroni
1, I. Tsandev, and C. P. Slomp
Utrecht University, Faculty of Geosciences, The Netherlands
1. Introduction
Evidence from sediment core records and model studies suggest that increased nutrient supply played a key role in the initiation of the Cenomanian-Turonian oceanic anoxic event 2 (OAE2; 94 Ma). However, the relative roles of nitrogen (N) and phosphorus (P) availability in controlling primary productivity during the event are not fully understood. Low nitro- gen-isotope ratios of OAE2 sediments in the proto-North Atlantic suggest an important role for both N2-fixation [1] and recycling of ammonium (NH4+) [2] in supplying N for primary producers.
2. Aim
1) Assess the effect of changes in ocean oxygen, P availability and circulation on N dynamics in the proto-North Atlantic during OAE2
2) Identify the spatial trends in denitrification and N2-fixation 3) Compare the role of upwelling of NH4+ to N2-fixation
3. Model
We expand an existing multi-box model of the coupled cy- cles of water (fig. 1) , P, carbon and oxygen in the proto- North Atlantic [3] with the marine N cycle. The model in- cludes both the coastal and open ocean. All initial rates are defined for pre-OAE2 conditions. Key processes for N are:
1) N uptake for phytoplankton growth
2) Degradation of particulate organic N (PON) 3) Conversion of NH4+ to nitrate (nitrification) 4) Export and burial of PON
5) Denitrification (sediment and water column)
6) N2-fixation
5. Results and Discussion
4. Numerical experiments STANDARD (for OAE2):
- Enhanced P supply by weathering (0.03 Tmol y-1)
- P (1.4 Tmol y-1) and oxygen input ([O2]=10 μmol L-1) from the Pacific Ocean
- Meridional ocean overturning of 12 Sv (70% the original)
Experiment 1: STANDARD + change in P input of Pacif- ic bottom waters (from 0.6 to 1.9 Tmol y-1).
Experiment 2: STANDARD + change in oxygen concen- trations of Pacific bottom waters (from 0 to 200 μmol L-1 ).
Experiment 3: STANDARD + change in ocean circula- tion (from 100 to 30% of the original model)
Fig. 4: N:P ratios in surface waters
While in pre-OAE2 conditions, all N:P ratios are close to Redfield
(N:P=16:1; horizontal white line), in all experiments for OAE2, N:P ra- tios are lower than Redfield. This makes N the limiting nutrient for primary productivity. During OAE2, the ratios show a strong spatial variability due to local differences in transport, sources and sinks of nutrients. The lowest ratios are found in the southern coast (W5) where euxinia develops. Highest ratios are observed in the central open ocean (W1).
Utrecht University
Fig. 5: N dynamics
N dynamics are significantly affected by in- puts of P and oxygen from the Pacific Ocean and by ocean circulation. The increase in P supply during OAE2 leads to a N-deficit that favours N2-fixation (especially in W7s).
There is a strong relationship between anox- ia, N2-fixation, denitrification and upwelling of NH4+. The strong regional variability in nu- trient availability and oxygen concentra- tions in the proto-North Atlantic leads to large spatial differences in N dynamics.
Fig. 2: Oxygen distribution
Oxygen concentrations in the proto-North Atlantic are higly sensitive to input of oxygen and P from the Pacific Ocean and to ocean circulation. Anoxia in the central open bottom waters (W1b), as expected from obser- vations, can be obtained only by further increasing the P supply (or reducing the oxygen concentrations) from the Pacific Ocean relative to the STANDARD run.
Fig. 3: Relative increase in burial of POC
The geological record suggests at least a doubling of particu- late organic carbon (POC) burial from pre OAE2 to OAE2 con- ditions (dashed grey line) in the proto-North Atlantic. This can be somewhat reproduced in the STANDARD run. However, burial of POC is particularly sensitive to increases in P supply from the Pacific Ocean. An input of at least 1.7 Tmol P y-1 leads to a doubling of POC burial in all coastal basins. This suggests that P supply was a major forcing for anoxia in the proto- North Atlantic during OAE2. In our model, high burial of POC is possible because of high N2-fixation rates which compensate for N-losses due to denitrification.
Fig. 6: OAE Scenario
During OAE2, large P inputs enhanced not only primary productivity, but also N2- fixation. Total N2-fixation in the proto-North Atlantic could have been as high as mod- ern global rates (10-24 Tmol y-1). The N sourc- es and sinks are similar in magnitude to upwelled NH4+. There is a strong spatial variability in N fluxes.
While high N2-fixation rates are widespread, denitrification rates are most important in the central open ocean (W1), the Western Inte- rior (W3) and the northern coast (W4). NH4+ contributes to primary productivity mainly in upwelling regions and is highest in the south- ern proto-North Atlantic, where euxinia de- velops.
upwelling upwelling
6. Conclusions
Our model confirms that upwelling of NH4+ is important in supplying N to surface waters during OAE2 with rates that are of the same order of magnitude as N2-fixation. N dynamics in the proto-Atlantic are highly sensitive to changes in the strength of the circulation and changes in oxygen and P supply from the Pacific Ocean.
References:
[1] Kuypers, M. M., van Breugel, Y., Schouten, S., Erba, E., and Sinninghe Damste, J. S.: N2-fixing cyanobacteria supplied nutrient N for Cretaceous oceanic anoxic events, Geology, 32, 853–856, 2004.
[2] Higgins, M. B., Wolfe-Simon, F., Robinson, R. S., Qin, Y., Saito, M. A., and Pearson, A.: Paleoenvironmental implications of taxonomic variation among 15N values of chloropigments, Geochimica et Cosmochimica Acta, 75, 7351–7363, 2011.
[3] Ruvalcaba-Baroni, I. Topper, R. P. M., van Helmond, N. A. G. M., Brinkhuis, H. and Slomp, C. P.: Was the North Atlantic Ocean well-ventilated during Oceanic Anoxic Event 2 in the mid-Cretaceous?, Biogeosciences Discussions, 10, 13 231–13 276
[4] Topper, R. P. M., Trabucho Alexandre, J., Tuenter, E., and Meijer, P. Th.: A regional ocean circulation model for the mid-Cretaceous North Atlantic Basin:
implications for black shale formation, Climate of the past, 7, 277–297, 2011.
1
I.RuvalcabaBaroni@uu.nl
c) a)
b)
OAE2
Tethys Gateway Western
Interior
Pacific Ocean
central Open Ocean
proto-North Atlantic (OAE2)
W5 W2i W2b W1b W3 W4 Anoxic\Euxinic
W6 W1i W7
Low oxygenated\Anoxic Oxic Uncertain
From OAE2 observations
Fig. 1: Model configuration
a) Model water cycle during OAE2: The proto-North Atlantic is divided into 7 boxes based on the locations of upwelling/down- welling regions and bathymetric features [4].
b) Bathymetry of the proto-North Atlantic during OAE2 [4].
c) Simplified trend in oxygen as de- duced from geological records and as cap- tured by the original box-model (boxe index:
i=intermediate, b=bottom).
Experiment 1 Experiment 2 Experiment 3
Input of P from Pacific (Tmol y −1 )
Basins 1.2 x modern [SRP]
STANDARD
W1i W1b W2i W2b W3 W4 W5 W6 W7
0.6 0.8 1 1.2 1.4 1.6 1.8
Basins NPSTANDARD
[O 2] from Pacific (µmol L−1 )
STANDARD
W1i W1b W2i W2b W30 W4 W5 W6 W7 20
40 60 80 100 120 140 160 180 200
Basins
[O2] µmol L−1
STANDARD
Ocean Circulation (%)
W1i W1b W2i W2b W3 W4 W5 W6 W7 30
40 50 60 70 80 90 100
0 20 40 60 80 100
Experiment 1 Experiment 2 Experiment 3 120
Acknowledgements: This research was funded by a ``Focus & Massa project´´ granted to C. P. Slomp and H. Brinkhuis by Utrecht University and by the European Research Council under the European Communitys Seventh Framework Program, ERC Starting Grant #278364. Additional financial support was provided by Statoil.
OAE2 Scenario
N budget for the proto-North Atlantic
Tmol N y−1
10 N 40 No
o
W1
0 5 10 15
In Out N recycle PON burial
0 10 20 30 40
Tmol N y−1
NH4+ up.
NO3− up.
Lateral transport River Burial N -fixation2 Denitrification
(water)
Denitrification
(sediments)
10
5 W3
W2 W4
W5
W6 W7
0 30 TOTAL Tmol y−1
Basins NH4+ upwelled
W2 W3 W4 W5 W7
0 0.01 0.02 0.03 0.04 0.05
Tmol y−1
TOTAL 0 30
Input of P from Pacific (Tmol y−1 )
Basins N2−fixation
W1s W2s W3s W3b W4s W4b W5s W5b W6s W6b W7s W7b 0.6
0.8 1 1.2 1.4 1.6 1.8
Experiment 1
Basins Denitrification
W1i W1b W2i W2b W3 W4 W5 W6 W7 Input of P from Pacific (Tmol y −1 )
0.6 0.8 1 1.2 1.4 1.6 1.8
10 x mol m y−1 −3 −1 mol m−3 y−1
0 30
TOTAL Tmol y−1
Experiment 1
OAE2 scenario
OAE2 scenario Mainly water column
denitrification
Mainly sediment denitrification
(it stops in anoxic conditions)
Both sediment and water column
denitrification
POC burial OAE2 / POC burial pre−OAE2
0 2 4 6 8 10
12 Experiment 1: SRP from Pacific
0 2 4 6 8 10
12 Experiment 2: [O2] from the Pacific
−1
W1 W2 W3 W4 W5 W6 W7
0 2 4 6 8 10 12
Basins
Experiment 3: Ocean circulation
0.6 Tmol P y STANDARD
µmol L µmol L
1.7 Tmol P y 1.9 Tmol P y
-1 -1
2000 −1
100 % 30 %
Experiment 1
Experiment 2
Experiment 3
