Observations indicate an expansion of oxygen minimum zones (OMZs) over the
past 50 years, likely related to ongoing deoxygenation caused by reduced
oxygen solubility, changes in stratification and circulation, and a potential
acceleration of organic matter turnover in a warming climate. The overall
area of ocean sediments that are in direct contact with low-oxygen bottom
waters also increases with expanding OMZs. This leads to a release of
phosphorus from ocean sediments. If anthropogenic carbon dioxide emissions
continue unabated, higher temperatures will cause enhanced weathering on
land, which, in turn, will increase the phosphorus and alkalinity fluxes into
the ocean and therefore raise the ocean's phosphorus inventory even further.
A higher availability of phosphorus enhances biological production,
remineralisation and oxygen consumption, and might therefore lead to further
expansions of OMZs, representing a positive feedback. A negative feedback
arises from the enhanced productivity-induced drawdown of carbon and also
increased uptake of CO
Oxygen minimum zones (OMZs) have more than quadrupled over the past 50 years
and it has been suggested that this expansion is related to recent climate
change (Stramma et al., 2008; Schmidtko et al., 2017). However, current
CO
The major source of P for the ocean is river input (Filippelli, 2008; Payton and McLoughlin, 2007; Föllmi, 1996; Palastanga et al., 2011; Froelich et al., 1982), which is determined by terrestrial weathering of apatite (Filippelli, 2002; Föllmi, 1996). The main factors controlling terrestrial weathering are temperature, precipitation and vegetation. Higher temperatures are generally associated with enhanced precipitation and occur in many places with higher terrestrial net primary productivity (Monteiro et al., 2012), which all tend to increase weathering rates (Berner, 1991).
It is difficult to determine how much of the globally weathered P enters the
ocean in a bioavailable form. Today, about 0.09–0.15 Tmol a
After taking up the bioavailable P for photosynthetic production of biomass,
a large fraction of the newly produced organic matter is exported out of the
euphotic zone as detritus (6.42 Tmol P a
The processes of burial and release of P are redox dependent. Under oxic
conditions the burial rate is high, while under suboxic conditions the
benthic release of P is elevated (Ingall and Jahnke, 1994; Kraal et al.,
2012; Wallmann, 2010; Slomp and Van Cappellen, 2007; Floegel et al., 2011;
Lenton and Watson, 2000; Tsandev and Slomp, 2009). The redox-dependent
release of P into the water column and the decrease in marine oxygen due to
remineralisation therefore represent a positive feedback loop on marine
biological production (see Fig. 1). Although the feedbacks between ocean and
atmosphere are complex (Sabine et al., 2004), we assume that an enhanced
detritus export into the ocean interior results in an increased marine uptake
of atmospheric CO
Possible feedbacks in the global phosphorus cycle under climate warming conditions.
These redox-dependent benthic P fluxes have been investigated in a previous study with the HAMOCC global ocean biogeochemistry model by Palastanga et al. (2011). Palastanga et al. (2011) show that doubling the input of dissolved P from rivers results in an increased benthic release of P. This leads to a rise in primary production as well as in oxygen consumption, which in turn affects the oxygen availability in sediments. The benthic release of P acts therefore as a positive feedback on expanding oxygen minimum zones on timescales of 10 000 to 100 000 years (Palastanga et al., 2011).
Other studies on marine oxygen deficiency focused on the geological past, especially the mid-Cretaceous warm period (120–80 Ma) (Tsandev and Slomp, 2009; Handoh and Lenton, 2003; Bjerrum et al., 2006; Föllmi et al., 1996). Several periods of oceanic oxygen depletion have been inferred from sediment data of black shales (Schlanger and Jenkyns, 1976), for example, for the Cretaceous oceanic anoxic event 2 (OAE) at the Cenomanian–Turonian boundary (93.5 Myr). Whether processes such as surface warming, sea-level rise (Handoh and Lenton, 2003), and possibly a slow-down of the ocean overturning circulation and vertical mixing (Monteiro et al., 2012; Tsandev and Slomp, 2009; Ruvalcaba Baroni et al., 2014) – as assumed for the Cretaceous – will lead to widespread oxygen depletion in the future is a reason of concern. Consequently, a better understanding of biogeochemical processes associated with Cretaceous OAE might help assess the risk of possible future events of low marine oxygen concentrations (Tsandev and Slomp, 2009).
In contrast to previous studies that focus on the geological past, we
investigate possible future changes over the next 1000 years using an Earth
System Climate Model of intermediate complexity to investigate the feedbacks
between the P cycle and OMZs under the extended Representative Concentration
Pathways Scenario 8.5 (RCP8.5) of the Intergovernmental Panel on Climate
Change (IPCC) AR5 report. The RCP8.5 scenario is characterised by an
increase in atmospheric CO
The University of Victoria Earth System Climate model (UVic ESCM) version 2.9
(Weaver et al., 2001; Eby et al., 2009) is a model of intermediate
complexity and consists of a terrestrial model based on TRIFFID and MOSES
(Meissner et al., 2003) including weathering (Meissner et al., 2012), an
atmospheric energy–moisture balance model (Fanning and Weaver, 1996), a
CaCO
Global and
annual mean time series of
Earlier applications of the UVic ESCM assumed a fixed marine P inventory. We included a representation of the dynamic P cycle for this study. It consists of a modified terrestrial weathering module (Meissner et al., 2012) and a redox-sensitive transfer function for burial and benthic release of P (Floegel et al., 2011; Wallmann, 2010).
The continental weathering module developed earlier for fluxes of dissolved
inorganic carbon (DIC) and alkalinity (Meissner et al., 2012; Lenton and
Britton, 2006) is based on the following equations:
We added the following flux to account for P weathering (
The rain rate of POP (RR
Two model simulations were performed. Our control simulation, called simulation REF hereafter, includes neither weathering, benthic release nor burial of P. The global amount of P in the ocean is therefore conserved in this simulation over time. The second simulation, called WB, includes P weathering as well as benthic burial and release of P but excludes additional anthropogenic input. The spin-up was performed by computing the burial and benthic release according to Eq. (6). The weathering fluxes were set to a value to compensate the burial rate (Eq. 4) during the spin-up but not thereafter.
After a spin-up of 20 000 years under pre-industrial boundary conditions, we
forced the model with anthropogenic CO
The UVic ESCM has been validated under present-day and pre-industrial conditions in numerous studies (Eby et al., 2009; Weaver et al., 2001). In particular, Keller et al. (2012) recently compared results of its ocean biogeochemical component to observations and previous model formulations. We therefore concentrate our validation on the new model component in this study, the P cycle.
Estimates of pre-industrial burial rates vary over a wide range in the
literature. The comprehensive review by Slomp (2011) reported a burial rate
of 0.032–0.35 Tmol P a
To conserve marine P during long model spin-ups, the dissolved weathering
flux of P under pre-industrial conditions is set equal to the diagnosed total
burial rate during the spin-up: 0.38 Tmol P a
Global values for benthic release under pre-industrial conditions equal
0.78 Tmol P a
The global mean atmospheric surface temperature, as simulated by the WB run,
increases until year 2835 and peaks at 23.1
Given that the response in temperature is similar for both simulations compared to considerable differences in biological productivity (see below), differences in oxygen concentration mainly originate from biogeochemical changes, which will be discussed in Sect. 3.3.
The weathering rate (see Fig. 3b) and associated flux of P into the ocean via river discharge more than doubles relative to the pre-industrial situation in our WB simulation and leads to an enhancement in global mean oceanic P concentrations by 27 % over 1000 years (see Fig. 2b). At the same time, benthic burial acts as the only P sink in our model (see the Supplement, Fig. S1), mitigating the total increase in marine P. The P concentration remains constant in the control run REF.
The weathering input in the WB simulation is largest north of 30
Increasing P concentrations as well as climate warming result in an increase
in net primary production in the ocean (ONPP). Globally integrated ONPP
ranges between 43.8 Tmol P a
Due to enhanced P inventory and enhanced ONPP, the WB simulation also has a
higher export rate (8.6 Tmol P a
The globally integrated remineralisation rate in the aphotic zone (results
not shown) ranges between 5.1 Tmol P a
The P burial in the WB simulation equals 0.38 Tmol P a
Difference (year 3005 minus year 1775) in
Oxygen concentration in mol O
The benthic P release in the WB simulation increases by 119 % until year
3005 to 1.7 Tmol P a
In our model simulations, both the weathering-induced P flux into the ocean (see Fig. 2c) as well as the net P released from the sediments (see Fig. 2c) show a strong increase under continued global warming, which explains the increase in the marine P inventory in the WB simulation (see Fig. 2b). However, the simulated increase in the weathering input has a much stronger (about 4 times larger) impact on the P budget and therefore on the expansion of OMZs than the benthic release feedback (see Fig. 2c). We note that even at the end of the 1000-year simulation, the P cycle has not yet reached a new steady state in experiment WB. Weathering rates are high in the warm climate and burial of P has not increased to counteract the supply by weathering (see Figs. 3b and S1). The release of P from sediments also adds to this imbalance. As a result, the marine P inventory is still increasing almost linearly at the end of our simulation. Extending the simulation until year 10 000 reveals that the ocean – as well as the coastal regions – does not become anoxic despite a more than 3-fold increase in oceanic P inventory (see Sect. 3.3 and Fig. S2) while the P cycle still exhibits a strong imbalance between sources and sinks.
The black contours in Fig. 5 indicate the lateral extent of OMZs for a depth
of 300 m (see Fig. S3 for a depth of 900 m). In year 1775, the suboxic
volume, defined here as waters with oxygen concentrations of less than
5 mmol m
During our transient simulations, we find a considerable expansion of OMZs
until year 3005 in both simulations (see Figs. 2d and 5). The expansion of
the suboxic volume between 300 and 900 m is particularly pronounced in the
WB simulation where the OMZs account for 4.85
The sea-floor area in contact with suboxic bottom waters, which directly
impacts the redox-sensitive benthic burial and P release, shows an increase
by more than a factor of 19
(WB
Somewhat unexpectedly, in our study, an increase in continental weathering
does not result in an anoxic ocean under current topography and seawater
chemistry – at least not until year 10 000. At the pre-industrial state
(year 1775), 0.12 % of all coastal margins are characterised by oxygen
concentrations below 0.005 mol m
Although the model's subcomponents for weathering, burial and benthic release rates are highly simplified in this study, the simulated global P fluxes fall within the range suggested by earlier studies and observational estimates (Palastanga et al., 2011; Filippelli, 2002; Baturin, 2007; Wallmann, 2010). The weathering fluxes are calibrated against global mean burial rates under an implicit steady-state assumption, although it is unclear whether the pre-industrial P cycle in the ocean was in equilibrium (Wallmann, 2010). The relatively high P weathering fluxes as well as the assumed indefinite P reservoir in the shelf sediments in our simulations might lead to an overestimation of the effects on the P cycle and OMZs.
In our model, the increase in the P inventory results in a strong increase in
ONPP. Contrary to other studies, e.g. Gregg et al. (2005) or Boyce et
al. (2010), in our study the temperature effect overcompensates the
stratification effect as described by Sarmiento et al. (2004), Taucher and
Oschlies (2011) and Kvale et al. (2015), and thus leads to an increase in
ONPP also in the reference run. While the net effect of warming on ONPP is
not well constrained and differs considerably among models, the impact of
changing environmental conditions on export production appears to be better
constrained (Taucher and Oschlies, 2011). In agreement with simulations by
other models, experiment REF shows a stratification-induced decline in export
production, while the increase in P induces an increase in export production
in WB. Although we use a coarse-resolution model, the applied sub-grid-scale
bathymetry allows the calculation of more accurate benthic burial and release
fluxes than otherwise possible with such a model. It should also be noted
that the benthic release feedback on OMZs might have been more efficient
under Cretaceous boundary conditions because the shelf area was considerably
larger due to higher sea levels (late Cretaceous shelf area:
46
Filippelli (2002) showed in his study that due to the anthropogenic
activities the global, total present-day river input of P has doubled in the
last 150 years. In our study, the direct anthropogenic influence, such as
agricultural input of P into the system, is excluded and should be considered
in future studies even though the human impact is projected to decrease
until year 3500 (Filippelli, 2008). Filippelli (2008) and Harrison et
al. (2005) estimated a rate of 0.03 Tmol P a
This study constitutes a first approach to estimate the potential impact of changes in the marine P cycle on the expansion of global ocean OMZs under global warming on millennial timescales. Model simulations show that the warming-induced increase in terrestrial weathering (see Fig. 3b) leads to an increase in marine P inventory (see Fig. 2b) resulting in an intensification of the biological pump, corroborating the findings by Tsandev and Slomp (2009). As a consequence, oxygen consumption as well as the volume of OMZs increase in our simulations by a factor of 12 over the next millennium (see Figs. 2d and 5).
The positive feedback involving redox-sensitive benthic P fluxes – where the
expansion of OMZs leads to an increase in benthic release of P (see Figs. 2c
and S1), which in turn enhances biological production and subsequent oxygen
consumption (Wallmann, 2010) – has only limited relevance for the
expansion of OMZs in this study. Instead, a negative feedback dominates,
which involves enhanced weathering and P supply to the ocean, an intensification of
the biological carbon pump and associated marine uptake of atmospheric
CO
The model data and model code are available at
The authors declare that they have no conflict of interest.
This work is a contribution to the Sonderforschungsbereich (SFB) 754 “Climate-Biogeochemical Interactions in the Tropical Ocean” and the BMBF project PalMod. We thank M. Eby for his excellent help with the UVic ESCM and K. F. Kvale and J. Getzlaff for proofreading. Katrin J. Meissner is thankful for UNSW Science Silver- and Goldstar Awards. The article processing charges for this open-access publication were covered by a Research Centre of the Helmholtz Association. Edited by: A. Levermann Reviewed by: two anonymous referees