Search

Despite intense efforts, the mechanisms that drive glacial–interglacial changes in atmospheric pCO2 are not fully understood. Here, we aim at quantifying the potential contribution of aeolian dust deposition changes to the atmospheric pCO2 drawdown during the Last Glacial Maximum (LGM). To this end, we use the Max Planck Institute Ocean Model (MPIOM) and the embedded Hamburg Ocean Carbon Cycle model (HAMOCC), including a new parameterization of particle ballasting that accounts for the acceleration of sinking organic soft tissue in the ocean by higher-density biogenic calcite and opal particles, as well as mineral dust. Sensitivity experiments with reconstructed LGM dust deposition rates indicate that the acceleration of detritus by mineral dust played a small role in atmospheric pCO2 variations during glacial–interglacial cycles – on the order of 5 ppmv, compared to the reconstructed ∼80 ppmv rise in atmospheric pCO2 during the last deglaciation. The additional effect of the LGM dust deposition, namely the enhanced fertilization by the iron that is associated with the glacial dust, likely played a more important role; although the full iron fertilization effect can not be estimated in the particular model version used here due to underestimated present-day non-diazotroph iron limitation, fertilization of diazotrophs in the tropical Pacific already leads to an atmospheric pCO2 drawdown of around 10 ppmv. [ABSTRACT FROM AUTHOR]

Temperature-dependent remineralization of organic matter is, in general, not included in marine biogeochemistry models currently used for Coupled Model Intercomparison Project Phase 5 (CMIP5) climate projections. Associated feedbacks have therefore not been quantified. In this study we aim at investigating how temperature-dependent remineralization rates ( Q10 = 2) in a warming ocean impact on the marine carbon cycle, and if this may weaken the oceanic sink for anthropogenic CO2. We perturb an Earth system model used for CMIP5 with temperature-dependent remineralization rates of organic matter using representative concentration pathway (RCP)8.5-derived temperature anomalies for 2100. The result is a modest change of organic carbon export but also derived effects associated with feedback processes between changed nutrient concentrations and ecosystem structure. As more nutrients are recycled in the euphotic layer, increased primary production causes a depletion of silicate in the surface layer as opal is exported to depth more efficiently than particulate organic carbon. Shifts in the ecosystem occur as diatoms find less favorable conditions. Export production of calcite shells increases causing a decrease in alkalinity and higher surface pCO2. With regard to future climate projections, the results indicate a reduction of oceanic uptake of anthropogenic CO2 of about 0.2 PgC yr−1 toward the end of the 21st century in addition to reductions caused by already identified climate-carbon cycle feedbacks. Similar shifts in the ecosystem as identified here, but driven by external forcing, have been proposed to drive glacial/interglacial changes in atmospheric pCO2. We propose a similar positive feedback between climate perturbations and the global carbon cycle but driven solely by internal biogeochemical processes. [ABSTRACT FROM AUTHOR]

The climate and marine biogeochemistry of the Indian Ocean are simulated with a globalcoupled atmosphere-ocean model (KCM-PISCES). Forcing is by orbital parametersand greenhouse gas concentrations for the Holocene from 9.5ka B.P. (9500 modelyears) and an idealized 1% p.a. atmospheric CO2 increase until two times CO2and stabilization thereafter for a further 1000 years. The analysis focuses on thelong term changes for the Indian Ocean and the comparison of natural (Holocene)and anthropogenic (CO2 increase experiment) variability. Changes in interannualand seasonal variability as well as remote influence from the global ocean are alsoaddressed. Climatic conditions for the Holocene are fairly stable, with relativelylow temperatures during the mid-Holocene and a modest warming during the lateHolocene. With regard to biogeochemistry, an expanding oxygen minimum zone in theIndian Ocean is simulated during the Holocene in both the Arabian Sea and the Bayof Bengal. The decreasing oxygen concentrations can be attributed mainly to aslowdown in large scale circulation as indicated by increasing water mass ages and acorresponding increase in apparent oxygen utilization (AOU). Marine primary andexport production remain fairly stable. Doubling the atmospheric CO2 results ina rapid warming of up to 5oC in the surface Indian Ocean followed by a furtherincrease of 1-2oC during stabilization. Surprisingly, when the climate warms inresponse to the atmospheric CO2 increase, O2 concentrations in the subsurfacenorthern Arabian Sea do initially increase, due to local changes in circulation resultingin younger water mass ages. This behaviour is different from further south in theArabian Sea and the Bay of Bengal and other oxygen depleted regions of the globalocean, where, more expected, O2 concentrations decrease rapidly for the futurescenario and oxygen minimum zones expand. During stabilization, also the northernArabian Sea loses oxygen as solubility decreases with increasing temperatures. [ABSTRACT FROM AUTHOR]

The influence of interannual variability of physical forcing on the mean ocean oxygen state is tested in two model simulations using the PISCES ocean biogeochemical model. PISCES is forced by a monthly output from (1) 500 years of interannually varying physical fields (IAV), and (2) 500 years of the constant climatology calculated from the IAV physical fields (CLIM). Unexpectedly, while the 500 years mean of the physical fields is identical, we find that the 500 years mean biogeochemical fields of the two experiments differ. These differences must originate from non-linearities in either physical or biogeochemical processes. On a global annual average, both experiments slightly overestimate oxygen concentrations in the upper ocean and underestimate observed oxygen concentrations in the deep ocean. In some oceanic regions, the inclusion of interannual variability in the forcing considerably improves the representation of the temporal mean oxygen concentration. This is especially found in the eastern equatorial Pacific (EEP), which harbors one of the largest oxygen minimum zones (OMZ) of the ocean. The better (lower) EEP oxygen concentrations in IAV may be explained by larger differences in the subduction regions of the Southern Ocean, the main oxygen source ventilating the EEP. Here in the IAV experiment oxygen concentrations are considerably lower in almost the entire water column, except for the very surface layer, so that less oxygen is entering the ocean. We find that in IAV the surface ocean is more stratified with regard to biogeochemical variables (e.g. oxygen, nutrients, ideal age). Especially an increase in the ideal age of water masses in the subsurface Southern Ocean, accompanied by stronger oxygen depletion, strongly points towards a higher sensitivity of ocean mixing to phases of stronger stratification (compared to climatological mean conditions) and a weaker influence of anomalously strong (deep) mixing, which would tend to reduce vertical gradients. Additional sensitivity experiments, systematically isolating the impact of interannual variability on either ocean physics or biogeochemistry, indeed reveal that the non-linear physical processes (mixing) play a dominant role in driving this difference between the two runs, whereas biogeochemical processes (e.g. oxygen solubility, temperature dependent remineralization) play a minor role. Overall our experiments suggest that the common strategy to spin-up biogeochemical models with a constant climatological forcing has some caveats as the internal variability of the circulation fields may result in a different oxygen mean state. This may lead to the erroneous interpretation of interannually forced (e.g. transient) model experiments that are started from a biogeochemical spin-up forced by a constant climatology. Furthermore, a change in the mean biogeochemical state of the ocean is not necessarily associated with a change in the mean climate, but could also be driven by changed climate variability. [ABSTRACT FROM AUTHOR]