Biogeochemical Ocean Models

During the four most recent glacial cycles, atmospheric
CO2 during glacial maxima has been lowered
by about 90–100 ppm with respect to interglacials. There is
widespread consensus that most of this carbon was partitioned
in the ocean. It is, however, still debated which processes
were dominant in achieving this increased carbon storage.
In this paper, we use an Earth system model of intermediate
complexity to explore the sensitivity of ocean carbon
storage to ocean circulation state. We carry out a set of simulations
in which we run the model to pre-industrial equilibrium,
but in which we achieve different states of ocean circulation
by changing forcing parameters such as wind stress,
ocean diffusivity and atmospheric heat diffusivity. As a consequence,
the ensemble members also have different ocean
carbon reservoirs, global ocean average temperatures, biological
pump efficiencies and conditions for air–sea CO2
disequilibrium. We analyse changes in total ocean carbon
storage and separate it into contributions by the solubility
pump, the biological pump and the CO2 disequilibrium component.
We also relate these contributions to differences in
the strength of the ocean overturning circulation. Depending
on which ocean forcing parameter is tuned, the origin of the
change in carbon storage is different. When wind stress or
ocean diapycnal diffusivity is changed, the response of the biological
pump gives the most important effect on ocean carbon
storage, whereas when atmospheric heat diffusivity or
ocean isopycnal diffusivity is changed, the solubility pump
and the disequilibrium component are also important and
sometimes dominant. Despite this complexity, we obtain a
negative linear relationship between total ocean carbon and
the combined strength of the northern and southern overturning
cells. This relationship is robust to different reservoirs
dominating the response to different forcing mechanisms. Finally,
we conduct a drawdown experiment in which we investigate
the capacity for increased carbon storage by artificially
maximising the efficiency of the biological pump in our ensemble
members.We conclude that different initial states for
an ocean model result in different capacities for ocean carbon
storage due to differences in the ocean circulation state
and the origin of the carbon in the initial ocean carbon reservoir.
This could explain why it is difficult to achieve comparable
responses of the ocean carbon pumps in model intercomparison
studies in which the initial states vary between
models. We show that this effect of the initial state is quantifiable.
The drawdown experiment highlights the importance
of the strength of the biological pump in the control state for
model studies of increased biological efficiency.

Abstrak Pada penelitian ini kita akan mencari sensitivitas interaksi dalam ekosistem tropic pertama terhadap nilai Ф (Detritus kompocition) dan kadar Do (Detritus/Organik Mater). Ekosistem model yang digunakan adalah NPZD. Terdapat korelasi positif pariabel Ф terhadap pariabel pitoplakton hingga maksimal Ф = 3 mmol m _3. Pada nutrient pariabel Ф hanya berpengaruh pada saat awal saja dengan kisaran Ф = 2 mmol m _3 ≤. Sedangkan pada zooplankton tidak terdapat korlasi apapun.terdapat korelasi positif antara pariabel Do dengan nutrient, Do dengan pitoplakton dan Do dengan Zooplakton. Kemudian tidak ada korelasi antara Do dengan bedafase nutrient ke pitoplakton, tetepi ada korelasi negative antara Do dengan bedafase pitoplakton ke zooplankton. Terakhir titik maksimal Do adalah 46.9 mmol m _3 .

Cobalt is an important micronutrient for ocean microbes as it is present in vitamin B 12 and is a co-factor in various metalloenzymes that catalyze cellular processes. Moreover, when seawater availability of cobalt is compared to biological demands, cobalt emerges as being depleted in seawater, pointing to a potentially important limiting role. To properly account for the potential biological role for cobalt, there is therefore a need to understand the processes driving the biogeochemical cycling of cobalt and, in particular, the balance between external inputs and internal cycling. To do so, we developed the first cobalt model within a state-of-the-art three-dimensional global ocean biogeochemical model. Overall, our model does a good job in reproducing measurements with a correlation coefficient of >0.7 in the surface and >0.5 at depth. We find that continental margins are the dominant source of cobalt, with a crucial role played by supply under low bottom-water oxygen conditions. The basin-scale distribution of cobalt supplied from margins is facilitated by the activity of manganese-oxidizing bacteria being suppressed under low oxygen and low temperatures, which extends the residence time of cobalt. Overall, we find a residence time of 7 and 250 years in the upper 250 m and global ocean, respectively. Importantly, we find that the dominant internal resupply process switches from regeneration and recycling of particulate cobalt to dissolution of scavenged cobalt between the upper ocean and the ocean interior. Our model highlights key regions of the ocean where biological activity may be most sensitive to cobalt availability. Plain Language Summary Biological activity in the sea requires cobalt, primarily due to its role in vitamin B12 but also because it is required in other cellular enzymes. While our observations of cobalt distributions in the ocean is growing, we do not have a quantitative understanding of the role of different external sources of cobalt or how it is internally processed by different biological and chemical processes in the ocean. To answer these questions, we built the first ever global ocean cobalt model that coupled the cycling of cobalt to the major biogeochemical processes occurring in the ocean. Using this model, we identified that sediments are the major cobalt source and that the combination of oxygen levels and scavenging removal by bacteria allow externally supplied cobalt to pervade the ocean as a whole. We find that in certain regions of the upper ocean, cobalt levels may be low enough to affect biological activity but that to quantify this requires further work on how we represent cellular biochemistry.

During the four most recent glacial cycles, atmospheric CO<sub>2</sub> during glacial maxima has been lowered by about 90–100 ppm with respect to interglacials. There is widespread consensus that most of this carbon was partitioned in the ocean. It is however still debated which processes were dominant in achieving this increased carbon storage. In this paper, we use an Earth system model of intermediate complexity to constrain the range in ocean carbon storage for an ensemble of ocean circulation equilibrium states. We do a set of simulations where we run the model to pre-industrial equilibrium, but where we achieve different ocean circulation by changing forcing parameters such as wind stress, ocean diffusivity and atmospheric heat diffusivity. As a consequence, the ensemble members also have different ocean carbon reservoirs, global ocean average temperatures, biological pump efficiencies and conditions for air-sea CO<sub>2</sub> disequilibriu...