Description: The low surface nitrate concentration and high atmospheric iron input in the tropical eastern North Atlantic provide beneficial conditions for N-2 fixation. Varying abundances of diazotrophs have been observed and an Fe- and P-colimitation of N-2 fixation was reported in this ocean region. It is however unclear, how different limiting factors control the temporal variability of N-2 fixation and what the role of Fe-limitation is in a region with high fluxes of dust deposition. To study the environmental controls on N-2 fixation, an one-dimensional ecosystem model is coupled with a physical model for the Tropical Eastern North Atlantic Times-series Station (TENATSO), north of the Cape Verde Islands. The model describes diazotrophy according to the physiology of Trichodesmium, taking into account a growth dependence on light, temperature, iron, dissolved inorganic (DIP) and organic phosphorus (DOP). The modelled total Chl a is compared with satellitederived total Chl a and modelled Trichodesmium (Tri) compared with satellite-derived cyanobacterial Chl a as well as with High Performance Liquid Chromatography data. Model results show a complex pattern of competitive as well as mutually beneficial interactions between diazotrophs and non-diazotrophic phytoplankton. High DOP availability after spring blooms of non-diazotrophic phytoplankton and the ability of Trichodesmium to take up DOP are crucial for allowing a maximal abundance of Tri in autumn. Part of the reactive nitrogen newly fixed by diazotrophs is directly excreted or released through mortality, significantly fuelling the growth of non-diazotrophic phytoplankton in autumn and winter. Fe consumption by non-diazotrophic phytoplankton earlier in the year makes Fe limitation of Tri in late summer more acute, whereas Tri growth in surface waters reduces phytoplankton abundance deeper in the water column by light limitation. Overall, the atmospheric iron input at the TENATSO site is required to enable diazotrophic growth and to support the observed abundance of non-diazotrophic phytoplankton

Description: A significant decrease of dissolved iron (DFe) concentration has been observed after dust addition into mesocosms during the DUst experiment in a low Nutrient low chlorophyll Ecosystem (DUNE), carried out in the summer of 2008. Due to low biological productivity at the experiment site, biological consumption of iron can not explain the magnitude of DFe decrease. To understand processes regulating the observed DFe variation, we simulated the experiment using a one-dimensional model of the Fe biogeochemical cycle, coupled with a simple ecosystem model. Different size classes of particles and particle aggregation are taken into account to describe the particle dynamics. DFe concentration is regulated in the model by dissolution from dust particles and adsorption onto particle surfaces, biological uptake, and photochemical mobilisation of particulate iron. The model reproduces the observed DFe decrease after dust addition well. This is essentially explained by particle adsorption and particle aggregation that produces a high export within the first 24 h. The estimated particle adsorption rates range between the measured adsorption rates of soluble iron and those of colloidal iron, indicating both processes controlling the DFe removal during the experiment. A dissolution timescale of 3 days is used in the model, instead of an instantaneous dissolution, underlining the importance of dissolution kinetics on the short-term impact of dust deposition on seawater DFe. Sensitivity studies reveal that initial DFe concentration before dust addition was crucial for the net impact of dust addition on DFe during the DUNE experiment. Based on the balance between abiotic sinks and sources of DFe, a critical DFe concentration has been defined, above which dust deposition acts as a net sink of DFe, rather than a source. Taking into account the role of excess iron binding ligands and biotic processes, the critical DFe concentration might be applied to explain the short-term variability of DFe after natural dust deposition in various different ocean regions.

Description: We analyzed 214 new core-top samples for their CaCO3 content from shelves all around Antarctica in order to understand their distribution and contribution to the marine carbon cycle. The distribution of sedimentary CaCO3 on the Antarctic shelves is connected to environmental parameters where we considered water depth, width of the shelf, sea-ice coverage and primary production. While CaCO3 contents of surface sediments are usually low, high (〉 15%) CaCO3 contents occur at shallow water depths (150–200 m) on the narrow shelves of the eastern Weddell Sea and at a depth range of 600–900 m on the broader and deeper shelves of the Amundsen, Bellingshausen and western Weddell Seas. Regions with high primary production, such as the Ross Sea and the western Antarctic Peninsula region, have generally low CaCO3 contents in the surface sediments. The predominant mineral phase of CaCO3 on the Antarctic shelves is low-magnesium calcite. With respect to ocean acidification, our findings suggest that dissolution of carbonates in Antarctic shelf sediments may be an important negative feedback only after the onset of calcite undersaturation on the Antarctic shelves. Macrozoobenthic CaCO3 standing stocks do not increase the CaCO3 budget significantly as they are two orders of magnitude lower than the budget of the sediments. This first circumpolar compilation of Antarctic shelf carbonate data does not claim to be complete. Future studies are encouraged and needed to fill data gaps especially in the under-sampled southwest Pacific and Indian Ocean sectors of the Southern Ocean.

Description: Stratospheric ozone depletion and emission of greenhouse gases lead to a trend of the southern annular mode (SAM) toward its high-index polarity. The positive phase of the SAM is characterized by stronger than usual westerly winds that induce changes in the physical carbon transport. Changes in the natural carbon budget of the upper 100 m of the Southern Ocean in response to a positive SAM phase are explored with a coupled ecosystem-general circulation model and regression analysis. Previously overlooked processes that are important for the upper ocean carbon budget during a positive SAM period are identified, namely, export production and downward transport of carbon north of the polar front (PF) as large as the upwelling in the south. The limiting micronutrient iron is brought into the surface layer by upwelling and stimulates phytoplankton growth and export production but only in summer. This leads to a drawdown of carbon and less summertime outgassing (or more uptake) of natural CO2. In winter, biological mechanisms are inactive, and the surface ocean equilibrates with the atmosphere by releasing CO2. In the annual mean, the upper ocean region south of the PF loses more carbon by additional export production than by the release of CO2 into the atmosphere, highlighting the role of the biological carbon pump in response to a positive SAM event.

Description: We combined data sets of measured sedimentary calcium carbonate (CaCO3) and satellite-derived pelagic primary production to parameterize the relation between CaCO3 content on the Antarctic shelves and primary production in the overlying water column. CaCO3 content predicted in this way was in good agreement with the measured data. The parameterization was then used to chart CaCO3 content on the Antarctic shelves all around the Antarctic, using the satellite-derived primary production. The total inventory of CaCO3 in the bioturbated layer of Antarctic shelf sediments was estimated to be 0.5 Pg C. This quantity is comparable to the total CO2 uptake by the Southern Ocean in only one to a few years (dependent on the uptake estimate and area considered), indicating that the dissolution of these carbonates will neither delay ocean acidification in this area nor augment the Southern Ocean CO2 uptake capacity.

Description: Dissolution of calcium carbonate neutralizes anthropogenic CO2. An upward shift of the calcite and aragonite saturation horizons exposes carbonate deposits to dissolution which is an important carbon sink reaction on a time scale of several thousand years for the world oceans. In the Southern Ocean, the surface calcite and aragonite saturation states are naturally low due to cold temperatures. They are further reduced by the uptake of anthropogenic carbon which is strongest in the top 1000 m. Undersaturation at the surface might occur even before the underlying water column is completely undersaturated. Therefore, carbonate sediments on Antarctic shelves are likely to be be among the first to dissolve due to man-made acidification. Obviously, we need to know the inventory of CaCO3 in the bioturbated layer of the Antarctic shelf sediments to quantify the capacity of this negative feedback mechanism. Here, we present a technique that allows us to spatially interpolate CaCO3 data on the Antarctic shelves. We derive quantitative relationships between nearly 400 measurements of CaCO3 on the Antarctic shelves, water depth and satellite-derived primary production in the overlying water column. This confirms that primary production mainly determines the CaCO3 distribution on the Antarctic shelves: On the one hand, there is hardly any CaCO3 production when primary production is low. On the other hand, dissolution due to CO2 produced by remineralization of organic matter dominates in high primary production regions; this constrains CaCO3 accumulation and preservation to regions with an optimum primary production level. These relationships between sedimentary CaCO3, primary production, and water depth are then applied to produce a map of CaCO3 on all Antarctic shelves. The inventory, calculated from this interpolated map of CaCO3, amounts to 4 Pg CaCO3, capable to neutralize about 0.5 Pg C. This, however, is in the same range as estimates of the annual anthropogenic CO2 uptake in the Southern Ocean. The dissolution of CaCO3 is limited by slow reaction kinetics, otherwise CaCO3 could disappear from the Antarctic shelves in only one to a few years. Our analysis suggests that deposits of CaCO3 will dissolve without releasing a significant buffering signal and that Antarctic acidification will proceed without being slowed down by dissolution of carbonates from Antarctic shelves.