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Nearly 75 years ago, Alfred C. Redfield observed a similarity between the elemental composition of marine plankton in the surface ocean and dissolved nutrients in the ocean interior 1 . This stoichiometry, referred to as the Redfield ratio, continues to be a central tenet in ocean biogeochemistry, and is used to infer a variety of ecosystem processes, such as phytoplankton productivity and rates of nitrogen fixation and loss [2][3][4] . Model, field and laboratory studies have shown that different mechanisms can explain both constant and variable ratios of carbon to nitrogen and phosphorus among ocean plankton communities. The range of C/N/P ratios in the ocean, and their predictability, are the subject of much active research [5][6][7][8][9][10][11][12] . Here we assess global patterns in the elemental composition of phytoplankton and particulate organic matter in the upper ocean, using published and unpublished observations of particulate phosphorus, nitrogen and carbon from a broad latitudinal range, supplemented with elemental data for surface plankton populations. We show that the elemental ratios of marine organic matter exhibit large spatial variations, with a global average that differs substantially from the canonical Redfield ratio. However, elemental ratios exhibit a clear latitudinal trend. Specifically, we observed a ratio of 195:28:1 in the warm nutrient-depleted low-latitude gyres, 137:18:1 in warm, nutrient-rich upwelling zones, and 78:13:1 in cold, nutrient-rich high-latitude regions. We suggest that the coupling between oceanic carbon, nitrogen and phosphorus cycles may vary systematically by ecosystem.A. C. Redfield first noted the similarity between the particulate nutrient ratios of plankton living in the surface ocean and that of dissolved nutrients in the deep ocean 1 . He predicted that deep-ocean nutrient ratios were controlled by the uniform nutrient requirements of sinking, and subsequently remineralized, surface plankton. This concept remains a central tenet in our understanding of ocean biogeochemistry. However, model, field and laboratory studies have shown that different mechanisms can explain both constant and variable C/N/P ratios among ocean plankton communities, but the range of ratios in the ocean and the relative importance of each mechanism are subject to much research [5][6][7][8][9][10]13,14 . Despite this continued lack of understanding of global patterns, the N/P ratio is routinely used to draw conclusions about ocean ecosystem processes, particularly related to nutrient limitation of phytoplankton production and the net magnitude of N 2 fixation and denitrification. 2,3 . Alas, these ratios are of critical importance to study, model and predict ocean biogeochemical cycles. Equally important is the C/P ratio of ocean plankton

As particulate organic carbon rains down from the surface ocean it is respired back to carbon dioxide and released into the ocean's interior. The depth at which this sinking carbon is converted back to carbon dioxide-known as the remineralization depth-depends on the balance between particle sinking speeds and their rate of decay. A host of climate-sensitive factors can affect this balance, including temperature 1 , oxygen concentration 2 , stratification, community composition 3,4 and the mineral content of the sinking particles 5 . Here we use a three-dimensional global ocean biogeochemistry model to show that a modest change in remineralization depth can have a substantial impact on atmospheric carbon dioxide concentrations. For example, when the depth at which 63% of sinking carbon is respired increases by 24 m globally, atmospheric carbon dioxide concentrations fall by 10-27 ppm. This reduction in atmospheric carbon dioxide concentration results from the redistribution of remineralized carbon from intermediate waters to bottom waters. As a consequence of the reduced concentration of respired carbon in upper ocean waters, atmospheric carbon dioxide is preferentially stored in newly formed North Atlantic Deep Water. We suggest that atmospheric carbon dioxide concentrations are highly sensitive to the potential changes in remineralization depth that may be caused by climate change.The downward flux of particulate organic carbon, F , at a depth z below the base of the euphotic zone z c is often represented by the power law function F (z) = F (z c ) × (z/z c ) −b (ref. 6) where the exponent b controls the efficiency of transfer of particulate organic carbon to depth. The global mean value of the exponent b estimated using ocean biogeochemical tracer data is 0.9-1.0 (refs 7-9). Sediment trap observations indicate that the downward transfer efficiency may vary regionally and temporally 10 with the exponent b ranging from 0.5 to 2.0 (refs 4, 11-14). Previous studies using three-dimensional global ocean biogeochemistry models showed that the remineralization depth has an important role in controlling the global distributions of nutrients and carbon 7,8 . In ref. 7, a large change in atmospheric pCO 2 (partial pressure of carbon dioxide) of ∼100 ppm was obtained when the exponent b was changed from 0.9 to 2.0. We show here that even a small change in remineralization depth results in a substantial change in atmospheric pCO 2 and we explore the underlying mechanisms of this large sensitivity.We performed a systematic sensitivity analysis of the distributions of nutrients and carbon to changes in remineralization depth using a 3D global ocean biogeochemistry model 8 (see the Methods section) coupled with a one-box model of the atmosphere. Because the response of the new production of organic matter to nutrient supply is uncertain, we bracketed the ocean's response to changes in the remineralization profile by considering two extreme cases: a

Climate change projections to the year 2100 may miss physical-biogeochemical feedbacks that emerge later from the cumulative effects of climate warming. In a coupled climate simulation to the year 2300, the westerly winds strengthen and shift poleward, surface waters warm, and sea ice disappears, leading to intense nutrient trapping in the Southern Ocean. The trapping drives a global-scale nutrient redistribution, with net transfer to the deep ocean. Ensuing surface nutrient reductions north of 30°S drive steady declines in primary production and carbon export (decreases of 24 and 41%, respectively, by 2300). Potential fishery yields, constrained by lower-trophic-level productivity, decrease by more than 20% globally and by nearly 60% in the North Atlantic. Continued high levels of greenhouse gas emissions could suppress marine biological productivity for a millennium.

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The ocean is the largest sink for anthropogenic carbon dioxide (CO), having absorbed roughly 40 per cent of CO emissions since the beginning of the industrial era. Recent data show that oceanic CO uptake rates have been growing over the past decade, reversing a trend of stagnant or declining carbon uptake during the 1990s. Here we show that ocean circulation variability is the primary driver of these changes in oceanic CO uptake over the past several decades. We use a global inverse model to quantify the mean ocean circulation during the 1980s, 1990s and 2000s, and then estimate the impact of decadal circulation changes on the oceanic CO sink using a carbon cycling model. We find that during the 1990s an enhanced upper-ocean overturning circulation drove increased outgassing of natural CO, thus weakening the global CO sink. This trend reversed during the 2000s as the overturning circulation weakened. Continued weakening of the upper-ocean overturning is likely to strengthen the CO sink in the near future by trapping natural CO in the deep ocean, but ultimately may limit oceanic uptake of anthropogenic CO.

In order to expand our understanding of the diversity and biogeography of Prochlorococcus ribotypes, we PCR-amplified, cloned and sequenced the 16S/23S rRNA ITS region from sites in the Atlantic and Pacific oceans. Ninety-three per cent of the ITS sequences could be assigned to existing Prochlorococcus clades, although many novel subclades were detected. We assigned the sequences to operational taxonomic units using a graduated scale of sequence identity from 80% to 99.5% and correlated Prochlorococcus diversity with respect to environmental variables and dispersal time between the sites. Dispersal time was estimated using a global ocean circulation model. The significance of specific environmental variables was dependent on the degree of sequence identity used to define a taxon: light correlates with broad-scale diversity (90% cut-off), temperature with intermediate scale (95%) whereas no correlation with phosphate was observed. Community structure was correlated with dispersal time between sample sites only when taxa were defined using the finest sequence similarity cut-off. Surprisingly, the concentration of nitrate, which cannot be used as N source by the Prochlorococcus strains in culture, explains some variation in community structure for some definitions of taxa. This study suggests that the spatial distribution of Prochlorococcus ecotypes is shaped by a hierarchy of environmental factors as well dispersal limitation.

[1] Ice flow models used to project the mass balance of ice sheets in Greenland and Antarctica usually rely on the Shallow Ice Approximation (SIA) and the Shallow-Shelf Approximation (SSA), sometimes combined into so-called "hybrid" models. Such models, while computationally efficient, are based on a simplified set of physical assumptions about the mechanical regime of the ice flow, which does not uniformly apply everywhere on the ice sheet/ice shelf system, especially near grounding lines, where rapid changes are taking place at present. Here, we present a new thermomechanical finite element model of ice flow named ISSM (Ice Sheet System Model) that includes higher-order stresses, high spatial resolution capability and data assimilation techniques to better capture ice dynamics and produce realistic simulations of ice sheet flow at the continental scale. ISSM includes several approximations of the momentum balance equations, ranging from the two-dimensional SSA to the three-dimensional full-Stokes formulation. It also relies on a massively parallelized architecture and state-of-the-art scalable tools. ISSM employs data assimilation techniques, at all levels of approximation of the momentum balance equations, to infer basal drag at the ice-bed interface from satellite radar interferometry-derived observations of ice motion. Following a validation of ISSM with standard benchmarks, we present a demonstration of its capability in the case of the Greenland Ice Sheet. We show ISSM is able to simulate the ice flow of an entire ice sheet realistically at a high spatial resolution, with higher-order physics, thereby providing a pathway for improving projections of ice sheet evolution in a warming climate.Citation: Larour, E., H. Seroussi, M. Morlighem, and E. Rignot (2012), Continental scale, high order, high spatial resolution, ice sheet modeling using the Ice Sheet System Model (ISSM),