Changes in the distribution of the preservation and burial of calcium carbonate (CaCO 3 ) in deep ocean sediments and associated atmospheric p CO 2 response to the shutdown of the Atlantic meridional overturning circulation (MOC) are examined using an Earth system model. We find that shutdown of the Atlantic MOC forced by the freshwater inflow significantly decreases the CaCO 3 content in North Atlantic sediments. This is a consequence of a decrease in bottom‐water carbonate ion concentrations and reduction in sea‐surface CaCO 3 production. The main sedimentary impacts of these two effects are separated in time, however, with reduced CaCO 3 production dominating the decrease in CaCO 3 burial during the first 1000 years after the forcing is applied. In the absence of significant overturning circulation in the Atlantic, atmospheric p CO 2 increases by 11 ppm, largely due to a decrease in POC export and a weakening biological pump. The change in p CO 2 induced by reorganization of CaCO 3 burial in deep‐sea sediments is small, only 1 ppm, because increased preservation of CaCO 3 in the Pacific largely efficiently buffers decreased preservation in the Atlantic, leaving the global burial and ocean alkalinity minimally changed at equilibrium.
Abstract Using a global ocean biogeochemistry model, we examined three drivers of global ocean production C:N:P ratio: flexible phytoplankton stoichiometry, phytoplankton community composition, and regional production shifts. For a middle-of-the-road warming scenario (SSP2), the model predicts a substantial increase in the global export C:P ratio from 113:1 to 119:1 by the year 2100. The most important physiological driver of this stoichiometric change is the effect of the worldwide warming on cyanobacteria, followed by the effect of phosphate depletion on eukaryotes in the Southern Ocean. Also, there is a modest global shift in the phytoplankton community in favor of cyanobacteria at the expense of eukaryotes with a minimal effect on the global production stoichiometry. We find that shifts in the regional production, even in the absence of any change in phytoplankton stoichiometry or taxonomy, can change the global production C:N:P ratio. For example, enhancing the production in the polar waters, which typically have low C:N:P ratios, will have the effect of lowering the global ratio. In our model, the retreat of Antarctic sea ice has this very effect but is offset by production changes downstream and elsewhere. This study thus provides an understanding of how regional production changes can affect the global production C:N:P ratio. However, the current literature indicates substantial uncertainty in the future projections of regional production changes, so it is unclear at this time what their net effect is on the global production C:N:P ratio. Finally, our model predicts that the overall increase in the carbon content of organic matter due to flexible C:N:P ratio helps to stabilize carbon export in the face of reduced nutrient export (i.e. the decrease in C export is ~30% smaller than expected from the decrease in P export by 2100) but has a minimal effect on atmospheric CO 2 uptake (~1%).
Abstract Modern observations indicate that variations in marine phytoplankton stoichiometry correlate with the boundaries of major surface waters. For example, phytoplankton in the oligotrophic subtropical gyres typically have much higher C:N:P ratios (i.e., higher C:P and higher N:P ratios) than those in eutrophic upwelling regions and polar regions. Such a spatial pattern points to nutrient availability as a key environmental driver of stochiometric flexibility. Environmental dependence of phytoplankton C:N:P opens unexplored possibilities for modifying the strength of the biological pump under different climate conditions. Here we present a power law formulation of C:N:P flexibility that is driven by nutrients, temperature, and light. We embed the formulation in a global ocean carbon cycle model with multiple phytoplankton types and explore biogeochemical implications under glacial conditions. We find three key controls on export C:N:P ratio: phytoplankton physiology and community structure as well as the balance in regional production at the global level. Glacial inputs of iron and sea ice expansion are important modifiers of these three controls. We also find that global export C:N:P increases substantially under glacial conditions, and this strongly buffers global carbon export against decrease and draws down approximately 20 μatm of atmospheric CO 2 . These results point to the importance of including phytoplankton C:N:P flexibility in a mix of mechanisms that drive atmospheric CO 2 over glacial‐interglacial time scale. Finally, our simulations indicate decoupling of nutrients, which may provide a resolution to the longstanding disagreement regarding nutrient utilization in the glacial Southern Ocean derived from different nutrient proxies.
AbstractIt is now well understood that the global surface ocean, whose pH has been reduced by ~0.1 in response to rising atmospheric CO2 since industrialization, will continue to become more acidic as fossil fuel CO2 emissions escalate. However, it is unclear how uncertainties in climate sensitivity to future CO2 emissions will alter the manifestation of ocean acidification. Using an earth system model of intermediate complexity, this study performs a set of simulations that varies equilibrium climate sensitivity by 1.0°–4.5°C for a given CO2 emissions scenario and finds two unexpected and decoupled responses. First, the greater the climate sensitivity, the larger the surface mixed layer acidification signal but the smaller the subsurface acidification. However, taken throughout the ocean, the highest climate sensitivity will paradoxically cause greater global warming while buffering whole-ocean pH by up to 24% on centennial time scales. Second, this study finds a large decoupling between pH and carbonate...
Phytoplankton in the Antarctic deplete silicic acid (Si(OH) 4 ) to a far greater extent than they do nitrate (NO 3 − ). This pattern can be reversed by the addition of iron which dramatically lowers diatom Si(OH) 4 :NO 3 − uptake ratios. Higher iron supply during glacial times would thus drive the Antarctic towards NO 3 − depletion with excess Si(OH) 4 remaining in surface waters. New δ 30 Si and δ 15 N records from Antarctic sediments confirm diminished Si(OH) 4 use and enhanced NO 3 − depletion during the last three glaciations. The present low‐Si(OH) 4 water is transported northward to at least the subtropics. We postulate that the glacial high‐Si(OH) 4 water similarly may have been transported to the subtropics and beyond. This input of Si(OH) 4 may have caused diatoms to displace coccolithophores at low latitudes, weakening the carbonate pump and increasing the depth of organic matter remineralization. These effects may have lowered glacial atmospheric pCO 2 by as much as 60 ppm.
List of 64 studies used in the meta-analysis.Abbreviations of the studies used in figures are listed on the left-hand column.All the studies here are included in the main bibliography.
Abstract We use the transport matrices of a data‐constrained circulation model to efficiently compute the steady state distribution of the deep ocean dissolved organic carbon (DOC) at a 1° horizontal resolution by propagating the surface DOC boundary conditions into the ocean interior. An equivalent simulation in the traditional forward modeling approach would be prohibitively computationally expensive. Our model simulates the total DOC as the sum of two DOC pools, the refractory and the semi‐labile. The model is able to simulate the large‐scale features of the deep ocean DOC without local sources or sinks of DOC in the ocean interior. The deep ocean DOC in the model is sensitive to the preformed DOC concentrations in the formation sites of deep and bottom waters, where observations are lacking. Furthermore, our model experiments indicate that the deep Atlantic DOC gradient is sensitive to the mixing of deep waters with different concentrations of preformed refractory DOC, the transport of semi‐labile DOC from the surface North Atlantic, and the decay rate of semi‐labile DOC. These, combined with the observation that much of the deep ocean DOC gradient is in the Atlantic, suggests that the semi‐labile DOC may be an important component of the deep Atlantic DOC. Finally, we show that DOC export depends substantially on the depth level where it is evaluated.
Cover: The Betsiboka River, Madagascar, during wet season discharge at the RN4 bridge in the vicinity of Maevatanana. The age‐old adage, regarding rivers as the ‘arteries of the Earth,’ could rarely be imbued more strongly than by the floodwaters of the second largest river in Madagascar. See Marwick et al . [pp. 122–137; doi: 10.1002/2014GB004911 ].
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