292 research outputs found
Ventilation of the North Atlantic Ocean during the Last Glacial Maximum: A comparison between simulated and observed radiocarbon ages
The distribution of radiocarbon during simulations of the Last Glacial Maximum with a coupled ocean-atmosphere-sea ice model is compared with sediment core measurements from the equatorial Atlantic Ceara Rise, Blake Ridge, Caribbean Sea, and South China Sea. During these simulations we introduce a perturbation of North Atlantic freshwater fluxes leading to varying strengths of the Atlantic meridional overturning. The best fit with the observations is obtained for an overturning weakened by 40% compared with today. Further, we simulate the phenomenon of an âage reversalâ found in deep sea corals, but we suggest that this indicates rather a sudden interruption of deep water formation instead of an increase in ventilation, which was suggested earlier
Isotopic constraints on the pre-industrial oceanic nitrogen budget
The size of the bio-available (i.e. "fixed") nitrogen inventory in the ocean influences global marine productivity and the biological carbon pump. Despite its importance, the pre-industrial rates for the major source and sink terms of the oceanic fixed nitrogen budget, N2 fixation and denitrification, respectively, are not well known. However, these processes leave distinguishable imprints on the ratio of stable nitrogen isotopes, ÎŽ15N, which can therefore help to infer their patterns and rates. Here we use ÎŽ15N observations from the water column and a new database of seafloor measurements to constrain rates of N2 fixation and denitrification predicted by a global three-dimensional Model of Ocean Biogeochemistry and Isotopes (MOBI). Sensitivity experiments were performed to quantify uncertainties associated with the isotope effect of denitrification in the water column and sediments. They show that the level of nitrate utilization in suboxic zones, that is the balance between nitrate consumption by denitrification and nitrate replenishment by mixing (dilution effect), significantly affects the isotope effect of water column denitrification and thus global mean ÎŽ15NO3â. Experiments with lower levels of nitrate utilization within the suboxic zone (i.e. higher residual water column nitrate concentrations, ranging from 20â32 ÎŒM) require higher ratios of benthic to water column denitrification (BD:WCD = 0.75â1.4, respectively), to satisfy the global mean NO3â and ÎŽ15NO3â constraints in the modern ocean. This suggests that nitrate utilization in suboxic zones play an important role in global nitrogen isotope cycling. Increasing the net fractionation factor for benthic denitrification (Ï”BD = 0â4â°) requires even higher ratios of benthic to water column denitrification (BD:WCD = 1.4â3.5, respectively). The model experiments that best reproduce observed seafloor ÎŽ15N support the middle to high-end estimates for the net fractionation factor of benthic denitrification (Ï”BD = 2â4â°). Assuming a balanced fixed nitrogen budget, we estimate that pre-industrial rates of N2 fixation, water column denitrification, and benthic denitrification were approximately 195â345, 65â75, and 130â270 Tg N yrâ1, respectively. Although uncertainties still exist, these results suggest that previous estimates of N2 fixation have been significantly underestimated and the residence time for oceanic fixed nitrogen is between ~ 1500â3000 yr
A Three-Dimensional Model of the Marine Nitrogen Cycle during the Last Glacial Maximum Constrained by Sedimentary Isotopes
Nitrogen is a key limiting nutrient that influences marine productivity and carbon sequestration in the ocean via the biological pump. In this study, we present the first estimates of nitrogen cycling in a coupled 3D ocean-biogeochemistry-isotope model forced with realistic boundary conditions from the Last Glacial Maximum (LGM) ~21,000 years before present constrained by nitrogen isotopes. The model predicts a large decrease in nitrogen loss rates due to higher oxygen concentrations in the thermocline and sea level drop, and, as a response, reduced nitrogen fixation. Model experiments are performed to evaluate effects of hypothesized increases of atmospheric iron fluxes and oceanic phosphorus inventory relative to present-day conditions. Enhanced atmospheric iron deposition, which is required to reproduce observations, fuels export production in the Southern Ocean causing increased deep ocean nutrient storage. This reduces transport of preformed nutrients to the tropics via mode waters, thereby decreasing productivity, oxygen deficient zones, and water column N-loss there. A larger global phosphorus inventory up to 15% cannot be excluded from the currently available nitrogen isotope data. It stimulates additional nitrogen fixation that increases the global oceanic nitrogen inventory, productivity, and water column N-loss. Among our sensitivity simulations, the best agreements with nitrogen isotope data from LGM sediments indicate that water column and sedimentary N-loss were reduced by 17â62% and 35â69%, respectively, relative to preindustrial values. Our model demonstrates that multiple processes alter the nitrogen isotopic signal in most locations, which creates large uncertainties when quantitatively constraining individual nitrogen cycling processes. One key uncertainty is nitrogen fixation, which decreases by 25â65% in the model during the LGM mainly in response to reduced N-loss, due to the lack of observations in the open ocean most notably in the tropical and subtropical southern hemisphere. Nevertheless, the model estimated large increase to the global nitrate inventory of 6.5â22% suggests it may play an important role enhancing the biological carbon pump that contributes to lower atmospheric CO2 during the LGM
Evaluation of a present-day climate simulation with a new coupled atmosphere-ocean model GENMOM
We present a new, non-flux corrected AOGCM, GENMOM, that combines the GENESIS version 3 atmospheric GCM (Global Environmental and Ecological Simulation of Interactive Systems) and MOM2 (Modular Ocean Model version 2) nominally at T31 resolution. We evaluate GENMOM by comparison with reanalysis products (e.g., NCEP2) and three models used in the IPCC AR4 assessment. GENMOM produces a global temperature bias of 0.6 °C. Atmospheric features such as the jet stream structure and major semi-permanent sea level pressure centers are well simulated as is the mean planetary-scale wind structure that is needed to produce the correct position of stormtracks. Most ocean surface currents are reproduced except where they are not resolvable at T31 resolution. Overall, GENMOM captures reasonably well the observed gradients and spatial distributions of annual surface temperature and precipitation and the simulations are on par with other AOGCMs. Deficiencies in the GENMOM simulations include a warm bias in the surface temperature over the southern oceans, a split in the ITCZ and weaker-than-observed overturning circulation
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Complementary constraints from carbon ÂčÂłC and nitrogen Âčâ”N isotopes on the glacial ocean's soft-tissue biological pump
A three-dimensional, process-based model of the oceanâs carbon and nitrogen cycles, including 13C and 15N isotopes, is used to explore effects of idealized changes in the soft-tissue biological pump. Results are presented from one preindustrial control run (piCtrl) and six simulations of the Last Glacial Maximum (LGM) with increasing values of the spatially constant maximum phytoplankton growth rate ÎŒmax, which accelerates biological nutrient utilization mimicking iron fertilization. The default LGM simulation, without increasing ÎŒmax and with a shallower and weaker Atlantic Meridional Overturning Circulation and increased sea ice cover, leads to 280 Pg more respired organic carbon (Corg) storage in the deep ocean with respect to piCtrl. Dissolved oxygen concentrations in the colder glacial thermocline increase, which reduces water column denitrification and, with delay, nitrogen fixation, thus increasing the oceanâs fixed nitrogen inventory and decreasing ÎŽ15NNO3 almost everywhere. This simulation already fits sediment reconstructions of carbon and nitrogen isotopes relatively well, but it overestimates deep ocean ÎŽ13CDIC and underestimates ÎŽ15NNO3 at high latitudes. Increasing ÎŒmax enhances Corg and lowers deep ocean ÎŽ13CDIC, improving the agreement with sediment data. In the modelâs Antarctic and North Pacific Oceans modest increases in ÎŒmax result in higher ÎŽ15NNO3 due to enhanced local nutrient utilization, improving the agreement with reconstructions there. Models with moderately increased ÎŒmax fit both isotope data best, whereas large increases in nutrient utilization are inconsistent with nitrogen isotopes although they still fit the carbon isotopes reasonably well. The best fitting models reproduce major features of the glacial ÎŽ13CDIC, ÎŽ15N, and oxygen reconstructions while simulating increased Corg by 510â670 Pg compared with the preindustrial ocean. These results are consistent with the idea that the soft-tissue pump was more efficient during the LGM. Both circulation and biological nutrient utilization could contribute. However, these conclusions are preliminary given our idealized experiments, which do not consider changes in benthic denitrification and spatially inhomogenous changes in aeolian iron fluxes. The analysis illustrates interactions between the carbon and nitrogen cycles as well as the complementary constraints provided by their isotopes. Whereas carbon isotopes are sensitive to circulation changes and indicate well the three-dimensional Corg distribution, nitrogen isotopes are more sensitive to biological nutrient utilization
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Climatic Consequences of a Pine Island Glacier Collapse
An intermediate-complexity climate model is used to simulate the impact of an accelerated Pine Island Glacier mass loss on the large-scale ocean circulation and climate. Simulations are performed for preindustrial conditions using hosing levels consistent with present-day observations of 3000 mÂł sâ»Âč, at an accelerated rate of 6000 mÂł sâ»Âč, and at a total collapse rate of 100 000 mÂł sâ»Âč, and in all experiments the hosing lasted 100 years. It is shown that even a modest input of meltwater from the glacier can introduce an initial cooling over the upper part of the Southern Ocean due to increased stratification and ice cover, leading to a reduced upward heat flux from Circumpolar Deep Water. This causes global ocean heat content to increase and global surface air temperatures to decrease. The Atlantic meridional overturning circulation (AMOC) increases, presumably owing to changes in the density difference between Antarctic Intermediate Water and North Atlantic Deep Water. Simulations with a simultaneous hosing and increases of atmospheric COâ concentrations show smaller effects of the hosing on global surface air temperature and ocean heat content, which the authors attribute to the melting of Southern Ocean sea ice. The sensitivity of the AMOC to the hosing is also reduced as the warming by the atmosphere completely dominates the perturbationsKeywords: Geographic location/entity,
Coupled models,
Ice sheets,
Meridional overturning circulation,
Thermocline circulation,
Models and modeling,
Circulation/ Dynamic
Response of a climate model to tidal mixing parameterization under present day and last glacial maximum conditions
Author Posting. © Elsevier B.V., 2007. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Ocean Modelling 19 (2007): 125-137, doi:10.1016/j.ocemod.2007.06.009.Experiments with a climate model were conducted under present day and last
glacial maximum conditions in order to examine the modelâs response to a vertical
mixing scheme based on internal tide energy dissipation. The increase in internal
tide energy flux caused by a 120 m reduction in sea level had the expected effect on
diffusivity values, which were higher under lower sea level conditions. The impact
of this vertical diffusivity change on the Atlantic meridional overturning is not
straightforward and no clear relationship between diffusivity and overturning is
found. There exists a weak positive correlation between overturning and changes to
the power consumed by vertical mixing. Most of the climatic response generated by
sea level change was not related to alterations in the internal tide energy flux but
rather to the direct change in sea level itself.Funding received from CFCAS through the CLIVAR and Polar Climate Stability Research networks. SRJ was supported by the U.S. National Science Foundation under Grant No. OCE-0241061
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Isotopic constraints on the pre-industrial oceanic nitrogen budget
The size of the bioavailable (i.e., âfixedâ) nitrogen inventory in the ocean influences global marine productivity and the biological carbon pump. Despite its importance, the pre-industrial rates for the major source and sink terms of the oceanic fixed nitrogen budget, Nâ fixation and denitrification, respectively, are not well known. These processes leave distinguishable imprints on the ratio of stable nitrogen isotopes, ÎŽÂčâ”N, which can therefore help to infer their patterns and rates. Here we use ÎŽÂčâ”N observations from the water column and a new database of seafloor measurements to constrain rates of Nâ fixation and denitrification predicted by a global three-dimensional Model of Ocean Biogeochemistry and Isotopes (MOBI). Sensitivity experiments were performed to quantify uncertainties associated with the isotope effect of denitrification in the water column and sediments. They show that the level of nitrate utilization in suboxic zones, that is the balance between nitrate consumption by denitrification and nitrate replenishment by circulation and mixing (dilution effect), significantly affects the isotope effect of water column denitrification and thus global mean ÎŽÂčâ”NOâ-. Experiments with lower levels of nitrate utilization within the suboxic zone (i.e., higher residual water column nitrate concentrations, ranging from 20 to 32 ÎŒM) require higher ratios of benthic to water column denitrification, BD:WCD=0.75â1.4, to satisfy the global mean NOâ- and ÎŽÂčâ”NOâ- constraints in the modern ocean. This suggests that nitrate utilization in suboxic zones plays an important role in global nitrogen isotope cycling. Increasing the net fractionation factor "BD for benthic denitrification ("BD = 0â4 â°) requires even higher ratios, BD:WCD=1.4â3.5. The model experiments that best reproduce observed seafloor ÎŽÂčâ”N support the middle to high-end estimates for the net fractionation factor of benthic denitrification (ΔBD =2â4 â°). Assuming a balanced fixed nitrogen budget, we estimate that preindustrial rates of Nâ fixation, water column denitrification, and benthic denitrification were between 195â350 (225), 65â 80 (76), and 130â270 (149) TgNyrâ1, respectively, with our best model estimate (ΔBD = 2-4 â°) in parentheses. Although uncertainties still exist, these results suggest that marine Nâ fixation is occurring at much greater rates than previously estimated and the residence time for oceanic fixed nitrogen is between ~1500 and 3000 yr
Benthic foraminiferal stable carbon isotope constraints on deglacial ocean circulation and carbon-cycle changes
How does deep-ocean circulation influence atmospheric CO2 across deglacial transitions? Although biogeochemical and physical processes complicate interpretation of foraminiferal stable carbon isotope data, these complications can be addressed with expanded data compilations, multiproxy approaches, and model-data assimilation efforts.Fil: Peterson, Carlye D.. University of California Riverside; Estados UnidosFil: Gebbie, G.. Woods Hole Oceanographic Institution; Estados UnidosFil: Lisiecki, L. E.. University of California Santa Barbara; Estados UnidosFil: Lynch Stieglitz, J.. School of Earth and Atmospheric Sciences; Estados UnidosFil: Oppo, D.. Woods Hole Oceanographic Institution; Estados UnidosFil: Muglia, Juan. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - Centro Nacional PatagĂłnico. Centro para el Estudio de Sistemas Marinos; ArgentinaFil: RepschlĂ€ger, Janne. Max Planck Institute for Chemistry; AlemaniaFil: Schmittner, A.. University of Oregon; Estados Unido
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Abrupt change in atmospheric COâ during the last ice age
During the last glacial period atmospheric carbon dioxide and temperature in Antarctica varied in a similar fashion on millennial time scales, but previous work indicates that these changes were gradual. In a detailed analysis of one event we now find that approximately half of the COâ increase that occurred during the 1500-year cold period between Dansgaard-Oeschger (DO) events 8 and 9 happened rapidly, over less than two centuries. This rise in COâ was synchronous with, or slightly later than, a rapid increase of Antarctic temperature inferred from stable isotopes
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