We use a state-of-the-art ocean general circulation and biogeochemistry model to examine the impact of changes in ocean circulation and biogeochemistry in governing the change in ocean carbon-13 and ...atmospheric CO2 at the last glacial maximum (LGM). We examine 5 different realisations of the ocean's overturning circulation produced by a fully coupled atmosphere-ocean model under LGM forcing and suggested changes in the atmospheric deposition of iron and phytoplankton physiology at the LGM. Measured changes in carbon-13 and carbon-14, as well as a qualitative reconstruction of the change in ocean carbon export are used to evaluate the results. Overall, we find that while a reduction in ocean ventilation at the LGM is necessary to reproduce carbon-13 and carbon-14 observations, this circulation results in a low net sink for atmospheric CO2. In contrast, while biogeochemical processes contribute little to carbon isotopes, we propose that most of the change in atmospheric CO2 was due to such factors. However, the lesser role for circulation means that when all plausible factors are accounted for, most of the necessary CO2 change remains to be explained. This presents a serious challenge to our understanding of the mechanisms behind changes in the global carbon cycle during the geologic past.
New radiocarbon and chlorofluorocarbon‐11 data from the World Ocean Circulation Experiment are used to assess a suite of 19 ocean carbon cycle models. We use the distributions and inventories of ...these tracers as quantitative metrics of model skill and find that only about a quarter of the suite is consistent with the new data‐based metrics. This should serve as a warning bell to the larger community that not all is well with current generation of ocean carbon cycle models. At the same time, this highlights the danger in simply using the available models to represent the state‐of‐the‐art modeling without considering the credibility of each model.
Helium isotopes (3He, 4He) are useful tracers for investigating the deep ocean circulation and for evaluating ocean general circulation models, because helium is a stable and conservative nuclide ...that does not take part in any chemical or biological process. Helium in the ocean originates from three different sources, namely, (i) gas dissolution in equilibrium with atmospheric helium, (ii) helium-3 addition by radioactive decay of tritium (called tritiugenic helium), and (iii) injection of terrigenic helium-3 and helium-4 by the submarine volcanic activity which occurs mainly at plate boundaries, and also addition of (mainly) helium-4 from the crust and sedimentary cover by α-decay of uranium and thorium contained in various minerals. We present the first simulation of the terrigenic helium isotope distribution in the whole Mediterranean Sea using a high-resolution model (NEMO-MED12). For this simulation we build a simple source function for terrigenic helium isotopes based on published estimates of terrestrial helium fluxes. We estimate a hydrothermal flux of 3.5 mol3 He yr−1 and a lower limit for the crustal flux at 1.6 × 10−7 4He mol m−2 yr−1. In addition to providing constraints on helium isotope degassing fluxes in the Mediterranean, our simulations provide information on the ventilation of the deep Mediterranean waters which is useful for assessing NEMO-MED12 performance. This study is part of the work carried out to assess the robustness of the NEMO-MED12 model, which will be used to study the evolution of the climate and its effect on the biogeochemical cycles in the Mediterranean Sea, and to improve our ability to predict the future evolution of the Mediterranean Sea under the increasing anthropogenic pressure.
The extratropical sources of equatorial undercurrent (EUC) water have been identified for an ocean circulation model using Lagrangian trajectory analysis. It has been found that the EUC waters ...emenate from a wide range of latitudes in the Pacific basin, with its densest constituent watermass being Subantarctic Mode Water (SAMW) from 50°S. Further analysis of the basin‐scale circulation fields has revealed significant advective diapycnal mass fluxes associated with intergyre exchange. As a result of these diapycnal mass fluxes, the EUC transport as a function of density at 151°W (an Eulerian diagnostic) looks quite different from the original subduction rate as a function of density for the same collection of water particles. This implicates diapycnal vertical mixing as an important player in determining the preferred density horizon of maximum EUC transport along the equator. In summary, these results illustrate an important interdependence between advective and diapycnal mixing processes associated with basin‐scale inter‐gyre and inter‐basin exchange in determining the mean equatorial stratification and EUC structure.
The neodymium (Nd) isotopic composition (
ϵ
Nd) of seawater is a quasi-conservative tracer of water mass mixing and is assumed to hold great potential for paleoceanographic studies. Here we present a ...comprehensive approach for the simulation of the two neodymium isotopes
143Nd, and
144Nd using the Bern3D model, a low resolution ocean model. The high computational efficiency of the Bern3D model in conjunction with our comprehensive approach allows us to systematically and extensively explore the sensitivity of Nd concentrations and
ϵ
Nd to the parametrisation of sources and sinks. Previous studies have been restricted in doing so either by the chosen approach or by computational costs. Our study thus presents the most comprehensive survey of the marine Nd cycle to date.
Our model simulates both Nd concentrations as well as
ϵ
Nd in good agreement with observations.
ϵ
Nd covaries with salinity, thus underlining its potential as a water mass proxy. Results confirm that the continental margins are required as a Nd source to simulate Nd concentrations and
ϵ
Nd consistent with observations. We estimate this source to be slightly smaller than reported in previous studies and find that above a certain magnitude its magnitude affects
ϵ
Nd only to a small extent. On the other hand, the parametrisation of the reversible scavenging considerably affects the ability of the model to simulate both, Nd concentrations and
ϵ
Nd. Furthermore, despite their small contribution, we find dust and rivers to be important components of the Nd cycle. In additional experiments, we systematically varied the diapycnal diffusivity as well as the Atlantic-to-Pacific freshwater flux to explore the sensitivity of Nd concentrations and its isotopic signature to the strength and geometry of the overturning circulation. These experiments reveal that Nd concentrations and
ϵ
Nd are comparatively little affected by variations in diapycnal diffusivity and the Atlantic-to-Pacific freshwater flux. In contrast, an adequate representation of Nd sources and sinks is crucial to simulate Nd concentrations and
ϵ
Nd consistent with observations. The good agreement of our results with observations paves the way for the evaluation of the paleoceanographic potential of
ϵ
Nd in further model studies.
We compared the 13 models participating in the Ocean Carbon Model Intercomparison Project (OCMIP) with regards to their skill in matching observed distributions of CFC-11. This analysis characterizes ...the abilities of these models to ventilate the ocean on timescales relevant for anthropogenic CO
2 uptake. We found a large range in the modeled global inventory (±30%), mainly due to differences in ventilation from the high latitudes. In the Southern Ocean, models differ particularly in the longitudinal distribution of the CFC uptake in the intermediate water, whereas the latitudinal distribution is mainly controlled by the subgrid-scale parameterization. Models with isopycnal diffusion and eddy-induced velocity parameterization produce more realistic intermediate water ventilation. Deep and bottom water ventilation also varies substantially between the models. Models coupled to a sea-ice model systematically provide more realistic AABW formation source region; however these same models also largely overestimate AABW ventilation if no specific parameterization of brine rejection during sea-ice formation is included. In the North Pacific Ocean, all models exhibit a systematic large underestimation of the CFC uptake in the thermocline of the subtropical gyre, while no systematic difference toward the observations is found in the subpolar gyre. In the North Atlantic Ocean, the CFC uptake is globally underestimated in subsurface. In the deep ocean, all but the adjoint model, failed to produce the two recently ventilated branches observed in the North Atlantic Deep Water (NADW). Furthermore, simulated transport in the Deep Western Boundary Current (DWBC) is too sluggish in all but the isopycnal model, where it is too rapid.
The marine biological carbon pump is dominated by the vertical transfer of particulate organic carbon (POC) from the surface ocean to its interior. The efficiency of this transfer plays an important ...role in controlling the amount of atmospheric carbon that is sequestered in the ocean. Furthermore, the abundance and composition of POC is critical for the removal of numerous trace elements by scavenging, a number of which, such as iron, are essential for the growth of marine organisms, including phytoplankton. Observations and laboratory experiments have shown that POC is composed of numerous organic compounds that can have very different reactivities. However, this variable reactivity of POC has never been extensively considered, especially in modelling studies. Here, we introduced in the global ocean biogeochemical model NEMO-PISCES a description of the variable composition of POC based on the theoretical reactivity continuum model proposed by Boudreau and Ruddick (1991). Our model experiments show that accounting for a variable lability of POC increases POC concentrations in the ocean's interior by 1 to 2 orders of magnitude. This increase is mainly the consequence of a better preservation of small particles that sink slowly from the surface. Comparison with observations is significantly improved both in abundance and in size distribution. Furthermore, the amount of carbon that reaches the sediments is increased by more than a factor of 2, which is in better agreement with global estimates of the sediment oxygen demand. The impact on the major macronutrients (nitrate and phosphate) remains modest. However, iron (Fe) distribution is strongly altered, especially in the upper mesopelagic zone as a result of more intense scavenging: vertical gradients in Fe are milder in the upper ocean, which appears to be closer to observations. Thus, our study shows that the variable lability of POC can play a critical role in the marine biogeochemical cycles which advocates for more dedicated in situ and laboratory experiments.
In this paper we set forth a 3-D ocean model of the radioactive trace isotopes .sup.230 Th and .sup.231 Pa. The interest arises from the fact that these isotopes are extensively used for ...investigating particle transport in the ocean and reconstructing past ocean circulation. The tracers are reversibly scavenged by biogenic and lithogenic particles.
Recently, major progress has been made in the simulation of the ocean
dynamics of the Mediterranean using atmospheric and oceanic models with
high spatial resolution. High resolution is essential to ...accurately capture the
synoptic variability required to initiate intermediate- and deep-water
formation, the engine of the Mediterranean thermohaline circulation (MTC).
In paleoclimate studies, one major problem with the simulation of regional
climate changes is that boundary conditions are not available from
observations or data reconstruction to drive high-resolution regional
models. One consistent way to advance paleoclimate modelling is to use a
comprehensive global-to-regional approach. However, this approach needs
long-term integration to reach equilibrium (hundreds of years), implying
enormous computational resources. To tackle this issue, a sequential
architecture of a global–regional modelling platform has been developed for
the first time and is described in detail in this paper. First of all, the
platform is validated for the historical period. It is then used to
investigate the climate and in particular, the oceanic circulation, during
the Early Holocene. This period was characterised by a large reorganisation
of the MTC that strongly affected oxygen supply to the intermediate and deep
waters, which ultimately led to an anoxic crisis (called sapropel). Beyond
the case study shown here, this platform may be applied to a large number of
paleoclimate contexts from the Quaternary to the Pliocene, as long as
regional tectonics remain mostly unchanged. For example, the climate
responses of the Mediterranean basin during the last interglacial period (LIG), the
Last Glacial Maximum (LGM) and the Late Pliocene all present interesting
scientific challenges which may be addressed using this numerical platform.
Understanding the ocean circulation changes associated with abrupt climate events is key to better assessing climate variability and understanding its different natural modes. Sedimentary Pa/Th, ...benthic delta.sup.13 C and Î.sup.14 C are common proxies used to reconstruct past circulation flow rate and ventilation. To overcome the limitations of each proxy taken separately, a better approach is to produce multiproxy measurements on a single sediment core. Yet, different proxies can provide conflicting information about past ocean circulation. Thus, modelling them in a consistent physical framework has become necessary to assess the geographical pattern and the timing and sequence of the multiproxy response to abrupt circulation changes.
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