Aluminium and manganese in the West Atlantic Ocean:A model study

Het doel van deze studie is om tot een beter begrip te komen van het gedrag van de spoormetalen aluminium (Al) en mangaan (Mn) in de oceaan, door middel van het simuleren van de distributies van opgelost Al en opgelost Mn met biogeochemische modellen die ingebed zijn in driedimensionale oceaanmodellen. Recent zijn vele monsters van zeewater geanalyseerd binnen het GEOTRACES-programma dat tot doel heeft de biogeochemische kringlopen beter te begrijpen, met als focus de metingen van spoormetalen in monsters zeewater. Die monsters zijn geverifieerd aan de hand van internationale referentiemonsters en de consensuswaarden van het GEOTRACES-programma. In dit proefschrift zijn deze gemeten concentraties van opgelost Al en Mn vergeleken met gesimuleerde opgeloste Al- en Mn-concentraties in een computermodel. Op hun beurt kunnen de resultaten van deze simulaties worden gebruikt voor het verder ontwikkelen van deze modellen om de verdelingen van opgelost Al en Mn preciezer te simuleren, en/of ons begrip van de processen te verbeteren. Er zijn twee redenen waarom het gedrag van Al in de oceaan wordt onderzocht. Op de eerste plaats wordt Al opgenomen door diatomeeën, een type fytoplankton met een extern skelet van kiezel (SiO₂), als plaatsvervangend spoormetaal voor silicium (Si). De verhouding van opname is in de orde van circa één atoom Al per 500–1000 atomen Si. Naarmate kiezel een hogere concentratie Al bevat, wordt de oplosbaarheid ervan lager, wat een effect heeft op de beschikbaarheid van Si in de fotische zone waar diatomeeën kunnen groeien. Op de tweede plaats kan de concentratie van opgelost Al in het oppervlak van de oceaan worden gebruikt als indicator voor beschikbare nutriënten van stofdeeltjes die vanaf het land via de lucht in het oppervlaktewater van de oceanen terecht komen. Deze stofdeeltjes bevatten zowel Al als diverse spoornutriënten, met name ijzer (Fe) en mangaan (Mn). Het biogeochemische Al-model in dit proefschrift heeft twee bronnen: de atmosfeer (stof) en het sediment in de oceaan. Interne processen zijn adsorptie van Al aan kiezel, sedimentatie en circulatie. De belangrijkste kenmerken van de wereldwijde distributie van opgelost Al in de oceaan zijn gesimuleerd in overeenstemming met de beschikbare observaties. In het bijzonder hebben we de GEOTRACES-observaties in de West-Atlantische Oceaan en de Atlantische sector van de Zuidelijke Oceaan redelijk goed kunnen reproduceren met het model. Mangaan wordt gebruikt voor fotosynthese. Wegens deze belangrijke functie willen we graag weten hoe Mn verdeeld is in de oceaan. Mangaan komt binnen in de oceaan via atmosferisch stof, rivieren, onderwatervulkanen en diffusie uit het sediment. Mangaan wordt verwijderd door oxidatie en zinken van geoxideerd Mn, en het zinken van door plankton opgenomen Mn. Dit proefschrift bevat het eerste onderzoek waarin de driedimensionale distributies van opgelost Mn en mangaanoxides zijn gemodelleerd. Veel kenmerken van de distributie van opgelost Mn zijn gereproduceerd door het model. Deze kenmerken bevatten de over het algemeen hogere concentraties van opgelost Mn onder en stroomopwaarts van locaties van stofdepositie, en de verhoogde concentraties nabij vulkanische bronnen

Long-term climate simulation in NorESM: burst-coupling the sediment in the BLOM/iHAMOCC ocean module

In this report we set forth a simulation method for long-term simulations of NorESM, the Norwegian Earth System Model. In this the sediment is repeatedly decoupled and coupled to the ocean model (BLOM/iHAMOCC), a process called burst coupling. Through this, the ocean (seawater and sediment) is brought into an approximate steady state. We show that just the model has to run at least 50000 yr to get in an approximate steady state. With burst coupling this can be done in a computationally reasonable time (wall time in the order of one week). The method can be used to generate the sediment over hundreds of thousands of years, so it is useful not only for present-day simulations but also for paleo-climatological studies

Evaluation of ocean dimethylsulfide concentration and emission in CMIP6 models

Characteristics and trends of surface ocean dimethylsulfide (DMS) concentrations and fluxes into the atmosphere of four Earth system models (ESMs: CNRM-ESM2-1, MIROC-ES2L, NorESM2-LM, and UKESM1-0-LL) are analysed over the recent past (1980–2009) and into the future, using Coupled Model Intercomparison Project 6 (CMIP6) simulations. The DMS concentrations in historical simulations systematically underestimate the most widely used observed climatology but compare more favourably against two recent observation-based datasets. The models better reproduce observations in mid to high latitudes, as well as in polar and westerlies marine biomes. The resulting multi-model estimate of contemporary global ocean DMS emissions is 16–24 Tg S yr−1, which is narrower than the observational-derived range of 16 to 28 Tg S yr−1. The four models disagree on the sign of the trend of the global DMS flux from 1980 onwards, with two models showing an increase and two models a decrease. At the global scale, these trends are dominated by changes in surface DMS concentrations in all models, irrespective of the air–sea flux parameterisation used. In turn, three models consistently show that changes in DMS concentrations are correlated with changes in marine productivity; however, marine productivity is poorly constrained in the current generation of ESMs, thus limiting the predictive ability of this relationship. In contrast, a consensus is found among all models over polar latitudes where an increasing trend is predominantly driven by the retreating sea-ice extent. However, the magnitude of this trend between models differs by a factor of 3, from 2.9 to 9.2 Gg S decade−1 over the period 1980–2014, which is at the low end of a recent satellite-derived analysis. Similar increasing trends are found in climate projections over the 21st century

Global Ocean Sediment Composition and Burial Flux in the Deep Sea

Quantitative knowledge about the burial of sedimentary components at the seafloor has wide-ranging implications in ocean science, from global climate to continental weathering. The use of 230Th-normalized fluxes reduces uncertainties that many prior studies faced by accounting for the effects of sediment redistribution by bottom currents and minimizing the impact of age model uncertainty. Here we employ a recently compiled global data set of 230Th-normalized fluxes with an updated database of seafloor surface sediment composition to derive atlases of the deep-sea burial flux of calcium carbonate, biogenic opal, total organic carbon (TOC), nonbiogenic material, iron, mercury, and excess barium (Baxs). The spatial patterns of major component burial are mainly consistent with prior work, but the new quantitative estimates allow evaluations of deep-sea budgets. Our integrated deep-sea burial fluxes are 136 Tg C/yr CaCO3, 153 Tg Si/yr opal, 20Tg C/yr TOC, 220 Mg Hg/yr, and 2.6 Tg Baxs/yr. This opal flux is roughly a factor of 2 increase over previous estimates, with important implications for the global Si cycle. Sedimentary Fe fluxes reflect a mixture of sources including lithogenic material, hydrothermal inputs and authigenic phases. The fluxes of some commonly used paleo-productivity proxies (TOC, biogenic opal, and Baxs) are not well-correlated geographically with satellite-based productivity estimates. Our new compilation of sedimentary fluxes provides detailed regional and global information, which will help refine the understanding of sediment preservation

A global scavenging and circulation ocean model of thorium-230 and protactinium-231 with improved particle dynamics (NEMO–ProThorP 0.1)

International audienceIn this paper we set forth a 3-D ocean model of the radioactive trace isotopes 230 Th and 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. Our simulations of 230 Th and 231 Pa are based on the NEMO-PISCES ocean biogeochemistry general circulation model, which includes biogenic particles, namely small and big particulate organic carbon, calcium carbonate and bio-genic silica. Small and big lithogenic particles from dust de-position are included in our model as well. Their distributions generally compare well with the small and big lithogenic particle concentrations from recent observations from the GEO-TRACES programme, except for boundary nepheloid layers for which, as of today, there are no non-trivial prognostic models available on a global scale. Our simulations reproduce 230 Th and 231 Pa dissolved concentrations: they compare well with recent GEOTRACES observations in many parts of the ocean. Particulate 230 Th and 231 Pa concentrations are significantly improved compared to previous studies, but they are still too low because of missing particles from nepheloid layers. Our simulation reproduces the main characteristics of the 231 Pa/ 230 Th ratio observed in the sediments and supports a moderate affinity of 231 Pa to biogenic silica as suggested by recent observations relative to 230 Th. Future model development may further improve understanding , especially when this will include a more complete representation of all particles, including different size classes, manganese hydroxides and nepheloid layers. This can be done based on our model as its source code is readily available

Output from a global scavenging and circulation ocean model of thorium-230 and protactinium-231 (ProThorP 0.1)

This archive contains all output from the PISCES-v2 model, containing 24 carbon, nitrate, phosphate and iron tracers. The model is extended with models of lithogenic particles and thorium-230 and protactinium-231, adding eight additional tracers, namely small and big lithogenic particles, and dissolved, small and big Th-230 and Pa-231 (referred to as ProThorP). Thus the model contains 32 different tracers (ptrc) for two different simulations, as well as diagnostic data (diad). The model is executed as part of the global ocean general cirulation model NEMO in the ORCA2 configuration (2° x 2° cos(phi) x 31 layers)

Variable reactivity of particulate organic matter in a global ocean biogeochemical model

International audienceThe 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 scaveng-ing, 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 macronutri-ents (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

Manganese in the west Atlantic Ocean in the context of the first global ocean circulation model of manganese

Dissolved manganese (Mn) is a biologically essential element. Moreover, its oxidised form is involved in removing itself and several other trace elements from ocean waters. Here we report the longest thus far (17 500 km length) full-depth ocean section of dissolved Mn in the west Atlantic Ocean, comprising 1320 data values of high accuracy. This is the G Lambda 02 transect that is part of the GEO-TRACES programme, which aims to understand trace element distributions. The goal of this study is to combine these new observations with new, state-of-the-art, modelling to give a first assessment of the main sources and redistribution of Mn throughout the ocean. To this end, we simulate the distribution of dissolved Mn using a global-scale circulation model. This first model includes simple parameterisations to account for the sources, processes and sinks of Mn in the ocean. Oxidation and (photo) reduction, aggregation and settling, as well as biological uptake and remineralisation by plankton are included in the model. Our model provides, together with the observations, the following insights: The high surface concentrations of manganese are caused by the combination of photoreduction and sources contributing to the upper ocean. The most important sources are sediments, dust, and, more locally, rivers. Observations and model simulations suggest that surface Mn in the Atlantic Ocean moves downwards into the southward-flowing North Atlantic Deep Water (NADW), but because of strong removal rates there is no elevated concentration of Mn visible any more in the NADW south of 40 degrees N. The model predicts lower dissolved Mn in surface waters of the Pacific Ocean than the observed concentrations. The intense oxygen minimum zone (OMZ) in subsurface waters is deemed to be a major source of dissolved Mn also mixing upwards into surface waters, but the OMZ is not well represented by the model. Improved high-resolution simulation of the OMZ may solve this problem. There is a mainly homogeneous background concentration of dissolved Mn of about 0.10-0.15nM throughout most of the deep ocean. The model reproduces this by means of a threshold on particulate manganese oxides of 25 pM, suggesting that a minimal concentration of particulate Mn is needed before aggregation and removal become efficient. The observed distinct hydrothermal signals are produced by assuming both a strong source and a strong removal of Mn near hydrothermal vents

On the effects of circulation, sediment resuspension and biological incorporation by diatoms in an ocean model of aluminium, link to model data

The distribution of dissolved aluminium in the West Atlantic Ocean shows a mirror image with that of dissolved silicic acid, hinting at intricate interactions between the ocean cycling of Al and Si. The marine biogeochemistry of Al is of interest because of its potential impact on diatom opal remineralisation, hence Si availability. Furthermore, the dissolved Al concentration at the surface ocean has been used as a tracer for dust input, dust being the most important source of the bio-essential trace element iron to the ocean. Previously, the dissolved concentration of Al was simulated reasonably well with only a dust source, and scavenging by adsorption on settling biogenic debris as the only removal process. Here we explore the impacts of (i) a sediment source of Al in the Northern Hemisphere (especially north of ~ 40° N), (ii) the imposed velocity field, and (iii) biological incorporation of Al on the modelled Al distribution in the ocean. The sediment source clearly improves the model results, and using a different velocity field shows the importance of advection on the simulated Al distribution. Biological incorporation appears to be a potentially important removal process. However, conclusive independent data to constrain the Al / Si incorporation ratio by growing diatoms are missing. Therefore, this study does not provide a definitive answer to the question of the relative importance of Al removal by incorporation compared to removal by adsorptive scavenging