Deep Mixing Research Articles

Deep mixed ocean volume in the Labrador Sea in HighResMIP models

Simulations from seven global coupled climate models performed at high and standard resolution as part of the high resolution model intercomparison project (HighResMIP) are analyzed to study deep ocean mixing in the Labrador Sea and the impact of increased horizontal resolution. The representation of convection varies strongly among models. Compared to observations from ARGO-floats and the EN4 data set, most models substantially overestimate deep convection in the Labrador Sea. In four out of five models, all four using the NEMO-ocean model, increasing the ocean resolution from 1° to 1/4° leads to increased deep mixing in the Labrador Sea. Increasing the atmospheric resolution has a smaller effect than increasing the ocean resolution. Simulated convection in the Labrador Sea is mainly governed by the release of heat from the ocean to the atmosphere and by the vertical stratification of the water masses in the Labrador Sea in late autumn. Models with stronger sub-polar gyre circulation have generally higher surface salinity in the Labrador Sea and a deeper convection. While the high-resolution models show more realistic ocean stratification in the Labrador Sea than the standard resolution models, they generally overestimate the convection. The results indicate that the representation of sub-grid scale mixing processes might be imperfect in the models and contribute to the biases in deep convection. Since in more than half of the models, the Labrador Sea convection is important for the Atlantic Meridional Overturning Circulation (AMOC), this raises questions about the future behavior of the AMOC in the models.

Low-Level Atmospheric Flow at the Central North Coast of Brazil

The planetary-boundary-layer (PBL) flow and above is investigated for the central north coast of Brazil, an equatorial region spanning from 8° to 2°S. The daily PBL flow is controlled by vertical entrainment of horizontal momentum from a southerly large-scale flow associated with the Hadley cell, and by a mesoscale pressure gradient force (PGF) created by the differential heating between land and ocean. Near the coast, the flow is from the north-east quadrant comprising a small rotation, probably caused by a weak mesoscale PGF and a weak Coriolis force. Inland it is north-easterly in the morning, but deep mixing during the afternoon brings down momentum from above causing it to become south-easterly. The mesoscale PGF executes a daily 360° rotation at most of the stations. In the afternoon it points to land due to continuous heating of the land, and a sea breeze develops in the presence of the background flow. Once convection dies out, the transfer of horizontal momentum is reduced, and the marine-air layer can flow faster into the continent as a nocturnal jet. As the stable boundary layer grows thicker, this flow tends to be eliminated at the surface. By morning, the mesoscale PGF points north, forcing the inland flow to become south-easterly, while on the coast flow becomes almost easterly. This scenario repeats during dry and wet seasons and can be understood as a consequence of the south–north propagation of an atmospheric circulation resembling a helix with its rotation axis oriented parallel to the shoreline.

The subsurface biological structure of Southern Ocean eddies revealed by BGC-Argo floats

Mesoscale eddies play several key roles in the Southern Ocean by redistributing momentum, potential vorticity, heat and salt. However, the role of eddies in Southern Ocean biogeochemistry has received less attention. Previous studies have been based on satellites or models. In this study, Southern Ocean eddies were matched with BGC-Argo float profiles to investigate the subsurface biological structure of Southern Ocean eddies. We focused on four frontal regions in the Indian sector of the Southern Ocean where data density is greater than in the other basins. The seasonality and regional variability of subsurface chlorophyll and backscatter structures were revealed by calculating anomalies in eddies relative to outside of eddies, in each season and frontal region. Chlorophyll anomalies in the mixed layer were mostly positive for both cyclones and anticyclones in all regions in spring, summer and autumn. Anomalies in winter were close to zero. Eddy pumping was the most likely mechanism sustaining high chlorophyll in cyclones. Eddy-induced Ekman pumping, in combination with deep vertical mixing was likely the driver of enhanced chlorophyll in anticyclones. Backscatter anomalies were mostly consistent with chlorophyll anomalies. Chlorophyll to backscatter ratios (chlorophyll:bbp) were also calculated to help understand the role of light acclimation in the phytoplankton. There were strong subsurface maxima for in-eddy chlorophyll:bbp in spring and summer. Such structures were weak in autumn and winter, but chlorophyll:bbp in the upper 30 m in autumn and winter was mostly higher than spring and summer in the same region. Photo-acclimation to low light at depth was evident for both cyclones and anticyclones in spring and summer in the euphotic zone, while in autumn and winter it was observed throughout the upper water column. The majority of positive chlorophyll anomalies were due to both an increase of biomass and photo-acclimation while few cases were due to photo-acclimation only.

Internal mixing of rotating stars inferred from dipole gravity modes

During most of their life, stars fuse hydrogen into helium in their cores. The mixing of chemical elements in the radiative envelope of stars with a convective core is able to replenish the core with extra fuel. If effective, such deep mixing allows stars to live longer and change their evolutionary path. Yet localized observations to constrain internal mixing are absent so far. Gravity modes probe the deep stellar interior near the convective core and allow us to calibrate internal mixing processes. Here we provide core-to-surface mixing profiles inferred from observed dipole gravity modes in 26 rotating stars with masses between 3 and 10 solar masses. We find a wide range of internal mixing levels across the sample. Stellar models with stratified mixing profiles in the envelope reveal the best asteroseismic performance. Our results provide observational guidance for three-dimensional hydrodynamical simulations of transport processes in the deep interiors of stars.

Optimization of deep soil mixing mass for construction of tunnel cross-passage in Bangkok

A cross-passage from a tunnel to an intervention shaft in Bangkok subsoils was modelled using three-dimensional finite element analysis. The optimization was performed for reducing the volume of soil-cement mass while maintaining ground stability. The result showed that the factor of safety increases linearly with the thickness of improved ground. The relationship between the improved groundmass and ground surface settlement curve was made. It was found that the predicted settlement profile can be captured by the formula proposed by Peck and O’Reilly, providing that the empirical coefficient of 0.6 is fit well with the results from the 3D simulation. This value was obtained from the average of empirical coefficient value 0.4 and 0.7, which are the empirical coefficient values of stiff clay and soft clay, respectively based on the overburden thickness.

A 30 ‐Year Time Series of Transient Tracer‐Based Estimates of Anthropogenic Carbon in the Central Labrador Sea

AbstractWe use a 30‐year time series (1986–2016) of dichlorodifluoromethane (CFC‐12) concentrations with a refined transit time distribution (TTD) method, to estimate the temporal variation of anthropogenic carbon (Cant) in the Central Labrador Sea. We determined that the saturation of CFC‐12 and sulfur hexafluroide (SF6) in newly‐formed Labrador Sea Water had departed significantly from 100% and varied systematically with time. Multiple linear regression of the time‐varying saturation, with the tracer's atmospheric growth rate and the wintertime mixed layer depth as independent variables, allowed reconstruction of the saturation history of CFC‐12 and SF6 in wintertime surface waters, which was implemented in the TTD method. Use of the time‐varying saturation for CFC‐12 gave Cant concentrations ∼7 μmol kg−1 larger than estimates obtained assuming a constant saturation of 100%. The resulting Cant column inventories were ∼20% larger and displayed lower interannual variability compared to conventional TTD‐based estimates. The column inventory of Cant increased at an average rate of 1.8 mol m−2 y−1 over the 30‐year period. However, the accumulation rate of Cant was higher than this average in the early 1990s and since 2013, whereas inventories remained almost unchanged between 2003 and 2012. The variation in the Cant accumulation rate is shown to be linked to temporal variability in the relative layer thickness of the annually ventilated Labrador Sea Water and the underlying Deep Intermediate Water. The non‐steady Cant accumulation highlights the importance of sampling frequency, especially in regions of variable deep mixing and high carbon inventories, and potential misinterpretation of Cant dynamics.

Photoacclimation State of Thalassiosira weissflogii is not Affected by Changes in Optical Depth Under A Fluctuating Light Regime Simulating Deep Mixing1.

Satellite-based remote sensing allows for global estimates of phytoplankton primary productivity by converting measurements of ocean color or photon absorption into units of carbon fixation. Models which perform this conversion often require an estimate of phytoplankton photoacclimation state such as the carbon to chlorophyll a ratio (C:Chl). Recently, our group developed a new photoacclimation model that can be applied to models of primary production. The model assumes that the phytoplankton photoacclimation state is not affected by periods of darkness during deep mixing beneath the photic zone, due to reduction in the plastoquinone pool in darkness and the subsequent deactivation of the signal for chlorophyll synthesis. In this study, we tested these assumptions by culturing the marine diatom Thalassiosira weissflogii under fluctuating light conditions simulating three different optical depths with progressively increasing deep mixing periods. The photoacclimation state, measured by the ratio of C:Chl, in T. weissflogii was not affected by changes in the length of simulated deep mixing periods. In addition, analysis of photosynthesis vs. irradiance (PE) curves showed that increases in optical depth caused decreases in both the maximum Chl-normalized rate of photosynthesis (Pbmax ) and in the slope of light-limited photosynthesis (αb ), but had no effect on the half-saturation irradiance (Ek , another metric of photoacclimation). However, measurements of chlorophyll fluorescence during simulated deep mixing did not support the hypothesis that the PQ pool was reduced during dark periods. Thus, our findings support the use of the photoacclimation model for estimating primary production while suggesting the need for further research into the mechanisms controlling photoacclimation in the upper mixed layer environment of the ocean.

Prediction of Unconfined Compressive Strength and Flexural Strength of Cement-Stabilized Sandy Soils: A Case Study in Vietnam

Stabilization of sand by the deep mixing method increases its unconfined compressive strength (UCS) so that it can be used as a foundation for infrastructure. This paper presents research work on physicomechanical properties of soil–cement columns installed in the central area of Vietnam, which consists of sandy clay, fine sand, medium sand, and coarse sand. Two types of cement were mixed with these soils in amounts of 150–350 kg/m3 at a water-cement ratio of 0.6. Approximately 80 soil–cement samples were prepared and cured at standard moist conditions and then tested for unconfined compressive strengths and flexural strengths at 7, 14, 28, and 56 days. The maximum unconfined compressive strength of the soil–cement was 10 MPa after 56-day curing for the coarse sand mixed with 350 kg/m3 cement. The Bayesian Model Averaging and the Principal Component Analysis Methods were used to evaluate the influence of different variables on the UCS of the cement-stabilized soil. The analysis shows that the UCS of the soil–cement depended on the contents of SiO2, SO3, K2O in the soil, the soil grain size, the cement content, and the curing time. The prediction equations allow the determination of unconfined compressive strength and flexural strength of cement-stabilized sand by the wet mixing method used as a ground improvement method for foundations.

Rapid photoacclimation during episodic deep mixing augments the biological carbon pump

Episodic deep mixing events are one component of the biological carbon pump that physically transports organic carbon into the mesopelagic. Episodic deep mixing also disrupts summertime thermal stratification thereby changing the light field and nutrient concentrations available for phytoplankton growth. Phytoplankton survival and growth below the mixed layer following restratification depends on how rapidly cells can employ a variety of photoacclimation processes in response to the environmental changes. To compare the relative timescales of summertime episodic deep mixing events with the timescales of phytoplankton photoacclimation processes, we first analyzed autonomous float data to survey the frequency and magnitude of deep mixing events in the western North Atlantic Ocean. Next, we simulated a sustained deep mixing event in the laboratory and measured rates of acclimation processes ranging from light harvesting to growth in a model diatom and green alga. In both algae increases in chlorophyll (Chl) were coupled to growth, but growth of the green alga lagged the diatom by about a day. In float profiles, significant increases in Chl and phytoplankton carbon (C phyto) were detected below the mixed layer following episodic deep mixing events. These events pose a previously unrecognized source of new production below the mixed layer that can significantly boost the amount of carbon available for export to the deep ocean.

An Extension to Barcelona Basic Model Predicting the Behavior of Unsaturated Soils

Slope failures along highways can highly affect the stability of road structure. In this respect, the analysis of the wetting process in unsaturated media is of great importance to slopes and pavements. In this study, the Barcelona basic model (BBM) was implemented in finite difference-based program to investigate the geomechanical behavior of fine-grained unsaturated soils subjected to wetting. This numerical modeling has been verified against some data in the literature. The application of the model was investigated for the response of a compacted fill slope involving suction-induced problems. The study also considered the effect of the over-consolidation ratio (OCR) in unsaturated soils, especially stress paths, as a modification to BBM. The simulations also include using BBM and modified BBM in FLAC to evaluate the response of deep soil mixing (DSM) improvement method in subsurface areas of the pavements in unsaturated media. Finally, modified BBM codes in the finite-difference program were presented for dealing with problems associated with soil slope and the DSM method to improve the foundation of pavements. Also, over-consolidation ratio has been added to BBM to optimize its application in suction various processes. Overall, it has been found that the modified BBM leads to more occurred results.

Emission reduction through preconstruction and utilization of alternative materials in infrastructure projects

Construction have a significant impact on the amount of emitted green-house gas (GHG) and a potential to cut emissions is quite substantial. In this article, possibilities provided by preconstruction activities and utilization of alternative materials are discussed. Preconstruction means actions to be taken in advance when soils in the construction area are too soft. These actions may be e.g. deep mixing, replacement or preloading. Considering that constructed subgrades, ground improvement and some other earth works required for the infrastructure project can be very energy-intensive, emissions can be reduced substantially only if preconstruction options, areal mass balance and potentiality of alternative materials are identified early enough. Examined case studies also focus on possibilities to reduce emissions related to certain materials e.g. deep mixing binding agents. In deep mixing, blend of cement and quick lime is the most traditional binder component. Therefore, reducing the amount of cement + lime and selecting the cement type have a great impact on CO2 emissions. Cement amount can be reduced by alternative materials such as fly ash, blast furnace slag or gypsum. Possibilities to reduce CO2 emissions were studied in various infrastructure projects and reflected to climate strategies. The studies were focused on areas and operations where changes were possible such as transportation, construction materials or subgrade construction methods. This article describes the case studies related to construction of various infrastructure projects and identified possibilities to reduce emissions.

Deep soil mixing for stabilising deep excavations

Excavating tunnels in Scandinavia can be difficult and costly due to deep layers of soft clay. To reduce cost and risk, traditional steel struts can be replaced by deep soil mixing (DSM) struts, as is done at West Link project in Gothenburg. DSM struts are made by mixing columns of lime and cement with the clay in the shaft, usually in rows or a grid pattern. Retaining walls can be designed more lightly while reducing deformations that might affect adjacent buildings. Also, earthworks can be performed more easily. However, the safety of the shaft is dominated by the acquired undrained shear strength of the DSM columns. This strength is hard to guarantee compared to steel struts. DSM strength has a reputation of being ‘poorly understood’ or ‘variable’, so authorities apply restrictions to DSM application as bottom struts. This article comprehensively studies the acquired strength of DSM for excavation support. Undrained shear strengths measured in 315 tests were investigated and two full scale test excavations were back calculated.It has been shown that although the achieved shear strength depends on many different input parameters, when these inputs are taken into account, the acquired shear strength can be predicted. Important inputs are curing time and temperature, soil natural shear strength, confining stress, binder quantity and binder composition. Particularly the curing time has a great impact and improvements are suggested for dealing with measurements made with incomplete curing. It is proven that the acquired DSM shear strength can be much higher than current restrictions of 100 kPa and a strength prediction formula is proposed that takes these inputs into account.

Applications of deep mixing technology for mitigating earth-quake disaster in Urban Area

Many earthquakes take place every year in Japan, in which the 1995 Hyogoken-Nambu earthquake and the 2011 Tōhoku earthquake induced both a humanitarian crisis and massive economic impacts. As liquefaction, which occurs in a loose and saturated sand layer, induces quite large damages of infrastructures, the importance of liquefaction mitigation has been emphasized to minimize earthquake disasters for many years. Many kinds of ground improvement techniques based on various improvement principles have been developed for earthquake disaster mitigation. Among them, the deep mixing technology has been often applied to liquefaction mitigation. In the techniques, in-situ soil is mixed with binder by several types of methods: mechanical mixing, high pressure injection mixing and combined mixing. The high applicability of the method was confirmed in several earthquakes, including the 1995 Hyogoken-Nambu earthquake and the 2011 Tōhoku earthquake. In this paper, the outline of the deep mixing method and some applications of the deep mixing method to earthquake disaster mitigation are briefly introduced.

Railway embankment behaviour due to increased axle loads - A numerical study

Due to an increase in axle loads, the development of excess pore water pressure and settlement in a railway track foundation of fine-grained subgrade soil can be observed. A thorough understanding of the mechanism of development of excess pore water pressure is essential for understanding the development of settlements and the design of potential ground improvement. In this paper, a three dimensional numerical study is presented, which investigates the effects of an increase in axle loads of trains on both excess pore water pressure and settlement. Special attention is given to a soft soil layer beneath the embankment and the influence of ground improvement (deep soil mixing columns). As a result, an increase in axle loads leads to a considerable increase in both excess pore pressures and settlement in the subgrade layer. This increase is more significant in the case of heavy axle load (32.5 tons) than that of the light axle load (16 tons). In addition, cyclic loading can lead to a considerable increase in both vertical displacements and excess pore water pressure. The use of deep soil mixing columns reduces excess pore water pressures and settlements significantly.

The Formation of Subtropical Phytoplankton Blooms Is Dictated by Water Column Stability During Winter and Spring in the Oligotrophic Northwestern North Pacific

AbstractSubtropical phytoplankton blooms were observed in winter and late spring (rather than in early spring, as is typical) via shipboard observations in an area south of the Kuroshio Extension in the northwestern North Pacific subtropical gyre. Satellite‐based observations revealed that these submesoscale blooms occurred in warmer water masses in winter and in cooler water masses in late spring. The fact that winter blooms occurred in warmer areas suggests that they depend on water column stratification caused by solar heating. In contrast, the fact that the late‐spring blooms occurred in cooler areas suggests a breakdown of stratification due to a recurrence of convective mixing. Mesoscale blooms occurred at intermediate water temperatures in early spring, suggesting a repeating sequence of stratification and mixing during this period. Wintertime deep convective mixing in the northwestern North Pacific subtropical gyre creates Subtropical Mode Water. The behavior of the wintertime mixed‐layer depth, which determines the thickness of Subtropical Mode Water, also dictates the characteristics of subtropical blooms such as their magnitude and the timing of onset and demise. Based on in situ observations and a model analysis, we conclude that deeper winter mixing, which increases nutrient concentrations, will intensify early‐spring blooms and facilitate the formation of episodic blooms in winter and late spring. On the other hand, shallower winter mixing should increase stratification and thus facilitate the formation of smaller blooms, even in winter.

18th Nordic Geotechnical Meeting NGM2020 Urban Geotechnics Preface

The 18th Nordic Geotechnical Meeting NGM 2020 was arranged as a virtual conference on 18th-19th January 2021. The conference was organized by Finnish Geotechnical Society and Finnish Association of Civil Engineers, both from Helsinki. The Nordic Geotechnical Meetings are the oldest regional geotechnical conference series starting from Stockholm since 1950. As the name suggests this event is rather a more informal meeting than an actual formal conference even though normal conference content is followed. The 18th Nordic Geotechnical Meeting, originally scheduled to take place in Helsinki in May 2020, had to be postponed due to COVID-19 pandemic. The pandemic and travel restrictions were the reason to organize a virtual conference on 18th-19th January 2021, arranged in parallel with the 14th Baltic Sea Region Geotechnical Conference. The 18th NGM attracted total of 271 participants from 32 countries, successfully allowing to share insights and discuss ideas related to Urban Geotechnics, the main topic of the meeting.The 18th NGM proceedings contain 78 scientific papers presented during the conference alongside 16 industry presentations. In the proceedings, the papers are divided into sections related to:• Case histories• Soil and rock investigations (incl. geophysical methods)• Slope stability and other geohazards• Climate change and circular economy• Eurocode and standardization• Modelling, monitoring and digital solutions• Ground improvement methods• Foundations and deep excavations• Transport infrastructure• Statistical methods, risk analysis and reliability-based design• Cold regions – frost.Furthermore, three keynote speakers presented during the 18th NGM. The signature keynote entitled “From soft soil modelling to engineering application” was given by Professor Minna Karstunen from the Chalmers University of Technology. The other two keynotes were awarded jointly with 14th BSGC: Professor Masaki Kitazume “Applications of deep mixing technology for mitigating earthquake disaster in Urban Area” from Tokyo Institute of Technology, Japan, and professor Jean-Sebastien L’Heureux “On the use of benchmark test sites for research and development in geotechnical engineering” from the Norwegian University of science and technology. The conference had 66 oral presentations and 26 on-demand prerecorded presentations. For both conferences, four parallel streams were created. The conference platform used was ExOrdo. After each presentation there was about five minutes possibility to ask questions. Also chat possibility was available during presentations. The conference platform has been open for two months after the conference. The audience has been given possibility to continue discussion during these two months through the ExOrdo platform.Organizers asked participants to fill a feedback survey and the conference was considered successful as the participants gave 3.9 for overall conference experience and 3.8 for the virtual platform of the event (scale 1 poor … 5 excellent).We wish to express our thanks to all the authors and keynote speakers for their valuable contribution to the success of the conference. We want also to thank our sponsors, session chairs, stage managers, members of scientific board and organization committee, without whom this event would not be possible.Leena Korkiala-TanttuNGM 2020 scientific board chair, EditorAnne Tuomela and Anssi RauhalaCo-editors

Evidence of a rapid phosphorus-induced regime shift in a large deep reservoir

Ecological regime shift studies in freshwater systems are mainly limited to shallow lakes and reservoirs, while abrupt changes in deeper lakes are often attributed to climate change. Here, we demonstrate the application of regime shift theory to one of California's newest and deepest reservoirs, Diamond Valley Lake (DVL), which in recent years showed an unexpected rapid departure from its water quality conditions of the previous decade. The reservoir shifted from a well oxygenated condition with low phytoplankton growth to a hypoxic, phytoplankton-dominated turbid system. We statistically identified the critical stressor (phosphorus (P)), switch points, and its load threshold and characterized its transition to an alternative stable state and the stabilizing mechanisms contributing to hysteresis. We analyzed long-term environmental, chemical and flow data, conducted a hydrographic survey, and developed a hydrodynamic model to characterize the factors that contributed to regime shift and to evaluate different management strategies that might reverse this shift. Our findings indicate that large deep systems exhibit different transition dynamics in the presence of an acute stressor compared to regime shifts in shallow systems. A cumulative external TP load threshold of 4.6 mg m−2 d−1 added to the reservoir over nearly 11 months was identified as the critical stressor. For large deep systems, inherent morphometric features such as large relative depth combine with external stressors to drive regime shifts. Light winds, morphometric conditions impeding deep mixing, and a stable stratification that lasts up to 9 months makes DVL more susceptible to hypolimnetic hypoxia, an intrinsic factor accelerating regime shift. Results also suggest regime shift occurred in 2013, when new limnological processes were established to reinforce the new alternative stable state and existing ecosystem services were impaired. Interactions between hypoxia, internal P loading (~2.1 mg m−2 d−1), and seasonal cyanobacterial blooms were identified as mechanisms perpetuating the new alternative state.

The role of internal feedbacks in shifting deep lake mixing regimes under a warming climate

Abstract Climate warming is causing changes in the physics of deep lakes, such as longer summer stratification, increased water column stability, reduced ice cover, and a shallower depth of winter overturns. An ultimate consequence of warming would be a transition to a different mixing regime. Here we investigate the role of physical, chemical, and biological feedback mechanisms that unfold during a shift in mixing regime, and whether these feedbacks could prompt and stabilise the new regime. Although climate, interannual temperature variation, and lake morphometry are the main determinants of a mixing regime, when climate change causes shifts in mixing regime, internal feedback mechanisms may gain in importance and modify lake ecosystem functioning. We review the role of these feedbacks in three mixing regime shifts: from polymictic to seasonally stratified, from dimictic to monomictic, and from holomictic to oligomictic or meromictic. Polymictic lakes of intermediate depth (c. 3–10 m mean depth) could experience seasonal stratification if a stratification event triggers phytoplankton blooms or dissolved organic matter release, reducing transparency and therefore further heating the surface layer. However, this feedback is only likely to have influence in small and clear lakes, it would be easily disturbed by weather conditions, and the resulting stratified state does not remain stable in the long term, as stratification is lost in winter. The ice‐albedo feedback might cause an accelerated shift from ice‐covered (dimictic) to ice‐free (monomictic) winters in sufficiently deep (mean depth 50 m or more) lakes, where temperature memory is carried over from one winter to the next. Nevertheless, there is an ongoing debate into whether this process can persist during natural weather variations and overcome self‐stabilising mechanisms such as thermal insulation by snow. The majority of studies suggest that a gradual transition from dimictic to monomictic is more likely than an abrupt transition. A shift from a holomictic to a meromictic regime can occur if anoxia is triggered by incomplete mixing and an increase in deep‐water density—through the accumulation of solutes—exceeds a density decrease by hypolimnetic warming. A shift to meromixis would strongly alter the biology of a lake and might be difficult to reverse. If solutes accumulate only minimally in the hypolimnion, an oligomictic regime is formed, in which years with complete and incomplete mixing alternate. Understanding the importance of feedback mechanisms and the role of biogeochemistry when lakes shift in mixing regime could lead to a better understanding of how climate change affects lake ecosystems.

Enhancement of the North Atlantic CO<sub>2</sub> sink by Arctic Waters

Abstract. The North Atlantic north of 50∘ N is one of the most intense ocean sink areas for atmospheric CO2 considering the flux per unit area, 0.27 Pg-C yr−1, equivalent to −2.5 mol C m−2 yr−1. The northwest Atlantic Ocean is a region with high anthropogenic carbon inventories. This is on account of processes which sustain CO2 air–sea fluxes, in particular strong seasonal winds, ocean heat loss, deep convective mixing, and CO2 drawdown by primary production. The region is in the northern limb of the global thermohaline circulation, a path for the long-term deep-sea sequestration of carbon dioxide. The surface water masses in the North Atlantic are of contrasting origins and character, with the northward-flowing North Atlantic Drift, a Gulf Stream offspring, on the one hand and on the other hand the cold southward-moving low-salinity Polar and Arctic waters with signatures from Arctic freshwater sources. We have studied by observation the CO2 air–sea flux of the relevant water masses in the vicinity of Iceland in all seasons and in different years. Here we show that the highest ocean CO2 influx is to the Arctic and Polar waters, respectively, -3.8±0.4 and -4.4±0.3 mol C m−2 yr−1. These waters are CO2 undersaturated in all seasons. The Atlantic Water is a weak or neutral sink, near CO2 saturation, after poleward drift from subtropical latitudes. These characteristics of the three water masses are confirmed by data from observations covering 30 years. We relate the Polar Water and Arctic Water persistent undersaturation and CO2 influx to the excess alkalinity derived from Arctic sources. Carbonate chemistry equilibrium calculations clearly indicate that the excess alkalinity may support at least 0.058 Pg-C yr−1, a significant portion of the North Atlantic CO2 sink. The Arctic contribution to the North Atlantic CO2 sink which we reveal was previously unrecognized. However, we point out that there are gaps and conflicts in the knowledge about the Arctic alkalinity and carbonate budgets and that future trends in the North Atlantic CO2 sink are connected to developments in the rapidly warming and changing Arctic. The results we present need to be taken into consideration for the following question: will the North Atlantic continue to absorb CO2 in the future as it has in the past?

Radium-228-derived ocean mixing and trace element inputs in the South Atlantic

Abstract. Trace elements (TEs) play important roles as micronutrients in modulating marine productivity in the global ocean. The South Atlantic around 40∘ S is a prominent region of high productivity and a transition zone between the nitrate-depleted subtropical gyre and the iron-limited Southern Ocean. However, the sources and fluxes of trace elements to this region remain unclear. In this study, the distribution of the naturally occurring radioisotope 228Ra in the water column of the South Atlantic (Cape Basin and Argentine Basin) has been investigated along a 40∘ S zonal transect to estimate ocean mixing and trace element supply to the surface ocean. Ra-228 profiles have been used to determine the horizontal and vertical mixing rates in the near-surface open ocean. In the Argentine Basin, horizontal mixing from the continental shelf to the open ocean shows an eddy diffusion of Kx=1.8±1.4 (106 cm2 s−1) and an integrated advection velocity w=0.6±0.3 cm s−1. In the Cape Basin, horizontal mixing is Kx=2.7±0.8 (107 cm2 s−1) and vertical mixing Kz = 1.0–1.7 cm2 s−1 in the upper 600 m layer. Three different approaches (228Ra diffusion, 228Ra advection, and 228Ra/TE ratio) have been applied to estimate the dissolved trace element fluxes from the shelf to the open ocean. These approaches bracket the possible range of off-shelf fluxes from the Argentine Basin margin to be 4–21 (×103) nmol Co m−2 d−1, 8–19 (×104) nmol Fe m−2 d−1 and 2.7–6.3 (×104) nmol Zn m−2 d−1. Off-shelf fluxes from the Cape Basin margin are 4.3–6.2 (×103) nmol Co m−2 d−1, 1.2–3.1 (×104) nmol Fe m−2 d−1, and 0.9–1.2 (×104) nmol Zn m−2 d−1. On average, at 40∘ S in the Atlantic, vertical mixing supplies 0.1–1.2 nmol Co m−2 d−1, 6–9 nmol Fe m−2 d−1, and 5–7 nmol Zn m−2 d−1 to the euphotic zone. Compared with atmospheric dust and continental shelf inputs, vertical mixing is a more important source for supplying dissolved trace elements to the surface 40∘ S Atlantic transect. It is insufficient, however, to provide the trace elements removed by biological uptake, particularly for Fe. Other inputs (e.g. particulate or from winter deep mixing) are required to balance the trace element budgets in this region.