Deep-water Renewal Research Articles

Deep-water renewal in the Skagerrak during the last 1200 years triggered by the North Atlantic Oscillation: evidence from benthic foraminiferal δ8O

Benthic foraminiferal tests of a sediment core from southwestern Skagerrak (northeastern North Sea, 420 m water depth) were investigated for their ratio of stable oxygen isotopes. During modern times sudden drops in temperature and salinity of Skagerrak deep waters point to advection-induced cascades of colder and denser central North Sea waters entering the Skagerrak. These temperature drops, which are recorded in benthic foraminiferal tests via the stable oxygen isotopic composition, were used to reconstruct deep-water renewal in the Skagerrak. In a second step we will show that, at least during the last 1200 years, Skagerrak deep-water renewal is triggered by the negative phase of the North Atlantic Oscillation (NAO). The NAO exerts a strong influence on the climate of northwestern Europe. It is currently under debate if the long-term variability of the NAO is capable of influencing Northern Hemisphere climate on long timescales. The data presented here cannot reinforce these speculations. Our data show that most of the ‘Little Ice Age’ was dominated by comparably warm deep-water temperatures. However, we did find extraordinary strong temperature differences between central North Sea waters and North Atlantic water masses during this time interval.

The Holocene record of Loch Etive, western Scotland: Influence of catchment and relative sea level changes

Two sediment cores from inner Loch Etive, a deep fjord basin on the west coast of Scotland, reveal a continuous sediment sequence spanning the last 10,000 yr. Benthic foraminiferal assemblages indicate that marine conditions prevailed in Loch Etive throughout the Holocene. However, changes in sediment grain size composition and magnetic susceptibility suggest that the strength and frequency of deep water renewal events has varied through time. This exchange with coastal water is controlled mainly by relative sea level changes (influencing sill depth) and the influx of freshwater to the loch. The very early Holocene sediments are well-sorted coarse silt–fine sands, with a diverse assemblage of calcareous foraminifera indicating frequent renewal of bottom waters. During this period, immediately following the withdrawal of Younger Dryas glaciers, denudation rates were high in the catchment area. The period 9–7 ky BP, during which relative sea level rose in this part of W Scotland, is characterised by highly variable environments. The grain size data indicate an enhanced transport of sandy bed load material from the ‘Bonawe Sill’ region to the basin deep during the mid-Holocene relative sea level high (8–9 m higher than present). High relative sea levels at this time would explain the occurrence of frequent, vigorous deep water renewal events and an increased tidal current regime in the loch. The late Holocene record is characterised by an upward fining of sediment grain size, disappearance of calcareous benthic foraminifera and increasing organic content; suggesting a development towards a more restricted fjordic circulation in response to the progressive lowering of relative sea level. A sudden acceleration in this late Holocene trend took place during the last millennium, during which time human land use and deforestation have influenced the sediment and organic material flux to Loch Etive. It appears that average Holocene sediment accumulation rates in the ‘Bonawe Deep’ were about 0.01–0.03 g cm − 2 yr − 1 , which is far below modern estimates for the site. This can be explained by vigorous bottom current activity during much of the Holocene, leading to a low net depositional rate.

Mo–total organic carbon covariation in modern anoxic marine environments: Implications for analysis of paleoredox and paleohydrographic conditions

Sedimentary molybdenum, [Mo]s, has been widely used as a proxy for benthic redox potential owing to its generally strong enrichment in organic‐rich marine facies deposited under oxygen‐depleted conditions. A detailed analysis of [Mo]s–total organic carbon (TOC) covariation in modern anoxic marine environments and its relationship to ambient water chemistry suggests that (1) [Mo]s, while useful in distinguishing oxic from anoxic facies, is not related in a simple manner to dissolved sulfide concentrations within euxinic environments and (2) patterns of [Mo]s‐TOC covariation can provide information about paleohydrographic conditions, especially the degree of restriction of the subchemoclinal water mass and temporal changes thereof related to deepwater renewal. These inferences are based on data from four anoxic silled basins (the Black Sea, Framvaren Fjord, Cariaco Basin, and Saanich Inlet) and one upwelling zone (the Namibian Shelf), representing a spectrum of aqueous chemical conditions related to water mass restriction. In the silled‐basin environments, increasing restriction is correlated with a systematic decrease in [Mo]s/TOC ratios, from ∼45 ± 5 for Saanich Inlet to ∼4.5 ± 1 for the Black Sea. This variation reflects control of [Mo]s by [Mo]aq, which becomes depleted in stagnant basins through removal to the sediment without adequate resupply by deepwater renewal (the “basin reservoir effect”). The temporal dynamics of this process are revealed by high‐resolution chemostratigraphic data from Framvaren Fjord and Cariaco Basin sediment cores, which exhibit long‐term trends toward lower [Mo]s/TOC ratios following development of water column stratification and deepwater anoxia. Mo burial fluxes peak in weakly sulfidic environments such as Saanich Inlet (owing to a combination of greater [Mo]aq availability and enhanced Mo transport to the sediment‐water interface via Fe‐Mn redox cycling) and are lower in strongly sulfidic environments such as the Black Sea and Framvaren Fjord. These observations demonstrate that, at timescales associated with deepwater renewal in anoxic silled basins, decreased sedimentary Mo concentrations and burial fluxes are associated with lower benthic redox potentials (i.e., more sulfidic conditions). These conclusions apply only to anoxic marine environments exhibiting some degree of water mass restriction (e.g., silled basins) and are not valid for low‐oxygen facies in open marine settings such as continent‐margin upwelling systems.

Hydrography and Circulation of Ice-Marginal Lakes at Bering Glacier, Alaska, U.S.A

ABSTRACTAn extensive suite of physical oceanographic, remotely sensed, and water quality measurements, collected from 2001 through 2004 in two ice-marginal lakes at Bering Glacier, Alaska—Berg Lake and Vitus Lake—show that each has a unique circulation controlled by their specific physical forcing within the glacial system. Conductivity profiles from Berg Lake, perched 135 m a.s.l., show no salt in the lake, but the temperature profiles indicate an apparently unstable situation, the 4°C density maximum is located at 10 m depth, not at the bottom of the lake (90 m depth). Subglacial discharge from the Steller Glacier into the bottom of the lake must inject a suspended sediment load sufficient to marginally stabilize the water column throughout the lake. In Vitus Lake, terminus positions derived from satellite imagery show that the glacier terminus rapidly retreated from 1995 to the present resulting in a substantial expansion of the volume of Vitus Lake. Conductivity and temperature profiles from the tidally influenced Vitus Lake show a complex four-layer system with diluted (~50%) seawater in the bottom of the lake. This lake has a complex vertical structure that is the result of convection generated by ice melting in salt water, stratification within the lake, and freshwater entering the lake from beneath the glacier and surface runoff. Four consecutive years, from 2001 to 2004, of these observations in Vitus Lake show little change in the deep temperature and salinity conditions, indicating limited deep water renewal. The combination of the lake level measurements with discharge measurements, through a tidal cycle, by an acoustic Doppler Current Profiler (ADCP) deployed in the Seal River, which drains the entire Bering system, showed a strong tidal influence but no seawater entry into Vitus Lake. The ADCP measurements combined with lake level measurements established a relationship between lake level and discharge, which when integrated over a tidal cycle, gives a tidally averaged discharge ranging from 1310 to 1510 m3 s−1.

Deep-Water Flow over the Lomonosov Ridge in the Arctic Ocean

Abstract The Arctic Ocean likely impacts global climate through its effect on the rate of deep-water formation and the subsequent influence on global thermohaline circulation. Here, the renewal of the deep waters in the isolated Canadian Basin is quanitified. Using hydraulic theory and hydrographic observations, the authors calculate the magnitude of this renewal where circumstances have thus far prevented direct measurements. A volume flow rate of Q = 0.25 ± 0.15 Sv (Sv ≡ 106 m3 s−1) from the Eurasian Basin to the Canadian Basin via a deep gap in the dividing Lomonosov Ridge is estimated. Deep-water renewal time estimates based on this flow are consistent with 14C isolation ages. The flow is sufficiently large that it has a greater impact on the Canadian Basin deep water than either the geothermal heat flux or diffusive fluxes at the deep-water boundaries.

Basin-wide circulation and ventilation study in the East Sea (Sea of Japan) using chlorofluorocarbon tracers

Abstract Near-synoptic basin-wide distributions of the chlorofluorocarbon (CFC) tracers were observed during the summer of 1999 in the East Sea (Sea of Japan) to study its circulation and ventilation. The CFCs penetrate throughout the water column, and the bottom water (BW) CFC concentrations have increased over measurements made in 1995 and 1996, indicating a continuous weak renewal of deep waters in the East Sea during this recent period. The CFC-11 water-column inventory estimated in 1999 (2000–5500 nmol m−2) is substantially higher than that of the adjacent North Pacific Ocean. A striking anticorrelation between the BW dissolved oxygen and CFC concentrations in the Japan Basin is attributed to more active vertical mixing along the basin boundary, a dynamically isolated deep eastern Japan Basin, and recent changes in deep water ventilation. The BW of the eastern Japan Basin with the lowest CFC content is the oldest water parcel in the East Sea, although it is colder and contains higher oxygen concentration than the surrounding BW. Active entrainment of overlying deep water is inferred from the unexpectedly higher BW CFC concentrations in the Yamato Basin. This may explain why the Yamato Basin BW is warmer and less oxygenated than the other basins in the East Sea, although it is believed to be a terminus of the ventilated DWs. Mesoscale anticyclonic eddies, which have been fused with other deeply convected waters in the northwestern East Sea, seem to play an important role in transferring surface-water properties to depths greater than 2500 m. The subsurface CFC maximum, centered within the typical East Sea Intermediate Water (ESIW) layer, is more widely distributed south of the subpolar front than previously reported. The ESIW consists primarily of the colder and fresher winter surface water in the northern region that has subducted below the subpolar front.

On Deep-Water Renewals in Indian Arm, British Columbia: Sensitivity to the Production of Turbulent Kinetic Energy Caused by Horizontal Variations in the Flow Field

Abstract A two-dimensional (i.e., laterally averaged) numerical model of the circulation in Burrard Inlet and Indian Arm near British Columbia, Canada, is used to examine the sensitivity of deep-water renewal events in Indian Arm to the turbulent mixing in the lee of the narrow sills in Burrard Inlet. Horizontal variations in the flow field can have an important influence on the production of turbulent kinetic energy near the sills and therefore also on the renewal events in Indian Arm. An ad hoc modification to the expression for the production of turbulent kinetic energy, required to obtain an acceptable simulation downstream of Second Narrows in Burrard Inlet, also results in a reasonable simulation of the observed circulation in Indian Arm. The modified laterally averaged model can reproduce the main features of the circulation away from the narrow sills. However, it seems that a three-dimensional model will be required if the circulation is to be simulated with greater accuracy and without the ad hoc modification, which has a free parameter.

A preliminary investigation of basin water response to climate forcing in a Scottish fjord: evaluating the influence of the NAO

The sea lochs (fjords) of NW Scotland bridge the land–ocean interface in a region of Europe which is particularly well situated to monitor changes in westerly air streams. Inter-annual atmospheric circulation changes at this latitude are largely governed by the North Atlantic Oscillation (NAO). Comparing two recent extreme NAO years, a two-dimensional model of Loch Sunart, NW Scotland, is used to examine the potential effects of climate oscillations on the magnitude and frequency of deep-water renewal events and the resulting water properties in the fjord basins. In the upper basin of the fjord, meteorological forcing during the high NAO index year (1988–89) resulted in less-frequent deep-water renewal, greater variability in basin salinity and a lower annual-mean salinity (by 0.52) than that predicted for the low NAO index year (1995–96). In the main basin, variations in meteorological forcing had much less effect on basin water properties. In both basins, predicted deep water inflow was significantly greater during the negative phase NAO, with annual inflow to the upper and main basins being respectively 50% and 300% greater during 1995–96 relative to 1988–99. Through a sensitivity analysis, the NAO is shown to affect upper basin water properties through the influence of low-frequency anomalies in the meteorological forcing, particularly the enhanced westerly wind stress associated with positive phases, which inhibits deep water renewal over the winter months. The salinity of the main and upper basins respond differently to the boundary forcing due to differential tidal mixing above the respectively sub- and super-critical entrance sills. Predictions of basin water isotope ratios are made by applying the salinity: δ 18O water mixing line for the region to the salinity results; the combination of the weak dependence of δ 18O water on salinity and the stable salinity predicted for the main basin suggests that some fjord basins may provide ideal sites for palaeotemperature studies.

Cold intrusions in Lake Baikal: Direct observational evidence for deep-water renewal

We studied cold, deep-water intrusions in the South Basin of Lake Baikal on the basis of 2 yr of data (December 1995-November 1997) from near-bottom and near-surface thermistor strings, monthly conductivity-temperature-depth (CTD) profiles, and a near-bottom current meter, all collected near the South Basin maximum depth of 1,461 in. The data show intrusions into the greatest depths with temperatures of 0.08-0.20degreesC below ambient (similar to3.33 to similar to3.38degreesC at maximum depth). The intrusions were observed three times per year between January and June, when surface water is always cooler than deep water, with durations of a few (at least 1-3) days. The estimated water input ranged from 1 to 10 km(3) per event, and the annually accumulated volume was estimated to be 10-30 km(3), which is significantly less than the steady-state indirect estimates of 30-70 km(3) yr(-1) to the permanently stratified deep water (depth > 300 in). This indicates that not all of the cold intrusions reach the deepest area. Because the cooling of the near-bottom waters was not accompanied by a significant increase in ion or particle content and because deep sediment traps did not contain significant enrichments of minerogenic particles, we concluded that Selenga River inflow is not a possible source of the cold intrusions. Two CTD profiles in June 1996 and 1997 showed lower temperature throughout the deep water, as expected from thermobaric instabilities. The required downwelling is definitely not occurring in the pelagic interior but most probably by near-coast counterclockwise currents. The source of the regularly occurring deep intrusions is clearly cold surface water, but the actual mechanism is uncertain.

Climate variations, an overlooked factor influencing the recent marine environment. An example from Gullmar Fjord, Sweden, illustrated by benthic foraminifera and hydrographic data

Like most sill fjords, Gullmar Fjord on the Swedish west coast, is subject to periods of stagnation. Deep water is usually renewed annually, but since the late 1970s several low-oxygen events have been documented in the deepest part of the fjord. These events occurred during a time when the North Atlantic Oscillation (NAO) was in a highly positive phase. We investigated how the benthic environment, in the deepest part of the fjord, has varied during the 20th century, using benthic foraminifera and an extensive history of instrumental hydrographic data. The foraminifera have undergone one major faunal change and two minor modifications during this time. The major faunal change occurred in the late 1970s to early 1980s, when the common Skagerrak-Kattegat fauna was replaced by one dominated by the opportunistic, low-oxygen-tolerant species,Stainforthia fusiformis. This major faunal change appears to be related to the severe low-oxygen event in 1979–1980. In the latter part of the 1990s the fauna changed again; the concentration ofS. fusiformis was still high, but other low-oxygen-tolerant species also became important. This minor faunal modification occurred in connection with the 2-yr stagnation period between 1996 and 1998 when a low-oxygenevent evolved, the most severe recorded in Gullmar Fjord. Between 1930 and 1980, there was little faunal variation, and a stable fjord environment is indicated. During this time, negative NAO indexes dominated and climate was more continental, with an increase in winds from the northeast and east. In connection with a climate transition indicated by the NAO index switching from positive to negative, a minor faunal change occurred in the late 1920s to early 1930s: the concentration of the Skagerrak-Kattegat fauna increased markedly in the fjord. The fauna characterizing the positive NAO phase between 1900 and the late 1920s is very different from the present positive NAO fauna. The foraminiferal record laid down between approximately 1914 and 2001 indicates that between 1930 and 1980 Gullmar Fjord was a stable fjord environment. During the last 20 yr, it experienced conditions that were more fluctuating and changing. For the most part, changes in the foraminiferal fauna are caused by changes in the deep-water renewal, their extent and frequency, which in turn are caused by climatic oscillations.

The rate of inflow and mixing during deep‐water renewal in a sill fjord

We obtained high‐resolution data on a deep‐water renewal in the basin of Gullmar Fjord, Sweden, using an autonomous profiling platform. In the middle of the fjord, where the platform is anchored, renewal starts with the passage of a gravity current front and continues with a steady thickening of the new, oxygen‐rich, low‐nitrate bottom layer and an associated lifting of the old, oxygen‐depleted, high‐nitrate bottom water. The basin continuously fills to sill level during a period of 10 d. At the mouth of the fjord, a three‐layer structure develops. Renewal is driven by the density difference between the intermediate water inside and the new deep water outside the fjord. The volume flux is well predicted by a hydraulic exchange model in which the upper layer plays a passive role. Local upwelling and downwelling of the upper halocline cause fluctuating baroclinic currents during renewal, but these seem to have little influence on the average volume flux of new deep water. Entrainment rates are small, and the associated volume flux increase seems to be balanced by detrainment.

The multi‐year development of long‐lived convective chimneys in the Greenland Sea

The first convective chimney in the Greenland Sea to have its temperature and salinity structure fully mapped was discovered in March 2001 near 75°N 0°W [Wadhams et al., 2002]. Later cruises have shown that this remarkable feature has survived for a further 26 months, being remapped in summer 2001, winter 2002, summer 2002 and most recently in April–May 2003, making it the longest‐lived chimney yet seen in the world ocean. The chimney has an anticyclonically rotating core and experiences an annual cycle in which it is uniform in properties from the surface to 2500 m in winter, but is capped by lower‐density water in summer. The latest cruise also discovered a second chimney, 70 km NW of the first, during a thorough survey of 15,000 km2 of the gyre centre which left the existence of further chimneys unlikely. We conclude that the 75°/0° chimney is not unique, but that Greenland Sea chimneys are rare and are probably rarer than in 1997, when several such features were discovered by a float survey. This has implications for deep water renewal and for the role of Greenland Sea convection in the North Atlantic circulation.

Red Sea deep water circulation and ventilation rate deduced from the 3He and 14C tracer fields

3He is injected in the deep ocean at plate boundaries in relation with hydrothermal activity. Its oceanic distribution, which is at a steady-state, shows appreciable vertical and horizontal gradients. Hence, 3He conservation equations may be inverted to determine the flow field and mixing coefficients within the ocean. Here, we use this technique to investigate the deep circulation of the Red Sea, whose deep thermohaline circulation is comparable to that of a miniature world ocean. 3He data, which are a combination of the Geosecs, Meseda and Merou cruises, are inverted using a linear inverse box model. The present study allows us to draw the details of the internal circulation. The latter is characterized by (i) a descending branch in the northernmost part of the basin, which corresponds to the sinking and subsequent north–south movement of dense surface waters and of additional water entrained by the sinking plume, (ii) by an internal counterclockwise recirculating loop with a northward return flow at intermediate depth. In the literature, the rate and modes of renewal of the Red Sea deep waters are poorly constrained, with bulk residence time estimates ranging from a few decades to a few centuries. In our model, the deep water renewal rate is directly dependent on the magnitude of the 3He sources. The global 3He flux is estimated using two independent approaches. The first method is based on the calculation of the mean 3He transfer flux at the air–sea interface. The second approach relies on recent estimates of the global terrestrial 3He flux. Both methods agree within their respective uncertainties. The circulation scheme defined by the inversion is further constrained by simulating the bomb 14C distribution. Both isotopes, one steady-state tracer ( 3He) and one transient tracer ( 14C), lead to reasonable agreement. The renewal time corresponding to the sinking of newly formed deep water in the northern part of the basin is 60 years. However, the global ventilation rate, deduced from the simulation of the decay rate of a numerical tracer, is much faster (26 years). This result shows, as already pointed out by others, that deep water formation is not the only process by which the deep Red Sea is ventilated. Although this mechanism is important in our model, the basin wide deep circulation coupled with efficient vertical exchange between the deep basin and the upper layers appear to be an even more powerful mode of ventilation.

Organic matter remineralisation and biogenic silica dissolution in a deep fjord in British Columbia, Canada: a regression analysis of upper ocean sediment-trap fluxes

Remineralisation of sinking particulate matter can be inferred from sediment-trap studies where fluxes decrease with depth. However, during a 4-year experiment of monthly sediment-trap deployments at two stations in Jervis Inlet, British Columbia, Canada, fluxes often increased with depth, a common observation in coastal and some oceanic settings. A regression analysis that simultaneously estimates the composition of ‘additional’ fluxes to deep sediment traps and inverse decay length (IDL) scales of the primary (operationally termed ‘anticipated’) flux is applied to the data from Jervis Inlet. IDL scales for organic carbon (OC), nitrogen (N) and biogenic silica (BSi) decreased with depth in a manner well described by the power function, and are used to generated curves of flux versus depth in the absence of additional fluxes. The curve for OC flux is comparable to previously published models, but in Jervis Inlet remineralisation was somewhat higher in the upper water column, and lower in deep waters, than implied from open-ocean curves. The first upper-ocean estimates for dissolution rates of sinking BSi are presented; conversion to time-dependent rate constants shows that BSi dissolution in Jervis Inlet was similar to results from other studies where dissolution was rapid. A fitting of the flux curves to the reactive continuum model demonstrates that settling organic matter in Jervis Inlet had 3–4 reactive components, while BSi had 6–7 reactive components and over-all was less susceptible to decay. Additional fluxes to deep sediment traps in Jervis Inlet appear to have resulted partly from resuspended sediment increasingly winnowed with distance from topographic boundaries. Resuspended fluxes were particularly abundant during deepwater renewals, and another source supplied additional fluxes to mid-depth and deep traps on a continual basis. Included in the analysis is an error term, which is interpreted as the degree to which anticipated fluxes were over- or under-represented in deep sediment traps. Although for the deep depth intervals in Jervis Inlet errors of regression analyses were negligible, for the more shallow intervals they were not. An enigma of the results from Jervis Inlet is that it appears OC, N and Al were generally over-collected at mid-depth sediment traps, while BSi was under-collected.

Long‐term changes in dissolved oxygen concentrations in the ocean caused by protracted global warming

In the Earth's geological record massive marine ecological change has been attributed to the occurrence of widespread anoxia in the ocean [Jahren, 2002; White, 2002; Wignall and Twitchett, 1996]. Climate change projection till the end of this century predict a 4 to 7% decline in the dissolve oxygen in the ocean [Bopp et al., 2002; Matear et al., 2000; Plattner et al., 2001; Sarmiento et al., 1998] suggesting the potential for global warming to eventually drive the deep ocean anoxic. To examine the multicentury impact of protracted global warming on oceanic concentrations of dissolved oxygen, we use a climate system model and a low‐order oceanic biogeochemical model. The models are integrated for an atmospheric equivalent CO2 concentration, which is specified to triple according to a standard scenario from the late nineteenth to the late twenty‐first century, and then is subsequently held constant at that elevated level for an additional 6 centuries. For the present day, the model successfully reproduced the large‐scale features of the dissolved oxygen field in the ocean. In the global warming simulation, the physical model displays marked changes in high‐latitude oceanic stratification and overturning, including near‐cessation of deep water renewal for depths greater than about 1.5 km during the period of elevated stable CO2 concentration. Our model predicts a decline in oxygen concentration through most of the subsurface ocean. Concentration changes in the thermocline waters result mainly from solubility changes in the upstream source waters, while changes in the deep waters result mainly from lack of ventilation and ongoing consumption of oxygen by remineralization of sinking particulate organic matter. Changes in the upper 2 km of the ocean generally show signs of equilibration by the end of the integration, but at greater depths, there occurs a slow but steady decline through to the end of the integration. By the end of the integration, we simulate a doubling of the volume of hypoxic water (less than 10 μmol/kg) in the thermocline of the eastern equatorial Pacific Ocean. During the integration deep ocean oxygen concentrations generally decline by between 20 and 40%, but, significantly, no extensive deep ocean anoxia develops during the period of integration, nor does it appear that it would likely do so for at least a further 4000 years of integration. Subsurface oxygen decline is moderated by an overall reduction in export production of particulate organic matter, which reduces oxygen consumption in the ocean interior due to the remineralization of this material.

Time series study of CFC concentrations in the Labrador Sea during deep and shallow convection regimes (1991–2000)

Annual measurements of concentrations of chlorofluorocarbons (CFCs) along the Labrador Sea WOCE repeat section (AR7W) during 1991–2000 revealed increasing CFC‐12 concentrations in the surface layer (<100 m), the newly ventilated Labrador Seawater (LSW), North East Atlantic Deep Water (NEADW), and Denmark Strait Overflow Water (DSOW). In newly ventilated LSW, CFC concentrations were influenced less by CFC concentrations in the atmosphere than by convection regimes, including the deep convection years of 1991–1994 and the shallow convection years of 1995–2000. The average saturation levels for CFC‐12 and CFC‐11 in the newly ventilated LSW also showed large variations between two convection regimes, which would suggest using caution in estimating ventilation ages for the LSW away from the source. A time series of the CFC‐12 inventory in the Labrador Sea demonstrated the enhanced storage of CFC‐12 during the years of deep convection, while a lower rate of inventory increase was observed during the shallow convection period of late 1990s. This suggests that a periodic renewal of intermediate and deep waters by deep convection influences the uptake of anthropogenic gases such as CFC‐12 and presumably anthropogenic carbon dioxide.

The origin of modern agglutinated foraminiferal assemblages: evidence from a stratified fjord

Loch Etive, a silled 145 m deep fjord on the Scottish west coast, provides an example of modern benthic foraminiferal assemblages at intermediate depths (i.e., below the intertidal zone and above the CCD) consisting almost exclusively of organic-cemented agglutinated forms. Since such faunas from intermediate depths are rare in modern oceans but relatively common in the fossil record, the present study allows new insights into one kind of ancient environment for which there are few modern analogues. The strong dominance of agglutinated forms (both living and in some dead assemblages of foraminifera to the exclusion of calcareous taxa) is attributed to the unusual oceanographic conditions. These include a combination of restricted deep-water renewals and strong influence of freshwater which drains through large areas (relative to the size of the loch) of vegetated land. The result is calm bottom water conditions with commonly occurring oxygen depletion (although not anoxic), brackish water throughout the water column (salinity 28 in the deeper parts), and organic-rich (mostly terrestrially derived) sediments with geochemical properties, which, to a much larger degree than open marine ones, are controlled by local input. This environment supports low abundance and low diversity live assemblages, mainly restricted to the surface 1 cm of sediment. The dead assemblages show similar faunal characteristics, but the calcareous components are, due to carbonate dissolution, even more reduced. One of the calcareous species in Loch Etive is Elphidium albiumbilicatum. Its occurrence is the first record in British waters and it matches the previously suggested southern limit of its distribution. Analysis of a 90 cm long core representing sediments deposited over the past two centuries shows the presence of a calcareous dominated assemblage, including more marine species, with a higher diversity, in the lower part. This suggests that Loch Etive is in the process of going from a marine, to a more terrestrial dominated environment. The relatively high sedimentation rate (0.5 cm per yr), the apparent lack of smearing through bioturbation, and the presence of faunal changes in response to reduced marine influence over the past centuries, shows that Loch Etive has a good potential for performing high-resolution climatic studies.

A high-resolution dinoflagellate cyst record from latest Holocene sediments in Koljö Fjord, Sweden

A high-resolution dinoflagellate cyst record is detailed for the very latest Holocene sediments preserved in a silled fjord from western Sweden. Koljö Fjord is characterised by brackish water conditions together with intermittent deep-water renewal and oxygen depletion. The data provide information derived from the phytoplankton populations living in the surface waters, including possible changes to the nutrient availability and salinity regimes using an actualistic ecological approach. The cyst record provides evidence that the dinoflagellate populations within the surface waters of the fjord over the last 155 years or so have fluctuated markedly. The dinoflagellate cyst record from Core KG1A demonstrates a 10-fold increase in both total cyst numbers and Lingulodinium polyedrum since c. 1938, and a shift from assemblages with high Pentapharsodinium dalei to those with high L. polyedrum and Protoceratium reticulatum from about 1980. These fluctuations are singly and/or collectively indicative of possible cultural changes within the fjord; the effects of the North Atlantic Oscillation on both deep-water renewal and seasonality; nutrient enhancement (eutrophication?); and increased water column stability.

The thermohaline structure and evolution of the deep waters in the Canada Basin, Arctic Ocean

Below the sill depth (at about 2400 m) of the Alpha-Mendeleyev ridge complex, the waters of the Canada Basin (CB) of the Arctic Ocean are isolated, with a 14C isolation age of about 500 yr. The potential temperature θ decreases with depth to a minimum θ m≈−0.524°C near 2400 m, increases with depth through an approximately 300 m thick transition layer to θ h≈−0.514°C, and then remains uniform from about 2700 m to the bottom at 3200–4000 m. The salinity increases monotonically with depth through the deep θ m and transition layer from about 34.952 to about 34.956 and then remains uniform in the bottom layer. A striking staircase structure, suggestive of double-diffusive convection, is observed within the transition layer. The staircase structure is observed for about 1000 km across the basin and has been persistent for more than a decade. It is characterized by 2–3 mixed layers (10–60 m thick) separated by 2–16 m thick interfaces. Standard formulae, based on temperature and salinity jumps, suggest a double-diffusive heat flux through the staircase of about 40 mW m −2, consistent with the measured geothermal heat flux of 40–60 mW m −2. This is to be expected for a scenario with no deep-water renewal at present as we also show that changes in the bottom layer are too small to account for more than a small fraction of the geothermal heat flux. On the other hand, the observed interfaces between mixed layers in the staircase are too thick to support the required double-diffusive heat flux, either by molecular conduction or by turbulent mixing, as there is no evidence of sufficiently vigorous overturns within the interfaces. It therefore seems, that while the staircase structure may be maintained by a very weak heat flux, most of the geothermal heat flux is escaping through regions of the basin near lateral boundaries, where the staircase structure is not observed. The vertical eddy diffusivity required in these near-boundary regions is O(10 −3) m 2 s −1. This implies Thorpe scales of order 10 m. We observe what may be Thorpe scales of this magnitude in boundary-region potential temperature profiles, but cannot tell if they are compensated by salinity. The weak stratification of the transition layer means that the large vertical mixing rate implies a local dissipation rate of only O(10 −10) W kg −1, which is not ruled out by plausible energy budgets. In addition, we discuss an alternative scenario of slow, continuous renewal of the CB deep water. In this scenario, we find that some of the geothermal heat flux is required to heat the new water and vertical fluxes through the transition layer are reduced.

Deep‐water renewal in Lake Issyk‐Kul driven by differential cooling

During two field campaigns on Lake Issyk‐Kul in March and August 2001, 179 high‐resolution conductivity‐temperature‐depth profiles were measured, along with profiles of dissolved oxygen and light transmission. On the basis of this extensive data set, we investigated the large‐scale vertical advective processes responsible for deepwater renewal in Lake Issyk‐Kul. At some locations sampled in March, variable peak structures in the tracer profiles were observed that indicate horizontally localized intrusions. These intrusions have their origin in density plumesthat propagate along the bottom of the channels in the eastern shelf region. The cold dense water at the bottom of the channels is most likely generated by differential cooling of the water in the shallow shelf regions during winter. During summer, vertical advective processes appear to be limited to the upper 200 m, where upwelling in the open water leads to a doming structure. The August data support the results of earlier investigations that suggested that dynamic forcing by a basinwide gyre is responsible for the upwelling in the open water.