Bottom Water Renewal Research Articles

Reflooding and repopulation of the Mediterranean Sea after the Messinian Salinity Crisis: Benthic foraminifera assemblages and stable isotopes of Spanish basins

Benthic foraminiferal, sedimentological, and stable isotope analyses performed on early Zanclean sediments from Alboran Basin ODP Site 976 and southern Spanish land-based sections in the Malaga, Nijar and Sorbas basins have enabled the reconstruction of Mediterranean environmental conditions immediately after the Messinian Salinity Crisis. The presence at the Miocene – Pliocene boundary of dark layers, often enriched in organic matter, suggests that the Zanclean reflooding has created water column stratification, and reduced bottom-water oxygen levels. Considering that such dark layers are recorded at both deep and marginal settings far away from the Gibraltar gateway/s, a Mediterranean-scale water-mass stratification must have occurred. This stratification could be the result of saline Atlantic waters sinking into a less saline Mediterranean Basin still under the influence of the Paratethys. Our early Zanclean benthic δ18O data show that the Mediterranean water budget was indeed less negative than at present, explaining the lower salinity of the basin. However, the Atlantic values of the benthic δ13C registered in the Alboran basin suggest that bottom-water renewal rates were quite high during the early Zanclean, preventing the reduction of δ13C at the seafloor as observed in the Messinian records. Zanclean benthic foraminiferal repopulation sequences show similarities with recovery from low-oxic episodes during sapropel deposition. These observations, paired with the gradual deepening of the basins, suggests that the Zanclean reflooding led to a progressive shift from stressed and unstable environments towards benthic associations typical of efficient circulation and bottom water ventilation.

Simulation of Winter Deep Slope Convection in Peter the Great Bay (Japan Sea)

In wintertime, a high-density water forms on the shallow shelf in the vast Peter the Great Bay (Japan Sea). The steep continental slope with deep canyons and cold winters in the area provide suitable conditions for the implementation of deep slope convection—an important phenomenon in the formation of intermediate and bottom waters that occurs at a few locations in some semi-enclosed seas, including the Japan Sea. The descent of dense shelf water down the continental slope of Peter the Great Bay usually occurs to 1000–1200 m; however, in anomalously cold winters, it has been observed at greater than 2000 m depth supporting renewal and deep ventilation of intermediate and bottom waters in the Japan Sea. The deep slope convection is a rare episodic phenomenon with durations ranging from several hours to several days, that has never been simulated in Peter the Great Bay with a realistic numerical model of circulation. We apply the Regional Ocean Modeling System (ROMS) with a 600 m horizontal resolution to simulate the deep slope convection in the anomalously cold winter of 2001 when it has been observed in cruises. The results are compared with propagation of deep shelf water in the regular winter of 2010 when hydrological characteristics of this water were recorded by a profiler “Aqualog” installed at the shelf break. Using Lagrangian methods, we track and analyze the formation of dense shelf water, its advection to the slope edge in the bottom layer and descent down the slope. Special attention is payed to the role of coastal eddies arising due to a symmetric instability. These eddies promote the cross-shelf transport of the dense shelf water towards the continental slope edge. The simulation results are compared with rare observations of the deep slope convection in Peter the Great Bay.

Different responses to artificial ventilation in two stratified coastal basins

We studied the effects of pumping surface water down through the pycnocline (i.e. artificial ventilation) on hydrodynamics, oxygen concentrations, hydrogen sulfide, and nutrients in two anoxic coastal basins (Lännerstasundet and Sandöfjärden). In addition, in a corresponding laboratory aquarium experiment, pumping of less saline surface water entrained dense bottom water with a mixing ratio of 6.8 and illustrated dispersal below the pycnocline. Oxygen saturation increased from 0 to 20%; oxygen penetrated poorly into the sediment of the aquarium. In the salinity-stratified Lännerstasundet basin, ventilation also oxidized the anoxic bottom water. The ventilation removed hydrogen sulfide and decreased the sub-pycnocline water pools of phosphorus and ammonium, which was not observed in a neighboring reference basin. The summertime ventilation warmed and made the sub-pycnocline water less saline. In the autumn, the inflows of cooled water from the surroundings with equal or higher salinity promoted its sinking in the relatively warm ventilated basin. The inflows maintained oxygen concentrations between 4 and 8 mg L−1 for months after the ventilation ended. In contrast to Lännerstasundet, ventilation did not prevent formation of anoxia and release of nutrients in the temperature-stratified Sandöfjärden. Here, the ventilation capacity was less than that in Lännerstasundet and ventilation expanded the sub-thermocline bottom area, warmed the bottom sediments, and probably displaced oxic water from the experimental area. The ventilation did not promote density conditions for inflows and no marked inflow-induced oxidation was observed after midsummer. We conclude that a significant amount of anoxic water was entrained into the ascending plume which reduced the oxygen content below the pycnocline ventilation in aquarium experiment. Additionally, summertime ventilation may improve the status of the salinity-stratified basins for further oxidation. The improvement occurs due to autumn cooling and favorable basin topography, which promote inflows of oxic water with larger density and thereby, renewal of bottom water in the pumped basin. The semi-enclosed and temperature-stratified basin cannot form such favorable density conditions for inflows and thus ventilation is less efficient.

Geothermal heating and episodic cold-seawater intrusions into an isolated ridge-flank basin near the Mid-Atlantic Ridge

Six-year records of ocean bottom water temperatures at two locations in an isolated, sedimented deep-water (∼4500 m) basin on the western flank of the mid-Atlantic Ridge reveal long periods (months to >1 year) of slow temperature rises punctuated by more rapid (∼1 month) cooling events. The temperature rises are consistent with a combination of gradual heating by the geothermal flux through the basin and by diapycnal mixing, while the sharper cooling events indicate displacement of heated bottom waters by incursions of cold, dense bottom water over the deepest part of the sill bounding the basin. Profiles of bottom water temperature, salinity, and oxygen content collected just before and after a cooling event show a distinct change in the water mass suggestive of an incursion of diluted Antarctic Bottom Water from the west. Our results reveal details of a mechanism for the transfer of geothermal heat and bottom water renewal that may be common on mid-ocean ridge flanks.

Revisiting the Role of Convective Deep Water Formation in Northern Baltic Sea Bottom Water Renewal

AbstractDeep water renewal and ventilation of the Baltic Sea is commonly considered to occur solely via saltwater inflows from the North Sea. However, recent analysis of geophysical and sediment proxy data suggests convective deep water formation during wintertime as a second process that has contributed to the ventilation of the northern and central Baltic Sea bottom waters during cold climate periods. Here, we investigate the role of deep water formation in the northern Baltic Sea in a regional ocean circulation model. Selecting the particularly cold winter 1986/1987 as a reference period, we perform sensitivity experiments in which the atmospheric temperature forcing is changed and/or localized brine rejection on subgrid scales is accounted for through a parameterization. We study the sinking and circulation of water masses via passive tracers introduced into the model. Generally, our model results support the established view that convective deep water formation does not play a major role in Baltic Sea deep water renewal. While a reduction of air temperatures does not qualitatively change the sinking and circulation of water masses in our experiments, brine rejection could potentially lead to localized deep water formation. However, the impact is too weak to possibly change the large‐scale deep water exchange between the Baltic proper and the northern Baltic sub‐basins. Although being in line with established knowledge, through consideration of the model limitations, our results provide insights on how and under which circumstances convective deep water formation could potentially have occurred during cold climate periods.

Major and trace element characterization of Oceanic Anoxic Event 1d (OAE 1d): Insight from the Umbria-Marche Basin, central Italy

The Pialli Level in the Umbria-Marche Basin (central Italy) correlates with the lowermost part of the positive carbon isotopic excursion characterizing the late Albian–early Cenomanian Oceanic Anoxic Event 1d (OAE 1d). High-resolution litho-, bio- and chemostratigraphic data from the Monte Petrano and Le Brecce sections allow for a bed-by-bed comparison of the two successions, and discriminate local from basin-scale signals. We present new X-ray fluorescence, ICP-MS and TOC data for both the limestones and the Pialli Level shales integrated with available carbonate carbon isotopes and nannofossil temperature and nutrient indices. Data indicate a homogenous background sedimentation dominated by pelagic carbonates, biogenic silica with little contribution by clays. The limited variation in lithogenic elements points to an essentially homogeneous detrital source area with limited fluvial terrigenous input. Higher Mn concentrations coupled with low enrichments in redox-sensitive elements, such as U, Fe, S, Re, Mo, Ag, suggest that the Pialli Level shales represent temporary suboxia without reaching anoxia. Furthermore, P, authigenic Ba, Cd and Ni enrichments, together with nannofossil nutrient index indicate generally low primary productivity conditions along the entire succession, with only minor increases for some of the black-to-dark grey shales of the Pialli Level. The nannofossil temperature index highlights a warm climate during the OAE 1d, with the warmest conditions experienced during the deposition of the Pialli Level shales. During the late Albian, the warm and humid climate was interrupted by brief episodes of relatively warmer and less saline surface waters ensuring slower rates of bottom water renewal and producing temporary suboxic conditions. Such paleoceanographic dynamics would be the continuation of episodic warmer and humid pulses characterizing the late Albian interval in the Umbria-Marche Basin. As such, the Pialli Level can be considered the result of a last episode closing a cycle before the establishment of a steadier climate during the early Cenomanian.

Tracing bottom-water redox conditions during deposition of Lower and Upper Jurassic organic-rich sedimentary rocks in the Lusitanian Basin (Portugal): Insights from inorganic geochemistry

The Lower Jurassic Água de Madeiros (AM Fm) and Vale das Fontes (VF Fm) Formations, and the Upper Jurassic Cabaços Formation (Cab Fm) are considered the most important units with petroleum source-rock potential in the Lusitanian Basin, and the likely sources for its widespread hydrocarbon occurrences. Nonetheless, the bottom-water redox conditions during their deposition remain debatable. To resolve this controversy, we investigated whole-rock inorganic geochemical data of the most significant organic-rich levels of the AM, VF and Cab Fms cropping out at basin scale. Significant variations in TOC, TS and redox-sensitive elements contents and their ratios are evident within and between the organic-rich levels of the studied units. A positive covariation between primary productivity proxies, TOC enrichment and paleo-redox proxies for the Lower Jurassic units clearly suggest an interdependence between enhanced primary productivity, organic matter accumulation and bottom-water redox conditions. For the AM Fm, the paleo-redox proxies indicate that the black shales deposition took place beneath bottom-waters that ranged from suboxic to strongly euxinic conditions, with a clear predominance of the latter. A TOC threshold of ~6–7 wt% clearly separates samples deposited under suboxic to anoxic conditions from those deposited under true euxinic conditions, and is interpreted as the minimum amount of organic matter (OM) accumulation within sediments required to sustain H2S production at rates that outpaced its consumption through precipitation of sulphides and/or OM sulphurization, allowing the diffusion of H2S to the bottom-waters. For the Vale das Fontes Fm, the paleo-redox proxies suggest a prevalence of suboxic to anoxic bottom-waters, despite all black shales displaying TOC contents greater than the above threshold. The less reducing conditions relative to the AM Fm euxinic black shales are attributed to: i) a larger proportion of continental OM; ii) lower reactivity of the OM reaching the sediment-water interface (decreasing oxygen consumption); and iii) higher availability of reactive iron (increasing the buffer effect to H2S produced through OM remineralization by sulphate-reducing bacteria). Both units display Mo-EF vs U-EF covariation patterns that deviate from the field of sediments deposited under unrestricted open-marine conditions, towards higher Mo enrichment relative to that of U, indicating an accelerated transport of aqueous Mo to the sediments, consistent with the operation of a metal-oxyhydroxide particulate shuttle linked to Mn and Fe redox cycling within the water column in a weakly-restricted basin. The Mo-COT regression-line slope indicates a moderate degree of bottom-water mass restriction, with [Mo]aq concentration of ~60–70% of the present-day seawater value and bottom-water renewal times of ~10–20 years, akin to the present-day Cariaco Basin. The redox conditions of the Cab Fm organic-rich levels are dependent on the location of the depositional environment in the basin, with deposition taking place beneath oxygenated bottom-waters at Cabo Mondego, and under suboxic conditions at Vale de Ventos and Pedrógão, the latter denoting a larger degree of oxygen depletion than the former. In this context, the overall potential for OM preservation should increase from Cabo Mondego to Vale de Ventos and Pedrógão.

Dynamical analysis of the oceanic circulation in the Gulf of San Jorge, Argentina

Abstract This study identifies the dynamical mechanisms driving seasonal variations in oceanic circulation and water mass characteristics of the Gulf of San Jorge (GSJ) and its exchanges with the Patagonian Shelf (PS). A suite of process-oriented numerical experiments indicates that GSJ circulation is mainly driven by tidal forcing and modulated by wind forcing and intrusions from the PS. During late spring and summer, stratification decouples the upper and deeper layers of the gulf, leading to a shallow, wind-forced surface circulation and a deeper, density driven, cyclonic geostrophic flow. The subsurface circulation is induced by differential tidal mixing in coastal and deep areas and its intensity is strongly affected by the temporal variability of the atmospheric heat flux, which increases from spring to summer and fades from fall to winter. As stratification weakens, circulation patterns are replaced by wind-driven anticyclonic gyres in the south and an open cyclonic loop in the north. Passive tracer diagnostics show that in summer, surface and subsurface waters from the GSJ northern coast are exported and replaced by waters from the coastal portion of PS currents. The renewal of bottom waters is slower: A small portion upwells in the southwestern coast but most are ventilated by winter convection in the southern region and by intrusions of PS waters in the northern region.

Hypoxia and dissolved oxygen trends in the northeastern Adriatic Sea (Gulf of Trieste)

Physical and chemical data collected in three stations, with time series ranging from 1983, 1986 and 1989 to 2016, were analyzed in order to detect trends and frequency of occurrence of hypoxia events in bottom waters of the Gulf of Trieste (Adriatic Sea). The results of the analysis of 30-years data show a tendency toward increasing oxygen concentration in the bottom waters, nevertheless two hypoxic events were recorded during the summers of 2015 and 2016 even in a relatively shallow area of the Gulf. The spatial and temporal extent of these events was analyzed by coupling oceanographic surveys with automatic oceanographic measurements. During both summers, the area was characterized by high seawater temperature (up to 28.4 °C at the surface) and salinity (38.1 at the bottom) and a marked stratification of the water column, which prevented the mixing of oxygen-rich surface water with oxygen-poor deep water. The main contribution to oxygen depletion in the bottom waters was attributed to plankton respiration (54–61%) and to benthic oxygen consumption (39–46%), which exceeded the oxygen produced by planktonic and benthic microalgae and the one diffused from the overlying oxygenated water. These events of marked oxygen depletion in shallow coastal ecosystems are possibly favored by the positive temperature trend in bottom waters, coupled with the increase in riverine discharges in late spring, limiting vertical mixing and bottom water renewal.

Coastal Freshening Prevents Fjord Bottom Water Renewal in Northeast Greenland: A Mooring Study From 2003 to 2015

AbstractThe freshwater content of the Arctic Ocean and its bordering seas has recently increased. Observing freshening events is an important step toward identifying the drivers and understanding the effects of freshening on ocean circulation and marine ecosystems. Here we present a 13 year (2003–2015) record of temperature and salinity in Young Sound‐Tyrolerfjord (74°N) in Northeast Greenland. Our observations show that strong freshening occurred from August 2005 to August 2007 (−0.92 psu or −0.46 psu yr−1) and from August 2009 to August 2013 (−0.66 psu or −0.17 psu yr−1). Furthermore, temperature‐salinity analysis from 2004 to 2014 shows that freshening of the coastal water (~range at sill depth: 33.3 psu in 2005 to 31.4 psu in 2007) prevented renewal of the fjord's bottom water. These data provide critical observations of interannual freshening rates in a remote fjord in Greenland and in the adjacent coastal waters and show that coastal freshening impacts the fjord hydrography, which may impact the ecosystem dynamics in the long term.

Seasonal variations in the vertical structure of pelagian water stratum in the Southern Baikal

For the first time, T,S-analysis was used to determine the specifics of seasonal variations in the vertical structure of Lake Baikal active layer. In the under-ice period, the active layer includes the under-ice, top winter, and upper intermediate water masses. The under-ice water mass, unlike other masses, shows an increase in mineralization to 100.74 mg/kg, which corresponds to a release of 71.1 g salt under 1 m2 of water surface in a layer 0–40 m in the process of salt freezing out during ice cover formation and accretion. In the phases of mixing (homothermy), the water masses of the active layer transform into a surface homogeneous mass. In summer and autumn, surface and upper intermediate water masses, separated by a water mass of summer thermocline can be identified. A specific feature of the summer thermocline water mass is the increased sum of ions because of an increase in HCO 3 - concentration at the decay of organic matter accumulating in the bottom part of the thermocline. The existence of the under-ice water mass and the water mass of summer thermocline was established in Lake Baikal for the first time. In the deep-water zone (>250 m), except for the bottom parts, the lower water masses (the lower intermediate and the deep) are permanent, their characteristics remaining stable during the year. The changes in the bottom water mass are due to the character of the processes of bottom water renewal.

Warming and Weakening of the Abyssal Flow through Samoan Passage

AbstractThe abyssal flow of water through the Samoan Passage accounts for the majority of the bottom water renewal in the North Pacific, thereby making it an important element of the meridional overturning circulation. Here the authors report recent measurements of the flow of dense waters of Antarctic and North Atlantic origin through the Samoan Passage. A 15-month long moored time series of velocity and temperature of the abyssal flow was recorded between 2012 and 2013. This allows for an update of the only prior volume transport time series from the Samoan Passage from WOCE moored measurements between 1992 and 1994. While highly variable on multiple time scales, the overall pattern of the abyssal flow through the Samoan Passage was remarkably steady. The time-mean northward volume transport of about 5.4 Sv (1 Sv ≡ 106 m3 s−1) in 2012/13 was reduced compared to 6.0 Sv measured between 1992 and 1994. This volume transport reduction is significant within 68% confidence limits (±0.4 Sv) but not at 95% confidence limits (±0.6 Sv). In agreement with recent studies of the abyssal Pacific, the bottom flow through the Samoan Passage warmed significantly on average by 1 × 10−3°C yr−1 over the past two decades, as observed both in moored and shipboard hydrographic observations. While the warming reflects the recently observed increasing role of the deep oceans for heat uptake, decreasing flow through Samoan Passage may indicate a future weakening of this trend for the abyssal North Pacific.

Water fluxes and renewal rates at Pertuis d'Antioche/Marennes-Oléron Bay, France

Water renewal is crucial for removal of pollutants (e.g. oil), organic loadings, and oxygen supply and within shellfish farms. This study presents a 3-D hydrodynamic model with local resolution of 500 m used to quantify water renewal and water fluxes at Pertuis d'Antioche/Marennes-Oléron Bay, France. Open boundary conditions were based on MyOcean products (http://www.myocean.eu/) downscaled using four nested domains with increasing resolution from the large to the local scale. Results from a six-month model run were compared with data giving a good agreement. The spatial variation of water renewal was characterized by dividing the coastal waters into boxes which grouped the main shellfish areas. A Lagrangian model was coupled to the hydrodynamic model to track the history of water renewal. The boxes were filled with tracers and the pattern of particles passing through them was quantified. Model results were integrated over the volume of the boxes to calculate the water fluxes across box boundaries. The results of the integration provide horizontal and vertical fluxes of water which were used to assess the renewal of surface and bottom waters in the bay. Depending on tide and freshwater conditions, the water renewal ranges between 2 and 18 days. Main current patterns were investigated by drawing streamlines in the residual velocity field. It was found that eddies can slow down renewal rates, and either favour transport of tracers or locally trap them inside the bay. An assessment of typical trajectories of water particles released at different locations and depths provided insights into the potential dispersion of passive tracers, including spilled oil, and larvae.

Pathways, Volume Transport, and Mixing of Abyssal Water in the Samoan Passage

AbstractThe flow of dense water through the Samoan Passage accounts for the major part of the bottom water renewal in the North Pacific and is thus an important element of the Pacific meridional overturning circulation. A recent set of highly resolved measurements used CTD/LADCP, a microstructure profiler, and moorings to constrain the complex pathways and variability of the abyssal flow. Volume transport estimates for the dense northward current at several sections across the passage, calculated using direct velocity measurements from LADCPs, range from 3.9 × 106 to 6.0 × 106 ± 1 × 106 m3 s−1. The deep channel to the east and shallower pathways to the west carried about equal amounts of this volume transport, with the densest water flowing along the main eastern channel. Turbulent dissipation rates estimated from Thorpe scales and direct microstructure agree to within a factor of 2 and provide a region-averaged value of O(10−8) W kg−1 for layers colder than 0.8°C. Associated diapycnal diffusivities and downward turbulent heat fluxes are about 5 × 10−3 m2 s−1 and O(10) W m−2, respectively. However, heat budgets suggest heat fluxes 2–6 times greater. In the vicinity of one of the major sills of the passage, highly resolved Thorpe-inferred diffusivity and heat flux were over 10 times larger than the region-averaged values, suggesting the mismatch is likely due to undersampled mixing hotspots.

Nitrate elimination and regeneration as evidenced by dissolved inorganic nitrogen isotopes in Saanich Inlet, a seasonally anoxic fjord

In this study, we used natural abundance isotope measurements of dissolved inorganic nitrogen (N) species to evaluate the effect of different oxygenation regimes on N transformation and elimination in Saanich Inlet, a seasonally anoxic fjord in British Columbia, Canada. We analyzed dissolved nutrient concentrations and the N (and O) isotope composition of nitrate (NO3−) and ammonium (NH4+) at different depths throughout the water column near the mouth of the inlet between April 2008 and April 2009. A gradual increase in both the NO3− δ15N and δ18O, associated with a decrease in NO3− concentration and an increase in biological excess N2, was observed after bottom water renewal events in August–October 2008, indicating NO3− consumption by denitrifying bacteria in an expanding suboxic water column. An increase in the δ15N of NH4+ with depth toward the suboxic/hypoxic transition, indicated net consumption of NH4+, most likely by micro-aerobic or anaerobic NH4+ oxidation and dissimilatory consumption by microorganisms. Deviations from a 1:1 correlation between the NO3− δ15N and δ18O (Δ(15,18)) that appears characteristic for both assimilatory and dissimilatory NO3− consumption in the ocean, were observed in surface waters and close to the hypoxic/suboxic transition. Lowered Δ(15, 18) values can most plausibly be explained by aerobic nitrification of newly remineralized NH4+ and/or low δ15N–NO3− inputs from atmospheric precipitation in the surface mixed layer, and NO3− regeneration through NH4+ oxidation and/or the reoxidation of nitrite (NO2−) in deeper waters. Closed and open system model-derived N isotope effects for NO3− consumption calculated from time-series samples collected near the sediments in anoxic bottom waters were significantly lower (as low as ~11‰) than the biological N isotope effects of ~20–30‰ for water column denitrification reported in other studies. We argue that the reduced N isotope effect is mainly due to the combined effects of water column and sediment denitrification, the latter occurring with a highly suppressed N isotope fractionation at the ecosystem level. We estimated that ~40–60% of the denitrification occurs within the sediments of the inlet.

Spatial Distribution of Radiocarbon in the Southwestern Japan/East Sea Immediately After Bottom Water Renewal

In April 2001, immediately after bottom water renewal in the Japan/East Sea, 7 vertical profiles of radiocarbon were obtained from the area, including the Japan Basin and the Tsushima/Ulleung Basin. The bottom water Δ14C of the Tsushima/Ulleung Basin increased by 24% during 1979–2001 and was 10–15% higher than the values reported for the Japan Basin and Yamato Basin during 1995–2002. Within the depth interval 800–2000 m, Δ14C values at the southernmost station in the Japan Basin were much higher than at other stations, with the maximum difference in Δ14C values being more than 50%. Based on a combination of physical observations with analysis of the Δ14C data, we concluded that the bottom water, which was formed by the sinking of surface water off Vladivostok, Russia, in late January-early February of 2001, had been widely distributed to depths below 2000 m within the southernmost Japan Basin in a timespan shorter than 2 months. However, there was no evidence that the water had been transported into the Tsushima/Ulleung Basin during the same time interval.

Renewal of deep waters of Lake Baikal revisited

The patterns of renewal of bottom waters in Lake Baikal under the influence of deep convection and intrusion of cold waters have been considered based on the data of temperature surveys of Lake Baikal conducted in 1993–2009. The volumes of the cold bottom layer with the maximums of 200–470 km3 in individual years and the values of its total cooling (−20–60 × 109 MJ) have been determined for South, Middle, and North Baikal. The renewal process is asynchronous and proceeds with different activity in these parts of the lake, which indicates that the mechanisms that cause deep convection in the context of the great latitudinal length and differences in the climate and hydrological processes manifest themselves regionally. The volume of intrusions has been determined. Its average value for the period was higher in South Baikal (20 km3) than in Middle Baikal (9.8 km3) and North Baikal (8.6 km3). The volume of the intrusions in these parts of the lake was 30–70 km3 in some years.

Specific features of cold bottom intrusion in Baikal according to observations in 1993–2009

The specific features of cold bottom intrusion in Baikal are considered based on the results of observations of 1993–2009. It is shown that the intrusion-related renewal of bottom water takes place not regularly and not simultaneously in different hollows. Its maximums were recorded in Southern Baikal in 1997 and 2007, in Middle Baikal in 1995, 2006, and 2009, and in Northern Baikal in 1997 and 2009. In these years, the volume of cold bottom layer in some hollows reached 200–470 km3 and its total cooling reached −20 to −60 MJ 109. Cold intrusions were more frequent and had greater effect on the bottom layer in Southern Baikal than in other parts of the lake. The intrusions, especially in the years of their active development, are shown to cause water cooling in both the bottom layer and the major part of the deep-water zone of all hollows.

Impact of deep-water renewal events on fixed nitrogen loss from seasonally-anoxic Saanich Inlet

We interpreted profiles of N 2/Ar ratios, δ 15 N – N 2 , and O 2 concentration collected in Saanich Inlet, British Columbia, Canada over an annual cycle. Our measurements and data from a regional cabled observatory indicated that four deep- or bottom-water renewal events occurred over our study period. Each event was correlated with a period of weak tidal currents, such that very low tidal mixing allowed inflowing water to retain its high density as it moved across the sill and into the deeper basin. By quantifying the concentration of excess N 2 in each month and the vertical diffusion rate, we determined that the N 2 production rate ranged from 1.7 ± 0.3 mmol N 2 m − 2 d − 1 in summer to 8.1 ± 2.8 mmol N 2 m − 2 d − 1 in winter. This depth-integrated estimate accounts for all pathways resulting in fixed (bioavailable) nitrogen loss as N 2 gas, including denitrification and anammox, and incorporates any benthic production of N 2 that diffuses into the overlying water column. In spring and summer, the maximum N 2 excess corresponded to the maximum δ 15 N – N 2 , indicating that denitrification approached completion. In these months, the average isotopic composition of the fixed N consumed was 7.5 ± 1.2‰. Following bottom-water renewal in fall, which brought in nutrient-rich, low-N 2 water, the N 2 concentration increased and became progressively more enriched in 15N. The high rates of N 2 production in Saanich Inlet likely exist in other anoxic basins that undergo periodic deep-water renewal by nitrate-rich waters.

Optical-thermal fronts under ice cover of Lake Baikal and their connection with the bottom water renewal

This work gives the measurement data of deep-water profiles of transparency and temperature in Lake Baikal in March–April, 2006. The optical-thermal manifestations of vertical exchange between surface and deep waters in the front zones of Baikal during the under-ice period have been considered. Mechanisms of surface water penetration into bottom ones over the period of inverse temperature stratification have been established.