Mixing Of Different Water Masses Research Articles

Characterizing dissolved inorganic and organic nutrients in the oligotrophic Kuroshio Current off eastern Taiwan during warm seasons

This study conducted sensitive and precise analyses of dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) concentrations and trace concentrations of nitrate and nitrite (N+N) and soluble reactive phosphorus (SRP) in seawater. The methods were applied to investigate the distributions and controls of N+N, SRP, DON, and DOP in the oligotrophic Kuroshio Current (KC) area off eastern Taiwan during warm seasons. The water in the studied area was classified into four major types: typical Kuroshio water (KW), KW influenced by the East China Sea water, KC influenced by the South China Sea (SCS) water, and KW influenced by the SCS water and river plumes, which is confined to the coastal zone. Nutrient distributions in KC revealed very low N+N (0.002−0.40 μM) and SRP (0.015−0.125 μM) concentrations but high DON (<8 μM) and DOP (<0.3 μM) concentrations above the nutricline depth, which accounted for >80% of TDN and TDP, respectively; theses concentrations can primarily be attributed to strong, permanent surface stratification. Among the water types, KW had the lowest N+N, SRP, DON, and DOP concentrations but greatest chlorophyll maximum depth and nutricline depth, except for in locations influenced by island-induced upwelling. The concentrations of all nutrients increased by various degrees in the other water types, which was attributed to the exchange and mixing of different water masses and coastal uplift of subsurface waters. KW was not only highly oligotrophic but also N+N-limited reflected from very low [N+N]/[SRP] ratio (0.02−0.15) in the mixed layer (ML). Overall, the N+N limitation and high nitrate anomaly value (N*: 2.47 ± 0.16 μM) above the nutricline depth strongly indicate prevailing N2 fixation at the surface of KW. Very high DON/DOP ratio in KW (16.9−69.1) probably resulted from the release of N-rich organic nutrients from phytoplankton including N2 fixers at the surface and faster recycling of DOP than DON in deep waters. Persistent coastal uplift of subsurface water occurs everywhere over the shore-side region of the KC, resulting in increasing surface concentrations of nutrients and chlorophyll a. Overall, the aforementioned physical and biogeochemical processes determined the upper-ocean distributions of nutrient species in warm seasons.

Investigating Seawater Intrusion in Republic of South Africa’s Heuningnes, Cape Agulhas Using Hydrogeochemistry and Seawater Fraction Techniques

The Heuningnes Catchment in the Republic of South Africa was used as a case study in this research to describe the application of saltwater fraction/quantification and hydrogeochemistry methods to evaluate the extent of saline intrusion in the coastal aquifers. The argument of the research is that the presence of seawater incursion may be conclusively determined by combining the examination of the major ions, seawater fraction, stable isotopes of water, bromide, and geochemical modeling. Using stable isotopes of oxygen (18O) and deuterium (2H), major ions chemistry, seawater composition, and geochemical modeling, the genesis of salinity and mixing of different water masses were examined. Twenty-nine (29) samples of groundwater were examined. All samples showed water facies of the Na-Cl type, indicating a seawater-related origin. The significance of mixing in coastal aquifers under natural conditions was shown by the hydrogeochemical characteristics of key ions derived from ionic ratios, which demonstrated substantial adherence to mixing lines among endmembers for freshwater as well as saltwater (seawater). The quantification of seawater contribution in groundwater percentages varied from 0.01 to 43%, with three samples having concentrations of seawater above 50%. It was clear from the hydrogeochemical analysis and determination of the proportion of saltwater that the seawater intrusion impacted the coastal fresh groundwater. In addition, the chloride concentration in the groundwater ranged from 81.5 to 26,557.5 mg/L, with the corresponding δ18O values ranging from −5.5‰ to −0.9‰, which suggested that freshwater and saltwater were mixing. The Br−/Cl− ratios showed that evaporation had played a part in elevating groundwater salinity as well. Since saturation indices were below zero, the mineral dissolution could also contribute to the salinization of groundwater. Further proof of seawater incursion in the investigated catchment was supplied by geochemical modeling and bromide. Even though such tools were not verified in multiple coastal aquifers for widespread generalization, the study offered a scientifically significant understanding of the application of such tools on seawater intrusion in coastal aquifers and has useful recommendations for the aquifer setting of similar environments.

Using dual water isotopes to quantify the mixing of water masses in the Pearl River Estuary and the adjacent northern South China Sea

The mixing of different water masses is important for local physical and biogeochemical processes as well as for ecosystems in the ocean. In this study, a new dataset of stable water isotopes (δD and δ18O) combined with temperature–salinity profiles was used to quantitatively understand the mixing of water masses in the Pearl River Estuary (PRE) and the adjacent northern South China Sea (SCS). Based on hydrographic characteristics and the isotope–salinity relationships in the water column, three water masses, namely, low isotopic values (<1.5‰ for δD and <0‰ for δ18O) with a salinity of <33.20 for PRE water (PREW), high isotopic values (>2.0‰ for δD and >0.6‰ for δ18O) with a salinity of >34.60 for SCS Kuroshio Branch (SCSKB), and higher isotopic values (>3.0‰ for δD and <0.4‰ for δ18O) with a salinity of >33.30 for SCS water (SCSW), were identified in the PRE and the adjacent SCS. The mixing of the three water masses in the PRE and the adjacent SCS was mainly from SCSW (71%), followed by the SCSKB (23%), and the proportion of PREW only accounted for 6%. However, different water layers and regions are affected differently by these three water masses. The surface water is mainly influenced by the PREW, whereas the subsurface water is mainly influenced by the intrusion of SCSKB (100–300 m). The mixing process of water masses in the west side of the study area (<115°E) is mainly contributed by the SCSW (86%), whereas the contributions of PREW and SCSKB are only 4% and 10%, respectively. By contrast, the mixing of water masses in the east side (>115°E) is mainly influenced by the Kuroshio intrusion (50%). This study reveals that dual water isotopes are exquisitely sensitive to determine the complex hydrological process in the PRE and the adjacent SCS, and water masses on marine environment should deserve more attention.

High‐resolution distribution pattern of surface water nitrous oxide along a cruise track from the Okhotsk Sea to the western Arctic Ocean

AbstractNitrous oxide in the surface water was measured using an automatic underway system, along with measurements of surface water pCO2, dissolved oxygen, salinity, temperature, and sea ice coverage, along a cruise track through the Bering Sea and Chukchi Sea to the Canadian Basin during the 7th Chinese National Arctic Research Expedition. The results show that, overall, the regions along the cruise track are net sources of N2O to the atmosphere. Several N2O oversaturation maxima were observed along the cruise track, with an absolute maximum of approximately 60%. According to the hydrographic setting and the distribution patterns of pCO2, dissolved oxygen, and sea ice coverage in the study area, it was concluded that the N2O oversaturation maxima may result from hydrographic processes, such as mixing of different water masses, upwelling or convection, and possible production beneath the sea ice. Additionally, the lowest value of approximately 90% may result from dilution related to the melting of sea ice. An evaluation of the air–sea flux along the cruise track shows that the continental shelf and upwelling region are N2O sources, whereas the study area in the open Arctic Ocean does not show obvious source or sink characteristics.

Dissolved organic carbon dynamics in the East China Sea and the northwest Pacific Ocean

Abstract. Oceanic dissolved organic carbon (DOC) represents one of the largest carbon reservoirs on Earth, and its distribution and biogeochemical cycles play important roles in carbon cycling and other biogeochemical processes in the ocean. We report the distribution and concentrations of DOC for water samples collected from the shelf-edge and slope regions in the East China Sea (ECS) and the Kuroshio Extension (KE) in the northwestern North Pacific during two cruises in 2014–2015. The DOC concentrations were 45–88 µM in the ECS and 35–65 µM in the KE. In addition to biological processes that are estimated to account for 7 % and 8 %–20 % in shaping the DOC distribution in the ECS and KE regions, respectively, the DOC distribution is largely controlled by hydrodynamic mixing of different water masses. By comparing the DOC results with dissolved inorganic carbon (DIC) and dissolved inorganic radiocarbon (Δ14C-DIC) measured from the same water samples, we further demonstrate that the intrusion of the Kuroshio Current could dilute the DOC concentrations at stations in the outer shelf and slope regions of the ECS. The concentrations of DOC in the KE were significantly lower in surface waters than in the ECS, and a relatively low and stable DOC level (∼40 µM) was found in deep water (below 1500 m) at all stations. Based on the previously reported DIC and Δ14C-DIC values for the stations, the observed spatial variations of DOC in the upper 700 m among the stations in the KE were mainly influenced by mixing of the two water masses carried by the Kuroshio and Oyashio, the two dominant western boundary currents in the region. The hydrodynamic processes are thus important factors in the distribution and dynamics of DOC in the KE region.

Non-conservative patterns of dissolved organic matter degradation when and where lake water mixes

In this study, we experimentally investigated the degradation of dissolved organic matter (DOM) during lateral and vertical mixing of different water masses in a peri-alpine lake. River intrusions and vertical winter turnover in Lake Geneva (Switzerland, France) were simulated through short-term laboratory incubations by mixing riverine and lacustrine waters or lacustrine waters collected at different depths in winter. Dissolved organic carbon (DOC) degradation was monitored by dissolved oxygen (DO) measurements and changes in DOM composition were tracked by fluorescence spectroscopy in pure and mixed treatments during 72 h. Initial DOC content and DOM composition were relatively similar between end-members. The amount of DOC respired at the end of the incubation was similar between treatments, but decay constants in mixed treatments derived from a first-order decay model were significantly higher than expected values calculated based on a simple mass balance model. Fluorescent dissolved organic matter (FDOM) in mixed treatments followed non-conservative patterns that could not be predicted based on observations made in pure treatments. Hence, one protein-like and one microbial-like fluorophore that were consumed in lake waters were continuously produced in mixed treatments although lake waters represented 90% of the mix. No relationships were observed between the rate of DOC consumption and the initial DOM composition, suggesting that other factors such as nutrients and/or interactions in microbial communities were involved. Moreover, no relationships were found between DOC and FDOM patterns during incubations, suggesting that these measurements targeted different facets of microbial metabolism of DOM, respectively microbial respiration (catabolism) and microbial production (anabolism). While additional investigations are required in order to identify the drivers of these changes, this study provides evidence of non-conservative behavior of DOM degradation in mixing zones in lakes.

Marine biogeochemical cycling of cadmium and cadmium isotopes in the extreme nutrient-depleted subtropical gyre of the South West Pacific Ocean

The Cd isotopic analysis of waters from an increasing number of oceanic regions has provided a wealth of new information on the oceanic cycling of Cd, revealing the complex interplay of a multitude of different biogeochemical processes. In this study, paired Cd concentration and Cd isotopic measurements were made on samples collected during the GEOTRACES GP-13 zonal section in the South West Pacific Ocean. The South Pacific subtropical gyre is the most oligotrophic gyre in the global ocean and a unique area to study the Cd isotope systematics associated with phytoplankton productivity under ultra-low nutrient concentrations. The dissolved Cd and PO4 concentrations of the study area are well correlated and can be expressed by two different linear relationships, as observed in other oceanic regions. The near quantitative biological uptake of Cd in the upper water column and the mixing of different water masses with different pre-formed Cd/PO4 ratios likely produces the ‘kink’ in the Cd–PO4 relationship. Across the GP-13 zonal section, the Cd isotopic composition of deep waters is relatively constant, as observed in other regions, and is centred around a δ114Cd value of 0.26 ± 0.11‰ (2 SD, n=40). In contrast, across the thermocline depth range, extending from 150 to 1500 m depth, the Cd concentration and δ114Cd values are negatively correlated and best described by Cd isotope fractionation under open-system conditions with continuous replenishment of the Cd source/s and a fractionation factor of 1.0006 ± 0.0002. This contrasts with the closed-system conditions without Cd replenishment that have been used to describe some other open ocean settings. Below 500 m depth, the Cd isotope systematics can largely be explained by three component mixing between key water masses with different pre-formed Cd isotope signatures. However, above 500 m, the Cd isotope systematics appear to be influenced by both water mass mixing and the biological uptake of isotopically light Cd in local and remote surface waters and the regeneration of Cd from sinking organic material deeper in the water column. Additionally, an unusual positive correlation was observed between Cd concentration and Cd isotopic composition in the upper water column of the South Pacific Ocean, from 15 to 150–200 m depth. These systematics can potentially be explained by one, or a combination, of the following processes: (i) a dominant role of supply-limited conditions during Cd uptake, (ii) partitioning of Cd into ligand phases, and/or (iii) atmospheric sources of Cd in these oligotrophic waters. Although subject to uncertainty, flux calculations suggest that atmospheric input could contribute 10–83% of the total Cd input to the surface waters of the subtropical South Pacific gyre.

Controls on δ18O, δD and δ18O-salinity relationship in the northern Indian Ocean

Oxygen isotopic composition (δ18O) and salinity (S) of the surface ocean are useful tracers for studying imprints of physical processes (such as runoff, sea ice melting, brine release caused by sea ice formation and large-scale ocean mixing) in the modern ocean, while δ18O-S relation is important to estimate paleo-salinity. Effects of Precipitation (P), Runoff (R) and Evaporation (E) (i.e., P+R – E budget) on δ18O, δD (hydrogen isotopic composition), S and their relations (δ18O-δD and δ18O-S) in the surface ocean are relatively well understood but understanding of such relations (e.g., δ18O-S) in subsurface ocean waters remains elusive. Based on a new dataset of 352 water samples, which includes vertical profiles of δ18O estimates, we have attempted to understand the effect of surface boundary currents and lateral mixing of different water masses on the δ18O-S and δD-δ18O relation in the northern Indian Ocean. Significantly stronger δ18O-S and δD-δ18O relations were observed during the winter compared to that during the spring with seasonal variability in the δ18O-S relation in surface water (δ18O = 0.14 (±0.01) × S – 4.58 (±0.48), R2 = 0.43, spring; δ18O = 0.28 (±0.01) × S – 9.10 (±0.30), R2 = 0.83, winter; δ18O = 0.26 (±0.01) × S – 8.55 (±0.22), R2 = 0.83, combined spring and winter). The higher slope and lower intercept of the δ18O-S relation during winter with respect to the spring implied that the runoff affects the δ18O-S relation during winter. We observed a significant correlation between d-excess and δ18O in the surface waters during the winter, while there was no correlation in this relationship during the spring. Slope and intercept of the δ18O-S relation [δ18O = 0.13 (±0.01) × S – 4.21 (±0.28), R2 = 0.84] in the subsurface waters off the Andaman Islands and in the surface Bay of Bengal were similar during spring. The δ18O-S relation for deeper water samples with lower slope and higher intercept compared to the mixed zone characterizes the presence of Persian Gulf Water mass at 200–500 m depth in the Andaman Sea.

Dissolved methane in the East China Sea: Distribution, seasonal variation and emission

We present a dataset of dissolved methane (CH4) in the East China Sea (ECS) during five cruises in March, May, August, October and December 2011. CH4 distribution in this region showed pronounced spatial and seasonal variability due to the complex mixing of different water masses and other variables. Surface CH4 concentrations gradually decreased from the coast to the open sea, with maximum values occurring near Changjiang estuary or outside the Hangzhou Bay. The annual mean CH4 concentration of the surface layer was 9.1 ± 1.6 nmol L-1 in the coastal area, which was nearly twice as large as that in the open sea (4.3 ± 1.3 nmol L-1). CH4 was distributed evenly from the surface to the bottom in the shelf region during March and December, while it increased gradually with depth during May and October. CH4 depth profiles exhibited various distribution features along the coast, in the middle and on the edge of continental shelf. CH4 levels at the bottom were generally higher than at the surface during all seasons, indicating obvious CH4 sources from sediments. Incubation experiments of sediment cores onboard suggested that sediment release was an important source of CH4 in the water column of the ECS. We estimated that the annual average CH4 release rate from sediments was about 1.11 μmol·m-2·d-1 on the continental shelf of the ECS. The maximum CH4 concentration and sediment-water CH4 flux both occurred in summer, which might be related with the occurrence of hypoxia in the bottom water. Surface seawater of the ECS was oversaturated with CH4 relative to the atmosphere over most of the five cruises, indicating that the ECS was a net source of atmospheric CH4. The annual mean area-weighted sea-air flux density of CH4 in the ECS was estimated to be about 10.7 μmol·m-2·d-1 in 2011. Accordingly, an area-weighted, seasonally adjusted annual rate of CH4 efflux was determined to be 2.98 × 109 mol yr-1 (∼0.05 Tg CH4 yr-1) from the ECS to the atmosphere.

Mercury and methylmercury in the Atlantic sector of the Southern Ocean

Oceans constitute one of the most important reservoirs for mercury. In order to provide a first insight into the concentrations of Hg species in the Atlantic sector of the Southern Ocean a sampling campaign was carried out south of the Polar Front.Water samples taken at discrete depths from the surface down to 300m at six stations were analysed for total Hg (HgT), methylmercury (MeHg) and other interpretative parameters such as salinity, temperature, dissolved and particulate organic carbon, dissolved oxygen, chlorophyll and inorganic nutrients.Results showed a high spatial variability in the concentrations of HgT and MeHg. HgT (0.93±0.69ngL−1) and MeHg (0.26±0.12ngL−1) levels were similar or higher than those reported in previous works in high latitude studies. The highest values were found at a location (−53°, 10°E) south of the South Polar Front, an area of strong gradients caused by the mixing of different water masses. Vertical profiles showed a great variability even for those stations sampled at the same location or an area dominated by the same oceanographic features. A decrease of HgT and a consequent increase in MeHg with depth was observed in some sites, suggesting the occurrence of Hg-methylation process, while at other stations, a concurrent decrease or increase of both mercury species was observed. In spite of these differences, an overall positive correlation between HgT and MeHg was observed. Differences between vertical profiles of Hg species were attributed to favourable environmental conditions for Hg methylation. The highest proportion of MeHg (% of HgT) was observed in sites with low dissolved oxygen or highest estimated remineralization rates.The results obtained in this study show that the Hg distribution and speciation in the Atlantic sector of the SO is comparable (or in some sites higher) to the ones published for the other open ocean regions. However, the concentrations of MeHg in this area are more dependent on the environmental conditions than on the total concentration of Hg present in the water.

Three Gorges Dam alters the Changjiang (Yangtze) river water cycle in the dry seasons: Evidence from H-O isotopes

As the largest hydropower project in the world, the Three Gorges Dam (TGD) has attracted great concerns in terms of its impact on the Changjiang (Yangtze) River and coastal marine environments. In this study, we measured or collected the H-O isotopic data of river water, groundwater and precipitation in the mid-lower Changjiang catchment during the dry seasons of recent years. The aim was to investigate the changes of river water cycle in response to the impoundment of the TGD. Isotopic evidences suggested that the mid-lower Changjiang river water was ultimately derived from precipitation, but dominated by the mixing of different water masses with variable sources and isotopic signals as well. The isotopic parameter “deuterium excess” (d-excess) yielded large fluctuations along the mid-lower mainstream during the initial stage of the TGD impoundment, which was inherited from the upstream water with inhomogeneous isotopic signals. However, as the reservoir water level rising to the present stage, small variability of d-excess was observed along the mid-lower mainstream. This discrepancy could be explained that the TGD impoundment had significantly altered the water cycle downstream the dam, with the rising water level increasing the residence time and enhancing the mixing of reservoir water derived from upstream. This eventually resulted in the homogenization of reservoir water, and thus small fluctuations of d-excess downstream the dam after the quasi-normal stage (2008 to present). We infer that the retention effect of large reservoirs has greatly buffered the d-excess natural variability of water cycle in large river systems. Nevertheless, more research attention has to be paid to the damming effect on the water cycle in the river, estuarine and coastal areas, especially during the dry seasons.

Optical assessment of colored dissolved organic matter and its related parameters in dynamic coastal water systems

Prediction of the curve of the absorption coefficient of colored dissolved organic matter (CDOM) and differentiation between marine and terrestrially derived CDOM pools in coastal environments are hampered by a high degree of variability in the composition and concentration of CDOM, uncertainties in retrieved remote sensing reflectance and the weak signal-to-noise ratio of space-borne instruments. In the present study, a hybrid model is presented along with empirical methods to remotely determine the amount and type of CDOM in coastal and inland water environments. A large set of in-situ data collected on several oceanographic cruises and field campaigns from different regional waters was used to develop empirical methods for studying the distribution and dynamics of CDOM, dissolved organic carbon (DOC) and salinity. Our validation analyses demonstrated that the hybrid model is a better descriptor of CDOM absorption spectra compared to the existing models. Additional spectral slope parameters included in the present model to differentiate between terrestrially derived and marine CDOM pools make a substantial improvement over those existing models. Empirical algorithms to derive CDOM, DOC and salinity from remote sensing reflectance data demonstrated success in retrieval of these products with significantly low mean relative percent differences from large in-situ measurements. The performance of these algorithms was further assessed using three hyperspectral HICO images acquired simultaneously with our field measurements in productive coastal and lagoon waters on the southeast part of India. The validation match-ups of CDOM and salinity showed good agreement between HICO retrievals and field observations. Further analyses of these data showed significant temporal changes in CDOM and phytoplankton absorption coefficients with a distinct phase shift between these two products. Healthy phytoplankton cells and macrophytes were recognized to directly contribute to the autochthonous production of colored humic-like substances in variable amounts within the lagoon system, despite CDOM content being partly derived through river run-off and wetland discharges as well as from conservative mixing of different water masses. Spatial and temporal maps of CDOM, DOC and salinity products provided an interesting insight into these CDOM dynamics and conservative behavior within the lagoon and its extension in coastal and offshore waters of the Bay of Bengal. The hybrid model and empirical algorithms presented here can be useful to assess CDOM, DOC and salinity fields and their changes in response to increasing runoff of nutrient pollution, anthropogenic activities, hydrographic variations and climate oscillations.

Tracing the Seepage of Subsurface Sinkhole Vent Waters into Lake Huron Using Radium and Stable Isotopes of Oxygen and Hydrogen

Exchange of water between groundwater and surface water could alter water quality of the surface waters and thereby impact its ecosystem. Discharges of anoxic groundwater, with high concentrations of sulfate and chloride and low concentrations of nitrate and oxygen, from three sinkhole vents (El Cajon, Middle Island and Isolated) in Lake Huron have been recently documented. In this investigation, we collected and analyzed a suite of water samples from these three sinkhole vents and lake water samples from Lake Huron for Ra, radon-222, stable isotopes of oxygen and hydrogen, and other ancillary parameters. These measurements are among the first of their kind in this unique environment. The activities of Ra are found to be one to two orders of magnitude higher than that of the lake water. Isotopic signatures of some of the bottom lake water samples indicate evidences for micro-seeps at distances farther from these three vents. A plot of δD versus δ18O indicates that there are deviations from the Global Meteoric Line that can be attributed to mixing of different water masses and/or due to some subsurface chemical reactions. Using the Ra isotopic ratios, we estimated the transit times of the vent waters from the bottom to the top of the vent (i.e., sediment–water interface) to be 4–37 days. More systematic studies on the distribution of the radioactive and stable isotope studies are needed to evaluate the prevalence of micro-seeps in Lake Huron and other Great Lakes system.

The influence of water mass mixing on the dissolved Si isotope composition in the Eastern Equatorial Pacific

Silicon isotopes are a powerful tool to investigate the cycling of dissolved silicon (Si). In this study the distribution of the Si isotope composition of dissolved silicic acid (δ30Si(OH)4) was analyzed in the water column of the Eastern Equatorial Pacific (EEP) where one of the globally largest Oxygen Minimum Zones (OMZs) is located. Samples were collected at 7 stations along two meridional transects from the equator to 14°S at 85°50′W and 82°00′W off the Ecuadorian and Peruvian coast. Surface waters show a large range in isotope compositions δ30Si(OH)4 (+2.2‰ to +4.4‰) with the highest values found at the southernmost station at 14°S. This station also revealed the most depleted silicic acid concentrations (0.2 μmol/kg), which is a function of the high degree of Si utilization by diatoms and admixture with waters from highly productive areas. Samples within the upper water column and the OMZ at oxygen concentrations below 10 μmol/kg are characterized by a large range in δ30Si(OH)4, which mainly reflects advection and mixing of different water masses, even though the highly dynamic hydrographic system of the upwelling area off Peru does not allow the identification of clear Si isotope signals for distinct water masses. Therefore we cannot rule out that also dissolution processes have an influence on the δ30Si(OH)4 signature in the subsurface water column. Deep water masses (>2000 m) in the study area show a mean δ30Si(OH)4 of +1.2±0.2‰, which is in agreement with previous studies from the eastern and central Pacific. Comparison of the new deep water data of this study and previously published data from the central Pacific and Southern Ocean reveal substantially higher δ30Si(OH)4 values than deep water signatures from the North Pacific. As there is no clear correlation between δ30Si(OH)4 and silicic acid concentrations in the entire data set the distribution of δ30Si(OH)4 signatures in deep waters of the Pacific is considered to be mainly a consequence of the mixing of several end member water masses with distinct Si isotope signatures including Lower Circumpolar Deep Water (LCDW) and North Pacific Deep Water (NPDW).

Water masses and 129I distribution in the Nordic Seas

The application of the radioactive isotope iodine-129 as a tracer of water circulation in the oceans has provided interesting information with respect to sources and mixing of different water masses. We here present results of 129I distribution in water profiles located in the Nordic Seas and use the isotope to fingerprint water masses in the region. The samples were collected by the US research vessel Knorr in May–June 2002. 129I signatures along the Norwegian Sea reflect a mixing of 129I-rich surface water along the Scandinavian continental slope and 129I-poor North Atlantic surface water. These two water masses become less segregated along the Fram Strait where apparent 129I enrichment penetrates the return Arctic flow into the East Greenland Current. The 129I data further suggest existence of a water mass that is not entirely labeled with respect to origin at the Denmark Strait bottom water. This water parcel probably originates from the Iceland Sea. 129I data also shed light on the major deep water outflow from the Nordic Seas located at the Faeroe Bank Channel.

Mixing along continental slopes affects North Atlantic Deep Water

In the northern Atlantic Ocean, cold salty water from the Arctic Ocean sinks to form North Atlantic Deep Water, a current that hugs the bottom of the ocean as it travels south toward Antarctica. The formation of this water mass is vital to large‐scale global ocean circulation and the spread of heat and nutrients across the planet's waters—when deep waters surface, they form warm surface currents such as the Gulf Stream, which are swept by wind back to the North Atlantic, where they cool and sink, beginning the cycle anew. In the Faroe‐Shetland Channel, so named because it lies between Scotland's Shetland Islands and Denmark's Faroe Islands, deep water masses that are tributaries to North Atlantic Deep Water abut the steep continental slopes that bracket the channel. The ebb and flow of ocean tides there produce horizontal and vertical waves that travel long distances and may break on the slopes like waves on a beach. These waves, called internal tides, were monitored and modeled by Hall et al., who sought to understand how they affect the mixing of different water masses that pass through the channel. They found that strong mixing due to internal tides on the slopes that bound the Faroe‐Shetland Channel may be an important mechanism by which North Atlantic Deep Water acquires the physical and chemical properties that it carries across the globe. (Journal of Geophysical Research‐Oceans, doi:10.1029/2010JC006213, 2011)

Distribution of size fractionated dissolved iron in the Canary Basin

The distribution of size fractionated dissolved iron (DFe, <0.2 μm) species was determined in the upper water column (0–150 m) of the Canary Basin (25–32°N and 18–24°W) on a research cruise in October 2002. A DFe concentration gradient resulting from a decrease in both soluble iron (SFe, <0.02 μm) and colloidal iron (CFe, 0.02–0.2 μm) was shown to extend from the coast of North West Africa into the oligotrophic gyre (varying from ∼1 nM in the shelf region to 0.15 nM in the most off shore waters). At the time of this study, the dominant dissolved Fe input to the region was deduced to be the advection of shelf and upwelled waters rather than Saharan dust deposition. SFe and CFe fractions had mean concentrations (± one standard deviation) of 0.25 ± 0.11 and 0.21 ± 0.16 nM, respectively ( n = 58). Colloidal iron formed a highly variable fraction of DFe (ca. 0–80%, mean of 42%) in the region but was less variable in the low iron, oligotrophic intermediate waters (0.18 ± 0.06 nM, 31.7°N, 22.0°W, 0–1300 m depth). The high variability found at the most productive near-shelf stations was driven by biological processing and mixing of different water masses. In contrast, less variability between SFe and CFe at the remote off shore stations suggested that vertical variations in the water column were controlled more by chemical partitioning and vertical particle fluxes with evidence of preferential biological uptake and/or removal of SFe in the most remote surface waters.

The lidar investigation of the upper water layer fluorescence spectra of the Baltic Sea

Marine water in coastal areas and open zone of the Baltic Sea is characterized by complex biophysical structure caused by mixing of different water masses and high human impact. Our lidar investigation of the Baltic upper seawater layer allows obtaining, in real-time and without disturbing the aquatic medium, the continuous fluorescence spectra of seawater in visible range of light. These spectra provide the information about concentration of Chlorophyll a (Chl a) and humus-type (HS) of colour dissolved organic matter (CDOM) as well as allow distinguishing coastal and open-sea waters.

Factors influencing the distribution, biomass, and productivity of phytoplankton in the Scotia Sea and adjoining waters

During January and February 2000 four research vessels,f rom Russia,t he UK, Japan, and the United States, conducted an oceanographic survey with 137 hydrographic stations within the Scotia Sea and adjoining waters as part of a survey sponsored by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) to estimate the biomass and distribution of Antarctic krill in the Scotia Sea. Chlorophyll-a (Chl-a) measurements showed great variability in phytoplankton biomass within the Scotia Sea,wi th some areas having among the lowest Chl-a concentrations found in Antarctic waters (o0.1mgm_3 in surface waters) while other areas were among the richest with 410mgm_3. This paper describes the distribution and concentration of Chl-a in the upper 100m of the water column and relates the Chl-a profiles at individual stations to profiles of upper water-column stability,t o the depth of the upper mixed layer, and to the mixing of different water masses. The 58 stations with the lowest Chl-a values in surface waters also had low values for integrated Chl-a (33.9+-9.5mgm_2) and a Chl-a maximum at depths of between 70 and 90m,in contrast to all other stations where deep Chl-a maxima did not occur. The T/S diagrams at many of these stations were indicative of Antarctic Circumpolar Current (ACC) waters. The central Scotia Sea and areas to the west and north of South Georgia had significantly higher integrated Chl-a values (98.1+-46.0mgm_2, n ¼ 57),i n addition to five stations with very high Chl-a values (mean of 359+-270mgm_2). The mean rate of integrated primary production,wh ich was estimated using the Chl-a data and the mean incident solar radiation measured from previous cruises as well as from satellite data,w as estimated to be 994mg carbon m_2 day_1. The temperature profiles at these stations suggested that considerable interleaving and mixing of water types had occurred,w hich was also evident in the T/S diagrams,w hich indicated mixing of ACC waters with coastal waters originating from Bransfield Strait or theWeddell Sea. There was no significant correlation between integrated Chl-a values and the profiles of upper water column stability or the depth of the upper mixed layer. The spatial variability in phytoplankton biomass within the Scotia Sea is discussed in relation to the hypothesis that low iron concentrations are the major factor controlling phytoplankton biomass in these pelagic Antarctic waters and that concentrations of iron available for phytoplankton uptake are strongly influenced by fronts and the mixing of different water masses.

Improved method for calculating anthropogenic CO2 in the upper layer of the North Pacific Subtropical Gyre along 175°E

Seawater samples were collected in the North Pacific along 175°E during a cruise of the Northwest Pacific Carbon Cycle Study (NOPACCS) program in 1994. Many properties related to the carbonate system were analyzed. By using well-known ratios to correct for chemical changes in seawater, the CO2 concentration at a given depth was back calculated to its initial concentration at the time when the water left the surface in winter. We estimated sea-surface CO2 and titration alkalinity (TA) in present-day winter, from which we evaluated the degree of air-sea CO2 disequilibrium in winter was. Using a correction factor for air-sea CO2 disequilibrium in winter, we reconstructed sea-surface CO2 in pre-industrial times. The difference between the back-calculated initial CO2 and sea-surface CO2 in pre-industrial times should correspond to anthropgenic CO2 input. Although the mixing of different water masses may cause systematic error in the calculation, we found that the nonlinear effect induced by the mixing of different water masses was negligible in the upper layer of the North Pacific subtropical gyre along 175°E. The results of our improved method of assessing the distribution of anthropogenic CO2 in that region show marked differences from those obtained using the previous back-calculation method.