Seawater Endmember Research Articles

Dissolved organic matter composition and characteristics during extreme flood events in the Yangtze River Estuary

Understanding the molecular composition and fate of dissolved organic matter (DOM) during transport in estuaries is essential for gaining a comprehensive understanding of its role within the global biogeochemical cycle. In 2020, a catastrophic flood occurred in the Yangtze River basin. It is currently unknown whether differences in hydrologic conditions due to extreme flooding will significantly impact the estuarine to oceanic DOM cycle. We determined the DOM composition in the Yangtze River estuary (YRE) to the East China Sea by using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) during the high discharge and the flood period (monthly average discharge was 1.2 times higher) on the same trajectory. Our study found that the composition of DOM is more diverse, and more DOM molecules were introduced to the YRE during the flood, especially in the freshwater end member. The result revealed that the DOM was significantly labile and unstable during the flood period. A total of 1840 unique molecular formulas were identified during the flood period, most of which were CHON, CHONS, and CHOS compounds, most likely resulting from anthropogenic inputs from upstream. Only 194 of these molecules were detected in the seawater end member after transporting to the sea, suggesting that the YRE served as a ‘filter’ of DOM. However, the flood enhances the transport of a group of terrigenous DOM, that is resistant to photodegradation and biodegradation. As a result, YRE experienced ~1.6 times higher terrigenous DOC flux than high discharge period. Considering the increased frequency of future floods, our study provides a preliminary basis for further research on how floods affect the composition and characteristics of estuarine DOM. With the help of the FT-ICR MS technique, we can now better understand the dynamic of DOM composition and characteristics in large river estuaries.

Unraveling the photochemical reactivity of dissolved organic matter in the Yangtze river estuary: Integrating incubations with field observations

Dissolved organic matter (DOM) sustains a substantial part of the organic matter transported seaward in large estuaries, where photochemical reactions significantly influence its transformation and fate. Irradiation experiments can provide valuable information on the photochemical reactivity (photo-labile, photo-resistant, and photo-product) of molecules. However, previous research paid less attention to exploring the controls of the initial DOM chemistry to irradiation experiments and examining the applicability of their further integration with field research. Here, we conducted irradiation experiments for samples from the freshwater and seawater endmember of the Yangtze River Estuary (YRE), which receives organic matter transport from the largest river in China, the Yangtze River. Molecules that occurred before and after irradiation experiments were characterized by the Fourier transform ion cyclotron resonance mass spectrometry. Results show that both post-irradiation samples have the lower aromaticity degree and reduced oxidation state, while the freshwater endmember sample exhibits more dramatic changes, indicating the controls of parent molecules to the effect of irradiation experiments. Integrating with the “molecular matching” approach, we compared the molecules occurring in field samples with the classified molecules (photo-resistant, photo-labile, and photo-product) acquired from performed irradiation experiments and correlated the relative intensity of photochemical reactivity types with salinity. When applying results from different experiments to conduct “molecular matching”, the photo-resistant and photo-labile relative intensity possess consistently positive and negative trends with increasing salinity, respectively. This suggests their reliability for molecular matching applications, while the inconsistent trends for the photo-product relative intensity with salinity suggest its uncertainty in assessing the photo-induced effects. Moreover, the molecular composition within the photochemical reactivity types in field samples also evolved along the salinity gradient and showed similar trends with the DOM changes after experimental irradiation. Despite various factors influencing estimations, it is revealed that a fraction of aromatic molecules and majority of carboxyl-rich alicyclic molecules considered with biologically persistent nature in the YRE freshwater zone are simultaneously not susceptible to photochemical transformation to potentially constitute a long-term marine carbon sink. This study emphasizes the importance and limitations of the combination of field research and laboratory-controlled experiments to provide a better understanding of the crucial role of photochemical reactions in affecting carbon cycling in large estuaries.

Groundwater springs in the German Wadden Sea tidal flat: A fast-track terrestrial transfer route for nutrients and dissolved organic matter

Submarine groundwater discharge (SGD) connects fresh groundwater and marine ecosystems and conveys terrestrially derived dissolved organic matter (DOM) and nutrients from land to sea. The connectivity of terrestrial and marine ecosystems via SGD depends strongly on local environmental settings. For example, SGD composition is modified on its transit through the coastal aquifer, with spring-type SGD from highly permeable aquifers presumably being less affected than diffuse discharge systems from sedimentary environments. In our study, we investigated spring-type SGD near Sahlenburg/Cuxhaven, Northern Germany, which passes through fine, unconsolidated tidal sediments before entering the coastal ocean. We characterized groundwater, surface water and seawater endmembers from different seasons and assessed the potential of tidal sediments impacting the biogeochemistry of “fast-track”, point-source groundwater discharge systems. In addition to physicochemical parameters and nutrients, we analyzed the DOM molecular composition via ultrahigh-resolution mass spectrometry (FT-ICR-MS). Our data revealed a widespread physicochemical and geochemical influence of the groundwater springs on the tidal flat, producing low salinity and low dissolved organic carbon (DOC), and high nitrate and high oxygen concentrations not only in the springs, but also in adjacent porewater. From near- to offshore, salinity and DOC concentrations in springs decreased whereas nitrate and oxygen concentrations increased, resembling an inverse estuarine pattern. Furthermore, high nitrate values suggest anthropogenic sources (e.g., agricultural influence) in the surrounding watershed and may stimulate primary productivity in the tidal flat. Humic-like fluorescent DOM (FDOM) abundances and DOM molecular fingerprints indicated inputs of terrestrial DOM from nearshore saltmarsh plants, as well as from the nearby Elbe and Weser estuaries. Our study demonstrated that SGD had a strong geochemical impact even in the vicinity of large rivers, with productive springs actively hindering sulfate and nitrate reduction by flushing otherwise anoxic systems with oxygen. We posit that the geochemical influence of groundwater springs in tidal flats is underestimated because it can extend far beyond their visual discharge points.

Distribution and size fractionation of nickel and cobalt species along the Amazon estuary and mixing plume

The Amazon River has the largest drainage basin in the world, making it a major source of trace elements and dissolved organic matter (DOM) to the Atlantic Ocean. However, despite the increasing anthropogenic impacts to the Amazon basin, few recent studies exist quantifying trace element data in this region. The aim of the study was to analyze the input and removal processes that influence the transport of Ni and Co species in the Amazon and Pará River estuaries and mixing zone. Toward this goal, this work provides a comprehensive mixing and speciation study for the trace elements Ni and Co. Samples were collected during a period of high river discharge on the RV Meteor cruise M147 (Amazon – GEOTRACES process study GApr11) in the Amazon and Pará River outflow regions, as well as the aging mixing plume to the north, a mangrove belt to the southeast and the North Brazil Current (NBC) seawater endmember. Here we present the results for labile particulate (>0.2 μm), labile and total dissolved (<0.2 μm), large colloidal (0.015–0.2 μm), soluble (<0.015 μm) and ultrafiltered (<1 and < 10 kDa) fractions of Ni and Co in surface waters (towed-fish) and along the water column at different depths (CTD) using comparative approaches by adsorptive cathodic stripping voltammetry (AdCSV) and inductively coupled plasma-mass spectrometry (ICP-MS). We observed good agreement between AdCSV and ICP-MS measurements for Ni, and to a lesser extent Co. In general, dissolved and soluble Ni and Co decreased with increasing salinity, however additional non-conservative removal was also observed and attributed to possible biological uptake and colloidal flocculation. Shipboard AdCSV measurements showed that dissolved Ni was present mostly in the “reactive” form as weak complexes, suggesting high bioavailability, while reactive dissolved Co was absent, indicating the presence of strong organic Co complexes. In both Ni and Co, an elevated colloidal fraction was observed at low salinity, suggesting removal of dissolved Ni and Co via colloidal flocculation upon seawater mixing, while the soluble species were transported to the Atlantic Ocean. At depth, the soluble phase dominated, and we observed concentration maxima at 500–1000 m, indicating the presence of Antarctic Intermediate Water (AIW) and possible biological regeneration. We also observed unique source signatures in dissolved and labile particulate Ni and Co species from the Amazon and Pará River outflow regions, in addition to a contribution from mangrove belt-associated groundwater.

Synoptic assessment of coastal total alkalinity through community science

Comprehensive sampling of the carbonate system in estuaries and coastal waters can be difficult and expensive because of the complex and heterogeneous nature of near-shore environments. We show that sample collection by community science programs is a viable strategy for expanding estuarine carbonate system monitoring and prioritizing regions for more targeted assessment. ‘Shell Day’ was a single-day regional water monitoring event coordinating coastal carbonate chemistry observations by 59 community science programs and seven research institutions in the northeastern United States, in which 410 total alkalinity (TA) samples from 86 stations were collected. Field replicates collected at both low and high tides had a mean standard deviation between replicates of 3.6 ± 0.3 µmol kg−1 (σ mean ± SE, n = 145) or 0.20 ± 0.02%. This level of precision demonstrates that with adequate protocols for sample collection, handling, storage, and analysis, community science programs are able to collect TA samples leading to high-quality analyses and data. Despite correlations between salinity, temperature, and TA observed at multiple spatial scales, empirical predictions of TA had relatively high root mean square error >48 µmol kg−1. Additionally, ten stations displayed tidal variability in TA that was not likely driven by low TA freshwater inputs. As such, TA cannot be predicted accurately from salinity using a single relationship across the northeastern US region, though predictions may be viable at more localized scales where consistent freshwater and seawater endmembers can be defined. There was a high degree of geographic heterogeneity in both mean and tidal variability in TA, and this single-day snapshot sampling identified three patterns driving variation in TA, with certain locations exhibiting increased risk of acidification. The success of Shell Day implies that similar community science based events could be conducted in other regions to not only expand understanding of the coastal carbonate system, but also provide a way to inventory monitoring assets, build partnerships with stakeholders, and expand education and outreach to a broader constituency.

Hazardous Noxious Substance Detection Based on Ground Experiment and Hyperspectral Remote Sensing

With an increase in the overseas maritime transport of hazardous and noxious substances (HNSs), HNS-related spill accidents are on the rise. Thus, there is a need to completely understand the physical and chemical properties of HNSs. This can be achieved through establishing a library of spectral characteristics with respect to wavelengths from visible and near-infrared (VNIR) bands to shortwave infrared (SWIR) wavelengths. In this study, a ground HNS measurement experiment was conducted for artificially spilled HNS by using two hyperspectral cameras at VNIR and SWIR wavelengths. Representative HNSs such as styrene and toluene were spilled into an outdoor pool and their spectral characteristics were obtained. The relative ratio of HNS to seawater decreased and increased at 550 nm and showed different constant ratios at the SWIR wavelength. Noise removal and dimensional compression procedures were conducted by applying principal component analysis on HNS hyperspectral images. Pure HNS and seawater endmember spectra were extracted using four spectral mixture techniques—N-FINDR, pixel purity index (PPI), independent component analysis (ICA), and vertex component analysis (VCA). The accuracy of detection values of styrene and toluene through the comparison of the abundance fraction were 99.42% and 99.56%, respectively. The results of this study are useful for spectrum-based HNS detection in marine HNS accidents.

Inter-annual Variability of the Carbonate System in the Hypoxic Upper Pearl River Estuary in Winter

Hypoxia has become a universal environmental and ecological problem in recent decades. The Pearl River estuary (PRE), the largest estuary in Southern China, is hypoxic year-round in the upper estuary. This study reports the inter-annual variation between 2005 and 2019 in the carbonate system of the hypoxic upper PRE in winter. In January 2005, both DIC and TA concentrations were >3000 umol kg-1 at the upstream-most station and decreased sharply downstream. However, DIC and TA were lower, with concentrations of 2300 and 1950 umol kg-1, respectively, at the upstream-most in January 2019. At salinities >15, both DIC and TA were conservative and reached steady values at the downstream seawater end-member. The upstream-most station was taken as an example to quantify the influences of biogeochemical processes on DIC and TA, including CO2 degassing, organic carbon oxidation, pelagic nitrification, CaCO3 dissolution and benthic release. Among the biogeochemical process, a decrease in CaCO3 dissolution (from 734.4 umol kg-1 in 2005 to 168.9 umol kg-1 in 2019) was the major factor driving the decreases of DIC and TA in 2019. In the context of global change, inter-annual variability in biogeochemical process should receive more attention.

Behaviour of chromium and chromium isotopes during estuarine mixing in the Beaulieu Estuary, UK

Rivers are the principal source of chromium (Cr) to seawater and the Cr isotopic signatures of ancient marine sediments are widely considered to provide a record of the presence or absence of oxidative weathering processes on land. This assumes, however, that the δ53Cr value of river water is faithfully transferred to the oceans and is not modified in the estuarine mixing zone. To test this assumption we have determined the concentration and δ53Cr values of inorganic Cr (Cr(III) + Cr(VI)), and also Cr speciation for water samples collected within the estuarine mixing zone of the Beaulieu River, UK. The δ53Cr values of dissolved inorganic Cr ranged from −0.59 to 1.68‰, Cr(VI) concentrations from 0.39 to 1.83 nmolkg−1 and Cr(III) concentrations from 0.11 to 3.21 nmolkg−1. Both Cr(VI) concentrations and δ53Cr values increased linearly as a function of salinity, while Cr(III) concentrations decreased linearly with salinity. Thus δ53Cr, Cr(III) and Cr(VI) all showed conservative behaviour in the estuarine mixing zone, and the δ53Cr signature of Beaulieu River water was modified only by mixing between the river and seawater endmembers. The calculated average δ53Cr value of the river water endmember (−0.39 ± 0.08‰) was, however, lower than the range that has been observed in other rivers, which we attribute to input of organically-bound Cr(III) released by anoxic weathering processes. This is supported by the fact that Cr recovered by UV irradiation was found to have low δ53Cr values (−0.11 to −0.75‰). While input of Cr from anoxic weathering processes is unlikely to be an important source of Cr to the oceans today, this suggests that processes other than oxidative weathering may have an influence on the δ53Cr values of estuarine and coastal waters on the local scale. The δ53Cr value of the coastal seawater endmember (1.6 ± 0.4‰) was also higher than the range observed in the deep open ocean, due to in situ biogeochemical cycling of Cr. These factors need to be considered in the interpretation of marine sedimentary δ53Cr records.

Inorganic and organic iron and copper species of the subterranean estuary: Origins and fate

Subterranean estuaries (STEs) are land-ocean interfaces where meteoric fresh groundwater mixes with intruding seawater in a coastal aquifer, before discharging into the adjacent water column. In contrast to surface estuaries, STEs have the potential to amplify concentrations of constituents such as copper (Cu) and iron (Fe) due to long residence times and reductive dissolution of mineral phases along the groundwater flowpaths. However, oxidative precipitation of Fe and Mn at the sediment-water interface may scavenge many constituents again before they reach the coastal water column. Hence, the geochemical impact of the suboxic to anoxic submarine groundwater discharge (SGD) on the oxygenated coastal ocean relies on the capability of constituents such as Cu and Fe to stay in solution across redox boundaries. Here, we propose that dissolved organic matter (DOM) in the STE plays a pivotal role in the speciation of Cu and Fe through (i) fueling reductive dissolution and (ii) providing ligands to form stable metal-DOM complexes, increasing their transfer from the STE into the coastal ocean. We investigated the concentrations and speciation of Cu and Fe, and DOM chemical characteristics, in two beach STEs of a barrier island. By combining well-established techniques with novel quantification and speciation approaches from both the inorganic and organic geochemical realm (size-fractionation filtration, ferrozine detection, voltammetry, sequential DOM extraction, and ultra-high resolution mass spectrometry) we characterized metal-DOM associations down to the molecular level. Overall, pore water from both STEs was enriched with Cu and Fe compared to seawater, which indicated transfer potential for both trace metals across the sediment-water interface. However, Fe gradients from pore water to surface were steeper than those for Cu, indicating a larger net transfer of the latter compared to the former. Our voltammetry data showed that Cu was exclusively organically bound in both STEs and the water column, mostly in soluble form (<20 nm). The majority of >60 newly identified Cu-containing complexes had primarily aliphatic character and N and S in their molecular formulae resembling labile marine DOM, while two Cu-DOM complexes had polyphenol (“humic-like”) molecular formulae indicative of terrestrial vascular plant-derived material. In contrast to Cu, the Fe pool consisted of either reduced, soluble (<20 nm), likely free Fe(II) in the anoxic STE, or of larger colloids (<200 nm and >20 nm) in the fresh groundwater and seawater endmembers, likely as Fe(III)(hydr)oxides stabilized by DOM. Furthermore, while Fe and humic-like DOM seemed to share common sources, all directly identified mobile Fe-DOM complexes appeared to have marine origins. Therefore, organic forms of Fe in the STE may primarily consist of immobile humic-Fe coagulates, partially mobile Fe-nanocolloids, and mobile, N-containing, marine aliphatic Fe-complexes. Our study indicates that aliphatic, N-containing ligands may play an important role in the organic complexation and stabilization of Fe and particularly Cu in the STE, and enable them to cross redox boundaries at the sediment-water interface.

Chemical and stable isotope composition (18O/16O, 2H/1H) of formation waters from the Carabobo Oilfield, Venezuela

In this short note, we present the first data on stable isotope composition of the oilfield waters from Carabobo area of the Faja Petrolifera del Orinoco “Hugo Chavez” (Orinoco Oil Belt). From a chemical point of view, the formation waters show a main Na-Cl level (TDS up to 30g/l) with a dilution trend toward Na-HCO3 composition (down to 1g/l). Until now, such a clear net chemical compositional trend was ascribed to a meteoric dilution (fresh/ brackish bicarbonate) of the seawater endmember (the saltiest chloride). The isotope results of this study reveal that the seawater mother water was modified during a high-temperature thrusting event (120–125°C), forming 18O-enriched diagenetic water (up to +4‰), which was diluted in recent times by glacial meltwater and presentday meteoric water. The hypothetical presence of flood by a meteoric paleo-water also offers new hints to explain the low API gravity (<10°API biodegraded, extra heavy oil) and composition of the local crude.

Evolution of groundwater chemistry in coastal aquifers of the south-eastern White Sea area (NW Russia) using 14C and 234U-238U dating

The specific objectives of the study are to clarify the sources and characteristics of groundwater in the aquifers along the coast of the White Sea in northwestern Russia, and on this basis to perform a broad 14C and 234U/238U dating of all their types, taking into account the mixing processes. Investigation of an evolution of the groundwater chemistry revealed that the main evolutionary trends are the following: (1) Mixing Late Pleistocene brackish water end member (brackish1) and Mikulino seawater end member with strongly brackish and salty water in the Vpd aquifer (salty Vpd) formation. Groundwater dating showed the “brackish1” residence time in the aquifer of 32.96±2.3ka. Recharge of “brackish1” could have occurred in MIS 3. (2) Mixing Late Pleistocene freshwater end member (fresh LP) and “salty Vpd” end member with brackish water (brackish2) formation. Groundwater dating showed the “brackish2” residence time in the aquifer from 25.1±0.7 to 39.2±6.3ka. Recharge of “fresh LP” could have occurred ~ in MIS 3 also. (3) Mixing Middle Pleistocene-Holocene freshwater of melting glaciers (fresh MP-H) end member and brine end member with the strongly brackish and salty water in Vmz aquifer (salty Vmz) formation. Recharge of “fresh MP-H” could have occurred in Middle Pleistocene-Holocene during MIS 12–MIS 1. As a result of intensive and rapid recharge after the glacial melting, glacial fluids have penetrated at depth to >200m. The results of this study provide a better understanding of the interrelationship of various groundwater flows near the coasts and contribute to a more justified and efficient use of them for drinking water supply in large cities, balneological treatment and industrial extraction of iodine waters. They also allow assessment of the risks of dumping saline drainage water into the environment.

Behavior of rare earth elements and yttrium during simulation of arctic estuarine mixing between glacial-fed river waters and seawater and the impact of inorganic (nano-)particles

Rivers are the major source of many trace elements to the oceans. As a consequence of climate change, increasing volumes of glacial meltwater enter the oceans after being affected by estuarine processes. Although the behavior of high field strength elements such as the rare earth elements and yttrium (REY) has been intensively studied in tropical, temperate and boreal estuaries, little is known about arctic estuarine mixing of glacial-fed river waters that are poor in organic nanoparticles and colloids (NPCs), but rich in inorganic NPCs. Here we provide the first data set from estuarine mixing experiments with glacial-fed river waters and seawater. The glacial-fed river water endmembers originate from southern Iceland (sampled in 2010 and 2013) and from West Greenland (sampled in 2013); these endmembers are rich in NPCs and larger particles of (glassy) volcanic ash and of ultra-fine rock flour, respectively.For the mixing experiments, 0.2 μm-filtered glacial-fed river water was mixed with seawater in different ratios to cover the full range of estuarine low- to high-salinity conditions. All 2010 freshwater endmembers show higher concentrations of individual REY (due to their high NPC load) than the respective seawater endmember, whereas the 2013 freshwater endmembers have higher light REY, but lower heavy REY concentrations (due to significantly less NPCs). Admixture of minute amounts of seawater (5% and 10% seawater (SW) admixture, i.e. salinities of ~1.7 psu and of ~3.4 psu, respectively) already has a strong impact on REY concentrations and REY signatures. In all experiments, a large amount of REY (e.g., up to 98.2% of Nd and 98.2% of Yb) is removed at very low salinities. At intermediate to high salinities, however, remobilization of REY from aggregated NPCs occurs if NPC-rich river waters are used in the experiments, whereas if the endmember is poor in NPCs, the REY mix almost conservatively. The REY removal is dependent on the amount of NPCs present in the glacial-fed river waters; NPC-rich river water shows larger REY removal than NPC-poorer river water. After the initial drop in REY concentrations under low-salinity conditions, the experiment with glacial-fed river water from Greenland reveals that heavy REY mix almost conservatively, but light REY are again remobilized from aggregated NPCs. Our results suggest that aggregated NPCs composed of rock flour (Greenland) and volcanic ash (Iceland) may cause different trace element behavior during estuarine processes. However, all experiments show similar Y-Ho fractionation during estuarine mixing, indicating that the actual type of particle is not a major constraint on Y-Ho behavior, but rather the availability of particle surfaces. Estuarine processes may cause the development of negative Ce anomalies during remobilization of REY at higher salinity if Ce(IV) compounds of low solubility are present in the (nano-)particle load. However, if only Ce(III) is present, such as in fresh mafic volcanic ash, no fractionation of Ce from its trivalent REY neighbors occurs.

Colloidal size spectra, composition and estuarine mixing behavior of DOM in river and estuarine waters of the northern Gulf of Mexico

Flow field-flow fractionation (FlFFF) coupled on-line with UV absorbance and fluorescence detectors was used to examine the colloidal composition and size distribution of optically active dissolved organic matter (DOM) in the lower Mississippi River (MR), the East Pearl River (EPR), the St. Louis Bay (SLB) estuary, and coastal waters of the northern Gulf of Mexico. In addition to field studies, laboratory mixing experiments using river and seawater end-members were carried out to study the processes controlling the estuarine mixing behavior and size partitioning of colloids with different sizes and composition. The colloidal size spectra of chromophoric DOM and humic-like DOM showed one dominant peak in the 0.5–4nm size range, representing >75% of the total FlFFF-recoverable colloids. In contrast, protein-like DOM showed a bi-modal distribution with peaks at 0.5–4nm and 4–8nm, as well as a major portion (from ∼41% in the EPR to ∼72% in the Mississippi Bight) partitioned to the >20nm size fraction. Bulk DOM was lower in abundance and molecular-weight in the MR compared with the EPR, while the proportion of colloidal protein-like DOM in the >20nm size range was slightly larger in the MR compared with the EPR. These features are consistent with differences in land use, hydrological conditions, and water residence time between the two river basins, with more autochthonous DOM in MR waters. In the SLB estuary, different DOM components demonstrated different mixing behaviors. The abundance of colloidal chromophoric DOM decreased with increasing salinity and showed evident removal during estuarine mixing even though the bulk DOM appeared to be conservative. In contrast, colloidal humic-like DOM behaved conservatively inside SLB and during laboratory mixing experiments. The ratio of colloidal protein-like to humic-like DOM generally increased with increasing salinity, consistent with increasing autochthonous protein-like DOM and removal of terrestrially-derived humic-like DOM in estuarine and coastal waters. Similar mixing behavior for the bulk DOM and colloids was observed in short-term laboratory mixing experiments, suggesting that physicochemical processes are the major controlling factor for colloidal removal in the estuary. For the first time, this study showed direct evidence of contrasting estuarine mixing behavior for different size fractions of optically active colloidal DOM.

Seawater-derived rare earth element addition to abyssal peridotites during serpentinization

Serpentinized abyssal peridotites are evidence for active communication between the Earth's hydrosphere and the upper mantle, where exchange and retention of both major and trace elements occur. Bulk rock Nd isotopes in serpentinized abyssal peridotites imply interaction of seawater with the peridotite. In contrast, the Nd isotopes of clinopyroxenes from serpentinized abyssal peridotites retain their primary magmatic signature. It is currently unclear if, how and where seawater-derived Nd and other REE are being added or exchanged with the mantle peridotite minerals during serpentinization. To remedy this knowledge gap, we present in situ trace and major element concentrations, bulk rock and sequential leaching experiment trace element concentrations as well as Nd, Sr isotope data on refertilized and depleted serpentinized abyssal peridotites from the Southwest Indian Ridge. The secondary serpentine matrix and magnetite veins in these peridotites have elevated LREE concentrations, with variable negative Ce anomalies and large Rb, Sr, Pb and U enrichments that resemble seawater trace element patterns. The LREE concentrations in the serpentine phase are higher than those expected for the primary mantle mineralogy (olivine, orthopyroxene) based on data from relic clinopyroxenes and equilibrium partition coefficients. These data are consistent with seawater-derived REE addition to the peridotite during serpentinization. The bulk rocks have more radiogenic Sr and more unradiogenic Nd isotopes than their clinopyroxene (up to 8 εNd units lower than clinopyroxene). Sequential leaching experiments designed to mobilize secondary carbonates and Fe-oxides show even more unradiogenic Nd isotope ratios in the leachates than the bulk rock and clinopyroxene, approaching seawater compositions (up to 15 εNd units lower than clinopyroxene). Mass balance calculations using trace elements or Nd isotopes suggest that up to 30% of the bulk peridotite Nd budget is of seawater origin and was added to the peridotite, as opposed to being exchanged. These data allows for more accurate constraints of the water/rock mass exchange ratios during serpentinization in these rocks (~1000 to 17,000) that are at least an order of magnitude lower than using typical mantle and seawater endmembers. These data provide strong evidence that serpentinization may also be a sink for the light REE, with implications for the refertilization of the peridotite during serpentinization, and the cycling of these elements through the subduction system and into the upper mantle.

Nutrients in the Western Wadden Sea: Freshwater Input Versus Internal Recycling

At present, phosphorus (P) is seen as the main limiting nutrient for phytoplankton growth in the western Wadden Sea. Six cruises were performed for water sampling at selected stations covering a full tidal cycle for later determination of dissolved and particulate nutrient concentrations. The major P sources were identified on a seasonal basis, by comparing the contribution of freshwater discharge and sediment release, calculated in a previous study, to the concentrations in the water column. A close relationship was found between the pelagic concentrations of dissolved inorganic nutrients and chlorophyll a, with a concomitant decrease in nutrients and increase in chlorophyll. This was observed in early spring and was followed by a later increase in the nutrient concentrations in spring–summer. The low concentrations found for the freshwater and seawater end-members for this period ruled out their importance as nutrient sources, suggesting that this increase resulted mainly from internal recycling in the Wadden Sea. Even though P limitation was observed during most of the year, a potential seasonal change in the limiting nutrient, from P to silica, was observed. The comparison between P supply to the Wadden Sea by freshwater discharge and sediment release showed a much higher contribution of the latter, especially in April–November. To our knowledge, this is the first study clearly presenting internal recycling as the main nutrient source to the western Wadden Sea in spring–autumn, instead of freshwater discharge or the North Sea.

226Ra and 228Ra tracer study on nutrient transport in east coastal waters of Hainan Island, China

Material fluxes (e.g., nutrients) from coastal waters to offshore areas play an important role in controlling the water quality of the adjacent sea areas not only by increasing nutrient concentration but also by changing nutrient structures. In this study, naturally occurring isotopes, 226Ra and 228Ra, were measured with the alpha spectrometry in the Wenjiao-Wenchang and Wanquan estuaries and adjacent sea areas along the east coast of Hainan Island. The excess 226Ra and 228Ra activities were observed by comparison with the values derived from the conservative mixing of freshwater and seawater end-members in both estuaries. Using a one-dimensional diffusion model, the horizontal eddy diffusion coefficient of 3.16 x 105 cm2/s, for nutrients diffusing from their sources, was derived from 228Ra activities. Consequently, the corresponding nutrient fluxes flowing into the coastal waters were assessed. The results can provide useful information for the study of the mixing and exchange processes of coastal waters as well as dissoluble pollutant transport in this sea area.

Molybdenum isotopes in two Indian estuaries: Mixing characteristics and input to oceans

The distributions of dissolved and particulate Mo and their isotope composition (δ98Mo) have been measured in the Narmada and the Tapi estuaries draining into the Arabian Sea. During monsoon, the δ98Mo of dissolved Mo in the Narmada estuary ranges from 0.49‰ to 2.19‰ in the salinity range 0–17.2 practical salinity unit (psu) quite similar to that in the Tapi estuary, 0.99–2.36‰, in the salinity range 0–20.3psu. Mo concentration in suspended sediments of the Narmada estuary collected during monsoon average 512±44ng/g (range 459–602ng/g) similar to that measured in one sample from the Tapi estuary 560ng/g Mo. δ98Mo of particulate Mo in the Narmada ranges from −0.21‰ to 0.48‰ with an average −0.03±0.2‰. Dissolved Mo in the Narmada and the Tapi rivers display isotopically heavier Mο compared to that in basalts, the major lithology of their drainage. This could result from a variety of processes, preferential weathering of Mo rich sulphide minerals dispersed in the basalts, preferential removal of isotopically lighter Mo during transport or contribution from marine cyclic salts supplied via rain or chemical weathering of organic rich shales in the basins.The distribution of δ98Mo in the Narmada and the Tapi estuaries with salinity does not follow the theoretical mixing line between river and seawater endmembers suggesting its non-conservative behavior. Particulate Mo and δ98Mo show concomitant increase with salinity in the Narmada estuary indicating loss of dissolved Mo by adsorption onto Fe–Mn oxyhydroxide. Balancing the Mo budget along the course of these estuaries using inverse model suggests that in the Narmada estuary there could be loss up to 8% of the dissolved Mo and that in the Tapi supply from anthropogenic sources could be up to 27%. The results obtained in this study bring out the processes modifying riverine input of Mo and its δ98Mo in the estuaries, oxic sink in the Narmada and anthropogenic input in the Tapi. Repetitive adsorption and desorption of Mo in the Narmada estuary can modify the supply of dissolved Mo and its δ98Mo relative to riverine supply by up to 40%, this can significantly impact the Mo isotope budget of the oceans. In contrast, in the Tapi estuary there is enhancement in the dissolved supply of Mo relative to that from river due to anthropogenic input of Mo. The investigations in these two estuaries underscore the importance of solute particle interactions and anthropogenic input in determining the Mo flux and its δ98Mo to the open Arabian Sea.

Optical properties of estuarine dissolved organic matter isolated using cross-flow ultrafiltration

Dissolved organic matter (DOM) from freshwater, mid-salinity, and seawater endmember samples in the Jiulong River Estuary, China were fractionated using cross-flow ultrafiltration with a 10-kDa membrane. The colloidal organic matter (COM; 10 kDa–0.22 μm) retentate, low molecular weight (LMW) DOM (<10 kDa) permeate, and bulk samples were analyzed using absorption spectroscopy and three-dimensional fluorescence excitation-emission-matrix spectroscopy. The UV-visible spectra of COM were very similar to those obtained for permeate and bulk samples, decreasing monotonically with increasing wavelength. Most of the chromophoric DOM (CDOM, expressed as the absorption coefficient a 355 ) occurred in the LMW fraction, while the percentage of CDOM in the colloidal fraction was substantially higher in the freshwater endmember (13.4% of the total) than in the seawater endmember (6.8%). The bulk CDOM showed a conservative mixing behavior in the estuary, while there was removal of the COM fraction and a concurrent addition of the permeate fraction in the mid-salinity sample, implying that part of the colloidal CDOM was transformed into LMW CDOM. Two humic-like components (C1: <250, 325/402 nm; and C2: 265, 360/458 nm) and one protein-like component (C3: 275/334 nm) were identified using parallel factor analysis. The contributions of the C1, C2, and C3 components of the COM fraction to the bulk sample were 2.5%–8.7%, 4.8%–12.6%, and 7.4%–14.7%, respectively, revealing that fluorescent DOM occurred mainly in the LMW fraction in the Jiulong River Estuary. The C1 and C2 components in the retentate and permeate samples showed conservative mixing behavior, but the intensity ratio of C2/C1 was higher in the retentate than in the permeate fractions for all salinity samples, showing that the humic component was more enriched in the COM than the fulvic component. The intensity ratio of C3/(C1+C2) was much higher in the retentate than in the permeate fraction for mid-salinity and seawater samples, revealing that the protein-like component was relatively more enriched in COM than the humic-like component. The contribution of the protein-like component (C3) to the total fluorescence in the retentate increased from 14% in the freshwater endmember to 72% for the seawater endmember samples, clearly indicating the variation of dominance by the humic-like component compared to the protein-like component during the estuarine mixing process of COM.

Hydrochemical and isotopic characterization of the hydrological budget of a MAB Reserve: Mar Chiquita lagoon, province of Buenos Aires, Argentina

Mar Chiquita is a coastal lagoon located in the Argentine Buenos Aires province in South America. The aim of this study was to perform a hydrochemical and stable isotopes characterization in order to better the understanding of the hydrology of the Mar Chiquita lagoon’s catchment and its water budget. Groundwater samples were taken from 144 wells and 21 samples from main streams, and seven lagoon water samples were also collected. Chemical analyses were carried out using standard laboratory methods, and isotopic determinations were made through laser spectroscopy using a DLT-100 liquid–water isotope analyzer. Hydrochemical analysis permits a general classification of groundwater and streamwater as sodium bicarbonate waters, while the lagoon chemical composition shows an evolution toward seawater composition, from the north to its mouth, which is located southerly. Isotopic data show a source of aquifer recharge from rainfall and a groundwater domain into the streams’ flow. Three main components can be recognized as end members in a plot of electrical conductivity (EC) versus δ18O: seawater, streamwater and groundwater. Obtained EC values for groundwater in the discharge zone (EC average value = 3,516 μS/cm) allow minimizing its direct contribution and to take into consideration two dominating end members: streamwater and seawater. Mar Chiquita lagoon’s water falls close to the line between streamwater and seawater end members according to its EC and δ18O. The obtained seawater proportion for these samples ranges from 84 % in the lagoon’s mouth to around 0 % in the more distal area.

Ascending Freshwater-Mesohaline Mixing: A New Scenario for Dolomitization

Abstract The process of mixing-zone dolomitization has received enough criticism in recent years to suggest that it no longer be considered viable for making large amounts of dolomite. This study proposes an alternative model that still requires fluid mixing. In this model, ascending flow of freshwater mixes with evaporated seawater, leading to extensive dolomitization. The new model is constrained by data from upper Miocene strata of southeast Spain, where dolomite yields fluid-inclusion Tm-ice data that range from −0.2 to −2.3 °C, indicating precipitation from fluids of 4 to 43 ppt seawater salt equivalent, thus confirming fluid mixing. The δ13C and δ18O of the dolomite ranges from −4.5 to +3.0‰ VPDB and +0.9 to +6.0‰ VPDB, respectively, and displays positive covariance. Although such positive covariation may represent fluid mixing, the mixing of two dolomites during sampling, or an originally homogeneous dolomite that has been partially altered (recrystallized), fluid-inclusion data and cathodoluminescence petrography disprove the mixture during sampling and partial-alteration hypotheses. Modeling of the most positive dolomite δ18O values indicates an evaporated seawater endmember of 43 ppt salinity, identical to that of the most saline fluid inclusions. Because paleotopography is preserved in the study area, the spatial variation in dolomite geochemistry can be mapped to evaluate if it is consistent with the hydrogeology of a typical mixing-zone model or an alternative. The data reveal that both downdip and updip areas were influenced by high-salinity and low-salinity fluid endmembers, and that stratigraphically lower units have more depleted δ13C and δ18O than upper units. These results are consistent with a model of upward flow of freshwater that provides the mechanism of mixing of freshwater and evaporated seawater. Upward flow and degassing of CO2 from loss of pressure contributed to nearly platform-wide dolomitization in this succession, and may have analogs in other ancient successions.