Proxy For Salinity Research Articles

Evapotranspiration and Rainfall Effects on Post‐Storm Salinization of Coastal Forests: Soil Characteristics as Important Factor for Salt‐Intolerant Tree Survival

AbstractFlooding and salinization triggered by storm surges threaten the survival of coastal forests. After a storm surge event, soil salinity can increase by evapotranspiration or decrease by rainfall dilution. Here we used a 1D hydrological model to study the combined effect of evapotranspiration and rainfall on coastal vegetated areas. Our results shed light on tree root uptake and salinity infiltration feedback as a function of soil characteristics. As evaporation increases from 0 to 2.5 mm/day, soil salinity reaches 80 ppt in both sandy and clay loam soils in the first 5 cm of soil depth. Transpiration instead involves the root zone located in the first 40 cm of depth, affecting salinization in a complex way. In sandy loam soils, storm surge events homogeneously salinize the root zone, while in clay loam soils salinization is stratified, partially affecting tree roots. Soil salinity stratification combined with low permeability maintain root uptakes in clay loam soils 4/5‐time higher with respect to sandy loam ones. When cumulative rainfall is larger than potential evapotranspiration ETp (ETp/Rainfall ratios lower than 1), dilution promotes fast recovery to pre‐storm soil salinity conditions, especially in sandy loam soils. Field data collected after two storm surge events support the results obtained. Electrical conductivity (a proxy for salinity) increases when the ratio ETp/Rainfall is around 1.76, while recovery occurs when the ratio is around 0.92. In future climate change scenarios with higher temperatures and storm‐surge frequency, coastal vegetation will be compromised, because of soil salinity values much higher than tolerable thresholds.

Arctic Alaska deepwater organic carbon burial and environmental changes during the late Albian–early Campanian (103–82 Ma)

The middle Cretaceous greenhouse period experienced profound environmental change including episodes of enhanced global burial of organic carbon marked by carbon isotopic excursions (CIEs). However, the role and response of polar regions like the newly formed, partially enclosed Arctic Ocean Basin during middle Cretaceous carbon burial remains enigmatic. We present the first Arctic deepwater CIE record that characterizes conditions offshore of the Alaska margin north of 75°N paleolatitude. Organic carbon isotopes (δ13Corg) and 103–82 Ma ash zircon U-Pb dates from the distal Hue Shale record multiple Albian–Campanian CIEs during slow ∼3–15 m/Myr sediment accumulation rates. Average total organic carbon (TOC) increased substantially during large 2–3 ‰ CIEs of the ∼101 Ma Albian-Cenomanian boundary event (from 7 to 18 % TOC) and ∼94 Ma Cenomanian-Turonian boundary event (5 to 10 % TOC). Turonian TOC remained elevated (8–13 %) during high global sea levels and temperatures of the Cretaceous Thermal Maximum, followed by an increase from 7 to 11 % TOC during the ∼90 Ma late Turonian event 1.5 ‰ CIE. Average TOC subsequently decreased in the Coniacian–Campanian, but relative maxima occurred during subtle 0.5–1 ‰ CIEs interpreted as the ∼87 Ma late Coniacian event (increase from 4 to 7 % TOC), ∼85 Ma Horseshoe Bay event (3.5 to 4.5 % TOC), and ∼84 Ma Santonian-Campanian boundary event (3.5 to 5 % TOC). Increases in hydrogen index and productivity proxies (P, Ba, Nd) that accompanied each CIE episode with enhanced TOC suggest a strong link between marine productivity and organic carbon burial at short-term CIE timescales. However, long-term (>5–8 Myr) changes in trace metal redox (Mo, Fe, V) and salinity (B/Ga) proxies suggest shifts in prevailing environmental conditions at timescales longer than the CIEs. Late Albian–middle Turonian marine salinity occurred during euxinic (103–98 Ma) and suboxic (98–90 Ma) conditions with deposition interpreted to have occurred within and beneath an oxygen minimum zone, respectively. In contrast, late Turonian–early Campanian (90–82 Ma) freshening and restricted euxinic basin conditions may signal the start of widespread restriction known to characterize the Paleogene Arctic. Overall, these results highlight that middle Cretaceous Arctic deepwater remained a productive marine carbon sink coupled to the global carbon cycle despite evolving Arctic greenhouse conditions.

An insight into effect of soil salinity on vegetation dynamics in the exposed seafloor of the Aral Sea

The desiccation of the Aral Sea has precipitated significant ecological degradation, resulting in the progressive development of vegetation on the exposed seafloor. Soil salinity emerges as a pivotal determinant in this ecological succession process. Employing a comprehensive methodology integrating multi-source datasets spanning from 1986 to 2023, this study elucidates the temporal changes in vegetation dynamics and soil salinity levels. Satellite imagery (Landsat-4/5/7/8), field soil samplings, hydrological and topographic data were analyzed to understand their interactions with regression analysis. The results reveal a consistent increasing trend in the Normalized Difference Vegetation Index (NDVI) across the exposed seabed since 1986. However, NDVI demonstrates a non-linear relationship with elevation in the North Aral Sea region. Interestingly, NDVI levels near an elevation of 42 m on the exposed seabed approximate those observed during the pre-recession period in the 1960s. Conversely, in the South Aral Sea region, NDVI demonstrates a linear upward trend with increasing elevation. Furthermore, the spatial distribution of soil salinity on the exposed seabed was delineated with linear regression analysis. It revealed water salinity levels at the time of sea recession can serve as a proxy for soil salinity in cases where direct soil data is unavailable. Through establishing a robust correlation between NDVI and soil salinity, the range of stable NDVI values on the exposed seabed was delineated. Lastly, three hypothetical scenarios of rising water levels were considered to evaluate changes in stable NDVI across different elevation gradients. If the water level returns to 45 m, the salt-desert area would decrease by 4.5 × 104 km2, accounting for 23 % of the total area in 1960. At this water level, it is anticipated that lake hydrological conditions and ecological environments may restore to those observed in 1981. This study provides a long-term perspective on environmental changes in the Aral Sea region by integrating multiple data sources and analytical methods. The predictive insights from the scenario analysis offer valuable guidance for future water management and ecological restoration efforts. Compared with previous studies, it presents a detailed and comprehensive picture of the interplay between vegetation dynamics and soil salinity, highlighting the critical impact of water level changes on the region's ecosystem.

Astronomically forcing salinity variations in a marginal-marine environment, Bohai Bay Basin, NE China

Exploring the interdependence between watermass conditions and climate change in marginal marine regions is essential for gaining insights into contemporary environmental challenges arising from global warming. Salinity, a key parameter of watermass conditions in coastal areas, plays a crucial role in influencing nutrient availability, stratification, and the spatial-temporal distribution of redox conditions. Here, we focus on the cyclostratigraphic analysis of gamma-ray (GR) data obtained from Eocene strata in the Luo-69 drillcore from the Bohai Bay Basin (NE China). This analysis establishes a high-resolution astronomical time scale (ATS) for the lower third member of the Shahejie Formation (Es3l), spanning 39.04 Ma to 41.46 million years ago (Ma). Various proxies for watermass conditions, such as salinity, sedimentary structures, mineral content, total organic carbon (TOC), organic carbon isotopes and nitrogen isotopes (δ13Corg and δ15N), exhibit a strong correlation with 405-kyr long orbital eccentricity climate forcing. Notably, spectral analysis of salinity data (represented by the Sr/Ba proxy) reveals a clear astronomical control, with 405-kyr eccentricity cycles that align closely with variations in the GR data. Peaks in paleosalinity, as indicated by Sr/Ba (as well as B/Ga and S/TOC), correspond with maxima in 405-kyr long orbital eccentricity cycles in the GR data. As such, we infer that astronomical forcing regulated long-term changes in watermass conditions, likely through mediation of the East Asia monsoon system. Specifically, during eccentricity maxima, the basin would have experienced a warm and humid climate with pronounced seasonality and a robust summer monsoon. During such periods, increased continental runoff would lead to a rise in lake-levels, facilitating better connections with the ocean, and hence increased salinity. By contrast, during eccentricity minima, cooler and more arid conditions with weaker seasonality would result in lower lake levels which together with a global eustatic lowstand would result in as weakened connection with the open ocean restricting the ingress of seawater into the basin lowering salinity. Our dataset spans the Middle Eocene Climatic Optimum (MECO, ∼40.60–40.10 Ma), identified by elevated GR values and a conspicuous negative excursion in oxygen isotopes. This interval is characterized by a significant decline in salinity, likely attributed to enhanced freshwater runoff driven by an extremely warm and humid climate. The paleosalinity fluctuation recorded by the proxies B/Ga, Sr/Ba, and S/TOC in Bohai Bay Basin demonstrate the sensitivity of marginal-marine environment to subtle changes in climate driven by astronomical forcing. Our results provide new insights into how elemental salinity proxies can be utilized in paleoenvironmental reconstruction studies.

Cod otoliths document accelerating climate impacts in the Baltic Sea

Anthropogenic deoxygenation of the Baltic Sea caused major declines in demersal and benthic habitat quality with consequent impacts on biodiversity and ecosystem services. Using Baltic cod otolith chemical proxies of hypoxia, salinity, and fish metabolic status and growth, we tracked changes from baseline conditions in the late Neolithic (4500 BP) and early twentieth century to the present, in order to understand how recent, accelerating climate change has affected this key species. Otolith hypoxia proxies (Mn:Mg) increased with expanding anoxic water volumes, but decreased with increasing salinity indexed by otolith Sr:Ca. Metabolic status proxied by otolith Mg:Ca and reconstructed growth were positively related to dissolved oxygen percent saturation, with particularly severe declines since 2010. This long-term record of otolith indicators provides further evidence of a profound state change in oxygen for the worse, in one of the world’s largest inland seas. Spreading hypoxia due to climate warming will likely impair fish populations globally and evidence can be tracked with otolith chemical biomarkers.

Changes in Indo-Pacific Warm Pool hydroclimate and vegetation during the last deglaciation

Drying across much of the Indo-Pacific Warm Pool (IPWP) during the Last Glacial Maximum (LGM) has been widely recognized from interpretations of sedimentological, geochemical, and paleoecological records. Reconstructions of precipitation isotopic compositions have emerged as a powerful tool to reconstruct rainfall amount in many tropical regions, yet it has proven difficult to reconcile precipitation isotope records from the IPWP with records of widespread drying. To evaluate the signals preserved in precipitation isotope records, we produced new hydrogen and carbon isotope records from Lake Towuti and Lake Matano in Sulawesi, Indonesia using long-chain n-alkanes. We then compared these new records to existing n-alkanoic acid records from the same lakes and compiled available marine runoff, salinity, leaf wax hydrogen isotope (δ2Hwax), and leaf wax carbon isotope (δ13Cwax) records in the IPWP. During the last deglaciation, marine runoff and salinity proxies reveal that precipitation amount began to increase dramatically between ∼12.3 ka at the end of the Younger Dryas. A principal component analysis of precipitation isotope records indicates a shift from more 2H-enriched to 2H-depleted waxes at ∼12.3 ka as sea level rise inundates most of the Sunda and Sahul shelves, coincident with runoff and salinity proxies, suggesting precipitation isotopes respond strongly to rainfall amount in this region. Over 70% of IPWP δ13Cwax records show a transition from more to less 13C-enriched waxes beginning about 19.9 ka, prior to the reconstructed increases in precipitation. We suggest that vegetation shifted in response to the changing seasonality of precipitation. The dramatic changes in the IPWP during the last deglaciation highlight the capacity for the region to experience dynamic changes in precipitation, vegetation, and atmospheric circulation.

High-resolution water temperature and salinity evolutions and associated drivers of the North Yellow Sea during the Late-Holocene

The mud deposit areas on continental shelf have developed in a relatively steady sedimentary condition with high sedimentation rates, thus rendering them ideal regions for exploring high-resolution paleo-sedimentary environment records. Since the sedimentary environment of the continental shelf is subject to the compound influence of multiple factors, the reconstruction of water salinity and temperature and comprehensive analysis of their response to global climate change remains challenging. Therefore, the present study characterized water salinity and temperature of the North Yellow Sea (NYS) using proxies, such as ratios of halogen elements of sediments and stable isotopic compositions of benthic foraminifera, and found the halogen elements of Cl in sediments was good proxy for water salinity. The evolutions of water salinity and temperature of the NYS for the past 3000 years were reconstructed, while the evolution stages and drivers of the sedimentary environment were explored. From approximately 3000 cal yr BP, the sea bottom salinity and temperature of the NYS were found to follow the same evolution trends predominantly consisting of the three stages: relatively low seawater salinity and temperature during 3000−2000 cal yr BP; relatively high seawater salinity and temperature during 2000−460 cal yr BP; and rapid changes of seawater salinity and temperature since 460 cal yr BP. The changes in seawater salinity and temperature of the NYS were largely driven by the East Asian winter monsoon (EAWM) as well as the influence of the Kuroshio Current. As the EAWM fluctuated, changes occurred in the flux of low temperature, low salinity coastal current water into the Yellow Sea, with concomitant changes in bottom temperature and salinity. Strengthening of the Kuroshio Current promoted the development of the Yellow Sea Warm Current, while the influx of warm, salty water into the Yellow Sea influenced the respective sea region. The abrupt global climate changes such as the “Little Ice Age” and the Medieval Warm Period first affected the East Asian monsoon, followed by the respective sea region.

Low-salinity conditions in the “marine” Late Triassic-Early Jurassic Neuquén Basin of Argentina: Challenges in paleosalinity interpretation

Salinity, a key property of watermasses, is difficult to reconstruct in paleodepositional systems, and errors in assigning salinity facies to epicratonic and marginal-marine deposits may be common. In this study, we evaluate the salinity conditions of mudstones of the Arroyo Malo Formation (AMF), spanning the Triassic-Jurassic transition (TJT) of the Neuquén Basin, Argentina. Although the AMF has long been considered marine based on its fossil fauna, we find that three elemental salinity proxies (B/Ga, Sr/Ba, S/TOC) are consistent in indicating dominantly freshwater influence, with occasional excursions to low-brackish conditions. In this context, the sparse, fragmented, and poorly preserved assemblage of bioclasts in the AMF do not accurately represent its salinity facies. We infer that, during the TJT, the Neuquén Basin was largely isolated from the Panthalassic Ocean, or perhaps intermittently weakly connected to it. The same elemental salinity proxies indicate low-brackish conditions in a Lower Jurassic unit previously assumed to be marine and fully marine conditions in Lower Cretaceous mudstone formations of the Neuquén Basin. This trend documents the gradual establishment of open communication with the Panthalassic Ocean, with fully marine conditions prevailing by the Middle to Late Jurassic. Standard redox proxies show that the AMF watermass was not markedly reducing, demonstrating that the impoverishment of the fossil biota was not due to unfavorable redox conditions. These findings demonstrate that inferences of marine conditions based on fossil content are potentially erroneous, and they emphasize the general need to make use of salinity proxies in analysis of deep-time depositional systems.

Testing the salinity of Cambrian to Silurian epicratonic seas

Ancient epicratonic formations, which represent the bulk of pre-Jurassic sedimentary rocks, have been widely interpreted as marine deposits, but recently developed bulk-shale elemental proxies for palaeo-watermass salinity (i.e. B/Ga, Sr/Ba and S/TOC, where TOC is total organic carbon) have shown this inference to be frequently incorrect. Here, we use these proxies to test the salinity conditions of 22 representative shale and marl formations of early Cambrian to early Silurian age from five cratons (Laurentia, Avalonia, Baltica, Iran and South China) in the context of formation-specific palaeogeographical and palaeoclimatic reconstructions. Our dataset shows that around half of these formations were probably deposited under brackish or mixed brackish–marine conditions rather than fully marine conditions (as previously inferred), and that one of them represents a freshwater facies (previously interpreted as marine mainly on the basis of Cruziana traces). In most cases, the development of reduced-salinity conditions can be related to the coastal and/or humid climate belt setting in which the formation of interest was deposited. Our dataset also reveals systematically low Sr/Ba values (i.e. relative to modern brackish and marine facies), suggesting that seawater Sr concentrations were lower during the Early Paleozoic than at present. Our findings suggest that re-evaluation of the salinity characteristics of many ancient epicratonic shale and marl formations is necessary. Supplementary material: A supplementary figure and tables are available at https://doi.org/10.6084/m9.figshare.c.7063365 Thematic collection: This article is part of the Chemical Evolution of the Mid-Paleozoic Earth System and Biotic Response collection available at: https://www.lyellcollection.org/topic/collections/chemical-evolution-of-the-mid-paleozoic-earth-system

The history of serpentinisation at mid-ocean ridges: Insights from in situ trace elements coupled with oxygen and boron isotopes

Oceanic serpentinisation is the principal process of water incorporation into the oceanic lithosphere, thus playing a significant role in the elements cycle. The conditions of serpentinisation vary in temperature, water-rock ratio, and fluid composition, but the investigation of the interplay of these factors, rather than their individual variation, is rarely attempted. This study examines mid-ocean ridge (MOR) serpentinite samples from Mid-Atlantic Ridge 15°20′N Fracture Zone (Leg 209, Sites 1272 and 1274) and from Hess Deep (Leg 147, Site 895D and 895E) to gain insight into the combined effects of these variables. Various in situ geochemical tools are used to evaluate the mutual influence of the different factors in the successive stages of serpentinisation. The Cl/B content of serpentine is employed as a proxy for fluid salinity, enabling a more precise temperature calculation when the constant Cl/B correlates with varying δ18O. The combined in situ temperature-dependent oxygen and boron isotope compositions reveal instances of localised fluctuations in fluid pH that impact the boron isotope composition of serpentine. The δ18O compositions of serpentine vary between 0.8 and 7.8 ‰, implying temperature variations within the range of ∼115 °C to 290 °C. Where the δ11B compositions of serpentine exceed the expected temperature-related variations of circa 9–13 ‰, we propose that pH variations during progressive serpentinisation are the cause (δ11B up to 22 ‰ variability observed between serpentine textures from a single sample). Moreover, this study emphasises the divergence in serpentinisation conditions between MOR and passive margin settings. Serpentinisation can occur at higher temperatures (up to 290 °C) with a more saline fluid (Cl/B > 25) under variable pH conditions in MOR settings, while the serpentinisation in PaMa settings takes place at lower temperatures (< 200 °C) with a less saline fluid (Cl/B < 25) and probably more alkaline conditions.

The variability of Intermediate water intrusion into the South China Sea during the last deglaciation

Millennial-scale variations of the intermediate water in the South China Sea (SCS) have not been well understood due to limited data. Here we investigate the temperature and δ18O of seawater (δ18Osw, a proxy of salinity) evolution history of the intermediate water during the last deglaciation by analyzing Mg/Ca and oxygen isotopes of planktonic foraminifera Globorotalia inflata in the southern SCS. The results indicate that intermediate water was warm and salty during the Heinrich 1 (H1) and Younger Dryas (YD) cold events, while it was cold and fresh during the Bølling-Alleød (B-A) warm event in the southern SCS. By comparison with other paleorecords, we find that there was a weak intrusion of the low-salinity intermediate water from the western Pacific into the SCS during H1 and YD, whereas the opposite occurred during the B-A. Our study reveals that the variability in the intrusion intensity of intermediate water into the SCS from the western Pacific was primarily influenced by the changes in winter wind intensity during the last deglaciation.

Glacial-interglacial lake hydrochemistry in step with the Pleistocene Indian summer monsoon at the southeastern Tibetan Plateau

The lake state and chemistry are sensitive to climate change at short timescales, but it is unclear how lake hydrochemistry responds to monsoonal climate, especially over glacial-interglacial cycles, owing to lack of well-dated and continuous sedimentary records. The types and geochemical compositions of authigenic carbonates from lacustrine sediments can be used to reconstruct past hydrochemistry. Yet, little is known about the significance and influencing factors of the Mg/Ca and Sr/Ca ratios of authigenic carbonates over glacial-interglacial cycles. Here, trace-element data of a 666 m long sediment sequence from the Heqing basin, a paleo-lake situated at the southeastern margin of the Tibetan Plateau, provide detailed glacial-interglacial hydrochemical conditions and their association with Indian summer monsoon (ISM) evolution during the entire Pleistocene. In interglacials, variation in both Mg/Ca and Sr/Ca ratios can serve as proxies for salinity as a result of precipitation of low-Mg calcite, constrained by enhanced ISM intensity and catchment weathering product. Instead, in glacials, high Mg/Ca and Sr/Ca ratios reflect high-Mg calcite and aragonite precipitation under low lake level conditions. There is a nonlinear response of the Sr/Ca ratio of lacustrine sediment to salinity variation when high-Mg calcite and aragonite precipitate in a hydrologically closed and evaporative lake. Based upon the sedimentary Sr/Ca and Mg/Ca ratios in the Heqing paleo-lake, the lake hydrochemistry during the Pleistocene experienced obvious glacial-interglacial ISM cycles at the life span of the lake itself: (1) in the old interval (2640–1840 ka), the salinity was low with cycles at the early stage of lake development owing to strong ISM; (2) in the middle interval (1840–920 ka), the lake was at a balance state and its salinity was more constrained by weathering solute, rather than ISM; and (3) in the young interval (920–130 ka), the salinity fluctuated with large amplitude over glacial-interglacial cycles and subsequent increase paced the weakened ISM. This study highlights the sensitivity of the types of lacustrine authigenic carbonates and lake chemistry to ISM changes over glacial-interglacial cycles.

Modeling and characterization of shallow aquifer water based on ion concentrations to salinity variations using multivariate statistical approach

Water chemistry data (1965–2015) from the Chicot and Evangeline aquifers in Nueces and Kleberg counties of Texas were analyzed to assess the hydrogeochemical processes based on variations in ion concentrations affecting groundwater salinity levels in those coastal aquifers. A further aim of this study was to identify a proxy using major-ion concentrations to predict groundwater salinity. Correlation analysis suggests that the hydrogeochemical processes operating in these aquifers differ significantly. In principal component analyses, the first three principal components explain 91% and 94.8% of the total variabilities of the variation of groundwater chemistry in the Evangeline and Chicot aquifers, respectively. The rotated factors (factors 1, 2, and 3) using the varimax rotation method implied that depth control and salinity variations predominantly cause the highest variabilities in water chemistry in the Chicot and Evangeline aquifers in the study area. Results showed depth control on water quality parameters was more pronounced in the Evangeline aquifer compared to the Chicot aquifer. Stepwise elimination of the least significant predictors to identify the proxy for groundwater salinity revealed chloride (Cl) could be the most significant predictor to estimate groundwater salinity variations in both aquifers. However, regression models generated from 75% of the training datasets predicted total dissolved solid (TDS) variations with 78% and 43% accuracies in Chicot and Evangeline aquifers, indicating that Cl can be considered the proxy for the Chicot aquifer only but not suitable for the Evangeline aquifer in the study area.

A late refugium for Classopollis in the Paleocene Lower Wilcox Group along the Texas Gulf Coast

Abstract We report a new ecological refugium for the Cheirolepidiaceae family (pollen form genus Classopollis) in the Paleocene Lower Wilcox Group in the Gulf Coast of southeastern Texas based on palynological analysis of four wells. The Cheirolepidiaceae were once thought to have gone extinct at the Cretaceous–Paleogene (K/Pg) boundary or earlier in North America; however, similar ecological refugia for this family in the Paleocene have previously been reported in China, Argentina, and potentially the Rocky Mountains of the United States. The highest relative abundances of Classopollis pollen were found in delta front, lagoon, and shoreface depositional paleoenvironments marked by high mud-fraction Sr/Ba (a geochemical proxy for salinity), and abundances generally increased down section in older Paleocene strata. The high relative abundance of Classopollis pollen in the well samples, the rarity of reworked Mesozoic palynomorphs, the generally good preservation of Classopollis pollen, and the similarity of Classopollis fluorescence spectra to other in situ Paleocene pollen all provide strong evidence for the survival of the Cheirolepidiaceae family in the coastal salt marshes of Texas through at least the late Paleocene.

Glacial–interglacial seawater isotope change near the Chilean Margin as reflected by δ2H values of C37 alkenones

Abstract. Stable hydrogen isotopic compositions of long-chain alkenones with 37 carbon atoms (δ2HC37) have been shown to reflect seawater salinity in culture and environmental studies, and this potential sea surface salinity proxy has been applied to several downcore records from different regions. However, previous studies were based solely on a single sediment core and often suggested unlikely large changes in salinity based on existing proxy calibrations. Here we present a new δ2HC37 record, in combination with oxygen isotopes of benthic foraminifera from the same samples, from a sediment core from the Chilean Margin (ODP Site 1235). The observed negative shift in δ2HC37 of 20 ‰ during the last deglaciation was identical to that of a previously published δ2HC37 record from the nearby, but deeper, ODP Site 1234, suggesting a regionally consistent shift in δ2HC37. This change translates into a negative hydrogen isotope shift in the surface seawater of ca. 14 ‰, similar to glacial–interglacial reconstructions based on other δ2HC37 records. The reconstructed bottom seawater oxygen isotope change based on benthic foraminifera during the last deglaciation is approximately −0.8 ‰, in line with previous studies. When translated into hydrogen isotopes of bottom seawater using the modern open-ocean water line, this would suggest a negative change of ca. 5 ‰, smaller than the reconstructed surface seawater shift based on alkenones. The larger change in surface water isotopes suggests that it experienced more freshening during the Holocene than bottom waters, either due to increased freshwater input, reduced evaporation, or a combination of the two.

Salinity is an important factor in carbon emissions from an inland lake in arid region

Saline lakes, serving as the ultimate destination for most hydrological systems, accumulate substantial amounts of nutrients and organic matter from basins, and act as vast carbon reservoirs. These lakes exhibit exceptionally active biogeochemical cycling processes of carbon dioxide (CO2) and methane (CH4), and constitute integral components of the global carbon cycle. However, understanding of greenhouse gas emissions from saline lakes remains unclear mostly due to scarce data. In this study, we obtained CO2 and CH4 diffusive fluxes and biogeochemical parameters during ice-free period of 2021 at Bosten Lake, which is a representative inland saline lake located in China's arid region. Results revealed that Bosten Lake was a significant source of atmospheric gas carbon emissions, with average diffusion emissions of 12.645 ± 3.475 mmol m−2 d−1 for CO2 and 0.279 ± 0.069 mmol m−2 d−1 for CH4. Temporally, field measurements found a positive correlation between conductivity (Spc, a proxy of salinity) and CO2 emissions (R2 = 0.50, p < 0.01). Furthermore, the CH4 diffusive fluxes increased with the trophic state index (TSI, R2 = 0.31, p < 0.01). Spatially, exogenous inputs led to the spatial heterogeneity of carbon emissions. Our results highlighted that temporal variations in salinity constitute a crucial factor influencing CO2 emissions, and the saline lake has greater global warming potential compared to freshwater. The study provides an in-depth analysis of greenhouse gas emissions and driving factors in saline lakes of arid regions, and supports a further understanding of the carbon cycle in different types of lakes.

Palmitic Acid Is Not a Proper Salinity Proxy in Baffin Bay and the Labrador Sea but Reflects the Variability in Organic Matter Sources Modulated by Sea Ice Coverage

AbstractPalmitic acid (PA) is ubiquitous in the biosphere and its hydrogen isotopic composition (δ2HPA) was proposed as a potential paleoenvironmental proxy for salinity, with δ2HPA values increasing with salinity. In this study, we analyzed 40 surface sediment samples from Baffin Bay and the Labrador Sea to examine the isotopic composition of PA in relation to local environmental variables, including salinity. In contrast to expectations, our results show a negative relationship between the δ2HPA and sea‐surface salinity, raising questions about its pertinence and usefulness as a salinity proxy. Instead, our results suggest that the relative abundance of distinct organisms that employ different metabolisms is the key in determining the hydrogen isotopic fractionations in PA. While we show that PA is mostly produced through photoautotrophic metabolisms by diatoms and dinoflagellates, varying contributions from heterotrophic metabolisms may obscure the stable isotope composition of PA. Surprisingly, we found no correlation between the stable carbon isotopic composition of the sedimentary organic matter (δ13Corg) and PA (δ13CPA), implying major differences in either the dominant organisms producing sedimentary PA or in carbon isotope fractionation during lipid biosynthesis. We also found that the presence of extended sea‐ice cover leads to enriched carbon and hydrogen isotopic compositions in PA. These enriched values suggest heterotrophic biodegradation in the water column and/or in the sediment as well as an increase in grazing activities. We propose that sea‐ice cover and surface water oxygenation modulate the relative impact of phototrophic and heterotrophic metabolisms, and therefore the isotopic composition of marine sedimentary PA.

Behaviour of barium in the tropical estuaries: Implications to its marine budgets and paleo-oceanographic applications

Marine Barium (Ba) is extensively used as a tracer for biogeochemical processes and paleo-oceanographic proxy for past river discharge, salinity, and long-term hydrological changes. However, poor constrain on the riverine Ba flux to oceans due to its modification in the estuary hinder such applications. The prerequisite for an accurate Ba flux estimate is to have a detailed understanding of Ba behaviour, extent of modifications in the estuary, and net input to the ocean. A comprehensive study of Ba was conducted in four Indian estuaries draining through diverse geological/tectonic settings under different climatic regimes, such as the Narmada, Tapi, and the Mandovi falling into the Arabian Sea and the Hooghly (a distributary of the Ganga) into the Bay of Bengal. Dissolved Ba distributions in the surface waters of these estuaries exhibit nonconservative behaviour along the salinity gradient, exhibiting a systematic gain of Ba at low (∼2.5–7) salinity in the Hooghly and mid-salinity (∼8–18) in the estuaries linked to the Arabian Sea. The Ba gain is attributed to its release from the particulates and contributions via submarine groundwater discharge (SGD). The Ba gain in the Narmada and the Hooghly estuary was higher during nonmonsoon despite lower riverine sediment supply and SGD compared to monsoon, suggesting that tidally induced resuspension of estuary sediments and solute-particulate interactions that release Ba through the ion exchange process are the primary driver in determining surface water Ba distributions in these estuaries. The estimate of excess Ba supply in these estuaries is much higher than its riverine supply, ∼4 times higher in the Hooghly to ∼30 times in the Narmada estuary. The estimate of excess Ba supply from the global estuaries ∼4.5 Gmol yr−1 is comparable to global riverine supply∼5.5 G mol yr−1, highlighting its significant role in global marine budgets. Mass balance calculation of Ba suggests that SGD and particulate release contribute an average of ∼17% and ∼ 32% of the total dissolved Ba in the Hooghly estuary, whereas ∼10% and ∼ 76% in the Narmada estuary respectively. The Ba/Ca ratio shows inverse relation with salinity and is compatible with the exponential fits. Extrapolation of the Ba/Ca-salinity relation to open ocean Ba/Ca ratio yields salinity of ∼35 in the Hooghly, close to the average open ocean salinity within ∼10% uncertainty, whereas it yields salinity ∼41 in the Narmada (pre-monsoon), which translates to large uncertainty (∼20%) due to huge release of particulate Ba. This uncertainty could further amplify in longer time scales (e.g. glacial-interglacials) due to sea level and climate changes and its impact on estuarine process. Therefore, regional calibrations in conjunction with other constraining proxies are the way forward to employ Ba/Ca ratio as a paleo-oceanographic proxy in coastal oceans.

Impacts of repeated coastal flooding on soil and groundwater following managed dike realignment

Coastal defense structures (e.g., dikes, seawalls) protect vulnerable communities along marine coastlines and estuaries from the physical and chemical influences of adjacent water bodies. These structures are susceptible to overtopping or breaching by tides and waves, with risks amplified by climate change-induced sea-level rise. Repeated inundation by saline water can contaminate freshwater resources and salinize soil, impacting land-use activities, including agricultural productivity. Managed ecosystem-based dike realignment and salt marsh restoration can provide alternatives to traditional coastal adaptation approaches. We assess the changes to soil salinity at a managed dike realignment project prior to the transformation from a diked terrestrial environment to an estuarine environment. Baseline data are compared to conditions following 8–10 months of intermittent flooding at spring tides.Results show that an increase in salinity occurred over the entire site in the shallow subsurface, with the most significant contamination occurring in low-lying areas. Bulk soil electrical conductivity (salinity proxy) measured from geophysical surveys increased from the previous freshwater condition of ∼300 μS/cm to over 6000 μS/cm following <20 flood events, while successive flooding resulted in increased soil moisture as infiltrated floodwater propagated to greater depths. Sediment deposition occurred at high rates, with up to 4 cm of sediment deposited per flood, converting much of the previously cultivated land into tidal mudflats. Deeper sediments and groundwater (i.e., >1.8 m depth) were not impacted over the time scale of this research. This study demonstrates that intermittent shallow flooding can rapidly increase moisture content and soil salinity in surficial sediments and, in turn, adversely impact conditions suitable for agricultural crop production. The realignment zone serves as an engineered analog of coastal flooding, presenting an opportunity to investigate how low-lying coastal environments may experience regular flooding in the future due to sea-level rise and intensifying coastal storms.

Holocene Water Balance Variations in Great Salt Lake, Utah: Application of GDGT Indices and the ACE Salinity Proxy

AbstractGreat Salt Lake (GSL), Utah, is a hypersaline terminal lake in the Great Basin, and the remnant of the late glacial Lake Bonneville. Holocene hydroclimate variations cannot be interpreted from the shoreline record, but instead can be investigated by proxies archived in the sediments. GLAD1‐GSL00‐1B was cored in 2000 and recently dated by radiocarbon for the Holocene section with the top 11 m representing ∼7 ka to present. Sediment samples every 30 cm (∼220 years) were studied for the full suite of microbial membrane lipids, including those responsive to temperature and salinity. The Archaeol and Caldarchaeol Ecometric (ACE) index detects the increase in lipids of halophilic archaea, relative to generalists, as salinity increases. We find Holocene ACE values ranged from 81 to 98, which suggests persistent hypersalinity with &lt;50 g/L variability across 7.2 ka. The temperature proxy, MBTʹ5Me, yields values similar to modern mean annual air temperature for months above freezing (MAF = 15.7°C) over the last 5.5 ka. Several glycerol dialkyl glycerol tetraether metrics show a step shift in microbial communities and limnology at 5.5 ka. Extended archaeol detects elevated salinity during the regional mid‐Holocene drought, not readily detected in the ACE record that is often near the upper limit of the index. We infer that the mid‐Holocene GSL was shallower and saltier than the late Holocene. The current drying may be returning the lake to conditions not seen since the mid‐Holocene.