Freshwater Input Research Articles

Warm surface waters increase Antarctic ice shelf melt and delay dense water formation

Melting ice shelves around Antarctica control the massive input of freshwater into the ocean and play an intricate role in global heat redistribution. The Amery Ice Shelf regulates wintertime sea-ice growth and dense shelf water formation. We investigated the role of warm Antarctic Surface Water in ice shelf melting and its impact on dense shelf water. Here we show that the coastal ocean in summer 2016/17 was almost sea-ice free, leading to higher surface water temperatures. The glacial meltwater fraction in surface water was the highest on record, hypothesised to be attributable to anomalous ice shelf melting. The excess heat and freshwater in early 2017 delayed the seasonal evolution of dense shelf water. Focused on ice shelf melting at depth, the importance and impacts of warming surface waters has been overlooked. In a warming climate, increased surface water heating will reduce coastal sea-ice production and potentially Antarctic Bottom Water formation.

Trophic Niche Partitioning of Five Sciaenidae Species Sampled in Zhoushan Archipelago Waters via Stable Isotope Analysis

Sciaenid fishes are usually associated with large freshwater inputs and are the most important economic fish on the coastal shelf off mainland China. To compare the differences in ecological niche and resource sharing among different populations of Sciaenidae species, we collected samples of Larimichthys polyactis, Collichthys lucidus, Johnius belangerii, Nibea albiflora, and Miichthys miiuy from Zhoushan Archipelago waters from 2019 to 2021 and investigated the carbon and nitrogen isotopic values in muscle tissues, the contribution of each food resource, and trophic niche widths and overlaps. Significant differences were observed in both isotopes in the muscles of the five Sciaenid species. Zooplankton was a key food resource for all Sciaenid species. In addition to zooplankton, J. belangerii, N. albiflora, and M. miiuy also fed on benthos organisms. C. lucidus presented a wide trophic niche width and had extensive habitat use. The trophic niche occupied by N. albiflora and M. miiuy was narrow; they presented a high trophic level, with a high degree of trophic niche overlap. This study showed that sciaenid fishes have overlapping trophic niches due to their common feeding on zooplankton, and differences in body size, migration, habitat, and feeding choices led to the reasonable sharing of resources among the five sciaenid fishes, allowing the coexistence of these species.

Characterizing mauka-to-makai connections for aquatic ecosystem conservation on Maui, Hawaiʻi

Mauka-to-makai (mountain to sea in the Hawaiian language) hydrologic connectivity – commonly referred to as ridge-to-reef – directly affects biogeochemical processes and socioecological functions across terrestrial, freshwater, and marine systems. The supply of freshwater to estuarine and nearshore environments in a ridge-to-reef system supports the food, water, and habitats utilized by marine fauna. In addition, the ecosystem services derived from this land-to-sea connectivity support social and cultural practices (hereafter referred to as socio-cultural) including fishing, aquaculture, wetland agriculture, religious ceremonies, and recreational activities. To effectively guide island resource management, a better understanding of the linkages from ridge-to-reef across natural and social usages is critical, particularly in the context of climate change, with anticipated increasing temperature and shifting precipitation patterns. The objective of this study was to identify spatial linkages that promote multiple and diverse uses, following the ridge-to-reef concept, at an island-wide scale to identify regions of high conservation importance for aquatic resources. We selected the Island of Maui as a study representative of many Pacific islands. Diverse datasets, including agricultural lands within watersheds, wetland locations, presence of stream species, indicators of freshwater input from streams, coral cover, nearshore fish biomass, socio-cultural data such as fishpond locations, wetland taro cultivation, beach recreation use, and lastly the dynamically downscaled Coupled Model Intercomparison Project Phase (CMIP5) future climate projections scenarios (Representative Concentration Pathway (RCP) 4.5 & 8.5) were used to examine the spatial linkages through hydrological connectivity from land to the sea. Zonation spatial planning software was used to prioritize areas of high management and conservation value and to help inform aquatic resources management. The resulting prioritized areas included many minimally disturbed watersheds in east Maui and western nearshore and coastal zones that are adjacent to diverse coral reefs. These results are driven by the importance of fish biomass and coral reef distribution as well as traditional wetland taro cultivation and coastal access points for recreation. These results underline the importance of examining ridge-to-reef systems for aquatic resource management and including important social and cultural values in resource management upon planning adaptation strategies for climate change. Improving our understanding of diverse natural and socio-cultural influences on habitat conditions and their values in these areas provides an opportunity to strategically plan future management and conservation actions.

Dissolved and Particulate Barium Distributions Along the US GEOTRACES North Atlantic and East Pacific Zonal Transects (GA03 and GP16): Global Implications for the Marine Barium Cycle

AbstractProcesses controlling dissolved barium (dBa) were investigated along the GEOTRACES GA03 North Atlantic and GP16 Eastern Tropical Pacific transects, which traversed similar physical and biogeochemical provinces. Dissolved Ba concentrations are lowest in surface waters (∼35–50 nmol kg−1) and increase to 70–80 and 140–150 nmol kg−1 in deep waters of the Atlantic and Pacific transects, respectively. Using water mass mixing models, we estimate conservative mixing that accounts for most of dBa variability in both transects. To examine nonconservative processes, particulate excess Ba (pBaxs) formation and dissolution rates were tracked by normalizing particulate excess 230Th activities. Th‐normalized pBaxs fluxes, with barite as the likely phase, have subsurface maxima in the top 1,000 m (∼100–200 μmol m−2 year−1 average) in both basins. Barite precipitation depletes dBa within oxygen minimum zones from concentrations predicted by water mass mixing, whereas inputs from continental margins, particle dissolution in the water column, and benthic diffusive flux raise dBa above predications. Average pBaxs burial efficiencies along GA03 and GP16 are ∼37% and 17%–100%, respectively, and do not seem to be predicated on barite saturation indices in the overlying water column. Using published values, we reevaluate the global freshwater dBa river input as 6.6 ± 3.9 Gmol year−1. Estuarine mixing processes may add another 3–13 Gmol year−1. Dissolved Ba inputs from broad shallow continental margins, previously unaccounted for in global marine summaries, are substantial (∼17 Gmol year−1), exceeding terrestrial freshwater inputs. Revising river and shelf dBa inputs may help bring the marine Ba isotope budget more into balance.

Geomorphic and Climatic Drivers Are Key Determinants of Structural Variability of Mangrove Forests along the Kenyan Coast

Mangrove forests occur across a diversity of coastal landforms that influence their structural development and productivity. Preliminary studies in Kenya indicate that mangroves growing in the region north and south of Tana River delta have different structural attributes. We hypothesise a close relationship between mangrove distribution, climate and landform types. Floristic composition of mangroves along the coast of Kenya was characterised and differences illustrated using non-metric multidimensional scaling (nMDS). Other structural properties of mangroves such as tree height, basal area, stand density and standing biomass were also assessed and their differences tested using analysis of variance (ANOVA). A hierarchical cluster analysis was used to compare mangrove species based on structural properties. Additionally, a regression fit model was used to investigate the relationship between mangrove standing biomass and possible drivers of variability. The study revealed significant differences in mangrove tree diameter, tree height, basal area, stand density and standing biomass across the sampled sites. High values of structural complexity were observed in estuarine and deltaic settings with high influence of freshwater input whereas low levels of structural complexity were observed for peri-urban with direct human influence. Our findings suggest that structural variability of mangroves in Kenya is highly influenced by geomorphological and climatic variability along the coast as well as the past and present management regimes of the forest.

A Song of Wind and Ice: Increased Frequency of Marine Cold‐Spells in Southwestern Patagonia and Their Possible Effects on Giant Kelp Forests

AbstractIn contrast to other coastal regions of the world, the giant kelp (Macrocystis pyrifera) ecosystem in southwestern Patagonia has been persistent in area and associated biodiversity in the last decades. In this ecoregion, sea surface temperature (SST) records have consistently remained below the upper thermal threshold for kelp survival, however, no studies have analyzed the spatiotemporal variability of SSTs and their anomalies across the geographical diversity of the southwestern Patagonian coastline. We explored the geographical distribution of extreme warm and cold events in this region from latitudes 47°–56°S in a range of ∼1,000 km, identifying the dates and spatial distribution of marine heatwaves (MHWs) and marine cold‐spells (MCSs) from 1982 to 2020. Results show that a peak in the number of MHWs occurred in the great El Niño year of 1998. Additionally, the 2014–2019 period has had more severe and extreme MCSs than the previous decades. We discuss the origin of these events with a focus on three main processes: (a) geographically constrained cold events caused by glacier melting, (b) regional cold events caused by extreme winds linked to the position of the polar front, and (c) extensive SST anomalies linked to planetary‐scale events such as El Niño and La Niña. Overall, those processes were conductive to counteract global warming trends locally/regionally, highlighting southwestern Patagonia as a possible climatic refugium for the giant kelp ecosystem. Despite this, the effects of freshwater inputs and storm turbulence on the exposed coasts facing the Southern Ocean may cause new kinds of stress on this ecosystem.

Diatom-based paleoenvironmental reconstruction from the coast of Northern Patagonia, Argentina

Estuaries and deltas are common along modern coasts and are well represented in the geologic record because of the high preservation of their deposits. Worldwide, estuaries are more abundant than deltas because of the relatively short time available for Holocene coastal sedimentation due to sea level variations. The evolution of estuaries and deltas during the recent geological past can be reliably reconstructed. Among others, paleostudies in estuaries aim to reconstruct sea level and freshwater input variations, and the timing and frequency of extreme events (e.g., tsunamis). A significant number of studies have been published in the past 20 years, which used preserved remains of microorganisms accumulated during long intervals in sediments. The preserved record of fossil remains evidences the response of microorganisms to environmental changes at the time of their deposition. Among the proxies preserved in sediments, diatoms are one of the most reliable bioindicators to trace environmental changes. Due to rapid reproduction rate under favourable conditions and their siliceous frustule, they are abundant in recent and older sediments from estuaries and deltas. In this manuscript, results from previous studies were reviewed, integrated and statistically analyzed with the aim to reconstruct Holocene coastal changes in relation to sea level curves. These studies were conducted along a latitudinal transect on four estuarine mesotidal systems from Northern Patagonia, Argentina: Colorado River delta (39°40′S; 62°08′W), San Blas Bay (40°33′S; 62°13′W), Negro River (40°57′S; 62°49′W) and Chubut River estuaries (43°22′S; 65°03′W). The focus is on the preserved diatom assemblages and the paleoecological information they represent. Seven coastal sedimentary cores were correlated. Based on multivariate analysis (NMDS), fossil and modern diatom assemblages were integrated and compared. These estuary systems are dominated by littoral drift, where the plains became flooded by the sea about 7000 14C years BP ago. When the sea transgression occupied the area, Colorado delta and San Blas Bay evolved from tidal channels flooded to tidal flats, sand flats and marshes during the late Holocene regression. At Colorado River, the gradually increase of the fluvial influence is associated with the eastward progradation of the deltaic lobe during the last ca. 7000 14C years BP. At the Negro River estuary, a shallow vegetated brackish/freshwater environment evolved into marine conditions, tidal channels and marshes during the last 2000 14C years BP. A marine coastal environment (estuary) at ca. 4400 14C years BP that evolved into a salt marsh was inferred at the inlet of the Chubut River. Diatom salinity-preferences showed how connections between the wetland and the sea varied over time with sea level changes and gradual infilling. Diatom-inferred paleoenvironments and radiocarbon dates allowed reconstruction of the coastline position to the east in the Early Holocene and to the west during transgressive-regressive events.

High‐frequency pH time series reveals pronounced seasonality in Arctic coastal waters

AbstractThe accelerated rate of climate change in the Arctic Ocean occurs in conjunction with a system known for its extreme seasonal variability. Here, we present 2 years of continuous pH, salinity, and temperature data from the north Arctic coast of Alaska from instruments deployed in a kelp bed at 4.5–6 m depths in Stefansson Sound. At the innermost site, which receives freshwater runoff from the nearby Sagavanirktok River, short‐term pH variability in late spring and summer produced pH values up to 8.67. The pH values of the deeper offshore site were less affected by freshwater input, although biological (heterotrophy) and physiochemical (ice formation) processes dominated during the winter months, driving pH down to 7.47 during the 8‐month period of ice cover. These long‐term physicochemical measurements reveal the natural but critical influence of changing river inputs on the pH of Arctic nearshore waters which support highly productive communities and subsistence fisheries.

Impact of Rainfall on Tropical Cyclone‐Induced Sea Surface Cooling

AbstractTropical cyclones (TCs) are often accompanied by strong winds and torrential rains. While the winds associated with TCs tend to enhance mixing in the upper ocean, the freshwater input from rain can stratify the water column and limit mixing. However, the extent to which the stabilizing effect of rainfall can compete with wind‐induced mixing and to what degree it modulates TC‐induced sea surface cooling remain unknown. Here we show, using a suite of observations, that heavy rains under weak TCs can significantly reduce the magnitude of cold wakes induced by them. When compared to storms with low rain rates, the ocean surface under TCs with high rain rates freshens significantly and cools less. High‐resolution climate model simulations and idealized experiments with an ocean mixed layer model support these results and reveal that oceanic mixing processes are primarily responsible for reduced cooling under TCs, with a lesser role for surface fluxes.

Taphonomic bias on calcareous micro and nannofossils and paleoenvironmental evolution across the Messinian Salinity Crisis onset: Insights from the Sorbas Basin (SE Spain)

During the Messinian Salinity Crisis (MSC, 5.97–5.33 Ma) the synergy between tectonic and climatic processes promoted the deposition of evaporites throughout the Mediterranean Basin. The disappearance of calcareous nannofossils and foraminifers at the MSC onset has been traditionally interpreted as the result of the establishment of hypersaline conditions in the water column, which were lethal for most marine eukaryotes. Here, we investigate the calcareous micro and nannofossil assemblages from the Sorbas Basin (SE Spain) spanning the onset and the first phase of the MSC (~ 5.9–5.7 Ma) in order to test whether their disappearance was related to adverse environmental conditions and/or to preferential dissolution of biogenic calcite. Micropaleontological analyses and petrographic observations suggest that the disappearance of calcareous fossils was the result of a taphonomic bias related to paleoceanographic changes that favoured the oxidation of both pyrite and organic matter at the sea floor. A poorly preserved benthic and planktic assemblage and aragonitic laminated sediment characterize the deposits of the first phase of the MSC from 5.93 Ma onward; aragonite deposition was likely bio-mediated in response to enhanced photosynthesis, as suggested by its δ13C signature. The fossil assemblage preserved in these deposits indicates a shallowing upward trend of the basin with respect to the pre-evaporitic phase, and the establishment of a lagoon-like environment characterized by episodic improvement of the connections with the open sea. We conclude that the temporary disappearance of calcareous fossils approximately at the MSC onset was related to ongoing basin restriction, which increased the sensitivity of the basin to fresh-water input and temperature changes, promoting marked fluctuations of the redox conditions in bottom sediments. This finding challenges the view of a marine biotic crisis related to hypersaline conditions at the MSC onset and opens new perspectives on the paleoenvironmental conditions that characterized this time interval.

Increases in temperature and freshwater inputs will shift grazing patterns of a coastal mesograzer on foundation species

AbstractFoundation species, and the roles that they play in structuring ecosystems, are threatened by global change. For example, charophytes are a refuge for zooplankton and stabilize sediments, but they are also a food source for various animal species (water birds, fishes, and invertebrates). Particularly, the introduction of new species, such as Gammarus tigrinus, into the Baltic Sea led to yet not understood changes in the food web. Furthermore, future projections point to increased water temperatures and freshwater inputs affecting species capacity to acclimatize to changing abiotic factors. In this study, we investigated the influence of temperature and salinity on the grazing pressure of G. tigrinus on two charophyte species: Chara aspera and Chara tomentosa. The grazing experiments were conducted in a full‐factorial design with the factors salinity (3–13 g kg−1), temperature (5–30°C), and charophyte species. Grazing rates were determined as mass deviation within 48 h considering biomass changes in the presence and absence of gammarids. Grazing rates were further used to calculate charophyte losses in two coastal lagoons with different salinity concentrations for recent and future time periods. Increasing freshwater inputs can buffer charophyte biomass loss at higher temperatures. Gammarids had a higher grazing impact on C. aspera than on C. tomentosa. The potential grazing peak of about 24°C is not yet reached in these coastal waters but may be reached in the near future as shown by our future projection results. However, a temperature increase and decrease in salinity will cause a shift in seasonal individual grazing patterns from summer to spring and autumn. An increase in temperature and freshwater input can lead to a shift in optimal habitats for G. tigrinus in the future. These interactions between abiotic and biotic factors will affect the future spatial distribution that charophytes can exploit as foundation species.

Organic matter enrichment in a terrestrial-marine transitional environment driven by global/regional climate recorded in the Upper Permian succession from the Qiangtang Basin, northern Tibet

Compared to organic matter (OM) accumulation in marine environments, OM enrichment mechanism in terrestrial-marine transitional environments remain unclear. The Late Permian was an important coal-forming period in China. Late Permian coal-bearing mudstones are important petroleum source rocks in the Qiangtang Basin. In this study, the coal-bearing rocks in the Late Permian Nayixiong Formation from the Qiangtang Basin are selected to address OM accumulation process in transitional environments. Our work demonstrates three-stage climatic changes during the deposition of coal-bearing sedimentary rocks: stages I, II and III, corresponding to a semi-warm and semi-humid climate, warm and humid climate, and cold and arid climate, respectively. Based on degree of pyritization (DOP), enrichment factor (EF) of elements, the Late Permian coal-bearing rocks were deposited under oxic-dysoxic environments. Low excessive barium (Baxs) and biogenic silica (Sibio) contents in the coal-bearing rocks indicates a low primary productivity. We suggest that warm and humid conditions, freshwater and terrestrial plants input are the primary factors controlling OM accumulation during the Late Permian coal-bearing deposition. We summarized a global/regional climate-driven model for OM accumulation in transitional environments. The warming climate created a habitable environment for both terrestrial and marine organisms. Subsequent freshwater input brought abundant nutrients and terrestrial OM to the aqueous system to build high OM accumulation.

Wind‐Driven Freshwater Export at Cape Farewell

AbstractIncreased freshwater input to the Subpolar North Atlantic from Greenland ice melt and the Arctic could strengthen stratification in deep convection regions and impact the overturning circulation. However, freshwater pathways from the east Greenland shelf to deep convection regions are not fully understood. We investigate the role of strong wind events at Cape Farewell in driving surface freshwaters from the East Greenland Current to the Irminger Sea. Using a high‐resolution model and an atmospheric reanalysis, we identify strong wind events and investigate their impact on freshwater export. Westerly tip jets are associated with the strongest and deepest freshwater export across the shelfbreak, with a mean of 37.5 mSv of freshwater in the first 100 m (with reference salinity 34.9). These wind events tilt isohalines and extend the front offshore, especially over Eirik Ridge. Moderate westerly events are associated with weaker export across the shelfbreak (mean of 15.9 mSv) but overall contribute to more freshwater export throughout the year, including in summer, when the shelf is particularly fresh. Particle tracking shows that half of the surface waters crossing the shelfbreak during tip jet events are exported away from the shelf, either entering the Irminger Gyre, or being driven over Eirik Ridge. During strong westerly wind events, sea ice detaches from the coast and veers toward the Irminger Sea, but the contribution of sea ice to freshwater export at the shelfbreak is minimal compared to liquid freshwater export due to limited sea ice cover at Cape Farewell.

The Role of Hydraulic Failure in a Massive Mangrove Die-Off Event.

Between late 2015 and early 2016, more than 7,000 ha of mangrove forest died along the coastline of the Gulf of Carpentaria, in northern Australia. This massive die-off was preceded by a strong 2015/2016 El Niño event, resulting in lower precipitation, a drop in sea level and higher than average temperatures in northern Australia. In this study, we investigated the role of hydraulic failure in the mortality and recovery of the dominant species, Avicennia marina, 2 years after the mortality event. We measured predawn water potential (Ψpd) and percent loss of stem hydraulic conductivity (PLC) in surviving individuals across a gradient of impact. We also assessed the vulnerability to drought-induced embolism (Ψ50) for the species. Areas with severe canopy dieback had higher native PLC (39%) than minimally impacted areas (6%), suggesting that hydraulic recovery was ongoing. The high resistance of A. marina to water-stress-induced embolism (Ψ50 = −9.6 MPa), indicates that severe water stress (Ψpd < −10 MPa) would have been required to cause mortality in this species. Our data indicate that the natural gradient of water-stress enhanced the impact of El Niño, leading to hydraulic failure and mortality in A. marina growing on severely impacted (SI) zones. It is likely that lowered sea levels and less frequent inundation by seawater, combined with lower inputs of fresh water, high evaporative demand and high temperatures, led to the development of hyper-salinity and extreme water stress during the 2015/16 summer.

Genetic Model of Mud-Clast Conglomerates in Salt Rocks and Their Significance for Salt Mineralization in Mohei Area, Simao Basin, China: A Case Study of Well L-2.

The Simao Basin is one of the most important Mesozoic salt basins in China, and the salt rocks generally contain mud-clast conglomerates. The characteristics of mud-clast conglomerates can provide effective information indicating the evolution process of the salt deposits. A combined analysis of trace elements and clay minerals was performed to characterize the genetic model of mud-clast conglomerates in the salt rocks. The results show that the sedimentary materials are felsic rocks from the Upper Crust. Ternary plot diagrams show that the tectonic settings are between the continental margin and the continental island arc. The clay minerals in the samples mainly contain illite and illite–smectite mixed layers. A dry and hot climate prevailed during the deposition of the Mengyejing Formation, and the warm-humid climate that also occurred is interspersed in some periods. Evaporation makes the water bodies shallower, and the concentrated brine starts to precipitate salt under the arid and hot climate conditions. Redox proxies indicate that most of the samples were deposited in relatively oxic conditions. The Sr/Ba ratios (average of 0.5) and paleosalinity (average of ∼35‰) during the depositional period indicate an increase in terrigenous freshwater input and carry of clasts into the evaporation basin. Furthermore, the freshwater supply enhances the hydrodynamics; as a result, the detritus that deposited in the early stage is broken into mud-clast conglomerates and co-deposited with the salt rocks. The results provide a geochemical basis for further study of mud-clast conglomerates in the Simao Basin and supply recommendations for the origin of salt deposits in similar basins around the world.

The Raman Spectroscopy Approach to Different Freshwater Microplastics and Quantitative Characterization of Polyethylene Aged in the Environment

The aim of this paper is to contribute to the investigation of microplastics reaching the Baltic Sea with freshwater input. The scope of the paper was to analyze samples from several locations with different environmental characteristics. First, samples from urban areas differing in their degree of urbanization, a forest, a river and its watercourse were examined. Secondly, the ageing quantitative and qualitative characterization is discussed. Spectral techniques are crucial in identifying polymers, but the signal itself constitutes a valuable source of the crystallinity and density parameters of the polyethylene materials. The study indicates that polypropylene, polyethylene, polycarbonate and polystyrene are the most common types of microplastics in the investigated areas.

Beryllium isotope variations recorded in the Adélie Basin, East Antarctica reflect Holocene changes in ice dynamics, productivity, and scavenging efficiency

The Adélie Basin is a relatively small (∼1600 km2), semi-enclosed continental shelf bathymetric depression located adjacent to the Wilkes Subglacial Basin, a basin underlying a sector of the East Antarctic Ice Sheet that contains ∼3–4 m sea level equivalent of ice. Located within the Adélie Basin is a ∼184 m thick laminated sediment deposit, the Adélie Drift, ideal for examining regional changes in ice sheet and ocean dynamics. Here, we examine the ratio of reactive beryllium-10 to reactive beryllium-9 ((10Be/9Be)reac) in a marine sediment core obtained from the Adélie Drift to assess these changes during the Holocene epoch (11.7 ka BP to present). The (10Be/9Be)reac record provides insight into changes in freshwater input, primary productivity, and scavenging efficiency, while removing the influence of particle size on 10Be concentration. During the early Holocene, (10Be/9Be)reac ratios indicate increased meltwater discharge from ca. 11.7 to 10 ka BP, as grounded ice retreated from the Adélie Basin and adjacent bathymetric highs. After ∼10 ka BP, beryllium isotopes are influenced by scavenging efficiency and dilution controlled by ocean currents and accumulation rate, operating alongside meltwater input, suggesting there are additional factors to consider when using (10Be/9Be)reac as a proxy for ice shelf cover and glacial dynamics.

Controls on buffering and coastal acidification in a temperate estuary

AbstractEstuaries may be uniquely susceptible to the combined acidification pressures of atmospherically driven ocean acidification (OA), biologically driven CO2 inputs from the estuary itself, and terrestrially derived freshwater inputs. This study utilized continuous measurements of total alkalinity (TA) and the partial pressure of carbon dioxide (pCO2) from the mouth of Great Bay, a temperate northeastern U.S. estuary, to examine the potential influences of endmember mixing and biogeochemical transformation upon estuary buffering capacity (β–H). Observations were collected hourly over 28 months representing all seasons between May 2016 and December 2019. Results indicated that endmember mixing explained most of the observed variability in TA and dissolved inorganic carbon (DIC), concentrations of which varied strongly with season. For much of the year, mixing dictated the relative proportions of salinity‐normalized TA and DIC as well, but a fall season shift in these proportions indicated that aerobic respiration was observed, which would decrease β–H by decreasing TA and increasing DIC. However, fall was also the season of weakest statistical correspondence between salinity and both TA and DIC, as well as the overall highest salinity, TA and β–H. Potential biogeochemically driven β–H decreases were overshadowed by increased buffering capacity supplied by coastal ocean water. A simple modeling exercise showed that mixing processes controlled most monthly changes in TA and DIC, obscuring impacts from air–sea exchange or metabolic processes. Advective mixing contributions may be as important as biogeochemically driven changes to observe when evaluating local estuarine and coastal OA.

Paleohydrological history of Lake Allos (2200 m a.s.l) since 13 500 cal a bp in the Mediterranean Alps inferred from an ostracod δ18O record

ABSTRACTThis paper presents the first Lateglacial/Holocene (the last 13 500 cal a bp) ostracod δ18O record to infer hydroclimate variability in the Southern French Alps. Cytherissa lacustris (δ18Osp) shells extracted from the sediments of Lake Allos allowed a reconstruction of δ18Olake water (δ18Olw) except for the interval 5800–2800 cal a bp. The shape of the Younger Dryas (YD) clearly differed from records documented in the northern Alps. First, δ18Olw values remained close to modern values before a marked drop ca. 12 000 cal a bp. Then, after several oscillations, δ18Olw values increased, reaching the highest value of the record ca. 6100 cal a bp during a thermal optimum for this latitude. Finally, low δ18Olw values occurred from 450 to 100 cal a bp during the Little Ice Age (LIA) following the Medieval Climate anomaly. At the beginning of the YD, δ18Olw probably reflected changes in local glacier dynamics and precipitation sources. The following decrease in δ18Olw values was associated with higher freshwater inputs during the second half of the YD. During the LIA, the low δ18O values are consistent with a higher torrential activity and lower air temperatures.

The Long chain Diol Index: A marine palaeotemperature proxy based on eustigmatophyte lipids that records the warmest seasons

Long chain 1,13- and 1,15-diols are lipids which are omnipresent in marine environments, and the Long chain Diol Index (LDI), based on their distributions, has previously been introduced as a proxy for sea surface temperature. The main biological sources for long chain 1,13- and 1,15-diols have remained unknown, but our combined lipid and 23S ribosomal RNA (23S rRNA) analyses on suspended particulate matter from the Mediterranean Sea demonstrate that these lipids are produced by a marine eustigmatophyte group that originated before the currently known eustigmatophytes diversified. The 18S rRNA data confirm the existence of early-branching marine eustigmatophytes, which occur at a global scale. Differences between LDI records and other paleotemperature proxies are generally attributed to differences between the seasons in which the proxy-related organisms occur. Our results, combined with available LDI data from surface sediments, indicate that the LDI primarily registers temperatures from the warmest month when mixed-layer depths, salinity, and nutrient concentrations are low. The LDI may not be applicable in areas where Proboscia diatoms contribute 1,13-diols, but this can be recognized by enhanced contributions of C28 1,12 diol. Freshwater input may also affect the correlation between temperature and the LDI, but relative C32 1,15-diol abundances help to identify and correct for these effects. When taking those factors into account, the calibration error of the LDI is 2.4 °C. As a well-defined proxy for temperatures of the warmest seasons, the LDI can unlock important and previously inaccessible paleoclimate information and will thereby substantially improve our understanding of past climate conditions.