Deep-water Renewal Research Articles

Intense lateral intrusion of offshore sub-surface waters in Halifax Harbour

The main physical processes affecting the circulation variability in Halifax Harbour (HH), including the intense lateral intrusion of offshore sub-surface waters into Bedford Basin (BB), are studied using a nested-grid modelling system (NGMS). The annual and monthly mean circulation in HH are characterized by a typical two-layer estuarine circulation, with seaward currents in the upper layer and landward currents in the lower layer, both showing strong temporal and spatial variability. The intense lateral intrusion of offshore sub-surface waters into BB occurs at time scales of a few days. The intense intrusion, in combination with winter connection, is important for the renewal of deep waters in BB. The model sensitivity experiments suggest that the persistent northwesterly winds strengthen the near-surface seaward currents and the lateral intrusion of offshore sub-surface waters into BB. The persistent southwesterly winds can also trigger the lateral intrusion, but are less effective than the northwesterly winds. The southeasterly winds, on the contrary, reduce significantly the seaward currents in the surface layer and prevent the lateral intrusion. The accumulative effects of winds and tides on the 3D hydrodynamics in HH are also examined. Local winds have dominant effects on the 3D currents and temperature/salinity, through driving vertical mixing and coastal upwelling. By comparison, tides play a secondly role in affecting the currents and temperature/salinity in BB but are important in the Narrows.

Deepwater Renewal in a Large, Deep Lake (Lake Geneva): Identifying and Quantifying Winter Cooling Processes Using Heat Budget Decomposition

AbstractWintertime deepwater renewal, which is important for heat–oxygen–nutrient exchange in lakes, is traditionally considered to be mainly driven by 1D vertical convective cooling. However, differential cooling between shallow and deep waters can produce density currents that flow into deep layers. In order to determine the role that these two cooling processes play in deepwater renewal, field measurements and 3D numerical modeling were combined to investigate heat content dynamics in Lake Geneva's large basin, the Grand Lac (maximum depth 309 m), during an exceptionally cold air spell in early 2012 where complete overturning had been reported. In a novel approach, the heat budget of the lake was decomposed, which allowed the identification and quantification of the heat budget components. The heat budget decomposition revealed that vertical convective cooling only penetrated to 200 m and that lateral advection was not only caused by density currents being discharged from the shallow littoral zone of the Grand Lac, but also from the Lake's shallow Petit Lac basin (maximum depth 75 m); the latter was found to be the main driver of heat content decrease in the deep layers of the Grand Lac below ∼200‐m depth. These findings provide unique insight into heat exchange processes that cannot be obtained from field data or numerical simulations alone. Heat budget decomposition proved to be a powerful, universally applicable tool for quantifying the contribution of alternative deepwater renewal processes. This is important, since deepwater renewal by convective cooling is weakening due to persistent global warming.

Coastal alterations influence deep water renewal in Norwegian sill fjords

Recent observations have heightened concerns that the reduced density of coastal water is leading to a decrease in the frequency of basin water renewal in Norwegian sill fjords. There are indications that coastal water is becoming lighter due to climate change. The renewal of fjord basin water constitutes the primary source of oxygenation in these fjords, and many sill fjords naturally undergo stagnation periods lasting up to several years. In this paper, we have mapped alterations in coastal water using time series data from hydrographic measurements along the coastline, utilizing them as input for a simplified two-layer fjord model.We demonstrate that the temperature has risen, salinity has decreased, and consequently, the density of coastal water has declined since approximately 1990. At a depth of 10 m, the rise in temperature and the reduction in salinity made similar contributions to the decrease in density. However, at depths of 50 and 100 m, the increasing temperature emerged as the primary factor. The density exhibited a more substantial decrease in the upper layers compared to the lower layers. Consequently, not only was the water less dense, but the stability of the water column had also increased.The estimated temperature in the fjord basins increased after 1990 in 98 out of 101 evaluated fjords, with an average increase of 0.6 °C. The altered coastal stratification had an impact on the estimated work against the buoyant forces in the fjord basins. In 78 out of the 101 fjords, the work against buoyant forces by diapycnal mixing increased. In most of the fjord basins where the work against buoyant forces increased, the rise was insufficient to compensate for the decreasing density at the coast. Consequently, basin water renewal occurred less frequently in 65 out of the 101 fjord basins along the Norwegian coast.

Trace-metal enrichment mechanisms in Bakken Formation black shales, Williston Basin, North Dakota, USA

The Bakken black shales represent a unique sedimentary system characterized by high total organic carbon contents (TOC; Lower Bakken shale = 10.8 and Upper Bakken shale = 11.8 avg. wt%) and enrichment factors (EF) of various trace metals (TMs) such as Cu (6.2–7.7), Mo (219.7–237.8), Ni (9.4–10.2), U (20.6–29.3), V (9.9–14.2), and Zn (10.4–12.2) that are considerably higher than many other Devonian-Mississippian (D–M) black shales. In this study, raw distributions of elemental concentrations combined with bivariate and principal component analysis (PCA) were used to clarify processes that contributed to high TM EF values in the Bakken shales. Total organic carbon shares heavier PCA component loadings (>0.445) and stronger Pearson correlation coefficients (r) with Cu, Mo, Ni, U, V, and Zn than with pyrite-associated (As, Co, Fe, and S) elements, suggesting that organic matter (OM) played a primary role in the scavenging and accumulation of TMs in the sediments. Reducing conditions within bottom waters or sediment pore waters may have accelerated the accumulation of redox-sensitive Cu, Mo, Ni, V, and Zn introduced into the sediments via an OM detritus host and most likely played a secondary role in their enrichment. Deep-water renewal times for the Bakken have been estimated to be ∼10–30 yrs. based on Mo/TOC results. The high TM EF values observed in the Bakken shales may reflect frequent resupply of TMs into basin waters, enhanced primary productivity that is necessary in scavenging TMs from the water column, the presence of H2S within sediment pore or bottom waters, or possibly secondary processes associated with basin-wide fluid and hydrocarbon migration. High TOC concentrations may be the result of a possible shallow water column (∼150 m), which would limit exposure time of OM to oxidation and degradation, coupled with intermittent fluctuations between dysoxic and anoxic/euxinic bottom-water redox conditions, the latter of which would reduce rates of OM degradation by benthic organisms.

Redox geochemical signatures in Mediterranean sapropels: Implications to constrain deoxygenation dynamics in deep-marine settings

Global warming and anthropogenic activity are boosting marine deoxygenation in many regions around the globe. Deoxygenation is a critical ocean stressor with profound implications for marine ecosystems and biogeochemical cycles. Understanding the dynamics and evolution of past deoxygenation events can enhance our knowledge of present-day and future impacts of climate change and anthropogenic pressure on marine environments. Many studies have reconstructed the evolution redox conditions of past deoxygenation events using geochemical proxies. In this regard, the present work focuses on understanding the paleoenvironmental significance of geochemical redox signals derived from the onset, evolution and termination of regional-scale deoxygenations in deep-marine settings, with a specific focus on sapropels in the Eastern Mediterranean (EM). Sapropels, rhythmic organic-rich sediments deposited in EM, offer a unique opportunity to investigate recent deoxygenation events linked to past climate changes. Sapropels serve as paleo-archives of past deoxygenation events and can provide insights into the potential impacts of ongoing climate change on marine ecosystems and biogeochemical cycles. By integrating previous sapropel geochemical studies with a detailed analysis of new geochemical data from five Quaternary sapropels (S1, S5, S6, S7 and S8) in three different EM deep-marine settings, this study enhances our understanding of the paleoenvironmental significance of geochemical redox signals produced by deoxygenation dynamics and postdepositional processes in different deep-marine settings. The study supports that certain trace elements, such as Mo, V, U, Co, and Ni, are identified as more reliable redox proxies compared to Cr, Cu, Pb, and Zn. Four recurrent geochemical intervals attributed to specific redox conditions and postdepositional processes have been identified. Moreover, internal calibration of redox proxies thresholds has been performed and demonstrates how local environmental conditions (e.g., productivity rate) and hydrogeographic features (e.g., water-depth, particulate-shuttling intensity, deep-water renewal and fluvial input) play crucial roles in controlling the authigenic uptake rates of redox-sensitive trace metals, and subsequently, redox thresholds values of geochemical redox proxies. The results also emphasize the importance of postdepositional processes to accurately interpret geochemical signals in paleoenvironmental studies. This research enhances our overall understanding of geochemical signals associated with regional-scale deoxygenation events in deep-marine settings, offering new insights into predicting biogeochemical changes in marine environments undergoing a transition towards anoxia. By comprehending the dynamics of past and present deoxygenation, we acquire valuable knowledge regarding the potential effects of climate variability in marine ecosystems.

Long-term dynamics, population structure and connectivity of the helmet jellyfish Periphylla periphylla in a Norwegian fjord and adjacent waters.

Mass occurrences of Periphylla periphylla in Norwegian fjords cause major concerns related to potential regime shifts that could affect ecosystem stability. 15years of trawl data (2006-2015), complemented with comprehensive sampling in different areas and seasons (2018-2021) allowed new insights on the dynamics, structure and connectivity of P. periphylla populations within and beyond Trondheimsfjorden. Despite assumed population bursts, no clear trend on P. periphylla population size in Trondheimsfjorden were identified. Sampling frequency and population size suggest a local reproduction of P. periphylla, especially in the inner part of the fjord where young-of-the-year (YOY) individuals occur. Size variations occurred in relation to sampling month, thus pointing at seasonal patterns in growth and reproduction. No distinct population structure of P. periphylla populations within Trondheimsfjorden and over larger spatial scales (> 100km) along the Norwegian coast was observed. Such poor geographic population structure provides evidence for a strong dispersal of P. periphylla, potentially triggered by frequent deep-water renewals of the fjords' basins that enable a high gene flow. Data on P. periphylla long-term dynamics, population structure and connectivity provide valuable information for ecosystem state assessments and enable the advancement of ecosystem management approaches, thus accounting for both stakeholder and ecosystem demands.

Water Renewal Simulation in Two Flow-Through Water Bodies in Western Greece

The basic hydrodynamic characteristics and water renewal of two flow-through water bodies in Western Greece, the Gulf of Patras and Lysimachia Lake, are studied via numerical simulations. The currents on the northern coasts of the Gulf are much stronger compared with the southern region, and rapid water renewal is achieved in the area of the Rio–Antirio strait (<1 month). In the northern part of the Gulf, the residence time varies from 1 to 4 months, while in the central and southern parts, it is estimated to exceed 6 months. Regarding the water renewal of deep waters (>60 m), which are enclosed between sills, the same pattern was observed, with residence times exceeding 6 months. In Lysimachia Lake, the effect of inflow waters from surrounding water bodies was analyzed over a time period of approximately 2 months. Gyres formation was observed due to local topography, and the numerically predicted results for water renewal were found to be in good agreement with those in the literature. Specifically, Lysimachia Lake seems to be replenished approximately 13 times per year.

Energetic Bottom Current at the Equatorial Gap of the Ninety East Ridge in the Indian Ocean Based on Mooring Data

AbstractBottom current at the equatorial gap of the Ninety East Ridge is revealed by long‐term bottom mooring observations from 24 December 2019 to 2 April 2021. It is found that the observed bottom current has significant intraseasonal variability (ISV) with a dominant time scale of ∼33 days, which becomes enhanced in the boreal summer and winter. Variability of the bottom current was explored by using the Mercator Ocean outputs, which revealed that the enhancement of ISV of the bottom current is closely related to the westward propagation of Rossby waves originating from the eastern boundary, with their ray paths vertically propagating into the deep layer. Considering the importance of water mass transport through the gaps in the Ninety East Ridge to the renewal of deep water and modification of deep circulation in the Central Indian Basin (CIB) and West Australian Basin (WAB), our study provides in‐situ evidence for previously unrevealed energetic variation of water mass exchange between the CIB and WAB, and it further helps us gain a comprehensive understanding of the pathway and variation of the deep meridional overturning circulation in the Indian Ocean.

What role does stratification play during winter in wind-induced exchange between the multi-depth basins of a large lake (Lake Geneva)?

Detailed studies of wind-driven interbasin exchange in lakes, focusing on the underlying driving forces and how they are affected by stratification, are presently lacking. We therefore investigated how stratification modifies wind-induced exchange between the Petit Lac (PL) (depth 75 m) and Grand Lac (GL) (depth 309 m) basins of Lake Geneva in winter, using field observations, 3D hydrodynamic modeling and particle tracking. Early, weakly-stratified (December) and late, fully-mixed (March) winter conditions in the PL were compared for a typical, strong along-axis wind forcing. During early winter, two-layer exchange develops between the basins, with downwind surface outflow into the GL balanced by intense bottom inflow of deep, cold hypolimnetic GL water into the PL which is enhanced by baroclinic pressure gradients caused by upwelling in the GL. Furthermore, the transversal water-level setup generates barotropic pressure gradients that balance Coriolis force acting on the outflow. This produces unidirectional along-wind epilimnion currents that strengthen interbasin exchange. In late winter, with the thermocline deeper than the PL bottom, upwelling in the GL does not reach the confluence and baroclinicity plays no role, resulting in weaker exchange currents with a depth-veering structure in the upper layers due to Coriolis force. In late winter, interbasin exchange decreases by 50 %, is more local, affects only waters near the confluence, and is more horizontal, with no deepwater upwelling from the GL. Our results suggest that prolonged winter stratification due to global warming will make wind-induced hypolimnetic interbasin-upwelling an increasingly important deepwater renewal process in large multi-depth basin lakes.

Life history traits of the deep-water medusa Periphylla periphylla as revealed through failure and recovery of recruitment

Extended recruitment failure and subsequent success of the coronate medusa Periphylla periphylla in a Norwegian fjord provided a unique opportunity to estimate age-related biological traits by combining individual size distribution in the population over time with previously published data. The average growth rate decreased exponentially with age, from 0.55% day−1 during the first year to <0.17% day−1 after four years. This species becomes mature at the third year, when females carry approximately 10 mature oocytes. At an age of 9 years, the medusa is on average 12 cm in diameter and carries over 1000 mature oocytes. Recruitment of juveniles, 0.5–1 cm diameter, was estimated to be 15 female−1 year−1 in Halsafjord; mortality from egg to juvenile 1 cm in diameter, using oocyte turnover times (T) of 1–12 months, was 93% (T = 12 months) to 99.95% (T = 1 month). Eggs and juveniles younger than 10 months are not capable of vertical migration and remain deeper than the sill depth where regular fjord circulation occurs. Therefore they are only vulnerable to deep-water renewal, and a fjord population of P. periphylla is estimated to gradually decrease recruitment with increased frequency of deep-water renewal and reach zero recruitment with renewal intervals of 10 months or less. It is hypothesized that differences in regular and intermittent deep-water renewal is the main factor causing variability in size (age) distribution between fjord populations.

The impact of local and climate change drivers on the formation, dynamics, and potential recurrence of a massive fish-killing microalgal bloom in Patagonian fjord

Harmful algal blooms (HABs) in southern Chile are a serious threat to public health, tourism, artisanal fisheries, and aquaculture in this region. Ichthyotoxic HAB species have recently become a major annual threat to the Chilean salmon farming industry, due to their severe economic impacts. In early austral autumn 2021, an intense bloom of the raphidophyte Heterosigma akashiwo was detected in Comau Fjord, Chilean Patagonia, resulting in a high mortality of farmed salmon (nearly 6000 tons of biomass) within 15 days. H. akashiwo cells were first detected at the head of the fjord on March 16, 2021 (up to 478 cells mL−1). On March 31, the cell density at the surface had reached a maximum of 2 × 105 cells mL−1, with intense brown spots visible on the water surface. Strong and persistent high-pressure anomalies over the southern tip of South America, consistent with the positive phase of the Southern Annular Mode (SAM), resulted in extremely dry conditions, high solar radiation, and strong southerly winds. A coupling of these features with the high water retention times inside the fjord can explain the spatial-temporal dynamics of this bloom event. Other factors, such as the internal local physical uplift process (favored by the north-to-south orientation of the fjord), salt-fingering events, and the uplift of subantarctic deep-water renewal, likely resulted in the injection of nutrients into the euphotic layer, which in turn could have promoted cell growth and thus high microalgal cell densities, such as reached by the bloom.

Frontogenesis and estuarine circulation at the shallow sill of a tropical fjord: Insights from Ambon Bay, eastern Indonesia

Surface water flushing of the tropical fjord of Ambon Bay is linked to stratification and frontogenesis that vary with tidal and seasonal cycles. Tidal- and seasonal-based​ deployments of CTD casts and bottom-mounted current meters in 2019 coupled with an analytical model were employed to investigate estuarine circulation (i.e. flood/ebb-mean transport) at the sill of Ambon Bay. Spring tides produce a large (flood/ebb-mean) horizontal salinity gradient, ∂S/∂x, (up to 13 × 10−4‰/m) across the sill, intensifying estuarine circulation. This intensification of ∂S/∂x is driven by strong (flood/ebb-mean) stratification at the inshore sill, associated with buoyant frontogenesis (indicated by densimetric Froude number, Fo2≥ 0.3) during the related strong flood tides. Estuarine circulation is stronger in the easterly monsoon (July) than in the transitional monsoon (October) since buoyant fronts are larger in thickness and horizontal extension along the sill in the easterly monsoon following high freshwater input. The estuarine circulation was intensified by a deep-water renewal process in which deep water from the adjacent open sea is upwelled to the sill by internal waves.Flushing of inner Ambon Bay (IAB, the fjord basin) due to estuarine circulation at the sill only occurs at the surface layer of IAB. Within a spring–neap sequence (∼2 weeks), the net oceanic inflow due to estuarine circulation at the sill replenished 73% and 41% volume of the surface layer of IAB in the easterly monsoon and the transitional season, respectively. The knowledge of flushing of IAB due to net oceanic inflow presented here has application for the mitigation of pollution build-up and for understanding of oceanic nutrient supply in the basin.

Vertical circulation due to deep-water renewal and phytoplankton blooms in the tropical fjord of Ambon Bay, eastern Indonesia

This study provides the first observational evidence on the role of deep-water renewal in triggering phytoplankton blooms in a rare shallow-silled tropical fjord (Ambon Bay). Seasonal variation in the tidal-induced deep-water intrusions into inner Ambon Bay (IAB, the fjord basin) upwardly displaces water from the IAB deep layer towards the surface. The budget method was employed to quantify vertical mixing in the deep layer of IAB (below sill depth) post-intrusions (stagnant periods). Within a spring-neap sequence (~ 2 weeks), deep-water intrusion pulses in IAB were observed to be more frequent in the easterly monsoon (July, eight pulses) than the transitional season (October, three pulses). These intrusion pulses uplifted the resident deep waters of IAB with rates of 2.4 - 4.0 m/day in the easterly monsoon and 1.5 - 1.8 m/day in the transitional season. Depth-averaged vertical diffusion coefficient (Kv) in the deep layer of IAB slightly varied between easterly monsoon (3.3 × 10-4 m2/s) and transitional season (3.7 × 10-4 m2/s). The parameterization of the vertical mixing-stratification relationship (Kv ∝ [N2]-β) in the IAB deep layer was found to be larger in easterly monsoon (β = 1.104) than in transitional season (β = 0.694). Chlorophyll-a concentration in the water column of IAB increased during uplifting events with phytoplankton bloom conditions (> 5 mg/m3) found only in the easterly monsoon. The described uplift mechanism as a driver of phytoplankton blooms should be included in future analyses of water quality for IAB.

The influence of mesoscale climate drivers on hypoxia in a fjord-like deep coastal inlet and its potential implications regarding climate change: examining a decade of water quality data

Abstract. Deep coastal inlets are sites of high sedimentation and organic carbon deposition that account for 11 % of the world's organic carbon burial. Australasia's mid- to high-latitude regions have many such systems. It is important to understand the role of climate forcings in influencing hypoxia and organic matter cycling in these systems, but many such systems, especially in Australasia, remain poorly described. We analysed a decade of in situ water quality data from Macquarie Harbour, Tasmania, a deep coastal inlet with more than 180 000 t of organic carbon loading per annum. Monthly dissolved oxygen, total Kjeldahl nitrogen, dissolved organic carbon, and dissolved inorganic nitrogen concentrations were significantly affected by rainfall patterns. Increased rainfall was correlated to higher organic carbon and nitrogen loading, lower oxygen concentrations in deep basins, and greater oxygen concentrations in surface waters. Most notably, the Southern Annular Mode (SAM) significantly influenced oxygen distribution in the system. High river flow (associated with low SAM index values) impedes deep water renewal as the primary mechanism driving basin water hypoxia. Climate forecasting predicts increased winter rainfall and decreased summer rainfall, which may further exacerbate hypoxia in this system. Currently, Macquarie Harbour's basins experience frequent (up to 36 % of the time) and prolonged (up to 2 years) oxygen-poor conditions that may promote greenhouse gas (CH4, N2O) production altering the processing of organic matter entering the system. The increased winter rainfall predicted for the area will likely promote the increased spread and duration of hypoxia in the basins. Further understanding of these systems and how they respond to climate change will improve our estimates of future organic matter cycling (burial vs. export).

Wind‐Induced Hypolimnetic Upwelling Between the Multi‐Depth Basins of Lake Geneva During Winter: An Overlooked Deepwater Renewal Mechanism?

AbstractCombining field observations, 3D hydrodynamic modeling, and particle tracking, we investigated wind‐induced interbasin exchange between the Petit Lac (PL) (depth 75 m) and Grand Lac (GL) (depth 309 m) basins of Lake Geneva in early winter. Following a strong 2.5‐day wind event, a two‐layer flow field developed, where the downwind surface drift into the GL was balanced by counterflowing hypolimnetic currents into the PL. Velocities in both layers exceeded 20 cm s−1, with the highest values (27 cm s−1) found near the bottom. For 3.5 days, hypolimnetic temperatures at the confluence decreased to values found in the deep GL hypolimnion at 180‐m depth. Approximately 1.5 days after the wind event ceased, currents reversed and upwelled waters returned into the deep GL hypolimnion. The Coriolis force strongly modified the interbasin exchange dynamics, which were well represented by the model. Particle tracking revealed a “current loop,” that is, water from below 150‐m depth first upwelled into the PL, intruded approximately 10 km (half its length), and then descended back into the GL hypolimnion. Model results showed that the PL hypolimnetic volume doubled during the upwelling. Low model‐based gradient Richardson numbers and temperature inversions in CTD profiles indicated turbulent mixing between the upwelled GL and surrounding PL waters. Our findings demonstrate that hypolimnetic upwelling between the two basins frequently occurs during winter and could potentially be an important, but as yet overlooked mechanism for hypolimnetic‐epilimnetic exchange and deepwater renewal in Lake Geneva, and probably in other multi‐depth basin lakes under similar wind conditions.

Iron and manganese shuttle has no effect on sedimentary thallium and vanadium isotope signatures in Black Sea sediments

Thallium and V isotopes in organic-rich sediments have recently been explored for reconstructing marine oxygenation by tracking the burial of Mn oxides either locally or globally. The ‘Fe and Mn shuttle’ is a well known mechanism that can transport elements associated with Fe and Mn oxide phases formed under mildly oxidizing shallow water environments both laterally and vertically to euxinic (sulfidic water column) deep waters. The Fe and Mn shuttle has been demonstrated to strongly affect Fe and Mo isotope compositions of sediments accumulating beneath anoxic and euxinic settings. Similar to Mo, Tl and V are also enriched in oceanic ferromanganese crust and nodules, which impart the largest isotopic offsets from modern seawater for Tl and V. Thus, the mechanism of an Fe and Mn shuttle has the potential to, but currently unconstrained, affect Tl and V isotopic compositions (ε205Tl and δ51V) in organic-rich sediments in the modern and ancient records.We have used Black Sea sediments that are known to preserve enrichment and sedimentary isotope signatures for an Fe shuttle. This is due to an Fe transport from the more oxic margin to the deep euxinic basin and corresponding enrichment in the underlying sediments. These samples were used to test for a potential Fe and Mn shuttle effect on ε205Tl and δ51V in sediments deposited under oxic, suboxic, and euxinic water columns. Authigenic ε205Tl from all three depositional settings is indistinguishable (–2.7 ± 0.3, –2.4 ± 0.5, and –2.4 ± 0.3), which is within error of the Black Sea surface seawater (–2.2 ± 0.3). In contrast, oxic and suboxic sediment have similar δ51V values (–1.06 ± 0.33‰ and –1.04 ± 0.30‰), which are significantly more negative than euxinic sediments (–0.57 ± 0.06‰).All the sediments deposited under oxic, suboxic, and euxinic water columns in the Black Sea capture ε205Tl of the surface seawater because there is almost no permanent burial of Mn oxides in this basin. This finding is consistent with the indistinguishable ε205Tl between the surface seawater and the riverine input. Sedimentary δ51V appears to be controlled predominantly by the redox state of the depositional environments with little effect from an Fe or Mn shuttle. The lack of Fe and Mn shuttle effect on ε205Tl and δ51V in the Black Sea is likely due to the shallow (∼100 m) chemocline in the strongly, 2000-m deep redox-stratified water column of this basin and very slow deep-water renewal rate. Iron and Mn in oxides are not delivered to the sediments, instead, they are dissolved and captured in the water column as iron sulfide and reduced Mn mineral phases. Our results suggest that Fe and Mn shuttle transport mechanisms may not strongly affect Tl and V isotopes, at least in extreme cases of water column redox stratification. Further work is required to constrain the isotopic signals using other modern and, by inference, diverse ancient settings for additional shuttling mechanisms.

Internal tidal waves and deep-water renewal in a tropical fjord: Lessons from Ambon Bay, eastern Indonesia

Seasonal-varying internal tidal dynamics have a critical role in deep-water renewal in tropical shallow-silled fjords (e.g. Ambon Bay, Indonesia). Seasonal- and tidal-based longitudinal CTD casts from the slope of outer Ambon Bay (OAB) to the sill of Ambon Bay coupled with bottom-mounted current meters at the sill and in inner Ambon Bay (IAB, the fjord basin), were employed to investigate the internal tidal-sill slope interaction and the characteristics of deep-water inflow. Weaker stratification in the easterly monsoonal season than the transitional monsoonal season (dT/dz = ~0.02 °C/m cf. dT/dz = ~0.06 °C/m) drives the predominant sub-critical slope condition at the OAB slope causing more frequent tidal upwelling events in the easterly monsoonal season (13 events in a spring-neap sequence) than in the transitional monsoonal season (5 events). The magnitude of tidal upwelling in Ambon Bay, as measured by the depth from which water upwells, is stronger in the easterly monsoonal season (up to 200 m) than in the transitional monsoonal season (maximum: 115 m). The influx of upwelled water across the sill was controlled by the tidal excursion of the deep-water plume in the easterly monsoonal season and by the deep-water/sill density difference in the transitional monsoonal season. A series of deep-water renewal events in IAB within a spring-neap sequence (~2 weeks) in the easterly monsoonal season supplied a total inflow volume of 0.09 ± 0.02 km3 which can replenish approximately 80% volume of the IAB deep layer. This total inflow volume was smaller in the transitional monsoonal season (0.05 ± 0.01 km3), hence, only flushing 40% volume of the IAB deep layer. The knowledge of internal tidal waves and deep-water inflow in Ambon Bay presented here will be a key reference for future studies focused on water transport in Ambon Bay and other shallow-silled tropical fjords.

Dissolved methane in the water column of the Saguenay Fjord

Near-shore environments are a significant source of atmospheric methane but the size of this source is poorly constrained, particularly for fjords and fjards. This study investigated the methane emission rates and the drivers controlling the dynamics of dissolved methane in the Saguenay Fjord, a deep, stratified, and well‑oxygenated subarctic fjord system in eastern Canada. Dissolved methane concentrations ([CH4]) in the water column were measured in October 2016 and June, October, and November 2017, with stable carbon isotope composition of methane (δ13CCH4) analyzed during the November 2017 survey. Surface-water [CH4] ranged from 16 to 184 nmol L−1 and decreased with increasing salinity in a bi-segment linear manner, inferring a temporally constant marine endmember but a freshwater discharge-dependent river endmember. The multi-cruises dataset yields a mean [CH4] saturation ratio of 12.7 (range: 4.5–48.7) and a mean emission rate of 53.4 μmol m−2 d−1 (range: 16.4–256.9 μmol m−2 d−1). [CH4] was generally higher in surface water than in deep water. However, sill-induced mixing could homogenize [CH4] near the mouth of the fjord and sedimentary input of biogenic methane (δ13CCH4: −57.660‰) in the fjord's head region increased [CH4] in the overlying bottom water up to 459 nmol L−1. The longitudinal pattern of [CH4] below the surface layer was primarily controlled by deep-water renewal events. Deep-water [CH4] declined with rising apparent oxygen utilization, suggestive of aerobic microbial methane oxidation at rates estimated to be <0.1 nmol L−1 d−1. The δ13CCH4 data yields a carbon isotopic fractionation factor of 1.08 in both the surface and deep waters that points to microbial oxidation dictating the carbon isotopic fractionation of methane in the fjord. Mass-balance budgeting reveals that river runoff accounts for 81% of the total methane input to the fjord (12.13 × 106 mol year−1) and that microbial oxidation of methane (4.45 × 106 mol year−1) is comparable to emission to the atmosphere (4.27 × 106 mol year−1). This study demonstrates the important roles of river runoff and deep-water renewal in controlling the dynamics of [CH4], δ13CCH4, and methane emission to air in fjords receiving large terrestrial freshwater discharges and experiencing frequent deep-water renewals. The areal methane emission rates for deep fjords obtained by this and earlier studies are one to two orders of magnitude higher than the mean flux estimate for global coastal oceans, placing fjords and fjards as a potentially significant contributor to coastal methane emission.

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

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

Rapid onset of ocean anoxia shown by high U and low Mo isotope compositions of sapropel S1

Authigenic uranium isotope compositions of Holocene sapropel S1 (δ238Uauth = +0.10 to +0.52 ‰; ODP core 967, 2550 mbsl) are significantly higher than the proposed upper boundary (+0.2 ‰) associated with the transport-porewater diffusion model for sediment uranium uptake. It is shown that these high δ238Uauth values are compatible with rapid initial slowdown of thermohaline overturning and the development of an anoxic water column. These conditions would favour U uptake in an organic-rich floccule layer overlying the sediment-water interface. The high δ238Uauth values correlate with low δ98Moauth values (+0.02 to −0.88 ‰), interpreted to reflect weakly euxinic conditions controlled by thiomolybdate–molybdate solution equilibria. The S1 data contrast markedly with published data from last interglacial sapropel S5 from the same core, which show δ238Uauth and δ98Moauth characteristics compatible with a restricted euxinic basin due to progressive slowdown in the thermohaline circulation. The U-Mo isotope data for S1 are similar to a range of published palaeo-settings. Sapropels are therefore shown to be useful templates for the unravelling of the interplay between productivity and deep water renewal times in ancient settings.