Sill Depth Research Articles

Controls on Early Cretaceous South Atlantic Ocean circulation and carbon burial – a climate model–proxy synthesis

Abstract. Black shale sediments from the Barremian to Aptian South Atlantic document the intense and widespread burial of marine organic carbon during the initial stages of seafloor spreading between Africa and South America. The enhanced sequestration of atmospheric CO2 makes these young ocean basins potential drivers of the Early Cretaceous carbon cycle and climate perturbations. The opening of marine gateways between initially restricted basins and related circulation and ventilation changes are a commonly invoked explanation for the transient formation and disappearance of these regional carbon sinks. However, large uncertainties in palaeogeographic reconstructions limit the interpretation of available palaeoceanographic data and prevent any robust model-based quantifications of the proposed circulation and carbon burial changes. Here, we present a new approach to assess the principal controls on the Early Cretaceous South Atlantic and Southern Ocean circulation changes under full consideration of the uncertainties in available boundary conditions. Specifically, we use a large ensemble of 36 climate model experiments to simulate the Barremian to Albian progressive opening of the Falkland Plateau and Georgia Basin gateways with different configurations of the proto-Drake Passage, the Walvis Ridge, and atmospheric CO2 concentrations. The experiments are designed to complement available geochemical data across the regions and to test circulation scenarios derived from them. All simulations show increased evaporation and intermediate water formation at subtropical latitudes that drive a meridional overturning circulation whose vertical extent is determined by the sill depth of the Falkland Plateau. The densest water masses formed in the southern Angola Basin and potentially reached the deep Cape Basin as Walvis Ridge Overflow Water. Palaeogeographic uncertainties are as important as the lack of precise knowledge of atmospheric CO2 levels for the simulated temperature and salinity spread in large parts of the South Atlantic. Overall temperature uncertainties reach up to 15 °C and increase significantly with water depth. The ensemble approach reveals temporal changes in the relative importance of geographic and radiative forcings for the simulated oceanographic conditions and, importantly, nonlinear interactions between them. The progressive northward opening of the highly restricted Angola Basin increased the sensitivity of local overturning and upper-ocean stratification to atmospheric CO2 concentrations due to large-scale changes in the hydrological cycle, while the chosen proto-Drake Passage depth is critical for the ocean dynamics and CO2 response in the southern South Atlantic. Finally, the simulated processes are integrated into a recent carbon burial framework to document the principal control of the regional gateway evolution on the progressive shift from the prevailing saline and oxygen-depleted subtropical water masses to the dominance of ventilated high-latitude deep waters.

Continued warming of deep waters in the Fram Strait

Abstract. The Fram Strait is the only deep gateway between the Arctic and the rest of the World Ocean, and it is thus a key region to understand how the deep Arctic will evolve. However, studies and data regarding the deep ocean are scarce, making it difficult to understand its role in the climate system. Here, we analyse oceanographic data obtained close to the Fram Strait sill depth of 2500 m by two long-term mooring locations (F11 and HG-FEVI) in the Fram Strait between 2010–2023 to investigate long-term changes in the hydrographic properties. For additional context, we compile hydrographic profile data from the 1980s for the adjacent basins: the Greenland Sea and the Eurasian Basin. At mooring F11 in the western Fram Strait, we find a clear seasonality, with increased Greenland Sea Deep Water (GSDW) presence during summer and increased Eurasian Basin Deep Water (EBDW) presence during winter. Evaluating long-term changes, we find a modest temperature increase of ∼ 0.1 °C for EBDW from the 1980s. For GSDW, south of the Fram Strait, we find a strong temperature increase of ∼ 0.4–0.5 °C for the same period. The different warming rates have led to GSDW becoming warmer than EBDW since ∼ 2017–2018. This means that the Greenland Sea is no longer a heat sink for the Arctic Ocean at depth but is rather a heat source. It is therefore possible that EBDW temperatures will increase faster in the future.

High-resolution seismic record of the Quaternary palaeoenvironments along a Dalmatian-type coast (Lošinj Channel, Adriatic Sea)

Coastal systems have changed considerably in response to Quaternary glacial–interglacial variability. Here, we offer the first detailed assessment of geomorphic features and late Quaternary sedimentation dynamics in the Lošinj Channel in the Adriatic Sea. The present study attempts to resolve the complex evolution of the environments by applying high-resolution seismic records that build upon sediment core data. Within this coastal karst basin, a significant amount of sediment accumulated during the Quaternary. Notably, the existence of several prominent erosional and depositional environments was revealed. We distinguished recurrent episodes of fluvio-lacustrine and marine sedimentation that probably occurred since Marine Isotope Stage (MIS) 6. The preserved record consists of erosional features and subparallel and oblique reflectors attributed to glacial periods (Seismic Units SU-II, SU-IV, and SU-V). Interglacial units are acoustically semitransparent and generally reflection-free (Seismic Units SU-I and SU-III). The mutual influence of sea level and climate change, sill depth, and karst hydrology enabled environmental changes in the Lošinj Channel. The area proves to be important for Quaternary studies because it contains a long sediment succession spanning at least the middle and late Pleistocene and Holocene. Our study has implications for the assessment of the long-term palaeoenvironmental evolution of similar coastal settings and reconstruction of the regional palaeogeography which is in turn crucial for understanding prehistoric human occupation and/or dispersal. This is one of the few well-preserved records in the Adriatic Sea that provides insight into submerged landscapes and sea levels since the penultimate glacial.

Current and density observations on a flow through a contraction and over a bottom elevation at the southern edge of the Cycladic Plateau in the Aegean Sea – East Mediterranean

A CTD/ADCP/surface-drifter survey in fall 2004 reveals the behaviour of a mesoscale unidirectional flow coming from the Cretan sea in the south with depths ∼1000 m and entering a channel-like area of the Cycladic shelf in the north, that forms a contraction which leads to a bottom elevation (sill depth ∼100 m), and finally returning into the Cretan Sea in the lee-side of the sill. The flow decelerates/accelerates upstream/downstream of the sill. The along-stream density contours near the sill bottom are raised prior to reaching the sill, while they deepen in the lee side of it indicating supercriticality. The long-wavelength internal wave speeds with realistic stratification and no-rotation are higher than the section averaged flow speeds and indicate subcriticality. A key element in this apparent paradox is the large height of the sill that potentially increases the body (drag) force exerted on the flow by the sill while flow blocking is also observed upstream of the sill.

Unravelling the excitation mechanism of very long-period (VLP) tremors in the Gulf of Guinea: evidence for vibrations of thin surface crustal plates

SUMMARY The Gulf of Guinea exhibits a continuous emission of narrow-band and long-period signals (16, 26 and 27 s) on teleseismic records, yet the underlying excitation mechanism remains unclear. This study establishes a connection between these tremors and the vibration of thin, decoupled crustal plates at unexplored volcanoes in the gulf. We first formulate the damped plate oscillation equation, by incorporating the vibration of the thin surface crustal plate and magma flow in the subsurface sill. The findings reveal that a fundamental-mode vibration with a period of several dozen seconds can be induced by a crustal plate that is less than 1.0 km thick but extends over tens of kilometres in both length and width, given a subsurface sill depth exceeding 10.0 cm. The thin plate hypothesis also allows for excitation of a few overtone modes, but such waves in higher frequencies diminish over long distances, leaving only the monotonous fundamental-mode vibration at teleseismic stations. The long duration of Guinea tremors at each recurrence is attributed to the presence of low viscosity basaltic magma, which influences the damping factor. Direct wave loads at the shallow gulf serve as the primary vibration source, accounting for seasonal variations and recurring patterns. Sporadic energy bursts may also occur due to large storms. Radiation patterns of Guinea tremors are linked to the geometric structure of the thin plate. Our theoretical estimates of tremor spectra closely align with observed data, confirming the model’s accuracy in capturing reported Guinea tremor characteristics. This study provides valuable insights into the origins of very long-period tremors at continental volcanoes.

Local forcing mechanisms challenge parameterizations of ocean thermal forcing for Greenland tidewater glaciers

Abstract. Frontal ablation has caused 32 %–66 % of Greenland Ice Sheet mass loss since 1972, and despite its importance in driving terminus change, ocean thermal forcing remains crudely incorporated into large-scale ice sheet models. In Greenland, local fjord-scale processes modify the magnitude of thermal forcing at the ice–ocean boundary but are too small scale to be resolved in current global climate models. For example, simulations used in the Ice Sheet Intercomparison Project for CMIP6 (ISMIP6) to predict future ice sheet change rely on the extrapolation of regional ocean water properties into fjords to drive terminus ablation. However, the accuracy of this approach has not previously been tested due to the scarcity of observations in Greenland fjords, as well as the inability of fjord-scale models to realistically incorporate icebergs. By employing the recently developed IceBerg package within the Massachusetts Institute of Technology general circulation model (MITgcm), we here evaluate the ability of ocean thermal forcing parameterizations to predict thermal forcing at tidewater glacier termini. This is accomplished through sensitivity experiments using a set of idealized Greenland fjords, each forced with equivalent ocean boundary conditions but with varying tidal amplitudes, subglacial discharge, iceberg coverage, and bathymetry. Our results indicate that the bathymetric obstruction of external water is the primary control on near-glacier thermal forcing, followed by iceberg submarine melting. Despite identical ocean boundary conditions, we find that the simulated fjord processes can modify grounding line thermal forcing by as much as 3 °C, the magnitude of which is largely controlled by the relative depth of bathymetric sills to the Polar Water–Atlantic Water thermocline. However, using a common adjustment for fjord bathymetry we can still predict grounding line thermal forcing within 0.2 °C in our simulations. Finally, we introduce new parameterizations that additionally account for iceberg-driven cooling that can accurately predict interior fjord thermal forcing profiles both in iceberg-laden simulations and in observations from Kangiata Sullua (Ilulissat Icefjord).

Dense Water Formation in the North–Central Aegean Sea during Winter 2021–2022

The evolution and drivers of dense water formation (DWF) in the North–Central Aegean Sea (NCAeg) during winter 2021–2022 are studied using observations from two Argo floats and the output of an operational data-assimilating model. Dense water with σθ>29.1 kgm−3 was produced over most of the NCAeg, except for the northeastern part covered by Black Sea water (BSW), where the maximum surface density was <29 kgm−3. The highest density waters were produced over the central and southern parts of the Lemnos Plateau and in the shallow coastal areas between Chios Island and the Edremit Gulf. Atmospherically driven transformation to the east of Lesvos Island resulted in the production of waters with anomalously high density and salinity, which flowed inside Skiros Basin, thus partly explaining its historically higher density and salinity compared to the rest of the NCAeg subbasins. The Skiros and Athos Basins were ventilated down to σθ∼29.35 kgm−3 horizons. The 29.1 kgm−3 isopycnal rose by ∼200 m, and the 29.25 kgm−3 isopycnal overflowed above the ∼400 m sill depth filling the southern depressions of the NCAeg. Combining data from Argo floats, vessel casts, gliders, and a fixed-point observatory, the dense water produced in the NCAeg was observed spreading in the deep layer of the Central Cretan Sea for at least one and a half years after the formation. The cyclonic circulation of the newly formed water in the NCAeg has been observed directly for the first time using deep-drifting floats. The Eastern Mediterranean warming and salinification signal has propagated below the NCAeg sill depth. The winter average buoyancy loss was comparable to that of the peak of the Eastern Mediterranean transient (EMT) and other known years of DWF in the NCAeg; however, the high temperature of the upper layers due to long-term warming prevented the widespread formation of denser water.

Impact of shallow sills on circulation regimes and submarine melting in glacial fjords

Abstract. The increased melting and rapid retreat of marine-terminating glaciers is a key contributor to sea-level rise. In glacial fjords with shallow sills common in Patagonia, Alaska, and other systems, these bathymetric features can act as a first-order control on the dynamics. However, our understanding of how this shallow bathymetry interacts with the subglacial discharge from the glacier and impacts the fjord circulation, water properties, and rates of submarine melting is limited. To address this gap, we conduct idealized numerical simulations using a coupled plume–ocean fjord model spanning a wide range of initial ocean conditions, sill depths, and subglacial discharge. A previously documented circulation regime leads to strong mixing and vertical transport over the sill, where up to ∼ 70 % of the colder water from the upper-layer outflow is refluxed into the deeper layer, cooling the incoming warm oceanic water by as much as 1 ∘C and reducing the stratification near the glacier front. When the initial stratification is relatively strong or the subglacial discharge is relatively weak, an additional unsteady circulation regime arises where the freshwater flow can become trapped below the sill depth for weeks to months, creating an effective cooling mechanism for the deep water. We also find that submarine melting often increases when a shallow sill is added to a glacial fjord due to the reduction of stratification – which increases submarine melting – dominating over the cooling effect as the oceanic inflow is modified by the presence of the sill. These results underscore that shallow-silled fjords can have distinct dynamics that strongly modulate oceanic properties and the melting rates of marine-terminating glaciers.

Effect of an Open Central American Seaway on Ocean Circulation and the Oxygen Minimum Zone in the Tropical Pacific From Model Simulations

AbstractThe tectonic closure of the Central American Seaway (CAS) during the mid‐Miocene to mid‐Pliocene (∼16–3 Ma BP) is thought of as a key interval for the onset of the present‐day tropical Pacific oxygen minimum zone (OMZ). In this study we investigate the impact of an open CAS on the ocean circulation and the OMZ in the tropical Pacific. We perform a series of sensitivity experiments with the Kiel Climate Model, where we vary the CAS sill depth from shallow to deep. We find that the eastern tropical Pacific OMZ was less developed during the period of an open CAS. This is driven mainly by an enhanced eastward subsurface current that facilitated an increased oxygen supply from the western tropical Pacific. In addition, a small decrease in net marine primary production and subsequent weaker export of particulate organic carbon induced less subsurface oxygen consumption in the Eastern Equatorial Pacific.

Dinocyst assemblages and water surface conditions in the Sea of Marmara during MIS 6 and 5 from two long cores

The Sea of Marmara is the connection between the vast Black Sea-Caspian Sea basin (Pontocaspian) and the Global Ocean via the Mediterranean Sea. Its water levels and water conditions has widely varied over times. Combining two cores in the Sea of Marmara (Turkey) and using organic-walled dinoflagellate cyst assemblages as the main proxy (combined with alkenones, diatoms and benthic foraminifera), allow qualitatively reconstructing water conditions during Marine Isotopic Stage (MIS) 6 and 5, such as salinity and oxygen level. A clear main marine phase is illustrated in MIS 5e. A minor marine incursion occurred during MIS 5c, mostly supported by alkenone data. The rest of the record indicates brackish Pontocaspian conditions, with more Spiniferites inaequalis in MIS 6 and more S. cruciformis in the non-marine parts of MIS 5.At the MIS 6/MIS 5 transition, an earlier initial marine flooding in the Sea of Marmara (dinocyst assemblages) in comparison to the Black Sea was highlighted. The marine reconnection occurred at different moments as seen in the terrestrial vegetation reconstructed from pollen analysis linking the two seas.The sapropels of the Sea of Marmara form when marine water penetrates at depth from the Aegean Sea beneath a layer of lower salinity water. Variations of the residence time of the marine deep water in a stratified Sea of Marmara are potentially an important factor underlying hypoxia/anoxia and sapropel deposition. When combining surface water proxies with benthic foraminifera (test assemblages and presence of inner organic lining), it appears that the successive MIS 5 sapropels formed under decreasing salinity and oxygen availability conditions as the marine inflow was increasingly restricted. Understanding the hydrologic evolution of the Sea of Marmara during MIS 5 implies taking into account that the Bosphorus and Dardanelle straits are hydrodynamically coupled and may not be simplified as on/off switches based on present day sill depths.

A new deglacial climate and sea-level record from 20 to 8 ka from IODP381 site M0080, Alkyonides Gulf, eastern Mediterranean

Records of relative sea-level rise for the last deglaciation are mostly limited to coral reef records and geophysical model estimates, but observational data from regions with temperate climates is sparse. We present a new relative climatic and regional sea-level rise record for glacial Termination 1 (Marine Isotope Stages [MIS] 2–1) based on ostracode paleoecology from the upper 8 m of the International Ocean Discovery Program (IODP) Site M0080 collected on Expedition 381, in the Gulf of Alkyonides, eastern Corinth basin of the Mediterranean Sea. Results show a series of major faunal transitions from lacustrine (Ponto-Caspian, Lake Corinth) glacial-age assemblages to fully marine (Mediterranean) interglacial assemblages between 20 and 8 ka. During glacial and early deglacial intervals, the Gulf of Alkyonides was characterized by non-marine lacustrine conditions with episodic sediment input from coastal, saline lake environments. Relatively stable lake shoreline conditions marked by the distinctive Tuberoloxoconcha sp. Existed from ∼17.5 to 15 ka. During the peak deglacial interval, the BØlling-AllerØd (B-A, ∼15–13.5 ka), rapid sea-level rise is indicated by a fully marine ostracode fauna colonization, which persisted from 13.5 to 7.5 ka (Late Pleistocene-Early to Middle Holocene).The transition from lacustrine to marine environments confirms that during the last glacial maximum (LGM) low sea level (130 - 125 m below present day), the Corinth-Alkyonides depocentres were lacustrine. Marine water breached the shallow Rion and Acheloos-Cape Pappas sills, which today are ∼50–60 m deep, separating the Mediterranean and Corinth-Alkyonides system beginning about 15 ka. Based on Alkyonides sedimentation rates, mean rates of sea-level rise during the B-A flooding of the Corinth-Alkyonides system are comparable to those obtained from coral reef sea level (SL) records, at least 10–20 mm yr−1. Changes in sedimentation and sill depths in this tectonically active region may have played a role in reconnection of the Mediterranean and Corinth/Alkyonides system over a prolonged period. However, the ages and scale of the faunal changes and their clear correspondence with previously published global sea-level curves and the regional sea-level curve based on deglacial land elevation changes predicted by the ICE-7G model suggests the M0080A deglacial is dominated by the glacio-eustatic sea-level rise and records details of global climate changes during Termination 1.

Sources and sinks of bottom water oxygen in a seasonally hypoxic fjord

Deoxygenation of the ocean has been occurring over the last half century, particularly in poorly ventilated coastal waters. In coastal and estuarine environments, both the water column and sediments play key roles in controlling oxygen variability. In this study, we focus on controls of oxygen concentration in Bedford Basin (BB), a 70 m deep, seasonally hypoxic semi-enclosed fjord on the West Atlantic coast in Nova Scotia. The basin is connected to the Scotian Shelf via a narrow 20 m deep sill that restricts the resupply of bottom water. Hypoxia was recorded seasonally in 2018, 2019 and 2021 with minimum oxygen concentration of 5, 6.7 and 2.7 μM, respectively. Using a 1-D benthic-pelagic coupled model we investigate oxygen consumption and resupply processes during these years. The model was constrained with weekly water column measurements of temperature, salinity, chlorophyll-a fluorescence and dissolved oxygen from a monitoring station in the central basin together with seasonal measurements of benthic diffusive oxygen uptake. Our model suggested that 29-81%, and up to 36% of bottom water re-oxygenation occurred during the winter mixing period and through summer/fall intrusions of Scotian Shelf water, respectively. Occasional shelf water intrusions occurred rapidly, on a timescale of a few hours, and delivered equivalent amounts of oxygen as winter mixing and were sufficient to end bottom water hypoxia. Collectively, these mechanisms supplied the majority of the oxygen delivered to the bottom water. Oxygen supply to bottom waters during periods of water column stratification accounted for 19-36% of the annual flux. The mean benthic uptake was 12 ± 8 mmol m-2 d-1 and contributed ~20% of the total oxygen consumption below the sill depth. In 2021, sea surface temperature (SST) was unusually high and likely resulted in 50% less bottom water oxygenation compared to 2018 and 2019 due to increased stratification; SST in BB was found to be increasing at a rate of 0.11 ± 0.02 °C/year. Climate control on water column stratification are discussed and numerical experiments are used to compare the effects of different water column mixing scenarios on bottom water oxygenation.

Mitigation of oxygen decline in fjords by freshwater injection

The exchange of water masses between deep fjords and the open ocean is commonly constrained by a topographical barrier called the sill. While fjord water above the sill depth communicates relatively freely with the open ocean, water below the sill depth is caught inside the fjord basin. This basin water may remain stagnant in deep fjords for many successive years. During these periods, the biological consumption of dissolved oxygen is larger than the supply of new oxygen, and the fjord basin might experience hypoxia and even anoxia. Such deoxygenation is natural but can be amplified by warming and human activities involving supplies of organic matter and other nutrients. Here, we use a general circulation model to explore how deoxygenation can be mitigated by injecting fresh water into the fjord basin. The freshwater injection causes density reduction of the basin water with subsequent water exchange and oxygenation. Our results suggest that the basin water of Masfjorden, a 480 m deep fjord with a basin volume of 4 × 109 m3, can avoid deoxygenation with a continuous freshwater injection of 0.05 m3 s−1. We conclude that freshwater injection might be an efficient tool to mitigate the deoxygenation of fjord basins.

Wind-Forced Seasonal Exchange between Marginal Seas and the Open Ocean

Abstract The circulation within marginal seas subject to periodic winds, and their exchange with the open ocean, are explored using idealized numerical models and theory. This is motivated by the strong seasonal cycle in winds over the Nordic Seas and the exchange with the subpolar North Atlantic Ocean through the Denmark Strait and Faroe Bank Channel. Two distinct regimes are identified: an interior with closed f/h contours and a shallow shelf region that connects to the open ocean. The interior develops a strong oscillating along-topography circulation with weaker ageostrophic radial flows. The relative importance of the bottom Ekman layer and interior ageostrophic flows depends only on ωh/Cd, where ω is the forcing frequency, h is the bottom depth, and Cd is a linear bottom drag coefficient. The dynamics on the shelf are controlled by the frictional decay of coastal waves over an along-shelf scale Ly = f0LsHs/Cd, where f0 is the Coriolis parameter, and Ls and Hs are the shelf width and depth. For Ly much less than the perimeter of the basin, the surface Ekman transport is provided primarily by overturning within the marginal sea and there is little exchange with the open ocean. For Ly on the order of the basin perimeter or larger, most of the Ekman transport is provided from outside the marginal sea with an opposite exchange through the deep part of the strait. This demonstrates a direct connection between the dynamics of coastal waves on the shelf and the exchange of deep waters through the strait, some of which is derived from below sill depth. Significance Statement The purpose of this study is to understand how winds over marginal seas, which are semienclosed bodies of water around the perimeter of ocean basins, can force an exchange of water, heat, salt, and other tracers through narrow straits between the marginal sea and the open ocean. Understanding this exchange is important because marginal seas are often regions of net heat, freshwater, and carbon exchange with the atmosphere. The present results identify a direct connection between processes along the coast of the marginal sea and the flow of waters through deep straits into the open ocean.

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.

Understanding the Role of Terrestrial and Marine Carbon in the Mid-Latitude Fjords of Scotland.

The sediments within fjords are critical components of the mid- to high-latitude coastal carbon (C) cycle, trapping and storing more organic carbon (OC) per unit area than other marine sedimentary environments. Located at the land-ocean transition, fjord sediments receive OC from both marine and terrestrial environments; globally, it has been estimated that 55%-62% of the OC held within modern fjord sediments originates from terrestrial environments. However, the mid-latitude fjords of the Northern Hemisphere have largely been omitted from these global compilations. Here we investigate the mechanism driving the distribution of OC originating from different sources within the sediments of 38 Scottish fjords. From an array of fjord characteristics, the tidal range and outer sill depth were identified as the main drivers governing the proportions of marine and terrestrial OC in the sediments. Utilizing this relationship, we estimate that on average 52%±10% of the OC held within the sediments of all Scotland's fjords is terrestrial in origin. These findings show that the Scottish fjords hold equivalent quantities of terrestrial OC as other global fjord systems. However, the analysis also highlights that the sediments within 29% of Scottish fjords are dominated by marine derived OC, which is driven by local fjord geomorphology and oceanography.

The physical oceanography of Fortune Bay, an overview

This paper describes the physical oceanography of Fortune Bay, a broad, mid-latitude fjord located in Newfoundland (Canada). Fortune Bay is subject to a strong seasonal stratification (0–16 °C sea-surface temperature range with up to 1 °C/m vertical gradient) influenced by local freshwater runoff, wind forcing and shelf inputs. Sea-ice is seldom present in the bay and unlikely to be of importance on the seasonal stratification and mixing processes. Fortune Bay is warmer than its adjacent shelf both at the surface (by about 2 °C) and at intermediate depths (by about 1 °C from 50–150 m). While the former is likely due to local freshwater runoff stratification influence, the latter is probably related to the warm, deep water input occurring in winter below sill depth and subsequently mixed with the intermediate layer via the input of a colder water mass flowing in summer and which eventually reaches the bottom as well. Currents are dominated by the ‘weather band’ (2–20 d) and characterized by energetic pulses associated with downwelling and upwelling events. Mean circulation is rather weak and the seasonal pattern obtained here did not reveal either the presence of a distinct estuarine circulation nor a strong influence of the main coastal current. Tidal currents are weak also and no inertial signal was observed. Estimates of water exchange between the inner and outer part of the bay were calculated using several methods and led to residence times of the order of a few to several months for the upper layers and of the order of a year for the bottom layer with a probable strong seasonal variability (larger residence time in summer for the upper layers). The “baroclinic pumping” processes, which include the downwelling/upwelling events, appear to be important players but more work is needed to better understand their nature and actual contribution.

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.

A song of volumes, surfaces and fluxes: The case study of the Central Mallorca Depression (Balearic Promontory) during the Messinian Salinity Crisis

AbstractThe Central Mallorca Depression (CMD) located in the Balearic Promontory (Western Mediterranean) contains a well‐preserved evaporitic sequence belonging to the Messinian Salinity Crisis (MSC) salt giant, densely covered by high‐ and low‐resolution seismic reflection data. It has been proposed recently that the MSC evaporitic sequence in the CMD could be a non‐deformed analogue of the key MSC area represented by the Caltanissetta Basin in Sicily. This presumed similarity makes the CMD an interesting system to better understand the MSC events. Physics‐based box models of the water mixing between sub‐basins, built on conservation of mass of water and salt, help constrain the hydrological conditions under which evaporites formed during the MSC. Those models have been widely used in the literature of the MSC in the past two decades. They have been mostly applied to the Mediterranean Sea as a whole focusing on the Mediterranean–Atlantic connection, or focusing on the influence of the Sicily Sill connecting the Western and Eastern Mediterranean Sea. In this study, we apply a downscaled version of such modelling technique to the CMD. First, we quantify the present‐day volumes of the MSC units. We then use a reconstructed pre‐MSC paleo‐bathymetry to model salinity changes as a function of flux exchanges between the CMD and the Mediterranean. We show that a persistent connection between the CMD and the Mediterranean brine near gypsum saturation can explain volume of Primary Lower Gypsum under a sea level similar to the present. For the halite, on the contrary, we show that the observed halite volume cannot be deposited from a connected CMD‐Mediterranean scenario, suggesting a drawdown of at least 850 m (sill depth) is necessary. Comparison between the deep basin halite volume and that of the CMD shows that it is possible to obtain the observed halite volume in both basins from a disconnected Mediterranean basin undergoing drawdown, although determining the average salinity of the Western Mediterranean basin at the onset of drawdown requires further investigation.

The effect of water depth on passage success of arctic grayling through two Denil fishways

Arctic grayling (Thymallus arcticus) are found throughout parts of North America, Europe and Asia. Denil fishways are being used in efforts to recover populations and provide passage for grayling through water diversions in Montana and elsewhere. This study evaluated how water depths in 1.83 m and 3.66 m Denil fishways affect upstream passage for grayling. The study was performed in a laboratory at the Bozeman Fish Technology Center (BFTC) in Bozeman, MT. Fifteen passage trials of different treatment combinations of approach depth and sill depth were performed in each ladder. A total of 310 grayling, in groups of 10 per trial, were allowed to volitionally attempt to swim upstream through each ladder. For the 1.83 m Denil, 81 of 137 participants successfully ascended resulting in an overall passage of 59.1% among all trials compared to 82 of 147 participants successfully ascending the longer structure for an overall passage of 55.8%. A quasi-binomial regression model was used to predict passage success for different combinations of approach and sill depths. Our study showed upstream passage success increased with deeper water depths at the sill for both ladders with the exception of the configuration with the shallowest approach depth and deepest water depth at the sill.