Deep Sill Research Articles

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.

Summer Hydrography and Circulation in Storfjorden, Svalbard, Following a Record Low Winter Sea‐Ice Extent in the Barents Sea

AbstractStorfjorden, Svalbard, hosts a polynya in winter and is an important source region of Brine‐enriched Shelf Water (BSW) that, if dense enough, feeds the Arctic Ocean deep water reservoir. Changes in the BSW production may thus have far‐reaching impacts. We analyze the water mass distribution and circulation in Storfjorden and the trough south of it, Storfjordrenna, using hydrographic sections occupied in July 2016, following a winter characterized by the lowest ice coverage recorded in the Barents Sea. These observations reveal an unusual hydrographic state, characterized at the surface by the near absence of Melt Water and Storfjorden Surface Water, replaced by a saltier water mass. At depth, BSW (maximum salinity of 34.95) was found from the bottom up to 90 m, above the 120‐m deep sill at the mouth to Storfjordrenna. However, no gravity driven overflow was observed downstream of the sill: the dome of BSW remained locked over the depression in a cyclonic circulation pattern consistent with a stratified Taylor column. Observations further reveal a previously unreported intrusion of Atlantic Water (AW) far into the fjord, promoting isopycnal mixing with entrapped Arctic Water. This intrusion was possibly favored by positive wind stress curl anomalies over Svalbardbanken and Storfjordrenna. The bottom plume exiting Storfjordrenna was weak, carrying Polar Front Water rather than BSW, too light to sink underneath the AW layer at Fram Strait. Whether Storfjorden switched durably to a new hydrographic state, following the observed Atlantification of the Barents Sea after 2005, remains to be established.

Remote Sensing of the Tautra Ridge: An Overview of the World’s Shallowest Cold-Water Coral Reefs

On the Tautra Ridge – a 39-100 m deep morainic sill located in the middle of the Trondheimsfjord, Norway – some of the world’s shallowest known occurrences of the scleractinian cold-water coral (CWC) Desmophyllum pertusum can be found. The earliest D. pertusum records from the Tautra Ridge date back to the 18th century, and since then, the location has provided easy access to physical coral specimens for numerous scientific studies. In 2013, the ridge was declared a marine protected area by the Norwegian Government due to its unique CWC reefs. However, few attempts have to our knowledge yet been made to characterize the distribution, extent and condition of these reefs extensively. The aim of the current study was therefore to add geospatial context to the Tautra CWC reef complex. In the study, data from multibeam echo sounding, synthetic aperture sonar imaging and underwater hyperspectral imaging are used to assess CWC reef occurrences from multiple perspectives. The study demonstrates how complementary remote sensing techniques can be used to increase knowledge generation during seafloor mapping efforts. Ultimately, predictive modeling based on seafloor geomorphometry is used to estimate both distribution and areal coverage of D. pertusum reefs along the majority of the Tautra Ridge. Our findings suggest that D. pertusum reef distribution on the Tautra Ridge is affected by several geomorphometric seafloor properties, and that the total reef extent in the area likely is close to 0.64 km2. Better description of current patterns across the Tautra Ridge will improve our understanding of the interaction between hydrography and geomorphology at the Tautra CWC reef complex in the future.

Hydrographic observations in the rift valley of Southwest Indian Ridge from 63°E – 69°E

The Southwest Indian Ridge (SWIR) is poorly understood in terms of local variability in hydrographical characteristics because of a relative lack of data. Furthermore, previous hydrographic studies in SWIR were restricted to the west of 60°E region. We present a new set of hydrographic data which were collected during a survey for the search of hydrothermal plume/activity in between Mt. Jourdanne and Rodrigues Triple Junction (RTJ) over SWIR. The primary goal of our study is to understand the segment-scale hydrography in and above the SWIR rift valley between 63°E to 69°E, with emphasis on water mass propagation and variability of their physical properties. CTD/Oxygen profiles were examined along a ~650 km distance of SWIR axial valley segments. The properties of Antarctic Bottom Water (AABW) were identified in the rift valley section which passes through deep sills from the Crozet basin. Along the ridge, the physical parameters especially at deep and bottom layer depths have observed gradients formed by the obstruction of flow at sills in the valley, along with a unidirectional density driven flow.

Ocean Modification and Seasonality in a Northern Ellesmere Island Glacial Fjord Prior to Ice Shelf Breakup: Milne Fiord

AbstractUnderstanding the impact of the break‐up of northern Ellesmere Island ice shelves on fjord dynamics is limited by a lack of ocean observations prior to ice loss. Based on profiling and mooring data collected between 2011 and 2019, we describe the oceanography of Milne Fiord prior to the 2020 breakup of the Milne Ice Shelf, including, sources of ambient water, under‐ice hydrography, glacial modification, and seasonality. Ambient waters originate in the Canada Basin but are modified by interaction with the 100 m thick ice shelf at the fjord entrance, and extensive glacier tongue at the head. Properties within the 436 m deep fjord are depth‐dependent, with freshwater surface runoff and subglacial discharge accumulating in the upper 50 m of the fjord each summer behind the ice shelf. Freshwater export is restricted to a basal channel in the ice shelf, resulting in the fjord being more stratified and warmer than waters offshore year‐round. Below the ice shelf and above a 260 m deep sill unrestricted exchange allows warm Atlantic Water to penetrate to the 150 m deep glacier grounding line where submarine melting occurs. Basal meltwater spreads down‐fjord close to its depth of origin due to the strong stratification. Interannual warming of fjord deepwater is likely driven by shoaling of the Arctic Ocean thermocline and spillover at the sill, highlighting the link between fjord properties and regional oceanography. Further breakup of the ice shelf is predicted to substantially alter fjord dynamics, with consequences for ocean forcing of the Milne Glacier.

Geochemistry of deep Tunguska Basin sills, Siberian Traps: correlations and potential implications for the end-Permian environmental crisis

A vast portion of the plumbing system of the Siberian Traps Large Igneous Province (STLIP) is emplaced in the Tunguska Basin, where borehole data reveal ubiquitous and abundant sills with great lateral extension. These intrusions intersect Cambrian–Ordovician evaporite, carbonate and siliciclastic series, and locally coal-bearing Permian host rocks, with a high potential for thermogenic gas generation. Here we present new geochemical data from 71 magmatic and 4 sedimentary rock samples from the Tunguska Basin center and periphery, recovered from 15 deep sills intercepted by boreholes. The studied samples are all low-Ti basalt and basaltic andesites, confirming absence of high-Ti and alkaline STLIP magmatism in the Tunguska Basin. The sills derive from picritic parental melts produced by extensive melting of a mantle source with recycled crustal components below a thinned lithosphere (50–60 km), within the spinel stability field. The mantle source was dominantly peridotitic, with enriched pyroxenitic domains formed by recycled lower crust, in agreement with previous models for the main tholeiitic STLIP phase. Limited amounts (up to 5%) of highly radiogenic granitoids or moderately radiogenic metapelites were assimilated in upper crustal magma reservoirs. After emplacement, sills intruded in Cambrian evaporites assimilated marlstones and interacted with the evaporitic host rocks, probably via fluids and brines. This is the first time that such process is described in subvolcanic rocks from all across the volcanic basin. The sills are correlated geochemically with the established chemostratigraphy for the on-craton STLIP lava piles and intrusions (Norilsk region). Sills correlated with the Morongovsky–Mokulaevsky Fm. and the Norilsk-type intrusions are the most voluminous, present all across the central Tunguska Basin, and bear the strongest evidence of interaction with evaporites. Massive discharge of thermogenic volatiles is suggested by explosive pipes and hydrothermal vent structures throughout the Tunguska Basin. We propose that this voluminous pulse of magmatism is a good candidate for the hitherto unidentified early intrusive phase of the STLIP, and may link the deep Tunguska basin sills to the end-Permian environmental crisis.

Ground Deformation After a Caldera Collapse: Contributions of Magma Inflow and Viscoelastic Response to the 2015–2018 Deformation Field Around Bárðarbunga, Iceland

AbstractImprovement of our understanding of the role of ground deformation due to viscoelastic relaxation following eruptions is important, as the generated signals can resemble renewed magma inflow. We study post‐eruptive unrest at the subglacial Bárðarbunga volcano, Iceland, after a caldera collapse and major magma drainage in 2014–2015. Elevated seismicity began about 6 months after the eruption ended, including nine Mlw > 4.5 earthquakes. Global Navigation Satellite System and Sentinel‐1 Interferometric Synthetic Aperture Radar geodesy are applied to evaluate post‐eruptive ground deformation from 2015 to 2018. Horizontal velocities locally exceed 10 cm/year and rapidly decay with distance away from the caldera. We explore two end‐member models and their combination to explain the post‐eruptive deformation field: 1) viscoelastic relaxation caused by the co‐eruptive caldera collapse and magma withdrawal, and 2) renewed magma inflow. We find parameter combinations for each model that explain the observed ground deformation. The purely viscoelastic relaxation model, consisting of a half‐space composed of a 7‐km thick elastic layer on top of a viscoelastic layer with a viscosity of 3.0 × 1018 Pa s reproduces broadly the observations. A simple magma inflow model consisting of a single point source with an inflow rate of 1 × 107 m3/year at 0.7 km depth broadly fits the observations, but may be unrealistic. A more elaborate model of magma inflow into a 10‐km deep sill combined with slip on the caldera ring fault explains the observations well. Our results suggest that the co‐eruptive deformation field is likely influenced by viscoelastic relaxation, renewed magma inflow, or a combination of both processes.

A Role for Gravity Currents in Cross‐Sill Estuarine Exchange and Subsurface Inflow to the Southern Strait of Georgia

AbstractMultiyear current meter records from five sites are used to examine subsurface renewal processes in the southern Strait of Georgia. The lower layer inflow of dense oceanic‐derived water over the ~100‐m deep sill in Boundary Pass near the entrance to the strait reaches peak velocities during spring tides and rarely reverses direction. Corresponding changes in water density over the sill are inversely correlated with inflow velocity and, it is assumed, with temporal (t) variations in bottom layer thickness, h(t), arising from the fortnightly variations in tidal‐mixing. The dense bottom water crossing the sill transforms into a gravity current whose velocity is dependent on both h and sill depth, H. Gravity flow velocities, which peak at h(t)/H ~ 0.3 and weaken for h → 0 (neap tides) and h → H (spring tides), modulate the cross‐sill exchange with the strait. The density anomaly, Δρ, of the gravity flow determines its terminal depth and along‐strait extent. Lower‐density gravity flows generated during spring tides separate from the sloping bottom of the southern strait, but the higher‐density flows formed during neap tides can penetrate over 100 km northward to the deep central basin. The most extensive bottom intrusions occur monthly near neap tides in summer, when the underlying estuarine circulation delivers dense upwelled water from the coast to the sill. These deep inflows are accompanied by strong middepth outflow. There are typically three to five monthly replacement episodes each summer, but fewer during strong El Niño years due to reduced coastal upwelling.

Overpressure transmission through interconnected igneous intrusions

In situ overpressures in sedimentary basins are commonly attributed to disequilibrium compaction or fluid expansion mechanisms, although overpressures in shallow sedimentary sequences may also develop by vertical transfer of pressure from deeper basin levels, for example, via faults. Mafic sill complexes are common features of sedimentary basins at rifted continental margins, often comprising networks of interconnected sills and dikes that facilitate the transfer of magma over considerable vertical distances to shallow basinal depths. Here, we document evidence for deep sills (depths >5 km [>16,000 ft]) hosting permeable, open fracture systems that may have allowed transmission of overpressure from ultradeep basinal (>7 km [>23,000 ft]) levels in the Faroe–Shetland Basin, northeast Atlantic margin. Most notably, well 214/28-1 encountered overpressured, thin ( 58 m.y.). We instead suggest that transgressive, interconnected sill complexes, such as those penetrated by well 214/28-1, may represent a previously unrecognized mechanism of transferring overpressures (and indeed hydrocarbons) laterally and vertically from deep to shallow levels in sedimentary basins and that they represent a potentially underrecognized hazard to both scientific and petroleum drilling in the vicinity of subsurface igneous complexes.

Pacific Abyssal Transport and Mixing: Through the Samoan Passage versus around the Manihiki Plateau

AbstractThe main source feeding the abyssal circulation of the North Pacific is the deep, northward flow of 5–6 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) through the Samoan Passage. A recent field campaign has shown that this flow is hydraulically controlled and that it experiences hydraulic jumps accompanied by strong mixing and dissipation concentrated near several deep sills. By our estimates, the diapycnal density flux associated with this mixing is considerably larger than the diapycnal flux across a typical isopycnal surface extending over the abyssal North Pacific. According to historical hydrographic observations, a second source of abyssal water for the North Pacific is 2.3–2.8 Sv of the dense flow that is diverted around the Manihiki Plateau to the east, bypassing the Samoan Passage. This bypass flow is not confined to a channel and is therefore less likely to experience the strong mixing that is associated with hydraulic transitions. The partitioning of flux between the two branches of the deep flow could therefore be relevant to the distribution of Pacific abyssal mixing. To gain insight into the factors that control the partitioning between these two branches, we develop an abyssal and equator-proximal extension of the “island rule.” Novel features include provisions for the presence of hydraulic jumps as well as identification of an appropriate integration circuit for an abyssal layer to the east of the island. Evaluation of the corresponding circulation integral leads to a prediction of 0.4–2.4 Sv of bypass flow. The circulation integral clearly identifies dissipation and frictional drag effects within the Samoan Passage as crucial elements in partitioning the flow.

Distinct Degassing Pulses During Magma Invasion in the Stratified Karoo Basin—New Insights From Hydrothermal Fluid Flow Modeling

AbstractMagma emplacement in organic‐rich sedimentary basins is a main driver of past environmental crises. Using a 2‐D numerical model, we investigate the process of thermal cracking in contact aureoles of cooling sills and subsequent transport and emission of thermogenic methane by hydrothermal fluids. Our model includes a Mohr‐Coulomb failure criterion to initiate hydrofracturing and a dynamic porosity/permeability. We investigate the Karoo Basin, taking into account host‐rock material properties from borehole data, realistic total organic carbon content, and different sill geometries. Consistent with geological observations, we find that thermal plumes quickly rise at the edges of saucer‐shaped sills, guided along vertically fractured high‐permeability pathways. Contrastingly, less focused and slower plumes rise from the edges and the central part of flat‐lying sills. Using a novel upscaling method based on sill‐to‐sediment ratio, we find that degassing of the Karoo Basin occurred in two distinct phases during magma invasion. Rapid degassing triggered by sills emplaced within the top 1.5 km emitted ~1.6·103 Gt of thermogenic methane, while thermal plumes originating from deeper sills, carrying a 13‐times‐greater mass of methane, may not reach the surface. We suggest that these large quantities of methane could be remobilized by the heat provided by neighboring sills. We conclude that the Karoo large igneous province may have emitted as much as ~22.3·103 Gt of thermogenic methane in the half million years of magmatic activity, with emissions up to 3 Gt/year. This quantity of methane and the emission rates can explain the negative δ13C excursion of the Toarcian environmental crisis.

A Century of Stability of Avannarleq and Kujalleq Glaciers, West Greenland, Explained Using High-Resolution Airborne Gravity and Other Data.

The evolution of Greenland glaciers in a warming climate depends on their depth below sea level, flow speed, surface melt, and ocean‐induced undercutting at the calving front. We present an innovative mapping of bed topography in the frontal regions of Sermeq Avannarleq and Kujalleq, two major glaciers flowing into the ice‐choked Torssukatak Fjord, central west Greenland. The mapping combines a mass conservation algorithm inland, multibeam echo sounding data in the fjord, and high‐resolution airborne gravity data at the ice‐ocean transition where other approaches have traditionally failed. We obtain a reliable, precision (±40 m) solution for bed topography across the ice‐ocean boundary. The results reveal a 700 m deep fjord that abruptly ends on a 100–300 m deep sill along the calving fronts. The shallow sills explain the presence of stranded icebergs, the resilience of the glaciers to ocean‐induced undercutting by warm Atlantic water, and their remarkable stability over the past century.

The T‐Reflection and the Deep Crustal Structure of the Vøring Margin, Offshore mid‐Norway

AbstractSeismic reflection data along volcanic passive margins frequently provide imaging of strong and laterally continuous reflections in the middle and lower crust. We have completed a detailed 2‐D seismic interpretation of the deep crustal structure of the Vøring Margin, offshore mid‐Norway, where high‐quality seismic data allow the identification of high‐amplitude reflections, locally referred to as the T‐Reflection. Using a dense seismic grid, we have mapped the geometry of the T‐Reflection in order to compare it with filtered Bouguer gravity anomalies and seismic refraction data. The T‐Reflection is identified between 7 and 10 s. Sometimes it consists of one single smooth reflection. However, it is frequently associated with a set of rough multiple reflections displaying discontinuous segments with varying geometries, amplitudes, and contact relationships. The T‐Reflection seems to be connected to deep sill networks and is locally identified at the continuation of basement high structures or terminates over fractures and faults. The T‐Reflection presents a low magnetic signal. The spatial correlation between the filtered positive Bouguer gravity anomalies and the deep dome‐shaped reflections indicates that the latter represent a high‐impedance boundary contrast associated with a high‐density and high‐velocity body. In ~50% of the outer Vøring Margin, the depth of the mapped T‐Reflection is found to correspond to the depth of the top of the Lower Crustal Body (LCB), which is characterized by high P wave velocities (>7 km/s). We present a tectonic scenario, where a large part of the deep crustal structure is composed of preserved upper continental crustal blocks and middle to lower crustal lenses of inherited high‐grade metamorphic rocks. Deep intrusions into the faulted crustal blocks are responsible for the rough character of the T‐Reflection, whereas intrusions into the ductile lower crust and detachment faults are likely responsible for its smoother character. Deep magma intrusions can be responsible for regional metamorphic processes leading to an increasing velocity of the lower crust to more than 7 km/s. The result is a heterogeneous LCB that likely represents a complex mixture of pre‐ to syn‐breakup mafic and ultramafic rocks (cumulates and sills) and old metamorphic rocks such as granulites and eclogites. An increasing degree of melting toward the breakup axis is responsible for an increasing proportion of cumulates and sill intrusions in the lower crust.

Equatorial Kelvin waves generated in the western tropical Pacific Ocean trigger mass and heat transport within the Middle America Trench off Costa Rica

AbstractSequences of correlated seafloor temperature, current velocity, and acoustic backscatter events recorded at Ocean Drilling Program (ODP) sites at 4300 m depth in the Middle America Trench have been inferred to result from tidally induced turbidity currents generated in the vicinity of the 3300 m deep sill at the southern end of the trench. New data from the borehole observatories extend the temperature records to 11 years (November 2002 to December 2013) and confirm the highly episodic nature of the events. We present satellite altimetry data and ocean circulation model results to show that event timing is correlated with intraseasonal Kelvin wave motions in the equatorial Pacific. The observed temperature events had a mean (±1 standard deviation) occurrence interval of 61 (±24) days, which spans the periods of the first two baroclinic modes. Lag times between peak bottom water temperatures at the ODP sites and the passage of eastward‐propagating Kelvin wave crests at locations in the eastern equatorial Pacific are consistent with the time for mode‐1 waves to propagate to the southern end of the trench at a mean phase speed of 2.0 m s−1. Findings indicate that Kelvin wave currents augment tidal motions in the vicinity of the sill, triggering turbidity currents that travel northwestward along the trench axis at mean speeds of ∼0.1 m s−1. We conclude that mode‐1 (or, possibly, mixed mode‐1 and mode‐2) baroclinic Kelvin waves generated by large‐scale atmospheric processes in the western tropical Pacific lead to heat and mass transport deep within Middle America Trench in the eastern tropical Pacific.

Buoyancy‐driven coastal current blocks ventilation of an anoxic fjord on the Pacific coast of Canada

AbstractShallow sills restrict the ventilation of deep coastal fjords. Dense oceanic water seaward of the sill and lower density water within the receiving basin are generally required for oxygenated water to cross the sill and descend deep into the fjord. Here we use concurrent 10 year time series from current meters in the fjord and on the continental shelf to examine ventilation of the 120 m deep, anoxic inner basin of Effingham Inlet on the west coast of Vancouver Island. Whereas density currents traverse the 40 m deep sill and flow into the inner basin at mid‐depth at quasi‐fortnightly tidal intervals, only five current intrusions descended to the bottom of the basin over the decade‐long measurement period. The deep intrusions had a mean (±SD) return interval of 1.75 (±1.33) years and induced bottom‐water changes that persisted for 1–2 months. We show that, in addition to conditions within the inlet, deep ventilation of the inner basin is dependent on a coordinated sequence of external processes: (1) the onset of upwelling winds along the outer coast; (2) reversal of the buoyancy‐driven coastal current that normally flows poleward over the inner shelf off Vancouver Island; and (3) reversal of the estuarine circulation in the channel connecting the inlet to the ocean. As the observed ventilation intervals are short compared to the decadal intervals derived from the spacing of “homogenites” (sedimentation sequences disrupted by intruding bottom currents), the use of homogenites as proxies for past upwelling conditions in the northeast Pacific may need to be reexamined.

Seasonal hydrography and surface outflow in a fjord with a deep sill: the Reloncaví fjord, Chile

Abstract. Seasonal data on temperature, salinity, dissolved oxygen (DO) and chlorophyll, combined with meteorological and river discharge time series, were used to describe the oceanographic conditions of the Reloncaví fjord (41°35′ S, 72°20′ W). The winds in the fjord valley mainly blow down-fjord during the winter, reinforcing the upper layer outflow, whereas the winds blow predominantly up-fjord during the spring and summer, contrary to the upper layer outflow. The fjord, with a deep sill at the mouth, was well stratified year-round and featured a thin surface layer of brackish water with mean salinities between 10.4 ± 1.4 (spring) and 13.2 ± 2.5 (autumn). The depth of the upper layer changed slightly among the different studied seasons but remained at 4.5 m near the mouth. This upper layer presented a mean outflow (Q1) of 3185 ± 223 m3 s−1, which implies a flushing time of 3 days for this layer. The vertical salt flux was ∼ 37 tons of salt per second, similar to the horizontal salt flux observed in the upper layer. These estimates will contribute to better management of the aquaculture in this region.

Seismic imaging of deep crustal melt sills beneath Costa Rica suggests a method for the formation of the Archean continental crust

Continental crust formed billions of years ago but cannot be explained by a simple evolution of primary mantle magmas. A multi-step process is required that likely includes re-melting of wet metamorphosed basalt at high pressures. Such a process could occur at depth in oceanic crust that has been thickened by a large magmatic event. In Central America, variations in geologically inferred, pre-existing oceanic crustal thickness beneath the arc provides an excellent opportunity to study its effect on magma storage, re-melting of meta-basalts, and the potential for creating continental crust. We use surface waves derived from ambient noise tomography to image 6% radially anisotropic structures in the thickened oceanic plateau crust of Costa Rica that likely represent deep crustal melt sills. In Nicaragua, where the arc is forming on thinner oceanic crust, we do not image these deep crustal melt sills. The presence of these deep sills correlates with more felsic arc outputs from the Costa Rican Arc suggesting pre-existing thickened crust accelerates processing of primary basalts to continental compositions. In the Archean, reprocessing thickened oceanic crust by subsequent hydrated hotspot volcanism or subduction zone volcanism may have similarly enhanced formation of early continental crust. This mechanism may have been particularly important if subduction did not initiate until 3 Ga.

TURBULENT MIXING IN OMBAI STRAIT

Ombai Strait is one of the exit passages of Indonesian Throughflow (ITF) which conveys hotspot of strong internal tidal energy. Internal tide is the one of main energy which causes mixing processes in the oceans and could lead to changes in water mass characteristics. The purpose of this research was to estimate the turbulent mixing by using Thorpe analysis. Nine CTD cast were obtained for one tidal cycle (24 hours) in Ombai Strait. The results showed the average value of the turbulent mixing is 833.5 x 10-4 m2s-1, the highest found in deep homogeneous layer (2383.4x 10-4 m2s-1), followed by mixed surface layer (103.0 x 10-4 m2s-1) and thermocline (14.2 x 10-4 m2s-1). This Turbulent mixing value is much higher than the previous measurement in Indonesian Sea. This is presumably due to the strong internal tidal energy and its interaction with existing deep sill in Ombai Strait. Keywords: Indonesian throughflow (ITF), Ombai Strait, turbulent mixing

The Dynamics of Greenland's Glacial Fjords and Their Role in Climate.

Rapid mass loss from the Greenland Ice Sheet has sparked interest in its glacial fjords for two main reasons: Increased submarine melting of glaciers terminating in fjords is a plausible trigger for glacier retreat, and the anomalous freshwater discharged from Greenland is transformed by fjord processes before being released into the large-scale ocean. Knowledge of the fjords' dynamics is thus key to understanding ice sheet variability and its impact on climate. Although Greenland's fjords share some commonalities with other fjords, their deep sills and deeply grounded glaciers, the presence of Atlantic and Polar Waters on the continental shelves outside the fjords' mouths, and the seasonal discharge at depth of large amounts of surface melt make them unique systems that do not fit existing paradigms. Major gaps in understanding include the interaction of the buoyancy-driven circulation (forced by the glacier) and shelf-driven circulation, and the dynamics in the near-ice zone. These must be addressed before appropriate forcing conditions can be supplied to ice sheet and ocean/climate models.

State analysis using the Local Ensemble Transform Kalman Filter (LETKF) and the three-layer circulation structure of the Luzon Strait and the South China Sea

A new circulation model of the western North Pacific Ocean based on the parallelized version of the Princeton Ocean Model and incorporating the Local Ensemble Transform Kalman Filter (LETKF) data assimilation scheme has been developed. The new model assimilates satellite data and is tested for the period January 1 to April 3, 2012 initialized from a 24-year simulation to estimate the ocean state focusing in the South China Sea (SCS). Model results are compared against estimates based on the optimum interpolation (OI) assimilation scheme and are validated against independent Argo float and transport data to assess model skills. LETKF provides improved estimates of the western North Pacific Ocean state including transports through various straits in the SCS. In the Luzon Strait, the model confirms, for the first time, the three-layer transport structure previously deduced in the literature from sparse observations: westward in the upper and lower layers and eastward in the middle layer. This structure is shown to be robust, and the related dynamics are analyzed using the results of a long-term (18 years) unassimilated North Pacific Ocean model. Potential vorticity and mass conservations suggest a basin-wide cyclonic circulation in the upper layer of the SCS (z > −570 m), an anticyclonic circulation in the middle layer (−570 m ≥ z > −2,000 m), and, in the abyssal basin (<−2,000 m), the circulation is cyclonic in the north and anticyclonic in the south. The cyclone–anticyclone abyssal circulation is confirmed and explained using a deep-layer reduced-gravity model as being caused by overflow over the deep sill of the Luzon Strait, coupled with intense, localized upwelling west of the strait.