Dynamics of wind-driven estuarine-shelf exchange in the Narragansett Bay estuary

Physical exchange between estuarine and continental shelf waters impacts flushing dynamics of the estuary and determines rates of ocean inputs of nutrients and plankton. To investigate the occurrence and propagation of shelf water intrusions into the Narragansett Bay estuary, we collected velocity data near the estuarine–shelf interface during three summer periods. These data were compared to environmental forcing factors, including wind velocity, tidal mixing and river discharge. Results suggest a background cyclonic flow within the two passages of the estuary based on mean inflow in the channel on the eastern side of the estuary and mean outflow on the western shoals. Within the lower estuary this background circulation was perturbed by channel-parallel winds. On the shelf outside of the estuary, winds parallel to the coast were associated with cross-shelf flow of deep water. Strong pulses in estuarine– shelf exchange flow fell into two wind-driven modes: responses to direct forcing by downestuary wind or rebounds following relaxation or reversal of up-estuary wind. Rebound events were common, providing the most dramatic perturbations to the mean background estuarine circulation. A reduction in exchange between Narragansett Bay and shelf waters during prevailing up-estuary winds in the summertime and short-lived pulses in exchange flow under wind reversal events are expected to affect nutrient fluxes and dynamics of hypoxia in the estuary.Keywords: Estuarine circulation, Inner shelf circulation, Hypoxia, Wind-driven currents, Southern New England shelf, Partially mixed estuar

Archaea dominate oxic subseafloor communities over multimillion-year time scales

Ammonia-oxidizing archaea (AOA) dominate microbial communities throughout oxic subseafloor sediment deposited over millions of years in the North Atlantic Ocean. Rates of nitrification correlated with the abundance of these dominant AOA populations, whose metabolism is characterized by ammonia oxidation, mixotrophic utilization of organic nitrogen, deamination, and the energetically efficient chemolithoautotrophic hydroxypropionate/hydroxybutyrate carbon fixation cycle. These AOA thus have the potential to couple mixotrophic and chemolithoautotrophic metabolism via mixotrophic deamination of organic nitrogen, followed by oxidation of the regenerated ammonia for additional energy to fuel carbon fixation. This metabolic feature likely reduces energy loss and improves AOA fitness under energy-starved, oxic conditions, thereby allowing them to outcompete other taxa for millions of years

Mid-Cenozoic tectonic and paleoenvironmental setting of the central Arctic Ocean

Drilling results from the Integrated Ocean Drilling Program’s Arctic Coring Expedition (ACEX) to the Lomonosov Ridge (LR) document a 26 million year hiatus that separates freshwater-influenced biosilica-rich deposits of the middle Eocene from fossil-poor glaciomarine silty clays of the early Miocene. Detailed micropaleontological and sedimentological data from sediments surrounding this mid-Cenozoic hiatus describe a shallow water setting for the LR, a finding that conflicts with predrilling seismic predictions and an initial postcruise assessment of its subsidence history that assumed smooth thermally controlled subsidence following rifting. A review of Cenozoic tectonic processes affecting the geodynamic evolution of the central Arctic Ocean highlights a prolonged phase of basin-wide compression that ended in the early Miocene. The coincidence in timing between the end of compression and the start of rapid early Miocene subsidence provides a compelling link between these observations and similarly accounts for the shallow water setting that persisted more than 30 million years after rifting ended. However, for much of the late Paleogene and early Neogene, tectonic reconstructions of the Arctic Ocean describe a landlocked basin, adding additional uncertainty to reconstructions of paleodepth estimates as the magnitude of regional sea level variations remains unknown

Crustal structure of the propagating TAMMAR ridge segment on the Mid-Atlantic Ridge, 21.5°N

Active ridge propagation frequently occurs along spreading ridges and profoundly affects ridge crest segmentation over time. The mechanisms controlling ridge propagation, however, are poorly understood. At the slow spreading Mid-Atlantic Ridge at 21.5°N a seismic refraction and wide-angle reflection profile surveyed the crustal structure along a segment controlled by rapid ridge propagation. Tomographic traveltime inversion of seismic data suggests that the crustal structure along the ridge axis is controlled by melt supply; thus, crust is thickest, 8 km, at the domed segment center and decreases in thickness toward both segment ends. However, thicker crust is formed in the direction of ridge propagation, suggesting that melt is preferentially transferred toward the propagating ridge tip. Further, while seismic layer 2 remains constant along axis, seismic layer 3 shows profound changes in thickness, governing variations in total crustal thickness. This feature supports mantle upwelling at the segment center. Thus, fluid basaltic melt is redistributed easily laterally, while more viscose gabbroic melt tends to crystallize and accrete nearer to the locus of melt supply. The onset of propagation seems to have coincided with the formation of thicker crust, suggesting that propagation initiation might be due to changes in the melt supply. After a rapid initiation a continuous process of propagation was established. The propagation rate seems to be controlled by the amount of magma that reaches the segment ends. The strength of upwelling may govern the evolution of ridge segments and hence ultimately controls the propagation length

Mesoscale Convective System Surface Pressure Anomalies Responsible for Meteotsunamis Along the U.S. East Coast on June 13th, 2013

Two destructive high-frequency sea level oscillation events occurred on June 13th, 2013 along the U.S. East Coast. Seafloor processes can be dismissed as the sources, as no concurrent offshore earthquakes or landslides were detected. Here, we present evidence that these tsunami-like events were generated by atmospheric mesoscale convective systems (MCSs) propagating from inland to offshore. The USArray Transportable Array inland and NOAA tide gauges along the coast recorded the pressure anomalies associated with the MCSs. Once offshore, the pressure anomalies generated shallow water waves, which were amplified by the resonance between the water column and atmospheric forcing. Analysis of the tidal data reveals that these waves reflected off the continental shelf break and reached the coast, where bathymetry and coastal geometry contributed to their hazard potential. This study demonstrates that monitoring MCS pressure anomalies in the interior of the U.S. provides important observations for early warnings of MCS-generated tsunamis

Dynamics of wind-driven estuarine-shelf exchange in the Narragansett Bay estuary

Physical exchange between estuarine and continental shelf waters impacts flushing dynamics of the estuary and determines rates of ocean inputs of nutrients and plankton. To investigate the occurrence and propagation of shelf water intrusions into the Narragansett Bay estuary, we collected velocity data near the estuarine-shelf interface during three summer periods. These data were compared to environmental forcing factors, including wind velocity, tidal mixing and river discharge. Results suggest a background cyclonic flow within the two passages of the estuary based on mean inflow in the channel on the eastern side of the estuary and mean outflow on the western shoals. Within the lower estuary this background circulation was perturbed by channel-parallel winds. On the shelf outside of the estuary, winds parallel to the coast were associated with cross-shelf flow of deep water. Strong pulses in estuarine-shelf exchange flow fell into two wind-driven modes: responses to direct forcing by down-estuary wind or rebounds following relaxation or reversal of up-estuary wind. Rebound events were common, providing the most dramatic perturbations to the mean background estuarine circulation. A reduction in exchange between Narragansett Bay and shelf waters during prevailing up-estuary winds in the summertime and short-lived pulses in exchange flow under wind reversal events are expected to affect nutrient fluxes and dynamics of hypoxia in the estuary

Persistent organic matter in oxic subseafloor sediment

Nearly half of the global seafloor is overlain by sediment oxygenated to the basement. Yet, despite the availability of oxygen to fuel aerobic respiration, organic carbon persists over million-year timescales. Identifying the controls on organic carbon preservation requires an improved understanding of the composition and distribution of organic carbon within deep oligotrophic marine sediments. Here we show that organic carbon in sediment from the oligotrophic North Atlantic and South Pacific is low (\u3c0.1%), yet stable to depths of 25 m and ages of 24 million years. This organic carbon is not bound in biomass and has a low carbon/nitrogen ratio. X-ray imaging and spectroscopic analyses reveal that the chemical composition of this old, deep organic carbon is dominated (40–60%) by amide and carboxylic carbon with a proteinaceous nature. We posit that organic carbon persists in oxic oligotrophic sediment through a combination of protective processes that involve adsorption to mineral surfaces and physical inaccessibility to the heterotrophic community. We estimate that up to 1.6 × 1022 g of organic carbon are sequestered on million-year timescales in oxic pelagic sediment, which exceeds current estimates of the total global sediment organic carbon and constitutes an important, previously overlooked carbon reservoir