New England Seamounts Research Articles

Impact of the New England Seamount Chain on Gulf Stream Pathway and Variability

Abstract The potential role of the New England seamount chain (NESC) on the Gulf Stream pathway and variability has been long recognized, and the series of numerical experiments presented in this paper further emphasize the importance of properly resolving the NESC when modeling the Gulf Stream. The NESC has a strong impact on the Gulf Stream pathway and variability, as demonstrated by comparison experiments with and without the NESC. With the NESC removed from the model bathymetry, the Gulf Stream remains a stable coherent jet much farther east than in the experiment with the NESC. The NESC is the leading factor destabilizing the Gulf Stream and, when it is not properly resolved by the model’s grid, its impact on the Gulf Stream’s pathway and variability is surprisingly large. A high-resolution bathymetry, which better resolves the New England seamounts (i.e., narrower and rising higher in the water column), leads to a tighter Gulf Stream mean path that better agrees with the observed path and a sea surface height variability distribution that is in excellent agreement with the observations.

Spatial Variability of Movement, Structure, and Formation of Warm Core Rings in the Northwest Atlantic Slope Sea

AbstractGulf Stream Warm Core Rings (WCRs) have important influences on the New England Shelf and marine ecosystems. A 10‐year (2011–2020) WCR dataset that tracks weekly WCR locations and surface areas is used here to identify the rings' path and characterize their movement between 55 and 75°W. The WCR dataset reveals a very narrow band between 66 and 71°W along which rings travel almost due west along ∼39°N across isobaths – the “Ring Corridor.” Then, west of the corridor, the mean path turns southwestward, paralleling the shelfbreak. The average ring translation speed along the mean path is 5.9 cm s−1. Long‐lived rings (lifespan >150 days) tend to occupy the region west of the New England Seamount Chain (NESC) whereas short‐lived rings (lifespan <150 days) tend to be more broadly distributed. WCR vertical structures, analyzed using available Argo float profiles indicate that rings that are formed to the west of the NESC have shallower thermoclines than those formed to the east. This tendency may be due to different WCR formation processes that are observed to occur along different sections of the Gulf Stream. WCRs formed to the east of the NESC tend to form from a pinch‐off mechanism incorporating cores of Sargasso Sea water and a perimeter of Gulf Stream water. WCRs that form to the west of the NESC, form from a process called an aneurysm. WCRs formed through aneurysms comprise water mostly from the northern half of the Gulf Stream and are smaller than the classic pinch‐off rings.

Origin of widespread Cretaceous alkaline magmatism in the Central Atlantic: A single melting anomaly?

The age and origin of the Late Cretaceous magmatism on the North American and Iberian-African margins and the adjacent northern Central and Southern North Atlantic ocean are not well constrained due to the lack of appropriate data. To solve this issue, we used the 40Ar/39Ar geochronology and Sr-Nd-Pb isotopes geochemistry of basalts from the New England Seamounts (NES) and the J-Anomaly Ridge (JAR) as these localities on the North American margin are still poorly investigated. We obtained a reliable age of 82.39 ± 0.12 Ma (2σ) for the Nashville Seamount (NES) and an alteration age of ca. 76 Ma for the JAR. Our new dates from the New England Seamounts, combined with those available from the Tore–Madeira Rise and SW Portugal, on the Iberia–African margins, confirm an overlapping period of activity around 105-80 Ma on both the North American and Iberian-African margins and the adjacent oceanic basins. Plate kinematic reconstructions indicate that these magmatic occurrences were located within a ~1000 km radius within the yet narrow Atlantic Ocean. The J-Anomaly Ridge samples were most likely formed at the mid-Atlantic ridge around ~120 Ma.The Sr-Nd-Pb initial isotopic ratios from the New England seamounts show similarities with the chemical signature of the Tore–Madeira Rise and, to a lesser extent, SW Portugal. Moreover, New England Seamounts display a trend toward EMI isotopic end-member, similar to those documented in at the Late Cretaceous Godzilla seamount on the Tore–Madeira Rise and sills from ODP Site 1276. The shared chemical signature is distributed across a torus-shaped area of ~2000 × 2000 km at a near-fixed location on Earth and is not temporally-controlled, suggesting a large-scale chemical anomaly in the shallow mantle. Therefore, geochronological, geochemical and plate reconstructions imply a large-scale, anomalously fertile mantle source that generated widespread magmatism during the Late Cretaceous in the northern Central Atlantic.

The mean flow and variability of the Gulf stream-slopewater system from MICOM

The slopewater region is influenced by surface wind-driven, deep buoyancy-driven and shelf currents, whose complex interactions can affect both the northward heat transport and southward return flows. The mean flow and variability of the Gulf Stream-slopewater system are studied using four year outputs of Miami Isopycnic Coordinate Ocean Model (MICOM) realistic high-resolution simulation of the North Atlantic circulation. Special attention is focused on the eastward Slopewater Jet (SJ), a surface current characterized by a mean path coinciding with the strong outcropping temperature front in the slopewater. The water mass, path and transport of the SJ in MICOM are found to be in reasonable agreement with the existing observations. The modeled SJ is associated in part with a GS’ secondary branch, induced by a Taylor column effect of the New England Seamount Chain (NESC) on the upper GS. This upper-ocean-topographic coupling results in a spatial GS bifurcation, and advection of GS waters into the slopewater region shortly downstream of the NESC. An EOF analysis of the pycnocline depth confirms this tendency, as the first mode displays a qualitative dependence of the GS fan-shape streamline dispersion on the strength and intersecting latitude of the incident GS. Additionally, the model displays a strong influence of the Deep Western Boundary Current (DWBC) on the path of the SJ, by acting as a potential vorticity barrier. Important interactions between the two currents are suggested by the statistical EOF of the slopewater column, as in observations. Downstream of the NESC, the SJ transport variability is seasonal in MICOM, due to the north-south annual oscillation of the GS path and mergings with anticyclonic eddies. However, the variability of the SJ velocity profile is dominated (49% eigenvalue) by lateral translations of the current, at a 9-month timescale characteristic of GS meander-intensity variability. South of the Grand Banks, the transport variations of both the SJ and Labrador Current (LC) are captured by the first mode of the upper-slope water (37% of the variability), with predominant timescales corresponding to the upstream variability of the GS, and seasonality of the LC. Both modes are also in reasonable agreement with observations.

Determining the mean Gulf Stream and its recirculations through combining hydrographic and altimetric data

The altimetric data from the first 2.5‐year Geosat Exact Repeat Mission were used to estimate the mean sea surface height (SSH) field in the region of the Gulf Stream and its recirculation gyres. Assuming the instantaneous surface velocity field is composed of an eastward flowing jet and two westward recirculating flows, we used the time‐varying surface data from the altimeter to determine the shape of the along‐track mean SSH profiles and the historical hydrographic data to constrain the net SSH difference across the Gulf Stream system. The two‐dimensional mean SSH field was determined by objectively mapping the mean height profiles along the ascending and descending tracks. The SSH jump across the mean Gulf Stream has a maximum of 1.15m around 65°W and drops to an almost constant 0.9m downstream of the New England Seamount Chain (NESC). While the SSH jump associated with the mean northern recirculating flow is mostly uniform, we found that the Gulf Stream's southern recirculation has two local gyres that are separated by the NESC. An attempt was then made to estimate the mean deep circulation in this region by comparing the mean SSH field derived from the altimetry data and the surface dynamic height field based on the historical hydrographic data. Despite the large uncertainties, the mean deep flow pattern thus estimated agrees favorably with the overall circulation pattern from the long‐term current meter observations. Like the well‐defined northern recirculation gyre, we found that a continuous southward flow exists along 57.5°W, which follows closely along the deep layer potential vorticity contours. To the south of the Gulf Stream, the deep circulation consists of two separated recirculation gyres; the recirculation gyre to the east of the NESC appears to be trapped around the Corner Rise.

On the Interaction between the Gulf Stream and the New England Seamount Chain

Abstract In the course of numerical simulations with a primitive equation regional model of the Gulf Stream, bottom topography and the New England Seamount Chain (NESC) in particular show significant influence on the variability and the energetics of the Gulf Stream system. The model is an eddy-resolving, coastal ocean model that includes thermohaline dynamics and a second-order turbulence closure scheme to provide vertical mixing coefficients; it is driven at the surface by observed monthly wind stress and heat fluxes. The surface and the deep variabilities obtained from the numerical simulations are in fair agreement with the observed variabilities inferred, for example, from the Geosat altimetry data and from measurements of eddy kinetic energy (EKE). To study how the NESC affects the Gulf Stream dynamics, a control run without the NESC (however, leaving the other topographic features such as the continental shelf and slope intact) is compared to simulation with full bottom topography. According to the...

Sea surface topographic variability near the New England Seamounts: An intercomparison among in situ observations, numerical simulations, and Geosat altimetry from the regional energetics experiment

Intercomparisons are made among three sets of results for two regions of the Gulf Stream, upstream and downstream of the New England Seamount Chain (NESC): observational records from 12 inverted echo sounders with pressure gauges (IES/PGs), concurrent Geosat altimeter crossover point differences, and results from a primitive equation model of the region. Standard deviations of sea surface topography show generally good agreement for the three data sources. No significant difference between the upstream and downstream regions is observed in the actual data. IES/PG records and Geosat topographic variability have values of about 30 cm on both sides of the NESC, although model results show a discrepancy in the eastern region. Barotropic fluctuations account for about 30% of the total surface topographic variability and are only partially correlated with baroclinic fluctuations.

Geochemical evolution of the New England seamount chain: Isotopic and trace-element constraints

Samples collected by dredge hauls and submersible dives from the New England Seamount (NES) chain, the most prominent of six seamount groups in the Northwest Atlantic Ocean, have been analyzed for Sr, Nd and Pb isotopic compositions and rare-earth and other trace-element contents. Unleached and HCl-leached whole-rock samples, and hornblende and clinopyroxene separates were analyzed in order to obtain isotopic data from the old and altered NES samples. Rare-earth and other trace-element contents were measured on unleached whole-rock samples. The results demonstrate that rimary (unaltered) geochemical data can be derived from older altered oceanic volcanics. The NES have isotopic and trace-element characteristics typical of alkaline volcanics from ocean islands, in particular those characterized by radiogenic Pb isotopic signatures (NES 206 Pb 204 Pb values range up to 20.65). The seamounts exhibit considerable interseamount variation, while intraseamount chemical heterogeneity appears to be substantially less. An increase in radiogenic Pb along the chain towards the southeast is suggested. No other simple geographic trends in chemistry are apparent. The isotopic characteristics of the NES necessitate at least three distinct source components, with substantial involvement of a “radiogenic Pb” type mantle. The isotopic similarities between the NES, Canaries, Azores and Ahaggar imply some type of systematic geographic distribution of this “radiogenic Pb” component.