ALBERT

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution December 1996

Description: Two-thirds of the surface of the Earth is created at mid-ocean ridges where magmas rise from the mantle and cool to form the oceanic crust. The objective of this Thesis is to examine the influence of magma supply and eruptive processes on axial morphology, crustal construction, and the properties of crustal magma chambers at intermediate and fast spreading ridges. Variations in magma supply on time scales of ~100 Kyr generate along-axis changes in crustal thickness and temperature. Magma sill properties and hydrothermal activity are closely linked to spreading events which occur on much shorter time scales (ca. 10-100 yr) than the longer-term variations in magma supply reflected in along-axis changes in ridge morphology. The seismically constrained depths of ridge crest magma sills (〉1-2 km) are considerably deeper than the level of neutral buoyancy (100-400 m). The apparent inverse relationship between magma sill depth and spreading rate suggests that a thermally controlled permeability boundary, such as the solidus horizon, controls the depth at which magma ponds beneath mid-ocean ridges. Recent thermo-mechanical models predict that, at intermediate spreading rates, rift valley and magma sill formation are sensitive to small changes in crustal thickness and mantle temperature. Analysis of gravity at an intermediate spreading ridge shows that small differences in crustal thickness (300-700 m) and mantle temperature (10-15°C) are indeed sufficient to produce major changes in lithospheric strength and axial morphology. A stochastic model for the emplacement of dikes and lava flows with a bimodal distribution of lava flows is required to satisfy geological and geophysical constraints on the construction of the extrusive section. Most dikes are intruded within a narrow zone at the ridge axis. Short flows build up approximately half the extrusive volume. Occasional flows that pond at a considerable distance off-axis build up the remainder of the extrusive section. This Thesis underlines the importance of eruption dynamics in the emplacement of the uppermost volcanic layer of the crust and of the crustal thermal structure in controlling local variations in magma sill depth and ridge morphology.

Description: Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 26 (2012): GB4018, doi:10.1029/2011GB004192.

Description: A series of seasonally distributed measurements from the six largest Arctic rivers (the Ob', Yenisey, Lena, Kolyma, Yukon and Mackenzie) was used to examine the magnitude and significance of Arctic riverine DIC flux to larger scale C dynamics within the Arctic system. DIC concentration showed considerable, and synchronous, seasonal variation across these six large Arctic rivers, which have an estimated combined annual DIC flux of 30 Tg C yr−1. By examining the relationship between DIC flux and landscape variables known to regulate riverine DIC, we extrapolate to a DIC flux of 57 ± 9.9 Tg C yr−1for the full pan-arctic basin, and show that DIC export increases with runoff, the extent of carbonate rocks and glacial coverage, but decreases with permafrost extent. This pan-arctic riverine DIC estimate represents 13–15% of the total global DIC flux. The annual flux of selected ions (HCO3−, Na+, Ca2+, Mg2+, Sr2+, and Cl−) from the six largest Arctic rivers confirms that chemical weathering is dominated by inputs from carbonate rocks in the North American watersheds, but points to a more important role for silicate rocks in Siberian watersheds. In the coastal ocean, river water-induced decreases in aragonite saturation (i.e., an ocean acidification effect) appears to be much more pronounced in Siberia than in the North American Arctic, and stronger in the winter and spring than in the late summer. Accounting for seasonal variation in the flux of DIC and other major ions gives a much clearer understanding of the importance of riverine DIC within the broader pan-arctic C cycle.

Description: Funding for this work was provided through NSF-OPP-0229302 and NSF-OPP-0732985. Additional support to SET was provided by an NSERC Postdoctoral Fellowship.

Description: 2013-06-14

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2013

Description: Large-scale thermal forcing and freshwater fluxes play an essential role in setting temperature and salinity in the ocean. A number of recent estimates of the global oceanic freshwater balance as well as the global oceanic surface net heat flux are used to investigate the effects of heat- and freshwater forcing at the ocean surface. Such forcing induces changes in both density and density-compensated temperature and salinity changes (’spice’). The ratio of the relative contributions of haline and thermal forcing in the mixed layer is maintained by large-scale surface fluxes, leading to important consequences for mixing in the ocean interior. In a stratified ocean, mixing processes can be either along lines of constant density (isopycnal) or across those lines (diapycnal). The contribution of these processes to the total mixing rate in the ocean can be estimated from the large-scale forcing by evaluating the production of thermal variance, salinity variance and temperature-salinity covariance. Here, I use new estimates of surface fluxes to evaluate these terms and combine them to generate estimates of the production of density and spice variance under the assumption of a linear equation of state. As a consequence, it is possible to estimate the relative importance of isopycnal and diapycnal mixing in the ocean. While isopycnal and diapycnal processes occur on very different length scales, I find that the surface-driven production of density and spice variance requires an approximate equipartition between isopycnal and diapycnal mixing in the ocean interior. In addition, consideration of the full nonlinear equation of state reveals that surface fluxes require an apparent buoyancy gain (expansion) of the ocean, which allows an estimate of the amount of contraction on mixing due to cabbeling in the ocean interior.

Description: The author would like to acknowledge support from the National Aeronautics and Space Administration, grant #NNX12AF59G and the National Science Foundation, grant #OCE-0647949.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2013

Description: Physical oceanographers have known for several decades the total amount of abyssal mixing and upwelling required to balance the deep-water formation, but are still working to understand the mechanisms and locations—how and where it happens. From observational studies, we know that areas of rough topography are important and the hundreds of Grand-Canyon sized canyons that line mid-ocean ridges have particularly energetic mixing. To better understand the mechanisms by which rough topography translates into energetic currents and mixing, I studied diffusive boundary layers over varying topography using theoretical approaches and idealized numerical simulations using the ROMS model. In this dissertation, I show a variety of previously unidentified characteristics of diffusive boundary layers that are likely relevant for understanding the circulation of the abyssal ocean. These boundary layers share many important properties with observed flows in abyssal canyons, like increased kinetic energy near topographic sills and strong currents running from the abyssal plains up the slopes of the mid-ocean ridges toward their crests. They also have a previously unknown capacity to accelerate into overflows for a variety of oceanographically relevant shapes and sizes of topography. This acceleration happens without external forcing, meaning such overflows may be ubiquitous in the deep ocean. These boundary layers also can force exchange of large volumes of fluid between the relatively unstratified boundary layer and the stratified far-field fluid, altering the stratification far from the boundary. We see these effects in boundary layers in two– and three–dimensions, with and without rotation. In conclusion, these boundary layer processes, though previously neglected, may be a source of a dynamically important amount of abyssal upwelling, profoundly affecting predictions of the basin-scale circulation. This type of mechanism cannot be captured by the kind of mixing parameterizations used in current global climate models, based on a bottom roughness. Therefore, there is much work still to do to better understand how these boundary layers behave in more realistic contexts and how we might incorporate that understanding into climate models.

Description: I gratefully acknowledge the financial support of the NSF Graduate Research Fellowship Program and WHOI Academic Programs.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1996

Description: We analyze bathymetric and gravity anomalies at five plume-ridge systems to constrain crustal and mantle density structure at these prominent oceanic features. Numerical models are then used to explore the physical mechanisms controlling plume-ridge interaction and to place theoretical constraints on the temperature anomalies, dimensions, and fluxes of the Icelandic and Galapagos plumes. In Chapter 1 we analyze bathymetric and gravity anomalies along the hotspot-influenced Galapagos Spreading Center. We find that the Galapagos plume generates along-axis bathymetric and mantle-Bouguer gravity anomalies (MBA) that extend 〉500 km east and west of the Galapagos Islands. The along-axis MBA becomes increasingly negative towards the plume center, reaching a minimum of ~-90 mGal near 91°W, and axial topography shallows by ~1.1 km toward the plume. These variations in MBA and bathymetry are attributed to the combined effects of crustal thickening and anomalously low mantle densities, both of which are due to a mantle temperature anomaly imposed beneath the ridge by the Galapagos plume. Passive mantle flow models predict a temperature anomaly of 50±25°C is sufficient to produce the 2-4 km excess crust required to explain the along-axis anomalies. 70-75% of the along-axis bathymetric and MBA variations are estimated to arise from the crust with the remaining 25-30% generated by the anomalously hot, thus low-density mantle. Along Cocos-plate isochrons, bathymetric and MBA variations increase with increasing isochron age, suggesting the subaxial mantle temperature anomaly was greater in the past when the plume was closer, to the ridge axis. In addition to the Galapagos plume-ridge system, in Chapter 2 we examine alongisochron bathymetric and MBA variations at four other plume-ridge systems associated with the Iceland, Azores, Easter and Tristan hotspots. We show that residual bathymetry (up to 4.7 km) and mantle-Bouguer gravity anomalies (up to -340 mGal) are greatest at on-axis plumes and decreases with increasing ridge-hotspot separation distance, until becoming insignificant at a plume-ridge separation of ~500 km. Along-isochron widths of bathymetric anomalies (up to 2700 km) decrease with increasing paleo-spreading rate, reflecting the extent to which plume material flows along-axis before being swept away by the spreading lithosphere. Scaling arguments suggest an average ridgeward plume flux of -2.2x106 km/my. Assuming that the amplitudes of the MBA and bathymetric anomalies reflect crustal thickness and mantle density variations, passive mantle flow models predict maximum subaxial mantle temperature anomalies to be 150-225°C for ridge-center plumes, which decrease as the ridges migrate away from the plumes. The dynamics of mantle flow and melting at ridge-centered plumes are investigated in Chapters 3 using three-dimensional, variable-viscosity, numerical models. Three buoyancy sources are examined: temperature, melt depletion, and melt retention. The width W to which a plume spreads along a ridge axis depends on plume volume flux Q, full spreading rate U, buoyancy number B = (QΔρg)/(48η0U2), and ambient/plume viscosity contrast ϒ according to W=2.37(Q/U)l/2(Bϒ)0.04. Thermal buoyancy is first order in controlling along-axis plume spreading while latent heat loss due to melting, and depletion and retention buoyancy forces contribute second order effects. Two end-member models of the Iceland-Mid-Atlantic Ridge (MAR) system are examined. The first endmember model has a broad plume source of radius 300 km, temperature anomaly of 75°C, and volume flux of 1.2xl07 km3/my. The second model has a narrower plume source of radius 60 km, temperature anomaly of l70°C, and flux of 2.1 x106 km3/my. The first model predicts successfully the observed crustal thickness, topographic, and MBA variations along the MAR, but the second model requires substantial along-axis melt transport in order to explain the observed along-axis variations in crustal thickness, bathymetry, and gravity. We favor this second model because it predicts a mantle P-wave velocity reduction in the plume of ~2% as consistent with recent seismic observations beneath Iceland. Finally in Chapter 4 we use three-dimensional numerical models to investigate the interaction of plumes and migrating midocean ridges. Scaling laws of axial plume spreading width Ware derived first for stationary ridges and off-axis plumes, which yield results consistent with those obtained from independent studies of Ribe [1996]. Wand the maximum plume-ridge interaction distance Xmax again scale with (Q/U)l/2 as in the case of ridge-centered plumes and increase with ϒ and buoyancy number. In the case of a migrating ridge, Xmax is reduced when a ridge migrates toward the plume due to excess drag of the faster-moving leading plate, and enhanced when a ridge migrates away from the plume due to reduced drag of the slower-moving trailing plate. Thermal erosion of the lithospheric boundary layer by the previously ridge-centered plume further enhances Wand Xmax but to a degree that is secondary to the differential migration rates of the two plates. Model predictions are compared with observed along-isochron bathymetric and MBA variations at the Galapagos plume-ridge system. The anomaly amplitudes and widths, as well as the increase in anomaly amplitude with age are predicted with a plume source temperature anomaly of 80-120°C, radius of 80-100 km, and volume flux of 4.5x106 km3/m.y. Our numerical models also predict crustal production rates of the Galapagos Islands consistent with those estimated independently using the observed island topography. Predictions of the geochemical signature of the plume along the present-day ridge suggest that mixing between the plume and ambient mantle sources is unlikely to occur in the asthenosphere or shallow crust, but most likely deeper in the mantle possibly by entrainment of ambient mantle as the plume ascends through the depleted portion of the mantle from its deep source reservoir.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution March 1997

Description: The formation of new oceanic crust is the result of a complex geodynamic system in which mantle rises beneath spreading centers and undergoes decompression melting. The melt segregates from the matrix and is focused to the rise axis, where it is eventually intruded and/or erupted to form the oceanic crust. This thesis combines surface observations with laboratory studies and geodynamic modeling to study this crustal-production system. Quantitative modeling of the crustal and mantle contributions to the axial gravity and topography observed at the East Pacific Rise shows that the retained melt fraction in the mantle is small (〈3%) and is focused into a narrow column extending up to 70 km beneath the ridge axis. Consistent with geochemical constraints, the extraction of melt from the mantle therefore appears to be efficiently focus melt toward the ridge axis. A combination of laboratory and numerical studies are used to constrain the pattern of mantle flow beneath highly-segmented ridges. Even when the buoyant component of mantle flow is constrained to be two-dimensional, laboratory studies show that a segmented ridge will drive three-dimensional mantle upwelling. However, using reasonable mantle parameters in numerical models, it is difficult to induce large-amplitude three-dimensional mantle upwelling at the relatively short wavelengths of individual segments (~50 km). Instead, a simple model of three-dimensional melt migration shows that the observed segment-scale variations in crustal thickness can be explained by focusing of melt as it upwells through a more two-dimensional mantle flow field. At the Reykjanes Ridge, the melt appears to accumulate in small crustal magma chambers, before erupting in small batches to form numerous overlapping hummocky lava flows and small volcanoes. This suggests that crustal accretion, particularly at slow-spreading centers, may be a highly discontinuous process. Long-wavelength variations in crustal accretion may be dominated by variations in mantle upwelling while short-wavelength, segment-scale variations are more likely controlled by a complex three-dimensional processes of melt extraction and magma eruption.

Description: During my first three years in the Joint Program, I was supported by an National Science Foundation Graduate Student Fellowship. Other support has been derived from National Science Foundation grants OCE-9296017, OCE-9224738, OCE-9215544, and EAR grant 93-07400.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May 1996

Description: The water mass distribution in the southwestern Barents Sea, the thermohaline structure of the western Barents Sea Polar Front, and the formation of local water masses are described based on an analysis of historical hydrographic data and a recent process-oriented field experiment. This study concentrated on the frontal region between Bj0rn0ya and Hopen Island where Arctic water is found on the Spitzbergen Bank and Atlantic Water in the Bear Island Trough and Hopen Trench. Distributions of Atlantic, Arctic, and Polar Front waters are consistent with topographic control of Atlantic water circulation. Seasonal buoyancy forcing disrupts the topographic control in the surface layer, altering the frontal structure, and affecting local water mass formation. In the winter, the topographic control is firmly established and both sides of the front are vertically well-mixed. Winter cooling creates sea-ice over Spitzbergen Bank and convectively formed Modified Atlantic Water in the Bear Island Trough and Hopen Trench. In the summer, heating melts the sea-ice, producing a surface meltwater pool that can cross the polar front, disrupting topographic control and substantially increasing the vertical thermohaline gradients in the frontal region. The meltwater pool produces the largest geostrophic shear in the region.

Description: Support for this work was provided by a Department of Defense National Defense Science and Engineering Graduate Fellowship and Office of Naval Research grant N00014- 90-J-1359.

Description: Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 13 (2012): Q08014, doi:10.1029/2012GC004163.

Description: Mapping and sampling of 18 eruptive units in two study areas along the Galápagos Spreading Center (GSC) provide insight into how magma supply affects mid-ocean ridge (MOR) volcanic eruptions. The two study areas have similar spreading rates (53 versus 55 mm/yr), but differ by 30% in the time-averaged rate of magma supply (0.3 × 106 versus 0.4 × 106 m3/yr/km). Detailed geologic maps of each study area incorporate observations of flow contacts and sediment thickness, in addition to sample petrology, geomagnetic paleointensity, and inferences from high-resolution bathymetry data. At the lower-magma-supply study area, eruptions typically produce irregularly shaped clusters of pillow mounds with total eruptive volumes ranging from 0.09 to 1.3 km3. At the higher-magma-supply study area, lava morphologies characteristic of higher effusion rates are more common, eruptions typically occur along elongated fissures, and eruptive volumes are an order of magnitude smaller (0.002–0.13 km3). At this site, glass MgO contents (2.7–8.4 wt. %) and corresponding liquidus temperatures are lower on average, and more variable, than those at the lower-magma-supply study area (6.2–9.1 wt. % MgO). The differences in eruptive volume, lava temperature, morphology, and inferred eruption rates observed between the two areas along the GSC are similar to those that have previously been related to variable spreading rates on the global MOR system. Importantly, the documentation of multiple sequences of eruptions at each study area, representing hundreds to thousands of years, provides constraints on the variability in eruptive style at a given magma supply and spreading rate.

Description: This work was supported by the National Science Foundation grants OCE08–49813, OCE08–50052, and OCE08– 49711.

Description: 2013-02-25

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2012

Description: The structure of the oceanic lithosphere results from magmatic and extensional processes taking place at mid-ocean ridges (MORs). The temporal and spatial scales of the variability of these two processes control the degree of heterogeneity of the oceanic lithosphere, represented by two end-member models: the classical Penrose Model exemplified by layered magmatic crust formed along fast-spreading MORs, e.g., East Pacific Rise (EPR); and the recently defined Chapman Model describing heterogeneous mafic and ultramafic lithosphere formed in settings of oceanic detachment faulting common along slow-spreading MORs, e.g., Mid-Atlantic Ridge (MAR). This thesis is using advanced marine geophysical methods (including finite-difference wave propagation modeling, 3D multi-channel seismic reflection imaging, waveform inversion, streamer tomography, and near-bottom magnetics) to study lithospheric accretion processes in MORs characterized by contrasting tectono-magmatic settings: the magmatically dominated EPR axis between 9°30'-10°00’N, and the Kane Oceanic Core Complex (KOCC), a section of MAR lithosphere (23°20’-23°38’N) formed by detachment faulting. At the EPR study area, I found that the axial magma chamber (AMC) melt sill is segmented into four prominent 2-4-km-long sections spaced every ~5- 10 km along the ridge axis characterized by high melt content (〉95%). In contrast, within the intervening sections, the AMC sill has a lower melt content (41-46%). The total magma volume extracted from the AMC sill was estimated of ~46 × 106 m3, with ~24 × 106 m3 left unerupted in the upper crust as dikes after 2005-06 eruption. At the KOCC, I used streamer tomography to constrain the shallow seismic velocity structure. Lithological interpretation of the seismic tomographic models provides insights into the temporal and spatial evolution of the melt supply at the spreading axis as the KOCC formed and evolved. Investigation of a magnetic polarity reversal boundary in crosssection at the northern boundary of KOCC suggests that the boundary (representing both a frozen isotherm and an isochron) dips away from the ridge axis along the Kane transform fault scarp, with a west-dipping angle of ~45° in the shallow (〈1 km) crust and 〈20° in the deeper crust.

Description: This thesis was funded by National Science Foundation grants OCE-9987004, OCE- 0621660 and OCE-0327885, WHOI Academic Program Office and WHOI Deep Ocean Exploration Institute.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2012

Description: The purpose of this study is to understand the interactions of tropical cyclones with ocean eddies. In particular we examine the influence of a cold-core eddy on the cold wake formed during the passage of Typhoon Fanapi (2010). The three-dimensional version of the numerical Price–Weller–Pinkel (PWP) vertical mixing model has previously been used to simulate and study the cold wakes of Atlantic hurricanes. The model has not been used in comparison with observations of typhoons in the Western Pacific Ocean. In 2010 several typhoons were studied during the Impact of Typhoons on the Ocean in the Pacific (ITOP) field campaign and Fanapi was particularly well observed. We use these observations and the 3DPWP to understand the ocean cold wake generated by Fanapi. The cold wake of Fanapi was advected by a cyclonic eddy that was south of the typhoon track. The 3DPWP model outputs with and without an eddy are compared with observations made during the field campaign. These observations are compared to model outputs with eddies in a series of positions right and left of the storm track in order to study effects of mesoscale eddies on ocean vertical mixing in the cold wake of typhoons.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1994

Description: This work investigates whether large-scale coherent vortex structures driven by wave-current interaction (Langmuir circulation) are responsible for maintaining the oceanic mixed layer. Langmuir circulations dominate the near-surface vertical transport of momentum and density when the characteristic scale for forcing (defined as the Craik-Leibovich instability parameter γCLS) is stronger than the characteristic scale for diffusive decay γdiff. Since the wave-current forcing is concentrated near the surface both terms depend on the cell geometry. Cells with long wavelengths penetrate more deeply into the water column. These cells grow more slowly than the fastest growing mode for most cases, but always dominate the solution in the absence of Coriolis forces. In the presence of Coriolis forces, the horizontal wavelength and thus the depth of penetration are limited. When a cell geometry is found such that γCLS » γdiff, the current profile produced by small-scale diffusion is unstable to Langmuir cells and the cells replace small-scale diffusion as the dominant vertical transport mechanism for momentum and density. The perturbation crosscell shear is predicted to scale as γCLS. Such a scaling is observed during two field experiments. The observed velocity profile during these experiments is more sheared than predicted by a model which implicitly assumes instantaneous mixing by large eddies, but less sheared than predicted by a model which assumes small-scale mixing by near-isotropic turbulence. The latter profile is unstable to Langmuir cells when waves are present. The inclusion of cells driven by wave-current interaction explains the failure of the mixed layer to restratify on two days with high waves and low wind. Wave-current interaction introduces a small but efficient source of energy for transporting density which goes as the surface stress times the Stokes drift.

Description: The Office of Naval Research supported me throughout graduate school, first as an ONR Graduate Fellow. and later as a research assistant under the Surface Waves Processes Program (ONR Grant N00014-90-J-1495).

Description: Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 117 (2012): C06010, doi:10.1029/2011JC007652.

Description: We propose a conceptual model for an Arctic sea that is driven by river runoff, atmospheric fluxes, sea ice melt/growth, and winds. The model domain is divided into two areas, the interior and boundary regions, that are coupled through Ekman and eddy fluxes of buoyancy. The model is applied to Hudson and James Bays (HJB, a large inland basin in northeastern Canada) for the period 1979–2007. Several yearlong records from instruments moored within HJB show that the model results are consistent with the real system. The model notably reproduces the seasonal migration of the halocline, the baroclinic boundary current, spatial variability of freshwater content, and the fall maximum in freshwater export. The simulations clarify the important differences in the freshwater balance of the western and eastern sides of HJB. The significant role played by the boundary current in the freshwater budget of the system, and its sensitivity to the wind-forcing, are also highlighted by the simulations and new data analyses. We conclude that the model proposed is useful for the interpretation of observed data from Arctic seas and model outputs from more complex coupled/climate models.

Description: We thank NSERC and the Canada Research Chairs program for funding. FS acknowledges support from NSF OCE–0927797 and ONR N00014-08-10490.

Description: 2012-12-20

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1989

Description: Argon measurements, obtained from three years of monthly detailed vertical profiles near Bermuda (Station S, 32°N 64°W), show a maximum in argon supersaturation of about 4% in the seasonal thermocline in late summer. Since the argon supersaturation is 3-4 times smaller than that of oxygen, most of the oxygen supersaturation is not of physical origin and hence must result from biological production. In the winter mixed layer, air injection produces argon supersaturation despite high gas exchange rates. During spring and summer, radiative heating, air injection, and an upward argon flux create an even larger supersaturation in the mixed layer. In the seasonal thermocline, radiative heating creates argon supersaturations that persist in spite of vertical mixing. The observed seasonal cycles of temperature, argon, helium, and oxygen are simulated with an upper ocean model. I linearize the model's response to variations in vertical diffusivity, air injection, gas exchange rate, and new production and then use an inverse technique (singular value decomposition) to determine the values of these parameters that best fit the data. Results for the 1985-1987 average are as follows: A vertical turbulent diffusivity of 1.0 ± 0.1 x 10-4 m2 s-1 is consistent with both the thermal history and subsurface argon distribution. The rate of air injection, determined to ±15%, is similar to previous estimates. The seasonally-averaged gas exchange rate, determined to ± 11%, is consistent within errors with that predicted by Liss and Merlivat (1986). I estimate a lower limit to depth-integrated new production below the mixed layer of 5.0 ± 1.0 moles 0 2 m-2 yr-1 , and obtain an estimate of 6.2 ± 0.9 moles 0 2 m-2 yr-1 if new production in the mixed layer is fixed at zero. The period 1985-1987 appears to be typical of the climatological mean conditions at Station S and comparable to the 1960-1970 average period analyzed by Jenkins & Goldman (1985) and Musgrave et al. (1988). I propose that a mesoscale anticyclonic eddy is responsible for excess 3He and nitrate in the euphotic zone observed at a July, 1986 occupation of the Station S site. Hydrographic profiles are consistent with a type of eddy observed by Brundage & Dugan (1986), characterized by an unusually thick lens of subtropical mode (18°C) water. Analysis of the 35 year hydrographic record suggests that such eddies may arrive at Station S with an average frequency of 2-6 times per year, mostly during the summer and in years of vigorous 18°C water formation. Their timing and character suggest that they may be formed during winter convection events in the northeastern Sargasso Sea, advected southwestward by the gyre-scale circulation, and eventually absorbed by the Gulf Stream. Their magnitude and frequency indicate that they may supply a significant portion of the 3He and nutrient flux into the euphotic zone near Bermuda, and suggest a mechanism by which newly formed subtropical mode water is incorporated within the gyre interior. However, enhanced new production in such eddies could account for only a small portion of the new production integrated over the Sargasso Sea.

Description: This project has been supported by grant OCE85-01171 from the National Science Foundation.

Description: Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 26 (2012): GB0E02, doi:10.1029/2012GB004299.

Description: While much of the dissolved organic carbon (DOC) within rivers is destined for mineralization to CO2, a substantial fraction of riverine bicarbonate (HCO3−) flux represents a CO2 sink, as a result of weathering processes that sequester CO2 as HCO3−. We explored landscape-level controls on DOC and HCO3− flux in subcatchments of the boreal, with a specific focus on the effect of permafrost on riverine dissolved C flux. To do this, we undertook a multivariate analysis that partitioned the variance attributable to known, key regulators of dissolved C flux (runoff, lithology, and vegetation) prior to examining the effect of permafrost, using riverine biogeochemistry data from a suite of subcatchments drawn from the Mackenzie, Yukon, East, and West Siberian regions of the circumboreal. Across the diverse catchments that we study, controls on HCO3− flux were near-universal: runoff and an increased carbonate rock contribution to weathering (assessed as riverwater Ca:Na) increased HCO3− yields, while increasing permafrost extent was associated with decreases in HCO3−. In contrast, permafrost had contrasting and region-specific effects on DOC yield, even after the variation caused by other key drivers of its flux had been accounted for. We used ionic ratios and SO4 yields to calculate the potential range of CO2 sequestered via weathering across these boreal subcatchments, and show that decreasing permafrost extent is associated with increases in weathering-mediated CO2 fixation across broad spatial scales, an effect that could counterbalance some of the organic C mineralization that is predicted with declining permafrost.

Description: Funding for this work was provided through NSF-OPP-0229302 and NSF-OPP-0732985. Additional support to S.E.T. was provided by an NSERC Postdoctoral Fellowship.

Description: 2013-02-21

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution October 1993

Description: In this thesis, production of dense water that feeds the dense overflows across the Greenland-Scotland Ridge has been considered. A new circulation scheme is developed which is consistent with the water masses, currents and air-sea fluxes in the region, and with the important observation that the dense overflows show little or no seasonal or interannual variability. An inverse box model has been constructed that shows that the new circulation scheme is consistent with conservation statements for mass, heat and salt as well. According to the new circulation scheme the major buoyancy is lost in the North Atlantic Current, which enters the Norwegian Sea between Iceland and Scotland, and flows northward towards the Arctic Ocean and the Barents Sea. The transformation is due to a large net annual heat loss over the North Atlantic Current, combined with a long residence time (2-3 years) and a large surface area. After subduction, one branch of the North Atlantic Current enters the Arctic Ocean, is modified in hydrographic properties into those associated with the Denmark Strait Overflow Waters in the western North Atlantic, exits the Arctic Ocean in the western Fram Strait and flows with the East Greenland Current towards the Denmark Strait Another branch of the North Atlantic Current recirculates directly in the Fram Strait and flows towards the Denmark Strait with the East Greenland Current This branch will not sink to the bottom of the North Atlantic as it is less compressible than the Arctic branch. The third branch of the North Atlantic Current enters the Barents Sea, continues to lose buoyancy, and enters the Arctic Ocean at intermediate depth. This branch exits the Arctic Ocean in the western Fram Strait, circulates around the Greenland Sea, enters the Norwegian Sea, and flows towards the Frer¢-Shetland Channel. The traditional view holds that the major sources of the dense overflows are the Iceland and Greenland gyres, west of the North Atlantic Current. Aside from the finding that the new circulation scheme is more likely in terms of water mass properties, currents etc., one fundamental problem with the old scheme lies with supplying a substantial overflow. There are indications that the production of dense water in the gyres is sensitive to the highly variable surface conditions and that indeed the production tends to shut on and off. The reservoirs in the gyres are so small that they would be drained within a few years if they were to supply the overflows during a shutdown period. Production of dense water within the North Atlantic Current is less sensitive to surface conditions. The density in the gyres is gained at a temperature around freezing, whereas in the North Atlantic Current the density is gained well above freezing. Therefore a freshwater anomaly in the two domains will have different consequences for vertical · overturning: within the North Atlantic Current the freshening can be overcome by further cooling, whereas in the gyres freezing will occur and the vertical overturning will cease. The observed lack of a significant seasonal signal associated with the dense overflows is consistent with the new circulations scheme. The net annual cooling dominates the seasonal oscillation in the atmospheric heat loss for time scales comparable with the residence time of the Atlantic Water within the domain. Thus winter formation of dense water within the North Atlantic Current does not induce a seasonal signal in the transport field of the dense water.

Description: Funding for this work was partly provided by a NASA Global Change Fellowship.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution November 1992

Description: Cosmogeruc P-32 (14.28 days) and P-33 (25.3 days) are powerful tracers of upper ocean P cycling, when coupled with time-series of the atmospheric sources. A method was developed to determine the low-level beta activities in rainwater and plankton. The wet deposition rates of P-32 and P-33 were determined during 12 months at a marine site, at Bermuda, coinciding with measurements of the activities and activity ratio P-33/P-32 in suspended particles and plankton tows at BATS station. The in situ production rates of radiophosphorus in the upper ocean were estimated by measuring the activities induced in Cl, K and S targets by cosmic rays. Knowledge of all the sources of radiophosphorus to the Sargasso Sea allowed the cycling of P-32 and P-33 in suspended particles and macrozooplankton to be studied. The study was based on the determination of the activity ratio P-33/P-32 in different particulate pools. The activity ratio was higher in particle collections dominated by higher levels in the food web. The increase in the ratio in plankton relative to rain allowed the determination of the turnover times of P in plankton and in situ grazing rates.

Description: Funding for this research was provided by NSF (grants OCE-8800957. OCE- 8817836 and OCE-902284), DOE (grant DE-FG02-88ER60681), Woods Hole Oceanographic Institution, Ocean Venture Funding of Woods Hole Oceanographic Institution and Scurlock Funds of Mr Arch Scurlock to MIT/WHOI Joint Program.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1993

Description: A turbulent mixing experiment was conducted to observe the dynamics and the energetics of layer formation along with the region of layer formation in the Reynolds number (Re) and the overall Richardson number (Rio) space. A salt stratified fluid was mixed uniformly throughout its depth with a vertical rod that moved horizontally at a constant speed. The evolution of density was measured with a conductivity probe. As the instability theory of Phillips (1972) and Posmentier (1977) shows, an initially uniform density profile turns into a series of steps when Rio is larger than a critical value Ric, which forms a stability boundary. For fixed Re, as Rio decreases to Ric, the steps get weaker; the density difference across the interface and the difference of density gradient between layers and interfaces become small. Ric increases as Re increases with a functional relation log Ric ≈ Re/900. The steps evolve over time, with small steps forming first, and larger steps appearing later through merging and decay of the interfaces. After some time the interior seems to reach an equilibrium state and the evolution of the interior steps stops. The length scale of the equilibrium step, ls, is a linear function of U /Ni, where U is the speed of the rod and Ni is the buoyancy frequency of the initial profile. The functional relationship is ls = 2.6U / Ni + l.Ocm. For Rio 〈 Ric, the mixing efficiency, Rf, monotonically decreases to the end of a run. However, for Rio 〉 Ric, the evolution of Rf is closely related to the evolution of the density field. Rf changes rapidly during the initiation of the steps. For Rio » Ric, R1 increases initially, while for Rio ≥ Ric, Rf ecreases initially. When the interior reaches an equilibrium state, Rf becomes uniform. Posmentier (1977) theorized that when steps reach an equilibrium state, a density flux is independent of the density gradient. The present experiments show a uniform density flux in the layered interior irrespective of the density structure, and this strongly supports the theory of Posmentier. The density flux generated in the bottom boundary mixed layer goes through the interior all the way to the top boundary mixed layer without changing the interior density structure. Thus, turbulence can transport scalar properties further than the characteristic length scale of active eddies without changing a density structure. When the fluid becomes two mixed layers, the relation between Rf and Ril was found for Ril 〉 1. Here, Ril is the local Richardson number based on the thickness of the interface. R, does decrease as Ril increases, which is the most crucial assumption of the instability theory.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 1995

Description: Fluctuations in light intensity due to vertical mixing in the open ocean surface layer will affect phytoplankton physiology. Conversely, indicators of phytoplankton photoacclimation will be diagnostic of mixing processes if the appropriate kinetics are known. A combination of laboratory and field experimental work, field observations, and theoretical models were used to quantify the relationship between vertical mixing and photoacclimation in determining the time and space evolution of single cell optical properties for the photosynthetic picoplankton, Prochlorococcus spp. Diel time-series observations from the Sargasso Sea reveal patterns in single-cell fluorescence distributions within Prochlorococcus spp. populations which appear to correspond to decreasing mixing rates and photoacclimation during the day, and increased mixing at night. Reciprocal light shift experiments were used to quantify the photoacclimation kinetics for Prochlorococcus spp. fluorescence. A laboratory continuous culture system was developed which could simulate the effects of mixing across a light gradient at the level of the individual cell. This system was operated at four different simulated diffusivities. Prochlorococcus marinus strain Med4 fluorescence distributions show distinct patterns in the mean and higher moments which are consistent with a simple quasi-steady turbulent diffusionphotoacclimation model. In both, daytime photoacclimation drove the development of a gradient in mean fluorescence, a decrease in variance overall, and skewing of distributions away from the boundaries. These results suggest that picophytoplankton single-cell fluorescence distributions could prove to be a useful diagnostic indicator of the mixing environment.

Description: This project received primary financial support from the Office of Naval Research, with additional support from the National Science Foundation, the Environmental Protection Agency, Sea Grant, M.I.T. Sloan funds and M.I.T. Department of Civil and Environmental Engineering funds. I also wish to acknowledge support from a Rockwell Fellowship and a National Science Foundation Graduate Fellowship.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 1995

Description: Inverse modeling activities in oceanography have recently been intensified, aided by the oncoming observational data stream of WOCE and the advance of computer power. However, interpretations of inverse model results from climatological hydrographic data are far from simple. This thesis examines the behavior of an inverse model in the WOCE CME (Community Modeling Effort) results where the physics and the parameter values are known. The ultimate hypotheses to be tested are whether the inferred circulations from a climatological hydrographic data set (where limited time means and spatial smoothing are usually used) represent the climatological ocean general circulations, and what the inferred "diffusion" coefficients really are. The inverse model is first tested in a non-eddy resolving numerical GCM ocean. Numerical/scale analyses are used to test whether the inverse model properly represents the GCM ocean. Experiments show how biased answers could result from an incorrect model, and how a correct model must produce the right answers. When the inverse model is applied to the time-mean hydrographic data of an eddy-resolving GCM ocean in the fine grid resolution of the GCM, the estimated horizontal circulation is statistically consistent with the EGCM time means in both patterns and values. Although the flow patterns are similar, the uncertainties for the GCM time means and the inverse model estimates are different. The former are very large, such that the GCM time-mean circulation has no significance in the deep ocean. The latter are much smaller, and with them the estimated circulations are well defined. This is consistent with the concept that ocean motions are very energetic, while variations of tracers (temperature, salinity) are low frequency. The inverse model succeeded in extracting the ocean general circulation from the "climatological" hydrographic data. The estimated vertical velocities are also statistically indistinguishable from the GCM time means. However, significant differences between the estimated "diffusion" coefficients and the EGCM eddy diffusion coefficients are found at certain locations. These discrepancies are attributed to the differences in physics of the inverse model and the EGCM ocean. The "diffusion" coefficients from the inversion parameterize not only the eddy fluxes, but also (part of) the temporal variation and biharmonic terms which are not explicitly included in the inverse model. Given the essentially red spectrum of the ocean, it makes sense to look for smooth solutions. Aliasing due to subsampling on a coarse grid and the effects of spatial smoothing are addressed in the last part of this thesis. It is shown that this aliasing could be greatly reduced by spatial smoothing. The estimated horizontal circulation from the spatially smoothed time-mean EGCM hydrographic data with a coarse grid resolution (2.4° longitude by 2.0° latitude) is generally consistent with the spatially smoothed EGCM time means. Significant differences only occur at some grid points at great depths, where the GCM circulations are very weak. The conclusions of this study are different from some previous studies. These discrepancies are explained in the concluding chapter. Finally, it should be pointed out that the issue of properly representing a GCM ocean by an inverse model is not identical to the issue of represent ing the real ocean by the same inverse model, since the GCM ocean is not identical to the real ocean. Numerical calculations show that both the non-eddy resolving and the eddy-resolving GCM oceans used in this work are evolving towards a statistical equilibrium. In the real ocean, the importance of temporal variation terms in the property conservation equations should also be analyzed when a steady mverse model is applied to a limited time-mean (the climatological) data set.

Description: This research was carried out under National Science Foundation grant OCE- 90-04396.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1991

Description: Based on the Levitus atlas, we find that the application of the Montgomery streamfunction to the isopycnal surfaces induces an error which can not be ignored in some regions in the ocean. The error arises from the sloping effect of the specific volume anomaly along isopycnal surfaces. By including the major part of this effect, new streamfunctions, namely the pressure anomaly and main pressure streamfunctions, are suggested for the use in potential density coordinates. By using the newly proposed streamfunction and by including the variations of specific volume anomaly along isopycnal surfaces, the inverse model proposed by Hogg (1987) is modified for increasing accuracy and applied to the Brazil Basin to study the circulation, diffusion and water mass balances. The equations in the model, i.e. the dynamic equation, continuity equation, integrated vorticity equation, and conservation equations for heat, salt and oxygen (in which a consumption sink term is allowed), are written in centered finite difference form with lateral steps of 2 degree latitude and longitude and 8 levels in the vertical. This system of equations with constraints of positive diffusivities and oxygen consumption rates is solved by the inverse method. The results indicate that the circulation in the upper oceans is consistent with previous works, but that in the deep ocean is quite different. In the NADW region, we find a coincidence of the flows with the tongues of water properties. The diffusivities and diapycnal velocities seem stronger in the region near the equator than in the south, with reasonable values. Diffusion plays an important role in the water mass balance. Examples show that similar property fields may results from different processes.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1998

Description: This thesis presents an investigation of the influence of surface waves on momentum exchange. A quantitative comparison of direct covariance friction velocity measurements to bulk aerodynamic and inertial dissipation estimates indicates that both indirect methods systematically underestimate the momentum flux into developing seas. To account for wave-induced processes and yield improved flux estimates, modifications to the traditional flux parameterizations are explored. Modification to the bulk aerodynamic method involves incorporating sea state dependence into the roughness length calculation. For the inertial dissipation method, a new parameterization for the dimensionless dissipation rate is proposed. The modifications lead to improved momentum flux estimates for both methods.

Description: This project was funded by the Oceanographer of the Navy.

Description: Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 13 (2012): Q04007, doi:10.1029/2011GC003990.

Description: Here we demonstrate with a study of the Lucky Strike hydrothermal field that image mosaicing over large seafloor areas is feasible with new image processing techniques, and that repeated surveys allow temporal studies of active processes. Lucky Strike mosaics, generated from 〉56,000 images acquired in 1996, 2006, 2008 and 2009, reveal the distribution and types of diffuse outflow throughout the field, and their association with high-temperature vents. In detail, the zones of outflow are largely controlled by faults, and we suggest that the spatial clustering of active zones likely reflects the geometry of the underlying plumbing system. Imagery also provides constraints on temporal variability at two time-scales. First, based upon changes in individual outflow features identified in mosaics acquired in different years, we document a general decline of diffuse outflow throughout the vent field over time-scales up to 13 years. Second, the image mosaics reveal broad patches of seafloor that we interpret as fossil outflow zones, owing to their association with extinct chimneys and hydrothermal deposits. These areas encompass the entire region of present-day hydrothermal activity, suggesting that the plumbing system has persisted over long periods of time, loosely constrained to hundreds to thousands of years. The coupling of mosaic interpretation and available field measurements allow us to independently estimate the heat flux of the Lucky Strike system at ~200 to 1000 MW, with 75% to 〉90% of this flux taken up by diffuse hydrothermal outflow. Based on these heat flux estimates, we propose that the temporal decline of the system at short and long time scales may be explained by the progressive cooling of the AMC, without replenishment. The results at Lucky Strike demonstrate that repeated image surveys can be routinely performed to characterize and study the temporal variability of a broad range of vent sites hosting active processes (e.g., cold seeps, hydrothermal fields, gas outflows, etc.), allowing a better understanding of fluid flow dynamics from the sub-seafloor, and a quantification of fluxes.

Description: This project was funded by CNRS/IFREMER through the 2006, 2008, 2009 and 2010 cruises within the MoMAR program (France), by ANR (France) Mothseim Project NT05-3 42213 to J. Escartín, and by grant CTM2010-15216/MAR from the Spanish Ministry of Science to R. Garcia and J. Escartín. T. Barreyre was supported by University Paris Diderot (Paris 7– France) and Institut de Physique du Globe de Paris (IPGP, France). E. Mittelstaedt was supported by the International Research Fellowship Program of the U.S. National Science Foundation (OISE-0757920).

Description: 2012-10-19

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1995

Description: Mixing and stirring of passive tracer and Lagrangian particles in the open ocean was studied through comparison of observations from the North Atlantic Tracer Release Experiment, a numerical model, and existing theory. Based on the observed distribution of tracer during the first six months of the NATRE field experiment, Ledwell et al. (1993) estimated that on scales of 1 to 10 km small-scale diffusivity κs ≈ 3 m2s-1 and rms strain rate γ ≈ 3 X 10-7 s-1 . From the observed tracer distribution after one year, Ledwell (personal communication) further estimated that on scales greater than the mesoscale the effective eddy diffusivity κe ≈ 1 x 103 m2s-1. In the present study, statistics of the NATRE float data and numerical simulations of Lagrangian particles and passive tracer were used to determine the biases and uncertainties associated with these estimates. The numerical model was calibrated so that the statistics of model floats agreed as closely as possible with the NATRE floats. It is found that observations of a passive tracer such as were made during the NATRE experiment may be used to determine the rms streak width, δs, and the rms strain rate and hence to estimate the effective small-scale diffusivity. However, caution must be exercised when estimating κs from the theoretical balance, δs = square root κs/γ, as this may introduce a bias which would lead to the over-estimation of κs. Of particular relevance to NATRE is that observations of δs may be biased toward larger estimated rms streak width due to the inability of the observer to distinguish individual streaks from those which have resulted from a recent merger of streaks. Numerical experiments show that such a bias could lead to the over-estimation of κs by up to a factor of 2 to 4, suggesting that the estimate of κs made by Ledwell et al., (1993) from the NATRE tracer observations has an associated uncertainty of similar magnitude. Analysis of NATRE float data indicates that the estimate κe ≈ 1 x 103 m2s-1 inferred from the tracer distribution in Spring, 1993 and Fall, 1994 is accurate to within a factor of 2.

Description: Support for this work was provided by NSF award number 9005738.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1998

Description: A freshwater plume often forms when a river or an estuary discharges water onto the continental shelf. Freshwater plumes are ubiquitous features of the coastal ocean and usually leave a striking signature in the coastal hydrography. The present study combines both hydrographic data and idealized numerical simulations to examine how ambient currents and winds influence the transport and mixing of plume waters. The first portion of the thesis considers the alongshore transport of freshwater using idealized numerical simulations. In the absence of any ambient current, the downstream coastal current only carries a fraction of the discharged fresh water; the remaining fraction recirculates in a continually growing "bulge" of fresh water in the vicinity of the river mouth. The fraction of fresh water transported in the coastal current is dependent on the source conditions at the river mouth. The presence of an ambient current augments the transport in the plume so that its freshwater transport matches the freshwater source. For any ambient current in the same direction as the geostrophic coastal current, the plume will evolve to a steady-state width. A key result is that an external forcing agent is required in order for the entire freshwater volume discharged by a river to be transported as a coastal current. The next section of the thesis addresses the wind-induced advection of a river plume, using hydrographic data collected in the western Gulf of Maine. The observations suggest that the plume's cross-shore structure varies markedly as a function of fluctuations in alongshore wind forcing. Consistent with Ekman dynamics, upwelling favorable winds spread the plume offshore, at times widening it to over 50 km in offshore extent, while downwelling favorable winds narrow the plume width to a few Rossby radii. Near-surface current meters show significant correlations between cross-shore currents and alongshore wind stress, consistent with Ekman theory. Estimates of the terms in the alongshore momentum equation calculated from moored current meter arrays also indicate an approximate Ekman balance within the plume. A significant correlation between alongshore currents and alongshore wind stress suggests that interfacial drag may be important. The final section of the thesis is an investigation of the advection and mixing of a surface-trapped river plume in the presence of an upwelling favorable wind stress, using a three-dimensional model in a simple, rectangular domain. Model simulations demonstrate that the plume thins and is advected offshore by the crossshore Ekman transport. The thinned plume is susceptible to significant mixing due to the vertically sheared horizontal currents. The first order plume response is explained by Ekman dynamics and a Richardson number mixing criterion. Under a sustained wind event, the plume evolves to a quasi-steady, uniform thickness. The rate of mixing slowly decreases for longer times as the stratification in the plume weakens, but mixing persists under a sustained upwelling wind until the plume is destroyed. Mixing is most intense at the seaward plume front due to an Ekman straining mechanism in which the advection of cross-shore salinity gradients balances vertical mixing. The mean mixing rate observed in the plume is consistent with the mixing power law suggested by previous studies of I-D mixing, in spite of the two-dimensional dynamics driving the mixing in the plume.

Description: This research was funded by a National Science Foundation graduate fellowship, and Gulf of Maine Regional Marine Research Program grants UM-S227 and UM-S276.

Description: Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 12 (2011): Q05008, doi:10.1029/2010GC003439.

Description: Variations in topography and seismic structure are observed along the Juan de Fuca (JdF) Ridge axis and in the vicinity of pseudofaults on the ridge flanks left by former episodes of ridge propagation. Here we analyze gravity data coregistered with multichannel seismic data from the JdF Ridge and flanks in order to better understand the origin of crustal structure variations in this area. The data were collected along the ridge axis and along three ridge-perpendicular transects at the Endeavor, Northern Symmetric, and Cleft segments. Negative Mantle Bouguer anomalies of −21 to −28 mGal are observed at the axis of the three segments. Thicker crust at the Endeavor and Cleft segments is inferred from seismic data and can account for the small differences in axial gravity anomalies (3–7 mGal). Additional low densities/elevated temperatures within and/or below the axial crust are required to explain the remaining axial MBA low at all segments. Gravity models indicate that the region of low densities is wider beneath the Cleft segment. Gravity models for pseudofaults crossed along the three transects support the presence of thinner and denser crust within the pseudofault zones that we attribute to iron-enriched crust. On the young crust side of the pseudofaults, a 10–20 km wide zone of thicker crust is found. Reflection events interpreted as subcrustal sills underlie the zones of thicker crust and are the presumed source for the iron enrichment.

Description: This work was supported by the National Science Foundation grants OCE‐0648303 to Lamont‐Doherty Earth Observatory, OCE‐0648923 to Woods Hole Oceanographic Institution.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution January 1999

Description: Today, deep waters produced in the North Atlantic are exported through the western South Atlantic. Antarctic intermediate water AAJW also enters the Atlantic in this region. Circumpolar deep water (CDW) fills the depths below AAIW and above and below northern source waters. A depth transect of cores from 1567-3909 m water depth in the western South Atlantic are ideally located to monitor inter-ocean exchange of deep water, and variations in the relative strength of northern versus southern source water production. Last glacial maximum (LGM) Cd/Ca and δ13C data indicate a nutrient-depleted intermediate-depth water mass. In the mid-depth western South Atlantic, a simple conversion of LGM δ13C data suggests significantly less nutrient enrichment than LGM Cd/Ca ratios, but Cd/Ca and δ13C data can be reconciled when plotted in CdW/δ13C space. Paired LGM Cd/Ca and δ13C data from mid-depth cores suggest increasingly nutrient rich waters below 2000 m, but do not require an increase in Southern Ocean water contribution relative to today. Cd/Ca data suggest no glacial-interglacial change in the hydrography of the deepest waters ofthe region. To maintain relatively low Cd/Ca ratios low nutrients in the deepest western South Atlantic waters, and in CDW in general, during the LGM requires an increased supply ofnutrient-depleted glacial North Atlantic intermediate water (GNA1W) and/or nutrient-depleted glacial Subantarctic surface waters to CDW to balance reduced NADW contribution to CDW. LGM Cd/Ca and δ13C data suggest strong GNA1W influence in the western South Atlantic which in turn implies export of GNAIW from the Atlantic, and entrainment of GNA1W into the Antarctic Circumpolar current.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1998

Description: The evolution of a coastal ocean undergoing uniform surface heat loss is examined. The dynamics of this ocean are initially modulated by the intense vertical mixing driven by surface cooling. The strong vertical mixing prevents the formation of geostrophic flows and inhibits the cross-shelf flux of heat. The vertical mixing is eventually suppressed by the advective transport of cold, dense water offshore. Once this happens, alongshore geostrophic flows form, and become baroclinically unstable. The surface heat flux is then balanced by a cross-shelf eddy heat flux. Scales are found for the cross-shelf density gradient which results from this balance. Solutions for linear internal waves are found for a wedge-shaped bathymetry with bottom friction. Bottom friction is capable of entirely dissipating the waves before they reach the coast, and waves traveling obliquely offshore are reflected back to the coast from a caustic. The internal wave climate near two moorings of the Coastal Ocean Dynamics Experiment observation program is analyzed. The high frequency internal wave energy levels were elevated above the Garrett and Munk spectrum, and the spectrum becomes less red as one moves to the shore. The wave field is dominated by vertical-mode one waves, and internal wave energy propagates shoreward.

Description: This work was funded by an Office of Naval Research fellowship and and Office of Naval Research AASERT fellowship, N00014-95:-1-0746.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1998

Description: The upper ocean crust contains a comprehensive record of the shallow geological processes active along the world's mid-ocean ridge system. This thesis examines the magnetic and seismic structure of the upper crust at two contrasting ridges-the fast spreading East Pacific Rise (EPR) and the slow spreading Mid-Atlantic Ridge (MAR)-to build a more complete understanding about the roles of volcanic emplacement, tectonic disruption and hydrothermal alteration in the near-ridge environment. A technique that inverts potential field measurements. directly from an uneven observation track is developed and applied to near-bottom magnetic data from the spreading segments north of the Kane transform on the MAR. It is concluded that the central anomaly magnetization high marks the locus of focused volcanic emplacement. A cyclic faulting model is proposed to explain the oscillatory magnetization pattern associated with discrete blocks of crust being transported out of the rift valley between intensely altered fault zones. Seismic waveform and amplitude analyses of the magma sill along the EPR reveal it to be a thin (〈100 m) body of partial melt. These characteristics have important implications for melt availability and transport within the cycle of eruption and replenishment. A genetic algorithm-based seismic waveform inversion technique is developed and applied to on- and near-axis multichannel data from 17°20'S on the EPR and the spreading segment south of the Oceanographer transform (MAR) to map and compare for the first time the detailed velocity structure of the upper crust at two different spreading rates. Combined with conventionally processed seismic profiles, our results show that, while final extrusive thickness is comparable at all spreading ridges (300-500 m), the style of thickening may vary. While a thin (≤100 m) extrusive carapace quadruples in thickness within 1-4 km of the EPR crest, the extrusive section at the MAR achieves its final thickness within the inner valley. Both show evidence for a narrow zone of volcanic emplacement. Vigorous hydrothermalism at the EPR may produce a more rapid increase in basement velocities relative to the MAR. Rapid modification of the extrusive/dike transition at both ridges indicates that hydrothermalism is enhanced in this interval. Along-axis transport of lavas may thicken the extrusive pile at slow spreading segment ends, strengthening the magnetic highs generated by lava chemistry.

Description: ONR graduate fellow. The magnetics portion of this thesis was also supported by NSF grants OCE-9204141, OCE-9200905 (M. A. Tivey & H. Schouten) and NERC GR3/7702 (R. C. Searle). Research for Chapter 4 was partially funded by NSF grant OCE-9402933 (R. S. Detrick). The final two science chapters were supported by NSF grants OCE-9012707, OCE-9300450 (R. S. Detrick), OCE-9401717 (G. M. Kent & R. S. Detrick), OCE-9400623 (M. A. Tivey) and the Education office.

Description: Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 38 (2011): L14307, doi:10.1029/2011GL047798.

Description: The heat output and thermal regime of fast and intermediate spreading centers are strongly controlled by boundary layer processes between the hydrothermal system and the underlying crustal magma chamber (AMC), which remain to be fully understood. Here, we model the interactions between a shallow two-dimensional cellular hydrothermal system at temperatures 〈700°C, and a deeper AMC at temperatures up to 1200°C. We show that hydrothermal cooling can freeze the AMC in years to decades, unless melt injections occur on commensurate timescales. Moreover, the differential cooling between upflow and downflow zones can segment the AMC into mush and melt regions that alternate on sub-kilometric length scales. These predictions are consistent with along-axis variations in AMC roof depth observed in ophiolites and oceanic settings. In this respect, fine-scale geophysical investigations of the structure of AMCs may help constrain hydrothermal recharge locations associated with active hydrothermal sites.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1998

Description: Efforts to understand the Arctic system have recently focused on the role in local and global circulation of waters from the Arctic shelf seas. In this study, steady-state exchanges between the Arctic shelves and the central basins are estimated using an inverse box model. The model accounts for data uncertainty in the estimates, and quantifies the solution uncertainty. Other features include resolution of the two-basin Arctic hydrographic structure two-way shelf-basin exchange in the surface mixed layer, the capacity for shelfbreak upwelling, and recognition that most inflows enter the Arctic via the shelves. Aggregate estimates of all fluxes across the Arctic boundary, with their uncertainties, are generated from flux estimates published between 1975 and 1997. From the aggregate estimates, mass-, heat-, and salt-conserving boundary flux estimates are derived, which imply a net flux of water from the shelves to the basins of 1.2±0.4 Sv. Due primarily to boundary flux data uncertainty, constraints of mass, heat, and salt conservation alone cannot determine how much shelf-basin exchange occurs via dense overflows, and how much via the surface mixed layer. Adding δ180 constraints, however, greatly reduces the uncertainty. Dense water flux from the shelves to the basins is necessary for maintaining steady state, but shelfbreak upwelling is not required. Proper representation of external sources feeding the shelves, rather than the basins, is important to obtain the full range of plausible steady solutions. Implications of the results for the study of Arctic change are discussed.

Description: This work was supported by National Science Foundation grant OPP-9422292 as part of the Arctic System Science ARCSS program, administered by the Office of Polar Programs.

Description: Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 116 (2011): B10102, doi:10.1029/2011JB008259.

Description: Crustal thickness variations at the ultraslow spreading 10–16°E region of the Southwest Indian Ridge are used to constrain melt migration processes. In the study area, ridge morphology correlates with the obliquity of the ridge axis with respect to the spreading direction. A long oblique “supersegment”, nearly devoid of magmatism, is flanked at either end by robust magmatic centers (Joseph Mayes Seamount and Narrowgate segment) of much lesser obliquity. Plate-driven mantle flow and temperature structure are calculated in 3-D based on the observed ridge segmentation. Melt extraction is assumed to occur in three steps: (1) vertical migration out of the melting region, (2) focusing along an inclined permeability barrier, and (3) extraction when the melt enters a region shallower than ∼35 km within 5 km of the ridge axis. No crust is predicted in our model along the oblique supersegment. The formation of Joseph Mayes Seamount is consistent with an on-axis melt anomaly induced by the local orthogonal spreading. The crustal thickness anomaly at Narrowgate results from melt extracted at a tectonic damage zone as it travels along the axis toward regions of lesser obliquity. Orthogonal spreading enhances the Narrowgate crustal thickness anomaly but is not necessary for it. The lack of a residual mantle Bouguer gravity high along the oblique supersegment can be explained by deep serpentization of the upper mantle permissible by the thermal structure of this ridge segment. Buoyancy-driven upwelling and/or mantle heterogeneities are not required to explain the extreme focusing of melt in the study area.

Description: This work was supported by grants OCE‐ 0623188 and OCE‐0937277 from the National Science Foundation.

Description: Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 35 (2008): L03402, doi:10.1029/2007GL032837.

Description: Arctic rivers transport huge quantities of dissolved organic carbon (DOC) to the Arctic Ocean. The prevailing paradigm is that DOC in arctic rivers is refractory and therefore of little significance for the biogeochemistry of the Arctic Ocean. We show that there is substantial seasonal variability in the lability of DOC transported by Alaskan rivers to the Arctic Ocean: little DOC is lost during incubations of samples collected during summer, but substantial losses (20–40%) occur during incubations of samples collected during the spring freshet when the majority of the annual DOC flux occurs. We speculate that restricting sampling to summer may have biased past studies. If so, then fluvial inputs of DOC to the Arctic Ocean may have a much larger influence on coastal ocean biogeochemistry than previously realized, and reconsideration of the role of terrigenous DOC on carbon, microbial, and food-web dynamics on the arctic shelf will be warranted.

Description: This material is based on work supported by the National Science Foundation under grant numbers OPP-0436106, OPP- 0519840, and EAR-0403962, and is a contribution to the Study of Environmental Arctic Change (SEARCH).

Description: Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 35 (2008): L08606, doi:10.1029/2008GL033532.

Description: Turbulent-scale temperature and conductivity were measured during the pan-arctic Beringia 2005 Expedition. The rates of dissipation of thermal variance and diapycnal diffusivities are calculated along a section from Alaska to the North Pole, across deep flat basins (Canada and Makarov Basins) and steep ridges (Alpha-Mendeleev and Lomonosov Ridges). The mixing rates are observed to be small relative to lower latitudes but also remarkably non-uniform. Relatively elevated turbulence is found over deep topography, confirming the dominant role of bottom-generated internal waves. Measured patterns of mixing in the Arctic are also associated with other mechanisms, such as double-diffusive structures and deep overflows. A better knowledge of the distribution of mixing is essential to understand the dynamics of the changing Arctic environment.

Description: This work was funded by the National Science Foundation through a Small Grant for Exploratory Research (ARC-0527874) and grant ARC-0612342 with additional support from the Doherty Foundation and internal WHOI Funds.

Description: Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 112 (2007): C05011, doi:10.1029/2006JC003899.

Description: In September 2004 a detailed physical and chemical survey was conducted on an anticyclonic, cold-core eddy located seaward of the Chukchi Shelf in the western Arctic Ocean. The eddy had a diameter of ∼16 km and was centered at a depth of ∼160 m between the 1000 and 1500 m isobaths over the continental slope. The water in the core of the eddy (total volume of 25 km3) was of Pacific origin, and contained elevated concentrations of nutrients, organic carbon, and suspended particles. The feature, which likely formed from the boundary current along the edge of the Chukchi Shelf, provides a mechanism for transport of carbon, oxygen, and nutrients directly into the upper halocline of the Canada Basin. Nutrient concentrations in the eddy core were elevated compared to waters of similar density in the deep Canada Basin: silicate (+20 μmol L−1), nitrate (+5 μmol L−1), and phosphate (+0.4 μmol L−1). Organic carbon in the eddy core was also elevated: POC (+3.8 μmol L−1) and DOC (+11 μmol L−1). From these observations, the eddy contained 1.25 × 109 moles Si, 4.5 × 108 moles NO3 −, 5.5 × 107 moles PO3 −, 1.2 × 108 moles POC, and 1.9 × 109 moles DOC, all available for transport to the interior of the Canada Basin. This suggests that such eddies likely play a significant role in maintaining the nutrient maxima observed in the upper halocline. Assuming that shelf-to-basin eddy transport is the dominant renewal mechanism for waters of the upper halocline, remineralization of the excess organic carbon transported into the interior would consume 6.70 × 1010 moles of O2, or one half the total oxygen consumption anticipated arising from all export processes impacting the upper halocline.

Description: This work was supported by the National Science Foundation, and office of Naval Research; DH OPP-0124900, NB OPP-0124868, DK OPP 0124872, RP N00014-02-1-0317.

Description: Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 112 (2007): G04S60, doi:10.1029/2006JG000371.

Description: Export of nitrate and dissolved organic carbon (DOC) from the upper Kuparuk River between the late 1970s and early 2000s was evaluated using long-term ecological research (LTER) data in combination with solute flux and catchment hydrology models. The USGS Load Estimator (LOADEST) was used to calculate June–August export from 1978 forward. LOADEST was then coupled with a catchment-based land surface model (CLSM) to estimate total annual export from 1991 to 2001. Simulations using the LOADEST/CLSM combination indicate that annual nitrate export from the upper Kuparuk River increased by ~5 fold and annual DOC export decreased by about one half from 1991 to 2001. The decrease in DOC export was focused in May and was primarily attributed to a decrease in river discharge. In contrast, increased nitrate export was evident from May to September and was primarily attributed to increased nitrate concentrations. Increased nitrate concentrations are evident across a wide range of discharge conditions, indicating that higher values do not simply reflect lower discharge in recent years but a significant shift to higher concentration per unit discharge. Nitrate concentrations remained elevated after 2001. However, extraordinarily low discharge during June 2004 and June–August 2005 outweighed the influence of higher concentrations in determining export during these years. The mechanism responsible for the recent increase in nitrate concentrations is uncertain but may relate to changes in soils and vegetation associated with regional warming. While changes in nitrate and DOC export from arctic rivers reflect changes in terrestrial ecosystems, they also have significant implications for Arctic Ocean ecosystems.

Description: This work was supported by the Arctic System Science Program of the National Science Foundation (OPP- 0436118) and by NSF funding for the Arctic LTER through a series of grants from 1987 to present.

Description: Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 112 (2007): C04S06, doi:10.1029/2006JC003643.

Description: A three-dimensional coupled ocean/ice model, intended for long-term Arctic climate studies, is extended to include tidal effects. From saved output of an Arctic tides model, we introduce parameterizations for (1) enhanced ocean mixing associated with tides and (2) the role of tides fracturing and mobilizing sea ice. Results show tides enhancing loss of heat from Atlantic waters. The impact of tides on sea ice is more subtle as thinning due to enhanced ocean heat flux competes with net ice growth during rapid openings and closings of tidal leads. Present model results are compared with an ensemble of nine models under the Arctic Ocean Model Intercomparison Project (AOMIP). Among results from AOMIP is a tendency for models to accumulate excessive Arctic Ocean heat throughout the intercomparison period 1950 to 2000 which is contrary to observations. Tidally induced ventilation of ocean heat reduces this discrepancy.

Description: This research is supported by the National Science Foundation Office of Polar Programs under cooperative agreements OPP-0002239 and OPP-0327664 with the International Arctic Research Center, University of Alaska Fairbanks.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2001

Description: This thesis studies interactions between mid-ocean ridges and mantle plumes using geophysics, geochemistry, and geodynamical modeling. Chapter 1 investigates the effects of the Marion and Bouvet hotspots on the ultra-slow spreading, highly-segmented Southwest Indian Ridge (SWIR). Gravity data indicate that both Marion and Bouvet impart high-amplitude mantle Bouguer anomaly lows to the ridge axis, and suggest that long-offset transforms may diminish along-axis plume flow. Building upon this observation, Chapter 2 presents a series of 3D numerical models designed to quantify the sensitivity of along-axis plume-driven mantle flow to transform offset length, spreading rate, and mantle viscosity structure. The calculations illustrate that long-offset transforms in ultra-slow spreading environments may significantly curtail plume dispersion. Chapter 3 investigates helium isotope systematics along the western SWIR as well as near a global array of hotspots. The first part of this study reports uniformly low 3HetHe ratios of 6.3-7.3 RlRa along the SWIR from 9°-24°E, compared to values of 8±1 Ra for normal mid-ocean ridge basalt. The favored explanation for these low values is addition of (U+ Th) into the mantle source by crustal and/or lithospheric recycling. Although high HetHe values have been observed along the SWIR near Bouvet Island to the west, there is no evidence for elevated 3HetHe ratios along this section of the SWIR. The second part of Chapter 3 investigates the relationship between 3HetHe ratios and geophysical indicators of plume robustness for nine hotspots. A close correlation between a plume's flux and maximum 3HetHe ratio suggests a link between plume upwelling strength and origination in the deep, relatively undegassed mantle. Chapter 4 studies 3D mantle flow and temperature patterns beneath oceanic ridge-ridge-ridge triple junctions (TJs). In non-hotspot- affected TJs with geometry similar to the Rodrigues TJ, temperature and upwelling velocity along the slowest-spreading of the three ridges are predicted to increase within a few hundred kilometers of the TJ, to approach those of the fastest-spreading ridge. Along the slowest-spreading branch in hotspot-affected TJs such as the Azores, a strong component of along-axis flow directed away from the TJ is predicted to advect a hotspot thermal anomaly away from its deep-seated source.

Description: Funding was provided by the National Science Foundation through grants OCE- 9811924 and OCE-9907630, and by a National Defense Science and Engineering Graduate Fellowship.

Description: Author Posting. © American Geophysical Union 2003. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 4 (2003): 8515, doi:10.1029/2003GC000609.

Description: Complete multibeam bathymetric coverage of the western Galápagos Spreading Center (GSC) between 90.5°W and 98°W reveals the fine-scale morphology, segmentation and influence of the Galápagos hot spot on this intermediate spreading ridge. The western GSC comprises three morphologically defined provinces: A Western Province, located farthest from the Galápagos hot spot west of 95°30′W, is characterized by an axial deep, rift valley morphology with individual, overlapping, E-W striking segments separated by non-transform offsets; A Middle Province, between the propagating rift tips at 93°15′W and 95°30′W, with transitional axial morphology strikes ∼276°; An Eastern Province, closest to the Galápagos hot spot between the ∼90°50′W Galápagos Transform and 93°15′W, with an axial high morphology generally less than 1800 m deep, strikes ∼280°. At a finer scale, the axial region consists of 32 individual segments defined on the basis of smaller, mainly 〈2 km, offsets. These offsets mainly step left in the Western and Middle Provinces, and right in the Eastern Province. Glass compositions indicate that the GSC is segmented magmatically into 8 broad regions, with Mg # generally decreasing to the west within each region. Striking differences in bathymetric and lava fractionation patterns between the propagating rifts with tips at 93°15′W and 95°30′W reflect lower overall magma supply and larger offset distance at the latter. The structure of the Eastern Province is complicated by the intersection of a series of volcanic lineaments that appear to radiate away from a point located on the northern edge of the Galápagos platform, close to the southern limit of the Galápagos Fracture Zone. Where these lineaments intersect the GSC, the ridge axis is displaced to the south through a series of overlapping spreading centers (OSCs); abandoned OSC limbs lie even farther south. We propose that southward displacement of the axis is promoted during intermittent times of increased plume activity, when lithospheric zones of weakness become volcanically active. Following cessation of the increased plume activity, the axis straightens by decapitating southernmost OSC limbs during short-lived propagation events. This process contributes to the number of right stepping offsets in the Eastern Province.

Description: This work was supported by NSF grants OCE98- 18632 to the University of Hawai’i and OCE98-19117 to the Woods Hole Oceanographic Institution; support was provided to M. B. by a CIW/DTM Postdoctoral Fellowship

Description: Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 9 (2008): Q04015, doi:10.1029/2007GC001611.

Description: Near-bottom magnetic data collected along the crest of the East Pacific Rise between 9°55′ and 9°25′N identify the Central Anomaly Magnetization High (CAMH), a geomagnetic anomaly modulated by crustal accretionary processes over timescales of ∼104 years. A significant decrease in CAMH amplitude is observed along-axis from north to south, with the steepest gradient between 9°42′ and 9°36′N. The source of this variation is neither a systematic change in geochemistry nor varying paleointensity at the time of lava eruption. Instead, magnetic moment models show that it can be accounted for by an observed ∼50% decrease in seismic Layer 2A thickness along-axis. Layer 2A is assumed to be the extrusive volcanic layer, and we propose that this composes most of the magnetic source layer along the ridge axis. The 9°37′N overlapping spreading center (OSC) is located at the southern end of the steep CAMH gradient, and the 9°42′–9°36′N ridge segment is interpreted to be a transition zone in crustal accretion processes, with robust magmatism north of 9°42′N and relatively low magmatism at present south of 9°36′N. The 9°37′N OSC is also the only bathymetric discontinuity associated with a shift in the CAMH peak, which deviates ∼0.7 km to the west of the axial summit trough, indicating southward migration of the OSC. CAMH boundaries (defined from the maximum gradients) lie within or overlie the neovolcanic zone (NVZ) boundaries throughout our survey area, implying a systematic relationship between recent volcanic activity and CAMH source. Maximum flow distances and minimum lava dip angles are inferred on the basis of the lateral distance between the NVZ and CAMH boundaries. Lava dip angles average ∼14° toward the ridge axis, which agrees well with previous observations and offers a new method for estimating lava dip angles along fast spreading ridges where volcanic sequences are not exposed.

Description: The research project was funded by National Science Foundation under grants OCE-9819261 and OCE- 0096468.

Description: Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 8 (2007): Q08013, doi:10.1029/2007GC001652.

Description: We report first evidence for hydrothermal activity from the southern Knipovich Ridge, an ultra-slow spreading ridge segment in the Norwegian-Greenland Sea. Evidence comes from optical backscatter anomalies collected during a systematic side-scan sonar survey of the ridge axis, augmented by the identification of biogeochemical tracers in the overlying water column that are diagnostic of hydrothermal plume discharge (Mn, CH4, ATP). Analysis of coregistered geologic and oceanographic data reveals that the signals we have identified are consistent with a single high-temperature hydrothermal source, located distant from any of the axial volcanic centers that define second-order segmentation along this oblique ridge system. Rather, our data indicate a hydrothermal source associated with highly tectonized seafloor that may be indicative of serpentinizing ultramafic outcrops. Consistent with this hypothesis, the hydrothermal plume signals we have detected exhibit a high methane to manganese ratio of 2–3:1. This is higher than that typical of volcanically hosted vent sites and provides further evidence that the source of the plume signals reported here is most probably a high-temperature hydrothermal field that experiences some ultramafic influence (compare to Rainbow and Logachev sites, Mid-Atlantic Ridge). While such sites have previously been invoked to be common on the SW Indian Ridge, this may be the first such site to be located along the Arctic ultra-slow spreading ridge system.

Description: Connelly and German were funded by NERC grant NER/B/S/ 2000/00755, NERC Core Strategic Funding at NOC, and the ChEss project of the Census of Marine Life.

Description: Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 9 (2008): Q08O10, doi:10.1029/2008GC001965.

Description: We use 2-D numerical models to explore the thermal and mechanical effects of magma intrusion on fault initiation and growth at slow and intermediate spreading ridges. Magma intrusion is simulated by widening a vertical column of model elements located within the lithosphere at a rate equal to a fraction, M, of the total spreading rate (i.e., M = 1 for fully magmatic spreading). Heat is added in proportion to the rate of intrusion to simulate the thermal effects of magma crystallization and the injection of hot magma into the crust. We examine a range of intrusion rates and axial thermal structures by varying M, spreading rate, and the efficiency of crustal cooling by conduction and hydrothermal circulation. Fault development proceeds in a sequential manner, with deformation focused on a single active normal fault whose location alternates between the two sides of the ridge axis. Fault spacing and heave are primarily sensitive to M and secondarily sensitive to axial lithosphere thickness and the rate that the lithosphere thickens with distance from the axis. Contrary to what is often cited in the literature, but consistent with prior results of mechanical modeling, we find that thicker axial lithosphere tends to reduce fault spacing and heave. In addition, fault spacing and heave are predicted to increase with decreasing rates of off-axis lithospheric thickening. The combination of low M, particularly when M approaches 0.5, as well as a reduced rate of off-axis lithospheric thickening produces long-lived, large-offset faults, similar to oceanic core complexes. Such long-lived faults produce a highly asymmetric axial thermal structure, with thinner lithosphere on the side with the active fault. This across-axis variation in thermal structure may tend to stabilize the active fault for longer periods of time and could concentrate hydrothermal circulation in the footwall of oceanic core complexes.

Description: Funding for this research was provided by NSF grants OCE-0327018 (M.D.B.), OCE-0548672 (M.D.B.), OCE- 0327051 (G.I.), and OCE-03-51234 (G.I.).

Description: Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 113 (2008): G02026, doi:10.1029/2007JG000470.

Description: Permafrost is a defining characteristic of the Arctic environment. However, climate warming is thawing permafrost in many areas leading to failures in soil structure called thermokarst. An extensive survey of a 600 km2 area in and around the Toolik Lake Natural Research Area (TLNRA) revealed at least 34 thermokarst features, two thirds of which were new since ∼1980 when a high resolution aerial survey of the area was done. Most of these thermokarst features were associated with headwater streams or lakes. We have measured significantly increased sediment and nutrient loading from thermokarst features to streams in two well-studied locations near the TLNRA. One small thermokarst gully that formed in 2003 on the Toolik River in a 0.9 km2 subcatchment delivered more sediment to the river than is normally delivered in 18 years from 132 km2 in the adjacent upper Kuparuk River basin (a long-term monitoring reference site). Ammonium, nitrate, and phosphate concentrations downstream from a thermokarst feature on Imnavait Creek increased significantly compared to upstream reference concentrations and the increased concentrations persisted over the period of sampling (1999–2005). The downstream concentrations were similar to those we have used in a long-term experimental manipulation of the Kuparuk River and that have significantly altered the structure and function of that river. A subsampling of other thermokarst features from the extensive regional survey showed that concentrations of ammonium, nitrate, and phosphate were always higher downstream of the thermokarst features. Our previous research has shown that even minor increases in nutrient loading stimulate primary and secondary production. However, increased sediment loading could interfere with benthic communities and change the responses to increased nutrient delivery. Although the terrestrial area impacted by thermokarsts is limited, the aquatic habitat altered by these failures can be extensive. If warming in the Arctic foothills accelerates thermokarst formation, there may be substantial and wide-spread impacts on arctic stream ecosystems that are currently poorly understood.

Description: The results presented in this report are based upon work supported by the U.S. National Science Foundation under grants to the Arctic Hyporheic project (OPP- 0327440) and the Arctic Long-Term Ecological Research Program (DEB- 9810222).

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution January 1977

Description: Millimeter scale fluctuations in refractive index recorded with a freely sinking shadowgraph system are correlated with finestructure profiles of temperature, salinity and density and compared to models of ocean turbulence. Images with vertically aligned periodic structure, called bands, are identified as salt fingers, while others with chaotic structure are turbulent. Images are found on interfaces that are 1-10m thick and have gradients at least several times the mean. From 6 profiles in the Mediterranean Outflow region of the eastern North Atlantic between 1.0 and 1.9 km depth, 398 interfaces have been identified and a significant fraction (about 1/3) of these have detectable images. High contrast images, including bands, are most often found below warm, saline intrusions and within stepped structure where there is a regular sequence of homogeneous mixed layers separated by interfaces. As the interfacial salinity gradient increases in the sense that allows salt finger convection, the fraction of interfaces with images increases. The horizontal spacing of bands (~5 mm) is consistent with calculated salt finger diameter. The calculated and observed length of ocean salt fingers (10-20 cm) is a small fraction of the interface thickness. High levels of small scale variability in the shadowgraphs is reflected in high levels of variance in the finestructure band of the temperature spectra. The temperature gradient spectra have a slope of -1, indicative of turbulence affected by buoyancy forces, and there is a relative peak at a wavelength near the observed salt finger length. The high contrast images are found at interfaces within the enhanced mean salinity gradient below saline intrusions. For very strong salinity gradients there is a solitary interface with intense images, but for weaker mean gradients the convection takes the form of stepped structure. The steps may evolve from the solitary interface as the salinity gradient is run down by salt finger convection. This study identifies parts of the ocean where salt finger convection is prevalent and includes the first comprehensive description of salt fingers in the ocean. Existing models of salt fingers are evaluated in light of ocean observations, and models of ocean turbulence are compared to measurements.

Description: Support from ONR contract N00014-66-C0241 NR. 083-004 is also acknowledged.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1999

Description: The bottom boundary layer is an important dynamical region of shallow water flows. In this thesis, the problem of turbulent mixing in the coastal bottom boundary layer is investigated with a unique set of field measurements of velocity and sound speed that span a significant fraction of the boundary layer obtained over a six-week long period in the late summer of 1996 on the New England shelf. The energetics of the turbulent fluctuations are investigated by testing simplified budgets for turbulent kinetic energy and scalar variance. The turbulent kinetic energy budget is locally balanced while the scalar variance budget is not, probably due to turbulent diffusion. The direct effects of stratification are consistently significant only in the outer part of the boundary layer, where the flux Richardson number is approximately equal to a critical value of 0.2. Turbulence closure is investigated in terms of non-dimensional profiles of velocity and sound speed. Close to the bottom, the results are consistent with Monin-Obukhov similarity theory, while in the outer part of the boundary layer other scales including the height of the boundary layer are important for setting the turbulent length scale.

Description: My doctoral work was supported by the Office of Naval Research under grants N000149S10373 and N0001496109S3.

Description: Author Posting. © American Geophysical Union, 2010. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 115 (2010): C10018, doi:10.1029/2009JC005660.

Description: Variations in the Arctic central Canada Basin mixed layer properties are documented based on a subset of nearly 6500 temperature and salinity profiles acquired by Ice-Tethered Profilers during the period summer 2004 to summer 2009 and analyzed in conjunction with sea ice observations from ice mass balance buoys and atmosphere-ocean heat flux estimates. The July–August mean mixed layer depth based on the Ice-Tethered Profiler data averaged 16 m (an overestimate due to the Ice-Tethered Profiler sampling characteristics and present analysis procedures), while the average winter mixed layer depth was only 24 m, with individual observations rarely exceeding 40 m. Guidance interpreting the observations is provided by a 1-D ocean mixed layer model. The analysis focuses attention on the very strong density stratification at the base of the mixed layer in the Canada Basin that greatly impedes surface layer deepening and thus limits the flux of deep ocean heat to the surface that could influence sea ice growth/decay. The observations additionally suggest that efficient lateral mixed layer restratification processes are active in the Arctic, also impeding mixed layer deepening.

Description: Support for the ITP program and this study was provided by the U. S. National Science Foundation and the Woods Hole Oceanographic Institution. Support for the IMB program came from the National Science Foundation and the National Oceanographic and Atmospheric Administration.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2010

Description: This paper begins to explore a previously neglected mechanism for abyssal ocean mixing using bottom boundary layer dynamics. Abyssal mixing and the associated upward buoyancy fluxes are necessary to balance the sinking of dense waters at high latitudes and to close the global overturning circulation. Previous studies have concentrated on the hypothesis that the primary mechanism for this mixing is breaking internal waves generated by tidal flows over rough topography. However, intriguing observations, particularly from the Brazil Basin Tracer Release Experiment, suggest that mixing in the flank canyons of the Mid-Atlantic Ridge generated when strong mean flows interact with the many sills and constrictions within the canyons may represent a dynamically important amount of abyssal mixing. The energy pathways and mechanisms of this mixing are much less clear than in the case of breaking internal waves. This study attempts to clarify this by suggesting an analogy with an idealized diffusive boundary layer over a sloping bottom. This boundary layer is characterized by up-slope flows powered by the buoyancy flux in the fluid far from the boundary. Here we explore the energy budget of the boundary layer, and find that the diffusive boundary layer provides flows that are generally consistent with those observed in submarine canyons. In addition, we derive the vertical velocity in the far-field fluid, analogous to an Ekman pumping velocity, that these boundary layers can induce when the bottom slope is not constant. Finally, we present both theoretical and numerical models of exchange flows between the bottom boundary and the far-field flow when the bottom slope is not constant. These exchange flows provide a mechanism by which boundary-driven mixing can affect the overall stratification and buoyancy fluxes of the basin interior.

Description: I was supported by a NSF Graduate Research Fellowship.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2010

Description: Mid-ocean ridge basalts (MORBs) exhibit a wide range of CO2 concentrations, reflecting saturation to supersaturation (and rarely undersaturation) relative to their emplacement depths. In this study, we explore the mechanisms of CO2 degassing and the implications this has for estimating the advance rates and durations of seafloor eruptions. We present dissolved volatile concentrations (mainly of CO2 and H2O) and vesicle size distributions (VSDs) for a unique suite of MORB glasses collected at the East Pacific Rise, ~9° 50′ N. These MORB glasses were collected at ~200 m intervals along an across-axis track over a single flow pathway within the recently emplaced 2005-06 eruption boundaries; systematic sample collection provides one of the first opportunities to characterize intra-flow geochemical and physical evolution during a single eruption at a fast-spreading ridge. Compared to measurements of MORB volatiles globally, dissolved H2O concentrations are relatively uniform (0.10 - 0.16 weight percent), whereas dissolved CO2 contents exhibit a range of concentrations (154 - 278 ppm) and decrease with distance from the EPR axis (i.e., eruptive vent). Ion microprobe analyses of dissolved volatiles within the MORB glasses suggest that the magma erupted supersaturated (pressure equilibrium with 920 - 1224 mbsf) and in near-equilibrium with the melt lens of the axial magma chamber (~1250 - 1500 mbsf), and degassed to near equilibrium (299 - 447 mbsf) with seafloor depths over the length of the flow. The decrease in CO2 concentrations spans nearly the full range of dissolved CO2 contents observed at the EPR and shows that the varying degrees of volatile saturation that have been observed in other MORB sample suites may be explained by degassing during emplacement. Vesicularity (0.1 - 1.2%) increases with decreasing dissolved CO2 concentrations. We use vesicle size distributions (VSDs)—vesicle sizes and number densities—to quantify the physical evolution of the CO2 degassing process. VSDs suggest that diffusion of CO2 into preexisting vesicles, and not nucleation of new vesicles, is the dominant mechanism of increasing CO2 in the vapor phase. We also use VSDs, along with estimates of vesicle growth rates, to constrain emplacement time of the 2005-06 eruption to 〈~24 hours and to resolve variations in advance rate with downflow distance.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1987

Description: This thesis consists of two loosely related problems, both of which analyze some consequences of the failure of Sverdrup relation. In the first part, Chapters 2 and 3, the Sverdrup relation is invalidated because substantial flow is obtained at the bottom where topography exists. The eddies play an essential role in transfering momentum vertically from the surface, where the forcing is applied, to the bottom, which is otherwise unforced. If the topography has a structure in the longitudinal direction, then the inviscid theory predicts the occurence of strong jets in the interior of the model ocean. According to the structure of the topography these internal jets can occur in both vertically homeogenous and baroclinic oceans. If the topographic slope changes sign, then one kind of jets is observed both in stratified and in homogeneous oceans. This phenomenon is robust to moderate amounts of dissipation and is not disturbed by the occurrence of recirculating gyres within the basin. If the topographic slope is constant, then another kind of internal jets is observed, and it occurs in stratified models only. I was unable to observe this kind of jets in the presence of weak dissipation. The reason for this failure is twofold: on one hand friction, especially interfacial friction, tends to make the flow more barotropic (and we believe that indeed this is one of the processes that the eddies accomplish in a stratified fluid) and therefore the phenomena that rely strongly on baroclinicity are discouraged. On the other hand, reduction of the dissipation leads to the onset of a strong recirculating, inertial gyre which, although confined in space, affects the global properties of the flow. In the second part of the thesis (Chapters 4 and 5) I developed a simple model of the recirculating, inertial gyre. Again the dynamics of this feature are far from being in Sverdrup balance. In this case inertia is responsible for the failure of Sverdrup relation, together with the eddy field which provides a mean for transfering momentum vertically and laterally into regions away from where the forcing is applied. In this model there is no direct forcing in the recirculation region, and the input of momentum is confined to the boundary currents surrounding the gyre, for example the separated Gulf Stream. One of the results of the recirculation model is the prediction of its transport. It is shown that most of the transport is depth independent, i.e. it can be calculated without detailed knowledge of the density structure of the ocean. It is also shown that the "barotropic" part of the transport increases as the cube of the meridional extent of the gyre.

Description: The thesis work has been supported by a National Foundation grant from the Office of Atmospheric Sciences.

Description: Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 9 (2008): Q08001, doi:10.1029/2007GC001922.

Description: Multichannel seismic and bathymetric data from the Juan de Fuca Ridge (JDFR) provide constraints on axial and ridge flank structure for the past 4–8 Ma within three spreading corridors crossing Cleft, Northern Symmetric, and Endeavour segments. Along-axis data reveal south-to-north gradients in seafloor relief and presence and depth of the crustal magma lens, which indicate a warmer axial regime to the south, both on a regional scale and within individual segments. For young crust, cross-axis lines reveal differences between segments in Moho two-way traveltimes of 200–300 ms which indicate 0.5–1 km thicker crust at Endeavour and Cleft compared to Northern Symmetric. Moho traveltime anomalies extend beyond the 5–15 km wide axial high and coincide with distinct plateaus, 32 and 40 km wide and 200–400 m high, found at both segments. On older crust, Moho traveltimes are similar for all three segments (∼2100 ± 100 ms), indicating little difference in average crustal production prior to ∼0.6 and 0.7 Ma. The presence of broad axis-centered bathymetric plateau with thickened crust at Cleft and Endeavour segments is attributed to recent initiation of ridge axis-centered melt anomalies associated with the Cobb hot spot and the Heckle melt anomaly. Increased melt supply at Cleft segment upon initiation of Axial Volcano and southward propagation of Endeavour segment during the Brunhes point to rapid southward directed along-axis channeling of melt anomalies linked to these hot spots. Preferential southward flow of the Cobb and Heckle melt anomalies and the regional-scale south-to-north gradients in ridge structure along the JDFR may reflect influence of the northwesterly absolute motion of the ridge axis on subaxial melt distribution.

Description: This work was supported by U.S. National Science Foundation grants OCE00-02488 to S.M.C., OCE06-48303 to S.M.C. and M.R.N., OCE-0648923 to J.P.C., and OCE00-02600 to G.M.K. and A.J.H.

Description: Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 113 (2008): C02018, doi:10.1029/2007JC004429.

Description: Radioisotope evaluation of a cold-core, anticyclonic eddy surveyed in September 2004 on the Chukchi Sea continental slope was used to determine its age since formation over the shelf environment. Because the eddy can be shown to have been generated near the shelf break, initial conditions for several age-dependent tracers could be relatively well constrained. A combination of 228Ra/226Ra, excess 224Ra, and 228Th/228Ra suggested an age on the order of months. This age is consistent with the presence of elevated concentrations of nutrients, organic carbon, suspended particles, and shelf-derived neritic zooplankton within the eddy compared to ambient offshore water in the Canada Basin but comparable to values measured in the Chukchi shelf and shelf-break environment. Hence this feature, at the edge of the deep basin, was poised to deliver biogeochemically significant shelf material to the central Arctic Ocean.

Description: This work was supported by National Science Foundation Polar Programs grants OPP-662690 and OPP-66040N to the University of Miami (DK), and Office of Naval Research grant N00014-02-1-0317 (RP).

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 1987

Description: This thesis studies the role of cross-isopycnal mixing in general circulation dynamics, from both the theoretical and observational points of view. The first two chapters discuss some theoretical aspects of cross-isopycnal mixing in the oceans. In chapter one, an integral constraint relating the interior stratification and air-sea heat fluxes is derived, based on the condition that the total mass of water of given density is constant in a steady state ocean. Two simple models are then used to examine the way the numerically small mixing, together with air-sea fluxes, determines the average vertical density stratification of the oceans, and the deep buoyancy driven circulation. In chapter two, a more complete model of a deep flow driven by cross isopycnal diffusion is presented, motivated by the Mediterranean outflow into the North Atlantic. Mixing in this model is responsible for the determination of the detailed structure of the flow and density field, while in the models of the first chapter it was allowed to determine only the average vertical density stratification. In chapter three, a hydrographic data set from the Mediterranean sea is analyzed by inverse methods. The purpose is to examine the importance of mixing when trying to explain tracer distributions in the ocean. The time-mean circulation and the appropriate mixing coefficients are calculated from the hydrographic data. We conclude that the numerically small cross isopycnal mixing processes are crucial to the dynamics, yet difficult to parameterize and measure using available hydrographic data.

Description: NSF grants OCE-8521685 and OCE-8017791 supported me during my studies in the joint program.

Description: Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 112 (2007): G04S54, doi:10.1029/2006JG000353.

Description: Dramatic changes have been observed in the Arctic over the last century. Many of these involve the storage and cycling of fresh water. On land, precipitation and river discharge, lake abundance and size, glacier area and volume, soil moisture, and a variety of permafrost characteristics have changed. In the ocean, sea ice thickness and areal coverage have decreased and water mass circulation patterns have shifted, changing freshwater pathways and sea ice cover dynamics. Precipitation onto the ocean surface has also changed. Such changes are expected to continue, and perhaps accelerate, in the coming century, enhanced by complex feedbacks between the oceanic, atmospheric, and terrestrial freshwater systems. Change to the arctic freshwater system heralds changes for our global physical and ecological environment as well as human activities in the Arctic. In this paper we review observed changes in the arctic freshwater system over the last century in terrestrial, atmospheric, and oceanic systems.

Description: The authors gratefully acknowledge the National Science Foundation (NSF) for funding this synthesis work. This paper is principally the work of authors funded under the NSF-funded Freshwater Integration (FWI) study.

Description: Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 109 (2004): C03051, doi:10.1029/2003JC001940.

Description: Arctic Ocean model simulations have revealed that the Arctic Ocean has a basin-wide oscillation with cyclonic and anticyclonic circulation anomalies (Arctic Ocean Oscillation (AOO)) that has a prominent decadal variability [Proshutinsky and Johnson, 1997]. This study explores how the simulated AOO affects the Arctic Ocean stratification and its relationship to the sea ice cover variations. The simulation uses the Princeton Ocean Model coupled to sea ice [Häkkinen and Mellor, 1992; Häkkinen, 1999]. The surface forcing is based on National Centers for Environmental Prediction/National Center for Atmospheric Research Reanalysis and its climatology, of which the latter is used to force the model spin-up phase. Our focus is to investigate the competition between ocean dynamics and ice formation/melt on the Arctic basin-wide freshwater balance. We find that changes in the Atlantic water inflow can explain almost all of the simulated freshwater anomalies in the main Arctic basin. The Atlantic water inflow anomalies are an essential part of AOO, which is the wind driven barotropic response to the Arctic Oscillation (AO). The baroclinic response to AO, such as Ekman pumping in the Beaufort Gyre, and ice melt/freeze anomalies in response to AO are less significant considering the whole Arctic freshwater balance.

Description: We gratefully acknowledge the support from National Science Foundation under Grant No OPP-0230184 (AP) and from NASA Headquarters (SH).

Description: Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 109 (2004): C03042, doi:10.1029/2003JC002007.

Description: Sea level is a natural integral indicator of climate variability. It reflects changes in practically all dynamic and thermodynamic processes of terrestrial, oceanic, atmospheric, and cryospheric origin. The use of estimates of sea level rise as an indicator of climate change therefore incurs the difficulty that the inferred sea level change is the net result of many individual effects of environmental forcing. Since some of these effects may offset others, the cause of the sea level response to climate change remains somewhat uncertain. This paper is focused on an attempt to provide first-order answers to two questions, namely, what is the rate of sea level change in the Arctic Ocean, and furthermore, what is the role of each of the individual contributing factors to observed Arctic Ocean sea level change? In seeking answers to these questions we have discovered that during the period 1954–1989 the observed sea level over the Russian sector of the Arctic Ocean is rising at a rate of approximately 0.123 cm yr−1 and that after correction for the process of glacial isostatic adjustment this rate is approximately 0.185 cm yr−1. There are two major causes of this rise. The first is associated with the steric effect of ocean expansion. This effect is responsible for a contribution of approximately 0.064 cm yr−1 to the total rate of rise (35%). The second most important factor is related to the ongoing decrease of sea level atmospheric pressure over the Arctic Ocean, which contributes 0.056 cm yr−1, or approximately 30% of the net positive sea level trend. A third contribution to the sea level increase involves wind action and the increase of cyclonic winds over the Arctic Ocean, which leads to sea level rise at a rate of 0.018 cm yr−1 or approximately 10% of the total. The combined effect of the sea level rise due to an increase of river runoff and the sea level fall due to a negative trend in precipitation minus evaporation over the ocean is close to 0. For the Russian sector of the Arctic Ocean it therefore appears that approximately 25% of the trend of 0.185 cm yr−1, a contribution of 0.048 cm yr−1, may be due to the effect of increasing Arctic Ocean mass.

Description: This material is based upon work supported by the National Science Foundation under grant 0136432.

Description: Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 109 (2004): B10310, doi:10.1029/2004JB003066.

Description: Multichannel seismic reflection data are used to infer crustal accretion processes along the intermediate spreading Galapagos Spreading Center. East of 92.5°W, we image a magma lens beneath the ridge axis that is relatively shallow (1.0–2.5 km below the seafloor) and narrow (∼0.5–1.5 km, cross-axis width). We also image a thin seismic layer 2A (0.24–0.42 km) that thickens away from the ridge axis by as much as 150%. West of 92.7°W, the magma lens is deeper (2.5–4.5 km) and wider (0.7–2.4 km), and layer 2A is thicker (0.36–0.66 km) and thickens off axis by 〈40%. The positive correlation between layer 2A thickness and magma lens depth supports the interpretation of layer 2A as the extrusive volcanic layer with thickness controlled by the pressure on the magma lens and its ability to push magma to the surface. Our findings also suggest that narrower magma lenses focus diking close the ridge axis such that lava flowing away from the ridge axis will blanket older flows and thicken the extrusive crust off axis. Flow of lava away from the ridge axis is probably promoted by the slope of the axial bathymetric high, which is largest east of 92.5°W. West of ∼94°W the “transitional” axial morphology lacks a prominent bathymetric high and layer 2A no longer thickens off axis. We detect no magma lens west of 94.7°W where a small axial valley appears. The above changes can be linked to the westward decrease in the magma and heat flux associated with the fading influence of the Galapagos hot spot on the Galapagos Spreading Center.

Description: This project was funded by NSF-OCE- 0002189.