A determination of the seismic velocity structure of sediments using both sources and receiver near the ocean floor

A trial experiment proves the power and practicality of using both sources and receivers near the ocean floor to make precise measurements of deep (∼6000 m) ocean sediment velocity structure. A digitally recording ocean bottom hydrophone receiver operating at a sampling rate of 1800 Hz recorded clear arrivals with bubble pulse frequencies of ∼500 Hz from 41b. explosive charges detonated at depths of 5500m along a 4 km long wide angle reflection profile. It is shown that corrections for changes in source depth may be computed without approximation and without prior knowledge of the velocity structure. The experiment was located at longitude 56° W in the trough of the Kane Fracture Zone. The velocity structure of the 1 km thick sedimentary section reveals a 310 m thickness of 3 km s⁻¹ material overlying igneous basement.     

Using inverse theory and seabeam bathymetry to improve seismic refraction data corrections

An array consisting of ocean bottom seismometer and on-bottom hydrophones, was used to conduct a seismic experiment on 0.4 Ma crust east of the Juan de Fuca Ridge. Seismic sources were large (>50 kg) explosive charges detonated by SUS devices set to explode at 1829 or 2438 m nominal depth. The objectives of the experiment were to determine the compressional wave velocity and attenuation structures of the uppermost 500 m depth. The relative positions of shots and receivers were originally determined by treating each shot-receiver pair independently, via raytracing of various water waves. Due to the reflection of some of these water waves by the rough bottom, significant scatter resulted, preventing a determination of a physically realizable velocity-depth function. A new method is described that co-locates shot and receiver positions, including receiver depths consistent withseabeam bathymetry, using only the water waves that do not interact with the bottom. Several potential pitfalls are outlined using this method. A stable solution could only be achieved by discarding shots located well outside the array. The water path corrections were applied to the refracted arrivals, again using theseabeam bathymetry. The joint inversion location procedure, along with the use of precise gridded bathymetry, reduced the travel time scatter to a level whereby a velocity-depth function could be determined. The results, using only the hydrophone data, indicate an initial velocity at the seafloor of 2.7 km s^-1 with gradients from 4.6 s^-1 slowly decreasing to 4.1 s^-1 at 679 m depth. This velocity is similar to others conducted over very young oceanic crust, and can be interpreted as being due to a high porosity at the surface, due to cracks, fissures, and open pores, which rapidly diminish with depth.     

Crustal structure of the South Florida Platform,eastern Gulf of Mexico: An ocean-bottom seismograph refraction study

Five seismic refraction lines, 70–90 km long, were shot in the South Florida Platform region of the Gulf of Mexico using digital ocean-bottom seismographs. Apparent velocities and depths were calculated from the refracted arrivals using a flat-layer model for the region. The two dominant refractors have apparent compressional-wave velocity ranges of 5.6 to 5.9 km s^–1 and 6.2 to 6.7 km s^–1. On the Sarasota Arch, the depth to the top of a 5.8–5.9km/s layer is 3–4 km below sea level. This depth corresponds to the depth to the crystalline basement. The basement dips to the north and to the south from the arch, with velocity of the upper crust increasing from 5.8–5.9 km s^–1 to a maximum of 6.7 km s^–1 at a depth of 6.3 km. Under the continental slope, the crust has presumably been thinned and extended. The deepest refractor has an apparent velocity of about 7.5 km s^–1 at a depth of 25 km. The thickness of the crustal section and the absence of any mantle arrivals in these long refraction profiles on the platform suggest that thick continental crust underlies the South Florida Platform. A north-south cross-section through the platform suggests the presence of two structural highs separated by a portion of the South Florida Basin, which contains at least 5 km of sediment.     

Tropospheric lapse rate and its relation to surface temperature from reanalysis data

Estimates of the tropospheric lapse rate γ and analysis of its relation to the surface temperature T _s in the annual cycle and interannual variability have been made using the global monthly mean data of the NCEP/NCAR reanalysis (1948–2001). The tropospheric lapse rate γ is about 6.1 K/km in the Northern Hemisphere (NH) as a whole and over the ocean and about 6.2 K/km over the continents. The value of γ decreases from 6.5 K/km at low latitudes to 4.5 K/km at polar latitudes. The values of dγ/dT _s, the parameter of sensitivity of γ to the variation of T _s for the NH in the interannual variability, are found to be about 0.04 km^?1 (0.041 km^?1 for the NH as a whole, 0.042 km^?1 over the ocean, and 0.038 km^?1 over the continents). This corresponds to an increase in γ of approximately 0.7% when the surface temperature of the NH is increased by 1 K. Estimates of dγ/dT _s vary from about 0.05 km^?1 in the subtropics to 0.10 km^?1 at polar latitudes. When dγ/dT _s is positive, the surface and tropospheric warming means a temperature decrease above a certain critical level H _cr. The height of the level H _cr with constant temperature, which is defined by the inverse value (dγ/dT _s)^?1, is about 25 km for the NH as a whole, i.e., above the tropopause. In the subtropics, H _cr is about 20 km. At polar latitudes, H _cr decreases to about 10 km. Positive values of dγ/dT _s characterize a positive climatic feedback through the lapse rate and indicate a general decrease in the static stability of the troposphere during global warming. Along with a general tendency of γ to increase with rising T _s, there are regional regimes with the opposite tendency, mainly over the ocean. The negative correlation of γ with T _s is found over the oceanic tropics and midlatitudes, in particular, over the oceanic belt around Antarctica.     

On the accuracy of upper ocean particulate organic carbon export fluxes estimated from 234Th/238U disequilibrium

An analysis of the ²³⁴Th method for determining the export flux of particulate organic carbon (P_POC) from the upper ocean using in situ pumps or water bottles shows that the accuracy of the method (the ratio of the experimental value of P_POC divided by the true POC flux, F_POC), defined as the p-ratio, is equal to the mean settling velocity of particulate ²³⁴Th divided by that of POC. Therefore, P_POC is equivalent to the true POC flux (F_POC) and the p-ratio is equal to unity if, and only if, POC and particulate ²³⁴Th have the same mean sinking velocities. A simple particle settling model is discussed that invokes Stokes’ Law settling velocities, volume:surface area (V:SA) fractionation of C:²³⁴Th and two assemblages of particles having different sizes and densities. The model is used to illustrate the ranges of parameter values that conform to values of the p-ratio sufficiently close to unity that the experimentally determined POC flux can be considered to be an accurate representation of the true POC flux.Despite the over-simplification of real systems implicit in the model, the results suggest that p-ratios<1, equivalent to an under-estimate of the POC flux, are representative of single particle settling regimes in which the larger particles dominate the vertical flux. This follows from the assumption that the ratio of C/²³⁴Th on particles is governed by the volume to surface area (V:SA) ratio of the particles. This results in a greater proportion of ²³⁴Th compared to C being associated with the smaller, more slowly settling particles and, as a result, normalization of the POC flux to the ²³⁴Th flux provides an under-estimate of the former quantity. However, when the smaller particle assemblage dominates the vertical flux, as could occur in open ocean regimes having high aeolian inputs of dense, rapidly settling, inorganic particles, then the p-ratio could exceed unity (p-ratio >1) resulting in an over-estimate of the POC flux using the ²³⁴Th method. High levels of flocculation associated with phytoplankton blooms in productive regions of the ocean are likely to produce p-ratios approaching unity, because flocs would tend to preserve the V:SA partitioning of the original particle size distribution and thereby minimize differences in the mean settling speeds of POC and particulate ²³⁴Th (Waite, A.W., Hill, P.S., 2006. Flocculation, phytoplankton and the accuracy of ²³⁴Th-based estimates of the vertical flux of particulate organic carbon in the sea. Marine Chemistry in press). Selective sampling of the large particle fraction using, for example, 53-μm screens can produce a more accurate estimate of the true POC flux, but may not entirely shift the p-ratio to a value of unity.     

Møre Margin: Crustal structure from analysis of Expanded Spread Profiles

Analysis in both the x—t and —p domains of high-quality Expanded Spread Profiles across the Møre Margin show that many arrivals may be enhanced be selective ray tracing and velocity filtering combined with conventional data reduction techniques. In terms of crustal structure the margin can be divided into four main areas: 1) a thicker than normal oceanic crust in the eastern Norway Basin; 2) expanded crust with a Moho depth of 22 km beneath the huge extrusive complex constructed during early Tertiary breakup; 3) the Møre Basin where up to 13–14 km of sediments overlie a strongly extended outer part with a Moho depth at 20 km west of the Ona High; and 4) a region with a 25–27 km Moho depth between the high and the Norwegian coast. The velocity data restricts the continent-ocean boundary to a 15–30 km wide zone beneath the seaward dipping reflector wedges. The crust west of the landward edge of the inner flow is classified as transitional. This region as well as the adjacent oceanic crust is soled by a 7.2–7.4 km s^–1 lower crustal body which may extend beneath the entire region that experienced early Tertiary crustal extension. At the landward end of the transect a 8.5 km s^–1 layer near the base of the crust is recognized. A possible relationship with large positive gravity anomalies and early Tertiary alkaline intrusions is noted.     

High short-term variability of CO2 fluxes during an upwelling event off the Chilean coast at 30°S

pH and alkalinity measurements from a coastal upwelling area located near 30°S (Coquimbo, Chile), are used to describe the short-term variations of CO₂ air–sea exchanges over a period of one week in summer 1996. A 180 km ocean–coastal transect, together with two almost-synoptic grid surveys off Coquimbo covering approximate 2500 km² each, showed that during and immediately after a 4 day long southwesterly wind event (24–28 January) a large area of cold surface water (≈14°C), highly supersaturated in CO₂ (fCO₂ up to 900 μatm), was located near the coast. Three days after the end of the event, the second grid survey showed that in most of the study area the surface temperature and pH had increased significantly (by 1–3°C and 0.05–0.2, respectively), and that the surface water was no longer supersaturated in CO₂. The CO₂-supersaturated water observed in the first grid survey was identified as upwelled subsurface equatorial water, a water mass with its core at about 200 m depth: the depth from which the water upwells is a major determinant of the surface water fCO₂. Integrated C fluxes within a 20 km wide coastal strip (1900 km²) indicate a strong outgassing of CO₂ from the ocean under upwelling conditions (Grid 1; 121 t C day^-1), while the net C exchange was directed to the ocean during the relaxation period (Grid 2; 19 t C day^-1). Estimates of CO₂ fluxes in upwelling areas based on surface water fCO₂ measurements must therefore take into account these short-term variations: reliance on longer-term averages and interpolation will lead to erroneous results.     

Persistently declining oxygen levels in the interior waters of the eastern subarctic Pacific

Fifty years of measurements at Ocean Station Papa (OSP, 50°N, 145°W) show trends in the interior waters of the subarctic Pacific that are both impacted by short term (few years to bi-decadal) atmospheric or ocean circulation oscillations and by persistent climate trends. Between 1956 and 2006, waters below the ocean mixed layer to a depth of at least 1000 m have been warming and losing oxygen. On density surfaces found in the depth range 100-400 m (σ_θ = 26.3-27.0), the ocean is warming at 0.005-0.012 °C y⁻¹, whereas oxygen is declining at 0.39-0.70 μmol kg⁻¹ y⁻¹ or at an integrated rate of 123 mmol m⁻² y⁻¹ (decrease of 22% over 50 years). During this time, the hypoxic boundary (defined as 60 μmol O₂ kg⁻¹) has shoaled from ∼400 to 300 m. In the Alaska Gyre, the 26.2 isopycnal occasionally ventilates, whereas at OSP 26.0σ_θ has not been seen at the ocean surface since 1971 as the upper ocean continues to stratify. To interpret the 50 year record at OSP, the isopycnal transport of oxygenated waters within the interior of the subarctic Pacific is assessed by using a slightly modified “NO” parameter [Broecker, W., 1974. “NO” a conservative water-mass tracer. Earth and Planetary Science Letters 23, 100-107]. The highest nitrate-oxygen signature in interior waters of the North Pacific is found in the Bering Sea Gyre, Western Subarctic Gyre and East Kamchatka Current region as a consequence of winter mixing to the ∼26.6 isopycnal. By mixing with low NO waters found in the subtropics and Okhotsk Sea, this signature is diluted as waters flow eastward across the Pacific. Evidence of low NO waters flowing north from California is seen along the coasts of British Columbia and SE Alaska. Oxygen in the subsurface waters of the Alaskan Gyre was supplied ∼60% by subarctic and 40% by subtropical waters during WOCE surveys, whereas such estimates are shown to periodically vary by 20% at OSP. Other features discernable in the OSP data include periods of increased ventilation of deeper isopycnals on an ∼18 year cycle and strong, short term (few month) variability caused by passing mesoscale eddies. The potential impacts of declining oxygen on coastal ecosystems are discussed.     

Velocity structure of the crust and upper mantle at the northern group of Kamchatka volcanoes (Based on the travel time of P-waves from volcanic earthquakes)

The results of a calculation of the P-wave (V _P ) velocity fields are presented on the basis of the method of the reversible wave and the TAU parameter characterizing the V _P /V _S ratio of seismic waves from the local volcanic earthquakes that occurred at the northern group of Kamchatka volcanoes in 2005–2007. The 3D velocity cross sections were constructed along the SW-NE-trending volcanic group from the Ploskii Tolbachik volcano in the southwest up to the Shiveluch volcano in the northeast. The change of velocity field in time and depth is found. The problems of relating these changes to volcanic activity is reviewed.     

地形和风速影响下的海气相互作用大涡模拟研究

当水流经过海洋地形时,水流的不稳定性会引起垂向混合并伴随大量湍流过程。针对传统海气耦合模式缺少在湍流尺度上讨论海洋地形与风速对海气相互作用影响的问题,使用并行大涡模拟海气耦合模式(the parallelized large eddy simulation model, PALM)在5 m/s的背景风场下,引入理想立方体地形,对比有无地形的影响;设置地形边长为L,高为3L (其中大气部分高L), L与水深H之比为L/H=1/2;然后保持地形条件不变。设置5、10和15 m/s三种风速,讨论风速对小尺度海气相互作用的影响。研究表明:地形在大气部分减弱顺风向速度,增强侧风向速度,影响0～5L的高度区域,而对垂向作用较小;无地形条件下湍流垂向涡黏系数K_m在-0.3L时,水深达到最大值0.024 m²/s,有地形条件下K_m在-0.8L时,达到最大值为0.16 m²/s,地形的存在使得上层海洋混合加强, K_m最大值增加1个数量级。随风速增大海洋和大气中的净热通量、淡水通量和浮力通量都相应...     

Seismic refraction observations in the Transkei Basin and adjacent areas

Some seismic refraction observations undertaken during the IGY are reported here together with a summary of other refraction studies carried out within the Transkei Basin, the Mozambique Ridge and the South African continental shelf area.A 2.5 km section of Cretaceous and younger rocks is associated with profiles observed on the continental shelf; directly below this group are rocks with velocities in the range 4.0–5.5 km s^-1, probably representatives of the Karroo and Cape supergroups. The basement material velocity variations were from 5.3 to 6.5 with an average of 5.9 km s^-1, and is correlated with granite or Malmesbury Formation plus granite. This crustal structure is similar to that found on the eastern continental shelf of southern South America.The profiles in the Transkei Basin show a thick layer of sediment with velocity range 1.50 to 3.50 km s^-1, underlain by a refracting layer in which the average velocity is 4.5 km s^-1. The velocity of 6.6 km s^-1 obtained for the oceanic layer is similar to the velocities of the crustal layer measured in the Argentine Basin. The mantle velocity (8.1 km s^-1) is consistent with the average mantle velocity for the Indian Ocean but significantly lower than the Pacific Ocean average of 8.20 km s^-1. The depth to Moho is about 12.0 km and the crustal section is typical oceanic. A plate tectonic model of the early opening of the South Atlantic is used to describe the evolution of the Transkei Basin.On the Mozambique Ridge the thin sediments (0.7 km) are underlain by rocks with velocities averaging 5.6 km s^-1. This is more than 1.0 km s^-1 faster than the velocity for layer 2 from the Transkei Basin and the Agulhas Plateau, indicating rocks of a younger age or of a different type. Moreover the crustal section of the Ridge has a thickness in excess of 22 km and is in isostatic equilibrium when compared with the adjacent Transkei Basin and Agulhas Plateau. DSDP site 249, situated on the Ridge, penetrated basalt at a depth of 0.4 km. Whether this is continental or oceanic basalt is not known; when this site 249 basalt was compared to the cored basalts of the adjacent Mozambique Basin, inconclusive results were obtained. The essential constitution of the Mozambique Ridge remains an enigma, but solution of this problem is vital for the proper understanding of the Mesozoic history of this oceanic region.     

Nickel utilization in phytoplankton assemblages from contrasting oceanic regimes

In most oceanic environments, dissolved nickel (Ni) concentrations are drawn down in surface waters with increasing concentrations at depth, implying a role for biology in the geochemical distribution of Ni. Studies with phytoplankton isolates from the surface ocean have established the biochemical roles of Ni in the assimilation of urea and oxidative defense. To determine if these requirements are relevant in natural marine planktonic assemblages, bottle-based fertilization experiments were used to test the effects of low-level additions of Ni, urea, or both Ni and urea to surface waters at several locations offshore of Peru and California, as well as in the Gulf of California. Urea and Ni+urea additions consistently promoted phytoplankton growth relative to control and +Ni treatments, except in a coastal upwelling site and Peruvian water. No effect was observed in the upwelling site, but in Peruvian waters urea additions resulted in increased phytoplankton pigments and phosphate drawdown only when Ni was added concurrently, suggesting a biochemically dependent Ni–urea colimitation. In the Gulf of California, Ni additions without urea resulted in increased abundances of cyanobacteria, picoeukaryotes, and the corresponding pigments. As urea additions showed the overall phytoplankton community was also urea-limited, it appears that the cyanobacteria and potentially the picoeukaryotes were colimited by Ni and urea in a biochemically independent fashion. In parallel, radiotracer-based uptake experiments were used to study the kinetics and spatial variation of biological Ni assimilation. In these experiments, the added radiotracer rarely equilibrated with the natural Ni present, precluding estimates a determination of in situ Ni uptake rates and suggesting that much of the natural Ni was not bioavailable. The lack of equilibration likely did not preclude the measurement of community Ni uptake kinetics, nor the comparison of measured rates between locations. The highest V_maxK_ρ^?1 values, which reflect a competitive advantage in Ni acquisition at low concentrations, were observed in stratified nitrogen-deplete communities, potentially linking Ni and nitrogen biogeochemistry in a manner consistent with the biochemical utilization of Ni. Overall, uptake rates were higher in the euphotic rather than non-euphotic zone communities, directly reconciling the nutrient-like depth profile of Ni. The Ni uptake rates observed at the nitrate-replete Fe-deplete Peru stations were an order of magnitude lower than the other sites. This result agrees with calculations suggesting that saturation of the cell surface with Ni and iron (Fe) transporters may limit uptake rates in low Fe waters.     

The Current Structure of the Tsushima Warm Current along the Japanese Coast

The branching of the Tsushima Warm Current (TWC) along the Japanese coast is studied based upon intensive ADCP and CTD measurements conducted off the Wakasa Bay in every early summer of 1995–1998, the analysis of the temperature distribution at 100 m depth and the tracks of the surface drifters (Ishii and Michida, 1996; Lee et al., 1997). The first branch of TWC (FBTWC) exists throughout the year. It starts from the eastern channel of the Tsushima Straits, flows along the isobath shallower than 200 m along the Japanese coast and flows out through the Tsugaru Strait. The current flowing through the western channel of the Tsushima Straits feeds the second branch of TWC (SBTWC) which develops from spring to fall. The development of SBTWC propagates from the Tsushima Straits to Noto Peninsula at a speed of about 7 cm sec⁻¹ following the continental shelf break with a strong baroclinicity. However, SBTWC cannot be always found around the shelf break because its path is influenced by the development of eddies. It is concluded that SBTWC is a topographically steered current; a current steered by the continental shelf break. Salient features at intermediate depth are the southwestward subsurface counter current (SWSCC) between 150 m and 300 m depths over the shelf region in 1995–1998 with the velocity exceeding about 5 cm sec⁻¹, although discrepancies of the velocity and its location are observed between the ADCP data and the geostrophic currents. This revised version was published online in August 2006 with corrections to the Cover Date.     

A 2—D section of 228Ra and 226Ra in the Northeast Pacific

Seawater samples collected in the northeast Pacific from 112° 50′W to 126° 36′W along a latitudinal band (21–25° N) have been analysed for ²²⁸RA and ²²⁶Ra. Both nuclides exhibit their characteristic distributions. In the surface water, the exponential-like decrease of ²²⁸ Ra away from Baja California can be interpreted by horizontal water mixing with eddy diffusion coefficients (K_x) of 1 × 10⁶ cm² s⁻¹ and 5 × 10⁷ cm² S⁻¹ for scale lengths of 200 km and 1000 km, respectively. In the bottom waters, the decrease of ²²⁸Ra away from bottom sediments can be modeled by vertical eddy diffusivities (K_z) of 15–30 cm² s⁻¹ except at one station (24° 16.9′ N, 115° 8.9′ W) where a value of 120 cm² s⁻¹ is obtained. The ²²⁸Ra-derived diffusivities were used to compute the mass balance of ²²⁶Ra using a two-box model. The model results show a mean mixing coefficient of 3.8 cm² s⁻¹ for the thermocline and a mean upwelling velocity of 7.7 m y⁻¹ in the study area, both are about two or three times higher than those generally quoted for the Pacific.     

Small Scale Field Study of an Ocean CO<Subscript>2</Subscript> Plume

We have carried out a small-scale (∼20 l) CO₂ sequestration experiment off northern California (684 m depth, ∼5°C, background ocean pH ∼7.7) designed as an initial investigation of the effects of physical forcing of the fluid, and the problem of sensing the formation of a low pH plume. The buoyant CO₂ was contained in a square frame 1.2 m high, exposing 0.21 m² to ocean flow. Two pH electrodes attached to the frame recorded the signal; a second frame placed 1.9 m south of the CO₂ pool was also equipped with two recording pH electrodes. An additional pH electrode was held in the ROV robotic arm to probe the fluid interface. Local water velocities of up to 40 cm sec⁻¹ were encountered, creating significant eddies within the CO₂ box, and forcing wavelets at the fluid interface. This resulted in rapid CO₂ dissolution, with all CO₂ being depleted in a little more than 2 days. The pH record from the sensor closest (∼10 cm) to the CO₂ showed many spikes of low pH water, the extreme value being ∼5.9. The sensor 1 m immediately below this showed no detectable response. The electrodes placed 1.9 m distant from the source also recorded very small perturbations. The results provide important clues for the design of future experiments for CO₂ disposal and biogeochemical impact studies. These include the need for dealing with the slow CO₂ hydration kinetics, better understanding of the fluid dynamics of the CO₂-water interface, and non-point source release designs to provide more constant, controlled local CO₂ enrichments within the experimental area. This revised version was published online in July 2006 with corrections to the Cover Date.     

Parameters relating to the distributions of planktonic organisms,especially euphausiids in the eastern tropical Pacific

Zooplankton was sampled through eight depth intervals above about 500 m along a transect of the eastern tropical Pacific (ETP), 23°N to 3°S, encompassing four environments. (1) The California Current—ETP transition off Baja California and the mouth of the Gulf of California is inhabited by California Current species at their southern limits, and by the galatheid ‘red crab’ Pleuroncodes planipes together with euphausiids (e.g. Euphausia eximia) of an abundance-based recurrent group of species, distinguished using the criterion of $\text{>}\text{x}$ abundance (Numbers under unit area of sea surface) at common localities, adapted to productive zones marginal to the O₂-deficient part of the ETP. Tropical species appear here where water with surface temperature > 26°C and [O₂] of < 0.1 ml l^?1 beneath a shoaling thermocline replaces the upwelling environment off Baja California. (1) The zone 22° to 10°N harbors euphausiids of two groups: the vertically migrating tropical species (e.g. Euphausia diomedeae) which tolerate intense O₂-deficiency at their daytime depths and enter the oxygenated mixed layer at night, and non-migrating Stylocheiron species which have vertical ranges extending up into the mixed layer. Nevertheless, most of these ‘ETP-adapted’ species are denser farther south, in the north equatorial countercurrent, but three ETP endemics (e.g. E. distinguenda), all vertical migrators belonging in one subgeneric division of Euphausia, are densest in the O₂-deficient regions. (3) The zone of the North Equatorial Countercurrent maintains high densities of three groups: the widely-ranging, ETP-adapted tropical species, the four common Stylocheiron species which, while recurring in abundance at the same localities, differ in depth and the mesopelagic tropical-subtropical species, not tolerant of O₂-deficiency, which occur here in easterly tongues of range. (4) At the equator (93°W), easterly ranging species (e.g. E. paragibba) and westerly Nyctiphanes simplex appear to migrate between equatorial currents which differ in direction with depth, thereby maintaining their narrow ranges along the equator. The ‘marginal proliferators’ such as E. eximia, prominent off Baja California, are again abundant here, availing of the equatorial divergence for high productivity and of the oppositely-directed currents for geographical stability.A second recurrent grouping of species, based on presence of their larvae at common localities, yielded groups also distinguishable by whether the larvae lived within or beneath the mixed layer.Ontogenetic strengthening of vertical migration capability is demonstrated by many species, with older larvae, juveniles and adults showing ranges, respectively, increasing from a few meters to up to 400 m. The pattern is the same in O₂-deficient regions as elsewhere.Regional distribution of euphausiid volume (wet displacement biomass) tended to agree with zooplankton volume, with maxima at the equator, 8°N, and at some localities off Baja California and the Gulf of California where red crab volume peaked. The depth at which euphausiid volume is equal in amount day and night, across which vertical migration takes place, is designated the equilibrium depth (EqD) for euphausiid volume. EqD for euphausiids generally agreed with EqD for zooplankton volume, indicating that euphausiids play a role in determining depth of EqD for zooplankton volume. Euphausiids comprised 13% ($\text{x}$) of zooplankton volume. 80% ($\text{x}$) of euphausiid volume migrated across EqD, the value showing no significant regional differences. 37% ($\text{x}$) of zooplankton volume engaged in such migration, but in the region south of 14°N encompassing the broad O₂-deficient zone, the value was 26%, which compares with 18% previously determined for biomass transferring in a comparable way between epiplankton and planktostad in the same region.     

Inorganic carbon in the central California upwelling system during the 1997–1999 El Niño–La Niña event

Sea surface pCO₂ was monitored during 49 cruises from February 1997 to December 1999 along a section perpendicular to the central California Coast. Continuous measurements of the ocean–atmosphere difference of pCO₂ were made on a mooring in the same region from July 1997 to December 1999. The El Niño/La Niña cycle of 1997–1999 had a significant influence on local ocean–atmosphere CO₂ transfer. During the warm anomaly associated with El Niño, upwelling was suppressed and average sea surface pCO₂ was below atmospheric level. High rainfall and river runoff in the late winter and early spring of 1998 produced areas where pCO₂ was depressed by as much as 100 μatm. A flux ranging from 0.3 to 0.7 mol C m⁻² y⁻¹ from the atmosphere into the ocean was estimated for the El Niño period from wind and ΔpCO₂ data. Temperatures and upwelling returned to near normal in the summer of 1998, but a cold anomaly developed during autumn of that year. Temperature and pCO₂ data indicate that upwelling continued throughout much of the 1998–1999 winter and intensified significantly in the spring of 1999. During strong upwelling events, the estimate of ocean to atmosphere flux approached rates of 50 mol C m⁻² y⁻¹. The estimate for the average CO₂ flux from July 1998 to July 1999 was 1.5–2.2 mol C m⁻² y⁻¹ from the ocean to the atmosphere. While the flux estimate for the El Niño time period may be applicable to a larger area, the high ocean to atmosphere fluxes during La Niña might be the result of sampling near a zone of intense upwelling.     

Propagating tsunami wave and subsequent resonant response signals detected by HF radar in the Kii Channel,Japan

Signals from the tsunami waves induced by the March 11, 2011 moment magnitude (M_w) 9.0 Tohoku-Oki earthquake and from subsequent resonances were detected as radial velocity variability by a high-frequency ocean surface radar (HF radar) installed on the eastern coast of the Kii Channel, at a range of about 1000 km from the epicenter along the eastern to southern coasts of Honshu Island. A time–distance diagram of band-passed (9–200 min) radial velocity along the beam reveals that the tsunami waves propagated from the continental shelf slope to the inner channel as progressive waves for the first three waves, and then natural oscillations were excited by the waves; and that the direction of the tsunami wave propagation and the axis of the natural oscillations differed from that of the radar beam. In addition, spectral analyses of the radial velocities and sea surface heights obtained in the channel and on the continental shelf slope suggest complex natural oscillation modes excited by the tsunami waves.     

Changes in deep-water CO2 concentrations over the last several decades determined from discrete pCO2measurements

Detection and attribution of hydrographic and biogeochemical changes in the deep ocean are challenging due to the small magnitude of their signals and to limitations in the accuracy of available data. However, there are indications that anthropogenic and climate change signals are starting to manifest at depth. The deep ocean below 2000 m comprises about 50% of the total ocean volume, and changes in the deep ocean should be followed over time to accurately assess the partitioning of anthropogenic carbon dioxide (CO₂) between the ocean, terrestrial biosphere, and atmosphere. Here we determine the changes in the interior deep-water inorganic carbon content by a novel means that uses the partial pressure of CO₂ measured at 20 °C, pCO₂(20), along three meridional transects in the Atlantic and Pacific oceans. These changes are measured on decadal time scales using observations from the World Ocean Circulation Experiment (WOCE)/World Hydrographic Program (WHP) of the 1980s and 1990s and the CLIVAR/CO₂ Repeat Hydrography Program of the past decade. The pCO₂(20) values show a consistent increase in deep water over the time period. Changes in total dissolved inorganic carbon (DIC) content in the deep interior are not significant or consistent, as most of the signal is below the level of analytical uncertainty. Using an approximate relationship between pCO₂(20) and DIC change, we infer DIC changes that are at the margin of detectability. However, when integrated on the basin scale, the increases range from 8–40% of the total specific water column changes over the past several decades. Patterns in chlorofluorocarbons (CFCs), along with output from an ocean model, suggest that the changes in pCO₂(20) and DIC are of anthropogenic origin.     

Effect of ocean acidification on coastal phytoplankton composition and accompanying organic nitrogen production

The effect of ocean acidification, caused by the increase in pCO₂ in seawater, on phytoplankton population and on related organic nitrogen production was experimentally examined by use of a natural coastal microbial population. pCO₂ and pH were controlled by aeration with air in which pCO₂ was at the current level (control), for which ambient air was used, and with air in which pCO₂ was ??800?? and ??1200?? ppm, in 500-L culture vessels. The experiment was continued for 15?days after addition of the inorganic nutrients such as nitrate, phosphate, and silicate. During most of the experimental period, a minor increase in phytoplankton biomass was noted, probably because of low irradiance, an increase in phytoplankton biomass was observed at the end of the experiment. Flow cytometric and microscopic observations revealed that this increase was because of Chrysochromulina sp. (Haptophyceae). The growth of Chrysochromulina sp. was most obvious in the control vessel, and tended to be obscured by increasing pCO₂ (decrease in pH), indicating the possibility that ocean acidification inhibits the growth of specific phytoplankton groups, for example Chrysochromulina sp. Production of particulate organic nitrogen (PON), determined by the ¹⁵N tracer method, also diminished under acidified conditions compared with that at the current level.