Modeling multiple wave systems in the eastern equatorial Pacific

Ocean Dynamics - While moderate wind and wave conditions prevail in the eastern equatorial Pacific, modeling waves in this area remains challenging due to the presence of multiple wave systems...     

Radium-226 measurements from three Geosecs intercalibration stations

This paper presents the results of²²⁶Ra intercalibration measurements made by three groups of investigators (LDGO, USC and SIO) on the seawater samples collected in profile at three Geosecs intercalibration stations. A common radium standard prepared from an NBS bulk standard for the Geosecs program has been adopted by all groups. At Geosecs-I station in the Northeast Pacific, the new²²⁶Ra results obtained from reoccupation of the station show that the agreement of the three groups has been significantly improved over the initial comparison made in 1970.At Geosecs-II in the Northwest Atlantic, the initial comparison of the²²⁶Ra profiles showed that the USC data were systematically higher than the SIO data by 1 radium unit of 10^?14 g/kg. This corresponds to a relative difference of 20% due to the very low radium content of Atlantic waters. The new results obtained from reoccupation of the station show that both the USC and SIO data are consistent with their previous data. Thus, a systematic difference of 1 radium unit still exists. However, the new LDGO profile falls in between those of USC and SIO, lying closer to the SIO profile.At Geosecs-III station in the Southwest Pacific, the²²⁶Ra measurements show that the LDGO data are systematically higher than the SIO data by about 10% above 3.4 km depth. Below this depth, both sets of data agree and show a sharp decrease in radium concentration. This radium discontinuity corresponds to the benthic front which is a density discontinuity separating the Deep and Bottom Water in the South Pacific.     

Meridional distribution of226Ra in the eastern Pacific along GEOSECS cruise tracks

We present the distribution of²²⁶Ra in eight vertical profiles from the eastern Pacific. The profiles are located along a meridional trend near 125°W, from 43°S to 29°N. Surface²²⁶Ra concentrations are about 7 dpm/100 kg, except for the two stations south of 30°S where the higher values are due to the Antarctic influence. Deep waters show a distinctive south-to-north increase in the²²⁶Ra content, from about 26 to 41 dpm/100 kg near the bottom. Unlike in the Atlantic and Antarctic Oceans, the effect of²²⁶Ra injection from bottom sediments is clearly discernible in the area. The presence of this primary²²⁶Ra can be traced up to at least 1–1.5 km above the ocean floor, making this part of the sea bed among the strongest source regions for the oceanic²²⁶Ra. Numerical solutions of a two-dimensional vertical advection-diffusion model applied to the deep (1.2–4 km)²²⁶Ra data give the following set of best fits: upwelling velocity(V_z) = 3.5m/yr, vertical eddy diffusivity(K_z) = 0.6cm²/s, horizontal (north-south) eddy diffusivity(K_y) = 1 × 10⁷cm²/s, and water-column regeneration flux of²²⁶Ra(J) = 3.3 × 10^?5dpmkg^?1yr^?1 as an upper limit. These parametric values are in general agreement with one-dimensional (vertical) model fits for the Ra-Ba system. However, consideration of²²⁶Ra balance leads us to suspect the appropriateness of describing the vertical exchange processes in the eastern Pacific with constantV_z and K_z. If future modeling is attempted, it may be preferable to treat the area as a diffusion-dominant mixing regime with depth-dependent diffusivities.     

Ferromanganoan sediments in the equatorial East Pacific

Ferromanganoan sediments containing little or no CaCO₃ have been found to occur extensively throughout the region between the East Pacific Rise and the Galapagos Rise. Concentrations of Fe and Mn of up to 18 and 6.5%, respectively, accompany low concentrations of Al and Ti. The concentrations of Cu, Ni, and Zn are also high relative to more typical pelagic sediments.While chemically similar to the non-carbonate fraction of metalliferous sediments previously described from the East Pacific Rise, the mineralogy is markedly different. A non-detrital smectite makes up the bulk of sediment (70 to 90%) and is the most important iron bearing phase. Fe and Mn oxides, occurring primarily as micro-nodules, comprise 10 to 20% of the sediment. Detrital material is relatively rare, amounting to less than 10% in all samples.     

Early diagenesis in sediments from the eastern equatorial Pacific,I. Pore water nutrient and carbonate results

Interstitial waters were extracted from cores at three locations in the eastern equatorial Pacific and analyzed for nutrients, dissolved carbonate species, Mn and Fe. From the depth variation in pore water chemistry, we infer that organic matter oxidation reactions occur with depth in the following sequence: O₂ reduction, NO₃^? and MnO₂ reduction, and then ferric iron reduction. From NO₃^? results we infer that O₂ is largely or totally consumed within the top few centimeters of sediment. NO₃^? is completely reduced at a sediment depth of 20 cm at a site near the crest of the East Pacific Rise, but is preserved at levels of 20–30 μmol/kg at 40 cm depth at a Guatemala Basin site.We have calculated the alkalinity for pore water samples assuming ions diffuse according to relative ionic diffusion coefficients, that the stoichiometry of organic matter oxidation reactions is that of “Redfield” organic matter, and that the pore waters are saturated throughout with respect to CaCO₃. The measured alkalinity increase is only about half of the predicted value. The difference is probably a result of either enhanced mixing of the pore water in the top few centimeters of sediments by biological or physical processes, or the occurrence of an inorganic reaction which consumes alkalinity.At depths of oxygen and nitrate reduction in the sediments, the ion concentration product of CaCO₃ is the same, within the analytical error, as the solubility product of Ingle et al. [34] at 1 atm and 4°C. This result indicates CaCO₃ resaturation on pressure change during coring. Where pore water Mn concentrations become measurable, the ion concentration product increases, indicating either supersaturation with respect to calcite or that another phase is controlling the carbonate solubility.     

Radium-226 chronology of Holocene travertine from Esanatoglia (Central Italy)

Here we investigate the feasibility of using alpha-spectrometric ²²⁶Ra methodology to date low U freshwater carbonate deposits (<0.1 μg g⁻¹) by analysing Holocene freshwater travertine deposits from Esanatoglia, Umbro-Marchean Apennines, Central Italy. Previously, such methods have been tested on hydrothermal deposits, which are characterised by high Ra and U. We calculate a ²²⁶Ra age estimate of 2.9±0.4 ka (1σ) for Holocene travertine that is in reasonable agreement with archaeological evidence by assuming that the ²²⁶Ra initial activity of the Holocene travertine is comparable with that of present-day carbonate deposits in the same area. We also investigate the use of Ba to normalise ²²⁶Ra activities. Comparison of age estimates based on Ra methods with ages derived from alpha-spectrometric ²³⁰Th/²³⁴U isochron techniques for the same co-eval sub-samples indicates that, where only alpha-spectrometric methods are available, the former are likely to be much more useful for Holocene travertine material, especially if there is a significant detrital component.     

Amino acids in equatorial Pacific Ocean water

Seawater samples from the Eastern Equatorial Pacific Ocean were analyzed for amino acids by a ligand-exchange chromatography technique. The results showed that dissolved free amino acids make up only 0.3% of DOC, much less than has been previously reported for open ocean waters. Substantially larger amounts of combined amino acids are present, however. Concentration variations of the combined amino acid fraction in the water column suggest their potential use as a water mass indicator.     

Reflection of equatorial Kelvin waves at eastern ocean boundaries Part II: Pacific and Atlantic Oceans

The effect of viscosity, non linearities, incident wave period and realistic eastern coastline geometry on energy fluxes are investigated using a shallow water model with a spatial resolution of 1/4 degree in both meridional and zonal directions. Equatorial and mid-latitude responses are considered. It is found that (1) the influence of the coastline geometry and the incident wave period is more important for the westward energy flux than for the poleward flux, and (2) the effect of the inclination of the eastern ocean boundary on the poleward energy flux, for the Pacific and Atlantic Oceans, decline as the period of the incident wave increases. Furthermore, the model simulations suggest that the poleward energy fluxes from meridional boundaries give plausible results for motions of seasonal and annual periods. For comparatively shorter periods, a realistic coastline geometry has to be included for more accurate results. It is recommended that any numerical model involving the reflection of baroclinic Rossby waves (of intraseasonal, seasonal or annual periods) on the eastern Pacific or Atlantic Oceans, should consider the effect of the coastline geometry in order to improve the accuracy of the results.     

226Ra in the Pacific Ocean

A total of 29 vertical Ra profiles has been measured from the Pacific as part of the GEOSECS program. These profiles are located on an east-west section along ～30°N, and a north-south section, close to the western boundary of the major basins in the western Pacific. Profiles from the northeast Pacific show a deep Ra maximum, with an excess concentration relative to the potential temperature and salinity. This maximum extends westward in the direction with decreasing Ra content, and finally vanishes completely in the northwest Pacific near Japan.Ra profiles along the western boundary show a mid-depth maximum around 3 km and a near-bottom minimum due to southward intrusion of the high-Ra Pacific Deep Water and a northward spreading of the low-Ra Antarctic Bottom Water. The contrast between the maximum and the minimum intensifies toward the south, where the benthic front has clearly separated these two water masses. Ra is thus a useful tracer for the studies of oceanic mixing and circulation in the Pacific.     

Response of the equatorial Pacific thermocline to climate warming

Climate models project a significant shoaling of the thermocline over the western equatorial Pacific Ocean under global warming, which has been generally regarded as a direct response to surface wind change. This study investigates the formation processes for the equatorial Pacific thermocline response to CO₂ quadrupling using the Community Earth System Model version 1 (CESM1). In particular, an overriding method is applied to isolate and quantify the wind stress effect and the direct radiative effect of CO₂ emissions. Results show that both effects of the wind stress and direct radiative forcing are equally important for shoaling the equatorial thermocline, with the former responsible for its upper portion change and the latter for its lower portion change. Further passive tracer experiments with the ocean component of the CESM1 verify the role of ocean surface warming in shoaling the equatorial thermocline and identify the ocean circulation change in response to the surface warming as its dynamic cause of formation.     

Non-steady state diagenetic record in eastern equatorial Atlantic sediments

The radiocarbon age profile of a sediment core from the eastern equatorial Atlantic shows a marked decrease in both clay and carbonate accumulation rates at the end of the last glacial period, with overall rates of 3.2 and 8.1 cm/kyr for the Holocene and late glacial respectively. The transition from glacial to postglacial conditions recorded in the sediments at 39 cm is above the visual Fe(II)/Fe(III) redox boundary at 48 cm which has been used in the past to identify the glacial section, and even further above the increase in preserved organic carbon content at 55 cm. It is concluded from the present organic carbon profile, and recent observations in this area and elsewhere, that organic diagenesis in these sediments is not steady state. Instead it is suggested that the uppermost glacial sediments lost carbon through oxic remineralisation at the change to postglacial conditions, and that this process is now continued with nitrate as the electron acceptor. These observations suggest that the double MnO_x solid phase peak which occurs in these and similar sediments is a direct consequence of an adjustment of the oxic layer depth following the decrease in carbon flux at the onset of the Holocene.     

A deep226Ra maximum in the Northeast Pacific

Seven vertical profiles of²²⁶Ra have been measured along an east-west traverse at about 30°N from San Diego to northwest of Hawaii. These profiles show that there is a distinct core of Ra maximum spreading westward as a tongue in the northeast Pacific deep water. This core starts in the east with 21.1 Ra units (1Ra unit= 10^?14g/kg) at 3.9 km depth at about 130°W, and deepens westward to 4.1 km with its Ra reduced to 18.3 units at 150°W. A similar core with some uncertainty due to possible sampling errors extends westward near the bottom at 5.2 km depth from 19.4 Ra units at 150°W to 15.9 units at about 180° longitude. In addition, these profiles appear to be correlated with each other in structure above the cores of Ra maximum. These cores indicate that the Ra input depends locally on the type and composition of sediments and so the flux varies over the ocean bottom. On the basis of a one-dimensional diffusion-decay model, a horizontal diffusion coefficient of 10⁶ cm²/sec has been computed along these cores. Although this value appears to be slightly lower, it is not inconsistent with those derived from other physical methods.     

A multitracer approach to the study of erosion in the northeast equatorial Pacific

Erosion of sediment at two sites within the siliceous-clay province of the northeast equatorial Pacific is investigated by examining radionuclide, radiolarian and percent smectite profiles of three ? cores. A numerical ? model of the sediment is presented that examines the effect of various particle rain rates, erosion rates and biological mixing parameters on these profiles.²³⁰Th, because of its longer half-life, is a more sensitive tracer of erosion than²³¹Pa. Because the input of these radionuclides into the sediment is not well defined however, conservative tracers, such as the percent smectite of the clay fraction and fossil radiolarians are used to constrain the rate of sediment recycling. Dissolution of radiolarians though can greatly affect the interpretation of the observed biostratigraphy.     

A226Ra section across the East Pacific Rise

Four vertical Ra profiles have been measured across the East Pacific Rise (EPR) from Callao to Tahiti. These profiles show that Ra in the deep water (below 2 km depth) increases toward the EPR. However, this increase does not necessarily indicate a Ra source on the EPR. The increase from Tahiti toward the EPR reflects the general trend of the Pacific Ra distribution. The decrease from the EPR eastward to the Peru Basin is probably due to the continental effect with higher sedimentation rates.The hydrography, especially potential temperature and oxygen, indicates significant differences below about 3 km depth between the east and west flanks of the EPR indicating the effect of the cold bottom water to the west of the EPR. The benthic front is identified at 3.9 km depth at the westernmost station near Tahiti. Silicate and salinity data are by no means unique and reflect a complicated local circulation and mixing pattern with a minor intrusion of the Antarctic Bottom Water from the south into the Peru Basin.The θ-Ra and Ra-Si relationships both indicate an enrichment of Ra in the deep water below 2 km depth probably due to input from the underlying sediments. Above 2 km depth, Ra covaries almost linearly with θ as well as Si, mimicking a stable conservative property. This suggests that the radiodecay rate is nearly balanced by the input rate within the water column between 1 and 2 km depth in which θ is linearly correlated withS.Simple vertical model calculations show that the in-situ production of Ra by particulate dissolution in the deep water is negligible within a reasonable range of upwelling rates from 2 to 12 m/yr. Thus the Ra profiles show a net decay effect and so the θ-Ra relations are not linear in the deep water. In fact, the composite θ-Ra plots show a break at 25 dpm/100 kg (at 2 km depth) rather than a smooth curve, while theθ-S plots are essentially linear. A maximum Ra production rate of about 8 × 10^?3 (dpm/100 kg) yr^?1 is obtained from all the profiles with minimum upwelling rates between 0.7 and 3.5 m/yr.     

Estimating thermohaline variability of the equatorial Pacific Ocean from satellite altimetry

The vertical thermohaline structure in the western equatorial Pacific is examined with a Gravest Empirical Mode (GEM) diagnosis of in-situ mooring measurements. The poor GEM performance in estimating deep thermohaline variability from satellite altimetry confirms a lack of vertical coherence in the equatorial ocean. Mooring observation reveals layered equatorial water with phase difference up to 6 months between thermocline and sub-thermocline variations. The disjointed layers reflect weak geostrophy and resemble pancake structures in non-rotating stratified turbulence. A coherency theorem is then proved, stating that traditional stationary GEM represents in-phase coherent structure and can not describe vertically out-of-phase variability. The fact that stationary GEM holds both spatial and temporal coherence makes it a unique tool to diagnose vertical coherent structure in geophysical flows. The study also develops a non-stationary GEM projection that captures more than 40% of the thermohaline variance in the equatorial deep water.     

Post-depositional metal enrichment processes inside manganese nodules from the north equatorial Pacific

Interiors of manganese nodules from siliceous ooze beneath the Pacific equatorial high-productivity region, when examined by scanning electron microscopy (SEM) and electron microprobe, display post-depositional recrystallization textures and metalliferous oxide bands (diameter 1–10 μm, 30–40 wt.% Mn, 4–5% Ni, 3–4% Cu). SEM has revealed biogenic siliceous matter in all stages of degradation and dissolution within nodule interiors, creating cavities and voids. Often these miniature vugs contain authigenic phillipsite crystallites which are coated with delicate clusters of crystalline Mn-Fe oxides (todorokite) containing significant amounts of Ni and Cu. We postulate the following diagenetic processes and mechanism of uptake of transition metals inside manganese nodules: (1) palagonite + biogenic silica + pelagic clay → phillipsite + montmorillonite; (2) biogenic matter + amorphous FeOOH or δ-MnO₂ → Fe_aq²⁺ and/or Mn^IIMn^IV oxide (todorokite); (3) aerated seawater or δ-MnO₂ + Fe_aq²⁺ → FeOOH and/or todorokite (deposited on phillipsite); (4) (Ni^II and Cu^II) organic chelates (adsorbed on clays, etc.) + amorphous FeOOH or δ-MnO₂ → Ni-Cu-todorokite + phillipsite, etc.This mechanism explains the well-known positive Mn-Ni-Cu and negative Fe-Ni, Fe-Cu correlations in nodules. By analogy with terrestrial todorokites, which require about 8 wt.% Mn to be in the divalent state to stabilize the crystal structure, as much as 8 wt.% (Ni + Cu) could be accommodated in todorokite-bearing deep-sea manganese nodules. However, although such nodules beneficiate Ni and Cu with respect to marine sediments and seawater, they remain undersaturated in these divalent cations.     

Oceanic crust and Moho of the Pacific Plate in the eastern Ogasawara Plateau region

Abstract   To show the structure of oceanic crust and Moho around the eastern Ogasawara Plateau, we have analyzed industry-standard two-dimensional multichannel seismic reflection data. To obtain improved velocity models, phase information of seismic signals was used for velocity analysis and velocity models for oceanic crust above Moho were determined. We apply this velocity analysis technique to seismic reflection data around the eastern Ogasawara Plateau, with the result of clear images of structures within oceanic crust and Moho. South of the Ogasawara Plateau, Moho deepens proximal to the Plateau. Moho distal to the Plateau is ca 7 km below sea floor (bsf), whereas it is ca 10 km bsf near the Plateau. The characters of oceanic crust and Moho differ significantly north and south of the Plateau. To the north, the structure of oceanic crust is ambiguous, the sea floor is shallower and less smooth, and Moho is discontinuous. To the south, structures within oceanic crust and Moho are imaged clearly, and the sea floor is deeper. A strong Moho reflection south of the Plateau might represent a sharp boundary between layered gabbro and peridotite. However, discontinuous Moho reflections north of the Plateau might represent rough topography because of intensive magmatism or a gradual downward increase in velocity within a thick Moho transition zone. A fracture zone north of the Plateau also appears to separate oceanic crust and Moho of different characters, suggesting vigorous magmatism between the Plateau and the fracture zone, and that the Ogasawara Plateau and the fracture zone influenced the genesis of oceanic crust and upper mantle. Differences in acoustic characteristics to the north and south of the Plateau are apparent in profiles illuminated by seismic attributes.     

Impact of winter thermal condition of the Tibetan Plateau on the zonal wind anomaly over equatorial Pacific

Using the improved CCM1 dynamic climate model, the impact of abnormal heat source and sink over the Tibetan Plateau in winter on the abnormal zonal wind over the Pacific Ocean is studied in this paper. The following new-findings are obtained: (1) When the at mospheric cold source during January—March on the Tibetan Plateau gets intensified, an abnormal anticyclone around the Tibetan Plateau will appear in lower troposphere. Abnormal northerly wind at the coastal area of the mainland of China and an abnormal cyclone will appear on the West Pacific in the following months. Then, abnormal west wind will appear over the equator of the West Pacific and extends to the East Pacific. (2) When the atmospheric cold source during January—March over the Tibetan Plateau is unusually weak, an abnormal cyclone around the Tibetan Plateau will appear at lower levels first, then abnormal anticyclone will appear on the West Pacific and move to the south and result in abnormal easterly wind over the equator of the West Pacific, which will extend to the east. Furthermore, abnormal changes of zonal wind on equatorial Indian Ocean can be caused by the intensity change of atmospheric cold source in winter and early spring over the Tibetan Plateau.

     

Reflection of equatorial Kelvin waves at eastern ocean boundaries Part I: hypothetical boundaries

A baroclinic shallow-water model is developed to investigate the effect of the orientation of the eastern ocean boundary on the behavior of equatorial Kelvin waves. The model is formulated in a spherical polar coordinate system and includes dissipation and non-linear terms, effects which have not been previously included in analytical approaches to the problem. Both equatorial and middle latitude response are considered given the large latitudinal extent used in the model. Baroclinic equatorial Kelvin waves of intraseasonal, seasonal and annual periods are introduced into the domain as pulses of finite width. Their subsequent reflection, transmission and dissipation are investigated. It is found that dissipation is very important for the transmission of wave energy along the boundary and for reflections from the boundary. The dissipation was found to be dependent not only on the presence of the coastal Kelvin waves in the domain, but also on the period of these coastal waves. In particular the dissipation increases with wave period. It is also shown that the equatorial β-plane approximation can allow an anomalous generation of Rossby waves at higher latitudes. Nonlinearities generally have a small effect on the solutions, within the confines of this model.