Ocean floor222Rn standing crops in the Atlantic and Pacific Oceans

Excess²²²Rn standing crops have been calculated from bottom profiles obtained at 119 locations in the Atlantic and Pacific Oceans during the GEOSECS cruises. They range over more than two orders of magnitude with clear north-south and east-west patterns. The standing crops are inversely related to the sedimentation rate and the zones of highest standing crops are also areas which have a high coverage of manganese nodules.     

ExcessHe in the Atlantic Ocean

³/He⁴He measurements at two stations in the Atlantic show that the deep water (> 2 km) contains far less excess³He than our previous measurements have shown for the Pacific Ocean. The³/He⁴He ratio anomaly (relative to atmospheric³/He⁴He) is approximately 5% for the deep Atlantic compared to about 20% for the deep Pacific. The North Atlantic³He profile shows much more structure than the South Atlantic profile, with maxima observed at 500 m, 1900 m, and 3200 m. The maxima at 500 m and 1900 m are probably due to in situ tritium decay, whereas the 3200 m maximum cannot be due to tritium, and is probably due to leakage of³He into the Atlantic water from the mantle. It seems significant that maxima in the trace elements Cu, Zn and Fe have also been observed at 3200 m at this station by Brewer, Spencer and Robertson.     

Dissolved chromium in the northwest Atlantic Ocean

Total dissolved chromium concentrations have been determined for four vertical profiles from Baffin Bay, the Labrador Sea and the northwest Atlantic Ocean. Chromium concentrations of 3.3 to 5.2 nM are found. While the vertical distribution of chromium in the study area is largely controlled by advective processes, the profiles show a small depletion in surface water with increase to a more constant level at depth. Surface depletion and correlations between chromium and nutrients indicate biogeochemical cycling of chromium. At one station, close to the Gibbs fracture zone, a distinct chromium maximum is observed. This feature centred at 3200 m is deeper than the core of the ambient water mass which is advected westward from the Eastern Basin of the Atlantic Ocean through the Gibbs fracture zone.     

The oceanic tides in the South Atlantic Ocean

The finite element ocean tide model of Le Provost and Vincent (1986) has been applied to the simulation of the M₂ and K₁ components over the South Atlantic Ocean. The discretisation of the domain, of the order of 200 km over the deep ocean, is refined down to 15 km along the coasts, such refinement enables wave propagation and damping over the continental shelves to be correctly solved. The marine boundary conditions, from Dakar to Natal, through the Drake passage and from South Africa to Antarctica, are deduced from in situ data and from Schwiderski’s solution and then optimised following a procedure previously developed by the authors. The solutions presented are in very good agreement with in situ data: the root mean square deviations from a standard subset of 13 pelagic stations are 1.4 cm for M₂ and 0.45 cm for K₁, which is significantly better overall than solutions published to date in the literature. Zooms of the M₂ solution are presented for the Falkland Archipelago, the Weddell Sea and the Patagonian Shelf. The first zoom allows detailing of the tidal structure around the Falklands and its interpretation in terms of a stationary trapped Kelvin wave system. The second zoom, over the Weddell Sea, reveals for the first time what must be the tidal signal under the permanent ice shelf and gives a solution over that sea which is generally in agreement with observations. The third zoom is over the complex Patagonian Shelf. This zoom illustrates the ability of the model to simulate the tides, even over this area, with a surprising level of realism, following purely hydrodynamic modelling procedures, within a global ocean tide model. Maps of maximum associated tidal currents are also given, as a first illustration of a by-product of these simulations.     

A230Th profile in the Atlantic Ocean

A²³⁰Th profile (dissolved + particulate) measured in the eastern central Atlantic at 24°45′N, 26°57′W displays a linear increase with depth. Values range between 0.2 dpm²³⁰Th/10⁴ l at the surface and 2.0 dpm²³⁰Th/10⁴ l at 4000 m. This concentration is 5 times lower than total²³⁰Th in the central Pacific and is similar to²³⁰Th measured on suspended matter in the Pacific and Indian Ocean. The lower concentration in the eastern Atlantic must either reflect a higher scavenging activity at the West African margin or be a consequence of a higher particle flux through the water column, yielding an average settling velocity of particulate matter of 6.2 × 10⁻³ cm/s.Despite the lower concentration, the equilibrium coefficient between dissolved and adsorbed²³⁰Th is similar to the one derived for the central Pacific. This shows that the equilibrium model for²³⁰Th scavenging from the water column derived by Nozaki et al. [1] and Bacon and Anderson [2] applies as well in the eastern Atlantic.     

Seabirds indicate changes in the composition of plastic litter in the Atlantic and south-western Indian Oceans

I compare plastic ingested by five species of seabirds sampled in the 1980s and again in 1999-2006. The numbers of ingested plastic particles have not changed significantly, but the proportion of virgin pellets has decreased 44-79% in all five species: great shearwater Puffinus gravis, white-chinned petrel Procellaria aequinoctialis, broad-billed prion Pachyptila vittata, white-faced storm petrel Pelagodroma marina and white-bellied storm petrel Fregetta grallaria. The populations sampled range widely in the South Atlantic and western Indian Oceans. The most marked reduction occurred in great shearwaters, where the average number of pellets per bird decreased from 10.5 to 1.6. This species migrates between the South and North Atlantic each year. Similar decreases in virgin pellets have been recorded in short-tailed shearwaters Puffinus tenuirostris in the Pacific Ocean and northern fulmars Fulmarus glacialis in the North Sea. More data are needed on the relationship between plastic loads in seabirds and the density of plastic at sea in their foraging areas, but the consistent decrease in pellets in birds suggests there has been a global change in the composition of small plastic debris at sea over the last two decades.     

Natural and anthropogenic radionuclide distributions in the northwest Atlantic Ocean

We have used in-situ pumps which filter large volumes of sea water through a 1 μm cartridge prefilter and two MnO₂-coated cartridges to obtain information on dissolved and particulate radionuclide distributions in the oceans. Two sites in the northwest Atlantic show subsurface maxima of the fallout radionuclides¹³⁷Cs,^239,240Pu and²⁴¹Am. Although the processes of scavenging onto sinking particles and release at depth may contribute to the tracer distributions, comparison of predicted and measured water column inventories suggests that at least 35–50% of the Pu and²⁴¹Am are supplied to the deep water by advection.The depth distributions of the naturally occurring radionuclides²³²Th,²²⁸Th and²³⁰Th reflect their sources to the oceans.²³²Th shows high dissolved concentrations in surface waters, presumably as a result of atmospheric or riverine supply. Activities of²³²Th decrease with depth to values 0.01 dpm/1000 l.²²⁸Th shows high activities in near surface and near bottom water, due to the distribution of its parent,²²⁸Ra. Dissolved²³⁰Th, produced throughout the water column from²³⁴U decay, increases with depth to 3000 m. Values in the deep water (> 3000 m) are nearly constant ( 0.6–0.7 dpm/1000 l), and the distribution of this tracer (and perhaps other long-lived particle-reactive tracers as well) may be affected by the advection inferred from Pu and²⁴¹Am data.The ratio of particulate to dissolved activity for both²³⁰Th and²²⁸Th is 0.15–0.20. This similarity precludes the calculation of sorption rate constants using a simple model of reversible sorption equilibrium. Moreover, in mid-depths²²⁸Th tends to have a higher particulate/dissolved ratio than²³⁰Th, suggesting uptake and release of²³⁰Th and²²⁸Th by different processes. This could occur if²²⁸Th, produced in surface water, were incorporated into biogenic particles formed there and released as those particles dissolved or decomposed during sinking.²³⁰Th, produced throughout the water column, may more closely approach a sorption equilibrium at all depths.²³⁰Th,²⁴¹Am and^239,240Pu are partitioned onto particles in the sequence Th > Am > Pu with 15% of the²³⁰Th on particles compared with 7% for Am and 1% for Pu. Distribution coefficients (K_d) are 1.3–1.6 × 10⁷ for Th, 5–6 × 10⁶ for Am and 7–10 × 10⁵ for Pu. The lower reactivity for Pu is consistent with analyses of Pu oxidation states which show 85% oxidized (V + VI) Pu. However, theK_d value for Pu may be an upper limit because Pu, like²²⁸Th, may be incorporated into particles in surface waters and released at depth only by destruction of the carrier phase.     

Age of Jurassic continental tholeiites of French Guyana, Surinam and Guinea: Implications for the initial opening of the Central Atlantic Ocean

A detailed ⁴⁰Ar/³⁹Ar study, of mineral separates from the Jurassic Atlantic Continental Tholeiites (JACT) of Guyana (French Guyana and Surinam, South America), and Guinea (West Africa) related to the initial opening of the Central Atlantic, has been carried out. In French Guyana, plateau ages of 196.0 ± 5.7 Ma and 196.1 ± 7.5 Ma were obtained on single, small amphibole grains from NNW—SSE trending dykes. In Guinea, single biotite grains from intrusive formations from the Kakoulima and Fouta Djalon areas yielded plateau ages of 200.4 ± 0.2 Ma and 194.8 ± 0.5 Ma, concordant with high temperature apparent ages on other biotites. The bulk plagioclase samples display disturbed age spectra due to alteration and excess argon. However, intermediate temperature, weighted mean plagioclase ages are similar in both regions of Guyana and Guinea, ranging from 200.2 ± 2.4 Ma to 188.7 ± 1.9 Ma, partly in agreement with the amphibole and biotite data.

These data, combined with previous ⁴⁰Ar/³⁹Ar and U/Pb results from the northern part of the Central Atlantic margins, indicate intense magmatic activity distributed over a large area from Iberia to Liberia (ca. 4500 km long) for a short period of time (204-195 Ma, perhaps less for the bulk of the magmatism) during the initial break-up of Pangea continent. These data do not support an initiation of the magmatism from a radial volcano-tectonic system centred in the south of the region, as suggested by May [1], and the initial break-up seems to affect the whole Central Atlantic during a period of 9 Ma.     

The distribution of particulate matter in the Atlantic Ocean

We have determined the dry weight of suspended particulate matter in seawater in a section through the western Atlantic Ocean from 75°N to 52°S. The concentrations, operationally defined as that weight retained on 0.6-μm and 0.4-μm pore size Nuclepore filters, contained in 1 kg of seawater, range from 5 to 300 μg/kg and show readily explainable regional features. High concentrations are found in surface waters and in association with radpidly moving bottom waters in the Denmark Straits overflow and in Antarctic bottom waters to 15°S. Low concentrations, <12 μg/kg, characterize the mid-water regions of the sub-tropical gyres. High concentrations are seen in sinking Labrador Sea water and in a plume extending at least a kilometer off the bottom at 35°N–40°N where the cruise track intersects the North Atlantic gyre. It is doubtful whether this important phenomenon could be observed by any means other than through particulate observations, either optical or gravimetric, and this provides a unique insight into the scale of vertical turbulent processes.     

The Kane fracture zone in the Central Atlantic Ocean

The Kane fracture zone has been traced as a distinct topographic trough from the Mid-Atlantic Ridge near 24°N to the 80-m.y. B.P. isochron (magnetic anomaly 34) on either side of the ridge axis for a total of approximately 2800 km. Major changes in trend of the fracture zone occur at approximately 72 m.y. B.P. (anomaly 31 time) and approximately 53–63 m.y. B.P. (anomaly 21–25 time) which are the result of major reorientations in spreading directions in the central Atlantic Ocean.     

The distribution of particulate iodine in the Atlantic Ocean

The iodine content of marine suspended matter obtained from thirteen stations in the Atlantic between 75°N and 55°S has been measured. The concentration of particulate iodine is high in the surface, up to 127 ng/kg of seawater being observed. Below the euphotic zone, it drops sharply to 1–2 ng/kg. The iodine-containing particles are probably biogenic. A simple box-model calculation shows that only 3% of the particulate iodine produced in the surface water may reach the deep sea and that the residence time of these particles in the surface water is about 0.1 year.     

大西洋中北部双频微地动特征

本文对大西洋中北部两侧五个地震台站2015年记录到的地震数据进行处理,计算噪声功率谱密度和概率密度函数,并通过极化分析对双频微地动不同周期的主导源区方位角分布进行了分析.研究结果显示:大西洋中北部台站双频微地动发生显著分裂,各台站的峰值周期各不同,且来自相同方向和不同方向的双频微地动都有可能产生双频微地动分裂;大西洋中...     

The Mutual Response of the Tropical Squall Line and the Ocean

—The Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) is used to investigate the mutual response of a tropical squall line and the ocean. Simulated squall line compares well with the observations, and consists of counterrotating vortices, and has a bow shape bulge toward the leading edge. In addition to these features, which are also shown in the previous numerical simulations, the unique results from the coupled simulation indicate that the air–sea interaction processes within the squall line are important. They affect both the atmosphere and the ocean locally. Simulated upper ocean displays significant response to the squall line with upwelling and baroclinicity. Depth of the ocean mixed layer in the coupled simulation becomes modified due to feedback processes. Ocean temperature acts as a destabilizing factor, and the salinity as a stabilizing factor. Surface turbulent fluxes from the coupled simulation are about 10% less than that of the uncoupled simulation. The SST in the coupled simulation decreases by about 0.21°C. Predicted squall line in the coupled simulation is weaker as compared to the uncoupled simulation. This is reflected in terms of differences in surface fluxes, cloud water, rain water and vertical velocities between the two simulations.     

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.     

The distribution of dissolved copper in the eastern Atlantic Ocean

Dissolved copper concentrations at five stations in the eastern Atlantic between 20 and 24°N are presented. The metal is depleted in surface waters with concentrations ranging from 1.1 to 1.7 nmol/l. Marked increases in concentration were found in near-bottom waters with levels up to 10.9 nmol/l; these are attributed to the release of dissolved copper from the surface sediment.     

On forecasting abnormal climatic events in the tropical Atlantic Ocean

Modelling and observational evidence indicate that interannual variabilities of dynamic height and sea surface temperature (SST) in the eastern part of the tropical Atlantic Ocean (Gulf of Guinea) are largely induced by preceding fluctuations in wind stress, mainly in the western equatorial basin. A wind-driven linear ocean model is used here to test the possibility of forecasting the abnormal dynamic heights. A control run of the model, forced by 1964–1993 wind stress monthly means, is first conducted. Yearly test runs (1964-1994) are subsequently performed from January to August by forcing the model with observed winds from January to May, and then by forcing with the May wind assumed to persist from June to August. During the last three decades the largest deviations of dynamic height simulated by the control run in the Gulf of Guinea in boreal summer would have been correctly forecast from wind data related only to conditions in May of each year. However, for weak climatic anomalies, the model may forecast overestimated values. For the most part (about 20 times during the last 30 years), the sign of the observed SST anomaly in the centre of the Gulf of Guinea during the boreal summer is identical to the sign of simulated anomalies of dynamic height deduced from both control and test runs. Along the eastern equatorial waveguide, the sea level forecasting skill slowly decreases from the first 2 weeks of June until the second 2 weeks of August, but remains high on both sides of the equator throughout boreal summer, as is expected from the adjustment in a linear ocean model. It is established that throughout the year in the Gulf of Guinea the accuracy of the 1-month forecast dynamic height anomaly provided by the simple linear method is greater than that of the 1-month forecast assuming persistence.     

Charge distribution on the atmospheric aerosol over the North Atlantic and the Indian Ocean

The distribution of electric charge on the marine aerosol was determined near the sea surface of the Indian Ocean and the North Atlantic during the final stage of the Snellius II-expedition. Mean values for small ion concentrationsn ⁺=455 cm^–3 andn ^–=340 cm^–3 were found over the Atlantic, whilen ⁺=310 andn ^–=250 cm^–3 were the mean values over the Indian Ocean. The ration ⁺/n^– increased from 1.2 to 2.0 with decreasing wind velocity. At wind velocities below 5 m/s 75% of the net space charge near the ocean surface was found to be carried by small ions.     

Lateral variation of Rayleigh wave dispersion character Part III: Atlantic Ocean,Africa and Indian Ocean

Summary In continuation of Part II, investigations of the lateral variation of group velocity dispersion character of Rayleigh waves were carried out for the Atlantic and Indian Oceans and for Africa. Maximum group velocity at such short periods as 25 to 30 seconds is observed in the Mid-Atlantic Ridge, in agreement with earlier observations from the East Pacific Rise, but was not observed for the Indian Rise. Less continental dispersion regions are found in Africa and in the whole of the Arabian Peninsula. Eastern half of the Indian Ocean seems to have a typical oceanic crustal structure. The dispersion patterns were checked in every area by earlier data. All results so far obtained by the present author on the lateral variation of dispersion characteristics have been compiled into a world map.     

The distribution of13C in the Atlantic Ocean

Individual vertical profiles and north-south sections for the distribution of theδ¹³C of total dissolved inorganic carbon are presented for the Atlantic stations of the GEOSECS program. In most cases theδ¹³C data parallel the distribution of dissolved O₂. Differences are attributed to in-situ oxidation of organic matter and dissolution of particles of CaCO₃. Antarctic Bottom and Intermediate Waters have aδ¹³C value of near 0.5‰ relative to the PDB isotopic standard. The lowest values in the Atlantic ocean were found in the Antarctic Circumpolar waters which haveδ¹³C values as low as 0.2‰. The core of the North Atlantic Deep Water has aδ¹³C value of 1.0‰.