Investigations of gram quantities of Atlantic and Pacific surface particulates

Particulates amounting to 0.1–2.0 g efficiently collected from large volumes of Atlantic and Pacific surface waters have been analyzed for carbonate, opal, quartz and several natural and man-made radioisotopes.The concentrations of particles range between 10 and 600 μg/kg. In the equatorial regions particle concentrations are low and similar in both the oceans. At higher latitudes (>30°N or S), the Atlantic waters, however, have higher concentrations of particles compared to those in the Pacific. The latitudinal distribution exhibits a north-south symmetry with higher concentrations in the 30°–60° belt. Based on the particulate abundance for CaCO₃ and opal and their sedimentation, we have estimated their production and in-situ integrated dissolution rates for a few regions.Radioisotopes having different source functions, namely¹⁴C and²³⁹Pu injected due to nuclear weapon tests,²³⁴Th,²³⁰Th and²²⁸Th produced in-situ in seawater,²³²Th which derives primarily from land,²¹⁰Pb introduced via wet precipitations and²²⁶Ra introduced through diffusion from deep-sea sediments have been measured in the particulates. The relative enrichment factors for these nuclides in particles vary as Th ? Pu > Pb > Ra. The atmospheric bomb fallout pattern is discernible in the surface particulates; the²³⁹Pu concentration increases with latitude in both the hemispheres; however, the values are about a factor of two lower in the southern hemisphere.The distribution pattern of radioisotopes is found to be complex, even for²³⁴Th whose source function in the oceans is uniform. In view of the differences in the source functions it becomes possible to delineate the principal geochemical/geophysical processes which determine the concentrations of these nuclides in surface waters.     

South Atlantic sea surface temperature anomalies and air-sea interactions: stochastic models

Data on the South Atlantic monthly sea surface temperature anomalies (SSTA) are analysed using the maximum-entropy method. It is shown that the Markov first-order process can describe, to a first approximation, SSTA series. The region of maximum SSTA values coincides with the zone of maximum residual white noise values (sub-Antarctic hydrological front). The theory of dynamic-stochastic climate models is applied to estimate the variability of South Atlantic SSTA and air-sea interactions. The Adem model is used as a deterministic block of the dynamic-stochastic model. Experiments show satisfactorily the SSTA intensification in the sub-Antarctic front zone, with appropriate standard deviations, and demonstrate the leading role of the abnormal drift currents in these processes.     

Transport,sloughing and settling rates of estuarine macrophytes: Mechanisms and ecological implications

The study of plant-bound nutrient transport has been largely neglected in estuaries. Lately however, it has been shown that nutrients bound to macroalgae and seagrasses can constitute a major part of the nutrient transport in shallow tidal estuaries. Organic detritus in estuaries comes from various sources. This paper looks into the source of detritus from sloughing, and the transport behaviour of plant detritus under unidirectional flows. In order to determine the extent of the sloughing of macrophytes, the threshold current velocities for the traction and resuspension of the most common submersed macrophytes in Venice Lagoon (Ulva lactuca, Enteromorpha sp., Ceramium rubrum, Cladophora sp., and Chaetomorpha linum) were studied in a laboratory flume. It was found that all macrophytes subjected to flows of 1.5–3.0 cm s⁻¹ move initially as bed load. The threshold for suspension of the macrophyte tissue was at current speeds >3 cm s⁻¹. The exception was the filamentous macroalgae, C. linum, which moved as bed load at all current speeds. This implies that the advection of plant-bound nutrients in Venice Lagoon is widespread and takes place over virtually all stages of the tide.     

A high-resolution satellite-derived sea surface temperature climatology for the western North Atlantic Ocean

Long-term and high-resolution (∼1.2 km) satellite-derived sea surface temperature (SST) fields of a monthly mean time series for the 1985–1999 period, and a daily climatology have been calculated for the North West Atlantic Ocean. The SST fields extend from 78°W to 41°W in longitude, and 30°N to 56°N in latitude, encompassing the region off Cape Hatteras, North Carolina, to the southern Labrador Sea. The monthly mean time series, consists of 180 cloud-masked monthly mean SST fields, derived from a full-resolution NOAA/NASA Pathfinder SST data set for the 1985–1999 period. The satellite-derived monthly mean SST fields, as compared with in situ monthly mean near-surface ocean temperatures from buoys located in the western North Atlantic, yield an overall RMS difference of 1.15 °C. The daily climatology, which consists of 365 fields, was derived by applying a least-squares harmonic regression technique on the monthly mean SST time series for the full study period. The monthly mean and daily climatological SST fields will be useful for studying inter-annual variability related to climate variability of SST over the study domain.     

Dissolution rates of calcium carbonate in the deep ocean; an in-situ experiment in the North Atlantic Ocean

An in-situ water circulator (ISWAC) was developed to allow accurate measurement of dissolution rates of various carbonate particles with minimal stagnation and mechanical weight loss. Three ISWAC packages were deployed at 3600, 4800 and 5518 m for 79 days in the Sargasso Sea (Northwest Atlantic). Weight losses for different particles during 79 days at 5518 m were as follows: pteropods and synthetic aragonite, 72.8%; reagent calcite, 57.5%; foraminifera, 23–36%; coccoliths, 11.3–24%, and diatoms, 12%. These weight losses are 2.5–7.5 times higher than those reported in earlier in-situ experiments. Normalization of weight losses with respect to BET specific surface area for different calcite particles yielded specific dissolution rates that differed by more than 2 orders of magnitude. Bleached biogenic particles dissolve significantly faster than non-bleached although their surface area is identical. We suggest that the BET surface area does not represent the reactive surface area available for dissolution, especially in biogenic calcite particles. Coatings, probably of organic matter, may reduce the reactive surface area and thus retard dissolution rates.The existence of a chemical lysocline in the Northwest Atlantic was confirmed. However, it seems that different kinds of particles have different lysoclines. The origin of the lysocline cannot be attributed to water flow or to the thermodynamic transition from supersaturation to undersaturation (Ω = 1). It seems to be a kinetic phenomenon.A simple model comparing the complete dissolution time and the residence time of a particle on the sediment-water interface suggests that coccoliths can be preserved in the sediments of the deep Northwest Atlantic below the CCD, in good agreement with SEM observations.     

Deglacial sea surface temperature and salinity increase in the western tropical Atlantic in synchrony with high latitude climate instabilities

A sediment core from the western tropical Atlantic covering the last 21,000 yr has been analysed for centennial scale reconstruction of sea surface temperature (SST) and ice volume-corrected oxygen isotopic composition of sea water (δ¹⁸O_ivc-sw) using Mg / Ca and δ¹⁸O of the shallow dwelling planktonic foraminifer Globigerinoides ruber (white). At a period between 15.5 and 17.5 kyr BP, the Mg / Ca SST and δ¹⁸O_ivc-sw, a proxy for sea surface salinity (SSS), reveals a warming of around 2.5 °C along with an increase in salinity. A second period of pronounced warming and SSS increase occurred between 11.6 and 13.5 kyr BP. Within age model uncertainties, both warming intervals were synchronous with air temperature increase over Antarctica and ice retreat in the southern South Atlantic and terminated with abrupt centennial scale SSS decrease and slight SST cooling in conjunction with interglacial reactivation of the meridional overturning circulation (MOC). We suggest that during these warm intervals, production of saline and warm water of the North Brazil Current resulted in pronounced heat and salt accumulation, and was associated with warming in the southern Atlantic, southward displacement of the intertropical convergence zone and weakened MOC. At the termination of the Younger Dryas and Heinrich event 1, intensification of cross-equatorial heat and salt transport caused centennial scale cooling and freshening of the western tropical Atlantic surface water. This study shows that the western tropical Atlantic served as a heat and salt reservoir during deglaciation. The sudden release of accumulated heat and salt at the end of Younger Drays and Heinrich event 1 may have contributed to the rapid reinvigoration of the Atlantic MOC.     

Satellite observation of Brazil Current inshore thermal front in the SW South Atlantic: Space/time variability and sea surface temperatures

A data set of 199 sea surface temperature maps derived from the Advanced Very High Resolution Radiometer for the period 2000–2002 was processed to derive the position of the surface inshore thermal front of the Brazil Current (BCIF) in the SE Brazilian coastal and oceanic area. After the derivation of the position of the BC front for each image, the ensemble of digital frontal vectors was processed using the algorithm of frontal density (FD). For each 5′×5′ cell in the domain the calculated FD provided an index expressing the presence and persistence of the front in the area or the probability of finding the front in the region. In the paper we present the results of the FD analysis to get a better view of the space and time variability of the BC front in the region. The highest values of FD were in general observed close to or at the shelf break zone (between 200 and 1000 m isobaths). From 20°S to 23°S there is a tendency of BCIF to be positioned over the outer shelf, inshore of the 200 m isobaths. SE of Cape Sao Tome and S of Cape Frio it was observed a bimodal spatial distribution of highest FD caused by the presence of two semi-permanent frontal eddies. After moving offshore near Cape Frio, the BCIF tends to return to the shelf break zone south of 24°S probably due to a potential vorticity conservation mechanism. The position of the highest FD values calculated for different seasons confirms previous studies in that BCIF is closer to the coast during the summer and furthest offshore in the winter. Statistical analysis of the SST data gave for the BCIF an average SST gradient of 0.31°C km⁻¹ with a standard deviation of 0.15°C km⁻¹. A mean frontal width of 6 km was inferred from the average SST gradient and typical temperatures near the front at both sides, at outer shelf and in the BC itself. A Weibull probability density function can be fitted to describe the BCIF SST gradients with scale factor c=0.3460°C km⁻¹ and shape factor k=2.1737. The BCIF SST gradient showed a seasonal variability with the smallest gradients in summer (~0.24 °C km⁻¹) and the highest in autumn (~0.33 °C km⁻¹). Using a three harmonic Fourier fit for the SST field near the BCIF, at the outer shelf and at interior of BC, it was possible to derive an analytical model for the time variability of the SST gradient of BCIF.     

Particulate aluminium, iron and manganese chemistry at the deep Atlantic boundary layer

Scanning electron microscopy and instrumental neutron activation analyses of filtered suspended matter from the Atlantic Ocean show that particulate aluminium (Al_p) is a sensitive measure of bottom derived or resuspended material. The proportion of Al_p in suspended particulate matter (SPM) increases slightly between surface and intermediate depths but shows large and steady increases in deep waters with approach to the bottom.

Fe_p/Al_p andMn_p/Al_p ratios are always higher than the crustal ratios throughout the water column. We show that the processes which can explain such enrichments are different for particulate matter in surface waters (scavenging, incorporation in biogenic particles) than for resuspended material (precipitation from interstitial waters on surficial sediments).

Close to the Mid-Atlantic Ridge, the bottom suspended matter exhibits higher Fe_p/Al_p andMn_p/Al_p ratios than in abyssal plains. A ridge crest source must be invoked to explain the striking enrichment of Mn_p. This source could also explain the enrichment of Fe_p, although primarily because the resuspended flux is small in that region, one cannot exclude the contribution of particles from the mid-water column.     

Linkage of deep sea rapid acidification process and extinction of benthic foraminifera in the deep sea at the Paleocene/Eocene transition

Ocean Drilling Program Leg 199 Site 1220 provides a continuous sedimentary section across the Paleocene/Eocene (P/E) transition in the carbonate‐bearing sediments on 56–57 Ma oceanic crust. The large negative δ¹³C shift in seawater is likely due to the disintegration of methane hydrate, which is expected to be rapidly changed to carbon dioxide in the atmosphere and well‐oxygenated seawater, leading to a reduction in deep‐sea pH. A pH decrease was very likely responsible for the emergence of agglutinated foraminiferal fauna as calcareous fauna was eliminated by acidification at the P/E transition at Site 1220. The absence of the more resistant calcareous benthic foraminifera and the presence of the planktonic foraminifera at Site 1220 is interesting and unique, which indicates that calcareous benthic foraminifera suffered greatly from living on the seafloor. Box model calculation demonstrates that, assuming the same mean alkalinity as today, pCO₂ must increase from 280 ppm to about 410 ppm for the calcite undersaturation in the deep ocean and for the oversaturation in the surface ocean during the P/E transition. The calculated increased pCO₂ coincides with paleo‐botanical evidence. The current global emission rate (～7.3 peta (10¹⁵) gC/y) of anthropogenic carbon input is approximately 30 times of the estimate at the P/E transition. The results at the P/E transition give an implication that the deep sea benthic fauna will be threatened in future in combination with ocean acidification, increased sea surface temperature and more stratified surface water.     

大西洋和西太平洋海域热带气旋海表面入流角和拖曳系数特征对比分析

本文利用大量全球定位下投式探空仪(dropsonde)资料,对15个大西洋飓风和11个西太平洋台风海域的入流角频数分布差异进行对比,探究在热带气旋不同强度和不同边界层平均风速下入流角的分布特征,分析了两个海域拖曳系数随入流角变化的趋势.结果表明,两海域在热带气旋背景下的入流角分布具有共同点:在强度更大的热带气旋里入流角...     

Oxygen isotope fractionation in dissolved oxygen in the deep sea

¹⁸O variations in dissolved oxygen have been measured at five stations from the eastern equatorial Pacific, at the GEOSECS-I and -II intercalibration stations in the North Pacific and North Atlantic, and along an Antarctic-South Pacific section from MONSOON expedition. Relative to atmospheric oxygen, dissolved oxygen in the ocean is enriched in¹⁸O up to a maximum of 14‰, the extreme enrichments occurring in the oxygen-minimum region of the eastern Pacific. The vertical diffusion-advection model has been used to determine the isotopic fractionation of deep-water in-situ oxygen consumption ascribed to bacterial metabolism. The single-stage enrichment, ε, in Pacific Deep Water below 1 km is 10‰ (α = 0.99,¹⁶O consumed preferentially). The model calculations show that the isotopic data cannot be fit without the introduction of a fractionation factor, just as the dissolved oxygen data cannot be fit without an in-situ consumption parameter. The consistency of the positive sign for ε and the negative source term for O₂, observed in all deep Pacific profiles analyzed to date, provide strong evidence that vertical transport and in-situ consumption terms dominate the horizontal tracer flux terms, and indicate the presence of a significant “deep metabolism” in abyssal ocean waters.     

Isotopic fractionations during oxygen consumption and carbonate dissolution within the North Atlantic Deep Water

The trajectory of the North Atlantic Deep Water is traced from 65°N to 20°N latitude. Along this track the dissolved O₂ decreases, theδ¹⁸O of the dissolved O₂ increases, and the¹⁴C content of the water decreases. From these observations the rate of in-situ O₂ utilization in the deep water is calculated to be 0.10 μmol kg^?1 yr^?1. As was observed previously in the Pacific, theδ¹⁸O data presented here indicate that the utilization is probably caused by bacterial respiration. Carbon dioxide is being added to the water at the rate of 0.07 μmol kg^?1 yr^?1 from the oxidation of this organic matter. An additional 0.12 μmol kg^?1 yr^?1 of CO₂ is derived from the dissolution of particles of CaCO₃.     

A scavenging model for trace elements in the deep sea

A vertical diffusion-advection model with a scavenging removal rate which is first-order in concentration is applied to the deep-water Geosecs-I profiles of four trace metals. The actual model scavenging rates are obtained by calibration of the vertical advection velocity with the radiocarbon profile from the same station — the¹⁴C data givew = 3.7m/yr. “Scavenging residence times”, τ_ψ, of 1400 years for Cu, 3200 years for Sb, and 2500 years for Sc are obtained. For these three elements the fit of the data to the scavenging-model equation is better than the fit to a conservative mixing model with no in-situ removal. TheF_χ comparison test is used to assess the probability that the improved fit given by the scavenging model is due merely to a random selection of data from an infinite set of values with a distribution governed by the stable-conservative model. This probability is found to be only 0.6% for Cu and only 3% for Sb. For Sc, however, the probability is 19% so that the two models cannot be distinguished for this element. The Ni profile shows no effect of scavenging; a lower limit of 3000 years is estimated for τ_ψ but the value is probably much greater. The association of Cu and Pb as two elements with definite indication of scavenging in deep water is consistent with experimental adsorption studies; the lack of indication of scavenging of Ni is consistent with the hypothesis that clay minerals are the scavenging agent for Pb and Cu. The method of comparing scavenging and conservative mixing-model fits to the profiles provides a mechanism for defining a set of trace elements which show consistent scavenging effects over large ocean areas, so that the reality of the deep-sea scavenging process for non-radioactive elements can be tested.     

Particulate and soluble210Pb activities in the deep sea

Particulate and soluble,²¹⁰Pb activities have been measured by filtration of large-volume water samples at two stations in the South Atlantic. Particulate phase²¹⁰Pb (caught by a 0.4-μm filter) varies from 0.3% of total²¹⁰Pb in equatorial surface water to 15% in the bottom water. The “absolute activity” of²¹⁰Pb per unit mass of particulate matter is about 10⁷ times the activity of soluble²¹⁰Pb per unit mass of water, but because the mass ratio of particulate matter to water is about 10^?8, the particulate phase carries only about 10% of the total activity. In Antarctic surface water the particulate phase carries 40% of the total²¹⁰Pb activity; the absolute activity of this material is about the same as in other water masses and the higher fraction is due to the much larger concentration of suspended matter in surface water in this region.In the equatorial Atlantic the particulate phase²¹⁰Pb activity increases with depth, by a factor of 40 from surface to bottom, and by a factor of 4 from the Antarctic Intermediate Water core to the Antarctic Bottom Water. This increase with depth is predicted by our previously proposed particulate scavenging model which indicated a scavenging residence time of 50 years for²¹⁰Pb in the deep sea. A scavenging experiment showed that red clay sediment removes all the²¹⁰Pb from seawater in less than a week. The Antarctic particulate profile shows little or no evidence of scavenging in this region, which may be due to the siliceous nature of the particulate phase in circumpolar waters. Our previous observation that the²¹⁰Pb/²²⁶Ra activity ratio is of the order of 0.5 in the deep water is further confirmed by the two South Atlantic profiles analyzed in the present work.     

Seasonal fluctuations in the deep central equatorial Atlantic Ocean: a data–model comparison

Low-frequency current fluctuations in the deep central equatorial Atlantic are analyzed using current meter measurements recorded from November 1992 to November 1994. Current meters were located at about 14°W of longitude and 1° of latitude on both sides of the equator between 1,700 m depth and the ocean bottom. At all sampling depths, the velocity fluctuations are dominantly zonal and symmetrical with respect to the equator. At 1,700 and 2,000 m, the flow is dominated by annual period fluctuations, at 3,000 m, the velocity field amplitude presents a minimum, and at 3,750 and 3,950 m, the flow is modulated by annual and semiannual period variability. The annual signal exhibits an apparent upward phase propagation. When considering the phase and the amplitude of the seasonal fluctuations, the data compare well with the outputs of a realistic numerical simulation of the Atlantic Ocean. Together with a previous analysis of the model simulations, this supports the idea that the observed annual fluctuations are due to wind-forced vertically propagating Kelvin and Rossby waves. Data and model do not provide deciding evidences of the presence of semiannual equatorial waves deeper than 3,500 m depth in the central equatorial Atlantic Ocean.     

Plastic debris in the surface waters of the South Atlantic

Polyethylene and polypropylene pellets, together with tarballs, were found to be the most common contaminates of the sea surface in the Cape Basin area of the South Atlantic Ocean. This is an area far removed from any obvious source of such materials.