Regions Of High Productivity Research Articles

Authigenic barites and fluxes of barium associated with fluid seeps in the Peru subduction zone

Large deposits of barite were discovered in association with biological communities, indicative of active fluid seepage on the middle slope of Paita and in the Chiclayo Canyon, in the Peru margin. We postulate that the barium source for the deposits is associated with the high concentration of non-detrital barite buried in sediments from this high productivity region. Barite is remobilized within the sediment column due to sulfate depletion. Subsequent flushing of the barium-rich fluids from the sediment to the bottom water, leads to the formation of barite deposits at the cold vent sites. High barium concentrations measured in pore fluids of sediments are consistent with remobilization of barium sulfate below the zone of sulfate depletion. Fluid samples—collected in a time sequence using a benthic chamber in the Paita middle slope vent sites — document a contemporaneous release of barium to the bottom water at a rate of 23 μmol cm−2 yr−1.Fluid seepage in the Peru margin is not restricted to the middle slope of Paita and the Chiclayo Canyon where barite deposits occur, but is also evident in the upper and lower slopes of Paita and in the Chimbote upper slope. Deployment of a benthic chamber on the Chimbote upper slope site show no measurable release of barium; even though the dissolved barium concentration in the pore fluids is high. These observations indicate that the barite deposits associated with fluid seepage in the Peru margin are restricted to areas where slope failure has exposed sequences deep enough such that the barium-rich fluids do not encounter sulfate-bearing pore fluids before emanating at the seafloor.

Potential impact of iodine on tropospheric levels of ozone and other critical oxidants

A new analysis of tropospheric iodine chemistry suggests that under certain conditions this chemistry could have a significant impact on the rate of destruction of tropospheric ozone. In addition, it suggests that modest shifts could result in the critical radical ratio HO2/OH. This analysis is based on the first ever observations of CH3I in the middle and upper free troposphere as recorded during the NASA Pacific Exploratory Mission in the western Pacific. Improved evaluations of several critical gas kinetic and photochemical rate coefficients have also been used. Three iodine source scenarios were explored in arriving at the above conclusions. These include: (1) the assumption that the release of CH3I from the marine environment was the only iodine source with boundary layer levels reflecting a low‐productivity source region, (2) same as scenario 1 but with an additional marine iodine source in the form of higher molecular weight iodocarbons, and (3) source scenario 2 but with the release of all iodocarbons occurring in a region of high biological productivity. Based on one‐dimensional model simulations, these three source scenarios resulted in estimated Ix (Ix = I + IO + HI + HOI + 2I2O2 + INOx) yields for the upper troposphere of 0.5, 1.5, and 7 parts per trillion by volume (pptv), respectively. Of these, only at the 1.5 and 7 pptv level were meaningful enhancements in O3 destruction estimated. Total column O3 destruction for these cases averaged 6 and 30%, respectively. At present we believe the 1.5 pptv Ix source scenario to be more typical of the tropical marine environment; however, for specific regions of the Pacific (i.e., marine upwelling regions) and for specific seasons of the year, much higher levels might be experienced. Even so, significant uncertainties still remain in the proposed iodine chemistry. In particular, much uncertainty remains in the magnitude of the marine iodine source. In addition, several rate coefficients for gas phase processes need further investigating, as does the efficiency for removal of iodine due to aerosol scavenging processes.

A 340 year record of biogenic sulphur from the Weddell Sea area, Antarctica

Detailed records of methanesulphonic acid (MSA) and non-sea-salt sulphate (nss SO42−) have been obtained from ice cores drilled on Dolleman Island on the east coast of the Antarctic Peninsula (70°35.2′S, 60°55.5′W). Annual average concentrations of MSA are presented for the period 1652–1992. Over this time span, the mean annual concentration of MSA is 0.69 μeq l−1 (σ = 0.33, n = 340), the range is 0.13–2.35μeq l−1, and the MSA/nss-SO42− ratio is 0.22. The high MSA concentration reflects the proximity of the Weddell Sea, believed to be a region of high marine phytoplankton production. The overall mean nss-SO42− concentration is about 66% of the total sulphate deposited in the snowfall. Low-frequency variations of MSA and oxygen-isotope signals correlate closely, indicating that they may be modulated by similar atmospheric processes. Positive correlations are observed between the oxygen-isotope signature and both MSA (r = 0.41) and nss SO42− (r = 0.50), significant at the 99% level. A small negative correlation can be seen between both species and the annual duration of sea ice at Scotia Bay, Laurie Island in the South Orkneys, since 1902 (MSA r= –0.23, and nss SO42–r = –0.29; significant at 95% confidence). No significant link between high MSA concentrations and El Niño events is observed at this location.

A 340 year record of biogenic sulphur from the Weddell Sea area, Antarctica

Detailed records of methanesulphonic acid (MSA) and non-sea-salt sulphate (nss SO4 2−) have been obtained from ice cores drilled on Dolleman Island on the east coast of the Antarctic Peninsula (70°35.2′S, 60°55.5′W). Annual average concentrations of MSA are presented for the period 1652–1992. Over this time span, the mean annual concentration of MSA is 0.69 μeq l−1 (σ = 0.33, n = 340), the range is 0.13–2.35μeq l−1, and the MSA/nss-SO4 2− ratio is 0.22. The high MSA concentration reflects the proximity of the Weddell Sea, believed to be a region of high marine phytoplankton production. The overall mean nss-SO4 2− concentration is about 66% of the total sulphate deposited in the snowfall. Low-frequency variations of MSA and oxygen-isotope signals correlate closely, indicating that they may be modulated by similar atmospheric processes. Positive correlations are observed between the oxygen-isotope signature and both MSA (r = 0.41) and nss SO4 2− (r = 0.50), significant at the 99% level. A small negative correlation can be seen between both species and the annual duration of sea ice at Scotia Bay, Laurie Island in the South Orkneys, since 1902 (MSA r= –0.23, and nss SO4 2– r = –0.29; significant at 95% confidence). No significant link between high MSA concentrations and El Niño events is observed at this location.

A dynamic box model of bioactive elements in the southern Taiwan Strait

A dynamic box model was applied to study the characteristics of biogeochemical cycling of PO4−P, NO3−N, AOU, POC and PON in the southern Taiwan Strait region based on the field data of the “Minnan Taiwan Bank Fishing Ground Upwelling Ecosystem Study” during the period of Dec. 1987–Nov. 1988. According to the unique hydrological and topographical features of the region, six boxes and three layers were considered in the model. The variation rates and fluxes of elements induced by horizontal current, upwelling, by diffusion, sinking of particles and biogeochemical processes were estimated respectively. Results further confirmed that upwellings had important effects in this region. The nearshore upwelling areas had net input fluxes of nutrients brought by upwelling water, also had high depletion rates of nutrients and production rates of particulate organic matter and dissolved oxygen. The abnormal net production of nutrients in the middle layer, (10–30 m) indicated the important role of bacteria in this high production region. The phytoplankton POC contributed about 28% of the total POC. POC settling out from the euphotic zone was estimated to be 2×10−6 g/(m2·s) which was about 35% of the primary production.

A quantitative study of microbial decomposition of biopolymers in Recent sediments from the Peru Margin

The relationship between depth changes in total organic matter (TOC), biopolymers (“carbohydrates”, “proteins” and lipids) and bacterial populations and their activity, were investigated on two cruises to an upwelling site on the Peru Margin. Samples were obtained from the sediment surface to 8020 cm below sea floor (cbsf). Bacterial concentrations increased from near surface to a maximum at 12.5 cbsf, then rapidly decreased to 20 cbsf and a slower decrease to 8020 cbsf. The bacterial population at 8020 cbsf was 9.5% of the near surface value, but at 3.3 × 10 8 cells/cm 3 it was still very significant. The presence of dividing bacterial cells indicated that a portion of the population was active and this was confirmed by radiotracer measurements. Biopolymers were overestimated in the surface 4 cm (138% of the TOC). By 5.5 cbsf, however, organic matter uncharacterized as biopolymers was present. This increased rapidly to 17% of the TOC at 10 cbsf and then more slowly to 79% at 2229 cbsf. Bacterial populations and activity were significantly correlated with depth changes in both TOC and biopolymers ( p < 0.002). Calculation of rates of anaerobic carbon metabolism from the measured rates of sulphate reduction and methanogenesis demonstrated that these processes could account for all (101%) of the decrease in “carbohydrate” and “protein” in the 2.5 to 22.5 cbsf interval, where bacterial activity was intense. These data confirm the importance of anaerobic bacterial processes in these high organic matter sediments. At deeper intervals, 22.5–480 and 480–8020 cbsf, the decrease in “carbohydrate” and “protein” only accounted for 40% and 15% respectively of the anaerobic bacterial metabolism and hence the significant bacterial population present must have been utilizing a portion of the increasing uncharacterized organic matter. There was a negative relationship between the total bacterial population and the percentage of uncharacterized organic matter, suggesting that a portion of the uncharacterized organic matter may have come from dead bacterial cells. Estimates of bacterial necromass production from rates of thymidine incorporation were significantly correlated with increases in uncharacterized organic matter ( p < 0.05, 2.5–197.5 cbsf) and could account for 16% of the increase within the top 25 cm. However, as the thymidine incorporation technique underestimates bacterial productivity in anoxic sediments it is likely that a much greater proportion of the increase in uncharacterized organic matter should be attributed to “bacterial necromass” production. It is suggested that bacterial necromass production may be an important mechanism for the production of recalcitrant, and hence preserved, organic matter in sediments in high productivity regions.

Biogeography of Recently Extinct Marine Species: Implications for Conservation

At least fifteen marine and coastal animal species have become extinct since the end of the Pleistocene. Analyses of the number of marine biogeographical provinces occupied by these species show that, contrary to the prevailing view that extinction is usually associated with a small range, at least five species (33%) had large ranges encompassing parts of two or more provinces. At least eight species occurred in areas that served as geographical refuges for taxa during the Pliocene and early Pleistocene. These refuges, in which primary planktonic productivity is generally very high, include the northwestern Atlantic and North Pacific. Extinctions in the northwestern Atlantic (four species) may be partly responsible for the subsequent success of human‐introduced species in subtidal and open‐coast intertidal habitats. Regions of high productivity deserve priority among marine areas to be protected from overexploitation and habitat destruction.

Modern and Ancient Continental Shelf Anoxia

Drs. R. V. Tyson and T. H. Pearson convened a symposium on modern and ancient shelf anoxia in May, 1989 under the aegis of the Gologica] Society Marine Studies Group. This volume is the product of that meeting. The meeting was multidisciplinary including geologists, biologists and chemists. The examination and interpretation of anoxic sedimentary environments is of significance in our understanding of the interplay of organic production, water circulation and sediment accumulation. When the flux of degradable organic material exceeds the oxygen supply, then anoxia and the exclusion of macrobenthos removes the more common bioturbation effects preserving primary depositional structures. It has also been postulated that the anoxic environment permits preservation of a larger proportion of the incoming organic matter. Studies in a variety of modern environments has tended to argue against that hypothesis although certainly the role of sulfate reducing bacteria versus aerobic bacterial processes is a major factor in the ultimate sedimentary product. Within the broad spectrum of anoxic environments, shelf anoxia is a special case. One complicating factor for the stratigrapher is that the anoxic condition in several shelf environments discussed in these papers is temporary and ephemeral. Thus a primary question is what artifacts of these periods of low or zero oxygen will remain in the sedimentary record? As the convenors point out in their opening review, the modern effects impact fisheries and with the overprinting of anthropogenic effects produce some complex questions for the marine biologist. They also aver that much of the hydrocarbon-rich sedimentary resources were laid down and developed in organic-rich and oxygen-depleted depositional environments. As noted earlier, the sometimes higher organic content of such sediments reflects the common association of high productivity regions (upwelling regions) with oxygen-deficient depositional environments. Even if degradation and removal rates are similar for all depositional environments, certainly the net input under high production areas will be larger. Major factors influencing oxygendeficiency in shelf regions are the higher biological production in margins as compared to open ocean, the short fall path in margins compared to open oceans and resulting higher sediment input rates, higher sediment accumulation rates and thus thicker mass accumulation and higher burial rates for carbon. Bioturbation is also operating at higher rates, and fluctuations in depositional regime are larger and faster. The editors also introduce a suggested standard terminology for such environments in which the environmental facies are termed oxic, dysoxic (with subdivisions), suboxic and anoxic with respective bottom water oxygen contents of > 2.0 ml/L, 0.2-2.0 ml/L, 0-0.2 ml/L and <0.0 ml/L (increasing dissolved H2S). Biofacies terms for these are respectively aerobic, dysaerobic, quasi-anaerobic and anaerobic at the same oxygen boundaries. Physiological regimes would be Normoxic, Hypoxic (no matching term for Suboxic) and anoxic respectively. The volume is subdivided into modern shelf anoxia studies, and ancient shelf anoxia studies. The papers in the two sections number 11 and 16 respectively and space does not permit review of each paper. The modern studies examine the northern Gulf Of Mexico shelf associated with or adjacent to the Mississippi River discharge (papers by Boesch and Rabalais; Rabalais, Turner, Wiseman and Boesch; Harper, McKinney, Nance and Salzer)) which discuss effects on benthos, recovery of communities and general oceanographic characteristics of the environment. This area may well be one in which the anoxia has an anthropogenic source as noted in the paper by Rabalais and others. Contrasts with other areas are noted. The effects are of the order of 4-5 months in duration in the delta front area. A paper by Van der Zwaan and Jorissen compares the effects in the northern Adriatic, Orinoco-Paria shelf and the Mississippi locale. All are influenced by high river discharges and pollution effects. The authors utilize benthic foraminifera as indi-

Experimental Study of an Embedded Longitudinal Vortex Interacting with a Three-Dimensional Turbulent Boundary Layer. 3rd Report, Triple Products and Production Mechanism.

The present research is undertaken to further the physical understanding of the behavior of an embedded vortex in a three-dimensional turbulent boundary layer. The vectors of instantaneous transport of turbulent kinetic energy are large above the vortex and in the downwash region, in Case 2. The production terms are mainly contributed to the values of V2 and W2. Then these profiles show an elliptic shape centered on the peak vorticity point. Both the production and advection terms of turbulent kinetic energy fall rapidly between the first and second measurement stations. In Case 2, a region of high production develops in the upwash region. The contribution of production and advection results in a uniform high value of turbulent kinetic energy in the entire region above the vortex.

Aspects of the geochemistry of sedimentary sterols in the Chang Jiang estuary

Sediment samples have been collected during the Donghai 1 cruise in January 1986 in the Chang Jiang estuarine region for which solvent extractable sterols have been analysed by GC and GC/MS and the data examined using Correspondence Factorial Analysis (CFA). The main autochthonous sterols in these sediments were cholesterol, 24-methylcholesta-5,22-dien-3β-ol and 24-methylcholesta-5,24(28)-dien-3β-ol, whereas 24-ethylcholesterol, 24-ethylcholesta-5,22-dien-3β-ol and 24-methylcholesterol were principally allochthonous. Autochthonous sterols prevailed in regions of high primary production observed in summer, and allochthonous ones dominated the sterol distribution in the accumulation region of fine sediments just outside the river mouth and in the reaches of coastal currents. The cholesterol/cholesterol ratio values exhibited higher values close to the river mouth and in the region where the turbidity maximum extends. A comparison between sediment and suspended matter indicated a great difference in the sterol content in regions of high autochthonous production which suggested that sterols have been transformed in the course of sedimentation and at the sediment/water interface. In contrast, sedimentary and suspended sterol concentrations were comparable close to the mouth of the Chang Jiang River and in the region where the turbidity maximum extends, a feature that may be attributable to sediment resuspension episodes and lower production of autochthonous sterols. These results demonstrate the distinct impact of biogeochemical processes on the sediment sterol features in different zones of the estuary.

Box models for the evolution of atmospheric oxygen: an update

A simple 3-box model of the atmosphere/ocean system is used to describe the various stages in the evolution of atmospheric oxygen. In Stage I, which probably lasted until redbeds began to form about 2.0 Ga ago, the Earth's surface environment was generally devoid of free O 2, except possibly in localized regions of high productivity in the surface ocean. In Stage II, which may have lasted for less than 150 Ma, the atmosphere and surface ocean were oxidizing, while the deep ocean remained anoxic. In Stage III, which commenced with the disapperance of banded iron formations around 1.85 Ga ago and has lasted until the present, all three surface reservoirs contained appreciable amounts of free O 2. Recent and not-so-recent controversies regarding the abundance of oxygen in the Archean atmosphere are identified and discussed. The rate of O 2 increase during the Middle and Late Proterozoic is identified as another outstanding question.

Box models for the evolution of atmospheric oxygen: an update

A simple 3-box model of the atmosphere/ocean system is used to describe the various stages in the evolution of atmospheric oxygen. In Stage I, which probably lasted until redbeds began to form about 2.0 Ga ago, the Earth's surface environment was generally devoid of free O 2, except possibly in localized regions of high productivity in the surface ocean. In Stage II, which may have lasted for less than 150 Ma, the atmosphere and surface ocean were oxidizing, while the deep ocean remained anoxic. In Stage III, which commenced with the disapperance of banded iron formations around 1.85 Ga ago and has lasted until the present, all three surface reservoirs contained appreciable amounts of free O 2. Recent and not-so-recent controversies regarding the abundance of oxygen in the Archean atmosphere are identified and discussed. The rate of O 2 increase during the Middle and Late Proterozoic is identified as another outstanding question.

Inter-Regional Variations in Labour Productivity in Indian Agriculture : A Simultaneous Equation Approach

With the help of a simultaneous equation model this paper examines the factors influencing variations in the components of labour productivity in Indian agriculture across 53 NSS agro-climatic regions in three triennia viz., 1962-65, 1970-73 and 1975-78. The model is estimated separately for high and low labour productivity regions. The results show that cropping pattern, irrigation and percentage of area under rice crop were the dominant factors determining variations in yield, cropping intensity and man-land ratio respectively. However, the importance of these factors varied according as whether the region is a high or a low productivity region.

Worldwide environmental productivity indices and yield predictions for a cam plant, Opuntia ficus-indica, including effects of doubled CO 2 levels

The productivity of Opuntia ficus-indica was predicted for 253 regions on a worldwide basis using data from 1464 weather stations within 60° of the equator. First, the climatic data were used to calculate daily values of a PAR index, a temperature index and a water index. These indices, each of which has a maximum value of unity when that environmental factor is not limiting net CO 2 uptake by O. ficus-indica over a 24-h period, were multiplied together to give an environmental productivity index, which indicates how the three environmental factors limit net CO 2 uptake and hence productivity. The photosynthetically active radiation (PAR) index for a canopy of closely spaced plants that have a high productivity per unit ground area was ⩾0.20 for 25% of the earth's land surface. The temperature index annually was ⩾0.50 for 81% of the earth's land surface, indicating that local temperatures do not greatly limit net CO 2 uptake by this species. The water index was ⩾0.50 for 79% of the earth's surface for O. ficus-indica, which exhibits Crassulacean acid metabolism with its accompanying high water-use efficiency. Predicted productivities for O. ficus- indica without irrigation were at least 10 metric tons ha −1 year −1, the value for many important annual agronomic crops, for 41% of the earth's land area. Irrigation increased such high productivity regions to 77% of the earth's land surface area within 60° of the equator. For simulations that included the worldwide changes in PAR, temperature and rainfall patterns that will most likely accompany a doubling in the ambient CO 2 level, the high productivity of 10 tons ha −1 year −1 was predicted to occur for 54% of the earth's land surface area. Under elevated CO 2, the predicted productivity of O. ficus-indica increased for most of the U.S.A. and a productivity of 32 tons ha −1 year −1 was predicted for western South America.

Marine humic and fulvic acids: Their effects on remote sensing of ocean chlorophyll

Marine humic and fulvic acids were concentrated from about 1,400 liters of seawater from the Gulf of Mexico, and specific absorption coefficients were measured for each from 240 to 675 nm. Spectral absorption coefficients were then calculated for Gulf of Mexico stations where earlier data on humic and fulvic acid concentrations were available. Marine humic and fulvic acid values have low molecular weights consistent with extrapolations from soil#x2010;derived curves of their specific absorption coefficients vs. molecular weight. Marine fulvic and humic acids appear to account for most if not all water color or Gelbstoff in the offshore regions of the Gulf of Mexico. Based on a remote#x2010;sensing reflectance model, it appears that the increase in the Gelbstoff: chlorophyll ratio for waters adjacent to and downstream from regions of high primary productivity accounts for much of the deviation found for such waters from the global chlorophyll algorithm of the Coastal Zone Color Scanner.

Tomales Bay, California

The Land‐Sea Interface (LSI) is a region of particular importance to human endeavor. The region includes the exposed outer coast, coastal waters, bays, estuaries, and deltas. Its area is small in comparison with that of the adjacent open ocean or land, but the LSI is at once a region of high biological productivity and complexity and the portion of the ocean most heavily impacted by human activities.The LSI also has a profound effect on the transport of materials between the land and the sea. Chemically reactive materials transported from the land to the sea undergo intense biogeochemical cycling and net transformations within the LSI. There are, for example, important sorption reactions associated with the transition from freshwater to saltwater. These reactions alter the distribution and bioavailibility of metals and may affect their toxicity and bioaccumulation.

Diatom productivity peak and increased circulation during latest Quaternary: Alboran Basin (Western Mediterranean)

Diatoms were studied quantitatively in six latest Quaternary (⋍70 kyr B.P. to Recent) piston cores from the westernmost Mediterranean, the Alboran Basin, and the Atlantic region immediately to the west of the Straits of Gibraltar. The Atlantic cores completely lack diatoms. In the Alboran Basin, diatoms are common from late Stage 3 (⋍27.5 kyr B.P.) to Termination Ib (9 kyr B.P.) and in Recent core tops, but are absent in the other latest Quaternary intervals. Maximum accumulation of diatoms and highest abundance of species normally in sediments associated with increased productivity occurred during the latest Quaternary deglaciation, in the first phase of Termination I (⋍14.8 kyr B.P.). In the modern Alboran Basin, a region of high biological productivity occurs immediately east of the Gibraltar Straits. This high productivity results from upwelling associated with the interaction between the Atlantic inflow and the bottom topography near the Spanish coast. The upwelled nutrient-rich waters are then advected to the east and southeast by the surficial anticyclonic gyral circulation. Late Quaternary variations in diatom abundance are considered to reflect changes in this upwelling intensity with highest diatom abundances inferred to result from increased upwelling associated with an intensification of the anticyclonic gyral circulation. Highest inferred upwelling rates occurred during the first phase of latest Quaternary deglaciation. It is possible that an intensification of circulation within the Mediterranean Basin as a whole occurred from late Stage 3 to mid Termination I because widespread hiatus formation has been reported at this time in the Straits of Sicily due to an increase in the formation of intermediate waters. Diatoms were not preserved in other latest Quaternary intervals due to insufficient productivity to counterbalance their dissolution.

Satellite passive microwave studies of the Sea of Okhotsk ice cover and its relation to oceanic processes, 1978–1982

This paper describes the use of the Nimbus 7 Scanning Multichannel Microwave Radiometer (SMMR) satellite data to study the ice cover in the Sea of Okhotsk during the four winters of 1978–1982. The ice concentration estimates obtained from SMMR are used to study two different polynyas: one over the Kashevarova Bank, and the other over the northwest continental shelf. For the Kashevarova polynya this study shows that the polynya, which may be related to the upwelling observed in historic summer cruises, occurs directly over the bank. For the continental shelf, which because of strong offshore winds and cold temperatures is a region of high ice production, the polynya areas are combined with heat flux estimates from weather station data to yield the production rates of ice, salt, and dense shelf water. As a previous study by Kitani (1973) shows, the mixing of this dense shelf water with Pacific water yields a water mass unique to the Okhotsk, which is the layer of cold, low‐salinity intermediate water between depths of 150 and 800 m. From our calculations, we estimate that the dense shelf water is produced at an annual rate of about 0.5 Sv, and that the intermediate water is produced at a rate of 1–2 Sv, which yields a renewal time for this layer of 10–40 years.

Chemistry of organic traces in air IX: Evidence of natural marine sources for chloroform in regions of high primary production

Chemistry of organic traces in air IX: Evidence of natural marine sources for chloroform in regions of high primary production

Oceanographic control of diatom abundances and species distributions in surface sediments of the tropical and southeast Atlantic

This paper is a descriptive account of the diatomaceous component of pelagic sediments from the tropical and southeast Atlantic Ocean. Geographic patterns of diatom abundances (whole and fragment forms) per gram, major species relative abundances and diatom assemblages (resolved by Q-mode factor analysis) are presented. Comparisons drawn to sea-surface temperature, salinity and productivity distributions suggest that: (i) diatom abundances primarily reflect surface productivity with no apparent dissolution effects due to water depth; (ii) relative abundances of major species vary in their ecologic dependence but do reflect major oceanographic features of the surface waters; and (iii) of the five diatom assemblages resolved, four finely delineate the complexity of the low-latitude oceanography. These assemblages, in decreasing order of statistical importance, are associated with: (1) warm waters of moderate to high primary productivity; (2) the oceanward margin of the Benguela Current, a region of high primary production with a maximum in austral summer; (3) low-salinity waters off the mouths of the Zaire and Niger rivers; and (4) subtropical to tropical waters of lower productivity, mostly in the western equatorial Atlantic. The fifth factor represents the equatorward advection of Subantarctic to Antarctic diatoms by deep-ocean currents.