Heterotrophic bacteria perform two major functions in the transformation of organic matter: They produce new bacterial biomass (bacterial secondary production [BP]), and they respire organic C to inorganic C (bacterial respiration [BR]). For planktonic bacteria, a great deal has been learned about BP and its regulation during the past several decades but far less has been learned about BR. Our lack of knowledge about BR limits our ability to understand the role of bacteria in the carbon cycle of aquatic ecosystems. Bacterial growth efficiency (BGE) is the amount of new bacterial biomass produced per unit of organic C substrate assimilated and is a way to relate BP and BR: BGED (BP)=(BPC BR). Estimates of BGE for natural planktonic bacteria range from <0.05 to as high as 0.6, but little is known about what might regulate this enormous range. In this paper we review the physiological and ecological bases of the regulation of BGE. Further, we assemble the literature of the past 30 years for which both BP and BR were measured in natural planktonic ecosystems and explore the relationship between BGE and BP. Although the relationship is variable, BGE varies systematically with BP and the trophic richness of the ecosystem. In the most dilute, oligotrophic systems, BGE is as low as 0.01; in the most eutrophic systems, it plateaus near 0.5. Planktonic bacteria appear to maximize carbon utilization rather than BGE. A consequence of this strategy is that maintenance energy costs (and therefore maintenance respiration) seems to be highest in oligotrophic systems.

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Bacterial growth efficiency in natural aquatic systems

The geology of northern and central Africa is characterized by broad plateaux, narrower swells and volcanism occurring from ∼45 Myr ago to the present. The greatest magma volumes occur on the >1,000-km-wide Ethiopian and east African plateaux, which are transected by the Red Sea, Gulf of Aden and east African rift systems, active since the late Oligocene epoch. Evidence for one or more mantle plumes having impinged beneath the plateaux comes from the dynamic compensation inferred from gravity studies, the generally small degrees of extension observed and the geochemistry of voluminous eruptive products1,2,3,4. Here we present a model of a single large plume impinging beneath the Ethiopian plateau that takes into account lateral flow and ponding of plume material in pre-existing zones of lithospheric thinning5. We show that this single plume can explain the distribution and timing of magmatism and uplift throughout east Africa. The thin lithosphere beneath the Mesozoic–Palaeogene rifts and passive margins of Africa and Arabia guides the lateral flow of plume material west to the Cameroon volcanic line and south to the Comoros Islands. Our results demonstrate the strong control that the lithosphere exerts on the spatial distribution of plume-related melting and magmatism.

Cenozoic magmatism throughout east Africa resulting from impact of a single plume

The study of noble gases in oceanic basalts is central to understanding chemical heterogeneity of the Earth’s mantle and origin of the atmosphere. In the terrestrial environment the abundances of noble gases are quite low because they were excluded from solid materials during planetary formation in the inner solar system. This low background inventory helps to make the noble gases excellent tracers of mantle reservoirs. In this context, mid-ocean ridge and ocean island basalts provide valuable windows into the Earth’s mantle. These oceanic basalts are not prone to the degree of contamination often observed in continental lavas that results from their passage through thick continental lithosphere and crust. Mid-ocean ridge basalts (MORBs) form by partial melting as the ascending mantle beneath spreading ridges reaches its solidus temperature, and MORBs are generally accepted to represent a broad sampling of the convecting upper mantle. Ocean island basalts (OIBs) represent melting ‘anomalies’ that are generally related to mantle upwelling. The extent to which ocean islands are derived from a thermal boundary layer in the deep mantle (e.g., as a mantle plume) or from chemical heterogeneities embedded within the mantle convective flow (e.g., as a mantle ‘blob’) has been debated for decades, and is not currently resolved. The isotope compositions of noble gases in oceanic basalts bear significantly on such debates over the chemical structure of the mantle. When oceanic basalts erupt as submarine lavas, their quenched rims of glass may contain high volatile abundances (especially when they are deeply erupted under elevated hydrostatic pressure), providing the best available opportunity for precisely characterizing the noble gas composition of the Earth’s mantle. In favorable cases, inclusions of melt or fluids trapped within magmatic phenocrysts and mantle xenoliths can also be precisely analyzed for noble gas composition.

Measurable changes in the isotope compositions of noble gases …

Noble Gas Isotope Geochemistry of Mid-Ocean Ridge and Ocean Island Basalts: Characterization of Mantle Source Reservoirs

The East African rift system

Ozone and other secondary photochemical pollutants: chemical processes governing their formation in the planetary boundary layer

We report on the rare earth and Nd-Sr-Pb isotopic composition of basalts dredged along the Sheba Ridge axis in the Gulf of Aden and its extension into the Gulf of Tadjoura and subaerial basalts from the Ardoukoba Rift in east Afar. The sampling profile provides a means to study the evolutionary nature of the mantle sources involved in the melting process associated with the interaction of the head of a starting mantle plume with continental lithosphere and an ocean basin at a nascent stage of formation. An 800-km-long Nd-Sr-Pb isotopic and La/Sm gradient, sinusoidally modulated, is apparent from the Afar eastward. The first enrichment peak occurs in the Gulf of Tadjoura, where diffuse extension of the Danakil-Aisha continental lithospheric block and westward rift propagation is currently progressing. The second enrichment peak at 46°E is associated with a mantle buoyancy anomaly and related constructional volcanism. East of 48°E, the MORBs are typically light rare earth element depleted, whereas 206Pb/204Pb and 87Sr/86Sr slightly increase, suggesting recent decoupling. In Nd-Sr-Pb isotope ratio space, three distinct vector trends are observed within a plane. The mixing vectors point toward three mantle source end-members which can be interpreted as Pan-African continental lithosphere along the Gulf of Tadjoura (a hybrid EM-l-EM-2), a mantle plume (relatively young HIMU-like) which dominates the 46°E anomaly, and the depleted asthenosphere east of 48°E (DUPAL-like). Combined data from the Gulf of Aden-Red Sea-Afar-Ethiopian rifted zones suggest a radial pattern of geochemical and isotopic variation about the Afar. A working dynamical-thermal model is presented for the past 30–40 m.y. history of the Horn of Africa. It invokes both passive rifting/seafloor spreading in the Red Sea/Gulf of Aden and the flattening and interaction of the starting head of a toruslike thermal mantle plume with the Pan-African continental lithosphere which is slowly moving northeastward with the plume head attached at its base. The plume flattened into a pancakelike form, twice the diameter of the original head which is estimated to be of the order of 700 km in diameter. The thinning of the lithosphere by stretching and thermal erosion by the mantle plume has not yet been completed. A working ternary mixing model constrained by the isotope data indicates that within the 800–1000 km radius of influence of the Afar mantle plume, melting of the lithosphere mantle and the depleted asthenosphere apparently entrained by the ascending mantle plume dominates initially. Only along the three rifting zones intersecting the flattened plume ring, 450±150 km in radius, composed of original HIMU-like plume material does the original plume component play a more dominant role. Judging from the spatial isotopic composition variation of the basalts, the plume torus may be apparent along (1) the 46°E Gulf of Aden anomaly where seafloor spreading is now well established; (2) the 13°–16°N southern Red Sea segment, which represents a rift zone at a transient stage of either development or abandonment (overlapping with the Afar NW neovolcanic zone), where ocean island alkali volcanism dominates and diffuse lithosphere extension may still operate; (3) the high alkaline field of the Aden Volcanic Series; and (4) the Ethiopian Rift around 8°N in a purely continental setting. The NW Afar neovolcanic zone, which is essentially at a nascent stage of seafloor spreading and is overlapping the ring and the center of the pancakelike flattened mantle plume, is dominated by tholeiites derived from depleted asthenospheric material entrained by the plume during its original ascent. Plate reconstructions further suggest that the original center of the flattened mantle plume head has moved with the lithosphere some 900 km northeastward. The stem feeder of the plume has now been drawn or tilted toward the Afar as a result of the migration of the Gulf of Aden/Red Sea spreading centers which act as sinks of asthenospheric material and the likelihood that the feeder of the mantle plume is encountering with time an African lithosphere increasing in age, thickness, and rigidity.

Nd‐Sr‐Pb isotopic variations along the Gulf of Aden: Evidence for Afar Mantle Plume‐Continental Lithosphere Interaction

We report on a Pb-Nd-Sr isotope and rare earth study of Mid-Atlantic Ridge (MAR) basalt glasses collected across the equatorial fracture zones from 7°S to 5°N (65 stations). The 1600-km-long profile reveals two mixing zones in the mantle that are isotopically distinct but cover the same range of (La/Sm)n ratios (0.3–2), with a gradational boundary between the Romanche and the Chain fracture zones. The potential mantle temperature profile inferred from Na2O content is also quite distinct. The north zone is dominated by a major, La/Sm and HIMU type Pb isotope anomaly centered at 1.7°N±300 km, which is flanked by two zones mildly radiogenic in Pb but depleted in light REE. A kinematic and evolutionary model describing the dispersion and interaction of the Sierra Leone plume with the asthenosphere and the MAR in the last 75 m.y. is proposed for this zone, which includes St. Paul and St. Peter's Rocks. In contrast, over the south zone the isotope/geochemical profiles are well correlated at all length scales and opposite in sign from the inferred potential mantle temperature profile and mean percent fusion. Broad negative gradients are observed between the Romanche and the Charcot fracture zones, superimposed by spikelike anomalies at the intersection with the eastern part of the Romanche and Chain transform faults, where cold plate edge effects prevail. The heterogeneous mantle model of Sleep [1984] and Langmuir and Bender [1984] is applicable to this zone, that is the volatile and radiogenic Pb-rich lumps are preferentially melted during mantle decompression and passively sampled. The lumps may reflect the early dispersion of the St. Helena or Ascension mantle plumes under a thick lithosphere, followed by redistribution due to intense shearing, continental lithosphere delamination, and secondary mantle convection. The presence of a depleted asthenosphere unpolluted by plumes along the 400-km-long MAR segment between the Charcot and Ascension fracture zones is also apparent in the data.

Influence of the Sierra Leone mantle plume on the equatorial Mid-Atlantic Ridge : a Nd-Sr-Pb isotopic study

We report on the major element composition of basaltic glasses from the Mid-Atlantic Ridge transecting the equatorial mega-fracture zones from 7°S to 5°N (65 stations, 10–20 km sampling intervals, 3.5–5 km water depth range). Many of the basaltic glasses are Na2O, SiO2, and MgO rich, similar to other basalt glasses erupted along the deepest regions of the mid-ocean ridge system, suggesting melt generation by relatively low degrees of partial melting at rather shallow depth in the upper mantle. Along the ridge axis, the compositional variations show regular and systematic long-wavelength trends with a major discontinuity at the complex St. Paul transform fault, just south of St. Peter and Paul islets. A corresponding long-wavelength trend in upper mantle potential temperature, mean pressure, and degree of melting and crustal thickness variation is inferred using parameterized petrologic decompression melting models. A 600-km-long, nearly linear negative gradient in these parameters is apparent from the Charcot fracture zone (FZ) to the St. Paul FZ. Over the length of this gradient, the upper mantle potential temperature drops by about 70°C, the mean degree of partial melting changes from 7% to 10%, and the inferred crustal thickness varies between 3 and 6 km. The gradient along the ridge axis is unaffected by the mega-transform fault offsets, implying that a broad (approximately 2000 km wide across-axis and 600 km long along-axis) cold zone is present in the upper mantle just south of the equator. At the discontinuity across the complex St. Paul transform fault, the gradients in inferred potential temperature, mean degree of partial melting, and crustal thickness abruptly change sign, respectively increasing by 80°C, rising from 7% to 10%, and changing from 3 to 6 km. The discontinuity is clearly related to the Sierra Leone plume affecting the Mid-Atlantic Ridge around 1.7°N, as also evident from Pb, Nd, and Sr isotopic variations previously reported on the same glasses (Schilling et al., 1994) and the K2O variation reported here. The cause of the petrologically inferred cold zone and large gradient in the upper mantle south of St. Peter and Paul islets remains more speculative. On the basis of a passive mantle upwelling flow model (Phipps Morgan and Forsyth, 1988) applied to the specific geometry of the equatorial Atlantic, we reject the simplest hypothesis that the cold zone is produced by the compounding cooling effect caused by the very long and densely distributed transform fault offsets in the equatorial Atlantic. The result of this test remains paradoxical in view that good correlations exist between segment length, maximum along-ridge axis relief per segment, mean segment depth, and per segment average bulk compositions of the erupted basalts, and corresponding mean degrees of melting. Other possible causes for the gradational cold zone are briefly explored. These include the evolutionary history of the region with respect to adjacent continental mantle, lithosphere age, thickness, and temperature, and tectonic mode of opening of the Atlantic, as well as large-scale convective motion associated with continental dispersion. No definite conclusions can be reached. However, we emphasize that the petrologically inferred upper mantle thermal structure in the equatorial Atlantic is quite robust and independent of the petrologic decompression melting models considered and their underlying detailed assumptions. Large seismic S wave velocity variations are predicted over the 0–150 km depth range of the upper mantle, based on the reported correlation of Na8 with S wave velocity reported by Yan et al. (1989). Thus detailed seismic tomographic mapping could be used to test further the cold upper mantle zone hypothesis for the equatorial Atlantic.

Thermal structure of the mantle beneath the equatorial Mid-Atlantic Ridge: Inferences from the spatial variation of dredged basalt glass compositions

Five methods for the measurement of CH2O vapor were compared under remote tropospheric conditions. The techniques included an aqueous-scrubber enzyme fluorescence method (URIcoil), TDLAS (UNI), 2,4-dinitrophenylhydrazine (DNPH) impregnated-cartridges (BNL/WSUcart), an aqueous-scrubber DNPH method (BNL/WSUcoil), and an unpublished aqueous-scrubber immobilized-enzyme fluorescence method (NCARbound). This was part of the Mauna Loa Observatory Photochemistry Experiment 2 (MLOPEX 2) which was performed in four ∼30-day intensives. In MLOPEX 2a (fall 1991), the URIcoil technique indicated higher concentrations relative to the BNL/WSUcoil which were higher than UNI. The limited number of NCARbound measurements for MLOPEX 2a were greater than the other measurements and least reliable. During MLOPEX 2b (winter 1992), URIcoil, UNI, BNL/WSUcoil, and NCAR-bound gave comparable measurements of CH2O with the latter again being consistently higher than the first three methods and its performance and reliability was improved over MLOPEX 2a. URIcoil and BNL/WSUcoil were comparable in MLOPEX 2c (spring 1992) and 2d (summer 1992), whereas BNL/WSUcart was consistently higher than both for these intensives. The major source of difference between techniques was ascribed to variations in the analytical-procedural field blanks. Calibration differences were the most likely cause of URIcoil being higher than BNL/WSUcoil and BNL/WSUcoil being higher than UNI during MLOPEX 2a. Lower free troposphere median concentrations were between 100 and 150 ppt for all seasons and without a seasonal trend. These values were a factor of 2 to 3 lower than model estimates for this study site and were in keeping with earlier work.

Formaldehyde methods comparison in the remote lower troposphere during the Mauna Loa Photochemistry Experiment 2

From the fall of 1991 through the summer of 1992 the Mauna Loa Observatory Photochemistry Experiment 2 (MLOPEX 2) took place in Hawaii. The experiment consisted of four intensive research periods, each approximately 1 month in duration. We report a comparison of measurements of atmospheric hydroperoxide and methylhydroperoxide as measured during MLOPEX 2a (September 16 to October 23, 1991) and MLOPEX 2b (January 15 to February 15, 1992). The analytical techniques used for measuring hydroperoxides included tunable diode laser infrared absorption (TDLAS) and fluorescence from an enzymatically produced dimer. The TDLAS technique measured hydrogen peroxide (H2O2) only. Three different procedures were utilized to distinguish between H2O2 and organic hydroperoxides in the fluorescence analytical techniques. These were high performance liquid chromatographic (HPLC) separation, enzymatic discrimination, and differential aqueous solubility. The measured concentrations ranged from 0 to 2 parts per billion by volume (ppbv) for H2O2 and 0 to ppbv for methylhydroperoxide (CH3OOH). Depending on the time period of measurement, the agreement varied between 20% and a factor of 2 or more for the measurement of H2O2. The discrepancy for the CH3OOH data was larger.

B. McCully

Papers

Nd‐Sr‐Pb isotopic variations along the Gulf of Aden: Evidence for Afar Mantle Plume‐Continental Lithosphere Interaction

Influence of the Sierra Leone mantle plume on the equatorial Mid-Atlantic Ridge : a Nd-Sr-Pb isotopic study

Thermal structure of the mantle beneath the equatorial Mid-Atlantic Ridge: Inferences from the spatial variation of dredged basalt glass compositions

Formaldehyde methods comparison in the remote lower troposphere during the Mauna Loa Photochemistry Experiment 2

Comparison of hydroperoxide measurements made during the Mauna Loa Observatory Photochemistry Experiment 2