Vertical Transport Rates Research Articles

A bound on the vertical transport of heat in the ‘ultimate’ state of slippery convection at large Prandtl numbers

AbstractAn upper bound on the rate of vertical heat transport is established in three dimensions for stress-free velocity boundary conditions on horizontally periodic plates. A variation of the background method is implemented that allows negative values of the quadratic form to yield ‘small’ ($O\left(1/ \mathit{Pr}\right)$) corrections to the subsequent bound. For large (but finite) Prandtl numbers this bound is an improvement over the ‘ultimate’$R{a}^{1/ 2} $scaling and, in the limit of infinite$Pr$, agrees with the bound of$R{a}^{5/ 12} $recently derived in that limit for stress-free boundaries.

The Role of Turbulent Mixing in an Overturning Circulation Maintained by Surface Buoyancy Forcing

Abstract The role of externally imposed rates of small-scale mixing in an overturning circulation forced by differential surface buoyancy fluxes is examined in a laboratory experiment. The circulation occupies the full volume and involves a dense turbulent plume against the endwall and a broad upwelling throughout the interior. For strong externally imposed stirring, turbulent diffusion is the primary means of vertical density transport in the flow, and the dependence of the equilibrated circulation on the mixing rate accords with a theoretical model; the overturning rate increases as the ¼ power of the turbulent diffusivity. For weak externally imposed stirring, advection is the dominant mechanism of vertical density transport, and the circulation is independent of the rate of external stirring. The rate of vertical density transport is parameterized as a bulk diffusivity obtained from different methods, including one from a Munk-like advection–diffusion balance and another from the transport of buoyancy across the surface. For strong stirring, the bulk diffusivities returned by the various methods agree with the externally imposed mixing rate. However, the parameterizations implicitly include a nondiffusive component of vertical transport associated with advection of the density field and it is shown that, for weak stirring, the bulk diffusivities exceed the externally imposed mixing rate. For the oceans, results suggest that the primary effect of mixing (with energy sourced from winds, tides, and convection) is to deepen the thermocline, thereby influencing the entrainment and consequent vertical transport of density in the dense sinking regions. It is concluded that this advective transport of density, and not vertical mixing, is crucial for coupling the surface to the abyss.

Variation in monsoon precipitation drives spatial and temporal patterns of Larrea tridentata growth in the Sonoran Desert

Summary 1. Broad-scale patterns of above-ground net primary production (ANPP) are closely coupled to climate features, particularly the distribution and magnitude of rainfall. In arid and semi-arid regions, however, the precipitation regime, together with local geomorphology and plant life history traits, combine to regulate soil water availability and patterns of growth, complicating simple correlations with climatic variables. 2. To better understand the drivers of plant growth in desert ecosystems, we characterized the rates and spatial heterogeneity of stem elongation by the dominant perennial shrub, creosote bush (Larrea tridentata) in the northern Sonoran Desert of Arizona (USA). Estimates of stem growth were made over a 5-year period (2006–2010) from 60 plots at 15 sites spanning c. 120 km across the Central Arizona–Phoenix (CAP) Long-term Ecological Research (LTER) area. 3. We observed both the highest and lowest rates of stem growth during summer, and these patterns were related to the amount of monsoon rainfall and local rates of water infiltration. The relationship between growth and precipitation in the summer was nonlinear, with rates increasing more than eightfold at plots receiving more than 100 mm of seasonal rainfall. Conversely, growth during the winter/spring was intermediate in magnitude, similar among years and poorly correlated with seasonal precipitation. 4. The spatial heterogeneity of stem growth also differed markedly between seasons and was greater both within and among sites during summer compared to winter/spring growing periods. At broad scales, spatial heterogeneity of shrub growth was correlated with seasonal changes in the spatial variability of rainfall across the study area. At small spatial scales, greater heterogeneity during the summer appears linked to local soil properties that influence infiltration and water availability following monsoon storms. 5. Overall, the strong, nonlinear growth response by L. tridentata to monsoon rainfall suggests that the recurrence interval of wet summer seasons is an important driver of ANPP for this long-lived shrub. More generally, our results illustrate how regional climate variability and local soil properties can interact to influence the rates and heterogeneity of desert plant growth at multiple scales.

Time lag between the tropopause height and the levels of7Be concentration in near surface air

The concentration of 7 Be at near surface air has been determined over 2009, a year of a deep solar minimum, in the region of Thessaloniki, Greece at 40 ◦ 62 � N, 22 ◦ 95 � E. In geomagnetic latitudes over 40 ◦ N, the elevation of the tropopause during the warm summer months and the vertical exchange of air masses within the troposphere cause greater mixture of the air masses resulting in higher concentration levels for 7 Be in surface air. The positive correlation between the monthly activity concentration of 7 Be and the tropopause height (0.94, p< 0.0001), and also between 7 Be concentration and the temperature T ( ◦ C) (R =0 .97, p< 0.001), confirm that the increased rate of vertical transport within the troposphere, especially during warmer summer months, has as a result the descent to surface of air masses enriched in 7 Be. However, the 7 Be concentration levels in near surface air are not expected to respond immediately to the change of elevation of the tropopause. It was found that there’s a time lag of ∼ 3 days between the change in the daily surface concentrations of 7 Be the change in the elevation of the tropopause. ∗ E-mail: anta@physics.auth.gr

7Be atmospheric concentration at mid latitudes (40°N) during a year of solar minimum

In this paper we present the variation of 7Be concentration in the surface air of Thessaloniki, Greece (40°62′N, 22°95′E) over the year 2009, a year of a deep solar minimum, and, as a consequence, a year of maximum concentration of 7Be in surface air. The mean annual activity concentration of 7Be for the year 2009 was 6.0 mBq m−3. The relative variability of 7Be surface concentration related to the solar cycle was calculated to be deviated by about 20% from maximum to mean. A positive correlation (R = 0.97) was revealed between the activity of 7Be and the temperature T (°C), confirming that the increased rate of vertical transport within the troposphere, especially during warmer months, that make descend air masses enriched in 7Be down to the surface layer. The anticorrelation (R = –0.65) with RH% is due to intense condensation during high relative humidity conditions, which results in increased aerosol particle sizes and as a consequence in higher scavenging rate of aerosols and lower concentration of 7Be in the atmosphere. The influence of precipitation on the 7Be atmospheric concentration variability was approximately 10%, with greater the influence of rainfall events of low precipitation rate e.g. drizzling.

Investigation into approaches to reduce excessive vertical transport over complex terrain in a regional photochemical grid model

Past photochemical modeling of the western United States (US) using the Comprehensive Air quality Model with extensions (CAMx) and the Community Multiscale Air Quality (CMAQ) model has resulted in large springtime ozone over predictions in the complex high-elevation terrain of the western United States (US). Comparisons against rural measurement data have shown that both models over predicted ozone levels by 20 ppb or more. A systematic investigation using CAMx revealed that excessive vertical transport in mountainous terrain draws down upper tropospheric ozone introduced by the lateral boundary conditions (developed by a global chemistry model), which can routinely exceed 1 ppm near the tropopause. This is not an unreasonable concentration at such altitudes during the spring, and there is observational evidence that stratospheric ozone intrusions result in occasional large ground-level concentrations in the western US, but not at the frequency and intensity simulated by the CAMx and CMAQ models. Past versions of CAMx and CMAQ possess similar algorithms to diagnose vertical velocity, and similar first-order accurate vertical advection algorithms. These similarities, in conjunction with poor vertical resolution of the upper troposphere, have resulted in similar ozone performance issues. Numerous approaches were explored with CAMx in an attempt to externally reduce the rates of vertical transport over complex terrain. Ultimately, we formulated and tested a new vertical advection methodology that included improvements to how vertical velocities are determined and introduced a second-order accurate advection solver technique. Together these improvements proved to yield the most successful results in reducing upper tropospheric ozone transport to the surface. CAMx was then run to simulate ozone throughout the western US for a full year to evaluate the effects of the new vertical transport algorithm on model performance. Ozone performance improvements exceeded those achieved through the application of arbitrary reductions in the upper tropospheric/stratospheric lateral boundary conditions.

7Be in the lower atmosphere at a mid-latitude (40°N) during the year 2009 of a particular minimum of solar activity

7Be activity concentrations were measured in the lower atmosphere at Thessaloniki, Northern Greece (40°38′N, 22°58′E) over the year 2009, a year of a particular minimum of solar activity. The mean annual activity concentration of 7Be at that year was 6.01 mBq m−3. The variability of 7Be surface concentrations related to the solar cycle appeared to be deviated about 40% between the maximum and the minimum values. A positive correlation (R = 0.97) was revealed between the activity concentrations of 7Be and the temperature, confirming that the increased rates of vertical transport within the troposphere, especially during the warmer months, resulted in carrying down to the surface layer air masses enriched in 7Be. Relatively high values of 7Be activity concentrations were observed by increasing of the tropopause height. A negative correlation (R = −0.65) between the 7Be activity concentrations and the relative humidity was due to the condensation process in the lower atmosphere which resulted in increased aerosol particle sizes with higher scavenging rates of aerosols and low activity concentrations of 7Be in the atmosphere. Influence of precipitation on the changes of 7Be activity concentrations was also observed.

The effect of surface flooding on the physical–biogeochemical dynamics of a warm-core eddy off southeast Australia

Warm-core eddies (WCEs) formed from the East Australian Current (EAC) play an important role in the heat, mass and biogeochemical budgets of the western Tasman Sea. The development and separation of an EAC WCE during July–December 2008 was observed using remotely sensed temperature, ocean colour and sea-level elevation, three Argo floats, a shipboard CTD, a shelf mooring array and a 15-day deployment of a Slocum glider. The eddy formed from an EAC meander during the first half of 2008 and in late August had a ∼ 275 m deep surface mixed layer. In the two months before separation in early December, fresher and warmer EAC water flooded the top of the eddy, submerging the winter mixed layer. The rate of vertical transport due to submergence was estimated to be between 1 and 6 Sv, at the time accounting for a significant fraction of the mean southward flow of the EAC. The core of the eddy had a surface chlorophyll a concentration of < 0.4 mg m − 3 throughout the observations. A 20–40 m thick pycnocline formed at the interface of the flooding surface waters and the submerged layer. Chlorophyll a concentration in the pycnocline ranged from 0.5 to 2 mg m −3, with depth-integrated concentration ranging between 25 and 75 mg m −2. The development of a sub-surface maximum suggests that flooding increased light levels in the pycnocline. Elevated levels of coloured dissolved organic matter in the submerged layer correspond to oxygen depletion, suggesting respiration of organic matter. A comparison is made with observations from WCEs in 1978 and 1997 in which, unusually, surface flooding did not occur, but solar heating stratified the top 50 m. In the two eddies with surface capping, surface chlorophyll a concentrations were an order of magnitude higher than the 2008 flooded eddy, but depth-integrated chlorophyll a was similar. These findings suggest that EAC WCEs with relatively shallow surface flooding contain more phytoplankton biomass than surface images would suggest, with the vertical position of the chlorophyll a maximum depending on whether, and to what depth, the winter surface mixed layer is submerged.

Vertical transport rates and concentrations of OH and Cl radicals in the Tropical Tropopause Layer from observations of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and halocarbons: implications for distributions of long- and short-lived chemical species

Abstract. Rates for large-scale vertical transport of air in the Tropical Tropopause Layer (TTL) were determined using high-resolution, in situ observations of CO2 concentrations in the tropical upper troposphere and lower stratosphere during the NASA Tropical Composition, Cloud and Climate Coupling (TC4) campaign in August 2007. Upward movement of trace gases in the deep tropics was notably slower in TC4 than during the Costa Rica AURA Validation Experiment (CR-AVE), in January 2006. Transport rates in the TTL were combined with in situ measurements of chlorinated and brominated organic compounds from whole air samples to determine chemical loss rates for reactive chemical species, providing empirical vertical profiles for 24-h mean concentrations of hydroxyl radicals (OH) and chlorine atoms in the TTL. The analysis shows that important short-lived species such as CHCl3, CH2Cl2, and CH2Br2 have longer chemical lifetimes than the time for transit of the TTL, implying that these species, which are not included in most models, could readily reach the stratosphere and make significant contributions of chlorine and/or bromine to stratospheric loading.

Budget Analysis of Escherichia coli at a Southern Lake Michigan Beach

Escherichia coli (EC) concentrations at two beaches impacted by river plume dynamics in southern Lake Michigan were analyzed using three-dimensional hydrodynamic and transport models. The relative importance of various physical and biological processes influencing the fate and transport of EC were examined via budget analysis and a first-order sensitivity analysis of model parameters. The along-shore advective flux of EC (CFU/m(2).s) was found to be higher compared to its cross-shore counterpart; however, the sum of diffusive and advective components was of a comparable magnitude in both directions showing the importance of cross-shore exchange in EC transport. Examination of individual terms in the EC mass balance equation showed that vertical turbulent mixing in the water column dominated the overall EC transport for the summer conditions simulated. Dilution due to advection and diffusion accounted for a large portion of the total EC budget in the nearshore, and the net EC loss rate within the water column (CFU/m(3).s) was an order of magnitude smaller compared to the horizontal and vertical transport rates. This result has important implications for modeling EC at recreational beaches; however, the assessment of the magnitude of EC loss rate is complicated due to the strong coupling between vertical exchange and depth-dependent EC loss processes such as sunlight inactivation and settling. Sensitivity analysis indicated that solar inactivation has the greatest impact on EC loss rates. Although these results are site-specific, they clearly bring out the relative importance of various processes involved.

The impact of tropical recirculation on polar composition

Abstract. We derive the tropical modal age of air from an analysis of the water vapor tape recorder. We combine the observationally derived modal age with mean age of air from CO2 and SF6 to create diagnostics for the independent evaluation of the vertical transport rate and horizontal recirculation into the tropics between 16–32 km. These diagnostics are applied to two Global Modeling Initiative (GMI) chemistry and transport model (CTM) age tracer simulations to give new insights into the tropical transport characteristics of the meteorological fields from the GEOS4-GCM and the GEOS4-DAS. Both simulations are found to have modal ages that are in reasonable agreement with the empirically derived age (i.e., transit times) over the entire altitude range. Both simulations show too little horizontal recirculation into the tropics above 22 km, with the GEOS4-DAS fields having greater recirculation. Using CH4 as a proxy for mean age, comparisons between HALOE and model CH4 in the Antarctic demonstrate how the strength of tropical recirculation affects polar composition in both CTM experiments. Better tropical recirculation tends to improve the CH4 simulation in the Antarctic. However, mean age in the Antarctic lower stratosphere can be compromised by poor representation of tropical ascent, tropical recirculation, or vortex barrier strength. The connection between polar and tropical composition shown in this study demonstrates the importance of diagnosing each of these processes separately in order to verify the adequate representation of the processes contributing to polar composition in models.

A novel method for estimating vertical eddy diffusivities using diurnal signals with application to western Long Island Sound

We present an approach that allows the estimation of vertical eddy diffusivity coefficients from buoy measurements made at two or more depths. By measuring the attenuation and phase lag of a scalar signal generated periodically at the surface as it propagates downwards, the vertical eddy diffusivity coefficients can be calculated as K v = ωΔz 2/2ln 2(α 2/α 1), where α 2/α 1 is the ratio of the real amplitudes at frequency ω at the two depths separated by Δ z = z 2 − z 1; as K V = ωΔ z 2/ 2φ 2, where φ is the phase lag at the frequency ω; or as K v = iωΔ z 2/ln 2( U 2/ U 1), where U 2/ U 1 is the ratio of the complex signal amplitudes at the two depths. The method requires that horizontal fluxes be small at the ω frequency and that the signal-to-noise ratios at the two depths allow the determination of the amplitude and phase of ω. Application of this method to summertime 2004 western Long Island Sound oxygen and temperature buoy measurements at two depths provides a time-series of two-day average vertical eddy diffusivity estimates. Using these eddy diffusivities in conjunction with measured vertical concentration gradients, we obtain a time-series of vertical transport rates for oxygen and heat and estimate mean downward fluxes for June and July as 150–260 mMol m − 2 day − 1 and 100–400 W m − 2 respectively. These estimates are of a similar magnitude to sub-pycnocline O 2 and heat demands of 240 ± 200 mMol m − 2 day − 1 and 180 ± 60 W m − 2 that we infer from simple budgets, implying that vertical transport is significant to both budgets. The eddy coefficients obtained from the independent O 2 and temperature measurements have a 68% correlation, and the O 2 flux estimates show a correlation of 41% to measured rates of change in bottom dissolved oxygen levels. Our results indicate that extended time-series of eddy diffusivity coefficients can be obtained from in situ buoy measurements and the method shows promise as a way to constrain the vertical transport variability in budgets of dissolved materials in estuaries.

Nonlinear dynamics in arid and semi-arid systems: Interactions among drivers and processes across scales

We discuss a new conceptual framework for arid and semi-arid systems that accounts for nonlinear dynamics and cross scale interactions in explaining landscape patterns and dynamics. Our framework includes a spatial and temporal hierarchy, and five key interacting components that connect scales of the hierarchy and generate threshold behaviors: (1) historical legacies that include climate, disturbance, and management regimes, (2) dynamic template of patterns in ecological variables and spatial context, (3) vertical and horizontal transport processes (fluvial, aeolian, animal), (4) rate, direction, and amount of resource redistribution between high and low resource areas, and (5) feedbacks among plants, animals, and soils. We illustrate how this framework can be used to understand, forecast, and manage ecological systems that exhibit nonlinear dynamics across a range of spatial and temporal scales. This paper provides the foundation for a series of papers from the Jornada Experimental Range ARS-LTER research site in southern New Mexico, USA that support this new conceptual framework.

Temporal changes of 7Be and 210Pb concentrations in surface air at temperate latitudes (40°N)

Atmospheric concentrations of 7Be and 210Pb were measured for 15 years (1987–2001) in ground-level air at Thessaloniki, Northern Greece (40°38′N, 22°58′E). Mean activity concentrations of 7Be and 210Pb were 5.02 mBq m −3 and 664 μBq m −3, respectively, characteristic of the latitude of 40°N. Monthly atmospheric concentrations of 7Be showed a strong seasonal trend with the highest values being observed in the summer and the lowest in the winter period. Multiple regression analysis of the data of 7Be concentrations and a number of meteorological parameters revealed that the sunspot number and temperature are the most significant parameters affecting the concentrations of 7Be in surface air. The observed strong positive correlation between the mean monthly concentrations of 7Be and the temperature confirms that the increased rate of vertical transport within the troposphere, especially during the warm months, has as a result to carry down to the surface layer air masses enriched in 7Be. Highest values of the mean monthly atmospheric concentrations of 210Pb were observed in the autumn and lowest in the spring period. The positive correlation that was observed between 210Pb and 7Be concentrations during the summer months suggests that these two radionuclides could be used together as tracers of environmental processes.

Frequency and magnitude of active‐layer detachment failures in discontinuous and continuous permafrost, northern Canada

AbstractActive‐layer detachment failures triggered weeks to months after forest fire in the central Mackenzie Valley (65°N, discontinuous permafrost zone) are compared to others generated almost immediately by summer meteorological conditions on the Fosheim Peninsula, Ellesmere Island (80°N, continuous permafrost zone). Preferred long‐axis orientations in both zones vary in relation to valley geometry and ground ice distribution: differential insolation plays no direct role in detachment failure distribution. Rates of geomorphic work over periods of one to two centuries are of the same order of magnitude. Threshold meteorological conditions for initiating failures on the Fosheim Peninsula can be incorporated into a surface heating index, but pre‐conditioning of the active layer remains important because rapid thaw does not always initiate activity. Slope pre‐conditioning does not occur at the fire‐affected sites because the failure zone is within formerly perennially frozen ground. Long‐term rates of unit vertical transport at the most active site on the Fosheim Peninsula are similar to those due to debris flow and slushflow in a nearby mountain range. The frequency of potential triggering events at the Ellesmere Island sites is expected to increase if summer climate warms, providing low percentage cloud cover is maintained during periods of high air temperatures. Copyright © 2005 John Wiley &amp; Sons, Ltd.

226Ra/230Th excess generated in the lower crust: Implications for magma transport and storage time scales

Excesses of 226 Ra in arc magmas have been interpreted as resulting from flux melting of the mantle above subducting slabs, followed by fast ascent rates of magma from slab to surface (up to 1000 m/yr). However, we demonstrate that incongruent melting of the lower crust could either maintain or augment mantle-derived 226 Ra excesses, and so reduce inferred vertical transport rates. We developed an incongruent, continuous melting model, and both the incongruent melting reaction and ingrowth effects contribute to the 226 Ra excess. In particular, we found that dehydration melting of amphibolite can produce modeled 226 Ra excesses greater than 300%. Mixtures of such amphibolite dehydration melts with mantle melts will likely retain a 238 U excess (subducted slab) signature. This amphibolite dehydration melting process will also produce elevated ratios of light rare earth elements to heavy rare earth elements, similar to those observed in several arc settings, which may distinguish these magmas from those with 226 Ra excesses produced by slab dewatering alone.

A time-resolved model of the mesospheric Na layer: constraints on the meteor input function

Abstract. A time-resolved model of the Na layer in the mesosphere/lower thermosphere region is described, where the continuity equations for the major sodium species Na, Na+ and NaHCO3 are solved explicity, and the other short-lived species are treated in steady-state. It is shown that the diurnal variation of the Na layer can only be modelled satisfactorily if sodium species are permanently removed below about 85 km, both through the dimerization of NaHCO3 and the uptake of sodium species on meteoric smoke particles that are assumed to have formed from the recondensation of vaporized meteoroids. When the sensitivity of the Na layer to the meteoroid input function is considered, an inconsistent picture emerges. The ratio of the column abundance of Na+ to Na is shown to increase strongly with the average meteoroid velocity, because the Na is injected at higher altitudes. Comparison with a limited set of Na+ measurements indicates that the average meteoroid velocity is probably less than about 25 km s-1, in agreement with velocity estimates from conventional meteor radars, and considerably slower than recent observations made by wide aperture incoherent scatter radars. The Na column abundance is shown to be very sensitive to the meteoroid mass input rate, and to the rate of vertical transport by eddy diffusion. Although the magnitude of the eddy diffusion coefficient in the 80–90 km region is uncertain, there is a consensus between recent models using parameterisations of gravity wave momentum deposition that the average value is less than 3×105 cm2 s-1. This requires that the global meteoric mass input rate is less than about 20 td-1, which is closest to estimates from incoherent scatter radar observations. Finally, the diurnal variation in the meteoroid input rate only slight perturbs the Na layer, because the residence time of Na in the layer is several days, and diurnal effects are effectively averaged out.

Anatomy of turbulence in a narrow and strongly stratified estuary

[1] More than 400 profiles of (salinity) density and velocity microstructure with 923 turbulent patches were collected over 4 days from an anchored boat with the Portable Flux Profiler (PFP) in the Swan River estuary during the wet season. By using the PFP the buoyancy flux was measured directly and estimated indirectly for this flow. Overturns were suppressed by the strong stratification, and large-scale anisotropy was correlated with turbulent Froude number. The direct measurements of the buoyancy flux exhibited both positive (downgradient) and negative (upgradient) values within the turbulent patches. The net average mixing efficiency was 0.04, less than the commonly used value of 0.17. Therefore the rate of vertical transport was small even though the dissipation levels within the turbulent patches were relatively high. The magnitude of the above mentioned competing fluxes increased with decreasing turbulent Froude numbers even though the net flux when bin-averaged with respect to turbulent Froude number remained close to zero.

Vertical budgets for organic carbon and biogenic silica in the Pacific sector of the Southern Ocean, 1996–1998

Abstract During the 1996–1998 Antarctic Environment and Southern Ocean Process Study (AESOPS), a component of US JGOFS, we obtained seasonal or longer-term data sets on the rates of production, vertical transport, remineralization and burial of particulate organic carbon (POC) and biogenic silica (BSiO2) in the Southern Ocean at 170°W between 55°S and 68°S. The AESOPS data records enable us to construct vertical C and Si budgets for the water column and upper sediments, with all estimates derived from direct measurement of the relevant fluxes. We constructed annual C and Si budgets for each of four ecologically distinct zonal bands within the system. For both POC and BSiO2 the greatest annual delivery to the sea floor (∼200 and 1400 mmol m−2 yr−1, respectively) and burial (∼6 and 160 mmol m−2 yr−1, respectively) were observed in the southern Antarctic Circumpolar Current (ACC) between 61.5°S and 65.5°S. That pattern is consistent with our observation that a diatom bloom propagated southward through the southern ACC during the spring and summer of 1997–1998, following the receding ice edge, and that this bloom was the main source of both POC and BSiO2 in the system on an annual basis. In the other zones the annual fluxes of POC and BSiO2 to the sea floor ranged from 19% to 67% of those in the zone traversed by the summer diatom bloom. The higher benthic fluxes of both POC and BSiO2 in the southern ACC imply that blooms similar to the one we observed in 1997–1998 occur commonly in the southern ACC, and that their high-productivity signature is transmitted to the sea floor. The data show preferential preservation of BSiO2 over POC throughout the water column and upper seabed. In the four zonal bands we consider, BSiO2 and POC are produced in mole ratios of 0.1–0.4, exported from the upper 100 m in ratios of 0.2–0.6, arrive at 1000 m in ratios of 1.5–4.5, reach the sea floor in ratios of 2.2–7.6, and are buried in ratios of 11.6–28. Despite the preferential preservation of BSiO2, accumulation of opal-rich sediments beneath the ACC does not result from unusually efficient preservation of siliceous material. The estimated BSiO2 preservation efficiency (burial-production) ranges from 1.2% to 5.5%, indistinguishable from the global average of 3%. Instead, opal-rich sediment accumulation in this region reflects very high annual rates of BSiO2 production in surface waters, along with very low accumulation rates of other sedimentary components (e.g., CaCO3 and detrital material). The observed high ratios of BSiO2 production to POC production in surface waters are consistent with the known tendency for the Si/C ratio of diatoms to increase when [Fe] is low. If greater Fe availability during the last glacial maximum permitted diatoms in the Southern Ocean to grow with lower, more normal Si/C ratios, export of diatom-produced POC could have occurred at 2–3 times its present rate. A corresponding increase in opal export or opal sediment accumulation is unlikely because silicic acid is almost totally depleted north of 65°S under present conditions. Thus, even large increases in POC production and export during glacial periods would not be reflected in the opal accumulation record.

Disequilibrium Chemistry in a Brown Dwarf's Atmosphere: Carbon Monoxide in Gliese 229B

The distribution of CO in Gliese 229B’s atmosphere reveals how disequilibrium processes establish the atmospheric composition. The CO abundance derived from its spectral signature exceeds the equilibrium value by several orders of magnitude. Our investigation of the source for CO considers disequilibrium mechanisms common to planetary atmospheres and concludes that the CO abundance is sensitive to atmospheric dynamics. Convection is not required to explain the observed abundance; instead, the vertical transport rate at »6 bars may be similar to that in planetary stratospheres. Subject headings: convection — planets and satellites: general — stars: atmospheres — stars: individual (Gliese 229B) — stars: low-mass, brown dwarfs