Robert F. Anderson

Wind-Driven Upwelling in the Southern Ocean and the Deglacial Rise in Atmospheric CO2

Wind-driven upwelling in the ocean around Antarctica helps regulate the exchange of carbon dioxide (CO2) between the deep sea and the atmosphere, as well as the supply of dissolved silicon to the euphotic zone of the Southern Ocean. Diatom productivity south of the Antarctic Polar Front and the subsequent burial of biogenic opal in underlying sediments are limited by this silicon supply. We show that opal burial rates, and thus upwelling, were enhanced during the termination of the last ice age in each sector of the Southern Ocean. In the record with the greatest temporal resolution, we find evidence for two intervals of enhanced upwelling concurrent with the two intervals of rising atmospheric CO2 during deglaciation. These results directly link increased ventilation of deep water to the deglacial rise in atmospheric CO2.

A review of the Si cycle in the modern ocean: recent progress and missing gaps in the application of biogenic opal as a paleoproductivity proxy

Abstract Due to the major role played by diatoms in the biological pump of CO2, and to the presence of silica-rich sediments in areas that play a major role in air–sea CO2 exchange (e.g. the Southern Ocean and the Equatorial Pacific), opal has a strong potential as a proxy for paleoproductivity reconstructions. However, because of spatial variations in the biogenic silica preservation, and in the degree of coupling between the marine Si and C biogeochemical cycles, paleoreconstructions are not straitghtforward. A better calibration of this proxy in the modern ocean is required, which needs a good understanding of the mechanisms that control the Si cycle, in close relation to the carbon cycle. This review of the Si cycle in the modern ocean starts with the mechanisms that control the uptake of silicic acid (Si(OH)4) by diatoms and the subsequent silicification processes, the regulatory mechanisms of which are uncoupled. This has strong implications for the direct measurement in the field of the kinetics of Si(OH)4 uptake and diatom growth. It also strongly influences the Si:C ratio within diatoms, clearly linked to environmental conditions. Diatoms tend to dominate new production at marine ergoclines. At depth, they also succeed to form mats, which sedimentation is at the origin of laminated sediments and marine sapropels. The concentration of Si(OH)4 with respect to other macronutrients exerts a major influence on diatom dominance and on the rain ratio between siliceous and calcareous material, which severely impacts surface waters pCO2. A compilation of biogenic fluxes collected at about 40 sites by means of sediment traps also shows a remarkable pattern of increasing BSi:Corg ratio along the path of the “conveyor belt”, accompanying the relative enrichment of waters in Si compared to N and P. This observation suggests an extension of the Si pump model described by Dugdale and Wilkerson (Dugdale, R.C., Wilkerson, F.P., 1998. Understanding the eastern equatorial Pacific as a continuous new production system regulating on silicate. Nature 391, 270–273.), giving to Si(OH)4 a major role in the control of the rain ratio, which is of major importance in the global carbon cycle. The fate of the BSi produced in surface waters is then described, in relation to Corg, in terms of both dissolution and preservation mechanisms. Difficulties in quantifying the dissolution of biogenic silica in the water column as well as the sinking rates and forms of BSi to the deep, provide evidence for a major gap in our understanding of the mechanisms controlling the competition between retention in and export from surface waters. The relative influences of environmental conditions, seasonality, food web structure or aggregation are however explored. Quantitatively, assuming steady state, the measurements of the opal rain rate by means of sediment traps matches reasonably well those obtained by adding the recycling and burial fluxes in the underlying abyssal sediments, for most of the sites where such a comparison is possible. The major exception is the Southern Ocean where sediment focusing precludes the closing of mass balances. Focusing in fact is also an important aspect of the downward revision of the importance of Southern Ocean sediments in the global biogenic silica accumulation. Qualitatively, little is known about the duration of the transfer through the deep and the quality of the material that reaches the seabed, which is suggested to represent a major gap in our understanding of the processes governing the early diagenesis of BSi in sediments. The sediment composition (special emphasis on Al availability), the sedimentation rate or bioturbation are shown to exert an important control on the competition between dissolution and preservation of BSi in sediments. It is suggested that a primary control on the kinetic and thermodynamic properties of BSi dissolution, both in coastal and abyssal sediments, is exerted by water column processes, either occuring in surface waters during the formation of the frustules, or linked to the transfer of the particles through the water column, which duration may influence the quality of the biogenic rain. This highlights the importance of studying the factors controlling the degree of coupling between pelagic and benthic processes in various regions of the world ocean, and its consequences, not only in terms of benthic biology but also for the constitution of the sediment archive. The last section, first calls for the end of the “NPZD” models, and for the introduction of processes linked to the Si cycle, into models describing the phytoplankton cycles in surface waters and the early diagenesis of BSi in sediments. It also calls for the creation of an integrated 1-D diagnostic model of the Si:C coupling, for a better understanding of the interactions between surface waters, deep waters and the upper sedimentary column. The importance of Si(OH)4 in the control of the rain ratio and the improved parametrization of the Si cycle in the 1-D diagnostic models should lead to a reasonable incorporation of the Si cycle into 3-D regional circulation models and OGCMs, with important implications for climate change studies and paleoreconstructions at regional and global scale.

The Last Glacial Termination

Warming Up For the past half-million years, our planet has passed through a cycle of glaciation and deglaciation every 100,000 years or so. Each of these cycles consists of a long and irregular period of cooling and ice sheet growth, followed by a termination—a period of rapid warming and ice sheet decay—that precedes a relatively short warm interval. But what causes glacial terminations? Denton et al. (p. 1652) review the field and propose a chain of events that may explain the hows and whys of Earths emergence from the last glacial period. Pulling together many threads from both hemispheres suggests a unified causal chain involving ice sheet volume, solar radiation energy, atmospheric carbon dioxide concentrations, sea ice, and prevailing wind patterns. A major puzzle of paleoclimatology is why, after a long interval of cooling climate, each late Quaternary ice age ended with a relatively short warming leg called a termination. We here offer a comprehensive hypothesis of how Earth emerged from the last global ice age. A prerequisite was the growth of very large Northern Hemisphere ice sheets, whose subsequent collapse created stadial conditions that disrupted global patterns of ocean and atmospheric circulation. The Southern Hemisphere westerlies shifted poleward during each northern stadial, producing pulses of ocean upwelling and warming that together accounted for much of the termination in the Southern Ocean and Antarctica. Rising atmospheric CO2 during southern upwelling pulses augmented warming during the last termination in both polar hemispheres.

Seasonality in the supply of sediment to the deep Sargasso Sea and implications for the rapid transfer of matter to the deep ocean

The flux of particles approaching the sea floor near Bermuda has been sampled by sediment trap nearly continuously for more than two years. The trap was placed at a depth of 3200 m, 1000 m above the bottom, and samples were recovered at two-month intervals. All major components of the sediment (biogenic carbonate and silicate, and organic matter) and a minor, presumably aeolian, clay component, as well as all size fractions (after sieving) were delivered in seasonally fluctuating amounts. The flux variations appear closely tied to the annual cycle of primary production in the surface water, which in the Sargasso Sea peaks in early spring and reaches a low in late fall. The total particulate flux varied by a factor of three (20 to 60 mg m−2 d−1), but some components varied by more than an order of magnitude. The close synchroneity between surface production and deep-water arrival of even fine particles, which presumably sink as components of larger aggregates, indicates extremely rapid settling of the bulk of the sediment. The evidence that even the flux of inorganic particles varies in phase with the primary production cycle suggests that an efficient mechanism exists for rapid removal from the mixed layer and transfer to deep water of many chemicals, including pollutants, which are associated with, or scavenged by, biogenic and aeolian particles.

Authigenic molybdenum formation in marine sediments: a link to pore water sulfide in the Santa Barbara Basin

Pore water and sediment Mo concentrations were measured in a suite of multicores collected at four sites along the northeastern flank of the Santa Barbara Basin to examine the connection between authigenic Mo formation and pore water sulfide concentration. Only at the deepest site (580 m), where pore water sulfide concentrations rise to .0.1 mM right below the sediment water interface, was there active authigenic Mo formation. At shallower sites (550, 430, and 340 m), where pore water sulfide concentrations were consistently ,0.05 mM, Mo precipitation was not occurring at the time of sampling. A sulfide concentration of ;0.1 mM appears to be a threshold for the onset of Mo-Fe-S co-precipitation. A second threshold sulfide concentration of;100 mM is required for Mo precipitation without Fe, possibly as Mo-S or as particle-bound Mo. Mass budgets for Mo were constructed by combining pore water and sediment results for Mo with analyses of sediment trap material from Santa Barbara Basin as well as sediment accumulation rates derived from 210 Pb. The calculations show that most of the authigenic Mo in the sediment at the deepest site is supplied by diffusion from overlying bottom waters. There is, however, a non-lithogenic particulate Mo associated with sinking particles that contributes #15% to the total authigenic Mo accumulation. Analysis of sediment trap samples and supernant brine solutions indicates the presence of non-lithogenic particulate Mo, a large fraction of which is easily remobilized and, perhaps, associated with Mn-oxides. Our observations show that even with the very high flux of organic carbon reaching the sediment of Santa Barbara Basin, active formation of sedimentary authigenic Mo requires a bottom water oxygen concentration below 3 mM. However, small but measurable rates of authigenic Mo accumulation were observed at sites where bottom water oxygen ranged between 5 and 23 mM, indicating that the formation of authigenic Mo occurred in the recent past, but not at the time of sampling. Copyright

Geochemistry of barium in marine sediments : Implications for its use as a paleoproxy

Abstract Variations in the accumulation rate of barium in marine sediments are thought to be indicative of variations in marine biological productivity through time. However, the use of Ba as a proxy for paleoproductivity is partly dependent upon its being preserved in the sediment record in a predictable or consistent fashion. Arguments in favor of high Ba preservation are partly based on the assumption that sediment porewaters are generally at saturation with respect to pure barite. The idea is that because nondetrital sedimentary Ba predominantly exists as barite, porewater saturation would promote burial. We present sediment porewater, sediment solid phase, and benthic incubation chamber data suggesting that solid-phase Ba preservation may be compromised in some geochemical settings. We propose that under suboxic diagenetic conditions, characterized by low bottom water oxygen and high organic carbon respiration rates, Ba preservation may be reduced. Independent of the mechanism, if this assertion is true, then it becomes important to know when the Ba record is unreliable. We present evidence demonstrating that the sedimentary accumulation of authigenic U may serve as a proxy for when the Ba record is unreliable. We then provide an example from the Southern Ocean during the last glacial period where high authigenic U concentrations coincide with high Pa:Th ratios and high accumulation rates of biogenic opal, but we find low accumulation rates of sedimentary Ba. Thus, for the study sites presented here during the last glacial, we conclude that Ba is an unreliable productivity proxy.

Concentration, oxidation state, and particulate flux of uranium in the Black Sea

In contrast to its unreactive behavior in the open ocean, uranium is removed from seawater to sediments of anoxic marine basins. Four biogeochemical processes have been proposed to account for this removal: active uptake by organisms whose remains are preserved by the anoxic conditions of the sediments; complexation of U(VI) by particulate organic matter; chemical reduction of soluble U(VI) to insoluble U(IV) which is scavenged to the sediments by settling particles; and precipitation of uranium within the sediments themselves. The Black Sea is the largest anoxic marine basin in the world today. Waters below 150–200 m are anoxic and H2S levels build up to about 400 μmol 1−1 in the bottom water (maximum depth ~2200 m). Concentrations of total U and the oxidation state of dissolved U were measured throughout the water column while particulate U fluxes were determined from time series sediment trap samples collected in the deep anoxic zone. Bottom waters were depleted in U by ~40% relative to the initial U content of the water. Two independent methods gave residence times of dissolved U in the deep-water column of about 103 years. Uranium occurred throughout the water column as soluble, chemically labile (reducible) U(VI) rather than the thermodynamically favored U(IV). Uranium is neither reduced to U(IV) by the high levels of H2S nor is it rapidly scavenged from the water column as would be expected for strongly hydrolyzed tetravalent actinides like U(IV). Fluxes of particulate authigenic U measured with sediment traps are at least two orders of magnitude less than the rate of authigenic U burial in Black Sea sediments. Therefore, the first three processes listed above contribute negligibly to the deposition of U in the sediments of anoxic marine basins such as the Black Sea. The dominant removal process must involve precipitation of U in the sediments rather than scavenging from the water column.

Removal of 230Th and 231Pa at ocean margins

Abstract Uranium, thorium and protactinium isotopes were measured in particulate matter collected by sediment traps deployed in the Panama Basin and by in-situ filtration of large volumes of seawater in the Panama and Guatemala Basins. Concentrations of dissolved Th and Pa isotopes were determined by extraction onto MnO 2 adsorbers placed in line behind the filters in the in-situ pumping systems. Concentrations of dissolved 230 Th and 231 Pa in the Panama and Guatemala Basins are lower than in the open ocean, whereas dissolved 230 Th/ 231 Pa ratios are equal to, or slightly greater than, ratios in the open ocean. Particulate 230 Th/ 231 Pa ratios in the sediment trap samples ranged from 4 to 8, in contrast to ratios of 30 or more at the open ocean sites previously studied. Particles collected by filtration in the Panama Basin and nearest to the continental margin in the Guatemala Basin contained 230 Th/ 231 Pa ratios similar to the ratios in the sediment trap samples. The ratios increased with distance away from the continent. Suspended particles near the margin show no preference for adsorption of Th or Pa and therefore must be chemically different from particles in the open ocean, which show a strong preference for adsorption of Th. Ocean margins, as typified by the Panama and Guatemala Basins, are preferential sinks for 231 Pa relative to 230 Th. Furthermore, the margins are sinks for 230 Th and, to a greater extent, 231 Pa transported by horizontal mixing from the open ocean.

Removal of230Th and231Pa from the open ocean

Concentrations of230Th and231Pa were measured in particulate matter collected by sediment traps deployed in the Sargasso Sea (Site S2), the north equatorial Atlantic (site E), and the north equatorial Pacific (Site P) as well as in particles collected by in situ filtration at Site E. Concentrations of dissolved Th and Pa were determined by extraction onto manganese dioxide adsorbers at Site P and at a second site in the Sargasso Sea (site D). Dissolved230Th/231Pa activity ratios were 3–6 at Sites P and D. In contrast, for all sediment trap samples from greater than 2000 m, unsupported230Th/231Pa ratios were 22–35 (average 29.7). Ratios were lower in particulate matter sampled at shallower depths. Particles filtered at 3600 m and 5000 m at Site E had ratios of 50 and 40. Results show that suspended particulate matter in the open ocean preferentially scavenges Th relative to Pa. Most of the230Th produced by decay of234U in the open ocean is removed by adsorption to settling particulate matter. In contrast, less than 50% of the231Pa produced by decay of235U is removed from the water column by this mechanism. Mixing processes transport the remainder to other sinks.

Fluxes of particles and constituents to the eastern United States continental slope and rise: SEEP—I

Seventeen time-series sediment traps were deployed for a year from September 1983 to October 1984 along 70°55′W longitude in a two-dimensional array in water depths from 500 m on the upper continental slope to 2750 m on the upper continental rise as part of the SEEP—I (Shelf Edge Exchange Processes) experiment. Fluxes of total particulate mass, organic carbon and nitrogen, calcium carbonate and 210Pb were measured in the almost 300 samples that were recovered. Based on 210Pb inventories in the slope sediments, the total mass fluxes measured in the traps near the bottom of the slope water column are representative of the longer-term (∼100 y) accumulation rates of slope sediments. In addition to the vertical flux of particles coming from near-surface waters of the slope, both fresh biogenic particulate matter and resuspended sediment derived from the shelf and upper slope are transported downslope and added to the burden of particles settling vertically to the slope and upper rise sediments. The fluxes and composition of the trapped particles vary seasonally as a result of effects resulting from the spring bloom in plant biomass on the shelf, and as a result of the succession of the spring-bloom effects by carbonate-secreting organisms in the late summer and autumn. On a much shorter time scale variations in fluxes and composition are caused by more local biological events and by events that resuspend and transport sediment from the shelf across the shelf-break and down the slope. The scavenging of particle-reactive radionuclides (210Pb and 234Th) and, by analogy, particle-reactive pollutants, is closely tied to the flux of fresh biogenic debris. Comparison of the fluxes of organic carbon on the slope with the estimates of primary productivity over the shelf, indicates that <10% of the biogenic carbon fixed on the shelf is exported to the slope in the SEEP—I area. Comparison of the fluxes of 210Pb in the slope traps with the known atmospheric input to the shelf and slope surface waters implies that of the order of 20% of both biogenic and abiogenic particles from the shelf are exported to the slope on a time scale commensurate with the half life of210Pb. Comparison of the SEEP—I organic carbon and 210Pb fluxes to the slope at 1200 m with those fluxes ∼250km to the south at 975 m suggests that the shelf-to-slope export may increase by as much as three-fold for organic carbon particles and by two-fold for biogenic and abiogenic particles with which 210Pb is associated.