Werner G. Deuser

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.

Seasonal changes in species composition, numbers, mass, size, and isotopic composition of planktonic foraminifera settling into the deep sargasso sea☆

Abstract Planktonic foraminifera recovered from six successive sediment-trap samples collected over fourteen months at a depth of 3200 m near Bermuda show varied responses to seasonal changes in the near-surface hydrography. These include changes in species composition and within species, changes in number of individuals collected per unit time, average size and weight of individuals, and in oxygen and carbon isotopic composition. A total of seventeen species were identified and eleven were sufficiently abundant for isotopic analysis in at least some of the samples. The accumulations on the sea floor of some of these species can be expected to contain a strong seasonal component in their oxygen isotopic composition which may be utilized in paleoclimatic and paleo-oceanographic reconstructions. Best suited to this purpose are those species which form their tests in isotopic equilibrium with surface water and whose abundance is confined to a portion of the year only. The best examples are Pulleniatina o bliguiloculata, Globigerinoides conglobatus, and Neogloboquadrina dutertrei. Globigerinoides ruber is a good estimator of the average surface-water temperature because it occurs in rather even abundance throughout the year. Hastigerina pelagica, though very abundant in surface water, is practically eliminated, even before arrival on the sea floor, by partial resorption and structural weakening of the tests during gametogenesis. Orbulina universa, Globorotalia truncaltulinoides and Globorotalia hirsuta show morphological and istopic evidence for contibnued calcification in deeper water after initial test formation near ther surface. Globigerina bulloides represents the most striking example of oxygen isotopic disequilibrium with sea water.

Polycyclic aromatic hydrocarbons in an anoxic sediment core from the Pettaquamscutt River (Rhode Island, U.S.A.)

Abstract Fifteen sections from an anoxic sediment core were analyzed for polycyclic aromatic hydrocarbons (PAH). Two types of PAH were observed: (a) those from combustion sources such as pyrene and chrysene and (b) those from natural sources such as retene and perylene. The combustion PAH levels in core sections dated between 1900 and 1970 were much higher than in earlier sections; this indicated an anthropogenic origin of these PAH at this location. The perylene and retene core profiles show significant anomalies during the period 1850–1880. Organic carbon does not fluctuate markedly but δC-13 of organic carbon shows several unexplained excursions; one of which correlates with the perylene and retene anomalies.

Trajectories of sinking particles in the Sargasso Sea: modeling of statistical funnels above deep-ocean sediment traps

Abstract Characteristics of statistical funnels above moored deep-ocean sediment traps at the Oceanic Flux Program (OFP) site in the Sargasso Sea were determined by Lagrangian analysis of particles sinking through a realistic horizontal velocity field. Stochastic simulations support previous assertions that the trajectories of sinking particles are, for the most part, far from vertical, and that traps sample particles from rather large “catchment” areas when evaluated over long time scales (much greater than 1 year). The dimensions and geographic centers of these catchment areas are determined by the characteristics of the ocean flow field, particle sinking speeds and depth of the traps. The predicted extent and center of the 3200 m OFP trap is nearly identical to that inferred from a previous analysis of OFP trap fluxes and Coastal Zone Color Scanner (CZCS) imagery. Traps moored at different depths may collect particles originating in widely separated areas at the sea surface. This is an important issue when the catchment area is assessed over short time scales (less than 60 days), which typify the collection times of most moored sediment traps. Given the typically patchy distribution of particles and particle producers, this can result in short-period flux measurements that show little or no coherence between collections made at the same location for adjacent times or those made simultaneously but at different depths. The effects of eddy dispersion will be greater for most other oceanic regions, as the Sargasso Sea is characterized by relatively low levels of both eddy and mean kinetic energy. The results of this study demonstrate that an understanding of the temporal and spatial characteristics of the flow field above deep-ocean sediment traps is just as important to the interpretation of flux measurements as is the analysis of the material collected by the traps.

Biological control of the removal of abiogenic particles from the surface ocean

Concurrent measurements of particle concentrations in the near-surface water and of particle fluxes in the deep water of the Sargasso Sea show a close coupling between the two for biogenic components. The concentrations of suspended matter appear to follow an annual cycle similar to that of primary production and deepwater particle flux. Although the concentration of particulate aluminum in the surface water appears to vary randomly with respect to that cycle, the removal of aluminum to deep water is intimately linked to the rapid downward transport of organic matter.

Seasonality in the fluxes of sugars, amino acids, and amino sugars to the deep ocean: Sargasso sea

The fluxes of sugars, amino acids, and amino sugars as released by acid hydrolysis were determined in the < 37-μm fraction of samples collected during successive two-month sediment trap deployments in the deep Sargasso Sea (3200 ± 100 m) from April 1978 to December 1981. All fluxes varied seasonally and in phase with the flux of the < 37-μm fraction, which has been shown to vary in phase with primary productivity in the surface layers. During the investigation the fluxes were in the range of 0.03 to 1.7 mg m−2 d−1. They contributed 13 to 34% of the measured organic carbon, and 30 to 53% of the measured total nitrogen could be accounted for by amino acids and amino sugars. The relative abundances of sugars and amino acids were, in general, similar to those reported for mineralized tissues of carbonate and silica producers and the cell walls of non-biomeneralizing organisms. However, the amounts of non-protein amino acids, i.e., β-alanine and γ-aminobutyric acid, of aspartic and glutamic acids, and of amino sugars relative to total amino acids varied seasonally. Relative abundances of these compounds appear to indicate the nature and source of organic matter arriving at the sediment trap.

Mooring line motions and sediment trap hydromechanics: in situ intercomparison of three common deployment designs

Trap array characteristics were monitored concurrent with particle collections for surface-tethered and bottom-moored cones and cylinders (MultiPITs) at the North Atlantic OFP-JGOFS site in the Sargasso Sea. At depths ranging from 145 to 3200 m, velocities of approaching fluid and those inside the traps were recorded at 5 Hz in bursts of 3–10 min every half hour during particle collections. A thermistor, a high resolution pressure gauge and two inclinometers concurrently monitored trap movements. Burst-averaged slip velocities experienced by both shallow and deep tethered traps reached 37 cm s−1, while a bottom-moored trap recorded 10-day averaged speeds of 4 cm s−1. Independent of deployment technique, for both cones and cylinders, flow cells inside the traps led to an intense flushing of fluid and particles. None of the surface-tethered traps tilted more than 8 degrees from vertical, even under strong flow accelerations. Tether-line motions, induced by the surface waves, generated high flow acceleration peaks of trap arrays at all depths, even for bungie-cord decoupled MultiPIT arrays. The flow cells inside traps were thus agitated with the result of intense turbulence prevailing close to the collection cup in the apex of tethered cones. Moored cone arrays recorded less dynamic environments. Trap fluxes by tethered cones were up to a factor of 8 smaller than by tethered MultiPITs at the same depth and time, cones collected more material with higher approaching fluid flows (untested so far for cylinders), and for the same conical geometry tethered traps collected less material than bottom-moored traps. The in situ deployments revealed substantial flow- and geometry-related differences in collection behavior among the different trap arrays, all of which deviated from steady-state flume simulation results. The diameter of the retention cup at the trap apex rather than the trap mouth diameter may be a controlling design parameter of particle collection rates for conical traps. Efforts to link trap and in situ fluxes require that hydrodynamics of individual trap arrays at depth are monitored, including line motions. Drift velocities rarely coincided with trap-experienced approach velocities. Trap simulation studies utilizing steady-state flume flows may be accurate only under very specialized conditions. Our data provide a hydrodynamic rationale for earlier recommendations by others of cylinders with adequate length-width ratio.

Surface-ocean color and deep-ocean carbon flux: how close a connection?

Abstract Seven years of simultaneous, quasi-continuous data collected by the Nimbus-7 Coastal Zone Color Scanner and by a deep-ocean sediment trap in the Sargasso Sea allow the derivation of empirical relationships between remotely sensed ocean color and the sinking of particulate carbon into the deep sea. In agreement with earlier observations, the results indicate a 1.5-month lag between surface-ocean events observed by the satellite and arrival of a record of those events, carried by sinking particles, at a depth of 3200 m. In addition, the results suggest that the sea-surface area most influential on particle-flux characteristics recorded by the sediment trap in the Sargasso Sea lies to the northeast of the traps mooring site. The results point towards possible ways of quantifying the role of marine biota in the regulation of atmospheric carbon dioxide through use of satellite observations.

Carbon isotope relationships between plankton and sea water

Marine plankton shows a variation in δC13 from −9 to about −30 %.. The fractionation with respect to the oceanic inorganic carbon appears to be due only to the photosynthetic process in the phytoplankton. The wide range can be explained on the basis of now laboratory experiments as being due to environmental differences in temperature, pH and carbon availability.

Methane in Lake Kivu: New Data Bearing on Its Origin

Lake Kivu, an African rift lake, contains about 50 cubic kilometers of methane (at standard temperature and pressure) in its deep water. Data resulting from two recent expeditions to the lake and a reevaluation of earlier data suggest that most of the methane was formed by bacteria from abiogenetic carbon dioxide and hydrogen, rather than being of volcanic origin or having formed from decomposing organic matter.