High Organic Matter Flux Research Articles

Rapid formation and sedimentation of large aggregates is predictable from coagulation rates (half-lives) of transparent exopolymer particles (TEP)

Two hypotheses have been proposed to account for the precipitous formation of large, rapidly settling aggregates at the termination of phytoplankton blooms in nature; aggregation due primarily to cell-cell collisions, and aggregation resulting from the presence of abundant transparent exopolymer particles (TEP), a recently discovered class of particles formed from polysaccharides excreted by phytoplankton. The hypothesis of TEP-driven coagulation in three disparate systems, a freshwater lake, a coastal ocean, and a saltwater mesocosm was evaluated, by comparing TEP abundance to several related factors including phytoplankton concentrations, measured sediment fluxes, and abundances of large aggregates. The timing of large aggregate formation and sedimentation events was related to coagulation rates expressed in terms of particle half-lives, t 1 2 , calculated as the time for TEP or phytoplankton to decrease to half their concentration through shear coagulation. While TEP have been previously investigated only in marine systems, it is reported here that TEP also can be present in high concentrations (860 ml −1) in freshwater lakes (Lake Constance, Germany) and that high fluxes of particulate organic matter at depth coincide with the disappearance of abundant TEP from overlying waters. The half-lives of TEP in the three different systems indicate that large aggregate formation and massive sedimentation events following diatom blooms occur when the TEP half-life decreases to less than a few days. By comparing TEP and phytoplankton half-lives in these systems, it is concluded that the formation of rapidly sinking aggregates following blooms of mucous-producing diatoms is primarily controlled by concentrations of TEP, not phytoplankton.

Evidence from nitrogen isotope ratios for enhanced productivity during formation of eastern Mediterranean sapropels

THE formation of horizons of organic-rich sediment (sapropels) in the eastern Mediterranean during the Holocene and Upper Pleistocene1 has been ascribed2 to enhanced preservation of organic carbon in bottom waters rendered anoxic by the restriction of deep-water renewal, either by lowered sea level during glacial maximum conditions3 or by the presence of low-salinity surface waters derived from increased runoff2,4. A second possibility is that the sapropels formed as a consequence of higher settling fluxes of organic matter caused by increased primary production connected with the increased runoff5,6. Here we report that in a sediment core from close to the mouth of the Nile, the sapropels have significantly lighter nitrogen isotope ratios (15N/<14N) than the intercalated marl oozes. We adduce evidence that these large differences cannot be caused either by variable mixtures of marine and terrestrial organic matter having different isotopic compositions, or by differences in the extent and type of post-depositional alteration. The differences are consistent, however, with a higher utilization of dissolved nitrate during sapropel formation, implying that the sapropels were ultimately formed as a consequence of high plankton production, which led to high fluxes of organic matter to the sea floor.

Heavy metal content in coastal sediments off Dar es Salaam, Tanzania

Anthropogenic input of Fe, Mn, Cu, Zn, Pb, Cr, Cd, and organic carbon into the Dar es Salaam coastal sediments is assessed in relation with human activities in the watershed and within the respective water body. The geochemically available fraction of the sediments from the Dar es Salaam harbor channel is enriched with iron and lead compared with the location off the harbor. Other elements such as copper, zinc, manganese chromium, and cadmium in the harbor sediments show moderate to weak signals in comparison with sediments outside the harbor area. Surprisingly, the sediments close to the outer anchorage zone for ships waiting to off-load cargo is strongly contaminated with Cd and Cr. In the case of Cd, the concentration is more than double that of the harbor sediments. There is a high flux of organic matter into the sediments in the harbor area.

Lack of enhanced preservation of organic matter in sediments under the oxygen minimum on the Oman Margin

The impingement of oxygen minima on continental margins is widely thought to promote the accumulation of sedimentary facies enriched in well-preserved organic matter. It is shown here, however, that such a relationship does not clearly apply to the productive Oman Margin in the Arabian Sea, which hosts one of the most severe oxygen minima in the oceans. Measurements made on the 0–1 cm depth interval from fourteen box cores collected from the outer shelf-upper continental slope area off Oman show that 1. (1) deposited organic matter is overwhelmingly of marine origin, 2. (2) there is no significant correlation between the abundance of sedimentary organic carbon (C org) and the bottom- water O 2 concentration, 3. (3) there is no relation between the sedimentary C org:N ratio and bottom-water O 2, 4. (4) there is no correlation between the hydrogen index (HI) of the organic matter and bottom water oxygen. There are, however, significant correlations between the C org:N ratio and the I:C org, Cr:Al, and Zr:Al ratios, as well as between the C org:N ratio and the hydrogen index. These relationships are interpreted as being the result of winnowing and attendant reworking which produce a progressive increase in the C org:N ratio and a decrease in the HI and the I:C org ratio as organic matter becomes progressively degraded. Commensurate rises in the Cr:Al and Zr:Al ratios result from the increased proportion of heavy minerals associated with winnowed lag deposits. Overall, these data suggest that the bottom water oxygen concentration has little effect in governing either the distribution or the degree of preservation of organic matter on this margin. Thus, the generally high but spatially variable C org content of the sediments on the Oman Margin may not reflect the occurrence of an oxygen minimum but instead be the result of a high settling flux of organic matter, supported by monsoon-driven upwelling, and post-depositional redistribution of the organic material by hydrodynamic influences.

Benthic foraminiferal assemblages from the eastern Weddell Sea between 68 and 73°S: Distribution, ecology and fossilization potential

Surface sediment samples taken with a vented ☐ corer from the eastern Weddell Sea on four profiles perpendicular to the continental margin have been investigated for their benthic foraminiferal content. The live fauna was differentiated from empty tests comprising the foraminiferal death assemblage. Based on the dead assemblages, potential fossil assemblages were calculated to facilitate the analogy with late Neogene core material. Five distinct live assemblages inhabit the continental margin today. Six dead assemblages and five potential fossil assemblages, respectively, correspond to these biocoenoses. A predominantly calcareous live fauna dominated by Trifarina angulosa is correlated with strong bottom currents and sandy sediments at the shelf break and on the uppermost continental slope. Below this, on the upper slope down to 2000 m water depth, the predominantly calcareous Bulimina aculeata assemblage coincides with the core of warm (>0°C) Weddell Deep Water and with fine and more organic-rich sediments. These calcareous live assemblages completely change composition during early diagenesis because of calcite dissolution within the uppermost sediment, which depends largely on the grain size distribution of the sediment and the fluxes of organic matter. Therefore, a still calcareous T. angulosa-dominated fossil assemblage indicates the sandy substrates on the shelf break and the upper slope, whereas the deeper slope with hemipelagic calm sedimentation and with high fluxes of organic matter is indicated by Martinottiella nodulosa, the characteristic arenaceous fossil remnant of the former predominantly calcareous live B. aculeata fauna. On a continental terrace between 2500 and 3500 m water depth Cribrostomoides subglobosus dominates the live fauna, but because of rapid disintegration of the empty tests of this agglutinated species a predominantly calcareous fauna characterized by Oridorsalis umbonatus and Epistominella exigua comprises the dead assemblage and the potential fossil assemblage, respectively. On the lower continental slope, between the carbonate lysocline (3500 m) and the carbonate compensation depth (4000 m), tests of Nuttallides umbonifer are the characteristic dead and potential fossil remnants of a former predominantly arenaceous live fauna, which is associated with the lower part of the Antarctic Bottom Water (AABW). This corroborates earlier investigations suggesting a relationship between the carbonate-corrosiveness of water masses and the distribution of N. umbonifer. This is important for inferring paleo-routes and estimates of paleo-production rates of AABW during the Neogene.

Bacterial origin of East Australian continental margin phosphorites

Many models of the origin of marine phosphorites require a sediment rich in organic matter, which by decomposition releases phosphorus for the precipitation of carbonate fluorapatite1–7. This concept is useful in explaining contemporary phosphorite in areas of very high productivity off the coasts of Peru–Chile and South West Africa8–10 where there are high organic matter fluxes to the sea floor4–7,11. It does not explain the origin of marine phosphorite deposits which formed in regions of limited oceanic upwelling and productivity12–14. The East Australian continental margin, an area of phosphogenesis throughout the late Pleistocene and Holocene13,14, is a modern analogue of an ‘East Coast’ phosphogenic province15–17 with low productivity over the upper slope region where the most recent phosphorites have been found18. Evidence is reported here for the carbonate fluorapatite in the East Australian phosphorites being located within bacterial cellular structures, and a model proposed for the origin of these deposits through the slow bacterial assimilation of phosphorus from seawater in an area of restricted sedimentation.

Euphausiacea (Crustacea) from Tropical West Africa : Boden, B. P., 1961. Atlantide Rep. 6: 251–262

The influence of different primary productivity regimes on live (Rose Bengal stained) and dead benthic foraminiferal distribution, as well as on the stable carbon isotopic composition of foraminiferal tests, was investigated in sediment surface samples (0–1 cm) from the upwelling region off Morocco between Cape Ghir (31°N) and Cape Yubi (27°N). A combination of factor analysis, detrended correspondence analysis (DCA) and canonical correspondence analysis (CCA) was applied to the benthic foraminiferal data sets. Five major assemblages for both the live and dead fauna were revealed by factor analysis. In the cape regions organic matter fluxes are enhanced by high chlorophyll-α concentrations in the overlying surface waters. Here, benthic foraminiferal faunas are characterized by identical live and dead assemblages, high standing stocks, and low species δ13C values, indicating constant year-round high productivity. Bulimina marginata dominates the unique fauna at the shallowest station off Cape Ghir indicating highest chlorophyll-a concentrations. Off both capes, the succession of the Bulimina aculeata/Uvigerina mediterranea assemblage, the Sphaeroidina bulloides/Gavelinopsis translucens assemblage, and the Hoeglundina elegans assemblage from the shelf to the deep sea reflects the decrease in chlorophyll-a concentrations, hence the export flux. In contrast, the area between the capes is characterized by differently composed live and dead assemblages, low standing stocks, and less depleted δ13C values, thus reflecting low primary productivity. High foraminiferal numbers of Epistominella exigua, Eponides pusillus, and Globocassidulina subglobosa in the dead fauna indicate a seasonally varying primary productivity signal. Significantly lower mean δ13C values were recorded in Bulimina mexicana, Cibicidoides kullenbergi, H. elegans, U. mediterranea and Uvigerina peregrina. Cibicidoides wuellerstorfi is a faithful recorder of bottom water δ13C in the Canary Islands regions. The mean δ13C signal of this species is not significantly influenced by constant high organic matter fluxes. The species-specific offset between live and dead specimens is the same.