Detmar Schnitker

The deep waters of the western North Atlantic during the past 24,000 years, and the re-initiation of the Western Boundary Undercurrent

Abstract Changes in composition of benthic foraminiferal faunas in two cores from the North American continental rise (at ∼ 3000 m and ∼ 4000 m depth) and one core from the center of the North American Basin (at ∼ 4600 m depth) reflect changes in the nature of bottom water in the western North Atlantic during the past 24,000 years. During late glacial times the portion of the basin below 4500 m depth was filled with cold, well oxygenated water, probably of local North Atlantic origin. This water was overlain by more than 1500 m of “old”, oxygen-deficient water, of probably southern (Antarctic) origin. A basinwide change of deep water occurred first at about 12,500 yr. B.P., which resulted in a strong vertical differentiation of the water masses. As the deepest water became isolated and was losing oxygen, a western boundary undercurrent became established which carried well-oxygenated water at ∼ 3000 m depth along the continental rise. “Old” oxygen-deficient water reoccupied the levels between these two. A second basinwide change occurred shortly after 9000 yr. B.P., marking the beginning of modern deep-water conditions. The Western Boundary Undercurrent enlarged, moved downslope and flushed out the “old” glacial-age deep water. Norwegian Sea overflow reached its maximum about 4000 years ago. The center of the basin experienced the second change with a delay of about 1000 years, when it filled with Norwegian Sea Overflow Water, and later (after 5000 yr. B.P.) received large proportions of Antarctic Bottom Water. These deep-water events are correlated with the initial retreat of the polar front into the Norwegian Sea, its southward readvance, and the final opening of the Norwegian Sea to the inflow of North Atlantic Drift water.

Postglacial Emergence of the Gulf of Maine

Sonic subbottom profiles reveal that glacial deposits off the Kennebec River, all shallower than 65 m, show an erosional topography with a relief of up to 30 m. At greater depths, in the area between Cape Small and Damariscove Island, these deposits are flat surfaced. A possible berm is present at the transition from dissected to undissected deposits, and the entire area is covered by a veneer of recent sediments. The erosional topography is thought to have formed subaerially during a postglacial period of emergence. A curve has been constructed for the isostatic change of the Gulf of Maine area during the past 13,000 yr. Crustal rebound commenced at a rate of 19 cm per yr, exceeding the rate of eustatic sea-level rise, so that by 12,300 yr B.P. the sea had retreated to the present coastline. Maximal emergence was reached about 8,500 yr B.P. when the crustal subsidence commenced; this settling is still continuing.

Deep-Sea Benthic Foraminifers: Food and Bottom Water Masses

Benthic foraminifers are an abundant, nearly cosmopolitan and easily preserved component of the deep-sea meiofauna. The benthic foraminiferal faunas are very strongly structured on regional scales. In the western North and South Atlantic the distribution of benthic assemblages follows the distribution of named “water masses,” leading to the expectation that fossil benthic foraminifers from sediment cores allow a reconstruction of past deep-water circulation patterns. However, in many other areas the faunal composition is strongly correlated with the productivity of the overlying surface waters. Such results demonstrate that the benthic meiofauna responds to the integral of the deep environment and it is the limiting or overwhelming influence that determines the success or failure of individual species and thus the composition of the fauna. Analysis of available data clearly indicates that benthic foraminifers are unequivocal indicators of productivity in areas where productivity is high. In areas of low or very uniform productivity the composition of the benthic fauna clearly carries the imprint of deep water mass structure as the dominant feature. Faunal sequences from late Quaternary cores from the Atlantic and Pacific oceans show very strong differentiations. Not only is the glacial/postglacial transition shown, but also much superimposed fine structure. Faunal composition is as sensitive to deep-water environmental change as are geochemical indicators. The faunal composition allows at least some appreciation of the cause for the major changes, but a quantitative differentiation between productivity or water mass changes is not yet possible. If, however, an independent assessment of productivity change is available, then the benthic foraminiferal faunas are exquisite indicators of changes in deep water circulation.

Loss of calcareous microfossils from sediments through gypsum formation

Abstract Laboratory experiments with fresh marine sediments demonstrate the dissolution of calcareous microfossils (foraminifers) and the production of “authigenic” gypsum. Slow oxidation of iron sulfide in the presence of oxygen drives the following reactions: 1. (1) 4FeS + 9O2 + 6H2O = 4FeO(OH) + 4SO2−4 + 8H+ or 2 FeS 2 + 7 1 2 O 2 + 5 H 2 O = 2 FeO ( OH ) + 4 SO 2− 4 + 8 H + 2. (2) H+ + CaCO3 = Ca2+ + HCO−3 3. (3) Ca2+ + SO2−4 + 2H2O = CaSO4·2H2O Three-month experiments resulted in minor to total dissolution of different species from estuarine and open ocean sediments. Elphidium ustilatum was the most resistant to dissolution whereas Elphidium subarcticum was the first species to disappear completely. Agglutinated foraminifers are nearly completely destroyed largely because of bacterial decay of the test binding agent. Loss or partial loss of species may occur either in conditions of storage after collection or because of changing environmental conditions (primarily oxygen levels) at the sediment—water interface.

Enhancement of diatom frustule dissolution by iron oxides

Abstract Several reports in the literature indicate the possibility that iron oxyhydroxides in sediments enhance the dissolution of diatom frustules. We tested this possibility by conducting a laboratory experiment in which various amounts of iron were added to sediment spiked with undissolved frustules. A visual dissolution index was developed to assess the extent of dissolution. A concentration of 300 mg-Fe/g-sediment was found to accelerate dissolution relative to 0, 3, and 30 mg-Fe/g-sediment additions. Such abnormally high exposure to iron oxyhydroxides may be encountered by frustules over time, particularly in aluminosilicate sediments subjected to frequent redox transitions.

Climatic variability and deep ocean circulation: Evidence from the North Atlantic

Abstract Rhythmic fluctuations of the faunal composition of deep-sea benthic foraminifers during the past 24,000 years occurred at periodicites of about 2500, 1500, 1000, and 650 years. These faunal fluctuations apparently are recordings of global climate fluctuations which have left similar records in climate sensitive biotic and abiotic indicators. The reddenings of the power spectra occur in a manner very similar to those produced by the models of Hasselmann and Lemke, with slopes of approximately −2 on a log—log plot, which could imply that the observed climatic variance is stochastic in origin. The presence of significant spectral peaks suggests that the propagation of variance from high to low frequencies is held up or accelerated at frequencies characteristics of turnover times of the oceanic deep-water circulation. The most sensitive triggering point to influence the deep thermohaline circulation lies in the northern North Atlantic, where the rate of bottom water formation and its physical and chemical characteristics are easily modulated by local atmospheric and oceanographic conditions. Any change in the deep-water characteristics propagate through the ocean basins with delays determined by the speed and length of deep-water flow. Feedback to the North Atlantic is provided by the direct return flow of some of these water masses as surface currents, and by the influence that changes in the rate and character of upwelling deep waters may have upon shaping surface climate. Variations of the rates and temperatures of North Atlantic downwelling and North Pacific upwelling influences the overall energy balance. The steering effect of North Pacific sea surface temperature anomalies upon the jet stream, as noted by Namias, provides an atmospheric teleconnection from the North Pacific to North America and the North Atlantic, which controls the regional energy distribution and in turn, the modulation of North Atlantic downwelling.

The direct titration of water-soluble sulfide in estuarine muds of Montsweag Bay, Maine☆

Abstract The use of an antioxidant buffer allows potentiometric titrations of dissolved sulfide, providing cadmium or lead rather than silver is used as titrant. The method yields a measure of water-soluble as contrasted with acid-soluble sulfide. The vertical distribution of water-soluble sulfide has been determined in eight sediment cores from Montsweag Bay, Maine. Concentration profiles are characterized by one or two distinct maxima near the surface at depths ranging from 30 cm in areas of slow sedimentation, to 120 cm in areas of rapid sediment accumulation. Concentrations of sulfide at the maxima range from 1 to 8 moles sulfur per cubic meter of in situ mud, or from 0.6 to 4. millimoles sulfur per liter of pore water. The sulfide-rich upper portion of the mud is poorly consolidated and custardy in texture. As they are likely to be resuspended during proposed channel modifications, it is appropriate to inquire as to the oxidative demand of these sediments arising from their hydrogen sulfide content alone. Using a conservative estimated mean of 5 moles sulfide/m 3 -mud, it is shown that a mixing ratio of less than 30 volumes of oxygen saturated seawater per volume of mud will render the water anoxic.

Late Glacial and Holocene Diatom Successions in the Gulf of Maine: Response to Climatologic and Oceanographic Change

The diatom floras of a 16 meter long piston core from the Wilkinson Basin in the Gulf of Maine were analyzed quantitatively and their composition compared with diatom floras contained in surface sediment samples from the Canadian Arctic to Cape Harteras.

Das Magnet-Tonbandgerät als Hilfsmittel in der Mikropaläontologie

ZusammenfassungEin schnelles Auszählen von großen und verschiedenen Populationen von Mikroorganismen wird durch den Gebrauch eines Magnet-Tonbandgerätes ermöglicht. Das Diktieren der Namen von den im Blickfeld erscheinenden Arten erlaubt dem Beobachter, mit den Augen an den Okularen zu bleiben. Das Zurückspielen des Bandes und das Aufzeichnen der Auszählung beansprucht nur einen Bruchteil der Zeit, die für die ursprüngliche Registrierung benötigt wurde.SummaryLarge and diverse populations of microorganisms can be counted rapidly through the use of a tape recorder. The microphone being attached to the microscope permits the observer to keep his eyes at the oculars while he dictates the names of the species that pass through the field of view. A foot switch allows to stop the tape in case short manipulations have to be performed or when barren parts of the sample pass through view. The playback and tallying requires only a fraction of the time needed for the recording.