Franz Gingele

Clay mineral distribution in surface sediments of the South Atlantic: sources, transport, and relation to oceanography

Abstract Surface samples, mostly from abyssal sediments of the South Atlantic, from parts of the equatorial Atlantic, and of the Antarctic Ocean, were investigated for clay content and clay mineral composition. Maps of relative clay mineral content were compiled, which improve previous maps by showing more details, especially at high latitudes. Large-scale relations regarding the origin and transport paths of detrital clay are revealed. High smectite concentrations are observed in abyssal regions, primarily derived from southernmost South America and from minor sources in Southwest Africa. Near submarine volcanoes of the Antarctic Ocean (South Sandwich, Bouvet Island) smectite contents exhibit distinct maxima, which is ascribed to the weathering of altered basalts and volcanic glasses. The illite distribution can be subdivided into five major zones including two maxima revealing both South African and Antarctic sources. A particularly high amount of Mg- and Fe-rich illites are observed close to East Antarctica. They are derived from biotite-bearing crystalline rocks and transported to the west by the East Antarctic Coastal Current. Chiorite and well-crystallized dioctaedral illite are typical minerals enriched within the Subantarctic and Polarfrontal-Zone but of minor importance off East Antarctica. Kaolinite dominates the clay mineral assemblage at low latitudes, where the continental source rocks (West Africa, Brazil) are mainly affected by intensive chemical weathering. Surprisingly, a slight increase of kaolinite is observed in the Enderby Basin and near the Filchner-Ronne Ice shelf. The investigated area can be subdivided into ten, large-scale clay facies zones with characteristic possible source regions and transport paths. Clay mineral assemblages of the largest part of the South Atlantic, especially of the western basins are dominated by chlorite and illite derived from the Antarctic Peninsula and southernmost South America and supported by advection within the Circumantarctic Deep Water flow. In contrast, the East Antarctic provinces are relatively small. Assemblages of the eastern basins north of 30°S are strongly influenced by African sources, controlled by weathering regimes on land and by a complex interaction of wind, river and deep ocean transport. The strong gradient in clay mineral composition at the Brazilian slope indicate a relatively low contribution of tropically derived assemblages to the western basins.

Discrete barite particles and barium as tracers of paleoproductivity in South Atlantic sediments

Geochemical analyses as well as X ray diffraction measurements were carried out on five sediment cores from the eastern Angola Basin and the equatorial divergence of the South Atlantic. Barite concentrations were calculated from total barium concentrations by subtracting the estimated barium background supplied by “nonbarite” barium carriers. Barite concentrations assessed by this geochemical method show a good correspondence to barite concentrations determined by quantitative X ray diffraction measurements. Barite proved to be an important carrier of barium in the pelagic cores, contributing up to 90% of the total barium concentrations in the sediment, while clastic material provides an important source of barium at sites closer to the African continent. Barite accumulation rates were calculated in order to eliminate the diluting effects of varying inputs of terrigenous and biogenic material. Barite accumulation rates show cyclic variations with maxima corresponding to glacial and minima to interglacial stages. Absolute paleoproduction rates were computed from barite accumulation rates. At the Congo fan and the equatorial divergence they are consistent with calculations based on total organic carbon (TOC) accumulation. At the Walvis Ridge, glacial-interglacial cycles contrast to a constant paleoproductivity computed from TOC accumulation.

Simultaneous formation of iron-rich layers at different redox boundaries in sediments of the Amazon Deep-Sea Fan

High resolution examinations of the solid phase have been performed on a gravity core that shows the typical Amazon fan stratigraphy of the last 20,000 years. The sedimentary record is characterized by two pronounced enrichments of solid phase Fe. The first Fe peak, from 0.34 to 0.37 m sediment depth, consists of an indurated Fe oxyhydroxide crust, located close to the Pleistocene/Holocene boundary. This peak represents a widespread, nonsteady-state, diagenetic feature on the fan. The second Fe peak is a pronounced enrichment of authigenic Fe sulfides, located between 6.15 and 6.45 m in the sediment. The formation of this maximum is attributed to sulfate reduction that is mainly driven by anaerobic methane oxidation. Based on SO42− pore water profiles and calculations of the time of formation of the Fe sulfide enrichment, we conclude that both Fe-rich layers formed more or less simultaneously. The condition that caused both Fe enrichments is a period of nonsteady-state diagenesis, induced by a pronounced decrease in sedimentation and organic carbon accumulation rates between the Pleistocene and the Holocene. During this depositional change, the redox boundaries and reaction fronts were fixed at particular sediment levels for a prolonged time, thus producing higher concentrations of the authigenic minerals than possible under steady-state diagenetic conditions.

Holocene climatic optimum in Southwest Africa—evidence from the marine clay mineral record

Abstract Sediment composition, grain size and clay mineral record of a high-resolution sediment core from the continental slope off Namibia was investigated to gain information on the deposition of terrigenous matter in this part of the Southwest African continental margin during the last 18 k.y. The depositional processes involved are fluvial input by the Kunene River and eolian input from the Namib and Kalahari deserts, each supplying characteristic mineral suites. During low sea level, erosion of the exposed shelf yields additional material. The amount of eolian or fluvial matter depends on the strength of the transport process, which is a function of aridity or humidity of the source area, thus allowing paleoclimatic interpretations. Arid conditions prevailed during a low sea level from 18 to 15 ka and unconsolidated shelf sediment was mobilized and supplied to the slope by short-distance transport by southerly winds. A dramatic increase in the accumulation of terrigenous sediment is recorded from 15 to 10 ka without major changes in sediment composition, which is attributed to increased runoff of the Kunene River and fluvial erosion of shelf sediments. This period coincides with a strengthening of the monsoonal system during a precessional minimum, which is observed in numerous sites in Central Africa and indicates an intensified influence of the monsoon at the Kunene headwaters. A distinct shift in clay mineralogy towards river-derived material marks a second period of increased river runoff—during high sea level—from 9 to 5 ka, indicating maximum humidity in the source area from 6 to 5 ka. This corresponds to the Holocene climatic optimum observed in the arid belts of Northern Africa. The present balance between fluvial and eolian input was reached approximately at 4 ka.

Palaeoproductivity at the Antarctic continental margin: opal and barium records for the last 400 ka

Abstract Records of biogenic opal and barium were measured in sediment cores at the Antarctic continental margin in the area of the Weddell, Lazarev and Cosmonaut seas. These records provide a qualitative and quantitative tool to estimate changes in palaeoproductivity over the last 400 ka. The stratigraphy of the investigated cores is calibrated to a lithostratigraphy, adjusted to a stable isotope record from the eastern Weddell Sea, which is supported by a Th-dating method. We present evidence that interglacial productivity along the Antarctic continental margin is twice as high compared to subantarctic sites near South Orkney. A glacial/interglacial pattern with high productivity during peak warm stages can be observed back to 400 ka. High interglacial productivity is linked to a reduced sea-ice coverage, which is regulated by the heat flux introduced by North Atlantic Deep Water (NADW) to the Antarctic Ocean. Generally, good correlations between the barium and opal records of the sediment cores indicate that dissolution of opal in the water column and the sediment does not obscure the surface productivity signal. Therefore, in this area biogenic opal in combination with other proxies, can be used for palaeoproductivity estimates. Palaeoproductivity is also assessed quantitatively from the barium record using the approaches of Dymond et al. (Dymond, J., Suess, E., Lyle, M., 1992. Barium in deep-sea sediments: a geochemical proxy for paleoproductivity. Paleoceanography 7, 163–181) and Francois et al. (Francois, R., Honjo, S., Manganini, S.J., Ravizza, G.E., 1995. Biogenic barium fluxes to the deep sea: implications for paleoproductivity reconstructions. Glob. Biochem. Cycles 9(2), 289–303). Palaeoproductivity rates obtained by both methods show a good temporal correspondence. In peak warm stages, higher values are computed with the approach of Dymond et al. (1992). Though some barium may be provided by lateral advection of material, as indicated by Th data near South Orkney, both methods provide values, which are representative of a high-productivity area. They are drastically reduced during glacial times. The extent and duration of sea-ice coverage and the persistence of coastal polynyas is considered to be of primary importance in controlling the flux of biogenic material to sediments of the Antarctic continental margin.

Clay Mineral Fluctuations in Late Quaternary Sediments of the Southeastern South Atlantic: Implications for Past Changes of Deep Water Advection

Downcore clay mineral fluctuations in Late Quaternary sediment cores from the southeastern South Atlantic and adjoining Southern Ocean are of low amplitude. North of the Antarctic Circumpolar Current/Weddell Gyre boundary, small-scale variations, particularly of clay mineral ratios, essentially monitor cyclic changes of deep water advection in response to climatic oscillations.

Biogenic Barium as a Proxy for Paleoproductivity: Methods and Limitations of Application

The obvious link between sedimentary barium and past and present ocean productivity has propagated numerous studies on this tracer in the past years. In this paper a summary is given on what is known of the generation of the barium signal and its link to the flux of organic carbon. We describe procedures to assess barium contents, distinguish biogenic from detrital barium and calculate barium accumulation rates. Two approaches to estimate absolute paleoproductivity rates from barium contents are described and compared. A considerable part of the paper is dedicated to synsedimentary and early diagenetic constraints involved in the application of the barium method. It is demonstrated that the most significant diagenetic overprinting of the barium signal occurs in sulfate depleted sediments by dissolution of barite. Examples from selected sites in the Atlantic Ocean show that barium may be a reliable proxy in this part of the world ocean to assess changes in ocean productivity, whereas other proxies are restricted to certain sedimentary environments. Recent studies suggest that for the oligotrophic areas of the Atlantic Ocean barium might be a more reliable tracer for past productivity than organic carbon. This is attributed to the action of oxidation fronts which resulted in a very efficient degradation of the organic matter initially present within the sediment. Nevertheless, quantitative paleoproductivity reconstructions are based on regional studies and future investigations must be carried out in a range of environments before a worldwide application can be obtained.

Clay mineral assemblages in the western Baltic Sea: recent distribution and relation to sedimentary units

Abstract Surface samples and nine cores from the western Baltic Sea and marginal water bodies were investigated for clay mineral composition. The clay mineral assemblages of recent sediments are rather homogeneous. Variations result mainly from the erosion of different glacial source deposits. High percentages of illite and low kaolinite/chlorite and quartz/feldspar ratios are characteristic for this glacial source. Advection of kaolinite-rich suspensions from the North Sea is believed to account for higher kaolinite/chlorite ratios in the Mecklenburg Bight. A contribution of the rivers Trave and Oder to the western Baltic Sea is indicated by increased smectite values in marginal water bodies. They correspond to increased kaolinite/chlorite and quartz/feldspar ratios. In the main basins the river signal is diluted beyond recognition. Cores from the Arkona, Bornholm and Gotland Basins penetrate through post-Littorina muds and sediments of the Ancylus Lake/Yoldia Sea into Late Glacial sediments of the Baltic Ice Lake. Clay mineral assemblages are characterized by an increase in kaolinite/chlorite ratios from Late Glacial to Holocene sediments, with a distinct shift at each facies change. This allows the distinction and core to core correlation of main lithological units with kaolinite/chlorite ratios. Kaolinite enrichment of Holocene muds corresponds to a brackish-marine facies and may reflect influx of kaolinite-rich suspensions from the North Sea. Cores from the lagoon of the Oderhaff show fluctuations in the contributions of the two main sediment sources: river suspension and glacial deposits during the Late Glacial and Postglacial sequence. Lacustrine sediments, which were deposited prior to 5500 years B.P. are characterized by smectite, kaolinite and quartz from the drainage area of the Oder river. Erosion of coastal and offshore glacial boulder clays with the Littorina transgression supplied a marine component rich in illite, chlorite and feldspars to the brackish muds of the Oderhaff.

Solid-phase manganese in Southeast Atlantic sediments: Implications for the paleoenvironment

Abstract In many deep-sea sediments Mn-rich layers are observed at various levels. In order to investigate their formation and preservation as well as their use to reconstruct past changes in the depositional environment downcore profiles of manganese and manganese carbonate from different early diagenetic environments in the Southeast Atlantic Ocean were examined by geochemical analysis, X-ray diffraction and SEM-analysis. The results were compared to pore water data and productivity regimes. In sediments underlying areas of high to moderate surface water fertility Mn is reduced and recycled to overlying waters or precipitated at the oxic/suboxic boundary within the sediment, forming subsurface Mn-spikes. Manganese carbonate is a characteristic feature of these early diagenetic Mn-enrichments. In the cores from the Equatorial Divergence where glacial-interglacial productivity changes have been recorded by geochemical and isotopical evidence manganese carbonate peaks occur at deglaciation phases (decreased supply of organic matter) though the total manganese contents are nearly constant indicating that manganese carbonate survives burying to anoxic sediments better than Mn(IV)-phases. In sediments of the central Walvis Ridge underlying nutrient-poor surface waters peaks of Mn(IV) and manganese carbonate occur in glacial sections thus giving evidence of higher reduction rates through increased supply of the reducing organic matter. The higher flux of organic matter is attributed to a higher surface water productivity during glacials on the central Walvis Ridge. Since the evaluation of paleoproductivity proxies in that area is ambigous, the interpretation of manganese and manganese carbonate profiles is an important qualitative tool supporting glacial-interglacial changes in paleoproductivity.

Anomalous South Atlantic lithologies confirm global scale of unusual mid-Pleistocene climate excursion

Abstract Carbonate-rich nannofossil oozes are the typical sediments deposited under the low-productivity regime of the subtropical gyre in the South Atlantic. Surface productivity has remained virtually constant during the late Pleistocene [Schmiedl and Mackensen, Palaeogeogr. Palaeoclimatol. Palaeoecol. 130 (1997) 43–80], increasing slightly only during the most extreme glacial periods. Recently, the puzzling occurrence of thick monospecific diatom layers ( Ethmodiscus rex ) has been reported within the carbonaceous sequences of two sediment cores in the South Atlantic Gyre [Schmieder et al., Earth Planet. Sci. Lett. 179 (2000) 539–549]. The layers, dated 544–534 ka, are isochronous and record a period of increased organic carbon flux. Above them, at 530–524 ka, unusually high carbonate contents are recorded. This double feature of anomalous lithologies is unique in the sediment record of the last 1500 kyr and can be related to the terminal event of the mid-Pleistocene climate transition (MPT). The MPT is characterized by reduced thermohaline circulation and stronger carbonate dissolution [Schmieder et al., Earth Planet. Sci. Lett. 179 (2000) 539–549]. Reduced circulation and a more stratified water column may have trapped nutrients in deep nitrate pools and may have fostered favorable conditions for the development of deep-dwelling populations of E. rex . The onset of more turbid conditions led to the rapid settling of diatom populations, subsequently introduced nutrients to the surface layer, and improved calcite preservation, thus creating a biogenic silica–carbonate double peak. The carbonate peak is synchronous with an unusual mid-Pleistocene sapropel observed in the Mediterranean, which is attributed to a ‘massive odd monsoon’ over Central Africa [Rossignol-Strick et al., Nature 392 (1998) 269–272]. Since summer insolation was relatively low during this time the formation of this thick sapropel cannot be explained in terms of current understanding of astronomical forcing [Rossignol-Strick et al., Nature 392 (1998) 269–272]. A non-linear response to a 400-kyr low-frequency climatic cycle as discussed for the Mediterranean sapropel is not confirmed by data from our South Atlantic core records.