Paleoproductivity Changes Research Articles

Geochemistry of a piston core from Ontong Java Plateau (western equatorial Pacific): evidence for sediment redistribution and changes in paleoproductivity

Geochemistry of a piston core from Ontong Java Plateau (western equatorial Pacific): evidence for sediment redistribution and changes in paleoproductivity

Geochemistry of a piston core from Ontong Java Plateau (western equatorial Pacific): evidence for sediment redistribution and changes in paleoproductivity

We discuss geochemical proxies, reflecting processes of primary productivity, CaCO3 dissolution, and sediment redistribution in a piston core (RNDB 74P) from the Ontong Java Plateau. Due to the shallow water depth, biogenic carbonate is well preserved and a very goodδ18O stratigraphy is available down to isotopic stage 11.230Thex gives evidence that the sediment accumulation pattern is driven mainly by processes of sediment focusing or winnowing. Due to the constant production of230Th in the water column, the bulk sediment accumulation rates could be corrected for the particle rain deriving from the water column above. The230Thex0/CaCO3 ratio reflects the well-known Pacific CaCO3 preservation pattern with ice growth dissolution spikes and deglacial preservation spikes. The record of the grain size fraction >63 µm supports these results. The downcore concentrations and accumulation rates of barium (Ba) are on a higher level during interglacials and show several peaks. Normalization of Ba with230Thex0 delivers a more uniform level of the Ba accumulation rates throughout the core. This pattern suggests a constantly higher biological productivity (nearly tenfold) in this area throughout the past 200 kyr compared with an open ocean environment. Barium peaks observed at the climatic transitions 2/1 and 6/5 and in stage 5 are in contrast to a predicted reduction of interglacial productivity at this location. A possible explanation might be the onset of the modern circulation pattern. The transition from Ba-enriched deep water to lower contents in the Atlantic might have resulted in an enhanced deposition of Ba in the Pacific.

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

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.

The formation of Pliocene sapropels and carbonate cycles in the Mediterranean: Diagenesis, dilution, and productivity

High‐resolution micropaleontological (planktonic foraminifera and calcareous nannofossils) and geochemical (stable isotopes, organic carbon, Fe, P, S, Ca, Ba, Mn, and Al) records are presented for the first sapropel‐containing carbonate cycle in the Pliocene of Sicily. The carbonate cycle is characterized by a gray to white to beige to white color layering typical of the marls of the Trubi formation. A faintly laminated sapropel is intercalated in the gray‐colored bed of the carbonate cycle. CaCO3 content varies from 40% in the beige to 45‐50% in the white layers. Lowest CaCO3 content of 25–30% is found in the gray layer and sapropel. Variations in carbonate and organic matter percentages can best be explained by changes in paleoproductivity rather than by variations in dilution and dissolution. Total productivity was highest during deposition of the gray layer and sapropel, as indicated by high organic carbon and Ba contents and high abundance of Globorotalia puncticulata. Carbonate production reached its highest values, however, during deposition of the white layers, as evidenced by enhanced abundances of planktonic foraminifera and nannofossils. The low carbonate content in the gray layer and sapropel is explained in terms of a collapse in carbonate production caused by extreme changes in the physical and biochemical properties of the water column, which in turn resulted in siliceous plankton and opportunistic foraminifers such as Globorotalia puncticulata outcompeting most calcareous organisms. The beige layer represents a low‐productivity environment similar to the present‐day eastern Mediterranean basin. Several mechanisms have previously been proposed to explain variations in productivity in the eastern Mediterranean. Both sapropels and gray layers were deposited at times when perihelion occurred in northern hemisphere summer. We envisage that the increase in seasonal contrast resulting from this orbital configuration enhanced winter mixing and stabilization of the water column during summer, both leading to favorable conditions for intensification of the spring bloom. In addition, a decrease in excess evaporation, as can be deduced from the δ18O record, led to shoaling of the pycnocline and reduced circulation, thus enhancing the availability of nutrients in the photic zone. Finally, enhanced precipitation and associated runoff should have caused an increase in river‐borne nutrients.

Late Quaternary paleoproductivity changes off the Congo deduced from stable carbon isotopes of planktonic foraminifera

A detailed record (≈2-kyr intervals) of the difference in stable carbon isotopes ( Δδ 13C) between Globigerina bulloides and Globigerinoides ruber (pink) is used to reconstruct changes in upwelling intensity off the Congo River for the last 190,000 yr. Comparisons of the oxygen and carbon isotope data from this core with records from the Niger Fan and from pelagic cores in the eastern equatorial South Atlantic indicate that the Congo Fan isotope records do not contain a strong freshwater signal as is described off other major rivers. The temporal pattern of the Δδ 13C signal correlates with the marine organic carbon record from the Congo Fan. Thus the planktonic Δδ 13C record, reflecting past changes in upwelling intensity and nutrient content, corroborates the signal provided by sedimentary organic carbon, which is presumed to indicate changes in the amount of biological productivity and export flux to the seafloor. The planktonic Δδ 13C signal is characterized by a dominant 23-kyr periodicity which provides evidence for a strong response of upwelling fluctuations off the Congo to precessional forcing. Minima in the Δδ 13C record are aligned with periods of minimum boreal summer insolation over Central Africa reflecting an increase of upwelling and biological productivity off the Congo at periods of enhanced zonal intensity of southeast trades and corresponding weak southeast monsoon over the eastern South Atlantic. A strong response to changes in river discharge probably did not occur, indicating that fertilization by river-derived nutrients has played only a minor role with respect to Late Quaternary changes in the total amount of primary productivity off the Congo.

Beryllium 10, thorium 230, and protactinium 231 in Galapagos microplate sediments: Implications of hydrothermal activity and paleoproductivity changes during the last 100,000 years

Biogenic particle fluxes from highly productive surface waters, boundary scavenging, and hydrothermal activity are the main factors influencing the deposition of radionuclides in the area of the Galapagos microplate, eastern Equatorial Pacific. In order to evaluate the importance of these three processes throughout the last 100 kyr, concentrations of the radionuclides 10Be, 230Th, and 231Pa, and of Mn and Fe were measured at high resolution in sediment samples from two gravity cores KLH 068 and KLH 093. High biological productivity in the surface waters overlying the investigated area has led to 10Be and 231Pa fluxes exceeding production during at least the last 30 kyr and probably the last 100 kyr. However, during periods of high productivity at the up welling centers off Peru and extension of the equatorial high‐productivity zone, a relative loss of 10Be and 231Pa may have occurred in these sediment cores because of boundary scavenging. The effects of hydrothermal activity were investigated by comparing the 230Thex concentrations to the Mn/Fe ratios and by comparing the fluxes of 230Th and 10Be which exceed production. The results suggest an enhanced hydrothermal influence during isotope stages 4 and 5 and to a lesser extent during isotope stage 1 in core KLH 093. During isotope stages 2 and 3, the hydrothermal supply of Mn was deposited elsewhere, probably because of changes in current regime or deep water oxygenation. A strong increase of the Mn/Fe ratio at the beginning of climatic stage 1 which is not accompanied by an increase of the 230Thex concentration is interpreted to be an effect of Mn remobilization and reprecipitation in the sediment.

Response of South Atlantic Surface Circulation to African Monsoon Variability: Evidence from Late Quaternary Records of Geochemical Tracers for Sea-Surface Temperatures and Productivity

Paleoclimate reconstructions and results from atmospheric general circulation models have revealed that during the Late Quaternary the zonal strength of trade winds over the South Atlantic has significantly responded to precessional variations of boreal summer insolation because zonality of trades is tightly coupled to African Monsoon intensity. The most prominent indicator for this trade-wind response to precessional forcing via the monsoon system is the corresponding predominance of periodic 19 to 23kyr variations between paleorecords of proxies for continental climate and those for sea-surface temperatures (SST) and biological productivity in equatorial Atlantic surface waters (Pokras & Mix, 1985; Prell & Kutzbach, 1987; Mclntyre et al. 1989; De Menocal et al. 1993). In order to reconstruct Late Quaternary surface circulation in the eastern South Atlantic (10~ to 10~ 0 to 20~ we evaluated SST and paleoproductivity changes for the last 200,000 years. For this purpose records of alkenone, organic carbon and opal fluxes, as well as time series of planktonic foraminifera and organic carbon 613C were produced from Guinea and Angola Basin and Walvis Ridge sediments (Schneider 1991; Miiller et aL, 1994; Schneider et al., 1993; Schneider et al., 1994). Comparable to the results from the equatorial Atlantic mentioned before, all these records of geochemical tracers for productivity and SST are characterized by a significant 23-kyr periodicity which is coherent to precessional forcing. But phase relationships of periodic 23 kyr variations in SST and productivity estimates from the eastern South Atlantic with respect to boreal summer insolation changes document that surface waters north and south of about 15~ have responded in different way to precessional insolation changes. Generally, periods of warm SST's and low productivity cooccured with intervals of increased North African insolation, but there is a distinct phase lag of maximum SST and minimum productivity between the two areas. While periodic SST and paleoproductivity changes related to the 23 kyr frequency band from the central equatorial divergence zone and from the Walvis Ridge are in phase with changes in July insolation at 10~ those from the African continental margin in the Angola Basin lag insolation changes by about 4 kyr and are in fairly phase with SST changes in the Cape Basin. Since the equatorial upwelling and the Walvis Ridge area are located beneath the centre of the modern zonal trade-wind field, we suggest from the phase pattern described above that surface water oceanography under the field of northwestdirected trades was predominantly driven by monsoonal modulation of their zonal intensity. Low productivity and warm surface waters within this zone are in phase with July insolation maxima which indicates direct reponse to orbital forcing and maximum monsoon strength at periods of minimum Earth-Sun distance during boreal summer. The 4 kyr lag of precessional SST and paleoproductivity changes in the eastern Angola Basin suggests that this part of the surface waters in the South Atlantic was not directly effected by trade-wind variations and that the zonal-directed trade-wind zone in the southern hemisphere did not migrate northward by several degrees of latidude during glacials. Instead, the close correspondence of Angola Basin SST and productivity phasing with that from the Cape Basin SST's can be taken as evidence that surface water oceanography in the most eastern part of the South Atlantic was additionally influenced by periodic 23 kyr changes in the advection of cold surface waters from the Southern Ocean.

Late Quaternary history of the Agulhas Current

The Agulhas Current, located in the South West Indian Ocean, is one of the world's major western‐boundary currents. Vertical and lateral shifts in the Current's movements, especially during glacial intervals, would effect inter‐ and intra‐ocean heat transfers and thus affect the world's climate, particularly off southern Africa. Oxygen isotope ratios of two cores recovered from beneath the Agulhas Current record isotopic shifts probably induced by ice volume effects with little additional shift related to temperature changes, suggesting that temperature and mean position of the flow has been stable for at least 150,000 years. Changes in the percent abundances of coccolithophore species from the same cores suggest changes in paleoproductivity due to eddy formation, lowering of sea level or nutrient content from land sources. Absence of any cold water microfossils at our southern core location suggests that during the Last Glacial, the Agulhas Current probably transferred heat from low to high latitudes and probably reached as far south as 38°S before retroflecting close to its modern‐day position.

Mineralogy of sedimentary formations in the western North Atlantic Ocean: Preliminary results

Six sedimentary formations, and two separate members recently were described byJansa, et al. (1979) from the western North Atlantic Ocean basin. We have investigated the mineralogical composition of these lithostratigraphic units by qualitative X-ray diffraction analyses of about five hundred samples from DSDP Sites 105, 106, 386, 387, and 391. The sedimentary section studied, from bottom to top, consists of: argillaceous limestones (Cat Gap Formation); limestones (Blake-Bahama Formation); claystones and shales (Hatteras Formation); zeolitic claystones (Plantagenet Formation); nannofossil marls (Crescent Peaks Member); siliceous oozes, clays, and cherts (Bermuda Rise Formation), and hemipelagic muds (Blake Ridge Formation) that locally contain redeposited shallow-water carbonates (Great Abaco Member). The section ranges in age from Oxfordian at the base to Quaternary at the sea-floor. Most of the formations and members described byJansa, et al. (1979) have a characteristic mineralogical composition. Thus it is possible to recognize boundaries between formations and members by changes in mineral components, although these changes range from sharp to transitional. Within the same formation local differences in mineral spectra between sites can be explained by changing distance from terrigenous sources, lateral changes in surface paleoproductivity, and varying diagenetic conditions.