Liselotte Diester-Haass

Eocene-Oligocene transition in the Southern Ocean: History of water mass circulation and biological productivity

High-resolution records of carbon and oxygen isotopes and benthic foraminiferal accumulation rates for the Eocene-Oligocene section at Ocean Drilling Program Site 689(Maud Rise, Weddell Sea; paleodepth about 1500 m) were used to infer variations in paleoproductivity in relation to changes in climate and ventilation of the deeper-water column. The benthic foraminiferal abundance and isotope records show short-term fluctuations at periodicities of 100 and 400 ka, implying orbitally driven climatic variations. Both records suggest that intermediate-depth water chemistry and primary productivity changed in response to climate. During the Eocene, productivity increased during cold periods and during cold-to-warm transitions, possibly as a result of increased upwelling of nutrient-rich waters. In the Oligocene, in contrast, productivity maxima occurred during intervals of low d18O values (presumably warmer periods), when a proto–polar front moved to the south of the location of Site 689. This profound transition in climate-productivity patterns occurred around 37 Ma, coeval with rapid changes toward increasing variability of the oxygen and carbon isotope and benthic abundance records and toward larger-amplitude d18O fluctuations. Therefore, we infer that, at this time, temperature fluctuations increased and a proto–polar front formed in conjunction with the first distinct pulsations in size of the Antarctic ice sheet. We speculate that this major change might have resulted from an initial opening of the Drake Passage at 37 Ma, at least for surface- and intermediate-water circulation.

Paleoproductivity: The benthic/planktonic ratio in foraminifera as a productivity index

Abstract When the effects of differential dissolution are excluded, the benthic to planktonic ratio in deep-sea foraminifera can be read as a productivity index. For pelagic conditions we use the BZ index, which is the product of the benthic/planktonic ratio in percent, and the depth of deposition (which controls supply of organic matter). For hemipelagic conditions, we use BZ × √D, which is the product of the BZ value and a factor for proximity to the shore (square root of distance from land), which controls coastal productivity. Anomalous values readily stand out from the normal populations which cluster around typical values of BZ or BZ × √D. Off northwest Africa, wherever calcite dissolution can be excluded as a factor because of presence of aragonite, these anomalies are due to lateral supply of redeposited organic matter.

Significance of high C/N ratios in organic-carbon-rich Neogene sediments under the Benguela Current upwelling system

Corganic/Ntotal values change from ∼5 in middle Miocene sediments deposited under the Benguela Current upwelling system to ∼15 in latest Miocene and Pliocene sediments. Although the change appears to record a larger proportion of land-derived material in younger sediments, bulk δ13Corg values indicate a predominance of marine origin for organic matter throughout the sequence. Marine paleoproductivity, as represented by mass accumulation rates of CaCO3 and organic carbon, increased over this time interval. We postulate that the larger Corganic/Ntotal values record a shift towards greater recycling rates of nitrogen-rich, relative to carbon-rich, organic matter components as the flux of total organic matter to the seafloor increased. Furthermore, we propose that high Corganic/Ntotal values may record enhanced paleoproductivity in other sedimentary sequences dominated by marine organic matter.

The late Miocene onset of high productivity in the Benguela Current upwelling system as part of a global pattern

Abstract We have examined the history of the elevated primary productivity associated with the Benguela Current upwelling system off southwest Africa using sediments from 7.5 to 4.8 Ma at Ocean Drilling Program Site 1085 in the middle Cape Basin. Sedimentation rates are low until 6.9 Ma. Low accumulation rates of benthic foraminifers and organic carbon indicate that biological productivity was also low. Paleoproductivity dramatically increased at 6.7–6.5 Ma and was highly variable until 4.8 Ma with productivity maxima during cooler periods. The presence of radiolarian opal only between 5.8 and 5.2 Ma suggests an interlude of silica-rich intermediate water in the Cape Basin. The onset of heightened productivity under the Benguela Current is mirrored by similar increases reported between 6.9 and 6.7 Ma in the tropical eastern Pacific, the western and northern Pacific, and the Indian Ocean. The similarity between the patterns at Site 1085 and in the Pacific and Indian Oceans suggests that the dramatic productivity increase off southwest Africa is part of a global response to paleoceanographic changes.

The Benguela Current and associated upwelling on the southwest African Margin: a synthesis of the Neogene-Quaternary sedimentary record at DSDP sites 362 and 532

Abstract Sediments cored at DSDP Sites 362 and 532 on the Walvis Ridge provide a Neogene-Quaternary history of the development of the upwelling system on the southwest African margin. Upwelling occurs principally on the landward side of the Benguela Current. The upwelling centres have shifted northward since the Middle Miocene as the current has intensified and has flowed farther to the north. Changes in productivity are recorded in the types, proportions and preservation of foraminifera, radiolaria, diatoms, organic matter, and clay minerals in the sedimentary record. Prior to the Late Miocene (10 Ma), the Benguela Current did not reach the Walvis Ridge, and enhanced productivity is not evident in the sediments at this location. Between 10 to 5.2 Ma, upwelling was recorded in the DSDP sites in glacial periods, indicating that the Benguela Current intensified during glacial periods and transported evidence of upwelling to Sites 362/532 from near-coastal areas. During interglacial periods the current was not as strong and did not reach the Walvis Ridge, turning instead to the west within the Cape Basin. Strengthening of the current continued such that by the Pliocene and Quarternary the upwelling signal is contained in interglacial sediments. Sediments deposited in these more recent glacial times contain a weak or absent upwelling signal because glacial intensification shifts the Benguela Current system northward to reach the Angola Basin before it turns westward away from its coast-parallel direction.

The Eocene-oligocene preglacial-glacial transition in the atlantic sector of the Southern Ocean (ODP site 690)

Abstract High resolution interdisciplinary analyses of the clay and coarse (>40 μm) fractions of ODP Leg 113 Site 690 Eocene-Oligocene sediments on the flank of the Maud Rise, provide information on paleoproductivity, water masses, paleoclimate and erosion in the Antarctic range as well as on the cyclicity of these processes. Three time intervals are distinguished: 1. (1) The middle Eocene, characterized by (a) nearly pure smectites, (b) productivity varying between relatively high values in the 42–44 Ma interval to very low values after 42 Ma, and (c) cyclic variations which correspond to changes in clay mineral associations. A warm saline deep water mass is inferred to have protected carbonate shells against dissolution at Site 690. We interpret the abundant mica in the coarse fraction as distributed by intermediate currents. 2. (2) The latest middle Eocene-late Eocene, characterized by low productivity values increasing with time. Kaolinite and illite concentrations also increase. Since the middle/late Eocene boundary mixing in the water column, which starts during a cooling event, causes similar clay mineral assemblages at Site 690 and at Site 689 on top of Maud Rise. 3. (3) The Oligocene, separated from the Eocene by a hiatus, is characterized by high productivity, highly increased amounts of illite and other minerals originating from erosion and physical weathering of Antarctica, and by cyclic variations of clay mineral indices which appear to be synchronous with productivity variations with a 400–450 kyr cyclicity. Six periods of strong carbonate dissolution associated with low productivity levels are attributed to incursions of cold carbonate aggressive bottom water of a “Proto-AABW” type.

Neogene coastal upwelling history off northwest and southwest Africa

Abstract In Neogene sediments from DSDP Legs 14, 40 and 41 indications of the influence of coastal upwelling on sediment composition have been found in the Upper Miocene (SW Africa) and in the Lower and Middle Miocene (NW Africa). Parameters pointing to upwelling influence include well-preserved diatom and radiolarian assemblages, low values for the ratio of planktic to benthic foraminifers, increased amounts of fish debris and the presence of phosphorite grains. First indications of strong coastal upwelling at Site 369A, today situated at the northern boundary of the trade-wind belt, are probably associated with the Early Miocene location of that site within the trade-wind belt.

Late Eocene-Oligocene paleoceanography in the southern Indian Ocean (ODP Site 744)

The late Eocene through Oligocene changes in the paleoceanography of the southernmost Indian Ocean have been reconstructed by means of a coarse fraction analysis of closely spaced samples (20 cm = about 20 kyr) from ODP Site 744, Kerguelen Plateau. Surface water productivity, reconstructed from accumulation rates of opal skeletons and benthic foraminifers is low in the early late Eocene, increases at about 36 Ma and shows a sharp increase to maximum values in the earliest Oligocene. In the early late Oligocene it decreases gradually to a minimum and increases again to a maximum in the latest Oligocene. Beside this general trend productivity varies in short-term cycles of a duration of about 400 kyr (340 kyr in the latest Oligocene) with maxima in productivity in warmer Oligocene periods. These productivity variations are reflected by strong variations in carbonate dissolution. Changes in bottom water mass chemistry have been deduced from the degree of carbonate dissolution and its relation to productivity proxies. Following Kennett and Stott (1990), it is suggested that a proto-antarctic bottom water (proto-AABW) and an overlying warm, saline deep water (WSDW) from low latitudes shifted vertically and latitudinally with time and exerted their influence on the sediments of Site 744. Proto-AABW is detected by means of strong carbonate dissolution when productivity is low. WSDW is detected by means of an excellent carbonate preservation despite high productivity. Terrigenous material occurs as very coarse ice-rafted detritus (IRD) in the late Eocene (167–168.5 mbsf) and after a main shift in oxygen isotopes in the early Oligocene. Very low amounts of 40–125 μm sized mica and very few quartz grains occur only in high productivity periods and at the transition from low to high productivity periods. These occurrences are attributed to wind/current supply during warmer Oligocene intervals.

Quaternary accumulation rates of biogenous and terrigenous components on the east Atlantic continental slope off NW Africa

Abstract Accumulation rates of total sediment, CaCO 3 , non-carbonaceous material, planktonic foraminifers, radiolarians, total benthos and five different benthonic components have been calculated for Holocene sections of fifteen cores from the east Atlantic continental slope off NW Africa. Wurm and Eem accumulation rates were obtained from five cores. Accumulation rates of all components decrease with increasing water depth in a profile off the Sahara. Off Senegal, this is only true for benthonic foraminifers. The upper continental slope off the Sahara has higher accumulation rates of carbonate and of all biogenic particles than that off Senegal, probably due to stronger upwelling influence. Accumulation rates of total sediment and of insoluble material, however, are higher off Senegal because of supply of terrigenous mud from the Senegal river. During the Wurm, accumulation rates of terrigenous material north of 20° N increased by a factor of 2–4, due to a change from arid to humid climate. Accumulation rates of CaCO 3 and planktonic foraminifers increased only slightly. An important increase was found for radiolarians and benthonic organisms in these cores, probably due to strongly increased upwelling influence during the Wurm. Eem accumulation rates are similar to or slightly higher than those of the Holocene.

Paleoceanographic and paleoclimatic evolution in the Weddell Sea (Antarctica) during the middle Eocene-late Oligocene, from a coarse sediment fraction and clay mineral data (ODP Site 689)

Abstract Paleoceanographic and paleoclimatic variations from the middle Eocene to the late Oligocene in the Antarctic area, Atlantic sector, have been deduced from a coarse fraction and clay mineral analysis of ODP Site 689 (Leg 113) sediments. In the middle Eocene, smectite-rich sediments were a consequence of warm climate with alternating wet and dry seasons on the Antarctic margins. Productivity in the Maud Rise area of the Southern Ocean was low: no opal was deposited, preservation of carbonate is very good. Carbonate preservation shows cyclic changes. Clinoptilolite is a common mineral. At the middle/late Eocene boundary (40.8 Ma), productivity suddenly increased enough to allow preservation of opal skeletons. Clinoptilolite disappeared where opal appeared. Illite appearance points to cooling of Antarctic climates. The presence of detrital kaolinite indicates a change in oceanic circulation. In the late Eocene, cyclic changes in productivity (cycle length 0.42 Ma) were associated with clay mineral compositional variations: sediments from colder, highly productive periods generally contain more illite and less smectite than warmer ones. The early Oligocene was a period of very high productivity. Highest opal accumulation rates correspond to strongest carbonate dissolution. Cyclic changes in productivity—with a mean cycle duration of 0.4–0.5 Myr—were associated with cyclic changes in clay mineral composition. Highest productivity intervals are generally those with highest smectite content, because of strong erosion of sediments of the Antarctic margins. Sediments from less productive warmer periods most of the time contain more illite. In the late Oligocene, the intensity of physical weathering on Antarctica was stronger, as documented by increasing fluxes of illite, muscovite, amphiboles and talc. Ice expanded on the East-Antarctic continent. Lower productivity than in the early Oligocene is indicated by lower accumulation rates of opal and weaker carbonate dissolution. Cyclic variations in productivity correlate to cyclic variations in illite and smectite contents as in the early Oligocene.