Torsten Bickert

The δ13C in benthic foraminiferal tests of Fontbotia wuellerstorfi (Schwager) Relative to the δ13C of dissolved inorganic carbon in Southern Ocean Deep Water: Implications for glacial ocean circulation models

On a transect between 20° and 70°S in the eastern Atlantic Ocean and Weddell Sea, water samples from 19 hydrographic stations and bottom water from 55 surface sediment samples taken with a multiple corer were investigated for the stable carbon isotopic composition of the total dissolved inorganic carbon (δ13CΣCO2). These measurements were compared to δ13C values determined on live specimens of the benthic foraminifer Fontbotia wuellerstorfi and closely related genera from the same stations. In addition, at 16 stations the stable carbon isotope composition of sedimentary organic carbon was measured. General deepwater and bottom-water mass circulation patterns as inferred from the δ13CΣCO2 are in close agreement with those known from other nonconservative tracers. Very low δ13C values of upper Circumpolar Deep Water (<0.3‰ Pee Dee belemnite (PDB)) in the Polar Front region and the eastern limb of the Weddell gyre coincide with nutrient maxima. However, a significant decoupling of the dissolved phosphate signal from the δ13CΣCO2 signal is indicated in the abyssal Weddell Sea. We attribute this to temperature-dependent fractionation processes during gas exchange of surface waters with the atmosphere at sites of bottom-water formation. Multiple corer water from the sediment/water interface is slightly δ13C depleted relative to deepwater and bottom-water δ13ΣCO2. The surface sediment organic carbon δ13C is 3 to 4‰ lower south of the Polar Front than north of it, and the δ13Corg in freshly accumulated phytodetritus is 3 to 4‰ lower than surface sediment organic carbon δ13C. Comparison of live F. wuellerstorfi δ13C and related genera with bottom-water δ13CΣCO2 exhibits at most stations between the Subtropical Front (≈41°S) and the southern boundary of the Antarctic Circumpolar Current (≈55°S) a significant lowering of foraminiferal δ13C values. Compilation of a mean last glacial/interglacial δ13C amplitude (Δδ13C) from six published southern ocean cores results in a shift of −0.99± 0.13‰ PDB; this shift is greater than that in all other regions. However, all of these cores are from positions close to Recent oceanic fronts. Thus, for these peripheral areas of the southern ocean, we suggest about half of the glacial/interglacial shift can be explained by varying frontal zone positions and widths accompanied by a change in mode and height of export production.

Influence of the water content on X‐ray fluorescence core‐scanning measurements in soft marine sediments

The X-ray fluorescence (XRF) core scanner provides bulk-sediment chemistry data measured nondestructively at the split core sediment surface. Although this method is widely accepted, there is little known about the effects of physical properties such as density and water content on XRF core scanner data. Comparison of XRF scanner measurements from the sediment surface and dry powder samples of sediment core GeoB7920 indicates strongly reduced element intensities for the lighter elements Al and Si. We relate the lower element intensities of the measurements taken at the sediment surface to the amount of water in the sample volume analyzed by the XRF core scanner. The heavier elements K, Ca, Ti, and Fe remain relatively unaffected by the variation of any physical property within sediment core GeoB7920. Additionally, we successfully use the elemental intensity of Cl as a proxy for the seawater content in the sample volume analyzed by the XRF core scanner. This enables the establishment of a correction function for the elements Al and Si that corrects for the radiation absorption of the water content in sediment core GeoB7920 off Cape Blanc, NW Africa.

The Ireviken Event in the lower Silurian of Gotland, Sweden – relation to similar Palaeozoic and Proterozoic events

Abstract For a long time, the Silurian was thought to represent a time of stable environmental conditions in the greenhouse period that followed the Late Ordovician glaciation. During the past decade, knowledge about the Silurian has increased markedly and today it is known that the conditions in the Silurian were much more variable than previously assumed. Detailed isotopic investigations have revealed several distinct positive excursions in both carbon and oxygen isotope values. In low latitudes, these periods of high C- and O-isotope values are in many cases characterised by the growth of reefs and the formation of extended carbonate platforms. The sediments deposited during these excursions contain impoverished fossil assemblages, especially with respect to conodonts, graptolites, and trilobites. A conspicuous isotope excursion coincident with facies changes and a marked mass extinction is observed near the Llandovery/Wenlock boundary. This event is called the ‘Ireviken Event’ after its type locality on the island of Gotland, Sweden (Jeppsson, L., 1987. In: Palaeobiology of Conodonts. Ellis Horwood Ltd., Chichester, pp. 129–145). Here, isotope data from nine sections at the NW coast of Gotland are presented that cover the time interval of the Ireviken Event. The δ13C mean values rise from +1.4‰ to +4.5‰, and the δ18O values increase from −5.6‰ to −5.0‰. The relative timing of stable isotope development, extinctions, and facies development is discussed. It is shown that first extinctions precede the isotope excursion. This indicates that extinction events and stable isotope development are only indirectly connected but might reflect the same causes. Other events characterised by similar relationships between positive isotope excursions, mass extinctions, and facies development are found in younger parts of the Silurian (late Wenlock and Ludlow), in the late Ordovician, the late Cambrian, and, with some reservations, in the Proterozoic. The similarities between these events indicate analogous controlling mechanisms. For the Silurian, climatic changes between humid and arid conditions in low latitudes were postulated in an earlier study of the authors. A palaeoceanographic/climatic model was postulated which is consistent with most of the sedimentological, palaeontological, and geochemical data. Here we demonstrate that this model may be applicable also for the older events.

Late Quaternary Deep Water Circulation in the South Atlantic: Reconstruction from Carbonate Dissolution and Benthic Stable Isotopes

Carbonate dissolution data (sand contents) and δ13C records of the epibenthic foraminifer Cibicides wuellerstorfi from 12 gravity cores are used to reconstruct the history of deep water circulation in the South Atlantic for the last 360,000 years. The cores were selected from depth-sections in four basins (Brasil-, Guinea-, Angola- and Cape Basins) in water depths between 2900 m and 4600 m. The depth-transect approach allows removal of mean global shifts as well as local productivity effects from the paleo-property records and extraction of variations which are due to changes in deep water chemistry and/or circulation in the South Atlantic. As a result of the reduction of NADW during the last glacial maximum the Southern Component Water was higher in the water column and extended farther north than it does today. This glacial water mass can be divided into an upper part (USCW) with δ13C values between 0.2%o and 0.7%o and a lower part (LSCW) characterized by values of -0.2%o to 0.2%o. The boundary, marked also by the calcite lysocline, was at 3800 m water depth near the equator and rose slightly toward the Southern Ocean. The asymmetry observed in bottom water circulation today (LCDW in western basins and in the Cape Basin, NADW in eastern basins below 4000 m) was not present. From comparison to a deep western Pacific core (ODP 806B; Bickert et al. 1993) there is evidence that the nutrient-enriched but oxygen-depleted LSCW resembles the glacial Pacific Deep Water. This is also true for the older glacial stages 4, 6, 8 and 10.

Quaternary time scale for the Ontong Java Plateau: Milankovitch template for Ocean Drilling Program Site 806

A 2 m.y. oxygen isotope record of Globigerinoides sacculifer from the Ontong Java Plateau, based on cores from Ocean Drilling Program Leg 130, is dated by matching variations to an orbital template. The procedure allows us to present the most complete Quaternary record available for the western equatorial Pacific. The template-generating algorithm describes a balance between growth and melting of ice. Following basic Milankovitch theory, ice growth is taken as constant, while melting is taken to depend on summer insolation, current ice mass, and average past ice mass. Template settings must be changed once, between 1 and 1.2 Ma, to reflect a major shift in climate. Template fits are strikingly good over much of the record and can be used to detect and fill gaps from core breaks and other disturbances. One result of template dating is an exact age for the Brunhes-Matuyama magnetic reversal boundary, at 790 ±5 ka, as well as several other precise dates (900 ka for the middle Pleistocene climate shift; 1070, 1240, and 1450 ka for isotope stages 31, 37, and 47, respectively). Sedi- mentation rates fluctuate between 18 and 28 m/m.y., a ca. 400 ka cycle being the most prominent. Major anomalies arise within the transitional regime (1.2 to 1 Ma). The origin of the cycles is unknown; we propose productivity variations in the western equatorial Pacific.

Paleoenvironmental changes in the Silurian indicated by stable isotopes in brachiopod shells from Gotland, Sweden

Ratios of stable carbon and oxygen isotopes in brachiopod shells (more than 370 specimens, esp. Atrypa reticularis) from the Silurian of Gotland, Sweden, have been analysed. Preservation of biological skeletal ultrastructures, observed in SEM-micrographs, and cathodoluminescence analyses indicate that usually no diagenetical alteration occurs. The Silurian of Gotland consists of 440 m carbonate deposits, spanning the late Llandovery to late Ludlow epochs (431-411 m.y.). Repeatedly, uniform sequences of micritic limestones and marks are interrupted by complex-structured reefs and adjacent platform sediments. Previously, the alteration of facies is interpreted as the result of sea level fluctuations caused by a gradual regression with superimposed minor transgressive pulses. The Silurian sequence of Gotland exhibits principally parallel carbon and oxygen isotope records corresponding closely to the topostratigraphic units. Lower values occur in periods dominated by deposition of marly sequences. Higher values are observed in periods dominated by reefs and extended carbonate platforms. The isotope ratios are influenced by local as well as global factors. The oxygen isotope ratios are interpreted to reflect paleosalinity changes due to varying freshwater input, rather than changes in paleotemperature. Consequently, the facies distribution of Gotland is interpreted as resulting from changes in terrigenous input caused by different rates of continental weathering and freshwater runoff rather than by sea level fluctuations. Periods of arid climate and, therefore, anti-estuarine downwelling of oxygenated surface water appear as short episodes of reef growing (≤1.5 m.y.) in an epoche characterized by a tropic humid climate, which causes an estuarine circulation and the upwelling of CO2-rich deep water. Carbon isotope ratios are obviously connected to these changes in circulation by the advection of 13C-rich surface water (arid episodes) or upwelling of 13C-depleted deep water (humid episodes) of a Silurian ocean which itself reveals generally euxinic deep water conditions due to the burial of organic carbon in black shales.

Rapid palaeoceanographic changes in the Benguela Upwelling System for the last 160,000 years as indicated by abundances of planktonic foraminifera

Abstract Two sediment cores retrieved from the continental slope in the Benguela Upwelling System, GeoB 1706 (19°33.7′S 11°10.5′E) and GeoB 1711 (23°18.9′S, 12°22.6′E), reveal striking variations in planktonic foraminiferal abundances during the last 160,000 years. These fluctuations are investigated to assess changes in the intensity and position of the upwelling centres off Namibia. Four species make up over 95% of the variation within the core, and enable the record to be divided into episodes characterized by particular planktonic foraminiferal assemblages. The fossil assemblages have meaningful ecological significance when compared to those of the modern day and the relationship to their environment. The cold-water planktonic foraminifer, Neogloboquadrina pachyderma sinistral [N. pachyderma (s)], dominates the modern-day, coastal upwelling centres, and Neogloboquadrina pachyderma dextral and Globigerina bulloides characterize the fringes of the upwelling cells. Globorotalia inflata is representative of the offshore boundary between newly upwelled waters and the transitional, reduced nutrient levels of the subtropical waters. In the fossil record, episodes of high N. pachyderma (s) abundances are interpreted as evidence of increased upwelling intensity, and the associated increase in nutrients. The N. pachyderma (s) record suggests temporal shifts in the intensity of upwelling, and corresponding trophic domains, that do not follow the typical glacial-interglacial pattern. Periods of high N. pachyderma (s) abundance describe rapid, discrete events dominating isotope stages 3 and 2. The timing of these events correlates to the temporal shifts of the Angola-Benguela Front (Jansen et al., 1997) situated to the north of the Walvis Ridge. Absence of high abundances of N. pachyderma (s) from the continental slope of the southern Cape Basin indicates that Southern Ocean surface water advection has not exerted a major influence on the Benguela Current System. The coincidence of increased upwelling intensity with the movement of the Angola-Benguela Front can be interpreted mainly by changes in strength and zonality of the trade wind system.

Late Miocene stable isotope stratigraphy of SE Atlantic ODP Site 1085: Relation to Messinian events

Abstract High-resolution benthic oxygen isotope and XRF (Fe and Ca) records from Site 1085 drilled in the Mid-Cape basin (ODP Leg 175) are used to investigate global climate changes during the Late Miocene in relation to Messinian geological events. The cyclic fluctuations of the time series at Site 1085 enable us to establish a reliable chronology for the time interval 7.3–4.7 Ma. Spectral analysis of the δ18O record indicates that the 41-kyr period of orbital obliquity dominates the Late Miocene record. A global climate record was extracted from the oxygen isotopic composition of benthic foraminifera. Both long- and short-term variabilities in the climate record are discussed in terms of sea-level and deep-water temperature changes. The time interval 7.3–6.25 Ma characterized by low-amplitude δ18O variations is followed by a period marked by maximum in the δ18O values (6.25–5.57 Ma). At about 5.56 Ma, a rapid decrease in δ18O values is documented that may reflect a warming of deep-water temperature associated with a global warming period. Comparison between the timing of the oceanic isotope events and the chronology of the Mediterranean Salinity Crisis suggest that global eustatic processes were not essential in the Mediterranean Salinity Crisis history. From our data, we infer that the global warmth documented in the Early/mid-Pliocene probably started during the Late Miocene (at 5.55 Ma). At the same time, the onset of evaporite deposition in the central basin of the Mediterranean Sea took place. Sharp changes in the sedimentation rates, mainly driven by terrigenous input at this site, are observed during the Messinian Stage.

The Silurian of Gotland (Sweden): facies interpretation based on stable isotopes in brachiopod shells

The Silurian of Gotland, Sweden, consists of 440 m of carbonate deposits. Repeatedly, uniform sequences of micritic limestones and marls are interrupted by complex-structured reefs and by adjacent platform sediments. Generally, the alteration of facies is interpreted as the result of sea-level fluctuations caused by a gradual regression with superimposed minor transgressive pulses. The purpose of this study is a facies interpretation based on both field observations and stable isotope measurements of brachiopod shells. Approximately 700 samples from stratigraphically arranged localities in different facies areas have been investigated. The carbon and oxygen isotopes show principally parallel curves and a close relationship to the stratigraphic sequence. Lower values occur in periods dominated by deposition of marly sequences. Higher values are observed in periods dominated by reefs and extended carbonate platforms. The oxygen isotope ratios are interpreted to reflect paleosalinity changes due to varying freshwater input, rather than to paleotemperature. Carbon isotope ratios are believed to have been connected to global changes in the burial of organic carbon in black shales during periods of euxinic deep water conditions. Consequently, the facies succession on Gotland results from global paleoclimatic conditions. Changes in terrigenous input due to different rates of weathering and freshwater runoff, rather than sea-level fluctuations, control the carbonate formation of the Silurian on Gotland.

Coccoliths in sediment traps from the Norwegian Sea

Abstract The fine fraction of surface sediment samples in the Norwegian Sea shows an unexpectedly high amount of calcareous nannoplankton. Investigations, using time-series sediment traps in the Lofoten Basin (69°N, 1983/84), near Bear Island (75°N, 1984/85) and in the Fram Strait (78°N, 1984/85) provided information about the accumulation of this material in relation to the strong seasonality of biological production. Coccolith identification and counting, by means of a scanning electron microscope, indicated that the coccolith assemblages in the traps consist almost entirely of the two speciesEmiliania huxleyi andCoccolithus pelagicus. These species dominated the flux rate of the nannoplankton carbonate. A further eight species only made minor contributions to the flux. In the Lofoten Basin a distinct seasonality could be recognized in both standing crop and carbonate flux. Also the relationship between the two main species and the proportion of intact coccospheres showed an annual cycle. In the sample series of the two northerly traps the seasonality was less distinct. In general, the coccolith flux decreases towards the North. This was particularly evident for the smaller speciesE. huxleyi, while the number of massiveC. pelagicus coccoliths — and so the coccolith carbonate flux — diminished only slightly. Additional investigations on water samples from the Norwegian Sea revealed species compositions that differed greatly from those in the sediment traps. This suggests that selective processes change the relative species proportions during sedimentation.