Showing papers on "Foraminifera published in 1995"

Pliocene stable isotope stratigraphy of site 846

Abstract: Oxygen and carbon isotope ratios were measured in benthic foraminifers from the entire Pliocene and latest Miocene sections of Site 846, a 180-m section, at a sampling interval of 10 cm. This provides a temporal resolution of about 2500 yr. The documented continuity of the record is excellent. Using the time scale that was developed on the basis of orbital tuning of GRAPE density records, we observed a fairly constant phase relationship between δ 18 θ and variations in the obliquity of EaruYs rotational axis. A new numbering scheme for Pliocene isotope stages is proposed. This high-resolution δ 18 θ record clarifies several interesting aspects of late Neogene climatic evolution, including a "glacial" event that may have caused the final Messinian desiccation of the Mediterranean Sea; one or more "interglacial" events that might have caused refilling of the Mediterranean; a well-resolved couplet of glacial events at about the age of the Sidujfall Subchron; interglacial extremes in the early part of the Gauss that could have resulted from either significant deglaciation on Antarctica or from warming of deep water; and a gradual ramp of increasingly extreme "glacial" events, starting at about the Kaena Subchron and culminating with δ 18 θ stage 100 in the earliest Matuyama.

Middle–Late Cretaceous climate of the southern high latitudes: Stable isotopic evidence for minimal equator-to-pole thermal gradients

Abstract: A detailed δ 18 O and δ 13 C stratigraphy has been generated from analysis of well-preserved Albian–early Maastrichtian foraminifera from Deep Sea Drilling Project (DSDP) Sites 511 and 327 (Falkland Plateau; ≈58° S–62° S paleolatitude) in the southern South Atlantic, and Cenomanian and Coniacian–Santonian foraminifera from DSDP Site 258 (Naturaliste Plateau; ≈58° S paleolatitude) in the southern Indian Ocean. These results, when combined with previously published Maastrichtian stable isotope data from Ocean Drilling Program (ODP) Site 690 (Weddell Sea, ≈65° S paleolatitude), provide new insight into the climatic and oceanographic history of the southern high latitudes during middle–Late Cretaceous time. The planktonic foraminifer δ 18 O curves reveal a gradual warming of surface waters from the Albian through the Cenomanian followed by extremely warm surface waters from the Turonian through the early Campanian. Long-term cooling of surface waters began in the late early Campanian and continued through the end of the Maastrichtian. The benthic foraminifer δ 18 O record generally parallels changes in the oxygen isotopic curves defined by shallow-dwelling planktonic foraminifera. The vertical oxygen and carbon isotopic gradients were relatively low during the Albian–Cenomanian, high from the Turonian–early Campanian, and then low during the late Campanian and Maastrichtian. Foraminiferal oxygen isotopic data from published sources and this study are averaged for each site, corrected for latitudinal changes in salinity based on modern-day surface-water values, and plotted versus paleolatitude for the late Albian, Coniacian–Santonian, and late Maastrichtian. Differences between lowand high-latitude surface-water paleotemperatures are estimated at ≈14 ° C during the late Albian and late Maastrichtian, but the Coniacian–Santonian reconstruction reveals only a 0–4 ° C latitudinal temperature gradient. Uncertainty regarding Cretaceous salinity gradients and possible diagenetic alteration of δ 18 O values introduce error into our estimates of paleolatitudinal thermal gradients; however, apparent low equator-to-pole temperature differences could indicate much higher poleward heat transport than at present.

Benthic foraminiferal responses to estuarine pollution: a review

Abstract: Benthic foraminiferal distributions in polluted marine areas have been investigated over the last three to four decades, and several workers have pointed out that they provide one of the most sensitive and inexpensive markers available for indicating deterioration of marginal marine environments. Most investigations have been carried out in temperate regions, in areas exposed to several pollution sources. However, environments characterized by organic waste contamination (e.g., sewage or paper and pulp mills) have been addressed more frequently than areas exposed to oil, thermal and various other kinds of pollution. Among the most abundant species close to many outfall areas in temperate regions are Elphidium excavatum and/or Eggerella advena (NW At1antic)lEggerelloides scabrus (NE Atlantic). The dominant tolerant or opportunistic species seems to depend on local hydrographical properties rather than type of effluent. Increased abundance, due to increased nutrient concentrations and reduced predation and competition, is often recorded in areas having high organic inputs. Such abundance aureoles may be separated from outfall centers by an area of strongly reduced abundance or, in severe cases, by a dead zone. Characteristic features of proximal areas include decreased diversity and increased dominance of tolerant or opportunistic species compared to distal areas. Whether agglutinated or calcareous forms dominate seems to depend on the local hydrography, acidity of the sediment porewater and whether living, dead or total (living + dead) assemblages are considered. Test deformation in foraminifera is known from the geological record. In modern environments, deformation occurs more frequently in polluted than in non-polluted areas. Whether different kinds of test deformation develop under pollution- versus naturally-induced stress and what kind of stress properties cause deformations have not yet been established. Differential adaptions to the complex, and in many cases unique, hydrographical and physical conditions that characterize estuarine environments often make it difficult to separate natural faunal properties from pollution effects, especially in a temporal context. Consequently, pollution effects on the biota in estuaries can best be evaluated by comparing the natural, pre-pollution assemblages with those of the present day. The presence of empty foraminiferal tests in sediment cores penetrating through contaminated intervals provides this kind of information, but possible diagenetic alterations of the original assemblages must always be considered. The fossil record can also provide a comparative baseline for evaluating to what extent legislation, intending to cause environmental improvements, has had a positive effect.

Evidence for a higher pH in the glacial ocean from boron isotopes in foraminifera

Abstract: RECORDS of past changes in the pH of the oceans should provide insights into how the carbonate chemistry of the oceans has changed over time. The latter is related to changes in the atmospheric CO2 content, such as that which occurred during the last glacial-interglacial transition1. Previous studies2,3 have shown that the fractionation of boron isotopes between sea water and precipitated carbonate minerals is pH-dependent. This finding has been used to reconstruct the evolution of ocean pH over the past 20 million years by analyses of boron isotopes in the carbonate shells of foraminifera4. Here we use the same approach to estimate changes in ocean pH between the last glacial and the Holocene period. We estimate that the deep Atlantic and Pacific oceans had a pH 0.3±0.1 units higher during the last glaciation. The accompanying change in carbonate ion concentration is sufficient to account for the decrease in atmospheric pco2 during the glacial period1. These results are consistent with the hypothesis5 that the low CO2 content of the glacial atmosphere was caused by an increased ratio of organic carbon to carbonate in the 'rain' to the sea floor, which led to an increase in carbonate ion concentration (and thus in pH) of deep water without a corresponding increase in the lysocline depth.

A 20,000-year record of ocean circulation and climate change from the Santa Barbara basin

Abstract: MUCH of the evidence for climate-driven fluctuations in ocean circulation during the past 20,000 years has come from studies of the North Atlantic region1-6. The extent to which such interactions have occurred in other ocean basins, and any associated teleconnec-tions between basins, is poorly understood. Here we present high-resolution palaeoclimate and palaeoceanographic records from a 20,000-year sedimentary sequence from the Santa Barbara basin, on the eastern margin of the North Pacific Ocean. The sequence shows oscillations of the benthic environment between low-oxygen conditions (laminated sediments) during periods of warm climate, and higher-oxygen conditions (non-laminated, bioturbated sediments) during cool intervals. Age differences between coexisting benthic and planktonic foraminifers indicate climate-related changes in the age and source-and, hence, oxygen content-of basin bottom waters. Relatively young bottom waters are associated with the cooler intervals and are considered to reflect high proportions of intermediate waters derived from proximal sources. Conversely, older bottom waters are associated with the warmer intervals and were derived from more distal sources. These climate-driven variations in ocean circulation appear to be synchronous with the main ocean-climate fluctuations in the North Atlantic region1-6, suggesting that a tight coupling mechanism operates between the two basins.

Benthic foraminifer stable isotope record from Site 849 (0 - 5 Ma) : local and global climate changes

Abstract: Benthic foraminifer and δC data from Site 849, on the west flank of the East Pacific Rise (0° 11 'N, 110°3 l'W; 3851 m), give relatively continuous records of deep Pacific Ocean stable isotope variations between 0 and 5 Ma. The mean sample spacing is 4 k.y. Most analyses are from Cibicides wuellerstorfi> but isotopic offsets relative to Uvigerina peregrina appear roughly constant. Because of its location west of the East Pacific Rise, Site 849 yields a suitable record of mean Pacific Ocean δ'C, which approximates a global oceanic signal. The ~lOO-k.y.-period climate cycle, which is prevalent in δ 1 8 θ does not dominate the long-term δC record. For δC, variations in the -400and 41-k.y. periods are more important. Phase lags of δC relative to ice volume in the 41and 23-k.y. bands are consistent with δC as a measure of organic biomass. A model-calculated exponential response time of 1-2 k.y. is appropriate for carbon stored in soils and shallow sediments responding to glacial-interglacial climate change. Oceanic δC leads ice volume slightly in the 100-k.y. band, and this suggests another process such as changes in continental weathering to modulate mean river δC at long periods. The δC record from Site 849 diverges from that of Site 677 in the Panama Basin mostly because of decay of C-depleted organic carbon in the relatively isolated Panama Basin. North Atlantic to Pacific δC differences calculated using published data from Sites 607 and 849 reveal variations in Pliocene deep water within the range of those of the late Quaternary. Maximum δC contrast between these sites, which presumably reflects maximum influx of high-δC northern source water into the deep North Atlantic Ocean, occurred between 1.3 and 2.1 Ma, well after the initiation of Northern Hemisphere glaciation. Export of high-δC North Atlantic Deep Water from the Atlantic to the circumpolar Antarctic, as recorded by published δ'C data from Subantarctic Site 704, appears unrelated to the North Atlantic-Pacific δC contrast. To account for this observation, we suggest that deep-water formation in the North Atlantic reflects northern source characteristics, whereas export of this water into the circumpolar Antarctic reflects Southern Hemisphere wind forcing. Neither process appears directly linked to ice-volume variations.

Deep-sea foraminifera in the South Atlantic Ocean; ecology and assemblage generation

Abstract: Benthic foraminiferal assemblages of 237 high-quality surface sediment samples from the South Atlantic Ocean, including the Atlantic sector of the circumpolar ocean, were quantitatively analyzed. Comparisons between assemblages of live (stained with Rose Bengal) and dead specimens showed that potential fossil assemblages of dead specimens were markedly impoverished in fragile and non-resistant agglutinated forms compared to the living assemblages. The dead material was grouped into nine principal faunal end-members by multivariate statistics and quantitatively correlated with available environmental variables. This data set is planned to serve as a reference for interpreting benthic foraminifera fluctuations in Pleistocene Southern Atlantic core material and to constrain the reliability of proxies directly derived from benthic foraminiferal shell geochemistry. Multiple regression analysis in concert with the analysis of distinctive and limited distribution areas of specific faunas delineated ecological demands and preferences of well-known cosmopolitan species and the generation of characteristic assemblages. We differentiated between four principal but interdependent groups of environmental agents acting upon the generation and distribution of nine specific potential fossil assemblages: (I1) Lateral advection and bottom water ventilation, (2) primary productivity and organic carbon flux rates, (3) bottom water carbonate corrosiveness and (4) energetic state at the benthic boundary layer.

Late Paleocene to Eocene paleoceanography of the equatorial Pacific Ocean: Stable isotopes recorded at Ocean Drilling Program Site 865, Allison Guyot

Abstract: An expanded and largely complete upper Paleocene to upper Eocene section was recovered from the pelagic cap overlying Allison Guyot, Mid-Pacific Mountains at Ocean Drilling Program (ODP) Site 865 (18o26'N, 179o33'W; paleodepth 1300-1500 m). Reconstructions show that the site was within a few degrees of the equator during the Paleogene. Because no other Paleogene sections have been recovered in the Pacific Ocean at such a low latitude, Site 865 provides a unique record of equatorial Pacific paleoceanography. Detailed stable isotopic investigations were conducted on three planktonic foraminiferal taxa (species of Acarinina, Morozovella, and Subbotina). We studied benthic foraminiferal isotopes at much lower resolution on species of Cibicidoides and Lenticulina, Nuttallides truernpyi and Gavelinella beccariiformis, because of their exceptional rarity. The 5180 and 513C stratigraphies from Site 865 are generally similar to those derived from other Paleocene and Eocene sections. The planktonic foraminiferal records at Site 865, however, include significantly less short-term, single-sample variability than those from higher-latitude sites, indicating that this tropical, oligotrophic location had a comparatively stable water column structure with a deep mixed layer and less seasonal variability. Low-amplitude (0.1-0.8%o) oscillations on timescales of 250,000 to 300,000 years correlate between the/513C records of all planktonic taxa and may represent fluctuations in the mixing intensity of surface waters. Peak sea surface temperatures of 24o-25oC occurred in the earliest Eocene, followed by a rapid cooling of 3-6oC in the late early Eocene. Temperatures remained cool and stable through the middle Eocene. In the late Eocene, surface water temperatures decreased further. Vertical temperature gradients decreased dramatically in the late Paleocene and were relatively constant through much of the Eocene but increased markedly in the late Eocene. Intermediate waters warmed through the late Paleocene, reaching a maximum temperature of 10oC in the early Eocene. Cooling in the middle and late Eocene paralleled that of surface waters, with latest Eocene temperatures below 5oC. Extinction patterns of benthic foraminifera in the latest Paleocene were similar to those observed at other Pacific sites and were coeval with a short-term, very rapid negative excursion in/513C values in planktonic and benthic taxa as at other sites. During this excursion, benthic foraminiferal/5180 values decreased markedly, indicating warming of 4 to 6oC for tropical intermediate waters, while planktonic taxa show slight warming (1 oC) followed by 2oC of cooling. Convergence of/5180 values of planktonic and benthic foraminifera suggests that thermal gradients in the water column in this tropical location collapsed during the excursion. These data are consistent with the hypothesis that equatorial Pacific surface waters were a potential source of warm, higher salinity waters which filled portions of the deep ocean in the latest Paleocene. Oxygen isotopic data indicate that equator to high southern latitude sea surface thermal gradients decreased to as little as 4oC at the peak of the excursion, suggesting some fundamental change in global heat transport.

North Atlantic Deepwater Temperature Change During Late Pliocene and Late Quaternary Climatic Cycles

Abstract: Variations in the ratio of magnesium to calcium (Mg/Ca) in fossil ostracodes from Deep Sea Drilling Project Site 607 in the deep North Atlantic show that the change in bottom water temperature during late Pliocene 41,000-year obliquity cycles averaged 1.5°C between 3.2 and 2.8 million years ago (Ma) and increased to 2.3°C between 2.8 and 2.3 Ma, coincidentally with the intensification of Northern Hemisphere glaciation. During the last two 100,000-year glacial-to-interglacial climatic cycles of the Quaternary, bottom water temperatures changed by 4.5°C. These results show that glacial deepwater cooling has intensified since 3.2 Ma, most likely as the result of progressively diminished deepwater production in the North Atlantic and of the greater influence of Antarctic bottom water in the North Atlantic during glacial periods. The ostracode Mg/Ca data also allow the direct determination of the temperature component of the benthic foraminiferal oxygen isotope record from Site 607, as well as derivation of a hypothetical sea-level curve for the late Pliocene and late Quaternary. The effects of dissolution on the Mg/Ca ratios of ostracode shells appear to have been minimal.

Northeastern Atlantic benthic foraminifera during the last 45,000 years: Changes in productivity seen from the bottom up

Abstract: We studied benthic foraminifera from the last 45 kyr in the >63 mu m size fraction in Biogeochemical Ocean Flux Studies (BOFS) cores 5K (50 degrees 41.3'N, 21 degrees 51.9'W, depth 3547 m) and 14K (58 degrees 37.2'N, 19 degrees 26.2'W, depth 1756 m), at a time resolution of several hundreds to a thousand years. The deepest site showed the largest fluctuations in faunal composition, species richness, and benthic foraminiferal accumulation rates; the fluctuations resulted from changes in abundance of Epistominella exigua and Alabaminella weddellensis. In the present oceans, these species bloom opportunistically when a spring plankton bloom results in seasonal deposition of phytodetritus on the seafloor. The ''phytodetritus species'' had very low relative abundances and accumulation rates during the last glacial maximum. A strong increase in absolute and relative abundance of E. exigua and A weddellensis during deglaciation paralleled the decrease in abundance of the polar planktonic foraminifer Neogloboquadrina pachyderma (s), and the increase in abundance of warmer water planktonic species such as Globigerina bulloides. This strong increase in relative abundance of the ''phytodetritus species'' and the coeval increase in benthic foraminiferal accumulation rate were thus probably caused by an increase in the deposition of phytodetritus to the seafloor (and thus probably of surface productivity) when the polar front retreated to higher latitudes. The abundance of ''phytodetritus species'' decreased during the Younger Dryas, but not to the low levels of fully glacial conditions. During Heinrich events (periods of excessive melt-water formation and ice rafting) benthic accumulation rates were very low, as were the absolute and relative abundances of the ''phytodetritus species'', supporting suggestions that surface productivity was very low during these events. In both cores Pullenia and Cassidulina species were common during isotope stages 2, 3 and 4, as were bolivinid, buliminid and uvigerinid species. High relative abundances of these species have been interpreted as indicative either of sluggish deep water circulation or of high organic carbon fluxes to the seafloor. In our cores, relative abundances of these species are negatively correlated with benthic foraminiferal accumulation rates, and we can thus not interpret them as indicative of increased productivity during glacials. The percentage of these ''low oxygen'' species calculated on a ''phytodetritus species'' - free basis decreased slightly at deglaciation at 5K, but not at 14K. This suggests that decreased production of North Atlantic Deep Water during the last glacial might have slightly affected benthic foraminiferal faunas in the eastern North Atlantic at 3547 m depth, but not at 1756 m. In conclusion, major changes in deep-sea benthic foraminiferal faunas over the last 45,000 years in our cores from the northeastern Atlantic were the result of changes in surface water productivity, not of changes in deep water circulation; productivity was lower during the glacial, probably because of extensive ice cover.

Environmental control of living symbiotic and asymbiotic foraminifera of the California Current

Abstract: Plankton tows from the northern California Current constrain biological and physical influences on living planktonic foraminifera. In this region, the dominant factors controlling the size and distribution of symbiotic and asymbiotic species are light and food. Food decreases off- shore. Light, needed for symbiont photosynthesis, increases offshore as water turbidity lessens. Asymbiotic foraminifera (e.g., right-coiling Neogloboquadrina pachyderma, Globigerina quinqueloba, and Globigerina bulloides), which survive by grazing, dominate the coastal fauna. The most abundant of these species, right-coiling Neogloboquadrina pachyderma, did not change in size in response to increasing food. Species that benefit from symbiont photosynthesis (OrbMina universa, Neogloboquadrina dutertrei, Globigerinoides ruber, and Globigerinita glutinata) dominate the offshore fauna. Individuals of these species are rare and have smaller shells in turbid waters where light is limited. G. ruber, which is near its thermal tolerance limit of --14oC, is the only species to demonstrate a clear temperature response. Although temperature may control a foraminiferal species' distribution near the limits of its thermal tolerance, food and light appear to provide the primary control under more favorable thermal conditions. We infer that gradients in food and light can result in quantifiable sedimentary patterns related to oceanic productivity through changes in plankton biomass and turbidity.

Middle Miocene deepwater paleoceanography in the southwest Pacific: Relations with East Antarctic Ice Sheet development

Abstract: A suite of middle Miocene Deep Sea Drilling Project sites in the southwest Pacific reveals large-scale changes in deepwater circulation associated with East Antarctic Ice Sheet (EAIS) variations from ∼16.5 to 13.8 Ma. Oxygen and carbon isotopic records based on Cibicidoides benthic foraminifera from a depth transect (sites 590B, 588A, 591B, and 206 from 1200- to 3150-m paleodepth at ∼35°S paleolatitude) and from a meridional transect (sites 588A, 590B, 593, and 594 from 30° to 48°S paleolatitude at intermediate water depth) allow detailed examination of southwest Pacific deepwater circulation from ∼17.5 to 12 Ma. Significantly, intervals of low δ18O from 16.5 to 16.3 Ma and perhaps at 15.7 Ma were marked by similar δ18O values at upper bathyal (∼1200–1500 m; sites 588A and 590B) and midbathyal (∼2100 m; site 591B) water depths. Small vertical δ18O gradients during δ18O minima may indicate warm saline deep water (WSDW) at midbathyal depths in the southwest Pacific during intervals of inferred global warmth and low global ice volume. Increased vertical δ18O gradients after ∼15.6 Ma and especially after 13.8 Ma indicate increased production of Southern Component Water (SCW) in association with EAIS growth. These data are consistent with the hypothesis (Woodruff and Savin, 1989) that major EAIS growth was fostered by diminished meridional heat transport to the high southern latitudes related to the termination of Tethyan Indian Saline Water (TISW) and an increase in SCW production during the early middle Miocene after ∼15.6 Ma. Further, a maximum vertical carbon isotopic gradient of ∼0.8 ‰ at 13.6 Ma suggests that Southern Component Intermediate Water (SCIW) production and Pacific Deep Water (PDW) strength were each at a maximum at this time and were critical to major EAIS growth. The establishment of near-modern δ13C and δ18O gradients following major EAIS growth from ∼14.0 to 13.8 Ma marks a major step in the development of the Neogene ocean/cryosphere system.

A Heinrich‐like event, H‐0 (DC‐0): Source(s) for detrital carbonate in the North Atlantic during the Younger Dryas Chronozone

Abstract: In the North Atlantic we define H-0 as a Heinrich-like event which occurred during the Younger Dryas chron. On the SE Baffin shelf prior to 11 ka, surface water productivity was reasonably high, as measured by the numbers of diatom and planktic foraminifera per gram, but an abrupt increase in detrital carbonate (DC-0 event) (from approximately 15% up to 50% carbonate by weight) occurred at 11 ± 14C ka and continued to circa 10 ka. These deposits, 2–6 m thick, are dominated by detrital calcite and silt- and clay-sized sediments. During this event (DC-0/H-0), ice extended onto the inner shelf but did not reach the shelf break and probably originated from a center over Labrador-Ungava. As a consequence, the pattern of ice-rafted debris and sediment provenance shown by H-O in the North Atlantic is different from that during H-1 (14.5 ka) or H-2 (20 ka) when the ice sheet extended along the axis of Hudson Strait and may have reached the shelf break; for example, there is no concrete evidence for DC-O is cores on the floor of the Labrador Sea due east of Hudson Strait (HU75-55,-56), but H-O has been noted in cores off Newfoundland and west of Ireland. A coeval carbonate event to DC-0, but this one dominated by dolomite, occurs in HU82-SU5 on the west side of Davis Strait with a source either from northern Baffin Bay or Cumberland Sound. Although other sources for North Atlantic detrital carbonate cannot be totally excluded, our evidence suggests that H-0 represents the expression of glaciological instability of the Laurentide Ice Sheet within the general region of Hudson Strait and probably to the north (Cumberland Sound and northernmost Baffin Bay). There is one younger DC event, dated circa 8.4 ka, present in sediments along the Labrador margin and in Hudson Strait, which represents the final collapse of the ice sheet within Hudson Strait and Hudson Bay.

Response of deep-water agglutinated foraminifera to dysoxic conditions in the California Borderland basins

Abstract: Analysis of agglutinated benthic foraminifera from surface samples collected in the San Pedro and Santa Catalina Basins reveals a predictable relationship between the proportions of morphogroups with decreasing bottom water oxygen levels and with the TOC content of the surficial sediment Living (Rose Bengal stained) foraminiferal faunas from dysaerobic environments display low diversity and high dominance, suggesting stressed conditions There is an inverse relationship between oxygen and the relative abundance of deep infaunal morphogroups Samples collected from shallow stations above the oxygen minimum zone are comprised of epifaunal and shallow infaunal morphotypes At intermediate depths (~500 m), there is a peak in the abundance of suspension-feeding and "climbing" forms (watchglass-shaped trochamminids attached to Rhabdammina) Specimens from intermediate stations display the largest overall size Deeper in the San Pedro Basin the living fauna is dominated by a small, flattened, tapered, species that is interpreted as having a deep infaunal microhabitat In the dysaerobic environments off California the greatest degree of faunal change occurs when bottom water dissolved oxygen values drop from 05 ml/l to 02 ml/l The effect of TOC content on the benthic fauna is demonstrated at two stations from the same depth in the San Pedro Basin The station with the higher TOC content (42% vs 29%) contains greater proportions of the small, deep infaunal morphotype These faunal changes may be attributed to differences in the depth of the oxygenated zone within the sediment surface layer Agglutinated faunas from areas that experience seasonal anoxia are comprised of a large proportion of opportunistic forms such as Reophax and Psammosphaera These are the same taxa that colonised abiotic sediment trays in a recolonisation experiment in the Panama Basin This study further demonstrates that agglutinated foraminiferal morphotypes respond in a similar manner to calcareous benthic foraminifera in dysaerobic environments

Timescale and paleoceanographic implications of a 3.6 m.y. oxygen isotope record from the northeast Indian Ocean (Ocean Drilling Program Site 758)

Abstract: Numerous studies have shown that δ18O records from benthic and planktonic foraminifera, primarily a proxy of global ice volume variations, reflect Milankovitch periodicities. To study climatic response to orbital forcing at Ocean Drilling Program site 758, we have generated continuous δ18O and δ13C records from a single benthic foraminiferal species Cibicides wuellerstorfi for the last 3.6 m.y. and extended the planktonic foraminiferal isotope records of Farrell and Janecek (1991) (0-2.5 Ma, based on Globigerinoides sacculifer) to 3.6 Ma (Chen, 1994). We then constructed an age model by matching, correlating and tuning the benthic δ18O record to a model simulation of ice volume (Imbrie and Imbrie, 1980). The filtered 41- and 23-kyr signals based on the resultant astronomically tuned age model are highly correlated to obliquity (r=0.83) and precession (r=0.75), respectively. Although derived with methodology different from Shackleton et al. (1990) and Hilgen (1991a, b), our results generally agree with their published astronomical timescales for the time interval from 0 to 3.0 Ma, providing additional support for the newly emerging chronology based on orbital tuning. Slight discrepancies exist in the time interval from 3.0 to 3.6 Ma, suggesting several possibilities, including differences in the approaches of orbital tuning and the relatively low amplitude of δ18O variations in our record. However, even if the discrepancies are due to the relatively low amplitude of the isotope signals in our record at 3.0–3.6 Ma, our resultant timescale as a whole does not adversely affect our evaluation of the paleoclimatology and paleoceanography of the Indian Ocean, such as the evolution of the 100-, 41- and 23-kyr cycles, and variation of global ice volume and deepwater temperature during the past 3.6 m.y.

Cassidulina teretis Tappan and Cassidulina neoteretis new species (Foraminifera): stratigraphic markers for deep sea and outer shelf areas

Abstract: . Fossil versus Recent specimens allocated to Cassidulina teretis Tappan display slight differences in both ecological distribution and morphology. This has led to a re-examination of specimens from the North Atlantic region. The study was mainly based on scanning electron microscopy. It resulted in the division of the plexus into two species with different stratigraphical and partly different environmental distributions: Cassidulina teretis Tappan and Cassidulina neoteretis n. sp. A distinction between C. teretis and C. neoteretis provides a new biostratigraphical marker and may also prove useful in ecological studies. C. teretis had its first occurrence during the Middle to Upper Miocene and its last well-documented occurrence shortly after the palaeomagnetic Brunhes/Matuyama boundary in the Norwegian Sea, whilst it apparently disappeared from the North Atlantic as early as a little after the Gauss/Matuyama boundary. C. neoteretis presumably evolved from C. teretis between about 2.0 and 2.3 Ma in the northern North Atlantic and migrated northward inhabiting the Norwegian Sea as C teretis became extinct here at about 0.7 Ma. All Recent specimens belong to C. neoteretis. Specimens of C. teretis have been documented from both arctic and boreal regions in inner shelf to bathyal environments (between about 50 and 2000 m water depth), whereas the Recent distribution of C. neoteretis is slightly more limited: arctic and cold boreal regions at water depths between 150 and 3000 m, most commonly between 1000 and 1500 m.

Chronology for climate change: Developing age models for the biogeochemical ocean flux study cores

Abstract: We construct age models for a suite of cores from the northeast Atlantic Ocean by means of accelerator mass spectrometer dating of a key core, BOFS 5K, and correlation with the rest of the suite. The effects of bioturbation and foraminiferal species abundance gradients upon the age record are modeled using a simple equation. The degree of bioturbation is estimated by comparing modeled profiles with dispersal of the Vedde Ash layer in core 5K, and we find a mixing depth of roughly 8 cm for sand-sized material. Using this value, we estimate that age offsets between unbioturbated sediment and some foraminifera species after mixing may be up to 2500 years, with lesser effect on fine carbonate (<10 mu m) ages. The bioturbation model illustrates problems associated with the dating of ''instantaneous'' events such as ash layers and the ''Heinrich'' peaks of ice-rafted detritus. Correlations between core 5K and the other cores from the BOFS suite are made on the basis of similarities in the downcore profiles of oxygen and carbon isotopes, magnetic susceptibility, water and carbonate content, and via marker horizons in X radiographs and ash beds.

Calcitic foraminiferal data confirmed by cadmium in aragonitic hoeglundina: application to the last glacial maximum in the northern indian ocean

Abstract: A core-top study of cadmium uptake into the shells of the aragonitic benthic foraminifera Hoeglundina elegans demonstrates that the Hoeglundina Cd/Ca partition coefficient is close to 1.0 throughout the ocean. Cd uptake by Hoeglundina is far less depth dependent than that of calcitic benthic foraminifera. Furthermore, manganese carbonate does not precipitate on these aragonitic shells, allowing for the recovery of Cd estimates in some samples where calcitic species are spoiled by contaminating overgrowths. Because Cd incorporation into Hoeglundina shows little depth dependence, a comparison of calcitic and Hoeglundina Cd data can be used to verify the assumption that the depth dependence observed for calcitic benthic foraminifera is time invariant. This comparison has been undertaken in downcore and last glacial maximum (LGM) samples from the northern Indian Ocean. Aragonitic and calcitic foraminiferal estimates for Cd in the LGM ocean are in excellent agreement. This result indicates upper ocean estimates of LGM Cd are reliable despite a factor-of-two variation in Cd uptake by calcitic species over this depth range. Cd and δ13C data indicate that the uppermost waters of the LGM Arabian Sea were strongly nutrient depleted relative to today but that the deepest waters of the northern Bay of Bengal had nutrient concentrations at least as high as or higher than modern levels. A strong vertical gradient of increasing nutrients with increasing depth existed in both basins during the LGM. A west to east gradient of increasing nutrient concentrations also is evident at all depths. In the Bay of Bengal, there is evidence for an enhanced gradient of nutrients increasing from south to north. The LGM deep Arabian Sea and intermediate-depth Bay of Bengal were nutrient depleted compared to the modern ocean; however, Cd and δ13C data indicate that nutrient depletion in the northern Indian Ocean did not exceed that of the deep eastern tropical Pacific except during the latest part of oxygen isotope stage 2 and (according to Cd data only) the early stages of deglaciation. During oxygen isotope stage 3 and early stage 2, Cd in the northern Indian Ocean and Cd in the eastern tropical Pacific were indistinguishable, although both regions have lower Cd at these times compared to the Holocene.