Planktic Foraminiferal Species Research Articles

Variations in mid-latitude North Atlantic surface water properties during the mid-Brunhes (MIS 9–14) and their implications for the thermohaline circulation

Abstract. Stable isotope and ice-rafted debris records from three core sites in the mid-latitude North Atlantic (IODP Site U1313, MD01-2446, MD03-2699) are combined with records of ODP Sites 1056/1058 and 980 to reconstruct hydrographic conditions during the middle Pleistocene spanning Marine Isotope Stages (MIS) 9–14 (300–540 ka). Core MD03-2699 is the first high-resolution mid-Brunhes record from the North Atlantic's eastern boundary upwelling system covering the complete MIS 11c interval and MIS 13. The array of sites reflect western and eastern basin boundary current as well as north to south transect sampling of subpolar and transitional water masses and allow the reconstruction of transport pathways in the upper limb of the North Atlantic's circulation. Hydrographic conditions in the surface and deep ocean during peak interglacial MIS 9 and 11 were similar among all the sites with relative stable conditions and confirm prolonged warmth during MIS 11c also for the mid-latitudes. Sea surface temperature (SST) reconstructions further reveal that in the mid-latitude North Atlantic MIS 11c is associated with two plateaus, the younger one of which is slightly warmer. Enhanced subsurface northward heat transport in the eastern boundary current system, especially during early MIS 11c, is denoted by the presence of tropical planktic foraminifer species and raises the question how strongly it impacted the Portuguese upwelling system. Deep water ventilation at the onset of MIS 11c significantly preceded surface water ventilation. Although MIS 13 was generally colder and more variable than the younger interglacials the surface water circulation scheme was the same. The greatest differences between the sites existed during the glacial inceptions and glacials. Then a north – south trending hydrographic front separated the nearshore and offshore waters off Portugal. While offshore waters originated from the North Atlantic Current as indicated by the similarities between the records of IODP Site U1313, ODP Site 980 and MD01-2446, nearshore waters as recorded in core MD03-2699 derived from the Azores Current and thus the subtropical gyre. Except for MIS 12, Azores Current influence seems to be related to eastern boundary system dynamics and not to changes in the Atlantic overturning circulation.

DECIPHERING THE MODERN CALCIFICATION DEPTH OF GLOBIGERINA BULLOIDES IN THE SOUTHWESTERN INDIAN OCEAN FROM ITS OXYGEN ISOTOPIC COMPOSITION

The water depth at which the planktic foraminiferal species Globigerina bulloides calcifies its shell has been deciphered based on the oxygen isotopic composition of its shells in surface sediment samples collected from the southwestern Indian Ocean. The seawater temperature for water depths ranging from 0−200 m was estimated from G. bulloides δ 18 O by using various paleotemperature equations. Out of several paleotemperature equations, the one given by Epstein and others (1953) provided the best-fit results. Similarly, the estimated seawater salinity was calculated using the seawater δ 18 O calculated from G. bulloides δ 18 O, and measured seawater temperature taken from the Levitus database. A comparison of seawater salinity and temperature estimated from G. bulloides δ 18 O, with the measured seawater salinity and temperature at different water depths, shows that the calcification depth of G. bulloides varies latitudinally. From the equator to ~15°S, the calcification depth of G. bulloides was inferred to be ~75 m. Between 15°S and ~43°S, the estimated seawater temperature matched well with the seawater temperature at ~200 m water depth, indicating that the G. bulloides calcification depth was comparatively deeper in this region. Farther south of ~43°S, however, the calcification depth appears to be considerably shallower than in the northern part of the study area. The comparison also showed that irrespective of latitudinal region, the estimated seawater temperature matches well with the seawater temperature during the austral spring season suggesting that G. bulloides is abundant at that time. The findings will help in paleoclimatic reconstruction studies based on characteristics of G. bulloides.

NE Atlantic surface water mass changes over the last 15 kyr

Planktic foraminiferal faunas (> 150 µm) and stable isotopes from the last 15 kyr were studied in kasten core 13078#16 from the Porcupine Abyssal Plain, NE Atlantic (48°49.91 N, 16°29.94 W, water depth 4844 m) in order to document changes in surface waters. The faunas were dominated (> 80%) by four species: Neogloboquadrina incompta (= N. pachyderma dextral), Globigerina bulloides, N. pachyderma (= N. pachyderma sinistral) and Globorotalia inflata and their abundances fluctuated strongly over the last 15 kyr. Other species, G. truncatulinoides (right and left-coiling forms) and Globigerinoides ruber (white), had low abundances (typically < 6%). The polar species N. pachyderma dominated (~ 60%) the terminal glacial interval when surface conditions were cold, decreased to 25–35% during the deglaciation and remained low (< 2%) throughout the Holocene. The relative abundance of N. incompta increased over the last 15 kyr and shows an inverse relationship with N. pachyderma. The increase in δ 13C values of G. bulloides, G. inflata and G. truncatulinoides over the last 15 kyr is consistent with the whole ocean shift due to the expansion of the terrestrial biosphere, indicating that the upper oceanic δ 13C values did not change significantly during this time. Between the deglaciation and the Holocene, the negative shift (~ 1.0‰) in δ 18O values of G. inflata and G. truncatulinoides equates to the global ice volume shift suggesting that there was no significant warming of the deeper surface waters or that the species migrated to occupy a similar temperature water mass during this time. In contrast, the negative shift (~ 1.7‰) in δ 18O values of G. bulloides corresponds to a local shift of 0.7‰, equivalent to a ~ 3 °C warming of surface waters. Between 10 kyr and 5 kyr, the positive δ 18O increases in G. inflata and G. truncatulinoides suggest a decrease in deeper surface water temperatures in the order of 1.0–1.5 °C, a change not evident in the shallow surface water δ 18O record of G. bulloides. Water mass stratification changes, revealed by the Δ δ 18O G. inflata – G. bulloides and Δ δ 18O G. truncatulinoides – G. bulloides records, suggest that the waters of the upper water column were more mixed during the last deglaciation than during the Holocene, and that there was a trend towards increasing water stratification over the last 15 kyr with a peak at ~ 4 kyr. Intervals of enhanced carbonate dissolution at ~ 12–8.5 kyr and ~ 4 kyr previously reported are not clearly reflected in changes in solution-susceptible planktic foraminiferal species, such as G. ruber.

Depth habitats and seasonal distributions of recent planktic foraminifers in the Canary Islands region (29°N) based on oxygen isotopes

Seasonal depth stratified plankton tows, sediment traps and core tops taken from the same stations along a transect at 29°N off NW Africa are used to describe the seasonal succession, the depth habitats and the oxygen isotope ratios (δ 18O shell) of five planktic foraminiferal species. Both the δ 18O shell and shell concentration profiles show variations in seasonal depth habitats of individual species. None of the species maintain a specific habitat depth exclusively within the surface mixed layer (SML), within the thermocline, or beneath the thermocline. Globigerinoides ruber (white) and (pink) occur with moderate abundance throughout the year along the transect, with highest abundances in the winter and summer/fall season, respectively. The average δ 18O shell of G. ruber (w) from surface sediments is similar to the δ 18O shell values measured from the sediment-trap samples during winter. However, the δ 18O shell of G. ruber (w) underestimates sea surface temperature (SST) by 2 °C in winter and by 4 °C during summer/fall indicating an extension of the calcification/depth habitat into colder thermocline waters. Globigerinoides ruber (p) continues to calcify below the SML as well, particularly in summer/fall when the chlorophyll maximum is found within the thermocline. Its vertical distribution results in δ 18O shell values that underestimate SST by 2 °C. Shell fluxes of Globigerina bulloides are highest in summer/fall, where it lives and calcifies in association with the deep chlorophyll maximum found within the thermocline. Pulleniatina obliquiloculata and Globorotalia truncatulinoides, dwelling and calcifying a part of their lives in the winter SML, record winter thermocline (∼180 m) and deep surface water (∼350 m) temperatures, respectively. Our observations define the seasonal and vertical distribution of multiple species of foraminifera and the acquisition of their δ 18O shell.

Cretaceous climate, volcanism, impacts, and biotic effects

Cretaceous volcanic activities (LIPs and CFBPs) appear to have had relatively minor biotic effects, at least at the generic level. Major biotic stress during the Cretaceous was associated with OAEs and related to nutrient availability largely from weathering, greenhouse warming, drowning of platform areas, and volcanism. The biotic effects of OAEs were often dramatic at the species level, causing the extinction of larger specialized and heavily calcified planktonic foraminifera (rotaliporid extinction) and nannoconids (nannoconid crises), the temporary disappearances of other larger species, and the rapid increase in r-selected small and thin-walled species, such as the low oxygen tolerant heterohelicids and radially elongated taxa among planktic foraminifera and thin walled nannofossils. Biotic diversity increased during cool climates, particularly during the late Campanian and Maastrichtian, reaching maximum diversity during the middle Maastrichtian. High biotic stress conditions began during greenhouse warming and Deccan volcanism about 400 ky before the K-T boundary; it reduced abundances of large specialized tropical planktic foraminiferal species and endangered their survival. By K-T time, renewed Deccan volcanism combined with a large impact probably triggered the demise of this already extinction prone species group. Evidence from NE Mexico, Texas, and the Chicxulub crater itself indicates that this 170 km-diameter crater predates the K-T boundary by ∼300,000 years and caused no species extinctions. The Chicxulub impact, therefore, can no longer be considered a direct cause for the K-T mass extinction. However, the K-T mass extinction is closely associated with a global Ir anomaly, which is considered too large, too widespread, and too concentrated in a thin layer to have originated from volcanic activity, leaving another large impact as the most likely source. This suggests that a second still unknown larger impact may have triggered the K-T mass extinction.

Stable isotope clue to episodic sea ice formation in the glacial North Atlantic

Sea ice is a determinant parameter of the climate system, which acts as amplifier through positive ice-albedo feedbacks and induces deep-water formation through brine production. It is also one of the most elusive parameters in the paleoclimate record. Most proxies of sea ice provide information on its presence and extent of (e.g., diatoms or dinocysts), but do not permit assessment on its rate of formation. However, seemingly off-equilibrium, low δ18O values in mesopelagic planktic foraminifer species, as observed today in the Arctic Ocean, are thought to relate to the production of isotopically light brines during sea ice formation, and might thus provide a clue on the rate of sea ice growth. With reference to the isotopic properties of modern planktic foraminifers in the Arctic Ocean and using multi-proxy data sets from the last glacial stage in the northwest North Atlantic, we re-examined some of the current interpretations of planktic isotope records in the glacial NW North Atlantic. The large amplitude light isotopic excursions recorded in Neogloboquadrina pachyderma left coiling during Heinrich events 1, for example, correspond to extensive sea ice cover as reconstructed from dinocysts, and do not seem unequivocally linked to low-salinity pulses. Such light isotopic excursion more likely responded to enhanced rates of sea ice formation resulting in the production and sinking of isotopically light brines. On the contrary, the isotopically heavy planktic foraminifers of the last glacial maximum (LGM) interval stricto sensu would rather suggest relatively low rates of brine production, thus low sea ice growth rates in the area. This would imply that the LGM distribution of sea ice in the North Atlantic was primarily linked to spreading and drifting from marginal and Nordic source areas and not from enhanced in situ production. At the scale of glacial–interglacial cycles, isotopic distillation processes relating to sea ice production at high latitudes could be one of the factors controlling the 18O–salinity relationship in deep ocean water masses that might deserve closer examination, as it would be independent of continental ice volume changes.

Linkages between surface and deep circulations in the southern Japan Sea during the last 27,000 years: Evidence from planktic foraminiferal assemblages and stable isotope records

We analyzed planktic foraminiferal assemblages, oxygen and carbon isotope records, and the presence or absence of laminations to reconstruct the paleoenvironments of the southern Japan Sea since the last glacial period. Data were collected from two well-dated cores. One core (water depth 999 m) included thinly laminated mud layers, the other (water depth 283 m) contained nonlaminated sediments. Tephrochronology and accelerator mass spectrometry 14C dating of 14 horizons revealed that the two cores contained continuous records of the last 27 cal kyr. A total of 13 planktic foraminiferal species belonging to six genera were identified in down-core samples. The typical indicators of the Tsushima Current water, Globigerinoides ruber, Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, Globigerinoides tenellus, and Globigerinita glutinata occurred since 9.3 cal kyr BP. Neogloboquadrina incompta, which was the dominant species in the Tsushima Current region of the modern Japan Sea, first occurred at 8.2 cal kyr BP and dominated the assemblage since 7.3 cal kyr BP. These results clearly indicate that the warm Tsushima Current started to inflow into the Japan Sea at 9.3 cal kyr BP, and the modern surface conditions in the southern Japan Sea were essentially established at 7.3 cal kyr BP. Our data and comparison of the presence or absence of laminated sediments in three locations from the southern Japan Sea suggest that deep circulation during the deglacial period was weaker than that at present. In addition, deep circulation in the modern Japan Sea, which supplies oxygen-rich water to the entire basin, started probably in association with the first inflow of the Tsushima Current beginning at 9.3 cal kyr BP.

SEDIMENT TRAP RESULTS FROM THE JAPAN TRENCH IN THE KUROSHIO DOMAIN: SEASONAL VARIATIONS IN THE PLANKTIC FORAMINIFERAL FLUX

Time-series sediment traps were deployed at two depths at Station JT (Japan Trench) along the northern margin of the Kuroshio current system domain from March 1991 to May 1992. Total (>125 μm) foraminiferal fluxes were 334–1028 shells/m 2 /day in the shallow trap (1200 m) and 657–2130 shells/m 2 /day in the deep trap (3700 m). A total of 35 planktic foraminiferal species belonging to 13 genera were identified. The assemblage was dominated by Globigerina bulloides, Globigerinita glutinata, Globigerinoides ruber and Neogloboquadrina dutertrei, whose fluxes varied seasonally. Globigerina bulloides and Globigerina quinqueloba occurred during the early spring phytoplankton bloom. These taxa were replaced by Globigerinoides ruber, Globigerinoides sacculifer, N. dutertrei and Pulleniatina obliquiloculata during the late spring to early autumn, with the development of a sharp thermocline and a deep chlorophyll a maximum just below the thermocline. The maximum shell flux of Globigerinita glutinata, Globigerina falconensis, Globorotalia inflata and Globorotalia truncatulinoides occurred during the winter near the 18 °C isotherm in well-mixed surface water that resulted from the extensive vertical mixing induced by the winter monsoon. Temporal changes in the path of the Kuroshio current system caused by its meandering appeared to be the dominant flux control factor, based on the correlation between the meander types of the Kuroshio and the timing of large fluxes in the deep trap. During the trap experiments, major meanderings were observed from September to October 1991 and from mid-April to May 1992. During these two periods, prominent flux peaks of Globigerina bulloides and Neogloboquadrina incompta were observed in both shallow and deep traps. The major meander of the Kuroshio current system was near Station JT during these periods, and the nearshore side of the Kuroshio Front, which is characterized by high productivity in the euphotic zone, was very close to Station JT. These results suggest that these two species were produced on the nearshore side of the Kuroshio Front and collected in the sediment trap. The temporal occurrences of N. incompta at Station JT associated with the meanders of the Kuroshio are consistent with the high abundance of N. incompta in the surface sediment record from beneath the modern cold water mass off Kishu and Enshu-nada, between the Japanese coast and the axis of the Kuroshio current system.

Large age differences between planktic foraminifers caused by abundance variations and Zoophycos bioturbation

The downward transport of surface sediment deep into the sediment column by the Zoophycos‐producing animal leads not only to large age differences between the Zoophycos structure and surrounding host sediment but also to large differences in age between different foraminifer species found inside the trace fossil. In the late Quaternary material from the southwestern Portuguese continental slope examined in this study, age differences of up to 2590 years were observed between the planktic foraminifer species Globigerinoides ruber and Globigerina bulloides. These differences are caused by the mixing of surface and host material with different abundances of the two species. If there are differences in the abundance of the two species at the surface and/or in the host sediment, plenty of relatively young foraminifers may be mixed with few relatively old ones, or vice versa. The age differences between species caused by the combination of deep‐reaching bioturbation by the Zoophycos producer and abundance variations may be considerably larger than the age differences caused by the homogenizing bioturbation in the mixed layer.

Isotopic evidences of past upwelling intensity in the Arabian Sea

Oxygen and carbon isotopic analyses have been performed on the tests of Globigerina bulloides, Globigerinoides sacculifer, Neogloboquadrina dutertrei and Pulleniatina obliquiloculata to study the Δ δ 18O and Δ δ 13C of shallow and deeper depth living planktic foraminifera species. High and low Δ δ 18O obl-bul and Δ δ 13C sac-dut coincides respectively with the low and high flux of G. bulloides (established monsoon upwelling index). The tangible relationships between the flux of G. bulloides and oxygen and carbon isotope differences between the shallow and deeper depth habitat planktic foraminiferal species appear to suggest that Δ δ 18O and Δ δ 13C of surface and subsurface living foraminifera can be used as isotope indices of upwelling in the Arabian Sea.

Reconstructing past planktic foraminiferal habitats using stable isotope data: a case history for Mediterranean sapropel S5

A high-resolution stable O and C isotope study is undertaken on all planktic foraminiferal species that are reasonably continuous through an Eemian sapropel S5 from the western side of the eastern Mediterranean. The data are considered within a context of high-resolution isotope records for two further S5 sapropels from the central and easternmost sectors of the basin, alkenone-based sea surface temperature records for all three sapropels, and planktic foraminiferal abundance records for the same sample sets through all three sapropels. Results are compared with similar data for Holocene sapropel S1. The adopted approach allows distinction between species that are most suitable to assess overall changes in the climatic/hydrographic state of the basin, including depth-related differentiations and the main seasonal developments, and species that are most affected by variable biological controls or local/regional and transient physico–chemical forcings. It is found that a-priori assumptions about certain species’ palaeohabitats, based on modern habitat observations, may become biased when non-analogue conditions develop. In the case of Mediterranean sapropel S5, these consisted of enhanced freshwater dilution, elevated productivity, shoaling of the pycnocline between intermediate and surface waters, and stagnation of the subsurface circulation. Under these conditions, some species are found to ‘shift’ into habitat settings that differ considerably from those occupied today. The present multiple-species approach can identify such ‘anomalous responses’, and thus offers a sound background for further shell-chemistry investigations and quantitative interpretation of the isotopic profiles. We capitalise on the latter potential, and offer the first quantitative estimates of monsoon flooding into the Mediterranean during the deposition of Eemian sapropel S5.

Primary production in the Arabian Sea during the last 135 000 years

Variations in primary productivity (PP) have been reconstructed in eutrophic, mesotrophic and oligotrophic parts of the Arabian Sea over the past 135 000 years applying principal component analysis and transfer function to planktic foraminiferal assemblages. Temporal variation in paleoproductivity is most pronounced in the mesotrophic northern (NAST site) and oligotrophic eastern (EAST site) Arabian Sea, and comparatively weak in the western eutrophic GeoB 3011-1 site in the upwelling area off Oman. Higher PP during interglacials (250–320 g C m −2 year −1) than during cold stages (210–270 g C m −2 year −1) at GeoB 3011-1 could have been caused by a strengthened upwelling during intensified summer monsoons and increased wind velocities. At NAST, during interglacials, PP is estimated to exceed 250 g C m −2 year −1, and during glacials to be as low as 140–180 g C m −2 year −1. These fluctuations may result from a (1) varying impact of filaments that are associated to the Oman coastal upwelling, and (2) from open-ocean upwelling associated to the Findlater Jet. At EAST, highest productivity of about 380 g C m −2 year −1 is documented for the transition from isotope stage 5 to 4. We suggest that during isotope stages 2, 4, 5.2, the transition 5/4, and the end of stage 6, deep mixing of surface waters was caused by moderate to strong winter monsoons, and induced an injection of nutrients into the euphotic layer leading to enhanced primary production. The deepening of the mixed layer during these intervals is confirmed by an increased concentration of deep-dwelling planktic foraminiferal species. A high-productivity event in stage 3, displayed by estimated PP values, and by planktic foraminifera and radiolaria flux and accumulation rate, likely resulted from a combination of intensified SW monsoons with moderate to strong NE monsoons. Differential response of Globigerina bulloides, Globigerinita glutinata and mixed layer species to the availability of food is suited to subdivide productivity regimes on a temporal and spatial scale.

Guembelitria-dominated late Maastrichtian planktic foraminiferal assemblages mimic early Danian in central Egypt

During the late Maastrichtian (66.8–65.5 Ma) the Asyut Basin in central Egypt experienced a breakdown of the surface to bottom gradient of the 13C/ 12C ratio with planktic δ 13C values 0.2–0.8‰ lighter than benthic values. Planktic foraminiferal species diversity was reduced by more than 50%, with assemblages dominated (60–90%) by the opportunistic blooms of the disaster species Guembelitria cretacea, which mimic the early Danian. The prolonged breakdown in productivity occurred during a time of tectonic activity and increased terrestrial runoff that may have resulted in highly eutrophic waters, coupled with a sea-level regression (65.5 Ma) that led to restricted circulation. Increased productivity during the short climate warming between 65.4 and 65.2 Ma is associated with increased species diversity, abundant rugoglobigerinids and common heterohelicids. At the end of the Maastrichtian, decreased productivity coincided with the K/T impact and mass extinction, followed by characteristically early Danian low diversity assemblages. The similarity of the late Maastrichtian and post-K/T impact Guembelitria-dominated assemblages reveals that the planktic foraminiferal response to the K/T catastrophe was not unique, but followed a predictable pattern of response to severe environmental perturbations.

High-stress paleoenvironment during the late Maastrichtian to early Paleocene in Central Egypt

Biostratigraphic, mineralogical, geochemical and isotopic analyses of the Gebel Qreiya section in the Asyut Basin of central Egypt indicate a depositional environment interrupted by periods of erosion due to local tectonic activity exacerbated by eustatic sea-level fluctuations, and by high-stress environmental conditions akin to those normally experienced during the Cretaceous–Tertiary boundary transition. During the late Maastrichtian (66.8–65.4 Ma) this region experienced a breakdown of the biologically mediated surface to bottom gradient of the 13C/ 12C ratio with planktic δ 13C values 0.2–0.8‰ lighter than benthic values. Planktic foraminiferal species diversity was reduced by more than 50%, with faunal assemblages dominated (75–90%) by the opportunistic disaster species Guembelitria cretacea, which alternate with abundance of small, low oxygen-tolerant heterohelicids ( Heterohelix navarroensis, H. dentata, H. globulosa). This prolonged breakdown in ocean primary productivity occurred during a time of global climate cooling and sea-level regressions (at 66.8 and 65.5 Ma), though clay mineralogy suggests that locally low seasonality warm, wet, tropical and subtropical conditions prevailed. The high detrital influx suggests that the biologically high-stress environment was primarily linked to the existing shallow shelf conditions in southern Egypt, and possibly to local tectonic activity and restricted circulation. A normal carbon isotope gradient was briefly re-established during the short climate warming and rising sea level between 65.4 and 65.2 Ma, a time of increased species diversity, peak abundance of rugoglobigerinids and common heterohelicids. During the last 200 000 years of the Maastrichtian, increased precipitation and terrestrial runoff (increased phyllosilicates and kaolinite) and increasing total organic carbon values are associated with Heterohelix-dominated planktic foraminiferal assemblages. The K/T boundary is marked by a red clay layer and Ir anomaly of 5.4 ppb. During the early Danian, planktic foraminiferal populations and stable isotope data indicate that similarly fluctuating high-stress conditions prevailed in central Egypt as elsewhere in the marginal eastern Tethys.

Middle Eocene–Early Pliocene Subantarctic planktic foraminiferal biostratigraphy of Site 1090, Agulhas Ridge

The analysis of planktic foraminiferal assemblages from Site 1090 (ODP Leg 177), located in the central part of the Subantarctic Zone south of South Africa, provided a geochronology of a 330-m-thick sequence spanning the Middle Eocene to Early Pliocene. A sequence of discrete bioevents enables the calibration of the Antarctic Paleogene (AP) Zonation with lower latitude biozonal schemes for the Middle–Late Eocene interval. In spite of the poor recovery of planktic foraminiferal assemblages, a correlation with the lower latitude standard planktic foraminiferal zonations has been attempted for the whole surveyed interval. Identified bioevents have been tentatively calibrated to the geomagnetic polarity time scale following the biochronology of Berggren et al. (1995). Besides planktic foraminiferal bioevents, the disappearance of the benthic foraminifera Nuttallides truempyi has been used to approximate the Middle/Late Eocene boundary. A hiatus of at least 11.7 Myr occurs between ∼78 and ∼71 m composite depth extending from the Early Miocene to the latest Miocene–Early Pliocene. Middle Eocene assemblages exhibit a temperate affinity, while the loss of several planktic foraminiferal species by late Middle to early Late Eocene time reflects cooling. During the Late Eocene–Oligocene intense dissolution caused impoverishment of planktic foraminiferal assemblages possibly following the emplacement of cold, corrosive bottom waters. Two warming peaks are, however, observed: the late Middle Eocene is marked by the invasion of the warmer water Acarinina spinuloinflata and Hantkenina alabamensis at 40.5 Ma, while the middle Late Eocene experienced the immigration of some globigerinathekids including Globigerinatheka luterbacheri and Globigerinatheka cf. semiinvoluta at 34.3 Ma. A more continuous record is observed for the Early Miocene and the Late Miocene–Early Pliocene where planktic foraminiferal assemblages show a distinct affinity with southern mid- to high-latitude faunas.

The effect of upwelling on the distribution and stable isotope composition of Globigerina bulloides and Globigerinoides ruber (planktic foraminifera) in modern surface waters of the NW Arabian Sea

Hydrographic changes in the NW Arabian Sea are mainly controlled by the monsoon system. This results in a strong seasonal and vertical gradient in surface water properties, such as temperature, nutrients, carbonate chemistry and the isotopic composition of dissolved inorganic carbon ( δ 13C DIC). Living specimens of the planktic foraminifer species Globigerina bulloides and Globigerinoides ruber, were collected using depth stratified plankton tows during the SW monsoon upwelling period in August 1992 and the NE monsoon non-upwelling period in March 1993. We compare their distribution and the stable isotope composition to the seawater properties of the two contrasting monsoon seasons. The oxygen isotope composition of the shells ( δ 18O shell) and vertical shell concentration profiles indicate that the depth habitat for both species is shallower during upwelling (SW monsoon period) than during non-upwelling (NE monsoon period). The calcification temperatures suggest that most of the calcite is precipitated at a depth level just below the deep chlorophyll maximum (DCM), however above the main thermocline. Consequently, the average calcification temperature of G. ruber and G. bulloides is lower than the sea surface temperature by 1.7±0.8 and 1.3±0.9 °C, respectively. The carbon isotope composition of the shells ( δ 13C shell) of both species differs from the in situ δ 13C DIC found at the calcification depths of the specimens. The observed offset between the δ 13C shell and the ambient δ 13C DIC results from (1) metabolic/ontogenetic effects, (2) the carbonate chemistry of the seawater and, for symbiotic G. ruber, (3) the possible effect of symbionts or symbiont activity. Ontogenetic effects produce size trends in Δ δ 13C shell–DIC and Δ δ 18O shell–w: large shells of G. bulloides (250–355μm) are 0.33‰ ( δ 13C) and 0.23‰ ( δ 18O) higher compared to smaller ones (150–250 μm). For G. ruber, this is 0.39‰ ( δ 13C) and 0.17‰ ( δ 18O). Our field study shows that the δ 13C shell decreases as a result of lower δ 13C DIC values in upwelled waters, while the effects of the carbonate system and/or temperature act in an opposite direction and increase the δ 13C shell as a result lower [CO 3 2−] (or pH) values and/or lower temperature. The Δ δ 13C shell–DIC [CO 3 2−] slopes from our field data are close to those reported literature from laboratory culture experiments. Since seawater carbonate chemistry affects the δ 13C shell in an opposite sense, and often with a larger magnitude, than the change related to productivity (i.e. δ 13C DIC), higher δ 13C shell values may be expected during periods of upwelling.

Biostratigraphy and palaeoecological interpretation of the Miocene–Pleistocene sequence at El-Dabaa, northwestern Egypt

Abstract. Thirteen shallow boreholes were drilled by the Qattara Project Authority (QPA) on the top of the second limestone ridge in the El-Dabaa area, along the Mediterranean coast, northwest Egypt. Five foraminiferal biozones could be recognized in ascending stratigraphic order: the Heterostegina costata, Praeorbulina sicana, Borelis melo melo, Globigerinoides obliquus extremus zones and an un-named zone, embracing the Middle Miocene Marmarica Formation and the Pliocene–Pleistocene Alexandria Formation.The deposition of the fossiliferous carbonate rocks of the Marmarica Formation probably took place in a warm water, inner shelf environment (0–20 m palaeodepth) subjected to some current activity, and with salinity ranging from normal to slightly hypersaline (35–50‰). The sediments of the oncolitic/shelly rocks were deposited in slightly deeper water than the overlying fossiliferous rocks and in less agitated conditions, as indicated from the high percentages of rotaliids and rarity of miliolids.The foraminiferal associations in the Alexandria Formation indicate a clear, relatively agitated, near-shore marine depositional environment in a vegetated inner neritic zone (10–25 m palaeodepth) with 25°C surface water isotherms. On the other hand, the accumulation of the clayey sediments and some geosoils (basal part of the Alexandria Formation) occurred in less agitated, intertidal water conditions, as deduced from the occurrence of planktic foraminiferal species as Globigerinoides trilobus trilobus, G. trilobus immaturus, G. obliquus extremus, G. obliquus obliquus, Globorotalia inflata, and Orbulina universa.

The Toba Volcanic Event and Interstadial/Stadial Climates at the Marine Isotopic Stage 5 to 4 Transition in the Northern Indian Ocean

The Toba volcanic event, one of the largest eruptions during the Quaternary, is documented in marine sediment cores from the northeastern Arabian Sea. On the crest of the Murray Ridge and along the western Indian continental margin, we detected distinct concentration spikes and ash layers of rhyolithic volcanic shards near the marine isotope stage 5–4 boundary with the chemical composition of the “Youngest Toba Tuff.” Time series of the Uk′37-alkenone index, planktic foraminiferal species, magnetic susceptibility, and sediment accumulation rates from this interval show that the Toba event occurred between two warm periods lasting a few millennia. Using Toba as an instantaneous stratigraphic marker for correlation between the marine- and ice-core chronostratigraphies, these two Arabian Sea climatic events correspond to Greenland interstadials 20 and 19, respectively. Our data sets thus depict substantial interstadial/stadial fluctuations in sea-surface temperature and surface-water productivity. We show that variable terrigenous (eolian) sediment supply played a crucial role in transferring and preserving the productivity signal in the sediment record. Within the provided stratigraphic resolution of several decades to centennials, none of these proxies shows a particular impact of the Toba eruption. However, our results are additional support that Toba, despite its exceptional magnitude, had only a minor impact on the evolution of low-latitude monsoonal climate on centennial to millennial time scales.

Micropaleontology and sedimentology across the Cretaceous/Tertiary boundary at La Ceiba (Mexico): impact-generated sediment gravity flows

A micropaleontological and sedimentological study across the Cretaceous/Tertiary boundary-officially Cretaceous/Paleogene (K/P) boundary from the La Ceiba section (Mexico) was performed to examine the K/P planktic foraminiferal biostratigraphy, the sedimentology of a controversial K/P clastic unit, and the benthic and planktic foraminiferal assemblages turnover across this boundary. The clastic unit is stratigraphically placed between two pelagic marly units (Méndez and Velasco Formations) and displays a fining-upward gradation similar to a turbidite sequence. This K/P clastic unit contains a basal subunit consisting of calcareous marls rich in millimeter-sized spherules (microtektites) altered to clay minerals, abundant detrital quartz, mica minerals, and shocked quartz. According to the K/P stratotype definition from El Kef (Tunisia), the K/P boundary at La Ceiba must be placed at the base of the clastic (microspherules) unit since it is equivalent to the base of the boundary clay at El Kef. A short hiatus affects the lower part of the Danian, including the Guembelitria cretacea and Parvularugoglobigerina eugubina biozones and the lower part of the Parasubbotina pseudobulloides biozone. Nearly all commonly recorded Maastrichtian planktic foraminiferal species were found in the uppermost Maastrichtian interval, and there was no support for a gradual mass extinction pattern in the terminal Cretaceous. Benthic foraminiferal assemblages suggest that the La Ceiba section was deposited at lower bathyal depths. Oscillating megatsunami waves and/or a sea-level lowstand cannot explain the nature of the clastic deposits because of the observed deposition paleodepth (more than 1000m). There is also evidence that the clastic unit was deposited under a high-sedimentation rate in upper flow regimes and that was emplaced as a single-pulse event as turbidites. This datum and other sedimentological features support a sediment gravity flow genesis for the clastic unit. All these results are consistent with the K/P impact theory and the asteroid impact on the Yucatan Peninsula.

Stable isotope composition (δ18O, δ13C) of living planktic foraminifers in the outer Laptev Sea and the Fram Strait

The upper water column in the Fram Strait and the outer Laptev Sea was sampled for water column isotopes and living planktic foraminifer species Neogloboquadrina pachyderma (sinistral coiling) (Ehrenberg), 1861 and Turborotalita quinqueloba (Natland),1938. Their shell δ18O and δ13C values are compared to water oxygen and dissolved inorganic carbon isotope data to determine the environmental influence on the foraminifers' isotopic ratio. Major controls on the oxygen isotope composition of both species are the shallow depth habitat under permanent ice coverage, the low salinity surface layer, and the rate of metabolic activity. None of the specimens precipitated its shell in isotopic equilibrium with the ambient sea water. They are all depleted in 13C and 18O, attributed to a species-specific vital effect. For nonencrusted N. pachyderma (sin.) in the 125–250μm size class, this vital effect amounts to 1.3‰ in δ18O and 2.0‰ in δ13C. It increases to higher values in waters under permanent ice cover. T. quinqueloba reveals a mean vital effect of about 1.3‰ in δ18O and 2.6‰ in δ13C. The general isotopic trends are similar for N. pachyderma (sin.) and T. quinqueloba. Differences in the species' isotope ratio at the same sites are caused by different calcification depths and metabolic activity. The oxygen isotope composition of N. pachyderma (sin.) shows a relationship to salinity measurements and indicates that it is a good quantitative proxy for salinity reconstructions, while no relationship exists in this region between N. pachyderma (sin.) oxygen isotopes and water temperature.