Showing papers on "Foraminifera published in 1996"

Pore fluid constraints on the temperature and oxygen isotopic composition of the glacial ocean

Abstract: Pore fluids from the upper 60 meters of sediment 3000 meters below the surface of the tropical Atlantic indicate that the oxygen isotopic composition (δ 18 O) of seawater at this site during the last glacial maximum was 0.8 ± 0.1 per mil higher than it is today. Combined with the δ 18 O change in benthic foraminifera from this region, the elevated ratio indicates that the temperature of deep water in the tropical Atlantic Ocean was 4°C colder during the last glacial maximum. Extrapolation from this site to a global average suggests that the ice volume contribution to the change in δ 18 O of foraminifera is 1.0 per mil, which partially reconciles the foraminiferal oxygen isotope record of tropical sea surface temperatures with estimates from Barbados corals and terrestrial climate proxies.

Evolution patterns of radiolaria and organic matter variations: A new approach to identify sea-level changes in mid-Cretaceous pelagic environments

Abstract: The relation between sea-level changes, plankton productivity, and evolution, as well as the occurrence of anoxic sediments, provides an interesting avenue of paleooceanographic research. In this context, we examined mid-Cretaceous radiolarian faunas, carbonate isotopic and organic matter type data of Deep Sea Drilling Project and Ocean Drilling Program cores from the North Atlantic, and samples from outcrops of the western Tethys from central and northern Italy. Former studies indicate that an expansion of the oxygen minimum zone caused plankton extinctions at the Cenomanian-Turonian boundary. An expanded oxygen minimum zone would destroy deeper habitats of planktic foraminifera, causing the extinction of deeper dwelling forms. Although this model is well established for the Cenomanian-Turonian boundary, not much is known about the causes of extinctions and radiations during the entire mid-Cretaceous (Aptian-Turonian). We demonstrate that the dimension of the oxygen minimum zone, which depends on the relative sea level and the corresponding nutrient supply, causes the complex pattern of evolution and radiation of planktic protozoa and the sedimentation of black shales in the mid-Cretaceous. This new depositional model allows correlation of micropaleontologic data and different types of black shales in the pelagic realm within a sequence stratigraphic framework.

Rapid diversification of planktonic foraminifera in the tropical Pacific (ODP Site 865) during the late Paleocene thermal maximum

Abstract: The planktonic foraminiferal generaMorozovellaandAcarininarapidly (in ;10 k.y.) diversified during the late Paleocene thermal maximum (LPTM), giving rise to such morphotypes asM. allisonensis(new species),M. africana, andA. sibaiyaensis. Single-specimen isotopicanalysisconfirmsthatM.allisonensisandA.sibaiyaensisarerestrictedtotheLPTM carbon isotope excursion recorded at Ocean Drilling Program Site 865 (equatorial Pacific Ocean). The short-lived (50 to several 100 k.y.) ‘‘excursion’’ taxa attest to the ephemeral effectsoftheLPTMonthecalcareousplankton.Single-specimenoxygenisotopedatashow that evolution of M. allisonensis and A. sibaiyaensis was accompanied by migration to deeper water depths. AncestralM. velascoensisandA. soldadoensiswere extremely rare or absent during the early stages of the LPTM, but immigrated back into the study area to coexist with their descendants in later LPTM horizons. Photosymbiosis may have facilitatedthemorozovellidandacarininiddiversificationsduringtheoligotrophicconditionsof the LPTM.

Seasonal oxygen depletion in continental-shelf waters of Louisiana: Historical record of benthic foraminifers

Abstract: A strong spring and summer oxygen depletion is induced in nearshore bottom waters of the Louisiana continental shelf by density stratification and by the carbon flux from phytoplankton production, which, in turn, is related to the nutrient load of the Mississippi and Atchafalaya rivers. In an attempt to read the historical record of this shelf hypoxia during the past two centuries, we compared the stratigraphic signals of benthic foraminifera (as reflected in a relative-dominance index for two common species of Ammonia and Elphidium ) in 210 Pb-dated cores, and we found evidence of an overall rise in oxygen stress (in intensity or duration), especially in the past 100 yr. This implies a progressive increase in the influence of river-borne nutrients, particularly anthropogenically influenced nitrates. Judging by our results, foraminiferal indices based on appropriate species ratios should prove useful in testing hypotheses about long-term environmental stresses, including eutrophication and paleohypoxia, on other marine shelves.

Pre‐bomb radiocarbon and the reservoir correction for calcareous marine species in the Southern Ocean

Abstract: Four Antarctic marine mollusc shells, which were collected alive between 1917 and 1940, were analyzed by accelerator mass spectrometry to provide the first pre-bomb radiocarbon measurements of biogenic carbonates from the Southern Ocean. After correcting for the impact of fossil fuel combustion (Suess Effect), radiocarbon activities of the pre-bomb shells averaged −149.8±10.4‰. In contrast, the Δ 14C values for post-bomb molluscs, echinoderms, brachiopods and foraminifera averaged −96.1±25.2‰. These biogenic carbonate Δ 14C values are nearly identical to pre-bomb estimates (-148‰ to −152‰) and post-bomb measurements (−98.4±22.0‰) of the surface waters in the Southern Ocean. Average radiocarbon ages of the biogenic carbonates before and after 1950 (1303±84 years and 811±205 years, respectively), along with those from seals and penguins, indicate that the Antarctic marine radiocarbon reservoir has decreased in age by nearly 500 years during the second half of the 20th century. Marine species and seawater measurements firmly place the radiocarbon reservoir correction at 1300±100 years for calcareous marine fossils which are widespread, abundant and well-preserved organic materials for interpreting ice-sheet, climate and sea level impacts on the Antarctic marine ecosystem during the Holocene.

Rapid changes in surface and deep water conditions at the Faeroe Margin during the last 58,000 years

Abstract: A high-resolution piston core, ENAM93-21, from a water depth of 1020 m near the Faeroe-Shetland Channel is investigated for variations in magnetic susceptibility, surface oxygen isotopes, grain size distribution, content of ice-rafted detritus (IRD), and distribution of planktonic and benthic foraminifera. The core, covering the last 58,000 years, is correlated with the Greenland ice cores and compared with paleorecords from the Norwegian Sea and the North Atlantic Ocean. All fifteen Dansgaard-Oeschger climatic cycles recognized from the investigated time period in the Greenland ice cores have been identified in the ENAM93-21 core. Each cycle is subdivided into three intervals on the basis of characteristic benthic and planktonic faunas. Interstadial intervals contain a relatively warm planktonic fauna and a benthic fauna similar to the modern fauna in the Norwegian Sea. This indicates thermohaline convection as at present, with a significant contribution of deep water to the North Atlantic Deep Water (NADW). Transitional cooling intervals are characterized by more cold water planktonic foraminfera and ice-related benthic species. The benthic fauna signifies restricted bottom water conditions and a reduced contribution to the NADW. The peak abundance of N. pachyderma (s.) and the coldest surface water conditions are found in the stadial intervals. The benthic fauna is dominated by species with an association to Atlantic Intermediate Water, suggesting an increased Atlantic influence in the Norwegian Sea, and there was probably no contribution to the NADW through the Faeroe-Shetland Channel. The three different modes of circulation can be correlated to paleoceanographic events in the Norwegian Sea and the North Atlantic Ocean.

Environmental change in eastern Greenland during the last 1300 years: evidence from foraminifera and lithofacies in Nansen Fjord, 68°N

Abstract: Lithofacies and benthic foraminiferal analyses of two sediment cores (BS1191-K13B and K14) from Nansen Fjord, eastern Greenland, show evidence of changing oceanographic and sea-ice conditions from ...

Drilling and dating New Jersey oligocene-miocene sequences: Ice volume, global sea level, and Exxon records

Abstract: Oligocene to middle Miocene sequence boundaries on the New Jersey coastal plain (Ocean Drilling Project Leg 150X) and continental slope (Ocean Drilling Project Leg 150) were dated by integrating strontium isotopic stratigraphy, magnetostratigraphy, and biostratigraphy (planktonic foraminifera, nannofossils, dinocysts, and diatoms). The ages of coastal plain unconformities and slope seismic reflectors (unconformities or stratal breaks with no discernible hiatuses) match the ages of global δ18O increases (inferred glacioeustatic lowerings) measured in deep-sea sites. These correlations confirm a causal link between coastal plain and slope sequence boundaries: both formed during global sea-level lowerings. The ages of New Jersey sequence boundaries and global δ18O increases also correlate well with the Exxon Production Research sea-level records of Haq et al. and Vail et al., validating and refining their compilations.

A High-resolution record of Late Quaternary upwelling along the Oman Margin, Arabian Sea based on planktonic foraminifera

Abstract: Planktonic foraminiferal abundances, fluxes, test sizes, and coiling properties are influenced in various ways by the southwest monsoon winds and associated upwelling in the western Arabian Sea. To determine the short-term changes in the southwest monsoon, we have carried out a high-resolution time-series analysis of three upwelling indices (total flux of planktonic foraminiferal tests and flux and relative abundance of the planktonic foraminiferal species Globigerina bulloides) from Ocean Drilling Program (ODP) Site 723A (Oman Margin, western Arabian Sea) spanning the last 19 kyr. In addition, we have determined the relationships between upwelling intensity and the relative abundance, fluxes, and shell concentrations of various planktonic foraminiferal species. Upwelling indices suggest that from 19 to 16 ka (22 to 18.2 cal kyr B.P.) the SW monsoon was relatively strong compared to the period 15.8 to 12.5 ka (17.8 to 13.8 cal kyr B.P.). The intensification of the SW monsoon took place at 12 ka (13.1 cal kyr B.P.) and reached a peak between 10 and 5 ka (10.6 and 4.8 cal kyr B.P.). The high-resolution data further demonstrate that the SW monsoon has started weakening from 5 ka (4.8 cal kyr B.P.) and the weakest phase was in place at 3.5 ka (3 cal kyr B.P.), which coincides with evidence of an arid climate in western Tibet. Fluxes and shell concentrations of many of the planktonic foraminiferal species increased between 12 and 5 ka in response to the intensification of the SW monsoon winds after the last glacial period. Globigerina bulloides shows a fivefold to tenfold increase in flux during this period of intense upwelling. The other species whose fluxes are influenced by this upwelling change are (in order from strongest to weakest response) Globigerinita glutinata, Globigerinoides ruber, Neogloboquadrina dutertrei, Globigerinella aequilateralis, Globigerina falconensis, and Globigerinoides sacculifer. The relative abundances of G. bulloides and G. ruber increased during intense upwelling, whereas the relative abundances of G. glutinata, N. dutertrei, G. falconensis, and G. sacculifer did not increase during this period, which might be due to differences in the productivity of various species in relation to upwelling change. Therefore the fluxes and shell concentrations provide better and more reliable information about the changes in the monsoon system in the Arabian Sea than relative abundance data.

Latest Paleocene benthic foraminiferal extinction and environmental changes at Tawanui, New Zealand

Abstract: A major extinction of intermediate-water (500–1000 m) benthic foraminiferal species coincided with a major decrease in δ13C (2.8‰) of terrestrial organic matter (n-C29 alkane) and δ34S (20‰) of whole rock sulfide in a continuous siltstone sequence in the Tawanui Section (46°S paleolatitude) along the Akitio River, southeastern North Island, New Zealand, in the middle part of the uppermost Paleocene nannofossil zone (CP8). The benthic extinction (25% of species) occurred over ∼3 kyr at ∼55.5 Ma. Increases in kaolinite/illite and kaolinite/smectite ratios and in terrestrial organic carbon percentages started ∼3 kyr before the major benthic extinctions, lasted over ∼40 kyr, and probably reflect warmer climate and increased rainfall. The productivity of planktonic foraminifera and calcareous nannoplankton decreased ∼3 kyr prior to the major extinctions and recovered at the time of benthic extinctions. These events that started ∼3 kyr before the extinction can be best explained by warming, increased rainfall, reduced salinity of surface waters, and increased influence of warm saline deep water (WSDW). Benthic foraminiferal oxygen indices indicate a strong decrease in dissolved oxygen levels within the intermediate water from low oxic (1.5–3.0 mL/L O2) to suboxic (0.3–1.5 mL/L O2) conditions coinciding with the benthic extinctions. Increases in total organic carbon (TOC) and in the hydrocarbon-generating potential of kerogen (measured as the hydrogen index (HI)) agree with the interpretation of decreased dissolved oxygen levels of the intermediate water. The lowest oxygen conditions lasted ∼40 kyr and coincided with a decrease in calcareous benthic foraminiferal productivity, highest TOC levels, and lowest δ13C of terrestrial organic carbon. Dominant formation of WSDW or sluggish intermediate-water circulation caused by warming and high rainfall in high-latitude areas most likely led to the ∼3-kyr time lag between events on land and in surface waters preceeding the extinction and the development of dysaerobia in the sea, coinciding with the major benthic extinction and decrease in δ13C and δ34S in New Zealand. Global warming of deep and intermediate waters may have caused decomposition of methane hydrate in sediments, resulting in a strongly decreased δ13C of marine carbonates, promoting dysaerobia in the ocean, and warming global climate by increased methane concentrations in the atmosphere. Upwelling of WSDW, occurring soon after it became dominant in high-latitude areas, is likely responsible for the recovery of normal salinity and the concomitant recovery of planktonic foraminifera and calcareous nannoplankton productivity in high-latitude surface waters. Minor benthic foraminiferal extinctions (9% of species) occurred ∼40 kyr after the major extinctions, lasted ≤ ∼6 kyr, and coincided with the initiation of environmental recovery.

Hydrographic changes of the Southern Ocean (southeast Indian Sector) Over the last 230 kyr

Abstract: Hydrographical changes of the southern Indian Ocean over the last 230 kyr, is reconstructed using a 17-m-long sediment core (MD 88 770; 46°01′S 96°28′E, 3290m). The oxygen and carbon isotopic composition of planktonic (N. pachyderma sinistra and G. bulloides) and benthic (Cibicidoides wuellerstorfi, Epistominella exigua, and Melonis barleeanum) foraminifera have been analysed. Changes in sea surface temperatures (SST) are calculated using diatom and foraminiferal transfer functions. A new core top calibration for the Southern Ocean allows an extension of the method developed in the North Atlantic to estimate paleosalinities (Duplessy et al., 1991). The age scale is built using accelerator mass spectrometry (AMS) 14C dating of N. pachyderma s. for the last 35 kyr, and an astronomical age scale beyond. Changes in surface temperature and salinity clearly lead (by 3 to 7 kyr) deep water variations. Thus changes in deep water circulation are not the cause of the early response of the surface Southern Ocean to climatic changes. We suggest that the early warming and cooling of the Southern Ocean result from at least two processes acting in different orbital bands and latitudes: (1) seasonality modulated by obliquity affects the high-latitude ocean surface albedo (sea ice coverage) and heat transfer to and from the atmosphere; (2) low-latitude insolation modulated by precession influences directly the atmosphere dynamic and related precipitation/ evaporation changes, which may significantly change heat transfer to the high southern latitudes, through their control on latitudinal distribution of the major frontal zones and on the conditions of intermediate and deep water formation.

Comparisons of the ecology and stable isotopic compositions of living (stained) benthic foraminifera from the Sulu and South China Seas

Abstract: Significant differences are observed between living (Rose Bengal stained) deep-sea benthic foraminifera found in 14 box cores (510–4515 m) from the thermospheric (> 10°C) environments of the Sulu Sea and the psychrospheric ( 2‰ range and are similar to those presented by previous workers, but have no consistent relationship with microhabitat preferences. Vertical distribution patterns and carbon isotope compositions of species, however, reflect microhabitat preferences and are consistent with previous observations from other regions. Epifaunal species (0–1 cm interval) such as Cibicidoides pachyderma, Cibicidoides wuellerstorfi, Hoeglundina elegans and Anomalinoides colligera, have higher δ13C values than taxa which have the ability to live deeper within the sediments. Infaunal taxa that live in the upper 2–3 cm, including Uvigerina peregrina, Uvigerina proboscidea, and Bulimina mexicana, have lower δ13C values than epifaunal species, and the deep infaunal species, Chilostomella oolina, has the lowest δ13C. Cibicidoides bradyi and Oridorsalis umbonatus are found between 0 and ∼ 4 cm and have lower carbon isotope values (by > 1.4‰ in some cores) than epifaunal Cibicidoides species. Exceptions to this pattern include the aragonitic species, Gavelinopsis lobatulus, (0–4 cm) which produces significantly lower δ13C values than deep infaunal taxa, and the shallow infaunal species, Ceratobulimina pacifica (also aragonitic) and Bolivinopsis cubensis (deep infaunal), which yield higher carbon isotopic values than epifaunal taxa. These exceptions are found primarily in only one core, and additional samples are needed to confirm the relationship between their distribution patterns and isotopic compositions. Each of the species examined has a relatively consistent δ13C value throughout its distribution within the sediments that may result from heterogeneity of microhabitats within the intervals sampled. Intrageneric differences in δ13C of Cibicidoides, and possibly Uvigerina and Bulimina, are evident. The isotopic differences between C. bradyi and many other Cibicidoides species are related to differences in microhabitat preferences between species. The δ13C results confirm the influence of microhabitat preferences on the carbon isotopic composition of deep-sea benthic foraminifera and reaffirm the importance of assessing the microhabitat preferences of species used for isotopic analyses.

Earliest evolution associated with closure of the Tropical American Seaway

Abstract: Oceanographic changes caused by the emerging Central American isthmus, which completely severed connections between the Caribbean Sea and tropical Pacific Ocean about 3.5 million years ago, began to stimulate evolution of Caribbean reef corals and benthic foraminifera in the Late Miocene. At that time, first appearances of benthic foraminifera increased, especially those species strongly associated with carbonate-rich substrata; reef corals diversified dramatically; and the carbonate content of southern Caribbean deep-sea sediments increased. We suggest that the changes in marine environments caused by the constricting seaway and resulting in increasing carbonate content of sediments induced accelerated origination in reef corals and carbonate-associated benthic foraminifera.

Three hundred eighty thousand year long stable isotope and faunal records from the Red Sea: Influence of global sea level change on hydrography

Abstract: Stable isotope and faunal records from the central Red Sea show high-amplitude oscillations for the past 380,000 years. Positive δ18O anomalies indicate periods of significant salt buildup during periods of lowered sea level when water mass exchange with the Arabian Sea was reduced due to a reduced geometry of the Bab el Mandeb Strait. Salinities as high as 53‰ and 55‰ are inferred from pteropod and benthic foraminifera δ18O, respectively, for the last glacial maximum. During this period all planktonic foraminifera vanished from this part of the Red Sea. Environmental conditions improved rapidly after 13 ka as salinities decreased due to rising sea level. The foraminiferal fauna started to reappear and was fully reestablished between 9 ka and 8 ka. Spectral analysis of the planktonic δ18O record documents highest variance in the orbital eccentricity, obliquity, and precession bands, indicating a dominant influence of climatically - driven sea level change on environmental conditions in the Red Sea. Variance in the precession band is enhanced compared to the global mean marine climate record (SPECMAP), suggesting an additional influence of the Indian monsoon system on Red Sea climates.

Living planktic foraminifera: tracers of circulation and productivity regimes in the central equatorial Pacific

Abstract: Planktic foraminifera (shelled protozoans from ∼0.01 to 1 mm in size) respond to equatorial circulation and ecosystem dynamics. In the JGOFS survey I cruise of the equatorial Pacific (9°N-12°S, 140°W, in February–March 1992), responses to upwelling, advection, and biological activity occurred in spite of little upper-ocean temperature contrast. Rather than being abundant within the entire productive equatorial zone, foraminifera concentrated off the equator at convergent fronts. For example, non-spinose, mostly herbivorous species (G. conglomerata, G. tumida, P. obliquiloculata, and N. dutertrei) dominated near 3°N, in the convergence between the South Equatorial Current and the North Equatorial Countercurrent. Juvenile forms outnumbered adults within the convergence, indicating that these foraminifera succeeded and reproduced here (rather than passively accumulating by advection) perhaps by maintaining buoyancy to stay within the convergent, food-rich zone. The South Equatorial Current was favored by spinose, endosymbiont-bearing G. aequilateralis and non-spinose, herbivorous G. glutinata, G. menardii, and P. obliquiloculata, perhaps an advected assemblage. Species hosting dinoflagellate endosymbionts (G. sacculifer, G. ruber, and G. conglobatus) prevailed in food-poor oligotrophic regions, perhaps because they obtain nutrition from their symbionts. Distributions of living foraminifera suggest that paleoceanographic transfer functions to estimate primary productivity in the geological record have merit, but controls of foraminiferal species distributions also include food stocks, light intensity, and advection.

The progressive intensification of northern hemisphere glaciation as seen from the North Pacific

Abstract: Ocean Drilling Project (ODP) site 882 (50°22′N, 167°36′E) provides the first high-resolution GRAPE density, magnetic susceptibility, carbonate, opal and foraminifera (planktonic and benthic) stable isotopes records between 3.2 and 2.4 Ma in the Northwest Pacific. We observed a dramatic increase in ice rafting debris at site 882 at 2.75 Ma, which is coeval with that found in the Norwegian Sea, suggesting that the Eurasian Arctic and Northeast Asia were significantly glaciated from 2.75 Ma onwards. Prior to 2.75 Ma planktonic foraminifera δ18O records indicate a warming or freshening trend of 4°C or 2‰ over 80 ka. If this is interpreted as a warm pre-glacial Pliocene North Pacific, it may have provided the additional moisture required to initially build up the northern hemisphere continental ice sheet. The dramatic drop in sea surface temperatures (SST>7.5°C) at 2.75 Ma ended this suggested period of enhanced SST and thus the proposed moisture pump. Moreover, at 2.79 and 2.73 Ma opal mass accumulation rates (MAR) decrease in two steps by five fold and is accompanied by a more gradual long-term decrease in CaCO3 MARs. Evidence from the Southern Ocean (ODP site 704) indicates that just prior to 2.6 Ma there is a massive increase in opal MARs, the opposite to what is found in the North Pacific. This indicates that the intensification of northern hemisphere glaciation was accompanied by a major reorganisation of global oceanic chemical budget, possibly caused by changes in deep ocean circulation. The initiation of northern hemisphere glaciation occurred in the late Miocene with a significant build up of ice on southern Greenland. However, the progressive intensification did not occur until 3.5–3 Ma when the Greenland ice sheet expanded to include northern Greenland. Following this stage we suggest that the Eurasian Arctic and Northeast Asia glaciated at 2.75 Ma, approximately 100 ka before the glaciation of Alaska (2.65 Ma) and 200 ka before the glaciation of the North East American continent (2.54 Ma).

Extinction and survivorship of southern Tethyan Benthic foraminifera across the Cretaceous/Palaeogene boundary

Abstract: Abstract The benthic foraminiferal record from the Cretaceous/Palaeogene boundary stratotype of El Kef, Tunisia, shows a succession of three distinct assemblages. The late Maastrichtian upper bathyal assemblage is highly diversified and shows no prominent signs of gradual change towards the boundary. The earliest Palaeocene is marked by the disappearance of more than 50% of the taxa, resulting in a strongly impoverished fauna, tolerant to low oxygen conditions and with a shallower water affinity. Sequential (re-) appearance of many taxa in the early Palaeocene signifies the restoration towards normal Palaeocene upper bathyal faunas. The faunal changes reflect major perturbations in redox and trophic conditions at the sea-floor. At least locally, and perhaps even on a regional Tethyan scale, the extinctions can be related to a sharp decrease in oxygen supply, in combination with strongly reduced nutrient resources. It is suggested that a prolonged reduction in surface fertility and food flux to the sea-floor invoked worldwide (but diachronous) benthic extinctions. In particular endo-benthic deposit feeders and other taxa adapted to high and perhaps heterogeneous nutrient resources suffered extinction.

Decadal-scale changes in benthic foraminiferal assemblages off Key Largo, Florida

Abstract: Assemblages of foraminiferal tests in sediments sampled off Key Largo, Florida, in 1982, 1991, and 1992 were significantly different from assemblages sampled along the same traverses in 1959–1961. Larger, algal symbiont-bearing taxa, primarily Soritidae, comprised 50–80% of the specimens in samples collected in 1959–1961, whereas Miliolidae and Rotaliidae comprised 65-90% of the specimens collected in 1991 and 1992. Test abundance in 1992 samples ranged from 1.0 × 102/g to 8.1 × 104/g; tests were least abundant in coarse, well-sorted sediments. The lack of test-density data for the 1959-1961 samples prevented assessment of whether densities of smaller foraminifera have increased, symbiotic foraminifera have decreased, or both. Between 1982 and 1992, densities of smaller foraminifera appear to have increased. Although the causes of these changes in foraminiferal assemblages are not known, possible factors include nutrient loading inshore, winnowing and transport of tests by storm activity, and disease. The shift in dominance from long-lived, algal symbiont-bearing taxa in 1959--1961 to small, fast-growing, heterotrophic taxa in 1992 is consistent with predictions of community response to gradually increasing nutrient flux into south Florida's coastal waters. This study indicates that published accounts of foraminiferal assemblages from sediments collected 30 or more years ago can be valuable resources in efforts to determine if biotic changes have occurred in coastal ecosystems. This study also indicates that family-level identifications may be sufficient to detect decadal-scale changes in foraminiferal assemblages in reef-tract sediments.

Latest Paleocene benthic extinction event on the southern Tethyan shelf (Egypt): Foraminiferal stable isotopic (δ13C, δ18O) records

Abstract: The dramatic global extinction of 35%–50% of benthic foraminifera species in the deep sea in the latest Paleocene and associated negative excursions in δ 13 C and δ 18 O may be related to spreading of warm, saline bottom water from subtropical Tethyan shallow regions over the sea floor worldwide. Our study of neritic sections in Egypt shows that in the southern shallow Tethys, a prominent long-term change in bottom-water chemistry, sedimentation, and benthic foraminifera fauna was initiated at the time when the deep-sea benthic extinction event (BEE) took place. Bottom-water δ 13 C values on the Tethyan shelf show a sudden 3.0‰ negative shift at this event; however, contrary to the deep sea, in which the δ 13 C excursion was of short duration, Tethyan δ 13 C values did not fully return to preboundary values, but remained depressed by ∼1.5‰ for at least 1 m.y. The δ 13 C values at the Egyptian shelf during the BEE are much lower than would be expected if this was a source region for global deep water. The δ 18 O values indicate no significant change in bottom-water salinity or temperature at the BEE. The long-lasting environmental changes that began on the Egyptian shelf at the BEE may be related to, for example, gateway reorganization along the Tethyan seaway. Paleogeographic changes possibly also triggered a change in the loci of global deep-water formation; however, these loci must be sought in another part of the Tethys.

Ventilation changes in the northeast Pacific during the Last Deglaciation

Abstract: Under present climate conditions, convection at high latitudes of the North Pacific is restricted to shallower depths than in the North Atlantic. To what extent this asymmetry between the two ocean basins was maintained over the past 20 kyr is poorly known because there are few unambiguous proxy records of ventilation from the North Pacific. We present new data for two sediment cores from the California margin at 800 and 1600 m depth to argue that the depth of ventilation shifted repeatedly in the northeast Pacific over the course of deglaciation. The evidence includes benthic foraminiferal Cd/Ca, 18O/16O, and 13C/12C data as well as radiocarbon age differences between benthic and planktonic foraminifera. A number of features in the shallower of the two cores, including an interval of laminated sediments, are consistent with changes in ventilation over the past 20 kyr suggested by alternations between laminated and bioturbated sediments in the Santa Barbara Basin and the Gulf of California [Keigwin and Jones, 1990; Kennett and Ingram, 1995; Behl and Kennett, 1996]. Data from the deeper of the two California margin cores suggest that during times of reduced ventilation at 800 m, ventilation was enhanced at 1600 m depth, and vice versa. This pronounced depth dependence of ventilation needs to be taken into account when exploring potential teleconnections between the North Pacific and the North Atlantic.