Showing papers on "Foraminifera published in 2000"

Cenozoic deep-sea temperatures and global ice volumes from Mg/Ca in benthic foraminiferal calcite

Abstract: A deep-sea temperature record for the past 50 million years has been produced from the magnesium/calcium ratio (Mg/Ca) in benthic foraminiferal calcite. The record is strikingly similar in form to the corresponding benthic oxygen isotope (δ18O) record and defines an overall cooling of about 12°C in the deep oceans with four main cooling periods. Used in conjunction with the benthic δ18O record, the magnesium temperature record indicates that the first major accumulation of Antarctic ice occurred rapidly in the earliest Oligocene (34 million years ago) and was not accompanied by a decrease in deep-sea temperatures.

Past temperature and d18O of surface ocean waters inferred from foraminiferal Mg/Ca ratios.

Abstract: Determining the past record of temperature and salinity of ocean surface waters is essential for understanding past changes in climate, such as those which occur across glacial-interglacial transitions. As a useful proxy, the oxygen isotope composition (delta18O) of calcite from planktonic foraminifera has been shown to reflect both surface temperature and seawater delta18O, itself an indicator of global ice volume and salinity. In addition, magnesium/calcium (Mg/Ca) ratios in foraminiferal calcite show a temperature dependence due to the partitioning of Mg during calcification. Here we demonstrate, in a field-based calibration experiment, that the variation of Mg/Ca ratios with temperature is similar for eight species of planktonic foraminifera (when accounting for Mg dissolution effects). Using a multi-species record from the Last Glacial Maximum in the North Atlantic Ocean we found that past temperatures reconstructed from Mg/Ca ratios followed the two other palaeotemperature proxies: faunal abundance and alkenone saturation. Moreover, combining Mg/Ca and delta18O data from the same faunal assemblage, we show that reconstructed surface water delta18O from all foraminiferal species record the same glacial-interglacial change--representing changing hydrography and global ice volume. This reinforces the potential of this combined technique in probing past ocean-climate interactions.

Foraminifera in the Arabian Sea oxygen minimum zone and other oxygen-deficient settings: taxonomic composition, diversity, and relation to metazoan faunas

Abstract: Previous work has shown that some foraminiferal species thrive in organically enriched, oxygen-depleted environments. Here, we compare ‘live’ (stained) faunas in multicorer samples (0–1 cm layer) obtained at two sites on the Oman margin, one located at 412 m within the oxygen minimum zone (OMZ) (O2=0.13 ml l?1), the other located at 3350 m, well below the main OMZ (O2~3.00 ml l?1). While earlier studies have focused on the hard-shelled (predominantly calcareous) foraminifera, we consider complete stained assemblages, including poorly known, soft-shelled, monothalamous forms. Densities at the 412-m site were much higher (16,107 individuals.10 cm?2 in the >63-m fraction) than at the 3350-m site (625 indiv.10 cm?2). Species richness (E(S100)), diversity (H?, Fishers Alpha index) and evenness (J?) were much lower, and dominance (R1D) was higher, at 412 m compared with 3350 m. At 412 m, small calcareous foraminifera predominated and soft-shelled allogromiids and sacamminids were a minor faunal element. At 3350 m, calcareous individuals were much less common and allogromiids and saccamminids formed a substantial component of the fauna. There were also strong contrasts between the foraminiferal macrofauna (>300-m fraction) at these two sites; relatively small species of Bathysiphon, Globobulimina and Lagenammina dominated at 412 m, very large, tubular, agglutinated species of Bathysiphon, Hyperammina, Rhabdammina and Saccorhiza were important at 3350 m. Our observations suggest that, because they contain fewer soft-shelled and agglutinated foraminifera, a smaller proportion of bathyal, low-oxygen faunas is lost during fossilization compared to faunas from well-oxygenated environments. Trends among foraminifera (>63 m fraction) in the Santa Barbara Basin (590 and 610 m depth; O2=0.05 and 0.15 ml l?1 respectively), and macrofaunal foraminifera (>300 m) on the Peru margin (300–1250 m depth; O2=0.02–1.60 ml l?1), matched those observed on the Oman margin. In particular, soft-shelled monothalamous taxa were rare and large agglutinated taxa were absent in the most oxygen-depleted ( Foraminifera often outnumber metazoans (both meiofaunal and macrofaunal) in bathyal oxygen-depleted settings. However, although phylogenetically distant, foraminifera and metazoans exhibit similar population responses to oxygen depletion; species diversity decreases, dominance increases, and the relative abundance of the major taxa changes. The foraminiferal macrofauna (>300 m) were 5 times more abundant than the metazoan macrofauna at 412 m on the Oman margin but 16 times more abundant at the 3350 m site. Among the meiofauna (63–300 m), the trend was reversed; foraminifera were 17 times more abundant than metazoan taxa at 412 m but only 1.4 times more abundant at 3350 m. An abundance of food combined with oxygen levels which are not depressed sufficiently to eliminate the more tolerant taxa, probably explains why foraminifera and macrofaunal metazoans flourished at the 412-m site, perhaps to the detriment of the metazoan meiofauna.

The influence of rivers on marine boron isotopes and implications for reconstructing past ocean pH.

Abstract: Ocean pH is particularly sensitive to atmospheric carbon dioxide content1,2,3. Records of ocean pH can therefore be used to estimate past atmospheric carbon dioxide concentrations. The isotopic composition of boron (δ11B) contained in the carbonate shells of marine organisms varies according to pH, from which ocean pH can be reconstructed4,5,6,7,8,9,10,11. This requires independent estimates of the δ11B of dissolved boron in sea water through time. The marine δ11B budget, however, is still largely unconstrained. Here we show that, by incorporating the global flux of riverine boron (as estimated from δ11B measurements in 22 of the world's main rivers), the marine boron isotope budget can be balanced. We also derive ocean δ11B budgets for the past 120 Myr. Estimated isotope compositions of boron in sea water show a remarkable consistency with records of δ11B in foraminiferal carbonates9,10,11, suggesting that foraminifera δ11B records may in part reflect changes in the marine boron isotope budget rather than changes in ocean pH over the Cenozoic era.

Ecological significance of benthic foraminifera: 13C labelling experiments

Abstract: We examined the ability of foraminifera in taking up freshly deposited algal carbon in intertidal estuarine sediments. Lyophylized C-13-labelled axenic Chlorella was added to intact sediment cores containing natural benthic biota. The response of the system as a whole was rapid; similar to 5% of the added carbon was respired to CO2 within 6 h. Bacteria assimilated similar to 2 to 4 % of the added carbon within 12 h. Among the foraminifera, the dominant foraminifer Ammonia exhibited rapid uptake and it is estimated that similar to 1 to 7% of the added carbon was ingested within 3 to 53 h. This rapid, substantial uptake by Ammonia and the significant contribution of foraminifera to meiobenthic abundance and biomass (37 and 47% respectively) suggest that foraminifera play an important role in the carbon cycle in these sediments. [KEYWORDS: meiobenthos; benthic foraminifera; bacterial biomarkers; algal carbon; carbon cycle; CO2; C-13 labelling Deep-sea; organic-carbon; sedimentation event; metazoan meiofauna; adriatic sea; fatty-acid; phytodetritus; community; microalgae; deposition]

The response of benthic foraminifera and ostracods to various pollution sources: a study from two lagoons in egypt

Abstract: A study of foraminiferal assemblages was carried out at two Egyptian Nile Delta lagoons. Analysis of surficial sediment samples from Manzalah Lagoon shows enrichment in heavy metals (Pb, Zn, Cu, Cr and Cd). The environment has become so lethal to foraminifera that no species can currently survive. Among ostracods, only one species ( Cyprideis torosa ) was found living and able to invade the polluted lagoon region. Samples from Edku Lagoon, which receives only agricultural drainage water, show heavy metal concentrations close to natural baseline levels, and yield living foraminifera. The frequent occurrence of deformed specimens in Manzalah Lagoon, comparable to Edku Lagoon, reveals that: (1) benthic foraminifera are more sensitive to industrial wastes containing heavy metals; (2) agricultural wastes do not significantly harm benthic foraminifera; (3) Ammonia beccarii forma parkinsoniana is less resistant to pollution than forma tepida ; (4) morphological abnormalities of the foraminiferal tests depend upon the nature of the pollutant; and (5) benthic foraminifera are less tolerant to pollution than ostracods and molluscs.

Using foraminiferal transfer functions to produce high-resolution sea-level records from salt-marsh deposits, Maine, USA

Abstract: Quantitative analyses of foraminiferal distributions along the coast of Maine are used to provide transfer functions for reconstruction of sea-level changes from fossil foraminiferal assemblages. The indicative meaning of fossil assemblages is most accurately predicted by a training set that incorporates live and dead foraminifera and that uses flooding duration, not height, as the predictor variable. Sea-level records are produced for two sites in Maine (USA): a middle marsh site in Scarboro and a high marsh site in Machiasport. These records offer temporal resolution of five to seven years and span the past century. They are compared with instrumental data from a nearby tidal station to assess their accuracy. Strong correlations are found between the geologic records and the observational data, in particular between the foraminiferal record from Machiasport and the tide-gauge record from Eastport (r2 = 0.83, p = 0.005). Thus a foraminifera-based transfer-function approach offers great potential for reso...

Recent Anthropogenic Impacts on the Bilbao Estuary, Northern Spain: Geochemical and Microfaunal Evidence

Abstract: The distributions of a range of elements (including137Cs and210Pb) have been studied in surface sediments and 0·5 m cores from the Bilbao Estuary (Cantabrian coast, northern Spain) to determine recent and historical sediment contamination. Benthic foraminifera have also been examined to infer recent ecosystem changes. High concentrations of a range of metals are observed in three cores and in surface-scrapes. Observed concentrations depend on the proximity to sources of pollutants. Living foraminifera are absent from surface sediments in the upper estuary, and are scarce in the middle and lower estuary, due to persistent anoxia in the estuarine channel, and possibly, high pollutant concentrations. High metal concentrations in sediment core profiles, combined with the sporadic presence of foraminifera, indicate that environmental degradation has persisted in this estuary for at least the last 40 years. This degradation has been caused by the discharge of untreated industrial and domestic effluents. Surface sediments show a few transported living foraminifera of species that have been identified as dominant in nearby, less-polluted estuarine environments. Successful recolonization by these species may occur if the sediment conditions improve sufficiently. Foraminiferal assemblages could therefore be used as environmental quality indicators to assess the effectiveness of current regeneration schemes implemented under strategic local authority plans.

Paleo‐sea surface temperature calculations in the equatorial east Atlantic from Mg/Ca ratios in planktic foraminifera: A comparison to sea surface temperature estimates from U37K′, oxygen isotopes, and foraminiferal transfer function

Abstract: We present two ∼270 kyr paleo-sea surface temperature (SST) records from the Equatorial Divergence and the South Equatorial Current derived from Mg/Ca ratios in the planktic foraminifer Globigerinoides sacculifer. The present study suggests that the magnesium signature of G. sacculifer provides a seasonal SST estimate from the upper ∼50 m of the water column generated during upwelling in austral low-latitude fall/winter. Common to both down-core records is a glacial-interglacial amplitude of ∼3°–3.5°C for the last climatic changes and lower Holocene and glacial oxygen isotope stage 2 temperatures compared with interglacial stage 5.5 and glacial stage 6 temperatures, respectively. The comparison to published SST estimates from alkenones, oxygen isotopes, and foraminiferal transfer function from the same core material pinpoints discrepancies and conformities between methods.

Seasonal dynamics of benthic foraminifera in a glacially fed fjord of svalbard, european arctic

Abstract: Seasonal variations in benthic foraminiferal populations from a sediment-laden fjord were analyzed in order to provide insights into arctic foraminiferal ecology and to improve the interpretation of the late-glacial record. The fjord is 25 km long and 100 m deep with a large tidewater glacier at the fjord head. A pilot transect of eight stations sampled in August 1995 revealed the typical off-glacier sequence of foraminiferal taxa. Unidentified allogromiids were abundant in the vicinity of the ice front. Further down the fjord Elphidium excavatum f. clavatum and Cassidulina reniforme co-dominated the glacier-proximal fauna. Nonionellina labradorica and Islandiella norcrossi characterized the glacier-distal setting. In 1996 three glacier-proximal stations were then sampled in March, May, July, August, September and November. Compared to the summer of 1995 the summer of 1996 was colder, resulting in weaker glacial meltwater discharge, and the foraminiferal fauna became less influenced by glaciers. This is portrayed by an increase in glacier-distal N. labradorica and a decrease in glacier-proximal C. reniforme and especially E. excavatum. Taxonomic diversity was higher in winter, possibly reflecting a more stable environment in the absence of the turbid meltwater plume, the source of ecological stress.

Importance of foraminifera for the formation and maintenance of a coral sand cay: Green Island, Australia

Abstract: CaCO3 production by reef-building organisms on Green Island Reef in the Great Barrier Reef of Australia is estimated and compared with the contribution of benthic foraminifera to the sediment mass of the vegetated sand cay. Major constituents of the cay are benthic foraminifera (mainly Amphistegina lessonii, Baculogypsina sphaerulata, and Calcarina hispida), calcareous algae (Halimeda and coralline algae), hermatypic corals, and molluscs. Among these reef-building organisms, benthic foraminifera are the single most important contributor to the sediment mass of the island (ca. 30% of total sediments), although their production of CaCO3 is smaller than other reef-building organisms. Water current measurements and sediment traps indicate that the velocity of the current around Green Island is suitable for transportation and deposition of foraminiferal tests. Abundant foraminifera presently live in association with algal turf on the shallow exposed reef flat, whose tests were accumulated by waves resulting in the formation and maintenance of the coral sand cay.

The isotopic signature of planktonic foraminifera from NE Atlantic surface sediments: implications for the reconstruction of past oceanic conditions

Abstract: The stable isotope compositions of the planktonic foraminifera Globigerina bulloides, Globigerinoides ruber (white and pink varieties), Globigerinoides trilobus, Globorotalia inflata and Globorotalia truncatulinoides (right‐ and left‐coiling types) were examined as recorders of North Atlantic surface water properties based on 40 box‐core surface sediments between 60° and 30°N. While G. ruber (white and pink varieties) and G. trilobus mainly reflect summer surface water conditions in their oxygen isotope composition, G. bulloides reflects temperatures of the northward‐migrating spring bloom, February–March in the south to May–June in the north. Our data show that G. bulloides cannot be regarded as an indicator for summer temperatures as deduced from Duplessy et al. ’s data. Gt. inflata and Gt. truncatulinoides (right‐ and left‐coiling) build their shells in the coldest waters compared with the other species and reflect temperatures between 100 and 400 m water depth. The difference in oxygen isotope composition between G. bulloides and G. inflata serves as a proxy for water mass stratification. G. bulloides is the only species that gives a distinct pattern in its carbon isotopic composition showing a high correlation with the surface water phosphate values along the transect and may serve as a proxy for palaeonutrients and/or productivity.

Orbitally Induced Climate and Geochemical Variability Across the Oligocene/Miocene Boundary

Abstract: To assess the influence of orbital-scale variations on late Oligocene to early Miocene climate and ocean chemistry, high-resolution (∼5 kyr) benthic foraminiferal carbon and oxygen isotope and percent coarse fraction time series were constructed for Ocean Drilling Program site 929 on Ceara Rise in the western equatorial Atlantic. These time series exhibit pervasive low- to high-frequency variability across a 5-Myr interval (20.5–25.4 Ma). The records also reveal several large-scale secular variations including two positive (∼1.6‰) oxygen isotope excursions at 22.95 and 21.1 Ma, suggestive of large but brief glacial maxima (Mi-1 and Mi-1a events of Miller et al. [1991]), and a long-term cyclical increase in the carbon isotopic composition of seawater (shift of ∼1.52‰) that reaches a maximum coincident with peak δ18O values at 22.95 Ma. Lower-resolution (∼25 kyr) records constructed from benthic and planktonic foraminifera as well as bulk carbonate at a shallower site on Ceara Rise (site 926) for the period 21.7–24.9 Ma covary with site 929 δ18O values reflecting changes in Antarctic ice-volume. Likewise, covariance among carbon isotopic records of bulk sediment, benthic, and planktonic foraminifera suggest that the low-frequency cycles (∼400 kyr) and long-term increase in δ13C values represent changes in the mean carbon composition of seawater ΣCO2. The time series presented here constitute the longest, most continuous, and highest-resolution records of pre-Pliocene climate and oceanography to date. The site 929 carbon and oxygen isotope power spectra show significant concentrations of variance at ∼400, 100, and 41 kyr, demonstrating that orbitally induced oscillations have been a normal characteristic of the global climate system since at least the Oligocene, including periods of equable climate and times with no apparent Northern Component Water production.

Geochemical characteristics of deep-sea sediments from the Arabian Sea: a high-resolution study

Abstract: Five deep-sea cores from the Arabian Sea, covering a time interval of 170 ky, were sampled at high resolution and analysed for major and trace elements. The stratigraphy of the cores was obtained by comparing the Ba/Al ratio with the SPECMAP data. This method is based on the close correlation between Ba concentrations (or Ba/Al ratios) and oxygen isotope ratios (δ18O) of foraminifera in Arabian Sea sediments. Ba/Al ratios, used as a productivity proxy, are variable but high in all deep-sea cores of the Arabian Sea, indicating a basin-wide influence of nutrient-rich water masses. Compared to glacial intervals the warmer periods are characterised by higher element/Al ratios of proxies, which are directly (Ba, Ca, Sr, P, i.e. hard parts of organisms) or indirectly (U, 230Thex) related to biological productivity. 230Thex provides evidence for intense boundary scavenging caused by high productivity due to enhanced upwelling and terrigenous input from the Indus fan. The Mn distribution in a core from the western Arabian Sea shows enrichments during interglacial periods and may indicate Mn export owing to the presence of a stronger oxygen-minimum zone. Several processes limit the applicability of proxies. Intercalated turbidites in the deep-sea cores can be identified by means of Si/Al, Ti/Al and Zr/Al ratios versus depth due to the enrichment of quartz, Ti-minerals and zircon in the basal layer. These elements are also important for the reconstruction of the history and extent of the eolian dust input. Diagenetic redistribution of redox sensitive elements occurs at the boundaries between turbidites and “normal” pelagic sedimentation and may mask or destroy primary signals.

Interannual variability of planktic foraminiferal populations and test flux in the eastern North Atlantic Ocean (JGOFS)

Abstract: Planktic foraminiferal assemblages vary in response to seasonal fluctuations of hydrographic properties, between water masses, and after periodical changes and episodic events (e.g. reproduction, storms). Distinct annual variability of the planktic foraminiferal flux is also known from sediment trap data. In this paper we discuss the short-term impacts on interannual flux rates based on data from opening-closing net hauls obtained between the ocean surface and 500 m water depth. Data were recorded during April, May, June, and August at around 47°N, 20°W (BIOTRANS) in 1988, 1989, 1990, 1992, 1993, and during May 1989 and 1992 at 57°N. 20-22°W. Species assemblages closely resemble each other when comparing the mixed layer fauna with the fauna of the upper 100 m and the upper 500 m of the water column. In addition, species assemblages > 100 μm are almost indistinguishable from assemblages that are > 125 μm in test size. The standing stock of planktic foraminifers at BIOTRANS can vary by more than one order of magnitude over different years; however, species assemblages may be similar when comparing corresponding seasons. Early summer assemblages (June) are distinctly different from late summer assemblages (August). Significant variations in the species composition during spring (April/May) are independent of the mixed layer depth. Spring assemblages are characterized by high numbers of Globigerinita glutinata In particular, day-to-day variations of the number of specimens and in species composition may have the same order of magnitude as interannual variations. This appears to be independent of the reproduction cycle. Species assemblages at 47°N and 57°N are similar during spring, although surface water temperatures and salinities differ by up to 10°C and 0.7 (PSU). We suggest that the main factors controlling the planktic foraminiferal fauna are the trophic properties in the upper ocean productive layer. Planktic foraminiferal carbonate flux as calculated from assemblages reveals large seasonal variations, a quasi-annual periodicity in flux levels, and substantial differences in timing and magnitude of peak fluxes. At the BIOTRANS station, the average annual planktic foraminiferal CaCO 3 fluxes at 100 and 500 m depth are estimated to be 22.4 and 10.0 g m -2 yr -1 , respectively.

Historical trends in Chesapeake Bay dissolved oxygen based on benthic foraminifera from sediment cores

Abstract: Environmentally sensitive benthic foraminifera (protists) from Chesapeake Bay were used as bioindicators to estimate the timing and degree of changes in dissolved oxygen (DO) over the past five centuries. Living foraminifers from 19 surface samples and fossil assemblages from 11 sediment cores dated by210Pb,137Cs,14C, and pollen stratigraphy were analyzed from the tidal portions of the Patuxent, Potomac, and Choptank Rivers and the main channel of the Chesapeake Bay.Ammonia parkinsoniana, a facultative anaerobe tolerant of periodic anoxic conditions, comprises an average of 74% of modern Chesapeake foraminiferal assemblages (DO-0.47 and 1.72 ml l−1) compared to 0% to 15% of assemblages collected in the 1960s. Paleoecological analyses show thatA. parkinsoniana was absent prior to the late 17th century, increased to 10–25% relative frequency between approximately 1670–1720 and 1810–1900, and became the dominant (60–90%) benthic formaniferal species in channel environments beginning in the early 1970s. Since the 1970s, deformed tests ofA. parkinsoniana occur in all cores (10–20% ofAmmonia), suggesting unprecedented stressful benthic conditions. These cores indicate that prior to the late 17th century, there was limited oxygen depletion. During the past 200 years, decadal scale variability in oxygen depletion has occurred, as dysoxic (DO=0.1–1.0 ml l−1), perhaps short-term anoxic (DO<0.1 ml l−1) conditions developed. The most extensive (spatially and temporally) anoxic conditions were reached during the 1970s. Over decadal timescales, DO variability seems to be linked closely to climatological factors influencing river discharge; the unprecedented anoxia since the early 1970s is attributed mainly to high freshwater flow and to an increase in nutrient concentrations from the watershed.

Variation of foraminiferal Sr/Ca over Quaternary glacial‐interglacial cycles: Evidence for changes in mean ocean Sr/Ca?

Abstract: [1] Records of Sr/Ca changes in planktonic and benthic foraminifera from diverse hydrographic settings reveal coherent variability (5 ± 1%) between ocean basins and between surface and deep waters over the past 300 kyr. There is a general increase in foraminiferal Sr/Ca over the penultimate glaciation declining to minimum values during stage 5 and an increase in Sr/Ca from stage 5 through stage 2. Coincident changes in benthic foraminifera records from the Atlantic and Pacific basins imply that Sr/Ca variations are not dominated by dissolution. Planktonic culturing data provide evidence that the downcore Sr/Ca variations are not controlled by temperature changes and suggest only a small influence of salinity and pH. Variation common to the records is most readily explained by changes in mean ocean Sr/Ca. If fossil foraminifera reliably record higher glacial seawater Sr/Ca, coral Sr paleothermometry would underestimate sea surface temperature during glacialepisodes.

Distribution, biomass and diversity of benthic foraminifera in relation to sediment geochemistry in the Arabian Sea

Abstract: The distribution, biomass, and diversity of living (Rose Bengal stained) deep-sea benthic foraminifera (>30 [mu]m) were investigated with multicorer samples from seven stations in the Arabian Sea during the intermonsoonal periods in March and in September/October, 1995. Water depths of the stations ranged between 1916 and 4425 m. The distribution of benthic foraminifera was compared with dissolved oxygen, % organic carbon, % calcium carbonate, ammonium, % silica, chloroplastic pigment equivalents, sand content, pore water content of the sediment, and organic carbon flux to explain the foraminiferal patterns and depositional environments. A total of six species-communities comprising 178 living species were identified by principal component analysis. The seasonal comparison shows that at the western stations foraminiferal abundance and biomass were higher during the Spring Intermonsoon than during the Fall Intermonsoon. The regional comparison indicates a distinct gradient in abundance, biomass, and diversity from west to east, and for biomass from north to south. Highest values are recorded in the western part of the Arabian Sea, where the influence of coastal and offshore upwelling are responsible for high carbon fluxes. Estimated total biomass of living benthic foraminifera integrated for the upper 5 cm of the sediment ranged between 11 mg Corg m-2 at the southern station and 420 mg Corg m-2 at the western station. Foraminifera in the size range from 30 to 125 [mu]m, the so-called microforaminifera, contributed between 20 and 65% to the abundance, but only 3% to 28% to the biomass of the fauna. Highest values were found in the central and southern Arabian Sea, indicating their importance in oligotrophic deep-sea areas. The overall abundance of benthic foraminifera is positively correlated with oxygen content and pore volume, and partly with carbon content and chloroplastic pigment equivalents of the sediment. The distributional patterns of the communities seem to be controlled by sand fraction, dissolved oxygen, calcium carbonate and organic carbon content of the sediment, but the critical variables are of different significance for each community.

Fluxes of planktic foraminifera in response to monsoonal upwelling on the Somalia Basin margin.

Abstract: Sediment trap samples collected off Somalia (bi)weekly from early June 1992 through mid February 1993 show large seasonal variations in the shell flux and species composition of planktic foraminifera. These variations mirror the monsoon-driven circulation, which resulted in massive upwelling and offshore eddy transport in summer during the SW Monsoon (June–September) and accounted for nearly 90% of the estimated annual shell flux of planktic foraminifera. G. bulloides dominated the SW Monsoon along with G. glutinata, T. quinqueloba, N. dutertrei and T. iota in a fauna-rich species but with a low diversity and equitability. During the autumn intermonsoon (November–December) the ocean became stratified and the nutrient exhausted, whereas during the NE Monsoon (January–March) a warm surface mixed layer developed with the nutrients entrained by deep wind mixing. It was dominated by G. ruber in association with G. tenella, G. aequilateralis, G. trilobus/G. sacculifer and G. menardii in a fauna poorer in species but with a higher diversity and equitability. Consequently, the ratio in the abundance of G. bulloides and G. ruber follows the monsoonal cycle closely, as both species persist throughout the year. Only the rare G. theyeri seems to be restricted to the SW Monsoon. In October at the end of the SW Monsoon, the trap at 265 m above the bottom intercepted massive amounts of fine-grained carbonate ( μm ) resuspended from the shelf and upper slope. It contained extreme fluxes of bioclastic fragments, benthic foraminifera and benthic ostracods as well as enhanced fluxes of small-sized planktic foraminifera μm , notably G. rubescens. Integrated over the year, the species composition intercepted by the trap shows a very good match with the shell fauna in a core top sediment from the same site. Apparently, the year-estimated monsoonal record of planktic foraminifera, which is strongly dominated by the SW Monsoon production, survives burial intact. Consequently, the species composition of planktic foraminifera and measures such as the bulloides/ruber ratio can be applied as a proxy for past changes in the intensity of the SW Monsoon off Somalia compared to the modern condition.