S.R. Troelstra

Organic flux control on bathymetric zonation of Mediterranean benthic foraminifera

A data set of benthic foraminiferal faunas counted in 138 surface samples from the Mediterranean Sea has been used to investigate whether the bathymetrical distribution of the dominant taxa is controlled by the amount of labile organic matter transported to the sea floor. We find that most of the major taxa show a clear W to E shallowing of their upper or lower depth limit, coinciding with a W to E decrease in the surface water primary production, and in the estimated flux of the labile organic matter to the sea floor. This observation implies that the bathymetrical succession of these taxa is indeed determined by the organic flux. In the western Mediterranean, we find successions from more oligotrophic taxa at greater water depths to more eutrophic taxa in more shallow water. Towards the eastern Mediterranean, most eutrophic taxa tend to become increasingly rare, or even to disappear, whereas the more oligotrophic taxa show a clear shoaling of their depth range. Deep infaunal taxa are mainly limited to the western part of the Mediterranean. This is explained by their dependency on a relatively elevated organic flux, and by the fact that the bacterial stocks on which they feed may become unattainable when the redox front is positioned too deep in the sediment. The close similarity between the flux level controlling our main faunal boundary, and the flux levels coinciding with important faunal changes in other parts of the world ocean, suggests that a flux level of about 2‐3 g labile Cm 22 y 21 level corresponds to a benthic ecosystem threshold value of global importance. q 2000 Elsevier Science B.V. All

Larger foraminifera distribution on a mesotrophic carbonate shelf in SW Sulawesi (Indonesia)

Larger symbiont bearing foraminifera typically live in shallow tropical seas. In this study the fauna composition of patch reefs scattered over the Spermonde Shelf (SW Sulawesi, Indonesia), a mesotrophic carbonate shelf, is examined. The foraminiferal fauna of the Spermonde Shelf is characterised by three assemblages, a seaward slope, a leeward slope and a reef base assemblage. This is similar to the fauna composition at Palau except that Palau’s reef flat assemblage is absent. The samples group in six clusters: the reef base, deep exposed slope, deep leeward slope, shallow leeward slope, shallow exposed slope and the southern near shore cluster. Of the four ecological zones that have been recognised on the Spermonde Shelf in corals and marine vegetation, we cannot distinguish between the two outer shelf zones, but we do recognise two near shore zones, a northern and a southern one. This latter part of the shelf within 4 km of the mainland coast has a less diverse foraminiferal fauna, in which the reef base fauna is absent. Compared to areas with a deeper euphotic zone, the deepest living species are absent whereas most other species occur in much shallower water. Neorotalia calcar and Calcarina gaudichaudii, however, have been found in deeper water, due to an absence of suitable habitat in the shallow areas. Other species, like Peneroplis planatus and Calcarina hispida have only been found in the deepest part of their depth range, as reported at Okinawa. These species show a greater flexibility in their habitat preference than has been observed so far. We used our results to test Hallock’s trophic resource continuum model with actual occurrences of larger foraminifera. The predictions of the model are met, but patterns, for example, in calcarinids and peneroplids show that not all taxa react in the same way to the described environmental parameters. Seasonal climatic conditions in the Spermonde Archipelago mainly affect species with a long life cycle and deep living species. A model is presented that explains why deep living species are more affected by seasonal changes in water transparency than shallow living species.

Millennial-scale glacial variability versus Holocene stability: changes in planktic and benthic foraminifera faunas and ocean circulation in the North Atlantic during the last 60 000 years

Two piston cores, DS97-2P from the Reykjanes Ridge in the central North Atlantic Ocean (1685 m water depth) and ENAM33 from southwest of the Faeroe Islands in the NE Atlantic (1217 m water depth), have been investigated for their planktic and benthic foraminiferal content. DS97-2P is situated near the Subarctic Front and productivity measured by accumulation rates of benthic and planktic foraminifera has been generally high during the Holocene. The productivity shows a clear decrease from an early Holocene maximum to a late Holocene minimum. Coeval changes in the benthic faunas indicate that the food supply changed from large, irregular pulses during the early Holocene to a more sustained flux during the late Holocene. Presumably in concert with decreasing bottom current activity oxygen conditions in the bottom water became poorer. Another feature of the late Holocene is an increasing instability of the North Atlantic thermohaline circulation regime. Nevertheless, the changes in faunal composition and productivity during the Holocene were gradual as compared to the discontinuous distribution patterns and abrupt productivity shifts during the glacial. The glacial shifts were on a millennial time scale and correlate with the interstadial-stadial phases of the Dansgaard-Oeschger cycles in the Greenland ice cores. The faunas of the warm interstadial phases resembled the Holocene faunas, and both surface and bottom productivity was high. The faunas suggest that the interstadial circulation pattern was very similar to the modern system with convection in the Nordic seas and generation of North Atlantic Deep Water. The planktic faunas during the cold stadials and Heinrich events were completely dominated by the polar species Neogloboquadrina pachyderma s, and surface conditions were cold and the productivity low. The benthic faunas were dominated by species that presently thrive in areas with a low amount of food and reduced oxygen content. The water column was probably stratified with low saline, cold surface water overlying poorly aerated, intermediate water masses.

Ocean circulation and iceberg discharge in the glacial North Atlantic: Inferences from unmixing of sediment size distributions

Variability in iceberg discharge and deep-ocean circulation in the North Atlantic during the last glacial period is inferred from the grain-size distribution and trace elemental composition of terrigenous sediments in a deep-sea core taken on Reykjanes Ridge, south of Iceland. End-member modeling of the grain-size distributions is used to unmix the signals of varying bottom-current speed and iceberg discharge. The size distribution within the silt fraction appears to be influenced by both factors. Based on Th-Sc-La relationship, we established that during the ice-rafted detritus events, continental material of likely Greenlandic origin increased to 87%, and that bottom-current–derived material contains to 40% mid-oceanic ridge fines, probably of Icelandic origin. Our results have important implications for the use of silt grain size as an indicator for paleocurrent speed in the glacial North Atlantic. We show that reconstructions of variations in bottom-current speed based on the raw grain-size data are opposite to inferences from the unmixed record. The latter indicates that deep-water convection decreased during periods of enhanced iceberg discharge, which is in general agreement with paleoceanographic reconstructions of the North Atlantic.

Salt marsh foraminifera from the Great Marshes, Massachusetts: environmental controls

Abstract By using sites in the Great Marshes at Barnstable (Massachusetts, USA) this study examines the effects of a set of environmental parameters on the foraminiferal distribution. The studied parameters are: elevation above mean high water; salinity of the porewater; various sediment characteristics; vegetation; and food source. Relations between the environmental parameters and foraminiferal properties (frequencies, densities and diversities) are quantified with correlation coefficients. For the first time Siphotrochammina lobata and Balticammina pseudomacrescens are documented in the New England region. The following species show a significant correlation with one or more of the studied parameters and are designated as key-species: Haplophragmoides manilaensis, Jadammina macrescens, Balticammina pseudomacrescens, Miliammina fusca and Tiphotrocha comprimata . Based on cluster analysis and the presence, absence or dominance of the key-species characteristic associations are distinguished. The distribution of three associations is indicative of specific marsh environments: the marsh fringe, the middle marsh and the marsh edge. These three marsh units are separated by their own salinity regime, flooding and sediment characteristics. The marsh fringe is typified by the H. manilaensis Association and experiences freshwater input (seepage, surface runoff and rainwater) and only slight marine influence, resulting in low salinity values (2.5–20‰). The width of the marsh fringe is variable, dependent on the amount of seepage which in turn is controlled by the permeability of the basement and the peat. The J. macrescens Association characterizes the middle marsh where salinities are controlled by infiltration of sea- and rainwater and by evaporation. Salinity values are higher than 20‰, while temporarily salinity can reach extreme high values during periods without flooding and high evaporation rates (e.g., 44‰). The fully marine M. fusca Association occupies the daily flooded marsh edge where the salinities have the same values as Cape Cod bay water (ca. 28‰). Unlike many other salt marshes the distribution of foraminiferal assemblages in the Great Marshes does not show a vertical zonation with respect to mean high water. This shows that a worldwide applicable model for paleoenvironmental studies in salt marshes based on foraminifera is not feasible. Each salt marsh has its own characteristics. Regional factors such as climate play an important role in the salinity regime, while the local upland characteristics determine if seepage takes place. Thus each marsh has its own foraminiferal fingerprint showing the opportunistic behaviour of the salt marsh agglutinants. A surface study is an indispensable first step in assessing the value of foraminifera as paleo-ecological indicators.

Mechanisms forcing abrupt fluctuations of the Indian Ocean Summer Monsoon during the last deglaciation

Abstract A piston core from the Somali upwelling area has been studied at high resolution for its dinoflagellate cyst content. Variations in cyst association are inferred to reflect changes in Indian Ocean summer monsoon intensity. Several abrupt fluctuations in monsoon intensity are detected for the interval between 20 and 10 ka BP. Comparison of these fluctuations with changes in contrast between 30°N and 30°S July insolation and the δ 18 O GRIP ice-core values suggests that different mechanisms influenced monsoon intensity at different time intervals. A general trend in monsoon intensity follows variations in insolation contrast, lagging minimum contrast by 7.5 ka and maximum contrast by 4 ka. This phase lag difference can be explained by assuming the existence of an ice/snow cover over central Asia/Tibet during glacial times. Between 18.7 and 12.5 ka BP fluctuations in SW-monsoon intensity may be forced by variations in the thermohaline ocean circulation. A rapid transition. towards strong SW-monsoons at 12.5 ka BP is possibly the result of variations in atmospheric circulation and melting of the snow/ice fields in central Asia/Tibet. Variations in glacial-interglacial boundary conditions related to temperature change at northern latitudes are likely to have influenced SW-monsoon intensity between 12.5 and 10 ka BP, whereas after 10 ka BP variations in tropical land surface boundary conditions may be the dominant forcing factor. 0 1997 Elsevier Science Ltd. All rights reserved.

Late Quaternary sedimentary processes and ocean circulation changes at the Southeast Greenland margin

A study has been made of late Quaternary depositional processes and bottom current activity on the Southeast Greenland margin, using seismic, sub-bottom profiling and deep-tow side-scan sonar data as well as sediment core information. The seabed data demonstrate the occurrence of strong, southerly bottom currents prevailing on the slope and rise. Well-defined longitudinal bedforms indicate maximum mean near-bottom current velocities of up to at least 1.0 m/s at the depth stratum of Labrador Sea water (800–1500 m). Similarly strong currents occur in Denmark Strait overflow water (DSOW) at the base of the slope, whereas more basinward the maximum DSOW flow speed is lower. Iceberg plow marks were found down to about 700 m water depth. Both at the shelf edge and on the lower slope and rise the seafloor morphology is indicative of downslope sediment transport and mass flow deposition, which is concluded to be a typically glacial feature. After generally more sluggish deep-water circulation during the last glacial maximum, DSOW basin ventilation was re-established shortly before 13.3 ka. On the shelf, in front of the retreating Greenland ice margin, permanent or semi-permanent sea ice conditions prevailed until about 12.5 ka. At that time increased Irminger Current activity had resulted in warming, and East Greenland Current (EGC)-controlled iceberg drift increased. No evidence was found for a return to extreme glacial conditions or a ceasing of DSOW flow during the Younger Dryas. Abundant coarse IRD collected at greater water depth from shallow sub-seabed strata has a provenance mainly in the northern part of East Greenland (68–73°N), which demonstrates the existence of a pre-Holocene EGC system initially extending to beyond the shelf edge.

A size analysis of planktic foraminifera from the Arabian Sea

Abstract Planktic foraminiferal faunas from different environments in the Arabian Sea were size fractionated using 14 sieves with meshes between 100 and 710 μm, to assess the effect of the sieve mesh size cut off level on the faunal composition and to determine the size frequency distribution of individual species. Nine samples from a plankton pump and a towed net, a sediment trap, a box-core and a piston core were selected, to cover living and settling flux faunas as well as fossil faunas from the sediment. In living faunas, most species show an exponential size frequency distribution, with highest numbers in the finest interval of the size spectrum. In sediment trap and core samples, individual species size frequency distributions may consist of: (1) an exponential distribution of relatively small pre-adult specimens; (2) a Gaussian-shaped distribution of larger specimens, which may be classified as adult or terminal; or (3) a combination of both. The distributions are separated using a best fit technique. The composition of the total planktic foraminiferal fauna strongly changes along the size spectrum. Dominant taxa in >355 μm fractions are Orbulina universa, Globorotalia menardii, Globorotalia tumida, Globigerinella siphonifera and Globigerinoides sacculifer, in 125–355 μm fractions Globigerina bulloides, Globigerinoides ruber, Neogloboquadrina dutertrei and Globigerinita glutinata, and in 100 μm) is present in the 100–125 μm fraction and 1–6% is larger than 250 μm. In samples representing a settling flux (sediment trap and sediment samples) 29–57% of the fauna is present in the 100–125 μm fraction, while 6–23% is larger than 250 μm. Size frequency distributions of the dextral Neogloboquadrina complex (= Neogloboquadrina dutertrei and Neogloboquadrina pachyderma + P–D intergrades) show a bimodal pattern; a smaller peak reflecting dextral Neogloboquadrina pachyderma, and a larger peak of adult Neogloboquadrina dutertrei. By applying a best fit technique to the data, the two species may be separated from each other. In size fractions larger than 150 μm most species have reached the adult stage of ontogeny and we recommend this mesh size for standard faunal analysis. In addition, sieve mesh sizes of 125 and 250 μm have to be used to obtain a reliable estimate of the abundance of small and large species, respectively.

Genotypic variability in subarctic Atlantic planktic foraminifera

Specimens of the planktic foraminiferal morphospecies, Globigerina bulloides, Turborotalita quinqueloba, Neogloboquadrina pachyderma (dextral) and Globigerinita uvula, were collected along a subarctic Atlantic transect. Partial sequences of the small subunit (SSU) ribosomal (r) RNA gene were obtained and a distance-based foraminiferal phylogeny constructed. The low latitude morphospecies, Globigerina falconensis, was included to improve within cluster resolution. G. bulloides, G. falconensis and T. quinqueloba cluster together as a distinct group within the molecular phylogeny. The diversification of these three morphospecies from their common ancestor is clearly later than the main planktic spinose radiation, consistent with current interpretations of the fossil record. G. bulloides and G. falconensis are highly divergent from one another, supporting palaeontological and biological evidence that they are separate species. N. pachyderma (dextral) clusters with Neogloboquadrina dutertrei within the benthic and non-spinose planktic region of the tree. G. uvula also clusters within the benthic and non-spinose planktic region of the tree, adjacent to Globigerinita glutinata, a member of the same genus, though resolution is too low to provide evidence of a sister–taxon relationship. The Globigerina bulloides and Turborotalita quinqueloba morphospecies comprise a complex of distinct SSU rDNA genetic types. These fall into two groups, representing high and low latitude genotypes. Along the subarctic transect, G. bulloides and T. quinqueloba were each represented by two distinct genotypes. Neogloboquadrina pachyderma (dextral) and Globigerinita uvula were each represented by a single genotype. Genotypes of a morphospecies exhibit distinctive and different distribution patterns. In the case of Globigerina bulloides, the genotype distribution is suggestive of differing adaptation. However, the Turborotalita quinqueloba genotype distribution was complicated by their co-existence in the same water column throughout the eastern sector. Further investigation will be required to determine whether they occupy a different niche within the water column. Although only T. quinqueloba Type IIa was found in the western region, sampling density was low and inconclusive. The Neogloboquadrina pachyderma (dextral) genotype was found across the entire transect. Further investigation of genotype distribution and genotype/habitat relationships could provide new high-resolution proxies for past oceanographic/climate reconstructions.

Living Neogloboquadrina pachyderma sin and its distribution in the sediments from Oman and Somalia upwelling areas

Abstract Microfaunal analysis of plankton nets collected offshore Oman/Yemen and sediment traps offshore Somalia shows that Neogloboquadrina pachyderma sin is abundant during the SW monsoon upwelling. The sediment traps recorded the highest flux (136 specimens m−2 day−1) and the highest relative frequency (3.55% of the total foraminifera assemblage) of the species during this period. During the intermonsoon it became less abundant and decreased in size, and only very few N. pachyderma sin were found in the water column during the NE monsoon. Sediment trap and Recent sediment data collected along a downslope transect off Somalia show that the species frequency decreases offshore. The highest concentrations of N. pachyderma sin in plankton nets off Oman/Yemen were found at a depth of 300–500 m. However, the oxygen isotope compositions of N. pachyderma sin at the depth of 300–500 m and from the surface 8 m are identical, and also similar to that of Globigerina bulloides which reflects properties of the upper 25 m of the water column. The carbon isotope values are relatively consistent within the studied samples from both areas except for the specimens from the surface water samples offshore Oman/Yemen. The data show that the δ18O ratio of N. pachyderma sin is in equilibrium with the ambient water while the δ13C values are at least 0.8‰ lower. We infer that N. pachyderma sin in the Arabian Sea reproduces, grows and calcifies in the upper 25 m of the water column, but possibly descends into deeper waters later during its life cycle.