Bathymetric Position Research Articles

Response of megabenthic assemblages to different scales of habitat heterogeneity on the Mauritanian slope

The topographically complex deep seabed on the Mauritanian slope, from 990 to 1460m water depth, was imaged with video in an extensive quantitative survey of 17,199m2 of seafloor using a Remote Operated Vehicle (ROV). This study investigated the influence of habitat heterogeneity at two scales on the megafaunal assemblages observed by ROV. Changes in megafaunal assemblages on the Mauritanian slope were assessed at a broad scale, within depth zones, and at a finer scale, in response to changes in local geomorphology associated with submarine landslides. Geomorphology was determined by classification of habitat parameters (slope, aspect, bathymetric position, curvature, fractal dimension and ruggedness) derived from an autonomous underwater vehicle-based multibeam bathymetry survey. Habitat parameters were classified by Iterative Self Organizing Clustering into six major geomorphological groups, four of which were assessed in the ROV video survey. A total of 29 megafaunal taxa were observed along the entire survey, with an overall average faunal density of 0.344 indm−2. Megafaunal assemblage density, species richness and evenness varied significantly across the depth range of the survey in the most common geomorphological zone (sedimentary plains of low slope and complexity). Characteristic species inhabited the shallow areas (asteroid, ophiuroid, anemone, small macrourid), intermediate areas (Benthothuria funabris, black cerianthid, squat lobster) and deeper areas (the holothurians Enypniastes eximia and Elipidia echinata). Megafaunal density, species richness and evenness were not significantly different between geomorphogical groups within one depth zone (1300–1400m). However, the steepest zone, on the edge of a major headwall feature, had four unique taxa (Parapagurus pilosimanus, a comatulid crinoid, a gorgonian and its associated ophiuroid).

A multi-method approach for benthic habitat mapping of shallow coastal areas with high-resolution multibeam data

The coastal waters of the Maltese Islands, central Mediterranean Sea, sustain a diversity of marine habitats and support a wide range of human activities. The islands’ shallow waters are characterised by a paucity of hydrographic and marine geo-environmental data, which is problematic in view of the requirements of the Maltese Islands to assess the state of their coastal waters by 2012 as part of the EU Marine Strategy Directive. Multibeam echosounder (MBES) systems are today recognised as one of the most effective tools to map the seafloor, although the quantitative characterisation of MBES data for seafloor and habitat mapping is still an underdeveloped field. The purpose of this study is to outline a semi-automated, Geographic Information System-based methodology to map the distribution of habitats in shallow coastal waters using high-resolution MBES data. What distinguishes our methodology from those proposed in previous studies is the combination of a suite of geomorphometric and textural analytical techniques to map specific types of seafloor morphologies and compositions; the selection of the techniques is based on identifying which geophysical parameter would be influenced by the seabed type under consideration.We tested our approach in a 28km2 area of Maltese coastal waters. Three data sets were collected from this study area: (i) MBES bathymetry and backscatter data; (ii) Remotely Operated Vehicle imagery and (iii) photographs and sediment samples from dive surveys. The seabed was classified into five elementary morphological zones and features – flat and sloping zones, crests, depressions and breaks of slope – using morphometric derivatives, the Bathymetric Position Index and geomorphometric mapping. Segmentation of the study area into seagrass-covered and unvegetated seafloor was based on roughness estimation. Further subdivision of these classes into the four predominant types of composition – medium sand, maërl associated with sand and gravel, seagrass settled on sand and gravel, and seagrass settled on bedrock – was carried out through supervised classifications of morphometric derivatives of the bathymetry and textural indices of backscatter, based on information from training stations. The resulting morphologic and seabed composition maps were combined to plot the distribution of the predominant habitats in the coastal waters offshore NE Malta, some of which are of high conservation value. Ground-truthing of the habitat map using ROV imagery and dive observations confirms that our approach produces a simplified and accurate representation of seafloor habitats while using all the information available within the MBES data sets.

Mesophotic coral buildups in a prodelta setting (Late Eocene, southern Pyrenees, Spain): a mixed carbonate–siliciclastic system

AbstractLower Priabonian coral bioherms and biostromes, encased in prodelta marls/clays, occur in the Aínsa‐Jaca piggyback basin, in the South Central Pyrenean zone. Detailed mapping of lithofacies and bounding surfaces onto photomosaics reveals the architecture of coral buildups. Coral lithosomes occur either isolated or amalgamated in larger buildups. Isolated lithosomes are 1 to 8 m thick and a few hundred metres wide; clay content within coral colonies is significant. Stacked bioherms form low‐relief buildups, commonly 20 to 30 m thick, locally up to 50 m. These bioherms are progressively younger to the west, following progradation of the deltaic complex. The lowermost skeletal‐rich beds consist of bryozoan floatstone with wackestone to packstone matrix, in which planktonic foraminifera are abundant and light‐related organisms absent. Basal coral biostromes, and the base of many bioherms, consist of platy‐coral colonies ‘floating’ in a fine‐grained matrix rich in branches of red algae. Corals with domal or massive shape, locally mixed with branching corals and phaceloid coral colonies, dominate buildup cores. These corals are surrounded by matrix and lack organic framework. The matrix consists of wackestone to packstone, locally floatstone, with conspicuous red algal and coral fragments, along with bryozoans, planktonic and benthonic foraminifera and locally sponges. Coral rudstone and skeletal packstone, with wackestone to packstone matrix, also occur as wedges abutting the buildup margins. Integrative analysis of rock textures, skeletal components, buildup anatomy and facies architecture clearly reveal that these coral buildups developed in a prodelta setting where shifting of delta lobes or rainfall cycles episodically resulted in water transparency that allowed zooxanthellate coral growth. The bathymetric position of the buildups has been constrained from the light‐dependent communities and lithofacies distribution within the buildups. The process‐product analysis used here reinforces the hypothesis that zooxanthellate corals thrived in mesophotic conditions at least during the Late Eocene and until the Late Miocene. Comparative analysis with some selected Upper Eocene coral buildups of the north Mediterranean area show similarities in facies, components and textures, and suggest that they also grew in relatively low light (mesophotic) and low hydrodynamic conditions.

When the species is also a habitat: Comparing the predictively modelled distributions of Lophelia pertusa and the reef habitat it forms

Internationally there is political momentum to establish networks of marine protected areas for the conservation of threatened species and habitats. Practical implementation of such networks requires an understanding of the distribution of these species and habitats. Predictive modelling provides a method by which continuous distribution maps can be produced from limited sample data. This method is particularly useful in the deep sea where a number of biological communities have been identified as vulnerable ‘habitats’, including Lophelia pertusa reefs. Recent modelling efforts have focused on predicting the distribution of this species. However the species is widely distributed where as reef habitat is not. This study uses Maxent predictive modelling to investigate whether the distribution of the species acts as a suitable proxy for the reef habitat. Models of both species and habitat distribution across Hatton Bank and George Bligh Bank are constructed using multibeam bathymetry, interpreted substrate and geomorphology layers, and derived layers of bathymetric position index (BPI), rugosity, slope and aspect. Species and reef presence records were obtained from video observations. For both models performance is fair to excellent assessed using AUC and additional threshold dependant metrics. 7.17% of the study area is predicted as highly suitable for the species presence while only 0.56% is suitable for reef presence, using the sensitivity–specificity sum maximisation approach to determine the appropriate threshold. Substrate is the most important variable in the both models followed by geomorphology in the RD model and fine scale BPI in the SD model. The difference in the distributions of reef and species suggest that mapping efforts should focus on the habitat rather than the species at fine (100 m) scales.

Bathymetric segregation of spawning stages in the Atlantic sardine Sardina pilchardus

The present study analyzed the spatio-temporal pattern of spawning aggregations of the Atlantic sardine Sardina pilchardus using a large set of reproductive data derived from 5 daily egg production method (DEPM) surveys conducted off Portugal and the Gulf of Cadiz. Similar to previous results for the Mediterranean sardine, sardines in the Atlantic were shown to form ephemeral spawning aggregations in which actively spawning females separated spatially from the remaining population, taking with them a large proportion of males. These aggregations were shown to take place at depth; specifically, bathymetric positioning of reproductively active individuals changed during the spawning cycle in such a way that imminent spawners were located at greater depths compared to recent spawners or non-spawning females. This pattern of bathymetric segregation of sardine spawning stages is in agreement with previously published evidence that sardines spawn at depth, either from indirect acoustic sampling data or from the occurrence of early embryonic stages at greater depths. A number of physiological mechanisms that may explain changes in the bathy metric positioning of actively spawning individuals are also suggested.

Prediction of benthic biotopes on a Norwegian offshore bank using a combination of multivariate analysis and GIS classification

Abstract Mortensen, P. B., Dolan, M., and Buhl-Mortensen, L. 2009. Prediction of benthic biotopes on a Norwegian offshore bank using a combination of multivariate analysis and GIS classification. – ICES Journal of Marine Science, 66: 2026–2032. This study is part of the multidisciplinary seabed mapping programme MAREANO (Marine AREAdatabase for NOrwegian coast and sea areas). The mapping programme includes acquisition of multibeam bathymetry and acoustic backscatter data together with a comprehensive, integrated biological and geological sampling programme. The equipment used includes underwater video, boxcorer, grab, hyperbenthic sled, and beam trawl. The Tromsøflaket offshore bank was used as a case-study area to develop suitable methods for mapping habitats and biotopes. A procedure for producing maps of predicted biotopes is described that combined information on the distribution of biological communities with environmental factors and indicators. Detrended correspondence analysis (DCA) was used to relate bottom environment [including multiscale physical descriptors of the seabed derived from multibeam echosounder (MBES) data] and faunal distribution to find the best physical biotope descriptors. DCA of 252 video samples (sequences 200 m long) revealed six groups of locations representing different biotopes. These were characterized by different compositions of species, substrata, depths, and values for terrain parameters. Prediction of biotope distribution was performed using a supervised GIS classification with the MBES-derived physical seabed descriptors with the strongest explanatory ability (depth, backscatter, and broad-scale bathymetric position index) identified by the DCA. The species diversity of the identified biotopes was described from the content of the bottom samples. For future MAREANO cruises, an important task will be to ground-truth predictions of habitat and biotopes and to test the reliability of these predictions in the wider MAREANO area.

Quantifying relationships between video observations of cold-water coral cover and seafloor features in Rockall Trough, west of Ireland

Cold-water coral distributions are only partially understood even in the most well- studied areas. This is partly due to the only recent development of appropriate technology, and partly to the high cost and time associated with coral mapping, particularly in deep water. One way to opti- mise mapping is to develop predictive habitat models as proxies for the actual distribution of corals. These models may provide objective criteria for the selection of prioritised coral mapping areas. In this study, we quantified the relationship between observed cold-water coral distribution and terrain attributes as an important step in developing predictive habitat models. We estimated deep-water coral percentage cover from remotely operated vehicle video and demonstrate how such data can be used to examine quantitative relations between coral cover and terrain parameters (slope, aspect, rugosity and bathymetric position index) derived from ship-borne multibeam swath acoustic data. We show that, at carbonate mound provinces within sites on the Irish margin, coral abundance is corre- lated with terrain that is strongly sloping and irregular to a varying degree, depending on spatial scale. It is likely that terrain variations influence the hydrodynamic setting, resulting in a varying food supply. A similar approach may be applicable for other fauna in a variety of benthic environ- ments.

An ArcGIS Seabed Characterization Toolbox Developed for Investigating Benthic Habitats

Using data sets collected north of San Francisco Bay (CA) an ArcGIS classification toolset was developed using supervised image classification tools to characterize potential shallow marine benthic habitats. First-derivative images and a topographic algorithm, called Bathymetric Position Index were created from the bathymetry data set using ArcGIS Spatial Analyst tools. Backscatter intensity was also analyzed by creating training samples based on the collected sediment samples and then applying multivariate statistical tools to delinate textural classes. The data collected revealed a rugged and complex seafloor and imaged in detail basement and bedrock outcrops, sand and gravel bedforms, and flat sediment covered seabed.

Hydrothermal sulfide deposits of the Lucky Strike vent field, Mid-Atlantic Ridge

Several hydrothermal sulfide structures were sampled using Mir manned submersibles in the relatively shallow Lucky Strike vent field, Mid-Atlantic Ridge; the bathymetric position of these structures varies by approximately 100 m. The investigation of the chemical and mineral compositions of hydrothermal ore occurrences led to the conclusion that the initial high-temperature ore-bearing solution ascending toward the surface became unstable and experienced phase separation beneath the ocean floor. The phase separation was responsible for the bathymetric control of hydrothermal ore formation in the ocean.

Rhodolith assemblages from the lower Tortonian carbonate ramp of Menorca (Spain): Environmental and paleoclimatic implications

Lower Tortonian distally steepened carbonate ramp of Menorca mostly consists of foramol and rhodalgal facies deposited in inner-middle ramp, ramp slope and outer ramp settings. Red algae are abundant from the middle ramp to the lower part of the slope and their taxonomic assemblages are clearly related to the bathymetric position. Melobesioids percentage increases basinward, passing from 55.8% in the middle ramp to 97% in the toe of slope. Mastophoroids are more abundant in the middle ramp (43.1%) and decrease toward deeper paleoenvironments (1.2%). Lithophylloids and sporolithaceans appear as accessory components from the middle ramp to slope settings. The percentage of melobesioids and mastophoroids observed in the middle ramp suggest that growth of the rhodoliths started in a water depth below 10–20 m. The deepest occurrence of the rhodoliths is in the ramp slope environment, where the dominance of melobesioids and the low percentage of shallower-water subfamily suggests a water depth range of 70 to 100 m. Shape and structure of rhodoliths are indicative of high-energy conditions in all ramp settings and they do not reflect a decrease in hydrodynamic energy related to water depth increase. These conditions are related to the presence of unidirectional currents that produced cross-bedded grainstones existing in the middle ramp, ramp slope and at the toe of the ramp slope settings. The high percentage of mastophoroids in the shallowest environments and the presence of Lithoporella and Sporolithon in the coralline assemblages suggests that carbonate production took place in tropical waters.

Biological reference points in the management of North American sea urchin fisheries

The precautionary approach calls for both target and limit reference points (TRPs and LRPs, respectively) and notes the vulnerability of developing fisheries to excess effort. LRPs should reflect a population's ability to continue to persist, which depends on lifetime egg production (LEP) and current abundance. The unique characteristics of sea urchin fisheries, such as (i) tight ecological coupling, (ii) their being roe fisheries, (iii) protection of juveniles under adult spine canopies, and (iv) broadcast spawning, can influence their management. Most North American sea urchin fisheries developed rapidly. Their reference points include (i) several LRPs and TRPs based on the logistic or surplus production model, (ii) a unique TRP involving direct monitoring of the bathymetric position of their macro phyte food, and (iii) an LRP based on the fraction of natural, unfished LEP. The dominant effect of fishing down and serial depletion on catch and effort data from developing sea urchin fisheries adversely affects fits to the logistic model. Reference points based on tight ecological coupling will be useful only where food webs are simple and one-dimensional. Sea urchin fisheries developed in the future should consider the fraction of natural LEP as an LRP and attempt to limit early excess fishing capacity.

Trace fossils as a paleoceanographic tool: evidence from Late Quaternary sediments of the southwestern Iberian margin

Biogenic structures in Late Quaternary sediments from the southwestern Iberian continental slope were studied by using X-ray images from two cores from 580 and 1750 m water depth. Eight different ichnocoenoses were observed: indistinct bioturbation, Planolites-dominated, Thalassinoides-dominated, Chondrites-dominated, Planolites and Thalassinoides-dominated, pyritized microburrows such as Trichichnus and ‘ Mycellia’-dominated, Chondrites, Trichichnus, and ‘ Mycellia’-dominated and Zoophycos. Variations of the ichnocoenoses within the cores show a striking correlation with climatically induced changes in the hydrographical regime, i.e. current strength and bathymetric position of the Mediterranean Outflow Water (MOW). Comparison of the response of the ichnocoenoses to changes in bottom-water conditions and substrate between the two cores studied indicate that bottom-water oxygenation and enrichment of particulate organic matter at the base of the MOW layer are the primary factors controlling the ichnocoenoses. The traces even recorded short-term climatic changes such as the Younger Dryas cold event. The spatial and temporal distribution patterns are in good agreement with earlier models of the MOW history, which gives reason to see a refinement of trace fossils as a complementary tool for paleoceanographical studies.

Eocene siliceous sponges from the Ebro Basin (Catalonia, Spain)

The Eocene (Bartonian) marls of the La Guixa Member and Gurb Member, Vic Marls Formation (Ebro Basin, Catalonia, Spain), contain a very rich and diversified siliceous sponge fauna. The fauna is dominated by hexactinellids; lithistids and other demosponges are rare. It consists of 16 species representing 16 genera. Eleven new species and two new genera are proposed for these sponges: Reguantella cavernosa nov. gen. nov. sp., Regadrella concinna nov. sp. (both Hexactinellida, Lyssacinosa), Eurete clava nov. sp., Pleuroguettardia iberica nov. sp., Aphrocallistes almeriae nov. sp., Hexactinella informis nov. sp. (all Hexactinellida, Hexactinosa), Brachiolites munterensis nov. sp., Centrosia viquensis nov. sp., Callicylix eocenicus nov. sp., Rhizocheton robustus nov. sp. (all Hexactinellida, Lychniscosa), Propetrosia pristina nov. gen. nov. sp. (Demospongia, Haplosclerida). Some genera of sponges in this fauna are still extant, but, in general, the predominant ones are very close in morphology, and, without doubt, closely related to the Late Cretaceous sponges. This fauna also differs considerably, in terms of composition, from most other described faunas of Tertiary sponges from the Mediterranean region, which are dominated by lithistid sponges. Lithistid sponges are rare in this investigated assemblage, which seems most similar to an as yet undescribed Eocene fauna from Italy. There is ecological differentiation in the proportions of particular sponges in various outcrops and/or stratigraphical levels that is clearly associated with water-depth-related controlling factor(s): Munter, Tona and Sta. Cecilia represent the deepest facies, Gurb is intermediate, and St. Roc and Vespella are the most shallow. The exact bathymetric position of the sponge fauna is difficult to estimate, but it seems that 100 m (but probably 200 m and more in the case of the deepest parts) of water depth may be inferred for this facies.

Species identification of pelagic fish schools using acoustic descriptors and ancillary information

It is shown that schools of similarly sized and behaving pelagic fish species can be accurately identified to species over a large spatial area, on the order of the broad spatial-scales typical of standard assessment surveys, and over a time-span of three years. Acoustic measurements of morphometric, energetic, and bathymetric features of anchovy, sardine, and round herring schools were extracted using commercially available software from acoustic data collected by a conventional single-frequency narrow-band echosounder during trawling operations of pelagic stock surveys of the South African continental shelf (November 1997, 1998 and 1999). Discriminant function analysis of 18 descriptors of the 214 schools for which the species composition was satisfactorily determined by trawl samples indicated that schools could be correctly identified to species in 88.3% of all cases (94.9% for anchovy, 82.6% sardine, 82.6% round herring). The three species were differentiated primarily on the basis of school bathymetric position and backscattered energy. Including ancillary information (latitude, longitude, sea surface temperature, bottom depth, and time of day) in analysis improved the accuracy of school identification to 94.9%, since such variables allowed discrimination on the basis of interspecific differences in habitat use, as well as in schooling behavior. a)Currently at MIT/Woods Hole Joint Program.

Age of settlement and accumulation rate of submarine “coralligène” (−10 to −60 m) of the northwestern Mediterranean Sea; relation to Holocene rise in sea level

Morphological and chronological studies have been carried out on coralline algal buildups (“coralligène”) situated between 10 and 60 m depth near Marseilles, and in Corsica (France). Despite the presence of occasionally sizeable quantities of iron hydroxide, these constructions prove to be a reliable material for radiocarbon dating. Ages obtained using this method range from 640±120 yrs B.P. (Corsica, Scandola Natural Reserve, −15 m) to 7760±80 yrs B.P. (Marseilles, Grand Congloue, −52 m). Internal erosion surfaces within the buildups give evidence of discontinuous development. The accumulation rate of the coralligène constructions is very low (0.006–0.83 mm yr −1 according to the depth and time period). The higher values (0.53–0.83 mm yr −1) were recorded for the deeper constructions. They correspond to a period between 8000 and 6000 yrs B.P. After 6000 yrs B.P., the only appreciable accumulation rates (0.11–0.42 mm yr −1) were recorded for constructions situated between 10 and 35 m depth, whereas the accumulation rates of deep coralligène (> 50 m) appear to be low or zero. The age of the large constructions (overhang: ⩾ 80 cm in width) is positively correlated with depth ( r = 0.95; p < 0.005). Their development occurred during the Flandrian transgression. The oldest structures, today situated at 50 to 60 m depth, started to develop when water depth was probably no greater than 10 to 15 m. Apart from in strongly shaded fissures on rocky coasts and areas subjected to heavy sedimentation, the main framework building algal species was initially Mesophyllum lichenoides (Ellis) Lemoine, a high tolerant species to light, hydrodynamic energy and temperature. With the rise in sea level, the coralligène structure gradually became available to other less tolerant algal species ( Lithophyllum, Lithothamnion), and the crustforming population diversified. Because of the good preservation of coralligène structures, the reliability of radiocarbon dating and the correlation between the age and bathymetric position of the large coralligène structures (except in areas of heavy sedimentation and fissures in shallow rocky coasts), these buildups are considered to be of use as biological indicators of variations in sea level.

Selectivity of end-Cretaceous marine bivalve extinctions.

Analyses of the end-Cretaceous or Cretaceous-Tertiary mass extinction show no selectivity of marine bivalve genera by life position (burrowing versus exposed), body size, bathymetric position on the continental shelf, or relative breadth of bathymetric range. Deposit-feeders as a group have significantly lower extinction intensities than suspension-feeders, but this pattern is due entirely to low extinction in two groups (Nuculoida and Lucinoidea), which suggests that survivorship was not simply linked to feeding mode. Geographically widespread genera have significantly lower extinction intensities than narrowly distributed genera. These results corroborate earlier work suggesting that some biotic factors that enhance survivorship during times of lesser extinction intensities are ineffectual during mass extinctions.

Climatic change and CaCO3 preservation: An 800,000 year bathymetric Reconstruction from the central equatorial Pacific Ocean

Sixteen deep‐sea cores from the central equatorial Pacific are used to reconstruct a continuous 800,000‐year (800‐kyr) record of bathymetric variations in carbonate preservation as measured by calcium carbonate (CaCO3) content. The depth of the sedimentary lysocline has fluctuated markedly in conjunction with late Pleistocene climate cycles while the carbonate critical depth (CCrD), which is the water depth where the sediments contain 10% CaCO3, has remained relatively constant. As a result, the depth of the lysocline controls the bathymetric position and thickness of the CaCO3 transition zone, defined as the depth from the lysocline to the CCrD. Modern and interglacial‐aged sediments show poor CaCO3 preservation and thick CaCO3 transition zones. Glacial‐aged sediments show good preservation and deep, thin zones due to the deepening of the lysocline. Detailed comparison of the CaCO3 preservation and oxygen isotope records from the central equatorial Pacific confirms the observation that preservation maxima and minima tend to occur during the latter half of glacial and interglacial stages and on climate transitions rather than during the middle of climatic stages. During the nine major glacial stages of the last 800 kyr, the lysocline deepened by at least 400 to 800 m. This deepening indicates an increase in the abyssal carbonate ion concentration ([CO3=]) and a depression of the calcite saturation horizon best explained by the deeper presence of a more carbonate‐saturated water mass. The bottom of the transition zone has remained at a relatively constant depth during the Brunhes Chron, indicating a balance between CaCO3 sedimentation and dissolution in the deepest waters of the central equatorial Pacific.

New discoveries of massive sulfides on the East Pacific Rise

Newly discovered massive sulfide occurrences were mapped on the East Pacific Rise between 18° and 24°S. Large occurrences of sulfides are restricted to zones in which there is more tectonic strain than usual for sea-floor spreading centers. These sites are also characterized by a relatively shallow bathymetric position and greater coverage with sheet lava than with pillow lava. Several hydrothermal sources with no sulfide precipitate on the sea floor suggest precipitation of sulfide below the sea floor (stockwork mineralization). Animal communities that depend on sulfide oxidation are scarce and small — at least, that is, the fauna that is large enough to be recognized in underwater photographs. The animal communities around hydrothermal sources which profit from the larger food supply provided by the primary fauna are present in dense populations over relatively large areas. The enrichment of the hydrothermal component in the sediments adjacent to a spreading center is not typical of hydrothermal fields along the East Pacific Rise. It can be concluded from the evaluation of the sediment that the hydrothermal activity increases northwards.

Recent abyssal benthic foraminiferal biofacies of the eastern equatorial Indian Ocean

Study of Recent abyssal benthic foraminifera from core-top samples in the eastern equatorial Indian Ocean has identified distinctive faunas whose distribution patterns reflect the major hydrographic features of the region. Above 3800 m, Indian Deep Water (IDW) is characterized by a diverse and evenly-distributed biofacies to which Globocassidulina subglobosa, Pyrgo spp., Uvigerina peregrina, and Eggerella bradyi are the major contributors. Nuttalides umbonifera and Epistominella exigua are associated with Indian Bottom Water (IBW) below 3800 m. Within the IBW fauna, N. umbonifera and E. exigua are characteristic of two biofacies with independent distribution patterns. Nuttalides umbonifera systematically increases in abundance with increasing water depth. The E. exigua biofacies reaches its greatest abundance in sediments on the eastern flank of the Ninetyeast Ridge and in the Wharton-Cocos Basin. The hydrographic transition between IDW and IBW coincides with the level of transition from waters supersaturated to waters undersaturated with respect to calcite and with the depth of the lysocline. Carbonate saturation levels, possibly combined with the effects of selective dissolution on the benthic foraminiferal populations, best explain the change in faunas across the IDW/IBW boundary and the bathymetric distribution pattern of N. umbonifera. The distribution of the E. exigua fauna cannot be explained with this model. Epistominella exigua is associated with the colder, more oxygenated IBW of the Wharton-Cocos Basin. The distribution of this biofacies on the eastern flank of the Ninetyeast Ridge agrees well with the calculated bathymetric position of the northward flowing deep boundary current which aerates the eastern basins of the Indian Ocean.

Paleoecologic Determination of Bathymetric Position of Organic Buildups--Example from Lower Paleozoic of Appalachians: ABSTRACT

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