High Organic Matter Flux Research Articles

Evolution and dynamics of the Arabian Sea oxygen minimum zone: Understanding the paradoxes

The Arabian Sea hosts a perennial and intense oxygen minimum zone (OMZ) at 150–1200 m depths with O2 concentrations <0.5 ml/l. It is generally believed that the oxygen-depleted conditions at mid-water depths result from high rate of O2 consumption due to monsoon-driven productivity generating a high organic matter flux, combined with slow renewal of thermocline waters in the region. With global warming and increasing hypoxia, there is growing interest to better understand the various factors controlling oxygen conditions in the thermocline waters and the impact on the nutrient cycling and climate. In this contribution, we provide an overview of new advances in understanding the basin-wide changes of the OMZ, and highlight new perspectives on the relative roles of ocean and atmospheric circulations in modulating the OMZ intensity through the late glacial-Holocene period. Comprehension of the existing and new proxy records (δ15N, aragonite preservation, δ13C of benthic foraminifera) from the productive western and oligotrophic eastern and north-eastern Arabian Sea provides insights into the regional heterogeneity in basin-wide changes of the OMZ, denitrification and carbonate (aragonite) lysocline, and their links to the seasonal monsoon variability and reorganisation of thermocline circulation. We also highlight the limitations of the existing proxy data to address the important questions of how circulation and chemical properties of intermediate/deep water masses contributing to the Arabian Sea thermocline waters changed in the past. Hence, more detailed proxy data are required to characterise sources of water masses, past changes in their pathways and vertical extents in the Arabian Sea, which are crucial to better constrain the temporal evolution of thermocline ventilation basin-wide.

HELMINTHOPSISANDCYLINDRICHNUSICHNOGUILDS FROM MIOCENE THIN-BEDDED TURBIDITES, TIERRA DEL FUEGO, ARGENTINA

AbstractMiocene thin-bedded turbidites from Tierra del Fuego record scarce graphoglyptids and two unusual ichnoguilds composed of diminutive elite trace fossils. The first, a monoichnospecific Cylindrichnus ichnoguild, consists of crowded, post-depositional burrows formed in surface sediments during the final phase of turbidite deposition. The second, a pre-depositional Helminthopsis ichnoguild, consists of dense aggregates of simple trails, mainly Helminthopsis and Helminthoidichnites, occupying a very shallow tier in organic-rich mud covering the sea floor prior to turbidite deposition. The trace makers of Cylindrichnus were opportunistic suspension/detritus feeding organisms, probably polychaetes, which bloomed during high flux of labile organic matter brought to internal and external levees by turbidity currents. The trace makers of Helminthopsis and Helminthoidichnites were probably nematodes that grazed on organic-rich muddy sediments with abundant disseminated pyrite associated with Kinneyia-like and other problematic wrinkle structures, suggesting sulfur-cycling chemosynthetic microbial communities originated during interturbidite phases. The rhythmical alternation of the Cylindrichnus and Helminthopsis ichnoguilds clearly differentiate the thin-bedded turbidites of the Viamonte Formation from channel-levee complexes elsewhere, stressing the point that ichnoassemblages reflect sets of environmental parameters and not necessarily particular depositional settings.

Palaeoceanographic and palaeoenvironmental controls on late Quaternary benthic foraminifera of the western continental slope of South Africa

Benthic foraminifera from two gravity cores recovered on the western continental slope (3522 and 3631 m water depth) of South Africa were studied to determine bottom water conditions and benthic environments over late Quaternary glacial-interglacial cycles. Benthic foraminifera assemblages and the carbon isotopic composition (δ13C) of Cibicidoides wuellerstorfi and Globoconella inflata were used to reconstruct palaeoenvironmental conditions. The assemblages reflect influences from substrate, productivity (seasonal and annual variations in the flux of organic matter, such as phytodetritus to the seafloor) and bottom water oxygenation. Uvigerinid forms were found to be distributed along bathymetric depths that also reflect oxygenation of bottom waters, where hispid and hispidocostate forms were more abundant at depths of oxygen-enriched bottom waters on the lower slope. These uvigerinids and other benthic species such as Epistominella exigua were also found to benefit from periods of high productivity. Palaeoproductivity increased during glacial periods but decreased sharply by a difference of ∼1‰ in δ13C during glacial terminations, reflecting a less nutrient-rich bottom water signal. During interglacial periods, epifaunal foraminifera and taxa adapted to oxygen-enriched environments, such as Cibicidoides spp. and high oxygen assemblages increased in abundance and were correlated to the less radiogenic regional εNd signals, while the abundance of high-productivity fauna and assemblages, associated with the nutrient-rich Southern Component Water, and that are adapted to a high organic matter flux and low oxygen conditions, decreased. Interglacial periods were reflected to exhibit benthic environments that are more oligotrophic, while glacial periods were interpreted to reflect more eutrophic conditions.

A one-million-year isotope record from siderites formed in modern ferruginous sediments

Abstract Ancient iron formations hold important records of environmental conditions during the Precambrian eons. Reconstructions of past oceanic systems require investigation of modern ferruginous analogs to disentangle water column and diagenetic signals recorded in iron-bearing minerals. We analyzed oxygen, iron, and carbon isotopes in siderite, a ferrous carbonate phase commonly used as an environmental proxy, from a 100-m-long record spanning a 1 Ma depositional history in ferruginous Lake Towuti, Indonesia. Combining bulk sediment and pore water geochemistry, we traced processes controlling siderite isotope signatures. We show that siderite oxygen isotope compositions (δ18O) reflect in-lake hydrological and depositional conditions. Low iron isotope values (δ56Fe) record water column oxygenation events over geological timescales, with minor diagenetic partitioning of Fe isotopes by microbial iron reduction after deposition. The carbon isotope compositions (δ13C) reflect the incorporation of biogenic HCO3–, which is consistent with sediment organic matter remineralization lasting over ca. 200 ka after burial. Positive δ13C excursions indicate an increased production of biogenic methane that escaped the sediment during low lake levels. Diffusion across the sediment–water interface during initial formation of siderites tends to align the isotope signatures of bottom waters to those of pore waters. As microbial reduction of ferric iron and oxidation of organic matter proceed and saturate pore water conditions with respect to siderite, overgrowth on nuclei partially mutes the environmental signal inherited from past bottom waters over ca. 1 Ma. Because high depositional fluxes of ferric iron and organic matter in early oceans would have promoted similar microbial processes in ferruginous deposits prior to lithification, the environmental record contained in siderite grains can successively integrate depositional and early diagenetic signals over short geological timescales.

Comparing paleo-oxygenation proxies (benthic foraminiferal surface porosity, I/Ca, authigenic uranium) on modern sediments and the glacial Arabian Sea

Oceanic oxygen reconstructions of the last glacial period are needed to understand the mechanisms of glacial deep ocean carbon storage and to validate climate model simulations. However, existing bottom-water oxygen (BWO) reconstructions are ambiguous due to limitations of each paleo-BWO proxy. Here we present data on three proxies for BWO: benthic foraminiferal surface porosity, benthic foraminiferal iodine/calcium (I/Ca), and authigenic uranium (aU), from globally distributed core-top samples, and we evaluate the potential advantages and limitations of these BWO proxies on global and regional scales. All three proxies are most sensitive to changes at relatively low BWO concentrations (<∼50 µmol/kg). Data from globally-distributed core tops confirm that foraminiferal surface porosity is correlated with BWO between 0 and 100 µmol/kg. Our analysis further confirms that benthic surface porosity is predominantly controlled directly by BWO rather than other potential functionalities of surface pores such as organic carbon uptake and respiratory CO2 release. Low benthic I/Ca can identify low BWO (<50 µmol/kg), whereas higher benthic I/Ca values are not associated with specific BWO, possibly due to additional dependence of I/Ca on temperature, salinity, carbonate ion concentration, or water mass mixing at higher BWO. The relationship between aU and BWO is regionally dependent. In the Arabian Sea, variable aU enrichments occur only within the Oxygen Minimum Zone (OMZ), driven by high export production and organic matter fluxes to the sediment. Finally, we combine foraminiferal surface porosity, I/Ca and aU to generate the first quantitative glacial-Holocene BWO reconstructions using a sediment core taken from within the modern Arabian Sea OMZ. All three proxies consistently suggest BWO < 50 μmol/kg in the shallow Arabian Sea during the last ∼30 kyr, with relatively higher BWO during the glacial period than the Holocene. A comparison with the benthic carbon isotope gradient proxy (Δδ13C) confirms that Δδ13C over-estimates BWO in low-oxygen settings possibly due to sediment diagenesis impacts. Our study provides new insights on the merits and limitations of these BWO proxies and confirms the importance of multi-proxy reconstructions for more reliable paleo-BWO estimates.

Calcium Carbonate Dissolution Triggered by High Productivity During the Last Glacial–Interglacial Interval in the Deep Western South Atlantic

Studies reconstructing surface paleoproductivity and benthic environmental conditions allow us to measure the effectiveness of the biological pump, an important mechanism in the global climate system. In order to assess surface productivity changes and their effect on the seafloor, we studied the sediment core SAT-048A, spanning 43–5 ka, recovered from the continental slope (1,542 m water depth) of the southernmost Brazilian continental margin, deep western South Atlantic. We assessed the sea surface productivity, the organic matter flux to the seafloor, and calcite dissolution effects, based on micropaleontological (benthic and planktonic foraminifers, ostracods), geochemical (benthic δ13C isotopes), and sedimentological data (carbonate and bulk sand content). Superimposed on the induced changes related to the last glacial–interglacial transition, the reconstruction indicates a significant and positive correlation between the paleoproductivity proxies and the summer insolation. From the reconstructed data, it was possible to identify high (low) surface productivity, high (low) organic matter flux to the seafloor, and high (low) dissolution rates of planktonic Foraminifera tests during the glacial (postglacial). Furthermore, within the glacial, enhanced productivity was associated with higher insolation values, explained by increased northeasterly summer winds that promoted meandering and upwelling of the nutrient-rich South Atlantic Central Water. Statistical analyses support the idea that productivity is the main cause for seafloor calcium carbonate dissolution, as opposed to changes in the Atlantic Meridional Overturning Circulation (at least for the 25–4 ka period). Further efforts must be invested in the comprehension and quantification of the total organic matter and biogenic carbonate burial during time intervals with an enhanced biological pump, aiming to better understand their individual roles.

Anammox bacteria drive fixed nitrogen loss in hadal trench sediments

Benthic N2 production by microbial denitrification and anammox is the largest sink for fixed nitrogen in the oceans. Most N2 production occurs on the continental shelves, where a high flux of reactive organic matter fuels the depletion of nitrate close to the sediment surface. By contrast, N2 production rates in abyssal sediments are low due to low inputs of reactive organics, and nitrogen transformations are dominated by aerobic nitrification and the release of nitrate to the bottom water. Here, we demonstrate that this trend is reversed in the deepest parts of the oceans, the hadal trenches, where focusing of reactive organic matter enhances benthic microbial activity. Thus, at ∼8-km depth in the Atacama Trench, underlying productive surface waters, nitrate is depleted within a few centimeters of the sediment surface, N2 production rates reach those reported from some continental margin sites, and fixed nitrogen loss is mainly conveyed by anammox bacteria. These bacteria are closely related to those known from shallow oxygen minimum zone waters, and comparison of activities measured in the laboratory and in situ suggest they are piezotolerant. Even the Kermadec Trench, underlying oligotrophic surface waters, exhibits substantial fixed N removal. Our results underline the role of hadal sediments as hot spots of deep-sea biological activity, revealing a fully functional benthic nitrogen cycle at high hydrostatic pressure and pointing to hadal sediments as a previously unexplored niche for anaerobic microbial ecology and diagenesis.

Feeding Characteristics of Deep-Sea Acorn Worm (Hemichordata, Enteropneusta, Torquaratoridae) from the Bering Sea.

Deep-sea hemichordates Torquaratoridae gen. sp. reach high abundance up to 12 spec. m-2 at the depths of 1830-2130 m on the slope of the Volcanologists Massif in the south-western part of the Bering Sea, dominating in the benthic community at these depths. Their abundance exceeds by two orders the values recorded earlier. In order to clarify this phenomenon, we examined the gut contents of Torquaratoridae gen. sp. The detritus particles and frustules of planktonic diatoms Thalassiosira, Coscinodiscus, Actinocyclus, Chaetoceros, Neodenticula, and Grammatophora were the most common in the gut, as well as the remains of skeletons of benthic invertebrates with little admixture of mineral particles. According to obtained data, Torquaratoridae gen. sp. are mobile deposit feeders with high selectivity to fresh phytodetritus, able to compete with holothurians occupying similar trophic niche. Unusually high abundance of acorn worms is apparently related to high organic matter flux to the seafloor as a result of spring phytoplankton bloom in the surface water layer of the Bering Sea.

Palaeoenvironmental changes after the Messinian Salinity Crisis in the Mediterranean Almería-Níjar Basin (SE Spain) recorded by benthic foraminifera

In this study, the early Pliocene palaeoenvironmental context of the Mediterranean Almería-Níjar Basin (SE Spain) is analysed using benthic foraminifera, with special focus on environmental conditions after the Messinian Salinity Crisis (MSC). A deep upper-slope setting (274 m water depth), with high oxygen levels and low organic matter fluxes, is inferred for the earliest Pliocene sediments over the Miocene-Pliocene boundary. In this setting, the westernmost and earliest recolonization of the species Siphonina reticulata across the Mediterranean is reported. This finding, along with the presence of intensively bioturbated sediments and highly diverse benthic foraminiferal assemblages at —or just above— the Miocene-Pliocene boundary might indicate the earliest establishment of well-ventilated fully marine conditions in the Mediterranean after the MSC. Subsequent shallowing from 274 to 220 m water depth during the earliest Pliocene is likely related to a global sea-level drop. Palaeobathymetry remains stable around 220–230 m water depth after this sea-level fall. However, three successive palaeoenvironmental subintervals could be defined within this upper slope setting. Increasing continental organic matter and sediment inputs indicate higher fluvial discharge during the first subinterval. The second subinterval records a decrease in both terrestrial organic matter and sediment supply due to lower riverine discharge. Finally, reduced input of continental organic matter suggests low river runoff during the third subinterval, in which a slight increase in transported shelf foraminiferal taxa may be related to vertical tectonic movements of the basin margins.

Oceanic water chemistry evolution and its implications for post-glacial black shale formation: Insights from the Cryogenian Datangpo Formation, South China

The Cryogenian Nanhua Basin in South China comprises one of the best-preserved Neoproterozoic glacial-interglacial sedimentary fills. The Datangpo Formation (ca. 663–654 Ma) was deposited after the Sturtian glaciation and is characterised by extensive organic-rich black shales and interbedded manganese deposits. Trace element data suggest overall anoxic waters with varying degrees of euxinia. These anoxic and sulfidic conditions, as inferred by high ratios of FeHR/FeT and Fepy/FeT and significant enrichments of molybdenum, developed due to high organic carbon loading, resulting in the formation of the black shales. The high organic matter flux was likely the result of increased nutrient supply from continental weathering, upwelling nutrient-rich deep seawater, and seawater exchange due to elevated sea levels after deglaciation, which resulted in a temporary connection with the open sea. An episode of oxygenation during deposition of the manganese-bearing interval shortly after deglaciation is indicated by a negative excursion in the iron speciation data, and generally low enrichments of trace elements. Muted sulfate reduction rates, and thus inferred less euxinic conditions, occurred due to decreased organic matter loading during deposition of the upper-part of the black shales. The lower marine primary productivity was likely a result of suppressed bio-essential elements due to progressively weaker chemical weathering under cooler climatic conditions. Superheavy pyrite with δ34Spy up to +61‰ for the Datangpo Formation is consistent with a compilation of published data. We propose that multiple processes contributed to the superheavy pyrite formation, and suggest a new, more accurate way of recalibration of the sulfate reservoir in sedimentary basins with similar geological and/or geochemical settings. This dataset and model serves as a reference for better understanding the formation of post-glacial organic-rich black shales, including the temporal evolution of oceanic conditions after low-latitude glaciations.

Is volcanic ash responsible for the enrichment of organic carbon in shales? Quantitative characterization of organic-rich shale at the Ordovician-Silurian transition

Abstract Subaerial volcanism and atmospheric volcanic ash deposition have been recognized as factors that can greatly affect the nutrient content of the surface ocean and the redox conditions of the water column. Black siliceous, organic-rich mudstone and shale containing numerous volcanic ash layers were deposited in the South China Block during the Ordovician-Silurian transition. Although this association has been observed in other regions, whether there is a relationship between volcanic ash and the organic carbon contents and the effect of volcanic ash remains unclear. Based on analysis of the concentrations of major elements, trace elements, and total organic carbon in the volcanic ash and shale, we found that anoxic and high-productivity environments existed during the Ordovician-Silurian transition and that organic matter was preferentially preserved under these conditions. For the volcanic ash, we quantitatively estimated the depletion of the nutrient elements Fe, Si, and P (in percentages). The calculated results show that leaching removed 25–75% of the Fe, Si, and P in most of the ash samples in the study area, potentially leading to high marine primary productivity in the surface water. Redox conditions also played a major role in the preservation of organic matter. The trace element analysis results show that although productivity was high during the Ordovician-Silurian transition, organic matter was preferentially preserved in the Lower Silurian strata. Therefore, high organic matter flux and good preservation conditions both contributed to the formation of the organic-rich shale, and volcanic ash was the dominant source of nutrients for primary productivity.

Changes in western Mediterranean thermohaline circulation in association with a deglacial Organic Rich Layer formation in the Alboran Sea

The accumulation of an Organic Rich Layer (ORL) during the last deglaciation in the Alboran Sea (western Mediterranean Sea) and its link to changes in deep and intermediate water circulation are here investigated. Benthic foraminiferal assemblages and the shallow infaunal foraminifer Uvigerina peregrina δ13C record support the establishment of sustained high organic matter fluxes, and thus eutrophic conditions at the sea floor, during the late phase of the ORL (Younger Dryas to early Holocene periods). Since organic matter fluxes were lower (mesotrophic conditions) during the Bølling-Allerød period, they cannot be solely responsible for the ORL initiation. Geochemical, sedimentological and micropalaeontological proxies support a major weakening of the deep-water convection in the Gulf of Lion as the main driver for the development of poorly-ventilated conditions from intermediate depths (946 m) to the deep western Mediterranean basin that promoted the beginning of the ORL deposition. Nevertheless, a better ventilation at intermediate depths was established during the late ORL, while the deep basin remained poorly ventilated. We propose that our data reflect the arrival of a new better-ventilated intermediate water mass analogue to the current Levantine Intermediate Water (LIW) and/or a new intermediate water mass from the Gulf of Lion. The ultimate source of this water mass needs to be further explored but chronologies of the changes recorded here indicate that intermediate and deep ventilation phases were decoupled between the western and eastern Mediterranean basins during the deglaciation and early-middle Holocene.

Organic Matter Sources in North Atlantic Fjord Sediments

AbstractTo better constrain the global carbon cycle fundamental knowledge of the role of carbon cycling on continental margins is crucial. Fjords are particularly important shelf areas for carbon burial due to relatively high sedimentation rates and high organic matter fluxes. As terrigenous organic matter is more resistant to remineralization than marine organic matter, a comprehensive knowledge of the carbon source is critical to better constrain the efficiency of organic carbon burial in fjord sediments. Here we investigated highly productive fjords in northern Norway and compare our results with both existing and new organic carbon to organic nitrogen ratios and carbon stable isotope compositions from fjords in mid‐Norway, west Svalbard, and east Greenland. The marine organic carbon contribution varies significantly between these fjords, and the contribution of marine organic carbon in Norwegian fjords is much larger than previously suggested for fjords in NW Europe and also globally. Additionally, northern Norwegian fjords show very high marine carbon burial rates (73.6 gC · m‐2 · year‐1) suggesting that these fjords are probably very distinct carbon burial hotspots. We argue that the North Atlantic Current inflow sustains these high burial rates and changes in the current strength due to ongoing climate change are likely to have a pronounced effect on carbon burial in North Atlantic fjords.

Are light-dark coupled laminae in lacustrine shale seasonally controlled? A case study using astronomical tuning from 42.2 to 45.4 Ma in the Dongying Depression, Bohai Bay Basin, eastern China

As a typical sedimentary structure in fine-grained rocks, laminae are widely distributed in shales and mudstones in sedimentary basins. The Shahejie Formation (42.2–45.4 Ma) formed during the Eocene of the Paleogene in the Dongying Depression of China is recognized as a typical area to study lacustrine shale. According to the composition of different lamina, four types of couplets are identified, including carbonate-clay couplets, carbonate-organic couplets, clay-organic couplets and carbonate-clay-organic triplets. All couplets are composed of light and dark layers. A combination of core images, microscopic observations, mineral compositions, geochemical data, carbon and oxygen isotopes, and strontium isotopes verifies that the laminae are primarily developed in a saline and anoxic, or even euxinic environment, with a high organic matter (OM) flux. Through astronomical cycle analysis of the natural gamma ray (GR) curve from sample site well NY1, the data suggest that the lacustrine shale laminae are formed annually with an average duration of 1.34 yr. The formation of laminae is affected by the season, which is closely related to the seasonal growth and death of algae. During the period of algae growth, a large number of light-colored carbonate laminae are deposited, whereas when the algae dies, organic matter accumulates at the bottom of lakes and forms dark organic-rich laminae. The interbedded and lenticular laminae, which are subdivided by layering characteristics, are primarily affected by diagenesis in postdepositional processes. Study of the laminae is helpful in understanding the formation process of lacustrine shale, and it provides invaluable sources of information for paleoclimate reconstruction.

The distinct roles of two intertidal foraminiferal species in phytodetrital carbon and nitrogen fluxes – results from laboratory feeding experiments

Abstract. Benthic foraminifera play a major role as primary consumers and detrivores redistributing organic carbon and nitrogen in intertidal environments. Here we compared the differences of phytodetrital carbon and nitrogen intake and turnover of two dominant intertidal foraminifera, Ammonia tepida and Haynesina germanica. Their lifestyles in relation to feeding behavior (feeding preferences, intake and turnover of phytodetrital carbon and nitrogen) and temperature adaptations were compared to obtain a closer definition of their specific roles in intertidal organic matter processing. For this comparison, we carried out a series of short-term laboratory incubations with stable-isotope-labeled (13C and 15N) detritus as the food source. We compared the response of the two species to diatom detritus at three different temperatures (15, 20, 25 ∘C). Ammonia tepida showed a very high, temperature-influenced intake and turnover rates with more excessive carbon turnover, compared to nitrogen. The fairly low metabolic nitrogen turnover in H. germanica was not affected by temperature and was higher than the carbon turnover. This might be related with the chloroplast husbandry in H. germanica and its lower demands for food-derived nitrogen sources. Ammonia tepida prefers a soft chlorophyte food source over diatom detritus, which is harder to break down. In conclusion, A. tepida shows a generalist behavior that links with high fluxes of organic matter (OM). Due to its high rates of OM processing and abundances, we conclude that A. tepida is an important key player in intertidal carbon and nitrogen turnover, specifically in the short-term processing of OM and the mediation of dissolved nutrients to associated microbes and primary producers. In contrast, H. germanica is a highly specialized species with low rates of carbon and nitrogen budgeting.

Metabolic specialization of denitrifiers in permeable sediments controls N2 O emissions.

Coastal oceans receive large amounts of anthropogenic fixed nitrogen (N), most of which is denitrified in the sediment before reaching the open ocean. Sandy sediments, which are common in coastal regions, seem to play an important role in catalysing this N-loss. Permeable sediments are characterized by advective porewater transport, which supplies high fluxes of organic matter into the sediment, but also leads to fluctuations in oxygen and nitrate concentrations. Little is known about how the denitrifying communities in these sediments are adapted to such fluctuations. Our combined results indicate that denitrification in eutrophied sandy sediments from the world's largest tidal flat system, the Wadden Sea, is carried out by different groups of microorganisms. This segregation leads to the formation of N2 O which is advectively transported to the overlying waters and thereby emitted to the atmosphere. At the same time, the production of N2 O within the sediment supports a subset of Flavobacteriia which appear to be specialized on N2 O reduction. If the mechanisms shown here are active in other coastal zones, then denitrification in eutrophied sandy sediments may substantially contribute to current marine N2 O emissions.

Benthic foraminiferal paleoecology of the Maastrichtian succession at the Kharga Oasis, Western Desert, Egypt

The present work deals with the benthic foraminiferal investigations and analyses of the Maastrichtian succession from two sections in the Kharga Oasis (Naqb El-Rufuf and Gabal Teir), to evaluate the prevailing paleobathymetric and paleoenvironmental conditions. Three benthic foraminiferal assemblages have been distinguished by the R-mode cluster analysis. The first two clusters are dominated by simple-walled and complex-walled non-calcareous agglutinated foraminifera, respectively; while the third assemblage is characterized by calcareous agglutinated and calcareous benthic foraminiferal species. The Q-mode Detrended Correspondence Analysis (DCA) of the studied samples indicates five main groups designated as five biofacies (IV), each with a characteristic faunal structure. The overall dominance of the non-calcareous agglutinated foraminifera and the sporadic occurrence of the calcareous benthic foraminifera suggest deposition in a shallow water environment of restricted brackish, lagoonal, and littoral setting with occasional marine incursions. The distribution of the identified benthic foraminiferal species suggests deposition under oxygen-depleted conditions with high organic matter flux. Surficial epifaunal taxa dominate the benthic assemblages at certain intervals suggesting episodes of stagnation and elevated position of the redox boundary relative to the water/sediment interface.

Heterogeneity of environments in a coastal lagoon mouth by the comparison between living and dead benthic foraminiferal assemblages (Ria de Aveiro Portugal)

This study provides a comparison between living (LAs) and dead (DAs) assemblages of benthic foraminifera in 137 sites located at the mouth region of the Aveiro Lagoon (N Portugal). This coastal lagoon, commonly named Ria de Aveiro, is under strong anthropogenic influence due to changes in natural hydrodynamics through engineering structures and by the input of contaminants. The sedimentary environments of the mouth area are characterized by biotic (benthic foraminifera) and sedimentological (grain size, trace metals and total organic carbon concentrations) data. The pollution load index (PLI) identifies areas of increased concentrations of potentially toxic trace metals. An innovative combination of several statistical methods and extremely detailed data analyses allowed to verify that the dead associations (DAs) and the living assemblages (LAs) are absent or reduced in the deep and most hydrodynamically active zones along the navigable waterways of the channels. Breakwaters reducing the bottom currents allow the development of larger and more diversified LAs. The most suitable areas for LAs are located close to the South Jetty and in Mira Channel excluding the navigable channel and the eastern margin. LAs have in general little similarity with the DAs. Confined zones and areas with high organic matter flux and trace metals concentrations, where the sediments are being accumulated, are particularly unfavourable environments for living foraminifera. The obtained results evidenced that marked differences in hydrodynamics determine large dissimilarities between LAs and DAs. In some environments the taphonomic effects caused by the removal and high accumulation of foraminiferal tests can significantly misrepresent the faunal composition in the fossil registry, and lead to misinterpretations about the paleoenvironmental evolution.

Eocene (Bartonian) benthic foraminifera and paleoenvironmental changes in the Western Tethys

The Eocene was a period of intense climate variability and the response of deep-sea biota is still poorly understood, especially across certain understudied intervals from the middle Eocene. We present new benthic foraminiferal data from a Bartonian marine sequence deposited in the western Tethys Ocean (Torre Cardela section, Spain), and determine the biotic and paleoenvironmental turnover. The assemblages indicate a middle to lower bathyal depth of deposition, and they contain allochthonous taxa (e.g. asterigerinids) that were transported from shallower settings. A Detrended Correspondence Analysis (DCA) performed on the total assemblage differentiates the autochthonous and allochthonous taxa, supporting the idea of different provenience. We suggest that the latter were likely transported downslope by currents. Quantitative analyses of the autochthonous assemblages show a strong dominance of calcareous infaunal taxa, mainly bolivinids, which point to a high flux of organic matter to the seafloor. Bolivinoides crenulata, the most common species, is associated with inputs of refractory organic matter and high-energy environments. Five intervals were differentiated based on changes in the relative abundance of taxa. Two intervals (B and D) show the highest percentages of allochthonous taxa and B. crenulata. We suggest that enhanced current activity not only transported allochthonous taxa, but also brought in refractory organic matter to the seafloor, which was consumed by opportunistic taxa during these two intervals. We conclude that benthic foraminiferal assemblages at Torre Cardela were strongly controlled by the amount and type of organic matter reaching the seafloor, which were in turn affected by sedimentary and, ultimately, by climatic factors.

Environmental controls on the distribution of living (stained) benthic foraminifera on the continental slope in the Campos Basin area (SW Atlantic)

Abstract Living (stained) benthic foraminifera from deep-sea stations in the Campos Basin, southeastern Brazilian continental margin, were investigated to understand their distribution patterns and ecology, as well as the oceanographic processes that control foraminiferal distribution. Sediments were collected from 1050 m to 1950 m of water depth during the austral winter of 2003, below the Intermediate Western Boundary Current (IWBC) and the Deep Water Boundary Current (DWBC). Based on statistical analysis, vertical flux of particulate organic matter and the grain size of sediment seem to be the main factors controlling the spatial distribution of benthic foraminifera. The middle slope (1050 m deep) is characterized by relatively high foraminiferal density and a predominance of phytodetritus-feeding foraminifera such as Epistominella exigua and Globocassidulina subglobosa. The occurrence of these species seems to reflect the Brazil Current System (BCS). The above-mentioned currents are associated with the relatively high vertical flux of particulate organic matter and the prevalence of sandy sediments, respectively. The lower slope (between 1350 and 1950 m of water depth) is marked by low foraminiferal density and assemblages composed of Bolivina spp. and Brizalina spp., with low particulate organic matter flux values, muddy sediments, and more refractory organic matter. The distribution of this group seems to be related to episodic fluxes of food particles to the seafloor, which are influenced by the BCS at the surface and are deposited under low deep current activity (DWBC).