High Surface Water Productivity Research Articles

Shallow-water whale-fall communities: Evidence from the middle–late Eocene basilosaurid whale bones, Wadi El-Hitan, Fayum, Egypt

The present study examines the basilosaurid whale bone specimens collected from the Sandouk El-Borneta section, Wadi El-Hitan (Valley of Whales), Fayum, Egypt. These specimens are embedded in highly fossiliferous calcareous sandstones of the middle Eocene Gehannam and the upper Eocene Birket Qarun formations. These whale bones display some post-mortem alterations, representing good signs of at least three distinct stages in what is called the whale fall. The co-occurrence of shark, ray, and crocodile fossil specimens with the examined whale bones may indicate the first mobile-scavenger stage. Borings of the siboglinid polychaete genus Osedax represent the most common worm type that deeply bioeroded the whale bones, contributing to their rapid degradation, and representing the enrichment-opportunist stage. These fossil traces of the bone-eating worm Osedax represent the first record in the Eocene Epoch regionally and inter-regionally, filling the gap between the Late Cretaceous and the Oligocene occurrences. Subsequently, the studied whale bones served as hard substrates for some calcareous tube-dwelling encrusting polychaetes, balanoid barnacles, sheet‐like cheilostome bryozoans, and scleractinian corals. Furthermore, a bioerosion structure produced by polychaete annelids was also observed. The presence of these sclerobionts assemblage confirms the well-developed final reef stage with prolonged exposure and colonisation of these whale bones prior to final burial. Based on the recorded post-mortem alterations, together with other sedimentological and palaeontological data, the studied whale bones were deposited in a shallow open marine bay to sheltered gulf environments, which were characterised by low depositional energy, low to moderate rate of sedimentation, and high surface water productivity.

Sulfate distribution and sulfate reduction in global marine sediments

The quantification of anaerobic oxidation of organic matter in the global seabed is to a large extent based on transport-reaction modeling of pore water ions involved in the mineralization processes. As the predominant of these processes, sulfate reduction can be modeled from the depth distribution of sulfate and other relevant solutes. An active organoclastic sulfate reduction is typically characterized by sulfate profiles with a concave-down shape. We use here a comprehensive database to show that half of all competent sulfate profiles have an opposite, concave-up shape, which may appear inconsistent with this concept. We also show that there is a strong discrepancy between sulfate profiles obtained by gravity coring and by IODP coring, with generally much higher sulfate flux and shallower sulfate penetration depth in the gravity cores (the term IODP here covers the international ocean drilling programs, DSDP, ODP and IODP). The discrepancy is correlated with a selectivity for coring sites in the published studies, by which gravity coring is focused towards the ocean margins and continental shelf and towards high surface water productivity and high sedimentation rate. In contrast, IODP coring is more frequently done in low sedimentation regions and is largely absent from the continental shelf. As a result, estimates of sulfate reduction in the global seabed may be biased by a selection of only IODP core data for the modeling. Furthermore, the modeling of pore water profiles shows net rates of sulfate reduction, while 35S-radiotracer experiments show the higher gross rates of the process. Estimates from 35S-radiotracer measurements suggest that dissimilatory sulfate reduction oxidizes 77 Tmol yr−1 of organic carbon globally, which is 3- to 4-fold higher than recent data based on modeling of IODP data. The potential biases by global estimates of sulfate reduction in marine sediments are discussed.

Contrasting particle fluxes and composition in a submarine canyon affected by natural sediment transport events and bottom trawling

Submarine canyons are important conduits of sediment and organic matter to deep-sea environments, mainly during high-energy natural events such as storms, river floods, or dense shelf water cascading, but also due to human activities such as bottom trawling. The contributions of natural and trawling-induced sediment and organic matter inputs into Palamós Canyon (NW Mediterranean) were assessed from three instrumented moorings deployed in the axis and northern flank of the canyon covering the trawling closure (February) and the trawling season (March-December) of 2017. During the trawling closure, large sediment fluxes with high contents of labile marine organic matter content were registered in the canyon axis, associated to storm resuspension on the shelf that coincided with dense shelf water cascading and high surface water productivity. Although no major natural sediment transport events occurred during the following spring and summer months, near-daily trawling-induced sediment gravity flows were recorded in the northern flank mooring, placed directly below a fishing ground, which sometimes reached the canyon axis. Compositionally, the organic matter transferred by trawling resuspension was impoverished in the most labile biomarkers (fatty acids, amino acids, and dicarboxylic acids) and had a high degree of degradation, which was similar to surficial sediment from the adjacent fishing ground. Trawling resuspended particles masked the transfer of organic matter enriched in labile biomarkers that naturally occur during the quiescent summer months. Overall, bottom trawling enhances the magnitude of particle fluxes while modifying its organic carbon composition, increasing the re-exposure and transfer of degraded organic carbon and potentially affecting benthic communities that rely on the arrival of fresh organic matter.

Laterally-continuous dolomite layers of the Miocene Pisco Formation (East Pisco Basin, Peru): A window into past cyclical changes of the diagenetic environment

Along the Peruvian coast, the sedimentary succession of the East Pisco Basin is exposed in the Ica Desert. At Cerro Los Quesos, laterally continuous dolomite layers characterise the diatomaceous sediments of the P2 sequence of the Miocene Pisco Formation, where a large number of marine vertebrates are exceptionally preserved, many enclosed in dolomite nodules. In this work, cemented layers from this sequence were described and sampled for petrographic, chemical, microscopic and isotopic analyses. Dolomite occurs in continuous 10–50 cm thick well cemented layers, formed by sediment of different nature: biogenic, terrigenous, volcanoclastic, and phosphatic. The underlying sediments exhibit a yellow layer with sparse dolomite crystals, a black layer with abundant Mn-oxides, and a reddish layer rich in Fe-oxides, indicating redox-related fronts. Two generations of dolomite can be recognised: an early diagenetic microcrystalline one, and a sparry one, filling the large cavities. As observed in both thin sections and on broken surfaces, microcrystalline dolomite also fills the inner spaces of the diatom areolae replicating their finest ultrastructure, such as foramina and cribra and replace calcite shells of foraminifera. δ18O and δ13C values from the microcrystalline dolomite of two layers, selected based on the absence of other carbonate phases (e.g. calcite) and the lack of sparry cement, are in agreement with those reported for the Peru margin and fall in the fields of either sulphate-reduction or methanogenesis.All the data point to dolomite precipitation associated with low-temperature early diagenesis that typically occurs in upwelling settings, where high surface water productivity is responsible for high rates of organic carbon flux to the sea bottom and for the cyclical oxygen depletion at the bottom. Such conditions also promote high abundances of marine vertebrates and the exceptional preservation of their skeletons in the sediments.

High productivity promoted exceptional fossil preservation of the early Middle Triassic Luoping biota of Yunnan Province, China

The early Middle Triassic (Anisian) Luoping biota is representative of marine ecosystems following their full recovery after the Permian-Triassic mass extinction; however, its exceptional preservation remains poorly understood. In this paper, we report multiple geochemical proxies (TOC, TN, P/Al, Cuxs, Nixs, Baxs, δ13Ccarb, and δ13Corg) from Member II of the Middle Triassic Guanling Formation at the Xiangdongpo section in Luoping County, Yunnan Province, China, to assess the relationship between primary productivity and exceptional preservation of the Luoping biota. Variations in TOC, TN, P/Al, Cuxs, and Nixs values show that exceptional preservation coincides with two intervals of high productivity. Relatively low C/N ratios and distributed Δ13Ccarb-org values indicate that primary productivity was dominated by eukaryotic algae and prokaryotic microbes. Based on geochemical analyses and regional correlations, we conclude that, following sea level rise, nutrients were supplied to the Luoping area by upwelling from the open ocean and facilitated the blooming of marine communities in an open platform setting. In the intra-platform depression, increased nutrients supply also fueled high primary productivity in surface waters and oxygen consumption in the water column, causing bottom water anoxia. The lack of oxygen in the bottom water reduced the rate of biomass degradation and bioturbation, promoting growth of microbial mats, leading to the exceptional preservation of macrofauna. This study highlights the important role of elevated primary productivity in exceptional fossil preservation by triggering anoxia of bottom waters. Our findings confirm the widespread development of anoxic conditions during the middle Anisian (Pelsonian), from the eastern Tethys to the Panthalassa, and also reveal the interactions between organisms and the environment after Permian-Triassic mass extinction.

Deglacial to Mid Holocene environmental conditions on the northeastern Greenland shelf, western Fram Strait

The western Fram Strait is the only deep-water gateway connecting the Arctic Ocean and North Atlantic. Consequently, today, the western Fram Strait and the Northeast Greenland shelf receive cold, low saline Polar and Arctic Atlantic waters from the Arctic Ocean incorporated in the East Greenland Current, together with warmer, saline recirculated Atlantic Water masses derived from the West Spitsbergen Current. We present a multiproxy study (grain size distribution, XRF core scans, and benthic foraminiferal assemblages) based on the lower part of the radiocarbon dated marine sediment core DA17-NG-ST03-039G, covering the period from 13.3 to 3.9 cal ka BP. We reconstruct the deglacial conditions of the Northeast Greenland shelf together with Early to Mid Holocene fluctuations in subsurface Atlantic Water and Polar Water advection. The results show that the outer Northeast Greenland shelf was deglaciated and marine conditions were established prior to c. 13.3 cal ka BP. At this time, our data show Atlantic Water masses flowing beneath an extensive sea-ice cover in a glaciomarine setting, potentially related to the Bølling-Allerød warm period. Around 12.9 cal ka BP, the onset of high surface and bottom water productivity may be associated with the Younger Dryas onset. This was followed by a transition towards warmer bottom and subsurface water conditions from c. 11.7 until 10.2 cal ka BP caused by enhanced advection of Atlantic-derived water masses. A cold period with marginal ice zone conditions and enhanced East Greenland Current incursion is evident from 10.2 to 9.4 cal ka BP, succeeded by harsher sea-ice conditions and Atlantic Water inflow, prevailing until c. 7.5 cal ka BP. Holocene Thermal Maximum conditions, characterized by high surface and subsurface water productivity, were promoted by enhanced Atlantic Water flow to the shelf from c. 7.5 to 6.7 cal ka BP. In contrast, the transition towards a cold period with increased drift-ice transport via a strong East Greenland Current is recorded from c. 6.2 cal ka BP and to the end of our record at 3.9 cal ka BP; it was associated with the onset of the Neoglaciation.

Geochemical characterisation of offshore New Zealand phosphorites, and mechanisms for their formation

Phosphate deposits can form in a range of environments, and despite similar P contents, their mineralogy, and hence major and trace element chemistry, can vary substantially. Continental margins with high surface-water productivity are recognised as a major environment for the formation of extensive phosphate deposits. The geochemical characterisation of continental-margin-derived phosphorites from Chatham Rise and south-eastern offshore New Zealand, as well as newly discovered seamount phosphorites from Bollons Seamount (first documented here), offer insights into the different mechanisms of phosphorite formation from these two different environments. Though less studied, seamounts are increasingly recognised for their phosphorite deposits as well as Fe-Mn crusts and Mn nodules. We propose the first model for the formation of nodules at Bollons Seamount, and an updated model for Chatham Rise nodules (and south-eastern offshore NZ phosphorites) is presented. Suboxic-to-anoxic enrichments of U and V are observed in Chatham Rise and south-eastern offshore phosphorites that contrast with oxic enrichments of Mn, Ce, Co, and Cr in Bollons Seamount phosphorites. Differences in uptake of Light Rare Earth Elements (LREE) vs. Heavy Rare Earth Elements (HREE) and variations in mineralogy (e.g., presence of glauconite vs. Fe-Mn crusts), allow phosphate deposits that formed in organic-rich, upwelling continental-margin environments to be distinguished from those that formed in oxic-suboxic organic-poor seamount environments.

Late pleistocene paleoceanographic changes in the Ross Sea – Glacial-interglacial variations in paleoproductivity, nutrient utilization, and deep-water formation

The outer Ross Sea continental shelf has experienced large variations in ice sheet extent over the Pleistocene that are theorized to be largely driven by changes in the westward-flowing Antarctic Slope Current (ASC) at the continental shelf break. This current regulates southward incursions of warm modified Circumpolar Water, and it is thought to have triggered past marine ice sheet retreat. Additionally, expansions of grounded ice sheets on the continental shelf have fundamentally altered the Ross Sea water mass formation processes, influencing surface water salinity, sea ice cover, nutrient utilization, deep-water ventilation, and primary productivity. Here, we report the geochemical, physical properties, grain size, bulk δ15N, and diatom records during the late Pleistocene from two sediment cores from the Iselin Bank on the outermost continental shelf in the Ross Sea. These core sites were not overridden by grounded ice sheets during the late Pleistocene glacial-interglacial cycles, allowing for a continuous archive of glacimarine environments that were influenced by the ASC. Interglacial periods are typically characterized by high surface water productivity and nutrient utilization, with Chaetoceros resting spores indicating nutrient limitation under open ocean conditions, and glacial periods are typically characterized by low surface water productivity and nutrient utilization, with sea ice diatoms and planktonic foraminifers indicating light limitation under extensive sea ice/ice margin proximal conditions. A grain size analysis indicates coarse-skewed distributions and winnowing in the Iselin Bank region during cold periods. The winnowing may be related to enhanced ASC flow instead of density driven shelf water outflow.

Organic geochemical signals of freshwater dynamics controlling salinity stratification in organic-rich shales in the Lower Permian Irati Formation (Paraná Basin, Brazil)

The Lower Permian Irati Formation in the northeastern and central eastern Paraná Basin (southern Brazil) was investigated in terms of bulk and molecular organic geochemistry in order to enlighten the complex depositional paleoenvironment during deposition of Irati black shales. The geochemical data reflect temporal and spatial variations of freshwater incursions promoting salinity stratification and high primary productivity in surface waters with the establishment of photic zone euxinia. Overall, highly concentrated C17 long-chain alkylnaphthalenes point to algal blooms as a result of freshwater inflows into the Irati Sea, mostly in the central eastern basin segment, and resemble concentration variations of pristane, phytane and nC17. The freshwater inflows were also retraced by increasing chroman ratios toward the top of the investigated units along with the general decrease of further palaeosalinity indicators, e.g., gammacerane, tetracyclic (C24), β-carotane, and the herein suggested γ-carotane and lexane indexes. The existence of a water column stratification promoting organic matter preservation is supported by the detection of tetrahydrophenanthrene at high concentrations, in addition to the known prominent occurrence of gammacerane. The detection of C10 to C31 aryl isoprenoids in addition to C16 and C18 pseudohomologue aryl isoprenoids points to the presence of photic zone euxinia. In general, samples from the central eastern basin were deposited in a deeper marine setting with lower salinity than samples from the northeastern basin, which displays signals of a shallower marine environment with reducing bottom water conditions, both deposited under freshwater influxes.

Primary productivity in the western tropical Atlantic follows Neogene Amazon River evolution

The Amazon River nutrient-rich plume currently triggers large-scale phytoplankton blooms in the otherwise oligotrophic western tropical Atlantic Ocean. Little is known about the onset and development of this high productivity system, although a direct link to the transcontinental Amazon River evolution can be expected. The Amazon submarine fan, located on the Brazilian Equatorial Margin (BEM), contains a unique sediment archive of the river's history and associated environmental changes in the marine realm. This study represents the first marine microfossil multi-proxy approach applied to any sedimentary record in the submarine fan area for the time interval encompassing the onset and development of the transcontinental Amazon River system. To reconstruct Miocene to Pleistocene changes in surface- and bottom water conditions we analyzed organic-walled dinoflagellate cyst- and benthic foraminiferal assemblages, respectively. Moreover, terrestrial- and freshwater palynomorph abundances were studied to provide a link between fluvial input and marine environmental changes. In addition, a planktonic foraminiferal biostratigraphy is constructed to verify the available calcareous nannofossil-based age model. Our data show that in the early- mid Miocene up to ~13 Ma limited fluvial input reached the BEM and primary productivity was elevated, after which distinctly low productivity conditions prevailed. After the birth of the transcontinental Amazon at ~9 Ma surface water productivity initially increased slightly. Consistently high surface water productivity and decreased bottom water oxygenation followed increasing terrestrial input after the (early) Pliocene. The temporal consistency between records from the Amazon Fan and the more distant Ceará Rise reflects large-scale marine environmental changes followed the development of the Amazon River, likely related to increased climatic variability in the Amazon Basin during the Plio-Pleistocene.

Influence of upwelling on the sedimentation and biota of the segmented margin of the western Neotethys: a case study from the Middle Triassic of the Balaton Highland (Hungary)

In the Middle Anisian, extensional tectonic movements led to the development of a small isolated carbonate platform in the middle part of the Balaton Highland, Transdanubian Range, Hungary. In the Late Illyrian, a condensed pelagic carbonate succession with phosphorite horizons was formed on the top of the already drowned platform. These strata contain an extraordinarily diverse ammonite fauna. This unit is overlain by radiolarian-rich carbonates, locally with radiolarite interbeds. We suggest that the drowning process and the post-drowning sediment deposition were controlled partly by regional factors, i.e., the onset of opening of the Neotethys Ocean, and partly by local factors such as the bottom topography and related current activity, which may also be connected with the opening of the ocean. The predominance of the radiolarian-rich sediments suggests eutrophic surface water, which may be explained by a monsoon-driven upwelling model. The segmented sea-floor topography together with the high-fertility surface water conditions may have provided favorable habitats for the ammonites, which may have adapted to various ecological conditions, leading to extreme diversification of this group. Since similar Middle to Late Anisian evolution was reported from many other units of the western Neotethys margin, regional factors such as the establishment of an extensional tectonic regime and related marginal basin formation, monsoon-driven upwelling, and related high surface water productivity seem to be of critical importance in controlling the depositional conditions.

Roveacrinids (Crinoidea, Roveacrinida) from the Cenomanian-Turonian of southwest Algeria (Saharan Atlas and Guir Basin)

In the southwestern part of Algeria, the Cenomanian-Turonian marine deposits build up a prominent ledge in a perched syncline (Ksour Mountains, western Saharan Atlas) or at high radius of curvature (Guir Basin). The petrographical analysis of the Cenomanian-Turonian deposits of the Ksour Mountains and of the Guir Basin reveals unexpected assemblages of roveacrinoidal ossicles comparable with those formerly reported from the Tinrhert area. For the first time, isolated ossicles of genuine and undisputable Roveacrinidae are illustrated. Three sections, Djebel Rhoundjaia (western Saharan Atlas), Berridel and Kénadsa (Guir Basin), were scrutinized to recognize the microcrinoidal sections within the carbonate microfacies and to compile the successive occurrence of respective roveacrinid taxa (besides the classical search for standard index microfossils) in an attempt to pinpoint more precisely the position of the Cenomanian-Turonian boundary (C/T B). These assemblages are particularly morphologically and taxonomically diverse with three species of genus Roveacrinus and one of genus Orthogonocrinus. The presence of Saccocomidae (Applinocrinus) is especially unusual in such stratigraphic levels. The relative abundance and diversity of Roveacrinidae evidence a peak when approaching the C/T B. Such an event is recurring in the latest Cenomanian in various Tethyan and Atlantic areas. These fluctuations are consistent with a high surface-water productivity just before the C/T B.

Diatom bloom‐derived bottom water hypoxia off the Changjiang estuary, with and without typhoon influence

AbstractDuring the summers of 2009 and 2013, seawater pH and concentrations of dissolved oxygen, inorganic carbon, and nutrients were measured off the Changjiang estuary in the East China Sea. The 2009 cruise captured the effects of Typhoon Morakot; the 2013 cruise sampled more typical conditions (no typhoon). Data from both years indicate a close correlation between high primary productivity in surface waters and hypoxia in bottom waters. Based on these observations, we developed a conceptual model to guide an exploration of processes contributing to the formation of summertime bottom hypoxia. A mixing‐model analysis of the 2009 data identified a surface diatom bloom as the major (70–80%) source of the organic carbon that decomposed and ultimately led to bottom water hypoxia. Within the Changjiang River plume, depth‐integrated net biological production in the water column was 1.8 g C m−2 d−1, indicating strong autotrophic production, which in turn led to a high respiration rate of 1.2 g C m−2 d−1 in the bottom water. During both cruises, strong surface‐to‐bottom physical and metabolic coupling was evident. In 2009, storm‐driven inputs of nutrients from elevated river discharge and strong vertical mixing helped to fuel the rapid development of a surface diatom bloom. Afterwards, stratified conditions re‐established, newly formed labile organic matter sank, and bottom water oxygen was quickly consumed to an extent that hypoxia and acidification developed. To our knowledge, the observed rate of hypoxia and acidification development (within 6 d) is the fastest yet reported for the Changjiang River plume.

First evidence of denitrification vis-\xe0-vis monsoon in the Arabian Sea since Late Miocene

In the Arabian Sea, South Asian monsoon (SAM)-induced high surface water productivity coupled with poor ventilation of intermediate water results in strong denitrification within the oxygen minimum zone (OMZ). Despite the significance of denitrification in the Arabian Sea, we have no long-term record of its evolution spanning the past several million years. Here, we present the first record of denitrification evolution since Late Miocene (~10.2 Ma) in the Eastern Arabian Sea, where the SAM generates moderate surface water productivity, based on the samples retrieved during the International Ocean Discovery Program (IODP) Expedition 355. We find that (i) the SAM was persistently weaker from ~10.2 to 3.1 Ma; it did not intensify at ~8 Ma in contrast to a few previous studies, (ii) on tectonic timescale, both the SAM and the East Asian Monsoon (EAM) varied synchronously, (iii) the first evidence of denitrification and productivity/SAM intensification was at ~3.2–2.8 Ma that coincided with Mid-Pliocene Warm Period (MPWP), and (iv) the modern strength of the OMZ where denitrification is a permanent feature was attained at ~1.0 Ma.

Characterization of “dead-zone” eddies in the eastern tropical North Atlantic

Abstract. Localized open-ocean low-oxygen “dead zones” in the eastern tropical North Atlantic are recently discovered ocean features that can develop in dynamically isolated water masses within cyclonic eddies (CE) and anticyclonic mode-water eddies (ACME). Analysis of a comprehensive oxygen dataset obtained from gliders, moorings, research vessels and Argo floats reveals that “dead-zone” eddies are found in surprisingly high numbers and in a large area from about 4 to 22° N, from the shelf at the eastern boundary to 38° W. In total, 173 profiles with oxygen concentrations below the minimum background concentration of 40 µmol kg−1 could be associated with 27 independent eddies (10 CEs; 17 ACMEs) over a period of 10 years. Lowest oxygen concentrations in CEs are less than 10 µmol kg−1 while in ACMEs even suboxic (< 1 µmol kg−1) levels are observed. The oxygen minimum in the eddies is located at shallow depth from 50 to 150 m with a mean depth of 80 m. Compared to the surrounding waters, the mean oxygen anomaly in the core depth range (50 and 150 m) for CEs (ACMEs) is −38 (−79) µmol kg−1. North of 12° N, the oxygen-depleted eddies carry anomalously low-salinity water of South Atlantic origin from the eastern boundary upwelling region into the open ocean. Here water mass properties and satellite eddy tracking both point to an eddy generation near the eastern boundary. In contrast, the oxygen-depleted eddies south of 12° N carry weak hydrographic anomalies in their cores and seem to be generated in the open ocean away from the boundary. In both regions a decrease in oxygen from east to west is identified supporting the en-route creation of the low-oxygen core through a combination of high productivity in the eddy surface waters and an isolation of the eddy cores with respect to lateral oxygen supply. Indeed, eddies of both types feature a cold sea surface temperature anomaly and enhanced chlorophyll concentrations in their center. The low-oxygen core depth in the eddies aligns with the depth of the shallow oxygen minimum zone of the eastern tropical North Atlantic. Averaged over the whole area an oxygen reduction of 7 µmol kg−1 in the depth range of 50 to 150 m (peak reduction is 16 µmol kg−1 at 100 m depth) can be associated with the dispersion of the eddies. Thus the locally increased oxygen consumption within the eddy cores enhances the total oxygen consumption in the open eastern tropical North Atlantic Ocean and seems to be an contributor to the formation of the shallow oxygen minimum zone.

Geochemical paleoredox indicators in organic-rich shales of the Irati Formation, Permian of the Paraná Basin, southern Brazil

ABSTRACT: Paleoredox indicators were used to assess the depositional environment of the Permian Irati Formation, and discuss the controls on the accumulation of organic-rich sediments. Geochemical data were measured for 43 drill-core samples in southern Paraná Basin. We infer that redox boundary was located at the sediment-water interface during the accumulation of these sediments. Trace-element data demonstrate that Irati shales were similar to average shales, except during deposition of two organic-rich beds in which U and Mo are enriched, suggesting fully anoxic conditions. The variable degree of oxygenation of bottom waters is also attested by DOPT, FeT/Al, and sedimentological indicators of episodic ventilation. DOPT and Fe and S correlation points to an iron-limited environment, where the main sulfur sink is organic matter rather than pyrite. Cu and Cu/Mo ratio suggests bottom-water anoxia driven by increased organic flux. The Irati Formation was probably deposited in a salinity-stratified body of water, with positive water balance. The main control on the accumulation of the organic-rich sediments was the high organic carbon flux resultant from high primary productivity in surface waters. Anoxia arose as a consequence of increased productivity, not as the primary driver in the formation of these organic-rich rocks.

Records of past mid-depth ventilation: Cretaceous ocean anoxic event 2 vs. Recent oxygen minimum zones

Abstract. Present day oceans are well ventilated, with the exception of mid-depth oxygen minimum zones (OMZs) under high surface water productivity, regions of sluggish circulation, and restricted marginal basins. In the Mesozoic, however, entire oceanic basins transiently became dysoxic or anoxic. The Cretaceous ocean anoxic events (OAEs) were characterised by laminated organic-carbon rich shales and low-oxygen indicating trace fossils preserved in the sedimentary record. Yet assessments of the intensity and extent of Cretaceous near-bottom water oxygenation have been hampered by deep or long-term diagenesis and the evolution of marine biota serving as oxygen indicators in today's ocean. Sedimentary features similar to those found in Cretaceous strata were observed in deposits underlying Recent OMZs, where bottom-water oxygen levels, the flux of organic matter, and benthic life have been studied thoroughly. Their implications for constraining past bottom-water oxygenation are addressed in this review. We compared OMZ sediments from the Peruvian upwelling with deposits of the late Cenomanian OAE 2 from the north-west African shelf. Holocene laminated sediments are encountered at bottom-water oxygen levels of < 7 μmol kg−1 under the Peruvian upwelling and < 5 μmol kg−1 in California Borderland basins and the Pakistan Margin. Seasonal to decadal changes of sediment input are necessary to create laminae of different composition. However, bottom currents may shape similar textures that are difficult to discern from primary seasonal laminae. The millimetre-sized trace fossil Chondrites was commonly found in Cretaceous strata and Recent oxygen-depleted environments where its diameter increased with oxygen levels from 5 to 45 μmol kg−1. Chondrites has not been reported in Peruvian sediments but centimetre-sized crab burrows appeared around 10 μmol kg−1, which may indicate a minimum oxygen value for bioturbated Cretaceous strata. Organic carbon accumulation rates ranged from 0.7 and 2.8 g C cm−2 kyr−1 in laminated OAE 2 sections in Tarfaya Basin, Morocco, matching late Holocene accumulation rates of laminated Peruvian sediments under Recent oxygen levels below 5 μmol kg−1. Sediments deposited at > 10 μmol kg−1 showed an inverse exponential relationship of bottom-water oxygen levels and organic carbon accumulation depicting enhanced bioirrigation and decomposition of organic matter with increased oxygen supply. In the absence of seasonal laminations and under conditions of low burial diagenesis, this relationship may facilitate quantitative estimates of palaeo-oxygenation. Similarities and differences between Cretaceous OAEs and late Quaternary OMZs have to be further explored to improve our understanding of sedimentary systems under hypoxic conditions.

High resolution Holocene record in the southeastern Bay of Biscay: Global versus regional climate signals

Core KS10b (North East Atlantic, 43°50′N, 2°03′W, water depth 550m) was recovered at a location of high sediment deposition, and hence includes a high-resolution palaeoclimatic record for the southeastern Bay of Biscay over the last 9000calyr BP. The effect of global and regional climatic forcing factors (e.g., orbital forcing, North Atlantic Oscillation: NAO) and their subsequent control on water temperature, sea-level, hydrology and continental influence were investigated by studying planktic and benthic foraminiferal faunas (>150μm), sedimentology, and stable isotopes. Results indicate probable episodic long-term incursions of the warm and salty Iberian Poleward Current (IPC) into the Bay of Biscay during the Holocene by the intermittent presence of subtropical Globigerinoides ruber in the otherwise temperate fauna. These incursions seem to be triggered by negative NAO-like conditions. Our data show five main climatic periods, which are in general agreement with the literature data on the climatic variability in the North Atlantic and the Iberian Peninsula: The early Holocene (~9000–7400calyr BP) is characterized by low sea-level, significant downslope transport, eutrophic benthic settings and high surface water productivity. These environmental characteristics are probably due to the general wet and warm climate under a prevailing negative NAO, increasing precipitation and river runoff and favouring the incursion of the IPC into the Bay of Biscay. The climatic optimum appears between ~7400 and 6000calyr BP under a generally positive NAO index. In the following (~6000–3500calyr BP), surface water cooling is matching a steep decline of the Northern Hemisphere summer insolation. Meanwhile, benthic settings are changing progressively towards present day trophic conditions. The time interval from ~3500 to 1800calyr BP encompasses the Iberian–Roman Humid period characterized by warm and humid conditions, and increasing primary production under a prevailing negative NAO index. The last 1800years were characterized by stable cool conditions, reflecting a change towards a positive NAO state.

Chemostratigraphy of the early Pliocene diatomite interval from MIS AND-1B core (Antarctica): Palaeoenvironment implications

The AND-1B drill core (1285m-long) was recovered, inside the ANDRILL (ANtarctic geological DRILLing) Program, during the austral summer of 2006/07 from beneath the floating McMurdo Ice Shelf. Drilling recovered a stratigraphic succession of alternating diamictites, diatomites and volcaniclastic sediments spanning about the last 14Ma. A core portion between 350 and 480mbsf, including a 80m-thick diatomite interval recording the early Pliocene warming event, was investigated in term of opal biogenic content and element geochemistry. Across the diatomite interval, in spite of the lithological uniformity, a fluctuating biogenic opal profile mirrors the δ18O record, testifying a decrease in productivity when temperature drops as a consequence of small glacial fluctuations. The comparison of biogenic opal data with Chaetoceros spp. abundances from Konfirst et al. (2012) documents alternations between periods of high primary productivity in stratified surface waters and of enhanced terrigenous input in ice-free conditions. Cluster analysis discriminates elements associated to terrigenous input from those subject to biogenic control. Further separation in sub-cluster was interpreted in term of different element response to changes in provenance but also to depositional/early diagenetic conditions at the seafloor. Whilst K and Ti are related to different sediment sources confirming previous studies from the same interval, V, Zn and, to a lesser extent, Fe, document reducing/anoxic conditions during the diatomites deposition (in particular in 400–460mbsf interval). Mg, Sr and Mn contents are related to authigenic carbonate precipitation whilst Ba is interested by nonsteady-state processes leading to local peaks of barium below the sulphate-rich/sulphate-poor pore water boundary where generally the low degree of barite saturation is responsible for Ba remobilization. Such alteration in depositional dynamics, responsible of the precipitation of an oxygen-depleted barium phase, was probably induced by change in sedimentation rate and/or in palaeoenvironmental conditions.

Response of eastern Indian Ocean (ODP Site 762B) benthic foraminiferal assemblages to the closure of the Indonesian seaway

Pliocene-Pleistocene deep sea benthic foraminifera from ODP Site 762B in the eastern Indian Ocean were examined to understand the tectonically/climatically induced palaeoceanographic changes. In addition to already published data on this site by Rai & Singh (2001), some more faunal parameters were considered in the present work. Characteristic benthic foraminiferal assemblages as well as more diverse fauna during the early Pliocene (before 3.5Ma) reflected relatively oligotrophic and warm bottom water conditions. At the beginning of the late Pliocene (i.e. ∼ 3±0.5Ma) relative abundances of Uvigerina proboscidea, infaunal taxa and high productivity taxa increased, whereas faunal diversity showed a distinct decline, suggesting the development of pronounced upwelling resulting in higher surface water productivity. The strongly reduced inflow of warm and oligotrophic water masses as the South Equatorial Current (SEC) from the South Pacific to the eastern Indian Ocean due to the effective closure of the Indonesian seaway increased the surface water productivity. The closing of the Indonesian seaway during the late Pliocene was also responsible for the cessation of the warm, southward-flowing Leeuwin Current (LC) and the greater influence of the cold, deep and northward-flowing Western Australian Current (WAC) in the eastern Indian Ocean.