Blake Nose Research Articles

Transient micropaleontological turnover across a late Eocene (Priabonian) carbon and oxygen isotope shift on Blake Nose (NW Atlantic)

Abstract. The Gulf Stream, a western boundary current transporting warm water into the North Atlantic, plays a key role in climate regulation and oceanographic stability at a regional and global scale as part of the Atlantic Meridional Overturning Circulation (AMOC). Evidence suggests that an ancestral Gulf Stream has existed since the Mesozoic, and it has altered its course repeatedly over Cenozoic times. In this study, we focus on the upper Eocene (Priabonian, ca. 36 Ma) from Ocean Drilling Program Site 1053 on Blake Nose (subtropical North Atlantic). Bulk carbon and oxygen stable isotopes, as well as benthic foraminiferal and calcareous nannofossil assemblages, provide an integrated assessment of the palaeoceanographic changes impacting the area through the water column to the seafloor. Micropaleontological assemblages suggest changes in surface ocean stratification and nutrient supply to the seafloor coeval with a paired negative carbon and oxygen isotope excursion and the return to background conditions higher up in the study section. These transitory changes are compatible with the longitudinal displacement of the proto-Gulf Stream and its related eddies. Our results build on previous work and support the hypothesis that links palaeoceanographic changes in the Blake Nose area with shifts in the proto-Gulf Stream during the middle and late Eocene.

New middle Eocene radiolarian species (Rhizaria, Polycystinea) from Blake Nose, subtropical western North Atlantic Ocean

Abstract Diverse and well-preserved radiolarian assemblages were recovered from the middle Eocene sedimentary sequences drilled at Ocean Drilling Program Site 1051 (Leg 171B; western subtropical Atlantic). In addition to biostratigraphically important species, several unknown morphotypes were observed in this material, leading to the description of three new spumellarian species and 18 new nassellarian species. Described herein are: Periphaena petrushevskayae n. sp. (Phacodiscidae), Stylodictya oligodonta n. sp. (Trematodiscidae), Excentrosphaerella delicata n. sp. (Heliodiscidae), Eucyrtidium granatum n. sp. (Eucyrtidiidae), Dictyoprora echidna n. sp., Spirocyrtis matsuokai n. sp. (Artostrobiidae), Elaphospyris cordiformis n. sp., Elaphospyris quadricornis n. sp. (Cephalospyrididae), Ceratocyrtis oconnori n. sp. (Lophophaenidae), Botryocella? alectrida n. sp., Pylobotrys? bineti n. sp. (Pylobotrydidae), Lychnocanium cheni n. sp., Lychnocanium cingulatum n. sp., Lychnocanium croizoni n. sp., Lychnocanium forficula n. sp. (Lithochytrididae), Apoplanius hyalinus n. sp., Apoplanius cryptodirus n. sp. (Lophocyrtiidae), Albatrossidium messiaeni n. sp., Phormocyrtis microtesta n. sp., Cryptocarpium? judoka n. sp. (Pterocorythidae), and Thyrsocyrtis kamikuri n. sp. (Theocotylidae). Biostratigraphic information is provided for each new species. In addition, we re-describe and illustrate the morphological variability of a remarkable Pterocyrtidium species formerly published by Bütschli (1882a). UUID: http://zoobank.org/a01f7f03-73b0-458a-af7b-b85dc4666cc2

New taxa of the Rhaphoneidaceae Forti (Bacillariophyta) from the middle Eocene

Rhaphoneis amphiceros (Ehrenberg) Ehrenberg is one of the commonest extant rhaphoneid diatoms and has been reported from multiple fossil deposits, but our work suggests this taxon may not be as common in the fossil assemblages as indicated by the literature. Indeed, the original description of the species, made by Ehrenberg, is not as detailed as modern descriptions, which can lead to confusion when identifying taxa. Recent studies show that diatom species previously reported to occur over time intervals of tens of millions of years may actually represent groups of taxa lumped together due to inaccurate identifications. In this article, we present two new species that, without careful SEM examination, can be confused with R. amphiceros. These new species, interpreted to represent two new genera in the Rhaphoneidaceae, were found in middle Eocene sediments from Ocean Drilling Program (ODP) Hole 1050A (30°06’N, 76°14.1’W), drilled on Blake Nose, western North Atlantic. Rhomboneis shtapovii Figus & Witkowski and Rosaforma paillesiae Figus & Witkowski show unique morphological characteristics that lead us to create new genera. Rhomboneis shtapovii has two rimoportulae per valve and an unusual type of velum occluding the areolae. Rosaforma paillesiae has a peculiar asymmetric apical pore field occupying half the valve pole, the other half contains a rimoportula with two apical pores.

Biotic turnover and carbon cycle dynamics in the early Danian event (Dan-C2): New insights from Blake Nose, North Atlantic

Short-lived (104–105 years) carbon isotope excursions (CIEs), many of which are associated with some degree of ocean warming, are a feature of the warm climates of the early Paleogene. Here we present new calcareous nannofossil and geochemical data through the first of these Paleogene carbon cycle perturbations, known as the DAN-C2 event (65.8–65.7 Ma), from ODP Site 1049C on Blake Nose, in the western sub-tropical North Atlantic. Increased Hg/TOC (ppb/%) and Hg/Al (ppb/cps) ratios recorded at 65.9 Ma strongly suggest that volcanic activity, likely related to Deccan Traps, preceded the DAN-C2 event. Approximately 20 kyr after the onset of DAN-C2, Shannon diversity (H) index shows increased nannofossil species richness, with greater abundances of eutrophic and high fertility species, most likely a response to more intense weathering during the event. During the DAN-C2 event, there is a shift towards smaller morphotypes of the dominant placolith species, Coccolithus pelagicus and Cruciplacolithus primus, together with a calcium carbonate dissolution interval. Finally, we suggest that surface ocean currents dynamics, influenced by an eccentricity maxima cycle, is likely a potential mechanism to explaining the strong δ13C (∼1.3 ‰) negative excursion observed in Blake Nose.

Artificial intelligence applied to the classification of eight middle Eocene species of the genus Podocyrtis (polycystine radiolaria)

Abstract. This study evaluates the application of artificial intelligence (AI) to the automatic classification of radiolarians and uses as an example eight distinct morphospecies of the Eocene radiolarian genus Podocyrtis, which are part of three different evolutionary lineages and are useful in biostratigraphy. The samples used in this study were recovered from the equatorial Atlantic (ODP Leg 207) and were supplemented with some samples coming from the North Atlantic and Indian Oceans. To create an automatic classification tool, numerous images of the investigated species were needed to train a MobileNet convolutional neural network entirely coded in Python. Three different datasets were obtained. The first one consists of a mixture of broken and complete specimens, some of which sometimes appear blurry. The second and third datasets were leveled down into two further steps, which excludes broken and blurry specimens while increasing the quality. The convolutional neural network randomly selected 85 % of all specimens for training, while the remaining 15 % were used for validation. The MobileNet architecture had an overall accuracy of about 91 % for all datasets. Three predicational models were thereafter created, which had been trained on each dataset and worked well for classification of Podocyrtis coming from the Indian Ocean (Madingley Rise, ODP Leg 115, Hole 711A) and the western North Atlantic Ocean (New Jersey slope, DSDP Leg 95, Hole 612 and Blake Nose, ODP Leg 171B, Hole 1051A). These samples also provided clearer images since they were mounted with Canada balsam rather than Norland epoxy. In spite of some morphological differences encountered in different parts of the world's oceans and differences in image quality, most species could be correctly classified or at least classified with a neighboring species along a lineage. Classification improved slightly for some species by cropping and/or removing background particles of images which did not segment properly in the image processing. However, depending on cropping or background removal, the best result came from the predictive model trained on the normal stacked dataset consisting of a mixture of broken and complete specimens.

Biotic Turnover and Carbon Cycle Dynamics in the Early Danian Event (Dan-C2): New Insights from Blake Nose, North Atlantic

Biotic Turnover and Carbon Cycle Dynamics in the Early Danian Event (Dan-C2): New Insights from Blake Nose, North Atlantic

Calcareous plankton biostratigraphic fidelity and species richness during the last 10 m.y. of the Cretaceous at Blake Plateau, subtropical North Atlantic

Species distributions of well-preserved and diverse assemblages of planktonic foraminifera and calcareous nannofossils spanning the last 10 m.y. of the Cretaceous (middle Campanian through Maastrichtian) are analyzed from samples taken across a 1400 m depth transect at Blake Nose in the western subtropical North Atlantic (Ocean Drilling Program Sites 1049, 1050 and 1052). Age models constructed by integrating foraminiferal, calcareous nannofossil, and magnetic polarity datum events provide a reliable framework for temporal correlation of the sites. This framework enables comparisons of species richness and abundance among sites and evaluation of the reliability of first and last appearance datums for regional and global correlation. Among the standard primary zonal marker datums, six of nine planktonic foraminifer and six of seven calcareous nannofossil events are considered reliable for constraining the age-depth models. Secondary datum ages calculated for 17 planktonic foraminiferal events suggest correlation offsets among the three sites of <0.1 m.y. for four species (including Pseudoguembelina praehariaensis Tur and Huber n. sp.), 0.1–0.5 m.y. for six events, and 0.6–1.0 m.y. for eight species. Secondary calcareous nannofossil datum ages calculated for six species show less reliability, with offsets of <0.1 m.y. for one species, 0.7 m.y. for one species, and 1.0–1.5 m.y. for the remaining four species. The distinctly identifiable new species Trinitella suturis Tur and Huber has lowest occurrences that are diachronous by as much as 1.73 m.y. among the Blake Nose sites. Occurrence rarity is the most likely explanation for the age offsets of this and other diachronous species.Planktonic foraminiferal assemblages show no significant differences in species composition and relative abundance among the three sites, suggesting the sites were all located below a single oligotrophic surface water mass. Species richness counts pooled in 200 kyr bins for the Blake Nose sites reveals high species origination rates from the late Campanian through early Maastrichtian and highest species richness (51–63 species) during the Maastrichtian. The only significant extinction pulse during the mid-Campanian through Maastrichtian occurs between 66.2 and 66.4 Ma with loss of five species representing ∼9% of the assemblage. These extinctions occur at the same time as a globally recognized warming event correlated with a pulse of eruptions at the Deccan Traps in India.Calcareous nannofossil assemblages show no significant change in relative abundance among the three sites. Two significant extinction events are documented: one from 75.20 to 75.40 Ma with a loss of six species representing ∼6% of the assemblage and one from 66.2 to 66.4 Ma with a loss of nine species representing ∼9% of the assemblage. The former event is associated with a hiatus at the base of Zone CC24, and the latter corresponds to Deccan Trap warming.Hiatuses are identified at all three Blake Nose sites near the base of the Maastrichtian (∼71.5 Ma; lowermost Pseudoguembelina palpebra/CC24 Zone) and only at the deeper Sites 1049 and 1050 in the mid-Maastrichtian (∼67.2 Ma; Pseudoguembelina hariaensis/CC26b Zone) and the mid-Campanian (∼75.9 Ma; base of Radotruncana calcarata/CC22 Zone). Slumping from across the shelf and slope could have caused the early Maastrichtian hiatus while changes in the pattern and strength of deep-water circulation may have been responsible for the mid-Maastrichtian and mid-Campanian hiatuses.

North Atlantic marine biogenic silica accumulation through the early to middle Paleogene: implications for ocean circulation and silicate weathering feedback

Abstract. The Paleogene history of biogenic opal accumulation in the North Atlantic provides insight into both the evolution of deepwater circulation in the Atlantic basin and weathering responses to major climate shifts. However, existing records are compromised by low temporal resolution and/or stratigraphic discontinuities. In order to address this problem, we present a multi-site, high-resolution record of biogenic silica (bioSiO2) accumulation from Blake Nose (ODP Leg 171B, western North Atlantic) spanning the early Paleocene to late Eocene time interval (∼65–34 Ma). This record represents the longest single-locality history of marine bioSiO2 burial compiled to date and offers a unique perspective into changes in bioSiO2 fluxes through the early to middle Paleogene extreme greenhouse interval and the subsequent period of long-term cooling. Blake Nose bioSiO2 fluxes display prominent fluctuations that we attribute to variations in sub-thermocline nutrient supply via cyclonic eddies associated with the Gulf Stream. Following elevated and pulsed bioSiO2 accumulation through the Paleocene to early Eocene greenhouse interval, a prolonged interval of markedly elevated bioSiO2 flux in the middle Eocene between ∼46 and 42 Ma is proposed to reflect nutrient enrichment at Blake Nose due to invigorated overturning circulation following an early onset of Northern Component Water export from the Norwegian–Greenland Sea at ∼49 Ma. Reduced bioSiO2 flux in the North Atlantic, in combination with increased bioSiO2 flux documented in existing records from the equatorial Pacific between ∼42 and 38 Ma, is interpreted to indicate diminished nutrient supply and reduced biosiliceous productivity at Blake Nose in response to weakening of the overturning circulation. Subsequently, in the late Eocene, a deepwater circulation regime favoring limited bioSiO2 burial in the Atlantic and enhanced bioSiO2 burial in the Pacific was established after ∼38 Ma, likely in conjunction with re-invigoration of deepwater export from the North Atlantic. We also observe that Blake Nose bioSiO2 fluxes through the middle Eocene cooling interval (∼48 to 34 Ma) are similar to or higher than background fluxes throughout the late Paleocene–early Eocene interval (∼65 to 48 Ma) of intense greenhouse warmth. This observation is consistent with a temporally variable rather than constant silicate weathering feedback strength model for the Paleogene, which would instead predict that marine bioSiO2 burial should peak during periods of extreme warming.

Early Paleogene biosiliceous sedimentation in the Atlantic Ocean: Testing the inorganic origin hypothesis for Paleocene and Eocene chert and porcellanite

The widespread occurrence of lower Eocene chert and porcellanite has been viewed as a major paleoceanographic issue since the advent of ocean drilling, and both biotic and abiotic forcings have been proposed to explain it. We present a reconstruction of indurated siliceous sediment (ISS) and preserved biosiliceous sediment (PBS) occurrences in the Atlantic Ocean through the Paleocene and Eocene (~66 through 34 Ma). ISS and PBS distributions reveal dissimilar temporal trends, with the peak of ISS occurrences coinciding with the Early Eocene Climatic Optimum, in line with previous studies. PBS occurrences show a generally increasing trend culminating between 44 and 43 Ma. The common co-occurrence of ISS and PBS, and their coherent geographic distribution lends strong support to the biogenic origin of the precursor to the widespread Paleogene ISS, and argues against an inorganic mode of early Cenozoic chert and porcellanite precipitation. Weight per cent biogenic opal records and trends in linear sedimentation rates indicate two plausible modes of silicification: 1) silicification due to prolonged exposure of biogenic opal-rich sediments to corrosive bottom waters; and 2) silicification due to elevated pressures and temperatures caused by rapid burial of biogenic opal-rich deposits. The confinement of ISS and PBS to proximal sites along continental margins points to the reliance of siliceous sedimentation through the Paleocene and Eocene on terrestrial supply of dissolved silicon. Consistent with this, quantitative siliceous microfossil assemblage records from the Blake Nose in the NW Atlantic indicate that the nutrient-rich marginal rather than oligotrophic pelagic settings hosted the majority of siliceous plankton production through the early Paleogene.The inorganic SiO2 precipitation model is unlikely to have been the dominant mechanism responsible for ubiquitous occurrences of early Paleogene ISS. We favor the biogenic ISS precursor scenario and reconcile it with the low-productivity early Cenozoic oceans by showing that large volumes of biogenic silica were supplied to the western North Atlantic Ocean from the North American margin through the Paleocene and Eocene. Dissolution of this surplus silica was facilitated by an early southwestward flow of young, SiO2-depleted waters from the North Atlantic. All these factors contributed to ISS and PBS focusing in the western North Atlantic through the early Paleogene.

Rethinking the chronology of early Paleogene sediments in the western North Atlantic using diatom biostratigraphy

Ocean Drilling Program (ODP) Site 1051 (Blake Nose, western North Atlantic) is of crucial importance for reconstructing diatom evolution and biosiliceous sedimentation patterns through the early Cenozoic period of extreme greenhouse warmth followed by the progressive global cooling. The magnetostratigraphy in Hole 1051A, however, has been subject to divergent interpretations resulting in multi-million-year age control uncertainties, especially for events surrounding the early to middle Eocene transition. To resolve these uncertainties, we compare the stratigraphy of Hole 1051A to the neighboring Holes 1050A, C. We compile the published biomagnetostratigraphic data for both sites and identify three possible magnetostratigraphic solutions for Hole 1051A, the difference being the number of hiati and their duration. In order to identify the most plausible magnetostratigraphic solution for Hole 1051A, we employ the graphic correlation method, in which we compare the depth of individual magnetic reversals in both study sites against an independent proxy, i.e., 49 diatom evolutionary events identified in Holes 1050A,C and 1051A. The distribution of diatom bioevents lends strong support to the presence of two major hiati in both study sites: the upper hiatus juxtaposes magnetozone C21n on C22n, and the lower hiatus juxtaposes magnetozone C23n on C24n, eliminating the record of the initial part of the Early Eocene Climatic Optimum. Diatom, calcareous nannofossil and foraminiferal biostratigraphic markers also indicate that Hole 1051A terminated within magnetozone C28n rather than C27n. This age interpretation is strongly supported by the alignment of high-resolution weight percent biogenic SiO2 records from both study sites. The revised age models developed here for Holes 1050A,C and 1051A have profound consequences for interpretations of western North Atlantic paleoceanographic and paleoclimatic history through the early Paleogene. We propose a revised labelling of the early Eocene carbon cycle perturbations identified to date in Hole 1051A, and show how published Blake Nose δ30Si records change when data from Sites 1050 and 1051 are rescaled to the age models proposed herein. We emphasize that these refinements to the stratigraphy of Sites 1050 and 1051 are based on a study of diatom bioevents, which are here identified and calibrated to the Geomagnetic Polarity Timescale. This successful application of diatom biostratigraphy is a substantial advance toward their future utility in providing high-resolution age control for Paleogene deep-sea sites.

Latest Cretaceous/Paleocene deep-sea ostracode fauna at IODP Site U1407 (western North Atlantic) with special reference to the Cretaceous/Paleogene boundary and the Latest Danian Event

Abstract During the Paleocene, the marine plankton community recovered its diversity after a severe diversity loss at the Cretaceous/Paleogene (K/Pg) boundary. In general, changes in Paleocene deep-sea ostracode faunas are poorly understood. It is still unclear how the deep-sea ostracode faunas changed during compelling events such as the K/Pg boundary (66.04 Ma) and the Latest Danian Event (LDE: ~ 62.18 Ma), a transient Paleocene warming event. In this report, we describe a study on the latest Cretaceous–Late Paleocene ostracodes from Integrated Ocean Drilling Program Site U1407, off Newfoundland, in the northwestern Atlantic. Thirty-six ostracode taxa were obtained from 202 discrete samples that were collected from the interval between 66.3 and 57.3 Ma. Near the K/Pg boundary, 8% (2 of 25) of the Maastrichtian taxa became extinct. The extinction rate is estimated to be at most 0.12 E MSY − 1 , which is equal to extinction rate of 12% per one million years and is in the same range as the background-level extinction rate. Comparing the extinction rates between different ocean basins such as Maud Rise and Blake Nose, the ostracode extinction at the K/Pg boundary displays striking geographical heterogeneity. Between the Maastrichtian and Danian, the ostracode faunas change from a biofacies dominated by shallow bathyal taxa to one dominated by deep bathyal taxa. During the LDE, ostracode faunas display an increase in their benthic ostracode accumulation rates and taxonomic diversity as well as an increase in the relative abundance of Krithe crassicaudata simultaneously with a decline in carbonate content. A similar change is commonly documented in fauna from the northeastern Atlantic during the Paleocene–Eocene Thermal Maximum (PETM; 56.0 Ma). In contrast to the PETM fauna, an extinction event is not recognized. Near the Danian/Selandian boundary, the ostracodes show an increase in export productivity. After the Danian, the ostracodes form a community that appears to tolerate warm and acidified water. Export productivity remains high during the Selandian and Thanetian.

Muted calcareous nannoplankton response at the Middle/Late Eocene Turnover event in the western North Atlantic Ocean

Key extinctions in two major planktonic foraminiferal groups and high taxic turnover in radiolarians have led to the identification of the Middle/Late Eocene Turnover (MLET) and point towards significant palaeoclimatic and/or palaeoceanographic changes at around 38 million years ago. Here we present quantitative calcareous nannofossil data from Ocean Drilling Program Site 1052 (Blake Nose, western North Atlantic) in order to investigate the response of phytoplankton during the MLET. Our data show only minor shifts in taxon abundance, with no strong trends identified through the interval and no nannofossil origination or extinction events associated with the MLET. The assemblages are characterised by the dominance of neritic braarudosphaerids and eurytopic reticulofenestrids. The increased abundance of warm to temperate and mesotrophic nannofossils (Reticulofenestra reticulata, Reticulofenestra bisecta and Coccolithus pelagicus) in and around the MLET occur against a backdrop of cooling, as indicated by oxygen isotopes, suggesting that changing nutrient conditions was the principle driver of these shifts in the nannoplankton assemblage. This is further supported by an increase in radiolarian accumulation rates at this time. The lack of response in the calcareous nannoplankton relative to the zooplanktonic planktonic foraminifera and radiolaria demonstrates the contrasting sensitivity to environmental change in these different plankton groups, with radiolarians showing the highest degree of change at the MLET and the nannoplankton showing little or none.

A silicon depleted North Atlantic since the Palaeogene: Evidence from sponge and radiolarian silicon isotopes

Despite being one of Earth's major geochemical cycles, the evolution of the silicon cycle has received little attention and changes in oceanic dissolved silica (DSi) concentration through geologic time remain poorly constrained. Silicon isotope ratios (expressed as δ30Si) in marine microfossils are becoming increasingly recognised for their ability to provide insight into silicon cycling. In particular, the δ30Si of siliceous sponge spicules has been demonstrated to be a useful proxy for past DSi concentrations. We analysed δ30Si in radiolarian tests and sponge spicules from the Blake Nose Palaeoceanographic Transect (ODP Leg 171B) spanning the Palaeocene–Eocene (ca. 60–30 Ma). Our δ30Si results range from +0.32 to +1.67‰ and −0.48 to +0.63‰ for the radiolarian and sponge records, respectively. Using an established relationship between ambient dissolved Si (DSi) concentrations and the magnitude of silicon isotope fractionation in siliceous sponges, we demonstrate that the Western North Atlantic was DSi deplete during the Palaeocene–Eocene throughout the water column, a conclusion that is robust to a range of assumptions and uncertainties. These data can constitute constraints on reconstructions of past-ocean circulation. Previous work has suggested ocean DSi concentrations were higher than modern ocean concentrations prior to the Cenozoic and has posited a drawdown during the Early Palaeogene due to the evolutionary expansion of diatoms. Our results challenge such an interpretation. We suggest here that if such a global decrease in oceanic DSi concentrations occurred, it must predate 60 Ma.

Remarks on the identification of the Albian/Cenomanian boundary and taxonomic clarification of the planktonic foraminifera index speciesglobotruncanoides, brotzeniandtehamaensis

AbstractThe planktonic foraminifera assemblage across the Global Boundary Stratotype Section and Point (GSSP) for the base of the Cenomanian Stage defined at Mont Risou (Haute-Alpes in France) is here restudied to clarify the identification and stratigraphic distribution ofThalmanninella globotruncanoides( =Rotalipora globotruncanoidesSigal, 1948) andPseudothalmanninella tehamaensis( =Rotalipora tehamaensisMarianos &amp; Zingula, 1966) whose appearance levels are primary and secondary criteria for placing the Albian/Cenomanian boundary. Since the ratification of the GSSP in 2002, the identification of the foraminifera index species across the Albian/Cenomanian boundary has been reported to be sometimes difficult either because of their rarity or uncertainty in the taxonomic identifications. We discuss the taxonomic status ofThalmanninella brotzeniSigal 1948, a species regarded for a long time to be a junior synonym ofTh. globotruncanoides, through images of Sigal's type materials deposited at the Musée National d’Histoire Naturelle (Paris), and elucidate the taxonomically important characters that enable unequivocal identification ofTh. brotzeni, Th. globotruncanoidesandPs. tehamaensis. Planktonic foraminifera marker species from Mont Risou are compared with well-preserved specimens from Blake Nose Plateau (ODP 171B, North Western Atlantic Ocean) to verify the reliability and stratigraphic distribution of these marker taxa outside the Mediterranean Tethyan area.

REASSESSMENT OF THE EARLY-MIDDLE EOCENE PLANKTIC FORAMINIFERAL BIOMAGNETOCHRONOLOGY: NEW EVIDENCE FROM THE TETHYAN POSSAGNO SECTION (NE ITALY) AND WESTERN NORTH ATLANTIC OCEAN ODP SITE 1051

We examined a planktic foraminiferal biostratigraphic record from the Tethyan Possagno section (Venetian Prealps of northeastern Italy) spanning the lower–middle Eocene transition, from Chron C24r to the base of Chron C20r and from Zones E1 to lower E8. Our results provide an updated magnetochronological calibration from the Tethyan realm of the lowest and highest occurrences of the zonal markers. All the foraminiferal biostratigraphic events utilized in the most recent and widely used zonal schemes have been recognized. Most of them are in reasonable agreement with the known magnetochronological calibration, such as those at the Paleocene/Eocene boundary and lowermost Eocene. However, a number of events, specifically the bases of Morozovella aragonensis, Acarinina cuneicamerata, and Guembelitrioides nuttalli, occur at a significantly lower position at Possagno than in the most recent zonal schemes. A comparable magnetostratigraphic position has been recorded for the first two bioevents in the western North Atlantic Ocean ODP Site 1051 (Blake Nose, Atlantic Ocean). Furthermore, the base of A. cuneicamerata and the top of Morozovella subbotinae occur in reverse order with respect to previous schemes, thus negating the effectiveness of Zone E6 as now used. We propose an alternative definition of the Zone E6/E7a boundary by substituting the base of Astrorotalia palmerae for that of A. cuneicamerata . This proposal avoids combining E6 and E7a into a single zone. Additionally, we recorded a significantly different magnetostratigraphic position for the base of Turborotalia frontosa , a critical event for the Ypresian-Lutetian boundary placement. Our biostratigraphic dataset is unique in the integration of deep-benthic foraminiferal events with magnetostratigraphy and calcareous plankton data. Because biostratigraphic data of planktic foraminifera from the Possagno section derive from the same sample set analyzed for calcareous nannofossils, we were able to outline an integrated, updated, calcareous plankton stratigraphy of the lower–basal middle Eocene from a Tethyan location.

Magnetotactic bacterial response to Antarctic dust supply during the Palaeocene–Eocene thermal maximum

Distinct magnetic properties of marine sediments that record the Palaeocene–Eocene thermal maximum (PETM) have been suggested to be due to a bacterial magnetofossil signal that is linked to enhanced weathering conditions during the PETM. We document the dominance of bacterial magnetite in deep-sea sediments from southern Kerguelen Plateau (Ocean Drilling Program Hole 738C, southern Ocean) not only during the PETM, but also before and after the thermal event. This occurrence of magnetofossils throughout the PETM indicates that the occurrence of bacterial magnetosomes is not due to a preservation effect. Instead, we suggest that it is due to sustained mild iron-reducing conditions that dissolved the most labile aeolian-derived iron, which favoured continued magnetotactic bacterial activity without being strong enough to dissolve the less reactive magnetite and haematite. Enhanced aeolian haematite abundances at the beginning of the PETM indicate drier conditions on the neighbouring Antarctic continent at those times. Our results provide evidence that iron fertilisation by aeolian dust was the main limiting factor that conditioned proliferation of magnetotactic bacteria in the deep sea at the southern Kerguelen Plateau, with the exception of two short periods of rapidly changing palaeoenvironmental conditions at the onset and termination of the PETM. Increased iron supply from aeolian dust, that enhanced oceanic primary productivity and subsequent delivery of organic carbon to the seafloor, along with mild iron-reducing diagenetic conditions, seem to have been necessary to provide the iron needed for magnetite biomineralization by magnetotactic bacteria to drive their marked increase in abundance in the studied PETM record from southern Kerguelen Plateau. Our analyses of a deep-sea PETM record from Hole 1051B at Blake Nose (Atlantic Ocean) failed to identify magnetofossils despite evidence for the occurrence of magnetite and haematite of probable aeolian origin. Contrasting magnetic properties at these PETM sections indicate that further work is needed to understand the palaeoenvironmental and diagenetic factors whose interactions lead to production and preservation of magnetofossils in deep-sea sediments.

Water mass circulation on Demerara Rise during the Late Cretaceous based on Nd isotopes

Circulation in the North Atlantic during the Late Cretaceous has implications for poleward heat transport and nutrient distribution during an extreme greenhouse interval with episodes of ocean anoxia. Nd isotopes of fossil fish teeth and debris represent one of the few water mass tracers that can be used to reconstruct deep ocean circulation. We present Nd isotopic data interpreted as bottom water values for 290 samples from three Ocean Drilling Program sites on Demerara Rise (Sites 1258, 1260, and 1261) along with 102 analyses from four other North Atlantic sites (Cape Verde, Goban Spur, Bermuda Rise and Blake Nose) that provide additional geographic and bathymetric control. Our results confirm the presence of a water mass with low εNd values (−14 to −17) that are believed to be influenced by continental material during local water mass formation at low latitudes. This Demerara Bottom Water (DBW) is the primary water mass in the region from the Cenomanian to Coniacian and from the late Campanian through early Maastrichtian following a hiatus of ~10my. A positive 8 εNd unit excursion occurs during Ocean Anoxic Event 2 (OAE2) that cannot be explained by changes in weathering inputs, diagenesis or magmatic sources of Nd; instead, it appears to represent an influx of bottom waters sourced from the Tethys or North Atlantic. This replacement of DBW during OAE2 argues for decreased production of DBW or enhanced production of Tethys/North Atlantic waters during peak greenhouse conditions. From the late Campanian through early Maastrichtian, DBW becomes the only water mass recorded at Demerara Rise and it appears to expand to abyssal depths at Cape Verde in the Campanian. This water mass is ultimately replaced by waters that appear to be sourced from the North Atlantic starting in the late Maastrichtian. Observed variations in circulation in the tropical North Atlantic during the Late Cretaceous can account for extensive deposition of black shales in the region prior to the hiatus, as well as local warming during a global cooling event in the Maastrichtian.

Paleotemperature and paleosalinity inferences and chemostratigraphy across the Aptian/Albian boundary in the subtropical North Atlantic

Geochemical analyses of extraordinarily well preserved late Aptian–early Albian foraminifera from Blake Nose (Ocean Drilling Program Site 1049) reveal rapid shifts of δ18O, δ13C, and 87Sr/88Sr in the subtropical North Atlantic that may be linked to a major planktic foraminifer extinction event across the Aptian/Albian boundary. The abruptness of the observed geochemical shifts and their coincidence with a sharp lithologic contact is explained as an artifact of a previously undetected hiatus of 0.8–1.4 million years at the boundary contact, but the values before and after the hiatus indicate that major oceanographic changes occurred at this time. 87Sr/88Sr increase by ∼0.000 200, δ13C values decrease by 1.5‰ to 2.2‰, and δ18O values decrease by ∼1.0‰ (planktics) to 0.5‰ (benthics) across the hiatus. Further, both 87Sr/88Sr ratios and δ18O values during the Albian are anomalously high. The 87Sr/88Sr values deviate from known patterns to such a degree that an explanation requires either the presence of inter‐basin differences in seawater 87Sr/88Sr during the Albian or revision of the seawater curve. For δ18O, planktic values in some Aptian samples likely reflect a diagenetic overprint, but preservation is excellent in the rest of the section. In well preserved material, benthic foraminiferal values are largely between 0.5 and 0.0‰ and planktic samples are largely between 0.0‰ to −1.0‰, with a brief excursion to −2.0‰ during OAE 1b. Using standard assumptions for Cretaceous isotopic paleotemperature calculations, the δ18O values suggest bottom water temperatures (at ∼1000 –1500 m) of 8–10°C and surface temperatures of 10–14°C, which are 4–6°C and 10–16°C cooler, respectively, than present‐day conditions at the same latitude. The cool subtropical sea surface temperature estimates are especially problematic because other paleoclimate proxy data for the mid‐Cretaceous and climate model predictions suggest that subtropical sea surface temperatures should have been the same as or warmer than at present. Because of their exquisite preservation, whole scale alteration of the analyzed foraminifera is an untenable explanation. Our proposed solution is a high evaporative fractionation factor in the early Albian North Atlantic that resulted in surface waters with higher δ18O values at elevated salinities than commonly cited in Cretaceous studies. A high fractionation factor is consistent with high rates of vapor export and a vigorous hydrological cycle and, like the Sr isotopes, implies limited connectivity among the individual basins of the Early Cretaceous proto‐Atlantic ocean.

Depth-habitat reorganization of planktonic foraminifera across the Albian/Cenomanian boundary

New mid-Cretaceous stable isotope (δ18O and δ13C) records of multiple planktonic foraminiferal species and coexisting coccoliths from Blake Nose (western North Atlantic) document a major depth-ecology reorganization of planktonic foraminifera. Across the Albian/Cenomanian boundary, deep-dwellingPraeglobotruncana stephaniandRotalipora globotruncanoidesadapted to living at a shallower depth, while, at the same time, the population of surface-dwellingParacostellagerina libycadeclined. Subsequently, the opportunistic speciesHedbergella delrioensisshifted to a deep environment, and the deep-dwelling formsRotalipora montsalvensisandRotalipora reichelifirst appeared. The primary paleoenvironmental cause of the observed changes in planktonic adaptive strategies is uncertain, yet their coincidence with an earliest Cenomanian cooling trend reported elsewhere implicates the importance of reduced upper-ocean stratification. Although there has been an implicit assumption that the species-specific depth habitats of fossil planktonic foraminifera were invariant through time, planktonic paleoecology is a potential variable. Accordingly, the possibility of evolutionary changes in planktonic foraminiferal depth ecology should be a primary consideration (along with other environmental parameters) in paleoceanographic interpretations of foraminiferal stable isotope data.

Blake Nose stable isotopic evidence against the mid-Cenomanian glaciation hypothesis

Detailed multitaxon stable isotope (δ 18 O and δ 13 C) data from Blake Nose (western North Atlantic) argue against a mid-Cenomanian glaciation event during the mid-Cretaceous greenhouse. Results generated are precisely correlated to sea-level changes inferred from European sequence stratigraphy using the twin δ 13 C excursions mid-Cenomanian event (MCE) Ia and MCE Ib. Microfossils analyzed (surface-dwelling to deep-dwelling planktonic foraminifera, benthic foraminifera, coccoliths) show remarkably consistent intertaxon δ 18 O and δ 13 C offsets; comparative scanning electron microscope and Sr/Ca analyses allow some δ 18 O data to be eliminated because of selective diagenesis. Across MCE Ia, the proposed interval of major glacioeustatic regression, the planktonic δ 18 O values are constant for each taxon. The absence of a mean seawater δ 18 O shift contradicts predictions for the mid-Cenomanian glaciation episode. The benthic δ 18 O records show significant fluctuations during MCE I, implying short-term variability in North Atlantic intermediate-water and deep-water circulation patterns and/or sources at that time.