Jörg Mutterlose

Stable organic carbon isotope stratigraphy across Oceanic Anoxic Event 2 of Demerara Rise, western tropical Atlantic

Ocean Drilling Program (ODP) Leg 207 recovered expanded sections of organic-carbon-rich laminated shales on Demerara Rise (western tropical Atlantic). High-resolution organic carbon isotope and total organic carbon (TOC) records are presented, which span the Cenomanian-Turonian boundary interval (CTBI), including the Oceanic Anoxic Event (OAE) 2, from four sites oriented along a NW striking depth transect. These records represent the first high-resolution carbon isotope records across OAE 2 from the South American margin of the tropical Atlantic. Due to the scarcity of age significant fossils, the main purpose of this study was to develop a detailed carbon isotope stratigraphy in order to correlate the CTBI across the depth transect and to tie this to biostratigraphically well-defined sections in the Western Interior Basin (Pueblo, USA), boreal shelf seas (Eastbourne, England), and western Tethys (Oued Mellegue, Tunisia). All four sections studied document a 6‰ increase of ?13Corg values at the base of the CTBI, which is followed by an interval of elevated ?13Corg values and a subsequent decrease. Our results supply an important stratigraphic base for subsequent paleoceanographic studies on Late Cenomanian to Early Turonian sediments from Demerara Rise and elsewhere.

The Cenomanian - Turonian of the Wunstorf section - (North Germany): global stratigraphic reference section and new orbital time scale for Oceanic Anoxic Event 2

The Cenomanian–Turonian Boundary Event (CTBE) is reflected by one of the most extreme carbon cycle perturbations in Earths history and is characterized by the widespread occurrence of sediments indicating oxygen deficiency in oceanic waters (Oceanic Anoxic Event 2 = OAE 2). At Wunstorf (northern Germany) the CTBE is represented by a 26.5 m thick sedimentary succession consisting of rhythmically bedded laminated black shales, dark organic-rich marls and marly limestones yielding abundant micro- and macrofossils, making the locality particularly well suited to serve as an international standard reference section for the CTBE. In 2006 a newly drilled continuous core recovered 76 m of middle Cenomanian to middle Turonian sediments. A high-resolution carbonate δ13C curve derived from core samples resolves all known features of the positive δ13C anomaly of OAE 2 with high accuracy. Throughout the middle Cenomanian – middle Turonian succession, the δ13C curve shows numerous small-scaled positive excursions, which appear to be cyclic. High-resolution borehole geophysics and XRF core scanning were performed to generate two time series of gamma-ray data and Ti concentrations for the CTBE black shale succession. Hierarchical bundling of sedimentary cycles as well as spectral analysis and Gaussian filtering of dominant frequencies reveal cycle frequency ratios characteristic for short eccentricity modulated precession (100 kyr, 21 kyr). This new orbital time scale provides a time estimate of 430–445 kyr for the duration of OAE 2 and refines the existing orbital age models developed at localities in the English Chalk, the Western Interior Basin and the Tarfaya Basin. Based on the new age model and high-resolution carbon isotope correlation, our data allow for the first time a precise basin-wide reconstruction of the palaeoceanographic modifications within the European shelf sea during OAE 2.

Isotopic evidence for Late Jurassic^Early Cretaceous climate change

Abstract Strontium-, oxygen- and carbon-isotope ratios have been determined from Late Jurassic–Early Cretaceous belemnites from the Volga Basin, Russia, and Kawhia Harbour, New Zealand. 87Sr/86Sr ratios derived from well-preserved belemnites from the Volga Basin support a Middle Tithonian age derived from the analysis of the endemic ammonite fauna. The Kawhia Harbour section records a gradual rise in 87Sr/86Sr values and in comparison with the published 87Sr/86Sr curve suggests that the lower part of the section is latest Oxfordian in age, whilst the upper part of the section correlates well with the biostratigraphic correlation suggestion of an Early–Middle Tithonian age. Although the published strontium calibration curve shows a degree of scatter, our new data confirm the uniform rise in 87Sr/86Sr values from the Late Jurassic into the Early Cretaceous. Such an increase may result from either a decrease in mid-oceanic ridge spreading and/or an increase in weathering rates and flux of radiogenic strontium, although a eustatic drop in sea level and concurrent Western Cordillera uplift suggests that weathering may have been the controlling factor of Late Jurassic seawater strontium-isotope composition. Palaeotemperatures derived from the well-preserved belemnite δ18Ocarb values from the Volga Basin indicate that the Middle Volgian (Late Kimmeridgian) was warm (∼14–20°C), followed by a slight cooling and a subsequent gradual increase to the Jurassic–Cretaceous boundary. The δ18Ocarb values from New Zealand (located at a palaeolatitude of ∼80°S), if interpreted in terms of palaeotemperature, indicate a high degree of variability. Such variability may not be related to palaeotemperature, but to changes in oceanic chemistry resulting from the formation and dissolution of an ice-sheet and/or snow during the Oxfordian–Kimmeridgian. Carbon-isotope trends for the Late Jurassic show a fall in values from the Oxfordian with lowest values occurring in the Early–Middle Tithonian, before rising but without reaching values obtained in the Oxfordian. The overall low δ13Ccarb may be related to a global increase in continental weathering and/or upwelling of cooler oceanic water enriched in oxidised organic carbon (12C-enriched).

Calcareous nannofossils from the Aptian–Lower Albian of southeast France: palaeoecological and biostratigraphic implications

Abstract The Aptian–lower Albian succession of the Vocontian Basin (SE France) consists of marine hemipelagic sediments including several black shale horizons. The latter are partly of regional and partly of global distribution. This sedimentary succession records the nannoplankton evolution of the Aptian–early Albian interval and thus provides an excellent opportunity to calibrate the calcareous nannofossil record with Tethyan ammonite and planktic foraminiferal biostratigraphy. The calcareous nannofossil biostratigraphy presented in this paper supports previous zonations, but it also provides a much higher resolution and thus improves the correlation of different black shale horizons on a supraregional scale. Up to 23 major (supraregionally significant) and minor (regionally significant) first and last occurrences of calcareous nannofossil taxa are recognized. Nannoconid abundances decrease rapidly in the upper Lower Aptian (nannoconid crisis I, NCI) and in the middle Upper Aptian (nannoconid crisis II, NCII). Both decreases correlate with carbonate–platform drowning events. The upper Lower Aptian interval above the NCI is characterized by high abundances of large specimens of Assipetra infracretacea and Rucinolithus terebrodentarius probably of supraregional significance. The uppermost Aptian–Lower Albian is characterized by high abundances of the calcareous nannoplankton taxon Repagulum parvidentatum, reflecting boreal influence on the Tethyan Realm. This suggests a temporary decrease in surface-water temperatures in the Vocontian Basin.

The impact of calcareous nannofossils on the pelagic carbonate accumulation across the Jurassic–Cretaceous boundary

Abstract Calcareous nannofossils were important producers of pelagic carbonates in Mesozoic oceans. In order to better understand the origin of Mesozoic pelagic carbonates we studied upper Jurassic to lower Cretaceous sediments in the Central Atlantic Ocean (DSDP Sites 105, 367, 534A) with respect to their content of calcareous nannofossils. The interval under investigation is characterized by a significant increase in the deposition of carbonate-rich sediments, a trend that goes along with the rapid radiation of calcareous nannofossils. The assemblage composition and the size variation of common taxa were analyzed, and subsequently the contribution of this phytoplankton group to the pelagic carbonate accumulation was calculated. Results reveal two long-term changes of the nannofossil assemblage composition. (1) The early Tithonian coccolith assemblages are of low diversity ( Watznaueria spp., Cyclagelosphaera spp., Zeugrhabdotus spp.), whilst the mid- to late Tithonian assemblage is dominated by nannoliths ( Conusphaera mexicana , Polycostella beckmannii , Nannoconus spp.) and large-sized Watznaueria . (2) The early Berriasian is characterized by a shift from the late Tithonian nannolith-rich assemblage to a highly diverse coccolith assemblage. Morphometric studies of the placolith genus Watznaueria show for all three DSDP sites large-sized forms of this genus in the mid- and late Tithonian, followed by a decrease of up to 2 μm for the mean size in the earliest Berriasian. At DSDP Site 105 the studied nannolith taxa show an increase in size during the mid-Tithonian to Berriasian interval. The records of both nannofossil carbonate estimates and the measured bulk-rock carbonate reveal two periods of increase in the nannofossil carbonate record of DSDP Site 105. A first significant increase of the carbonate accumulation is observed in the mid- and late Tithonian, probably caused by mass occurrences of strongly calcified taxa ( C. mexicana , P. beckmannii , Nannoconus spp., Watznaueria cf. manivitae ). This interval is here named ‘Nannofossil Calcification Event’ (NCE). The second carbonate maximum in the late Berriasian is related to a rise in absolute abundances of nannofossils. This peak is amplified by an overall increase of the sedimentation rate. The calculated accumulation rates of nannofossils, nannofossil carbonate and bulk-rock carbonate for the late Berriasian are on the same scale as values from recent ocean surface sediments. A comparison of nannofossil carbonate values with the bulk-rock carbonate content shows that on average only 27% of the total carbonate can be explained by our nannofossil carbonate estimates. This discrepancy is most likely caused by the high amount of unidentifiable micrite and fragments of calcareous nannofossils. Other factors contributing to the error are possible inaccuracies in the determination of absolute abundances and nannofossil volume calculations. The NCE occurs during a long-term sea-level fall, dry climate and presumed low pCO 2 levels. A decline in abundance of strongly calcified nannofossils coincides with the opening of the Pacific–Atlantic seaway via Central America, which may have had a substantial impact on the palaeoceanographic situation in the Central Atlantic Ocean. These changes are here considered to have at least partly caused the shifts in abundance, size and assemblage composition of calcareous nannofossils observed across the Jurassic–Cretaceous boundary interval.

Early Cretaceous calcareous nannofossils from high latitudes: implications for palaeobiogeography and palaeoclimate

Abstract In order to better understand the palaeoceanography and palaeoclimate of the Early Cretaceous, nannofossil data have been obtained from high-latitudinal sites from offshore mid-Norway and from the Barents Sea. No consistent nannofossil data are yet available for Early Cretaceous high latitudes, which should more clearly reflect possible palaeoclimatic changes, as recorded in fluctuations of diversity and abundance. Existing data from France, Italy, Romania, Poland and NW Europe (Germany, North Sea, England) have been complemented by material from higher latitudes. In order to record the Arctic–Boreal nannofossil assemblages, 400 samples from the Norwegian Shelf and the Barents Sea have been analysed. Sixty samples, covering the Berriasian–Barremian interval, yielded calcareous nannofossils. These are derived from cores between 63 and 73°N of present-day latitudes. The nannofossil assemblages recorded are generally of a low diversity and characterized by abundant Watznaueria barnesae and Crucibiscutum salebrosum , whereas Biscutum constans is less common. C. salebrosum , rare or absent at low latitudes, is extremely common in the Norwegian samples. It shows a bipolar distribution especially during Valanginian–Hauterivian times. In the early Valanginian, a distinctive latitudinal gradient in the abundance of C. salebrosum , impoverished in low latitudes and abundant in high latitudes, possibly reflects latitudinal differences in temperature. Restricted palaeoceanographic settings, caused by a sea-level lowstand, may have amplified these thermal gradients. Palaeoclimatically this implies the existence of climatic belts throughout parts of the Early Cretaceous (early Valanginian, Hauterivian), resulting from considerable temperature gradients from north to south. The palaeobiogeographic patterns discussed resulted in the formation of distinctive latitudinally bound nannofossil zones, which, to a certain extent, are similar to those of modern oceans. The palaeobiogeographic patterns described support the idea of an ice-house phase for the early Valanginian and disagree with the suggestion of a globally warm equable climate during that time.

Crystallographic texture and microstructure of terebratulide brachiopod shell calcite : An optimized materials design with hierarchical architecture

Abstract We analyzed the microstructure, microchemistry, and microhardness variations across the architectural elements of the shells of the brachiopod species Megerlia truncata and Terebratalia transversa with scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), laser-ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS), and Vickers microhardness indentation (VMHI). The brachiopod valves consist of two principal layers of distinct calcite biomineralization: a thin, nanocrystalline, outer, hard protective layer with VMHI values exceeding 200 HV and a much thicker, inner, secondary layer of a hybrid organic-inorganic fiber composite material. The secondary layer is further structured into two sublayers, an outer part with VMHI values varying between 110 and 140 HV, and a softer inner part (70 < HV < 110). Whereas the size of the calcite crystals within the primary layer varies between a few tens of nanometers and 2 μm, calcite crystals within the secondary layer are fibrous, commonly reaching lengths exceeding 150 μm. Cross sections of these fibrous crystals are spade shaped, their dimensions being about 5 × 20 μm. The fibers are aligned parallel to each other. They are single crystals with their morphological fiber axes pointing almost parallel to the shell vault. The crystallographic orientation of the morphological fiber axes, however, is arbitrary within the a-b plane of the calcite lattice, whereas the c-axis (hexagonal unit-cell setting) is perpendicular to the morphological fiber axes and thus parallel to the radial vector of the valve vault. This morphology strongly indicates that fibrous growth is controlled by confinement within a cell in an organic matrix and not by attachment of biomolecules to specific crystallographic faces. We observe inhomogeneous Sr2+ and Mg2+ concentrations in the shell calcite within the 0.1.0.9 wt% range. Design of the shell appears to be highly optimized for mechanical performance. Crystal morphology and orientation as well as incorporated organic matter are structured hierarchially at different length levels forming a hybrid organic-inorganic fiber composite architecture.

Mesozoic calcareous nannofossils state of the art.

Calcareous nannofossils originated in the Triassic, radiated in the Jurassic and became a dominant component of the marine biosphere from the earliest Jurassic onward. They can be considered as one of the most important “innovations” of the Mesozoic oceans. Their basic morphology allows the differentiation of three different groups: coccoliths, nannoliths and calcispheres (= calcareous dinocysts). Only coccoliths and nannoliths are discussed in this article in some detail. Coccoliths and nannoliths have contributed greatly in the Interpretation of Mesozoic marine Systems through biostratigraphy and palaeoecology/palaeoceanography. Ever since the late 1960s both coccoliths and nannoliths have proven to be useful and reliable zonal markers for biostratigraphic schemes, allowing detailed zonations for the Jurassic and Cretaceous. Though affected by palaeobiogeographic provincialism, coccoliths and nannoliths have supplied many cosmopolitan biostratigraphic markers. These allow a global correlation of marine sedimentary units both from onshore sections in the classical European and North American areas and pelagic sequences recovered in the course of the DSDP/ODP drilling from the world’s oceans. Thus research on calcareous nannofossils Covers both, regional and global aspects. Research in the last 15 years concentrated on palaeoecological aspects. Apart from dinoflagellates, coccolithophores were the most important primary producers in Mesozoic oceans. As such they heavily relied on autecological factors such as light, nutrients and temperature. Variations in the assemblage composition of these groups may thus be viewed as a key for understanding palaeoecological, palaeoceanographic and palaeoclimatic changes of the past.KurzfassungKalkige Nannofossilien sind in der Trias entstanden, erlebten eine Radiation im Jura und sind seit dem Ober-Jura eine dominante Organismengruppe der marinen Biosphäre. Bei dieser Phytoplanktongruppe handelt es sich um eine der wichtigsten biologischen Neuerungen in den mesozoischen Ozeanen. Die Grundmorphologie erlaubt eine Unterscheidung von drei Gruppen: Coccolithen, Nannolithen und Calcisphären (= kalkige Dinoflagellatenzysten). Im vorliegenden Artikel werden nur die Coccolithen und die Nannolithen eingehender behandelt. Für zwei Bereiche haben Coccolithen und Nannolithen wichtige Informationen geliefert, die zu einer Neudeutung mesozoischer mariner Systeme geführt haben: 1. Biostratigraphie, 2. Paläoökologie/Paläoozeanographie. Seit der zweiten Hälfte der 60-er Jahre des 20. Jahrhunderts haben sich sowohl Coccolithen als auch Nannolithen als wichtige und nützliche Zonenleitfossilien für biostratigraphische Gliederungen erwiesen. Diese ermöglichen eine detaillierte Zonengliederung des Jura und der Kreide. Obwohl für Coccolithen und Nannolithen biogeographischer Provinzialismus bekannt ist, haben beide Gruppen viele kosmopolitische Leitformen geliefert. Mit Hilfe dieser Leitformen ist eine globale Korrelation mariner Sedimentabfolgen der klassischen On-shore Profile Europas und Nord Amerikas mit den pelagischen Abfolgen möglich, die im Rahmen des DSDP/ODP Programmes erbohrt wurden. Forschungsaktivitäten im Bereich des kalkigen Nannoplankton decken somit sowohl regionale als auch globale Aspekte ab. Die Forschung der letzten 15 Jahren fokussiert sich auf die Rolle dieser Gruppe als wichtige Primärproduzenten in den mesozoischen Ozeanen; neben Dinoflagellaten waren die Coccolithen die wichtigsten Primärproduzenten. Damit ist eine klare Abhängigkeit von autökologischen Faktoren wie Licht, Nährstoffen und Temperatur gegeben. Variationen in den Florenvergesellschaftungen sind somit ein wesentlicher Schlüssel zum Verständnis von paläoökologischen, paläoozeanographischen und paläoklimatischen Veränderungen der Vergangenheit.

Biostratigraphy and palaeobiogeography of Early Cretaceous calcareous nannofossils

Abstract Nineteen nannofossil zones are recognizable in the Lower Cretaceous of north-west Europe. Some of these can be found in other boreal regions, but correlation with the Tethys and the Indo-Pacific is not possible before the Aptian. The nannofossil assemblages of the Early Cretaceous show a distinctive palaeobiogeographic distribution pattern for the intervals Berriasian-Barremian on the one hand and for the Aptian-Albian on the other. The Berriasian-Barremian period is characterized by provinciality on the generic level and it is possible to differentiate between a Boreal Realm and a Tethyan Realm. The Tethyan Realm is subdivided into a Mediterranean Province and an Indo-Pacific Province. The Aptian-Albian was marked by a nannofloral (and faunal) turnover, causing a worldwide homogenization of nannofloras. The changes of the distribution patterns are best explained by (1) increased sea floor spreading; (2) a sea-level highstand in the Aptian; (3) temperature control of some species.

The Greenland-Norwegian Seaway: a key area for understanding Late Jurassic to early Cretaceous paleoenvironments

The paleoclimatology and paleoceanology of the Late Jurassic and Early Cretaceous are of special interest because this was a time when large amounts of marine organic matter were deposited in sediments that have subsequently become petroleum source rocks. However, because of the lack of outcrops, most studies have concentrated on low latitudes, in particular the Tethys and the “Boreal Realm,” where information has been based largely on material from northwest Germany, the North Sea, and England. These areas were all south of 40°N latitude during the Late Jurassic and Early Cretaceous. We have studied sediment samples of Kimmeridgian (∼154 Ma) to Barremian (∼121 Ma) age from cores taken at sites offshore mid-Norway and in the Barents Sea that lay in a narrow seaway connecting the Tethys with the northern polar ocean. During the Late Jurassic-Early Cretaceous these sites had paleolatitudes of 42–67°N. The Late Jurassic-Early Cretaceous sequences at these sites reflect the global sea-level rise during the Volgian-Hauterivian and a climatic shift from warm humid conditions in Volgian times to arid cold climates in the early Hauterivian. The sediments indicate orbital control of climate, reflected in fluctuations in the clastic influx and variations in carbonate and organic matter production. Trace element concentrations in the Volgian-Berriasian sediments suggest that the central part of the Greenland-Norwegian Seaway might have had suboxic bottom water beneath an oxic water column. Both marine and terrigenous organic matter are present in the seaway sediments. The Volgian-Berriasian strata have unusually high contents of organic carbon and are the source rocks for petroleum and gas fields in the region. The accumulation of organic carbon is attributed to restricted conditions in the seaway during this time of low sea level. It might be that the Greenland-Norwegian segment was the deepest part of the transcontinental seaway, bounded at both ends by relatively shallow swells. The decline in organic matter content of the sediments in the Valanginian-Hauterivian indicates greater ventilation and more active flow through the seaway as the sea level rose. The same benthic foraminifera assemblages are encountered throughout the seaway. Endemic assemblages of arenaceous foraminifera in the Volgian-Berriasian give way to more diverse and cosmopolitan Valanginian-Hauterivian benthic communities that include calcareous species. The foraminiferal assemblages also suggest low oxygen content bottom waters during the earlier Cretaceous, changing to more fully oxygenated conditions later. The calcareous nannoplankton, particularly Crucibiscutum salebrosum, which is rare at low latitudes and abundant in high latitudes, reflect the meridional thermal gradient. They indicate that the Greenland-Norwegian segment of the seaway was north of a subtropical frontal zone that acted as a barrier between the Tethyan and Boreal Realms. This implies the existence of stable climatic belts during the early Valanginian and Hauterivian, significant meridional temperature gradients, and moderate “ice house” conditions.