André Bornemann

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.

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.

Stable isotope and benthic foraminiferal records of the Latest Danian Event at ODP Site 1262 (Walvis Ridge)

The Latest Danian Event (LDE – aka Top Chron 27n Event) is characterized by a >1h negative benthic foraminiferal CIE in various sections in Egypt, which has been correlated with δ13C shifts of ~0.7h in Zumaia (Spain), Wombat Plateau (ODP 761B, Indian Ocean) and Shatsky Rise (ODP 1209, Pacific Ocean) (Bornemann et al., 2009; Westerhold et al., 2011). A concurrent ~0.5h δ18O excursion suggests a 2◦C bottom water temperature rise during this event at Shatsky Rise, suggesting a hyperthermal nature for this event (Westerhold et al., 2011).

Deep-sea benthic foraminiferal turnovers in the early Eocene: The role of the PETM and ETM2

PETM, taxonomy Throughout the Cenozoic, deep-sea benthic foraminiferal communities have faced many periods of environmental turmoil. Three major, yet relatively gradual faunal turnovers occurred during the Eocene-Oligocene, middle Miocene and middle Pleistocene - all periods of pronounced cooling and increases in polar ice volume. In contrast, the PaleoceneEocene Thermal Maximum (PETM; ~56 Ma) – a transient global warming event or hyperthermal - is characterized by a rapid extinction of 30-50% of all deep-sea benthic foraminiferal species (Thomas, 1998; 2007). So far, the exact cause(s) of this severe extinction event that devastated bathyal and abyssal faunas is not known. It is likely that a change in food-web structure, affected by high temperatures, a decrease in the oxygenation state of the oceans, calcite undersaturation, primary productivity or ocean current circulation changes played an important role in the benthic foraminiferal extinction (BFE) and in the establishment of the opportunistic fauna that characterizes the PETM itself (Thomas, 1998, 2007). Historically, the earliest Eocene benthic foraminiferal associations were considered as a transitional fauna, recovering from the BFE and gradually recolonizing vacant niches and habitats. This transition from a Cretaceous-Paleocene Velascotype fauna into a typical Eocene Barbados-type fauna appeared to be characterized by the gradual appearance of new taxa (Tjalsma & Lohmann, 1983; Berggren & Miller, 1989). With the discovery of several Eocene hyperthermals similar to the PETM (e.g. Eocene Thermal Maximum 2; ETM2; ~53.7 Ma), the question arises of what role these events played in the development of early Eocene benthic communities. For instance, how did the impoverished benthic fauna cope with the environmental perturbations associated with these successive and smaller Eocene hyperthermals on short time scales? Did it become more or less sensitive to climatic and oceanographic changes? One can also wonder what the effects were on longer time scales. Did early Eocene hyperthermals hamper or stimulate recolonization of the benthic realm, and if so, in what way?