Expedition 306 summary

The overall aim of the North Atlantic paleoceanography study of Integrated Ocean Drilling Program Expedition 306 is to place late Neogene–Quaternary climate proxies in the North Atlantic into a chronology based on a combination of geomagnetic paleointensity, stable isotope, and detrital layer stratigraphies, and in so doing generate integrated North Atlantic millennial-scale stratigraphies for the last few million years. To reach this aim, complete sedimentary sections were drilled by multiple advanced piston coring directly south of the central Atlantic “ice-rafted debris belt” and on the southern Gardar Drift. In addition to the North Atlantic paleoceanography study, a borehole observatory was successfully installed in a new ~180 m deep hole close to Ocean Drilling Program Site 642, consisting of a circulation obviation retrofit kit to seal the borehole from the overlying ocean, a thermistor string, and a data logger to document and monitor bottom water temperature variations through time

Radiolaires des sédiments messiniens à diatomées de Falconara (Sicile, Italie)

Les sédiments à diatomées de Falconara (agés de 6,93 à 6,08 millions d\u27années) ont une épaisseur totale d\u27environ 27 m. La séquence est composée de 41 paires diatomite/argile. Le dépôt de ces sédiments biosiliceux est apparemment modulé par le cycle de précession astronomique (avec une période de 21000 ans) et semble avoir eu lieu dans un bassin peu profond; les sédiments à diatomées et argileux correspondent respectivement à des périodes de bas et haut niveau marin. La composition des assemblages à radiolaires polycystines et l\u27abondance de ceux-ci varient beaucoup d\u27un cycle à l\u27autre, d\u27absent dans le cycle 2 jusqu\u27à plus de 142000 radiolaires/g dans le cycle 26. Les radiolaires, ainsi que les diatomées, sont généralement absents des échantillons argileux. Cependant, une argile (S11) a une concentration élevée (&gt; 43000 radiolaires/g) et neuf autres ont une faible concentration. Nous avons compté 68 morphotypes. Les données brutes ont été soumises à une analyse factorielle en mode Q. Avec cinq facteurs, nous pouvons expliquer 83,65 % de la variance cumulée, tandis que neuf facteurs sont nécessaires pour rendre compte de 96,52 % de cette variance. Ces neufs facteurs montrent des maximums bien définis qui peuvent être utilisés pour naviguer le long du profil et pour corréler les différentes coupes. Une autre méthode permettant une bonne résolution stratigraphique pourrait être fondée sur la faune particulière à chacune de ces diatomites. Dans une étude pilote, nous avons choisi trois diatomites et nous avons comparé la composition de leurs assemblages et leur abondance en radiolaires. Le cycle 11 est dominé par Stichocorys delmontensis et Lithomitra lineata, avec des maximums à environ 50 % et 36 % respectivement. Dans le cycle 21, S. delmontensis and Botryostrobus auritus/australis sont les plus communs, avec des maximums à environ 40 % et 30 % respectivement. Pour ces deux cycles, les nassellaires forment le groupe dominant. Dans le cycle 26, Larcoidea sp., Porodiscus sp., et Spongotrochus glacialis sont les taxons dominants.The diatomitic sediments in Falconara (6.93 to 6.08 Ma) have a total thickness of c. 27 m, and the sequence is composed of 41 diatomite/claystone couplets. The deposition of these biosiliceous sediments, seemingly modulated by the astronomical precession cycle (21,000 years period), has been suggested to have taken place in a shallow basin with diatomaceous and claystone sediments being deposited during low and high sea level stands, respectively. Polycystine radiolarians show major changes in the assemblage compositions and the total abundances between the different cycles, from being barren in Cycle 2 till &gt; 142,000 radiolarians/g in Cycle 26. Radiolarians are usually not present in the claystone samples, this is also the case with diatoms. However, one claystone (S11) had a significant high number, &gt; 43,000 radiolarians/g sediment, while nine others had a low content of radiolarians. We have in our counts recognised 68 morphotypes. Q-Mode Factor analysis has been used on the raw counting data. By using five factors we were able to explain 83.65% of the cumulative variance, while using nine factors allowed us to explain 96.52% of the variance. These nine factors displayed well-defined peaks that can be used to navigate within the profile and to correlate between sections. Another high resolution stratigraphical tool could be represented by the peculiar faunal composition of each of the diatomites. In our pilot study we selected three diatomites and compared their assemblage composition and radiolarian abundances. Cycle 11 is dominated by Stichocorys delmontensis and Lithomitra lineata, c. 50 and 36% maximum values respectively. In Cycle 21 S. delmontensis and Botryostrobus auritus/australis are most common, with 40 and 30% as maximum values respectively. In both these cycles nassellarians are the dominant group. In Cycle 26 Larcoidea sp.,Porodiscus sp., and Spongotrochus glacialis are the dominant taxa.</p

The radiolarian fauna during the Younger Dryas–Holocene transition in Andfjorden, northern Norway

We report on the changing radiolarian faunas from the Younger Dryas (ca.12.9 calibrated thousands of years BP [cal. Ky BP]) and into the Holocene (ca. 10.3 cal. Ky BP) in the core JM99-1200 from Andfjorden, northern Norway. Temperature reconstructions using both the Q-mode factor analysis and modern analogue technique methods show stable, cold temperatures below ca. 410 cm core depth, followed by abrupt warming into a relatively stable Holocene temperature regime. Age-depth modelling with three different methods gives an age of ca. 11.9–12.0 cal. Ky BP at this core depth, clearly older than the Younger Dryas–Holocene transition at ca. 11.65 cal. Ky BP according to ice core chronology. Considering that the age models may be insufficiently informed in an interval without radiometric dates, it is possible that the base of the Holocene is indeed at 410 cm, as indicated by the radiolarian and other core data. Such a change in the chronology would have implications for previously published work on the JM99-1200 core. Alternatively, the abrupt warming in Andfjorden predated the Younger Dryas–Holocene transition by a few hundred years. A distinct cold pulse at ca. 315–335 cm, or 11.4–11.5 cal. Ky BP is interpreted as the Preboreal Oscillation. The Preboreal Oscillation has not previously been detected in temperature curves based on planktic foraminifera in the same core, indicating that Radiolaria may be a more sensitive temperature indicator in this region

Modern incursions of tropical Radiolaria into the Arctic Ocean

Plankton samples obtained by the Norwegian Polar Institute (August, 2010) in an area north of Svalbard contained an unusual abundance of tropical and subtropical radiolarian taxa (98 in 145 total observed taxa), not typically found at these high latitudes. A detailed analysis of the composition and abundance of these Radiolaria suggests that a pulse of warm Atlantic water entered the Norwegian Sea and finally entered into the Arctic Ocean, where evidence of both juvenile and adult forms suggests they may have established viable populations. Among radiolarians in general, this may be a good example of ecotypic plasticity. Radiolaria, with their high species number and characteristic morphology, can serve as a useful monitoring tool for pulses of warm water into the Arctic Ocean. Further analyses should be followed up in future years to monitor the fate of these unique plankton assemblages and to determine variation in northward distribution and possible penetration into the polar basin. The fate of this tropical fauna (persistence, disappearance, or genetic intermingling with existing taxa) is presently unknown. The current event may not be unique, nor a consequence of global warming, because analyses of sediment samples suggest that several natural pulses of warm water of this kind occurred in the prior century and, indeed, there may be more in years to come

A high-resolution radiolarian-derived paleotemperature record for the Late Pleistocene-Holocene in the Norwegian Sea

Polycystine radiolarians are used to reconstruct summer sea surface temperatures (SSSTs) for the Late Pleistocene-Holocene (600-13,400 C-14 years BP) in the Norwegian Sea. At 13,200 C-14 years BP, the SSST was close to the average Holocene SSST (similar to12degreesC). It then gradually dropped to 7.1degreesC in the Younger Dryas. Near the Younger Dryas-Holocene transition (similar to10,000 C-14 years BP), the SSST increased 5degreesC in about 530 years. Four abrupt cooling events, with temperature drops of up to 2.1degreesC, are recognized during the Holocene: at 9340, 7100 ("8200 calendar years event''), 6400 and 1650 C-14 years BP. Radiolarian SSSTs and the isotopic signal from the GISP2 ice core are strongly coupled, stressing the importance of the Norwegian Sea as a mediator of heat/precipitation exchange between the North Atlantic, the atmosphere, and the Greenland ice sheet. Radiolarian and diatom-derived SSSTs display similarities, with the former not showing the recently reported Holocene cooling trend

Radiolaria and Phaeodaria (siliceous Rhizaria) in south-western and northern Norwegian fjords during late summer 2016: dominant species and biomass in shallow-water assemblages

To determine the present-day community composition of siliceous Rhizaria (Radiolaria and Phaeodaria) in Norwegian fjords, plankton tows were conducted in south-western and northern Norwegian fjords in September 2016. The mean total abundance of radiolarians was 306 m–3 in the Sognefjord complex, which was the southern research site, and, in the north, 945 m–3 in Malangen and 89 m–3 in Balsfjord, both above the Arctic Circle. Sticholonche zanclea was the most abundant radiolarian in the Sognefjord complex and Malangen, accounting for 78–100% (mean 89%) of radiolarian abundance. The mean total abundance of phaeodarians was 1554 m–3 in the Sognefjord complex, 51 m–3 in Malangen and 11 m–3 in Balsfjord. Medusetta arcifera was the most abundant phaeodaria in the Sognefjord complex, accounting for >99% of phaeodarian abundance, but was absent in Malangen and Balsfjord, where Protocystis tridens accounted for >96% of phaeodarian abundance. The carbon biomass of S. zanclea and M. arcifera was 188 and 438 µg C m–3, respectively, which is similar to and 8.6 times higher than, respectively, that of phaeodarians >1 mm in the western North Pacific, suggesting that M. arcifera contributes to organic carbon transport in the Sognefjord complex. Amphimelissa setosa (Nassellaria, Radiolaria), which was a dominant species in the study area in 1982–83, was absent in the present study in all sampled fjords. This could have been caused by the approximately 2 °C increase in water temperature that has occurred since 1990 and can be taken as evidence of a climate-change-associated local temperature rise linked to the warming of advected Atlantic Water

Late Pleistocene-Holocene radiolarian paleotemperatures in the Norwegian Sea based on Artificial Neural Networks 224 (2005) 311 332

Artificial Neural Networks (ANN) were trained by using an extensive radiolarian census dataset from the Nordic (Greenland, Norwegian, and Iceland) Seas. The regressions between observed and predicted Summer Sea Temperature (SST) indicate that lower error margins and better correlation coefficients are obtained for 100 m (SST100) compared to 10 m (SST10) water depth, and by using a subset of species instead of all species. The trained ANNs were subsequently applied to radiolarian data from two Norwegian Sea cores, HM 79-4 and MD95-2011, for reconstructions of SSTs through the last 15,000 years. The reconstructed SST is quite high during the Bolling-Allerod, when it reaches values only found later during the warmest phase of the Holocene. The climatic transitions in and out of the Younger Dryas are very rapid and involve a change in SST100 of 6.2 and 6.8 degrees C, taking place over 440 and 140 years, respectively. SST100 remains at a maximum during the early Holocene, and this Radiolarian Holocene Optimum Temperature Interval (RHOTI) predates the commonly recognized middle Holocene Climatic Optimum (HCO). During the 8.2 ka event, SST100 decreases by ca. 3 degrees C, and this episode marks the establishment of a cooling trend, roughly spanning the middle Holocene (until ca. 4.2 ka). Successively, since then and through the late Holocene, SST100 follows instead a statistically significant warming trend. The general patterns of the reconstructed SSTs agree quite well with previously obtained results based on application of Imbrie and Kipp Transfer Functions (IKTF) to the same two cores for SST0. A statistically significant cyclic component of our SST record (period of 278 years) has been recognized. This is close to the de Vries or Suess cycle, linked to solar variability, and documented in a variety of other high-resolution Holocene records