Low-temperature magnetic remanence and hysteresis measurements on sediment core GeoB4901-8 from the Niger deep-sea fan

Low-temperature (LT) magnetic remanence and hysteresis measurements, in the range 300-5 K, were combined with energy dispersive spectroscopy (EDS) in order to characterize the magnetic inventory of strongly diagenetically altered sediments originating from the Niger deep-sea fan. We demonstrate the possibility of distinguishing between different compositions of members of the magnetite-ulvöspinel and ilmenite-hematite solid solution series on a set of five representative samples, two from the upper suboxic and three from the lower sulfidic anoxic zone of gravity core GeoB 4901. Highly sensitive LT magnetic measurements were performed on magnetic extracts resulting in large differences in the magnetic behavior between samples from the different layers. This emphasizes that both Fe-Ti oxide phases occur in different proportions in the two geochemical environments. Most prominent are variations in the coercivity sensitive parameter coercive field (BC). At room-temperature (RT) hysteresis loops for all extracts are narrow and yield low coercivities (6-13 mT). With decreasing temperature the loops become more pronounced and wider. At 5 K an approximately 5-fold increase in BC for the suboxic samples contrasts a 20-25-fold increase for the samples from the anoxic zone. We demonstrate that this distinct increase in BC at LT corresponds to the increasing proportion of the Ti-rich hemoilmenite phase, while Fe-rich (titano-)magnetite dominates the magnetic signal at RT. This trend is also seen in the room-temperature saturation isothermal remanent magnetization (RT-SIRM) cycles: suboxic samples show remanence curves dominated by Fe-rich mineral phases while anoxic samples display curves clearly dominated by Ti-rich particles. We show that the EDS intensity ratios of the characteristic Fe Kalpha and Ti Kalpha lines of the Fe-Ti oxides may be used to differentiate between members of the magnetite-ulvöspinel and ilmenite-hematite solid solution series. Furthermore it is possible to calculate an approximate composition for each grain if the intensity ratios of natural particles are linked to well-known standards. Thus, element spectra with high Fe/Ti intensity ratios were found to be rather typical of titanomagnetite while low Fe/Ti ratios are indicative of hemoilmenite. The EDS analyses confirm the LT magnetic results, Fe-rich magnetic phases dominate in the upper suboxic environment whereas Ti-rich magnetic phases comprise the majority of particles in the lower anoxic domain: The mineral assemblage of the upper suboxic environments is composed of magnetite (~19%), titanomagnetite (~62%), hemoilmenite (~17%) and ~2% other particles. In the lower anoxic sediments, reductive diagenetic alteration has resulted in more extensive depletion of the (titano-)magnetite phase, resulting in a relative enrichment of the hemoilmenite phase (~66%). In these strongly anoxic sediments stoichiometric magnetite is barely preserved and only ~5% titanomagnetite was detected. The remaining ~28% comprises Ti-rich particles such as pseudobrookite or rutile.

Age determination and foraminiferal faunas in sediment core MD99-2275

A high-resolution study of palaeoceanographic changes off North Iceland during the time period 8600-5200 cal year BP is based on benthic and planktonic foraminiferal assemblages. The core material (MD99-2275) was obtained from about 440 m water depth on the eastern part of the North Icelandic shelf. Changes in the faunal composition are interpreted to be mainly caused by variations in the strength of the relatively warm, high-salinity Irminger Current and the cold East Icelandic Current, which have been shown to be linked to the climatic changes in the North Atlantic region. Environmental proxies at that site are particularly sensitive to palaeoceanographic changes due to its position close to the marine Polar Front. Benthic assemblages show that relatively cold conditions prevailed at the base of the record. An increase in the influence of Atlantic water masses at the sea floor is seen at around 8400 cal year BP, whereas the surface waters were relatively warm already at 8600 cal year BP. The warming was interrupted by a cold event at around 8100-8000 cal year BP, registered both in the bottom and surface waters and correlated with the so-called 8.2 kyr cooling event. Both the benthic and the planktonic faunal compositions indicate that the Irminger Current had maximum influence in the area between 8000 and about 7300 cal year BP, followed by a gradually decreasing influence through the remaining part of the studied time interval. It is suggested that the contribution of Atlantic water masses from the east and north-east to the Arctic Surface waters off North Iceland increased after around 7000 cal year BP, and that this was further intensified after 6200 cal year BP. At present, the Arctic Surface Water north of Iceland consists of Polar waters, intermittently with direct influence from the East Greenland Current, mixed with Atlantic waters derived from the eastern part of the Nordic Seas. A comparison of the mean values of selected environmental proxies in the interval 8600-5200 cal year BP with the upper part of the same core shows that the water masses north of Iceland were considerably warmer during the Holocene thermal maximum than during the last 2000 cal year. In general, results from core MD99-2275 are in accordance with other marine records from the North Icelandic shelf and the northern North Atlantic region, although a detailed comparison on a centennial time scale is hampered by problems with spatial as well as temporal changes in the marine reservoir ages in the region.

Paleomagnetic and rock magnetic characterization of ODP Leg 210 cores

We conducted an integrated paleomagnetic and rock magnetic study on cores recovered from Ocean Drilling Program Sites 1276 and 1277 of the Newfoundland Basin. Stable components of magnetization are determined from Cretaceous-aged sedimentary and basement cores after detailed thermal and alternating-field demagnetization. Results from a series of rock magnetic measurements corroborate the demagnetization behavior and show that titanomagnetites are the main magnetic carrier. In view of the normal polarity of magnetization and radiometric dates for the sills at Site 1276 (~98 and ~105 Ma, both within the Cretaceous Normal Superchron) and for a gabbro intrusion in peridotite at Site 1277 (~126 Ma, Chron M1), our results suggest that the primary magnetization of the Cretaceous rocks is likely retained in these rocks. The overall magnetic inclination of lithologic Unit 2 in Hole 1277A between 143 and 180 meters below seafloor is 38°, implying significant (~35° counterclockwise, viewed to the north) rotation of the basement around a horizontal axis parallel to the rift axis (010°). The paleomagnetic rotational estimates should help refine models for the tectonic evolution of the basement. The mean inclinations for Sites 1276 and 1277 rocks imply paleolatitudes of 30.3° ± 5.1° and 22.9° ± 12.0°, respectively, with the latter presumably influenced by tectonic rotation. These values are consistent with those inferred from the mid-Cretaceous reference poles for North America, suggesting that the inclination determinations are reliable and consistent with a drill site on a location in the North America plate since at least the mid-Cretaceous. The combined paleolatitude results from Leg 210 sites indicate that the Newfoundland Basin was some 1800 km south of its current position in the mid-Cretaceous. Assuming a constant rate of motion, the paleolatitude data would suggest a rate of 12.1 mm/yr for the interval from ~130 Ma (Site 1276 age) to present, and 19.6 mm/yr for the interval from 126 Ma (Site 1277 age) to recent. The paleolatitude and rotational data from this study are consistent with the possibility that Site 1276 may have passed over the Canary and Madeira hotspots that formed the Newfoundland Seamounts in the mid-Cretaceous.

Data compilation on the biological response to ocean acidification: environmental and experimental context of data sets and related literature

The exponential growth of studies on the biological response to ocean acidification over the last few decades has generated a large amount of data. To facilitate data comparison, a data compilation hosted at the data publisher PANGAEA was initiated in 2008 and is updated on a regular basis (doi:10.1594/PANGAEA.149999). By January 2015, a total of 581 data sets (over 4 000 000 data points) from 539 papers had been archived. Here we present the developments of this data compilation five years since its first description by Nisumaa et al. (2010). Most of study sites from which data archived are still in the Northern Hemisphere and the number of archived data from studies from the Southern Hemisphere and polar oceans are still relatively low. Data from 60 studies that investigated the response of a mix of organisms or natural communities were all added after 2010, indicating a welcomed shift from the study of individual organisms to communities and ecosystems. The initial imbalance of considerably more data archived on calcification and primary production than on other processes has improved. There is also a clear tendency towards more data archived from multifactorial studies after 2010. For easier and more effective access to ocean acidification data, the ocean acidification community is strongly encouraged to contribute to the data archiving effort, and help develop standard vocabularies describing the variables and define best practices for archiving ocean acidification data.

Seawater carbonate chemistry and biological processes during experiments with clownfish Amphiprion percula, 2009

The persistence of most coastal marine species depends on larvae finding suitable adult habitat at the end of an offshore dispersive stage that can last weeks or months. We tested the effects that ocean acidification from elevated levels of atmospheric carbon dioxide (CO2) could have on the ability of larvae to detect olfactory cues from adult habitats. Larval clownfish reared in control seawater (pH 8.15) discriminated between a range of cues that could help them locate reef habitat and suitable settlement sites. This discriminatory ability was disrupted when larvae were reared in conditions simulating CO2-induced ocean acidification. Larvae became strongly attracted to olfactory stimuli they normally avoided when reared at levels of ocean pH that could occur ca. 2100 (pH 7.8) and they no longer responded to any olfactory cues when reared at pH levels (pH 7.6) that might be attained later next century on a business-as-usual carbon-dioxide emissions trajectory. If acidification continues unabated, the impairment of sensory ability will reduce population sustainability of many marine species, with potentially profound consequences for marine diversity.

B, d11B, K and d18O data for altered oceanic crust at DSDP Sites 417/418

Samples of drilled oceanic crust, from DSDP Holes 417A, 417D and 418A and ODP Hole 735B, and oceanic crust from the Oman and Cyprus ophiolites, were analyzed for B contents and d11B. Composite samples from DSDP Holes 417A, 417D and 418A were used to represent the upper 550 m of altered oceanic crustal Layer 2A. Whole-rock samples from the Troodos ophiolite, Cyprus, and the Oman ophiolite were selected to represent crustal Layer 2B dikes. Composite samples from ODP Hole 735B were used to represent crustal Layer 3. The B content of the DSDP composites ranges from 7.2 ppm to 104 ppm and correlates with both d1818O and K, showing that it is a good indicator of the extent of low temperature alteration. The d11B of the DSDP composites varies between -2.5? and 5.4?. The B content of the samples from the Troodos ophiolite ranges from 2.4 ppm to 8.1 ppm; d11B varies from -0.9? to 7.8?. The B content of the Oman ophiolite samples ranges from 5.0 ppm to 11.1 ppm; d11B varies from -1.6? to 16.9?. The B content of the samples from ODP Hole 735B ranges from 1.1 ppm to 7.1 ppm; d11B varies from -4.3? to 24.9?. The general pattern displayed by these samples is one of greatest (and most variable) B enrichment at the top of the crust and least enrichment at the bottom of the section. All of these samples are enriched compared to unaltered MORB, which is believed to have a B content of approximately 0.5 ppm. The d11B values of deeper samples, from Layers 2B and 3, are more variable and generally higher than those from Layer 2A. Boron contents and d11B are not correlated. The data from the DSDP Site 417/418 composites indicate that the d11B of fluid circulating in the upper crust changes only slightly during alteration, increasing by an average of 5.1? with an accompanying decrease in B concentration of 7%. Low temperature alteration appears to be a water-dominated process resulting in minor modification of circulating seawater. A minimum water-rock ratio of 400 is calculated for these samples, implying a minimum low-temperature seawater flux through the upper oceanic crust of 3.4?10**14 l/y. The average B content of altered oceanic crust, as represented by these samples, is 5.2+/-1.7 ppm and the average d11B is 3.4+/-1.1?. This average isotopic composition is measurably different from the apparent average of oceanic sediments, supporting the idea that d11B could be useful for identifying the source(s) of B in island arcs.