ALBERT

Description: As the planet cooled from peak warmth in the early Cenozoic, extensive Northern Hemisphere ice sheets developed by 2.6 Ma ago, leading to changes in the circulation of both the atmosphere and oceans. From not, vert, similar2.6 to not, vert, similar1.0 Ma ago, ice sheets came and went about every 41 ka, in pace with cycles in the tilt of Earth’s axis, but for the past 700 ka, glacial cycles have been longer, lasting not, vert, similar100 ka, separated by brief, warm interglaciations, when sea level and ice volumes were close to present. The cause of the shift from 41 ka to 100 ka glacial cycles is still debated. During the penultimate interglaciation, not, vert, similar130 to not, vert, similar120 ka ago, solar energy in summer in the Arctic was greater than at any time subsequently. As a consequence, Arctic summers were not, vert, similar5 °C warmer than at present, and almost all glaciers melted completely except for the Greenland Ice Sheet, and even it was reduced in size substantially from its present extent. With the loss of land ice, sea level was about 5 m higher than present, with the extra melt coming from both Greenland and Antarctica as well as small glaciers. The Last Glacial Maximum (LGM) peaked not, vert, similar21 ka ago, when mean annual temperatures over parts of the Arctic were as much as 20 °C lower than at present. Ice recession was well underway 16 ka ago, and most of the Northern Hemisphere ice sheets had melted by 6 ka ago. Solar energy reached a summer maximum (9% higher than at present) not, vert, similar11 ka ago and has been decreasing since then, primarily in response to the precession of the equinoxes. The extra energy elevated early Holocene summer temperatures throughout the Arctic 1–3 °C above 20th century averages, enough to completely melt many small glaciers throughout the Arctic, although the Greenland Ice Sheet was only slightly smaller than at present. Early Holocene summer sea ice limits were substantially smaller than their 20th century average, and the flow of Atlantic water into the Arctic Ocean was substantially greater. As summer solar energy decreased in the second half of the Holocene, glaciers re-established or advanced, sea ice expanded, and the flow of warm Atlantic water into the Arctic Ocean diminished. Late Holocene cooling reached its nadir during the Little Ice Age (about 1250–1850 AD), when sun-blocking volcanic eruptions and perhaps other causes added to the orbital cooling, allowing most Arctic glaciers to reach their maximum Holocene extent. During the warming of the past century, glaciers have receded throughout the Arctic, terrestrial ecosystems have advanced northward, and perennial Arctic Ocean sea ice has diminished. Here we review the proxies that allow reconstruction of Quaternary climates and the feedbacks that amplify climate change across the Arctic. We provide an overview of the evolution of climate from the hot-house of the early Cenozoic through its transition to the ice-house of the Quaternary, with special emphasis on the anomalous warmth of the middle Pliocene, early Quaternary warm times, the Mid Pleistocene transition, warm interglaciations of marine isotope stages 11, 5e, and 1, the stage 3 interstadial, and the peak cold of the last glacial maximum.

Description: Highlights: ► We imaged a 3 × 5-km giant fluid seep structure, the Giant Gjallar Vent, off mid-Norway. ► We combined neural network analysis and sandbox modeling. ► We define the internal geometries of the underlying pipe. ► The Giant Gjallar Vent may be a proto-fluid seep at an early stage of its development. An exploration 3D seismic data set from the Gjallar Ridge off mid-Norway images a giant fluid seep structure, 3 × 5 km wide, which connects to late Palaeocene magmatic sills at depth. Two of the pipes that have developed as hydrothermal vents reach all the way to the modern seafloor implying that they either were active much longer than the original hydrothermal activity or have been reactivated. We combine detailed seismic analysis of the northern pipe and sandbox modeling to constrain pipe initiation and propagation. Although both the seismic data and the sandbox models suggest that fluids at depth are focused through a vertical conduit, sandbox models show that fluids ascend and reach a critical depth migration where focused migration abruptly transforms into distributed fluid flow through unconsolidated sediments. This indicates that at this level the sediments are intensely deformed during pipe propagation, creating a V-shaped structure, i.e. an inverted cone at depth and a positive relief anomaly, 5 to 10 m high, at the seafloor, which is clearly identified on 3D seismic data. Comparison of the geometries observed in sandbox modeling with the seismically observed geometries of the Giant Gjallar Vent suggests that the Giant Gjallar Vent may be a proto-fluid seep at an early stage of its development, preceding the future collapse of the structure forming a seafloor depression. Our results imply that the Gjallar Giant Vent can be used as a window into the geological processes active in the deep parts of the Vøring Basin.

Description: We present a magnesium (Mg) and strontium (Sr) record from an aragonitic speleothem (Grotte de Piste, Morocco, 34‬°N; 04°W) providing a reconstruction of effective rainfall from 619 to 1962 AD. The corresponding drip site was monitored over 2 yr for drip water Mg/Ca and Sr/Ca ratios. Results show evidence for prior aragonite precipitation, which can explain negative correlations between speleothem Mg and Sr concentrations. The data shown here have important climate implications concerning the evolution of the North Atlantic Oscillation (NAO). A comparison of the stalagmite data from Grotte de Piste with an updated tree ring based drought reconstruction from Morocco and other NAO related proxy records confirms that the Medieval Warm Period (MWP) was dominated by NAO+ conditions. The stalagmite record and multiple proxy records from the Iberian Peninsula, however, suggest that considerable rainfall variability occurred during the MWP. This implies that the NAO has been more variable during the MWP than formerly suggested.

Description: Highlights • The complex geodynamic structure of the area is reflected in the wide range of compositions of the emitted fluids. • High salinity waters with hydrocarbon gases and a purely crustal He component were collected from deep wells. • Hyperalkaline waters with CH4- and H2-dominated gases are found in the ophiolite complex. • Shallow meteoric groundwaters in the southern part of the basin show a prevailing atmospheric component for dissolved gases. • A significant mantle component (He and C) is found in the dissolved gases of the northeastern sites. Abstract We investigated the geochemical features of the fluids circulating over the Amik Basin (SE Turkey–Syria border), which is crossed by the Northern extension of the DSF (Dead Sea Fault) and represents the boundary area of three tectonic plates (Anatolian, Arabian and African plates). We collected 34 water samples (thermal and cold from natural springs and boreholes) as well as 8 gas samples (bubbling and gas seepage) besides the gases dissolved in the sampled waters. The results show that the dissolved gas phase is a mixture of shallow (atmospheric) and deep components either of mantle and crustal origin. Coherently the sampled waters are variable mixtures of shallow and deep ground waters, the latter being characterised by higher salinity and longer residence times. The deep groundwaters (from boreholes deeper than 1000 m) have a CH4-dominated dissolved gas phase related to the presence of hydrocarbon reservoirs. The very unique tectonic setting of the area includes the presence of an ophiolitic block outcropping in the westernmost area on the African Plate, as well as basalts located to the North and East on the Arabic Plate. The diffuse presence of CO2-enriched gases, although diluted by the huge groundwater circulation, testifies a regional degassing activity. Fluids circulating over the ophiolitic block are marked by H2-dominated gases with abiogenic methane and high-pH waters. The measured 3He/4He isotopic ratios display contributions from both crustal and mantle-derived sources over both sides of the DSF. Although the serpentinization process is generally independent from mantle-type contribution, the recorded helium isotopic ratios highlight variable contents of mantle-derived fluids. Due to the absence of recent volcanism over the western side of the basin (African Plate), we argue that CO2-rich volatiles carrying mantle-type helium and enriched in heavy carbon, are degassed by deep-rooted regional faults rather than from volcanic sources.

Description: This paper is an introduction to and an overview of papers presented in the Special Issue of Marine Geology “Methane seeps at the Hikurangi Margin, New Zealand”. In 2006 and 2007, three research cruises to the Hikurangi Margin at the east coast of New Zealand's North Island were dedicated to studying methane seepage and gas hydrates in an area where early reports suggested they were widespread. Two cruises were carried out on RV TANGAROA and one on RV SONNE using the complete spectrum of state-of-the-art equipment for geophysics (seismic, sidescan, controlled source electromagnetics, ocean bottom seismometers and hydrophones, singlebeam and multibeam), seafloor observations (towed camera systems, ROV), sediment and biological sampling (TV-guided multi-corer, gravity-corer, grab, epibenthic sled), deployment of in-situ observatories (landers) as well as water column sampling and oceanographic studies (CTD, moorings). The scientific disciplines involved ranged from geology, geophysics, petrography, geochemistry, to oceanography, biology and microbiology. These cruises confirmed that a significant part of the Hikurangi Margin has been active with locally intense methane seepage at present and in the past, with the widespread occurrence of dead seep faunas and knoll-forming carbonate precipitations offshore and on the adjacent land. A close link to seismically detected fluid systems and the outcropping of the base of the gas hydrate stability zone can be found at some places. Pore fluid and free gas release were found to be linked to tides. Currents as well as density layers modulate the methane distribution in the water column. The paper introduces the six working areas on the Hikurangi Margin, and compiles all seep locations based on newly processed multibeam and multibeam backscatter data, water column hydroacoustic and visual data that are combined with results presented elsewhere in this Special Issue. In total, 32 new seep sites were detected that commonly show chemoherm-type carbonates or carbonate cemented sediment with fissures and cracks in which calyptogenid clams and bathymodiolid mussels together with sibloglinid tube worms live. White bacterial mats of the genus Beggiatoa and dark gray beds of heterotrophic ampharetid polychaetes typically occur at active sites. Bubble release has frequently been observed visually as well as hydroacoustically (flares) and geochemical analyses show that biogenic methane is released. All seep sites, bubbling or not, were inside the gas hydrate stability zone. Gas hydrate itself was recovered at three sites from the seafloor surface or 2.5 m core depth as fist-sized chunks or centimeter thick veins. The strong carbonate cementation that in some cases forms 50 m high knolls as well as some very large areas being paved with clam shells indicates very strong and long lasting seep activity in the past. This activity seems to be less at present but nevertheless makes the Hikurangi Margin an ideal place for methane-related seep studies in the SW-Pacific.

Description: Marine Protected Areas (MPAs) now form an important part of marine conservation and fisheries management; hence, there is broad interest in developing procedures that optimize their design. We used data collected over a 10-year period (2003–2012) from direct surveys and 〉100 adult male and female loggerhead sea turtles (Caretta caretta ) tracked with devices, includ ing GPS loggers and Fastloc GPSArgos, to consider the optimum design for a MPA at a globally important breeding area, where there is already an existing national marine park aiming to protect the population (Zakynthos, Greece). Turtles primarily used areas very close to shore (approx. 7 km in length by 1 km in width, within the 〈10 m isobath) for breeding and foraging activity at different times of the year. We calculated that this small nearshore coastal zone encompassed 72% of all turtle GPS locations recorded in the MPA, and is therefore important for conservation management. We developed an index to evaluate the suitabili ty of the existing and proposed conserv ation zones based on (1) home range area use by turtles in these zones versus (2) zone size, so that the benefit to turtles could be maximized while minimizing the negative impacts to other stakeholders (e.g., boat operators). With this evidence-based approach, we propose a modification to the existing MPA that might both enhance local economic tourism activities and better safeguard this key sea turtle breeding population. The approaches used here will have general application for the design of MPAs used by mobile species that can be tracked.

Description: Subduction of oceanic lithosphere is a key feature of terrestrial plate tectonics. However, the effect of this recycled crustal material on mantle composition is debated. Ocean island basalts (OIB) provide direct insights into the composition of Earth's mantle. The distinct composition of the HIMU (high 238U/204Pb)- and EM (enriched mantle)-type OIB mantle sources may be due to either recycling of oceanic crust and sediment into the mantle or metasomatic processes within the mantle. Chlorine derived from seawater or crustal fluids potentially provides a tracer for recycled material. Previously reported δ37Cl values for mid-ocean ridge basalts (MORB) range from ca. −3.0 to near 0‰. In contrast to MORB, we find a larger variation in OIB glasses representing HIMU- and EM-type mantle sources based on replicate SIMS analyses with δ37Cl values ranging from −1.6 to +1.1‰ for HIMU-type and −0.4 to +2.9‰ for EM-type lavas. These δ37Cl values correlate positively with 87Sr/86Sr ratios for both the HIMU- and EM-type samples. The negative δ37Cl values of some HIMU-type lavas overlap with those of altered oceanic lithosphere, which is assumed to be present in the HIMU source. The EM lavas have high 87Sr/86Sr and primarily positive δ37Cl values. We hypothesize that subducting sediments may have developed high δ37Cl values by expelling 37Cl-depleted pore fluids, thus accounting for the positive δ37Cl values recorded in the EM-type lavas.

Description: Highlights • High-resolution AUV bathymetry is used to identify hydrothermal sulfides on the seafloor. • 1.2 Mt of massive sulfide is identified along 24 km of mid-ocean ridge length. • Massive sulfide accumulated at a rate of ∼400 t/yr. • Efficiency of sulfide deposition is ∼5% of total mobilized metals and reduced sulfur. • Current global seafloor sulfide estimates likely underestimate amount present by about a factor of four. Abstract Hydrothermal sulfide deposits that form on the seafloor are often located by the detection of hydrothermal plumes in the water column, followed by exploration with deep-towed cameras, side-scan sonar imaging, and finally by visual surveys using remotely-operated vehicle or occupied submersible. Hydrothermal plume detection, however, is ineffective for finding hydrothermally-inactive sulfide deposits, which may represent a significant amount of the total sulfide accumulation on the seafloor, even in hydrothermally active settings. Here, we present results from recent high-resolution, autonomous underwater vehicle-based mapping of the hydrothermally-active Endeavour Segment of the Juan de Fuca Ridge, in the Northeast Pacific Ocean. Analysis of the ridge bathymetry resulted in the location of 581 individual sulfide deposits along 24 km of ridge length. Hydrothermal deposits were distinguished from volcanic and tectonic features based on the characteristics of their surface morphology, such as shape and slope angles. Volume calculations for each deposit results in a total volume of 372,500 m3 of hydrothermal sulfide–sulfate–silica material, for an equivalent mass of ∼1.2 Mt of hydrothermal material on the seafloor within the ridge's axial valley, assuming a density of 3.1 g/cm3. Much of this total volume is from previously undocumented inactive deposits outside the main active vent fields. Based on minimum ages of sulfide deposition, the deposits accumulated at a maximum rate of ∼400 t/yr, with a depositional efficiency (proportion of hydrothermal material that accumulates on the seafloor to the total amount hydrothermally mobilized and transported to the seafloor) of ∼5%. The calculated sulfide tonnage represents a four-fold increase over previous sulfide estimates for the Endeavour Segment that were based largely on accumulations from within the active fields. These results suggest that recent global seafloor sulfide resource estimates, which were based mostly on the sizes and distribution of hydrothermally-active deposits, may be similarly underestimating the amount of sulfide along the global submarine neovolcanic zones.

Description: Highlights: • The first Nd isotopic data on the lateritic surface cover (Cover Horizon) of western Equatorial Africa. • Clearly different Nd isotopic signatures between the Cover Horizon and underlying basement. • A consistent model attributing the Cover Horizon to the settling of aeolian particles derived from the Namib desert. Abstract: Surficial formations in Gabon, as well as in other places of western Central Africa include a ubiquitous, homogeneous and 1–3 m-thick clayey to sandy lateritic surface cover known as the ‘Cover Horizon’. From 14C radiometric dating it has been concluded that the emplacement of this unit was correlative with a major environmental crisis which affected Central Africa c. 3000–2000 years ago. 10Be and Nd-isotopic analyses have been performed to provide new constraints on the age and origin of this layer. Six samples from two depth profiles investigated for 10Be exhibit an almost constant concentration consistent with a very recent deposition age. Nd-isotopic analyses performed on the silt to clay fraction of eleven samples from widely spaced locations over Gabon attest for mildly radiogenic signatures (εNd = −23 to −17) in ten of them, and a slightly radiogenic signature (εNd = −9) in one sample. TDM model ages range from 1.6 to 2.6 Ga, and a perfect discrimination is observed between the Nd-isotopic signature of the Cover Horizon and that of the underlying Congo Craton. This makes an aeolian origin as the most probable for the Cover Horizon. The average εNd (c. −20) is however rather unusual for aeolian sediments or aerosols. A possible source of particles is therefore tested by considering the present-day atmospheric flux over Gabon and adjacent regions. Combined atmospheric modeling and Nd-isotopes leads to the conclusion that the fine fraction of the Cover Horizon could have originated from the northern part of the Namib desert.

Description: Silver is one of the most toxic elements for the marine microbial and invertebrate community. However, little is known about the distribution and behaviour of dissolved silver in marine systems. This paper reports data on dissolved and sediment-associated silver in European estuaries and coastal waters which have been impacted to different extents by past and present anthropogenic inputs. This is the first extended dataset for dissolved silver in European marine waters. Lowest dissolved silver concentrations were observed in the Gullmar Fjord, Sweden (8.9 ± 2.9 pM; x ± 1σ), the Tamar Estuary, UK (9.7 ± 6.2 pM), the Fal Estuary, UK (20.6 ± 8.3 pM), and the Adriatic Sea (21.2 ± 6.8 pM). Enhanced silver concentrations were observed in Atlantic coastal waters receiving untreated sewage effluent from the city of A Corũna, Spain (243 ± 195 pM), and in the mine-impacted Restronguet Creek, UK (91 ± 71 pM). Anthropogenic wastewater inputs were a source of dissolved silver in the regions studied, with the exception of the Gullmar Fjord. Remobilisation of dissolved silver from historically contaminated sediments, resulting from acid mine drainage or sewage inputs, provided an additional source of dissolved silver to the estuaries. The ranges in the log particle-water partition coefficient (Kd) values of 5-6 were similar for the Tamar and Mero estuaries and agreed with reported values for other estuaries. These high Kd values indicate the particle reactive nature of silver with oxic sediments. In contrast, low Kd values (1.4-2.7) were observed in the Fal system, which may have been due to enhanced benthic inputs of dissolved silver coupled to limited scavenging of silver on to sediments rich in Fe oxide. © 2010 Elsevier Ltd.