BENTHIC FORAMINIFERA RESPONSE TO METHANE RELEASE INAN ADRIATIC SEA POCKMARK

The presence of methane-bearing shallow sediments in the Adriatic Sea has been known since the fifties, but little is known about the benthic foraminiferal assemblages associated with them. One seep and two control cores were collected in the Bonaccia field (central Adriatic Sea) at a water depth of 80 m to investigate the possible influence of the release of gas from marine reservoirs on these foraminiferal assemblages. The seep core was drilled inside a pockmark in the vicinity of an active mud-volcano. Two control cores were collected in nearby areas unaffected by presence of methane. Benthic foraminiferal assemblages from the seep core are comparable to those from the seep-free cores and are composed of species common in the central Adriatic Sea. However, foraminiferal density in the seep-core is remarkably lower than in the control ones cores. Besides, calcitic foraminiferal tests from the seep core revealed unusual trends in stable carbon isotope composition. Even though not within the same range, a similar trend was observed throughout the seep core for the ä13C values of Gavelinopsis lobatulus, Cassidulina carinata, and Bulimina marinata. In particular, negative carbon isotope values were recorded for Gavelinopsis lobatulus at the top of the core where methane seeps were detected and deep inside the core sediments (50 cm). These findings seem to point to temporal variations in seep activity, proving that the ä13C values of foraminiferal tests reflect hydrocarbon release and may hence be used to reconstruct seep activity history.Table 1 (all.): Number of benthic foraminifera from GAB1core samplesTable 2 (all.): Foraminiferal percent of the total benthic foraminiferal fauna from GAB1 core samplesTable 3 (all.): Number of benthic foraminifera from GAB2 core samples.Table 4 (all.): Foraminiferal percent of the total benthic foraminiferal fauna from GAB2 core samples.Table 5 (all.): Number of benthic foraminifera from GAB3 core samples.Table 6 (all.): Foraminiferal percent of the total benthic foraminiferal fauna from GAB3 core samples

Living benthic foraminifera from methane seep environments: a case study from the Adriatic Sea

Abstrac

HgCl2 addition to pore water samples from cold seeps can affect the geochemistry of dissolved inorganic carbon ([DIC], δ13CDIC)

The conventional use of mercuric chloride (HgCl2) as an antimicrobial agent in water samples for dissolved inorganic carbon (DIC) analysis poses health and environmental risks related to its handling and disposal. Even though there is an increasing interest in quantifying pore water DIC contribution to the ocean carbon cycle and deep-sea acidification, the paucity of comparative studies on marine pore waters prevents the modification of sampling protocols and HgCl2 still remains widely used. Here, we compared DIC concentrations and δ13CDIC composition in pore water samples from methane seepage areas in the Barents Sea and offshore N. Svalbard. Samples were extracted using 0.15 μm rhizon filters and split into two aliquots with 2–3 replicates each. Only one aliquot was treated with 10 μL saturated HgCl2(aq) and all samples were stored in the dark at 4 ◦C, prior to measurements ~30 days later. The samples yielded extremely wide ranges of DIC concentrations, from 1.8 mM to 20.1 mM, and δ13CDIC values, from − 36.0‰ to − 1.6‰ (VPDB), due to variable contributions of methane-derived DIC to the pore water system. Overall, we obtained lower [DIC] (77% samples; N = 26) and heavier δ13C values (79% samples; N = 42) in Hg-treated samples. Isotopic and concentration differences larger than the uncertainties on the mean of replicated measurements (±0.2–0.5‰; ± 0.5 mM) and analytical precision (0.15‰; 0.71 mM) represent the 38% and 19% of the samples, with offsets of up to 7.4‰ and 1.9 mM, respectively. The largest offsets are in agreement with our CO2-degassing model, suggesting an interaction between mercuric chloride and dissolved hydrogen sulfide released by sulfate-driven methane oxidation. We therefore caution against further use of HgCl2 for DIC studies of marine pore waters from cold seeps

Biogeochemical investigations of methane seepage at the ultraslow-spreading Arctic mid-ocean ridge: Svyatogor ridge, Fram Strait

The Arctic continental shelves are important reservoirs of methane stored in gas hydrates and deeper geological formations. However, little is known about methane dynamics in deeper oceanic settings such as mid-ocean ridges, mainly due to operational challenges related to their remoteness. This study investigates a recently discovered methane seepage environment at Svyatogor Ridge, a sediment-covered transform fault on the western flank of the Arctic mid-ocean ridge west of Svalbard. Svyatogor Ridge was previously hypothesized to host deep gas hydrates and active fluid flow systems, which is a unique combination for this setting and requires geochemical evidence. Based on sediment and foraminiferal geochemistry, we demonstrate that Svyatogor Ridge hosts a shallow methane cycle with anaerobic oxidation of microbial methane, sustaining chemosynthetic communities at the seafloor. Our geochemical datasets also suggest that methane fluxes were higher in the past than today and long-lasting, with episodes of intense methane oxidation recorded in pre-Holocene sediments. Methane seepage is still ongoing at this location, although no evidence exists that methane is reaching the sea surface. The results provide the first evidence of a methane seepage environment ever reported from the ultra-slow spreading Arctic mid-ocean ridge, thus calling for a reevaluation of the role of this type of ridge in the ocean methane cycle

Mediterranean megaturbidite triggered by the AD 365 Crete earthquake and tsunami

Historian Ammianus Marcellinus documented the devastating effects of a tsunami hitting Alexandria, Egypt, on July 21, AD 365. "The solidity of the earth was made to shake … and the sea was driven away. The waters returning when least expected killed many thousands by drowning. Huge ships… perched on the roofs of houses… hurled miles from the shore….”. Other settlements around the Mediterranean were hit at roughly the same time. This scenario is similar to that of the recent Sumatra and Tohoku tsunamis. Based on geophysical surveys and sediment cores from the Ionian Sea we show that the 20–25 m thick megaturbidite known in the literature as Homogenite/Augias was triggered not by the Santorini caldera collapse but by the 365 AD Cretan earthquake/tsunami. An older similar megaturbidite was deposited after 14.590 ± 80 yr BP, implying a large recurrence time of such extreme sedimentary events in the Mediterranean Sea

Ice-sheet melt drove methane emissions in the Arctic during the last two interglacials

Circum-Arctic glacial ice is melting in an unprecedented mode, and release of currently trapped geological methane may act as a positive feedback on ice-sheet retreat during global warming. Evidence for methane release during the penultimate (Eemian, ca. 125 ka) interglacial, a period with less glacial sea ice and higher temperatures than today, is currently absent. Here, we argue that based on foraminiferal isotope studies on drill holes from offshore Svalbard, Norway, methane leakage occurred upon the abrupt Eurasian ice-sheet wastage during terminations of the last (Weichselian) and penultimate (Saalian) glaciations. Progressive increase of methane emissions seems to be first recorded by depleted benthic foraminiferal δ13C. This is quickly followed by the precipitation of methane-derived authigenic carbonate as overgrowth inside and outside foraminiferal shells, characterized by heavy δ18O and depleted δ13C of both benthic and planktonic foraminifera. The similarities between the events observed over both terminations advocate for a common driver for the episodic release of geological methane stocks. Our favored model is recurrent leakage of shallow gas reservoirs below the gas hydrate stability zone along the margin of western Svalbard that can be reactivated upon initial instability of the grounded, marine-based ice sheets. Analogous to this model, with the current acceleration of the Greenland ice melt, instabilities of existing methane reservoirs below and nearby the ice sheet are likely

Hand-written letters and photo albums linking geoscientists with school classes

Do we miss something about &laquo;traditional&rdquo; media such as handwritten letters and photography before the digital age? Some of the authors remember this age fondly, and we wanted to see if this fondness could be translated into a science dialogue project with school classes. We designed and carried out a communication process with 4 classes at different schools across Europe. During this process, each class would interact with a single scientist primarily via hand-written questions &amp; letters, and a Polaroid photo album. The scientists would make this unique, one-of-a-kind album whilst on board a research expedition in the Barents Sea. We asked the question whether this process might show any benefits to the school students involved. To answer this, we asked the students to write up their thoughts on communicating with a scientist in this way. We analysed the texts and found that most students thought the letters and polaroid albums were a &ldquo;beautiful experience&rdquo;. Others commented on how important it is to actually put pen to paper and write, since they use (almost) only digital media these days. Most importantly, the students learnt different elements of the science connected to the research expedition, but also about the scientific process in general. And, equally important, some of the students were surprised and thankful that the scientists took the time to communicate with them in such a personal way. These results could possibly have been achieved using other media, however the hand-written letters and Polaroids worked very well. They also seemed to conjure up some of the personal memories that we have about communication not so long ago. Maybe there is something to be said for slowing things down with our science communication projects and making them more personal and unique. This is something that snail-mail and making photo albums forces us to do.</p

The Impact of Methane on Microbial Communities at Marine Arctic Gas Hydrate Bearing Sediment

Cold seeps are characterized by high biomass, which is supported by the microbial oxidation of the available methane by capable microorganisms. The carbon is subsequently transferred to higher trophic levels. South of Svalbard, five geological mounds shaped by the formation of methane gas hydrates, have been recently located. Methane gas seeping activity has been observed on four of them, and flares were primarily concentrated at their summits. At three of these mounds, and along a distance gradient from their summit to their outskirt, we investigated the eukaryotic and prokaryotic biodiversity linked to 16S and 18S rDNA. Here we show that local methane seepage and other environmental conditions did affect the microbial community structure and composition. We could not demonstrate a community gradient from the summit to the edge of the mounds. Instead, a similar community structure in any methane-rich sediments could be retrieved at any location on these mounds. The oxidation of methane was largely driven by anaerobic methanotrophic Archaea-1 (ANME-1) and the communities also hosted high relative abundances of sulfate reducing bacterial groups although none demonstrated a clear co-occurrence with the predominance of ANME-1. Additional common taxa were observed and their abundances were likely benefiting from the end products of methane oxidation. Among these were sulfide-oxidizing Campilobacterota, organic matter degraders, such as Bathyarchaeota, Woesearchaeota, or thermoplasmatales marine benthic group D, and heterotrophic ciliates and Cercozoa

Implications of transient methane flux on associated biological communities in high-arctic seep habitats, Storbanken, Norwegian Barents sea

The continental margins of the Arctic Ocean basin contain methane seeps, where transient fluxes of seafloor methane are released due to the thermal dissociation of gas hydrates. An increase in shallow methane seeps identified over the past decade, potentially due to enhanced warming of the Arctic Ocean bottom water and associated destabilization of hydrate structure. Biological communities associated with methane release east of Svalbard in the Barents Sea (Storbanken Crater site, 76° 46.7′N, 35° 43.5′E, depths between 120 m–300 m depths) were investigated using towed camera imagery and ship-based platforms during a 2017 CAGE17-2 cruise on the RV Helmer Hanssen. We analyzed relationships among methane flux data, seafloor habitat characteristics, and biological community structure (i.e., presence and distribution of megafauna and expression of microbial mats) from a total of 14 surveys (6827 images and 40 multicore sediment cores) within the Storbanken Crater area and compared it to 2015 data. Unlike seep expressions at deeper sites (∼1200 m) in the Norwegian margin region, no seep-endemic, chemosynthetic-associated megafaunal species were observed at the shallow surveyed sites and all sites hosted similarly diverse communities of non-seep species, including commercially important fish and crustaceans. Methane concentrations did not markedly differ between the crater and non-crater sites. Rates of methane gas advection through sediments (in the form of flares) were relatively low and concentration of methane was even lower in porewater samples at the crater site. We present the first evidence of methane flare flux and intermittent microbial mat distribution with associated folliculinid ciliates, which suggests a long history of methane emissions and a transient seep environment in spatial and temporal flux. Together, this study presents a critical baseline on the temporal release of arctic methane and benthic biological communities to initiate temporal studies that identify future changes and predict the impact of climate chang