Methane cycling within sea ice: Results from drifting ice during late spring, north of Svalbard

Summer sea ice cover in the Arctic Ocean has declined sharply during the last decades, leading to changes in ice structures. The shift from thicker multi-year ice to thinner first-year ice changes the methane storage transported by sea ice into remote areas far away from its origin. As significant amounts of methane are stored in sea ice, minimal changes in the ice structure may have a strong impact on the fate of methane when ice melts. Hence, sea ice type is an important indicator of modifications to methane pathways. Based on measurements of methane concentration and its isotopic composition on a drifting ice floe, we report on different storage capacities of methane within first-year ice and ridged/rafted ice, as well as methane supersaturation in the seawater. During this early melt season, we show that ice type and/or structure determines the fate of methane and that methane released into seawater is a predominant pathway. We suggest that sea ice loaded with methane acts as a source of methane for polar surface waters during late spring.publishedVersio

Identification Of Mitotically Competent SOX2+ Cells In White Matter Of Normal Human Adult Brain

SOX2 expression is linked to the undifferentiated state of stem cells in mammalian neurogenic niches. While its expression has been reported in the adult human subventricular zone (SVZ), to date it has not been detected in adult human white matter. Here we describe a population of SOX2+ cells from the white matter of the adult human temporal lobe, which proliferate and express glial markers in vitro

Climate relevant trace gases (N2O and CH4) in the Eurasian Basin (Arctic Ocean)

The concentration of greenhouse gases, including nitrous oxide (N2O), methane (CH4), and compounds such as total dimethylsulfoniopropionate (DMSPt), along with other oceanographic variables were measured in the icecovered Arctic Ocean within the Eurasian Basin (EAB). The EAB is affected by the perennial ice-pack and has seasonal microalgal blooms, which in turn may stimulate microbes involved in trace gas cycling. Data collection was carried out on board the LOMROG III cruise during the boreal summer of 2012. Water samples were collected from the surface to the bottom layer (reaching 4300 m depth) along a South-North transect (SNT), from 82.19°N, 8.75°E to 89.26°N, 58.84°W, crossing the EAB through the Nansen and Amundsen Basins. The Polar Mixed Layer and halocline waters along the SNT showed a heterogeneous distribution of N2O, CH4 and DMSPt, fluctuating between 42-111 and 27–649% saturation for N2O and CH4, respectively; and from 3.5 to 58.9 nmol L−1 for DMSPt. Spatial patterns revealed that while CH4 and DMSPt peaked in the Nansen Basin, N2O was higher in the Amundsen Basin. In the Atlantic Intermediate Water and Arctic Deep Water N2O and CH4 distributions were also heterogeneous with saturations between 52% and 106% and 28% and 340%, respectively. Remarkably, the Amundsen Basin contained less CH4 than the Nansen Basin and while both basins were mostly under-saturated in N2O. We propose that part of the CH4 and N2O may be microbiologically consumed via methanotrophy, denitrification, or even diazotrophy, as intermediate and deep waters move throughout EAB associated with the overturning water mass circulation. This study contributes to baseline information on gas distribution in a region that is increasingly subject to rapid environmental changes, and that has an important role on global ocean circulation and climate regulation

Deciphering the Properties of Different Arctic Ice Types During the Growth Phase of MOSAiC: Implications for Future Studies on Gas Pathways

The increased fraction of first year ice (FYI) at the expense of old ice (second-year ice (SYI) and multi-year ice (MYI)) likely affects the permeability of the Arctic ice cover. This in turn influences the pathways of gases circulating therein and the exchange at interfaces with the atmosphere and ocean. We present sea ice temperature and salinity time series from different ice types relevant to temporal development of sea ice permeability and brine drainage efficiency from freeze-up in October to the onset of spring warming in May. Our study is based on a dataset collected during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) Expedition in 2019 and 2020. These physical properties were used to derive sea ice permeability and Rayleigh numbers. The main sites included FYI and SYI. The latter was composed of an upper layer of residual ice that had desalinated but survived the previous summer melt and became SYI. Below this ice a layer of new first-year ice formed. As the layer of new first-year ice has no direct contact with the atmosphere, we call it insulated first-year ice (IFYI). The residual/SYI-layer also contained refrozen melt ponds in some areas. During the freezing season, the residual/SYI-layer was consistently impermeable, acting as barrier for gas exchange between the atmosphere and ocean. While both FYI and SYI temperatures responded similarly to atmospheric warming events, SYI was more resilient to brine volume fraction changes because of its low salinity (< 2). Furthermore, later bottom ice growth during spring warming was observed for SYI in comparison to FYI. The projected increase in the fraction of more permeable FYI in autumn and spring in the coming decades may favor gas exchange at the atmosphere-ice interface when sea ice acts as a source relative to the atmosphere. While the areal extent of old ice is decreasing, so is its thickness at the onset of freeze-up. Our study sets the foundation for studies on gas dynamics within the ice column and the gas exchange at both ice interfaces, i.e. with the atmosphere and the ocean

Toward High-Resolution Vertical Measurements of Dissolved Greenhouse Gases (Nitrous Oxide and Methane) and Nutrients in the Eastern South Pacific

In this study, in situ, real-time and high-resolution vertical measurements of dissolved greenhouse gases (GHGs) such as nitrous oxide (N2O) and methane (CH4) and nutrients are reported for the eastern South Pacific (ESP); a region with marked zonal gradients, ranging from highly productive and suboxic conditions in coastal upwelling systems to oligotrophic and oxygenated conditions in the subtropical gyre. Four high-resolution vertical profiles for gases (N2O and CH4) and nutrients (NO3- and PO43-) were measured using a Pumped Profiling System (PPS), connected with a liquid degassing membrane coupled with Cavity Ring-Down Spectroscopy (CRDS) and a nutrient auto-analyzer, respectively. The membrane-CRDS system maintains a linear response over a wide range of gas concentrations, detecting N2O and CH4 levels as low as 0.0774 ± 0.0004 and 0.1011 ± 0.001 ppm, respectively. Continuous profiles for gases and nutrients were similar to those reported throughout the ESP, with pronounced N2O and CH4 peaks at the upper oxycline and at the base of the euphotic zone and pycnocline, respectively, in the coastal zone; but almost constant depth profiles in the subtropical gyre. Additionally, other vertical gas and nutrient structures were observed using continuous sampling, which would not have been detected by discrete sampling. Our results demonstrate that continuous measurements can be a potentially useful methodology for future GHGs cycle studies

Biogeochemical data during RV POLARSTERN cruise PS109

The concentration and carbon isotope delta (δ13C) of methane (CH4), the water mass tracer δ18O(H2O), and nutrient concentrations were measured in seawater during the Polarstern expedition PS109. The expedition took place from 12 September to 14 October 2017, on the Northeast Greenland continental shelf. Seawater samples were collected using a shipboard Sea-Bird Scientific SBE911plus CTD (Conductivity-Temperature-Depth) profiler, integrated into a SBE32 Carousel Water Sampler with 24 Niskin bottles of 12 L each (Kanzow et al., 2018). The analysis of CH4 concentrations and carbon isotope delta, 13C, were done using a gas chromatograph (Agilent GC 7890B) with a Flame Ionization Detector (FID) and a Thermo Finnigan Delta plus XP mass spectrometer, respectively (Verdugo et al., 2022). The δ18O(H2O) samples were analyzed using a Finnigan MAT Delta S mass spectrometer with two equilibration units. Unfiltered nutrient samples were taken in 15 mL Falcon tubes at the same depth as CH4 concentrations and (δ13C) of CH4 samples, stored at -20°C, and were analyzed for nitrate+nitrite, phosphate, nitrite, and silicate on a four-channel SEAL Analytical nutrient AutoAnalyser 3

Methane pathways within sea ice and seawater in the Arctic Ocean: an observational study

This PhD thesis highlights the role of the cycles of sea ice formation and melt on the methane (CH4) pathways in the Arctic Ocean, considering the release of dissolved CH4 into the ocean as an alternative pathway to the ice-air CH4 flux. In this dissertation, CH4 concentration and its stable carbon isotopic signature (13C-CH4) were combined with oceanographic data, and the stable oxygen isotopic signature of seawater (18O-H2O) as a water mass tracer to follow the pathways of CH4. The CH4 pathways were traced within sea ice and at the sea ice-seawater interface in the Arctic Ocean and on an Arctic shelf region, to infer its potential sources and main sinks at the sea ice-ocean-atmosphere interface. Methane is a potent greenhouse gas contributing to global warming. Because of a phenomenon called Arctic amplification (warming is twice as large in the Arctic than in the global mean), the most notable changes due to global warming are currently happening in the Arctic. Arctic amplification of global warming is causing a rapid decrease in summer sea ice extent in the Arctic Ocean. Thin and more fragile first-year ice (FYI), which is more susceptible to oceanic and atmospheric forcing, is replacing complex structured and thick multi-year ice. Sea ice contributes to control the gas exchange between the ocean and the atmosphere. Hence, changes in the ice regime are likely to influence the CH4 pathways in the Arctic Ocean. In Publication I, we analyzed the CH4 concentration and the 13C-CH4 in both sea ice and seawater. The expedition PS106.1 took place on a drifting ice floe north of Svalbard during late spring 2017. We report on different storage durations of CH4 within FYI and ridged/rafted ice and supersaturation (CH4 excess) in surface waters. We found that the ice types and/or structures influence the fate of CH4. During the early melt season, when basal melting starts and the ice still holds impermeable ice layers at its surface, CH4 released from sea ice into the seawater is the predominant pathway. The excess of CH4 in surface waters depends on the degree of ice melt, which regulates the amount of CH4 ultimately diluted by meltwater via mixing. We suggest that sea ice loaded with CH4 acts as a source of CH4 for polar surface waters during late spring. In late summer, melting ice forms a low salinity freshwater layer that enhances the water stratification, but it also contributes to CH4 dilution. Therefore, this led to the question of what is the impact of meltwater discharge on the marine CH4 pathways. In Publication II, we present the effects of glacier and ice melt on marine CH4 pathways on the Northeast Greenland (NEG) continental shelf, a site of intense water mass transformation involving both glacier dynamics and sea ice processes. The expedition PS109 took place on the NEG continental shelf during the transition from late summer to early autumn 2019. Based on measurements of CH4 concentration, the 13C-CH4, the water mass tracer 18O-H2O, and the physical properties of the water masses, we constrained the marine CH4 pathways to infer its potential sources and main sinks. We report on CH4 excess in surface waters influenced by sea ice, i.e., in the winter mixed layer and meltwater layer. We determined that this CH4 excess is induced by brine release during sea ice formation in the previous winter season. In the following summer, the CH4 excess is sustained throughout the melt season with the help of an enhanced stratification, which restricts the efflux from the ocean surface into the atmosphere. This CH4 excess is susceptible to dilution by surrounding shelf waters influenced by different meltwater types, as the predominant pathway. In deeper waters uninfluenced by sea ice, the basal glacial meltwater further dilutes the CH4 excess, indicating that the ocean acts as a sink for this excess on the NEG continental shelf. We suggest that CH4 dilution by meltwater needs to be considered as a marine CH4 sink. In Publication III, we investigated the effect of freeze-up on CH4 inclusions in sea ice. Based on measurements of the CH4 concentration in sea ice and seawater on the drifting ice floe during the MOSAiC drift expedition 2019-2020, we followed the CH4 pathways at the ice-seawater interface during the freeze-up phase in autumn, to determine the fate of CH4 in the early stages of ice growth. We detected different stages of CH4 undersaturation in the cold halocline layer and the surface mixed layer, indicating on one side ongoing CH4 oxidation within seawater, and on the other, a decoupling between the ocean surface and the atmosphere. We traced the uptake of dissolved CH4 in seawater into permeable sea ice during the onset of freezing. During the ongoing freeze, surface ice shifts from permeable layers into impermeable, and the CH4 concentration at its surface decreases. The residual CH4 in sea ice remains stored in disconnected brine pockets, where CH4 concentration is controlled by brine dynamics. This PhD thesis advances the understanding of the CH4 exchange between Arctic sea ice, seawater, and atmosphere and demonstrates that during late spring when basal sea ice melting starts, dissolved CH4 released from sea ice into the seawater is the predominant pathway. We determine that sea ice loaded with CH4 acts as a source for supersaturation in surface waters. During late summer, different types of meltwater create a dilution of the CH4 excess in sea ice-influenced water. We suggest CH4 dilution as a marine CH4 sink in polar regions restricting sea-air flux. Finally, the extended fraction of FYI in the Arctic Ocean will influence the CH4 pathways at the sea ice-ocean-air interface and more meltwater discharge may enhance the sink capacity of the ocean

Nutrient concentrations in sea ice during POLARSTERN cruise PS106/1 (ARK-XXXI/1.1)

Nutrient concentration in sea ice during the Polarstern expedition PS106.1. The ice drift took place between 4 - 15 June 2017, north of Svalbard (Macke and Flores, 2018). Sea ice cores were collected using a Kovacs Mark II 9cm drill ice corer. In all ice stations, the first ice core was used for in situ ice temperature measurements, and the second one for methane concentration, stable carbon isotopic signature of methane, bulk ice salinity and nutrient concentrations (Verdugo et al., 2021)