G04_03

Quantifying fossil carbon utilization and release from eroding permafrost coastlines – results from an incubation experiment

Ruben M^1,2, Marchant H^3,4, Wietz M^1,4, Genz T¹, Mollenhauer G^1,2,3

¹Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung, Bremerhaven, Germany, ²Universität Bremen, Bremen, Germany, ³MARUM - Zentrum für Marine Umweltwissenschaften der Universität Bremen, Bremen, Germany, ⁴Max-Planck/Institute for Marine Microbiology, Bremen, Germany

The carbon-rich permafrost bounded coastlines of the Arctic represent around a third of the total global coastline. Rising sea-level and temperatures are increasing erosion of these coastlines by tens of meters annually. Coastal erosion results in the mobilization of large quantities of previously freeze-locked fossil organic carbon, which then may become degraded, potentially causing a positive feedback loop. Despite the tremendous impact the mobilized permafrost organic carbon may have on atmospheric greenhouse gas levels, the extent to which eroded fossil permafrost organic matter can be utilized by microbes in the Arctic Ocean is poorly constrained. Hence, previous studies, models, and eventually decisions of policy makers have relied largely on assumptions on the strength of this permafrost carbon feedback.

To tackle this issue, we incubated permafrost soil from the Lena delta in natural coastal sea water collected in the Arctic Ocean in the eastern Kara Sea. Using a multi-disciplinary approach combining biogeochemical analyses (C, N, & P), DNA sequencing of bacterial communities and radiocarbon dating, we are now for the first time able to prove and quantify fossil carbon utilization and establish tentative links to  microbial communities. Our data clearly indicate that fossil permafrost organic carbon is highly bio-available to water column microorganisms, indicating that coastal permafrost erosion is a source of fossil carbon emissions, thus constituting a self-enhancing positive feedback loop of Arctic climate change.

G04_04

Radiocarbon age of organic matter of supraglacial systems of mountain glaciers using the example of the Garabashi Glacier (Northern Caucasus)

Zazovskaya E¹, Mergelov N¹, Dolgikh A¹, Shishkov V¹, Turchinskaya S¹, Dobryanskiy A¹, Goryachkin S¹

¹Institute of geography RAS, Moscow, Russian Federation

In modern terms, glaciers constitute a large terrestrial biome, unique in that they integrate autotrophic-heterotrophic ecosystems with the most significant contribution from abiotic processes. This view of glacial systems makes it particularly interesting to consider how old carbon can accumulate in glaciers, what its sources are, and what contribution this carbon makes to the formation of ecosystems, including soils, during modern glacial melt. The object of our study is the Garabashi mountain-valley glacier (43o 18' N, 42o 28'E, North Caucasus). Between 1997 and 2020 the glacier decreased by 27%. The content of organic carbon, nitrogen, their isotopic composition, and radiocarbon age (AMS) were determined in the  cryoconite, moraines and soils. The resulting radiocarbon ages of the cryoconites range from 850 to 7500 ¹⁴C years BP. It is important to note that we didn’t obtain a modern date for any sample of cryoconite. The age of soil organic matter on the  40 years old moraine is  about 1000 ¹⁴C years BP, indicating an inherited carbon character. The age of organic matter from nearby moraines also varies widely, ranging from 1000 to 4000 ¹⁴C years BP. The obtained data on the age of cryoconites correlate with the results obtained earlier by different researchers for Arctic and Antarctic cryoconites. However, in general there is still no explanation for such an ancient age of cryoconite material. The main problem in the dating and interpretation of radiocarbon data obtained from the cryoconite material is the inability to confidently interpret the source of OM.

G04_05

Insight in high alpine soil carbon dynamics from compound-specific and soil fraction radiocarbon analysis on a glacier forefield chronosequence

Smittenberg R^1,2, Schwab V³, Gierga M², Bernasconi S², Hajdas I⁴, Wacker L⁴, Trumbore S^3,5, Xu X⁵

¹Department of Geological Sciences, Stockholm University, Stockholm, Sweden, ²ETH Zurich, Geological Inst., Zurich, Switzerland, ³Department Biogeochemical Processes, Max-Planck-Institute for Biogeochemistry, Jena, Germany, ⁴Laboratory of Ion Beam Physics, ETH Zurich, Zurich, Switzerland, ⁵Department of Earth System Science, University of California, Irvine, USA

The ecosystem carbon balance of high latitude and high altitude ecosystems are particularly sensitive to climate change, where increasing temperatures generally lead to a rise of the ecosystem carbon balance, but also increasing carbon turnover times. In this study, we investigated the carbon dynamics of the 150 year-long Damma Glacier forefield chronosequence, Switzerland. Specifically, we performed radiocarbon analysis of total soil carbon, supposedly 'stable' carbon pools (fine mineral-bound, and peroxide-resistant carbon), respired CO₂, dissolved soil organic carbon (DOC), hydrophobic leaf wax-derived alkanes, and microbial-derived fatty acids. Comparison of our results with the penetration of the radiocarbon bomb spike and the increase of soil and ecosystem carbon over the chronosequence allowed us to make the following inferences: (i) A small but persistent contribution of ancient carbon is present in forefield, which is particularly visible in the hydrophobic leaf wax ¹⁴C data. From this we conclude that this old carbon pool is at least in part a remnant of ancient soil carbon from a previous warm and glacier-free period, besides a potential contribution of fossil-fuel derived black carbon deposition. (ii) There is a significant portion of soil carbon with a decadal-scale carbon turnover rate, and (iii) mineral-bound carbon clearly has a lower turnover time. (iv) Microbial lipids, soil CO₂ and DOC ¹⁴C content reflect different carbon sources: in younger soils, relatively low ¹⁴C contents indicate a higher relative contribution of ancient carbon decomposition, while in older soils this signal is swamped by decomposition freshly photosynthesized organic matter.

G04_06

Export of pre-aged carbon to the Bay of Biscay at the end of the LGM

Queiroz Alves E¹, Wang Y², Hefter J¹, Grotheer H¹, Zonneveld K³, Mollenhauer G^1,3

¹Alfred Wegener Institute (AWI), Bremerhaven, Germany, ²NORCE Norwegian Research Centre, Bergen, Norway, ³MARUM-Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany

The last deglaciation was the most recent relatively well-documented period of pronounced and fast climate warming. As such, it holds important information for our understanding of the climate system. Notably, the mechanisms leading to rapid atmospheric CO₂  changes during this period are incompletely defined. While research into terrestrial organic carbon reservoirs has been instrumental in exploring the possible sources of atmospheric CO₂ during these periods of rapid change, the underlying processes are not yet fully understood. Here we investigate the mobilization of organic carbon to the Bay of Biscay at the mouth of  the English Channel, where an enhanced terrigenous input has been reported for the last glacial-interglacial transition.  We have established an accurate and robust chronological framework for this deposition, showing enhanced rates of sediment accumulation from approximately 20.2 to 15.8 cal ky BP. The compound-specific radiocarbon  dating of n-alkanoic acids isolated from the sedimentary archive disclosed the deposition of pre-aged carbon with pre-deposition ages of up to ca. 30,000 yr, constituting the first direct evidence for the presence of ancient organic matter at the core location.  In the light of what has been reported for other regions with present or past permafrost conditions on land, this result points to the possibility of permafrost and/or petrogenic carbon export to the ocean, caused by processes that likely furthered the observed changes in atmospheric CO₂.

G04_P01

Exploiting radiocarbon to investigate the fate of permafrost organic matter supply to the Canadian Beaufort Sea

Bröder L^1,2, Lattaud J¹, Juhls B³, Eulenburg A³, Priest T⁴, Fritz M³, Matsuoka A⁵, Pellerin A⁶, Bossé-Demers T⁷, Rudbäck D⁸, O'Regan M⁸, Whalen D⁹, Haghipour N¹, Eglinton T¹, Overduin P³, Vonk J²

¹Swiss Federal Institute of Technology, Zürich, Switzerland, ²Vrije Universiteit, Amsterdam, The Netherlands, ³Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Potsdam, Germany, ⁴Max-Planck-Institute for Marine Microbiology, Bremen, Germany, ⁵University of New Hampshire, Durham, USA, ⁶Université du Québec à Rimouski, Rimouski, Canada, ⁷Université Laval, Québec City, Canada, ⁸Stockholm University, Stockholm, Sweden, ⁹Natural Resources Canada, Halifax, Canada

The Canadian Beaufort Sea receives large quantities of sediment, organic carbon and nutrients from rapid coastal erosion and permafrost thaw. In addition, the Mackenzie River, the largest North American Arctic river, discharges great amounts of freshwater, dissolved solids and suspended sediments to the Beaufort Sea. Current changes in these fluxes in response to the warming climate have uncertain consequences for the carbon budget on the shelf and in the deep ocean. To investigate the movement and transformation of organic matter along the land-ocean continuum, we collected water and surface sediment samples across the Beaufort Sea during fall 2021. Sampling locations span from shallow, coastal, sites with water depths ≤ 20 m, to shelf-break and deep-water settings on the continental slope (water depths of ≥1000 m). For this study, we use radiocarbon analyses of dissolved inorganic (DIC), dissolved organic (DOC) and particulate organic carbon (POC) for surface and bottom waters, as well as surface sediments, in order to compare, contrast and constrain the relative source contributions and ages of these different forms of carbon. Our results will help to better understand the fate of permafrost organic matter in the marine environment and to ultimately improve assessments of the Canadian Beaufort Sea shelf as a carbon source or sink and its potential trajectory with ongoing environmental changes.

G04_P02

Radiocarbon age of plant remains in massive ground ice of the Barrow Permafrost Tunnel, Alaska

Iwahana G¹, Uchida M², Mantoku K², Kobayashi T²

¹University Of Alaska Fairbanks, Fairbanks, United States, ²National Institute for Environmental Studies, Tsukuba, Japan

Permafrost provides paleoenvironmental information from organic matter, gas, water, and sediment contents captured in the perennially frozen ground.  Syngenetic ice wedges that grow laterally in frost cracks of the permafrost sediments are expected to be an alternative paleoenvironmental proxy where information from nearby glacier/ice sheet core or lake sediments is unavailable. Massive ground ice found in the Barrow Permafrost Tunnel at the depth range between 3 and 7 m from the surface has been interpreted as ice-wedge and used to reconstruct environmental changes in the early Holocene. To better understand the development of the massive ground ice, we conducted Radiocarbon dating of plant remains and stable isotope analysis of the ice along with two profiles. Combining with previous results, we mapped the radiocarbon age distribution within the massive ground ice. The age distribution from our dense sampling showed two ice regions with similar ages centering 11,200 and 10,200 yBP divided by a relatively narrow region of intermediate age along with the 5-m profile parallel to the tunnel long-axis. From the other sampling profile that is perpendicular to the tunnel, the youngest age (8,451 yBP) was found from the NW end of the profile. The water stable isotopes from the profile perpendicular to the tunnel showed the lowest anomaly at the SE end, which contradicts the ice-wedge origin assumption. Our results indicate the existence of unknown processes in the massive ice growth or large randomness of cracking locations during ice-wedge development.

G04_P03

Multiple radiocarbon dating of POC, DOC, DIC, and plant remains in ground ice of Siberian permafrost

Minami M¹, Sato R¹, Iwahana G², Hiyama T¹

¹Nagoya University, Nagoya, Japan, ²University of Alaska Fairbanks, Fairbanks, USA

For understanding paleoclimate changes and hydrological environmental changes preserved in ground ice, it is important to determine the chronology of the ice formation. To examine which carbon fraction in ground ice shows the most true formation age, we performed multiple ¹⁴C dating of some carbon fractions in ground ice: particulate organic carbon (POC), dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and plant remains. The ground ice samples used are permafrost outcrops collected at Syrdakh and Churapcha, near the city of Yakutsk, Russia. The thawing samples were filtered through a 0.7-μm quartz filter, and the filtrate was ultrafiltered through a 10,000 MWCO (Molecular Weight Cut-Off) followed by a 3,000 MWCO (Vivaspin Turbo, Sartorius). The ¹⁴C ages of POC in the ground ice samples were 40−27 kyr BP, which is about 10,000 years older than the plant age of 24−22 kyr BP, while the ¹⁴C ages of DOC varied with molecular size: 28−19 kyr BP for the 0.7 μm−10,000 MW and 10,000−3,000 MW fractions, and a younger age of 18−12 kyr BP for the <3000 MW, which is similar to ¹⁴C ages for DIC.

We will discuss the result of the multiple ¹⁴C dating to determine which carbon fraction in the ground ice is most suitable for accurate dating of the ice formation.