Alfred Wegener Institute for Polar and Marine Research
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Holocene Terrestrial Permafrost Contributes More Highly Reactive Organic Matter to the Laptev Sea Shelf Than Pleistocene Permafrost
Full text linkAbstract Warming can lead to mobilization of organic matter (OM) initially stored in circumarctic permafrost and subsequent greenhouse gas release to the atmosphere. Our understanding remains limited regarding how the extent of carbon release, that is, OM reactivity, varies across terrestrial permafrost types and how it changes during transport from land to marine shelves. In this study, we measured bulk organic (TOC, C/N), isotopic ( δ 13 C, Δ 14 C), and thermogravimetric properties (TGA) as proxies of OM reactivity on bulk and water‐soluble fractions (leachates) from terrestrial Holocene and Pleistocene permafrost, bulk surface sediments from the Laptev Sea, and sediment cores from the western Laptev Sea. Bulk OM from terrestrial Pleistocene permafrost exhibited lower reactivity compared to Holocene permafrost, as indicated by its lower thermoreactivity and more advanced degradation state, reflected in higher δ 13 C values and lower C/N ratios. Marine surface sediments showed relatively old radiocarbon ages and reduced OM thermoreactivity in the eastern Laptev Sea shelf compared to the central and western Laptev Sea shelf. This likely resulted from a higher contribution of Pleistocene permafrost‐derived OM. In the central and western Laptev Sea, a rapid decrease in OM thermoreactivity was observed near the coast, followed by a more gradual decline offshore. Downcore analyses revealed that the reduction in OM thermoreactivity primarily reflected degradation during cross‐shelf transport rather than after burial. Our results advance the understanding of OM reactivity differences between Pleistocene and Holocene permafrost, as well as changes in terrestrial permafrost OM thermoreactivity during transport and post‐burial.
Plain Language Summary Warming in the Arctic is releasing carbon from permafrost as microbes break down frozen soil, plants, and animal remains. To better understand this process, we studied how easily organic matter (OM) from both young and old terrestrial permafrost breaks down, as well as OM in surface and deeper sediments from the Laptev Sea shelf. We measured how easily the OM in these materials breaks down by examining the amount of the OM that decomposed at lower versus higher temperatures. On land, we found that old permafrost is more resistant to breakdown than young permafrost. In the eastern Laptev Sea shelf, surface sediments contained more OM from old, less degradable terrestrial permafrost. Moving westward, OM degradability decreases with increasing distance from the coast, suggesting rapid OM breakdown nearshore. Deeper sediment layers revealed that most OM degradation happened during transport from land to sea and across the shelf, with relatively little breakdown after burial. These findings improve our understanding of the differing vulnerabilities of young and old permafrost to degradation and emphasize the importance of transport processes in shaping OM reactivity distributions on Arctic Ocean marginal shelves.
Key Points Bulk organic matter and leachates from terrestrial Pleistocene permafrost exhibit lower thermoreactivity than Holocene permafrost Organic matter source and transport distance on shelf collectively control organic matter thermoreactivity in shelf surface sediment The majority of reactive organic matter from terrestrial permafrost is degraded during transport on land or in nearshore coastal area
Global, multi-scale standing deadwood segmentation in centimeter-scale aerial images
Full text linkWith tree mortality rates rising across many regions of the world, efficient methods to map dead trees are becoming increasingly important to monitor forest dieback, assess ecological impacts, and guide management strategies. Deep learning-based pattern recognition combined with the high spatial detail of aerial images from drones or airplanes provides an avenue for mapping dead tree crowns or partial canopy dieback, collectively referred to as standing deadwood. However, current methods for mapping standing deadwood are limited to specific biomes or image resolutions. Here, we present a transformer-based semantic segmentation model that generalizes across forest biomes and a wide range of image resolutions (1–28 cm) for mapping both dead tree crowns and partial canopy dieback. Our approach combines a SegFormer-based transformer architecture for image feature extraction and Focal Tversky Loss to mitigate class imbalance. We used a globally distributed crowd-sourced dataset of 434 high-resolution aerial images and manual delineations of standing deadwood of vastly varying quality. The orthophotos span all major forest biomes and cover 10,778 hectares. To further mitigate imbalances across biomes, resolutions, deadwood occurrence, and image sources, we developed a four-dimensional sampling scheme that ensures balanced representation during training. The models were trained and evaluated using heterogeneous crowd-sourced data, which, as expected, negatively affects the F1-scores. A visual inspection on independent data highlights the very precise quality of the segmentation. Our analysis revealed resolution-dependent performance variations across biomes, suggesting a relationship between optimal mapping resolution and biome-specific characteristics. We make both our model and a machine-learning-ready dataset publicly available on deadtrees.earth to support future research in tree mortality mapping
Of sequences and images - diversity and quantity of Arctic epipelagic zooplankton by an integrative approach
Full text linkDue to the high sensitivity of zooplankton to environmental fluctuations, monitoring their taxonomic composition, abundance and biomass is of high priority to identify changes in the ecosystem. Recent advances in imaging and molecular technologies promise to greatly accelerate the processing of samples to determine both the diversity and quantity of the zooplankton community. In our study, we analyzed the diversity and quantity of an epipelagic Arctic zooplankton community using multi-marker metabarcoding and imaging analysis (ZooScan). We identified a total of 11 phyla and 58 species in the northern Barents Sea and the Nansen Basin. Metabarcoding identified more taxa than image analysis, while imaging provided quantitative information on abundance and biomass. Multivariate analyses revealed overall the same significant environmental drivers (temperature and percentage of Polar Surface Water in the sampling depth layer) explaining the similarity and spatial distribution of the zooplankton community. For all approaches, similar spatial patterns of the zooplankton community were found. Abundance, biovolume and biomass decreased with increasing latitude within the analyzed regions. Based on this study, we recommend ZooScan image analysis in combination with COI metabarcoding for future monitoring of Arctic zooplankton diversity and quantification to ensure the detection of changes in both aspects of these communities
Fading of a sulfate-methane transition in deep and hot subseafloor sediments from the Nankai Trough
No full textBiogeochemical processes in subseafloor sediments change significantly over geological timescales due to changing oceanographic, climatic or depositional conditions. Using dynamic reactive transport modeling, we reconstructed the evolution of biogeochemical processes over the past 5.5 million years in ~1.2-km deep and up to 120°C hot sediments from International Ocean Discovery Program Site C0023 in the Nankai Trough, which records a complex depositional and thermal history. A distinctive feature is an inverse sulfate-methane transition (SMT) with a broad overlap zone between sulfate and methane of ~100 m, located in 80° to 85°C hot sediments. This temperature coincides with the known temperature limit of anaerobic methane-oxidizing microbial communities. Based on the reactive transport model, we show that the inverse SMT was established ~2.5 million years ago (Ma) after the onset of biogenic methanogenesis and anaerobic oxidation of methane (AOM) as a consequence of increased organic carbon burial. Depth-integrated AOM rates decreased markedly since the beginning of trench-style deposition and an associated rapid heating of ~50°C across the sediment column ~0.4 Ma. We argue that the activity of anaerobic methane-oxidizing communities at the inverse SMT has already started to cease and that the SMT is in the process of disappearing. This is the first study that documents the successive fading of an SMT and the decrease in the efficiency of this microbial methane sink as a result of sediment temperature increasing beyond the threshold of being suitable for anaerobic methane-oxidizing microbial communities
Time matters: Transcriptomic insights into temporally regulated reproductive and physiological processes in the life cycle of salps
Full text linkDespite the increasing importance of salps and the recognition of their role as important players in food webs and biogeochemical cycles, their life cycle characteristics and physiology remain mysterious. This uncertainty encourages oversimplifying modeling approaches, leading to inaccuracies that may affect population dynamics results. This lack of knowledge is critical, making it difficult to adequately assess their sensitivity to global warming and their impact on ecosystems if their abundance and distribution change with rising seawater temperatures. Therefore, we generated a de novo transcriptome of Salpa fusiformis to further investigate the physiological processes involved in the life cycle of salps. We examined differentially expressed genes between both reproductive forms, blastozooids and oozoids, and detected a general form-specific difference among Salpa. Furthermore, we identified mainly temporally driven processes (energy delivery, cell communication, spermatogenesis) by studying gene expression profiles of different developmental stages of blastozooids of Salpa fusiformis. A life cycle that physiologically prepares blastozooids during the potential reproductive period, regardless of their fertilization status, may favor rapid response to favorable conditions and formation of salp blooms. Understanding the processes involved will contribute to a better assessment of their sensitivity to environmental change and support the implementation of their role in ecosystem models. In addition, the generated transcriptome was used to select and validate a set of potential reference genes for future qPCR applications. This will facilitate molecular studies of tunicates generally and stimulate future physiological studies on salps
First record of Atelecyclus undecimdentatus (Herbst, 1783) (Decapoda: Brachyura: Atelecyclidae) along the Dutch North Sea coast
Full text linkQuantifying thaw subsidence in a permafrost landscape (Bayelva basin, Svalbard)
Full text linkRising temperatures have led to permafrost degradation throughout the Arctic. The melting of excess ground ice leads to a loss of structural support and consolidation of soils. As a consequence, the surface subsides seasonally
when the active layer thaws, or long-term when the active layer deepens. Therefore, inter-annual thaw subsidence is an important metric for monitoring permafrost degradation. With temperature rise reaching twice the Arctic
average, warming trends in Svalbard are particularly high, leading to severe impacts on permafrost conditions and periglacial landscapes. However, knowledge on subsurface permafrost changes in Svalbard is mostly limited to a few in-situ observations. In this study, I spatially expanded research on permafrost degradation by applying a two-fold approach to quantify thaw subsidence in the Bayelva basin, northwestern Svalbard. Firstly, I oregistered and differencedhigh-resolution digital elevation models (DEMs) for a period of more than 80 years (1936, 1995, 2008, 2010, 2019, and 2020). Secondly, during
a field campaign in summer 2023, I measured Global Navigation Satellite System (GNSS) positions and calculated subsidence rates since a previous
GNSS survey in 2019. In addition, I analysed how elevation change relates to landscape, terrain forms, vegetation cover, and timing of snow disappearance.
Finally, I compared thaw subsidence in the Bayelva basin to the surrounding Brøgger peninsula. My findings show a clear elevation loss trend in the Bayelva basin that has persisted for decades and stretches across the
entire study area. I found that periglacial terrain subsided at rates of −2.6 to −6.4 cm/year in DEM-based and −0.7 cm/year in GNSS-based analyses with larger subsidence during the 20th century than during recent observation periods. Within the periglacial landscape, I observed slightly larger subsidence in topographic depressions and for areas that are longer snow-covered. My
study further suggests that the Bayelva basin is a representative subregion of the northern Brøgger peninsula as subsidence rates and patterns are generally
similar. In this study, I demonstrated the challenges of thaw subsidence quantification in periglacial areas with a lack of stable reference terrain. At the same time, my results highlight the great potential of multi-temporal
DEMs and repeated GNSS measurements for monitoring long-term elevation changes. This study represents a pioneering effort in the area, revealing ongoing
subsidence in the Bayelva basin for the first time and indicating widespread ground ice loss, a process notoriously difficult to detect. I conclude that thaw subsidence is a crucial yet often underestimated component of permafrost landscapes in the warming Arctic
Reconstruction of Neogene circulation in Princess Elizabeth Trough, Southern Ocean – Indications for locations of carbon uptake in the past?
No full textThe atmospheric CO2 content has been discussed as one of the major factors influencing global climate. In the framework of the deep ocean forming the main reservoir of carbon dioxide, the Southern Ocean plays a crucial role in partitioning carbon between the atmosphere and the deep ocean. The processes resulting in the variability of atmospheric CO2 and carbon uptake in the deep ocean have not yet been fully identified. Sedimentary structures imaged with seismic reflection data in Princess Elizabeth Trough, the deep water gateway between the Kerguelen Plateau and Prydz Bay, are interpreted regarding direction and intensity of pathways of deep/bottom water masses transported in gyres, eddies, and boundary currents to contribute to the knowledge on potential locations of carbon subsidence. Under the assumption that the general circulation scheme has been similar during the Neogene, i.e., driven by gyres, the positions and sizes of palaeo-gyres have been reconstructed, which, combined with information from ODP Leg 188 Sites 1165 and 1167, were then interpreted regarding the potential intensity of carbon uptake. This has been compared with published reconstructions of warming/cooling trends of the global climate. While the method applied is equivocal, it links observed sedimentary structures with the development of gyres, thus potential locations of carbon uptake. This way the presented reconstruction provides pieces to the climate variability puzzle, which can be tested using numerical simulation
Fram Strait Marine Cold Air Outbreaks in CARRA and ERA5: Effects on Surface Turbulent Heat Fluxes and the Vertical Structure of the Troposphere
No full textMarine Cold Air Outbreaks (MCAOs) have a profound influence on atmospheric conditions and
the surface‐atmosphere heat exchange in Fram Strait and Svalbard. Comparing the global reanalysis ERA5 to its
novel Arctic counterpart CARRA for November–March 1991–2020, we investigate the surface turbulent heat
fluxes and the spatial characteristics associated with MCAO conditions. We find that the sensible heat flux from
the surface to the atmosphere is substantially higher in CARRA while the latent heat flux is higher in ERA5. For
sensible heat flux, the differences scale with the magnitude, leading to maximum disagreement over the ice‐free ocean where the heat flux is high. Accounting for the varying heat flux magnitude over different surface types, we find the largest relative disagreement over sea ice. During MCAOs, negative anomalies in temperature and specific humidity are present throughout most of the troposphere in both reanalyses. Over the studied period, sea ice decline has caused positive trends in the surface‐atmosphere potential temperature difference forming the
basis of the MCAO index, leading to higher heat fluxes around much of Svalbard. In ice‐free Fram Strait however, both reanalyses show negative trends in the MCAO index and the heat fluxes in January, when the increase in potential temperature is larger at 850 hPa than at the surface. Finally, large differences between two individual MCAO cases are found in reanalysis heat flux variability and radiosonde‐derived atmospheric conditions over Svalbard