Spatial and temporal organic carbon burial along a fjord to coast transect: A case study from Western Norway

We investigated spatial and temporal changes in accumulation rate and source of organic carbon on a gradient along the Lysefjord and the more coastal Høgsfjord, Western Norway. This was achieved through analysis of total organic carbon and nitrogen content of sediment cores, which were radiometrically dated to the early 19th and 20th centuries for the Høgsfjord and Lysefjord, respectively. Benthic foraminifera (protists) were utilized to determine changes in organic carbon supply and Ecological Quality Status (EcoQS) by their accumulation rate (benthic foraminiferal accumulation rate (BFAR)), assemblage composition, species diversity, individual species responses and the composition of stable carbon isotopes of the tests (shells) of Cassidulina laevigata, Hyalinea balthica and Melonis barleeanus. Organic carbon accumulation rates were greatest closest to the river Lyse at the head of the Lysefjord (83–171 g C m−2 yr−1). The organic carbon at the head of the fjord is mainly terrestrial in origin, and this terrestrial influence becomes progressively less seaward. The δ13C in H. balthica tests as well as the benthic foraminiferal assemblage composition also showed a clear fjord to coast gradient. Organic carbon accumulation rates were lower and less variable at the seaward study sites (13–61 g C m−2 yr−1). We observe no temporal trend in organic carbon, carbon isotopes, EcoQS or foraminiferal assemblage composition in the Lysefjord. In contrast, in the Høgsfjord, there seems to have been an increase in organic carbon accumulation rates during the 1940s. Subsequent accumulation rates are stable. The foraminiferal assemblages in the surface sediments reflect a recent transition from good/moderate to moderate/bad EcoQS.publishedVersio

Vulnerability of the North Water ecosystem to climate change

High Arctic ecosystems and Indigenous livelihoods are tightly linked and exposed to climate change, yet assessing their sensitivity requires a long-term perspective. Here, we assess the vulnerability of the North Water polynya, a unique seaice ecosystem that sustains the world’s northernmost Inuit communities and several keystone Arctic species. We reconstruct mid-to-late Holocene changes in sea ice, marine primary production, and little auk colony dynamics through multi-proxy analysis of marine and lake sediment cores. Our results suggest a productive ecosystem by 4400–4200 cal yrs b2k coincident with the arrival of the first humans in Greenland. Climate forcing during the late Holocene, leading to periods of polynya instability and marine productivity decline, is strikingly coeval with the human abandonment of Greenland from c. 2200–1200 cal yrs b2k. Our long-term perspective highlights the future decline of the North Water ecosystem, due to climate warming and changing sea-ice conditions, as an important climate change risk

Rapid response of Helheim Glacier in Greenland to climate variability over the past century

Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Geoscience 5 (2012): 37-41, doi:10.1038/ngeo1349.During the early 2000s the Greenland Ice Sheet experienced the largest ice mass loss observed on the instrumental record1, largely as a result of the acceleration, thinning and retreat of major outlet glaciers in West and Southeast Greenland2-5. The quasi-simultaneous change in the glaciers suggests a common climate forcing and increasing air6 and ocean7-8 temperatures have been indicated as potential triggers. Here, we present a new record of calving activity of Helheim Glacier, East Greenland, extending back to c. 1890 AD. This record was obtained by analysing sedimentary deposits from Sermilik Fjord, where Helheim Glacier terminates, and uses the annual deposition of sand grains as a proxy for iceberg discharge. The 120 year long record reveals large fluctuations in calving rates, but that the present high rate was reproduced only in the 1930s. A comparison with climate indices indicates that high calving activity coincides with increased Atlantic Water and decreased Polar Water influence on the shelf, warm summers and a negative phase of the North Atlantic Oscillation. Our analysis provides evidence that Helheim Glacier responds to short-term (3-10 years) large-scale oceanic and atmospheric fluctuations.This study has been supported by Geocenter Denmark in financial support to the SEDIMICE project. CSA was supported by the Danish Council for Independent Research│Nature and Universe (Grant no. 09-064954/FNU). FSt was supported by NSF ARC 0909373 and by WHOI’s Ocean and Climate Change Institute and MHRI was supported by the Danish Agency for Science, Technology and Innovation.2012-06-1

Hundred years of genetic structure in a sediment revived diatom population

This paper presents research on the genetic structure and diversity of populations of a common marine protist and their changes over time. The bloom-forming diatom Skeletonema marinoi was used as a model organism. Strains were revived from anoxic discrete layers of a 210Pb-dated sediment core accumulated over more than 100 y, corresponding to >40,000 diatom mitotic generations. The sediment core was sampled from the highly eutrophic Mariager Fjord in Denmark. The genetic structure of S. marinoi was examined using microsatellite markers, enabling exploration of changes through time and of the effect of environmental fluctuations. The results showed a stable population structure among and within the examined sediment layers, and a similar genetic structure has been maintained over thousands of generations. However, established populations from inside the fjord were highly differentiated from open-sea populations. Despite constant water exchange and influx of potential colonizers into the fjord, the populations do not mix. One fjord population, accumulated in 1980, was significantly differentiated from the other groups of strains isolated from the fjord. This differentiation could have resulted from the status of Mariager Fjord, which was considered hypereutrophic, around 1980. There was no significant genetic difference between pre- and posteutrophication groups of strains. Our data show that dispersal potential and generation time do not have a large impact on the genetic structuring of the populations investigated here. Instead, the environmental conditions, such as the extreme eutrophication of the Mariager Fjord, are deemed more important

Organic matter coating defines threshold of motion in natural sediments

The onset of sediment erosion in is governed by sediment properties such as grain size, density, and environmental controls such as current strength. Investigating the relevance of each of these parameters has been an ongoing part of experimental sedimentology, resulting in several empirical threshold of motion curves [1,2]. These are used for different geotechnical applications, but so far, none of these include the effects of organic matter (OM) on particle motion, a draw-back that has been identified early on and limits the applicability in natural systems [e.g., 3]. We perform sediment erosion experiments on natural, untreated OM-rich sediments to investigate the impact of high OM concentrations on the sediment threshold of motion. Six sediment cores from Swiss lakes were inserted to EROMES, a resuspension chamber using a propeller to produce and control shear stress [4,5]. By incrementally increasing propeller rotation rates, the first and second erosion thresholds were identified. Measuring OC (%) revealed the fluff eroded at er I has higher concentrations than the suspended particles at er II and consists of labile aggregates and free OM (e.g., leaves). Moreover, the presence of benthic organisms (tube worms) resulted in a measurable strengthening of the sediment surface. The measurements of er I and II are plotted against calibration measurements performed with standardized (quartz) grains of known grain size distribution, which reveals the lower threshold of motion of particles associated with OM. Consequently, we argue for the recalibration of threshold motion curves to include low-density, OM-rich particles and the stabilising effects of benthic organisms [6]. Key words: SLOB hypothesis, sediment threshold of motion, organic matter, EROMES erosion chamber [1] A. Shields, Anwendung der Ähnlichkeitsmechanik und der Turbulenzforschung auf die Geschiebebewegung, Technische Hochschule Berlin, 1936. [2] F. Hjulström, Studies of the Morphological Activity of Rivers as Illustrated by the River Fyris, Geogr. Ann. 18 (1936) 121. https://doi.org/10.2307/519824. [3] M.C. Miller, I.N. McCave, P.D. Komar, Threshold of sediment motion under unidirectional currents, Sedimentology. 24 (1977) 507–527. https://doi.org/10.1111/j.1365-3091.1977.tb00136.x. [4] T.J. Andersen, E.J. Houwing, M. Pejrup, On the erodibility of fine-grained sediments in an infilling freshwater system, in: Proc. Mar. Sci., Elsevier B.V., 2002: pp. 315–328. https://doi.org/10.1016/S1568-2692(02)80024-9. [5] T.J. Tolhurst, K.S. Black, D.M. Paterson, H.J. Mitchener, G.R. Termaat, S.A. Shayler, A comparison and measurement standardisation of four in situ devices for determining the erosion shear stress of intertidal sediments, Cont. Shelf Res. 20 (2000) 1397–1418. https://doi.org/10.1016/S0278-4343(00)00029-7. [6] E.T. Bruni, T.M. Blattmann, N. Haghipour, D. Louw, M. Lever, T.I. Eglinton, Sedimentary Hydrodynamic Processes Under Low-Oxygen Conditions: Implications for Past, Present, and Future Oceans, Front. Earth Sci. 10 (2022) 1–18. https://doi.org/10.3389/feart.2022.886395

Sharp contrasts between freshwater and marine microbial enzymatic capabilities, community composition, and DOM pools in a NE Greenland fjord

Increasing glacial discharge can lower salinity and alter organic matter (OM) supply in fjords, but assessing the biogeochemical effects of enhanced freshwater fluxes requires understanding of microbial interactions with OM across salinity gradients. Here, we examined microbial enzymatic capabilities—in bulk waters (nonsize-fractionated) and on particles (≥ 1.6 μm)—to hydrolyze common OM constituents (peptides, glucose, polysaccharides) along a freshwater–marine continuum within Tyrolerfjord-Young Sound. Bulk peptidase activities were up to 15-fold higher in the fjord than in glacial rivers, whereas bulk glucosidase activities in rivers were twofold greater, despite fourfold lower cell counts. Particle-associated glucosidase activities showed similar trends by salinity, but particle-associated peptidase activities were up to fivefold higher—or, for several peptidases, only detectable—in the fjord. Bulk polysaccharide hydrolase activities also exhibited freshwater–marine contrasts: xylan hydrolysis rates were fivefold higher in rivers, while chondroitin hydrolysis rates were 30-fold greater in the fjord. Contrasting enzymatic patterns paralleled variations in bacterial community structure, with most robust compositional shifts in river-to-fjord transitions, signifying a taxonomic and genetic basis for functional differences in freshwater and marine waters. However, distinct dissolved organic matter (DOM) pools across the salinity gradient, as well as a positive relationship between several enzymatic activities and DOM compounds, indicate that DOM supply exerts a more proximate control on microbial activities. Thus, differing microbial enzymatic capabilities, community structure, and DOM composition—interwoven with salinity and water mass origins—suggest that increased meltwater may alter OM retention and processing in fjords, changing the pool of OM supplied to coastal Arctic microbial communities