Alfred Wegener Institute for Polar and Marine Research
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Depositional controls and budget of organic carbon burial in fine-grained sediments of the North Sea – the Helgoland Mud Area as a natural laboratory
No full textAbstract. The burial of organic matter (OM) within fine-grained continental shelf sediments represents one of the major long-term sinks of carbon. We investigated the key factors controlling organic carbon burial in sediments of the North Sea by using the Helgoland Mud Area (HMA) as a natural test field. The HMA represents the most significant depocentre of fine-grained and organic-rich sediments in the German Bight (SE North Sea). We examined factors including sedimentation and accumulation rate, sediment-mixing rate, grain size, total organic carbon (TOC) content, and aerobic remineralisation rate. Highest sedimentation rates (SRs) of up to ∼ 4.5 mm yr−1 and average TOC contents of 2 wt % were found in the southern part of the HMA, which is under the influence of the Elbe River outflow, reaching organic carbon burial efficiencies of >65 %. Sedimentation rates 4 times lower and the lowest TOC contents (0.7 wt %–1.0 wt %) were found in the shallow eastern part of the research area, with the lowest organic carbon burial efficiencies being 30 %. High sedimentation rates are known to limit oxygen exposure time, thereby enhancing OM preservation. Our data support this finding, demonstrating and confirming that sedimentation rate is the key factor determining organic carbon burial efficiency (OC BE) and long-term sedimentary carbon storage. In the southern part of the HMA, close to the outflow of the Elbe River, the OM being degraded is primarily of terrigenous origin, while, in the central and northern parts of the HMA, a mixture of marine and terrigenous OM is remineralised. At the sites dominated by the degradation of marine organic matter, as found in the western and northwestern HMA, the organic carbon burial efficiency is lower and fluctuates around 55 %. The burial efficiency of OM is highest in sedimentary habitats characterised by high sedimentation rates and OM of terrigenous sources. Sediment-mixing rates were highest in the northwestern HMA, where the highest bottom-trawling activity is also reported. The comparison of sites similar in depositional characteristics but different in bottom-trawling intensity suggests that, in the area of intense bottom trawling in the northwestern HMA, the sequestration of OM is reduced by around 30 %. The annual burial flux of organic carbon in the HMA amounts to an average of 22.5 g C m−2 yr−1. Considering the strong tidal currents in the shallow HMA, the burial flux is exceptionally high and even compares with those reported for the deeper Skagerrak and Norwegian Trough (∼ 10 to 66 g C m−2 yr−1), which are the main depocentres for fine-grained and organic-rich sediments in the North Sea. For the entire HMA, the total annual organic carbon accumulation amounts to 0.011 Tg C yr−1. These findings highlight the importance of depocentres for fine-grained sediments as important carbon sinks: while the area of the HMA represents only 0.09 % of the North Sea, it stores 0.76 % of the total annual accumulated organic carbon in this shelf sea area
Towards a strategy for offshore installations to enhance the environmental status of coastal seas: Multi-use concepts for ecosystem restoration
Get PDFIn European coastal and shelf seas, concurrent and sometimes conflicting economic and conservation needs call for innovative spatial management approaches that take account of new use concepts. In highly degraded environments, large areas contemplated for offshore wind farm (OWF) development could be actively used for different ecosystem enhancement concepts such as habitat restoration or the establishment of artificial reefs as part of conventional scour protection systems. Simultaneously, different uses, such as extractive aquaculture or other offshore renewable energy could be located within OWFs to more efficiently use limited marine space while also maximizing the benefit of a site. However, to date the environmental and spatial enhancement potential of such multi-use approaches is rarely considered in OWF planning and development. One concern is that stronger focus on such enhancement approaches could lead to reduced efforts in other urgent nature protection needs such as Marine Protected Areas (MPAs). We argue that co-designed by knowledgeable stakeholders, and effectively implemented, appropriate forms of multi-use concepts could help with impact reduction of OWF areas and the improvement of the already floundering ecosystem status of coastal and shelf seas, all while maintaining urgently needed conservation schemes
Vertical and horizontal variability and representativeness of the water vapor isotope composition in the lower troposphere: insight from ultralight aircraft flights in southern France during summer 2021
Get PDFThe isotopic composition of water vapor can be used to track atmospheric hydrological processes and to evaluate numerical models simulating the water cycle. Accurate model–observation comparisons require understanding the spatial and temporal variability of tropospheric water vapor isotopes. The challenging task of obtaining highly resolved water vapor isotopic observations is typically addressed through airborne measurements performed aboard conventional aircraft, but these offer limited microscale insights. This study uses ultralight aircraft observations to investigate water vapor isotopic composition in the lower troposphere over southern France in late summer 2021. Combining observations with models, we identify key drivers of isotopic variability and detect short-lived, small-scale processes. The key findings of this study are that (i) at hourly and sub-daily scales, vertical mixing is the primary driver of isotopic variability in the lowermost troposphere above the study site; (ii) evapotranspiration significantly impacts the boundary layer water vapor isotopic signature, as revealed by the δ18O–δD relationship; and (iii) while water vapor isotopes generally follow large-scale humidity patterns, with separation distances that might range up to 100–300 km, they also reveal distinct small-scale structures (approximately hundreds of meters) that are not fully explained by humidity variations alone, highlighting sensitivity of water vapor isotopic composition to additional fine-scale processes. The latter are particularly evident for δD, which also exhibit the largest differences in horizontal and vertical gradients. Combined with other airborne datasets, our results support a simple model driven by surface observations to simulate tropospheric δD vertical profiles, improving surface–satellite comparisons
Processes and Palaeo‐Environmental Changes in the Arctic from Past to Present (PalaeoArc) – introduction
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Evaluating the role of physical mechanisms as possible triggers for turbidity currents in a deep ocean seamount
Get PDFTurbidity currents on continental margins are often attributed to cyclic climate variability and sea-level change, while the causes of deep ocean turbidites are as yet to be tested. The Atlantic Iberian margin provides a unique setting to contrast deep ocean and continental environments, including depression features that further protect from resuspension and erosion by along-slope bottom currents. We present records of low-frequency, non-periodic, climate-independent turbidites from three deep cores covering up to 426,000 years in the Tore seamounts area. By evaluating a range of physical oceanographic mechanisms, the breaking of internal waves and mesoscale Mediterranean-eddies against unstable slopes in the seamounts area arises as the most likely triggers that precondition the recurrence pattern of the observed deep ocean turbidites
Review article: AntArchitecture – building an age–depth model from Antarctica's radiostratigraphy to explore ice-sheet evolution
Get PDFAbstract. Radio-echo sounding (RES) has revealed an internal architecture within both the West and East Antarctic ice sheets that records their depositional, deformational and melting histories. Crucially, RES-imaged internal-reflecting horizons, tied to ice-core age–depth profiles, can be treated as isochrones that record the age–depth structure across the Antarctic ice sheets. These enable the reconstruction of past climate and ice dynamical processes on large scales, which are complementary to but more spatially extensive than commonly used proxy records (e.g. former ice limits constrained by cosmogenic dating or offshore sediment sequences) around Antarctica. We review the progress towards building a pan-Antarctic age–depth model from these data by first introducing the relevant RES datasets that have been acquired across Antarctica over the last 6 decades (focussing specifically on those that detected internal-reflecting horizons) and outlining the processing steps typically undertaken to visualise, trace and date (by intersection with ice cores or modelling) the RES-imaged isochrones. We summarise the scientific applications for which Antarctica's internal architecture has been used to date and present a pathway to expanding Antarctic radiostratigraphy across the continent to provide a benchmark for a wider range of investigations: (1) identification of optimal sites for retrieving new ice-core palaeoclimate records targeting different periods; (2) reconstruction of surface mass balance on millennial or historical timescales; (3) estimation of basal melting and geothermal heat flux from radiostratigraphy and comprehensive mapping of basal-ice units to complement inferences from other geophysical and geological methods; (4) advancement of the knowledge of volcanic activity and fallout across Antarctica; and (5) refinement of numerical models that leverage radiostratigraphy to tune time-varying accumulation, basal melting and ice flow, firstly to reconstruct past behaviour and then to reduce uncertainties in projecting future ice-sheet behaviour
Verborgene Welten unter dem Antarktis-Eis: Signifikanz für das Verständnis künftiger Klimazustände unserer Erde
Get PDFAuf einer Expedition mit dem deutschen Forschungseisbrecher Polarstern im Frühjahr 2017 wurde erstmals in der Antarktis das Meeresboden-Bohrgerät MARUM-MeBo70 eingesetzt, welches in bisher unerreichbare Sedimentablagerungen des westantarktischen Kontinentalschelfs des Amundsenmeeres eindringen konnte. In jahrelanger komplexer transdisziplinärer Arbeit unter Beteiligung diverser europäischer Institutionen konnten detaillierte Rekonstruktionen antarktischer Umweltbedingungen erstellt werden, die einzigartige Einblicke in die Klima- und Eisschildgeschichte der extrem klimasensitiven Westantarktis erlauben. Die offengelegten antarktischen Umweltbedingungen vergangener Extremklimazustände innerhalb der späten Kreide und des mittleren bis späten Paläogens, sowie Übergänge in und aus diesen, sind entscheidend, um künftige Klimadynamik unter warmen und CO2-reichen Bedingungen mit Hilfe numerischer Modellprognosen verlässlicher vorherzusagen