Alfred Wegener Institute for Polar and Marine Research
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Benthic microbial biogeographic trends in the North Sea are shaped by an interplay of environmental drivers and bottom trawling effort
Get PDFMicrobial composition and diversity in marine sediments are shaped by environmental, biological, and anthropogenic processes operating at different scales. However, our understanding of benthic microbial biogeography remains limited. Here, we used 16S rDNA amplicon sequencing to characterize benthic microbiota in the North Sea from the top centimeter of 339 sediment samples. We utilized spatially explicit statistical models, to disentangle the effects of the different predictors, including bottom trawling intensity, a prevalent industrial fishing practice which heavily impacts benthic ecosystems. Fitted models demonstrate how the geographic interplay of different environmental and anthropogenic drivers shapes the diversity, structure and potential metabolism of benthic microbial communities. Sediment properties were the primary determinants, with diversity increasing with sediment permeability but also with mud content, highlighting different underlying processes. Additionally, diversity and structure varied with total organic matter content, temperature, bottom shear stress and bottom trawling. Changes in diversity associated with bottom trawling intensity were accompanied by shifts in predicted energy metabolism. Specifically, with increasing trawling intensity, we observed a transition toward more aerobic heterotrophic and less denitrifying predicted metabolism. Our findings provide first insights into benthic microbial biogeographic patterns on a large spatial scale and illustrate how anthropogenic activity such as bottom trawling may influence the distribution and abundances of microbes and potential metabolism at macroecological scales
Evolutionary Developments in Alkenones from the Campanian to Paleocene Recorded in Sediments from the Transkei Basin (IODP Site U1581)
Get PDFCampanian to Paleocene organic-rich sediments (~74-60 Ma) recovered from the Transkei Basin (Hole U1581B), offshore South Africa, during IODP Expedition 392 contain suites of C37-C40 alkenones derived from haptophyte algae that extend the temporal continuity of their occurrences and expand their paleogeographic range to high southern latitudes (~58°S) during this time interval. Alkenone profiles are broadly similar throughout the stratigraphic section with the similarity between Maastrichtian and Danian samples indicating a conformity in biosynthetic pathways across the K/Pg boundary. Thus, the source haptophytes for alkenones survived and subsequently recovered after the extinction event, consistent with temporal trends for assemblages of calcareous nannoplankton from the southern hemisphere. The lineages of specific alkenones record evolutionary developments in their biosynthetic pathways. The occurrence of a methyl C39:2 alkenone in the Paleocene and both methyl and ethyl C38 and C39 alkenones in the Campanian extends the range of occurrence of alkenone with carbonyl groups at multiple positions, and the required duality in their biosynthetic pathways. The dominance of the C40 alkadien-3-one in several samples contrasts with its scarcity in Neogene marine sediments and presence among extant haptophytes. C40 alkenones are prevalent constituents of coastal and lacustrine species in phylogenic Group II, notably Isochrysis, but have only once been reported in marine species from phylogenic Group III. The sporadic prominence of C40 alkenones prior to the early Eocene seems to reflect a broader suite of active biosynthetic pathways than those expressed by extant marine haptophytes. Thus, Cretaceous through Paleocene marine sediments may reflect alkenone contributions from both Isochrysidaceae (Group II) and Noelaerhabdaceae (Group III) following their divergence in the Early Cretaceous. The accompanying C40:3 alkenone contrasts with the absence of other alkatrienones prior to the appearance of C37 and C38 components in the Paleocene. These data refute the hypothesis that alkatrienones represent a response in haptophyte producers to ocean cooling after the early Eocene Climatic Optimum (EECO) and suggest this biosynthetic innovation may have originated at high southern latitudes
Contributions of Polycyclic Aromatic Hydrocarbons (PAH) Derived from Angiosperms and Wildfires in Campanian to Paleocene Sediments from High Southern Latitudes
Get PDFCore samples recovered from the Transkei Basin (Hole U1581B), offshore South Africa, during IODP Expedition 392 include an expansive record of organic-rich sediments from the Campanian to Paleocene (~74-63 Ma). Investigation of the biomarker composition of this sequence revealed significant amounts of polycyclic aromatic hydrocarbons (PAH) reflecting terrestrial sources of organic matter (OM) coupled with evidence for wildfires. Perylene, derived from diagenetic alteration of terrestrial OM, is the dominant PAH in the Campanian sediments. Other prominent PAH components are tetrahydrochysenes and tetrahydropicenes, which are formed by early-stage diagenetic aromatization, with or without loss of the functionalized A-ring, of triterpenoid lipids. The co-occurrence of their precursor alkenes and ketones afforded further evidence for inputs of angiosperm-derived terrigenous OM, whereas the absence of diterpenoids suggests minimal contributions of terrestrial OM from gymnosperms. The dominance of OM originating from angiosperms throughout this interval suggests that these plants were well-established as the principal vegetation in southern Africa following their expansion during the Late Cretaceous. The presence of coronene in the sedimentary succession indicates that the sources of terrestrial OM also include pyrogenic material because this compound is formed during intense combustion and therefore serves as a sedimentary marker for wildfires. Coronene occurs as a minor PAH component in Campanian samples, but it is a prominent PAH component of the Paleocene samples. It is dominant in the basal Paleocene sample consistent with inputs from global wildfires at the K/Pg boundary, confirming their prevalence at high southern latitudes, whereas the lower proportion of perylene in this interval may reflect a diminished supply of unburnt lignin precursors. Coronene remains a substantive component in later Paleocene samples suggesting the protracted occurrence of wildfires. In addition, PAH generated during the K-Pg boundary event may provide a sustained detrital influx derived from erosion and weathering of burnt biomass, which is consistent with evidence that larger PAH are primarily transported by clastic detritus rather than airborne particulates
Seafloor weathering of Agulhas Plateau Large Igneous Province volcaniclastics as a driver of ocean chemistry
Get PDFSeafloor weathering of Agulhas Plateau Large Igneous Province volcaniclastics as a driver of ocean chemistr
Southern Weddell Sea surface freshwater flux modulated by icescape and atmospheric forcing
Get PDFAbstract. Sea ice formation dominates surface salt forcing in the southern Weddell Sea. Brine rejected in the process of sea ice production results in the production of High Salinity Shelf Water (HSSW) that feeds the global overturning circulation and fuels the basal melt of the adjacent ice shelf. The strongest sea ice production rates are found in coastal polynyas, where steady offshore winds promote divergent ice movement during the freezing season. We used the Finite Element Sea ice–ice shelf–Ocean Model (FESOM) forced by output from the regional atmospheric model COSMO-CLM (CCLM) with 14 km horizontal resolution to investigate the role of polynyas for the surface freshwater flux of the southern Weddell Sea (2002–2017). The presence of stationary icescape features (i.e., fast-ice areas and grounded icebergs) can influence the formation of polynyas and, therefore, impact sea ice production. The representation of the icescape in our model is included by prescribing the position, shape and temporal evolution of a largely immobile ice mélange formed between the Filchner–Ronne Ice Shelf (FRIS) and a major grounded iceberg based on satellite data. We find that 70 % of the ice produced on the continental shelf of the southern Weddell Sea is exported from the region. While coastal polynyas cover 2 % of the continental shelf area, sea ice production within the coastal polynyas accounts for 17 % of the overall annual sea ice production (1509 km3). The largest contributions come from the Ronne Ice Shelf and Brunt Ice Shelf polynyas and polynyas associated with the ice mélange. Furthermore, we investigate the sensitivity of the polynya-based ice production to the (i) representation of the icescape and (ii) regional atmospheric forcing. Although large-scale atmospheric fields determine the sea ice production outside polynyas, both the treatment of the icescape and the regional atmospheric forcing are important for the regional patterns of sea ice production in polynyas. The representation of the ice mélange is crucial for the simulation of polynyas westward/eastward of it, which are otherwise suppressed/overestimated. Compared to using ERA-Interim reanalysis as an atmospheric forcing data set, using CCLM output reduces polynya-based ice production over the eastern continental shelf due to weaker offshore winds, yielding a more realistic polynya representation. Our results show that the location and not just the strength of the sea ice production in polynyas is a relevant parameter in setting the properties of the HSSW produced on the continental shelf, which in turn affects the basal melting of the Filchner–Ronne Ice Shelf.
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Alkalinity generation from carbonate weathering in a silicate-dominated headwater catchment at Iskorasfjellet, northern Norway
Get PDFThe weathering rate of carbonate minerals is several orders of magnitude higher than for silicate minerals. Therefore, small amounts of carbonate minerals have the potential to control the dissolved weathering loads in silicate-dominated catchments. Both weathering processes produce alkalinity under the consumption of CO2. Given that only alkalinity generation from silicate weathering is thought to be a long-term sink for CO2, a misattributed weathering source could lead to incorrect conclusions about long- and short-term CO2 fixation. In this study, we aimed to identify the weathering sources responsible for alkalinity generation and CO2 fixation across watershed scales in a degrading permafrost landscape in northern Norway, 68.7–70.5∘ N, and on a temporal scale, in a subarctic headwater catchment on the mountainside of Iskorasfjellet, characterized by sporadic permafrost and underlain mainly by silicates as the alkalinity-bearing lithology. By analyzing total alkalinity (AT) and dissolved inorganic carbon (DIC) concentrations, as well as the stable isotope signature of the latter (δ13C-DIC), in conjunction with dissolved cation and anion loads, we found that AT was almost entirely derived from weathering of the sparse carbonate minerals. We propose that in the headwater catchment the riparian zone is a hotspot area of AT generation and release due to its enhanced hydrological connectivity and that the weathering load contribution from the uphill catchment is limited by insufficient contact time of weathering agents and weatherable materials. By using stable water isotopes, it was possible to explain temporal variations in AT concentrations following a precipitation event due to surface runoff. In addition to carbonic acid, sulfuric acid, probably originating from oxidation of pyrite or reduced sulfur in wetlands or from acid deposition, is shown to be a potential corrosive reactant. An increased proportion of sulfuric acid as a potential weathering agent may have resulted in a decrease in AT. Therefore, carbonate weathering in the studied area should be considered not only as a short-term CO2 sink but also as a potential CO2 source. Finally, we found that AT increased with decreasing permafrost probability, and attributed this relation to an increased water storage capacity associated with increasing contact of weathering agent and rock surfaces and enhanced microbial activity. As both soil respiration and permafrost thaw are expected to increase with climate change, increasing the availability of weathering agents in the form of CO2 and water storage capacity, respectively, we suggest that future weathering rates and alkalinity generation will increase concomitantly in the study area
MR imaging as a monitoring tool for gonadal growth in fish
Get PDFAn in vivo approach of magnetic resonance imaging (MRI) will be presented to follow the gonadal development of unanaesthetised male and female polar cod (Boreogadus saida). The first screening procedure was carried out on a set of reproductive polar cod every four weeks. Gonad maturation was followed from November until shortly before spawning in February. The resolution of the MR images allowed for specific sex determination and calculation of gonads volumes. The results of calculated gonad volume in vivo was in agreement with the in vitro gonad weight