Suchergebnisse

The Australian Government Research Training Program Scholarship funded Craig's Ph.D. candidature. An International Association of Sedimentologists Postgraduate Award Grant funded Craig's visit to Bangor University (Bangor, UK). The UK Natural Environment Research Council grant NE/1027223/1 (COHBED project) enabled this research to be undertaken using the flume facility built by Rob Evans (Bangor University). ; Sediment gravity flows are the primary process by which sediment and organic carbon are transported from the continental margin to the deep ocean. Up to 40% of the total marine organic carbon pool is represented by cohesive extracellular polymeric substances (EPS) produced by microorganisms. The effect of these polymers on sediment gravity flows has not been investigated, despite the economic and societal importance of these flows. We present the first EPS concentrations measured in deep-sea sediment, combined with novel laboratory data that offer insights into the modulation of the dynamics of clay-laden, physically cohesive sediment gravity flows by biological cohesion. We show that EPS can profoundly affect the character, evolution, and runout of sediment gravity flows and are as prevalent in deep oceans as in shallow seas. Transitional and laminar plug flows are more susceptible to EPS-induced changes in flow properties than turbulent flows. At relatively low concentrations, EPS markedly decrease the head velocity and runout distance of transitional flows. This biological cohesion is greater, per unit weight, than the physical cohesion of cohesive clay and may exert a stronger control on flow behavior. These results significantly improve our understanding of the effects of an unrealized biological component of sediment gravity flows. The implications are wide ranging and may influence predictive models of sediment gravity flows and advance our understanding about the ways in which these flows transport and bury organic carbon globally. ; Publisher PDF ; Peer reviewed

[EN]Southern Ocean waters are projected to undergo profound changes in their physical and chemical properties in the coming decades. Coccolithophore blooms in the Southern Ocean are thought to account for a major fraction of the global marine calcium carbonate (CaCO3) production and export to the deep sea. Therefore, changes in the composition and abundance of Southern Ocean coccolithophore populations are likely to alter the marine carbon cycle, with feedbacks to the rate of global climate change. However, the contribution of coccolithophores to CaCO3 export in the Southern Ocean is uncertain, particularly in the circumpolar subantarctic zone that represents about half of the areal extent of the Southern Ocean and where coccolithophores are most abundant. Here, we present measurements of annual CaCO3 flux and quantitatively partition them amongst coccolithophore species and heterotrophic calcifiers at two sites representative of a large portion of the subantarctic zone. We find that coccolithophores account for a major fraction of the annual CaCO3 export, with the highest contributions in waters with low algal biomass accumulations. Notably, our analysis reveals that although Emiliania huxleyi is an important vector for CaCO3 export to the deep sea, less abundant but larger species account for most of the annual coccolithophore CaCO3 flux. This observation contrasts with the generally accepted notion that high particulate inorganic carbon accumulations during the austral summer in the subantarctic Southern Ocean are mainly caused by E. huxleyi blooms. It appears likely that the climate-induced migration of oceanic fronts will initially result in the poleward expansion of large coccolithophore species increasing CaCO3 production. However, subantarctic coccolithophore populations will eventually diminish as acidification overwhelms those changes. Overall, our analysis emphasizes the need for species-centred studies to improve our ability to project future changes in phytoplankton communities and their influence on marine biogeochemical cycles. ; European Union's Horizon 2020, Marie Skłodowska-Curie Individual fellowship

Datos de investigación en: http://hdl.handle.net/10366/143074 ; [EN]Ocean acidifcation is expected to have detrimental consequences for the most abundant calcifying phytoplankton species Emiliania huxleyi. However, this assumption is mainly based on laboratory manipulations that are unable to reproduce the complexity of natural ecosystems. Here, E. huxleyi coccolith assemblages collected over a year by an autonomous water sampler and sediment traps in the Subantarctic Zone were analysed. The combination of taxonomic and morphometric analyses together with in situ measurements of surface-water properties allowed us to monitor, with unprecedented detail, the seasonal cycle of E. huxleyi at two Subantarctic stations. E. huxleyi subantarctic assemblages were composed of a mixture of, at least, four diferent morphotypes. Heavier morphotypes exhibited their maximum relative abundances during winter, coinciding with peak annual TCO2 and nutrient concentrations, while lighter morphotypes dominated during summer, coinciding with lowest TCO2 and nutrients levels. The similar seasonality observed in both time-series suggests that it may be a circumpolar feature of the Subantarctic zone. Our results challenge the view that ocean acidifcation will necessarily lead to a replacement of heavily-calcifed coccolithophores by lightly-calcifed ones in subpolar ecosystems, and emphasize the need to consider the cumulative efect of multiple stressors on the probable succession of morphotypes. ; European Union's Horizon 2020, Marie Skłodowska-Curie Individual fellowship