Beschreibung: Understanding present-day sediment provenance and transport processes is crucial for studies about the dynamics of ocean circulation, as well as for paleoclimate reconstructions in the Drake Passage (DP), a key area for Earth's global oceanic circulation and climate during past and future. Based on a comprehensive set of surface sediment samples, we used spatial variations in grain-size distribution, bulk sediment mineralogy, silt and clay mineralogy across the entire DP region to elucidate the terrigenous sources and transport mechanisms. The statistical evaluation of these data identifies southern Patagonia (carbonate, illite, chlorite, feldspar and quartz) and the Antarctic Peninsula (chlorite, smectite, and amphibole) as the main sources for terrigenous sediments in the DP region. Different current systems are transporting the sediment material. Here, we provide a new, robust flow speed calibration for silt grain-sizes to enable the reconstruction of Antarctic Circumpolar Current (ACC) dynamics in the DP sector of the Southern Ocean. We correlated the sortable silt mean grain-size records of surface sediments with adjacent long-term current meter data. A clear bottom current speed pattern shows the variability of the ACC in the DP responding to the dynamics of ocean fronts, in agreement with modern observation.

Beschreibung: The Antarctic Circumpolar Current is the world's largest current system connecting all major ocean basins of the global ocean. Its flow, driven by strong westerly winds, is constricted to its narrowest extent in the Drake Passage, located between South America and the Antarctic Peninsula. Due to the remoteness of the area, harsh weather conditions and strong bottom currents, sediment recovery is difficult and data coverage is still inadequate. Here, we report on the composition of 51 surface sediments collected during the R/V Polarstern PS97 expedition (Februaryâ-April 2016) across the western and central Drake Passage, from the Chilean/Argentinian continental margin to the South Shetland Islands and the Bransfield Strait (water depth: ∼100–4000 m). We studied microfossils (diatoms), bulk sediment composition and geochemical proxies (biogenic opal, organic carbon, calcium carbonate, carbon and nitrogen stable isotopes, sterols and photosynthetic pigments), and evaluated how they respond to, and reflect oceanic domains and polar to subpolar frontal systems in this region. Our multi-proxy approach shows a strong relationship between the composition of surface sediments and ocean productivity, terrigenous input, intensity of ocean currents, and ice proximity, clearly differentiating among 4 biogeographical zones. The Subantarctic Zone was characterized by warmer-water diatoms, high carbonate (〉45) and low organic carbon contents (avg. 0.26), as well as low concentrations of pigments (avg. 1.75 μg/g) and sterols (avg. 0.90 μg/g). A general N-S transition from carbonate-rich to opal-rich sediment was observed at Drake Passage sites of the Polar Front and Permanently Open Ocean Zone. These sites were characterized by low organic carbon content (0.22), high relative abundances of heavily silicified diatoms (≥60% Fragilariopsis kerguelensis), and abundant foraminifera at shallower stations. Approaching the Antarctic Peninsula in the Transitional Zone, an increase in the concentrations of pigments and sterols (avg. 2.57 μg/g and 1.44 μg/g, respectively) and a strong decrease in carbonate content was observed. The seasonal Sea-Ice Zone in the southern section of the study area, had the highest contents of biogenic opal (avg. 14.6) and organic carbon (avg. 0.7), low carbonate contents (avg. 2.4), with the occurrence of sea-ice-related diatoms and sterols. In all zones, terrigenous input was detected, although carbon/nitrogen ratios and δ13Corg suggest a predominance of marine-derived organic matter; lower values of δ13Corg occurred south of the Polar Front. The new results presented here constitute a highly valuable reference dataset for the calibration of microfossil and geochemical proxies against observational data and provide a useful regional baseline for future paleo-research.

Beschreibung: Human alteration of nutrient cycles has caused persistent and widespread degradation of water quality around the globe. In many regions, including Western Europe, elevated nitrate (NO3−) concentration in surface waters contributes to eutrophication and noncompliance with environmental legislation. Discharge, NO3− concentrations and the vulnerability of the aquatic ecosystems to eutrophication often exhibit a distinct seasonality. Understanding spatial patterns and long‐term trends in this seasonality is crucial to improve water quality management. Here, we hypothesized that NO3− concentrations during high‐flow periods would respond faster to changes in nutrient inputs than low‐flow concentrations because of greater connectivity of shallow diffuse NO3− sources with the river network. To test this hypothesis, we compiled long‐term NO3− and discharge time series from 290 Western European catchments. To characterize the long‐term trajectories of seasonal NO3− concentration, we propose a novel hysteresis approach comparing low‐ and high‐flow NO3− concentration in the context of multi‐decadal N input changes. We found synchronous winter maxima of NO3− and discharge in 84% of the study catchments. However, contrary to our hypothesis, there were surprisingly diverse long‐term trajectories of seasonal NO3− concentration. Both clockwise (faster high‐flow NO3− response) and counterclockwise hysteresis (faster low‐flow NO3− response) occurred in similar proportions, potentially due to a high complexity in the underlying processes. Spatial variability of seasonality in NO3− concentration across the catchments was more pronounced and better predictable than its long‐term variability. This work demonstrates the value of seasonal and inter‐annual hydrochemical analysis and provides new tools for water quality monitoring and management.

Beschreibung: Plain Language Summary: Nitrogen is an essential element of all living organisms and has thus often been used excessively as fertilizer to secure food production. However, surface waters can suffer from elevated nutrients inputs, causing toxic algal blooms and impairing drinking water quality, especially during summer low flows. To manage water quality, it is crucial to understand these seasonal variations of nitrogen and discharge and the underlying processes. We used data from 290 catchments in France and Germany to characterize average seasonality patterns and their long‐term evolution across the variety of landscapes and human influences. This allowed classifying catchment behavior and linking them to controls. As expected, both nitrogen and discharge peak during winter in most catchments (84%). However, there are well explainable deviations, for example, in mountainous regions. The long‐term evolution of seasonality was more diverse than expected suggesting a complex interplay of various processes with the long input history from fertilization and wastewater being part of the controls. We found that the differences among catchments were greater than the long‐term changes of seasonality within most catchments. By identifying catchment typologies, our study increases the understanding of nitrate seasonality patterns across a large extent and thus supports ecological water quality management.

Beschreibung: Key Points: Spatial patterns of nitrate and discharge seasonality are linked to topography and hydroclimate with winter maxima dominating for both. After decreasing nutrient inputs, cases with decreases in river nitrate preceding during low‐ and high‐flow seasons occurred equally often. Spatial variability of nitrate seasonality is greater and more predictable from catchment characteristics than its long‐term variability.

Beschreibung: Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659

Beschreibung: Helmholtz Association http://dx.doi.org/10.13039/501100009318

Beschreibung: US National Science Foundation (NSF)