Dissolved organic carbon measured on water bottle samples at time series station DYFAMED

Dissolved organic carbon (DOC) distribution and dynamics are investigated at the DYFAMED site (central Ligurian Sea, NW Mediterranean) in relation to hydrological and biological contexts, using a 4-year time-series dataset (1991-1994). The DYFAMED site is regarded as a one-dimensional station where simple hydrological mechanisms prevail and where the ecosystem is quite well understood. An average vertical profile of DOC concentration ([DOC]) indicates that maximal concentrations and variability are concentrated in the surface layers. For depths >800 m, the annual variations are on average similar to the analytical standard deviation (~2 µM). The "composite" [DOC] distribution (average distribution over a typical year, integrating about 40 monthly profiles) for surface waters (0-200 m) is closely related to hydrological and phytoplanktonic forcings. It exhibits summer DOC accumulation in surface waters, due to spring-summer stratification and successive phytoplanktonic events such as spring and summer blooms, and winter DOC removal to deeper waters, due to intense vertical mixing. The analysis of vertical [DOC] gradient at 100-m depth as a function of the integrated DOC content in the 0-100-m layer makes it possible to objectively distinguish three specific periods: the winter vertical mixing period, the period of stratification and spring phytoplankton bloom, and the period of stratification re-inforcement and summer-fall phytoplankton bloom. We recalculate the vertical DOC fluxes to deep waters using a larger original dataset, after the first direct calculation (Deep-Sea Res. 40 (10) (1993) 1963, 1972) that was reproduced for other oceanic areas. The seasonal variations of the "composite" [DOC] distribution in surface waters are significantly correlated to the apparent oxygen utilization distribution, but the biogeochemical significance of such a correlation is still under examination. The global significance of our local findings is presented and the role of the oceanic DOC in the global carbon cycle is emphasized, especially with respect to several current issues, such as the oceanic "missing sink" and the equivalence between new production and exported production.

(Table 2) Uranium series results of Villars Cave stalagmite Vil-stm14

We present a new high resolution speleothem stable isotope record from the Villars Cave (SW-France) that covers part of marine isotope stage (MIS) 3. The Vil14 stalagmite grew between ~52 and 29 ka. The d13C profile is used as a palaeoclimate proxy and clearly shows the interstadial substages 13, 12 and 11. The new results complement and corroborate previously published stalagmite records Vil9 and Vil27 from the same site. The Vil14 stalagmite chronology is based on 12 Th-U dating by MC-ICP-MS and 3 by TIMS. A correction for detrital contamination was done using the 230Th/232Th activity ratio measured on clay collected in Villars Cave. The Vil14 results reveal that the onset of Dansgaard-Oeschger (DO) events 13 and 12 occurred at ~49.8 ka and ~47.8 ka, respectively. Within uncertainties, this is coherent with the latest NorthGRIP time scale (GICC05-60 ka) and with speleothem records from Central Alps. Our data show an abrupt d13C increase at the end of DO events 14 to 12 which coincides with a petrographical discontinuity probably due to a rapid cooling. As observed for Vil9 and Vil27, Vil14 growth significantly slowed down after ~ 42 ka and finally stopped ~ 29 ka ago where the d13C increase suggests a strong climate deterioration that coincides with both North Atlantic sea level and sea surface temperature drop.

Artificial oxygen fluxes measured by the eddy correlation method using stirring-sensitive oxygen microsensor and oxygen optodes in a flume experiment

In the last decade, the aquatic eddy correlation (EC) technique has proven to be a powerful approach for non-invasive measurements of oxygen fluxes across the sediment water interface. Fundamental to the EC approach is the correlation of turbulent velocity and oxygen concentration fluctuations measured with high frequencies in the same sampling volume. Oxygen concentrations are commonly measured with fast responding electrochemical microsensors. However, due to their own oxygen consumption, electrochemical microsensors are sensitive to changes of the diffusive boundary layer surrounding the probe and thus to changes in the ambient flow velocity. The so-called stirring sensitivity of microsensors constitutes an inherent correlation of flow velocity and oxygen sensing and thus an artificial flux which can confound the benthic flux determination. To assess the artificial flux we measured the correlation between the turbulent flow velocity and the signal of oxygen microsensors in a sealed annular flume without any oxygen sinks and sources. Experiments revealed significant correlations, even for sensors designed to have low stirring sensitivities of ~0.7%. The artificial fluxes depended on ambient flow conditions and, counter intuitively, increased at higher velocities because of the nonlinear contribution of turbulent velocity fluctuations. The measured artificial fluxes ranged from 2 - 70 mmol m**-2 d**-1 for weak and very strong turbulent flow, respectively. Further, the stirring sensitivity depended on the sensor orientation towards the flow. Optical microsensors (optodes) that should not exhibit a stirring sensitivity were tested in parallel and did not show any significant correlation between O2 signals and turbulent flow. In conclusion, EC data obtained with electrochemical sensors can be affected by artificial flux and we recommend using optical microsensors in future EC-studies. Flume experiments were conducted in February 2013 at the Institute for Environmental Sciences, University of Koblenz-Landau Landau. Experiments were performed in a closed oval-shaped acrylic glass flume with cross-sectional width of 4 cm and height of 10 cm and total length of 54 cm. The fluid flow was induced by a propeller driven by a motor and mean flow velocities of up to 20 cm s-1 were generated by applying voltages between 0 V and 4 V DC. The flume was completely sealed with an acrylic glass cover. Oxygen sensors were inserted through rubber seal fittings and allowed positioning the sensors with inclinations to the main flow direction of ~60°, ~95° and ~135°. A Clark type electrochemical O2 microsensor with a low stirring sensitivity (0.7%) was tested and a fast-responding needle-type O2 optode (PyroScience GmbH, Germany) was used as reference as optodes should not be stirring sensitive. Instantaneous three-dimensional flow velocities were measured at 7.4 Hz using stereoscopic particle image velocimetry (PIV). The velocity at the sensor tip was extracted. The correlation of the fluctuating O2 sensor signals and the fluctuating velocities was quantified with a cross-correlation analysis. A significant cross-correlation is equivalent to a significant artificial flux. For a total of 18 experiments the flow velocity was adjusted between 1.7 and 19.2 cm s**-1, and 3 different orientations of the electrochemical sensor were tested with inclination angles of ~60°, ~95° and ~135° with respect to the main flow direction. In experiments 16-18, wavelike flow was induced, whereas in all other experiments the motor was driven by constant voltages. In 7 experiments, O2 was additionally measured by optodes. Although performed simultaneously with the electrochemical sensor, optode measurements are listed as separate experiments (denoted by the attached 'op' in the filename), because the velocity time series was extracted at the optode tip, located at a different position in the flume.