Analytical results from sediment core MD03-2698

It is well established that orbital scale sea-level changes generated larger transport of sediments into the deep-sea during the last glacial maximum than the Holocene. However, the response of sedimentary processes to abrupt millennial-scale climate variability is rather unknown. Frequency of distal turbidites and amounts of advected detrital carbonate are estimated off the Lisbon-Setúbal canyons, within a chronostratigraphy based on radiometric ages, oxygen isotopes and paleomagnetic key global anomalies. We found that: 1) Higher frequency of turbidites concurred with Northern Hemisphere coldest temperatures (Greenland Stadials [GS], including Heinrich [H] events). But more than that, an escalating frequency of turbidites starts with the onset of global sea-level rising (and warming in Antarctica) and culminates during H events, at the time when rising is still in its early-mid stage, and the Atlantic Meridional Overturning Circulation (AMOC) is re-starting. This short time span coincides with maximum gradients of ocean surface and bottom temperatures between GS and Antarctic warmings (Antarctic Isotope Maximum; AIM 17, 14, 12, 8, 4, 2) and rapid sea-level rises. 2) Trigger of turbidity currents is not the only sedimentary process responding to millennial variability; land-detrital carbonate (with a very negative bulk d18O signature) enters the deep-sea by density-driven slope lateral advection, accordingly during GS. 3) Possible mechanisms to create slope instability on the Portuguese continental margin are sea-level variations as small as 20 m, and slope friction by rapid deep and intermediate re-accommodation of water masses circulation. 4) Common forcing mechanisms appear to drive slope instability at both millennial and orbital scales.

Predicting bed form roughness: the influence of lee side angle

Flow transverse bedforms (ripples and dunes) are ubiquitous in rivers and coastal seas. Local hydrodynamics and transport conditions depend on the size and geometry of these bedforms, as they constitute roughness elements at the bed. Bedform influence on flow energy must be considered for the understanding of flow dynamics, and in the development and application of numerical models. Common estimations or predictors of form roughness (friction factors) are based mostly on data of steep bedforms (with angle-of-repose lee slopes), and described by highly simplified bedform dimensions (heights and lengths). However, natural bedforms often are not steep, and differ in form and hydraulic effect relative to idealised bedforms. Based on systematic numerical model experiments, this study shows how the hydraulic effect of bedforms depends on the flow structure behind bedforms, which is determined by the bedform lee side angle, aspect ratio and relative height. Simulations reveal that flow separation behind bedform crests and, thus, a hydraulic effect is induced at lee side angles steeper than 11 to 18° depending on relative height, and that a fully developed flow separation zone exists only over bedforms with a lee side angle steeper than 24°. Furthermore, the hydraulic effect of bedforms with varying lee side angle is evaluated and a reduction function to common friction factors is proposed. A function is also developed for the Nikuradse roughness (k s), and a new equation is proposed which directly relates k s to bedform relative height, aspect ratio and lee side angle.

pH and calcium change in the microenvironment of a benthic foraminifer (Ammonia sp.) and its size during experiments

Calcareous foraminifera are well known for their CaCO3 shells. Yet, CaCO3 precipitation acidifies the calcifying fluid. Calcification without pH regulation would therefore rapidly create a negative feedback for CaCO3 precipitation. In unicellular organisms, like foraminifera, an effective mechanism to counteract this acidification could be the externalization of H+ from the site of calcification. In this study we show that a benthic symbiont-free foraminifer Ammonia sp. actively decreases pH within its extracellular microenvironment only while precipitating calcite. During chamber formation events the strongest pH decreases occurred in the vicinity of a newly forming chamber (range of gradient about 100 µm) with a recorded minimum of 6.31 (< 10 µm from the shell) and a maximum duration of 7 h. The acidification was actively regulated by the foraminifera and correlated with shell diameters, indicating that the amount of protons removed during calcification is directly related to the volume of calcite precipitated. The here presented findings imply that H+ expulsion as a result of calcification may be a wider strategy for maintaining pH homeostasis in unicellular calcifying organisms.

Grain-size distribution and frictional behaviour of sediment cores from the Nice Slope

The upper shelf of the landslide-prone Ligurian Margin (Western Mediterranean Sea) off Nice well-known for the 1979 Airport Landslide is a natural laboratory to study preconditioning factors and trigger mechanisms for submarine landslides. For this study low-stress ring shear experiments have been carried out on a variety of sediments from >50 gravity cores to characterise the velocity-dependent frictional behaviour. Mean values of the peak coefficient of friction vary from 0.46 for clay-dominated samples (53 % clay, 46 % silt, 1 %) sand up to 0.76 for coarse-grained sediments (26 % clay, 57 % silt, 17 % sand). The majority of the sediments tested show velocity strengthening regardless of the grain size distribution. For clayey sediments the peak and residual cohesive strength increases with increasing normal stress, with values from 1.3 to 10.6 kPa and up to 25 % of all strength supported by cohesive forces in the shallowmost samples. A pseudo-static slope stability analysis reveals that the different lithologies (even clay-rich material with clay content >=50 %) tested are stable up to slope angles <26° under quasi-drained conditions.