New insights into lake responses to rapid climate change: the Younger Dryas in Lake Gościąż, central Poland

The sediment profile from Lake Gościąż in central Poland comprises a continuous, seasonally resolved and exceptionally well-preserved archive of the Younger Dryas (YD) climate variation. This provides a unique opportunity for detailed investigation of lake system responses during periods of rapid climate cooling (YD onset) and warming (YD termination). The new varve record of Lake Gościąż presented here spans 1662 years from the late Allerød (AL) to the early Preboreal (PB). Microscopic varve counting provides an independent chronology with a YD duration of 1149+14/–22 years, which confirms previous results of 1140±40 years. We link stable oxygen isotopes and chironomid-based air temperature reconstructions with the response of various geochemical and varve microfacies proxies especially focusing on the onset and termination of the YD. Cooling at the YD onset lasted ~180 years, which is about a century longer than the terminal warming that was completed in ~70 years. During the AL/YD transition, environmental proxy data lagged the onset of cooling by ~90 years and revealed an increase of lake productivity and internal lake re-suspension as well as slightly higher detrital sediment input. In contrast, rapid warming and environmental changes during the YD/PB transition occurred simultaneously. However, initial changes such as declining diatom deposition and detrital input occurred already a few centuries before the rapid warming at the YD/PB transition. These environmental changes likely reflect a gradual increase in summer air temperatures already during the YD. Our data indicate complex and differing environmental responses to the major climate changes related to the YD, which involve different proxy sensitivities and threshold processes.     

High-frequency and meso-scale winter sea-ice variability in the Southern Ocean in a high-resolution global ocean model

This study is on high-frequency temporal variability (HFV) and meso-scale spatial variability (MSV) of winter sea-ice drift in the Southern Ocean simulated with a global high-resolution (0.1°) sea ice-ocean model. Hourly model output is used to distinguish MSV characteristics via patterns of mean kinetic energy (MKE) and turbulent kinetic energy (TKE) of ice drift, surface currents, and wind stress, and HFV characteristics via time series of raw variables and correlations. We find that (1) along the ice edge, the MSV of ice drift coincides with that of surface currents, in particular such due to ocean eddies; (2) along the coast, the MKE of ice drift is substantially larger than its TKE and coincides with the MKE of wind stress; (3) in the interior of the ice pack, the TKE of ice drift is larger than its MKE, mostly following the TKE pattern of wind stress; (4) the HFV of ice drift is dominated by weather events, and, in the absence of tidal currents, locally and to a much smaller degree by inertial oscillations; (5) along the ice edge, the curl of the ice drift is highly correlated with that of surface currents, mostly reflecting the impact of ocean eddies. Where ocean eddies occur and the ice is relatively thin, ice velocity is characterized by enhanced relative vorticity, largely matching that of surface currents. Along the ice edge, ocean eddies produce distinct ice filaments, the realism of which is largely confirmed by high-resolution satellite passive-microwave data.     

Bacterial methane in the Atzbach-Schwanenstadt gas field (Upper Austrian Molasse Basin), Part I: Geology

Bacterial methane gas accumulations occur in Upper Oligocene to Early Miocene clastic deepwater sediments in the Austrian Molasse Basin. Methane gas is produced from the Upper Puchkirchen Fm. (Aquitanian) in the Atzbach-Schwanenstadt gas field which is one of the largest gas fields in this basin.     

Cross-section through the frontal Japan Trench subduction zone: Geochemical evidence for fluid flow and fluid–rock interaction from DSDP and ODP pore waters and sediments

Abstract Fluids and sediments from Deep Sea Drilling Project/Ocean Drilling Program Legs (56, 57, 87 and 186) along a transect extending from the subducting plate, across the midslope and upper slope of the Japan Trench forearc were analyzed for B and B isotopes in order to assess their composition and fluid–sediment interaction. At the reference Site 436 on the subducting plate, changes in B contents and B isotopes are controlled by the lithology and diagenesis only. The midslope Sites 440 and 584 showed stronger variations in the B geochemistry, which can be related to diagenesis and tectonic dewatering along faults. The strongest changes in the B geochemistry were observed on the upper slope Sites 1150 and 1151, where profound down‐hole freshening (chlorinities as low as ～310 mmol) coincides with a B enrichment (up to 9.3 × seawater concentration). The B isotope pore fluid profile of Site 1150 displayed a bimodal variation with depth, first increasing to values more positive than seawater, then shifting to lower signatures typical for deep‐seated fluids, whereas Site 1151 showed a constant B decrease with depth. Sites 1150 and 1151 sediments showed B increases with depth to values as high as ～164 p.p.m. and isotopic compositions ranging from ～+4 to ?9‰. A linear decrease in B_solid/B_fluid ratio, suggests that B geochemistry of the upper slope sites is controlled by fluid–rock interaction and deep‐seated fluid flow, whereas constant B_solid/B_fluid ratios were observed at the reference site on the incoming plate. This fluid overprint is probably caused by normal faults in the sediment cover which might be interconnected to deep thrusts in the underlying Cretaceous accreted wedge. This suggests that the erosive Japan Trench margin is characterized by back‐flux of deep‐seated, B‐enriched fluids into the ocean, which is facilitated by extensional normal faulting as a result of tectonic erosion and subsidence.     

Global methane emission through mud volcanoes and its past and present impact on the Earth's climate

Mud volcanism is an abundant, global phenomenon whereby fluid-rich, low-density sediments extrude both on land and offshore. Methane, which generally exceeds 90 vol% of the gas phase, is emitted at high rates during and after emplacement of the mud domes and is known for its high global warming potential (GWP). This comprehensive estimate of the annual contribution of mud volcano degassing assesses the significance of mud volcanism for the accumulation of greenhouse gases in the atmosphere. A first-order estimate for the earlier, pre-anthropogenic volume of methane released through mud volcanoes further supports their profound effect on the Earth's climate since at least the Paleozoic (570 Ma).     

Evidence for a warm-humid climate in arid northwestern China during 40–30 ka BP

Former shorelines and sedimentary records from several lake basins in northwestern China (Xinjiang, Qinghai) give evidence for warm and humid climatic conditions during 40–30 ka BP. Further indications of this favorable climate period are derived from palaeosols from the Ili loess and from cemented calcareous layers on the terraces of the Keriya River at the southern margin of the Tarim Basin and in the Badain Jaran Desert in Inner Mongolia. At that time, annual mean temperature in the Qaidam Basin was 2 °C higher and in the western part of Inner Mongolia even 2–3 °C higher than today. Precipitation in most parts of northwestern China was between 60–300 mm greater than today. These changes were probably a consequence of an increase in ocean surface temperature and evaporation resulting from a higher radiation input at middle and low latitudes caused by changes in the Earth's precessional cycle. As a result of these orbital changes, it is suggested that the intensified westerly circulation was responsible for increased moisture over northwestern China.     

Heat flow anomalies in the Gulf of Cadiz and off Cape San Vincente,Portugal

Heat flow anomalies provide critical information in active tectonic environments. The Gulf of Cadiz and adjacent areas are affected by the plate convergence between Africa and Europe, causing widespread deformation and faulting. Active thrust faults cause lateral movement and advection of heat that produces systematic variations in surface heat flow. In December 2003 new heat flow data were collected during the research vessel Sonne cruise SO175 in the Gulf of Cadiz over two sites of recent focused research activity: (i) the Gulf of Cadiz sedimentary prism and (ii) the Marques de Pombal escarpment. Both features have also been discussed as potential source areas of the Great Lisbon earthquake and tsunami of 1755. Background heat flow at the eastern terminus of the Horseshoe abyssal plain is about 52–59 mW/m². Over the Gulf of Cadiz prism, heat flow decreases from ∼57 mW/m² to unusually low values of 45 mW/m² roughly 120 km eastward. Such low values and the heat flow trend are typical for active thrusting, supporting the idea of an east-dipping thrust fault. Slip rates are 10 ± 5 mm per year, assuming that the fault dips at 2°. A fault dipping at 5°, however, would result into slip rates of 1.5–5 mm per year, suggesting that subduction has largely ceased. Based on seismic data, the Marques de Pombal fault is interpreted as part of an active fault system located ∼100 km westward of Cape San Vincente. Heat flow over the fault is affected by refraction of heat caused by the 1 km high escarpment. Thermal models suggest that the slip rate along the fault must either be small or shear stresses acting on the fault are rather high. With respect to other fault zones, however, it is reasonable to assume that the fault's slip rate is small.     

Real-time drilling mud gas monitoring for qualitative evaluation of hydrocarbon gas composition during deep sea drilling in the Nankai Trough Kumano Basin

Background

Riverine particles undergo a rapid transformation when they reach estuaries. The rapid succession of hydrodynamic and biogeochemical regimes forces the particles to flocculate, settle and enter the sediment pool. The rates and magnitudes of flocculation depend on the nature of the particles which are primarily affected by the types and quantities of organic matter (OM). Meanwhile, the OM characteristics vary widely between environments, as well as within a single environment due to seasonal climate and land use variability. We investigated the effect of the OM types and quantities through laboratory experiments using natural estuarine particles from the Mississippi Sound and Atchafalaya Bay as well as model mixtures of montmorillonite and organic molecules (i.e., biopolymers (guar/xanthan gums) and humic acid).

Results

Biopolymers promote flocculation but the magnitude depends on the types and quantities. Nonionic guar gum yields much larger flocs than anionic xanthan gum, while both of them exhibit a nonlinear behavior in which the flocculation is the most pronounced at the intermediate OM loading. Moreover, the effect of guar gum is independent of salinity whereas the effect of xanthan gum is pronounced at higher salinity. Meanwhile, humic acid does not affect flocculation at all salinity values tested in this study. These results are echoed in the laboratory manipulation of the natural estuarine particles. Flocculation of the humic acid-rich Mississippi Sound particles is unaffected by the OM, whereas that of biopolymer-rich Atchafalaya Bay particles is enhanced by the OM.

Conclusions

Flocculation is positively influenced by the presence of biopolymers that are produced as the result of marine primary production. Meanwhile, humic acid, which is abundant in the rivers that drain the agricultural soils of Southeastern United States, has little influence on flocculation. Thus, it is expected that humic acid-poor riverine particles (e.g., Mississippi River, and Atchafalaya River, to a lesser degree) may be prone to rapid flocculation and settling in the immediate vicinity of the river mouths when mixed with biopolymer-rich coastal waters. It is also expected that humic acid-rich riverine particles (e.g., Pearl River) may resist immediate flocculation and be transported further away from the river mouth.     

In situ pore-pressure evolution during dynamic CPT measurements in soft sediments of the western Baltic Sea

We present in situ strength and pore-pressure measurements from 57 dynamic cone penetration tests in sediments of Mecklenburg (n?=?51), Eckernförde (n?=?2) and Gelting (n?=?4) bays, western Baltic Sea, characterised by thick mud layers and partially free microbial gas resulting from the degradation of organic material. In Mecklenburg and Eckernförde bays, sediment sampling by nine gravity cores served sedimentological characterisation, analyses of geotechnical properties, and laboratory shear tests. At selected localities, high-resolution echo-sounder profiles were acquired. Our aim was to deploy a dynamic cone penetrometer (CPT) to infer sediment shear strength and cohesion of the sea bottom as a function of fluid saturation. The results show very variable changes in pore pressure and sediment strength during the CPT deployments. The majority of the CPT measurements (n?=?54) show initially negative pore-pressure values during penetration, and a delayed response towards positive pressures thereafter. This so-called type B pore-pressure signal was recorded in all three bays, and is typically found in soft muds with high water contents and undrained shear strengths of 1.6–6.4 kPa. The type B signal is further affected by displacement of sediment and fluid upon penetration of the lance, skin effects during dynamic profiling, enhanced consolidation and strength of individual horizons, the presence of free gas, and a dilatory response of the sediment. In Mecklenburg Bay, the remaining small number of CPT measurements (n?=?3) show a well-defined peak in both pore pressure and cone resistance during penetration, i.e. an initial marked increase which is followed by exponential pore-pressure decay during dissipation. This so-called type A pore-pressure signal is associated with normally consolidated mud, with indurated clay layers showing significantly higher undrained shear strength (up to 19 kPa). In Eckernförde and Gelting bays pore-pressure response type B is exclusively found, while in Mecklenburg Bay types A and B were detected. Despite the striking similarities in incremental density increase and shear strength behaviour with depth, gas occurrence and subtle variations in the coarse-grained fraction cause distinct pore-pressure curves. Gaseous muds interbedded with silty and sandy layers are most common in the three bays, and the potential effect of free gas (i.e. undersaturated pore space) on in situ strength has to be explored further.