ALBERT

Description: We analyze sedimentary charcoal records to show that the changes in fire regime over the past 21,000 yrs are predictable from changes in regional climates. Analyses of paleo- fire data show that fire increases monotonically with changes in temperature and peaks at intermediate moisture levels, and that temperature is quantitatively the most important driver of changes in biomass burning over the past 21,000 yrs. Given that a similar relationship between climate drivers and fire emerges from analyses of the interannual variability in biomass burning shown by remote-sensing observations of month-by-month burnt area between 1996 and 2008, our results signal a serious cause for concern in the face of continuing global warming.

Description: The NW Iberian continental margin is characterised by a complex morphology and by a sedimentary system which was highly dynamic over glacial to interglacial times. The sedimentary history of the continental slope was strongly influenced by the interaction of bottom currents with topographic highs of structural origin leading to the accumulation of several sediment drifts. A combined analysis of gravity cores from different water depth with hydroacoustic data reveals the vertical behaviour of the upper Mediterranean Outflow Water (MOW) core after the Last Glacial Maximum (LGM). A coarser grained interval during Deglacial and early Holocene times (17.2 to 9.9 cal ka BP) points to an increase in bottom current strength. This increase in velocity was probably related to oceanic density fronts, which migrated through the 300 m thick transition zone between the underlying Labrador Sea Water and the overlying MOW. Radiocarbon dates timed the current strengthening to 17.2 cal ka BP, and a following weakening of the bottom current to 13.3 cal ka BP at 1965 m water depth and to 9.9 cal ka BP at 1885 m water depth. The depth-dependent current weakening suggests an upward shifting of the transition zone by 80 m that was related either to an overall shallowing of MOW or a vertical contraction of this water mass. The upward movement happened over a time interval of approximately 3.4 thousand years. In addition sediment core analysis reveals significant lateral heterogeneities within cm to dm thick sediment layers in the contourite drift. These heterogeneities suggest a need of a detailed core coverage across current-influenced deposits for palaeoceanographic studies to minimize misinterpretations.

Description: The continental margin off the La Plata Estuary (SE South America) is characterized by high fluvial sediment supply and strong ocean currents. High-resolution sediment-acoustic data combined with sedimentary facies analysis, AMS-14C ages, and neodymium isotopic data allowed us to reconstruct late Quaternary sedimentary dynamics in relation to the two major sediment sources, the La Plata Estuary and the Argentine margin. Sediments from these two provinces show completely different dispersal patterns. We show that the northward-trending La Plata paleo-valley was the sole transit path for the huge volumes of fluvial material during lower sea levels. In contrast, material from the Argentine margin sector was transported northwards by the strong current system. Despite the large sediment volumes supplied by both sources, wide parts of the shelf were characterized by either persistent non-deposition or local short-term depocenter formation. The location and formation history of these depocenters were primarily controlled by the interplay of sea level with current strength and local morphology. The high sediment supply was of secondary importance to the stratigraphic construction, though locally resulting in high sedimentation rates. Thus, the shelf system off the La Plata Estuary can be considered as a hydrodynamic-controlled end-member.

Description: The continental shelf adjacent to the Río de la Plata (RdlP) exhibits extremely complex hydrographic and ecological characteristics which are of great socioeconomic importance. Since the long-term environmental variations related to the atmospheric (wind fields), hydrologic (freshwater plume), and oceanographic (currents and fronts) regimes are little known, the aim of this study is to reconstruct the changes in the terrigenous input into the inner continental shelf during the late Holocene period (associated with the RdlP sediment discharge) and to unravel the climatic forcing mechanisms behind them. To achieve this, we retrieved a 10 m long sediment core from the RdlP mud depocenter at 57 m water depth (GeoB 13813-4). The radiocarbon age control indicated an extremely high sedimentation rate of 0.8 cm per year, encompassing the past 1200 years (AD 750-2000). We used element ratios (Ti / Ca, Fe / Ca, Ti / Al, Fe / K) as regional proxies for the fluvial input signal and the variations in relative abundance of salinity-indicative diatom groups (freshwater versus marine-brackish) to assess the variability in terrigenous freshwater and sediment discharges. Ti / Ca, Fe / Ca, Ti / Al, Fe / K and the freshwater diatom group showed the lowest values between AD 850 and 1300, while the highest values occurred between AD 1300 and 1850. The variations in the sedimentary record can be attributed to the Medieval Climatic Anomaly (MCA) and the Little Ice Age (LIA), both of which had a significant impact on rainfall and wind patterns over the region. During the MCA, a weakening of the South American summer monsoon system (SAMS) and the South Atlantic Convergence Zone (SACZ), could explain the lowest element ratios (indicative of a lower terrigenous input) and a marine-dominated diatom record, both indicative of a reduced RdlP freshwater plume. In contrast, during the LIA, a strengthening of SAMS and SACZ may have led to an expansion of the RdlP river plume to the far north, as indicated by higher element ratios and a marked freshwater diatom signal. Furthermore, a possible multidecadal oscillation probably associated with Atlantic Multidecadal Oscillation (AMO) since AD 1300 reflects the variability in both the SAMS and SACZ systems.

Description: Altered pyroclastic (tephra) deposits are highly susceptible to landsliding, leading to fatalities and property damage every year. Halloysite, a low-activity clay mineral, is commonly associated with landslide-prone layers within altered tephra successions, especially in deposits with high sensitivity, which describes the post-failure strength loss. However, the precise role of halloysite in the development of sensitivity, and thus in sudden and unpredictable landsliding, is unknown. Here we show that an abundance of mushroom cap?shaped (MCS) spheroidal halloysite governs the development of sensitivity, and hence proneness to landsliding, in altered rhyolitic tephras, North Island, New Zealand. We found that a highly sensitive layer, which was involved in a flow slide, has a remarkably high content of aggregated MCS spheroids with substantial openings on one side. We suggest that short-range electrostatic and van der Waals interactions enabled the MCS spheroids to form interconnected aggregates by attraction between the edges of numerous paired silanol and aluminol sheets that are exposed in the openings and the convex silanol faces on the exterior surfaces of adjacent MCS spheroids. If these weak attractions are overcome during slope failure, multiple, weakly attracted MCS spheroids can be separated from one another, and the prevailing repulsion between exterior MCS surfaces results in a low remolded shear strength, a high sensitivity, and a high propensity for flow sliding. The evidence indicates that the attraction-detachment model explains the high sensitivity and contributes to an improved understanding of the mechanisms of flow sliding in sensitive, altered tephras rich in spheroidal halloysite.

Description: High-resolution sediment echosounder data combined with radiocarbon-dated sediment cores allowed us to reconstruct the Late Quaternary stratigraphic architecture of the Kveithola Trough and surrounding Spitsbergenbanken. The deposits display the successive deglacial retreat of the Svalbard-Barents Sea Ice Sheet. Basal subglacial till indicates that the grounded ice sheet covered both bank and trough during the Late Weichselian. A glaciomarine blanket inside the trough coinciding with laminated plumites on the bank formed during the initial ice-melting phase from at least 16.1 to 13.5 cal ka BP in close proximity to the ice margin. After the establishment of open-marine conditions at around 13.5 cal ka BP, a sediment drift developed in the confined setting of the Kveithola Trough, contemporary with crudely laminated mud, an overlying lag deposit, and modern bioclastic-rich sand on Spitsbergenbanken. The Kveithola Drift shows a remarkable grain-size coarsening from the moat towards the southern flank of the trough. This trend contradicts the concept of a separated drift (which would imply coarser grain sizes in proximity of the moat) and indicates that the southern bank is the main sediment source for the coarse material building up the Kveithola Drift. This depocenter represents, therefore, a yet undescribed combination of off-bank wedge and confined drift. Although the deposits inside Kveithola Trough and on Spitsbergenbanken display different depocenter geometries, time-equivalent grain-size changes imply a region-wide sediment-dynamic connection. We thus relate a phase of coarsest sediment supply (8.8-6.3 cal ka BP) to an increase in bottom current strength, which might be related to a stronger Atlantic Water inflow from the Southeast across the bank leading to winnowing and off-bank export of sandy sediments.

Description: The southern Tuaheni Landslide Complex (TLC) at the Hikurangi subduction margin displays distinctive morphological features along its distribution over the Tuaheni slope offshore Gisborne, New Zealand. The datasets provide geophysical (MSCL) and geochemical (XRF) core logs from a gravity core transect that systematically samples surficial sediments from the source area to the toe of this landslide complex.

Description: Over the Uruguayan shelf and uppermost slope the coalescence of northward flowing Subantarctic Shelf Water and southward flowing Subtropical Shelf Water forms a distinct thermohaline front termed the Subtropical Shelf Front (STSF). Running in a SW direction diagonally across the shelf from the coastal waters at 32°S towards the shelf break at ca. 36°S, the STSF represents the shelf-ward extension of the Brazil-Malvinas Confluence zone. This study reconstructs latitudinal STSF shifts during the Holocene based on benthic foraminifera d18O and d13C, total organic carbon, carbonate contents, Ti/Ca, and grain-size distribution from a high-accumulation sedimentary record located at an uppermost continental-slope terrace. Our data provide direct evidence for: (1) a southern STSF position (to the South of the core site) at the beginning of the early Holocene (〉9.4 cal ka BP) linked to a more southerly position of the Southern Westerly Winds in combination with restricted shelf circulation intensity due to lower sea level; (2) a gradual STSF northward migration (bypassing the core site towards the North) primarily forced by the northward migration of the Southern Westerly Winds from 9.4 cal ka BP onwards; (3) a relatively stable position of the front in the interval between 7.2 and 4.0 cal ka BP; (4) millennial-scale latitudinal oscillations close to 36°S of the STSF after 4.0 cal ka BP probably linked to the intensification in El Niño Southern Oscillation; and (5) a southward migration of the STSF during the last 200 years possibly linked to anthropogenic influences on the atmosphere.

Description: The Mar del Plata Canyon is located at the continental margin off northern Argentina in a key intermediate and deep-water oceanographic setting. In this region, strong contour currents shape the continental margin by eroding, transporting and depositing sediments. These currents generate various depositional and erosive features which together are described as a Contourite Depositional System (CDS). The Mar del Plata Canyon intersects the CDS, and does not have any obvious connection to the shelf or to an onshore sediment source. Here we present the sedimentary processes that act in the canyon and show that continuous Holocene sedimentation is related to intermediate-water current activity. The Holocene deposits in the canyon are strongly bioturbated and consist mainly of the terrigenous "sortable silt" fraction (10-63 µm) without primary structures, similarly to drift deposits. We propose that the Mar del Plata Canyon interacts with an intermediate-depth nepheloid layer generated by the northward-flowing Antarctic Intermediate Water (AAIW). This interaction results in rapid and continuous deposition of coarse silt sediments inside the canyon with an average sedimentation rate of 160 cm/kyr during the Holocene. We conclude that the presence of the Mar del Plata Canyon decreases the transport capacity of AAIW, in particular of its deepest portion that is associated with the nepheloid layer, which in turn generates a change in the contourite deposition pattern around the canyon. Since sedimentation processes in the Mar del Plata Canyon indicate a response to changes of AAIW contour-current strength related to Late Glacial/Holocene variability, the sediments deposited within the canyon are a great climate archive for paleoceanographic reconstructions. Moreover, an additional involvement of (hemi) pelagic sediments indicates episodic productivity events in response to changes in upper ocean circulation possibly associated with Holocene changes in intensity of El Niño/Southern Oscillation.

Description: The mechanical behavior of the plate boundary fault zone is of paramount importance in subduction zones, because it controls megathrust earthquake nucleation and propagation as well as the structural style of the forearc. In the Nankai area along the NanTroSEIZE (Kumano) drilling transect offshore SW Japan, a heterogeneous sedimentary sequence overlying the oceanic crust enters the subduction zone. In order to predict how variations in lithology, and thus mechanical properties, affect the formation and evolution of the plate boundary fault, we conducted laboratory tests measuring the shear strengths of sediments approaching the trench covering each major lithological sedimentary unit. We observe that shear strength increases nonlinearly with depth, such that the (apparent) coefficient of friction decreases. In combination with a critical taper analysis, the results imply that the plate boundary position is located on the main frontal thrust. Further landward, the plate boundary is expected to step down into progressively lower stratigraphic units, assisted by moderately elevated pore pressures. As seismogenic depths are approached, the décollement may further step down to lower volcaniclastic or pelagic strata but this requires specific overpressure conditions. High-taper angle and elevated strengths in the toe region may be local features restricted to the Kumano transect.