The trace of the Pacific‐Cocos‐Nazca triple junction in the Central Pacific and the formation of an overlapping spreading centre

The `plate tectonic mirror image' to the region of the Cocos and Nazca plates, which are currently being subducted beneath Central America, is preserved in the Central Pacific around 120°W just south of the equator. Cruise SO‐180 investigated this remote area during project CENTRAL and acquired new magnetic and bathymetric data. A plate tectonic model for the ‘mirror image’ is presented based on the newly acquired as well as reprocessed existing data. Discordant magnetic anomaly patterns and bathymetric structures indicate at least two major reorganization events (19.5 and 14.7 Myr), which can be detected both in the Cocos‐Nazca spreading system and in the East Pacific Rise. Irregularities in the anomaly pattern and curvilinear structures on the sea floor of the survey area are interpreted in terms of a fossil overlapping spreading centre at the location where the Farallon break‐up originated.     

Recycling of Proterozoic crust in the Andean Amazon foreland of Ecuador: implications for orogenic development of the Northern Andes

The retro‐arc foreland Andean Amazon Basin records sedimentary infill from the South American craton and the emerging Northern Andean chain from the middle Cretaceous until Present day. The U/Pb ages of detrital zircons indicate significant reworking of Archean‐Proterozoic (max. 2.9 Ga) and Paleozoic crust and sediments, which were eroded on both sides. Heavy mineral associations show that the material derived from Proterozoic craton was supplied by Cretaceous reworking of non‐metamorphosed (unannealed) Paleozoic and older sedimentary rocks, which cover the Amazon Craton. Following latest Cretaceous switch of the dominant sediment source to the Andean cordillera, the influx of Precambrian zircons persisted, and these zircons were derived from the metamorphosed basement and Paleozoic sediments of the Cordillera Real (Loja terrane). Re‐evaluation of existing detrital zircon fission‐track record proves that the rise of the Cordillera Real at the Cretaceous‐Tertiary transition was initiated by the collision of Caribbean Oceanic Plateau and associated arc elements from 75–65 Ma. A further important exhumation event also occurred in the Late Oligocene, which is correlated with the break‐up of the Farallon plate.     

Surface roughness and geological mapping at subhectometer scale from the High Resolution Stereo Camera onboard Mars Express

The quantitative measurement of surface roughness of planetary surfaces at all scales provides insights into geological processes. A characterization of roughness variations at the scale of a few tens of meters is proposed that complements the analysis of local topographic data of the martian surface at kilometer scale, as achieved from the Mars Orbiter Laser Altimeter (MOLA) data, and at the subcentimeter scale using photometric properties derived from multi-angular observations. Relying on a Gabor filtering process, an algorithm developed in the context of image classification for the purpose of texture analysis has been adapted to handle data from the High Resolution Stereo Camera (HRSC). The derivation of roughness within a wavelength range of tens of meters, combined with analyses at even longer wavelengths, gives an original view of the martian surface. The potential of this approach is evaluated for different examples for which the geological processes are identified and the geological units are mapped and characterized in terms of roughness.     

Assessment of annual high‐water events for the Mackenzie River basin,Canada

River ice break‐up is known to have important morphological, ecological and socio‐economic effects on cold‐regions river environments. One of the most persistent effects of the spring break‐up period is the occurrence of high‐water events. A return‐period assessment of maximum annual nominal water depths occurring during the spring break‐up and open‐water season at 28 Water Survey of Canada hydrometric sites over the 1913–2002 time period in the Mackenzie River basin is presented. For the return periods assessed, 13 (14) stations are dominated by peak events occurring during the spring break‐up (open‐water) season. One location is determined to have a mixed signal. A regime classification is proposed to separate ice‐ and open‐water dominated systems. As part of the regime classification procedure, specific characteristics of return‐period patterns including alignment, and difference between the 2 and 10‐year events are used to identify regime types. A dimensionless stage‐discharge plot allows for a contrast of the relative magnitudes of flows required to generate maximum nominal water‐depth events in the different regimes. At sites where discharge during the spring break‐up is approximately one‐quarter or greater than the magnitude of the peak annual discharge, nominal water depths can be expected to exceed those occurring during the peak annual discharge event. Several physical factors (location, basin area, stream order, gradient, river orientation, and climate) are considered to explain the differing regimes and discussed relative to the major sub‐regions of the MRB. Copyright © 2008 John Wiley & Sons, Ltd and Her Majesty the Queen in right of Canada.     

Geochemical and petrographic characteristics of impactites and Cretaceous target rocks from the Yaxcopoil‐1 borehole,Chicxulub impact structure,Mexico: Implications for target composition

Abstract— We present major and trace element data as well as petrographic observations for impactites (suevitic groundmass, bulk suevite, and melt rock particles) and target lithologies, including Cretaceous anhydrite, dolomite, argillaceous limestone, and oil shale, from the Yaxcopoil‐1 borehole, Chixculub impact structure. The suevitic groundmass and bulk suevite have similar compositions, largely representing mixtures of carbonate and silicate components. The latter are dominated by melt rock particles. Trace element data indicate that dolomitic rocks represented a significant target component that became incorporated into the suevites; in contrast, major elements indicate a strong calcitic component in the impactites. The siliceous end‐member requires a mafic component in order to explain the low SiO₂ content. Multicomponent mixing of various target rocks, the high alteration state, and dilution by carbonate complicate the determination of primary melt particle compositions. However, two overlapping compositional groups can be discerned—a high‐Ba, low‐Ta group and a high‐Fe, high‐Zn, and high‐Hf group. Cretaceous dolomitic rocks, argillaceous limestone, and shale are typically enriched in U, As, Br, and Sb, whereas anhydrite contains high Sr contents. The oil shale samples have abundances that are similar to the North American Shale Composite (NASC), but with a comparatively high U content. Clastic sedimentary rocks are characterized by relatively high Th, Hf, Zr, As, and Sb abundances. Petrographic observations indicate that the Cretaceous rocks in the Yaxcopoil‐1 drill core likely register a multistage deformation history that spans the period from pre‐ to post‐impact. Contrary to previous studies that claimed evidence for the presence of impact melt breccia injection veins, we have found no evidence in our samples from a depth of 1347–1348 m for the presence of melt breccia. We favor that clastic veinlets occur in a sheared and altered zone that underwent intense diagenetic overprint prior to the impact event.     

Holocene glacier and deep water dynamics,Adélie Land region,East Antarctica

This study presents a high-resolution multi-proxy investigation of sediment core MD03-2601 and documents major glacier oscillations and deep water activity during the Holocene in the Adélie Land region, East Antarctica. A comparison with surface ocean conditions reveals synchronous changes of glaciers, sea ice and deep water formation at Milankovitch and sub-Milankovitch time scales. We report (1) a deglaciation of the Adélie Land continental shelf from 11 to 8.5 cal ka BP, which occurred in two phases of effective glacier grounding-line retreat at 10.6 and 9 cal ka BP, associated with active deep water formation; (2) a rapid glacier and sea ice readvance centred around 7.7 cal ka BP; and (3) five rapid expansions of the glacier–sea ice systems, during the Mid to Late Holocene, associated to a long-term increase of deep water formation. At Milankovich time scales, we show that the precessionnal component of insolation at high and low latitudes explains the major trend of the glacier–sea ice–ocean system throughout the Holocene, in the Adélie Land region. In addition, the orbitally-forced seasonality seems to control the coastal deep water formation via the sea ice–ocean coupling, which could lead to opposite patterns between north and south high latitudes during the Mid to Late Holocene. At sub-Milankovitch time scales, there are eight events of glacier–sea ice retreat and expansion that occurred during atmospheric cooling events over East Antarctica. Comparisons of our results with other peri-Antarctic records and model simulations from high southern latitudes may suggest that our interpretation on glacier–sea ice–ocean interactions and their Holocene evolutions reflect a more global Antarctic Holocene pattern.     

Effects of subsurface soil characteristics on wetland–groundwater interaction in the coastal plain of the Chesapeake Bay watershed

Ecosystem services provided by depressional wetlands on the coastal plain of the Chesapeake Bay watershed (CBW) have been widely recognized and studied. However, wetland–groundwater interactions remain largely unknown in the CBW. The objective of this study was to examine the vertical interactions of depressional wetlands and groundwater with respect to different subsurface soil characteristics. This study examined two depressional wetlands with a low‐permeability and high‐permeability soil layer on the coastal plain of the CBW. The surface water level (SWL) and groundwater level (GWL) were monitored over 1 year from a well and piezometer at each site, respectively, and those data were used to examine the impacts of subsurface soil characteristics on wetland–groundwater interactions. A large difference between the SWL and GWL was observed at the wetland with a low‐permeability soil layer, although there was strong similarity between the SWL and GWL at the wetland with a high‐permeability soil layer. Our observations also identified a strong vertical hydraulic gradient between the SWL and GWL at the wetland with a high‐permeability soil layer relative to one with a low‐permeability soil layer. The hydroperiod (i.e., the total time of surface water inundation or saturation) of the wetland with a low‐permeability soil layer appeared to rely on groundwater less than the wetland with a high‐permeability soil layer. The findings showed that vertical wetland–groundwater interactions varied with subsurface soil characteristics on the coastal plain of the CBW. Therefore, subsurface soil characteristics should be carefully considered to anticipate the hydrologic behavior of wetlands in this region.     

From small sets of GPS trajectories to detailed movement profiles: quantifying personalized trip-dependent movement diversity

ABSTRACT

The ubiquity of personal sensing devices has enabled the collection of large, diverse, and fine-grained spatio-temporal datasets. These datasets facilitate numerous applications from traffic monitoring and management to location-based services. Recently, there has been an increasing interest in profiling individuals' movements for personalized services based on fine-grained trajectory data. Most approaches identify the most representative paths of a user by analyzing coarse location information, e.g., frequently visited places. However, even for trips that share the same origin and destination, individuals exhibit a variety of behaviors (e.g., a school drop detour, a brief stop at a supermarket). The ability to characterize and compare the variability of individuals' fine-grained movement behavior can greatly support location-based services and smart spatial sampling strategies. We propose a TRip DIversity Measure --TRIM – that quantifies the regularity of users' path choice between an origin and destination. TRIM effectively captures the extent of the diversity of the paths that are taken between a given origin and destination pair, and identifies users with distinct movement patterns, while facilitating the comparison of the movement behavior variations between users. Our experiments using synthetic and real datasets and across geographies show the effectiveness of our method.