Basement Topography Research Articles

2-D Spectral Analysis of Aeromagnetic Anomalies over Parts of Monguno and Environs, Northeastern Nigeria

2-D Spectral Analysis of aeromagnetic anomalies over parts of Monguno and environs, northeastern Nigeria was carried out to determine the depth to basement, establish the basement topography and determine the temperature at depths in the area. Data enhancement was carried out to delineate residual features and the more intense anomalies relative to the strong regional gradients. Typical techniques applied were computations of reduction to pole, trend surface analysis, and a number of filtering processes. Results of the 2-D spectral analysis of the aeromagnetic data revealed a two depth source model. The depth to the deeper magnetic source bodies ranges from 1.22 to 3.14 km, with an average depth of 2.341 km. This layer may be attributed to magnetic rocks of the basement, lateral variations in basement susceptibilities and intra- basement features like faults and fractures. The shallower magnetic sources range in depth from 0.12 to 0.93 km, with an average depth of 0.572 km and could be attributed to near surface magnetic sources, which are magnetic rocks that intruded into the sedimentary formations. Based on the computed average sediment thickness obtained in this study together with the temperature at depths in the area, some parts of the study area have been demarcated for detail hydrocarbon exploration in-line with the renewed interest in hydrocarbon exploration in the basin.

Basal topographic controls on rapid retreat of Humboldt Glacier, northern Greenland

AbstractDischarge from marine-terminating outlet glaciers accounts for up to half the recent mass loss from the Greenland ice sheet, yet the causal factors are not fully understood. Here we assess the factors controlling the behaviour of Humboldt Glacier (HG), allowing us to evaluate the influence of basal topography on outlet glacier response to external forcing since part of HG’s terminus occupies a large overdeepening. HG’s retreat accelerated dramatically after 1999, coinciding with summer atmospheric warming of up to 0.19°C a–1 and sea-ice decline. Retreat was an order of magnitude greater in the northern section of the terminus, underlain by a major basal trough, than in the southern section, where the bedrock is comparatively shallow. Velocity change following retreat was spatially non-uniform, potentially due to a pinning point near HG’s northern lateral margin. Consistent with observations, numerical modelling demonstrates an order-of-magnitude greater sensitivity to sea-ice buttressing and crevasse depth (used as a proxy for atmospheric warming) in the northern section. The trough extends up to 72 km inland, so it is likely to facilitate sustained retreat and ice loss from HG during the 21st century.

Sensitivity of the Weddell Sea sector ice streams to sub-shelf melting and surface accumulation

Abstract. A recent ocean modelling study indicates that possible changes in circulation may bring warm deep-ocean water into direct contact with the grounding lines of the Filchner–Ronne ice streams, suggesting the potential for future ice losses from this sector equivalent to ~0.3 m of sea-level rise. Significant advancements have been made in our knowledge of both the basal topography and ice velocity in the Weddell Sea sector, and the ability to accurately model marine ice sheet dynamics, thus enabling an assessment to be made of the relative sensitivities of the diverse collection of ice streams feeding the Filchner–Ronne Ice Shelf. Here we use the BISICLES ice sheet model, which employs adaptive-mesh refinement to resolve grounding line dynamics, to carry out such an assessment. The impact of realistic perturbations to the surface and sub-shelf mass balance forcing fields from our 2000-year "reference" model run indicate that both the Institute and Möller ice streams are highly sensitive to changes in basal melting either near to their respective grounding lines, or in the region of the ice rises within the Filchner–Ronne Ice Shelf. These same perturbations have little impact, however, on the Rutford, Carlson or Foundation ice streams, while the Evans Ice Stream is found to enter a phase of unstable retreat only after melt at its grounding line has increased by 50% of likely present-day values.

The evolution of Palaeolake Flixton and the environmental context of Star Carr, NE. Yorkshire: stratigraphy and sedimentology of the Last Glacial-Interglacial Transition (LGIT) lacustrine sequences

The basal topography and sediments recording the Last Glacial-Interglacial Transition (LGIT) from Palaeolake Flixton, North Yorkshire have been reinvestigated by a detailed auger and borehole survey. The data presented here, from the area to the north of Flixton Island and the River Hertford, suggests the basal topography of Palaeolake Flixton is irregular with deep and areally small basins interspersed within a gravel surface ca. 21m OD. At its shallowest the gravel surface is ca. 2m below current land surface and the deeper, steep-sided basins are up to 9m in depth. Examination of the sediment sequences indicates the deeper basins accumulate sediments from the Dimlington Stadial (DS), Windermere Interstadial (WI), Loch Lomond Stadial (LLS) and the Holocene, whilst the shallower sequences only record the Holocene. The configuration of the deposits in the basins suggest that lake levels declined during the WI from 24 to 23m OD and then fell further during the LLS to ca. 20.90m OD. The lake water levels then rose slowly during the Holocene to a height of between 23 and 24m OD. These fluctuations in lake water level at the transition from the LLS to Holocene perhaps indicate that the configuration of the water body during the resettlement of the area in the Mesolithic was different to previous reconstructions, with lower water levels and a greater area of land exposed for habitation. This highlights the potential for additional preservation of archaeological assemblages in the area of the former lake. Moreover the complex basal stratigraphy suggests that a systematic analyses of the Palaeolake Flixton sedimentary archive is required to resolve high-resolution palaeoclimate data from this lake sequence.

The ca 2.74 Ga Mopoke Member, Kylena Formation: a marine incursion into the northern Fortescue Group?

The northern part of the Fortescue Group consists of interbedded flood basalts and sedimentary rocks that were deposited on the southern margin of the Pilbara Craton, Western Australia, during one or more periods of continental rifting between ca 2.78 and ca 2.63 Ga. Well-preserved sedimentary intervals within the group have yielded stable carbon and sulfur isotope data that have been used to infer changes in geobiological processes in the Neoarchean. However, the Fortescue Group is notable for being a predominantly subaerial succession, and it remains unclear whether data obtained from these intervals should be interpreted in the context of deposition in marine environments, possibly recording changes in the global ocean/atmosphere system, or in local and restricted lacustrine settings. Here, we describe the sedimentology, stratigraphy, stromatolites and stable carbon isotope geochemistry of the ca 2.74 Ga Mopoke Member, Kylena Formation, the oldest stromatolitic horizon in the Fortescue Group. This unit differs in terms of internal stratigraphic relationships, sedimentology, carbonate mineralogy and stable isotope geochemistry when compared with intervals of probable lacustrine origin in the overlying Tumbiana and Maddina formations. In contrast, we suggest that parts of the Mopoke Member may have been deposited under open marine conditions, or alternatively, in a lacustrine environment characterised by differing water chemistry and basement topography. Stromatolitic microfabrics of the Mopoke Member are dominated by spar, dolospar and vertically aligned calcitic crusts, rather than the micritic microfabrics described from other Fortescue Group stromatolites. Mud-draped ripples are common sedimentary features in the Mopoke Member, suggesting a tidal influence. Mopoke Member δ13Ccarb values are generally slightly positive, but also include some significantly depleted values, which may relate to the reoxidation of 13C-depleted organic matter. δ13Corg values average –36.7‰, consistent with Neoarchean marine units reported from elsewhere, but significantly less 13C-depleted than values reported from overlying lacustrine intervals in the Fortescue Group. We conclude that some features of Fortescue Group datasets relevant to the field of geobiology may be facies dependent, and that more work focusing on the overall depositional environments of the Fortescue Group is needed in order to appropriately interpret geobiological data reported from that group.

Selective erosion beneath the Antarctic Peninsula Ice Sheet during LGM retreat

AbstractIn mountainous terrain, the relationship between ice sheet dynamics and basal topography is complex, with each component influencing the other. This paper investigates how the last glacial maximum Antarctic Peninsula Ice Sheet might have modified its bed both at maximum extent and during progressive grounding line retreat. Focussing on the Marguerite Trough Ice Stream we then examine the degree to which basal topographical conditions affected the rate of ocean-forced recession. Zones of peak subglacial erosion are preferentially located in areas of convergent flow and where horizontal strain rates are highest. During ice sheet retreat, potential erosion rates increase in these areas, but the foci remain fixed. This leads to selective and progressive deepening of subglacial basins. As grounding lines migrate landward, faster retreat tends to occur over subglacial basins, especially if flow is divergent, whereas slower retreat takes place on sloping beds and where the geometry of the outlet allows convergent flow and a non-negative flux balance. In conclusion the Antarctic Peninsula Ice Sheet selectively erodes its bed beneath linear outlets and, over successive glacial cycles, progressive deepening of subglacial basins may bring about non-linear retreat of the ice sheet margin.

Evaluating Marie Byrd Land stability using an improved basal topography

Prior understanding of the ice-sheet setting in Marie Byrd Land (MBL) was derived primarily from geologic and geochemical studies of the current nunataks, with very few geophysical surveys imaging the ice covered regions. The geologic context suggested that the ice rests on a broad regional high, in contrast to the deep basins and trenches that characterize the majority of West Antarctica. This assumed topography would favor long-term stability for the West Antarctic Ice Sheet (WAIS) in MBL. Airborne geophysical data collected in 2009 reveal a much deeper bed than previously estimated, including a significant trough underlying DeVicq Glacier and evidence for extensive glacial erosion. Using these data, we produce a new map of subglacial topography, with which we model the sensitivity of WAIS to a warming ocean using the ice-sheet model of Pollard and DeConto (2012b). We compare the results to estimates of ice loss during WAIS collapse using the previously defined subglacial topography, to determine the impact of the newly discovered subglacial features. Our results indicate that the topographic changes are not sufficient to destabilize the northern margin of MBL currently feeding the Getz Ice Shelf; the majority of ice loss occurs from flow toward the Siple Coast. However, despite only slight dynamic differences, using the new bed as a boundary condition results in an additional 8 cm of sea-level rise during major glacial retreat, an increase of just over 2%. Precise estimation of past and future ice retreat, as well as a complete understanding of the geologic history of the region, will require a higher resolution picture of the bed topography around the Executive Committee mountains.

Tectonic framework of the Barra de São João Graben, Campos Basin, Brazil: Insights from gravity data interpretation

The Barra de São João Graben (BSJG), shallow water Campos Basin, is part of the Cenozoic rift system that runs parallel to the Brazilian continental margin. This system was formed in an event that caused the reactivation of the main Precambrian shear zones of southeastern Brazil in the Paleocene. We proposed a new structural framework of BSJG based on gravity data interpretation. Magnetic data, one available 2D seismic line, and a density well-log of a nearby well were used as constraints to our interpretation. To estimate the top of the basement structure, we separated the gravity effects of deep sources from the shallow basement (residual anomaly). Then, we performed a 2D modeling exercise, in which we kept fixed the basement topography and the density of the sediments, to estimate the density of the basement rocks. Next, we inverted the residual anomaly to recover the depth to the top of the basement. This interpretation strategy allowed the identification of a complex structural framework with three main fault systems: a northeast–southwest-trending normal fault system, a northwest–southeast-trending transfer fault system, and an east–west-trending transfer fault system. These trends divided the graben into several internal highs and lows. Our interpretation was corroborated by the magnetic anomalies. The existence of ultradense and strongly magnetized elongated bodies in the basement was interpreted as ophiolite bodies that were probably obducted by the time of the shutdown of the Proterozoic Adamastor Ocean.

Anomalous South Pacific lithosphere dynamics derived from new total sediment thickness estimates off the West Antarctic margin

Paleotopographic models of the West Antarctic margin, which are essential for robust simulations of paleoclimate scenarios, lack information on sediment thickness and geodynamic conditions, resulting in large uncertainties. A new total sediment thickness grid spanning the Ross Sea–Amundsen Sea–Bellingshausen Sea basins is presented and is based on all the available seismic reflection, borehole, and gravity modeling data offshore West Antarctica. This grid was combined with NGDC's global 5 arc minute grid of ocean sediment thickness (Whittaker et al., 2013) and extends the NGDC grid further to the south. Sediment thickness along the West Antarctic margin tends to be 3–4km larger than previously assumed. The sediment volume in the Bellingshausen, Amundsen, and Ross Sea basins amounts to 3.61, 3.58, and 2.78millionkm3, respectively. The residual basement topography of the South Pacific has been revised and the new data show an asymmetric trend over the Pacific–Antarctic Ridge. Values are anomalously high south of the spreading ridge and in the Ross Sea area, where the topography seems to be affected by persistent mantle processes. In contrast, the basement topography offshore Marie Byrd Land cannot be attributed to dynamic topography, but rather to crustal thickening due to intraplate volcanism. Present-day dynamic topography models disagree with the presented revised basement topography of the South Pacific, rendering paleotopographic reconstructions with such a limited dataset still fairly uncertain.

Gravity analysis of the Precambrian basement topography associated with the northern boundary of Ghadames Basin (southern Tunisia)

Gravity data were analyzed to determine the structural development of the northern boundary of the Ghadames Basin in southern Tunisia. The Ghadames Basin which also occurs in eastern Algeria and northwestern Libya is one of the most prolific hydrocarbon producers in North Africa with several of the largest oil fields occurring along its northern boundary. The Ghadames Basin was formed during a series of tectonic events ranging from the Early Paleozoic to the Early Cenozoic. These tectonic events produced a basin in southern Tunisia that has a complex basement configuration which is not completely known. A residual gravity anomaly map constructed using polynomial trend surfaces, and vertical and horizontal gravity derivative maps indicate that the northern boundary contains a series of maxima and minima anomalies that trend in two prominent directions: northeast–southwest and east–west. The horizontal and vertical derivative gravity anomaly maps indicate that the width of the basement structures range between 10 and 20km in width. Three-dimensional (3D) Euler deconvolution and 3D forward modeling constrained by well data, one seismic reflection profile and remote sensing data confirm the width of the basement structures and indicates that the depth of basin varies between 1.5 and 5km, with deeper sections in general more numerous in the southern sections of the boundary. The gravity analysis constrained by the seismic reflection profile and well data implies that the basement topography may have been formed during the Pan African and/or late Mesozoic rifting. However, additional seismic reflection and well data are needed to confirm this conclusion. The discovery of the numerous basement structures suggests that there may exist additional hydrocarbon traps within the northern boundary of the Ghadames Basin.

Gravity inversion predicts the nature of the Amundsen Basin and its continental borderlands near Greenland

The high-Arctic Eurekan Orogeny was caused by a northward movement of Greenland relative to North America and Eurasia during the latest Cretaceous to late Eocene. While the Eurekan N–S shortening is well-documented in Ellesmere Island, North Greenland and Svalbard, the nature of the event is largely unknown in the ice-covered Arctic Ocean to the north of Greenland. In this contribution, we show that the tectono-physiographic evolution of the oceanic Amundsen Basin, the continental Lomonosov Ridge and the Morris Jesup Rise were all affected by significant Eurekan compression. We present the results of 3-D gravity inversion for predicting the sediment thickness and basement geometry within the Amundsen Basin and along its borderlands. We use the recently published LOMGRAV-09 gravity compilation and adopt a process-oriented iterative cycle approach that minimizes misfit between an Earth model and observations. The sensitivity of our results to lateral variations in depth and density contrast of the Moho is further tested by a stochastic inversion. Within their limitations, the approach and setup used herein provides the first detailed model of the sediment thickness and basement geometry in the Arctic Ocean north of Greenland. Our preferred result, using a C25 breakup scenario of the Amundsen Basin, correlates well with seismic observations along existing and several new marine seismic profiles. Breakup-related rift basins are predicted along the Lomonosov Ridge and a broad depocentre is predicted above high-relief basement in the central Amundsen Basin. Significantly, an up to 7 km deep elongated sedimentary basin is predicted along the northern edge of the Morris Jesup Rise. This basin continues into the Klenova Valley south of the Lomonosov Ridge and correlates with an offshore continuation of the Eurekan Mount Rawlinson Fault in Ellesmere Island. We compute the anomalous basement topography and show evidence of deformed oceanic and continental crust in relation to this fault zone (LKFZ), suggesting that pronounced Eurekan crustal shortening took place here.

Three-dimensional viscoelastic finite-difference code and modelling of basement focusing effects on ground motion characteristics

This paper consists of two parts. The first part of the paper is concerned with developing a 3D staggered grid time-domain finite-difference (FD) code with fourth-order spatial accuracy for simulating the responses of viscoelastic geological models with a continuous variable grid size in all the directions. The 3D FD code is written in Fortran 90. In the developed code, the realistic damping in the time-domain simulation is incorporated based on the generalized Maxwell body rheological model proposed by Emmerich and Korn (GMB-EK) Geophysics 52,1252–1264, 1987 with improvements made by Kristeck and Moczo Bull Seism Soc Am 93, 2273–2280, 2003. The accuracy of implementation of the realistic damping is validated by comparing the numerically computed phase velocity, quality factors and spatial spectral damping with the same based on GMB-EK model and Futterman’s relations. It is also concluded that the grid spacing ratio up to 5.0 can be used unharmed. The second part of the paper presents the application of the developed code to simulate the basement focusing effects on ground motion characteristics in an unbounded medium. The combined effects of sediment damping and the focusing caused by the hemi-spherical (HS) and hemi-cylindrical (HC) synclinal basement topography (SBT) on the ground motion characteristics are studied. The simulated responses and the computed snapshots revealed the SBT focusing and defocusing phenomenon, intense mode conversion and diffractions of the incident waves. A good match of the spectral amplifications caused by the HS and HC basement topography models at their focus with the same computed analytically also reveals the accuracy of the developed FD code. The response of elastic HS and HC basement topography models revealed an increase of spectral amplification with an increase of frequency. Further, an increase of rate of frequency-dependent amplification towards the focus was inferred. The average spectral amplification (ASA) caused by the elastic HS basement topography model at the focus is more than the square of the same caused by the elastic HC basement topography model, and this ratio of ASA is increasing with an increase of sediment damping.

Optimal initial conditions for coupling ice sheet models to Earth system models

AbstractWe address complications in the coupling of a dynamic ice sheet model (ISM) and forcing from an Earth system model (ESM), which arise because of the unknown ISM initial conditions. Unless explicitly accounted for during ISM initialization, the ice sheet is far from thermomechanical equilibrium with the surface mass balance forcing from the ESM. Upon coupling to ESM forcing, the result is a shock and unphysical and undesirable transients in ice geometry and other state variables. Under the assumption of thermomechanical equilibrium, we present an approach for finding ISM initial conditions—characterized by optimization of the basal sliding coefficient and basal topography fields—that balance a best fit to surface velocity and basal topography observations against the minimization of unphysical transients when coupling to surface mass balance forcing. A quasi‐Newton method is used to solve the resulting large‐scale, partial differential equation‐constrained optimization problem, where the cost function gradients with respect to the parameter fields are computed using adjoints. After studying properties of our approach on a synthetic test problem, we apply the method toward obtaining optimal initial conditions for a model of the Greenland ice sheet. Our results show that, in the presence of uncertainties in the basal topography, ice thickness should also be treated as an optimization variable. While the focus here is on the coupling between an ISM and ESM‐derived surface mass balance, the method is easily extended to include optimal coupling to forcing from an ocean model through submarine melt rates.

Development of arc–continent collision mélanges: Linking onshore geological and offshore geophysical observations of the Pliocene Lichi Mélange, southern Taiwan and northern Luzon arc, western Pacific

Although the Lichi mélange has been studied intensively over the last decade, its enigmatic nature has generated debates regarding its origin and evolution. Two prominent models have been proposed to explain the Lichi melange: the olistostrome model with slumping sediments and the tectonic collision model with intensive shearing. Neither model can explain what causes the interpreted slumping and complex faulting processes for the sediments. Here, we study the Lichi melange using a time–space equivalence approach, in which the tectonics of the offshore continuation of the Lichi mélange should represent an earlier stage in its evolution. Our study of marine multichannel seismic data suggested that the backthrusts in the accretionary prism propagate arcward above and within the deforming forearc and arc basement to incorporate the sediment and basement materials into the rear of the accretionary prism. Therefore, we proposed the “retrowedge evolution model” whose novel key feature is that the irregular topography of the arc basement affects the taper angle of the retrowedge. The retrowedge has a greater taper than the minimum taper of the prowedge, so the slope can be steeper, and thus favors gravitational failure. Such processes would generate complex faulting and slumping processes in the backthrusted forearc ridge and basin. In sum, this retrowedge model reconciles the mixture of slumping and faulting processes found in the Lichi mélange.

Detection of Lineaments in Denizli Basin of Western Anatolia Region Using Bouguer Gravity Data

The aim of this study is to investigate the geostructural boundaries of the eastern part of Western Anatolia. To achieve this, three methods, horizontal gradient, analytic signal, and tilt angle, were used. With the application of each method to the Bouguer gravity data, the common lineaments were determined using maximum values of the horizontal gradient, analytic signal maps, and zero contours of the tilt angle maps. The basement topography was also produced using the Parker–Oldenburg algorithm. Then, the produced lineaments were compared with the active fault map of the region. The results suggested that although a good agreement between the current work and earlier work exists, the new four lineament regions were also detected. We concluded that this work will lead to better understanding of Anatolian geostructural and its impact on the larger scale geological processes.

Basal terraces on melting ice shelves

AbstractOcean waters melt the margins of Antarctic and Greenland glaciers, and individual glaciers' responses and the integrity of their ice shelves are expected to depend on the spatial distribution of melt. The bases of the ice shelves associated with Pine Island Glacier (West Antarctica) and Petermann Glacier (Greenland) have similar geometries, including kilometer‐wide, hundreds‐of‐meter high channels oriented along and across the direction of ice flow. The channels are enhanced by, and constrain, oceanic melt. New meter‐scale observations of basal topography reveal peculiar glaciated landscapes. Channel flanks are not smooth, but are instead stepped, with hundreds‐of‐meters‐wide flat terraces separated by 5–50 m high walls. Melting is shown to be modulated by the geometry: constant across each terrace, changing from one terrace to the next, and greatly enhanced on the ~45° inclined walls. Melting is therefore fundamentally heterogeneous and likely associated with stratification in the ice‐ocean boundary layer, challenging current models of ice shelf‐ocean interactions.

A geophysical view of the Southeastern Brazilian margin at Santos Basin: Insights into rifting evolution

This study investigates the rifting structures of Santos Basin at the Southeastern Brazilian margin, based on an integrated geophysical approach. Our aim is to constrain the crustal basement topography of central and northern Santos basin, the presence of magmatism and the role of inherited structures in space and time through the rifting processes. We present a new high resolution aeromagnetic dataset, which in correlation with gravity anomalies enables us to interpret the tectonic trends and crustal basement structures. We calculated the magnetic basement depth for the central and northern Santos Basin using power spectrum analysis. The obtained depths range between 2 and 9 kms, and are comparable with results from previous works. From our integrated study, three margin domains could be identified, which display distinct rifting structures and are characterized by important lateral variation along the margin. The proximal domain displays trends and magnetic basement blocks NE–SW oriented, i.e., parallel to inherited onshore crustal basement with an inflexion to E–W oriented trends; the necking domain is characterized by oblique magnetic basement highs and lows (E–W and NW–SE) and a structural trend change. The trends and magnetic basement highs are bounded by NW–SE negative anomalies, interpreted as transfer zones. Oceanwards at the distal domain, the lineaments and transfer zones show a progressive structural inflexion to ENE and E–W, sub-parallel to adjacent South Atlantic Fracture Zones. The observed crustal basement architecture and segmentation suggest the reactivation of pre-rift structures at the proximal margin and the obliquity of rifting relative to them. From the proximal domain oceanwards the structural pattern may reflect the passage from a “continental type” domain, where lithospheric inheritance controls the deformation, to a distal margin where this influence diminishes and “new” structural trends are formed. We propose that northern Santos Basin show evidences of an intensely deformed zone, where rift evolved under oblique extension, similar to that observed at transform margin segments.

Fluvial system development and subsequent marine transgression in Yellow River (Huanghe) delta and its adjacent sea regions during last glacial maximum to early Holocene

Paleotopography of the Yellow River (Huanghe) delta area and the western Bohai Sea during the last glacial maximum (LGM) is important to understand fluvial activities of the Yellow River linked with the LGM climate. By integrating data sets of both onshore and offshore borehole cores and offshore high-resolution seismic profiles, we reconstructed the paleotopography of this area from the LGM to the early Holocene. The fluvial sediment facies of the LGM identified in these cores was characterized by poorly sorted medium- to coarse-grained sands, which shows chaotic patterns in seismic profiles. REE characteristics and clay mineral components of the fluvial sediments suggest that they were derived mostly from the paleo-Yellow River. The basal and top bounding surfaces of the fluvial sediments slope very gently toward the northeast, similar to the present morphology of the North China Plain formed by the Yellow River. No incised valley morphology is detected in the basal topography, because of the long distance from the study area to the paleoshoreline during the LGM, and also because of the very gentle gradient of the paleo-Yellow River from the northern part of North China Plain to the continental shelf area, with concave-upward morphology. Aggradational stacking of the fluvial sediments over the entire North China Plain and in the study area indicates that the Yellow River flowed in these areas during the LGM to the early Holocene. The Holocene marine and coastal sediments onlap onto the underlying fluvial sediments. These basal marine or brackish sediments are diachronous from offshore areas of the Bohai Sea to the Yellow River delta area, with older sediments in the east and younger sediments in the west, which clearly reflects the early Holocene marine transgression from the North Yellow Sea to the Bohai Sea.

Evolution of a supercooled Ice Shelf Water plume with an actively growing subice platelet matrix

AbstractWe use new observations in Western McMurdo Sound, combined with longitudinal hydrographic transects of the sound, to identify a northward‐flowing Ice Shelf Water (ISW) plume exiting the cavity of the McMurdo‐Ross Ice Shelf. We estimate the plume's net northward transport at 0.4 ± 0.1 Sv, carving out a corridor approximately 35 km wide aligned with the Victoria Land Coast. Basal topography of the McMurdo Ice Shelf is such that the plume is delivered to the surface without mixing with overlying warmer water, and is therefore able to remain below the surface freezing temperature at the point of observation beneath first‐year ice. Thus, the upper ocean was supercooled, by up to 50 mK at the surface, due to pressure relief from recent rapid ascent of the steep basal slope. The 70 m thick supercooled layer supports the growth and maintenance of a thick, semirigid, and porous matrix of platelet ice, which is trapped by buoyancy at the ice‐ocean interface. Continued growth of individual platelets in supercooled water creates significant brine rejection at the top of the water column which resulted in convection over the upper 200 m thick, homogeneous layer. By examining the diffusive nature of the intermediate water between layers of ISW and High Salinity Shelf Water, we conclude that the ISW plume must have originated beneath the Ross Ice Shelf and demonstrate that it is likely to expand eastward across McMurdo Sound with the progression of winter.

Estimation of Precambrian basement topography in Central and Southeastern Wisconsin from 3D modeling of gravity and aeromagnetic data

Abstract Increased concerns about groundwater resources in Wisconsin have brought about the need for better understanding of the subsurface geologic structure that leads to developing conceptual hydrogeologic models for numerical simulation of groundwater flow. Models are often based on sparse data from well logs usually located large distances apart and limited in depth. Model assumptions based on limited spatial data typically require simplification that may add uncertainty to the simulation results and the accuracy of a groundwater model. Three dimensional (3D) modeling of gravity and aeromagnetic data provides another tool for the groundwater modeler to better constrain the conceptual model of a hydrogeologic system. The area near the Waukesha Fault in southeastern Wisconsin provides an excellent research opportunity for our proposed approach because of the strong gravity and aeromagnetic anomalies associated with the fault, the apparent complexity in fault geometry, and uncertainty in Precambrian basement depth and structure. Fond du Lac County provides another opportunity to apply this approach because the Precambrian basement topography throughout the area is known to be very undulated and this uneven basement surface controls water well yields and creates zones of stagnant water. The results of the 3D modeling of gravity and aeromagnetic data provide a detailed estimation of the Precambrian basement topography in Fond Du Lac County and southeastern Wisconsin that may be useful in determining ground water flow and quality in this region.