Landscape evolution and origin of Lake Fúquene (Colombia): Tectonics, erosion and sedimentation processes during the Pleistocene

The Basin of Ubaté–Chichinquirá (5°28′N, 73°45′ W, c. 2580 m altitude) includes the Fúquene Valley and is located in the central part of the Eastern Cordillera of Colombia. Rocks and sediments were folded and faulted during the Miocene, uplifted during the (late) Pliocene, and affected by glaciers during the Pleistocene. Successive glacial and interglacial periods left significant marks in the landscape which were used to reconstruct six stages in the development of the landscape along a relative chronology. During early Pleistocene episode 1 glaciers formed U-shape valleys. Evidence of the impact of ice sheets has been found as far downslope as ca. 2900 m elevation. During episode 2 moraines developed which were cut by the present San José River. During episode 3 abundant sediment was produced by glacial erosion. It accentuated the sculpturing of hard rock and deepening of the drainage basin. The ancestral Ubaté–Suarez River constituted a dynamic erosive system that gave rise to deep V-shaped valleys and progressively formed a set of intricate valleys with a high sediment production. Finally, intense glacial and fluvio-glacial erosion led to a geomorphological system with high energy levels and intensive sediment transport leading to wide valleys. During episode 4 the Ubaté–Suarez River eroded and deepened its valley until it captured the old El Hato–San José Valley. It caused intense erosion of the moraine and the fluvio-glacial gravels. Deep V-shaped valleys stabilized in the high areas of the main drainage system and these valleys form the present-day fluvial sub-basins. During episode 5 the deep valley in the northern part of the Basin of Ubaté–Chichinquirá developed. During middle Pleistocene episode 6 colluvial sediments formed the Saboya dam and a lake was formed in the river valley of which the present Lake Fúquene is only a small remnant. Lithological changes indicate fluctuating water levels and Lake Fúquene must have expanded periodically up to an area 5 to 10 times the present-day surface.     

西峰蔡家咀黄土剖面记录的末次盛冰期到全新世最佳期气候变化

采用挖探井方式获取西峰蔡家咀剖面200 cm黄土沉积序列,以7个¹⁴C-AMS测年数据为年龄控制点,利用沉积物粒度模式建立最近20 ka的年代框架。磁化率、灰度、粒度等指标的综合对比表明该剖面记录了末次盛冰期增温以来几次重要气候事件,如Heinrich事件1(H₁)、Bølling-Allerød暖期(B-A)、新仙女木事件(YD)、8.2 ka事件等,说明西峰黄土剖面具有记录短尺度气候突变事件的潜力。     

Estimation of Maximum Earthquakes in Northeast India

We attempt to estimate possible maximum earthquakes in the northeast Indian region for four seismic source zones, namely EHZ, MBZ, EBZ, and SHZ, which encapsulates the various seismogenic structures of the region and also for combined source zones taken as a single seismic source regime. The latter case exhibits a high maximum earthquake estimate of M_W 9.4 (±0.85) through Bayesian interpretation of frequency magnitude distribution with Gamma function implicating a moderate deviation from the standard Gutenberg Richter model at the higher magnitudes. However, tapering Gutenberg Richter models with corner magnitudes at M_W 8.01, 8.7 and 9.1, respectively indicated maximum values corresponding to M_W 8.4, 9.0, and 9.3. The former approach was applied to each of the source zones wherein the data are presented in parts according to the data completeness, thereof. EHZ, MBZ, EBZ and SHZ are seen with maximum earthquakes of M_W 8.35 (±0.59), 8.79 (±0.31), 8.20 (±0.50), and 8.73 (±0.70), respectively. The maximum possible earthquakes estimated for each individual zone are seen to be lower than that estimated for the single regime. However, the pertaining return periods estimated for the combined zone are far less than those estimated for the demarcated ones.     

末次间冰期以来渭河上游气候演化的黄土记录研究

对渭河上游樊家台剖面末次间冰期以来黄土地层的粒度与CaCO3进行综合分析表明,末次间冰期时,受夏季风影响较大,粒度较细,CaCO3有较强的淋溶与淀积,气候呈现较为暖湿的特点;末次冰期时,受冬季风影响较强,粒度较粗,CaCO3淋溶淀积微弱,气候较为干冷。间冰期早期（相当于深海氧同位5e阶段）由于夏季风较强,粉尘堆积较少,古土壤向下发育,并与下部土壤融合,导致粒度较5a、5c阶段粗;2阶段的早期,由于粉尘源区出现了高湖面,环境湿度大,造成此时段早期黄土粒度较晚期细;表现出区域气候的特点。     

Structural, tectonic and glaciological controls on the evolution of fjord landscapes

The fjord landscape of South America, stretching ~ 1500 km between Golfo Corcovado (~ 43°S) and Tierra del Fuego (~ 56°S), is the largest continuous fjord landscape on Earth. This paper presents the results of new structural geological and geomorphological mapping of this landscape using optical satellite images and digital elevation models. First-order geological structures are represented by strike-slip faults forming lineaments up to hundreds of kilometres long. The strike-slip faulting has been active since Late Cretaceous times and is responsible for the presence of a conspicuous structural cleavage visible as lineaments up to ~ 10 km long. A detailed analysis of these second-order lineaments from digital image data was carried out in three sectors. In Sector 1, located northwest of the North Patagonian Icefield, there are three distinct mean orientations, characterized by a main nearly orogen-parallel orientation (az. ~ 145°) and two orogen-oblique secondary orientations (az. ~ 20° and az. ~ 65°). In Sector 2, located west of the South Patagonian Icefield, there are also three separate mean orientations, with most of the lineaments concentrated between azimuths 0° and 80° (mean at ~ 36°); and two other orogen-oblique means at azimuth ~ 122° and ~ 163°. In Sector 3, around the Cordillera Darwin, there is a single main orogen-parallel mean at ~ 100–115°. In all three sectors, mapped fjord orientations bear a striking similarity to the structural data, with fjords orientated preferentially in the same direction as structural lineaments. We infer that successive glaciations followed the same ice-discharge routes, widening and deepening pre-existing geological structures at the expense of the surrounding terrain to create the fjord landscape. This study has broader implications for ice sheet reconstructions and landscape evolution beneath ice sheets because we demonstrate that the primary control on fjord development in glaciated areas is geological and not glaciological.     

Summer temperatures during the Medieval Warm Period and Little Ice Age inferred from varved proglacial lake sediments in southern Alaska

For the heavily glaciated mountains of southern Alaska, few high-resolution, millennial-scale proxy temperature reconstructions are available for comparison with modern temperatures or with the history of glacier fluctuations. Recent catastrophic drainage of glacier-dammed Iceberg Lake, on the northern margin of the Bagley Icefield, exposed subaerial outcrops of varved lacustrine sediments that span the period 442–1998 AD. Here, an updated chronology of varve thickness measurements is used to quantitatively reconstruct melt-season temperature anomalies. From 1958 to 1998, varve thickness has a positive and marginally significant correlation with May–June temperatures at the nearest coastal measurement stations. Varve sensitivity to temperature has changed over time, however, in response to lake level changes in 1957 and earlier. I compensate for this by log-transforming the varve thickness chronology, and also by using a 400-year-long tree-ring-based temperature proxy to reconstruct melt-season temperatures at Iceberg Lake. Regression against this longer proxy record is statistically weak, but spans the full range of occupied lake levels and varve sensitivities. Reconstructed temperature anomalies have broad confidence intervals, but nominally span 1.1°C over the last 1500+ years. Maximum temperatures occurred in the late twentieth century, with a minimum in the late sixth century. The Little Ice Age is present as three cool periods between 1350 and 1850 AD with maximum cooling around 1650 AD. A Medieval Warm Period is evident from 1000 to 1100 AD, but the temperature reconstruction suggests it was less warm than recent decades—an observation supported by independent geological evidence of recent glacier retreat that is unprecedented over the period of record. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.

Probability distributions for maximum wave and crest heights

The paper discusses short- and long-term probability models of ocean waves. The Gaussian theory is reviewed, and nonlinear short-term probability distributions are derived from a narrow band second-order model. The nonlinearity has different impact on different measurement techniques, and this is further demonstrated for wave data from the WAVEMOD Crete measurement campaign and laser data from the North Sea. Finally, we give some examples on how the short-term statistics may be used to estimate the probability distributions for the maximum waves during individual storms as well as in a wave climate described by long-term distributions.     

Highstands in the interior of the Qinghai–Tibetan Plateau since the last interglacial: evidence from grain‐size analysis of lacustrine core sediments in Zabuye Salt Lake

Highstands in the Marine Isotope Stage (MIS) 3 based on ¹⁴C dating in the Qinghai–Tibetan Plateau (QTP) are widely documented. Recent records from shoreline sediments dated using U‐series and/or optically stimulated luminescence (OSL), however, reveal that the highstands originally dated in MIS 3 should now be considered to fall in MIS 5. This paper provides new evidence from the interior of the QTP, based on the grain‐size from a continuous lake core in the Zabuye Salt Lake, to verify the MIS 5 highstand in the QTP. Grain‐size analysis of the core sediments also distinguishes two other highstands in MIS 3 and MIS 2, respectively. The MIS 5 highstand is considered as the maximum lake level since the Last Interglacial, as cored sediments contain very low values of Median Diameter (Md) during MIS 5. Compared with the discontinuous records from lake shorelines sediments, the grain‐size records from the continuous lake centre core sediments provide a more complete dataset to infer lake level variations, and make it possible to make wider palaeoclimatic and palaeoenvironmental interpretation. In the interior of the QTP, highstands might have continued into cold climate periods due to the reduced evaporation rates in the latter. The influence of the moisture‐bearing southerly‐shifted Westerly wind pathway may also have contributed to the highstands in the glacial period. Copyright © 2015 John Wiley & Sons, Ltd.     

Glacial isostatic adjustment in Fennoscandia from GRACE data and comparison with geodynamical models

The Earth’s gravity field observed by the Gravity Recovery and Climate Experiment (GRACE) satellite mission shows variations due to the integral effect of mass variations in the atmosphere, hydrosphere and geosphere. Several institutions, such as the GeoForschungsZentrum (GFZ) Potsdam, the University of Texas at Austin, Center for Space Research (CSR) and the Jet Propulsion Laboratory (JPL), Pasadena, provide GRACE monthly solutions, which differ slightly due to the application of different reduction models and centre-specific processing schemes. The GRACE data are used to investigate the mass variations in Fennoscandia, an area which is strongly influenced by glacial isostatic adjustment (GIA). Hence the focus is set on the computation of secular trends. Different filters (e.g. isotropic and non-isotropic filters) are discussed for the removal of high frequency noise to permit the extraction of the GIA signal. The resulting GRACE based mass variations are compared to global hydrology models (WGHM, LaDWorld) in order to (a) separate possible hydrological signals and (b) validate the hydrology models with regard to long period and secular components. In addition, a pattern matching algorithm is applied to localise the uplift centre, and finally the GRACE signal is compared with the results from a geodynamical modelling. The GRACE data clearly show temporal gravity variations in Fennoscandia. The secular variations are in good agreement with former studies and other independent data. The uplift centre is located over the Bothnian Bay, and the whole uplift area comprises the Scandinavian Peninsula and Finland. The secular variations derived from the GFZ, CSR and JPL monthly solutions differ up to 20%, which is not statistically significant, and the largest signal of about 1.2 Gal/year is obtained from the GFZ solution. Besides the GIA signal, two peaks with positive trend values of about 0.8 Gal/year exist in central eastern Europe, which are not GIA-induced, and also not explainable by the hydrology models. This may indicate that the recent global hydrology models have to be revised with respect to long period and secular components. Finally, the GRACE uplift signal is also in quite good agreement with the results from a simple geodynamical modelling.     

Postglacial isostatic adjustment in a self-gravitating spherical earth with power-law rheology

Since microphysics cannot say definitively whether the rheology of the mantle is linear or non-linear, the aim of this paper is to constrain mantle rheology from observations related to the glacial isostatic adjustment (GIA) process—namely relative sea-levels (RSLs), land uplift rate from GPS and gravity-rate-of-change from GRACE. We consider three earth model types that can have power-law rheology (n = 3 or 4) in the upper mantle, the lower mantle or throughout the mantle. For each model type, a range of A parameter in the creep law will be explored and the predicted GIA responses will be compared to the observations to see which value of A has the potential to explain all the data simultaneously. The coupled Laplace finite-element (CLFE) method is used to calculate the response of a 3D spherical self-gravitating viscoelastic Earth to forcing by the ICE-4G ice history model with ocean loads in self-gravitating oceans. Results show that ice thickness in Laurentide needs to increase significantly or delayed by 2 ka, otherwise the predicted uplift rate, gravity rate-of-change and the amplitude of the RSL for sites inside the ice margin of Laurentide are too low to be able to explain the observations. However, the ice thickness elsewhere outside Laurentide needs to be slightly modified in order to explain the global RSL data outside Laurentide. If the ice model is modified in this way, then the results of this paper indicate that models with power-law rheology in the lower mantle (with A  10⁻³⁵ Pa⁻³ s⁻¹ for n = 3) have the highest potential to simultaneously explain all the observed RSL, uplift rate and gravity rate-of-change data than the other model types.