Airborne laser altimetry survey of Glaciar Tyndall, Patagonia

The first airborne laser altimetry measurements of a glacier in South America are presented. Data were collected in November of 2001 over Glaciar Tyndall, Torres del Paine National Park, Chilean Patagonia, onboard a Twin Otter airplane of the Chilean Air Force. A laser scanner with a rotating polygon-mirror system together with an Inertial Navigation System (INS) were fixed to the floor of the aircraft, and used in combination with two dual-frequency GPS receivers. Together, the laser–INS–GPS system had a nominal accuracy of 30 cm after data processing. On November 23rd, a total of 235 km were flown over the ablation area of Glaciar Tyndall, with 5 longitudinal tracks with a mean swath width of 300 m, which results in a point spacing of approximately 2 m both along and across track. A digital elevation model (DEM) generated using the laser altimetry data was compared with a DEM produced from a 1975 map (1:50,000 scale — Instituto Geográfico Militar (IGM), Chile). A mean thinning of − 3.1 ± 1.0 m a^− 1 was calculated for the ablation area of Glaciar Tyndall, with a maximum value of − 7.7 ± 1.0 m a^− 1 at the calving front at 50 m a.s.l. and minimum values of between − 1.0 and − 2.0 ± 1.0 m a^− 1 at altitudes close to the equilibrium line altitude (900 m a.s.l.). The thinning rates derived from the airborne survey were similar to the results obtained by means of ground survey carried out at  600 m of altitude on Glaciar Tyndall between 1975 and 2002, yielding a mean thinning of − 3.2 m a^− 1 [Raymond, C., Neumann, T.A., Rignot, E., Echelmeyer, K.A., Rivera, A., Casassa, G., 2005. Retreat of Tyndall Glacier, Patagonia, over the last half century. Journal of Glaciology 173 (51), 239–247.]. A good agreement was also found between ice elevation changes measured with laser data and previous results obtained with Shuttle Radar Topography Mission (SRTM) data. We conclude that airborne laser altimetry is an effective means for accurately detecting glacier elevation changes in Patagonia, where an ice thinning acceleration trend has been observed during recent years, presumably in response to warming and possibly also drier conditions.     

Leaky savannas: the significance of lateral carbon fluxes in the seasonal tropics

Globally, dissolved inorganic carbon (DIC) accounts for more than half the annual flux of carbon exported from terrestrial ecosystems via rivers. Here, we assess the relative influences of biogeochemical and hydrological processes on DIC fluxes exported from a tropical river catchment characterized by distinct land cover, climate and geology transition from the wet tropical mountains to the low‐lying savanna plains. Processes controlling changes in river DIC were investigated using dissolved organic carbon, particulate organic carbon and DIC concentrations and stable isotope ratios of DIC (δ¹³C_DIC) at two time scales: seasonal and diel. The recently developed Isotopic Continuous Dissolved Inorganic Carbon Analyser was used to measure diel DIC concentration and δ¹³C_DIC changes at a 15‐min temporal resolution. Results highlight the predominance of biologically mediated processes (photosynthesis and respiration) controlling diel changes in DIC. These resulted in DIC concentrations varying between 3.55 and 3.82 mg/l and δ¹³C_DIC values ranging from ?19.7 ± 0.31‰ to ?17.1 ± 0.08‰. In contrast, at the seasonal scale, we observed wet season DIC variations predominantly from mixing processes and dry season DIC variations due to both mixing processes and biological processes. The observed wet season increases in DIC concentrations (by 6.81 mg/l) and δ¹³C_DIC values of river water (by 5.4‰) largely result from proportional increases in subsurface inflows from the savanna plains (C₄ vegetation) region relative to inflows from the rainforest (C₃ vegetation) highlands. The high DIC river load during the wet season resulted in the transfer of 97% of the annual river carbon load. Therefore, in this gaining river, there are significant seasonal variations in both the hydrological and carbon cycles, and there is evidence of substantial coupling between the carbon cycles of the terrestrial and the fluvial environments. Recent identification of a substantial carbon sink in the savannas of northern Australia during wetter years in the recent past does not take into account the possibility of a substantial, rapid, lateral flux of carbon to rivers and back to the atmosphere. Copyright © 2015 John Wiley & Sons, Ltd.     

Indication of a geomagnetic low-inclination excursion in supposed Middle Weichselian interstadial marine clay at Rubjerg,Denmark

Palaeomagnetic measurements have been made on specimens from Late Pleistocene sediments of a piston boring at Rubjerg in Vendsyssel, northern Jutland. The stratigraphy of the deposits is based on content of foraminifera. A total of 70 relatively oriented specimens were investigated palaeomagnetically. Normal steep inclinations close to that of the axial dipole field were found in the Upper Saxicava Sand and in the Younger Yoldia Clay (radiocarbon dated at 14,650 ± 190?12,650 ± 180 B.P.), and a secular variation with an amplitude of 10–12° in the inclincation and a “period” roughly estimated at about 350–400 years was found in the Younger Yoldia Clay.Seventeen relatively oriented specimens from undisturbed older marine deposits revealed a stable low inclination of 11° with α₉₅ = 3°. The age of this apparent geomagnetic excursion falls somewhere between 23,000 and 40,000 B.P. (Older Yoldia Clay). Among other known geomagnetic excursions and events within this interval are Laschamp in France, Mono Lake in California and Lake Mungo in Australia. Until more definite ages have been obtained, the excursion is provisionally named the “Rubjerg Excursion”.     

Regular spacing of drainage outlets from linear mountain belts

Abstract Straight sections of many actively uplifting mountain belts have simple patterns of drainage, transverse to their main structural trend. Streams rising near or beyond the topographic ridgepole of these sections are spaced at seemingly regular intervals. To test whether this regularity exists, morphometric aspects of drainage networks were measured in 11 mountain belts. The spacing of drainage basins can be expressed using a spacing ratio, which in effect is the ratio of the length and the width of the catchments under consideration. Average spacing ratios for most linear mountain belts are within a narrow range of values between 1.91 and 2.23. A linear relationship exists between the spacing of catchment outlets and the distance between the main divide and the front of the mountain belt in which they have developed. The Nepalese Himalaya form an exception to this regular pattern. In this mountain belt drainage is blocked and diverted by structures that have developed in relation to the Main Boundary Thrust. Structural complications cause drainage patterns to become less regular, introducing important non trans verse components. The linear relationship between spacing of catchment outlets and half-width of the mountain belt may be expressed in an equation of the same general form as Hack's law of stream length and drainage basin area. It seems likely that the mechanism underlying Hack's law also explains the consistent regularity of drainage spacing in active mountain belts. However, no generally accepted explanation for Hack's law has been offered. The narrow range of spacing ratios found for drainage networks in active orogens may represent an optimal catchment geometry that embodies a ‘most probable state’ in the uplift-erosion system of a linear mountain belt. The linear relationship between the half-width of a mountain belt and spacing of catchment outlets has profound implications for the modelling of erosion of orogenic topography, and for the formation and filling of foreland basins.     

Transient heat flow in a stratified medium

The equation of heat conduction is solved for a horizontally stratified medium initially at constant temperature and subject to a step-function surface temperature change. The problem is solved by Laplace transformation and by applying a concept of further stratifications of the medium into unitary layers of constant ratio of thickness to the square root of thermal diffusivity. The solution takes the form of an infinite sum of complementary error functions, with coefficients given by recursion relations, is suitable for numerical applications and offers an attractive alternative to harmonic and quasi transient approaches in calculating the penetration of transient surface temperature variations into a layered medium. Use of the theorem of superposition yields a general expression for an arbitrary surface temperature function. The utility of the theory is illustrated by modelling examples of palaeoclimatically induced subsurface temperature and heat flow perturbations.     

Palaeomagnetic distortion modelling and possible recovery by inversion

A robust finite-element technique is presented for computation of both the internal demagnetization effects and magnetic terrain effects in bodies with arbitrary shape and arbitrary susceptibility distribution. This method facilitates a flexible analysis of the palaeomagnetic deflection problem. Tests on geologically realistic settings of highly magnetic rocks demonstrate that deflections of several degrees may occur even for relatively simple two-dimensional models. Similarly, the magnetic intensity may well be biased by 5-15% by demagnetization effects. The present paper focuses on deflections and intensity variations inside the magnetized body, where we find a systematic shallowing of inclination for bodies with a horizontal elongation. Because the bodies sampled at a typical palaeomagnetic site will have a dominant direction of elongation, the magnetic deflection effect will tend to impose a systematic bias which doesn’t average out. An inversion-based procedure for elimination of the deflection effect is presented. It requires that the magnetic body is quite homogeneous and that its surface geometry is known, as may be the case for historical lava flows. Tests demonstrate that in order to recover both ambient palaeofield direction and the effective susceptibility at blocking temperature it is necessary to sample near strong topographic elements in the magnetic body. Since the surface geometry rarely is known it is proposed as an alternative to inversion that an effective susceptibility is assessed and a horizontal slab correction is applied for samples taken far from topographical features. When shape geometry is unknown and no correction applied, palaeomagnetic conclusions must take into account the possible bias from internal demagnetization and magnetic terrain effects.     

New paleomagnetic pole and magnetostratigraphy of Faroe Islands flood volcanics, North Atlantic igneous province

A paleomagnetic sampling was carried out along four sections (altogether 86 lava flows, 548 samples) in the North Atlantic Igneous Province outcropping in Faroe Islands, Denmark. The four polarity zones in the 700-m-thick exposed part of the Faroes lower formation can be correlated with the geomagnetic polarity time scale as C26n-C25r-C25n-C24r. The seven lava flows erupted during C25n indicate a very low eruption rate in the upper part of the Faroes lower formation of ∼1/70 kyr. The Faroes middle and upper formations (composite thickness ∼2300 m) are all reversely magnetized corresponding to C24r. The eruption rate at the onset of middle formation volcanism was very high as evidenced by several thick lava sequences recording essentially spot readings of the paleomagnetic field. The shift in eruption rate between the Faroes lower and middle formations and evidence that onset of the Faroes middle formation volcanism took place in C24r are of particular importance, placing onset of middle formation volcanism in close temporal relation to North Atlantic continental break-up and the late Paleocene thermal maximum. After grouping flows recording the same field directions, we obtained 43 independent readings of the paleomagnetic field, yielding a paleomagnetic pole with coordinates 71.4°N, 154.7°E (A₉₅=6.0°, K=14, N=43); age 55-58 Ma. The pole is supported by a positive reversal test. Paleosecular variation, estimated as the angular standard deviation of the virtual geomagnetic pole distribution 21.7°+3.9°/−2.8°, is close to expected for the given age and paleolatitude. Our new Faroes paleomagnetic pole is statistically different from the majority of previously published poles from the British and Faroes igneous provinces, and we suggest that these older data should be used with care.     

Dissolved organic matter (DOM) export to a temperate estuary: seasonal variations and implications of land use

Inputs of dissolved carbon, nitrogen, and phosphorus were assessed for an estuary and its catchment (Horsens, Denmark). Seasonal patterns in the concentrations of DOM in the freshwater supply to the estuary differed depending on the soil and drainage characteristics of the area. In streams draining more natural areas the, patterns observed were largely driven by seasonal temperature fluctuations. The material exported from agricultural areas was more variable and largely controlled by precipitation events. Positive exponential relationships were found between the nitrogen and phosphorus loading, and the percentage of catchment area used for agriculture. Colored DOM (CDOM) loading measurements were found to be a good predictor of dissolved organic carbon (DOC) loading across the different subcatchments, offering a rapid and inexpensive alternative of operationally monitoring DOC export. For all the dissolved nutrient inputs to the estuary, dissolved inorganic nitrogen (DIN) and dissolved organic phosphorus dominated the loadings. Although 81% of the nitrogen annually supplied to the estuary was DIN, 83% of the nitrogen exported from the estuary was dissolved organic nitrogen (DON). Results show that increasing the area of the catchment covered by forest and natural pastures would have a positive effect on the trophic status of the estuary, leading to a considerable decrease in the phosphorus loading and a shift in the nitrogen loading from DIN to DON. Such a change in land use would also increase the export of DOC and CDOM to the estuary having the potential to increase oxygen consumption and reduce the photic depth.     

Geomagnetic variations recorded in Older (≫ 23 000 B.P.) and Younger Yoldia Clay (∼ 14 000 B.P.) at Nørre Lyngby, Denmark

Summary. Detailed palaeomagnetic results from a rapidly deposited 12.5-m Younger Yoldia Clay sequence of age around 14 000 B.P. at the coast cliff at Nørre Lyngby (northern Jutland, Denmark) and a 2-m Older Yoldia Clay sequence of age somewhere between 23 000 and 40 000 B.P. are presented and discussed. The Younger Yoldia Clay spans some 1000–1500 yr and shows swings in inclination and declination of about that period, and also more rapid oscillations which are particularly marked in inclination, showing that rapid secular variations as have occurred during historic times were indeed also present back in time. There exist easterly declinations of 80° to 90° in the upper half of the Younger Yoldia Clay which cause the virtual geomagnetic pole to migrate clockwise to around 50° away from the rotation pole. This we name the 'Nørre Lyngby declination excursion'.
In the Older Yoldia Clay, as well as secular variations in both declination and inclination, significant low inclination values are found, confirming the existence of the recently named 'Rubjerg low inclination excursion', with the virtual geomagnetic pole moving first in a clockwise then in an anticlockwise sense at 40° to 60° away from the rotation pole.
It is therefore inferred that models for the Earth's geomagnetic field should involve at least local rapid eastward as well as westward 'drift' of the non-dipole field components at various times in the past.