Cambridge deep Ocean geophone

We describe recent mechanical andeelectronic modifications to the Cambridge Ocean Bottom Hydrophone system, enabling it to record in addition three geophone channels from a deployed, disposable geophone package. Examples of data from seismic refraction experiments show good correspondence between records of ground motion detected by the hydrophone and the vertical geophone. Seismic signals are undistorted by noise from instrument related sources. Clear examples of P to S conversions just below the receiver are observed. Improved recording conditions are achieved by deploying the geophones in a small pressure vessel as far away as possible from the main instrument package.     

Diagnosing snow accumulation errors in a rain‐snow transitional environment with snow board observations

Diagnosing the source of errors in snow models requires intensive observations, a flexible model framework to test competing hypotheses, and a methodology to systematically test the dominant snow processes. We present a novel process‐based approach to diagnose model errors through an example that focuses on snow accumulation processes (precipitation partitioning, new snow density, and snow compaction). Twelve years of meteorological and snow board measurements were used to identify the main source of model error on each snow accumulation day. Results show that modeled values of new snow density were outside observational uncertainties in 52% of days available for evaluation, while precipitation partitioning and compaction were in error 45% and 16% of the time, respectively. Precipitation partitioning errors mattered more for total winter accumulation during the anomalously warm winter of 2014–2015, when a higher fraction of precipitation fell within the temperature range where partition methods had the largest error. These results demonstrate how isolating individual model processes can identify the primary source(s) of model error, which helps prioritize future research.     

Seismic‐stratigraphic constraints on the age of the Faroe Islands Basalt Group,Faroe–Shetland region,Northeast Atlantic Ocean

Lower Palaeogene extrusive igneous rocks of the Faroe Islands Basalt Group (FIBG) dominate the Faroese continental margin, with flood basalts created at the time of breakup and separation from East Greenland extending eastwards into the Faroe‐Shetland Basin. This volcanic succession was emplaced in connection with the opening of the NE Atlantic; however, consensus on the age and duration of volcanism remains lacking. On the Faroe Islands, the FIBG comprises four main basaltic formations (the pre‐breakup Lopra and Beinisvørð formations, and the syn‐breakup Malinstindur and Enni formations) locally separated by thin intrabasaltic sedimentary and/or volcaniclastic units. Offshore, the distribution of these formations remains ambiguous. We examine the stratigraphic framework of these rocks on the Faroese continental margin combining onshore (published) outcrop information with offshore seismic‐reflection and well data. Our results indicate that on seismic‐reflection profiles, the FIBG can be informally divided into lower and upper seismic‐stratigraphic packages separated by the strongly reflective A‐horizon. The Lower FIBG comprises the Lopra and Beinisvørð formations; the upper FIBG includes the Malinstindur and Enni formations. The strongly reflecting A‐horizon is a consequence of the contrast in properties of the overlying Malinstindur and underlying Beinisvørð formations. Onshore, the A‐horizon is an erosional surface, locally cutting down into the Beinisvørð Formation; offshore, we have correlated the A‐horizon with the Flett unconformity, a highly incised, subaerial unconformity, within the juxtaposed and interbedded sedimentary fill of the Faroe‐Shetland Basin. We refer to this key regional boundary as the A‐horizon/Flett unconformity. The formation of this unconformity represents the transition from the pre‐breakup to the syn‐breakup phase of ocean margin development in the Faroe–Shetland region. We examine the wider implications of this correlation considering existing stratigraphic models for the FIBG, discussing potential sources of uncertainty in the correlation of the lower Palaeogene succession across the Faroe–Shetland region, and implications for the age and duration of the volcanism.     

藏北高原地壳及上地幔导电性结构——超宽频带大地电磁测深研究结果

为了研究西藏中、北部壳、幔导电性结构,讨论高原中、北部岩石圈热状态,1998年和1999年（INDEPTH（Ⅲ） MT）在西藏中、北部完成了德庆—龙尾错（500线）和那曲—格尔木（600线）超宽频带大地电磁深探测剖面的研究.研究结果表明,西藏中、北部以昆仑山断裂为界,其南北壳、幔电性结构有很大差异.昆仑山断裂以北地壳和上地幔为高阻区.而昆仑山以南,地壳和上地幔的导电性有明显的分层结构：地壳上部以不连续的高阻体为主,夹有局部低阻异常体,沿南北方向上地壳的电性结构复杂,具有不连续、分块的特点;但中、下地壳为大范围的高导异常区,区内发育有大规模、不相连续、产状各异的高导体,其电阻率均小于4Ωm;在班公—怒江和金沙江缝合带之下,壳内高导体都具有向上地幔延伸的趋势,存在连通壳、幔的低阻通道.根据西藏高原中、北部壳、幔电性结构的研究推断：如同藏南一样,这里也普遍存在部分熔融体和热流体,它们的成因主要与班公—怒江和金沙江缝合带的壳-幔热交换、热活动有关,这是两期形成的壳-幔热交换通道.其中,班公—怒江缝合带的壳-幔热交换通道形成时间比金沙江缝合带早.因此,研究区壳、幔的热活动是从南边和西边开始,向北、向东扩展,导致现今西藏中、北部地壳和上地幔的热流分布由西向东、由南向北增大.     

Dissolved oxygen variations in alpine glacial meltwaters

Recent models of chemical weathering in alpine glacial meltwaters suggest that sulphide oxidation is a major source of solute in the distributed component of the subglacial hydrological system. This reaction requires O₂, and may lower dissolved oxygen levels to below saturation with respect to the atmosphere. This should result in an inverse association between SO₇^2- and dissolved oxygen saturation. However, measurements of O₂ saturation in bulk meltwaters draining the Haut Glacier d'Arolla, Switzerland, show that there is a positive association between SO₄^2- and O₂ saturation. The O₂ content of glacial meltwaters depends on the initial content of snow and icemelt, which may be controlled by the rate of melting, and the kinetic balance between O₂ losses (e.g. sulphide oxidation, microbial respiration) and gains (e.g. diffusion of O₂ into solution).     

Lower Pleistocene deltaic and marine sediments in boreholes from the central North Sea

Lower Pleistocene sediments recovered in boreholes from the Aberdeen Ground Formation in the central North Sea indicate that the unit was deposited in a delta front to prodelta/shallow, open shelf marine setting. Possible estuarine and clastic nearshore marine deposits have been identified on the western margin of the basin. The delta front sediments consist of interbedded, structureless to laminated sands and muds with organic debris, ferruginous nodules and common soft sediment deformation structures. Sporadic rippled and graded beds, basal scours to beds and starved ripples suggest periodic wave–current reworking. Prodelta/shelf marine sediments are predominantly argillaceous with only occasional thin sand beds and rare phosphatic bands. One exceptionally thick sand body or submarine channel-fill although this remains uncertain. The estuarine/clastic nearshore marine sediments include coarse channel-lag deposits and rippled and laminated subtidal sands. A rich microfossil assemblage recovered from the prodelta/shelf marine sequence indicates that deposition occurred under fluctuating climatic conditions.