Are response function representations of the global carbon cycle ever interpretable?

Response function models are often used to represent the behaviour of complex, high order global carbon cycle (GCC) and climate models in applications which require short model run times. Although apparently black-box, these response function models need not necessarily be entirely opaque, but instead may also convey useful insights into the properties of the parent model or process. By exploiting a transfer function (TF) framework to analyse the Lenton GCC model, this paper attempts to demonstrate that response function representations of GCC models can sometimes also provide structural information on the parent model from which they are identified and calibrated. We take a fifth-order TF identified from the impulse response of the Lenton model atmospheric burden, and decompose this to show how it can be re-expresses in a generic five-box form in sympathy with the structure of the parent model.     

A MODELLING RECONSTRUCTION OF THE LAST GLACIAL MAXIMUM ICE SHEET AND ITS DEGLACIATION IN THE VICINITY OF THE NORTHERN PATAGONIAN ICEFIELD, SOUTH AMERICA

ABSTRACT. A time-dependent model is used to investigate the interaction between climate, extent and fluctuations of Patagonian ice sheet between 45° and 48°S during the last glacial maximum (LGM) and its subsequent deglaciation. The model is applied at 2 km resolution and enables ice thickness, lithospheric response and ice deformation and sliding to interact freely and is perturbed from present day by relative changes in sea level and equilibrium line altitude (ELA). Experiments implemented to identify an LGM configuration compatible with the available empirical record, indicate that a stepped ELA lowering of 750 to 950 m is required over 15000 years to bracket the Fenix I-V suite of moraines at Lago Buenos Aires. However, 900 m of ELA lowering yields an ice sheet which best matches the Fenix V moraine (c. 23000 a BP) and Caldenius' reconstructed LGM limit for the entire modelled area. This optimum LGM experiment yields a highly dynamic, low aspect ice sheet, with a mean ice thickness of c. 1130 m drained by numerous large ice streams to the western, seaward margin and two large, fast-flowing outlet lobes to the east. Forcing this scenario into deglaciation using a re-scaled Vostok ice core record results in an ice sheet that slowly shrinks by 25% to c. 14500 a bp , after which it experiences a rapid collapse, loosing some 85% of its volume in c. 800 years. Its margins stabilize during the Antarctic Cold Reversal after which it shrinks to near present-day limits by 11 000 a bp .     

Facies descriptions and associations in ancient reworked (?transgressive) shelf sandstones: Cambrian and Cretaceous examples

Preferred facies trends in ancient shallow-marine sediments have been determined by Markov-chain and substitutability analyses of detailed sections measured in Lower Cambrian outcrops in the Southern Canadian Rocky Mountains (St Piran Formation, 600 m of vertical section; Fort Mountain Formation, 500 m of vertical section), and in a Cretaceous subsurface reservoir in south-central Alberta (Viking Formation in the Garrington Field: 69 cores). Those sections with more defined cycles are interpreted to result from progradation of shelf-ridge sands and gravels; examples with less defined order originated as inter-ridge deposits, in zones with complex palaeoflows and in areas of subdued palaeotopographic relief. The highest degree of facies organization (> 80% significant facies transitions) occurs in the Cretaceous Viking Formation sandstone and conglomerate ridges, interpreted to have had abundant sediment supply, water depths between fair- and storm-weather wave-base, and a strong littoral current system. A moderate degree of facies organization (70% significant facies transitions) occurs in the inter-ridge deposits of the Cambrian Fort Mountain Formation, interpreted to have had a variable sediment supply, water depths above storm-weather wavebase, and a variable, oscillating tidal current system. A poor degree of facies organization (< 45% significant facies transitions) occur in the ridge deposits of the Cambrian Fort Mountain and St Piran formations and in the inter-ridge deposits of the Cambrian St Piran and Cretaceous Viking formations. This facies disorganization occurs in sites where there was continuous and rapid subsidence (Cambrian) or in areas of subdued seafloor relief with complex current systems (Cretaceous). The pattern of facies relationships also varies. Relatively short linear patterns occur in all of the shelf-ridge deposits. This contrasts with the inter-ridge deposits in which there are long linear patterns or complex ring structures, interpreted to result from complex palaeoflow patterns in areas of subdued seafloor topography. Various statistical techniques (including Markov-chain and substitutability analyses) can aid the interpretation of very complex lithological successions, as shown by the shelf sandstones and conglomerates examined in this study.     

An electromagnetic device for automatic detection of bedload motion and its field applications

A new sensor for the continuous and unmanned detection and recording of bedload motion is described. The sensor acts in the same fashion as a conventional metal detector but is elongate and installed permanently in the bed of an alluvial channel. Artificial clasts are labelled with short lengths of ferrite rod and are seeded upstream from the sensor, replacing bed particles. The entrainment of seeded clasts takes them over the sensor where they distort the magnetic field and produce a change in inductance that is detected and recorded. A field installation demonstrates the value of the sensor by revealing for the first time in coarse-grained alluvium the spasmodic nature of particle motion reminiscent of kinematic waves. It also illustrates the importance of pebble clusters in delaying particle entrainment.     

Architectural analysis of a volcaniclastic jökulhlaup deposit, southern Iceland: sedimentary evidence for supercritical flow

The 1918 eruption of the glacially capped Katla volcano, southern Iceland, generated a violent jökulhlaup, or glacial outburst flood, inundating a large area of Mýrdalssandur, the proglacial outwash plain, where it deposited ca 1 km³ of volcaniclastic sediment. The character of the 1918 jökulhlaup is contentious, having been variously categorized as a turbulent water flow, a hyperconcentrated flow or as a debris flow, based on localized outcrop analysis. In this study, outcrop‐based architectural analyses of the 1918 deposits reveal the presence of lenticular and tabular bedsets associated with deposition from quasi‐stationary antidunes and down‐current migrating antidunes, and from regular based bedsets, associated with transient chute‐and‐pool bedforms, all of which are associated with turbulent, transcritical to supercritical water flow conditions. Antidune wavelengths range from 24 to 96 m, corresponding to flow velocities of 6 to 12 m sec^?1 and average flow depths of 5 to 19 m. This range of calculated flow velocities is in good agreement with estimates made from eyewitness accounts. Architectural analysis of the 1918 jökulhlaup deposits has led to an improved estimation of flow parameters and flow hydraulics associated with the 1918 jökulhlaup that could not have been achieved through localized outcrop analysis. The observations presented here provide additional sedimentological and architectural criteria for the recognition of deposits associated with transcritical and supercritical water flow conditions. The physical scale of sedimentary architectures associated with the migration of bedforms is largely dependent on the magnitude of the formative flow events or processes; sedimentary analyses must therefore be undertaken at the appropriate physical scale if reliable interpretations, regarding modes of deposition and formative flow hydraulics, are to be made.     

Climate change and 'anomalous' glacier fluctuations: the southwest outlets of Mrdalsjökull, Iceland

Evidence of past glacier fluctuations is valuable palaeoenvironmental data, but determining their relationship to climatic change is sometimes complex because of differing glacier sensitivities and patterns of response. In Iceland, a diverse range of glaciation creates changing geographical patterns of response to climatic changes. The outlet glaciers of the Márdalsjökull ice cap in southern Iceland have produced detailed, but differing, records of change. For a key southwestern sector of the ice cap, we specifically searched for evidence equivalent to the c . 4500 BP, c . 3100 BP and c . 1200 BP advances of Sólheimajökull reported earlier. A combination of geomorphological mapping and dating by tephrochronology and lichenometry was used to constrain the glacier advances and determine the relative magnitude of Neoglacial glacier episodes. This is a key step towards creating a record of the changes for the entire ice cap. Major glacier advances c . 4500–1000 BP previously identified on the southern margin of Márdalsjökull are shown not to have occurred in this sector, where Neoglacial maxima occur post-1755 AD.     

Late Quaternary ice sheet, lake and sea history of southwest Scandinavia – a synthesis

Based on a large number of new boreholes in northern Denmark, and on the existing data, a revised event‐stratigraphy is presented for southwestern Scandinavia. Five significant Late Saalian to Late Weichselian glacial events, each separated by periods of interglacial or interstadial marine or glaciolacustrine conditions, are identified in northern Denmark. The first glacial event is attributed to the Late Saalian c. 160–140 kyr BP, when the Warthe Ice Sheet advanced from easterly and southeasterly directions through the Baltic depression into Germany and Denmark. This Baltic ice extended as far as northern Denmark, where it probably merged with the Norwegian Channel Ice Stream (NCIS) and contributed to a large discharge of icebergs into the Norwegian Sea. Following the break up, marine conditions were established that persisted from the Late Saalian until the end of the Early Weichselian. The next glaciation occurred c. 65–60 kyr BP, when the Sundsøre ice advanced from the north into Denmark and the North Sea, where the Scandinavian and British Ice Sheets merged. During the subsequent deglaciation, large ice‐dammed lakes formed before the ice disintegrated in the Norwegian Channel, and marine conditions were re‐established. The following Ristinge advance from the Baltic, initiated c. 55 kyr BP, also reached northern Denmark, where it probably merged with the NCIS. The deglaciation, c. 50 kyr BP, was followed by a long period of marine arctic conditions. Around 30 kyr BP, the Scandinavian Ice Sheet expanded from the north into the Norwegian Channel, where it dammed the Kattegat ice lake. Shortly after, c. 29 kyr BP, the Kattegat advance began, and once again the Scandinavian and British Ice Sheets merged in the North Sea. The subsequent retreat to the Norwegian Channel led to the formation of Ribjerg ice lake, which persisted from 27 to 23 kyr BP. The expansion of the last ice sheet started c. 23 kyr BP, when the main advance occurred from north–northeasterly directions into Denmark. An ice‐dammed lake was formed during deglaciation, while the NCIS was still active. During a re‐advance and subsequent retreat c. 19 kyr BP, a number of tunnel‐valley systems were formed in association with ice‐marginal positions. The NCIS finally began to break up in the Norwegian Sea 18.8 kyr BP, and the Younger Yoldia Sea inundated northern Denmark around 18 kyr BP. The extensive amount of new and existing data applied to this synthesis has provided a better understanding of the timing and dynamics of the Scandinavian Ice Sheet (SIS) during the last c. 160 kyr. Furthermore, our model contributes to the understanding of the timing of the occasional release of large quantities of meltwater from the southwestern part of the SIS that are likely to enter the North Atlantic and possibly affect the thermohaline circulation.     

A method for the disaggregation of mudstones

We describe a method which effectively and gently disaggregates muds and mudstones by combining a saturation-freeze-thaw technique with treatment by ultrasonic probe. The method works well for samples containing <5% TOC and which have been buried to depths less than about 3500 m. It does not work for samples strongly cemented with carbonates or silicates. Scanning electron micrographs of size fractions and the strong mineralogical segregation between size fractions are offered as evidence of complete disaggregation.