Amorphous silica mobilization by inter‐rill erosion: insights from rainfall experiments

Amorphous silica (ASi) carried in suspension by rivers is an important component in the global Si budget. Water erosion processes in cultivated catchments are likely to drive ASi delivery to the river system. However, no studies have investigated the controls on ASi mobilization by erosional processes in croplands. Rainfall experiments were performed on split fields (i.e. a part conventionally ploughed and a part under reduced tillage) to simulate ASi mobilization by inter‐rill erosion in croplands, and identify its dependency on soil, field and rainfall characteristics. The ASi content of the soil and the inter‐rill erosion rate were determined as the major controls on ASi mobilization. Variables such as tillage technique and crop type did not have a consistent direct or indirect effect. Inter‐rill erosion is clearly selective with respect to ASi, indicating association of ASi with the fine soil fraction and with soil organic carbon. Our experiments demonstrate that erosion increases due to human perturbation will increase the delivery of reactive Si to aquatic systems. We estimate that globally, c. 7% of all reactive Si that enters aquatic systems is derived from erosion of agricultural soils. Copyright © 2015 John Wiley & Sons, Ltd.     

Readvance of the last British–Irish Ice Sheet during Greenland Interstade 1 (GI-1): the Wester Ross Readvance,NW Scotland

Fourteen samples obtained from Torridon sandstone boulders on four moraines marking the limit of the Wester Ross Readvance (WRR) in NW Scotland yielded tightly clustered ¹⁰Be exposure ages confirming contemporaneous or penecontemporaneous moraine deposition. Collectively, the 14 samples yield mean ages of 13.5 ± 1.2 ka to 14.0 ± 1.7 ka, depending on choice of geomagnetic scaling and sampling surface erosion rates. All fourteen moraine ages are significantly younger than an age of ca 16.3 ka previously proposed for the WRR, and also younger than most samples obtained from rock outcrops within the WRR limits. The ages obtained for the WRR moraines appear to confirm that a substantial cover of glacier ice persisted over low ground in NW Scotland during at least the early part of the Lateglacial Interstade (≈Greenland Interstade 1). We infer that the WRR probably occurred in response to rapid short-lived cooling during the Older Dryas climatic reversal (≈Greenland Interstade 1d), though the possibilities that the WRR represents ice-margin response to a later climatic reversal during the Lateglacial Interstade or stabilization and readvance of the ice margin following rapid offshore calving cannot be discounted.     

Sediment dynamics in the Eurasian Arctic Ocean during the last deglaciation — The clay mineral group smectite perspective

This study focuses on sedimentological investigations of sediment cores recovered during the international Arctic′91, expeditions with the German research ice breaker RV “Polarstern” to the European sector of the Arctic Ocean. Here, we deduce the last glacial/interglacial changes in transport mechanism and sedimentation from the clay mineral group smectite. We choose the smectites as an example of how sediment mineralogy can be linked with particular source regions (the Kara and Laptev seas), distinct transport mechanism (sea ice and surface currents) and sedimentation processes. Smectite contents in Arctic sediments discussed for two time slices, including the Last Glacial Maximum (LGM), and the last deglaciation (Termination I), reveal the highest variability subsequent to the retreat of the Eurasian ice sheets. Our results show that smectite anomalies in the Eurasian Basin are associated with distinct meltwater pulses and occurred around 13.5–13.0 ¹⁴C ka B.P. Compelling evidence is provided that these anomalies are deduced from sea-ice entrained sediments from the eastern Kara Sea that entered the Arctic Ocean after ice-sheet break-up and eventually flooding of the Kara Sea. We propose that smectite anomalies in sediments of the eastern Arctic Ocean can be utilized to identify deglacial events and to help decipher configurations of the Eurasian ice sheets. The identification of smectite maxima along the modern sea-ice edge in the Eurasian Basin further indicates biologically enhanced sedimentation from melting sea ice allowing the reconstruction of seasonally open water in the region. Hence, considering the poor preservation conditions of primary paleoceanographic proxies in the Arctic Ocean, the clay mineral contents, particularly the smectite group, may be one alternative tool for paleoclimatic reconstruction in the Eurasian Basin.     

Palaeoecological constraints on late Glacial and Holocene ice retreat in the Southern Andes (53°S)

Late Glacial to Holocene ice retreat was investigated along a 120 km long fjord system, reaching from Gran Campo Nevado (GCN) to Seno Skyring in the southernmost Andes (53°S). The aim was to improve the knowledge on regional and global control on glacier recession with special emphasis on latitudinal shifting of the westerlies. The timing of ice retreat was derived from peat and sediment cores, using mineralogical and chemical characteristics, and pollen as proxies. Stratigraphy was based on ¹⁴C-AMS ages and tephrochronology. The ice retreat of the Seno Skyring Glacier lobe is marked by an ice rafted debris layer which was formed around 18,300 to 17,500 cal. yr B.P. Subsequently, fast glacier retreat occurred until around 15,000 to 14,000 cal. yr B.P. during which around 84% of Skyring Glacier were lost. This fast recession was probably also triggered by an increase of the Equilibrium Line Altitude (ELA) from 200 to 300 m. Subsequently, the ice surface was lowered below the ELA in an area that previously made up more than 50% of the accumulation area. Much slower retreat and glacier fluctuations of limited extent in the fjord channel system northeast of GCN occurred between around 14,000 to 11,000 cal. yr B.P. during both the Antarctic Cold Reversal and the Younger Dryas. This slow down of retreat indicates a decline in the general warming trend and/or increased precipitation, due to a southward migration of the westerlies. After around 11,000 cal. yr B.P. pollen distribution shows evolved Magellanic Rainforest and similar climate as at present, which lasted throughout most of the Holocene. Only Late Neoglacial moraine systems were formed in the period 1220–1460 AD, and subsequently in the 1620s AD, and between 1870 and 1910 AD. The results indicate that the Gran Campo Nevado ice cap has reacted more sensitive and partly distinct to climate change, compared to the Patagonian Ice Field.     

Joint interpretation of the size and time distributions of seismic activity around the Hengill triple junction (SW Iceland) between 1993 and 1999

A newly developed clustering algorithm is described which uses the Unified Scaling Law for earthquakes to identify correlations between events that lead to clustering. This algorithm is applied to a dataset from the Hengill triple junction in south west Iceland to separate spatially and temporally overlapping earthquake sequences. We show that the algorithm successfully identifies spatio-temporally clustered events.A search for certain patterns in the identified clusters is performed. These patterns emerged in numerical simulations of seismicity performed by Hainzl (2003) when viscous coupling was introduced in order to reproduce features of swarm earthquakes. Namely we look for a combination of increased Gutenberg-Richter b-value and decreased exponent of the waiting-time distribution α. This pattern, indicating the strongest influence of viscous coupling on the characteristics of the seismicity, could be localized beneath an extinct volcano approximately 5 km north of an area of recent crustal uplift.