Rapid change in early Holocene environments inferred from Lake Pupuke,Auckland City,New Zealand

Sediment cores from Lake Pupuke in Auckland City, New Zealand, contain a high‐resolution millennial to centennial‐scale record of changing climate and catchment hydrology spanning the past ca. 10 000 years. Here, we focus on the period between 9500 ± 25 and 7000 ± 155 cal. yr BP during which grain size, diatom palaeoecology, biogenic silica concentrations, sediment elemental and carbon isotope geochemistry reflect changes in sediment sources and lake conditions, with a significant event commencing at ca. 8240 cal. yr BP, commensurate with a lowering of lake level, faster erosion rates and increased sediment influx with a duration of ca. 360 yrs. However, the changes in the lake are not reflected in the terrestrial vegetation, where the pollen record indicates that podocarp forest dominated the Auckland region, with apparent environmental stability during this part of the early Holocene. The synchronous change in most of the proxies between ca. 8240 and 7880 cal. yr BP at Lake Pupuke indicates the presence of a sustained episode of relatively low lake level and concomitant increased rate of erosion in the early Holocene that appears to be at least partly coeval with the 8200 cal. yr BP meltwater event proposed for the North Atlantic region. Copyright © 2008 John Wiley & Sons, Ltd.     

Evaluation of an empirical orthogonal function–based method to downscale soil moisture patterns based on topographical attributes

Soil moisture influences many hydrologic applications including agriculture, land management and flood prediction. Most remote‐sensing methods that estimate soil moisture produce coarse resolution patterns, so methods are required to downscale such patterns to the resolutions required by these applications (e.g. 10‐ to 30‐m grid cells). At such resolutions, topography is known to affect soil moisture patterns. Although methods have been proposed to downscale soil moisture based on topography, they usually require the availability of past high‐resolution soil moisture patterns from the application region. The objective of this article is to determine whether a single topographic‐based downscaling method can be used at multiple locations without relying on detailed local observations. The evaluated downscaling method is developed on the basis of empirical orthogonal function (EOF) analysis of space–time soil moisture data at a reference catchment. The most important EOFs are then estimated from topographic attributes, and the associated expansion coefficients are estimated on the basis of the spatial‐average soil moisture. To test the portability of this EOF‐based method, it is developed separately using four data sets (Tarrawarra, Tarrawarra 2, Cache la Poudre and Satellite Station), and the relationships that are derived from these data sets to estimate the EOFs and expansion coefficients are compared. In addition, each of these downscaling methods is applied not only for the catchment where it was developed but also to the other three catchments. The results suggest that the EOF downscaling method performs well for the location where it is developed, but its performance degrades when applied to other catchments. Copyright © 2011 John Wiley & Sons, Ltd.     

Assessment of δ13C and C/N ratios in bulk organic matter as palaeosalinity indicators in Holocene and Lateglacial isolation basin sediments,northwest Scotland

Carbon isotopes (δ¹³C) and C/N ratios from bulk organic matter have recently been used as alternative proxies for relative sea‐level (RSL) reconstruction where there are problems associated with conventional biological indictors. A previous study on a single isolation basin (Upper Loch nan Eala) in northwest Scotland has shown a clear relationship between δ¹³C, C/N ratios and palaeosalinity from Younger Dryas and Holocene aged sediments. In this paper we present results of δ¹³C and C/N ratio analyses from other isolation basins in northwest Scotland over the Holocene and the Lateglacial period in order to validate this technique. The results from the Holocene sequences support the earlier findings that this technique can be used to identify RSL change from isolation basins over the Holocene in this region. The relationship between δ¹³C, C/N ratios and RSL change is not apparent in sediments of Lateglacial age. Other environmental variables such as atmospheric CO₂ concentration, poor vegetation development and temperature influence δ¹³C values during this period. Copyright © 2007 John Wiley & Sons, Ltd.     

A finite element approach to the simulation of hydraulic fractures with lag

We presented a finite‐element‐based algorithm to simulate plane‐strain, straight hydraulic fractures in an impermeable elastic medium. The algorithm accounts for the nonlinear coupling between the fluid pressure and the crack opening and separately tracks the evolution of the crack tip and the fluid front. It therefore allows the existence of a fluid lag. The fluid front is advanced explicitly in time, but an implicit strategy is needed for the crack tip to guarantee the satisfaction of Griffith's criterion at each time step. We enforced the coupling between the fluid and the rock by simultaneously solving for the pressure field in the fluid and the crack opening at each time step. We provided verification of our algorithm by performing sample simulations and comparing them with two known similarity solutions. Copyright © 2012 John Wiley & Sons, Ltd.