Frequency-Dependent Climate Signal in Upper and Lower Forest Border Tree Rings in the Mountains of the Great Basin

We examine the relationships, over the past millennium, between tree-ring chronologies from long-lived pines at their upper and lower limits in four mountain ranges in and near to the semi-arid Great Basin. We confirm LaMarche's (1974a) finding, based on a single mountain range in this same region, and a much shorter period of comparison, that climate responses are frequency dependent. In particular, upper and lower forest border chronologies in each mountain range are strongly coherent at decadal periods and less, with particular strength in the 3–7 year band. Thisvariability is significantly correlated with precipitation. Conversely, we find no significant correlation between the low frequency fluctuations (60 years and longer) of upper and lower forest border chronologies. There are, however, significant correlations between the low-frequency components of the upper forest border chronologies in the different ranges, consistent with their containing a growing season temperature signal on decadal time scales. The four upper forest border chronologies all show an anomalous increase in growth since the late 19th century, and an apparent change in climate control of ring growth.     

Tree-Ring Reconstructed Winter Precipitation and Tropical Teleconnections in Durango, Mexico

Earlywood width chronologies from Douglas-fir tree rings were used to reconstruct winter (November–March) precipitation for more than 600 years over Durango, Mexico. The tree-ring data were obtained from two sites of long-lived Douglas-fir in northern and southern Durango and the seasonal climatic precipitation data were regionally averaged from five weather stations well distributed across the state. The averaged earlywood chronology accounted for 56% of the variance in instrumental November–March precipitation 1942–1983. We validated the reconstruction against independent precipitation records. The worst winter drought of the 20th century in Durango occurred 1950–1965. However, the reconstruction indicates droughts more severe than any witnessed in the 20th century, e.g., the 1850s–1860s, and the megadrought in the mid- to late-16th century. Reconstructed winter precipitation 1540–1579 shows 33 of 40 years were dry. Persistent drought may be linked to extended La Niña episodes. The Tropical Rainfall Index (TRI) correlates well with instrumental and reconstructed winter precipitation (r = 0.49 and 0.55, respectively), reflecting the strong ENSO modulation of cool season climate over northern Mexico. The ENSO teleconnection varies through time, with TRI-reconstructed precipitation correlations ranging from 0.78 to 0.27 in five periods 1895–1993. The 1942–1983 winter observed and reconstructed Durango data correlate well with the corresponding seasonalization of the All-Mexico Rainfall Index (AMRI; r=0.68, P<0.0001 and r=0.70, P<0.001, respectively), indicating that both the observed and the reconstructed precipitation often reflect broad-scale precipitation anomalies across Mexico. New long Douglas-fir and baldcypress tree-ring chronologies are now available for central and southern Mexico near major population centers, allowing the exploration of relationships between drought, food scarcity, and social and political upheaval in Mexican history.     

Efficient hydrologic tracer-test design for tracer-mass estimation and sample-collection frequency, 1, method development

Hydrological tracer testing is the most reliable diagnostic technique available for the determination of basic hydraulic and geometric parameters necessary for establishing operative solute-transport processes. Tracer-test design can be difficult because of a lack of prior knowledge of the basic hydraulic and geometric parameters desired and the appropriate tracer mass to release. A new efficient hydrologic tracer-test design (EHTD) methodology has been developed to facilitate the design of tracer tests by root determination of the one-dimensional advection-dispersion equation (ADE) using a preset average tracer concentration which provides a theoretical basis for an estimate of necessary tracer mass. The method uses basic measured field parameters (e.g., discharge, distance, cross-sectional area) that are combined in functional relationships that describe solute-transport processes related to flow velocity and time of travel. These initial estimates for time of travel and velocity are then applied to a hypothetical continuous stirred tank reactor (CSTR) as an analog for the hydrological-flow system to develop initial estimates for tracer concentration, tracer mass, and axial dispersion. Application of the predicted tracer mass with the hydraulic and geometric parameters in the ADE allows for an approximation of initial sample-collection time and subsequent sample-collection frequency where a maximum of 65 samples were determined to be necessary for describing the predicted tracer-breakthrough curve (BTC). Inclusion of tracer retardation and decay cause a net increase in tracer-mass estimates so that the preset average tracer concentration will be maintained and there will be a consequent steepening of the BTC, but retardation also causes BTC spreading and a delay in tracer arrival.     

An assessment of the Sr/Ca ratio in shallow water hermatypic corals as a proxy for sea surface temperature

The high precision measurement of the Sr/Ca ratio in corals has the potential for measuring past sea surface temperatures at very high accuracy. However, the veracity of the technique has been questioned on the basis that there is both a spatial and temporal variation in the Sr/Ca ratio of seawater, and that kinetic effects, such as the calcification rate, can affect the Sr/Ca ratio of corals, and produce inaccuracies of the order of 2-4 °C. In the present study, a number of cores of the massive hermatypic scleractinian coral Porites, from the central Great Barrier Reef, have been analyzed for Sr/Ca at weekly to monthly resolution. Results from a 24 year record from Myrmidon Reef show an overall variation from 22.7 °C to 30.4 °C. The record shows a warming/cooling trend with maximum warming centred on the 1986-1987 summer. While some bleaching was reported to have occurred at Myrmidon Reef in 1982, the Sr/Ca record indicates that subsequent summer temperatures were much higher. The 4.5 year record from Stanley Reef shows a maximum SST of 30 °C during the 1997-1998 El Niño event. The calibrations from Myrmidon and Stanley Reefs are in excellent agreement with previously published calibrations from nearby reefs. While corals do not calcify in equilibrium with seawater due to physiological control on the uptake of Sr and Ca into the lattice of coralline aragonite, it can be argued that, provided only a single genus such as Porites sp. is used, and that the coral is sampled along a major vertical growth axis, then the Sr/Ca ratio should vary uniformly with temperature. Similarly, objections based on the spatial and temporal variability of the Sr/Ca activity ratio of seawater can be countered on the basis that in most areas where coral reefs grow there is a uniformity in the Sr/Ca activity ratio, and there does not appear to be a change in this ratio over the growth period of the coral. Evidence from several corals in this study suggest that stress can be a major cause of the breakdown in the Sr/Ca-SST relationship. Thermal stress, resulting from either extremely warm or cool temperatures, can produce anomalously low Sr/Ca derived SSTs as a result of the breakdown of the biological control on Sr/Ca fractionation. It is considered that other stresses, such as increased nutrients and changes in light intensity, can also lead to a breakdown in the Sr/Ca-SST relationship. Two of the main issues affecting the reliability of the Sr/Ca method are the calibration of the Sr/Ca ratio with measured SST and the estimation of tropical last glacial maximum (LGM) palaeotemperatures. Instead of producing a constant calibration, just about every one published so far is different from the others. What is obvious is that for most calibrations while the slope of the calibration equation is similar, the intercepts are not. While the cause for this variation is still unknown, it would appear that corals from different localities around the world are responding to their own particular environment or that certain types of environments exert a control on the corals’ physiology. Sr/Ca derived SST estimates for the LGM and deglaciation of 5 °C-6 °C cooler than present are at odds with estimates of 2 °C-3 °C cooling by other climate proxies. The apparent lack of reef growth during the LGM suggests that SSTs were too cold in many parts of the tropics for reefs to develop. This would lend support to the idea that tropical SSTs were much cooler than what the CLIMAP data suggests.     

A detailed palaeointensity and inclination record from drill core SOH1 on Hawaii

A new record of absolute palaeointensity was obtained from drill core Scientific Observation Hole 1 (SOH1) on Kilauea volcano, Hawaii. Kilauea’s high eruption rate resulted in a relatively continuous record and stratigraphic constraints preserved the chronological order. Three hundred and sixty samples were studied with the Thellier-Thellier technique, which gave 195 successful palaeointensity and 271 successful inclination determinations. Three geomagnetic excursions were observed, which exhibited intensity reductions of about 50%. Initial age control from K-Ar and Ar/Ar dating only constrained the total age between 20 and 120 ka. The final age model was obtained by stretching the SOH1 record relative to other Hawaiian palaeomagnetic data. This gave an age range of 0-45 ka for the flows and identified the excursions as the Hilina Pali, Mono Lake and Laschamp events. The SOH1 record of the Hilina Pali event is the most detailed ever, incorporating data from around 40 flows. This age model suggests that Kilauea had a burst of activity at the SOH1 site around 20 ka. All available data was combined to form a composite record of palaeointensity and inclination on Hawaii for 0-45 ka.     

A new approach to soil characterisation for hydrology–stability analysis models

Landslide stability analysis increasingly utilises high-resolution coupled hydrology–slope stability models (CHASM) to improve stability assessments in areas subject to dynamic pore pressure regimes. In such environments, the estimation of soil hydraulic conductivity (K) is a key parameter but one which is not always readily available or determined with the required resolution. By using basic soil particle-size distribution (PSD) data, we evaluate the microscopic composition of the actual soil, and applying the analytical relations obtain by a Self-Consistent Method (SCM) approach, we determine an appropriate value of K. This is of importance in that it allows within-soil type variability to be reflected in terms of K and hence within the model structure. The SCM methodology is briefly reviewed and an illustrative application is undertaken for a slope typical of Hong Kong. The results show model output sensitivity in terms of moisture content and factor of safety (FOS) when comparing K values determined using the SCM approach and the conventional field determination. In attempting to determine slope hydrological processes and attendant stability conditions, we conclude that the application of SCM approach offers a novel methodology for potentially improving the parameterisation of hydrology–slope stability models.     

Development of spatial regression models for predicting summer river temperatures from landscape characteristics: Implications for land and fisheries management

There is increasing demand for models that can accurately predict river temperature at the large spatial scales appropriate to river management. This paper combined summer water temperature data from a strategically designed, quality controlled network of 25 sites, with recently developed flexible spatial regression models, to understand and predict river temperature across a 3,000 km² river catchment. Minimum, mean and maximum temperatures were modelled as a function of nine potential landscape covariates that represented proxies for heat and water exchange processes. Generalised additive models were used to allow for flexible responses. Spatial structure in the river network data (local spatial variation) was accounted for by including river network smoothers. Minimum and mean temperatures decreased with increasing elevation, riparian woodland and channel gradient. Maximum temperatures increased with channel width. There was greater between‐river and between‐reach variability in all temperature metrics in lower‐order rivers indicating that increased monitoring effort should be focussed at these smaller scales. The combination of strategic network design and recently developed spatial statistical approaches employed in this study have not been used in previous studies of river temperature. The resulting catchment scale temperature models provide a valuable quantitative tool for understanding and predicting river temperature variability at the catchment scales relevant to land use planning and fisheries management and provide a template for future studies.     

Quantification of contemporary storm-induced boulder transport on an intertidal shore platform using radio frequency identification technology

Extreme storm events are known to produce, entrain, transport and deposit sizable boulders along rocky coastlines. However, the extent to which these processes occur under moderate, fetch-limited wave conditions is seldom considered. In this study we quantify boulder transport at a relatively sheltered location subject to high-frequency, low-magnitude storm activity. This was achieved by deploying radio frequency identification (RFID) tags within 104 intertidal limestone boulders ranging in size from fine to very coarse (intermediate axis: 0.27–2.85 m). The study was conducted over 3 years (July 2015–July 2018) and encompassed numerous storm events. Tagged boulders were relocated during 17 field surveys and their positions recorded using a differential global positioning navigation satellite system (DGNSS). On completion, we identified boulder displacement in 69% of the tagged array. The accrued boulder transport distance amounted to 233.0 m from 195 incidents of displacement, including the movement of a boulder weighing an estimated 11.9 t. Transport was not confined to autumn and winter storms alone, as displacement was also recorded during summer months (April–September), despite the seasonally reduced wave magnitude. Boulder production by wave quarrying was documented in three tagged clasts, confirming observations that the shore platform is actively eroding. Incidents of overturning during transport were also recorded, including multiple overturning of clasts weighing up to 5 t. We further identify a statistically significant difference (maximum p-value ≤ 0.03) between the transport distances attributed to constrained and unconstrained boulders, suggesting that the pre-transport morphological setting exerts considerable control over boulder transport potential. The findings establish low to moderate storm waves as a key component in the evolution of the study site. More broadly, we claim that high-frequency, low-magnitude storms regularly modify these overlooked rocky coastal locations, suggesting that the hydrodynamic capability at such sites may previously have been underestimated. © 2020 John Wiley & Sons, Ltd.     

Farewell to Wynand Kleingeld (1946 to 2020)

Mathematical Geosciences -     

Saturated permeability measurements on pumice and welded-tuffaceous materials

Fluid flows in consolidated porous media of volcanic origin are being investigated to support such diverse efforts as the modeling of thermal/outgassing phenomena at Mount St. Helens and the hydrological modeling of tuffaceous rocks in support of the Department of Energy’s (DOE) Nevada Nuclear Waste Storage Investigations Project An experimental apparatus was designed and built to allow water-saturated permeabilities as low as 10⁻¹⁸ m² to be measured on cores of diameter 5 cm and length 10 cm under steady-state flow conditions. This same apparatus can also be utilized in a transient (pressure-decay) mode in order to measure permeabilities several orders of magnitude lower than the steady-state limit. Tests were conducted on samples of pumice, fractured welded tuff, and welded tuff, representing a permeability range of seven orders of magnitude Pumice was found to have a permeability of ∼3×10⁻¹² m², sufficiently high to allow the complete Darcy-to-Ergun regime to be investigated Welded (unfractured) tuff was tested in the transient mode, yielding a permeability of ∼5×10⁻¹⁹ m². Two, long-time-scale, steady-flow experiments were conducted on a core of welded tuff containing a single, through-going fracture. For the first experiment, the core was an integral cylinder containing a naturally occurring fracture. For the second experiment, the core was separated into two pieces along the existing fracture plane, then rejoined. Effects of essentially constant, as well as rapidly varied, circumferential stress were studied in both tests. Results showed core permeability to decay to 2×10⁻¹⁸ m² in both cases, independent of the initial fracture state (closed versus open). With a naturally occurring fracture, core permeability decreased by a factor of 2 over a 200-h test period. With an initially open fracture, core permeability decreased by a factor of 4 under the influence of a comparable 200-h load-time history, after 700 h of testing, core permeability was reduced by an order of magnitude from its initial level. Final effective hydraulic fracture aperture was calculated to be 10⁻⁶ m, corresponding to a calculated effective fracture permeability of 10⁻¹³ m² Fracture flow was thus estimated to account for 80% of the total flow rate through this core under final test conditions.