Danny McCarroll

Comparison of stable carbon isotope ratios in the whole wood, cellulose and lignin of oak tree-rings

Abstract The stable carbon isotope ratios (δ13C) of whole wood, cellulose and acid-insoluble lignin from annual latewood increments of Quercus robur L., from modern and sub-fossil wood, were measured and their potential use as palaeoenvironmental indicators examined. The resulting time series demonstrate a very high degree of coherence, with δ13C of cellulose isotopically enriched by approximately 3‰ compared to δ13C of lignin. The δ13C values of all three components are influenced by the climate of July and August. Modern whole wood retains the strongest climate signal, perhaps because its composition is closest to that of leaf sugars. In sub-fossil wood there is no evidence that differential decay leads to fractionation of carbon within either cellulose or lignin, but differential decay can alter the cellulose to lignin ratio.

THE LAST ICE SHEET IN NORTH-WEST SCOTLAND: RECONSTRUCTION AND IMPLICATIONS

Recent models of the last Scottish ice sheet suggest that nunataks remained above the ice surface in areas peripheral to the main centres of accumulation. This proposition has been investigated on 140 mountains over an area of 10,000 km2 in NW Scotland. Outside the limits of the later Loch Lomond Readvance in this area there is evidence for a single high-level weathering limit that separates glacially eroded terrain from higher areas of in situ frost debris. This limit occurs at altitudes ranging from 425 to 450 m in the Outer Hebrides to >950 m on the mainland, and is best developed on lithologies that resisted breakdown after ice-sheet downwastage. Interpretation of this weathering limit as a periglacial trimline cut by the last ice sheet at its maximum thickness is supported by: (1) joint-depth and Schmidt hammer measurements that indicate significantly more advanced rock breakdown above the weathering limit; (2) a much greater representation of gibbsite (a pre-Late Devensian weathering product) in the clay fraction of soils above the limit; (3) cosmogenic isotope dating of the exposure ages of rock outcrops above and below the limit; (4) the sharpness of the limit at some sites and its regular decline along former ice flowlines; and (5) shear stress calculations based on the inferred altitude and gradient of the former ice surface. Reconstruction of the ice surface based on trimline evidence indicates that the mainland ice shed lay near or slightly east of the present watershed and descended northwards from >900 m to ca. 550 m at the north coast. Independent dispersion centres fed broad ice streams that occupied major troughs. On Skye an ice dome >800 m deflected the northwestwards movement of mainland ice, but the mountains of Rum were over-ridden by mainland ice up to an altitude of ca. 700 m. The Outer Hebrides supported an independent ice cap that was confluent with mainland ice in the Minches. Extrapolation of the trimline evidence indicates that most reconstructions of ice extent are too conservative, and suggests that low-gradient ice streams extended across the Hebridean Shelf offshore. Wider implications of this research are: (1) that blockfields and other periglacial weathering covers are not all of the same age or significance, depending on the resistance of different lithologies to frost weathering; (2) that the contrasting degree of glacial modification in the Western and Eastern Highlands of Scotland may reflect a former cover of predominantly warm-based ice in the former and predominantly cold-based ice in the latter; and (3) that the approach and techniques developed in this study have potential application for constraining ice-sheet models, not only in areas peripheral to the main centres of ice accumulation in Britain and Ireland, but also in other mountain areas where nunataks protruded through warm-based Late Pleistocene ice masses.

Stable carbon isotope ratios of Pinus sylvestris from northern Finland and the potential for extracting a climate signal from long Fennoscandian chronologies

Stable carbon isotope ratios were measured on the latewood cellulose of 36 Scots pine trees from four sites in northern Finland. δ13C values were corrected for changes in the δ13C of air and de-trended to remove the effect of tree age. Simple linear and multivariate correlations were used to determine the nature and strength of any climate signal. At three sites, the dominant controls are, in descending order, summer sunshine, temperature and antecedent precipitation. At the fourth site, the dominant controls are antecedent precipitation and air relative humidity, while summer sunshine and temperature appear unimportant. δ13C values record changes in the concentration of CO2 in the stomatal chambers, which reflects the balance between stomatal conductance and photosynthetic rate. Photosynthetic rate is controlled primarily by photon flux (sunshine) and temperature, suggesting that this dominates at three of the sites, whereas stomatal conductance is controlled by air humidity and soil moisture status (antecedent precipitation), suggesting that it dominates at the fourth site. The balance between these controls varies spatially and it is likely to have varied locally during the Holocene. Without some independent estimate of either stomatal conductance or photosynthetic rate, there is limited potential for using δ13C alone to extract a clear and consistent climate signal from the long Fennoscandian pine chronologies.

European summer temperatures since Roman times

The spatial context is criticalwhen assessing present-day climate anomalies, attributing them to potential forcings and making statements regarding their frequency and severity in a long-term perspective. Recent international initiatives have expanded the number of high-quality proxy-records and developed new statistical reconstruction methods. These advances allow more rigorous regional past temperature reconstructions and, in turn, the possibility of evaluating climate models on policy-relevant, spatiotemporal scales. Here we provide a new proxy-based, annually-resolved, spatial reconstruction of the European summer (June-August) temperature fields back to 755 CE based on Bayesian hierarchical modelling (BHM), together with estimates of the European mean temperature variation since 138 BCE based on BHM and composite-plus-scaling (CPS). Our reconstructions compare well with independent instrumental and proxy-based temperature estimates, but suggest a larger amplitude in summer temperature variability than previously reported. Both CPS and BHM reconstructions indicate that the mean 20th century European summer temperature was not significantly different from some earlier centuries, including the 1st, 2nd, 8th and 10th centuries CE. The 1st century (in BHM also the 10th century) may even have been slightly warmer than the 20th century, but the difference is not statistically significant. Comparing each 50 yr period with the 1951-2000 period reveals a similar pattern. Recent summers, however, have been unusually warm in the context of the last two millennia and there are no 30 yr periods in either reconstruction that exceed the mean average European summer temperature of the last 3 decades (1986-2015 CE). A comparison with an ensemble of climate model simulations suggests that the reconstructed European summer temperature variability over the period 850-2000 CE reflects changes in both internal variability and external forcing on multi-decadal time-scales. For pan-European temperatures we find slightly better agreement between the reconstruction and the model simulations with high-end estimates for total solar irradiance. Temperature differences between the medieval period, the recent period and the Little Ice Age are larger in the reconstructions than the simulations. This may indicate inflated variability of the reconstructions, a lack of sensitivity and processes to changes in external forcing on the simulated European climate and/or an underestimation of internal variability on centennial and longer time scales.

Latewood Width, Maximum Density, and Stable Carbon Isotope Ratios of Pine as Climate Indicators in a Dry Subalpine Environment, French Alps

Abstract Pine latewood width, density, and stable carbon isotope ratios were measured at two sites, separated in altitude by 400 m, close to the forest limit on a south-facing slope in the western French Alps. The signal to noise ratio in the δ13C series from each site is higher than that of either of the growth proxies. When the sites are combined, the high-frequency climate signal in the δ13C series is enhanced, whereas in both the ring width and density series it is weakened. Because regional climate dominates over local site conditions, δ13C ratios from long pine chronologies will provide a better indicator of past climate than either ring widths or densities. At dry Alpine sites, δ13C values are controlled mainly by stomatal conductance, which is linked to summer moisture stress and thus antecedent precipitation.

POTENTIAL AND LIMITATIONS OF 1THE SCHMIDT HAMMER FOR RELATIVE-AGE DATING: FIELD TESTS ON NEOGLACIAL MORAINES, JOTUNHEIMEN, SOUTHERN NORWAY*

In order to assess the potential of the Schmidt hammer for relative-age dating, rebound (R) values were collected from more than 50 sites on two glacier forelands in Jotunheimen, southern Norway. Four factors are found to influence results: instrument errors, lithology, weathering, and boulder-surface roughness. The influence of each is examined and its potential as a source of error assessed. Particular attention is paid to roughness, which may reflect either transport history or the differential weathering of minerals. On Little Ice Age moraines, where differential weathering of minerals is insignificant, variation in R-values parallels variation in boulder-surface roughness, which in turn is related to transport history. Boulder roundness provides a surrogate measure of primary bouldersurface roughness (prior to weathering) and assists in interpreting sites which yield anomalously low R-values. Although the Schmidt hammer has some potential for measuring rock weathering, and thus for interpreting relative age, it should be viewed only as a rapid, inexpensive, and convenient technique for preliminary assessment.

A 500-year record of summer near-ground solar radiation from tree-ring stable carbon isotopes

Tree-ring stable carbon isotope ratios (d13C) in environments of low moisture stress are likely to be controlled primarily by photosynthetic rate. Therefore, sunshine, rather than temperature, represents the more direct controlling factor. Temperature reconstructions based on tree-ring d13C results thus rest on the assumption that temperature and sunshine are strongly coupled. This assumption is tested using a d13C series from pine trees in NW Norway, where there are long (>100 yr) records of both summer temperature and cloud cover. It is demonstrated that when summer temperature and d13C diverge, summer temperature and cloud cover also diverge, and that cloud cover/sunshine may provide a stronger and more consistent parameter with which to calibrate tree-ring d13C series in this area. When a 500-year reconstruction of summer cloudiness is compared with a published reconstruction of summer temperatures in northern Sweden based on tree-ring maximum densities, the two time-series are largely parallel, with high levels of annual—decadal coherence. We identify, however, three distinct periods of lower frequency divergence: two (AD 1600—1650 and AD 1900—1927) when we propose summers were cool but sunny and one during the first half of the sixteenth century when summers were warm but cloudy. These episodes where temperature and sunshine decouple may represent large-scale changes in circulation as recorded in the Arctic Oscillation (AO) index. Strongly negative values of the summer AO index, as occurred during the early twentieth century, are associated with persistent high pressure over northern Norway and Fennoscandia, bringing cool summers with clear skies. Long reconstructions of cloudiness (near-ground radiation), based on tree-ring d13C series from suitable sites, would be extremely valuable for testing General Circulation Models (GCMs), because the generation of cloud is a strong control on temperature evolution, but remains a major source of uncertainty.

Spatial variability and temporal trends in water‐use efficiency of European forests

The increasing carbon dioxide (CO2 ) concentration in the atmosphere in combination with climatic changes throughout the last century are likely to have had a profound effect on the physiology of trees: altering the carbon and water fluxes passing through the stomatal pores. However, the magnitude and spatial patterns of such changes in natural forests remain highly uncertain. Here, stable carbon isotope ratios from a network of 35 tree-ring sites located across Europe are investigated to determine the intrinsic water-use efficiency (iWUE), the ratio of photosynthesis to stomatal conductance from 1901 to 2000. The results were compared with simulations of a dynamic vegetation model (LPX-Bern 1.0) that integrates numerous ecosystem and land-atmosphere exchange processes in a theoretical framework. The spatial pattern of tree-ring derived iWUE of the investigated coniferous and deciduous species and the model results agreed significantly with a clear south-to-north gradient, as well as a general increase in iWUE over the 20th century. The magnitude of the iWUE increase was not spatially uniform, with the strongest increase observed and modelled for temperate forests in Central Europe, a region where summer soil-water availability decreased over the last century. We were able to demonstrate that the combined effects of increasing CO2 and climate change leading to soil drying have resulted in an accelerated increase in iWUE. These findings will help to reduce uncertainties in the land surface schemes of global climate models, where vegetation-climate feedbacks are currently still poorly constrained by observational data.

Stable carbon isotope ratios of latewood cellulose in Pinus sylvestris from northern Finland: variability and signal-strength

Stable carbon-isotope ratios were measured on the latewood cellulose of 25 Pinus sylvestris trees at five sites from the Arctic Circle to the northern limit of pine in Finland. A correlation-based analysis of variance was used to define within and between-site signal strengths. Expressed Population Signals (EPS) from samples of five trees are very high (0.89 to 0.95). Combining 15 trees from the three northernmost sites yields an EPS of 0.97. Despite the very strong common signal, different trees yield absolute δ13C values offset by >2%, which is similar in magnitude to the variability within individual series that includes the effect of climate. Samples of. 10 trees are necessary to obtain mean absolute values with confidence limits <1% wide (p < 0.05). The common practice of pooling four cores from four trees to yield absolute δ13C values is thus inadequate for palaeoclimate research. Because the δ13C values are homeoscedastic they can be indexed by differencing from the mean of each tree series, retaining all of the information on relative differences within each tree but yielding much narrower confidence limits when series are averaged. When five trees are used, 80% of annual confidence bands are, < ± 0.5%(P<0.05). Indexing effectively masks low-frequency variations when overlapping tree series are combined to produce long chronologies.

A 1200-year multiproxy record of tree growth and summer temperature at the northern pine forest limit of Europe

Combining nine tree growth proxies from four sites, from the west coast of Norway to the Kola Peninsula of NW Russia, provides a well replicated (> 100 annual measurements per year) mean index of tree growth over the last 1200 years that represents the growth of much of the northern pine timberline forests of northern Fennoscandia. The simple mean of the nine series, z-scored over their common period, correlates strongly with mean June to August temperature averaged over this region (r = 0.81), allowing reconstructions of summer temperature based on regression and variance scaling. The reconstructions correlate significantly with gridded summer temperatures across the whole of Fennoscandia, extending north across Svalbard and south into Denmark. Uncertainty in the reconstructions is estimated by combining the uncertainty in mean tree growth with the uncertainty in the regression models. Over the last seven centuries the uncertainty is < 4.5% higher than in the 20th century, and reaches a maximum of 12% above recent levels during the 10th century. The results suggest that the 20th century was the warmest of the last 1200 years, but that it was not significantly different from the 11th century. The coldest century was the 17th. The impact of volcanic eruptions is clear, and a delayed recovery from pairs or multiple eruptions suggests the presence of some positive feedback mechanism. There is no clear and consistent link between northern Fennoscandian summer temperatures and solar forcing.