T.H.E. Heaton

Holocene climate variability in Europe: Evidence from δ18O, textural and extension-rate variations in three speleothems

Time-series O isotope profiles for three U–Th dated stalagmites have revealed that for much of the Holocene, a site on the Atlantic seaboard (SW Ireland) exhibits first-order δ18O trends that are almost exactly out of phase with coupled δ18O curves from two southern European sites (SE France and NW Italy). In the Irish stalagmite (CC3 from Crag Cave, SW Ireland), low δ18O at 10,000 cal yr BP reflects cool conditions. By the early to mid-Holocene (9000–6000 cal yr BP) δ18O had increased, reflecting the onset of warmer conditions on the Atlantic seaboard. This shift to higher δ18O was accompanied by a marked increase in the stalagmite extension rate, reinforcing our interpretation that this was a period of relative warmth. Except for an episode of increased extension rate about 5500 yr ago, δ18O in the Crag stalagmite exhibits a gradual decrease, accompanied by declining extension rates between 7800 and 3500 cal yr BP, interpreted as a cooling trend. There is evidence for increases in both δ18O and stalagmite extension rate in the period from 3500 cal yr BP to the present day suggesting a return to warmer conditions on the Atlantic seaboard. In the stalagmite from NW Italy (ER76, Grotta di Ernesto, Trentino province) the early-Holocene (c. 9200-7800 cal yr BP) is characterised by high δ18O, probably indicative of warm and/or dry conditions. Exceptionally low δ18O from 7800 to 6900 cal yr BP at this site reflects a well-defined wet phase (Cerin wet phase). In the last three millennia, this stalagmite exhibits a shift to lower δ18O, interpreted as some combination of cooler and/or wetter conditions. Unlike the Irish stalagmite, the Italian sample does not show a correlation between δ18O and extension rate. Instead, its extension rate correlates roughly with δ13C, presumably reflecting a climate-driven vegetation change. In the early Holocene, δ18O in the French stalagmite (CL26, Grotte de Clamouse, Herault province, SE France) was low relative to its Holocene average. For much of the period since c. 3500 cal yr BP this stalagmite exhibits higher δ18O than in the early Holocene, suggesting warmer conditions. Like the Irish stalagmite, the French sample exhibits a well-defined correlation between δ18O and extension rate. Had drip-water availability been the dominant control on δ18O at this semi-arid site then higher δ18O would have been accompanied by lower, not higher extension rates. This suggests strongly that temperature rather than rainfall amount was the dominant control at this site. While conclusions regarding the patterns of climate variability on a continent scale must remain tentative because of the limited number of stalagmites studied we argue that early Holocene warm conditions on the Atlantic seaboard (Irish site) coincided with relatively cool conditions at the Clamouse site. By c. 3500 yr ago the pattern appears to have been reversed.

Crop manuring and intensive land management by Europe’s first farmers

The spread of farming from western Asia to Europe had profound long-term social and ecological impacts, but identification of the specific nature of Neolithic land management practices and the dietary contribution of early crops has been problematic. Here, we present previously undescribed stable isotope determinations of charred cereals and pulses from 13 Neolithic sites across Europe (dating ca. 5900–2400 cal B.C.), which show that early farmers used livestock manure and water management to enhance crop yields. Intensive manuring inextricably linked plant cultivation and animal herding and contributed to the remarkable resilience of these combined practices across diverse climatic zones. Critically, our findings suggest that commonly applied paleodietary interpretations of human and herbivore δ15N values have systematically underestimated the contribution of crop-derived protein to early farmer diets.

Potential canopy influences on the isotopic composition of nitrogen and sulphur in atmospheric deposition

Abstract Isotopic studies of nitrogen and sulphur inputs to plant/soil systems commonly rely on limited published data for the 15N/14N and 34S/32S ratios of nitrate, ammonium and sulphate in rainfall. For systems with well-developed plant canopies, however, inputs of these ions from dry deposition or particulates may be more important than rainfall. The manner in which isotopic fractionation between ions and gases may lead to dry deposition and particulates having 15N/14N or 34S/32S ratios different from those of rainfall is considered. Data for rainfall and throughfall in coniferous plantations are then discussed, and suggest that: (1) in line with expectations, nitrate washed from the canopy has 15N/14N ratios higher than those in rainfall; (2) the 15N/14N ratios of ammonium washed from the canopy are variable, with high ratios being found for canopies of higher pH in conditions of elevated ambient ammonia gas concentrations; and (3) in accord with expectations and previous work, 34S/32S ratios of sulphate washed from the canopy are not substantially different from those in rainfall. The study suggests that if atmospheric inputs are relevant to isotopic studies of the sources of nitrogen for canopied systems, then confident interpretation will require analysis of these inputs.

Faunal stability during the Early Oligocene climatic crash

Abstract Traditional Neo-Darwinism views species as highly flexible entities which adapt to climatic change by gradually evolving new morphologies. Of the 177 species of mammals from the upper Eocene-lower Oligocene (37-30 Ma) White River Group in the High Plains, most species are static for 2.4 million years on average, and some persist much longer. Only three examples of gradualism can be documented in the entire fauna, and these are mostly size changes. Contrary to expectations, most mammalian species show no change during the earliest Oligocene climatic crash (33.2 Ma), in spite of the fact that the vegetation changed from dense forests to open forested grassland, mean annual temperatures dropped 13°C, and conditions got much drier and more seasonal. Only a few mammalian lineages speciated, a few more went extinct, and the vast majority (62 out of 70) persisted through this climatic event with no observable response whatsoever. This evidence shows that organisms are much less responsive to the environment than short-term neontological studies suggest.

Last interglacial conditions in southern Europe: evidence from Ioannina, northwest Greece

A new record combining isotopic and palynological results over the interval 133-111 ka BP at 100-200 year resolution from a long lacustrine sequence at Ioannina, northwest Greece is presented. The sequence provides an opportunity to examine the nature of climate variability during the Last Interglacial in southern Europe where information has hitherto been relatively limited. The record shows that the frequency and amplitude of changes during the transitional late glacial and late interglacial phases were markedly higher than that of the full interglacial interval. These differences are probably a reflection of the relative size of ice sheets in the circum-North Atlantic and associated ice-rafting events and climatic perturbations during these periods. During intervals of increased ice volume, it appears that North Atlantic variability has a significant downstream impact, dominating the climate signal in northwest Greece. During intervals of minimum ice volume there may be a decoupling between the North Atlantic system and continental climates with other factors, such as insolation changes, becoming more important. The length of the Last Interglacial at Ioannina defined by the presence of forest is here estimated to be c. 15 ka, in agreement with recent results from Portugal, but in conflict with estimates of c. 10 ka for the duration of the Eemian in northwest Europe. In the absence of independent confirmation for these estimates, however, these differences and associated implications remain unresolved.

Carbon and oxygen isotope fractionation in non-marine ostracods: Results from a 'natural culture' environment

Abstract Carbon and oxygen isotope analysis of ostracods living in the near-constant conditions of spring-fed ponds in southern England allowed accurate determination of the ostracod’s calcite-water 13C/12C and 18O/16O fractionations. The 13C/12C fractionations of two species, Candona candida and Pseudocandona rostrata, correspond to values expected for isotopic equilibrium with the pond’s dissolved inorganic carbon at the measured temperature (11°C) and pH (6.9), whilst those of a third species, Herpetocypris reptans, would represent equilibrium at a slightly higher pH (7.1). The 18O/16O fractionations confirm two previous studies in being larger, by up to 3‰, than those ‘traditionally’ regarded as representing equilibrium. When the measured fractionations are considered in the context of more recent work, however, they can be explained in terms of equilibrium if the process of calcite formation at the ostracod lamella occurs at a relatively low pH (≤7) irrespective of the pH of the surrounding water. The pH of calcite formation, and therefore the calcite-water 18O/16O fractionation, may be species and stage (adult versus juvenile) specific, and related to the rate of calcification.

Oxygen and sulfur isotopic composition of volcanic sulfate aerosol at the point of emission

Volcanic sulfate aerosol is emitted from the vents of many active volcanoes, but its origin has not yet been explained. We report the first measurements of the isotopic compositions of near-vent volcanic aerosol sulfate and use the combined sulfur and oxygen isotope systematics of the aerosol and magma at Masaya volcano, Nicaragua, to draw preliminary conclusions about the production mechanism of near-source volcanic sulfate. The sulfate aerosol does not display a significant mass-independent oxygen or sulfur isotopic signature, which suggests that primary volcanic aerosol does not contribute to the larger mass-independent anomalies found in some ambient atmospheric aerosols and as preserved in ice cores. The oxygen isotope composition of the sulfate appeared to become heavier on the particle filters with increasing amount of sample collected and suggests that the δ 18O value of the sulfate aerosol is <8‰. The sulfur isotopic composition of the sulfate did not vary with sample loading on the filters and had a mean δ 34S value of 7.7 ± 0.8‰, similar to that of the magma (6.6 ± 0.2‰). The low Δ17O value, the δ 18O value of the magma (6.6 ± 0.3‰) and the high δ 18O value of atmospheric oxygen (23.5‰) suggest that known atmospheric oxidation pathways at ambient temperatures are not the major routes of production for this sulfate. Instead, the isotopic systematics of aerosol and magma are consistent with sulfate production either by high-temperature equilibration of the magmatic gas mixture with small amounts of atmospheric oxygen or by direct emission of SO4 2− from the magma.

Carbon and oxygen isotope variations among lacustrine ostracods: implications for palaeoclimatic studies.

Large ranges of 13C/12C and 18O/ 16O ratios are found among modem ostracods from lakes in Jamaica and Mexico. If their calcite carapaces grew in isotopic equilibrium with the lake water, then the main period of carapace formation was probably in the cooler and/or wetter months. Differences in δ13C and δ18 O values between ostracods from different sites relate to spatial variations in the composition of the lake water, and differences between species at the same site probably reflect differences in organic photosynthesis or decay among the micro-habitats occupied by the species. There nevertheless remain significant variations in both δ13C and δ18O among modern valves of the same species at the same site; a consequence of the temporal and spatial variation in lake water and the fact that ostracods form their carapace over a short period. Individual valves from a 1-cm section of a sediment core show that the isotopic effects of changes in the rainfall versus evaporation status of a lake over a period of c. 102 years can be of a similar magnitude to changes recorded by bulk, fine-grained carbonate throughout the late Quaternary. These findings emphasize the need for caution when using isotopic data for single ostracod valves in palaeoclimatic studies.

Integrating botanical, faunal and human stable carbon and nitrogen isotope values to reconstruct land use and palaeodiet at LBK Vaihingen an der Enz, Baden-Württemberg

In this paper we reconstruct the palaeodietary setting of LBK Vaihingen an der Enz, south-west Germany (later sixth millennium cal. bc) using δ13C and δ15N values of human and faunal bone collagen and of charred plant remains from cereal crops (e.g. emmer and einkorn wheat) and pulses (lentil and pea). Our examination of this Neolithic dietary ‘food web’ incorporates crop δ15N values within a linear-mixing model to examine the estimated proportions of animal and plant protein in the human diet. We interpret the stable isotope dietary model outcomes together with accompanying archaeobotanical and zooarchaeological evidence to shed light on the role of crops in land use strategies and human diet, and conclude that (manured) crops probably formed the dominant protein source.

Carbon and oxygen isotope variations within the shell of an African land snail (Limicolaria kambeul chudeauiGermain): a high-resolution record of climate seasonality?:

Stable isotope analyses at high-resolution intervals along the growth axis of a shell of the land snail Limicolaria kambeul chudeaui Germain, collected in Ethiopia, show a cyclic pattern of d18O variation. The cycles have regular periodicity but vary in amplitude. The shape and amplitude of the shell d18O cycles is consistent with seasonal changes in the amount and isotopic composition of the rainfall, coupled with tempera ture and evaporative effects. The number of cycles (10) corresponds to the estimated lifespan of Limicolaria. Generally constant d13C values suggest unvarying diet, environmental CO2 influences, and metabolism throughout the snails life, except for one interval of large variations in both d13C and d18O that may be related to the onset of reproductive activity. If similar results can be obtained for fossil shells, they may provide a high-resolution record of past seasonal climate variability.