Stable Isotope Proxies Research Articles

New insight from old bones: stable isotope analysis of fossil mammals

Stable isotope analysis of fossil materials has become an increasingly important method for gathering dietary and environmental information from extinct species in terrestrial and aquatic ecosystems. The benefits of these analyses stem from the geochemical fingerprint that an animal's environment leaves in its bones, teeth, and tissues. Ongoing study of living mammals has found the stable isotopic composition of several light (hydrogen, carbon, nitrogen, oxygen, and sulfur) and even a few heavy (calcium and strontium) elements to be useful tracers of ecological and physiological information; many of these can be similarly applied to the study of fossil mammals. For instance, the carbon isotopic composition of an animal's tissues tracks that of the food it eats, whereas the oxygen isotopic compositions of the carbonate and phosphate in an animal's bones and teeth are primarily controlled by that of the surface water it drinks or the water in the food it ingests. These stable isotope proxies for diet and habitat information are independent of inferences based on morphological characters and thus provide a means of testing ecological interpretations drawn from the fossil record. As such, when well-preserved specimens are available, any dietary study of fossil species should seriously consider including this approach. To illustrate the potential benefits associated with applying these methods to paleontological research, a review of current work on the ecological and evolutionary history of fossil mammals through geochemical analysis is presented. After a brief introduction to issues associated with the preservation of stable isotopic information in soft and mineralized tissues, a series of case studies involving the application of stable isotope analysis to fossil mammal research is discussed. These studies were selected to highlight the versatility of this analytical method to paleontological research and are complemented by a discussion of new techniques and instrumentation in stable isotope analysis (e.g., laser ablation and compound-specific isotope ratio mass spectrometry, and calcium and clumped isotopes), which represent the latest advances in the extension of these geochemical tools to the paleontology of fossil mammals.

Changes in methanogenic substrate utilization and communities with depth in a salt-marsh, creek sediment in southern England

A combined biogeochemical and molecular genetic study of creek sediments (down to 65 cm depth) from Arne Peninsula salt-marsh (Dorset, UK) determined the substrates used for methanogenesis and the distribution of the common methanogens, Methanosarcinales and Methanomicrobiales capable of metabolising these substrates. Methane concentrations increased by 11 cm, despite pore water sulphate not being removed until 45 cm. Neither upward methane diffusion or anaerobic oxidation of methane seemed to be important in this zone. In the near-surface sulphate-reduction zone (5–25 cm) turnover time to methane for the non-competitive methanogenic substrate trimethylamine was most rapid (80 days), and were much longer for acetate (7900 days), methanol (40,500 days) and bicarbonate (361,600 days). Methylamine-utilizing Methanosarcinales were the dominant (60–95%) methanogens in this zone. In deeper sediments rates of methanogenesis from competitive substrates increased substantially, with acetate methanogenic rates becoming ∼100 times greater than H 2/CO 2 methanogenesis below 50 cm. In addition, there was a dramatic change in methanogen diversity with obligate acetate-utilizing, Methanosaeta related sequences being dominant. At a similar depth methanol turnover to methane increased to its most rapid (1700 days). This activity pattern is consistent with deeper methanogen populations (55 cm) being dominated by acetate-utilizing Methanosaeta with H 2/CO 2 and alcohol-utilizing Methanomicrobiales also present. Hence, there is close relationship between the depth distribution of methanogenic substrate utilization and specific methanogens that can utilize these compounds. It is unusual for acetate to be the dominant methanogenic substrate in coastal sediments and δ 13C-CH 4 values (−74 to −71‰) were atypical for acetate methanogenesis, suggesting that common stable isotope proxy models may not apply well in this type of dynamic anoxic sediment, with multiple methanogenic substrates.

High resolution climate reconstructions of recent warming using instrumental and ice core records from coastal Antarctica

In-situ observational record of Antarctic surface temperatures is rather sparse. Proxy based ice core studies are thus critical for reconstructing the past climate change on centennial and decadal time scales. The present study review the available instrumental and proxy records from the Dronning Maud Land region of East Antarctica as well as report recent evidences of Antarctic climate change and its global linkages. The monthly mean air temperature records of the Novolazarevskaya (Novo) station, which is the longest (since 1961) and continuous meteorological record in this region, revealed a significant warming trend at a rate of 0.25 °C / decade. To understand the spatial and temporal consistency of this warming, well-dated ice cores from the coastal Dronning Maud Land region were assessed. All proxy records consistently suggest an enhanced warming up to +0.12 °C / decade. This is further supported by a recent assessment of stable oxygen and hydrogen isotope proxy records from two high resolution ice cores (IND-25/B5 and IND-22/B4) from this region. Among these records, the IND-25/B5 provided ultra-high-resolution data for the past 100 years (1905-2005) and the IND-22/B4 core represented the past ~470 years (1530-2002) of Antarctic change. These ice records provided insights on the influence of solar forcing on Antarctic climate system as well as its linkages with the tropical and mid-latitude climatic modes like the Southern Annular Mode (SAM) and El Niño Southern Oscillation (ENSO). The calculated surface air temperatures using these records showed a warming by 0.06-0.1 °C / decade, with greatly enhanced warming during the past several decades (~0.4 °C / decade). It is confirmed that the coastal areas of Dronning Maud Land are indeed warming and the trend is apparently enhancing in the recent decades.

Controls on the Spatial Variability of Modern Meteoric 18O: Empirical Constraints from the Western U.S. and East Asia and Implications for Stable Isotope Studies

We analyze a dataset of multiannual modern precipitation and meteoric water records from the western U.S. (n = 206) and east Asia (n = 478) to (1) determine which environmental parameters correlate best with and account for spatial variability of meteoric water isotopic (δ18O and δD) compositions and (2) assess the degree to which this variability is a function of physiography and climatology. Multivariate linear regression analysis including five environmental parameters [latitude, longitude, elevation, mean annual temperature (MAT), mean annual precipitation (MAP)] reveals that latitude and elevation are consistently strongly correlated with meteoric δ18O distributions throughout much of the western U.S. and east Asia, but correlations with site longitude, MAT, and MAP can also be significant, depending on region. Our analysis also indicates that isotope-environment relationships are region-dependent. Isotope-elevation gradients are reduced by a factor of two or more in continental interior rainshadows (for example, Basin and Range) and high elevation continental plateaus (for example, Tibetan Plateau) in comparison to isotope-elevation gradients observed in orographic settings with a single dominant moisture source and relatively simple storm track trajectories (for example, Sierra Nevada and Himalaya). Published global and continental predictive equations for the calculation of modern meteoric water isotopic compositions that do not account for this regional variability are shown to be characterized by significant predictive uncertainties for modern δ18O values (∼ ±3-4‰), particularly in regions of complex moisture source interactions, continental moisture recycling and increased convective storm activity where reduced isotope-elevation gradients are also observed. We present new empirical relationships between environmental parameters and isotopic compositions that inform stable isotope-based reconstructions of climate change, timing and location of groundwater recharge, and paleoaltimetry by providing quantitative constraints on the statistical relationship between precipitation δ18O and individual environmental parameters (for example, latitude, elevation, temperature) and identifying how isotopic relationships are influenced by physiography and climatology. In particular, paleoelevation estimates in regions of low-magnitude isotope-elevation gradients (Basin and Range, Tibet) are shown to be prone to significant uncertainties (in some cases > ±2 km). These relationships improve the interpretation of stable isotope proxy data, particularly in cases where the paleogeographic setting of proxy formation can be constrained.

Climate variability in the Early Pliocene Arctic: Annually resolved evidence from stable isotope values of sub-fossil wood, Ellesmere Island, Canada

Tree-ring analyses have contributed significantly to investigations of past climate. Stable isotope climate proxies (δ 18O, δD and δ 13C values) enhance traditional ring-width data, although poor preservation of ancient wood has tended to limit development of stable isotope proxy records to the Holocene and the Late Pleistocene. Here we apply stable isotope techniques to wood that represent the remains of Mixed-Coniferous Boreal Vegetation preserved in Early Pliocene (4–5 Ma) deposits at Strathcona Fiord, Ellesmere Island, Canada (ca. 78°N). Four well-preserved tree trunks, identified through wood anatomical characteristics as Larix (larch), from this high Arctic site provide annually resolved sequences of up to 250 years from which we developed a high-resolution record of Pliocene climate. Stable oxygen isotope values, in conjunction with ring-width measurements were used to derive annually resolved temperature records for this site. Our ring-width and isotope-based reconstructions provide an annually resolved record, up to 250 years, of temperature and indicate growing season (JJ) temperatures (15.8 ± 5.0 °C) 11.8 ± 5.1 °C, and mean annual temperatures (MAT) (− 1.4 ± 4.0 °C) 18.3 ± 4.1 °C warmer than present. Estimated isotope values of precipitation of − 16.3 ± 2‰ (δ 18O) and − 150.1 ± 8.9‰ (δD) were calculated from the isotopic values of wood cellulose. Relative humidity estimated from both δ 13C and δD records ranged from 60 to 80%. Paleotemperature, source water and humidity estimates are comparable to those of a modern Boreal Forest growing ca. 15–20° south of modern Ellesmere Island.

Sequence stratigraphy of the ANDRILL AND-2A drillcore, Antarctica: A long-term, ice-proximal record of Early to Mid-Miocene climate, sea-level and glacial dynamism

Present understanding of Antarctic climate change during the Early to Mid-Miocene, including major cycles of glacial expansion and contraction, relies in large part on stable isotope proxies from deep sea core drilling. Here, we summarize the lithostratigraphy of the ANDRILL Southern McMurdo Sound Project drillcore AND-2A. This core offers a hitherto unavailable ice-proximal stratigraphic archive from a high-accommodation continental margin setting, and provides clear evidence of repeated fluctuations in climate, ice expansion/contraction and attendant sea-level change over the period c. 20.2–14.2 Ma, with a more fragmentary record of Late Miocene and Pliocene time. The core is divided into seventy-four high-frequency (fourth- or fifth-order) glacimarine sequences recording repeated advances and retreats of glaciers into and out of the Victoria Land Basin. The section can be resolved into thirteen longer-term, composite (third-order) sequences, which comprise packages of higher frequency sequences that show a consistent stratigraphic stacking pattern (Stratigraphic Motif). The distribution of the six recognized motifs indicates intervals of less and more ice-proximal, and temperate to subpolar/polar climate, through the Miocene period. The core demonstrates a dynamic climate regime throughout the Early to Mid-Miocene that may be correlated to some previously-recognized events such as the Mid-Miocene Climatic Optimum, and provides a detailed reference point from which to evaluate stable isotope proxy records from the deep-sea.

Sedimentary geochemical response to human impact on Lake Nõmmejärv, Estonia

INTRODUCTION A number of palaeoecological investigations have proved that the geochemical composition of lake sediments serves as a valuable archive of information for the reconstruction of natural as well as human-induced processes that shape the present character of a lake's ecosystem (Engstrom & Wright, 1984; Birks & Birks, 2006; Battarbee et al., 2007). The use of chemical proxies is particularly important for analysis of sediments that accumulated during the last centuries when intensified human activities began to alter natural biogeochemical pathways, causing disturbance of natural systems at an elemental level (Boyle, 2001; Tylmann, 2005; Punning et al., 2007). Previous investigations show that during the last century the natural ecosystem of Lake Nommejarv in NE Estonia became affected by human activities that resulted in significant alterations in its sediment and water composition as well as in its ecological status (Vesiloo, 1987; Sagris, 1989; Varvas, 1994; Punning et al., 1997). As a result of the implementation of environmental protection measures and the political transition at the beginning of the 1990s, the industrial impact in the area connected with oil shale mining and power plants has become less pronounced. A complex palaeoecological study, which was carried out at the beginning of the 1990s, showed that this factor seems to positively contribute to a decrease of pollution connected with atmospheric fallout as well as to the overall tendency of the lake ecosystem to return to pre-disturbance conditions (Punning et al., 1997). Based on geochemical and stable isotope proxies, this study aims to describe the changes in the geochemical composition of the sediment as a response to the presumed reduction in the human impact on the lake during the last 20 years. It also attempts to detect what anthropogenic factors continue to control the present state of the lake. To document and understand the ways in which human impact on the ecosystems interacts with natural processes is one of the essential prerequisites for establishing sustainable and efficient land management (Oldfield & Dearing, 2003). STUDY AREA Lake Nommejarv is located in NE Estonia (58[degrees]03' N and 26[degrees]30' E) in the western part of the Kurtna lake district (Fig. 1). The lake is 15 ha in area and its maximum depth is 7 m (Varvas & Punning, 1993). Its catchment has a forested area in its western and eastern parts, peatland to the north, and arable land in the south (Fig. 1a). Human influence began to intensify at the end of the 19th century when drainage of the surrounding bogs and pastures started. During the 1920s, the natural flow of the Raudi Stream (Fig. 1a) was directed to the lake, changing it from a closed to an open system. Local human impact on the lake continued to increase with the construction of a military camp on the shore at the beginning of the 1930s, which was used then for military and later, including today, for tourist activities (Punning et al., 1997). At the beginning of the 1950s, oil shale mining began in the vicinity of the Kurtna lake district. Expansion of oil shale mining and related industries such as oil-shale-based power engineering became a strong industrial factor in the region (Erg, 2003). Local kukersite oil shale forms horizontal sequences in the limestone from which it has been extracted in underground mines. To facilitate the discharge of groundwater from mines, several Kurtna lakes have been utilized and interconnected with channels. In 1970 the Raudi Stream was completely transformed into an artificial channel and mine water started to flow through Lake Nommejarv. Variation in the discharge has been largely governed by seasonal fluctuations and the volume of mining waters (from 25 000 to 54 000 [m.sup.3] [h.sup.-1]) (Sagris, 1989). Water is also carrying mine clastic mineral debris, which consists of carbonate matter, siliciclastic (silica-bearing sedimentary rock) particles, and oil shale remains and is high in sulphate ions and calcium (Bauert & Kattai, 1997; Erg, 2003). …

Cenozoic migration of topography in the North American Cordillera

Continental topography is the result of complex interactions among mantle convection, continental dynamics, and climatic and erosional processes. Therefore, topographic evolution of mountain belts and continental interiors reflects directly upon the coupling between mantle and surface processes. It has recently been proposed that the modern topography of western North America is partly controlled by the removal of the subducting Farallon plate and replacement of lithospheric mantle by hot asthenosphere, creating surface uplift of the Colorado Plateau, the southwestern United States, and northern Mexico, while concomitant subsidence characterizes the central United States. How the topography of the Cenozoic North American Cordillera evolved in the past is largely unknown, yet currently debated tectonic models each have a predictable topographic response. Here we examine Cenozoic surface uplift patterns of western North America based on a record of ∼3000 stable isotope proxy data. This data set is consistent with Eocene north to south surface uplift in the Cordillera, culminating in the assembly of an Eocene–Oligocene highland 3–4 km in elevation. The diachronous record of surface uplift and associated magmatism further supports tectonic models calling for the convective removal of mantle lithosphere or removal of the Farallon slab by buckling along an east-west axis. The Eocene–Oligocene development of rainout patterns similar to present-day patterns along the flanks of the Cordilleran orogen is therefore unlikely to be the result of late Mesozoic crustal thickening and associated development of an Andean-style Altiplano.

A 150-year record of recent changes in human activity and eutrophication of Lake Wushan from the middle reach of the Yangze River, China

In order to determine baseline conditions (pre-impact) and recent changes to lakes on the middle reach of the Yangtze River, China, a lake sediment core was extracted from Lake Wushan covering the last ca 150 years. Detailed chemical, biological (subfossil chironomids), and physical analyses of the lake sediments were undertaken. The data showed consistent trends of increased productivity since the early 1920s, notably significant changes in the chironomid fauna which were associated with changes in the sedimentological and stable isotope proxies. More typically eutrophic chironomid taxa first appeared around this time that had not been present in the lake since at least the 1860s. Further increases in productivity occurred around the 1950s which coincided with the local decline and extirpation of some chironomid taxa, particularly macrophyte associated taxa, which had been present in the lake since at least the late 19th Century. A chironomid-inferred water total phosphorus (CI-TP) reconstruction produced accurate levels of water TP compared with contemporary measurements (207.4 μg L-1 TP), and suggested that levels for the late 19th Century were relatively low (50-60 μg L-1 TP). These reconstructions illustrate the baseline levels that existed pre-impact and provide potential targets for restoration, but they also show the magnitude of human impact in this region, which has increased the nutrient content of Lake Wushan fourfold within the last ca 100 years.

Stable isotope variation in tooth enamel from Neogene hippopotamids: monitor of meso and global climate and rift dynamics on the Albertine Rift, Uganda

The Neogene was a period of long-term global cooling and increasing climatic variability. Variations in African-Asian monsoon intensity over the last 7 Ma have been deduced from patterns of eolian dust export into the Indian Ocean and Mediterranean Sea as well as from lake level records in the East African Rift System (EARS). However, lake systems not only depend on rainfall patterns, but also on the size and physiography of river catchment areas. This study is based on stable isotope proxy data (18O/16O, 13C/12C) from tooth enamel of hippopotamids (Mammalia) and aims in unravelling long-term climate and watershed dynamics that control the evolution of palaeolake systems in the western branch of the EARS (Lake Albert, Uganda) during the Late Neogene (7.5 Ma to recent). Having no dietary preferences with respect to wooded (C3) versus grassland (C4) vegetation, these territorial, water-dependant mammals are particularly useful for palaeoclimate analyses. As inhabitants of lakes and rivers, hippopotamid tooth enamel isotope data document mesoclimates of topographic depressions, such as the rift valleys and, therefore, changes in relative valley depth instead of exclusively global climate changes. Consequently, we ascribe a synchronous maximum in 18O/16O and 13C/12C composition of hippopotamid enamel centred around 1.5–2.5 Ma to maximum aridity and/or maximum hydrological isolation of the rift floor from rift-external river catchment areas in response to the combined effects of rift shoulder uplift and subsidence of the rift valley floor. Structural rearrangements by ~2.5 Ma within the northern segment of the Albertine Rift are well constrained by reversals in river flow, cannibalisation of catchments, biogeographic turnover and uplift of the Rwenzori horst. However, a growing rain shadow is not obvious in 18O/16O signatures of the hippopotamid teeth of the Albertine Rift. According to our interpretation, this is the result of the overriding effect of evaporation on 18O/16O responding to aridification of the basin floor by a valley air circulation system through relative deepening of the valley. On the other hand, a synchronous arid pulse is not so clearly recorded in palaeosol data and mammalian fauna of the eastern branch of the EARS. This discrepancy indicates that rift mesoclimates may represent an underestimated aspect in previous palaeoclimate reconstructions from rift valley data and represent a clear limitation to attempts at global climate reconstructions. The results of this study also suggest that using 18O/16O data as a proxy to rain shadow evolution must take into account relative basin subsidence to properly document mountain range uplift.

Stable isotope characterization of mammalian predator–prey relationships in a South African savanna

This paper characterizes predator–prey interactions amongst African mammals from C4 savanna environments using stable carbon and nitrogen isotope proxies for diet. Stable carbon (δ13C) and nitrogen (δ15N) isotope data from hair and faeces of large African mammal carnivores, and herbivores as potential prey, are presented for a diverse range of taxa. Carbon-isotope data imply that most carnivores from the “lowveld” savanna of South Africa form part of C4 grass-based food webs. Nitrogen isotope data show clear differences between trophic levels, although it appears that the magnitude of these differences varies between predators feeding on invertebrates and vertebrates, respectively. Whilst the number of carnivore samples for which data are available is relatively few, and data for prey are restricted mainly to large ungulate herbivores, results clearly demonstrate the potential for future applications of this technique to predator–prey food webs in African savannas. In tandem with traditional approaches, stable isotopes can help elucidate patterns of predator impacts on prey populations, domestic livestock, and resolving similar food webs in palaeoenvironmental contexts.

Testing seasonal resolution trace element and stable isotope proxies of East Asian monsoon rainfall

Testing seasonal resolution trace element and stable isotope proxies of East Asian monsoon rainfall

Modern stable isotopic (δ18O, δ2H, δ13C) variation in terrestrial, fluvial, estuarine and marine waters from north‐central Sarawak, Malaysian Borneo

AbstractStable isotope data on humid tropical hydrology are scarce and, at present, no such data exist for Borneo. Delta18O, δ2H and δ13C were analysed on 22 water samples from different parts of the Sungai (river) Niah basin (rain, cave drip, rainforest pool, tributary stream, river, estuary, sea) in north‐central Sarawak, Malaysian Borneo. This was done to improve understanding of the modern stable isotope systematics of the Sungai Niah basin, essential for the palaeoenvironmental interpretation of the Late Quaternary stable isotope proxies preserved in the Great Cave of Niah. The Niah hydrology data are put into a regional context using the meteoric water line for Southeast Asia, as derived from International Atomic Energy Agency/World Meteorological Organization isotopes in precipitation network data. Although the Niah hydrological data‐set is relatively small, spatial isotopic variability was found for the different subenvironments of the Sungai Niah basin. A progressive enrichment occurs towards the South China Sea (δ18O −4·6‰; δ2H −29·3‰; δ13C −4·8‰) from the tributary stream (δ18O −8·4‰; δ2H −54·7‰; δ13C −14·5‰) to up‐river (δ18O c. −8‰; δ2H c. −51‰; δ13C c. −12‰) and down‐river values (δ18O c. −7·5‰; δ2H c. −45‰; δ13C c. −11‰). This is thought to reflect differential evaporation and mixing of different components of the water cycle and a combination of depleted biogenic δ13C (plant respiration and decay) with enriched δ13C values (due to photosynthesis, atmospheric exchange, mixing with limestone and marine waters) downstream. Cave drip waters are relatively enriched in δ13C as compared to the surface waters. This may indicate rapid degassing of the cave drips as they enter the cave atmosphere. Copyright © 2005 John Wiley & Sons, Ltd.

Shifts in depth of water extraction and photosynthetic capacity inferred from stable isotope proxies across an ecotone of Juniperus osteosperma (Utah juniper) and Artemisia tridentata (big sagebrush)

Summary 1 In western North America, juniper trees (Juniperus spp.) are apparently encroaching into numerous communities including sagebrush-dominated (Artemisia tridentata) valleys, where, as density of juniper increases, the density and condition of sagebrush decline but juniper condition appears unaffected. 2 We examined stable isotope proxies of plant gas exchange and relative depth of soil water extraction of Juniperus osteosperma and Artemisia tridentata as their relative densities changed across a transition zone in northern Utah, USA. Measurement of 13C and 18O of foliage allowed separation of the contributions of stomatal and biochemical factors to differences in mean intercellular CO2 concentration, while deuterium composition of stem water served as an indicator of the relative depth of water extraction. 3 Leaf δ13C of juniper and, to a lesser extent, of sagebrush increased with decreasing density of juniper. Foliage 18O did not vary significantly with juniper density, indicating that this was primarily caused by an increase in photosynthetic capacity rather than decreased supply of CO2. Increased foliage nitrogen concentration of both species with decreasing juniper density is also consistent with increased photosynthetic capacity. 4 The much greater mean age of juniper trees in the high juniper density plots may explain their lower photosynthetic capacity. 5 Averaged across densities, juniper always extracted water deeper in the profile than did sagebrush, but in June and August, only the high juniper density plots showed an effect. Sagebrush is either restricted from using water at greater depth in the presence of juniper at high density or was in such poor condition that deep-water sources were not necessary to maintain their reduced function. 6 Ecotones can serve as useful, spatially constrained stages for examining resource partitioning and relationships among plant species. We observed spatial and temporal variation in resource use and partitioning of limited resources that will enable better design of manipulative experiments to explicitly examine competition.

Hydrological factors in the interpretation of stable isotopic proxy data present and past: a European perspective

In the context of this paper, stable isotopic proxies are those preserving directly, or via known or calculable fractionation factors, a record of 18O/ 16O and/or 2H/ 1H in the water which contributed to their growth. Suitable proxies include amongst others tree-ring cellulose, bone phosphate, lake sediment minerals and speleothem fluid inclusions. A principal aim of studying isotopic proxies is to gain more information about past climates through a knowledge of how rainfall isotope compositions have changed over time. However, converting back from proxy to rainfall is not necessarily straightforward, because however well the isotopic fractionation due to proxy growth or formation is understood, a variety of modifying processes may have intervened between this and the actual rainfall events. Three main reservoirs of environmental water may contribute to the isotopic compositions of terrestrial proxy indicators: surface waters, soil moisture, and groundwater. Each of these can in principle be isotopically modified compared to rainfall in the locality. This review examines these effects and how they may change under different European climate types at the present day. It is concluded that while groundwater isotopic compositions typically represent closely the bulk composition of rainfall under a variety of climatic regimes, soil and particularly lake waters require detailed study if their isotope systematics are to be adequately understood. The review also finds that to a large extent “the present is the key to the past” in terms of recharge processes during the late Pleistocene, though glaciation may have had profound isotopic consequences in certain areas, while a more general aridity led to the apparent absence of recharge in most areas for much of the late Devensian.

Diagenetic overprinting of the sphaerosiderite palaeoclimate proxy: are records of pedogenic groundwater δ18O values preserved?

AbstractMeteoric sphaerosiderite lines (MSLs), defined by invariant δ18O and variable δ13C values, are obtained from ancient wetland palaeosol sphaerosiderites (millimetre‐scale FeCO3 nodules), and are a stable isotope proxy record of terrestrial meteoric isotopic compositions. The palaeoclimatic utility of sphaerosiderite has been well tested; however, diagenetically altered horizons that do not yield simple MSLs have been encountered. Well‐preserved sphaerosiderites typically exhibit smooth exteriors, spherulitic crystalline microstructures and relatively pure (> 95 mol% FeCO3) compositions. Diagenetically altered sphaerosiderites typically exhibit corroded margins, replacement textures and increased crystal lattice substitution of Ca2+, Mg2+ and Mn2+ for Fe2+. Examples of diagenetically altered Cretaceous sphaerosiderite‐bearing palaeosols from the Dakota Formation (Kansas), the Swan River Formation (Saskatchewan) and the Success S2 Formation (Saskatchewan) were examined in this study to determine the extent to which original, early diagenetic δ18O and δ13C values are preserved. All three units contain poikilotopic calcite cements with significantly different δ18O and δ13C values from the co‐occurring sphaerosiderites. The complete isolation of all carbonate phases is necessary to ensure that inadvertent physical mixing does not affect the isotopic analyses. The Dakota and Swan River samples ultimately yield distinct MSLs for the sphaerosiderites, and MCLs (meteoric calcite lines) for the calcite cements. The Success S2 sample yields a covariant δ18O vs. δ13C trend resulting from precipitation in pore fluids that were mixtures between meteoric and modified marine phreatic waters. The calcite cements in the Success S2 Formation yield meteoric δ18O and δ13C values. A stable isotope mass balance model was used to produce hyperbolic fluid mixing trends between meteoric and modified marine end‐member compositions. Modelled hyperbolic fluid mixing curves for the Success S2 Formation suggest precipitation from fluids that were < 25% sea water.

Variation of Labrador Sea Water formation over the Last Glacial cycle in a climate model of intermediate complexity

The variation of North Atlantic Deep Water (NADW) formation over the Last Glacial cycle, from Oxygen Isotopic Substage 5e (OIS-5e; the Eemian) to future global warming projections, is investigated using the UVic Earth System Climate Model. The results are compared with available micropaleontological and stable isotope proxy paleo-reconstructions. Equilibrium simulations for the Eemian ( 125 kyr BP) and the Last Glacial Maximum (LGM— 21 kyr BP) both reveal the absence of Labrador Sea Water (LSW) formation although NADW formation still occurs, albeit at a reduced rate relative to the modern times. For the Eemian, the location of convection in the eastern North Atlantic is similar to the present, although it is generally shallower and less extensive. In the case of the LGM, deep convection has moved southward to the western coast of Europe and is much more localised. The inferred inception of a modern-like circulation slightly before 7 kyr BP revealed by proxy reconstructions is not captured by the model unless the meltwater forcing from the Laurentide ice sheet is applied in a long transient simulation. This raises questions concerning the applicability of equilibrium simulations in capturing the early Holocene climate. In all global warming projections, the LSW formation initially ceases as atmospheric CO 2 rises, but recovers once the level is held fixed in the atmosphere. Convection in the north extends further into the Nordic Seas as the sea ice edge retreats. In all simulations convection remains active in the eastern North Atlantic, with its latitude depending on the position of the sea ice edge, suggesting that the formation of lower NADW is a robust feature of Late Quaternary climate. As the Labrador Sea is found to be very sensitive to the freshwater forcing, it suggests that this region represents an ideal location for the concentration of observational studies to monitor a possible oceanic response to anthropogenic climate change.

An Altitudinal And Stable Carbon Isotope Survey of C3 and C4 Graminoids On Mount Kenya

An altitudinal survey of grasses and sedges was conducted on the Sirimon and Chogoria tracks on Mount Kenya to supplement previous surveys restricted to the Timau track. Thirty seven grasses and twenty three sedges were recorded and stable carbon isotope analysis was used to identify the photosynthetic pathway (C3 or C4) used by these species. The occurrence of a group of C4 graminoids was confirmed close above the tree line, and the literature suggests this may also hold for the Aberdares Range and other East African mountain massifs. Lower altitude graminoids are C3 forest species, and this distribution upsets the assumption that C4 gives way to C3 with increasing altitude. The significance of this is discussed in relation to the interpretation of palaeoenvironments through carbon stable isotope proxies.