Changes In δ13C Research Articles

The evolution of deep‐ocean flow speeds and δ13C under large changes in the Atlantic overturning circulation: Toward a more direct model‐data comparison

AbstractTo investigate the dynamics of the Atlantic meridional overturning circulation (AMOC) on timescales longer than the observational records, model‐data comparisons of past AMOC variability are imperative. However, this remains challenging because of dissimilarities between different proxy‐based AMOC tracers and the difficulty of comparing these to model output. We present an iLOVECLIM simulation with tuned AMOC evolution and focus on AMOC tracers that are directly comparable to reconstructions: flow speeds and δ13C. We deduce their driving factors and show that they yield different but complementary information about AMOC changes. Simulated flow speed changes are only linked to AMOC changes in regions bathed by North Atlantic Deep Water; however, in those regions they do provide details on vertical migration and thickness changes of the water masses. Simulated δ13C changes in the North Atlantic Deep Water region are again related to AMOC changes. Yet in regions bathed by Antarctic Bottom Water or Antarctic Intermediate Water, the δ13C evolution is driven by Southern Hemisphere source water δ13C changes, while in the Nordic Seas and the two major overflow regions it is driven by Northern Hemisphere source water δ13C changes. This shows that AMOC changes are not necessarily recorded by δ13C and stresses the need for combining both tracers in paleoclimate studies. A preliminary model‐data comparison for Last Interglacial flow speeds and δ13C changes in the Deep Western Boundary Current shows that this integrated approach is far from straightforward and currently inconclusive on the Last Interglacial AMOC evolution. Nonetheless, the approach yields potential for more direct and in‐depth model‐data comparisons of past AMOC changes.

Changes in δ13C of dark respired CO2 and organic matter of different organs during early ontogeny in peanut plants

Carbon isotope composition in respired CO2 and organic matter of individual organs were measured on peanut seedlings during early ontogeny in order to compare fractionation during heterotrophic growth and transition to autotrophy in a species with lipid seed reserves with earlier results obtained on beans. Despite a high lipid content in peanut seeds (48 %) compared with bean seeds (1.5 %), the isotope composition of leaf- and root-respired CO2 as well as its changes during ontogeny were similar to already published data on bean seedlings: leaf-respired CO2 became 13C-enriched reaching −21.5 ‰, while root-respired CO2 became 13C-depleted reaching around −31 ‰ at the four-leaf stage. The opposite respiratory fractionation in leaves vs. roots already reported for C3 herbs was thus confirmed for peanuts. However, contrarily to beans, the peanut cotyledon-respired CO2 was markedly 13C-enriched, and its 13C-depletion was noted from the two-leaf stage onwards only. Carbohydrate amounts being very low in peanut seeds, this cannot be attributed solely to their use as respiratory substrate. The potential role of isotope fractionation during glyoxylate cycle and/or gluconeogenesis on the 13C-enriched cotyledon-respired CO2 is discussed.

Milk production and nutrient partitioning as measured by 13C enrichment of milk components during C3 and C4 plant feeding in purebred Holstein and in Charolais × Holstein F2 crossbred cows

Nutrient partitioning was investigated in cows with different genetic merits for milk production by measuring 13C/12C ratios (reported by delta values δ13C) in milk components in response to C3 (grass silage) and C4 diets (corn silage). We hypothesised that changes of δ13C in milk differ between Holstein (HOL; high milk production) and Charolais × Holstein cows with medium (CHM) and low (CHL) milk production. Changes of δ13C (Δδ13C) in milk components were estimated by calculating differences of δ13C due to switch from C3 to C4 feeding. After switch to C4 feeding, Δδ13C of lactose was greater in HOL than in CHL. Immediate Δδ13C of milk fat was the lowest in CHL. The maximal Δδ13C of casein was the lowest in HOL. The proportion of carbon in milk derived from diet increased with milk yield, indicating the main impact of the milk production level, but minor impact of breed, on nutrient partitioning towards the mammary gland.

Test stable carbon isotopic composition of soil organic matters as a proxy indicator of past precipitation: Study of the sand fields in northern China

Understanding the stable carbon isotopic composition (δ13C) of surface soil organic matter under various climates is crucial for quantitative reconstruction of past climatic changes. Although the relationship between precipitation and plant δ13C is well understood at global and regional scale, studies relating δ13C of surface soil organic matter to precipitation in northern China remain controversial. In this study, 90 surface soil samples were collected from the desert/loess transition zone of Northern China, where vegetation is sensitive to climatic changes. The stable carbon isotopic composition of organic matter in these soil samples was analyzed in order to examine its palaeoclimatic implications. Our results show that the δ13C varied from −26.56 to −19.41‰ among the investigated sites, with an average of −23.54‰. There was a strong negative correlation between the δ13C of surface soils and the mean annual precipitation (MAP) (R2 = 0.52, p < 0.05). Variation in the δ13C of C3 plants was the main factor that determined the δ13C of soil organic matter. The C4/C3 biomass ratio may also contribute to the δ13C changes, but this effect was relatively minor in our study area. Our findings suggest that a general transform function between the proxy index δ13C value and climates may not be applicable at one specific site, and that a comprehensive investigation is needed when reconstructing the past climate change using the δ13C of buried soils.

Variability of the stable carbon isotope ratio in modern and archaeological millets: evidence from northern China

Stable carbon isotopic analyses of human skeletal remains may provide fundamental evidence for human dietary reconstruction and subsistence strategies. Millet is closely associated with the emergence and development of agriculture-based societies in northern China. Although often overlooked, baseline values of millet seeds are essential for using stable isotope analysis to understand past human and animal diets. Here, we report spatial and temporal variations in the δ13C values of millets by analyzing modern samples, including seeds and leaves, as well as archaeological samples. The δ13C values of modern foxtail millet seeds range from −13.9 to −11.3‰, with a mean value of −12.3 ± 0.5‰ (1σ, n = 66), while δ13C values for modern common millet seeds vary between −14.3 and −12.0‰, with a mean value of −12.8 ± 0.6‰ (1σ, n = 19). There is an approximately 1‰ temporal change in δ13C for millet grains. Leaves have lower δ13C values than grains, implying that eaters living on different tissues of the same plant could show different isotopic values. These background δ13C values must be considered when reconstructing the dietary history of a millet-based society.

Structure of the carbon isotope excursion in a high-resolution lacustrine Paleocene–Eocene Thermal Maximum record from central China

The carbon isotope excursion (CIE) associated with the Paleocene–Eocene Thermal Maximum (PETM) has been recognized for the first time in the micritic carbonate, total organic carbon (TOC) and black carbon (BC) contained within the lacustrine sediments from the Nanyang Basin, central China. The remarkably large excursion (∼−6‰) in the δ13CTOC and δ13CBC values is possibly attributable to increased humidity and elevated pCO2 concentration. The ∼−4‰ CIE recorded in the δ13Ccalcite, reflecting the average isotope change of the watershed system, is consistent with that observed in planktonic foraminifera. This correspondence suggests that the true magnitude of the carbon isotope excursion in the ocean–atmosphere system is likely close to −4‰. The ∼10 m excursion onset in our multi-proxy δ13C records demonstrates that the large input of 13C-depleted carbon into the ocean–atmosphere system was not geologically instantaneous. Despite difference and somewhat smoothness in detailed pattern of the CIE due to localized controls on different substrates, inorganic and organic δ13C data generally depict a gradual excursion onset at least over timescales of thousands of years. In addition, continental temperature reconstruction, based on the distribution of membrane lipids of bacteria, suggests a warming of ∼4 °C prior to the PETM and ∼7 °C increase in temperature during the PETM. The temperature data are overall similar in pattern and trend to the δ13C change across the PETM. These observations, combined with pre-CIE warming, are in line with the idea that 13C-depleted carbon release operated as a positive feedback to temperature, suggesting supply from one or more large organic carbon reservoirs on Earth's surface.

The magnitude of the naturally occurring isotopic enrichment of 13C in exhaled CO2 is directly proportional to exercise intensity in humans

Due to various biochemical fractionation processes during lipid synthesis, the lipid molecules in the body contain substantially lower concentrations of 13C than the nonlipid molecules. Because of the isotopic differences between these two endogenous nutrient pools, any shift toward nonlipid fuel oxidation would be expected to increase in the δ13C of the exhaled breath. Interestingly, the possibility of whether or not an exercise-induced change actually occurs has been debated in literature for over two decades and researchers have still not reached a consensus. We measured ventilatory and metabolic variables before, during, and after exercise in forty-eight adults (n=25 females, n=23 males; 20.1±1.9years) assigned to either a resting treatment or one of three exercising treatments where they maintained a heart rate of 130, 150, or 170bpm for 56min. We found that the mean metabolic rates of the exercising groups increased 4.4-fold, 6.1-fold, and 7.7-fold above resting values, respectively. Exercise caused small increases in respiratory exchange ratios (e.g., from 0.83±0.08 to 0.86±0.10) indicative of increased carbohydrate oxidation, but these changes were too variable to be reliably correlated with exercise intensity. In contrast, the δ13C of the exhaled breath increased by 0.62±0.19‰, 1.14±0.29‰, and 1.79±0.50‰, respectively, for the three groups and was significantly correlated with the intensity of exercise. We also show that the isotopic difference of the lipid and nonlipids of the body is similar (~2.7‰) even when consuming bulk diets that are isotopically distinct (>8‰). If not corrected for, these exercise-induced changes in δ13C of the breath would be sufficiently large to skew the results of studies investigating the oxidative fates of exogenous nutritional supplements.

Spatial changes in carbon and nitrogen stable isotope ratios of sludge and associated organisms in a biological sewage treatment system

Carbon and nitrogen stable isotope ratios (δ13C and δ15N) have been utilized as powerful tools for tracing energy or material flows within food webs in a range of environmental studies. However, the techniques have rarely been applied to the study of biological wastewater treatment technologies. We report on the spatial changes in δ13C and δ15N in sludge and its associated biotic community in a wastewater treatment system. This system consisted of an upflow anaerobic sludge blanket (UASB) and a down-flow hanging sponge (DHS) which is a novel type of trickling filter. The results showed clear spatial changes in the δ13C and δ15N of suspended solids (SS), retained sludge, and macrofauna (oligochaetes and fly larvae) in the system. The δ13C and δ15N was used as a natural tracer to determine the SS dynamic throughout the system. The results imply that SS in the DHS effluent was mainly eluted from the retained sludge in the lower section of the DHS reactor. The δ15N of the retained sludge in the DHS reactor increased drastically from the inlet towards to the outlet, from −0.7‰ to 10.3‰. This phenomenon may be attributed to nitrogen conversion processes (i.e. nitrification and denitrification). The δ15N of oligochaetes also increased from the inlet to the outlet, which corresponded well to that of the retained sludge. Thus, the δ15N of the oligochaetes might simply mirror the δ15N of the retained sludge. On the other hand, the δ13C and δ15N of sympatric fly larvae differed from those of the oligochaetes sampled, indicating dietary differences between the taxa. Therefore δ13C and δ15N reflected both treatment and dietary characteristics. We concluded that δ13C and δ15N values are potentially useful as alternative indicators for investigating microbial ecosystems and treatment characteristics of biological wastewater treatment systems.

Seasonal and age-related changes in the stable isotope composition (15N/14N and 13C/12C) of millipedes and collembolans in a temperate forest soil

Seasonal changes in environmental conditions and biotic interactions are often ignored when using stable isotope analysis for reconstructing the trophic structure of soil communities in temperate ecosystems. In this study, we estimated seasonal and age-related changes in δ13C and δ15N values in three epigeic species of collembolans (Pogonognathellus longicornis, Orchesella flavescens and Isotoma viridis) and two litter-dwelling species of millipedes (Polydesmus denticulatus and Leptoiulus proximus) in deciduous and coniferous forest stands in central Russia. Age-related changes in δ13C or δ15N values were either absent or negligible (within 1‰) in L. proximus, but adult and subadult specimens of P. denticulatus were enriched in 15N compared to early larval stages. Since the adults of P. denticulatus were generally more enriched in 15N than adults of L. proximus, they presumably occupy more distinct trophic niches than juveniles do. Age-related changes in isotopic composition were small or absent in collembolans studied. Neither δ13C nor δ15N values of millipedes changed significantly during the vegetation season. In contrast, consistent seasonal changes in δ13C and δ15N values were found in collembolans. Increased δ13C values coincided with the period of minimum soil moisture and correlated with a decreased C/N ratio in collembolan tissues. These changes can largely be attributed to the depletion of lipid-rich storage tissues. Seasonal changes in δ15N values were similar among collembolan species, yet slightly varied between habitats. A general trend of increasing δ15N values from June to September–October may indicate either a reduced importance of non-vascular plants (algae and lichen) in collembolan diet or variation in the isotopic composition of these plants. Overall, our data show that seasonal variations should be taken into account when estimating the isotopic composition of epigeic collembolans in forest soils.

Habitat and body size effects on the isotopic niche space of invasive lionfish and endangered Nassau grouper

Species invasions are a significant threat to global biodiversity and ecosystem function, and yet our knowledge of consequences for native species remains poor. The problem is exacerbated in highly speciose ecosystems like coral reefs. The invasion of the wider Caribbean by predatory lionfish (Pterois spp.) is one of the most successful marine colonizations ever documented, and its impact is anticipated to be substantial on native species. However, despite the ecological and commercial importance of iconic Nassau grouper (Epinephelus striatus), the impacts of the invasion on this IUCN Red‐Listed species remain unexamined. Using data gathered from two critical habitats in the Bahamas, we investigate isotopic niche space overlap between lionfish, Nassau grouper and putative prey species. Despite their relatively small body size, we find that lionfish occupy the highest isotopic niche position on patch reefs, occupying much of the same space as the native apex predator. Contrary to expectation, lionfish trophic level (δ15N) does not increase with body size, contrasting with confamilials in their native range. However, we find that tissue carbon (δ13C) changes systematically with body size on deep forereef habitats, representing a length‐specific shift in food resources, with smaller individuals partitioning resources from larger individuals in this habitat but not on shallow patch reefs. We conclude that, despite the difference in body size, lionfish are capable of directly competing for food resources with Nassau grouper, and that impacts on guilds such as planktivores and invertivores may vary systematically by habitat. Our study contributes to the growing body of research aimed at understanding how a species that is relatively rare in its native range achieved the most successful fish invasion ever documented.

Synchronous mid-Miocene upper and deep oceanic δ13C changes in the east equatorial Pacific linked to ocean cooling and ice sheet expansion

We present mid-Miocene (∼12.0–16.0 Ma) high-resolution (∼3.8 kyr) deep thermocline planktonic foraminiferal δ18O and δ13C records at IODP Site U1337 from the eastern equatorial Pacific. The benthic and planktonic δ18O records of Site U1337 have a similar long-term cooling trend, but display obvious amplitude differences of glacial/interglacial cycles after 13.9 Ma. Planktonic δ18O cycles are usually larger than 1.0‰, nearly 2 times those of the benthic δ18O. The post-13.9 Ma change is probably caused by intensified upwelling along with ocean cooling, but may include upper ocean circulation changes. Both the benthic and planktonic δ13C records at Site U1337 reveal marked 400-kyr carbon isotope cycles during the MMCO (Middle Miocene Climate Optimum) with a trend toward lower δ13C from 16.0 Ma to 12.0 Ma. The similarity in response between surface and deep carbon isotopes indicates that the mid-Miocene carbon excursions in the east equatorial Pacific involved the whole ocean basin water column and thus are a global signal. Box model simulations reveal that the long eccentricity (400 kyr) paced carbon inputs from weathering changed the burial ratio of carbonates to organic carbon and helped to produce the significant 400-kyr cycles in oceanic δ13C. The increased weathering of carbonate, silicate and kerogen rocks are the major factors controlling the long-term decrease in oceanic δ13C after 16.0 Ma.

The curved 14C vs. δ13C relationship in dissolved inorganic carbon: A useful tool for groundwater age- and geochemical interpretations

Determination of the 14C content of dissolved inorganic carbon (DIC) is useful for dating of groundwater. However, in addition to radioactive decay, the 14C content in DIC (14CDIC) can be affected by many geochemical and physical processes and numerous models have been proposed to refine radiocarbon ages of DIC in groundwater systems. Changes in the δ13C content of DIC (δ13CDIC) often can be used to deduce the processes that affect the carbon isotopic composition of DIC and the 14C value during the chemical evolution of groundwater. This paper shows that a curved relationship of 14CDIC vs. δ13CDIC will be observed for groundwater systems if (1) the change in δ13C value in DIC is caused by a first-order or pseudo-first-order process, e.g. isotopic exchange between DIC and solid carbonate, (2) the reaction/process progresses with the ageing of the groundwater, i.e. with decay of 14C in DIC, and (3) the magnitude of the rate of change in δ13C of DIC is comparable with that of 14C decay. In this paper, we use a lumped parameter method to derive a model based on the curved relationship between 14CDIC and δ13CDIC. The derived model, if used for isotopic exchange between DIC and solid carbonate, is identical to that derived by Gonfiantini and Zuppi (2003). The curved relationship of 14CDIC vs. δ13CDIC can be applied to interpret the age of the DIC in groundwater. Results of age calculations using the method discussed in this paper are compared with those obtained by using other methods that calculate the age of DIC based on adjusted initial radiocarbon values for individual samples. This paper shows that in addition to groundwater age interpretation, the lumped parameter method presented here also provides a useful tool for geochemical interpretations, e.g. estimation of apparent rates of geochemical reactions and revealing the complexity of the geochemical environment.

Decadal (1994–2008) change in the carbon isotope ratio in the eastern South Pacific Ocean

AbstractWe determined the 14 year change in the anthropogenic CO2 inventory in the eastern South Pacific Ocean along the 110°W meridian from 67°S to 21°N, using seawater δ13C data sets collected in 1994 and 2008. The vertical integral of the 14 year δ13C change was assessed in five latitude bands and found to be greatest (−14.7‰ m yr−1) in the subpolar band (38°S–55°S) and smallest (−3.0‰ m yr−1) in the tropical band (21°N–18°S). The δ13C change in each of the latitudinal bands was primarily caused by inputs of anthropogenic CO2 via air‐sea exchange and transport. More than 50% of the total anthropogenic CO2 was added to the subpolar band via the northward movement of Antarctic Intermediate Water (AAIW) from the south, and the remaining 50% was added via air‐sea exchange. We also calculated the ratio of the temporal change in δ13C to the change in dissolved inorganic carbon, which is a measure of the efficiency of oceanic uptake of anthropogenic CO2. The ratio for AAIW in 1994 (−0.017‰ (µmol kg−1) −1) was greater than that in 2008 (−0.010‰ (µmol kg−1) −1) based on the change in preformed δ13C and dissolved inorganic carbon (DIC), indicating reduced efficiency of CO2 uptake by the Southern Ocean in 2008 relative to that in 1994. AAIW remained at the surface for a shorter period in 2008 relative to 1994, and thus would have taken up less atmospheric CO2 prior to subduction. The projected reduction in this ratio indicates a weakening of CO2 uptake by the Southern Ocean in the future.

Environmental change and seasonal behavior of mastodons in the Great Lakes region inferred from stable isotope analysis

We investigate seasonal variations in the diet and drinking water of four Great Lakes mastodon (Mammut americanum) specimens using stable isotope analysis of serially sampled inner-enamel bioapatite structural carbonate (δ13Csc, δ18Osc), and previously published bulk analyses. Isotopic analyses and thin section measurements showed that mastodon tooth enamel extension rates (~12–4mm/yr, decreasing toward the cervix) were lower than those of mammoths or modern elephants. Mastodons had distinct and highly regular seasonal variations in δ13Csc and δ18Osc, which we interpret in the context of local glacial history and vegetation changes. Seasonal variations in δ18O were large but variations in δ13C were small, and may have been obscured if coarser sampling methods than our inner-enamel sampling approach were used. Thus, our approach may be particularly useful for understanding relatively small seasonal changes in δ13C within C3 environments. The seasonal patterns, though not entirely conclusive, suggest that the Ontario mastodons did not migrate over very long distances. Rather, the climate and seasonal dietary patterns of mastodons within the region changed over time, from ~12,400 to 10,400 14CyrBP (~15,000 – 12,000calyrBP). Insights gained using these methods can contribute to a better understanding of megafaunal extinctions and Paleoamerican lifeways.

Estimating the impact of the cryptic degassing of Large Igneous Provinces: A mid-Miocene case-study

Large Igneous Provinces (LIPs) have been emplaced throughout Earth's history, erupting great quantities (>104 km3) of lava in long-lived (>105 y) events that have been linked to major environmental disruptions. The largest LIP eruptions (e.g. Siberian Traps) are widely considered to have had an impact on global climate through basalt CO2 degassing but the impact of the more numerous smaller LIPs is debated. Here we test the hypothesis that LIPs had a greater impact on Earth's climate history than previously estimated because of the ‘cryptic degassing’ of intruded and crust-contaminated magma, injecting extra CO2 over and above that coming from sub-aerial basalts. We use biogeochemical box models to investigate the potential impact of the Columbia River Basalts (CRB) during the mid-Miocene where multiple palaeorecords for this geologically relatively recent event enable more rigorous data-model comparison. We find that the effect on the long-term carbon cycle of basalt degassing from the CRB alone is negligible, but that a total CRB emission of 4090–5670 Pg of carbon with 3000–4000 Pg of this carbon emitted during the Grande Ronde Basalt eruptions, a flux within the acceptable estimated range when cryptic degassing is included, does well in reproducing the record of benthic δ13C and atmospheric CO2 change during the core of the Miocene Climatic Optimum. Nevertheless, mechanisms other than degassing are required to drive observed warmth before 16.3 Ma and to match observed calcite compensation depth behaviour after ∼15.4 Ma. Hence, our findings rule out the possibility that CRB emplacement alone can fully explain the mid-Miocene record but they demonstrate the enhanced climate impact that occurs when substantial cryptic degassing accompanies LIP emplacement.

Subduction zone metamorphic pathway for deep carbon cycling: I. Evidence from HP/UHP metasedimentary rocks, Italian Alps

Metamorphosed Jurassic oceanic sediment in the Italian Alps experienced peak P–T conditions (1.5–3.0GPa; 330–550°C) similar to those experienced by sediments subducting through forearcs in most modern subduction zones. Integrated field, petrologic, and geochemical study of the devolatilization history of these rocks provides evidence regarding the extents of loss and mobility of oxidized and reduced C during subduction of sediments to depths beneath volcanic fronts, thus constraining models of the release of initially subducted C into the atmosphere via arc volcanism or contributing to the C budget of the deeper mantle.In this suite, occurrences of lawsonite and grossular-rich garnet in higher grade calcschists indicate some decarbonation reaction along a P–T gradient of ~8°C/km. Across grade, carbonates largely retain δ13CVPDB values typical of marine carbonates (mostly ranging from −1.5 to +1.5‰), with some shifts to lower values in low-carbonate samples possibly in part reflecting decarbonation but likely largely due to isotopic exchange with abundant reduced C. Carbonaceous matter in some carbonate-poor samples shows increase in δ13C at higher grades, from values of −25 to −22‰ typical of marine organic matter, to values as high as −10‰, consistent with the expected effects of devolatilization involving release of varying fractions of the original reduced C into fluids as CH4. In more carbonate-rich samples, shifts in the carbonaceous matter to higher values, by up to 15‰, reflect varying degrees of exchange with the carbonate in these samples. Carbonate across grade has δ18OVSMOW of +17 to +22‰ significantly lower than values typical of marine carbonates (the latter +28 to +30‰), likely in part reflecting exchange with silicate phases. Such shifts in the more carbonate-rich samples cannot be explained by closed-system exchange with silicates and require exchange with an external reservoir. The observed extents of decarbonation in these carbonate-bearing sediments subducted to depths of up to 90–100km are consistent with limited open-system behavior, hence intermediate between models of Rayleigh-like CO2 loss and recent models supporting open-system flushing of carbonate-rich rocks by H2O-rich fluid from underlying subducting oceanic lithosphere. Based on a separate study of devolatilization history in the metapelites intercalated with the metacarbonates, and consideration of the O isotope compositions of the two lithologies, the metapelites appear to be sources for the H2O-rich fluid that drove decarbonation reactions and shifted the δ18O values of the carbonates to their present values. Calculated decarbonation histories of the calcschists, using the Perple_X software, predict significant release of CO2 related to the breakdown of CO2-bearing phases (dolomite, aragonite) and the growth of Ca-rich silicate phases (lawsonite, grossular-rich garnet). This decarbonation occurred at very low XCO2 (<0.01), for the P–T range considered here, perhaps attained during pulses of infiltration by H2O-rich fluids from the nearby metapelites.These results demonstrate that a large fraction of the initially subducted oxidized and reduced C in pelitic–carbonate sediment can be retained during forearc metamorphism and thus be available for loss beneath arcs or addition to the deeper mantle. In forearcs, the C inventory in these sediments experience minimal isotopic shift due to devolatilization, and isotopic reequilibration of the oxidized and reduced reservoirs during heating likely results in little net change in the bulk δ13C of the subducting section. Further attention should be paid to understanding the extent of C release, and possible isotope fractionation, during decarbonation, partial melting, and carbonate dissolution at the 80–100km depths over which subducting slabs experience greater heating by exposure to the convecting mantle wedge.

A continuous stable isotope record from the penultimate glacial maximum to the Last Interglacial (159–121 ka) from Tana Che Urla Cave (Apuan Alps, central Italy)

Relatively few radiometrically dated records are available for the central Mediterranean spanning the marine oxygen isotope stage 6–5 (MIS 6–5) transition and the first part of the Last Interglacial. Two flowstone cores from Tana che Urla Cave (TCU, central Italy), constrained by 19 U/Th ages, preserve an interval of continuous speleothem deposition between ca. 159 and 121ka. A multiproxy record (δ18O, δ13C, growth rate and petrographic changes) obtained from this flowstone preserves significant regional-scale hydrological changes through the glacial/interglacial transition and multi-centennial variability (interpreted as alternations between wetter and drier periods) within both glacial and interglacial stages. The glacial stage shows a wetter period between ca. 154 and 152ka, while the early to middle Last Interglacial period shows several drying events at ca. 129, 126 and 122ka, which can be placed in the wider context of climatic instability emerging from North Atlantic marine and NW European terrestrial records. The TCU record also provides important insights into the evolution of local environmental conditions (i.e. soil development) in response to regional and global-scale climate events.

A high-resolution benthic stable-isotope record for the South Atlantic: Implications for orbital-scale changes in Late Paleocene–Early Eocene climate and carbon cycling

The Late Paleocene and Early Eocene were characterized by warm greenhouse climates, punctuated by a series of rapid warming and ocean acidification events known as “hyperthermals”, thought to have been paced or triggered by orbital cycles. While these hyperthermals, such as the Paleocene Eocene Thermal Maximum (PETM), have been studied in great detail, the background low-amplitude cycles seen in carbon and oxygen-isotope records throughout the Paleocene–Eocene have hitherto not been resolved. Here we present a 7.7 million year (myr) long, high-resolution, orbitally-tuned, benthic foraminiferal stable-isotope record spanning the late Paleocene and early Eocene interval (∼52.5–60.5 Ma) from Ocean Drilling Program (ODP) Site 1262, South Atlantic. This high resolution (∼2–4 kyr) record allows the changing character and phasing of orbitally-modulated cycles to be studied in unprecedented detail as it reflects the long-term trend in carbon cycle and climate over this interval. The main pacemaker in the benthic oxygen-isotope (δ18O) and carbon-isotope (δ13C) records from ODP Site 1262, are the long (405 kyr) and short (100 kyr) eccentricity cycles, and precession (21 kyr). Obliquity (41 kyr) is almost absent throughout the section except for a few brief intervals where it has a relatively weak influence. During the course of the Early Paleogene record, and particularly in the latest Paleocene, eccentricity-paced negative carbon-isotope excursions (δ13C, CIEs) and coeval negative oxygen-isotope (δ18O) excursions correspond to low carbonate (CaCO3) and coarse fraction (%CF) values due to increased carbonate dissolution, suggesting shoaling of the lysocline and accompanied changes in the global exogenic carbon cycle. These negative CIEs and δ18O events coincide with maxima in eccentricity, with changes in δ18O leading changes in δ13C by ∼6 (±5) kyr in the 405-kyr band and by ∼3 (±1) kyr in the higher frequency 100-kyr band on average. However, these phase lags are not constant, with the lag in the 405-kyr band extending from ∼4 (±5) kyr to ∼21 (±2) kyr from the late Paleocene to the early Eocene, suggesting a progressively weaker coupling of climate and the carbon-cycle with time. The higher amplitude 405-kyr cycles in the latest Paleocene are associated with changes in bottom water temperature of 2–4 °C, while the most prominent 100 kyr-paced cycles can be accompanied by changes of up to 1.5 °C. Comparison of the 1262 record with a lower resolution, but orbitally-tuned benthic record for Site 1209 in the Pacific allows for verification of key features of the benthic isotope records which are global in scale including a key warming step at 57.7 Ma.

Effects of litter manipulation in a tropical Eucalyptus plantation on leaching of mineral nutrients, dissolved organic nitrogen and dissolved organic carbon

Although many studies have shown that soil solution chemistry can be a reliable indicator of biogeochemical cycling in forest ecosystems, the effects of litter manipulations on the fluxes of dissolved elements in gravitational soil solutions have rarely been investigated. We estimated the fluxes of NH4-N, NO3-N, K, Ca, Mg, Na, Cl, dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) over the first two years after re-planting Eucalyptus trees in the coastal area of Congo. Two treatments were replicated in two blocks after clear-cutting 7-year-old stands: in treatment R, all the litter above the mineral soil was removed before planting, and in a double slash (DS) treatment, the amount of harvest residues was doubled. The soil solutions were sampled down to a depth of 4m and the water fluxes were estimated using the Hydrus 1D model parameterized from soil moisture measurements in 4 plots. Isotopic and spectroscopic analytical techniques were used to assess the changes in dissolved organic matter (DOM) properties throughout the transfer in the soil. The first year after planting, the fluxes of NH4-N, K, Ca, Mg, Na, Cl and DOC in the topsoil of the DS treatment were 2–5 times higher than in R, which showed that litter was a major source of dissolved nutrients. Nutrient fluxes in gravitational solutions decreased sharply in the second year after planting, irrespective of the soil depth, as a result of intense nutrient uptake by Eucalyptus trees. Losses of dissolved nutrients were noticeably low in these Eucalyptus plantations despite a low cation exchange capacity, a coarse soil texture and large amounts of harvest residues left on-site at the clear cut in the DS treatment. All together, these results clarified the strong effect of litter manipulation observed on eucalypt growth in Congolese sandy soils. DOM fluxes, as well as changes in δ13C, C:N and aromaticity of DOM throughout the soil profile showed that the organic compounds produced in the litter layer were mainly consumed by microorganisms or retained in the topsoil. Below a depth of 15cm, most of the DOC and the DON originated from the first 2cm of the soil and the exchanges between soil solutions and soil organic matter were low.

Influence of long-term CO2 storage on δ13C in vials capped with butyl and butyl/PTFE caps and the relevance for ecological samples

The carbon (C) isotope signature of solid materials such as plants and soil, or gaseous samples (atmospheric or soil air), can be used as a useful tool for investigations of the C cycle. In gaseous samples, stability of δ13C of carbon dioxide (CO2) in air during storage represents a problem. We tested the long-term storage effect of δ13C originated from CO2 in vials both capped with butyl or butyl coated with polytetrafluoroethylene (PTFE) on the internal surface. Therefore, pure CO2, depleted in 13C, was stored for up to 736 days. In addition, the relevance of long-term storage for ecological soil air data collected from a free-air CO2 enrichment (FACE) experiment located in Stuttgart (Germany) during one growing period with a maximum storage period of 210 days was judged. With increasing storage time, a change in isotopic composition towards less depleted δ13C was observed. The changes in δ13C were highest at the beginning of the storage period and decreased over time, which could be described with an asymptotic model. The maximum change in δ13C was less than 2 ‰ and lower for vials capped with butyl/PTFE septa. In the FACE experiment, the comparison between corrected and uncorrected data showed that δ13C originated from soil air changed within this data set by up to 1 ‰. The calculation of the fraction of plant derived C resulted in an underestimation of up to 10 %. The storage effect should be taken into consideration when interpreting δ13C values in order to avoid miscalculations.