Drop In δ13C Research Articles

Hoof Growth Rates of the European Roe Deer (Capreolus capreolus) for Dating the Hoof’s Isotopic Archive

Hooves preserve the isotopic information laid down during their growth and may be used for reconstruction of animal feeding history. To assign certain positions along hooves to corresponding times, growth rates are required. Hoof growth rates are known for domestic animals; however, they cannot be obtained easily in wild animals. We estimated the hoof growth rate of the European roe deer (Capreolus capreolus L.) by using the immediate drop in δ13C along the hoof as a tag that is assigned to the date of maize (Zea mays L.) harvest. Keratin samples were taken each mm along 17 hooves and analyzed for their δ13C. A linear regression between (1) time differences of expected maize harvest to animal death and (2) distances between the points of the δ13C drop to the periople yielded the growth rate. Mean hoof growth rate was 0.122 mm/day (95% CI 0.014 mm/day) and 0.365%/day (±0.026%/day) of the hoof length and within the range of domestic animals. The method may be applied to determine growth rates of other incrementally growing tissues. Our estimated growth rate fosters dating isotopic information in hooves, facilitating research on feed resources and space use of roe deer.

Stable isotope and calcareous nannofossil assemblage record of the late Paleocene and early Eocene (Cicogna section)

Abstract. We present records of stable carbon and oxygen isotopes, CaCO3 content, and changes in calcareous nannofossil assemblages across an 81 m thick section of upper Paleocene–lower Eocene marine sedimentary rocks now exposed along the Cicogna Stream in northeast Italy. The studied stratigraphic section represents sediment accumulation in a bathyal hemipelagic setting from approximately 57.5 to 52.2 Ma, a multi-million-year time interval characterized by perturbations in the global carbon cycle and changes in calcareous nannofossil assemblages. The bulk carbonate δ13C profile for the Cicogna section, once placed on a common timescale, resembles that at several other locations across the world, and includes both a long-term drop in δ13C and multiple short-term carbon isotope excursions (CIEs). This precise correlation of widely separated δ13C records in marine sequences results from temporal changes in the carbon composition of the exogenic carbon cycle. However, diagenesis has likely modified the δ13C record at Cicogna, an interpretation supported by variations in bulk carbonate δ18O, which do not conform to expectations for a primary signal. The record of CaCO3 content reflects a combination of carbonate dilution and dissolution, as also inferred at other sites. Our detailed documentation and statistical analysis of calcareous nannofossil assemblages show major differences before, during and after the Paleocene–Eocene Thermal Maximum. Other CIEs in our lower Paleogene section do not exhibit such a distinctive change; instead, these events are sometimes characterized by variations restricted to a limited number of taxa and transient shifts in the relative abundance of primary assemblage components. Both long-lasting and short-lived modifications to calcareous nannofossil assemblages preferentially affected nannoliths or holococcoliths such as Discoaster, Fasciculithus, Rhomboaster/Tribrachiatus, Sphenolithus and Zygrhablithus, which underwent distinct variations in abundance as well as permanent evolutionary changes in terms of appearances and disappearances. By contrast, placoliths such as Coccolithus and Toweius, which represent the main component of the assemblages, were characterized by a gradual decline in abundance over time. Comparisons of detailed nannofossil assemblage records at the Cicogna section and at ODP Site 1262 support the idea that variations in the relative and absolute abundances, even some minor changes, were globally synchronous. An obvious link is through climate forcing and carbon cycling, although the linkages between variations in calcareous nannoplankton, changes in δ13C records and oceanography will need additional work.

Southwest Atlantic water mass evolution during the last deglaciation

AbstractThe rise in atmospheric CO2 during Heinrich Stadial 1 (HS1; 14.5–17.5 kyr B.P.) may have been driven by the release of carbon from the abyssal ocean. Model simulations suggest that wind‐driven upwelling in the Southern Ocean can liberate 13C‐depleted carbon from the abyss, causing atmospheric CO2 to increase and the δ13C of CO2 to decrease. One prediction of the Southern Ocean hypothesis is that water mass tracers in the deep South Atlantic should register a circulation response early in the deglaciation. Here we test this idea using a depth transect of 12 cores from the Brazil Margin. We show that records below 2300 m remained 13C‐depleted until 15 kyr B.P. or later, indicating that the abyssal South Atlantic was an unlikely source of light carbon to the atmosphere during HS1. Benthic δ18O results are consistent with abyssal South Atlantic isolation until 15 kyr B.P., in contrast to shallower sites. The depth dependent timing of the δ18O signal suggests that correcting δ18O for ice volume is problematic on glacial terminations. New data from 2700 to 3000 m show that the deep SW Atlantic was isotopically distinct from the abyss during HS1. As a result, we find that mid‐depth δ13C minima were most likely driven by an abrupt drop in δ13C of northern component water. Low δ13C at the Brazil Margin also coincided with an ~80‰ decrease in Δ14C. Our results are consistent with a weakening of the Atlantic meridional overturning circulation and point toward a northern hemisphere trigger for the initial rise in atmospheric CO2 during HS1.

Carbon isotope (δ13Ccarb) stratigraphy of the Lower–Middle Ordovician (Tremadocian–Darriwilian) in the Great Basin, western United States: Implications for global correlation

New stable carbon isotope data (δ13Ccarb) from Lower–Middle Ordovician (Tremadocian to Darriwilian) carbonate mudstone and wackestone rocks of the Pogonip Group are presented from two sections in the Great Basin region (USA) — Shingle Pass (east-central Nevada) and the Ibex area (western Utah). The Pogonip Group is a succession of mixed carbonate and siliciclastic rocks that accumulated on a carbonate ramp under normal marine conditions during the Late Cambrian (Furongian) to Middle Ordovician (Darriwilian). The Shingle Pass and Ibex area sections have been previously studied for their conodont biostratigraphy and contain a North American Midcontinent conodont fauna that range from the Cordylodus intermedius Zone (uppermost Cambrian) to the Phragmodus polonicus Zone (Darriwilian). The δ13C trend has four distinct characteristics recognized in both Great Basin sections: 1) a drop in δ13C from +1‰ at the base of the Ordovician (Tremadocian) to −0.7‰, 2) a 1 to 2‰ positive δ13C shift in the uppermost Rossodus manitouensis Zone during the late Tremadocian, 3) a gradual δ13C increase from −2‰ to ca. 0‰ during the end of the Early Ordovician (Floian), and 4) a steady δ13C decrease from 0‰ to −4 to −5‰ during Middle Ordovician (Dapingian–Darriwilian).In the Lower Ordovician, δ13C trends reported here from the Great Basin are not consistent with a causal mechanism involving sea level change and the migration of isotopically distinct water bodies. Instead, these Lower Ordovician isotope data most likely reflect primary seawater chemistry and changes in δ13C on a global scale. This interpretation is supported by the excellent correlation of δ13C in the Lower Ordovician to other δ13C trends reported from the sections in the Argentine Precordillera (La Silla and San Juan formations) and in western Newfoundland (St. George and Table Head groups). These correlations using δ13C are consistent with published biostratigraphic data and provide an integrated and high-resolution chemo-biostratigraphic framework for the Lower Ordovician sedimentary record of the Laurentian margin. The Middle Ordovician portion of the δ13C curves in the Great Basin represented by the Kanosh and Lehman formations shows significant isotopic depletion relative to the section in Argentina. Thus, although there is some indication that minima and maxima in the Middle Ordovician curves can be correlated, the Great Basin sections show clear evidence of overprinting by local variables related to both diagenesis (dolomitization) and platform restriction.

Isotopically depleted carbon in the mid‐depth South Atlantic during the last deglaciation

AbstractThe initial rise in atmospheric CO2 during the last deglaciation was likely driven by input of carbon from a 13C‐depleted reservoir. Here we show that high resolution benthic foraminiferal records from the mid‐depth Brazil Margin display an abrupt drop in δ13C during Heinrich Stadial 1 (HS1) that is similar to but larger than in the atmosphere. Comparing the Brazil Margin results to published records from the North Atlantic, we are unable to account for the South Atlantic δ13C data with conservative mixing between northern and southern component water masses. Rapid input of abyssal water from the Southeast Atlantic could account for deglacial δ13C anomalies at the Brazil Margin but it would require a reversal in deep water flow direction compared to today. A new mid‐depth water mass may explain similar HS1 δ13C values in both the North and South Atlantic, but contrasting oxygen isotopic values between the two basins do not support such a scenario. Instead, it appears that δ13C behaved non‐conservatively during the deglaciation, possibly reflecting the input of carbon from an isotopically depleted source.

Warm Eocene climate enhanced petroleum generation from Cretaceous source rocks: A potential climate feedback mechanism?

Earth surface temperatures, including in the deep sea increased by 5–10°C from the late Paleocene ca. 58 Myr ago to the Early Eocene Climatic Optimum (EECO) centered at about 51 Myr ago. A large (∼2.5‰) drop in δ13C of carbonate spans much of this interval. This suggests a long‐term increase in the net flux of13C‐depleted carbon to the ocean and atmosphere that is difficult to explain by changes in surficial carbon cycling alone. We reveal a relationship between surface temperature increase and increased petroleum generation in sedimentary basins operating on 100 kyr to Myr time scales. We propose that early Eocene warming has led to a synchronization of periods of maximum petroleum generation and enhanced generation in otherwise unproductive basins through extension of the volume of source rock within the oil and gas window across hundreds of sedimentary basins globally. Modelling the thermal evolution of four sedimentary basins in the southwest Pacific predicted an up to 50% increase in petroleum generation that would have significantly increased leakage of light hydrocarbons and oil degeneration products into the atmosphere. Extrapolating our modelling results to hundreds of sedimentary basins worldwide suggests that globally increased leakage could have caused a climate feedback effect, driving or enhancing early Eocene climate warming.

South Pacific intermediate water oxygen depletion at the onset of the Paleocene-Eocene thermal maximum as depicted in New Zealand margin sections

[1] Extreme global warming and massive input of 13C-depleted carbon into the exogenic carbon cycle characterize the Paleocene-Eocene thermal maximum (PETM) circa 55.5 Ma. Previous work indicates that dissolved O2 concentrations dropped in some regions of the ocean during this event, but spatial, temporal, and causal relationships between warming, carbon input, and O2 deficiency remain elusive. We present lithologic, carbon isotopic, and trace fossil results from outcrops in New Zealand that clarify this issue. Sections exposed at Mead and Dee Streams in eastern Marlborough, South Island, contain expanded sedimentary intervals that were deposited before, during, and after the PETM on an upper continental slope. The PETM in these intervals is characterized by enhanced siliciclastic deposition, a hallmark negative carbon isotope excursion, and a loss of bioturbation. Importantly, however, the latter only occurs during the prominent drop in δ13C and not during its recovery. Our results suggest that South Pacific intermediate waters became hypoxic coincident to carbon injection at the start of the PETM. While higher sea surface temperatures throughout the PETM would have decreased the dissolved oxygen content of sinking surface waters, we argue here that oxygen depletion was driven by some combination of elevated temperature, water column stratification, and intermediate water methane oxidation.

Evidence for northward expansion of Antarctic Bottom Water mass in the Southern Ocean during the last glacial inception

We investigated deep water changes in the Southern Ocean during the last glacial inception, in relationship to surface hydrology and global climatology, to better understand the mechanisms of the establishment of a glacial ocean circulation. Changes in benthic foraminiferal δ13C from three high‐resolution cores are compared and indicate decoupled intermediate and deep water changes in the Southern Ocean. From the comparison with records from the North Atlantic, South Atlantic, and the Southern Ocean, we show that the early southern deep water δ13C drop observed at the MIS 5.5–5.4 transition occurred before any significant reduction of North Atlantic Deep Water ventilation. We propose that this drop is linked to the northward expansion of poorly ventilated Antarctic Bottom Water (AABW) mass in the Southern Ocean. Associated with an early cooling in the high southern latitudes, the westerly winds and surface oceanic fronts would migrate equatorward, thus weakening the upwelling of Circumpolar Deep Waters. Reduced heat brought to Antarctic surface waters would enhance sea ice formation during winters and the deep convection of cold and poorly ventilated AABW.

The Late Pleistocene - Holocene palaeolimnology of Lake Victoria East Africa, based upon elemental and isotopic analyses of sedimentary organic matter

Three piston cores from Lake Victoria (East Africa) have been analysed for organic carbon (TOC) and nitrogen (TN) content, stable isotopes (δ13C and δ15N), and Hydrogen Index (HI). These data are combined with published biogenic silica and water content analyses to produce a detailed palaeolimnological history of the lake over the past ca. 17.5 ka. Late Pleistocene desiccation produced a lake-wide discontinuity marked by a vertisol. Sediments below the discontinuity are characterised by relatively low TOC and HI values, and high C/N, δ13C and δ15N, reflecting the combined influence of abundant terrestrial plant material and generally unfavourable conditions for organic matter preservation. A thin muddy interval with lower δ13C and higher HI and water content indicates that dry conditions were interrupted by a humid period of a few hundred years duration when the lake was at least 35 m deep. The climate changed to significantly more humid conditions around 15.2 ka when the dry lake floor was rapidly flooded. Abundant macrophytic plant debris and high TOC and δ13C values at the upper vertisol surface probably reflect a marginal swamp. δ13C values decrease abruptly and HI begins to increase around 15 ka BP, marking a shift to deeper-water conditions and algal-dominated lake production. C/N values are relatively low during this period, suggesting a generally adequate supply of nitrogen, but increasing δ15N values reflect intense utilisation of the lake's DIN reservoir, probably due to a dramatic rise in productivity as nutrients were released to the lake from the flooded land surface. An abrupt drop in δ13C and δ15N values around 13.8-13.6 ka reflects a period of deep mixing. Productivity increased due to more efficient nutrient recycling, and δ13C values fell as 12C-rich CO2 released by bacterial decomposition of the organic material was brought into the epilimnion. A weak drop in HI values suggests greater oxygen supply to the hypolimnion at this time. Better mixing was probably due to increased wind intensity and may mark the onset of the Younger Dryas in the region. After the period of deep mixing, the water column became more stable. TOC, C/N, δ13C and HI values were at a maximum during the period between 10 and 4 ka, when the lake probably had a stratified water column with anoxic bottom waters. A gradual decrease in values over the last 4000 yrs suggest a change to a more seasonal climate, with periodic mixing of the water column. Rising sediment accumulation rates and a trend to more uniform surface water conditions over the last 2000 yrs are probably a result of increased anthropogenic impact on the lake and its catchment. Following a maximum at the time of the rapid lake-level rise during the terminal Pleistocene, δ15N has remained relatively low and displays a gradual but consistent trend to lower values from the end of the Pleistocene to the present. TN values have risen during the same period. The lack of correlation between δ13C and δ15N, and the absence of any evidence for isotopic reservoir effects despite the rise in TN, suggests that the atmosphere, rather than the lake's dissolved nitrogen pool has been the principal source of nitrogen throughout the Holocene. The importance of atmospheric N fixation to Lake Victoria's nitrogen cycle thus predates by a very considerable margin any possible anthropogenic eutrophication of the lake.

Onset of the Mid‐Cretaceous greenhouse in the Barremian‐Aptian: Igneous events and the biological, sedimentary, and geochemical responses

Basalts and biostratigraphy dated at 125–120 Ma from the Ontong Java and Manihiki Plateaus in the western Pacific evidence the largest volcanic event in Earth history in at least the past 160 m.y. The intervening Nova‐Canton Trough rifted at about 121–118 Ma, and a number of guyots and seamounts formed concurrently or slightly later. Geological events that probably were responses to these volcanic/tectonic events occurred in the following chronostratigraphic order. Biotic fluctuations began at about 122.5 Ma. At about 122.0 Ma, 87Sr/86Sr began to decline slowly. Metal concentrations of Co, Mn, Pb, Yb, and Cu in sediments peaked at about 121.5–121.2 Ma. Changes in planktonic communities and sedimentation culminated in a nannoconid “crisis” just prior to 120.5 Ma and in the Selli black shale (OAE la) at about 120.5–119.5 Ma. A sharp drop in δ13C occurred at the beginning of the Selli event and rebounded into a longer positive excursion that reached a peak after the Selli event at about 119.5–118.5 Ma. At 120.5 Ma, 87Sr/86Sr declined rapidly and reached a minimum at about 116–113 Ma. We speculate that the intensity of these latter responses suggests a corresponding peak in volcanic/tectonic activity at about 121–119 Ma.

Influence of a [CO2(aq)] dependent biological C‐isotope fractionation on glacial 13C/12C ratios in the ocean

Planktonic foraminiferal shells buried in deep ocean sediments record lower δ13C values of surface water dissolved inorganic carbon during glacial times than during Holocene. In the Southern Ocean and at high northern latitudes, a drop of between 0.3‰ and 0.9‰ is observed, whereas the mean change in low and middle latitudes is only ±0.15‰. However, a stronger biological carbon pump sufficient to explain the 80 ppmv lower atmospheric pCO2 values during glacial times would raise the surface ocean δ13C values of dissolved inorganic carbon by about 1.0‰. Here the results of a three‐dimensional ocean circulation model study are presented which demonstrate that the increase of δ13C values in the sea surface due to a strengthening of the biological carbon pump is counteracted by processes which drive the δ13C values in the opposite direction. This was found by performing simulations employing the three‐dimensional Hamburg Model of the Oceanic Carbon Cycle (HAMOCC) combined with a [CO2(aq)] dependent parameterization of the biological carbon isotope fractionation. The difference in the biological carbon isotope fractionation between Glacial and Holocene is responsible for a lowering of δ13C values in surface water dissolved inorganic carbon by about 0.3‰. The additional effects of the glacially elevated CO32− concentration (0.25–0.50‰) combined with the 0.35‰ lowering of δ13C values for the whole ocean due to a transfer of terrestrial organic carbon from the biosphere to the ocean‐atmosphere reservoir also contribute to a further δ13C drop of 0.6–0.85‰. Hence a small glacial decrease of the planktonic foraminifera δ13C of the order of 0.25‰ instead of an increase is predicted.

Plankton ecology and the Proterozoic-Phanerozoic transition

Most modern marine ecology is ultimately based on unicellular phytoplankton, yet most large animals are unable to graze directly on even relatively large net phytoplankton; the repackaging effected by herbivorous mesozooplankton thus represents a key link in marine metazoan food chains. Despite the deep taphonomic biases affecting plankton fossilization, there is a clear record of phytoplankton from at least 1800 m.y ago. Proterozoic plankton are represented by small-to medium-sized sphaeromorphic acritarchs and probably do not include many/most of the unusually large acritarchs that characterize the Neoproterozoic. The first significant shift in phytoplankton diversity was therefore the rapid radiation of small acanthomorphic acritarchs in the Early Cambrian. The coincidence of phytoplankton diversification with the Cambrian radiation of large animals points compellingly to an ecological linkage between the two, particularly in light of recently discovered filter-feeding mesozooplankton in the Early Cambrian. The introduction of planktic filter feeders would have established the second tier of the Eltonian pyramid, potentially setting off the “self-propagating mutual feedback system of diversification” now recognized as the Cambrian explosion (Stanley 1973, 1976).By consuming significant percentages of net phytoplankton and suspending it as animal biomass and non-aggregating fecal pellets, mesozooplankton cause a netreductionin export production; a general introduction of zooplankton would therefore have reduced carbon burial and moderated the bloom and bust cycle that must have characterized Proterozoic populations of net phytoplankton. The effect of added trophic levels in Early Cambrian ecosystems can be viewed as a serial application of the trophic cascade process observed in modern lakes, whereby the introduction of higher trophic levels determines the accumulation of plant biomass at the base of the system. As such, the major biogeochemical perturbations that mark the onset of the Phanerozoic might be considered a consequence, rather than a cause, of the Cambrian explosion; reduced C export due to zooplankton expansion explains the otherwise anomalous drop in δ13C at the base of the Tommotian.Cambrian acanthomorphic acritarchs likely derived from planktic leiosphaerids exposed to mesozooplanktic grazing pressure, the ornamentation effectively increasing vesicle size without compromising buoyancy or surface-area:volume ratios. Alternatively, they may represent an escape into the plankton through a miniaturization of the much larger Neoproterozoic acanthomorphs. An invasion of small benthic herbivores into the water column to exploit the phytoplankton accounts for the origin of the mesozooplankton and may have been the key innovation in the Cambrian explosion.

Stable isotope evidence for gradual environmental changes and species survivorship across the Cretaceous/Tertiary Boundary

High‐resolution δ13C and δ18O records have been generated from analyses of the planktonic foraminiferal species Heterohelix globulosa and the benthonic foraminiferal taxon Lenticulina spp from 3 m of a cored section spanning the Cretaceous/Tertiary (K/T) boundary at Brazos River, Texas. These are the first stable isotope records across the K/T boundary based on monospecific and monogeneric foraminiferal samples. They show a gradual decrease in δ13C values of about 2.5 permil beginning at the K/T boundary, as defined by the first appearance of Tertiary planktonic foraminifera, and continuing 17–20 cm above the boundary, approximately 40,000 years later. Gradual 13C depletion contrasts with the sudden δ13C drop at the K/T boundary observed in many deep‐sea sections. The surface‐to‐bottom δ13C gradient decreased to less than zero approximately 25,000–30,000 years after the K/T boundary and remained negative for at least the next 140,000 years. Concomitant with change in δ13C values is a gradual decrease of about 2.5 permil in δ18C values which has not been observed at other localities. This 18O depletion suggests changes in temperature and/or salinity in the earliest Paleocene Gulf of Mexico. No extinction of foraminiferal species is associated with the K/T boundary or the onset of 18O and 13C depletions. Instead, two phases of Cretaceous species extinctions occur. One extinction phase is below the K/T boundary and below the tsunami bed of Bourgeois et al. [1988] and may be linked to sea level regression and environmental perturbations. The second extinction phase coincides with the minimum in δ13C and δ18O values in the Early Danian (Zone P0/Pla) and appears directly related to environmental changes reflected in the isotopic record. H. globulosa, which is commonly present in Maastrichtian and Danian sediments, exhibits significantly lower 18O/16O and 13C/12C ratios in Tertiary sediments relative to specimens from Maastrichtian sediments, demonstrating the survival of this important Cretaceous taxon after the K/T boundary event.

Permian-Triassic of the Tethys: Carbon isotope studies

Profiles of carbon isotopes were studied in marine limestones of Late Permian and Early Triassic age of the Tethyan region from 20 sections in Yugoslavia, Greece, Turkey, Armenian SSR, Iran, Pakistan, India, Nepal, and China. The Upper Permian sections continue the high positive values of δ13C previously found in Upper Permian basins in NW Europe and western USA. In the more complete sections of Tethys it can now be demonstrated that the values of δ13C drop from the Murgabian to the Dzhulfian Stages of the Upper Permian, then sharply to values near zero during the last two biozones of the Dorashamian. These levels of δ13C sample the Tethys Sea and the world ocean, and equal values from deep-water sediments at Salamis Greece indicate that they apply to the whole water column. We hypothesize that the high values of δ13C are a consequence of Late Paleozoic storage of organic carbon, and that the declines represent an episodic cessation of this organic deposition, and partial oxidation of the organic reservoir, extending over a period of several million years. The carbon isotope profile may reflect parallel complexity in the pattern of mass extinction in Late Permian time.

The Atmospheric δ13C Record as Derived from 56 Pinyon Trees at 14 Sites in the Southwestern United States

We have developed a master δ13C chronology from 14 pinyon pine sites in 6 states of the southwestern U S. Two of the individual isotopic chronologies, reported here for the first time, and 10 of those previously reported (Leavitt & Long, 1986; 1988) are from sites where cores from 4 trees were pooled prior to analysis, and the other 2 are merged from groups of 4 single-tree chronologies (sites) developed in an earlier phase of research (Leavitt & Long, 1985). Regressions of first differences of ring-width indices and δ13C values from each site were used to “correct” individual δ13C chronologies for climate effects which appear primarily related to high-frequency δ13C fluctuations, many of which are common among sites. These climate-corrected chronologies were normalized as deviations from their respective 1800–1849 δ13C means, and these normalized chronologies were averaged into the master. The overall δ13C drop from 1600 to the present is ca 1.2–1.4, consistent with recent ice-core data showing a drop of 1.14 ± 0.15% from 1740 to present (Friedli et al, 1986). However, the δ13C decline in the late 19th and early 20th centuries is greater in the pinyon chronology than that of the ice cores, thus supporting a greater biospheric CO2 input to the atmosphere than that indicated in the ice-core data.