École Normale Supérieure de Lyon
Laboratoire de Geologie de Lyon
We report new Nd, Hf, Sr, and high-precision Pb isotopic data for 44 lava and tephra samples from Erebus volcano. The samples cover the entire compositional range from basanite to phonolite and trachyte, and represent all three phases of... more
We report new Nd, Hf, Sr, and high-precision Pb isotopic data for 44 lava and tephra samples from Erebus volcano. The samples cover the entire compositional range from basanite to phonolite and trachyte, and represent all three phases of the volcanic evolution from 1.3 Ma to the present. Isotopic analyses of 7 samples from Mt. Morning and the Dry Valley Drilling Project (DVDP) are given for comparison. The Erebus volcano samples have radiogenic 206Pb/204Pb, unradiogenic 87Sr/86Sr, and intermediate 143Nd/144Nd and 176Hf/177Hf, and lie along a mixing trajectory between the two end-member mantle components DMM and HIMU. The Erebus time series data show a marked distinction between the early-phase basanites and phonotephrites, whose Nd, Hf, Sr, and Pb isotope compositions are variable (particularly Pb), and the current ‘phase-three’ evolved phonolitic lavas and bombs, whose Nd, Hf, Sr, and Pb isotope compositions are essentially invariant. Magma mixing is inferred to play a fundamental role in establishing the isotopic and compositional uniformity in the evolved phase-three phonolites. In-situ analyses of Pb isotopes in melt inclusions hosted in an anorthoclase crystal from a 1984 Erebus phonolite bomb and in an olivine from a DVDP basanite are uniform and identical to the host lavas within analytical uncertainties. We suggest that, in both cases, the magma was well mixed at the time melt inclusions were incorporated into the different mineral phases.
Changes in the orgC/CaCO3 ratio in particles sinking from the surface to the deep ocean have the potential to alter the atmospheric pCO2 over the span of a glacial/interglacial cycle. Recent paleoceanographic and modern observational... more
Changes in the orgC/CaCO3 ratio in particles sinking from the surface to the deep ocean have the potential to alter the atmospheric pCO2 over the span of a glacial/interglacial cycle. Recent paleoceanographic and modern observational studies suggest that silica is a key factor in the global carbon biogeochemical cycle that can influence the flux ratio, especially at low latitudes, through “silicic acid leakage” [Brzezinski, M., Pride, C., Franck, M., Sigman, D., Sarmiento, J., Matsumoto, K., Gruber, N., Rau, R., Coale, K., 2002. A switch from Si(OH)4 to NO3- depletion in the glacial Southern Ocean. Geophysical Research Letters 29, 5]. To test this hypothesis, we reconstruct biogenic fluxes of CaCO3, orgC and Si for three equatorial Pacific cores. We find evidence that a floral shift from a SiO2-based community to a CaCO3-based occurred, starting in mid-marine isotope stage (MIS) 3 (24–59 cal. ka) and declining toward MIS 2 (19–24 cal. ka). This could reflect the connection of the Peru upwelling system to the subantarctic region, and we postulate that excess silica was transported from the subantarctic via the deep Equatorial Undercurrent to the eastern equatorial Pacific. In the eastern equatorial Pacific only, we document a significant decrease in rain ratio starting mid-MIS 3 toward MIS 2. This decrease is concomitant with a significant decrease in silica accumulation rates at the seabed. This pattern is not observed in the Pacific influenced by equatorial divergence and shallow upwelling, where all reconstructed fluxes (CaCO3, orgC, and opal) increase during MIS 2. We conclude that the overall calcium carbonate pump weakened in the EEP under Peru upwelling influence.
The (231Pa/230Th)xs,0 records obtained from two cores from the western (MD97-2138; 1°25′S, 146°24′E, 1900 m) and eastern (Ocean Drilling Program Leg 138 Site 849, 0°11.59′N, 110°31.18′W, 3851 m) equatorial Pacific display similar... more
The (231Pa/230Th)xs,0 records obtained from two cores from the western (MD97-2138; 1°25′S, 146°24′E, 1900 m) and eastern (Ocean Drilling Program Leg 138 Site 849, 0°11.59′N, 110°31.18′W, 3851 m) equatorial Pacific display similar variability over the last 85,000 years, i.e., from isotopic stages 1 to 5a, with systematically higher values during the Holocene, isotopic stage 3, and isotopic stage 5a, and lower values, approaching the production rate ratio of the two isotopes (0.093), during the colder periods corresponding to isotopic stages 2 and 4. We have also measured the 230Th-normalized biogenic preserved and terrigenous fluxes, as well as major and trace elements concentrations, in both cores. The (231Pa/230Th)xs,0 results combined with the changes in preserved carbonate and opal fluxes at the eastern site indicate lower productivity in the eastern equatorial Pacific during glacial periods. The (231Pa/230Th)xs,0 variations in the western equatorial Pacific also seem to be controlled by productivity (carbonate and/or opal). The generally high (231Pa/230Th)xs,0 ratios (>0.093) of the profile could be due to opal and/or MnO2 in the sinking particles. The profiles of (231Pa/230Th)xs,0 and 230Th-normalized fluxes indicate a decrease in exported carbonate, and possibly opal, during isotopic stages 2 and 4 in MD97-2138. Using 230Th-normalized flux, we also show that sediments from the two cores were strongly affected by sediment redistribution by bottom currents suggesting a control of mass accumulation rates by sediment focusing variability.
The carbonate fraction of sediment core ODP 849, leg 138, located in the eastern equatorial Pacific, mostly consisting of coccoliths, was separated and analyzed for its Zn isotopic composition. The overall variation in Zn isotopic... more
The carbonate fraction of sediment core ODP 849, leg 138, located in the eastern equatorial Pacific, mostly consisting of coccoliths, was separated and analyzed for its Zn isotopic composition. The overall variation in Zn isotopic composition, as determined by multiple-collector, magnetic-sector, inductively coupled plasma mass spectrometry, was found to be on the order of 1‰ (expressed in δ66Zn, where δxZn=[(xZn/64Zn)sample/(xZn/64Zn)standard−1]×103 and x=66, 67 or 68) over the last 175 ka. The analytical precision was 0.04‰ and the overall reproducibility was usually better than 0.07‰. The Zn isotopic composition signal exhibits several marked peaks and a high-frequency variability. A periodogram of the δ66Zn signal showed two periodicities of 35.2 and 21.2 ka. We suggest that the latter is caused by the precession of the Earth’s axis of rotation. The periodogram exhibits a minimum at 41.1 ka, thus showing that the Zn isotopic composition is independent of the obliquity in the eastern equatorial Pacific. The range of δ66Zn values observed for the carbonate fraction of ODP 849 overlaps with the range observed for Fe–Mn nodules in the world’s oceans, which suggests that seawater/carbonate Zn isotope fractionation is weak. We therefore assume that most of the Zn isotope variability is a result of the selective entrainment of the light isotopes by organic matter in the surface ocean. The ODP 849 δ66Zn record seems to follow the changes in the insolation cycles. Changes in the late summer/fall equatorial insolation modulate the intensity of the equatorial upwelling, hence the mixing between deep and surface waters. We propose that during decreased summer/fall equatorial insolation, when a steep thermocline can develop (El Niño-like conditions), the surface waters cannot be replenished by deep waters and become depleted in the lighter Zn isotopes by biological activity, thus resulting in the progressive increase of the δ66Zn values of the carbonate shells presumably in equilibrium with surface seawater.
Difficulties in determining the 230Th and 231Pa concentration of seawater have hindered rapid progress in the application of these unique natural tracers of particle scavenging and ocean circulation. In response, we have developed an... more
Difficulties in determining the 230Th and 231Pa concentration of seawater have hindered rapid progress in the application of these unique natural tracers of particle scavenging and ocean circulation. In response, we have developed an ICP/MS analytical procedure combining a degree of sensitivity, precision and sample throughput that can facilitate the systematic measurement of basin-scale changes in 230Th and 231Pa seawater concentration, and provide important constraints on circulation and mixing rates in the deep ocean.
Seawater samples are spiked with 229Th and 233Pa and equilibrated before pre-concentration using conventional methods of Fe oxyhydroxide co-precipitation and anion exchange. Isotopic ratios are measured using a Finnigan MAT Element magnetic sector Inductively Coupled Plasma mass spectrometer (ICP/MS) equipped with a desolvating micronebulizer. Measurements are done on 10–20 l seawater samples with an internal precision of ∼2% and a reproducibility of ∼5% (95% confidence intervals (CI)) in deep water. After correction for procedural blank, 232Th tailing, and 232Th1H interference, the detection limits are ∼3 fg for 230Th and ∼0.4 fg for 231Pa. Applied to 20 l volumes, these detection limits correspond to concentrations of 0.15 fg/kg for 230Th and 0.02 fg/kg for 231Pa, which are 5–15 times lower than typical concentrations in surface water. The capability of this method is illustrated by two seawater profiles from the Equatorial Atlantic region that show systematic variations in 230Th and 231Pa concentration consistent with patterns of deep water circulation.
Seawater samples are spiked with 229Th and 233Pa and equilibrated before pre-concentration using conventional methods of Fe oxyhydroxide co-precipitation and anion exchange. Isotopic ratios are measured using a Finnigan MAT Element magnetic sector Inductively Coupled Plasma mass spectrometer (ICP/MS) equipped with a desolvating micronebulizer. Measurements are done on 10–20 l seawater samples with an internal precision of ∼2% and a reproducibility of ∼5% (95% confidence intervals (CI)) in deep water. After correction for procedural blank, 232Th tailing, and 232Th1H interference, the detection limits are ∼3 fg for 230Th and ∼0.4 fg for 231Pa. Applied to 20 l volumes, these detection limits correspond to concentrations of 0.15 fg/kg for 230Th and 0.02 fg/kg for 231Pa, which are 5–15 times lower than typical concentrations in surface water. The capability of this method is illustrated by two seawater profiles from the Equatorial Atlantic region that show systematic variations in 230Th and 231Pa concentration consistent with patterns of deep water circulation.
The eastern equatorial Pacific (EEP) is an important center of biological productivity, generating significant organic carbon and calcite fluxes to the deep ocean. We reconstructed paleocalcite flux for the past 30,000 years in four cores... more
The eastern equatorial Pacific (EEP) is an important center of biological productivity, generating significant organic carbon and calcite fluxes to the deep ocean. We reconstructed paleocalcite flux for the past 30,000 years in four cores collected beneath the equatorial upwelling and the South Equatorial Current (SEC) by measuring ex230Th-normalized calcite accumulation rates corrected for dissolution with a newly developed proxy for “fraction of calcite preserved.” This method produced very similar results at the four sites and revealed that the export flux of calcite was 30–50% lower during the LGM compared to the Holocene. The internal consistency of these results supports our interpretation, which is also in agreement with emerging data indicating lower glacial productivity in the EEP, possibly as a result of lower nutrient supply from the southern ocean via the Equatorial Undercurrent. However, these findings contradict previous interpretations based on mass accumulation rates (MAR) of biogenic material in the sediment of the EEP, which have been taken as reflecting higher glacial productivity due to stronger wind-driven upwelling.
We have analyzed by MC-ICP-MS the Zn isotopic composition of different components (seeds, leaves, and rhizome, stem and leaves) of lentils (Lens culinaris) and bamboos (Phyllostachys aurea), respectively. Zn isotopes are systematically... more
We have analyzed by MC-ICP-MS the Zn isotopic composition of different components (seeds, leaves, and rhizome, stem and leaves) of lentils (Lens culinaris) and bamboos (Phyllostachys aurea), respectively. Zn isotopes are systematically fractionated between seeds and leaves of lentil and between stem and leaves of bamboos. Leaves are enriched in light Zn isotopes compared to the other parts of plants. The range of the fractionation is up to 0.52‰ per amu and is clearly mass dependent. The observed Zn isotopic fractionation is consistent with that occurring during both diffusive processes and cross-cell membrane transport. Our study also shows a clear interspecies variability for Zn isotopic fractionation. We conclude that the Zn could be used as a tracer for biological activities.
Our study gives new constraints on the response of Atlantic Meridional Overturning Circulation (AMOC) export to various forcings during the Last Glacial Inception. The decay corrected excess sedimentary (231Pa/230Th) activity ratio... more
Our study gives new constraints on the response of Atlantic Meridional Overturning Circulation (AMOC) export to various forcings during the Last Glacial Inception. The decay corrected excess sedimentary (231Pa/230Th) activity ratio (hereafter referred to as (Pa/Th)) has been measured over that period in two deep cores from the Western (SU90-11, 44°04′N, 40°01′W, 3645 m) and Eastern (MD01-2446, 39°03′N, 12°37′W, 3547 m) basins of the North Atlantic. Both records display significant changes despite the relatively short half-life of 231Pa (∼ 32 kyr) compared to the period we investigate. The (Pa/Th) variability does not correlate to changes in local opal flux normalized to 230Th. Moreover, the (Pa/Th) profiles display a high degree of coherency with indirect proxies of AMOC activity such as the benthic foraminifera δ13C and the mid-latitude summer Sea Surface Temperature in nearby reference cores. These additional pieces of evidence support our interpretation of the (Pa/Th) as reflecting AMOC export. The (Pa/Th) repeatedly underwent rapid changes during the Last Glacial Inception associated with the extension of ice rafted detritus in the North Atlantic, highlighting the control of ice-sheet dynamics through freshwater forcing on AMOC export. AMOC export remains large during periods of ice-sheet growth and its decreases lag the Northern Hemisphere summer insolation forcing. AMOC modulation appears driven by ice-sheet dynamics, itself driven by the seasonal insolation gradient between low and high Northern Hemisphere latitudes and the associated intensity of the meridional oceanic and atmospheric circulation.
- by Sylvain Pichat and +1
- •
- Paleoclimatology, Paleoceanography, Pa/Th
Knowledge of the copper cycle in the plant–soil–water system is needed in order to better constrain proper plant micronutrient nutrition, control pollution, and determine sustainable soil management practices. Here, we report the Cu... more
Knowledge of the copper cycle in the plant–soil–water system is needed in order to better constrain proper plant micronutrient nutrition, control pollution, and determine sustainable soil management practices. Here, we report the Cu isotopic compositions of different components (seeds, germinated seeds, leaves, and stems) of the dicot, lentil (Lens culinaris), and of two monocots, Virginia wild rye (Elymus virginicus) and hairy-leaved sedge (Carex hirsutella). According to our data, the isotopic compositions of these plants are systematically enriched in the lighter isotope of Cu (63Cu) in comparison to the soil in which they grow. Furthermore, different components within the plants themselves are isotopically fractionated. The shoots (stems, leaves and seeds) are systematically lighter than the germinated seeds of the plants and the Cu isotopic compositions of individual leaves correlate with their heights on the plant. These results are similar to what has been observed for Zn isotopes, which are assumed to be transported through plants by means of diffusion and kinetic fractionation across cell membranes. Because of this similarity, we suggest that the same transport mechanisms are also responsible for the observed isotopic fractionation of Cu. As a side-note, the Cu isotopic variations measured in plants are similar in magnitude to the differences previously measured in various soils, and therefore should not be neglected while interpreting the isotopic composition of soils.
The δ66Zn (permil deviation of the 66Zn/64Zn ratio from a terrestrial standard) values for a suite of 20 non-Antarctic HED (howardite–eucrite–diogenite) meteorites and one mesosiderite, and for eight Antarctic eucrites and diogenites,... more
The δ66Zn (permil deviation of the 66Zn/64Zn ratio from a terrestrial standard) values for a suite of 20 non-Antarctic HED (howardite–eucrite–diogenite) meteorites and one mesosiderite, and for eight Antarctic eucrites and diogenites, were measured in order to determine the role of volatization in the formation of their presumed parent body, the asteroid 4-Vesta. The 20 non-Antarctic HEDs had δ66Zn values that ranged from −2.0‰ to +1.67‰, with a mean value of −0.01 ± 0.39‰ (2 se); this range likely represents a small-scale heterogeneity due to brecciation induced by multiple impacts. The non-Antarctic eucrites (δ66Zn = +0.00 ± 0.58‰ (2 se), n = 12) were isotopically the same as the diogenites (δ66Zn = −0.31 ± 0.80‰ (2 se), n = 4), and the howardites (δ66Zn = +0.26 ± 0.37‰ (2 se), n = 4). On average, non-Antarctic eucrite falls were isotopically heavier (+0.50‰) than non-Antarctic finds (−1.00‰). The Antarctic finds studied were all unbrecciated samples, and they were significantly heavier than the non-Antarctic samples with a δ66Zn range of +1.63‰ to +6.22‰ for four eucrites (mean, +4.32‰) and +0.94‰ to +1.60‰ for three diogenites (mean + 1.23‰), excluding one anomalous sample, while their Zn concentration is significantly lower than the brecciated samples. These data suggest that the unbrecciated eucrites probably represent the eucritic crust shortly after differentiation and cooling of the parent asteroid, at which time volatization of lighter zinc isotopes led to an isotopically heavy crust. Early impact events caused the ejection of these unbrecciated meteorites, which were subsequently spared from brecciation caused by multiple additional impacts on the much larger Vesta. The range of δ66Zn values and Zn concentration for the brecciated HEDs in this study supports a major contribution to the Vestan surface by chondritic impactors (−1.30 < δ66Zn < +0.76‰ for ordinary and carbonaceous chondrites). The anomalous eucrite PCA 82502 (δ66Zn = −7.75‰) is significantly isotopically lighter than the other HEDs and is the natural sample with the lightest Zn isotopic composition reported in the solar system to date. This meteorite most likely originated from a distinct parent body.
We investigate the time scales of magma genesis, melt evolution, crystal growth rates and magma degassing in the Erebus volcano magmatic system using measurements of 238U–230Th–226Ra–210Pb–210Po, 232Th–228Ra–228Th and 235U–231Pa–227Ac.... more
We investigate the time scales of magma genesis, melt evolution, crystal growth rates and magma degassing in the Erebus volcano magmatic system using measurements of 238U–230Th–226Ra–210Pb–210Po, 232Th–228Ra–228Th and 235U–231Pa–227Ac. These are the first measurements of 231Pa–227Ac in volcanic samples and represent the first set of data in a volcanic system to examine the entire suite of relevant 238U, 235U and 232Th decay series nuclides. Our sample suite consists of 22 phonolite volcanic bombs, erupted between 1972 and 2005, and five anorthoclase megacrysts separated from bombs erupted in 1984, 1989, 1993, 2004 and 2005. The 238U–230Th, 230Th–226Ra and 235U–231Pa systems are uniform over the 34 years examined. The anorthoclase megacrysts and phonolite glasses show complementary 226Ra/230Th disequilibria with (226Ra/230Th) ∼40 in the anorthoclase and ∼0·75 in the phonolite glass. In all samples, (210Pb/226Ra) is in radioactive equilibrium for both phases. In two phonolite glass samples (227Ac/231Pa) is unity. For the phonolite glasses (228Ra/232Th) is in equilibrium, whereas in the anorthoclase megacrysts it is significantly greater than unity. Instantaneous crystal fractionation, with magma residence times greater than 100 years and less than 10 kyr, can account for the measured 238U–230Th–226Ra–210Pb and 235U–231Pa–227Ac. However, the significant 228Ra/232Th disequilibria in the anorthoclase megacrysts preclude this simple interpretation. To account for this apparent discrepancy we therefore developed an open-system, continuous crystallization model that incorporates both nuclide ingrowth and decay during crystallization. This open-system model successfully reproduces all of the measured 238U and 232Th disequilibria and suggests that the shallow magma reservoir at Erebus is growing. The implication of this modeling is that when the time scale of crystallization is comparable with the half-life of the daughter nuclide of interest (e.g. 226Ra) the simple isochron techniques typically used in most U-series studies can provide erroneous ages. The observation that (210Pb/226Ra) and (227Ac/231Pa) are in radioactive equilibrium suggests that the residence time of the magmas is >100 years. When considering the effect of 222Rn degassing on 210Pb/226Ra, the data indicate that the majority of magma degassing is deep and long before eruption, consistent with melt inclusion data. Additionally, for the 2005 lava bomb, whose eruption date (16 December 2005) is known explicitly, 210Po was not completely degassed from the magma at the time of eruption. Incomplete degassing of 210Po is atypical for subaerially erupted lavas and suggests that the Erebus shallow magma degasses about 1% of its Po per day. The combined 238U and 232Th data further indicate that the pyroclasts ejected by Strombolian eruptions at Erebus have compositions that are close to what would be expected for a near-steady-state system, reflecting inmixing of degassed magmas, crystal fractionation, and aging.
The strength and geometry of the Atlantic meridional overturning circulation is tightly coupled to climate on glacial–interglacial and millennial timescales, but has proved difficult to reconstruct, particularly for the Last Glacial... more
The strength and geometry of the Atlantic meridional overturning circulation is tightly coupled to climate on glacial–interglacial and millennial timescales, but has proved difficult to reconstruct, particularly for the Last Glacial Maximum. Today, the return flow from the northern North Atlantic to lower latitudes associated with the Atlantic meridional overturning circulation reaches down to approximately 4,000 m. In contrast, during the Last Glacial Maximum this return flow is thought to have occurred primarily at shallower depths. Measurements of sedimentary 231Pa/230Th have been used to reconstruct the strength of circulation in the North Atlantic Ocean, but the effects of biogenic silica on 231Pa/230Th-based estimates remain controversial. Here we use measurements of 231Pa/230Th ratios and biogenic silica in Holocene-aged Atlantic sediments and simulations with a two-dimensional scavenging model to demonstrate that the geometry and strength of the Atlantic meridional overturning circulation are the primary controls of 231Pa/230Th ratios in modern Atlantic sediments. For the glacial maximum, a simulation of Atlantic overturning with a shallow, but vigorous circulation and bulk water transport at around 2,000 m depth best matched observed glacial Atlantic 231Pa/230Th values. We estimate that the transport of intermediate water during the Last Glacial Maximum was at least as strong as deep water transport today.
The Atlantic Meridional Overturning Circulation (AMOC) is a key feature of the climate system. However, its role during climate change is still poorly constrained particularly during an Interglacial to Glacial climate transition and the... more
The Atlantic Meridional Overturning Circulation (AMOC) is a key feature of the climate system. However, its role during climate change is still poorly constrained particularly during an Interglacial to Glacial climate transition and the associated global cooling. We present here the first reconstruction of the evolution of the vertical structure of the rate of the AMOC from the Last Interglaciation to the subsequent glaciation (128,000–60,000 years ago) based on sedimentary (231Pa/230Th) records. We show a deep AMOC during the interglacial warmth Marine Isotope Stage (MIS) 5.5 and a shallower glacial one during glacial MIS 4. The change between these two patterns occurred mostly during the glacial inception, i.e. the transition from MIS 5.5 to MIS 5.4. Our data show that AMOC was enhanced during this latter transition as a consequence of a large increase of the overturning rate of the Intermediate Waters, above 2500 m. We suggest that this AMOC pattern required a reinforced Gulf Stream-North Atlantic Current system that ultimately supported ice-sheet growth by providing heat and moisture to the Northern high latitudes. From MIS 5.4 to MIS 5.1, the AMOC was broadly continuous below 2000 m and supported periods of ice-sheet growth. As a result, a glacial AMOC is triggered at the beginning of MIS 4 due to the extension of ice-sheet and the subsequent reorganization of deep-water formation. This study highlights the role of intermediate waters as a major player during climate change.► Unprecedented (231Pa/230Th) records covering the Last Interglaciation. ► Reconstruction of the evolution of the vertical structure of the AMOC export. ► AMOC was enhanced due to increased intermediate water export. ► AMOC transported moisture to the Northern high latitudes feeding ice-sheet growth.
The isotopic compositions of major elements in soils can help understand the mechanisms and processes that control the evolution of soils and the nature and dynamics of the soil constituents. In this study, we investigated the... more
The isotopic compositions of major elements in soils can help understand the mechanisms and processes that
control the evolution of soils and the nature and dynamics of the soil constituents. In this study, we investigated
the variations of the Fe concentrations and isotopic compositions combined with classical soil parameters,
such as granulometry, pH, and C and N concentrations. We selected three soils submitted to different
hydrodynamic functioning along a toposequence: a well-drained Cambisol and two hydromorphic soils, an
Albeluvisol and a Gleysol. In the Cambisol, the isotopic variations were small indicating little redistribution
of Fe which we attributed to centimetric-scale exchanges from the Si-bound to the weakly-bound iron
pools and insignificant subsurface Fe export. In contrast, the hydromorphic soils showed an overall variation
of 0.37‰for δ56Fe (δ56Fe (‰) = [(56Fe / 54Fe)sample / (56Fe / 54Fe)IRMM-014 − 1] × 1000) and an inverse correlation
between the Fe isotopic compositions and the oxide-bound Fe concentrations. We suggest that, in
the uppermost horizon, the mobilisation of oxide-bound Fe was due to the reducing conditions and predominantly
involved the light Fe isotopes. Similarly, within the Bt horizon of the Albeluvisol, the fluctuations of
the water table level induced changes in the redox conditions and thus Fe dissolution and transport of isotopically
light Fe. The Fe isotopic composition profile in the B/C horizon of the Gleysol is dominated by the signature
of the parental material. Overall, the variations of the underground water table combined with
topography-driven water flow were suggested to be the main mechanisms of Fe translocation in these hydromorphic
soils. Finally, the comparison between Fe isotope profiles in worldwide soils allows us to show that
Fe isotopic variations can help discriminate between various mechanisms and scales of Fe transfer in soils
and, accordingly, provide information on the evolution of soils, when used in combination with pedological,
geochemical, geographical, and environmental characterisations.
control the evolution of soils and the nature and dynamics of the soil constituents. In this study, we investigated
the variations of the Fe concentrations and isotopic compositions combined with classical soil parameters,
such as granulometry, pH, and C and N concentrations. We selected three soils submitted to different
hydrodynamic functioning along a toposequence: a well-drained Cambisol and two hydromorphic soils, an
Albeluvisol and a Gleysol. In the Cambisol, the isotopic variations were small indicating little redistribution
of Fe which we attributed to centimetric-scale exchanges from the Si-bound to the weakly-bound iron
pools and insignificant subsurface Fe export. In contrast, the hydromorphic soils showed an overall variation
of 0.37‰for δ56Fe (δ56Fe (‰) = [(56Fe / 54Fe)sample / (56Fe / 54Fe)IRMM-014 − 1] × 1000) and an inverse correlation
between the Fe isotopic compositions and the oxide-bound Fe concentrations. We suggest that, in
the uppermost horizon, the mobilisation of oxide-bound Fe was due to the reducing conditions and predominantly
involved the light Fe isotopes. Similarly, within the Bt horizon of the Albeluvisol, the fluctuations of
the water table level induced changes in the redox conditions and thus Fe dissolution and transport of isotopically
light Fe. The Fe isotopic composition profile in the B/C horizon of the Gleysol is dominated by the signature
of the parental material. Overall, the variations of the underground water table combined with
topography-driven water flow were suggested to be the main mechanisms of Fe translocation in these hydromorphic
soils. Finally, the comparison between Fe isotope profiles in worldwide soils allows us to show that
Fe isotopic variations can help discriminate between various mechanisms and scales of Fe transfer in soils
and, accordingly, provide information on the evolution of soils, when used in combination with pedological,
geochemical, geographical, and environmental characterisations.
- by Sylvain Pichat and +2
- •
- Geochemistry, Soil Science, Pedogenesis
The influence of atmospheric dust on climate and biogeochemical cycles in the oceans is well understood but poorly quantified. Glacial atmospheric dust loads were generally greater than those during the Holocene, as shown, for example, by... more
The influence of atmospheric dust on climate and biogeochemical cycles in the oceans is well understood but poorly quantified. Glacial atmospheric dust loads were generally greater than those during the Holocene, as shown, for example, by the covariation of dust fluxes in the Equatorial Pacific and Antarctic ice cores. Nevertheless, it remains unclear whether these increases in dust flux were associated with changes in sources of dust, which would in turn suggest variations in wind patterns, climate or paleo-environment. Such questions can be answered using radiogenic isotope tracers of dust provenance.
Here, we present a 160-kyr high-precision lead isotope time-series of dust input to the Eastern Equatorial Pacific (EEP) from core ODP Leg 138, Site 849 (0°11.59'N, 110°31.18'W). The Pb isotope record, combined with Nd isotope data, rules out contributions from Northern Hemisphere dust sources, north of the Intertropical Convergence Zone, such as Asia or North Africa/Sahara; similarly, eolian sources in Australia, Central America, the Northern Andes and Patagonia appear insignificant based upon the radiogenic isotope data.
Fluctuations in Pb isotope ratios throughout the last 160 kyr show, instead, that South America remained the prevailing source of dusts to the EEP. There are two distinct South American Pb isotope end-members, constrained to be located in the south Central Volcanic Zone (CVZ, 22°S-27.5°S) and the South Volcanic Zone (SVZ, 33°S-43°S), with the former most likely originating in the Atacama Desert. Dust availability in the SVZ appears to be related to the weathering of volcanic deposits and the development of ash-derived Andosols, and influenced by local factors that might include vegetation cover. Variations in the dust fluxes from the two sources are in phase with both the dust flux and temperature records from Antarctican ice cores. We show that the forcing of dust provenance over time in the EEP overall is influenced by high-southerly-latitude climate conditions, leading to changes in the latitudinal position and strength of the South Westerlies as well as the coastal winds that blow northward along the Chilean margin. The net result is a modulation of dust emission from the Atacama Desert and the SVZ via a northward migration of the South Westerlies during cold periods and southward retreat during glacial terminations.
Here, we present a 160-kyr high-precision lead isotope time-series of dust input to the Eastern Equatorial Pacific (EEP) from core ODP Leg 138, Site 849 (0°11.59'N, 110°31.18'W). The Pb isotope record, combined with Nd isotope data, rules out contributions from Northern Hemisphere dust sources, north of the Intertropical Convergence Zone, such as Asia or North Africa/Sahara; similarly, eolian sources in Australia, Central America, the Northern Andes and Patagonia appear insignificant based upon the radiogenic isotope data.
Fluctuations in Pb isotope ratios throughout the last 160 kyr show, instead, that South America remained the prevailing source of dusts to the EEP. There are two distinct South American Pb isotope end-members, constrained to be located in the south Central Volcanic Zone (CVZ, 22°S-27.5°S) and the South Volcanic Zone (SVZ, 33°S-43°S), with the former most likely originating in the Atacama Desert. Dust availability in the SVZ appears to be related to the weathering of volcanic deposits and the development of ash-derived Andosols, and influenced by local factors that might include vegetation cover. Variations in the dust fluxes from the two sources are in phase with both the dust flux and temperature records from Antarctican ice cores. We show that the forcing of dust provenance over time in the EEP overall is influenced by high-southerly-latitude climate conditions, leading to changes in the latitudinal position and strength of the South Westerlies as well as the coastal winds that blow northward along the Chilean margin. The net result is a modulation of dust emission from the Atacama Desert and the SVZ via a northward migration of the South Westerlies during cold periods and southward retreat during glacial terminations.
The Crozet Plateau is a 54 Ma-old volcanic plateau that supports five islands characterized by recent volcanic manifestations that are the surface expression of a deep-mantle plume. Due to their remote location and difficult access, the... more
The Crozet Plateau is a 54 Ma-old volcanic plateau that supports five islands characterized by recent volcanic manifestations that are the surface expression of a deep-mantle plume. Due to their remote location and difficult access, the Crozet Islands are poorly sampled. Both the petrological descriptions and geochemical data are scarce. Thus, the sources of the Crozet plume are still under debate. Similarly, the interactions between the Southwest Indian Ridge (SWIR) and the Crozet plume remain questioned. Here, we present a new set of isotopes (Pb, Sr, Nd and He), major and trace elements data on basalts from three islands of the Crozet Archipelago: Penguins, East, and Possession Islands. Our main purpose is to characterize the sources of the Crozet plume and to test its influence at regional scale. Two groups of lavas can be distinguished based on the isotopic data: East and Possession lavas, and Penguins lavas. Principal component analyses on our high-precision Pb isotopes data and literature data show that two mantle sources can explain most of the geochemical variability measured in Crozet lavas. A third minor contribution is however needed to fully explain the data. The entire set of isotopic compositions (Pb, Sr, Nd and He) can be explained by a mixing between three mantle sources: (1) a FOZO (Focus Zone) component, with 206 Pb/ 204 Pb higher than 19.5 and high 207 Pb/ 204 Pb, 208 Pb/ 204 Pb 87 Sr/ 86 Sr, 143 Nd/ 144 Nd and R/R a (R/R a = 3 He/ 4 He sample / 3 He/ 4 He atmosphere) ratios, that is mainly sampled Penguins lavas, (2) a component called "East-Possession" that is mostly sampled by the East-Possession lava group and which presents Pb, Sr and Nd isotope signatures similar to those of the Reunion-Mauritius Islands, and (3) a third minor contribution of the local Depleted MORB Mantle (DMM). The new He isotopes data on the Crozet plume allow us to propose that Crozet plume material is present in the segment of the Southwest Indian Ridge located between the Indomed (ITF) and Gallieni (GTF) transform faults. This hypothesis is confirmed by a mixing model based on trace-elements and isotopes data from the ITF-GTF segment of the SWIR and Crozet. We propose that the shallow mantle below the ITF-GTF segment of the SWIR is contaminated by deep material from the Crozet plume.
Copper (Cu) and zinc (Zn) are naturally present and ubiquitous in soils and are important micronutrients. Human activities contribute to the input of these metals to soils in different chemical forms, which can sometimes reach a... more
Copper (Cu) and zinc (Zn) are naturally present and ubiquitous in soils and are important micronutrients.
Human activities contribute to the input of these metals to soils in different chemical forms, which can
sometimes reach a toxic level for soil organisms and plants. Isotopic signatures could be used to trace sources of anthropogenic Cu and Zn pollution. The aim of this paper is to determine whether it is possible
to identify (i) Cu and Zn contamination in soils and their sources, on the basis of their isotopic signatures
Copper (Cu) and zinc (Zn) are naturally present and ubiquitous in soils and are important micronutrients.Human
activities contribute to the input of these metals to soils in different chemical forms,which can sometimes reach
a toxic level for soil organisms and plants. Isotopic signatures could be used to trace sources of anthropogenic Cu and Zn pollution. The aim of this paper is to determine whether it is possible to identify (i) Cu and Zn contamination in soils and their sources, on the basis of their isotopic signatures, and (ii) situations that are a priori favorable or not for tracing Cu and Zn pollution using the isotopic approach. Therefore, we compiled data from the literature on Cu and Zn isotopes in soils, rocks and pollutants and added to this database the results of our own research. As only a few studies have dealt with agricultural contamination, we also studied a soil toposequence from Brittany, France, that experienced spreading of pig slurry for tens of years. In the surface horizons of the natural soils, the δ65Cu values vary from−0.15 to 0.44‰and the δ66Zn from−0.03 to 0.43‰. Furthermore, vertical variations along soil profiles range from−0.95 to 0.44‰ for δ65Cu and from −0.53 to 0.64‰ for δ66Zn values.We concluded that pedogenetic processes can produce isotopic fractionation, yet, it is not always discernible and can be overprinted by an exogenous isotopic signature. Furthermore, some contaminants are
enriched in heavy Cu or in light Zn compared to the rock or soil, but no generalization can be made. The anthropogenic inputs can be identified based on stable Cu and Zn isotope ratios if the isotope ratios of the sources are different from those of the soil, which needs to be tested for each individual case.
Human activities contribute to the input of these metals to soils in different chemical forms, which can
sometimes reach a toxic level for soil organisms and plants. Isotopic signatures could be used to trace sources of anthropogenic Cu and Zn pollution. The aim of this paper is to determine whether it is possible
to identify (i) Cu and Zn contamination in soils and their sources, on the basis of their isotopic signatures
Copper (Cu) and zinc (Zn) are naturally present and ubiquitous in soils and are important micronutrients.Human
activities contribute to the input of these metals to soils in different chemical forms,which can sometimes reach
a toxic level for soil organisms and plants. Isotopic signatures could be used to trace sources of anthropogenic Cu and Zn pollution. The aim of this paper is to determine whether it is possible to identify (i) Cu and Zn contamination in soils and their sources, on the basis of their isotopic signatures, and (ii) situations that are a priori favorable or not for tracing Cu and Zn pollution using the isotopic approach. Therefore, we compiled data from the literature on Cu and Zn isotopes in soils, rocks and pollutants and added to this database the results of our own research. As only a few studies have dealt with agricultural contamination, we also studied a soil toposequence from Brittany, France, that experienced spreading of pig slurry for tens of years. In the surface horizons of the natural soils, the δ65Cu values vary from−0.15 to 0.44‰and the δ66Zn from−0.03 to 0.43‰. Furthermore, vertical variations along soil profiles range from−0.95 to 0.44‰ for δ65Cu and from −0.53 to 0.64‰ for δ66Zn values.We concluded that pedogenetic processes can produce isotopic fractionation, yet, it is not always discernible and can be overprinted by an exogenous isotopic signature. Furthermore, some contaminants are
enriched in heavy Cu or in light Zn compared to the rock or soil, but no generalization can be made. The anthropogenic inputs can be identified based on stable Cu and Zn isotope ratios if the isotope ratios of the sources are different from those of the soil, which needs to be tested for each individual case.
During the last glacial period, Greenland's climate shifted between cold (stadial) and warm (interstadial) phases that were accompanied by ocean circulation changes characterized by reduced Atlantic Meridional Overturning Circulation... more
During the last glacial period, Greenland's climate shifted between cold (stadial) and warm (interstadial) phases that were accompanied by ocean circulation changes characterized by reduced Atlantic Meridional Overturning Circulation (AMOC) during stadials. Here we present new data from the western tropical Atlantic demonstrating that AMOC slowdowns preceded some of the large South American rainfall events that took place during stadials. Based on 231 Pa/ 230 Th and Ti/Ca measurements in the same sediment core, we determine that the AMOC started to slowdown 1420 ± 250 and 690 ± 180 (1σ) years before the onset of two large precipitation events associated with Heinrich stadials. Our results bring unprecedented evidence that AMOC changes could be at the origin of the large precipitation events observed in tropical South America during Heinrich stadials. In addition, we propose a mechanism explaining the differences in the extent and timing of AMOC slowdowns associated with shorter and longer stadials.