Glacial Interglacial Climate Cycles Research Articles

Variability in aerobic methane oxidation over the past 1.2 Myrs recorded in microbial biomarker signatures from Congo fan sediments

Methane (CH4) is a strong greenhouse gas known to have perturbed global climate in the past, especially when released in large quantities over short time periods from continental or marine sources. It is therefore crucial to understand and, if possible, quantify the individual and combined response of these variable methane sources to natural climate variability. However, past changes in the stability of greenhouse gas reservoirs remain uncertain and poorly constrained by geological evidence. Here, we present a record from the Congo fan of a highly specific bacteriohopanepolyol (BHP) biomarker for aerobic methane oxidation (AMO), 35-aminobacteriohopane-30,31,32,33,34-pentol (aminopentol), that identifies discrete periods of increased AMO as far back as 1.2Ma. Fluctuations in the concentration of aminopentol, and other 35-aminoBHPs, follow a pattern that correlates with late Quaternary glacial-interglacial climate cycles, with highest concentrations during warm periods. We discuss possible sources of aminopentol, and the methane consumed by the precursor methanotrophs, within the context of the Congo River setting, including supply of methane oxidation markers from terrestrial watersheds and/or marine sources (gas hydrate and/or deep subsurface gas reservoir). Compound-specific carbon isotope values of −30‰ to −40‰ for BHPs in ODP 1075 and strong similarities between the BHP signature of the core and surface sediments from the Congo estuary and floodplain wetlands from the interior of the Congo River Basin, support a methanotrophic and likely terrigenous origin of the 35-aminoBHPs found in the fan sediments. This new evidence supports a causal connection between marine sediment BHP records of tropical deep sea fans and wetland settings in the feeding river catchments, and thus tropical continental hydrology. Further research is needed to better constrain the different sources and pathways of methane emission. However, this study identifies the large potential of aminoBHPs, in particular aminopentol, to trace and, once better calibrated and understood, quantify past methane sources and fluxes from terrestrial and potentially also marine sources.

Lithium distribution and isotopic fractionation during chemical weathering and soil formation in a loess profile

Lithium (Li) is a fluid-mobile element and δ7Li in secondary deposits represents an excellent proxy for silicate weathering and authigenic mineral formation. The soil samples from 1205 to 1295cm in the Weinan profile, one of the best developed loess-paleosol sequences covering the last glacial–interglacial climatic cycle, were collected and chemically separated into detritus and carbonate fractions for subsequent analyses of Li, δ7Li, major and trace elements. Other desert specimens (i.e., Qaidam Desert, Tengger Desert, Badain Juran Desert and Taklimakan Desert) near the Chinese Loess Plateau (CLP) and various standard clays were analyzed for assisting provenance determination. The Li and δ7Li distributions in the detritus are rather homogeneous, 1.4–2.0μg/g and +2.5‰ to +4.7‰, respectively, compared with the carbonate fraction. The detrital δ7Li varies systematically with magnetic susceptibility and grain size changes, reflecting significant Li isotopic variation associated with sources and mineralogy of detrital material. On the other hand, Li and δ7Li in carbonates show large changes, 781–963ng/g and −4.1‰ to +10.2‰, respectively. These carbonate δ7Li correlated well with the estimated index of chemical weathering, as a result of Li mobilization and soil formation during chemical weathering.

Pleistocene range dynamics in the eastern Greater Cape Floristic Region: A case study of the Little Karoo endemic Berkheya cuneata (Asteraceae)

The glacial–interglacial climate cycles of the Pleistocene played a significant role in dramatically altering species distributions across the globe. However, the climate of the Greater Cape Floristic Region is thought to have been decoupled from global fluctuations and the current Mediterranean climate remained relatively buffered during this period. Here we explore the roles of climate stability and the topographic complexity of the region on the range history of an endemic Little Karoo plant, Berkheya cuneata, using ensemble species distribution modelling and multi-locus phylogeography. The species distribution models projected onto downscaled climate simulation of the Last Glacial Maximum demonstrated a considerable range contraction and fragmentation into the western and eastern Little Karoo, separated by the Rooiberg inselberg. This population fragmentation is mirrored in the phylogeographic structuring of both chloroplast and nuclear DNA. These results suggest that sufficient climatic buffering coupled with regionally complex topography ensured the localised population persistence during Pleistocene climate cycles but these features have also promoted population vicariance in this, and likely other, Little Karoo lowland species.

Fluvial terrace formation in the eastern Fenwei Basin, China, during the past 1.2 Ma as a combined archive of tectonics and climate change

The key to understanding the fluvial response to climate change and surface uplift is a thorough distinguishing between their roles in terrace formation. Previous studies have tended to attribute the fluvial behavior of deposit–incision alternation to climate cyclicity. A preliminary explanation is proposed here for terrace formation in the eastern part of the Fenwei Basin, China. The observed fluvial terrace sequence was developed by the Yellow River deeply downcutting into the Emei Platform, which was uplifted within the eastern Fenwei Basin, and therefore it probably records the tectonic history of this platform. On the basis of the magnetostratigraphy and optically stimulated luminescence (OSL) dating, a ca. 1.2Ma chronology was established for this terrace sequence. Pedostratigraphic analysis of the loess deposits accumulated on each tread reveals that the terrace deposits are overlain immediately by a paleosol bed, suggesting that the abandonment of these terraces due to fluvial incision occurred at the transitions from glacial to interglacial climates. The glacial–interglacial climate cycle probably has a temporal control on the fluvial behavior of deposit–incision alternation, even though the Yellow River develops in the subsiding Fenwei Basin. The terrace generation may be sporadic and in the form of unusual stacked pattern before the Emei Platform was uplifted within the eastern Fenwei Basin. The terrace staircases however, formed in synchrony with glacial–interglacial climate cycles since this platform began to uplift in the late Middle Pleistocene. This result indicates that uplift may be necessary in large terrace staircase genesis. It can force the river system to downcut deeply enough during climatic transitions to separate terrace levels adequately, favoring the generation and subsequent preservation of the terrace treads. The terrace sequence of the Yellow River in the eastern Fenwei Basin therefore can be considered as a combined archive of climate change and local tectonic activity.

A hypothesis linking sub-optimal seawater <I>p</I>CO<sub>2</sub> conditions for cnidarian-<I>Symbiodinium</I> symbioses with the exceedence of the interglacial threshold (>260 ppmv)

Abstract. Most scleractinian corals and many other cnidarians host intracellular photosynthetic dinoflagellate symbionts ("zooxanthellae"). The zooxanthellae contribute to host metabolism and skeletogenesis to such an extent that this symbiosis is well recognised for its contribution in creating the coral reef ecosystem. The stable functioning of cnidarian symbioses is however dependent upon the host's ability to maintain demographic control of its algal partner. In this review, I explain how the modern envelope of seawater conditions found within many coral reef ecosystems (characterised by elevated temperatures, rising pCO2, and enriched nutrient levels) are antagonistic toward the dominant host processes that restrict excessive symbiont proliferation. Moreover, I outline a new hypothesis and initial evidence base, which support the suggestion that the additional "excess" zooxanthellae fraction permitted by seawater pCO2 levels beyond 260 ppmv significantly increases the propensity for symbiosis breakdown ("bleaching") in response to temperature and irradiance extremes. The relevance of this biological threshold is discussed in terms of historical reef extinction events, glacial-interglacial climate cycles and the modern decline of coral reef ecosystems.

Rapid climate change and no-analog vegetation in lowland Central America during the last 86,000 years

Glacial–interglacial climate cycles are known to have triggered migrations and reassortments of tropical biota. Although long-term precessionally-driven changes in temperature and precipitation have been demonstrated using tropical sediment records, responses to abrupt climate changes, e.g. the cooling of Heinrich stadials or warmings of the deglaciation, are poorly documented. The best predictions of future forest responses to ongoing warming will rely on evaluating the influences of both abrupt and long-term climate changes on past ecosystems. A sedimentary sequence recovered from Lake Petén-Itzá, Guatemalan lowlands, provided a natural archive of environmental history. Pollen and charcoal analyses were used to reconstruct the vegetation and climate history of the area during the last 86,000 years. We found that vegetation composition and air temperature were strongly influenced by millennial-scale changes in the North Atlantic Ocean. Whereas Greenland warm interstadials were associated with warm and relatively wet conditions in the Central American lowlands, cold Greenland stadials, especially those associated with Heinrich events, caused extremely dry and cold conditions. Even though the vegetation seemed to have been highly resilient, plant associations without modern analogs emerged mostly following sharp climate pulses of either warmth or cold, and were paralleled by exceptionally high rates of ecological change. Although pulses of temperature change are evident in this 86,000-year record none matched the rates projected for the 21st Century. According to our findings, the ongoing rapid warming will cause no-modern-analog communities, which given the improbability of returning to lower-than-modern CO2 levels, anthropogenic barriers to migration, and increased anthropogenic fires, will pose immense threats to the biodiversity of the region.

Different orbital rhythms in the Asian summer monsoon records from North and South China during the Pleistocene

Here we construct a Pleistocene astronomical timescale for the Nihewan fluvio–lacustrine sediments (North China), via tuning a stacked summer monsoon index generated from grain size and low-field magnetic susceptibility records to orbital obliquity and precession. Combining the summer monsoon records retrieved from the Nihewan and the Chinese loess deposits in North China on the one hand, and those from the stalagmites and the marine deposits in South China on the other, the Asian summer monsoon records from North and South China appear to show different orbital rhythms during the Pleistocene. The monsoon records from both the Nihewan Basin and Chinese Loess Plateau are equally characterized by dominant obliquity (41 kyr) before ~ 0.9 Ma and dominant eccentricity (100 kyr) after this time, closely following the marine δ 18O record. In contrast, the δ 18O record of stalagmites from South China (Wang et al., 2001, 2008b; Cheng et al., 2009) and the iron oxide proxy record from the South China Sea (Zhang et al., 2007, 2009; Ao et al., 2011), which are considered as a proxy indicator of the Asian summer monsoon intensity in South China, reveal a dominant cyclicity of precession (23 kyr) over the past 1.8 Myr, closely following the solar insolation curve instead. We further present a possible interpretation of the different orbital rhythms in the Asian summer monsoon records from North and South China. The orbital rhythm in the summer monsoon records from North China is mainly modulated by the migration of the Asian summer monsoon due to changes in sea level and Northern Hemisphere ice volume during glacial–interglacial cycles. Strong summer monsoon may have reached North China mainly during interglacial periods. During glacial periods when the sea level was low and Northern Hemisphere ice volume was large, the southeasterly migration of summer monsoon would make North China beyond the reach of the summer monsoon. Thus the summer monsoon records from North China primarily show a cyclic oscillation similar to the glacial–interglacial climate cycles as indicated by cycle-by-cycle correlation between monsoon and marine oxygen isotope records. However, the summer monsoon always prevailed over South China during both glacial and interglacial periods. Therefore, the orbital-scale variability of the summer monsoon in South China shows a direct response to the orbital variations in the low-latitude summer insolation, without significant influence from the migration of monsoon during glacial–interglacial cycles.

An updated astronomical timescale for the Plio-Pleistocene deposits from South China Sea and new insights into Asian monsoon evolution

Here we present an improved astronomical timescale since 5 Ma as recorded in the ODP Site 1143 in the southern South China Sea, using a recently published Asian summer monsoon record (hematite to goethite content ratio, Hm/Gt) and a parallel benthic δ 18O record. Correlation of the benthic δ 18O record to the stack of 57 globally distributed benthic δ 18O records (LR04 stack) and the Hm/Gt curve to the 65°N summer insolation curve is a particularly useful approach to obtain refined timescales. Hence, it constitutes the basis for our effort. Our proposed modifications result in a more accurate and robust chronology than the existing astronomical timescale for the ODP Site 1143. This updated timescale further enables a detailed study of the orbital variability of low-latitude Asian summer monsoon throughout the Plio-Pleistocene. Comparison of the Hm/Gt record with the δ 18O record from the same core reveals that the oscillations of low-latitude Asian summer monsoon over orbital scales differed considerably from the glacial–interglacial climate cycles. The popular view that summer monsoon intensifies during interglacial stages and weakens during glacial stages appears to be too simplistic for low-latitude Asia. In low-latitude Asia, some strong summer monsoon intervals appear to have also occurred during glacial stages in addition to their increased occurrence during interglacial stages. Vice versa, some notably weak summer monsoon intervals have also occurred during interglacial stages next to their anticipated occurrence during glacial stages. The well-known mid-Pleistocene transition (MPT) is only identified in the benthic δ 18O record but not in the Hm/Gt record from the same core. This suggests that the MPT may be a feature of high- and middle-latitude climates, possibly determined by high-latitude ice sheet dynamics. For low-latitude monsoonal climate, its orbital-scale variations respond more directly to insolation and are little influenced by high-latitude processes, thus the MPT is likely not recorded. In addition, the Hm/Gt record suggests that low-latitude Asian summer monsoon intensity has a long-term decreasing trend since 2.8 Ma with increased oscillation amplitude. This long-term variability is presumably linked to the Northern Hemisphere glaciation since then.

Quaternary-scale evolution of sequences of talus flatirons in the hyperarid Negev

Talus flatiron sequences are ubiquitous landforms in arid and semiarid regions characterized by horizontal erodible rocks capped by more resistant rocks. Alternating phases of deposition and erosion lead to the formation of generations of talus flatirons in which ancient ones are located farther from the source cliff. The existing conceptual model of this systematic spatial distribution of talus flatirons is related to glacial–interglacial climatic cycles and to relatively high rates of cliff retreat. Three groups of talus flatirons were analyzed in the northeastern hyperarid Negev desert. The analyses include mapping, age determination using optically stimulated luminescence and cosmogenic nuclide exposure dating, electrical resistivity tomography and sedimentological and soil analyses. All talus flatirons contain gypsic-salic soil catena typical to hyperarid climate (< 80 mm yr − 1 ). No pedogenic indicators of past wetter environments such as buried calcic soil horizons, evidence of intense biogenic activity, or buried organic material were observed. 10Be exposure ages of the talus flatirons suggest that they were deposited during the middle Pleistocene; ~ 610 ka and ~ 170 ka for the oldest and intermediate-aged talus flatiron groups, respectively. These ages, combined with the present location of the talus flatirons relative to the source cliff yield retreat rates of 6–12 and ~ 200 m Ma − 1 for the bedrock cliff and talus flatiron apex, respectively, and in opposite directions. Our results show that (a) climatic changes at glacial–interglacial time scales are not the main controls over the formation of talus flatiron generation, and (b) that significant cliff retreat is not the main cause for the systematic spatial distribution of talus flatiron generations relative to the cliff. Our results indicate that the process of talus flatiron formation in the Negev desert must be associated with the balance between production, deposition, and removal of clasts under hyperarid conditions with only a minor cliff retreat.

Evolutionary drivers of phylogeographical diversity in the highlands of Mexico: a case study of the Crotalus triseriatus species group of montane rattlesnakes

Aim To assess the genealogical relationships of widespread montane rattlesnakes in the Crotalus triseriatus species group and to clarify the role of Late Neogene mountain building and Pleistocene pine–oak forest fragmentation in driving the diversification of Mexican highland taxa. Location Highlands of mainland Mexico and the south-western United States (Texas, New Mexico, and Arizona). Methods A synthesis of inferences was used to address several associated questions about the biogeography of the Mexican highlands and the evolutionary drivers of phylogeographical diversity in co-distributed taxa. We combined extensive range-wide sampling (130 individuals representing five putative species) and mixed-model phylogenetic analyses of 2408 base pairs of mitochondrial DNA to estimate genealogical relationships and divergence times within the C. triseriatus species group. We then assessed the tempo of diversification using a maximum likelihood framework based on the birth–death process. Estimated times of divergences provided a probabilistic temporal component and questioned whether diversification rates have remained constant or varied over time. Finally, we looked for phylogeographical patterns in other co-distributed taxa. Results We identified eight major lineages within the C. triseriatus group, and inferred strong correspondence between maternal and geographic history within most lineages. At least one cryptic species was detected. Relationships among lineages were generally congruent with previous molecular studies, with differences largely attributable to our expanded taxonomic and geographic sampling. Estimated divergences between most major lineages occurred in the Late Miocene and Pliocene. Phylogeographical structure within each lineage appeared to have been generated primarily during the Pleistocene. Although the scale of genetic diversity recognized affected estimated rates of diversification, rates appeared to have been constant through time. Main conclusions The biogeographical history of the C. triseriatus group implies a dynamic history for the highlands of Mexico. The Neogene formation of the Transvolcanic Belt appears responsible for structuring geographic diversity among major lineages. Pleistocene glacial–interglacial climatic cycles and resultant expansions and contractions of the Mexican pine–oak forest appear to have driven widespread divergences within lineages. Climatic change, paired with the complex topography of Mexico, probably produced a myriad of species-specific responses in co-distributed Mexican highland taxa. The high degree of genetic differentiation recovered in our study and others suggests that the Mexican highlands may contain considerably more diversity than currently recognized.

Limited hydrologic response to Pleistocene climate change in deep vadose zones — Yucca Mountain, Nevada

Understanding the movement of water through thick vadose zones, especially on time scales encompassing long-term climate change, is increasingly important as societies utilize semi-arid environments for both water resources and sites viewed as favorable for long-term disposal or storage of hazardous waste. Hydrologic responses to Pleistocene climate change within a deep vadose zone in the eastern Mojave Desert at Yucca Mountain, Nevada, were evaluated by uranium-series dating of finely layered hyalitic opal using secondary ion mass spectrometry. Opal is present within cm-thick secondary hydrogenic mineral crusts coating floors of lithophysal cavities in fractured volcanic rocks at depths of 200 to 300 m below land surface. Uranium concentrations in opal fluctuate systematically between 5 and 550 μg/g. Age-calibrated profiles of uranium concentration correlate with regional climate records over the last 300,000 years and produce time-series spectral peaks that have distinct periodicities of 100- and 41-ka, consistent with planetary orbital parameters. These results indicate that the chemical compositions of percolating solutions varied in response to near-surface, climate-driven processes. However, slow (micrometers per thousand years), relatively uniform growth rates of secondary opal and calcite deposition spanning several glacial–interglacial climate cycles imply that water fluxes in the deep vadose zone remained low and generally buffered from the large fluctuations in available surface moisture during different climates.

Links between eccentricity forcing and the 100,000-year glacial cycle

The 100,000-year glacial cycles are generally thought to be driven by the eccentricity of the Earth’s orbit. Statistical analyses of climate variability and orbital forcing over the past five million years indicate that the glacial cycles are the result of an internal climate oscillation phase locked to the 100,000-year eccentricity cycle. Variations in the eccentricity (100,000 yr), obliquity (41,000 yr) and precession (23,000 yr) of Earth’s orbit have been linked to glacial–interglacial climate cycles. It is generally thought that the 100,000-yr glacial cycles of the past 800,000 yr are a result of orbital eccentricity1,2,3,4. However, the eccentricity cycle produces negligible 100-kyr power in seasonal or mean annual insolation, although it does modulate the amplitude of the precession cycle. Alternatively, it has been suggested that the recent glacial cycles are driven purely by the obliquity cycle5,6,7. Here I use statistical analyses of insolation and the climate of the past five million years to characterize the link between eccentricity and the 100,000-yr glacial cycles. Using cross-wavelet phase analysis, I show that the relative phase of eccentricity and glacial cycles has been stable since 1.2 Myr ago, supporting the hypothesis that 100,000-yr glacial cycles are paced8,9,10 by eccentricity4,11. However, I find that the time-dependent 100,000-yr power of eccentricity has been anticorrelated with that of climate since 5 Myr ago, with strong eccentricity forcing associated with weaker power in the 100,000-yr glacial cycle. I propose that the anticorrelation arises from the strong precession forcing associated with strong eccentricity forcing, which disrupts the internal climate feedbacks that drive the 100,000-yr glacial cycle. This supports the hypothesis that internally driven climate feedbacks are the source of the 100,000-yr climate variations12.

Insolation and CO2 contribution to the interglacial climate before and after the Mid-Brunhes Event

Reconstructions of climate from marine sediment(1) and ice(2) cores show that the amplitude of glacial-interglacial climate cycles increased substantially after the Mid-Brunhes Event(3), about 430,000 years ago. Interglacial periods before the event seem to be characterized by larger continental ice sheets, lower sea level(4,5), cooler temperatures in Antarctica2 and lower atmospheric CO2 concentrations(6), relative to the more recent interglacials. Here we use an Earth system model of intermediate complexity to assess the contributions of insolation and greenhouse-gas concentrations to the climate associated with the peaks of all the interglacials over the past 800,000 years. Our simulations recreate the expected warmer interglacials after the Mid-Brunhes Event and suggest that later interglacials are warmer primarily because of increased global mean temperatures during Northern Hemisphere winters. This warmth arises from increased insolation during this season, relative to the interglacials that preceded the Mid-Brunhes Event, in conjunction with increased atmospheric greenhouse-gas concentrations. The effect of boreal winters and of the Southern Hemisphere, which is also warmer during austral winters, on the carbon cyle should be assessed when investigating the underlying causes of the higher CO2 concentrations during the later interglacials.

Sequences and genesis of the Yellow River terraces from Sanmen Gorge to Kouma

Based on field landscape investigations, thermoluminescence (TL), magnetostratigraphy and loess-paleosol sequence, we found that there are at least four Yellow River terraces, whose ages are 0.86 Ma, 0.62 Ma, 0.13 Ma and 0.05 Ma, in Yuxi Fault-Uplift (from Sanmen Gorge to Mengjin) and at least three Yellow River terraces, whose ages are 1.24 Ma, 0.25 Ma and 0.05 Ma, in Huabei Fault Depression (from Mengjin to Kouma). All the terraces have a similar structure that several meters of paleosols directly develop on the top of fluvial silt. It shows that the Yellow River incised and consequently abandoned floodplain converted to terrace during the interglacial period. Therefore, there may be a link between the formation of terraces and glacial-interglacial climatic cycles. However, the differences in the Yellow River terrace sequences and ages between Yuxi Fault-Uplift and Huabei Fault Depression indicate that the surface uplift should play an important role in the formation of these terraces.

An improved flow analysis–ion chromatography method for determination of cationic and anionic species at trace levels in Antarctic ice cores

A method was developed for the quantitative determination of cations and anions in Antarctic ice cores at μg L −1 and sub-μg L −1 levels by ion chromatography (IC), after ultra-clean decontamination procedures. Strict manipulation and decontamination procedures were used in sub-sampling, in order to minimise sample contamination. Na +, NH 4 +, K +, Mg 2+ and Ca 2+ were determined by 12-min isocratic elution (H 2SO 4 eluent). Contemporaneously, in a parallel device, F −, MSA (methanesulfonic acid), Cl −, NO 3 − and SO 4 2− were analysed in a single 12-min run with multiple-step elution using Na 2CO 3/NaHCO 3 as eluent. Melted ice samples were pumped from their still-closed containers (polystyrene accuvettes with polyethylene caps), shared between the two ion chromatographic systems, online filtered (0.45 μm Teflon membrane) and pre-concentrated (anions and cations pre-concentration columns) using a flow analysis system, thus avoiding uptake of contaminants from the laboratory atmosphere. Sensitivity, linear range, reproducibility and detection limit were evaluated for each chemical species. Anion or cation detection limits ranged from 0.01 to 0.15 μg L −1 by using a relatively small sample volume (1.5 mL). Such values are significantly lower than those reported in literature for almost all the components. These methods were successfully applied to the analysis of cations and anions at trace levels in the Dome C ice core. The composition of the atmospheric aerosol for the last 850 kyr was reconstructed by high-resolution continuous chemical stratigraphies. Concentration trends in the last nine glacial-interglacial climatic cycles were shown and briefly discussed.

Dating of erosion surface and terraces in the eastern Qilian Shan, northwest China

AbstractThe actively deformed foreland of eastern Qilian Shan (mountains) contains well‐preserved geomorphic features such as erosion surfaces, river terraces and tectonically uplifted alluvial fans, providing suitable archives for research on regional tectonic activities and palaeoclimatic changes. These geomorphic surfaces are well dated by using a combination of magnetostratigraphy, electron spin resonance, thermoluminescence, infra‐red stimulated luminescence, radiocarbon dating, and correlation with the well‐established loess–palaeosol sequences of China. Our results show that the erosion surface formed about 1·4 Ma ago, and the age of river terraces is 1·24 Ma, 820–860 ka, 780 ka, 420–440 ka, 230–250 ka, 140 ka, 60 ka and 10 ka, respectively. Valley incision rates of c. 0·09–0·25 m ka−1 have been identified. The repetitive stratigraphic and geomorphic pattern of these terraces indicates the fluvial sedimentation–incision cycles are tightly associated with the 100‐ka glacial–interglacial climatic cycles. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Palaeovalleys and fault-controlled depocentres in the Late-Ordovician glacial record of the Murzuq Basin (central Libya)

Abstract In the Gargaf area, the stratigraphic architecture of the Late-Ordovician glacial drift results from successive glacial erosion events, with the location of the main glacial valleys partly controlled by inherited Panafrican structural trends, and by the existence of glacio-isostatically induced fault-related depocentres. Four laterally discontinuous, depositional units correspond to the filling of palaeovalleys. Each of the corresponding basal bounding surfaces was incised during a major ice front advance, reaching at least the northern Gargaf (>28°S). The bulk of the glacial record is made up of fluvial to shallow-marine sediments deposited in relatively distal glacial environments. Each unit, which recorded a glacial-interglacial climatic cycle, can be used for correlation throughout the Murzuq Basin and even at the scale of the North Gondwana platform. To cite this article: J.-F. Ghienne et al., C. R. Geoscience 335 (2003).

Atmospheric CO2 variations over the last three glacial-interglacial climatic cycles deduced from the Dome Fuji deep ice core, Antarctica using a wet extraction technique

A deep ice core drilled at Dome Fuji, East Antarctica was analyzed for the CO 2 concentration using a wet extraction method in order to reconstruct its atmospheric variations over the past 320 kyr, which includes three full glacial–interglacial climatic cycles, with a mean time resolution of about 1.1 kyr. The CO 2 concentration values derived for the past 65 kyr are very close to those obtained from other Antarctic ice cores using dry extraction methods, although the wet extraction method is generally thought to be inappropriate for the determination of the CO 2 concentration. The comparison between the CO 2 and Ca 2+ concentrations deduced from the Dome Fuji core suggests that calcium carbonate emitted from lands was mostly neutralized in the atmosphere before reaching the central part of Antarctica, or that only a small part of calcium carbonate was involved in CO 2 production during the wet extraction process. The CO 2 concentration for the past 320 kyr deduced from the Dome Fuji core varies between 190 and 300 ppmv, showing clear glacial–interglacial variations similar to the result of the Vostok ice core. However, for some periods, the concentration values of the Dome Fuji core are higher by up to 20 ppmv than those of the Vostok core. There is no clear indication that such differences are related to variations of chemical components of Ca 2+ , microparticle and acidity of the Dome Fuji core. DOI: 10.1034/j.1600-0889.2003.00050.x

Atmospheric CO2 variations over the last three glacial’interglacial climatic cycles deduced from the Dome Fuji deep ice core, Antarctica using a wet extraction technique

A deep ice core drilled at Dome Fuji, East Antarctica was analyzed for the CO2 concentration using a wet extraction method in order to reconstruct its atmospheric variations over the past 320 kyr, which includes three full glacial—interglacial climatic cycles, with a mean time resolution of about 1.1 kyr. The CO2 concentration values derived for the past 65 kyr are very close to those obtained from other Antarctic ice cores using dry extraction methods, although the wet extraction method is generally thought to be inappropriate for the determination of the CO2 concentration. The comparison between the CO2 and Ca2× concentrations deduced from the Dome Fuji core suggests that calcium carbonate emitted from lands was mostly neutralized in the atmosphere before reaching the central part of Antarctica, or that only a small part of calcium carbonate was involved in CO2 production during the wet extraction process. The CO2 concentration for the past 320 kyr deduced from the Dome Fuji core varies between 190 and 300 ppmv, showing clear glacial—interglacial variations similar to the result of the Vostok ice core. However, for some periods, the concentration values of the Dome Fuji core are higher by up to 20 ppmv than those of the Vostok core. There is no clear indication that such differences are related to variations of chemical components of Ca2×, microparticle and acidity of the Dome Fuji core.

Neodymium isotopes in planktonic foraminifera: a record of the response of continental weathering and ocean circulation rates to climate change

The neodymium (Nd) isotope composition of ancient seawater is a potentially useful tracer of changes in continental inputs and ocean circulation on timescales of a few ka. Here we present the first Nd isotope record for seawater using sedimentary foraminifera cleaned using standard oxidative–reductive techniques. The data, along with Mn/Ca ratios, suggest that cleaned foraminifera provide a reliable record of Nd in seawater and hold out the prospect of using Nd in foraminifera to examine changes in seawater that accompany glacial–interglacial climatic cycles. The principal potential problem to be overcome with the use of forams as records of trace elements in ancient seawater is their diagenetic Fe–Mn coatings. These contain large amounts of Nd and other trace elements but can be cleaned off using highly reducing reagents. Mn/Ca ratios for the majority of the cleaned sedimentary foraminifera analysed here lie within the range (10–100 μmol/mol) that has yielded success in studies of transition elements in forams. Mass-balance modelling suggests that for residual Mn/Ca ratios <100 μmol/mol, Nd added to the foram in the coating will never shift the measured Nd isotope composition significantly away from the seawater value acquired by the foram test in the water column. Additionally, Nd concentrations measured in cleaned sedimentary foraminifera are comparable with those for a modern sample that has never encountered diagenetic fluids. Finally, core-top planktonic foraminifera for two sites have Nd isotope compositions that are identical to local surface seawater. The data we present here for Labrador Sea forams over the past 2.5 m.y. are interpreted in terms of changes in the seawater isotopic composition. The data show a pronounced shift from ε Nd values of ∼−12 to ∼−19 in the period 2.5–1.5 Ma. This change is interpreted to result from the initiation of Northern Hemisphere glaciation and the increased derivation of Labrador Sea Nd via ice-rafting from Archaean terranes in central Canada. In combination with stable isotope and foraminiferal relative species abundance data, the new Nd data are consistent with the surface hydrography of the Labrador Sea being dominated by a fluctuating balance between cold, polar waters containing unradiogenic Nd and warm, subtropical waters containing more radiogenic Nd. The major change in Labrador Sea Nd that is observed in the past 2.5 Ma can, on its own, account for the change in the Nd isotope composition of North Atlantic Deep Water over the same time period.