Geomorphology, Stratigraphy and 14C-Chronology of Ancient Tufas at Louie Creek, Northwest Queensland, Australia

Louie Creek is a karst spring-fed stream situated in the seasonally humid tropics of northwest Queensland, Australia. It rises as a series of small exsurgences along the eastern edge of the Barkly Tableland. As it enters the lowlands of the Carpentaria plain, the creek deposits tufa which produces a series of cascades. This modern tufa extends discontinuously for about 1.5 km. A series of ancient tufas, in places lying adjacent to sites of modern deposition, extends discontinously for about 8 km downstream. At least two ancient tufa units are preserved at one location, Little lndarri site. The older unit comprises a sequence of well-preserved barrages with an orientation transverse to present-day stream flow. In places, erosion has reduced these barrages to their calcrete substrate. The older tufa is overlain in places by sediment which has become cemented to form a second calcrete unit. This sediment is in turn succeeded by the younger ancient tufa. Subsequent river incision has removed part of the sediment from the older unit and exposed several contact points between the ancient tufa and calcrete units. Radiocarbon dating of the Little lndarri site tufas, as well as other ancient Louie Creek units, yielded apparent ages ranging from ~ 30 to ~ 14 ka BP, suggesting that conditions were sufficiently wet during the period immediately preceding and throughout the Last Glacial Maximum for tufa depostion to occur. However, ancient tufa formation occurred during a phase of net river aggradation. There is geomorphic evidence that such aggradation was a result of an increased sediment supply to the fluvial system, most likely in response to conditions drier than present. Results from studies elsewhere in the region support such a Late Pleistocene trend. Incision of Louie Creek, which postdates the youngest of the dated ancient tufas, is most likely to have resulted from a shift to wetter conditions during the early Holocene,Le Louie Creek est un ruisseau issue d'une source karstique dans les tropiques humides du nord-ouest du Queensland. En entrant dans la plaine de Carpentaria, Ie ruisseau dépose un tuf qui forme une série de cascades. Ce tuf moderne s'étend de façon discontinue sur environ 1,5 km. Une série de tufs anciens, parfois adjacents, s'étendent de façon discontinue sur environ 8 km en aval. Au moins deux unités de tuf ancien sont conservées à un endroit, le site de Little lndarri. L'unité la plus ancienne comprend une suite de barrages bien conservés dont l'orientation est transversale à l'écoulement actuel. Le tuf le plus ancien est parfois recouvert par un sédiment qui s'est cimenté pour former une deuxième unité d'encroûtement. Ce sédiment, à son tour, laisse place à un tuf ancien plus récent. L'encaissement subséquent du ruisseau a fait disparaître une partie du sédiment de l'unité plus ancienne. La datation au 14C des tufs de Little lndari ainsi que d'autres unités anciennes du Louie Creek a donné des âges entre ~ 30 et ~ 14 ka BP, indiquant que le climat était suffisamment humide au cours de la période précédente et tout au long du dernier maximum glaciaire pour permettre le dépôt de tuf. Toutefois, la formation du tuf ancien s'est produite pendant une phase d'alluvionnement important. Des indications géomorphologiques montrent qu'un tel alluvionnement était le résultat de l'accroissement de l'apport sédimentaire dans le réseau, tout probablement en réponse à un climat plus sec. Les résultats d'études entreprises ailleurs dans la région corrobore cette tendance au Pleistocene supérieur. L'encaissement du Louie Creek, survenu après la formation du plus récent des tufs anciens, résulte probablement d'un changement à des conditions plus humides survenu au cours de l'Holocène inférieur.Der Louie Creek ist ein im Frùhling genâhrtes Karst-FlùBchen in den saisonbedingt feuchten Tropen von Nordwest-Queensland. Er entspringt als eine Série von kleinen AusfluBstellen làngs des ôstlichen Randes des Barkley-Tafellandes. Sobald er in die Niederungen der Carpentaria-Ebene eintritt, lagert er Tuffe ab, welche eine Série von Wasserfâllen produzieren. Dieser moderne Tuff breitet sich diskontinuierlich ùber etwa 1,5 km aus. Eine Série von alten Tuffen, welche sich an Plâtzen neben der modernen Ablagerung befinden, erstreckt sich diskontinuierlich ùber etwa 8 km fluBabwàrts. Mindestens zwei alte Tuff-Einheiten sind an einer Stelle erhalten, am Platz von Little lndarri. Die altère Einheit umfaBt eine Folge von guterhaltenen Dâmmen, deren Orientierung quer zum heutigen Lauf liegt. Der altère Tuff ist an manchen Stellen von Sediment ùberlagert, das so verkittet ist, daB es eine zweite Verkrustungseinheit bildet. Auf dieses Sediment folgt wiederum ein alter Tuff neueren Datums. Die darauffolgende FlulB-Eindàmmung hat einen Teil des Sediments der âlteren Einheit ausgegraben. Radiokarbondatierung der Tuffe von Little lndarri sowie andere alte Louie Creek-Einheiten lieferten offensichtliche Alter zwischen -30 bis -14 ka v.u.Z. Sie lassen erkennen, daB das Klima wâhrend der Zeit unmittelbar vor und wâhrend des letzten glazialen Maximums feucht genug war, urn die Ablagerung von Tuff zu ermôglichen. Indessen geschah die alte Tuff-Bildung wâhrend einer Phase reiner FluBanschwemmung. Geomorphologische Belege zeigen, daB dièse Anschwemmung wegen einer verstârkten Sediment-Zufuhr zu dem FluB-System stattfand, hôchstwahrschein-lich als Reaktion auf trockenere klimatische Bedingungen als gegenwârtig. Die Eindâmmung des Louie Creek, welche nach der Bildung des jungsten der datierten alten Tuffe stattfand, ist wohl auf einen Wechsel zu feuchteren klimatischen Bedingungen wâhrend des frùheren Holozâns zurùckzufùhren

Seeking cyclonic activity records in speleothems from central Pacific: preliminary sample screening

PosterInternational audienceIdeal cyclone-sensitive speleothems would have the following properties: active at the time of sampling and recording the past few millenia, with fast growth rate, made of clean primary calcite, precipitated without kinetic fractionation, and fed by water with a short residence time allowing for the isotopic signal of short events like cyclones to be transferred but long enough for the drip water to be supersaturated. The screening for this type of speleothem is still in progress and the results presented here are very preliminary. U-Th dating: low U concentration (9-40ng/g; host rock is reef limestone) ; significant detrital content, involving large corrections and large final age uncertainties. Growth rate is highly variable, from ~0.12 mm/yr to ~0.03mm/yr. Tau12A: δ 18 O profile shows some quite large variations (~3‰), with several abrupt events that we will try to identify in other archives. δ 13 C signal amplitude is very large (~-4 to ~-14‰) suggesting the effect of prior calcite precipitation. The δ 18 O signal gets heavier when the δ 13 C does, which could reflect the rainfall amount in this context. Var12C: δ 18 O profile shows values between-3.1 and-6.3‰, with decadal or centennial fluctuations, superimposed on a general trend which is consistent with the one of the δ 13 C profile without being correlated, which implies that calcite precipitation could occur in conditions close to isotopic equilibrium. Once properly dated, this record should provide interesting information about past rainfall regimes on the island

Synchronous timing of abrupt climate changes during the last glacial period.

Abrupt climate changes during the last glacial period have been detected in a global array of palaeoclimate records, but our understanding of their absolute timing and regional synchrony is incomplete. Our compilation of 63 published, independently dated speleothem records shows that abrupt warmings in Greenland were associated with synchronous climate changes across the Asian Monsoon, South American Monsoon, and European-Mediterranean regions that occurred within decades. Together with the demonstration of bipolar synchrony in atmospheric response, this provides independent evidence of synchronous high-latitude-to-tropical coupling of climate changes during these abrupt warmings. Our results provide a globally coherent framework with which to validate model simulations of abrupt climate change and to constrain ice-core chronologies

The influence of Antarctic subglacial volcanism on the global iron cycle during the Last Glacial Maximum

Marine sediment records suggest that episodes of major atmospheric CO₂ drawdown during the last glacial period were linked to iron (Fe) fertilization of subantarctic surface waters. The principal source of this Fe is thought to be dust transported from southern mid-latitude deserts. However, uncertainty exists over contributions to CO₂ sequestration from complementary Fe sources, such as the Antarctic ice sheet, due to the difficulty of locating and interrogating suitable archives that have the potential to preserve such information. Here we present petrographic, geochemical and microbial DNA evidence preserved in precisely dated subglacial calcites from close to the East Antarctic Ice-Sheet margin, which together suggest that volcanically-induced drainage of Fe-rich waters during the Last Glacial Maximum could have reached the Southern Ocean. Our results support a significant contribution of Antarctic volcanism to subglacial transport and delivery of nutrients with implications on ocean productivity at peak glacial conditions

Synchronous timing of abrupt climate changes during the last glacial period

Many geographically dispersed records from across the globe reveal the occurrence of abrupt climate changes, called interstadial events, during the last glacial period. These events appear to have happened at the same time, but the difficulty of determining absolute dates in many of the records have made that proposition difficult to prove. Corrick et al. present results from 63 precisely dated speleothems that confirm the synchrony of those interstadial events. Their results also provide a tool with which to validate model simulations of abrupt climate change and calibrate other time series such as ice-core chronologies.Science, this issue p. 963Abrupt climate changes during the last glacial period have been detected in a global array of palaeoclimate records, but our understanding of their absolute timing and regional synchrony is incomplete. Our compilation of 63 published, independently dated speleothem records shows that abrupt warmings in Greenland were associated with synchronous climate changes across the Asian Monsoon, South American Monsoon, and European-Mediterranean regions that occurred within decades. Together with the demonstration of bipolar synchrony in atmospheric response, this provides independent evidence of synchronous high-latitude–to-tropical coupling of climate changes during these abrupt warmings. Our results provide a globally coherent framework with which to validate model simulations of abrupt climate change and to constrain ice-core chronologies

Western Pacific hydroclimate linked to global climate variability over the past two millennia

Interdecadal modes of tropical Pacific ocean-atmosphere circulation have a strong influence on global temperature, yet the extent to which these phenomena influence global climate on multicentury timescales is still poorly known. Here we present a 2,000-year, multiproxy reconstruction of western Pacific hydroclimate from two speleothem records for southeastern Indonesia. The composite record shows pronounced shifts in monsoon rainfall that are antiphased with precipitation records for East Asia and the central-eastern equatorial Pacific. These meridional and zonal patterns are best explained by a poleward expansion of the Australasian Intertropical Convergence Zone and weakening of the Pacific Walker circulation (PWC) between ~1000 and 1500 CE Conversely, an equatorward contraction of the Intertropical Convergence Zone and strengthened PWC occurred between ~1500 and 1900 CE. Our findings, together with climate model simulations, highlight the likelihood that century-scale variations in tropical Pacific climate modes can significantly modulate radiatively forced shifts in global temperature

Persistent influence of obliquity on ice age terminations since the Middle Pleistocene transition.

Radiometric dating of glacial terminations over the past 640,000 years suggests pacing by Earth's climatic precession, with each glacial-interglacial period spanning four or five cycles of ~20,000 years. However, the lack of firm age estimates for older Pleistocene terminations confounds attempts to test the persistence of precession forcing. We combine an Italian speleothem record anchored by a uranium-lead chronology with North Atlantic ocean data to show that the first two deglaciations of the so-called 100,000-year world are separated by two obliquity cycles, with each termination starting at the same high phase of obliquity, but at opposing phases of precession. An assessment of 11 radiometrically dated terminations spanning the past million years suggests that obliquity exerted a persistent influence on not only their initiation but also their duration

Effects of organic removal techniques prior to carbonate stable isotope analysis of lacustrine marls: a case study from palaeo‐lake Fucino (central Italy)

Rationale The suitability of organic matter (OM) removal pre‐treatments in isotopic studies of lacustrine carbonates is currently under debate. Naturally occurring OM seems to have a negligible effect on the bulk isotopic composition of carbonates compared with changes induced by pre‐treatments. This study provides further insights into the possible effects induced by commonly used pre‐treatments on natural lacustrine carbonates. Methods Sixteen samples from the Fucino F1–F3 lacustrine succession (Abruzzo, central Italy) were characterised for their mineralogical and geochemical composition and each was split into three identical aliquots. One aliquot was left untreated while the remaining two were treated with NaOCl and H2O2 dilutions. The same treatment was applied to an internal standard consisting of pure Carrara marble. The treated and untreated samples were analysed for their carbon (δ13C values) and oxygen (δ18O values) isotope compositions using an Analytical Precision AP2003 isotope ratio mass spectrometer. Results The samples had variable proportions of endogenic and detrital components, the detrital portion being more (less) abundant during colder (warmer) climate phases. We observed that neither the NaOCl nor the H2O2 treatment was able to completely remove OM and therefore there was selective removal of compounds within the OM pool. A possible effect of pre‐treatment is the loss of carbonates intimately interspersed within the OM, as suggested by the evolution of isotopic ratios towards the local detrital array. Conclusions Our study highlights sample‐specific changes in geochemistry associated with sample pre‐treatments; however, such changes do not seem to lead to either systematic and/or predictable isotopic shifts. We suggest that the suitability of NaOCl or H2O2 pre‐treatments for OM removal should be evaluated on a case‐by‐case basis. In the specific case of lacustrine marls from palaeo‐lake Fucino containing relatively low amounts of OM and in which both detrital and endogenic carbonates occur, both pre‐treatments should be avoided