Showing papers on "Speleothem published in 2010"

Orbital‐scale timing and mechanisms driving Late Pleistocene Indo‐Asian summer monsoons: Reinterpreting cave speleothem δ18O

Abstract: [1] Southeast China cave δ18O, often interpreted as a pure East Asian summer monsoon proxy, lags maximum northern hemisphere summer insolation by 2.9 ± 0.3 kyrs at the precession cycle. The Arabian Sea summer monsoon stack lags by 8 ± 1 kyr, consistent with 13 other Indian and East Asian summer monsoon proxies from marine, lake, and terrestrial archives. This 5 kyr phase difference cannot be attributed to age control inadequacies in the marine chronology; it requires reconciliation in the context of proxy interpretation. Both of these lags are incompatible with a direct response to northern hemisphere summer insolation, implicating additional forcing mechanisms. Analysis of heterodynes in the cave δ18O spectrum demonstrates that variance contained in the Arabian Sea summer monsoon proxies also resides in the cave δ18O record. This variance is subtracted from the cave δ18O record yielding a residual that is highly coherent and in phase with precession minima, reflecting the impact of winter temperature change on cave δ18O (meteorological precipitation under cold conditions). Thus, we argue that the timing of light cave δ18O peaks cannot be interpreted as reflecting the timing of strong summer monsoons alone. The 2.9 kyr precession band phase lag of cave δ18O reflects the combined influence of summer monsoon forcing with a phase lag of 8 kyrs relative to precession minima and winter temperature forcing that is in phase with precession minima. This interpretation is consistent with modern seasonality in the amount and isotopic composition of rainfall in southeast China.

Seasonal microclimate control of calcite fabrics, stable isotopes and trace elements in modern speleothem from St Michaels Cave, Gibraltar

Abstract: Detailed monitoring of three drip sites in New St Michael’s Cave, Gibraltar, reveals a strongly coherent seasonal pattern of dripwater chemistry despite each site having significantly different flow paths and discharge patterns. Calcite saturation is closely linked to regular seasonal variations in cave air pCO2 caused by seasonally reversing ventilation driven by temperature difference between the cave interior and the air outside. A coupled model of CO2 degassing and calcite precipitation links seasonal dC variations in coexisting dripwater, cave air CO2 and speleothem calcite to large variations in pCO2 that are driven by cave ventilation. The relationships between stable isotope ratios, Sr/Ca and speleothem fabrics across annually formed calcite laminae are consistent with a degassing–calcite precipitation process in which rapid degassing controls the dC of both drip water DIC and calcite whereas a much slower rate of calcite precipitation causes seasonal cycles of Sr in a more complex manner. By demonstrating the causes of laminated speleothem fabrics plus trace element and isotope cycles in modern speleothem from a closely monitored cave, this study provides clear links between the local microclimate and the proxy record provided by speleothem geochemistry. In Gibraltar, low cave air pCO2 in summer is unusual compared to what has been revealed by cave monitoring carried out elsewhere and shows that caution is needed when linking paired speleothem fabrics to specific seasons without knowledge of local processes operating in the cave. Speleothems provide continuous and precisely dated records of past environmental change which have advanced understanding of climate variability on timescales from glacial–interglacial cycles (Wang et al. 2008) down to seasonal patterns of precipitation (Borsato et al. 2007). Speleothem deposition in stable cave environments can record changes in surface climate as variations in properties such as extension rate, trace element abundances and stable isotopes (McDermott 2004; Fairchild et al. 2006a) but the causal relationships between these proxies and climate are not always fully understood. For some proxies they appear to be straightforward, for example the dependence of extension rate on the amount of rainfall (Baker et al. 2008), but for others such as stable isotopes interpretations have often been based on assumptions and guesswork regarding the aspects of climate that are most closely reflected. Ideally, any proxy-climate transfer function used should be based on a full understanding of the physico-chemical workings of the local climate–karst-cave system and its influences on the recording process. Careful, multi-annual monitoring of the cave microclimate, dripwater chemistry and calcite growth mechanisms reveals some of the local effects that may modify the climate recording process. Some of the important issues include the relationships between precipitation, recharge, drip rates and solute chemistry (Bottrell & Atkinson 1991; Genty & Deflandre 1998; Baker & Brunsdon 2003; Tooth & Fairchild 2003; Cruz et al. 2005; Baldini et al. 2006; Genty 2008), the role of seasonal ventilation and degassing (Ek & Gewelt 1985; Bar-Matthews et al. 1996; Spotl et al. 2005; Banner et al. 2007; Baldini et al. 2008) and the impact of kinetic factors such as fast degassing or crystal chemical effects on CaCO3 growth (Hendy 1971; Mickler et al. 2004, 2006). Knowledge of these local processes and their stability through time are a critical step in the derivation of reliable climate–proxy transfer functions that can be used for quantitative climate reconstruction. From: PEDLEY, H. M. & ROGERSON, M. (eds) Tufas and Speleothems: Unravelling the Microbial and Physical Controls. Geological Society, London, Special Publications, 336, 323–344. DOI: 10.1144/SP336.17 0305-8719/10/$15.00 # The Geological Society of London 2010. Several recent studies of speleothem records at high resolutions have revealed climate features on seasonal (Treble et al. 2003; Johnson et al. 2006; Banner et al. 2007; Mattey et al. 2008) or even synoptic time scales (Frappier et al. 2002) which provide the critical direct link between the local weather and how it is recorded during the speleothem deposition process. Speleothem calcite deposition in caves is commonly seen to be cyclical, resulting in the development of laminae defined by alternating pairs of fabrics (Baker et al. 2008; Genty & Quinif 1996). Using constraints from growth on dated artefacts and C analyses (Baldini et al. 2005; Genty et al. 2001; Tan et al. 2006; Mattey et al. 2008) cyclic laminae have sometimes been shown to be annual features and related to strong seasonality of the local climate. Annual growth laminae provide a means of deriving a chronology at the best possible precision, and may also preserve trace element and stable isotope patterns that can be related to the local climate and hydrological cycle. Our work in Gibraltar combines comprehensive multi-annual cave monitoring with high resolution analyses of fabric, trace elements and stable isotopes in modern speleothem. A recent study of a modern stalagmite from New St. Michaels Cave (Mattey et al. 2008) revealed annual growth laminae which preserve exceptionally well-defined seasonal dC and dO cycles linked to ventilation. We were able to identify the dO of winter dripwater from the complex seasonally resolved speleothem record and show excellent inter-annual correspondence with the dO of winter precipitation. In the present paper we present a more detailed overview of the results of the first 4 years of cave environment monitoring which includes local meteorology, cave and soil temperature, humidity and pCO2, and of drip discharge and monthly analysis of drip water for trace element and isotopic analysis. The monitoring data enable the seasonally resolved speleothem fabric, trace element and isotope record to be precisely linked to the nature and timing of local processes in the soil, cave air and local climate. We propose a coupled CO2 degassing–calcite precipitation model which links the development of annual cycles in dC and Sr with the effects of seasonal cave ventilation. Regional setting, monitoring techniques and analytical methods Old and New St Michaels Caves The Rock of Gibraltar, located where the Atlantic meets the Mediterranean at the junction of Europe and Africa, forms a North–South trending ridge 2.5 km long with a maximum elevation of 423 m (Fig. 1). The ridge is asymmetric, having a steep to near-vertical eastern slope which is partly banked by Pleistocene sand-dune deposits, and a western slope falling more steadily at 358 towards the town of Gibraltar near sea level. Above 100 m altitude the western slopes are covered in Mediterranean scrub forest with low rock outcrops of the Gibraltar limestone. The peninsula of Gibraltar links the Betic and Rif mountain chains, which form the southwestern end of the Mediterranean Alpine belt and is mainly composed of early Jurassic age limestone and dolomite which form the lower limb of an overturned nappe (Rose & Rosenbaum 1991). These beds dip steeply to the west and although there are no surface streams, swallets or resurgence features, the dolomites and limestones contain many solution caves located at altitudes ranging from below present sea level to near the summit ridge at over 400 m. Many caves have natural entrances exposed by erosion, but other significant caves have also been revealed though tunneling (Rosenbaum & Rose 1991). The location and a plan of St Michaels Cave is shown on Figures 1 and 2. Old St Michaels Cave (OSM) (Shaw 1955) has been known since Roman times and is open to tourists as a show cave. The cave has developed in faulted dolomitic limestone creating a large main chamber. Dissolution has also followed bedding planes, creating minor caves linked to OSM and forming natural entrances to the system. During World War II, a new access tunnel was driven into the lowest part of the show cave, known as the Hospital, exposing a lower series of solution rifts leading southwards along the strike of the Gibraltar limestone at an altitude of around 325 m (Fig. 2). This system, New St Michaels Cave (NSM) (Shaw 1954), rivals the old show cave system in terms of the scale of speleothem decoration, and also contains a 6 m deep lake which accumulates water from seepages and drips entering the southern part of NSM. Gibraltar caves such as the St Michaels system preserve evidence of phreatic origins and have since undergone several phases of draining and decoration with secondary speleothem deposits (Tratman 1971). Because the present altitude of the large St Michaels system is over 300 m asl, the phreatic features indicate that these caves have undergone significant uplift to their present position (Tratman 1971; Rose & Rosenbaum 1991; Rodrigues-Vidal et al. 2004). Tunnelling near sea level in the 19th century revealed more large natural caves such as the Ragged Staff system with similar overall morphology but with far less speleothem deposition. Ragged Staff Cave contains brackish lakes with water filled passages extending D. P. MATTEY ET AL. 324

Speleothem climate records from deep time? Exploring the potential with an example from the Permian

Abstract: Speleothems are well-proven archives of terrestrial climate variation, recording mean temperature, rainfall, and surface vegetation data at subannual to millennial resolution. They also form within the generally stable environment of caves, and thus may remain remarkably well preserved for many millions of years and, most important, can be dated radiometrically to provide robust chronologies that do not rely on orbital tuning, ice-flow modeling, or estimates of sediment deposition rates. The recent adaptation of the U-Pb dating technique to speleothems has greatly extended their potential as paleoclimate recorders back into the more distant geological past, well beyond the ∼500 k.y. limit previously imposed by U-series techniques, but the opportunities presented by these new methods have yet to be fully explored. As an extreme example, here we report on samples recovered from Permian cave fills, the oldest radiometrically dated speleothems so far documented. Using state of the art analytical techniques it is possible to determine not only their age and state of preservation, but also to extract apparently nearly pristine climate proxy data. Armed with these methods, it now seems reasonable to apply the lessons learned from more recent speleothems to ancient materials, wherever they can be found, and of whatever age, to generate snapshots of paleoclimate that can be used to greatly refine the records preserved within the sediments and fossils of the time.

Holocene climate change in arid Australia from speleothem and alluvial records

Abstract: New high-resolution MC-ICPMS U/Th ages and C and O isotopic analyses from a Holocene speleothem in arid south-central Australia provide evidence for increased effective precipitation (EP) relative to present at c. 11.5 ka and c. 8—5 ka, peak moisture at 7—6 ka, and onset of an arid climate similar to present by c. 5 ka. δ18O and δ13C time-series data exhibit marked (>+1‰) contemporaneous excursions over base-line values of −5.3‰ and −11.0‰, respectively, suggesting pronounced moisture variability during the early middle Holocene ‘climatic optimum’. Optically stimulated luminescence and 14C ages from nearby terraced aggradational alluvial deposits indicate a paucity of large floods in the Late Pleistocene and at least five large flood events in the last c. 6 kyr, interpreted to mark an increased frequency of extreme rainfall events in the middle Holocene despite overall reduced EP. Increased EP in south-central Australia during the early to middle Holocene resulted from (1) decreased El Nino-Southern Oscil...

Age frequency distribution and revised stable isotope curves for New Zealand speleothems: palaeoclimatic implications

Abstract: The occurrence of speleothems in New Zealand with reversed magnetism indicates that secondary calcite deposition in caves has occurred for more than 780 thousand years (ka). 394 uranium-series dates on 148 speleothems show that such deposition has taken place somewhere in the country with little interruption for more than 500 ka. A relative probability distribution of speleothem ages indicates that most growth occurred in mild, moist interglacial and interstadial intervals, a conclusion reinforced by comparing peaks and troughs in the distribution with time series curves of speleothem δO and δC values. The stable isotope time series were constructed using data from 15 speleothems from two different regions of the country. The greater the number of overlapping speleothem series (i.e. the greater the sample depth) for any one region, the more confidence is justified in considering the stacked record to be representative of the region. Revising and extending earlier work, composite records are produced for central-west North Island (CWNI) and north-west South Island (NWSI). Both demonstrate that over the last 15 ka the regions responded similarly to global climatic events, but that the North Island site was also influenced by the waxing and waning of regional subtropical marine influences that penetrated from the north but did not reach the higher latitudes of the South Island. Cooling marking the commencement of the last glacial maximum (LGM) was evident from about 28 ka. There was a mid-LGM interstadial at 23-21.7 ka and Termination 1 occurred around 18.1 ka. The glacial-interglacial transition was marked by a series of negative excursions in δO that coincide with dated recessional moraines in South Island glaciers. A late glacial cooling event, the NZ Late Glacial Reversal, occurred from 13.4-11.2 ka and this was followed by an early Holocene optimum at 10.8 ka. Comparison of δO records from NWSI and EPICA DML ice-core shows climatic events in New Zealand to lag those in Antarctica by several centuries to a thousand years. Waxing and waning of subantarctic and subtropical oceanic influences in the Tasman Sea are considered the immediate drivers of palaeoclimatic change.

Constraining Holocene sea levels using U-Th ages of phreatic overgrowths on speleothems from coastal caves in Mallorca (Western Mediterranean)

Abstract: Phreatic overgrowths on speleothems (POS) are carbonate formations deposited at the water table of caves in unique karstic coastal settings having morphologies that can be directly related to sea level at the time of formation. The U-Th ages of calcite and aragonite overgrowths collected from the modern water table in coastal caves on Mallorca (Cova de Cala Varques A and Cova des Pas de Vallgornera) were determined using high-precision MC-ICPMS techniques. U-Th ages indicate that phreatic carbonate deposition occurred between ca 2·8 and at least 0·6 ka BP and are in accord with an archeologically estimated age of 3·7–3·0 ka BP for a drowned prehistoric construction at a depth of 1 m below current sea level in a cave from the same area. Speleothem δ13C and δ18O and chemical composition of cave pools provide supportive evidence that POS reflect mixing between seawater and brackish water table. Copyright © 2010 John Wiley & Sons, Ltd.

Effects of intraseasonal variation of summer monsoon rainfall on stable isotope and growth rate of a stalagmite from northwestern Thailand

Abstract: [1] An annually laminated stalagmite from Namjang cave (19°40′30″N, 98°12′12″E), northwestern Thailand, has been analyzed to investigate the annual resolution climate signal persevered within its oxygen isotopic composition (δ18O) and growth rate parameters. The cave site is under the influence of Asian monsoon, and local rainfall shows noticeable variations through the monsoon season. Both δ18O and growth rate records, covering the last 105 years, exhibit persistent decadal-scale variability and can be compared with local instrumental data. Low δ18O values coincide with high growth rates in the specimen and correspond to higher relative amounts of rainfall in later monsoon season (August–October; hereinafter referred to as ASO rainfall) versus rainfall in early monsoon season (May–July; hereinafter referred to as MJJ rainfall). The strong correlation between the δ18O value and the 5 year averaged ratio of ASO to MJJ rainfall (r = −0.50, p < 0.001) indicates a significant imprint of intraseasonal variation of monsoonal rainfall on stalagmite δ18O. The close resemblance between the speleothem δ18O record and Western Pacific Warm Pool (WPWP) sea surface temperature (SST) implies that the WPWP may play an important role on the decadal variability of later monsoon rainfall in this region. Unique in its annual chronology, high-resolution δ18O, and direct comparison with instrumental data, our record shows for the first time that the climate in northwestern Thailand has undergone decadal-scale variability and speleothem δ18O is a robust proxy for regional monsoon intensity.

Humid climate during deposition of sapropel 1 in the Mediterranean Sea: Assessing the influence on the Alps

Abstract: article i nfo Cave and lake isotope records from the circum-Mediterranean realm show anomalously low O isotope values suggesting high rainfall intensity during the time of sapropel 1 deposition (9.5 to 6.5 ka; all ages are given before the year AD 2000, i.e. b2k), coincident with an interval of conspicuously low sea-surface salinities in the entire Mediterranean Sea. Speleothem data from Corchia Cave (Tuscany) currently provide the most precise terrestrial chronology and constrain the wettest interval to ca. 8.2 to 7.3 ka. We have traced this isotopic signal to the north and observe a synchronous isotopic change in stalagmites from southalpine and eastalpine caves, but in opposite direction. We attribute this to a shift in the local moisture balance, i.e. to a higher proportion of moisture advected from the Mediterranean Sea relative to the otherwise dominant northwesterly air masses in the Alps. This isotopic source effect can be traced up to the northern rim of the Alps, albeit with decreasing amplitude. Forest density at the treeline in the Central Alps decreased during this time interval indicating short vegetation periods consistent with rainy summers. The glaciers in the Eastern Alps, which did not show far-reaching advances during the preceding 8.2 ka event, responded strongly (positively) to this humid phase. Additionally, two of the largest alluvial fans in the Eastern Alps showed a massive accumulation peak radiocarbon dated to between ca. 8.3 to 7.4 ka and thus providing one of the strongest pieces of evidence for anomalously high rainfall intensities coeval with 'pluvial' conditions in the Mediterranean region.