Modification and preservation of environmental signals in speleothems

Speleothems are primarily studied in order to generate archives of climatic change and results have led to significant advances in identifying and dating major shifts in the climate system. However, the climatological meaning of many speleothem records cannot be interpreted unequivocally; this is particularly so for more subtle shifts and shorter time periods, but the use of multiple proxies and improving understanding of formation mechanisms offers a clear way forward. An explicit description of speleothem records as time series draws attention to the nature and importance of the signal filtering processes by which the weather, the seasons and longer-term climatic and other environmental fluctuations become encoded in speleothems. We distinguish five sources of variation that influence speleothem geochemistry: atmospheric, vegetation/soil, karstic aquifer, primary speleothem crystal growth and secondary alteration and give specific examples of their influence. The direct role of climate diminishes progressively through these five factors. We identify and review a number of processes identified in recent and current work that bear significantly on the conventional interpretation of speleothem records, for example: 1) speleothem geochemistry can vary seasonally and hence a research need is to establish the proportion of growth attributable to different seasons and whether this varies over time. 2) whereas there has traditionally been a focus on monthly mean Ã�´18O data of atmospheric moisture, current work emphasizes the importance of understanding the synoptic processes that lead to characteristic isotope signals, since changing relative abundance of different weather types might 1Corresponding author, fax +44(0)1214145528, E-mail: [email protected] control their variation on the longer-term. 3) the ecosystem and soil zone overlying the cave fundamentally imprint the carbon and trace element signals and can show characteristic variations with time. 4) new modelling on aquifer plumbing allows quantification of the effects of aquifer mixing. 5) recent work has emphasized the importance and seasonal variability of CO2-degassing leading to calcite precipitation upflow of a depositional site on carbon isotope and trace element composition of speleothems. 6) Although much is known about the chemical partitioning between water and stalagmites, variability in relation to crystal growth mechanisms and kinetics is a research frontier. 7) Aragonite is susceptible to conversion to calcite with major loss of chemical information, but the controls on the rate of this process are obscure. Analytical factors are critical to generate high-resolution speleothem records. A variety of methods of trace element analysis are available, but standardization is a common problem with the most rapid methods. New stable isotope data on Irish stalagmite CC3 compares rapid laser-ablation techniques with the conventional analysis of micromilled powders and ion microprobe methods. A high degree of comparability between techniques for Ã�´18O is found on the mm-cm scale, but a previously described high-amplitude oxygen isotope excursion around 8.3 ka is identified as an analytical artefact related to fractionation of the laser-analysis associated with sample cracking. High-frequency variability of not less than 0.5o/oo may be an inherent feature of speleothem Ã�´18O records

Northeastern Caribbean Rainfall Variability Linked to Solar and Volcanic Forcing

We present a 500-year precipitation-sensitive record based on co-varying speleothem delta 18O values and Mg/Ca ratios from Larga cave in Puerto Rico. This multi-proxy record shows that the evolution of rainfall in the northeastern Caribbean was characterized by alternating centennial dry and wet phases corresponding to reduced versus enhanced convective activity. These phases occurred synchronous with relatively cool and warm tropical Atlantic sea-surface temperatures (SSTs), respectively. While the observed pattern suggests a close link of northeastern Caribbean rainfall to the Atlantic Multidecadal Variability, a regional comparison reveals intermittent regional heterogeneity especially on decadal timescales, which may be related to a superimposing influence of the Pacific and Atlantic basins. Furthermore, the speleothem-based hydroclimate reconstruction indicates a significant volcanic impact during the past two centuries, and further reveals a potential solar signal in the preceding three centuries. We posit that the forcing likely shifted from solar to volcanic during the eighteenth century in being an important source of multidecadal to centennial Caribbean rainfall variability. The link between convective rainfall and natural forcing may be explained through a modulation of SST variations in the tropical Atlantic and Pacific oceans.Climate change is expected to affect rainfall throughout the Caribbean and Central America, where over 200 million people are heavily dependent upon rain as their main source of freshwater. We have looked at how the amount of rainfall in the Caribbean has changed through time, and possible reasons for these changes, to inform predictions of future rainfall patterns for the region. Cave mineral deposits, also known as speleothems, can be used to assess the amount of regional rainfall in the past by looking at their slight changes over time in the mineral's chemical composition. We created a record of Caribbean rainfall spanning the last five centuries using a speleothem from Puerto Rico. Our results indicate that rainfall changes since the sixteenth century were strongly influenced by changes in temperatures of the surface waters of the Atlantic Ocean. We further found that changes in the sun's brightness and volcanic eruptions may alter Atlantic Ocean sea-surface temperature, which in turn affect the overall changes in Caribbean rainfall patterns.Speleothem-based Puerto Rican rainfall reconstruction shows prominent multidecadal-to-centennial variability during the past five centuries Puerto Rican rainfall fluctuations are linked to solar variations before the eighteenth century and volcanic forcing thereafter Caribbean Sea-surface temperature anomalies as part of the Atlantic Multidecadal Variability emerge as robust precursor of rainfall amount

Exceptional Record of Mid-Pleistocene Vertebrates Helps Differentiate Climatic from Anthropogenic Ecosystem Perturbations

Mid-Pleistocene vertebrates in North America are scarce but important for recognizing the ecological effects of climatic change in the absence of humans. We report on a uniquely rich mid-Pleistocene vertebrate sequence from Porcupine Cave, Colorado, which records at least 127 species and the earliest appearances of 30 mammals and birds. By analyzing \u3e20,000 mammal fossils in relation to modern species and independent climatic proxies, we determined how mammal communities reacted to presumed glacial-interglacial transitions between 1,000,000 and 600,000 years ago. We conclude that climatic warming primarily affected mammals of lower trophic and size categories, in contrast to documented human impacts on higher trophic and size categories historically. Despite changes in species composition and minor changes in small-mammal species richness evident at times of climatic change, overall structural stability of mammal communities persisted \u3e600,000 years before human impacts

Partitioning of Mg, Sr, Ba and U into a subaqueous calcite speleothem

The trace-element geochemistry of speleothems is becoming increasingly used for reconstructing palaeoclimate, with a particular emphasis on elements whose concentrations vary according to hydrological conditions at the cave site (e.g. Mg, Sr, Ba and U). An important step in interpreting trace-element abundances is understanding the underlying processes of their incorporation. This includes quantifying the fractionation between the solution and speleothem carbonate via partition coefficients (where the partitioning (D) of element X (DX) is the molar ratio [X/Ca] in the calcite divided by the molar ratio [X/Ca] in the parent water) and evaluating the degree of spatial variability across time-constant speleothem layers. Previous studies of how these elements are incorporated into speleothems have focused primarily on stalagmites and their source waters in natural cave settings, or have used synthetic solutions under cave-analogue laboratory conditions to produce similar dripstones. However, dripstones are not the only speleothem types capable of yielding useful palaeoclimate information. In this study, we investigate the incorporation of Mg, Sr, Ba and U into a subaqueous calcite speleothem (CD3) growing in a natural cave pool in Italy. Pool-water measurements extending back 15 years reveal a remarkably stable geochemical environment owing to the deep cave setting, enabling the calculation of precise solution [X/Ca]. We determine the trace element variability of ‘modern’ subaqueous calcite from a drill core taken through CD3 to derive DMg, DSr, DBa and DU then compare these with published cave, cave-analogue and seawater-analogue studies. The DMg for CD3 is anomalously high (0.042 ± 0.002) compared to previous estimates at similar temperatures (∼8 °C). The DSr (0.100 ± 0.007) is similar to previously reported values, but data from this study as well as those from Tremaine and Froelich (2013) and Day and Henderson (2013) suggest that [Na/Sr] might play an important role in Sr incorporation through the potential for Na to outcompete Sr for calcite non-lattice sites. DBa in CD3 (0.086 ± 0.008) is similar to values derived by Day and Henderson (2013) under cave-analogue conditions, whilst DU (0.013 ± 0.002) is almost an order of magnitude lower, possibly due to the unusually slow speleothem growth rates (<1 μm a−1), which could expose the crystal surfaces to leaching of uranyl carbonate. Finally, laser-ablation ICP-MS analysis of the upper 7 μm of CD3, regarded as ‘modern’ for the purposes of this study, reveals considerable heterogeneity, particularly for Sr, Ba and U, which is potentially indicative of compositional zoning. This reinforces the need to conduct 2D mapping and/or multiple laser passes to capture the range of time-equivalent elemental variations prior to palaeoclimate interpretation

Evaluation of bulk carbonate delta C-13 data from Triassic hemipelagites and the initial composition of carbonate mud

Bulk carbonate samples of hemipelagic limestone\u2013marl alternations from the Middle and Upper Triassic of Italy are analysed for their isotopic compositions. Middle Triassic samples are representative of the Livinallongo Formation of the Dolomites, while Upper Triassic hemipelagites were sampled in the Pignola 2 section, within the Calcari con Selce Formation of the Southern Apennines in Southern Italy. Triassic hemipelagites occur either as nodular limestones with chert nodules or as plane-bedded limestone\u2013marl alternations which are locally silicified. In the Middle Triassic Livinallongo Formation, diagenetic alteration primarily affected the stable isotopic composition of sediment surrounding carbonate nodules, whereas the latter show almost pristine compositions. Diagenesis lowered the carbon and oxygen isotope values of bulk carbonate and introduced a strong correlation between d13C and d18O values. In the Middle Triassic successions of the Dolomites, bulk carbonate of nodular limestone facies is most commonly unaltered, whereas carbonate of the plane-bedded facies is uniformly affected by diagenetic alteration. In contrast to carbonate nodules, plane-bedded facies often show compaction features. Although both types of pelagic carbonate rocks show very similar petrographic characteristics, scanning electron microscopy studies reveal that nodular limestone consists of micrite (< 5 lm in diameter), whereas samples of the plane-bedded facies are composed of calcite crystals ca 10 lm in size showing pitted, polished surfaces. These observations suggest that nodular and plane-bedded facies underwent different diagenetic pathways determined by the prevailing mineralogy of the precursor sediment, i.e. probably high-Mg calcite in the nodular facies and aragonite in the case of the plane-bedded facies. Similar to Middle Triassic nodular facies, Upper Triassic nodular limestones of the Lagonegro Basin are also characterized by uncorrelated d13C and d18O values and exhibit small, less than 5 lm size, crystals. The alternation of calcitic and aragonitic precursors in the Middle Triassic of the Dolomites is thought to mirror rapid changes in the type of carbonate production of adjacent platforms. Bioturbation and dissolution of metastable carbonate grains played a key role during early lithification of nodular limestone beds, whereby early stabilization recorded the carbon isotopic composition of sea water. The bulk carbonate d13C values of Middle and Upper Triassic hemipelagites from Italy agree with those of Tethyan low-Mg calcite shells of articulate brachiopods, confirming that Triassic hemipelagites retained the primary carbon isotopic composition of the bottom sea water. A trend of increasing d13C from the Late Anisian to the Early Carnian, partly seen in the data set presented here, is also recognized in successions from tropical palaeolatitudes elsewhere. The carbon isotopic composition of Middle and Upper Triassic nodular hemipelagic limestones can thus be used for chemostratigraphic correlation and palaeoenvironmental studies

Possible influence of the &#948;13C variations on the Carnian/Norian conodonts diffusion.

Quantitative analyses of the absolute abundances of the species belonging to the five most widespread Upper Carnian/Lower Norian conodont genera (Paragondolella, Carnepigondolella, Metapolygnathus, Epigondolella, and Norigondolella) from the Pizzo Mondello section (Monti Sicani, Western Sicily, Italy), GSSP candidate for the Norian stage (Balini et al., 2008), show potential ecological competition between these genera. Cross checks of the quantitative curves evidence the presence of three major assemblage changes, named T events: at T1 Carnepigondolella is replaced by its descendant Epigondolella in an evolutionary step; at T2 Epigondolella is substituted by the mass occurrence of Metapolygnathus and at T3 Metapolygnathus is succeeded by advanced Epigondolellae and by Norigondolella. A comparison between these quantitative curves and coeval \u3b418O and \u3b413C isotopic curves, obtained from the carbonate bulk of the section, shows a correspondence between higher13C/12C ratios and event T2 (and partially event T3), but not with T1. Thus, while the shifts in the C isotopic composition of seawater do not affect conodont evolution, they have an influence on the diffusion of the studied genera, which react differently to the \u3b413C variations. Epigondolella and Carnepigondolella proliferate in fact when seawater \u3b413C is lower than 2.5\u2030, Metapolygnathus is instead limited to environmental conditions related to \u3b413C values higher than 2.5\u2030 (Mazza et al., 2009). The rapid expansion of photosynthetically active organisms in the Upper Carnian may explain these perturbations in the carbon cycle and have influenced the trophic chain of the different conodont animal genera, producing thus mutual local migrations

Palaeoenvironmental significance of carbon- and oxygen-isotope stratigraphy of marine Triassic-Jurassic boundary sections in SW Britain

Carbon-isotope stratigraphy is a useful tool for stratigraphic correlation, especially for strata deposited during major perturbations of the carbon cycle that affected the marine, terrestrial and atmospheric reservoirs. For the Triassic-Jurassic boundary, effectively defined by a first-order mass extinction, major fluctuations in carbon-isotope values have been well documented, but these datasets have generally been derived from bulk-rock samples. Hence, the extent to which features of the isotopic curve reflect diagenetic alteration or changing proportions of constituent materials is unconstrained. Here, carbon- and oxygen-isotope data are presented from well-preserved oyster shells (Liostrea) comprising low-magnesium calcite, a mineral species relatively resistant to diagenetic alteration. Samples were obtained from Lavernock Point, Glamorgan, Wales, a coastal section close to a candidate stratotype for the base of the Jurassic at St Audrie's Bay, Somerset, England. The carbon-isotope signature from St Audrie's Bay, previously defined on the basis of analysis of bulk organic matter, is confirmed by our new data. Major features are (1) the upper part of an 'initial' negative isotope excursion in the lowest part of the section, followed by (2) a pronounced positive excursion, and (3) an extended 'main' negative isotope excursion in the highest part of the section. The data confirm that the carbon-isotope stratigraphy previously documented from bulk organic matter in SW England records the chemical composition of the contemporaneous seawater. Bulk carbonates sampled over the same interval near Lyme Regis, England, show similar trends to those from oyster calcite in the lower part of the study section, but there are more 13C-depleted values up-section. These lower values probably result from an admixture of primary and diagenetic carbonate. Palaeotemperatures calculated from oxygen-isotope values from Lavernock Point oyster shells are relatively cool at the beginning of the positive carbon-isotope excursion, and increased by up to 10 8C during the main negative carbon-isotope excursion. The new results are compatible with the view that positive carbon-isotope excursions correspond to times of low atmospheric carbon dioxide content, whereas negative carbon-isotope excursions correspond to times of high atmospheric carbon dioxide content, as is also found to be the case during the Early Jurassic (Toarcian) Oceanic Anoxic Event. The Mg/Ca and Sr/Ca ratios and 18O of investigated Liostrea hisingeri show no correlation,supporting data from modern bivalves that indicate that incorporation of Mg and Sr is controlled mainly by factors other than temperature. © 2009 Geological Society of London

Holocene climate variability in Sicily from a discontinuous stalagmite record and the Mesolithic to Neolithic transition

Fabric and stable isotopic composition of a Holocene stalagmite (CR1) from a cave in northern Sicily record changes in paleorainfall in the early Holocene. High \u3b413C stable isotope values in the calcite deposited from ca. 8500 to ca. 7500 yr ago are interpreted as reflecting periods of high rainfall. The wet phase was interrupted by two periods of multi-century duration characterized by relatively cool and dry winters centered at ca. 8200 and ca. 7500 yr ago, highlighted by low \u3b413C and \u3b418O values. A high variability of \u3b413C values is recorded from ca. 7500 to ca. 6500 yr ago and indicates that the transition from a pluvial early Holocene to the present-day climate conditions was punctuated by decadal-scale periods of relatively dry winters. In northern Sicily, the traditional elements of the Neolithic appear at ca. 7700 yr ago. It is possible that changes in rainfall influenced the passage from hunter-gathering to farming and sheep-herding economies