Peter L. Smart

Wet periods in northeastern Brazil over the past 210 kyr linked to distant climate anomalies

The tropics are the main source of the atmospheres sensible and latent heat, and water vapour, and are therefore important for reconstructions of past climate. But long, accurately dated records of southern tropical palaeoclimate, which would allow the establishment of climatic connections to distant regions, have not been available. Here we present a 210,000-year (210-kyr) record of wet periods in tropical northeastern Brazil—a region that is currently semi-arid. The record is obtained from speleothems and travertine deposits that are accurately dated using the U/Th method. We find wet periods that are synchronous with periods of weak East Asian summer monsoons, cold periods in Greenland, Heinrich events in the North Atlantic and periods of decreased river runoff to the Cariaco basin. We infer that the wet periods may be explained with a southward displacement of the Intertropical Convergence Zone. This widespread synchroneity of climate anomalies suggests a relatively rapid global reorganization of the ocean–atmosphere system. We conclude that the wet periods probably affected rainforest distribution, as plant fossils show that forest expansion occurred during these intermittent wet intervals, and opened a forest corridor between the Amazonian and Atlantic rainforests.

Elevated and variable values of 13C in speleothems in a British cave system

δ 13C isotope variations in speleothems have been investigated for samples from the British Isles, where plants which use the Hatch-Slack or C4 photosynthetic pathway are not present. The range of δ 13C expected in speleothem carbonate formed in isotopic equilibrium with soil CO2 derived from the overlying C3 vegetation should thus fall in the range −12 to −6‰. Forty-one actively growing speleothem samples from low-discharge sites were analysed from Stump Cross Caverns, Yorkshire, England. Ten percent have δ 13C greater than −6%. In addition, a large range of δ 13C was observed (−8.06 ± 1.38‰, a 1 σ variability of 17%), with adjacent samples having δ 13C differing by a maximum of 4.74‰. Similar findings were obtained from a review of analyses of late Quaternary speleothem samples from the British Isles, with 75% of flowstone samples and 57% of high-flow stalagmite samples exhibiting elevated δ 13C. Three possible processes are proposed as possible causes of elevated δ 13C in speleothems. Firstly, fractionation may occur between the stalactite and stalagmite due to evaporation or degassing. Secondly, degassing of the groundwaters may have occurred within the aquifer before reaching the cave void, allowing release of some CO2 from the water whilst remaining saturated in calcium. Finally, the elevated δ 13C may be due to short water residence times in the soil, such that equilibrium between soil water and soil CO2 is not reached. Evidence presented here demonstrates that any one of these mechanisms may be important in the karst areas of the British Isles. Caution is needed before interpreting the δ 13C signal within speleothems in terms of palaeovegetation.

ANNUAL TRACE ELEMENT VARIATIONS IN A HOLOCENE SPELEOTHEM

High resolution secondary ionization mass spectrometry (SIMS) analysis along the growth axis of a Holocene speleothem from northern Scotland reveals high frequency oscillations and longer term trends in Mg/Ca, Sr/Ca and Ba/Ca. At the scale of the high frequency oscillations, Mg/Ca and Sr/Ca display a well-defined anticorrelation, while Sr/Ca and Ba/Ca are positively correlated. Imaging of elemental ratios in two dimensions reveals that the high frequency signal is well preserved and laterally continuous across the sample. TIMS 230Th ages and annual ultraviolet luminescence banding provide chronological control and demonstrate that the high frequency cyclicity is annual. Trace element variations in speleothem calcite are a reflection of hydrochemical processes in the unsaturated zone overlying the cave (supply effects) and partitioning at the water–calcite interface. Mg partitioning between water and calcite is temperature dependent. Calculations suggest that the annual Mg/Ca oscillations may be caused by seasonal temperature changes. However, the longer timescale variations in Mg/Ca are too great to be explained by this mechanism. Seasonal variations in water residence time in the unsaturated zone (which reflect effective precipitation) provide a more probable explanation for the observed annual oscillations and also explain the inverse relationship between Mg/Ca and Sr/Ca. This record suggests that trace elements in speleothems have the potential to provide the high resolution insights into interannual palaeoclimatic variability required for assessment of general circulation models.

Variations in the discharge and organic matter content of stalagmite drip waters in Lower Cave, Bristol

Six drip waters, which were actively depositing stalagmites in Lower Cave, Bristol, were analysed both for discharge and luminescence properties. Drip discharges were determined for two different years, and show a complex response to surface precipitation variations. Inter annual variability in drip discharge is demonstrated to be significantly higher than intra-annual variability, and discharge was demonstrated both to increase and decrease non-linearly with increased precipitation. Drip waters demonstrate a correlation between their luminescence intensity and drip discharge, with increased luminescence in winter as more organic matter is flushed through the aquifer. The strength of the relationship between luminescence intensity and discharge increases with increased discharge. The results presented here have implications for the palaeoenvironmental interpretation of annual growth laminae and the growth rates of stalagmite samples.

Northwest European palaeoclimate as indicated by growth frequency variations of secondary calcite deposits

A compilationn for the northwest of Europe of over 500 uranium-series speleothen and travertine dates is presented using a cumulative distributed error frequency approach. These secondary carbonate deposits require both significant groundwater supply and a biogenic soil carbon dioxide source for their growth. During glacial periods soil CO2 production is inhibited and the water supply is ice locked, therefore growth is slowed significantly if not stopped. Thus deposits can be used on a chronological basis to give a signal of glacial and non-glacial periods. However, sensitivity to the groundwater supply modifies this signal, with growth also limited in times of high aridity, the palaeoclimatic signal is thus complex. The compilation presented is used both as a chronology for comparison with the orbitally tuned marine oxygen isotope record, and as a palaeoclimatic indicator. Attention is focused on two periods of the last glacial / interglacial cycle where the record provides a significantly different palaeoclimatic record to other terrestrial and oceanic records. These are the isotope stages 5/4 transition, for which a relatively low cumulative growth frequency indicates an earlier increase in aridity than observed elsewhere; and isotope stage 3, the pleniglacial, where both statistically significant high (49–62 ka) and low (22–35, 44–46 ka) levels of growth are observed, and can be used to constrain the timing of many of the interstadial events within this period.

Active circulation of saline ground waters in carbonate platforms: Evidence from the Great Bahama Bank

Measurements of salinity, temperature, and ground-water discharges within the Great Bahama Bank provide evidence of active circulation of near-normal ocean water beneath North Andros Island. Elevated-salinity waters (38‰-42‰) derived by density refulx from the Great Bahama Bank flow eastward beneath the island and mix with normal-salinity cold ground waters (19-20 °C) from deeper than 250 m in the adjacent oceans. East to west flow of water from the Straits of Florida may be driven by head differences generated by the Florida Current, which impinges on the western margins of the platform. Alternatively, density differences between the reflux and Tongue of the Ocean sea waters may cause more local circulation on the eastern flanks. These flows have important diagenetic implications, particularly in the explanation of pervasive secondary dolomitization widely reported at shallow depths in the Bahamas.

Dating of late Pleistocene interglacial and interstadial periods in the United Kingdom from speleothem growth frequency

Abstract The growth of speleothems is indicative of interglacial and interstadial conditions in the United Kingdom, since their growth is dependent on two factors. First, the occurrence of significant diffuse groundwater recharge and, second, the biogenic production of carbon dioxide in the soil, both are dependent on temperature and water availability. The growth frequency of speleothems is examined using a cumulative distributed error frequency method applied to 341 uncontaminated uranium-series age determinations. The curves derived are shown to be statistically stable, and the ages of the peaks are interpreted as the best estimates of the ages of interglacial and interstadial periods. Ten such periods are recognized during the interval 220,000 to 20,000 yr B.P., considerably more than are currently recognized in the UK pollen- and coleoptera-based Quaternary stratigraphy. Correlations between the speleothem growth frequency peaks and last interglacial (Ipswichian) sites can be made, but correlations with last glacial (Devensian) interstadial sites are limited because of the paucity of dates. The speleothem growth frequency record provides a well-dated terrestrial chronology for the past 140,000 yr B.P., which directly reflects regional palaeoclimatic conditions in Britain, and should prove very useful in unravelling the complex stratigraphy of the Devensian and Ipswichian stages.

Geochemistry and isotope systematics of sulphur in the mixing zone of Bahamian blue holes

Abstraet--A profile of aqueous S species concentrations and stable isotope compositions is presented for the mixing zone of Cousteaus Blue Hole, North Andros, Bahamas, together with similar, but less detailed results from two other blue holes. These data show that S o and S 2- are produced by bacterially mediated sulphate reduction near the base of the mixing zone and that these species are reoxidized at shallower levels. Acidity generated by oxidation can contribute to corrosiOn of limestone walbrock. Estimated rates of such corrosion range up to 1200 mm wall-rock recession per 10 ka at Cousteaus Blue Hole (equivalent to 16% porosity generation per l0 ka) and are comparable with rates of dissolution caused by inorganic mixing corrosion.

Recent flowstone growth rates: Field measurements in comparison to theoretical predictions

Abstract The model of calcite precipitation kinetics of D. Buhmann and W. Dreybrodt, based on the rate laws of L.N. Plummer et al., is used to predict cave flowstone growth rates. These theoretically modelled growth rates are compared to actual growth rates of recent samples found in cave and mine sites in southwest England. A good agreement is found between modelled and actual growth rates within the 95% confidence level of the determinations, although in general modelled growth rates overestimate actual growth rate by between 2.4 and 4.7 times. Several reasons for this overestimation are discussed, including uncertainties arising from the experimental data of L.N. Plummer et al., seasonal shut-off of water flow onto the flowstones and significant variations in the growth rate determining parameters during the period of flowstone growth. For one flowstone an underestimation of growth rate is observed and is explained by the presence of rimstone pools which pond water on the sample surface.

Fate of reflux brines in carbonate platforms

Active reflux may occur during periods of platform-top brine generation, but the role and fate of these brines after reflux events are uncertain. We have used a numerical flow model to investigate and quantify the response of reflux brines to changes in platform-top salinity. Simulations suggest that reflux brines, originally concentrated to gypsum saturation (150‰), have a relatively long platform residence time, on the order of 100 times the duration of the reflux event. When brine-generating conditions cease, brines will continue to sink through the platform, entraining seawater, a variant of reflux circulation we term latent reflux. Mesosaline brines intercepted by drilling of carbonate margins by the Ocean Drilling Program may have originated from Pleistocene reflux event(s) on the adjacent platform top and be currently moving by latent reflux. Latent-reflux circulation could deliver a significant quantity of dissolved reactants to platform carbonates, including Mg for dolomitization.