George A. Brook

Environmental Controls on the Petrology of a Late Holocene Speleothem from Botswana with Annual Layers of Aragonite and Calcite

ABSTRACT A carbonate stalagmite from Drotskys Cave in northwestern Botswana consists of alternating layers of calcite and aragonite. Layer counts and radiocarbon ages indicate that the calcite-aragonite pairs are annual layers representing about 1500 years of deposition. The annual layering probably resulted from highly seasonal rainfall. Comparison of the uppermost layers of the speleothem with meteorological records shows that precipitation of CaCO3 in Drotskys Cave was controlled by climate. Thickness of calcite layers correlates with rainfall, suggesting that calcite precipitation was largely dependent on the quantity of water supplied to the speleothem. By contrast, thickness of aragonite layers correlates with temperature, although variation in temperature cannot explain greater aragonite abundance on the sides of the speleothem compared to its center. Mg/Ca ratios in calcite layers increase upward to the bases of overlying aragonite layers, and analyses of cave waters suggest that fluid Mg/Ca ratios reach levels sufficient to cause aragonite precipitation. Increasing evaporation, which caused greater ionic strength and supersaturation, resultant increasing Mg/Ca ratios in the fluid, and perhaps increasing temperature probably combined to cause aragonite precipitation. Detailed petrographic analysis suggests that each annual cycle of CaCO3 precipitation began with relatively intense fluid flow, sometimes sufficient to dissolve some of the underlying aragonite before precipitation of calcite. Calcite precipitation under a thick fluid layer allowed euhedral crystals to form at first, but thinning of the fluid to a film allowed only flatly terminated calcite crystals by seasons end. As fluid flow diminished, increasing evaporation, increasing Mg/Ca ratios in the fluid, and perhaps increasing temperature combined to cause aragonite precipitation to begin, particularly on the sides of the speleothem. In some years, fluid flow diminished to the point that dust accumulated on aragonite surfaces before the onset of the next years precipitation. /P>

Late Pleistocene wetting and drying in the NW Kalahari: an integrated study from the Tsodilo Hills, Botswana

Abstract The sediments and landforms at the Tsodilo Hills, in the northwestern Kalahari desert, provide an opportunity to directly investigate the late Quaternary wetting and drying of the region from evidence at a single site. Lacustrine carbonates, including incorporated molluscs and diatoms, a lake shoreline feature and stabalised linear dunes were investigated for their constituent palaeoenvironmental signals. Chronometric control is provided by calibrated 14 C , AMS and OSL dating. The evidence suggests that linear dune construction has not occurred since the Last Glacial Maximum, with particular development from 36 to 28 ka . Lake stands indicating wetter regional conditions than present occurred at 40– 32 ka , with more seasonal conditions from 36 ka , and at 27– 12 ka with a possible drying out at 22– 19 ka . Data are consistent with other independent studies from the region, and with recent evidence obtained from Atlantic cores off the coast of Namibia. It is concluded that careful consideration of multi-proxy data from a single location can assist in resolving discrepancies that arise from independent studies of lake, cave and dune records in the Kalahari.

A high-resolution proxy record of rainfall and ENSO since AD 1550 from layering in stalagmites from Anjohibe Cave, Madagascar

Two stalagmites from Anjohibe Cave have annual layers made up of inclusion-rich calcite over inclusion-free calcite or of darker aragonite over clear aragonite. Geochemical evidence indicates that the basal units are deposited slowly in the wet season and the upper units more rapidly in the dry season. For the period with rainfall and temperature data (ad 1951–1992), layer thickness correlates well with the Southern Oscillation Index (SOI), as well as rainfall, water surplus, and actual evapotranspiration (AET) at nearby Majunga. Com parison of the layer record for one stalagmite with 1866–1994 SOI data indicates that layer thickness correlates best with the frequency and intensity of warm, low-phase SO (El Niño) events, not with average SOI conditions. In addition, the 415-year layer thickness time-series from that speleothem agrees remarkably well with historical records of El Niño frequency, with Galápagos (Ecuador) coral records of sea-surface temperature in the eastern Pacific, and with accumulation rates on the Quelccaya Ice Cap of Peru, which are lower at times of high El Niño frequency.

Desert paleoenvironmental data from cave speleothems with examples from the Chihuahuan, Somali-Chalbi, and Kalahari deserts

Abstract Pollen-bearing lake, bog, and spring sediments are relatively scarce in many arid and semiarid regions of the world, and few are dateable beyond the 14C range. We have obtained pollen spectra from speleothems collected from caves in the Somali-Chalbi and Kalahari deserts suggesting that these deposits may be an important future source of desert paleovegetation data. As cave speleothems can be dated by the 230Th/234U method to c. 350,000 yr B.P., and by the TL and ESR methods potentially to 1 m.y. B.P., and can sometimes give paleotemperature and paleohydrologic data, they could provide a first glimpse of desert paleoenvironments during isotope stages 4–9. Ages of speleothems from the Chihuahuan, Kalahari, and Somali-Chalbi deserts suggest that there was more available moisture in the southwestern U.S.A. and in northwestern Bostwana during glacials and interstadials of the last c. 300,000 years, but that wetter conditions in the Horn of Africa corresponded with interglacials and perhaps to a lesser extent with interstadials. Pollen from three northern Somalia speleothems indicate more mesic conditions in the Horn of Africa at 10,000, 11,800, and 176,500 yr B.P., while speleothem pollen spectra from Matupi Cave in northeastern Zaire, presently surrounded by tropical rainforest, suggest a savanna grassland at this cave c. 14,000 yr B.P.

A Holocene pollen record for the Kalahari Desert of Botswana from a U-series dated speleothem

A 61 cm core through a speleothem column in Drotskys Cave, Botswana, has yielded a U-series dated pollen record of Holocene vegetation changes in the Kalahari Desert. Between c.10000-7000BP, the site was surrounded by an arid grassland with dry-adapted trees and shrubs such as Acacia and Commiphora. An increase in pollen of Combretaceae and Cyperaceae, and the appearance of pollen of such mesic savanna plants as Grewia between c. 7000—6000 BP, may indicate the onset of slightly wetter conditions. Combretaceae and other mesic pollen types increase between c. 6000-3000 BP, but the late Holocene record appears variable, with the period c. 5000-4000 BP being somewhat dryer than the millennia before and after. It would appear that Kalahari vegetation has changed less during the Holocene than many other African environments previously investigated. Despite the limitations imposed by pollen analysis of cave sites, the record from Drotskys Cave shows good agreement with regional trends derived from other studies. The use of U-series dated pollen spectra derived from speleothems holds potential for expanding the geographic scope of palynological studies to areas lacking suitable lakes and bogs, and for extending the dated pollen record back an order of magnitude beyond the 14C timescale.

The Ancient Maya Drought Cult: Late Classic Cave Use in Belize

ies have demonstrated that a dry period or drought was coincident with the ninthcentury A.D. Classic Maya collapse. These studies have utilized data sources such as lake cores from the Peten and Yucatán (Brenner et al. 2002; Curtis et al. 1996; Curtis et al. 1998; Dunning et al. 1997; Hodell et al. 1995; Hodell et al. 2001; Rosenmeier et al. 2002) as well as the highresolution sediments from the Cariaco Basin in Venezuela (Haug et al. 2003; Peterson and Haug 2005). Richardson Gill (2000) argued mightily that drought was the driving force behind the collapse, the Preclassic abandonment, and Maya hiatus. While the latter are still debated, it is difficult to disassociate regional drought from the Classic collapse due to the overwhelming cumulative scientific evidence derived from numerous regional studies. Agency theories remind us that it is not the event per se but the human response to it that causes changes in the social order or “collapse.” Recent studies (Aimers 2007; Demarest et al. 2004:546) question the usage of terms such as collapse or fall THE ANCIENT MAYA DROUGHT CULT: LATE CLASSIC CAVE USE IN BELIZE

Middle-to late-Holocene moisture changes in the desert of northwest Namibia derived from fossil hyrax dung pollen

New pollen results and radiocarbon dating from fossil hyrax middens derived from the edge of the northern Namib Desert address the shortage of continental palaeobotanical evidence in arid Namibia by providing evidence for the environmental conditions during the mid to late Holocene in the region. The results obtained reflect long-term stability in the area throughout most of the sequence. Higher than modern moisture availability is suggested between c. 6 and 1 ka BP by the increased abundance of Poaceae, Cyperaceae or Chenopodiaceae, which respond rapidly by flowering after modest quantities of rainfall. Around 1 ka and recently arid conditions seem to have prevailed, with a decrease in Poaceae and an increase in Acanthaceae. Other palaeoecological evidence from the local and regional surroundings that focused on fluvial deposits, marine palynology and mineralogy confirms this mid-Holocene increase in more effective rainfall.

Late pleistocene paleowind directions, Atlantic Coastal Plain, U.S.A

Abstract Parabolic dunes, transverse dunes and fields of irregular dunes are common along the left banks, the north or east sides, of streams of the Atlantic Coastal Plain from Georgia to Delaware and probably beyond. Similar dunes occur in the Appalachian Piedmont, but are less common. The features have been eroded, to some extent, and are normally recognizable on topographic maps only as areas of irregular topography with wet and dry depressions. The larger parabolic dunes can be recognized on aerial photographs and satellite images because the associated soils are poor and the vegetative cover relatively sparse. The dunes are of eolian origin and late Pleistocene in age. It is thought that most of the dunes are late Wisconsin in age and contemporary with the Carolina Bays. A small sample of paleowind directions based on average directions of parabolic dune axes indicates that late Wisconsin winds in Georgia were westerly and grain sizes suggest that the winds were stronger than present-day coastal winds of the area. Paleowind directions shift progressively toward the south across the Carolinas and are southwesterly in North Carolina. They change abruptly to the northwest in the Delmarva Peninsula. Following the theory that Carolina Bays are former lakes that are elongated perpendicular to the effective wind direction, the fact that paleowind directions derived from Carolina Bay orientations are virtually identical to those from parabolic dune orientations, strongly supports the wind orientation theory of Carolina Bay origin and confirms the validity of the small sample of directions obtained from parabolic dunes. By analogy with dunes of the eastern shore of Lake Michigan and the Atlantic and Pacific Coasts, the Atlantic Coast Plain dunes do not necessarily indicate arid conditions. The sediment load of some streams may have been sandier than at present and streams with sandy, braided channels may have been common.

Evapotranspiration in the Okefenokee Swamp watershed: a comparison of temperature-based and water balance methods

Abstract In the Okefenokee Swamp, actual evapotranspiration (AET) should approximate potential evapotranspiration (PET) as there is rarely a moisture deficit. Even during droughts, a high evapotranspiration rate is maintained owing to a large area of wetlands and a shallow groundwater table in the watershed. Therefore, if AET can be estimated in the Okefenokee Swamp watershed, it can be used to evaluate the accuracy of PET estimates for the region. In this study, temperature-based PET estimates were compared with AET obtained by the water balance method for the Okefenokee Swamp watershed. Swamp water level data were used to identify subperiods with identical storage at the beginning and end of the period, so that a steady-state water balance model could be used to estimate AET as the difference between precipitation and outflow runoff. When the PET estimates were regressed upon AET, Thornthwaite PET had the highest R 2 value (0.817), followed by Blaney-Criddle PET (0.781), and Holdridge PET proportioned by biotemperature (0.768). The Thornthwaite method also rendered a long-term average very close to that of AET. Seasonal errors in Thornthwaite PET were reduced by using pan evaporation and temperature data to partition annual values into monthly values. Holdridge PET overestimated evapotranspiration from the Okefenokee Swamp watershed. Using standard crop coefficients, the Blaney-Criddle method would have overestimated evapotranspiration in the humid Okefenokee Swamp watershed. However, by substituting a very low crop coefficient ( K = 1.5), the Blaney-Criddle method gave results for the Okefenokee region similar to those obtained by the Thornthwaite method with the lowest error among all the methods examined. Pan evaporation correlated well with AET ( R 2 = 0.628) once an appropriate pan coefficient (0.71) had been determined.

Hydrology of the Nahanni karst, northern Canada, and the importance of extreme summer storms

Abstract Discovered in August 1971, the Nahanni karst is the most complex high-latitude karst known. Mean annual temperature is −4.5°C and precipitation 566 mm. The most spectacular landforms occur on a structural col (the “north karst”) connecting south Nahanni and Ram Plateau. Between July 19 and 31, 1972, an extreme summer storm deposited 224 mm of rain on the area. First, Second and Third Poljes in the north karst flooded; maximum water depths were 8.5, 25 and 8 m, respectively, and Third Polje overflowed. The level of Raven Lake (0.25-0.5 km long) rose 49 m at an average 2.9 m per day. Flooding occurs through random perching of water above and below ground where conduits have been heavily alluviated. There is no highly integrated regional groundwater body; water moves along independent or poorly integrated conduits or multiple aquifers. Some depressions are inundated by groundwater entering through estavelles, others when surface and spring inputs exceed drainage. Although winter snowfall averages 213 cm, in most years spring snowmelt does not appear to cause prolonged flooding. However, it raises water levels in the aquifer leaving the area prone to flooding by frequent, intense summer rains. The magnitude and complexity of hydrologic activity in the subarctic Nahanni karst is remarkable, being comparable with that in tropical and temperature carbonate areas.