Augusto S. Auler

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.

Timing and structure of the 8.2 kyr B.P. event inferred from δ18O records of stalagmites from China, Oman, and Brazil

Oxygen isotope records of stalagmites from China and Oman reveal a weak summer monsoon event, with a double-plunging structure, that started 8.21 ± 0.02 kyr B.P. An identical but antiphased pattern is also evident in two stalagmite records from eastern Brazil, indicating that the South American Summer Monsoon was intensifi ed during the 8.2 kyr B.P. event. These records demonstrate that the event was of global extent and synchronous within dating errors of <50 years. In comparison with recent model simulations, it is plausible that the 8.2 kyr B.P. event can be tied in changes of the Atlantic Meridional Overturning Circulation triggered by a glacial lake draining event. This, in turn, affected North Atlantic climate and latitudinal position of the Intertropical Convergence Zone, resulting in the observed low-latitude monsoonal precipitation patterns.

Abrupt variations in South American monsoon rainfall during the Holocene based on a speleothem record from central-eastern Brazil

Well-dated high-resolution oxygen isotope records of speleothems in central-eastern Brazil spanning from 1.3 to 10.2 kyr B.P. reveal that the occurrence of abrupt variations in monsoon precipitation is not random. They show a striking match with Bond events and a significant pacing at ∼800 yr, a dominant periodicity present in sea surface temperature records from both the North Atlantic and equatorial Pacific Oceans that is possibly related to periods of low solar activity (high 14 C based on the atmospheric Δ 14 C record). The precipitation variations over central-eastern Brazil are broadly antiphased with the Asian and Indian Monsoons during Bond events and show marked differences in duration and structure between the early and late Holocene. Our results suggest that these abrupt multicentennial precipitation events are primarily linked to changes in the North Atlantic meridional overturning circulation (AMOC). Anomalous cross-equatorial flow induced by negative AMOC phases may have modulated not only the monsoon in South America but also affected El Nino–like conditions in the tropical Pacific during the Holocene.

Hydroclimate changes across the Amazon lowlands over the past 45,000 years

Reconstructing the history of tropical hydroclimates has been difficult, particularly for the Amazon basin—one of Earth’s major centres of deep atmospheric convection. For example, whether the Amazon basin was substantially drier or remained wet during glacial times has been controversial, largely because most study sites have been located on the periphery of the basin, and because interpretations can be complicated by sediment preservation, uncertainties in chronology, and topographical setting. Here we show that rainfall in the basin responds closely to changes in glacial boundary conditions in terms of temperature and atmospheric concentrations of carbon dioxide. Our results are based on a decadally resolved, uranium/thorium-dated, oxygen isotopic record for much of the past 45,000 years, obtained using speleothems from Paraíso Cave in eastern Amazonia; we interpret the record as being broadly related to precipitation. Relative to modern levels, precipitation in the region was about 58% during the Last Glacial Maximum (around 21,000 years ago) and 142% during the mid-Holocene epoch (about 6,000 years ago). We find that, as compared with cave records from the western edge of the lowlands, the Amazon was widely drier during the last glacial period, with much less recycling of water and probably reduced plant transpiration, although the rainforest persisted throughout this time.

Orbital and Millennial-Scale Precipitation Changes in Brazil from Speleothem Records

Paleorainfall variability on orbital and millennial time scales is discussed for the last glacial period and the Holocene, based on a multi-proxy study of speleothem records from Brazil. Oxygen isotope (δ18O) records from Botuvera and Santana caves, precisely dated by U-series methods, indicate stronger summer monsoon circulation in subtropical Brazil during periods of high summer insolation in the southern hemisphere. In addition, variations in Mg/Ca and Sr/Ca ratios from speleothems confirm that this monsoon intensification led to an increase in the long-term mean rainfall during insolation maxima. However, they also suggest that glacial boundary conditions, especially ice volume buildup in the northern hemisphere, promoted an additional displacement of the monsoon system to the south, which produced rather wet conditions during the period from approximately 70 to 17 ka B.P., in particular at the height of the Last Glacial Maximum (LGM).

Hypogene Karst Regions and Caves of the World

Signs of hypogenic speleogenesis have been detected in a number of caves of the Murcia Region (SE Spain), in some cases revealing active speleogenetic mechanisms rarely observed in hypogene cavities elsewhere in the world. Here, we investigate the hypogenic morphologies and speleothems of four caves in this region, namely Sima de la Higuera, Sima Destapada, Cueva del Agua and Cueva del Puerto. Also, other ten caves showing evidence for hypogenic speleogenesis has been preliminary described. Processes related to ancient and current hydrothermal activity, the discordance of permeability structures in the adjacent beds and the spatial arrangement of the regional hydrogeology have given rise to maze patterns and typical subaqueous hypogenic morphologies. These include spongework mazes, rising wall channels and shafts, feeders, bubble trails, solution pockets, megascallops and rising of chains cupolas, among others. Carbonic acid speleogenesis is responsible for the formation of most of these cave features; however, evidence of sulfuric acid speleogenesis (SAS) has been observed in Cueva del Puerto and Sima del Pulpo, which host massive secondary gypsum deposits. Speleothems typically linked to hydrothermal water upwelling and CO2 degassing close to the water table are present in most of these cavities, including folia, calcite spar crystals, cave clouds, calcite rafts deposits and several types of cave raft cones. The wide variety of hypogenic speleogenesis indicators and speleothems whose genesis is unconnected to meteoric water seepage reveals that the hydrothermal field of the Murcia Region hosts one of the densest active hypogenic subterranean networks in the world.

A large Cervidae Holocene accumulation in Eastern Brazil: an example of extreme taphonomical control in a cave environment

A remarkable cervid bone accumulation occurs at a single passage (named Cervid Passage; CP) at Lapa Nova, a maze cave in eastern Brazil. CP lies away from cave entrances, is a typical pitfall passage and contains bone remains of at least 121 cervids, besides few bats, peccaries and rodents remains. There is no evidence of water (or sediment) flow at the site and in general bones lack post depositional alterations and display anatomical proximity, suggesting that the majority of the remains found inside CP (mainly cervids) are due to animals that after entering the cave got trapped in the site. Observations suggest that two entrances could have provided access to cervids (and the few other animals, besides bats), either by falling inside the cave or by entering by their own free will. Once inside the cave, the maze pattern would make route finding difficult, and of all passage intersections, only the one leading to CP would result in a non-return situation, starving the animal to death. Radiocarbon dates suggest that animal entrapment occurred during at least 5 thousand years, during the Holocene. The reasons why mainly cervids were found are unknown but they are probably related to the biology of this group coupled with the fact that caves provide several specific taphonomic processes that may account for a strong bias in bone accumulation. Indeed, the frequent occurrence of Cervidae in both the fossil and sub-fossil record in Brazilian caves may be related to an overall high faunal abundance or may suggest that these animals were especially prone to enter caves, perhaps in search of nutrients (as cave saltpetre) or water.

Lagoa Santa Karst: Cradle of Brazilian Cave Studies

The Lagoa Santa Karst is the best known karst area in Brazil and has been studied since the early 1800s. This area is developed over Precambrian limestone of the Sete Lagoas Formation, Bambui Group, and displays scenic surface karst landforms, especially limestone cliffs, karst lakes, and karst plains, in addition to numerous solution dolines. Thick soil sequences derived from overlying phyllites cover a significant portion of the karst, but exhumation is presently taking place, exposing karst outcrops with subsoil karren features. Karst hydrology is primarily autogenic, with short groundwater flow routes. Limestone outcrops are vertically dissected, exposing ancient paragenetic caves intersecting cliff faces. More than 700 caves are known in the area, the majority of which are short ( 350 kyr). Lund was also the first to describe ancient human remains frequently found in caves and rock shelters. Lagoa Santa is now known to contain hundreds of archaeological sites dating from the Early Holocene/Late Pleistocene and has been at the center of the debate on the origin and age of human colonization in the Americas. The Lagoa Santa Karst faces severe environmental threats due to limestone mining and the expansion of the metropolis of Belo Horizonte and its surrounding towns. A number of preservation areas have now been established, improving the conservation status of this landmark Brazilian karst area.

Carajás National Forest: Iron Ore Plateaus and Caves in Southeastern Amazon

The Carajas National Forest in the Amazon is an outstanding area of conservation. The occurrence of mafic and felsic volcanic (Neoarchean) rocks is associated with banded iron formation (BIF) lenses, including jaspilites. Particularly on the BIF, there is the occurrence of ferruginous breccia which supports the tops of several mountain plateaus , generally referred to as the Carajas Ridge. Hills, drainage channels, and a number of closed depressions are commonly found on the surface, in addition to savannah surrounded by a luxuriant tropical forest. Subsurface rainwater drainage (throughflow) occurs via a network of small conduits on the tops of the mountain ranges, in addition to rainwater draining superficially in channels that are directed to closed depressions or to drainage headwaters. Several rocky scarps exist on the edges of the mountain ranges, in addition to talus deposits. Over 1,000 caves have been recorded in the area, and shallow, short-length (about 30 m) caves are predominant in this region. Longer caves, which may reach over 300 m, are normally present as single semicircular, funnelled, or straight chambers and are comprised of very irregular interconnecting passages in various sizes. Within the caves, clastic deposits are predominantly autogenic and originate from collapsed parts of the ceilings and walls. Chemical deposits are generally made up of small-sized features with a diverse mineralogy. Crusts and coralloids predominate in addition to draperies and micro-rimstone dams, and irregular pendulous forms known as pingentes hang from the ceilings, and are similar to stalactites. Speleothems are composed mostly of iron oxide–hydroxide and phosphates. The Carajas National Forest is a protected federal area that is used for different purposes, including the operation of the world’s largest iron ore mines. The savannah environment which is embedded amidst the tropical forest must be protected as conservation areas to represent the most expressive remnants of the original context. In addition, environmental compensation areas contiguous to the National Forest are being purchased, and these will become part of the Carajas protected area.

Peter Wilhelm Lund’s Scientific Contributions

Following Peter Wilhelm Lund’s fortuitous meeting with compatriot Peter Claussen and his first visit to the Maquine cave in 1834, Lund converted himself into a highly specialized scientist, formulating original concepts not only regarding the fossil bones but also the geology, speleology, and the age and modes of deposition of bones and cave sediments. Most of those topics were addressed in order to provide a robust framework in which the description of the fossil bones, his karst main interest, could be properly understood. In particular, Lund dedicated considerable efforts to understanding the depositional context of the cave bones. In his first forays into caves, Lund attempted to match his observations with the cataclysmic theory of his master, George Cuvier, which stated that a universal deluge would have suddenly killed all animals. However, Lund soon realized that living species were also found in caves, sometimes mixed with older ones. Lund cautiously distanced himself from Cuvier’s ideas, proposing five mechanisms of fossil emplacement in caves: (i) carried in by predators, (ii) the fall of animals in vertical passages, (iii) animals that wandered into caves and got lost, (iv) the death of species that live inside caves, and (v) washed by runoff into caves. The discovery of human fossil bones at the Sumidouro cave allowed Lund to propose the great antiquity of these bones and their contemporaneity with the extinct species; both ideas have been proved correct by modern studies. Lund’s contribution to various disciplines in the Lagoa Santa karst has retained its scientific value nearly 200 years later.