Ivo Karmann

Insolation-driven changes in atmospheric circulation over the past 116,000 years in subtropical Brazil

During the last glacial period, large millennial-scale temperature oscillations—the ‘Dansgaard/Oeschger’ cycles—were the primary climate signal in Northern Hemisphere climate archives from the high latitudes to the tropics. But whether the influence of these abrupt climate changes extended to the tropical and subtropical Southern Hemisphere, where changes in insolation are thought to be the main direct forcing of climate, has remained unclear. Here we present a high-resolution oxygen isotope record of a U/Th-dated stalagmite from subtropical southern Brazil, covering the past 116,200 years. The oxygen isotope signature varies with shifts in the source region and amount of rainfall in the area, and hence records changes in atmospheric circulation and convective intensity over South America. We find that these variations in rainfall source and amount are primarily driven by summer solar radiation, which is controlled by the Earths precessional cycle. The Dansgaard/Oeschger cycles can be detected in our record and therefore we confirm that they also affect the tropical hydrological cycle, but that in southern subtropical Brazil, millennial-scale climate changes are not as dominant as they are in the Northern Hemisphere.

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.

ESR dating of a toxodon tooth from a Brazilian karstic cave.

A pre-molar of a Toxodon platensis (sp), a large mammal that inhabited the forests of Southeast Brazil during quaternary times, was dated by electron spin resonance (ESR). Enamel and dentine were separated giving, the following archeological doses (AD): (4.0+/-0.8) and (11+/-2) Gy, respectively. This difference was explained by the higher content of Uranium in dentine (25.1+/-0.3) ppm as compared to enamel (0.37+/-0.03) ppm. Taking account of the internal dose contribution to the AD, the age obtained is (6.7+/-1.3) ky BP for dentine and (5.0+/-1.6) ky BP for enamel.

Timing and structure of Mega‐SACZ events during Heinrich Stadial 1

A substantial strengthening of the South American monsoon system (SAMS) during Heinrich Stadials (HS) points toward decreased cross-equatorial heat transport as the main driver of monsoonal hydroclimate variability at millennial time scales. In order to better constrain the exact timing and internal structure of HS1 over tropical South America, we assessed two precisely dated speleothem records from central-eastern and northeastern Brazil in combination with two marine records of terrestrial organic and inorganic matter input into the western equatorial Atlantic. During HS1, we recognize at least two events of widespread intensification of the SAMS across the entire region influenced by the South Atlantic Convergence Zone (SACZ) at 16.11–14.69 kyr B.P. and 18.1–16.66 kyr B.P. (labeled as HS1a and HS1c, respectively), separated by a dry excursion from 16.66 to 16.11 kyr B.P. (HS1b). In view of the spatial structure of precipitation anomalies, the widespread increase of monsoon precipitation over the SACZ domain was termed “Mega-SACZ.”

Centennial-scale solar forcing of the South American Monsoon System recorded in stalagmites.

The South American Monsoon System (SAMS) is generally considered to be highly sensitive to Northern Hemisphere (NH) temperature variations on multi-centennial timescales. The direct influence of solar forcing on moisture convergence in global monsoon systems on the other hand, while well explored in modeling studies, has hitherto not been documented in proxy data from the SAMS region. Hence little is known about the sensitivity of the SAMS to solar forcing over the past millennium and how it might compete or constructively interfere with NH temperature variations that occurred primarily in response to volcanic forcing. Here we present a new annually-resolved oxygen isotope record from a 1500-year long stalagmite recording past changes in precipitation in the hitherto unsampled core region of the SAMS. This record details how solar variability consistently modulated the strength of the SAMS on centennial time scales during the past 1500 years. Solar forcing, besides the previously recognized influence from NH temperature changes and associated Intertropical Convergence Zone (ITCZ) shifts, appears as a major driver affecting SAMS intensity at centennial time scales.

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).

Geomorphological map of the Serra da Bodoquena karst, west-central Brazil

Abstract Please click here to download the map associated with this article. The Serra da Bodoquena is a karst area situated on the southern edge of the Pantanal wetland region in the states of Mato Grosso and Mato Grosso do Sul, central-western Brazil. Morphological analysis, on a scale of 1:60,000, made it possible to identify such various karst features as dolines, caves, sinks and springs, as well as karst cones and corridors, among others, compartmentalized into six morphological units. To the west of the Serra, the labyrinth karst that was identified reflects a situation of extreme flattening and diffuse infiltration via vertical fractures, which is gradually limited by polygonal karst, to the extent that there is drawdown of the water level. To the north of the Serra, these features are obscured by a greater degree of uvial incision, which develops in the form of canyons and alluvial plains. To the east, there is a predominance of karst plains with dolines in a thick soil covering, associated with residual hills. There are tufa deposits along the current uvial drainage system, and extensive older deposits occur in Quaternary terraces. Sandstone plains with innumerable dolines occur to the southeast of the Serra, reecting the presence of subjacent karst. Geomorphological mapping of this area will be able to contribute to the Serra da Bodoquena National Park management plan, or assist in the development of anthropic soil use/occupation plans.

New Accelerator Mass Spectrometry (AMS) Ages Suggest a Revision of the Electron Spin Resonance (ESR) Middle Holocene Dates Obtained for a Toxodon platensis (Toxodontidae, Mammalia) from Southeast Brazil

In a paper published in Applied Radiation and Isotopes, Baffa et al. (2000) reported a Middle Holocene date (~6.5 kyr BP) for a specimen of Toxodon platensis from Ribeira do Iguape, southeast Brazil, using the emergent technique electron spin resonance (ESR). Through an accelerator mass spectrometry (AMS) procedure applied on tooth collagen, we provide a new set of dates to test the accuracy of the ages generated by ESR. We obtained 2 dates more than 4500 BP older than the previous one, suggesting a minimum Late Pleistocene age for the specimen.

A high-resolution history of the South American Monsoon from Last Glacial Maximum to the Holocene

The exact extent, by which the hydrologic cycle in the Neotropics was affected by external forcing during the last deglaciation, remains poorly understood. Here we present a new paleo-rainfall reconstruction based on high-resolution speleothem δ18O records from the core region of the South American Monsoon System (SAMS), documenting the changing hydrological conditions over tropical South America (SA), in particular during abrupt millennial-scale events. This new record provides the best-resolved and most accurately constrained geochronology of any proxy from South America for this time period, spanning from the Last Glacial Maximum (LGM) to the mid-Holocene.

Quaternary tufa in the Serra da Bodoquena Karst, West-Central Brazil: Evidence of wet period

The Serra da Bodoquena is one of the most extensive continuous karst areas in Brazil, located in the Mato Grosso do Sul State, in the central western part of Brazil, on the southern edge of the Pantanal wetland region. It consists of a north-south plateau that extends for approximately 200 km, which forms an important water divide with an altitude of approximately 800 m. The area has a humid tropical climate with average temperatures between 22 and 24oC and an average annual rainfall of 1,419 mm, with 1-3 dry months. In the Serra da Bodoquena karst occurs the best examples of present-day deposition of tufa, along most of the rivers of the region, in the river beds and along their margins, forming dams (Figure 1).