Nicolás M Stríkis

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.

The puzzle assembled: Ediacaran guide fossil Cloudina reveals an old proto-Gondwana seaway

ABSTRACTDuring the Ediacaran the Clymene Ocean separated the Lau-rentia, Amazonia, and Rio Apa cratons from several landmasses to the west forming the proto-Gondwana supercontinent. However, no clear evidence about the existence of Ediacaran epeiric seas over those landmasses has been found. Here we report and discuss the discovery of the Ediacaran guide fossil Cloudina sp. associated with other metazoan body and trace fossils in the Bambui Group (central eastern Brazil). The Ediacaran age of the Bambui Group and the paleogeographic position of Cloudina -bearing successions in Brazil, Antarctica, Namibia, and Argentina suggest a scenario of ocean con-nectivity among coeval intracratonic basins of South America, Africa, and Antarctica at the end of Neoproterozoic time. The new fi nding epitomizes one of the most important paleontological discoveries ever made in South America, helping to solve an old paleogeographic puz-zle of the Gondwana supercontinent.INTRODUCTION The Bambui Group crops out in central eastern Brazil, and is one of the most studied Neoproterozoic sedimentary successions in South America. It comprises an extensive, fl at-lying cratonic cover in erosional contact with Paleoproterozoic and Archean basement rocks of the Sao Francisco craton. In its eastern and westernmost occurrences, the Bam-bui Group is deformed and metamorphosed by the Brasilia and Aracuai mobile belts, respectively (Fig. 1A). The regional tectonostratigraphy has been interpreted as refl ecting deposition in a foreland basin related to Gondwanan collisions involving the Sao Francisco craton (Pimentel et al., 2011).The Bambui Group (BG) overlies rocks of the Macaubas Group and Jequitai Formation and the Carrancas conglomerate, units supposedly de-posited during Sturtian glaciation. The unit is 700–1000 m thick (Misi et al., 2007) and comprises limestones and dolomites of the Sete Lagoas For-mation at the base that grade upward to shales and siltstones of the Serra de Santa Helena Formation, followed by marls, siltstones, limestones, and sandstones of the Lagoa do Jacare, Serra da Saudade, and Tres Marias Formations (Fig. 1B).The precise age of the BG is not well established, precluding accurate correlation with other supposedly coeval successions such as the Corumba Group (Brazil), Itapucumi (Paraguay), Arroyo del Soldado (Uruguay), Si-erras Bayas (Argentina), and Nama Group (Namibia). The presence of

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.

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.

South American monsoon response to iceberg discharge in the North Atlantic

Significance Here, we present a precisely dated speleothem record of South American monsoon precipitation covering the period encompassed by the last six Heinrich Stadials. Our monsoon record allows us to determine the timing of regional hydroclimatic expression of Heinrich Stadials over tropical lowland South America. By comparing our record with sea-surface temperature reconstructions from the subtropical North Atlantic, our results provide evidence connecting South American monsoon precipitation and methane release with the events of iceberg discharge depicted by the deposits of ice-rafted detritus. These results are relevant to climate modelers and paleoclimatologists interested in abrupt climate change, tropical–extratropical climate teleconnections, and paleo-reconstructions of the monsoon and the tropical hydrologic cycle. Heinrich Stadials significantly affected tropical precipitation through changes in the interhemispheric temperature gradient as a result of abrupt cooling in the North Atlantic. Here, we focus on changes in South American monsoon precipitation during Heinrich Stadials using a suite of speleothem records covering the last 85 ky B.P. from eastern South America. We document the response of South American monsoon precipitation to episodes of extensive iceberg discharge, which is distinct from the response to the cooling episodes that precede the main phase of ice-rafted detritus deposition. Our results demonstrate that iceberg discharge in the western subtropical North Atlantic led to an abrupt increase in monsoon precipitation over eastern South America. Our findings of an enhanced Southern Hemisphere monsoon, coeval with the iceberg discharge into the North Atlantic, are consistent with the observed abrupt increase in atmospheric methane concentrations during Heinrich Stadials.

Two Millennia of South Atlantic Convergence Zone Variability Reconstructed From Isotopic Proxies

Most reconstructions of the South American Monsoon System (SAMS) over the last two millennia are based on δO records from locations at high-elevation sites in the Andes, which are not influenced by the South Atlantic Convergence Zone (SACZ). Yet the SACZ is a key driver of SAMS variability over much of Brazil. Here we use two new δO records from speleothems sampled in the central and southwestern portions of the SACZ core to show that the SAMS was not varying in phase over the entire tropical continent during the last two millennia. In fact, speleothem records located to the northeast of the SACZ record precipitation variations that are antiphased with similar records on the opposite side of the SACZ, in particular during the Little Ice Age period, while records close to the core of the SACZ axis show no significant departure from the mean state during this period. Plain Language Summary The South American Monsoon System (SAMS) is responsible for most rainfall occurring over tropical South America. Within this monsoon system the South Atlantic Convergence Zone (SACZ) is considered a key driver of SAMS variability over Brazil. By analyzing the chemical composition of stalagmites we can track the history of rainfall in the region where these stalagmites grew. Using stalagmites from caves in the SACZ region of Brazil that formed during the last two millennia, we can reconstruct the behavior of the SACZ, and consequently the rainfall distribution over Brazil during this period, which includes global climate changes that significantly affected human history, such as the Medieval Climate Anomaly and the Little Ice Age. Understanding the nature of these past changes in climate is fundamental for putting current climate changes in a longer-term perspective and for differentiating between natural and anthropogenic causes of current and future climate change.

Um sistema de exudação de metano permiano como um análogo paleoambiental para plataformas carbonáticas pré-surgimento dos metazoários

Following the rise of metazoans, the beginning of bioclasticity and substrate competition, the saga of microbial mats was in a fluctuating decline in the end of the Neoproterozoic era. Increases in diversity during the Phanerozoic and punctual upturns in the microbial carbonate production occured after the events of global mass extinctions. Gradually along the Phanerozoic, the microbial colonies occupied isolated niches and grazers-free environments, characterized by physically and/or geochemically stressful conditions, such as those found in saline bays, alkaline lakes and hydrothermal or cold seep vents. Here we report one of the oldest occurrences of a vent camp coupled with cold seepage of methane in the geologic record, associated with well-preserved microbialites and elephant skin structures. During the seep activity, oxygen depletion and high salinity conditions are prohibitive for complex animal life, clearing the way to microbial colonies to flourish. Due to the co-occurrence of high adaptability and low competitiveness of microbial forms, they became highly specialized in stressful conditions. We argue that the sporadic microbial mat upturns in Earth’s history are not restricted to geological periods, following massive death of metazoan species; they also may occur in response to punctual paleoenvironmental conditions that enable microbial colonies to growth. Indeed, the Phanerozoic geological record is punctuated of these examples, in a kind of hide-and-seek game of Precambrian times.

Um Olhar Crítico dos Alunos Sobre o Curso de Bacharelado em Geologia

The aim of this paper is to discuss the main problems in the curricular structure of the BachelorsA¢ Geology Course of the Geosciences Institute - USP, and in some subjects, from the studentsA¢ point of view. In discussions between a student representative and the CEPEGE, and in some assemblies during 2006, some problems were noticed and discussed, and proposals to correct them were considered. Some of the main topics were the indifference shown with the graduation course, the low quality of the subjects offered to the geology students by other institutes, a better use of the field excursions undertaken during the first two years of the course, and the concentration of many important subjects in third and forth years, overloading the course.