Lecture Notes in Earth Sciences 88 Editors: S. Bhattacharji, Brooklyn G. M. Friedman, Brooklyn and Troy H. J. Neugebauer, Bonn A. Seilacher, Tuebingen and Yale oluaS Rodrigues-Filho German rellitM A Holocene Sedimentary Record from Lake Silvana, SE Brazil .~ Evidence for Paleoclimatic Changes from Mineral, Trace-Metal and Pollen Data With 29 Figures and 11 Tables regnirpS Authors Dr. Saulo Rodrigues-Filho Geochemist CETEM/CNPq, Centro de Technologia Mineral Rua 4, Quadra D. Cidade Universitaria, 21941.590 Rio de Janeiro, Brazil Prof. Dr. Dr. h. c. mult. German Mtiller Ruprecht-Karls-Universit~it Heidelberg, Institut ftir Umweltgeochemie Im Neuenheimer Feld 236, D-69120 Heidelberg, Germany Cataloging-in-Publication data applied for Die Deutsche Bibliothek - CIP-Einheitsaufnahme Rodrigues-Fiiho, Saulo: A hoiocene sedimentary record from Lake Silvana, SE Brazil : evidence ibr paleoclimatic changes from mineral, trace metal and pollen data / Saulo Rodrignes-Filho ; German Miiller. - Berlin ; Heidelberg ; New York ; Barcelona ; Hong Kong ; London ; Milan ; Paris ; Singapore ; Tokyo : Springer, 1999 (Lecture notes in earth sciences ; 88) ISBN 3-540-66205-7 "For all Lecture Notes in Earth Sciences published till now please see final pages of the book" ISSN 0930-0317 ISBN 3-540-66205-7 Springer-Verlag Berlin Heidelberg New York This work is subject to copyright. 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Typesetting: Camera ready by author SPIN: 10736938 32/3142-543210 - Printed on acid-free paper To Andr6a Vilhena and Jfilia and Pedro Vilhena Rodrigues Preface In 1950 Martin Schwarzbach from Cologne University published a remarkable book ,,Das Klima der Vorzeit,,, followed by the second (1961) and third (1974) edition and an English version ,,Climates of the Past,, (1963), tracing the historical evolution of the earth's climate since Cambrian time. The catalytic effect of his book was enormous and encouraged worldwide research with the development and application of new methods, to mention only isotopic methods to reconstruct paleotemperatures. During the past two decades the climate question has become a new ecological dimension: Will the ,,greenhouse effect,,, related to the combustion of fossil fuels, influence our present climate? Will it remain as it is, will it become colder, or will it become warmer? And: how rapid will a change occur (if it occurs at all!)? Studies on paleoclimatic development to-day concentrate on Pleistocene sediments and on ice cores deposited during and between glacial transgressions and on post-glacial sediments laid down during the Holocene. The present study is an example of a sediment series in a lake formed some 10.000 years ago in a now tropical climate. Already by vision four different sediment types can be recognized which represent four paleoenvironmental zones. Mineralogical-sedimentological, geochemical and palynological investigations permit the establishment of a general climatic change from grassland to savanna type vegetation and a period of stronger rainfall leading to the present-day semideciduous forest. The study is also an example for a multidisciplinary approach which permits the connection of sediment data with the weathering and erosion history in the catchment. stnemegdelwonkcA We would like to thank the CAPES (Brazilian Coordination for Post-Graduation) for the financial support with a fellowship, the Institute of Environmental Geochemistry at the University of Heidelberg for analytical facilities and financial support, and the CETEM/CNPq (Center for Mineral Technology) for logistical support during the field work in Brazil. The first author is greatly indebted to Prof. Dr. Roberto C. Villas B6as for his support in drawing the outlines of this project, as well as to Dr. Fernando A. Freitas Lins and Adgo Benvindo da Luz for the institutional support from CETEM/CNPq. We are grateful to Dr. Hermann Behling, who kindly carried out the pollen analyses in lake sediments. Without the participation of Prof. Dr. Georg Irion in the sampling campaign, the coring operation would not have been possible. Suggestions by Dr. J6rg Matsehullat helped a great deal during the preparation of this manuscript. We are also grateful to Anne Marie De Grosbois for revising the text. The analytical support provided by Silvia and Stephan Rheinberger was a valuable help. We are pleased to acknowledge the friendly collaboration by Manfred Gastner in the mineralogical analyses. Dr. Hans-Peter Meyer helped in operating the X-ray fluorescence spectrometer. Dagmar Eggergluss and Ralf Ottenstein helped with chemical analyses and data processing. The first author would like to express his special thanks to his wife, Andr6a M. Gouthier de Vilhena and their children, Jfilia and Pedro Vilhena Rodrigues, as well as to his parents, Saulo Rodrigues Pereira and C61ia F. Rios Pereira, for the long-term and valuable support. Contents 1 Abstract 2 Introduction 5 2.1 Study area 8 2.1.1 Geology and geomorphology 9 2.1.2 Climate and vegetation 21 2.2 The dammed-valley lakes of Rio Doce 41 2.3 The lacustrine environment in the Rio Doce lakes and related redox processes 19 2.4 Chemical weathering in tropical regions 24 3 Methods 72 3.1 Sampling 27 3.2 Analyses 82 4 Results 33 4.1 Lake sediments - paleoenvironmental proxies from core SB 1 33 4.1.1 Paleoenvironmentat zone I (1270-1180 cm) 33 4.1.2 Paleoenvironmental zone II (I 180-1160 and 990-970 cm) 53 4.1.3 Paleoenvironmental zone III (1160-990 and 970-360 cm) 04 4.1.4 Paleoenvironmental zone IV (360-0 cm) 24 IIX 4.2 Weathering profiles EG1 and EG2 48 4.2.1 Mercury accumulation at the top of profiles 55 4.3 Physical and chemical parameters of the water column of Lake Silvana 57 5 Discussion 63 5.1 Correlation with global-scale climatic events 66 5.2 Siderite-inferred recurrence of climatic events 70 6 Conclusions 73 7 References 57 8 Appendices 87 1 Abstract Geochemical, mineralogical and palynological data from a 12.7-m-long sediment core demonstrate the relevance of an interdisciplinary approach to paleoenvironmental studies. The history of the Holocene sedimentation in Lake Silvana was reconstructed by tracing back the source of detrital sediments based on contrasting geochemical and mineralogical patterns within the pedologic mantle of the catchment. Pollen record has led to the reconstruction of the paleovegetation, while comparison of the geochemical and mineralogical composition of lake sediments with that from the weathering mantle provided evidence of changes in take level and slope-erosion activity since the early Holocene. Such evidence yields some insights into the Holocene landscape evolution and the origin of the lake itself. The lowermost core section reached the Holocene-Pleistocene boundary, according to radiocarbon age determinations with accelerator mass spectrometry (AMS). Manganese and Fe concentrations are shown to be good tracers for the formation of the authigenic component in the lake, while the allogenic component was well traced by ,1A Ti and Pb concentrations. Four major paleoenvironmental zones indicative of distinct phases of the hydrological cycle in Lake Silvana are consistent with vegetation changes indicated by pollen data since the early Holocene. In zone I, yellowish silty- clayey sediments present trace-metal concentrations and a mineralogical composition that resemble those from the lower weathering profile, indicating an environment characterized by low water level and oxic conditions. In zone II, the occurrence of well-sorted clayey sediments composed of kaolinite, gibbsite, goethite and siderite, and with slightly elevated concentrations of Mn and organic C, suggests anoxic conditions and the formation of a shallow lake. Sediment geochemistry and pollen data point to avery early stage of the climate-driven geomorphic process that culminated in the formation of Lake Silvana. The rapid sedimentation of a 7.8-m-thick sequence of slope-wash sediments, named zone III, appears to be related to the formation of Lake Silvana and the whole take system of the middle Rio Doce Valley, southeastern Brazil. Slope erosion has been favored by a pollen-indicated sparse vegetation (tropical savanna). Events of slope instability suggest sharply increased precipitation and correspond to two pollen-indicated climatic transitions from dry to moister conditions that culminated 8500 years ago. This age resembles that of forest expansion registered in southern and southeastern Brazil and in southeastern Africa, as well as the early to mid-Holocene climatic event observed in records from ice cores. The records from Greenland point to an inverse fluctuation relative to that indicated for the study area, that is a notable increase in aridity, probably reflecting climatic changes in the Northern Hemisphere, as the bigger size of the northern continents suggests. Increasing cold and dry conditions from 9000 to 6000 years B.P. in northern low latitudes have been explained elsewhere with the orbitally induced decrease in summer insolation (July to August), which would have the opposite effect in the southern tropics. For the phase preceding the natural damming that likely gave rise to Lake Silvana, represented by zone I, sediment composition indicates a source from lower horizons of the weathering mantle. After this phase, materials derived from the uppermost pedologic horizon started to accumulate in the lake basin, thus forming the zone II. For the phase of intense slope-erosion activity, represented by zone III, detrital sediments are likely derived from the mixing of horizons regardless of their topographic position. In turn, sediments deposited during phases of less intense erosion and lacustrine sedimentation show no evidence of source, as observed for zone IV, probably as a result of their major authigenic nature. Since 8500 years B.P., environmental changes were marked by the expansion of forest and increasing proportions of the authigenic component in sediments, which characterize the whole zone IV. The vertical distribution of siderite-rich layers, arranged at intervals of ~1 meter, points to the cyclic recurrence of