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Other Titles: Climate changes and variability in the last two thousand years in Northeast Brazilian Region: Lake Boqueirão - RN
Keywords: Anomalia Climática Medieval;  Pequena Idade do Gelo;  Função de Transferência;  Diatomáceas Nordeste Brasileiro;  Medieval Climate Anomaly;  Little Ice Age;  Transfer Function;  Diatom;  Northeast Brazilian Region
Issue Date: 25-Jun-2012
Abstract: The present study aims to evidence for climate variability over the last 2000 years in the Brazilian Northeast Region from Boqueirão Lake sedimentary records (mainly diatoms), Rio Grande do Norte, Brazil (5°14'42.51"S e 35°32'40.80"W). The chronology of Boqc0901 core was obtained by 2400 BP. Sediment core was divided in five main units. Between 418 BC and 388 AD (Sub-Unit V.c), particle size distribution oscillations may represent more transporting environments, with features of shallow water, which also can be indicated by the low relative abundances of Mastogloia smithii var. lacustris and high abundances of Brachysira brebissonii, Navicula cryptotenella, Nitzschia amphibia. Between 388 AD and 1113 AD (Sub-Unit V.b), in its beginning have shown a mark of limnological changes on Boqueirão Lake, where increase and dominance of M. smithii var. lacustris and no record of the main acidophilic and/or eutrophic species (ex. B. brebissonii and Nitzschia amphibia, respectively) lead to the interpretation of water and alkaline environments with lower levels of nutrients (especially phosphorus) suggesting response to the increase of the water level. This interpretation is also supported by lower sedimentation rates and the changing pattern where the particle size distributions began to tend mainly to the clay and fine silt. Between 1113 and AD 1444 AD (Sub-Unit V.a and Unit IV) there was a continuing high level of water, also indicated by low values of C/N and peaks of M. smithii var. lacustris and Cyclotella meneghiniana.Between 1444 AD and 1795 AD (Unit III) there was a reduction of the water column, mainly indicated by the downward trend of M. smithii var. lacustris abundances, return of acidophilic/eutrophic species records and higher values of C:N. However, this reduction in the water column on Unit III was not prominent, because the records of Aulacoseira ambigua var. ambigua and Discostella cf. stelligera have also suggested high water column. Between 1795 AD and 2009 AD (Unit II and I) there was a continuation of high waters mainly indicated by lower values of C/N. The reconstruction of Boqueirão Lake water depth (m) through transfer function (Weighted Average), processed data (r2 = 0.59 and rboot = 0.53) appeared as a plausible model, where distinct patterns were observed. These patterns corroborate previous interpretations, which can highlight the intervals between 418 BC and 370 AD, where the depth ranged from 3.62 ± 1.73 m and 4.59 ± 1.70 m and between 370 AD and 2009 AD, which occurred at depths greater than 5m, such as the intervals: between 713 AD and 1052 AD depths was greater than 7m for the first time (min: 7.07 ± 1.75 m and max: 7.24 ± 1.72); the interval 1052 AD (7.24±1,72) to 1250 AD (7.23±1,73) shown water level decline between 1195 AD and 1215 AD (6,17m±1,69 and 6,31m±1,70); Between 1329 AD (7,24m±1,74) and 1834 AD (5,53m±1,67) fluctuations of the depth was reduced. In the last 100 years the water level presented high values (eg 7,22m±1,68 in 1974 AD and 6,55±1,73 in 2009 AD). The results suggest distinct climatic variability over the last millennium, in particular during the Medieval Climate Anomaly - MCA (~ AD 900-1100) and Little Ice Age - LIA (~ 1400-1820 AD). The wet conditions during the MCA were assigned to La Niña like-conditions in the Pacific Ocean due to intensification of Atlantic Meridional Overturning Circulation (AMOC), caused by prolonged positive phase of NAO, despite the indications of position to the north of the Intertropical Convergence Zone (ITCZ). The pattern of Northern Hemisphere cooling during LIA and the weakening of the AMOC can influence the displacement of the ITCZ to the south of its climate, causing more rain in Northeast Brazil, however, South American Summer Monsoon stronger condition may have led more droughts in the Northeast region of Brazil during LIA.
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