Accumulation of metals and changes in composition of freshwater lake organic sediments during the Holocene

Abstract

The distribution of metals in the strata of sediments deposited in the freshwater environment formed during the Holocene in the territory of Latvia has been studied rather insufficiently. Considering the large volume of accumulated organic matter in sediments, studies on the content of metallic elements in sediments can help to determine the peculiarities of the metal accumulation process in limnic systems and the biogeochemical cycling of metals. Lake Pilvelis, which is located in the eastern part of Latvia (the northern part of Europe), was selected as the main study site. Lake Pilvelis is a small lake of a glacial origin, and >90% of its depression is filled with organic-rich sediments. The development stages of Lake Pilvelis since circa 10,000 cal BP are described in detail via reconstruction of the Holocene palaeoenvironmental conditions using multi-proxy sedimentary records. The elemental composition and content of metallic elements in sapropel are analysed with the main objectives of characterising the accumulation rate of metals in the full profile of sapropel and determining the anthropogenic impact depending on the evolution of the lake and the genesis of sediment organic matter. The correlation among the components characterising sediments such as sapropel and metallic elements revealed that the elemental distribution could be divided into three main groups according to the content of the elements associated with organic compounds (e.g., for Zn and Mn) and mineral compounds (for Ni, Co, Cr, K, Fe, Mg, and Cu). Ni and Co are partly associated with mineral compounds and partly associated with carbonates. A principal component analysis revealed the main factors indicating the origin of metallic elements and the accumulation tendencies of metals in sediment layers: the input of the mineral matter, the characteristics of organic matter, and human impact. The content of metallic elements in sapropel from Lake Pilvelis is mainly affected by the composition of mineral matter, not by the size of the particles.