A Lateglacial and Holocene organic carbon isotope record of lacustrine palaeoproductivity and climatic change derived from varved lake sediments of Lake Holzmaar, Germany

A composite varve-dated 11.4 m long sediment sequence from Lake Holzmaar, situated in the Eifel region of western Germany, was investigated for total organic matter content, total nitrogen content and stable organic carbon isotopes. Mean time resolution is 75 years for TN and 14 years for delta(13)C(org). On millennial time scales primary production of lacustrine algae strongly depends on the delivery of nutrients from the catchment. The respective carbon isotope record is characterized by marked variations of delta(13)C(org) ranging from -36.0parts per thousand to -27.0parts per thousand and includes a number of pronounced shifts. Reactions of the lacustrine system and the catchment to changes of environmental parameters, e.g. runoff, solar radiation and temperature, induce changes of algal associations and of lacustrine primary production which are reflected in the sediments as carbon isotope variations. Clear evidence of ecosystem reorganizations is detected by the carbon isotope record around 14,200, 10,400, 9600, 5500, 2700, 1700 and 900 varve years BP. In particular, the Holocene events of 9600, around 5500 and 2700 are interpreted as the expression of massive changes of the climate system. The steady rise of delta(13)C(org) values during the mid-Holocene is interpreted as a continuous climatic amelioration reaching an optimum around 6500 varve years BP. Rapid and large changes of delta(13)C(org) values from 2700 varve years BP to the present indicate major disturbances in the catchment area. These are most probably related to deforestation or reforestation and runoff changes, presumably in conjunction with human impact. Carbon isotopes, thus, characterize the Holzmaar ecosystem in time revealing lacustrine palaeoproductivity as well as providing palaeoenvironmental and palaeoclimatic information. (C) 2002 Elsevier Science Ltd. All rights reserved.