ORGANIC MATTER RECORDS IN SWEDISH LAKES: A KEY TO PALEOENVIRONMENTAL INTERPRETATION

Organic matter (OM) in lakes is a good indicator of paleoenvironmental changes, climatic variation, and affects of human activities. High latitude glacial lakes can provide a wealth of information because high sedimentation rates (~ 1m/1000 years) and good preservation of OM make them ideally suited for studying sedimentary records over 10 + ky periods.

We are investigating three lakes situated close to Stockholm. Lake Marviken is an isolated oligotrophic lake surrounded by boreal forests. It has high DOC values and reducing conditions that increase with depth. Lake Norrviken is a hard water eutrophic lake affected by Mn and sulfate reduction. The shallow sediments are more reducing compared to the deeper sediments. Lake Brunnsviken is eutrophic and connected to the Baltic Sea. The sediments are strongly reducing (-123 to -137 mV) with a sharp odor from H₂S. Undisturbed cores (ca. 50 cm) were collected by gravity coring from the deepest part of each lake and recording over several centuries of deposition. The cores were sectioned at 1 cm interval and freeze dried.

Lake Marviken: In the top 10 cm, C/N ratio varies (between 10-40 by wt%) suggesting a mixture of vascular plants and algal material, but sharply decreases with depth (10) due to degradation. Consistent with this TOC and P content decreases, and ¹³C/¹²C (-31.5 to -30‰) and ¹⁵N/¹⁴N (3 to 6‰) become heavier by several ‰. Lake Norrviken: C/N ratio is between 4-8 implying input from algae. The ¹³C/¹²C is around -30‰ and ¹⁵N/¹⁴N ranges from 3 to 6‰. Lake Brunnsviken: High C/N ratio occur between 1-10 cm (20-90) and 25-35 cm (20-30) suggesting input from surrounding pine and oak wood as the main source for OM. Consistent with this ¹³C/¹²C (-27 to -28‰), ¹⁵N/¹⁴N (6 to 8‰), and low P levels were observed. In contrast, low C/N ratio and high %N and P suggests algal material as the main source for OM between 11-24 cm and 36-45 cm. The heavier d¹³C_OMvalue in Brunnsviken may result from input of marine carbonates because it is connected to the Baltic Sea. Overall the sediments reflect periods of eutrophic conditions during which increased productivity probably occurred naturally.

Details of the paleoenvironmental changes evident in the bulk OM characteristics will be augmented by ongoing n-alkane and PAH analyses, and mass accumulation rates based on ²¹⁰Pb dating.