NJG75-2/3-06
1995
Sewage history in the anoxic sediments of the fjord Nordåsvannet, western Norway: (Il) The origin of the sedimented organic matter fraction
75
2, 3
pp. 146-155

Anoxic sediments retrieved from two locations in the deep parts of the fjord Nordåsvannet ( located near the city of Bergen, western Norway) contain a record of changing sewage discharge from AD 1700 to the present. Four periods of sewage discharge history are documented: (1) Before 1700: Minor sewage discharge. (2) From 1700 to 1969: Increasing shallow water sewage discharge. (3) From 1969 to 1988: Major deep water sewage discharge. ( 4) After 1988: No sewage discharge. Loss-on-ignition (LOI, organic carbon) analysis, ?13C and the C/N ratio of the total sedimentary organic matter fraction are used to distinguish between marine-produced, terrestrial and sewage-derived organic matter, and to relate these to the four periods. The sedimented organic matter composition shows three steps of development at both stations. (A) Below 15 cm: LOI = 26 ± 1%, ?13C = -23 ± 0.50/00 , C/N = 11 ± 0.5 corresponding to the time of no sewage discharge before 1700. (B) From 15 to 5 cm: LOI increases from 25 to 30%, ?13C varies between -23.7 and -25.80/00 , C/N between 10.6 and 13.7 corresponding to the period of shallow water sewage discharge from 1700 to 1969. (C) Above 5 cm: LOI decreases to 22 and 28%, ?13C and C/N kept stable at -25 ± 0.20/00 and 13 ± 0.6, respectively, corresponding to the period of deep water sewage discharge from 1969 to 1988 and of no sewage discharge from 1988 to 1991. Further, step (C) is characterized by a marked increase in grain size from silty clay to sandy/clayey silt, increasing LOI mass accumulation rates (from 12 to 27 gC/m2/y) and exponentially increasing trace metal

con tent.

0029-196X