SEWAGE HISTORY IN THE ANOXIC SEDIMENTS OF THE FJORD NORDASVANNET, WESTERN NORWAY .2. THE ORIGIN OF THE SEDIMENTED ORGANIC-MATTER FRACTION

Anoxic sediments retrieved from two locations in the deep parts of the fjord Nordasvannet (located near the city of Bergen, western Norway) contain a record of changing sewage discharge from AD 1700 to the pres ent. Four periods of sewage discharge history are documented: (I) Befo re 1700: Minor sewage discharge. (2) From 1700 to 1969: Increasing sha llow water sewage discharge. (3) From 1969 to 1988: Major deep water s ewage discharge. (4) After 1988: No sewage discharge. Loss-on-ignition (LOI, organic carbon) analysis, delta(13)C and the C/N ratio of the t otal sedimentary organic matter fraction are used to distinguish betwe en marine-produced, terrestrial and sewage-derived organic matter, and to relate these to the four periods. The sedimented organic matter co mposition shows three steps of development at both stations. (A) Below 15 cm: LOI = 26 +/- 1%, delta(13)C = -23 +/- 0.5 parts per thousand, C/N = 11 +/- 0.5 corresponding to the time of no sewage discharge befo re 1700. (B) From 15 to 5 cm: LOI increases from 25 to 30%, delta(13)C varies between -23.7 and -25.8 parts per thousand, C/N between 10.6 a nd 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%, delta( 13)C and C/N kept stable at -25 +/- and 0.2 parts per thousand and 13 +/- 0.6, respectively, corresponding to the period of deep water sewag e 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 accumu lation rates (from 12 to 27 gC/m(2)/y) and exponentially increasing tr ace metal content.