Holocene sediments of Lake Barrine, north-east Australia, and their implications for the history of lake and catchment environments

Lake Barrine (17° 15′ S, 145° 38′ E, 721 m a.s.l.) lies in a maar crater surrounded by tropical rainforest. Its surface area is 1 km² and its maximum depth 67 m. It has no inflow streams and an outlet only active in the wet summer months. The sediments underlying the lake have been investigated by numerous Mackereth cores and in-situ frozen slabs. In addition to lithological descriptions, selected samples have been analysed for grain size, bulk density and combustible content, mineralogy and chemistry, palaeomagnetism, pollen and charcoal. A chronology was established from radiocarbon dating of 211 sediment samples. The deposits are divisible into three main strata. Between about 16 ka and 9.7 ka cal BP they are mainly inorganic representing occasional, very energetic, inwash from the lightly vegetated inner walls of the crater; only a few small enclaves were perennially flooded. Between 9.7 and 5.4 ka cal BP sediments were, for the most part, finely grained with a substantial organic component. Their indications are of a water level rising overall but not at a constant rate. From time to time erosion and redeposition of marginal materials and older sediments took place. By 7.3 ka BP, as rainforest began to cover the catchment, the water level had risen sufficiently for reducing conditions to be established in its deeps and subsequent deposits indicated pedogenesis beneath the forest and reduced runoff to the lake. Between 9.7 and 7.3 ka cal BP the catchment erosion rate was 859 kg (dry wt) ha^{- 1} ka^{- 1} but this fell to 267 kg ha^{- 1} ka^{- 1} from 7.3 to 5.4 ka cal BP. From 5.4 ka cal BP to the present, the sediments beneath > 55 m of the modern lake are strongly and finely laminated. This was the result of intermittent relocation of detritus from shallower to deeper water when the lake's thermal stability was reduced by unusually low winter temperatures, the resultant lamination being preserved under a persistent monimolimnion. Lamination counts indicate that this has happened more than a thousand times in the last 5.4 ka. By 3.2 ka cal BP the water level had reached the modern outlet and its oscillations thereby minimised. Between 5.4 ka cal BP and 3.2 ka cal BP, the catchment sediment yield was 112 kg ha^{- 1} ka^{- 1}, rising to 146 kg ha^{- 1} ka^{- 1} thereafter. In the period between 9.7 and 3.2 ka cal BP the lake volume increased from virtually nothing to almost 36 Gl.