Organic carbon in floodplain alluvium: Signature of historic variations in erosion processes associated with deforestation, Waipaoa River basin, New Zealand

Abstract

We use C‐org and δ13C to trace the origin of the alluvium deposited on the Waipaoa River floodplain at McPhail's Bend between 1853 and 2002. The overbank deposits exhibit a more positive range of δ13C values (−26.3 ± 0.6) than contemporary suspended sediment in transport at intermediate flows (−27.8 ± 0.2) when gully erosion releases most material to stream channels, reflecting the greater contribution made by shallow landsliding in the headwaters during large precipitation events. Overbank sediment associated with recent floods contains a higher (∼50%) amount of organic carbon than alluvium deposited by pre‐1927 floods, which was derived from outcrops of weathered bedrock on steep, riparian hillslopes that were the primary sediment source before the soil‐mantled hillslopes elsewhere in the headwaters of the basin were destabilized by deforestation. The recent alluvium resembles the Bw horizon of soils present on these hillslopes. Sediment deposited during floods generated by localized storms and an overbank event that did not feature widespread shallow landsliding also has a distinctive signature that provides an indication of its provenance. The organic carbon associated with the alluvium appears to be old (4031 ± 40 B.P., in the case of the carbon associated with sediment deposited in 2002) and to be derived from weathered Cretaceous and Tertiary sedimentary rocks.