Signals of watershed change preserved in organic carbon buried on the continental margin seaward of the Waipaoa River, New Zealand

Abstract

Abstract Holocene sediments buried on the continental shelf seaward of the Waipaoa River, northeastern New Zealand, preserve a stratigraphic record of terrestrial environmental change. The well characterized, long-term record of storms, volcanism, and human disturbance in this region provides an opportunity to examine how such changes are reflected in the character of organic carbon (OC) buried on the continental margin. Complimentary evidence obtained from analyses of the 14 C content and the elemental and stable carbon isotopic composition of different sedimentary fractions, including charcoal, wood, and clay-sized isolates, indicates that these perturbations led to mobilization of OC components with variable storage histories in the watershed. Charcoal transported in the modern Waipaoa River and buried in offshore depocenters includes a highly aged component that has apparently been slowly released from storage in soil or alluvial terraces. The charcoal fraction ages become dramatically younger in sediments deposited after the Taupo volcanic eruption (1717 cal yr BP) and then Polynesian settlement (ca. 700 cal yr BP), both signaling biomass burning. The mean age of woody plant fragments and clay-bound OC deposited on the shelf also varies over time, with older material having accumulated in the middle Holocene and since human occupation. Deposition of older-than-average wood fragments and clay-bound OC between about 5 and 3.6 kyr BP correlates with evidence for a period of increased storm frequency in the region and may reflect the enhanced delivery of aged soil and sedimentary rock mobilized from hill slopes via earthflows and/or deep seated landslides. Similarly, the deposition of older wood and clay-bound OC commensurate with the first anthropogenic disturbance ca. 700 years ago is consistent with accelerated mass wasting due to deforestation. At the same time, a change in the elemental and stable isotopic composition of bulk and clay-bound OC buried on the shelf may reflect increased marine primary productivity and/or mobilization of OC from deep levels in soil profiles. Deforestation of the Waipaoa headwaters by European settlers beginning in the middle 19th century is evinced by a sharper rise in the age of clay associated OC buried offshore. Today, deep gully incision into fractured sedimentary bedrock is a major source of sediment and kerogen to the river, and this process has left its mark on the age of sedimentary OC delivered to the adjacent margin.