Carbon storage in a Holocene deltaic sequence: An example from the Nobi Plain, central Japan

We estimated the rate of accumulation and total amount of organic carbon (OC) stored in Holocene sediments of the Nobi Plain of central Japan by analyzing 2701 existing borehole sections and 218 14C ages, reconstructing the three-dimensional stratigraphic architecture of the delta system, calculating the volume of sediments in the delta, and using OC contents measured from sediments from eight boreholes on the plain. We calculated that a total of 167 Tg of OC was stored in deltaic deposits on the plain during the past 6000 years. A dramatic increase in the amount of stored OC since 1000 cal BP is probably a combined result of increases in sediment supply to the delta in response to human activities in the drainage basin and expansion of the area of subaerial delta as a result of progradation. OC accumulation rates for the Nobi Plain during the past 6000 years ranged from 15.3 to 37.5 g m−2 y−1, which are comparable to rates obtained from other Holocene delta, and to rates from floodplain and lake systems with higher OC contents in their sediments. The higher sedimentation rates of the deltaic deposits compensates for their lower OC contents.