Sedimentary C/S relationships in a large tropical estuary: Evidence for refractory carbon inputs from mangroves

Mud islands in the Fly River estuary were deposited under dominantly brackish conditions. Changes in C/S ratio with depth in sediment cores reflect geomorphic development of islands and/or changes in location of river channels. An unusual feature of the C/S relationships is positive carbon-axis intercepts, indicating the presence of an organic phase that is resistant to decomposition. Nonzero intercepts mean that the slope of the C/S plot, and not individual C/S values, must be used to determine paleoenvironments. Moreover, a nonzero intercept may be useful for indicating proximity to a source of refractory carbon, such as mangroves.