Table 1_Studying the temporal bias of the steady-state approximation of 234Th-derived carbon export during phytoplankton blooms.docx

The ²³⁴Th–²³⁸U pair technique is widely used in estimating particulate organic carbon (POC) flux, typically with a steady-state (SS) assumption. There is often a temporal bias between the SS-derived and actual POC fluxes caused by neglecting the temporal change in ²³⁴Th. However, this temporal bias has not been fully investigated due to the lack of continuous sampling of ²³⁴Th profiles and sediment traps. Here, we develop a radioactive trace model of ²³⁴Th built on a physical–biogeochemical model to simulate the scavenging of ²³⁴Th in the water column by POC sinking processes at the South East Asia time series (SEATS) site. The seasonal patterns of the ²³⁴Th profiles simulated by the model generally compared well with in situ observations. Analysis based on the model simulation suggests that the temporal bias can be depicted and reproduced in a simplified ²³⁴Th continuity equation. By obtaining an analytical solution for the SS-derived POC flux from the simplified ²³⁴Th equation, we found that the temporal bias results from the phase difference in time between the SS-derived and direct sinking POC fluxes. To provide a method that does not need repeated samplings to reduce this temporal bias for in situ observations, a modification term was constructed for the SS-derived POC flux from the analytical solution. Applying this term to the data obtained at the Bermuda Atlantic time series and SEATS reduced the bias by up to 67% and 34%, respectively. This study provides a feasible way to improve ²³⁴Th-derived POC flux under the SS assumption.