Dual carbon isotopes constrain the sources and age variations of terrestrial organic carbon in the middle Okinawa Trough since the last deglaciation

Abstract

The burial of terrestrial organic carbon (OC) in marine sediments is a crucial process in the global carbon cycle, as it transfers carbon from relatively active to inactive reservoirs. However, there remains a limited understanding of the spatial and temporal variations in the source and age characteristics of terrestrial OC in marine sediments, which determine its ability to serve as a contemporary carbon sink. In this study, radiocarbon isotope (14C, expressed as F14C) analysis was conducted on bulk OC from a sediment core (M063–05) collected from the middle Okinawa Trough, with core depositional ages spanning 1.5–16.1 ka. A novel mathematical approach was developed to differentiate the carbon isotope signals of terrestrial OC from those of bulk OC properties, enabling the investigation of the source, age, and burial flux of terrestrial OC in the Okinawa Trough. Our results indicate that sedimentary OC in the Okinawa Trough was predominantly composed of marine OC (average proportion, 58 %), with terrestrial OC present to a lesser extent (average proportion, 42 %). Terrestrial OC in core M063–05 was found to be primarily a mixture of terrestrial OC from the Huanghe (Yellow River) and petrogenic OC from Taiwan Island, and the apparent initial ages of terrestrial OC (AIRterrestrial) exhibited higher values of 7933–8886 yr during the mid-late Holocene compared to 3062–7677 yr during the marine transgression stage. The elevated AIRterrestrial during sea level highstand was attributed to (1) increased petrogenic OC input from Taiwan Island due to the intensified Kuroshio Current, which transports material from Taiwan northward along the Okinawa Trough, and (2) vigorous hydrodynamic processes in the continental shelves of the East China marginal seas, leading to significant aging of terrestrial OC during lateral transport. This innovative dual carbon isotope-based approach (enumeration method) elucidates the temporal variations in terrestrial OC sources and age characteristics in a typical continental slope within the western Pacific Ocean, providing a framework for investigating the sources and fate of terrestrial OC in marginal seas, especially in scenarios where potential changes in carbon isotope endmembers occur in response to climate and environmental changes.