Radiocarbon offsets between organic carbon and carbonates: implications for Holocene paleoceanographic reconstructions in the Cabo Frio upwelling system

Abstract

Understanding the temporal relationship between carbon pools in marine sediments is essential for reliable paleoenvironmental reconstructions, yet ¹⁴C-age discrepancies between organic and carbonate fractions remain poorly constrained. Here, we present the first assessment of the ¹⁴C-age offset between organic carbon (OC) and planktonic foraminifera from co-occurring stratigraphic layers of the Cabo Frio upwelling system (CFUS, off southeastern Brazil) over the past 5.8 kyr. Conventional ¹⁴C ages of the OC are, on average, 643 ± 186 yr older than those of co-occurring foraminifera, likely reflecting long-term storage and winnowing of organic matter (OM) on the mid-shelf driven by local upwelling-related hydrodynamic processes. Higher OC apparent initial ¹⁴C-ages (AIR_OC) coincide with periods of intensified upwelling and stronger bottom currents, which prolong OC residence time before burial, and greater delivery of pre-aged terrestrial OM. Conversely, reduced AIR_OC corresponds to weaker hydrodynamic conditions and higher sedimentation rates that promote more rapid OC deposition. The new carbonate-based chronology developed for the investigated core revises the timing of Holocene paleoceanographic changes previously inferred from OC-based age models. Intensified South Atlantic Central Water (SACW) upwelling in the mid-shelf now aligns with the mid-Holocene sea-level highstand, while the development of stronger stratification and reduced SACW penetration occurs entirely within the late Holocene. The establishment of modern upwelling conditions shifted to younger ages, improving consistency with wider-scale climate phenomena and regional NE-wind variability. These results refine the temporal and mechanistic interpretation of Holocene variability in the CFUS and underscore the importance of multi-fraction ¹⁴C dating in dynamic coastal upwelling systems.