Bridging Molecular Signatures from Terrestrial to Continental: A Critical Re-evaluation of EEM-PARAFAC as a Diagnostic Tool for Carbon Source Fingerprints

Accurately representing terrestrial carbon fluxes from local ecosystems to continental scales is a central challenge for land-ocean interface, particularly concerning the fate of carbon transformed during transportation. It is required to apply robust validation to accurately project continental carbon fluxes, yet the ultimate fate of carbon exported from land to ocean remains a key uncertainty. While marine sedimentary archives provide an integrated, long-term record of this flux, our ability to interpret those after degradation and diagenetic processes is paramount. Fluorescence EEM-PARAFAC is a promising technique for the source identification in water column, but we argue its application in sedimentary organic matter.

The conventional use of EEM-PARAFAC assigns fluorescent components (C, A, M) to specific terrestrial or marine sources, underpinning popular indices like the Humification Index (HIX). However, we show this source-centric view is incomplete for three key reasons. First, the diagenetic environment can overwrite source signals; anoxic processing of terrestrial matter, for example, can mimic a 'marine' low-HIX signature. Second, key indices like HIX are inherently sensitive to organic matter concentration, requiring careful re-calibration to separate measurement artifacts from true biogeochemical change. Third, thermal alteration during deep burial further degrades and distorts these fluorescent signals.

Instead of viewing these sensitivities as confounding factors, we propose repurposing them as diagnostic tools based on the results of indoor incubation monitoring and in-situ profiles combining paleoenvironmental analysis. In this new framework, fluorescent signatures become proxies for the environment itself with correction and background information: HIX, AC/M, and P/H can trace paleoenvironments, e.g., historical redox conditions, paleo-thermometers.

By embracing this holistic approach, we transform EEM spectroscopy from a simple source-tracker into a dynamic environmental recorder. This study unlocks a richer, multi-layered narrative of carbon's journey from source to sequestration, providing a powerful new set of process-based constraints.