Distinction of marine and terrestrial origin of humic acids in North Sea surface sediments by absorption spectroscopy

A suite of humic acid isolates from terrestrial and marine soils and sediments have been analysed by UV/visible spectroscopy. Marine samples are characterised by a maximum at 407 nm which is lacking in terrestrial ones. The absorbance ratio of 270/407 nm (A2 /A4) can be used to differentiate humic acid sources as is demonstrated for North Sea surface sediments. Here, the southern coastal samples had high A2/A4 ratios indicating higher terrestrial contributions to the humic acid pool. In contrast, samples from the northern North Sea had low A2/A4 ratios suggesting higher marine contributions. Analysis of core samples and an Eocene oil shale indicates that the A2/A4 ratio is unaffected by early diagenetic processes.