REASSESSING CORG/NTOT RATIOS IN PALEOCEANOGRAPHY

Corg/Ntot ratios are commonly used to distinguish the proportions of marine and continental organic matter in subaqueous sediments. Recently Calvert (2004) published a comment regarding the occurrence of linear relationships between Corg/Ntot with %TOC, HI and S2 data. Although we agree in principal with the findings of Calvert, we have employed the multitude of data present in the literature to reassess our understanding (or not) of Corg/Ntot values. Specifically, we have obtained elemental and isotopic data of sediments and sedimentary rocks from a myriad of marine and freshwater settings ranging in age from Paleozoic to modern. A preliminary assessment of the data by using simple x-y plots was conducted; this was followed by 3-D scatter plots. Significant relationships were recorded between Corg/Ntot with d15Ntot, carbonate percent with TOC, %N with TOC and TOC with d15Ntot. However, the relation between Corg/Ntot and TOC is not as clear as previously reported by Calvert and may, in fact, be dependent on important differences in environmental settings and organic matter sources. 3-D scatter plots confirm the relations indicated by x-y plots and yield distinct separations between previously indistinguishable environments. We suggest that 3-D plots are significantly more useful in making inferences about regional, environmental and chronological variation of these geochemical proxies in paleoceanographic and paleoclimatic reconstructions.