We examined the elemental (total nitrogen, total organic carbon, and C/N), isotopic (δ<sup>13</sup>C<sub>org</sub> and δ<sup>15</sup> N<sub>org</sub> ), and Rock-Eval pyrolysis (S<sub>2</sub>, S<sub>3</sub>, hydrogen index, oxygen index, and T<sub>max</sub> ) compositions of bulk marine sediments from Hole U1357A in the Adelie Basin, Antarctica, to assess the organic geochemical characteristics and vertical variations in the late Holocene diatomaceous-dominated sediments. Pyrolysis parameters indicate that most of the organic materials are immature and in the Type II stage, indicative of a marine source. In contrast, the results from a conventional cross-plot of δ<sup>13</sup>C<sub>org</sub> vs. total organic carbon/total nitrogen indicate organic materials that originated as freshwater organic carbon, not marine organic carbon. Given that <sup>13</sup>C-depleted phytoplankton are dominant in diatomaceous ooze-rich sediments of the Adelie Basin, the apparent dominance of freshwater organic matter is likely attributable to the extremely low δ<sup>13</sup>C<sub>org</sub> (ppm) values in the bulk sediments.