The uppermost about 500 m of basaltic ocean crust is permeable, and fluid flow is focused in specific areas at the contacts of lava flows or in brecciated zones. In these areas, seawater oxidizes young basaltic crust (younger than 10 Ma), and this interaction affects the microbial ecosystems. Iron cycling, both oxidation and reduction of iron, supports metabolic activity in basalts; however, the microorganisms responsible for Fe oxidation of basalts are not clear. In this study, carbon isotopic analyses of basalts and sediments at North Pond, the western flank of the mid-Atlantic Ridge, were conducted to understand the origin and formation of carbon compounds in relation to possible microbial activity in basaltic crust. Total carbon (TC) contents range approximately from 6 to 11 wt% for whole-sediment samples. Depth profiles of the carbon isotopic compositions (δ1C-TC) for sediments (approximately -0.04 to +1.93 ) are similar to those of TC. TC (approximately 0.01-0.37 wt%) and total organic carbon (TOC) (approximately 0.01-0.03 wt%) contents for basalts are almost constant with depth, whereas sediment breccias and carbonates contain more carbon than basalts (approximately 3.56-11.9 wt%). The value of δ13C-TC for basalts ranges approximately from -21.8 to +2.69. Sediment breccias and carbonates have larger δ13C-TC values, approximately from -18.6 to +2.82. The value of δ13C-TOC for hard rocks is lower at greater depths. The value of δ13C-kerogen is slightly smaller than that of δ13C-TOC.