"The decrease in δ13C as water flows from the Atlantic to the North Pacific is due mainly to the addition of organically produced material and its subsequent oxidation as it falls through the water column. Marine organic carbon has a δ13C of near -250/00, hence the addition of this material decreases the δ13C of the ΣCO2." (from conclusion)
P. M. Kroopnick, Deep-Sea Research Part a-Oceanographic Research Papers, 1985, 32, 57-84. https://www.sciencedirect.com/science/article/pii/0198014985900172