Discordant 14C-stratigraphies in upper Monterey Canyon: A signal of anthropogenic disturbance

Laminated fine-grained sediments drape the flanks and terraces on the sides of upper Monterey Canyon, where rapid sedimentation occurs in the relatively calm environment sandwiched between the wave-washed continental shelf and the sediment gravity-flow-scoured axial channel on the canyon floor. The isotopic composition of organic matter contained in these accumulating sediments shows that they are primarily of terrestrial origin and are apparently supplied by the Salinas and Pajaro Rivers that drain the adjacent agricultural valleys and discharge near the canyon head. esence of alien pollen grains within these fine-grained sediments indicates that > 5 m of sediment was deposited within historical times (post-1700) on the canyon flanks. Two trends are seen in the 14C content of the sedimentary organic carbon. The uppermost sediments on the canyon flanks contain pesticide residues indicating post-1945 deposition and have an average 14C content that is 82 ± 3.2 percent modern carbon. This is similar to 14C values for suspended sediments discharged from the Salinas River during floods. Disseminated bulk organic matter and carbonate sampled immediately below pesticide-bearing sediments on the flanks of Monterey Canyon are distinctly more 14C-depleted. The proportion of 14C in the disseminated bulk organic matter increases somewhat erratically with further depth in all nine cores with multiple measurements. These discordant 14C-stratigraphies are developed within sediments that are pesticide-free (pre-1945), but still of historical age based on the occurrence of alien pollen grains. Two factors may contribute to the discordant 14C-stratigraphies observed in upper Monterey Canyon: (1) the addition of bomb-produced 14C in post-1945 sediments, and (2) changes in the proportion of older soil carbon that was washed off agricultural land. Land use practices associated with the early stages of modern development of the Salinas and Pajaro Valleys caused rapid loss of virgin soils and a subsequent increase in the proportion of older soil carbon exported as deeper soil horizons were exhumed. A sharp decrease in the amount of older carbon being flushed off the land has occurred since the construction of dams and impoundment structures on the Salinas and Pajaro Rivers by the middle of the 20th century.