Coastal upwelling and radiocarbon - Evidence for temporal fluctuations in ocean reservoir effect off Portugal during the Holocene

This paper focuses on the use of the radiocarbon content of marine shells collected along the Portuguese coast as a proxy for the intensity of coastal upwelling off of Portugal. Differences in the C-14 ages of closely associated marine mollusk shells and terrestrial material (charcoal or bones) from several Portuguese archaeological contexts seem to be significant throughout the Holocene. Delta R values range from 940 +/- 50 to -160 +/- 40 C-14 yr. Five of these values are significantly higher than the modem value (250 +/- 25 C-14 yr), while the remaining values are lower. The modem value was calculated by measuring the 14C content of live-collected, pre-bomb marine mollusk shells. This value is in accordance with an active upwelling of strong intensity that currently occurs off of Portugal. Some primary observations based on data presented here can be made: i) during the Holocene important changes have occurred in the ocean reservoir effect off the Portuguese coast; ii) these fluctuations may be correlated with regional oceanographic changes, namely with changes in the strength of coastal upwelling; and iii) these changes suggest some sort of variability of the climatic factors forcing coastal upwelling off of Portugal.