THE MULTI-CENTENNIAL DYNAMICS OF AN UPWELLING SYSTEM REVEALED BY STABLE ISOTOPE AND TRACE ELEMENT SCLEROCHRONOLOGY OF AMINO-ACID DATED BIVALVE AND BRACHIOPOD SHELLS

This study is part of a concerted effort to understand the centennial to millennial-scale dynamics of upwelling cells. These zones in modern oceans support highly productive ecosystems and can help us to understand processes that led to the formation of commercially important petroleum source rocks. Yet, little is known of the temporal stability of upwelling systems beyond the time frames offered by direct monitoring and historical records.

To monitor the long-term dynamics of an upwelling system, we used shells dredged from two upwelling-influenced sites along the inner shelf of the Southeast Brazilian Bight (Southwest Atlantic). From each site, specimens of an aragonitic bivalve, and a calcitic brachiopod were individually dated using amino acid racemization calibrated with radiocarbon. The resulting time series have nearly-complete centennial resolution for the past millennium for each bio-mineralic system.

To reconstruct upwelling activity, dated shells were analyzed for multiple geochemical proxies (Sr, Ba, δ13C, δ18O) using sclerochronological profiles and ICP-MS laser ablation. We specifically targeted the variability of these proxies at multiple scales, from multi-centennial to single specimens. Pairwise comparisons between levels were evaluated to determine if the variability of any proxy was different across different time scales. Our expectation was that if upwelling activity fluctuated in the last millennium then shells from the same century should be less variable in their geochemical signature than those compared across different centuries.

Preliminary results indicate that upwelling activity has fluctuated in this area during the last millennium. Several centuries have higher variability in carbon and oxygen isotopes and Ba/Ca ratios than most other centuries. Specifically, modern shells, and those from 500 to 600 years BP, show a variability range of 4-5 per mil for carbon, which is twice the average from other centuries. These findings could indicate that this upwelling cell has been only intermittently active for the past millennium. This may have important implications for the recognition of ancient high-productivity systems as well as for better understanding of the long-term dynamics of upwelling-supported high-productivity ecosystems in the modern ocean.