A high resolution paleo-record of export production using deep-sea coral stable isotope values from a unique HNLC zone on the California Margin

Abstract:

The Sur Ridge, located ~30 km off the Big Sur coast of central California, represents a unique system within the highly productive California Current ecosystem. Its unique high nutrient, but low chlorophyll characteristics are not fully understood. Time series of bulk stable carbon (δ¹³C) and stable nitrogen (δ¹⁵N) isotopes can help us better understand past changes in nutrient dynamics and phytoplankton community baselines for this region in order to better predict future changes. Deep-sea proteinaceous corals are particularly powerful paleoarchives of past ocean conditions. These organisms serve as “living sediment traps,” incorporating the stable isotope values of exported particulate organic material (POM) from the surface into their growth layers. The longevity of bamboo corals (Isidella, up to 400 years) makes them excellent resources for creating high resolution, centennial time series of δ¹³C and δ¹⁵N dynamics. Bamboo corals used in this study were harvested during summer of 2014 from 1220 to 1300 m depths. Two corals were milled in sub-millimeter intervals to generate a 200 year time series at approximately three year temporal resolution. Over the past 200 years, deep-sea coral δ¹³C values ranged from -15.7 to -19.0‰ and δ¹⁵N values ranged from 14.4 to 15.9‰, consistent with earlier data from the CA margin. The δ¹³C records were characterized by long periods of remarkable stability, contrasted with several large shifts (~1900 and ~1960) in δ¹³C of approximately 1‰. We hypothesize that these shifts likely reflect changes in plankton composition or production associated with regional climate shifts. The δ¹⁵N data were more dynamic, including several large shifts (1940 - 1960), as well as periods of apparent decadal scale oscillation (1825 - 1925 and 1965 – present). These shifts may reflect changes in the source or utilization of nitrogen at the base of the food web. Together, these data give us a first look at baseline stability of biogeochemical systems in this unique region, and will be crucial in connecting potential future system changes in climate and upwelling to possible shifts in nutrient dynamics and phytoplankton species composition.