200 years of climate-driven biogeochemical shifts in the California Current System reconstructed from high-fidelity proteinaceous coral archives

Over the past several decades, fisheries in the highly productive California Current System (CCS) have undergone major shifts in productivity associated with modes of low-frequency climate variability such as the North Pacific Gyre Oscillation. However, the brevity of instrumental oceanographic and especially biological records limits our ability to understand decadal- to multidecadal- scale ocean climate variability and ecosystem responses. Here we present records of climate-linked biogeochemical shifts from the skeletons of deep-sea bamboo corals. The proteinaceous skeletons of certain long-lived deep-sea coral taxa, including bamboo corals from the cosmopolitan family Isididae, are unique high-fidelity archives of past environmental change with annual to decadal resolution. Since bamboo coral skeletal protein directly records the carbon and nitrogen isotopic composition of sinking particulate organic matter (POM), this approach offers insight into marine nitrogen cycling, surface productivity and trophic dynamics; however, numerous interrelated processes affect the isotopic composition of marine POM, and fractionation introduced during trophic transfer(s) further complicates interpretation of bulk isotope data. For this reason, we used compound-specific isotope analysis of amino acids (CSI-AA) to elucidate specific processes driving variability in bulk isotopic composition through time. Here, we present new coupled bulk and CSI-AA records from coral skeletal material collected at Sur Ridge, California (36.4°N, 122.3°W) spanning the last ~200 years. Specifically, we examined variability in 𝛿¹⁵N of “source” amino acids, which do not undergo trophic enrichment, compared to 𝛿¹⁵N of “trophic” amino acids to differentiate changes in source nitrate composition from shifts in the trophic structure of the planktic food web. These results provide novel insight into plankton ecosystem responses to ocean dynamics and suggest substantial shifts in marine nitrogen cycling in the central CCS.