Proteinaceous deep sea coral amino acid isotope records reveal climate-driven decadal-scale planktic ecosystem fluctuations

Decadal-scale climate variability drives important fluctuations in nutrient availability and productivity in highly productive eastern boundary current upwelling ecosystems, but the relatively brief duration of most monitoring efforts limits understanding of these dynamics. When applied to high-resolution paleoarchives such as deep-sea proteinaceous coral skeletons, stable carbon and nitrogen isotope (δ 13 C and δ 15 N) analysis can provide useful new insight into biogeochemical and ecological changes beyond the instrumental record. However, interpretation of bulk δ 13 C and δ 15 N records is often complicated by multiple possible drivers of variability. Here, we addressed these challenges by applying both bulk and compound-specific amino acid δ 15 N and δ 13 C analysis to two bamboo coral specimens from Sur Ridge on the central California margin, generating sub-decadal resolution records spanning c. 1810 to present. Our overarching goals were to first test amino acid δ 13 C and δ 15 N proxies in proteinaceous deep-sea bamboo coral archives, and second to investigate links between climate forcing and biogeochemical responses on the California margin over the Anthropocene. Together, comparison of deep-sea coral amino acid trophic position results to local sediment traps and endmember mixing analysis indicate that bamboo coral feed directly on exported sinking particles, which are comprised primarily of zooplankton fecal pellets (>70 %). This new evidence contradicts some past work based on bulk δ 15 N analysis alone and validates bamboo coral as archives of euphotic zone processes. Amino acid δ 15 N proxies also reveal that trophic position, not baseline δ 15 N of nitrate or phytoplankton production, is the primary driver of bulk δ 15 N variability in these coral records from a highly-productive coastal upwelling environment. Our approximately 200-year reconstruction shows overall long-term ecosystem stability since the pre-industrial period, overlain by major multidecadal-scale fluctuations in bamboo coral trophic position and δ 13 C of primary production. Relatively high (low) trophic position and low (high) δ 13 C values of primary production occurred during negative (positive) phases of the Pacific Decadal Oscillation over the 20th century. Counter to expectations, these results suggest lower primary production likely occurred during past periods of high nitrate availability in our study region. Modern satellite chlorophyll-a observations corroborate this finding. We hypothesize that offshore transport and subduction of nutrients and phytoplankton and/or precipitation-mediated changes in iron availability may link climate variability and planktic ecosystem dynamics in this region.