Local and Remote Forcing of Denitrification in the Northeast Pacific for the Last 2,000 Years

• Published in: Paleoceanography and paleoclimatology 2019-08, Vol.34 (8), p.1517-1533
• Main Authors: Wang, Yi, Hendy, Ingrid L., Thunell, Robert
• Format: Article
• Language: English
• Subjects: Advection; Aleutian low; Atmospheric circulation; California margin; Chlorophyll; Coastal upwelling; Coherence; Curl (vectors); Denitrification; Drought; High pressure; Ice ages; ITCZ; Little Ice Age; Nitrogen isotopes; Ocean circulation; Organic carbon; Oxygenation; Pressure cells; Primary production; Productivity; Records; Total organic carbon; Tracers; Tropical climate; Undercurrents; Upwelling; Water circulation; Water column; Water masses; Winds
• ISSN: 2572-4517; 2572-4525
• DOI: 10.1029/2019PA003577

Sedimentary δ15N (δ15Nsed) has been widely applied as a proxy for water column denitrification. When combined with additional productivity proxies, it provides insights into the driving forces behind long‐term changes in water column oxygenation. High‐resolution (~2 years) δ15Nsed and productivity proxy records (total organic carbon [TOC], Si/Ti, and Ca/Ti) from Santa Barbara Basin, California, were generated from a well‐dated Kasten core (SPR0901‐03KC). These records reveal the relationship between Southern California upwelling and oxygenation over the past 2,000 years. Inconsistencies between Si/Ti (coastal upwelling proxy) and TOC (total export productivity proxy) suggest wind curl upwelling influenced Southern California primary productivity, especially during intervals of weak coastal upwelling. Coherence between δ15Nsed, TOC, and drought indicators supports a local control of δ15Nsed by atmospheric circulation, as persistent northerly winds associated with an intensified North Pacific High pressure cell lead to enhanced coastal upwelling. In the northeast Pacific, δ15Nsed is used as a water mass tracer of denitrification signals transported north from the eastern tropical North Pacific (ETNP) via the California Undercurrent. A 1,200‐year δ15Nsed record from the Pescadero slope, Gulf of California, lies between denitrifying subsurface waters in the ETNP and Southern California. During the Medieval Climate Anomaly, coherence between Pescadero and Santa Barbara Basin δ15Nsed indicates connections between ETNP and Southern California on centennial timescales. Yet an out‐of‐phase relationship occurred when the Aleutian Low was anomalously strong during the Little Ice Age. We suggest intensified nutrient‐rich subarctic water advection might have transported high‐15N nitrate into Southern California when the California Undercurrent and ETNP denitrification weakened. Key Points Wind curl upwelling contributes to Southern California primary productivity, especially during weak coastal upwelling intervals Intensified NPH leads to stronger denitrification through enhanced coastal upwelling and reduced rainfall California receives relatively more tropical water during the Medieval Climate Anomaly and more subarctic water during the Little Ice Age