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

Abstract

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.

The Impacts of Flood, Drought, and Turbidites on Organic Carbon Burial Over the Past 2,000 years in the Santa Barbara Basin, California

“…Sediments from flood periods contained significantly lower TOC and had the most depleted δ 13 C (−24.34 ± 0.84‰) and δ 15 N (6.26 ± 0.84‰) values (Figures 2 and 3), consistent with nonmarine sources of OM (Meyers, 1994;Sweeney & Kaplan, 1980). The δ 15 N signature of flood sediments is significantly lower than background, turbidite, and drought sediments, with fluctuations of >1.5‰ that are substantially greater than the ±0.5‰ (Figure 3e) associated with changes in water column mixing, biological utilization, and oxygen availability (e.g., Wang, Hendy, & Thunell, 2019; see section 1.2). These flood sediments, however, are not as depleted as the~2‰ reported by Sweeney and Kaplan (1980) in terrestrial sewage effluent, suggesting a mixed marine and terrestrial sediment source.…”

“…Following SXRF, the core was sampled at 2 mm intervals and oven dried. An age model was developed using a combination of 14 C dating of planktonic foraminiferal carbonates and laminae counts (Hendy et al, 2013;Wang, Hendy, & Thunell, 2019). The age-depth model was generated using Bacon 2.2 (Blaauw & Christen, 2011;Du et al, 2018), where 14 C ages were converted to calendar ages using the Marine13 calibration curve (Reimer et al, 2013) with variable reservoir ages from Hendy et al (2013).…”

“…For the sediment core, the weight percent TOC (%TOC) and TN (%TN) were measured on Costech ECS 4010 Elemental Analyzer at University of Michigan using the methods of Wang, Hendy, and Thunell (2019). For the sediment traps, rivers, and kelp samples, the %TOC and %TN were measured on a PerkinElmer 2400 CHNS Elemental Analyzer.…”

Ongoing Increase in Eastern Tropical North Pacific Denitrification as Interpreted Through the Santa Barbara Basin Sedimentary δ15N Record

“…In parallel, a decrease in the strength of upwelling activity near Point Conception would decrease the frequency of flushing events in SBB and increase the potential for local denitrification. It should be noted that earlier intervals of decoupling between SBB Pescadero Basin δ 15 N records have also been documented and attributed to large‐scale perturbations in North Pacific circulation (Wang et al, ).…”

Changes in Deep‐Sea Oxygenation in the Northeast Pacific Ocean During 32–10 ka

“…Planktic foraminifer Globigerina bulloides is very sensitive to the wind‐driven upwelling in the coastal region (Gupta et al, 2003; Saravanan et al, 2019). A number of proxy records have produced intermediate water δ 13 C and δ 18 O records spanning the interval from 32 to 10 kiloyears before the present (ka BP) in the Santa Barbara Basin and Santa Monica basins, California (Balestra et al, 2018; Hendy & Kennett, 1999; Ohkushi et al, 2013; Pak et al, 2012; Taylor et al, 2014), as well as planktic and benthic foraminiferal faunal records (Balestra et al, 2018; Hendy & Kennett, 2000) and geochemical data from the region (Chang et al, 2014; Gombiner et al, 2016; McClymont et al, 2012; Wang et al, 2019). The changes in deep ocean ventilation (alteration of marine carbon cycle) influence the global climate via changes in the concentration of CO 2 in the atmosphere (Marchitto et al, 2007).…”