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) δ15Nsedand 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 δ15Nsedby atmospheric circulation, as persistent northerly winds associated with an intensified North Pacific High pressure cell lead to enhanced coastal upwelling. In the northeast Pacific, δ15Nsedis 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 δ15Nsedrecord 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 δ15Nsedindicates 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.
Decades of observations show that the world's oceans have been losing oxygen, with far‐reaching consequences for ecosystems and biogeochemical cycling. To reconstruct oxygenation beyond the limited scope of instrumental records, proxy records are needed, such as sedimentary δ15N. We combine two δ15N records from the Santa Barbara Basin (SBB), a 24‐year‐long, biweekly sediment trap time series, and a 114‐year, high‐resolution sediment core together spanning the years 1892–2017. These records allow for the examination of δ15N variability on seasonal to centennial timescales. Seasonal variability in SBB δ15N is consistent in timing with the poleward advection of a high δ15N signal from the Eastern Tropical North Pacific in the summer and fall. Strong El Niño events result in variable δ15N signatures, reflective of local rainfall, and neither the Pacific Decadal Oscillation nor North Pacific Gyre Oscillation impose strong controls on bulk sedimentary δ15N. Seasonal and interannual variability in sediment trap δ13Corgis consistent with local productivity as a driver; however, this signal is not retained in the sediment core. The time series from the sediment trap and core show that bulk sedimentary δ15N in SBB has now exceeded that measured for the past 2,000 years. We hypothesize that the change in δ15N reflects the increasing influence of denitrified waters from the Eastern Tropical North Pacific and ongoing deoxygenation of the Eastern Pacific. When juxtaposed with other regional δ15N records our results further suggest that SBB is uniquely situated to record long‐term change in the Eastern Tropical North Pacific.
more » « less- NSF-PAR ID:
- 10459546
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Paleoceanography and Paleoclimatology
- Volume:
- 34
- Issue:
- 9
- ISSN:
- 2572-4517
- Page Range / eLocation ID:
- p. 1554-1567
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Bulk sediment δ15N records from the eastern tropical Pacific (ETP) extending back to the last ice age most often show low glacial δ15N, then a deglacial δ15N maximum, followed by a gradual decline to a late Holocene δ15N that is typically higher than that of the Last Glacial Maximum (LGM). The lower δ15N of the LGM has been interpreted to reflect an ice age reduction in water column denitrification. We report foraminifera shell‐bound nitrogen isotope (FB‐δ15N) measurements for the two species
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