Disrupted Coherence Between Upwelling Strength and Redox Conditions Reflects Source Water Change in Santa Barbara Basin During the 20th Century

Abstract

AbstractVarved sediments from the center of the Santa Barbara Basin (SBB) off southern California are a valuable archive for high‐resolution climate reconstruction. Using mass spectrometry imaging of lipid biomarkers, this study examines interactions of perennial upwelling strength and oxygen‐driven redox conditions at the sediment‐water interface from 1900 CE to 2009. In the SBB, variations in upwelling are recorded by isoprenoidal tetraethers from planktonic archaea, while the redox‐sensitive C29 stanol/stenol ratio reflects oxygen content in the bottom water and surficial sediment. The changing coherence of these two proxies allows investigation of the interplay between upwelling, bioproductivity and redox conditions, and their dependence on SBB source water composition during the 20th century. Prior to a large‐scale oceanographic regime shift observed in the North Pacific in the 1970s, both proxies are positively correlated: periods of enhanced upwelling promoted mixing, resulting in increased oxygen availability at the sediment‐water interface; conversely periods of reduced upwelling favored development of oxygen‐depleted water at depth. In the wake of the basin‐wide regime shift, changing oceanographic conditions led to a reduction in the southward flowing California Current and a stronger influence of the poleward California Undercurrent, which increased stratification and the supply of warm, oxygen‐poor tropical water in the subsurface. As a result, oxygen availability in SBB bottom waters was predominantly regulated by upwelling‐induced productivity and subsequent oxygen‐consuming remineralization of organic matter.