Abstract
The oxygen minimum zone (OMZ) of the late Quaternary California margin experienced abrupt and dramatic changes in strength and depth in response to changes in intermediate water ventilation, ocean productivity, and climate at orbital through millennial time scales. Expansion and contraction of the OMZ is exhibited at high temporal resolution (107–126 year) by quantitative benthic foraminiferal assemblage changes in two piston cores forming a vertical profile in Santa Barbara Basin (569 m, basin floor; 481 m, near sill depth) to 34 and 24 ka, respectively. Variation in the OMZ is quantified by new benthic foraminiferal groupings and new dissolved oxygen index based on documented relations between species and water‐mass oxygen concentrations. Foraminiferal‐based paleoenvironmental assessments are integrated with principal component analysis, bioturbation, grain size, CaCO3, total organic carbon, and δ13C to reconstruct basin oxygenation history. Fauna responded similarly between the two sites, although with somewhat different magnitude and taxonomic expression. During cool episodes (Younger Dryas and stadials), the water column was well oxygenated, most strongly near the end of the glacial episode (17–16 ka; Heinrich 1). In contrast, the OMZ was strong during warm episodes (Bølling/Allerød, interstadials, and Pre‐Boreal). During the Bølling/Allerød, the OMZ shoaled to <360 m of contemporaneous sea level, its greatest vertical expansion of the last glacial cycle. Assemblages were then dominated by Bolivina tumida, reflecting high concentrations of dissolved methane in bottom waters. Short decadal intervals were so severely oxygen‐depleted that no benthic foraminifera were present. The middle to late Holocene (6–0 ka) was less dysoxic than the early Holocene.
Highlights
[2] Widespread shifts in oceanic oxygenation occurred across the Pacific Ocean basin during the last glacial cycle in concert with climatic and oceanographic change
[4] The linkage between oxygenation history and climate and water column change is well documented in Santa Barbara Basin, where a ~200 m core was recovered from Ocean Drilling Program (ODP) Site 893 in 1992 [Kennett, 1995]
[6] In this investigation, we have quantitatively examined benthic foraminiferal assemblages in two late Quaternary cores retrieved from Santa Barbara Basin (SBB) (Figure 1) and analyzed the same cores for total organic carbon (TOC), CaCO3, and grain size
Summary
[2] Widespread shifts in oceanic oxygenation occurred across the Pacific Ocean basin during the last glacial cycle in concert with climatic and oceanographic change. The current investigation in SBB extends this work in five ways: (1) quantification of benthic assemblages at much higher temporal resolution than previously published, (2) comprehensive quantitative analyses of assemblages to include all benthic foraminiferal taxa, (3) employment of hydrographic data to establish environmental tolerances of taxa to oxygenation concentrations, (4) upward extension of the depth transect to sill depth by the addition of core MD02-2504, and (5) comparison of changes in the foraminiferal assemblages with geochemical and grain-size data to better understand basin ventilation and the potential contribution of other environmental variations on the benthic faunal record. We aim to better understand changes in basin oxygenation and in the intensity and vertical distribution of the OMZ and their relations with orbital through millennial climate change
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