Quantified intermediate water oxygenation history of the NE Pacific: A new benthic foraminiferal record from Santa Barbara basin

Abstract

[1] 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, CaCO 3 , total organic carbon, and δ 13 C 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.

Vertical oxygen minimum zone oscillations since 20 ka in Santa Barbara Basin: A benthic foraminiferal community perspective

“…In spite of its above sill depth, MV0811‐15JC benthic foraminiferal assemblage composition oscillates similarly and synchronously to previously established faunal changes in the deeper basin ([ Cannariato et al ., ; Ohkushi et al ., ]; Figure ). Assemblages older than 14.7 ka are composed of Quinqueloculina spp., and Nonionella labradorica (65–85% of total abundance).…”

“…All faunal calculations included intervals of low abundances (Figure S1 of the supporting information). Assemblage composition for cores MD02‐2504 and MD02‐2503 is previously published [ Ohkushi et al ., ]. Metrics of community‐scale structure include: log density (individuals/cm −3 ), the Shannon diversity index ( H ′ = −∑ p i ln ( p i )) (equation (2)), where p i is the proportional abundance of a species, and Pielou's evenness index ( J ′ = H ′/ln ( s )) (equation (3)), where s is the number of species.…”

“…Sediment cores from SBB provide high‐resolution climate and environmental archives, which exhibit submillennial scale climate synchronicity between planktonic foraminiferal oxygen isotopes and the GISP2 ice core record [ Hendy et al ., ]. Sequences from the deep basin exhibit dramatic oscillations in seafloor oxygenation, as recorded in sedimentary proxies [ Behl , ] and foraminiferal community diversity [ Cannariato et al ., ; Ohkushi et al ., ]. Rapid deoxygenation of these deep basin sequences occurred during the abrupt warmings associated with interstadial events, the Bølling/Allerød (B/A) and the early Holocene; these data indicate that deoxygenation of the deep basin is synchronous with rapid warming in the high latitudes of the Northern Hemisphere.…”

“…In this study we present a new deglacial‐aged sediment record (piston core MV0811‐15JC; 418 m water depth) that builds upon previously established evidence of rapid biological reorganization in response to oxygenation changes over the last deglacial episode [e.g., Cannariato and Kennett , ; Ohkushi et al ., ]. MV0811‐15JC, in conjunction with data from two previously published cores at 481 m and 570 m, completes a 152 m depth transect through SBB, ranging from the basin center to 55 m above the current western sill [ Hill et al ., , ; Ohkushi et al ., ]. This contribution documents the vertical history of oxygenation in SBB using quantitative analyses of benthic foraminiferal assemblages (density, diversity, and evenness) and taxonomic affiliations to modern environmental regimes.…”

The simulated climate of the Last Glacial Maximum and insights into the global marine carbon cycle

“…The increase in dissolved oxygen in O-LGM was considerable but agreed well with proxy reconstructions for the upper ocean. The oxygen-poor intermediate waters of the western North Pacific (Ishizaki et al, 2009;Shibahara et al, 2007), eastern North Pacific (Cannariato and Kennett, 1999;Cartapanis et al, 2011;Chang et al, 2014;Dean, 2007;Nameroff et al, 2004;Pride et al, 1999;Ohkushi et al, 2013;van Geen et al, 2003), eastern South Pacific (Martinez et al, 2006;Muratli et al, 2010;Salvatteci et al, 2016), equatorial Pacific (Leduc et al, 2010) and Indian Ocean (Reichart et al, 1998;Suthhof et al, 2001;van der Weijden et al, 2006) were better oxygenated at the LGM relative to the PI climate. An important consequence of oxygenating the upper ocean is a reduction in the strength of denitrification in these regions.…”