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

Abstract

[1] Here we present a history of deoxygenation of upper intermediate waters during the last deglaciation from Santa Barbara Basin (SBB), based on quantitative analyses of benthic foraminiferal assemblages, from a new shallow piston core above basin sill depth (MV0811-15JC, 418 m), and previously described sequences in the deeper basin (MD02-2504, 481 m and MD02-2503, 570 m). We document a 152 m depth transect of benthic foraminiferal assemblages to extract changing community structure (density, diversity, and evenness) and improve paleoenvironmental interpretation of late Quaternary vertical oscillations in the upper boundary of the oxygen minimum zone (OMZ). Close interaction between changes in open margin OMZ and that of the restricted SBB is documented using these quantitative techniques. MV0811-15JC, while being unlaminated, contains strongly hypoxic foraminiferal assemblages (including species Bolivina tumida and Nonionella stella), coeval with preserved sediment laminations in the deeper cores. Last Glacial Maximum (LGM) assemblages across this transect contained oxic fauna and high diversity. At 14.7 ka, glacial termination IA, hypoxic benthic fauna appeared across the transect, recording hypoxic waters (<0.5 ml L À1 ) < 300 m from the ocean surface. Bølling/Allerød (B/A) assemblages uniquely stand out in the record, exhibited by low density, diversity, and evenness, and taxonomic composition reflecting extreme and stressful hypoxia and methane-rich environments. Younger Dryas assemblages were diverse and composed of oxic fauna, similar to LGM assemblages. Termination IB initiated another deoxygenation shift, followed by OMZ-associated faunal and density patterns. This analysis strengthens the quantitative assessment of oxygen concentrations involved in deglacial OMZ change and reveals the unexpected, remarkable shallowness of OMZ influence during the B/A.

Response of seafloor ecosystems to abrupt global climate change

“…S3) is consistent with an environment similar to the core of the modern OMZ (15,17). Simultaneously, foraminifera are extremely abundant and dominated by OMZ-associated fauna (20). Interestingly, the mid-Holocene community is most similar to the successional colonization and recovery of the late Allerød.…”

“…Core MV0811-15JC is a short (922 cm) deglacial archive with an independent age model constructed using radiocarbon dates and oxygen isotopic tie-points to ODP Site 893A (9,20). Core MV0811-15JC exhibits a "two-step" record of planktonic foraminiferal oxygen isotopic shifts during the last deglaciation with a relatively indistinct, highly variable cooling episode representing the Younger Dryas; similar characteristics to this episode are exhibited at nearby ODP 893A (9, 28) ( Fig.…”

“…At Termination 1A (14.7 ka), core MV0811-15C was at ∼300 m water depth, with sea level ∼110 m below that of the present day (29). At this shallow water depth, the foraminiferal biotic archive of core MV0811-15JC recorded vertical changes in the upper boundary of the regional California Current OMZ (20).…”

“…Peak densities of benthic Foraminifera (>1,000 individuals/cm 3 ), however, occur when hypoxic oxygen concentrations range between 0.5 and 0.1 mL·L −1 , in response to rich detrital food resources and a scarcity of invertebrate competitors and predators (19). Overall, changes in foraminiferal density, diversity, and relative abundances of hypoxic marker species are sensitive recorders of changes in OMZ strength (20).…”

“…S4). During this time, ocean and climate changes led to abrupt reorganization of the vertical zonation of ecosystems on the continental margin through the expansion of the OMZ (20,44,45). Long-term (>1,000 y) ecosystem recovery is demonstrated in the Younger Dryas oxygenation reversal, wherein differences in trophic structures and physiological adaptations, from chemosynthetic extremophiles to motile carnivores and deposit feeders, determine the successional process and its timing.…”