Foraminifera as indicators of methane-rich environments: A study of modern methane seeps in Santa Barbara Channel, California

Abstract

Methane hydrates have been implicated as a possible forcing mechanism for rapid climate change during the late Quaternary, via the release of methane from marine reservoirs. Carbon isotopic excursions of foraminifera observed in high-resolution sediment records from Santa Barbara Basin (ODP Site 893) have been interpreted as evidence of episodes of methane release during the late Quaternary. However, the potential relationship between methane in the modern environment and its effect on the stable isotopic composition of benthic and planktonic foraminifera is not well understood. Short cores taken inside a ∼500-m-diameter pockmark with active methane seeps in Santa Barbara Channel, CA, USA, yield information on the effect of high concentrations of environmental methane on foraminiferal assemblages and their stable isotopic composition. These seeps reside at ∼120 m water depth in the eastern portion of the channel, associated with faults and fractures in the Miocene Monterey Formation. Planktonic foraminiferal assemblages and radiocarbon ages show that surface sediments are of late Glacial age (∼25 300 calendar years). This finding suggests an absence of Holocene and deglacial sedimentation in the seep area, likely due the formation of the pockmarks and subsequent winnowing or lack of sedimentation. Benthic foraminiferal assemblages are marked by high abundance of Bolivina tumida, a species previously observed during interstadial episodes and the early Holocene in Santa Barbara Basin, interpreted to include episodes of high methane flux from the basin. While oxygen isotopic values of both benthic and planktonic foraminifera appear normal for cool, glacial conditions, negative carbon isotopic values are seen in benthic foraminifera, primarily in the near-surface intervals. In one interval (1–2 cm below surface), both benthic and planktonic species record highly negative carbon isotopic values, implying that isotopically light methane influenced the entire water column. This spike is of similar or larger magnitude than several earlier observed spikes in the late Quaternary record from ODP Site 893. Single specimen isotopic analyses give an indication of the total range of carbon isotopic values recorded (−0.01 to −25.23‰ for benthics, −0.07 to −22.22‰ for planktonics) in the spike interval. Since modern methane seeps are commonly associated with authigenic carbonate precipitation, scanning electron microscopy was used to assess the possibility that post-depositional alteration may have affected the isotopic composition of the test. These experiments indicate that authigenic carbonate is absent and therefore unable to account for the highly negative carbon isotopic signatures. These data improve our understanding of methane-associated environments by showing that foraminiferal isotopic composition and assemblages may be used to discern the flux of methane from the oceanic reservoir in the geologic record.