Intermediate Waters, Oceanic Methane and Abrupt
Climate Change:
Insights From Modern Methane Seeps and High-
Resolution Isotope Records

By
Tessa M. Hill
tmhill@ucdavis.edu

There exists ongoing debate in the paleoclimate
community about the forcing mechanisms and
consequences of abrupt warming in the past and
present. This dissertation research focused on the
causes and oceanic responses to abrupt climate
change during the late Quaternary utilizing four
approaches: 1) testing the role of oceanic methane
release in climate change via studies of d13C spikes in
the geologic record, 2) refining the use of foraminifera
as proxies of methane release in the past, 3) defining
the response of California margin surface and
intermediate waters to climatic change, and 4)
exploring the role and response of intermediate waters
to climate change, including feedbacks with the oxygen
minimum zone.

Foraminifera from methane seep environments in
Santa Barbara Channel, CA and Hydrate Ridge, OR
record the presence of methane via shifts in d13C
values and species composition to those tolerant of low
oxygen, organic rich environments. These studies
indicate that foraminifera can be useful indicators of
methane-rich environments. The d13C shifts from
methane seep environments are similar in magnitude
to those observed in late Quaternary sediment
sequences from Santa Barbara Basin. High-resolution
(~5yr) d13C studies of these d13C spikes indicated that
these excursions were <10 years in duration, and
synchronous in benthic and planktonic foraminifera,
implying the release of methane to the water column
and atmosphere.

Research using newly collected cores in the Santa
Barbara Basin investigated the record of deglacial
climate change from surface and intermediate waters.
Intermediate water temperatures warm synchronously
with surface waters, recording early warming on the
California margin approximately 2 ka prior to
Termination IA. Comparison of stable isotopic values of
benthic foraminifera and lamination strength from 440m
and 570m indicate interactions between intermediate
waters, the methane hydrate reservoir, and the oxygen
minimum zone during times of abrupt warming. These
findings are consistent with increases in the amount of
tar seepage to basin sediments during deglacial
warming. During time periods of abrupt warming, both
the hydrate and thermogenic methane reservoirs may
increase outputs of methane to the water column and
atmosphere.