Late Quaternary oxygen (18O)
and carbon (
13C)
isotopic records for the benthic foraminifer Uvigerina and the planktonic
foraminifer Globigerina bulloides are presented for the upper 20 meters
composite depth sediment sequence of Ocean Drilling Program Site 1014, Tanner
Basin, in the outer California Borderland province. The benthic oxygen isotopic
record documents a continuous >160-k.y. sequence from marine isotope Stage
(MIS) 6 to the present day. The record closely resembles other late Quaternary
North Pacific benthic isotope records, as well as the well-dated deep-sea
sequence (SPECMAP), and thus provides a detailed chronologic framework.
Site 1014 provides a
useful record of the California response to climate change as it enters the
southern California Border-land. Sedimentation rates are relatively constant and
high (~11.5 cm k.y. –1 ). The planktonic foraminiferal
record is well pre-served except during marine isotope Substages 5b and 5d, when
normally high G. bulloides abundance is strongly diminished as a result
of dissolution. The planktonic oxygen isotopic shift of ~3‰ between the last
glacial maximum and the Holocene suggests a surface water temperature shift of
<7°C, similar to estimates from Hole 893A (Leg 146) to the north. Unlike
Santa Barbara Basin, G. bulloides 18O
values during the last interglacial (MIS 5) at Site 1014 were significantly
higher than during the Holocene. In particular, marine isotope Substage 5e (Eemian)
was ~0.8‰ higher. This is unlikely to reflect a cooler Eemian but is instead
the result of preferential dissolution of thin-shelled (low
18O)
specimens during this interval. In this mid-depth basin, a large benthic
18O
shift during Termination I suggests dramatic temperature and salinity changes in
response to switches in the source of North Pacific Intermediate Water.
Although 13C
values of the planktonic foraminifer G. bulloides are in disequilibria
with seawater and hence interpretations are limited, the G. bulloides record
exhibits several negative
13C
excursions found at other sites in the region (Sites 1017 and 893). This
indicates a response of G. bulloides
13C
to regional surface water processes along the southern California margin. A
general increase in benthic carbon isotopic values (–1.75‰ to –0.75‰) in
Tanner Basin during the last 200 k.y. is overprinted with smaller fluctuations
correlated with climate change. The coolest intervals during the last glacial
maximum (MISs 2 and 4) exhibit lower benthic
13C
values, which correlate with global
13C
shifts. The opposite relationship is exhibited during the last interglacial
before 85 ka, when lower benthic
13C
values are associated with warmer intervals (marine isotope Substages 5c and 5e)
of the last interglacial. These time intervals were also marked by decreased
intermediate water ventilation. Increased dissolution and organic accumulation
during Substages 5b and 5d are anticorrelated with the benthic
13C
record. These results suggest that a delicate balance in intermediate water
13C
has existed between the relative influences of global
13C
and regional ventilation changes at the 1165-m water depth of Site 1014.
1Lyle, M., Koizumi, I., Richter, C., and Moore, T.C., Jr. (Eds.), 2000. Proc. ODP, Sci. Results,167 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/167_SR/167sr.htm>. [Cited YYYY-MM-DD]
2Department of Geological Sciences and Marine Science Institute, University of California, Santa Barbara CA 93106, USA. Correspondence author: hendy@magic.geol.ucsb.edu
Date of
initial receipt: 2 November 1998
Date of acceptance: 19 April 1999
Ms 167SR-205