USGS Open-File Report 94-588
North Atlantic deep water temperature change during late Pliocene and Quaternary climatic cycles
- G.S. Dwyer
- Department of Geology, Duke University
- T.M. Cronin
- U.S. Geological Survey, Reston, VA
We studied the Mg/Ca ratio of shells of the deep-sea ostracode genus
Krithe to estimate bottom water temperature (BWT) in the deep
Atlantic Ocean during late Pliocene and Quaternary climatic cycles. This
work was first presented in Dwyer et al (1994) and is described in detail
in Dwyer et al (submitted) and Cronin et al (in prep.). Using the
preliminary calibration equation T (°C) = 451.7 x (Mg/Ca Krithe)
- 2.2 derived from Correge (1993), we calculated bottom water
temperature in the PRISM time slice 3.15-2.85 Ma at the following DSDP and
ODP sites 610A, 607, 552A, 704A, 658, 659, 541, 502A, and 661 in the North
and South Atlantic (Cronin et al, in prep.). We also studied a late
Pliocene time series in the interval 3.2-2.3 Ma from DSDP Site 607 (41°N,
33°W, 3427 m water depth) and the late Quaternary interval 200 kyr-
present (oxygen isotope stages 7-1) from the nearby Chain core 82-24-4PC
(42°N, 33°W, 3427 m water depth) to examine bottom water temperature
variation during 41kyr obliquity and 100 kyr eccentricity cycles (Dwyer et
al, 1994, submitted). Finally, we carried out Mg/Ca and faunal analyses
of Krithe from transects through the thermocline and into the
deep-sea in the Arctic Ocean, Norwegian/Greenland Seas, Little Bahama
Banks, Florida-Hatteras Slope, Chilean Fjords, and off Japan in order to
improve existing calibrations of Mg/Ca to temperature.
Our preliminary conclusions are as follows.
- Analyses of ten Krithe species from site 607 indicate no
obvious interspecific vital effect in the Mg/Ca ratio.
- Intraspecific analyses of Krithe cf. pernoides from
high northern hemisphere latitudes showed no vital effect in Mg/Ca ratios
among adult individuals of the same species due to shell size, gender, or
right/left valve, but there is an ontogenetic variation of increasing
Mg/Ca ratios with younger age (ie., early instars).
- During the relatively warm interval from 3.2-2.8 Ma, BWT averaged 2.3°C,
ranging from a minimum of 1.65° to a maximum of 3.3°C. Minima and
maxima average 1.9° and 2.6°C respectively, with an average "glacial-
interglacial" amplitude of about 0.7°C.
- During the interval 2.8 to 2.3 Ma, which spans the initiation of
significant continental glaciation in the northern hemisphere, the average
amplitude of the glacial/interglacial BWT shift increased to 1.5°C, with
an average glacial BWT of 1.4°C; average interglacial BWT was 2.9°C. The
absolute glacial minima and interglacial maxima for this period are 0.9°
and 3.8°C, respectively, and the average interglacial BWTs for this
interval are similar to the present day temperature (2.6 °C).
- The Mg/Ca ratios and temperature estimates show a good correlation
with the benthic stable isotope records from 3.2-2.3 Ma; there is an
apparent 41 kyr periodicity to the Mg/Ca record (Dwyer et al., 1994) which
coincides with patterns of ostracode faunal assemblages from site 607.
- CH82-24-4PC was heavily sampled, especially in the interval
corresponding to the last glacial maximum, but our preliminary data
indicate a large glacial-interglacial shift in BWT (aproximately 2.25°C)
between isotope stages 7 and 6 and between stages 6 and 5. Interglacial
maxima are similar to those of the late Pliocene, averaging 2.7°C with an
absolute maximum of 2.9°C. Glacial BWT for isotope stage 6 was 0.7 °C,
the coldest observed. Our first approximation of late Quaternary glacial-
interglacial BWT change suggests that it was greater during these 100kyr
cycles than during 41kyr-cycles of the late Pliocene.
- For the 3 Ma time slice reconstruction, the preliminary Mg/Ca data
show a gradient in BWT from warmer temperatures in the North Atlantic to
cooler temperatures in the South Atlantic, as would be expected if
southern source deep bottom water was colder than northern source water.
However, this pattern must be confirmed with additional analyses.
The Mg/Ca-derived bottom water temperature estimates provide means by
which to separate the temperature component from the deep-sea benthic
foraminifer d18O record and reconstruct the seawater d18O record and an
ice volume curve. The resulting curve provides a detailed record of late
Pliocene sea level that is generally concordant with ocean margin records
of sea-level change. Additional work is underway to improve and expand
the Mg/Ca-derived BWT record and to improve our understanding of late
Pliocene and Quaternary sea level history. A more detailed analysis of
the late Quaternary is also warranted in light of the limited sampling of
the Chain core.
References Cited:
- Correge, T. 1993. Preliminary results of paleotemperature reconstruction using the magnesium to calcium ratio of deep-sea ostracode shells from the Late Quaternary of site 822, Leg 133 (western Coral Sea). In, J.A. McKenzie, P.J. Davies, A. Palmer-Johnson, eds., Proc. ODP Sci. Results, 133, College Station, Texas, p. 175-180.
- Cronin, T.M., Dwyer, G.S., and Dowsett, H.J. in prep. Deep-sea bottom water temperature reconstruction, Atlantic Ocean 3 Ma based on Mg/Ca ratios in the ostracode Krithe.
- Dwyer, G.S., Cronin, T.M., Baker, P.A., Raymo, M.E., Dowsett, H.J. 1994. Late Pliocene and Quaternary bottom water temperature changes in the deep North Atlantic. AGU Baltimore Meeting, April, 1994, p. 55-56.
- Dwyer, G.S., Cronin, T.M., Baker, P.A., Raymo, M.E., Dowsett, H.J., and Correge, T. submitted. Late Pliocene and Quaternary bottom water temperature change in the deep North Atlantic. (submitted to Nature July, 1994).
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