Geophysical Research Letters
Supporting Information for
The response of SST to insolation and ice-sheet variability from MIS 3 to MIS 11
Aleix Cortinaa, Francisco Javier Sierrob, José Abel Floresb, Belen Martrata, Joan O. Grimalta
a Department
b
of Environmental Chemistry, IDAEA-CSIC, 08034 Barcelona, Spain
Department of Geology, University of Salamanca, 37008 Salamanca, Spain
Contents of this file
Text S1 to S2
Figure S3 to S4
Introduction
This supporting information is composed by two texts and two figures. Text S1 focuses on the
methodology followed for extracting biomarkers from sediments and for estimating SST based
on the proportion of long chain di- and tri- unsaturated C37 alkenones. Text S2 explains the
methodology for calculating the StdT index. This index allowed us measuring the SST
amplitude changes at high-frequency variability during glacial stages. Figure S3 illustrates the
variance between the StdT index and sedimentation rate in order to discard differential
sedimentation rate as the main cause of StdT index variability. Figure S4 illustrates the
theoretical effect on SST of an equal ice-sheet variability depending on insolation.
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Text S1.
Samples were freeze-dried. We added an internal standard (n-nonadecan-1-ol, nhexatriacontane and n-dotetracontane) to about 2.5 g of sediment, which was extracted with
dichloromethane in an ultrasonic bath. The extract was saponified with 10% potassium
hydroxide in methanol to clean up interferences by carboxylic acids and ester wax. The phase
with the neutral lipids was extracted with hexane and evaporated to dryness under a
N2 atmosphere. Finally, the compounds obtained were redissolved with toluene, derivatized
with bis(trimethylsilyl) trifluoroacetamide ,and analyzed by gas chromatography. The samples
were analyzed with a Varian 3400 chromatograph equipped with a CPSIL-5 CB column coated
with 100% dimethylsiloxane (film thickness of 0.12 mm). Hydrogen was used as the carrier gas
(50 cm/s). The oven was programmed from 90ºC to 170ºC at 20ºC/min, then to 280ºC at
6ºC/min (holding time 35min), to 300ºC at 10ºC/min (holding time 7min) and finally to 320ºC at
10ºC/min (holding time 3min). The injector was programmed from 90ºC (holding time 0.3) to
320ºC at 200ºC/min (the final holding time was 20min). The detector was maintained with a
constant temperature of 320ºC.
From the chromatograms we identified long chain di- and tri- unsaturated C37 alkenones.
After integration, area was converted to concentration, taking into consideration sample
weight and the concentration of the internal standard.
The SST proxy was based on the Uk’37 index (Brassell et al., 1986; Prahl and Wakeham,
1987). The Uk’37 index results from calculating the relative abundance of different types of
alkenones with 37 carbons, varying in the number of double bonds.
Uk’37= [C37:2] / ([C37:2] + [C37:3]);
It can take values between 0 and 1, respectively, corresponding to temperatures between
0 and 26 ºC. To transform this index into annual mean SST values, we used the following
equation:
SST= (Uk’37 – 0.044)/0.033 (Müller et al., 1998)
References
Brassell, S. C., G. Eglinton, I. T. Marlowe, U. Pflaumann, and M. Sarnthein (1986), Molecular
stratigraphy: a new tool for climatic assessment, Nature, 320(6058), 129-133.
Müller, P. J., G. Kirst, G. Ruhland, I. von Storch, and A. Rosell-Melé (1998), Calibration of the
alkenone paleotemperature index U37K′ based on core-tops from the eastern South Atlantic
and the global ocean (60°N-60°S), Geochimica et Cosmochimica Acta, 62(10), 1757-1772.
Prahl, F. G., and S. G. Wakeham (1987), Calibration of unsaturation patterns in long-chain
ketone compositions for palaeotemperature assessment, Nature, 330(6146), 367-369.
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Text S2.
In order to evaluate SST amplitude at millennial-scale level during glacials, we used the
standard deviation. Since long-term variability (e.g. precession) influences temperature values,
and consequently their standard deviation, we first removed this long-term parameter by
subtracting the corresponded polynomial fit. Depending on the length of the glacial interval
and the long-term variability effect (mainly precession) we used different degrees of
polynomial fits.
MIS 3-4: 5th degree polynomial fit
MIS 6: 5th degree polynomial fit
MIS 8: 3rd degree polynomial fit
MIS 10: 2nd degree polynomial fit
The result of this subtraction was a new SST proxy (SST*) in which precession and longer
variability were removed, but higher-frequency variability remained unaltered. The standard
deviation was calculated for this SST* proxy for each of the glacial stages (MIS 3 and 4
computed together since they belong to the same glacial stage), providing a measurement of
the SST amplitude variability at high frequencies (StdT index). High (low) StdT index values
represented glacials with high (low) millennial-scale amplitude variability.
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Figure S3.
Differences in the sedimentation rate can affect the resolution of the SST record and hence the
StdT index. Low sedimentation rate may mask millennial-scale events, resulting in a smooth
SST record and the associated decrease in our StdT index. As mean of discarding this
differential sedimentation effect as the main cause of the differential StdT, we compared the
30-point-average moving curve of the sedimentation rate with a 30-point-average moving
curve of the StdT for the period encompassed between MIS 3-4. This interval was intentionally
chosen because it has the most reliable age model. We choose 30-point average window in
order to consider normal data distribution. Results do not reveal any significant correlation
between high sedimentation rate and StdT. It indicates that the SST signal responded to
external forcings and was not an artifact of differential sedimentation rate.
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Figure S4.
Effect of ice-sheet variability in the SST along the Gulf of Lions (GL). Both illustrations
show a theoretically equal ice-sheet variability transmitted to the GL by northwesterly
winds: (A) during high North Hemisphere summer insolation periods, producing a high
SST effect (higher dSST), and (B) during low North Hemisphere summer insolation
periods, producing a low SST effect (lower dSST).
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