Alternative reconstructions of sea level and temperature
The reconstructions presented in the paper are based on a marine oxygen isotope stack
of 57 records1, which in our view provides the best estimate for mean ocean conditions
since individual records are potentially influenced by local deep-water conditions2,3.
Still, one might wonder what the results would be if individual records were used
instead. To address this, we repeated the calculations for two 18O-records of sufficient
(> 1070 kyr) length, that is, DSDP 607 from the midlatitude mid-Atlantic Ocean4 and
ODP 846 from the equatorial Pacific Ocean5 (Suppl. Fig. 1a). In doing so, all model
parameters were kept at their original values for both cases. The resulting temperature
and sea level reconstructions are depicted in Suppl. Figs. 1b and 1c. Clearly, the
interglacial-glacial fluctuations are generally similar in both reconstructions, indicating
that both records carry a significant part of the global 18O-signal. Local water mass
variability presumably accounts for the differences, which occur mainly in the period
500-700 kyr BP. These local influences, often hard to quantify, are expected to have
averaged out in the 18O-stack that was used in the paper.
Marine oxygen isotope value ( 0/00 )
2.5
a
3
3.5
4
4.5
5
5.5
0
200
400
600
Time (kyr BP)
800
1000
10
b
Surface air temperature (°C)
deviation from present
5
0
-5
-10
-15
-20
0
200
400
600
800
1000
800
1000
Time (kyr BP)
Global sea level (m below present)
0
c
20
40
60
80
100
120
140
0
200
400
600
Time (kyr BP)
Supplementary Figure 1. Simulated 1070-kyr time series for two benthic oxygen
isotope records. We have used the midlatitude central Atlantic record DSDP
607 (ref. 4) (red) and record ODP 846 from the equatorial Pacific5 (blue). To
obtain equal interglacial values for both, 0.5 0/00 was added to the ODP 846
isotope record. a. Observed input oxygen isotope. b. Modelled NH surface air
temperature. c. Modelled global sea level.
Another issue is whether our new method, when used to quantify past global sea level
based on marine isotope data, provides a significant advantage over simple scaling the
18O-signal. To test this, we have linearly converted 18O-values into global sea levels
(minimum interglacial and maximum glacial values were assumed to represent 0 and –
127 m of global sea level, respectively). Suppl. Fig. 2 shows how modelled and scaled
sea levels compare. In particular during glacial inception stages, linear scaling would
overestimate sea level by as much as 20 m because the 18O-signal can then almost
completely be attributed to climate cooling, as argued and shown in the paper. The
reason for this is that ice sheets cannot always grow at the rate dictated by the 18Osignal, as ice accretion is maximized by the total accumulation rate. Even though the
comparison appears to hold reasonably well for the remainder of glacial epochs, our
results clearly demonstrate that the separation of the isotope signal into an ‘ice sheet’
and a ‘deep-water’ component is not constant in time (Fig. 4, middle), and that isotopescaled sea level estimates should therefore be interpreted with care. Note also that our
method not only yields global sea level but also quantifies the spatial distribution of the
paleo ice sheets.
140
18O-scaled global sea level
(m below present)
120
100
80
60
Inception
40
Deglaciation
20
0
0
20
40
60
80
100
120
140
Modelled global sea level
(m below present)
Supplementary Figure 2. 18O-scaled global sea level versus modelled global
sea level based on the 18O-stack1 used in the paper. Each green dot
represents one 0.1-kyr value of the 1070-kyr run.
1. Lisiecki, L. E., & Raymo, M. E. A Pliocene-Pleistocene stack of 57 globally
distributed benthic 18O records, Paleoceanography, 20, PA1003,
doi:10.1029/2004PA001071 (2005).
2. Waelbroeck, C., et al. Sea-level and deep water temperature changes derived from
benthic foraminifera isotopic records. Quat. Sci. Rev. 21, 295-305 (2002).
3. Lea, D. W., Martin, P. A., Pak, D. K. & Sperbo, H. J. Reconstructing a 350-kyr
history of sea level using planktonic Mg/Ca and oxygen isotope records from a Cocos
Ridge core. Quat. Sci. Rev. 21, 283-293 (2002).
4. Raymo, M. E., Ruddiman, W. F, Backman, J., Clement, B. M. & Martinson, D. G.
Late Pliocene variation in northern hemisphere ice sheets and North Atlantic deepwater
circulation. Paleoceanography 4, 412-446 (1989).
5. Mix, A. C., Le, J. & Shackleton, N. J. Benthic foraminiferal stable isotope
stratigraphy from Site 846: 0–1.8 Ma. Proc. Ocean Drill. Program Sci. Results, 138,
839-847 (1995).