Page 111 - Earth's Climate Past and Future

CHAPTER 5 • Greenhouse Climate 87

The best-constrained estimates of past changes in
Erosion eustatic sea level are those spanning the last 100 Myr
(Figure 5-7) because the geologic record of this interval
is relatively complete compared to earlier times. But
even for this interval, sea level estimates remain highly
uncertain.

Deposition 5-4 Causes of Tectonic-Scale Changes in Sea Level

In the Cretaceous world of 100 to 80 Myr ago, the
coastlines and interiors of most continents were flooded
A Low sea level
by an ocean situated well above modern level (see
Figure 5-1). Areas flooded included much of southern
Europe (Figure 5-8) and interior regions of North
Erosion inland
Deposition America penetrated by seaways linked to the Gulf of
Mexico and the Arctic Ocean. Since that time, sea level
Shelf has slowly fallen to its modern position, close to the
lowest level on record (except for glacial intervals with
more ice on land than there is now).
Slope Geoscientists disagree about how much higher sea
level was 80 to 100 Myr ago. Estimates have ranged
from 100 to 300 m above today’s level. This wide range
of uncertainty reflects complications that have arisen
B High sea level
during the tens of millions of years since the Cretaceous
FIGURE 5-6 Sea level (A) When sea level is low, the sediments were deposited. The water-rich sediments
coastline lies near the base of the continental shelf and have been compacted and lost their original thickness,
sediment is deposited on the continental slope. (B) When high the added weight of the sediments has caused the
sea levels flood the continental margin, more sediment is underlying rock crust to slowly subside, and ocean
trapped and deposited on the submerged shelf. water has moved in over both the compacted sediments

these times, erosion prevails on continental margins, and Height above present sea level (m)
most of the eroded sediment is carried to the continental 0 100 200 300
slope and dispersed down in the deeper ocean. When sea
level is high, the ocean floods the low-gradient continen-
tal margin to depths of 100 m or more (Figure 5-6B). At
20
such times, sediment is deposited on the submerged con-
tinental shelf and can survive as part of the geologic
record.
Local tectonic factors that cause uplift or subsidence 40
of the land can also affect the relative vertical position
of the ocean margin against the land, even in the Myr ago
absence of changes in global sea level. These regional 60
processes include mountain building, broad-scale warp-
ing of Earth’s surface connected to deep-seated heating,
and local depression and rebound of the land caused by
80
the weight of ice sheets. In this chapter, we ignore these
local effects and focus on changes in eustatic sea
level—changes that are global in scale.
The most persuasive evidence for higher global sea 100
levels in the past comes from the presence of marine sed- FIGURE 5-7 Sea level in the last 100 Myr Quantitative
iments simultaneously deposited on coastal margins and estimates of sea level change show higher sea levels 100 Myr
in shallow interiors of continents at levels well above pre- ago. The blue area defines the large range of estimates based
sent sea level. Deposition of marine sediments on several on different methods. (Adapted from M. Steckler, “Changes
continents at the same time indicates that the changes in in Sea Level,” in Patterns of Change in Earth Evolution, ed. H. H.
sea level are global in scale, not just local features. Holland and A. F. Trendall [Berlin: Springer-Verlag, 1984].)