Page 42 - Earth's Climate Past and Future

18 PART I • Framework of Climate Science

Climate Archives, Dating and Resolution continental basins that contain lakes; shallow interior
seas that at times flood low-lying land; and lens-shaped
Like written chronicles of human history, climate archives piles of sediment along continental shelves (the barely
hold stories of climate change for those who can read submerged coasts of continents) and steeper continental
them. For the immense span of Earth’s history prior slopes leading down into the deep ocean.
to the invention of instruments in recent centuries, Sediments are useful climate archives to the extent
sediments, ice, corals, and trees are the major climatic that their deposition is uninterrupted. Major disturbances
archives. come from wave action reaching several meters below
sea level and from occasional large storms that reach tens
2-1 Types of Archives
of meters deep in the water column and erode previ-
Although relatively recent climate changes can be stud- ously deposited layers. In addition, sediments deposited
ied in an array of archives, sediments—primarily con- on steep continental slopes are vulnerable to dislodgment
tinuous sequences of sediment deposited by water—are by disturbances such as earthquakes and tsunamis.
the major climate archive on Earth for over 99% of In the longer term, erosion tied to sea level change
geologic time. is a major factor that interrupts sediment deposition.
Sediments Rainfall and the runoff it produces erode Through time, the sea moves up and down along the
rocks exposed on the continents and transport the eroded continental margins over a total vertical range of about
debris in streams and rivers in both physical (granular) 200 meters. Sediments can be deposited on the upper
and chemical (dissolved) forms. The sediments are even- margins when sea level is high, but these deposits are
tually deposited in quieter waters where layer after layer often eroded by waves and storms and carried to the
of sediment is laid down in undisturbed succession. Most deep sea when sea level subsequently falls.
sediment is carried to the ocean either right after it is All these factors ultimately determine the types of
eroded or after temporary deposition on land followed climate records preserved in sediment archives (see
by one or more cycles of additional erosion and redeposi- Figure 2–1). Sediments deposited on continental shelves
tion. Sediment delivered to the seafloor may persist there when sea level is high form lens-shaped units separated
for tens of millions of years until tectonic processes by distinct surfaces where erosion has occurred. Deposi-
destroy it. The relentless action of these two processes, tion is often continuous within these sequences, with the
erosion and tectonic activity, decreases the likelihood highest rates in regions where rivers deliver sediment.
that older sedimentary records will be preserved as time Sediments deposited in interior seas on the continents
passes. during times when the ocean floods low-lying regions
For intervals prior to the last 170 million years, all form continuous sequences covering wide areas.
surviving sedimentary records come from the continents. Sediments deposited in lakes in continental basins
Under favorable conditions, sediments may be preserved conform to the structural framework of the depression
for a long time in the regions shown in Figure 2–1: in the bedrock. Deposition tends to be most continuous

Lake
Ocean
Bedrock Ocean
Bedrock
Bedrock
Bedrock
Lake
sediments
Interior sea Coastal
sediments margin Ocean
sediments

Ocean crust
FIGURE 2-1 Sediment archives
Layered sediments are major climate
archives on all time scales. The insets
Deep-ocean
sediments show typical sediment layering in
sediment archives from land and sea.