Questions 31-39
; }9 s/ ~& y$ u$ ~: jPerhaps one of the most dramatic and important changes that took place in the/ S5 F8 I8 a# g( H. d
Mesozoic era occurred late in that era, among the small organisms that populate the
* N3 W- H" K' A% A5 j5 b( iuppermost, sunlit portion of the oceans--the plankton. The term "plankton" is a broad
% L3 b- Y! X: }, ]+ uLine one, designating all of the small plants and animals that float about or weakly propel* w+ W! S' x% k3 V1 S
(5) themselves through the sea. In the late stages of the Mesozoic era. during the Cretaceous. E/ Q [5 ?/ \; ~: R3 G
period, there was a great expansion of plankton that precipitated skeletons or shells
8 z% L& P2 Y9 `3 O/ pcomposed of two types of mineral: silica and calcium carbonate. This development+ {6 {3 B$ x! c( r& U
radically changed the types of sediments that accumulated on the seafloor, because,+ ~! @8 E, |& D7 @
while the organic parts of the plankton decayed after the organisms died, their mineralized
* {& g. t7 @5 \* o4 K8 I(10) skeletons often survived and sank to the bottom. For the first time in the Earth's long
5 R, ]; i- R6 rhistory, very large quantities of silica skeletons, which would eventually harden into rock,
7 l3 h3 G, M) D2 {; g. \! y; Obegan to pile up in parts of the deep sea. Thick deposits of calcareous ooze made up of
* o! A& `% j- q, f$ N/ m! mthe tiny remains of the calcium carbonate-secreting plankton also accumulated as never
9 a4 K4 S5 }1 c4 Q! y- Xbefore. The famous white chalk cliffs of Dover, in the southeast of England, are just one( E7 |$ h0 B/ S, @
(15) example of the huge quantities of such material that amassed during the Cretaceous
* q5 ^2 u' }: N7 ~ K# M/ x: lperiod; there are many more. Just why the calcareous plankton were so prolific during2 L9 p* O8 n% o; h
the latter part of the Cretaceous period is not fully understood. Such massive amounts3 S0 H- r, S: N! H* B0 C
of chalky sediments have never since been deposited over a comparable period of time.
# x& F) `4 H8 A; R% CThe high biological productivity of the Cretaceous oceans also led to ideal conditions
( K* G9 T& X. k) ], x! {6 U" h(20) for oil accumulation. Oil is formed when organic material trapped in sediments is slowly
$ A( z2 T% x( Y6 H2 R) sburied and subjected to increased temperatures and pressures, transforming it into% ~* W1 Y7 O! `% c, O4 _; B
petroleum. Sediments rich in organic material accumulated along the margins of the
( }2 k2 m; G6 H6 Z6 vTethys Seaway, the tropical east-west ocean that formed when Earth's single landmass* ]3 W/ @" j- H" k" r
(known as Pangaea) split apart during the Mesozoic era. Many of today's important oil2 z. x& b' b8 q) V/ q
(25) fields are found in those sediments--in Russia, the Middle East, the Gulf of Mexico, and2 L1 r9 `5 } X9 ` e
in the states of Texas and Louisiana in the United States.% O q- r) @4 i$ |- W; G' n7 C
! |- ~* G: u6 r1 Y# `% ?6 d8 O( t& F i
1 ?% Q. b* L# X31. What does the passage mainly discuss?
; `# h: r# C$ }1 E. h(A) How sediments were built up in oceans during the Cretaceous period( r+ v6 `' v& n3 w2 E% G: T& i* M" B
(B) How petroleum was formed in the Mesozoic era) `/ M* Q, ?* K6 C8 P7 [- n" O8 e. H
(C) The impact of changes in oceanic animal and plant life in the Mesozoic era
& A6 B5 y: m! p# R% `& L% n$ m: n(D) The differences between plankton found in the present era and Cretaceous plankton |
发表于 2012-8-14 22:34:35
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