Questions 31-39( q" Y Y# l: w& u0 N3 D3 X
Perhaps one of the most dramatic and important changes that took place in the
- T. c8 o; ]8 O. z! h: uMesozoic era occurred late in that era, among the small organisms that populate the
$ u0 @% h1 K' r. a* auppermost, sunlit portion of the oceans--the plankton. The term "plankton" is a broad0 l( }# K: v( }; S( s# W7 t
Line one, designating all of the small plants and animals that float about or weakly propel
( ~1 u7 |, ?( i(5) themselves through the sea. In the late stages of the Mesozoic era. during the Cretaceous
% ~ N6 z5 y5 A5 b: Lperiod, there was a great expansion of plankton that precipitated skeletons or shells
3 ^- H& _; g3 K4 s( E0 Q: ycomposed of two types of mineral: silica and calcium carbonate. This development
' @+ X5 }+ G! W8 p5 Hradically changed the types of sediments that accumulated on the seafloor, because,
, A) y _% w; z& G8 ?while the organic parts of the plankton decayed after the organisms died, their mineralized
! X8 f4 ^4 F+ c! F(10) skeletons often survived and sank to the bottom. For the first time in the Earth's long
, P6 R ^9 i' {( B: Y) C) `! hhistory, very large quantities of silica skeletons, which would eventually harden into rock,0 \1 q! E: \- P" N" |2 J5 |
began to pile up in parts of the deep sea. Thick deposits of calcareous ooze made up of
$ M+ A2 k3 V' F% a5 bthe tiny remains of the calcium carbonate-secreting plankton also accumulated as never
: g$ ~' T1 u, M! d! Y0 i9 jbefore. The famous white chalk cliffs of Dover, in the southeast of England, are just one. }2 C: R" T2 N+ V& @: Z
(15) example of the huge quantities of such material that amassed during the Cretaceous" y8 R r' S4 f
period; there are many more. Just why the calcareous plankton were so prolific during: Y; Z1 N. O5 n2 P. ]7 a3 K
the latter part of the Cretaceous period is not fully understood. Such massive amounts
% t3 d. D9 c" P% @: ?$ Eof chalky sediments have never since been deposited over a comparable period of time.7 k" s' I5 [, ?* F
The high biological productivity of the Cretaceous oceans also led to ideal conditions4 n) l% `2 j2 C8 I
(20) for oil accumulation. Oil is formed when organic material trapped in sediments is slowly0 U' ]6 Z, K3 h4 ]
buried and subjected to increased temperatures and pressures, transforming it into% ^8 N0 w" c4 ~0 x$ @0 _$ X
petroleum. Sediments rich in organic material accumulated along the margins of the
/ K& h/ s, O% K7 `6 gTethys Seaway, the tropical east-west ocean that formed when Earth's single landmass' i4 v7 _1 ]( x4 U
(known as Pangaea) split apart during the Mesozoic era. Many of today's important oil
6 m8 A ]# e6 u/ `* j, q9 a6 F(25) fields are found in those sediments--in Russia, the Middle East, the Gulf of Mexico, and
: O3 H8 N& O6 l9 E& T' ein the states of Texas and Louisiana in the United States.
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0 n/ a( D K& h2 u$ m$ z; _ J, b/ `7 k V" Y
S! e T, k( b+ S5 G31. What does the passage mainly discuss?, _* ]/ _6 O- |' K- ]1 o+ E
(A) How sediments were built up in oceans during the Cretaceous period- u* m3 G! C7 E5 H, h
(B) How petroleum was formed in the Mesozoic era
: M' m. n% l& y. J% @3 S(C) The impact of changes in oceanic animal and plant life in the Mesozoic era
7 v' u: A6 f5 ^% ~! u& a5 w0 ~(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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