3 1 ( 1 ) , / / - / 6 Tap chi CAC KHOA HOC VE TRAI DAT 3-2009
CHU KY MILANKOVICH TRONG CAC SO LIEU DO TIT CAM DIA TANG TAI RANH GIpI TRAM TICH PERMI - TRIAS
LtJNG CAM ( H A GIANG)
LUU THI PHUONG LAN, BROOKS ELLWOOD, NGUYEN THANH DUNG
I. MO DAU
Chu ky Milankovitch l i tip hpp nhirng sir kien phan inh sir bie'n ddi thdi tie't theo chuyen ddng cua Trai Di't, bao gdm : dd lech tam, sir nghieng cua true va su tien ddng cua bien thien quy dao Trai Di't, dupe ky su din dung ngudi Serbi Milankovitch nghien ciru tir nhirng nam 1930. Su lien quan cua cic chu ky thdi tie't de'n cac chuye'n ddng ciia Trii Di't sau dd dupe ri't nhidu nhi nghien ciru tie'n hinh tie'p. Gin diy Berger | 2 | da khing dinh va phat trien ly thuyet cua Milankovitch khi nghien ciru su dn dinh cua cic tin sd Idn trong lich siif Trii Di't cho cic nghien ciru cd khi hiu. Cic ke't qua nghien ciru cho thi'y su phil hpp giira tinh toan ly thuye't vi cic chu ky thdi tiet thu dupe tren cic sd lieu cHa cac chudi trim tich trong khoang tudi tir Creta den Trias la 100 nghin nim, 41 nghin nam va 21 nghin nim.
Tie'n hanh phin tich phd Fourier cic sd lieu do dd tir cam, nhdm nghien ciru ngudi My da tim ra cic chu ky lech tim 100 nghin nam, chu ky nghieng quy dao41 nghin nam va chu ky tien ddng 21 nghin nim khi nghien ciru miu d khoang ranh gidi Cenoman - Touron tai mat cit GSSP Pueblo, Colorado, My vi d khoang ranh gidi Pleistocence - Eocene tai Ala- medilla, Tay Ban Nha|121.
Trong hai thap ky qua, mdt nhdm tic gia ngudi My dii tie'n hinh nhieu nghien ciru dd tir cam dia ting tren cic chudi trim tich. Cic nghien ciru |5, 6, 9| dii chi ra dudng bie'n thien cua dd tir cam cd hai dang die trung : dang thir nhi't lien quan tdi cac bie'n ddi khi hiu cd chu ky ngan, bien dd nhd, dupe diing de' xet tuong quan khu vuc | 3 , 5 | ; dang thir hai lien quan tdi cic bien ddi khi hiu cd chu ky dai, bien dd Idn. Tuy nhien, ti't ca cac cdng trinh trudc diy da sir dung su bie'n ddi chu ky tren gdc dd dinh tinh de' linh tuctng quan. Cac tie gia da dung die trung hinh thai bie'n thien cua cic chu ky : tang.
giam, sir chdng chip cac chu ky nhd tren chu ky Idn de tinh tuong quan dia ting. Gin day, viec nghien ciru niy da tie'p cin de'n tinh dinh lupng. Cic nhi nghien ciru da dting bie'n ddi Fourier de Ipe cic chu ky nhd va li'y ra cic chu ky dai |9, 12|. dd chinh la cac chu ky cHa Milankovitch,
Cdng trinh niy thdng bio ket qua nghien ciru dd tir cam tai mat cat ranh gidi Permi - Trias d Liing Cam, Ha Giang nham cung cip thdng tin dinh gia v6 cac chu ky bie'n ddng cd khi hiu dupe phan anh tren sd lieu do dd tir cam tai Viet Nam.
II. CO SO PHUONG P H A P NGHIEN CLTU /. Do tu cam
Dd tir cam (Magnetic Suscepbitility-MS) la tham sd dinh gii dd nhiy cam cila vQt chat khi chiing dupe dat trong tir trudng. Dd tir cam trong trim tich bie'n la mdt him sd phu thudc ndng dd cac vat lieu tir chira trong nd vi td hpp cic ye'u td ci'u true cua nd (thanh phin thach hpc, ci'u triic hat, hoac hinh dang ci'u triic).
Trong trudng tir thi'p, MS dupe xac dinh bdi ty sd cila moment tir cam M; vi bien dd cua trudng Hj (cudng dp thi'p) dat trong trudng tir dd.
Mi = Xij Hj (khd'i lupng rieng) Hoac Mi = kij HJ (the tich rieng)
Trong cic bieu thirc tren, MS do trong he SI dupe bie'u hien bang tham sd k, khi phep do dupe gin vdi the tich 1 met khd'i (m ) va la mdt dai luong khdng thir nguyen. MS se bie'u hien bing tham sd X, khi phep do gin vdi khd'i lupng 1 kg vi cd don vi la m'Acg. Ca hai tham sd k va x <leu li dai lupng bit ding hudng.
2. Suhinh thanh MS trong qua trinh tram tich bien
Trim tich bie'n tao bdi nhirng thinh phan cd ngudn gd'c Iuc dia, cd ngudn gd'c do gid v i nhirng thanh phin carbonat sinh vit hoac vd silic cila cic vit chi't htru co trong mdi trudng bien. Dd Idn ciia MS la do su ddng gdp cua cic thinh phin cd ngudn gd'c Iuc dia vi sinh vat, song cic thanh phin sinh vat chi la cac chat nghich tir ye'u, nen cic thinh phin cd ngudn gd'c Iuc dia la chil ye'u, Vi vay, cac ddng vat lieu cd ngudn gd'c Iuc dia di vio mdi trudng bie'n se quyet dinh nhung bien ddi ciia MS. Ne'u qui trinh dd cd tinh chu ky thi ciing se giy ra nhirng bie'n ddi cd tinh chu ky trong cac tram tich va dupe phan inh trong gii tri MS. Cd ri't nhieu ye'u td cd the lim bieh ddi nhirng diu hieu MS nay, cd the' giam ve cudng dd MS song nhirng die trung cd tinh chu ky vin dupe giir lai v i cd the tich ra. Chdng len nhtrng bie'n ddi cd tinh chu ky l i nhti'ng su kien cd tinh dj thudng, nhu hoat ddng cua nui lira hay cic tic ddng ve khi tupng. Nhung tic ddng nay cd the giy ra cic diu hieu cd tinh dia phuong, khu vuc hoac toan ciu | 3 | .
Trong qui trinh tao di, MS cila cic trim tich bien bi suy giam do cac hat sit tir chiem uu the trong cic trim tich chua thinh d i bi bien ddi thanh pha thuin tir trong sudt qui trinh chon vtii vi tao thinh di, chil ye'u do cac hoat ddng khu sunfat cua cac vi khuin tir. Qui trinh nay chuyen hda rit nhieu sit thinh pha thuin tir nhu sunfat sit vi carbonat. Tuy nhien, qui trinh nay khdng loai bd sit khdi di ma tdng luong sit trong d i vin dupe giii nguyen. Nhirng pha mdi niy sau dd ddng gdp ciing \a\ cac thinh phin thuin tir do vit chit bd rdi khac nhu set vi cic thinh phin sat cdn lai hoac tii sinh tao thanh MS trong trim tich tao da 110|. Qui trinh tao d i se lim bie'n ddi mot vii thinh phin thuin tir vdn la san phim cua cac trim tich bie'n cd ngudn gdc Iuc dia,
Trong qui trinh tao thinh d i sau ciing, MS trong hiu het cic da trim tich phan anh qua trinh vat ly trong cac thanh phin bd rdi do gid mang tdi cic be trim tich. Nhu nhirng quan sit tren trim tich trong mdi trudng chua thanh da, cac qui trinh thdi tiet, nui Ilia, kie'n tao, sir bie'n ddi muc nude bie'n vi sir cin bang ding tinh cung ghi nhan dupe trong mdi trudng trim tich tao da tai dia phuong v i khu vuc.
3. Su lien quan ciia chu ky thdi tiet voi nhirng bien thien MS khac
Ke't qua phin tich MS trong hiu he't cac nghien cuu tren di trim tich bien chi ra ri't nhieu mire chu
ky thdi tie't khac nhau. Cd mdt sd chu ky ri't dai dupe minh giai nhu la ke't qua cHa qua trinh bien tie'n va bie'n thoii do su thing giing cua mUc nude bien lien quan vdi c i c thdi ky bang ha. Nhti'ng loai chu ky khic ngin hon ri't nhieu dupe cho li ke't qua cila cac qua trinh bie'n ddi thdi tie't |6, 7|. Ke't qua nghien ciru v l chu ky MS tren cap d i vdi v i set vdi tai doan ranh gidi Dani - Selandi (paleocen) d Zuma- ia. Tiy Ban Nha dupe minh giai la ke't qua cua cic bie'n thien thdi tie't. Chu ky thdi tie't d diy do chuye'n ddng tie'n ddng 1 9 - 2 3 nghin nim.
Nam chdng len nhirng chu ky nay li nhirng xung MS ri't ndi trdi ; chiing thudng dupe giai thich bing sir ngip lut toan bd be mat v i bang ddng cic manh vun cha'y manh bit thin di vao cic bon trim tich. Khi bien dp MS rdng hay thay ddi ddt ngdt (the hien thdng qua nhieu diem sd lieu) thi dupe ly giai bing cic qui trinh dia chi't giy nen su bie'n ddi cin ban cila cic trim tich vinh bien. Vi du, sir bien ddi ve hinh dang tir MS thi'p trong Permi de'n MS cao trong Trias 113| da dupe giai thich bing su chiy dpt ngdt cac vat lieu manh vun vao cic bdn trim tich dai duong. la mdi trudng cd nhieu quin the thuc vat bie'n vao thdi gian niy 11 j. Mdt xung MS tuong tu da dupe gin vdi ranh gidi K-T (Creta - De Tam) va dupe ly giai bdi sir bie'n ddng cua thdi tie't vao thdi ky d d | 8 | .
Sir thay ddi khi hiu toan ciu cd mdt sd tic d6ng len cic chudi sd lieu do d6 tir cam. Khi khi hau a'm va am trong nude bie'n thudng hinh thinh cic hat tir ri't nhd nhu maghemit, magnetit, hematite vi gregrit.
Ngoai ra khi mua nhieu cung lim tang viec bao mdn vat lieu Iuc nguyen v i tang MS. Hien tupng bing tan nhieu din tdi qua trinh bie'n tie'n v i cung lai lim ting su bio mdn vit chit va do dd lam ting MS.
Cic nghien ciiu tren diy cho thi'y, nhii:ng sir kien thay ddi thdi tie't trong qua khir dupe the' hien tren .sd lieu do dd tir cam. Vi vay, nghien ciru dd tir cam cila vat cha't trong mdi trudng cd kha nang cho thdng tin ve cac su kien bie'n ddng thdi tie't.
III. KET QUA NGHIEN CUU MS TAI LUNG CAM VA MINH GlAl 1. Vi tri va phuong phap thu thap mJu Die'm la'y miu Lung Cim {hinh I) nam d ven qude Id 4C, cich thi tri'n Phd Bang (huyen Ddng Vin) khoang 4 km ve phia ddng nam cd toa dd dia ly 23°14'11" B, 10°13'02" D. Tai mat cit. cic miu
12
Hinh I.
So dd vi tri lay miu
CHUGAI:
ggSS:; Da voi -k Vi tri lay miu
li'y tren khoang ranh gidi Permi-Trias, khoang cich la'y miu cich deu 5 cm. Tai diy da thu thip lien tuc 101 miu MSEC tren chieu dai 5 m, gdm 30 lap. Chi tiet mat cit Liing Cim dupe md ta trong (3|.
2. Ket qua nghien cuu MS va ban luan Toin b6 tip hpp miu dupe do tai trudng dai hpc tdng hpp Louisiana (My). Ca sd lieu thd (dudng dirt net) va .sd'Iieu lim tron (dudng lien net) diu dupe the hien tren hinh 2a. So lieu lam tron diing de tinh toan va xiy dung cac dd thi tiep theo. Cac dd thi trinh bay dudi dang bin logarit, true logarit bie'u dien gia trj MS va true cdn lai bie'u dien chieu cao quy udc (so vdi miu sd 1), Ca sd lieu MS thd va sd lieu MS da lam tron ciing dupe bie'u dien tren cic dd thi bin logarit nay. Tren dd thi, xu hudng chu ky MS ting hoac giam qua hai die'm sd lieu hoac nhieu die'm hon se dupe coi li su bie'n ddi cd dd tin ciy, Dieu nhin thi'y rd rang tren hinh 2a li cac chu ky nhd nam chdng chip len chu ky Idn. Gii tri MS cd su nha'y bac rd rang khi di qua Idp 12 va 13.
Dudi Idp 11 cic gii tri MS deu bie'n ddi trong khoang lO'^ (m'/kg), cdn khi di qua Idp 12 gii tri MS ting len *10"(m'/kg).
Tren hinh 2a cic chu ky nhd dao ddng tren nen eac chu ky Idn. Ro ring li. theo be diy tich tu trim tich. cac gia tri tit cam da the' hien cic chu ky khic nhau. De lim rd su bie'n thien cila cic chu ky chiing Idi da chon cong thirc :
6MS ( M S - M S , h u i n ) / M S c h u i n
trong dd : MSchuan = 5,5 x 10' m/kg.
Gia tri MS sau bie'n ddi dupe the hien trong hinh 2h. Tren hinh ve cic chu ky Idn da the' hien ro net hon va bam sat theo cic chu ky Idn nay li sir dao ddng cila cic chu ky nhd.
De'nghienciru cac chu ky Idn, dii sir dung phuong phip Fourier de' tich cic chu ky nhd ra khdi cac chu ky Idn.
Trong cdng trinh nay chiing tdi ap dung phuong phip Fourier de xic dinh die trung tin sd tren tap hop sd lieu MS da dupe lam tron vdi gia dinh khoang cich gitra cic miu la tuyen tinh theo mdt khoang thdi gian, tire li Ax ~ At. Ke't qua phin tich Fourier trinh biy tren hinh 3.
Ne'u gia dinh xung diu tien cd chu ky 400 nghin nam thi lin lupt cic xung tie'p theo se cd chu ky 100 nghin nam, 41 nghin nam vi 21 nghin nam. Diy chinh li cac chu ky cila Milankovitch. Trong khi phin tich chiing tdi da chon xung thir hai, E2, cd gii tri 100 nghin nim (viing td dim dinh E2 cd 1,2 chu ky/m, hinh 3) bdi le chudi sd lieu miu MS tai mat cit Liing Cim chua dii dii cho tinh toin thd'ng ke tin xui't xui't hien chu ky cd dd chinh xic cao.
De' tinh tudi cila mat cit nghien ciru. cic tic gia da phin chia sd lieu MS thanh cic vting tir MSZ.
Viec phin chia viing tir nay dua tren co sd phin chia viing tir MSZ cila mat cit chuin GSSP Meishan. Cac viing ttr niy segidng nhau khi cd ciing xu hudng MS gid'ng nhau v i sir xui't hien cac gid'ng loai cd sinh nhu nhau. Ke't qua cho thi'y : khoang ranh gidi Pemii
1,0E-09 1.0E-08 Do ti/cam (m''/kg)
1 .OE-07 (1,2) (1,0) (0,8) (0,6) SMS
(0,4)
Hinh 2. Ke't qua do dd tiir cam tai mat cit Lung Cim
a) Dd thi sd lieu do MS nguyen thuy, b) Dd thi sd lieu do MS da dupe chuin hoa
(0.2)
<- Hinh 3.
Ke't qua khai trie'n Fourier tren sd lieu thd cua mat cit
Liing cim Dai qiiy dao El ~ 400 nghin nim va dai quy dao
E2 ~ 100 nghin nam
4 5 6 7 Tan sd (chu ky/m)
10
14
- Trias tai mat cit Lung Cim, H i Giang cd 11 vung tir irng vdi 5,5 chu ky. Cic viing tir nay dupe the hien tren hinh 4. Vi chu ky chpn de tinh trong phin
tich Fourier cd gia tri 100 nghin nim nen chieu dai mat cit tuong ung vdi 5.5 chu ky li: 550 ±50 nghin nam.
(1.2) (1.0) (0,8) (0,6) 6MS
(0,4) (0,2)
Hinh 4.
Ke't qua phin tich vting tir cam MS tren mat cat Lung Cim
KET LUAN
Ke't qua phin tich tip hop 100 sd lieu thd ve tir cam thu dupe tai ranh gidi Permi - Trias cua mat cat Lung Cim (Ha Giang) da the' hien rd cic dai chu ky thdi tie't Milankovitch : El ~ 400 nghin nim, va E2 ~ 100 nghin nim (lien quan deh ehuyeh ddng qiiy dao), chu ky 41 nghin nim (lien quan deh d6 nghieng cila true quay) va chu ky 21 nghin nam lien quan deh chuye'n d6ng tieh d6ng).
Tir nhirng chu ky MS binh dn dupe gin vdi .sd lieu E2 ~ 100 nghin nim cila chudi phin tich Fourier, da dinh gia dupe thdi khoang cua mat cit nghien ciru la 550 ±50 nghin nam.
Ldi cam dn : bai b i o dupe hoan thinh nhd su tai tro cila de tii NCCB ma sd 712006,
TAI LIEU DAN
111 T.J. ALGEO. B.B ELLWOOD, N.T.K. THOA H.
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| 2 | A, BERGER, M.F, LOLTRE, J. LASKER, 1992 ; Stability of the astronomical frequencies over the Earth's history for paleoclimate studies. Science 255, 560-566.
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| 6 | B.B. ELLWOOD, RE. CRICK, J. GARCIA-AL- CALDE FERNANDEZ, F. SOTO, M. TRTOLS-MASSONl, A. EL HASSAN], E. KOVAS, 2001 : Global correlation using magnetic susceptibility data from Lower Devo- nian rocks. Geology, July 2001 ; V. 29, 7, 583-586.
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[81 B.B. ELLWOOD, W.D. MACDONALD, C.
WHEELER, S.L BENOIST, 2003 : The K-T Boundary in Oman: Identified Using Magnetic Susceptibility Field Measurements with Geochemical Confirma- tion, Earth Planetary Science Letters 206, 529-540.
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1101 B.B, ELLWOOD, 2007 : Low-Field magnetic susceptibility (MS) of sediments and sedimentary rocks : A general overview of active research areas.
Encyclopedia of Geomagnetism and Paleomagne- tism, Springer, 566-572.
[Ill B.B. ELLWOOD, J.H, TOMKIN, KT, RATCLIF- FE, M. WRIGHT, A.M. KAFAFY, 2008 : High resolution Magnetic Susceptibility and Geochemistry for the Cenomanian/Turonian Boundary GSSP with Correla- tion to Time Equivalent Core. Palaeogeography, Palaeoclimatology, Palaeoecology doi : 10.1016/].
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[14| LUU THI PHUONG LAN, B.B. ELLWOOD, DOAN NHAT TRUING, DANG TRAN HUVEN, 2007 : Khoang ranh gidi Permi-Trias tai mat cit Liing Cam (Hi Giang) : so sinh vdi mat cat Mei.shan, Trung Qude. bang tip hop sd lieu tir cam dia ting. Tuyeh tip H6i nghj Dja vat ly toin qude 2007, 86-96.
[ 151 http://vi.Wikipedia.org/ wiki/Milankovitch - cycles : Chu ky Milankovitch.
SUMMARY
Milankovitch cycles from MS data at Permian - Triassic boundary at Lungcam, Hagiang The paper presents analysis results on the raw MS data from the P/T boundary section of Lungcam, Ha-giang (101 samples, with distance - 5 cm interval).
The results show somes cycles. The picks are : -400 kyr, -100 kyr, -41 kyr and -21 kyr, indicating Milanko- vitch cycles (essentricity-400 kyr (El), -100 kyr(E2), obliquity 0-41 kyr, and precession P -21 kyr). The conclusion is MS data sets of Lungcam sediments agreed with weather variation that were associated with movements of the Earth in the Solar system.
Ngdy nhan bdi : 04-8-2008 • ' Vien Vdt ly Dia cdu. Ha Noi, Viet Nam.
Trudng Dai hpc tdng hap Bang Louisiana, My