Ph^nll DjATU-- DIEN LY 19:
CHU KY MILANKOVICH TRONG CAC SO LIEU DO TlT CAM DIA TANG TAI RANH GIOI
T R A MTICK FRASNI-FAMEN
XOM NHA, QUANG BINH
Luu Thj PhuoTig Lan, Nguyen Thanh Dung
Vien Vdt Ly Dig cdu, 18 Hodng Qudc Viet, Cdu Gidy Hd Npi E-Mail: liiuphuonglan@gmail. com
Brooks B. Ellwood
Khoa Dia chdt dia vdt ly Truang Dgi hoc Tdng hap bang Louisiana, Baton Rouge. Louisiana 70803, My
Mail: ellwood'cvlsu.edu
Tom tdt: Chiing tdi da tiin hdnh tinh phd Fourier tren tap hap sd lieu tie cdm dig tdng duac xdc dinh trin cdc mdu ddt dd tgi rgnh gidi Frasni - Famen ndm a cica hang niu Len Dan, Xdm Nha, Qudng Binh. Tdng sd 147 mdu ddt dd duoc Idy tren chiiu ddi 7,3m vd kit qud do dp tie cdm dd duac cdng bd trong cdng trinh [8f. Kit qud cho thdy trin chuoi sd liiu dd thi hien phd chu ky thai tiit Milankovitch E2 -100.000 ndm, 0 ~ 41.000 ndm, vd P -20,000 ndm. Dieu ndy chieng td cdc chu ky vi biin dpng cd khi hau duac phdn dnh trin sd lieu do dp tic cdm. Kit qud ndy hodn todn phii hop vai cdc kit qud nghien cicu vi su biin ddi dp tie cdm trong trdm tich duac tiin hdnh tren thi gidi. Tic kit qua phdn tich FFT vd so sanh vdi mgt cdt chudn vi thai tiet chiing tdi dd tinh duoc khodng thai gian nghiin cicu mat cdt Xdm Nha Id 480 ± 20 nghin ndm.
L Mff dau
Vao nhung nam 30, khi nghien cim ve sy anh huong cua quy dao va sy quay ciia Trai dit quanh Mat tr6i dii voi khi hau va nhiet do tren Trai dit, Milankovitch [10], da diing CO hgc vu try dl tinh toan sy thay doi co chu ky ciia quy dao va sy van hanh ciia Trai dit nhu do nghieng gay ra diem phan (equinox) va chuyin dgng quay tul sai (precession). Ong tim ra dugc su thay d6i ve cirong do anh sang hire xa ciia Mat troi din Trai dit dua theo chu ky xoay tue sai ciia true xoay Trai dit (co 21000 nam), chu ky thay dii quy dao ellipse ciia Trai dat quanh Mat trai (100000 nam) va chu ky thay dii do nghieng ciia true quay Trai dat (co 41000 nam). Suthay dii cuong do birc xa anh sang Mat troi tai Trai dit vi the cung co thay d6i Ion nhit theo cac chu ky 100000 nam.
Chu ky Milankovitch dugc ra doi tir day.
Sy lien quan ciia cac chu ky thai tiet den cac chuyin dgng ciia Trai dit sau do dugc rit nhiiu nlia nghien cim tiln hanh tiep. Gan day Berger [1] da khing dinh va phat triin ly thuylt ciia Milankovitch khi nghien cim sy 6n djnh cua cac tin s l Ion trong lich sir Trai dit lien quan din cac bien d6i c6 khi hau. Cac kit qua nghien cim da cho thiy sy
194 Tuygn tap cac cong trinh nghien cCfu Vat ly Dja cau 2008
phii hgp giua tinh toan ly thuylt va cac chu ky thoi tiet thu dugc tren cac so lieu ciia cac chuoi trim tich trong khoang tuli tir Creta din Trias la 100 nghin nam, 41 nghin nam va 20 nghin nam.
Trong 2 thap ky qua, da c6 nhieu nghien cim rien hanh do do tir cam dia tang tren cac chuoi tram tich. Cac nghien cim [4; 5J cho thay rang duong bien thien ciia do tir cam c6 2 dang dac trung, dang thu nhat lien quan toi cac bien doi khi hau co chu ky ngan, bien do nh6, dugc dimg xet tuong quan khu virc [2, 4]. Dang thir 2 lien quan toi cac bien doi khi hau co chu ky dai, bien do Ion. Tuy nhien, tat ca cac cong trinh truoc day da sii' dung sy bien doi chu ky tren goc do dinh tinh de tinh tuong quan. Cac tac gia da diing dac trung hinh thai bien thien ciia cac chu ky cimg tang len, ciing giam xuong, cimg CO sy chong chap cac chu ky nh6 tren chu ky Ion de tinh tuong quan dia tang. Gan day viec nghien cim nay da tiep can den tinh dinh lugng. Cac tac gia da diing bien doi Fourier de Igc cac chu ky nho va lay ra cac chu ky dai [6, 7] chinh la cac chu ky Milankovitch. Nhom cac nha nghien ciai My da tim ra chu ky Milankovitch nay bang viec phan tich pho Fourier cac so lieu do MS lien tuc tren cac tram tich bien. Nhom nghien cim da tim ra cac chu ky lech tam 100 nghin nam, chu ky nghieng quy dao 41 nghin nam va chu ky tien dgng 21 nghin nam khi nghien cim mau a khoang ranh giai C!enoman - Touron tai mat cat GSSP Pueblo, Colorado, My va o khoang ranh giai Pleistocence - Eocene tai Alamedilla Tay Ban Nha [7]. O Viet Nam, khi nghien cim khoang ranh gioi Pecnii - Trias tai Lung Cam. Ha Giang, Luu Thi Phuong Lan va cs.
ciing da tim ra cac chu ky Milankovitch [9].
Cong trinh nay thong bao ket qua nghien ciru MS tai mat cat ranh gioi Frasni- Famen de cung cap thong tin ve cac chu ky bien dgng co khi hau dugc phan anh tren sl lieu MS tai mat cat X6m Nha.
2. Moi tuong quan giira do tir cam cua tram tich bien va cac chu trinh khi hau Tat ca cac hat khoang deu de tro' thanli bi nhiem tir trong truo'ng tir hien dai, va dp tir cam (MS) la chi so danh gia do nhay cam cua vat chat khi chimg duoc dat tron^ tir truong. Do tir cam trong tram tich bien la ham s6 ciia nong do cac vat lieu tir chira trong no va tl hgp cau tnic ciia no (thanh phan thach hgc, ciu true hat, hoac hiiili dang hgc).
Tram tich bien dugc tao boi nhung thanli phan co nguln glc lyc dia. c6 nguln glc do gio vci nhihig thanli phan cacbonat sinh vat va hoac v6 silixit ciia cac vat chit hiru co trong moi truong bien. Do Ion ciia MS ghi dugc tren cac da la kit qua dong gop ciia cac thanh phin co nguon goc luc dia va sinh vat. Nhung cac thanh phin sinh vat chi la cac chit nghich tir yeu. nen cac thanh phan co nguon glc lyc dia se dong gop chii ylu vao gia tri ciia MS. Do do cac d6ng vat lieu c6 nguon goc lyc dia dua vao moi truancy biln dong vai tr6 rat quan trgng doi voi nhirng bien dii MS. NghTa la nhung chu trinh ciia thai tiet gay x6i mon va van chu>en cac \at lieu tir luc dia toi cung se gay ra nhima biln doi CO tinli chu ky. Gia tri MS se tang cao khi khi hau am va im va khi do tromi nuac bien thuong hinh thanh cac hat tir rat nho nhu magneth, hematit, maghemit la nhCrncr hat c6 t\x tinli manh. Khi mua nhieu cung lam tang viec bao mon luc nguyen la nhuno vat chat CO chira nhieu khoang sat tir lam tirng MS. Khi bang tan nhilu cung lam tarTo su
Phin II: DjA T i r - DIEN LY 195
bao m6n vat chat dua toi lang dgng trong moi truong bien va lam tang gia tri MS. Khi thori tiet kho ban thu6ng di lien voi gia tri MS nh6 vi vat lieu luc nguyen de tich ty tram tich thuong it bon \ a cac hat tir co tir tinh manh ciing it dugc hinh thanh hon.
Sy bien doi vat lieu tram tich lang dgng kem theo cac hat tir tinh nay dugc ghi nhan trong gia tri MS. Co rat nhieu ylu t6 co the lam bien doi nhimg dau hieu MS nay. Tuy cac bien doi ky vgng co the giam vl cu6ng do song nhung dac tnmg co tinh chu ky van dugc giir lai va co the tach ra dugc. ChIng len nhiing bien doi co tinh chu ky la nhiing sy kien co tinh di thuong, nhu la sy phun ciia mil lira hay cac bien d6i ve khi hau [4]. Nhimg tac dgng nay c6 the gay ra cac dau hieu co tinh dia phuong, khu vyc hoac toan cau [4, 8].
3. Ket qua nghien ciiu MS tai xom Nha, Quang Binh
3. 1. Vi tri vd phuong phdp thu thiip mdu
Mat cit da v6i Xom Nha (17°48' 28" B; 105°56' 40" D) {hinh 1) nim tai cira hang da suofn Tay Nam nui da v6i Len Don, Xom Nha, Quang Binh. Cac mau dugc lay tren khoang ranh gioi Frasi - Famen voi khoang each lay mau each deu 5cm. Tai day da thu thap lien tuc 147 mau MS tren chieu dai 7,3ni, gom 24 lop. Chi tiet mat cat Xom Nha dugc mo ta trong van lieu [8].
Hinh 1: So- d l vi tri mat c i t ranh giai F/F Xom Nha tai khu vuc phia Tay tinh Quang Binh [9]
3. 2. Minh gidi ket qua nghien ciru
Toan bo tap hgp mau dugc do tai tru6ng dai hoc ting hgp Louisiana, My. Ca s l lieu tho (duong diit net) \'a so lieu lam tron (duong liln net) diu dugc thi hien tren hinli 2.
s l lieu lam tron da dugc diing de xa\- dung va tinli toan cac d l thi tilp theo. Cac d l tlii dugc trinh ba>' duoi dang ban logarit. true logarit bieu dien gia tri MS va true c6n lai bilu dien chilu cao qu> uoc (so voi mau s6 1). Ca so lieu MS tho va sl lieu MS da kun tron cimg dugc bieu dien tren cac do thj ban jogarit nay. Tren d l thi, xu huong chu ky MS tang hoac tziani qua hai diem so lieu hoiic nhieu diem hon se dugc coi la sy biln dii co tin cay.
Trontz cong trinh [8], dya vao ket qua do MS tai mat cit chuin GSSP La Serre C, cac tac tiia da chia ra cac \-iing tir MS ciia mat cat (hinh 2a). Ranh gioi F/F dugc vach ra
196 Tuyln tap cac cong trinh nghien ci>u Vat ly Dia c l u 2008
tai noi nao c6 su tuyet diet cua Pa. linguiformis (doi chi thi ciia Frassni) va su xuat h i p diu tien ciia Pa. triangularis (doi chi thi cua Famen). Tai ranh gioi nay chimg ta nhan thiy gia tri MS giam dot nggt tir 6 x 10'^ xuIng 4 x 10"^(m^/kg).
a) b)
L a Serre C , P b a p VCiug Iff '•'
: i -
l x .
O , ,
l.0H-(>3
D<> 1 it c4in (xo'/kig)
X d r a a h a , V l t i N a m
•-3L
i.<jEJ)a
D ^ i i r c a m CniVJtg)
1223
IP
1 ^
P r W
^1^3
wm
Hinh 2: K i t qua do iVlS va s u phan chia vung tu'
a) Tai mat c i t La Serre C , Phap b) Tai mat c i t Xom Nha, Viet Nam
O 0,8-
E 2'100,000 nSin,'
••rt 3 0,6
0,:
S'o.o / ,
41,000 nam pr:0,000 nan,'
J 5 6
Tan so (chu kv/ niiin)
\^,W/V^yy/i \ ^
Hinh 3: K i t qua khai t r i l n Fourier tren s l lieu tho ciia mat c i t Xom Nha; Dai quy dao E2 ~ 100 nghin nam va dai quy dao 0 ~ 41nghin nam, Dai quy dao P~ 20 nghin nam.
Cac kit qua do MS tai Xom Nha va ket qua nghien cim co sinh da dugc so sanh vod mat cit chuin GSSP va dan toi dugc phan chia \amg hi nhu trong hinh 2b. Tai mat cat Xom Nha ranh gioi F/F ciing dugc vach ra tai noi co sy hiyet diet ciia Pa. linguiformis (doi chi thi cua Frassni) va co sy xuat hien dau tien ciia Pa, triangularis (doi chi tlii ciia Famen). Tai ranh gioi nay chimg toi ciing nhan thay gia tri MS giam dot nggt mgt bac hi 3 x 10"^ xuong 4x10' (m^/kg).
Dilu nhan thay ro rang Uen ca hinh 2a va 2b la cac chu ky nho nam chong chap len chu
ky Ian: theo be day tich hi ham tich, cac gia hi MS da the hien cac chu ky khac nhau. .,., D I nghien cim cac chu ky Ion, chiing toi da sir dung phuong phap Fourier (FFT) de tach cac chu ky nho ra khoi cac chu ky Ion. Phuong phap Fourier dugc ap dung de xac
Phin II; DjA T i r - DIEN LY 197
dinh dac trung tan so tren tap hgp s l lieu MS da dugc lam tron voi gia dinh khoang each giua Ccic mau la tuyen tinh theo mgt Idioang thai gian, hic la Ax ~ At. Kit qua phan tich Fourier dugc trinh bay tren hinh 3.
Neu gia djnh xung diu tien c6 chu ky 100 nghin nam thi Ian lugt cac xung tiep theo se CO chu ky 41 nghin nam va 20 nghin nam. Day chinh la cac chu ky ciia Milankovitch.
Trong khi phan tich chimg toi da chgn xung thu hai, 0 co gia tri 41 nghin nam lam chuan voi suy nghT chuoi s l lieu tai mat cit Xom Nha chua dii dai de tinh toan thing ke voi tan xuat xuat hien chu ky dai 100 nghin nam (chu ky dau tien) co do chinh xac cao (hinh 3). Nhu vay 24 viing tir thu dugc tren mat cat Xom Nha (hinh 2b) se tuong ung voi 12 chu ky co do dai 40 nghin nam. Tir ket qua nay co the gia thiet la tuoi ciia mat cat nghien ciru Xom Nha da keo dai trong khoang th6i gian la 480 ± 20 nghin nam
Dya tren gia thiet de tinh FFT, khoang each giira cac mau la tuyen tinh theo th6i gian, chiing toi da xay dimg mgt thang chuan ve khi hau tuong img voi cac viing tir thu dugc.
De so sanh tuong quan t6c do tram tich trong mat cat nghien cim, chimg toi da xay dung do thi giira cac viing tir thu dugc tir so lieu do MS voi thang chuan ve khi hau (hinh 5).
Theo hinh nay, true tung the hien cac viing tir thu dugc tai mat cat Xom Nha, c6n true hoanh the hien thang chuan ve khi hau. Tren do thi cho thay ro c6 5 doan khac nhau rieng
~ biet: Doan 1 tir diem dau vimg tir FrT cho den diem dau vimg tir FrX, doan 2 tir diem dau viing tir FrT cho den bat dau viing tir FrZ, doan 3 tir diem dau vimg tir FrZ cho den diem dau viing tir Fa9, doan 4 tir diem dau viing tir Fa9 cho den diem dau viing tir Fa 12, doan 5 til' diem bat dau vimg tir Fal2 cho den cu6i mat cat nghien cim.
Chieu cao in-
Tliajia chuan dioi ti^Jt
Hinh 4: D I thi bieu d i l n s u ling dgng trim tich tai mat c i t Xom Nha so vai thang chuin v l khi hau.
198 Tuyln tap cac cong trinh nghien cu'u Vat ly Dja c^u 2008
Co thi thiy tren 5 doan nay la doan 1, 3 va 5 gin nhu song song voi nhau, con doan 2 va 4 thi die ban 3 doan tren. Dilu nay cho thiy ring toe do tram tich tai doan 1,3 va 5 la nhu nhau, nghia la dilu kien vl khi hau tai 3 giai doan nay tuang doi giong nhau. Tuy nhien tai doan 2 va 4 tic do trim tich tang dot nggt, dieu nay cho phep nghi toi la da co sy biln dgng Ion vl khi hau, ve mua gio hoac bao, cho nen co nhieu vat lieu ban dugc mang vao qua trinh lang dgng lam cho toe do lang dgng tram tich giai doan 2 va 4 nhilu ban 3 giai doan con lai. Day se la tien de de nghien ciru ve van dgng kiln tao khi ta co nhieu so lieu do MS tai ciing khoang thai gian 6 cac khu vyc khac nhau.
4. Ket luan
Ket qua phan tich tap hgp 147 so lieu do tir cam (cat qua 7,3m cua khoang ranh giai Frasni-Famen tai mat cat Xom Nha) cho thay c6 cac dai chu ky khi hau Milankovitch E2 ~ 100 nghin nam (lien quan den chuyen dgng qQy dao), chu ky 41 nghin nam (lien quan den do nghieng cua tryc quay) va chu ky 20 nghin nam lien quan den chuyen dgng tue sai cua true quay Trai dat).
Phan tich nhirng chu ky MS binh on gan voi so lieu 0 - 4 1 nghin nam ciia chuoi phan tich Fourier tai X6m Nha, Quang Binh, cac tac gia da danh gia dugc khoang tuoi cua mat cat nghien cim la trong khoang 480 ± 20 nghin nam.
Lai cdm an: Bdi bdo duac hodn thdnh nha su tdi tra ciia di tdi NCCB ma sd 712006, su giiip da vd tdi tra ciia GS. Nguyen Thi Kim Thoa. Cdc tdc gid xin chdn thdnh cdm an.
5. Tai lieu tham khao
1. Berger, A., Loiitre, M. F., Lasker, J., Stability of the astronomical frequencies over the Earth's history for paleoclimate studies. Science 255, (1992), 560-566.
2. Crick, R. E., B. B. Ellwood, El Hassani, A., Feist, R., and Hladil, J., Magnetosusceptibility event and Cyclostratigraphy (MSEC) of the Eifelian- Givetian GSSP and associated boundary sequences in North Africa and Europe:
Episodes, v. 20, (1997), p. 1.67-175.
3. Crick, R. E., B. B. Ellwood, R. Feist, A. El Hassani, E. Schindler, R. Dreesen, D. J.
Over and C. Girard, Magnetostratigraphy susceptibility of the Frasnian/Famennian boundary. Palaeogeography, Palaeoclimatology, Palaeoecology, 181, (2002), 67-90 4. Ellwood, B. B., Crick, R. E., El Hassani, A., The magnetosusceptibility event and
cyclostratigraphy (MSEC) method used in geological correlation of Devonian rock from Anti-Atlas Morocco; American Association of Petroleum Geologists Bulletin, V. 83,(1999), p. II19-1134;
5. Ellwood, B. B., Crick, R. E., Garcia-AIcalde Fernandez, J., Soto, F., Tryols- Massoni, M.. EI Hass.ani, A., Kovas E., Global correlation using magnetic
yrian II: OIA T i r - DIEN LY I 9 9
susceptibility data from Lower Devonian rocks. Geology; July 2001: v. 29' no. 7;
(2001), p:583-586, 11.
6. Ellwood, B. B., Balsam, W. L,. Roberts, H. H., Gulf of Mexico Sediment Sources and Sediment Transport Trends from Magnetic Susceptibility Measurements of Surface Samples, Marine Geology 230, (2006), 237-248.
7. Ellwood, B. B., Kafafy, A,, Kassab, A., Tomkin, J. H., Abdeldayem, A,, Obaidalla, N., Willson, K., Thompson, D, E,, Magnetostratigraphy Susceptibility Used for High Resolution Correlation Among Paleocene/Eocene Boundary Sequences in Egypt, Spain and the U.S.A. In Modern Stratigraphic techniques: Theories and Case Histories, eds. Ratcliffe. K. and B. Zaitlin, SEPM Special Publicarion, (2008A), in press.
8. Luu Thi Phuong Lan, Ellwood B, B,. Ta Hoa Phuong, Xac dinh ranh gioi F/F tren cac he tang da voi tai Xom Nha, Quang Binh bing phuong phap MSEC, Tap chi Cac Khoa hgc vl Trai dit. Tap 29 (1), (2007), 30-37.
9. Luu Thi Phuong Lan, Ellwood B, B., Nguyen Thanh Dung, Chu ky Milankovich trong cac so lieu do tir cam dia tang tai ranh gioi tram tich Pecmi- Trias Liing Cam Ha Giang, Tap chi Cac Khoa hoc ve Trai dat, (2008). dang in.
10. Wikipedia tieng Viet, chu kv Milankovitch
ABSTRACT
MILANKOVITCH CYCLES FROM MS DATA AT FRASNIAN - FAMENIAN BOUNDARY XOM NHA, QUANG BINH
I-
Authors present the analysis of FFT on the raw MS data from the F/F boundary section of Xom Nha, Quang Binh (147 samples, over 7,3m, with distance each another 5cm). The results show the cycles with follow picks: -100 kyr, -41 kyr and -21 kyr, as Milankovitch cycles (eccentricity -100 kyr (E2), obliquity 0-41 kyr, and precession P -20 kyr). The results allowed to say that MS data sets of Xomnha reflected paleoclimat 'yclic remain in the sediment of section relation with the movement of the earth in the 'un system. From the analysis of FFT we choosen second peak for age assignment '•elding an age range for the Xomnha section, that is - 0.48 myr.
