53. Jg.> Heft 21, 1966 547

Grain Size, Carbonate Content, and Carbonate Mineralogy of Recent Sediments of the Indian Ocean off the Eastern Coast of Somalia

GERMAN ~VIOLLER*

Mineralogisch-Petrographisches Inst i tut der Universit&t, Laboratorium It~r Sedimentforschung, Heidelberg

Recent 7ndian Ocean Sediments off the eastern coast of SomMia decrease progressively in grain size and carbonate content away from the coast. They form zones of various sediment types based on different sand-si l t -day ratios and a different carbonate content. These zones are approximately parallel to the coast.

The sediments' carbonate mineralogy is typical of the different depositional environments: deep-water sediments (globigerina ooze,

* Advanced report to "3Ieteor-Forsehungsberiehte, Indian Ocean Expedition 1964/65", to be published in 1967.

44 ~ 4 60 4 8o

deep-sea clay) are composed nearly exclusively of low-magnesian caIcite, whereas carbonate sands in the shelf area (mainly derived from reef debris) are rich in high-magnesian calcite and aragonite. Dolomite was found by X-ray diffraction analyses in almost half of the samples.

A. Introduction

In a preliminary report SCHOTT and v. STACKELBERG [1] gave a general account on Recent sediments of the

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Gu/ ,4do - " ' , . . . . . / �9 �9 . : -

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-b:;.:'~-- ~.'~ ~--.-'~---'T:'..~..~--- - - - ~ - - - - - " 123 118,124,140

MOGAD i S O H ~ "-"~-~.-. ' L - - - - - ' 2 ~ ' ~ =- A " / k 2! e. . " I . . . . . m ' ~ ' - g~ / " " \ / k ...-"[.~ . . . . . . . 7 ~ . . . . . ~132 -- - 104

]"--'~ --'--[ [----[ ~----" " - - ~ " ~ 139 lSO 121 " - - '~ \ \ 7 5 / 2 5

2' - - - ~ = -_-- , - - - - - s ~ _ ' ~ _ ~ , ~ 3 ~ \ \ \ \ so,qo m zo

i ~ . : 151__~'~- 0,063- 0,002 a m 132110g~ '~ OzO0 2 a m ~ , c ' _ [

H; [ , V - ' , 4.40 460 48':' 50 ~ 520 540 560

Fig. 1. Sand-silt-clay ratio of Recent Indian Ocean sediments off the eastern coast of Somalia

548 C-. M O L L E R : G r a i n S ize , C a r b o n a t e C o n t e n t , a n d C a r b o n a t e M i n e r a l o g y Naturwissemcha#en

Indian Ocean off East Africa. They distinguished five different facies that run parallel to the coast: a) Biogenic calcareous sand, consisting chiefly of reef debris, covering mainly the shelf of the East African coast, but was also found on the continental slope in a depth of 650 m. It contains varying proportions of non-carbonates.

e) Deep-sea clay. Reddish brown clay with a varying content of pelagic foraminifera makes up the sedi- ment in greater depths of the Somali Basin, between 4130 and 5050 m.

This report is concerned with grain size distribution, carbonate content, and carbonate mineralogy of the Recent sediments off the Eastern coast of Somalia,

440 460 48 ~ 500 520 540

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\'~\ rbonate content(~176

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~ ~~"~4-~- - ~ 90-75 - - 7+-+o

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Fig. 2. Carbonate content of Recent Indian Ocean sediments off the eastern coast of Somalia

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b) Olive-gray mud is present in the upper part of the continental slope, in depths between 540 and 980 m. It contains varying amounts of pelagic foraminifera and locally grades into the foraminiferal sand c). c) Foraminiferal sand appears in depths between 760 and t710m on the continental slope, east of Mom- bassa and Abbia, as well as south of Sokotra Island. The main constituents are pelagic foraminifera of the family globigerinidae. d) Globigerina ooze is present at the rise of the con- tinental slope, in depths between t 670 and 46t 0 m.

covering about 80 percent of the area investigated b y SCI{OTT and v. STACKELBERG. The samples taken with the REIZX!ECK box-sampler, the van Veen grab, and a piston corer were made available to us by the courtesy of Dr. H. E. Reineck, Wilhelmshaven. After desalting the samples were dried, sieved and the fraction < 0.063 mm (silt and clay) was sedimented in Atterberg-cylinders. The total carbonate content of the sediment was determined as the portion of Ca, Mg, and Sr (cal- culated as CaCQ, MgCQ, and SrCQ) soluble in

58. Jg., He/t 21, 1966 G. MOLLXR: Grain Size, Carbonate Content, and Carbonate Mineralogy 549

diluted HC1. I t was determined by titration, or, in the case of Sr, by atomic absorption spectroscopy. The relative amounts of the carbonate minerals low- magnesian calcite, high-magnesian calcite, and arago- nite were investigated by quantitative X-ray diffrac- tion analyses. The amount of dolomite and the MgCO a- content of low-magnesian and high-magnesian calcite were also determined by X-ray diffraction methods.

44 ~ 460 48 ~

"ADEN

12~ _ G u l f o f A d e n

The areal distribution of sediment types with a similar sand-silt-clay ratio is shown in Fig. t. According to this ratio the following sediment types (for nomen- clature see )/If#LLER [2]) c a n be distinguished: 1. sand 2. silty sand and silt-sand 3. sand-silt and sandy silt, the latter partly clayey 4. silt and clayey silt.

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.ow-M g Ca lc i te 105~04~2a(I other stations

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0 ~

I~ i . . . . . , , , , , , i . . . . .

A r a g o n i t e

440 480 48 ~ 50 o 520 Fig. 3. Carbonate Mineralogy of Recent Indian Ocean sediments off the eastern coast of Somalia

i / __ il "l'!l! i~ -2~ High-Mg Catci te

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Since dolomite contents of less than 2--3 percent in a sample cannot be detected by XRD, the presence or absence of dolomite traces cannot be ascertained.

B. Grai~ Size (Fig. t)

A detailed account of the results obtained from grain size analyses will be presented in a final report. Only the ratios of the fractions < 0 . 0 0 2 m m (clay), 0.002--0.063 mm (silt), and >0.063 mm (sand) are discussed here.

43b Naturwissenschaften t966

These sediment types form zones more or less parallel to the coast, whereby types (3) and (4) make up by far the greatest part of the investigated area. Type (3) covers a larger portion of the southern part of the region under study. It is notable, that even far away from the coast small amounts of sand occur in very fine-grained sediments. The sediment types cannot be directly equated with the facies types of SCttOTT and v. STACKELBERG. Generally, however, deep-sea clay and globigerina ooze can be related with the sediment types (4) and

550 Kurze Originalmitteilungen Nat~vwissensch~z/Xet*

(3), biogenic calcareous sand with t y p e (1), and fora- minifera l sand with type (~) or (2). The o l ive-gray m u d belongs to t ype (2) or (3) - - a c c o r d i n g to i ts sand port ion. The average grain size (median) decreases from abou t 0 A - - 0 . 2 m m to 0 . 0 0 2 - - 0 . 0 0 4 m m between t y p e (1) and (4). The fract ion 0.002 9.0063 m m (fine silt) makes up the bulk (30--50 percent) of the globiger ina ooze and the deep-sea clay. Sor t ing is best in the sands and poores t in the c layey silts.

C. Total Carbonate Content

Carbonate contents v a r y from 20.9 to t 00 percent . They decrease away from the coast and are lower in the southern pa r t of the area under s t u d y : terr igenous de t r i tus becomes more p rominen t in this area. The ca rbona te conten t of the sect ion south of Mogadischo is a lways less than 70 percent . A correla t ion of do- mains hav ing s imilar ca rbona te conten t wi th different g ranu lomet r ic or facies t ypes is impossible. However , the region wi th a ca rbona te content below 50 percent corresponds more or less to the deep-sea c lay region of SCHOTT and v. ~TACKELBERG.

D. Carbonate Mi~eralogy

Low-magnes ian calcite, h igh-magnes ian calcite, ara- gonite, and subord ina te dolomite make up the to ta l carbonate content . The low-magnesian Calcite contains 0 - -2 .2 mol per- cent MgCOa, the h igh-magnes ian calcite t 1 - - t 4 mol percent MgCO~ in solid solut ion. The re la t ive propor t ions of the i m p o r t a n t carbonates are p lo t t ed in a t e rna ry d iag ram (Fig. 3). I t is evident , t ha t h igh-magnes ian calci te and aragoni te decrease away from the coast wi th a s imul taneous increase in

low-magnesian calcite. This regular, progressive in- crease of low-magnesian calci te re la t ive to high- magnes ian calci te t oward the deeper par t s of the ocean is also shown b y chemical analyses which will be discussed in more de ta i l elsewhere. The domains of sediments wi th a s imilar ca rbona te minera logy (Fig. 3) are cor re la ted with the facies types of SCHOTT and v. STACKELBERG as follows: The region with less than 25 percent of low-magnesian calcite corresponds to the biogenic calcareous sand, t ha t of 25- -50 percent of low-magnesian calci te more or less to the facies of foraminiferal sand and the olive- g r ay mud, and t ha t of more than 75 percent of low- magnes ian calci te to the facies of gtobigerina ooze and of deep-sea clay. There is, therefore, a close re la t ion be tween the de- pos i t ional env i ronment and ca rbona te minera logy (also see FRIEDMAU [31): deep-sea sediments conta in ma in ly low-magnesian calcite, forming the tests of pelagic foraminifera ; shallow water sediments show p r e d o m i n a n t l y h igh-magnes ian calci te which is pro- duced ma in ly b y calcareous algae and other reef- bu i ld ing organisms. Foramin i fe ra l sand and ol ive-gray mud pa r t l y occur in the nar row t rans i t iona l zone be tween sediments rich or poor in low-magnesian calcite. Dolomite was de tec ted in about half of the samples. I t is most p rominen t a t s ta t ion 1 t t ( 5 - - 7 p e r c e n t approx. ) , and a t s ta t ions 1t2, t29, and t 4 t . I t is, therefore, not res t r ic ted to a specific g ranulomet r ic or facies type. I t s de t r i t a l origin is qui te cer ta in - - at least in the coarse-grained sediments .

[1] SC~tOTT, W., u. U. yon STACKELBERG: Erd~t u. Kohie 18, 945 (~965). -- [~] M ijLLER, G.: Neues Jahrb. Mineral., Monatsh. 1961, 148.- [8] FRIED~AN, G.M.: Geol. Soc. America Bull., 76, ~91

(1965).

Eingegangen am 27. Mat 1966

Kurze Originalmitteilungen Fiir die IKurzen Originalmitteilungen sind ausschlieBlich die Verfasser verantwortlich

Bestimmung der Aktivierungsenergie beim thermischen Abbau yon Kristallhydraten aus thermogravimetrischen Daten

I~LAUS HEIDE

Mineraiogisches Instilut der Universit.Xt, Jena

Zur 13estimmung der Aktivierungsenergie E + und der IReak- tionsordnung ~e beim thermischen Abbau yon Kristallhydraten wurde die yon FREEMAN und CARROLL [f] angegebene dyna- mische Methode verwendet. Nach Schaffung bestimmter Ver- suchsbedingungen konnten aus den thermogravimetrischen Analysen die zur graphischen Bestimmung notwendigen A (T-l) - und ZJg-Werte bestimmt werden (T = Temperatur, g = Gewichtsverlust). Die Thermogramme wurden mit dem Derivatographen aufgenommen. Auf besondere Probleme bei der Auswertung kann in diesem Zusammenhang nicht n~iher eingegangen werden. Erprobt wurde das Verfahren dutch Bestimmung der Aktivie- rungsenergie bet dem thermischen Zerfall des CaCO 8. Der ermittelte Weft yon 49,3 kcal/mol steht in guter (Jbereinstim-

mung mit Angaben in der Literatur (z. B. 51,7 kcal/mol bet [2]). Der dynamische thermische Abbau verschiedener Hydrate wurde fri~her eingehend untersucht [3]. Fi~r einige besonders interessante Reaktionen konnten die Aktivierungsenergien und iReaktionsordnungen bestimmt werden (T R = Reaktions- temperatur) :

~. MgSO4.H~O--~MgSO ~ +H~O Kieseritstruktur, E + und ~r wurden aus zwei unabh~ingigen Thermo- kurven in einem Diagramrn berechnet. Fundorte: a wasserklare Kristalle: Bartensleben. b feink/brnige Probe: Gebra-Lohra.

T~ = 364--438~ C; E+ =67,9 kcal/mol; n=0,5 2. MgSOc H,O--~MgSO~ q- H~O Hergestellt dutch thermische Zersetzung von MgSO4-7H20 ,,Mono- hydrat".

T/z =255--294 ~ C; E + = 14,2 kcal/inoI; ~r = 0,7 3. K2Mg(SO~)~4H~O--~K2Mg(SO~)g'2HeO +2H20 Leonitstruktur, gelbliche Krisiatte, Merkers/Rh~bn.

Tg = 140o C; E§ = 54,2 keal/mol; ~ = 0,6 bzw. 1,5 4. K2Mg(SO4)~'2H~O--+K2Mg(SO4)~ q-2H20