Geology Reference
In-Depth Information
enced by diagenesis, variation of the distribution coef-
ficient and Sr/Ca ratio in seawater, e.g. caused by varia-
tions in hydrothermal activity. Short-term variations are
controlled by sea-level fluctuations.
emphasize some applications in the context of inte-
grated facies analyses.
Current stable isotope analyses of carbonate rocks
aiming for paleoenvironmental or diagenetic interpre-
tations are based on the investigation of micrites, car-
bonate cements or fossils. Stable isotopes in fossils (pre-
dominantly foraminifera, mollusks and corals) are used
in reconstructing growth rates of organisms (Wefer and
Berger 1991), paleoclimatic fluctuations and paleo-
oceanographic circulation patterns. Isotope signals re-
corded in planktonic foraminifera offer excellent pos-
sibilities for recognizing climate cycles, paleoproduc-
tivity and changes in ocean circulation.
Carbonate oxygen isotope paleothermometry offers
a possibility to evaluate paleo-seawater temperatures.
The fractionation of oxygen isotopes is temperature
dependent. The ratio of the two most common isotopes
(
16
O and
18
O) in carbonate fossils can, in principle, be
used to reconstruct the temperatures of ancient oceans.
Cenozoic and Mesozoic foraminifera are commonly
analyzed in order to determine sea-surface, deep-water
and bottom-water temperatures but mollusk and bra-
chiopod shells and carbonate cements are used as well
(see Corfield 1995 for discussion).
Basics: Trace elements in carbonates
Brand, U. and Veizer, J. (1980): Chemical diagenesis of a
multicomponent carbonate system - I. Trace elements. -
Journal of Sedimentary Petrology,
50
, 1219-1236
Carpenter, S.J. and Lohmann, K.C. (1992): Sr/Mg ratios of
modern marine calcite: empirical indicators of ocean
chemistry and precipitation rate. - Geochimica et Cosmo-
chimica Acta,
56
, 1837-1849
Churnet, H.G., Misra, K.C. (1981): Genetic implications of
the trace distribution pattern in the upper Knox carbonate
rocks, Copper Ridge District, East Tennessee. - Sedimen-
tary Geology,
30
, 173-194
Kühl, A., Schreiber, A., Scheffler, E. (1996): Faziesanalyse
mit multivariatstatistischen und geostatistischen Methoden
des Dunklen Knotenkalkes von Wildenfels. - Freiberger
Forschungshefte, C,
462
, 212 pp.
Morse, J.W., Mackenzie, F.T. (1990): Geochemistry of sedi-
mentary carbonates. - 707 pp., Amsterdam (Elsevier)
Popp. B., Anderson, F.T., Sandberg, P.A. (1986): Textural,
elemental and isotopic variations among constituents in
Middle Devonian limestones, North America. - Journal
of Sedimentary Petrology,
56
, 715-727
Renard, M. (1986): Pelagic carbonate chemostratigraphy (Sr,
Mg,
18
O,
13
C). - Marine Micropaleontology,
16
, 117-164
Veizer, J. (1983): Chemical diagenesis of carbonates: theory
and application of trace element techniques. - In: Arthur,
M.A., Anderson, T.F., Kaplan, I.R., Veizer, J., Land, L.S.
(eds.): Stable isotopes in sedimentary geology. - Soc.
Econom. Paleont. Miner., Short Course,
10
, 3.1 - 3.100
Further reading:
K154, K155, K156
Over the past forty years, the
16
O/
18
O record in mi-
cro-organisms preserved in deep-sea sediments has pro-
vided basic data in establishing a history of long-term
oceanic temperature and global ice volume during the
Quaternary. Many investigations are concentrated on
the Cenozoic, but increasingly more studies are deci-
phering the isotopic history of Mesozoic and Paleo-
zoic oceans and carbonates. δ
1
3
C shifts during the Phan-
erozoic and excursions at or near boundaries are be-
lieved to represent changes in the global carbon cycle.
Phanerozoic isotope carbon curves are used for corre-
lations identifying climate variations, determining the
isotopic composition of seawater, and recognizing secu-
lar variations of organic carbon and carbonate organic
production/burial rates. Cenozoic and ancient (particu-
larly Paleozoic) isotope patterns exhibit considerable
differences, possibly due to primary differences in the
isotopic composition, e.g. caused by exchange between
sea water and oceanic ridges; differences in water tem-
peratures; different biological fractionation of ancient
organisms as compared with modern organisms;
changes in productivity, and diagenetic alterations.
13.2.4 Stable Isotopes
Marine and non-marine carbonates have been used ex-
tensively in documenting changes in the ratios of iso-
topic species. Stable isotope analysis is one of the most
powerful tools in deciphering the depositional and di-
agenetic history of carbonate rocks and fossils, and un-
derstanding environmental controls and global changes
of climate and oceanography. The studies are based on
oxygen isotopes (
18
O,
16
O), carbon isotopes (
13
C,
12
C)
and strontium isotopes (
87
Sr,
86
Sr; Elderfield 1987).
Delta values (‰) describe the difference in isotope ra-
tios between samples and standards. Standards are usu-
ally based on CaCO
3
(PDB standard; Pedee Formation
Belemnite) or H
2
O (SMOW standard; Standard Mean
Ocean Water). Measurements are whole rock data or
individual constituent data (crystals, grains) that require
microsampling techniques.
Basic information on the methods and use of stable
isotope analyses can be found in Arthur et al. (1983),
Faure (1986) and Hoefs (1997). The following notes
Well-preserved brachiopods (Wadleigh and Veizer
1992; Wenzel 1997; Samtleben et al. 2000) and car-
bonate cements (Lohmann 1985; Carpenter et al. 1991)
are assumed to be the most suitable phases for deci-
